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# Open Chemistry

### formerly Central European Journal of Chemistry

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Volume 15, Issue 1

# Recent Microextraction Techniques for Determination and Chemical Speciation of Selenium

Ahmed S. A. Ibrahim
• Corresponding author
• Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O.Box. 80207 Jeddah, Kingdom of Saudi Arabia
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• Other articles by this author:
• Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O.Box. 80207 Jeddah, Kingdom of Saudi Arabia
• Other articles by this author:
/ Usama Hawas
• Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O.Box. 80207 Jeddah, Kingdom of Saudi Arabia
• Other articles by this author:
/ Yasser Shaban
• Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, P.O.Box. 80207 Jeddah, Kingdom of Saudi Arabia
• Other articles by this author:
Published Online: 2017-05-25 | DOI: https://doi.org/10.1515/chem-2017-0013

## Abstract

Research designed to improve extraction has led to the development of microextraction techniques (ME), which involve simple, low cost, and effective preconcentrationof analytes in various matrices. This review is concerned with the principles and theoretical background of ME, as well as the development of applications for selenium analysis during the period from 2008 to 2016. Among all ME, dispersive liquid-liquid microextraction was found to be most favorable for selenium. On the other hand, atomic absorption spectrometry was the most frequently used instrumentation. Selenium ME have rarely been coupled to spectrophotometry and X-ray spectrophotometry methods, and there is no published application of ME with electrochemical techniques. We strongly support the idea of using a double preconcentration process, which consists of microextraction prior to preconcentration, followed by selenium determination using cathodic stripping voltammetry (ME-CSV). More attention should focus on the development of accurate, precise, and green methods for selenium analysis.

## 1 Introduction

Sample treatment is the most important step prior to analysis with a suitable instrument in order to attain optimal sensitivity, precision, accuracy, and detection limits for a targeted analyte. Among the most common preconcentration and separation techniques, microextraction (ME) techniques promise relatively high performance in the field of analytical chemistry. Recently developed ME methods have been applied to a variety of purposes and are intended to produce high-quality results and high enrichment factors using simple, automated, and inexpensive methods. At the same time, they follow principles of green chemistry in that they use very small amounts (microliter volumes) of organic solvents [1]. Twenty-six years ago, microextraction techniques were established. One form of ME, liquid phase microextraction (LPME), was introduced by Rezaee et al. [2], and a second form, solid phase microextraction (SPME), was introduced by Arthur and Pawliszyn [3]. Within these two classifications of ME, there are many subclasses of techniques with different modes and equipment used for preconcentration and separation [1].

## 2.1 Theoretical principles

SPME is based on the partitioning of an analyte between a solution phase and a fiber with a relatively large surface area in the microscale range. The concept is based on the distribution of that analyte between the two components by two mechanisms: partitioning between the aqueous phase and the fiber and adsorption of the analyte on the fiber surface. For both mechanisms, mathematical equations have been derived as illustrated in equation (1), where D is the distribution constant, Kd is the distribution coefficient, V2 is the volume of analyte in solution, and V1 is the volume of analyte in the fiber. $D=Kd∗V2/V1$(1)

The volume of analyte from this preconcentration step depends on the distribution constant. Therefore, the fraction of the extracted analyte Fex can be calculated using equation (2), where Fex represents the extracted fraction. $Fex=D/(1+D)=KdV2/(KdV2+V1)$(2)

## 2.2 Components Affecting SPME

For any ME technique, many factors can affect the extraction process. To evaluate feasibility and optimize the process, all factors should be investigated. To attain a high enrichment factor, the time to reach equilibrium should be short. It is also important to select a fiber material of suitable polarity; the fiber material that is the most effective for SPME and is readily available is polydimethylsiloxane (PDMS) [1]. Temperature is another important factor affecting SPME. Increasing temperature tends to yield higher coefficients of distribution between the solvent and the coating fiber. Longer extraction times are usually recommended in order to ensure complete extraction. After sample extraction is achieved for a certain analyte, then the extract is introduced to a suitable instrument for determination [1]. Therefore, the process of eluting the analyte from the fiber and the time elution takes should be optimized. The strength and time of sonication and magnetic stirring also should be optimized and considered. Lastly, the salt content that controls the ionic strength and hence analyte volatility also should be investigated [1].

## 3 Liquid Phase Microextraction (LPME)

LPME was developed from classical LPE, and the separation mechanism is similar for both techniques. The technique involves separation and preconcentration of certain analytes between Microliter quantities of organic (analyte acceptor) and aqueous (analyte donor) solvents [4]. Depending on the separation mode, LPME can be classified into three techniques [4]: single drop microextraction (SDME), dispersive liquid-liquid microextraction (DLLME), and hollow fiber-based liquid phase microextraction (HF-LPME).

## 3.1 Theoretical principles

The theory behind the basic LPME technique involves partitioning and adsorption as in SPME (see section 2.1), and some relevant equations are described below.

## 3.2 Components affecting LPME

For any technique, there are many factors that affect the separation and preconcentration processes. Briefly, in SDME, the nature and volume of the organic solvent are critical to optimal selectivity and enrichment factors. In addition, the extraction time and temperature must be optimized. The salt content and ionic strength as well as the pH of the aqueous phase should also be studied [4].

SDME calculations can be performed using equation (3), where CAaq is the concentration of an analyte in a sample, CA0 is the concentration of an analyte in a microdrop, V0 is the microdrop volume, CA,0aq is the initial concentration of an analyte in an aqueous sample, and Vaq is the sample volume [1]: $VaqCAaq+CA0V0=CA,0aqVaq$(3)

DLLME is a technique based on three solvent components (Fig. 1.) which include the sample’s aqueous solution, a disperser solvent, and an extraction solvent [4]. The mathematical equation used for DLLME calculations is displayed as equation (4), where EF is an enrichment factor, Csed is the concentration of an analyte in a sedimented organic solvent, and C0 is the concentration of essential analyte [1]. $EF=Csed/C0$(4)

Figure 1

Representation of dispersive liquid-liquid microextraction (DLLME), the most favorable technique for preconcentration and separation of selenium.

The experimental conditions to be considered for DLLME are the same conditions to be considered for SDME. In addition, in DLLME, the disperser solvent and the use of a complexing agent for metal ions also must be considered [4].

LPME is based on the solidification of a floating organic drop. This liquid-liquid microextraction technique uses a low-density organic solvent followed by freezing of the organic drop containing the analyte prior to the analysis step [5]. A hollow fiber technique that is based on LPME utilizes the partitioning of an analyte between organic and aqueous phases inside a fiber membrane and sample solution [6].

## 4 Analytical Selenium Microextraction Methods

Microextraction techniques have been developed and used to investigate many heavy metals in the environment. Selenium is a non-metal, but it displays important characteristics including toxicity that has made it the subject of recent development of preconcentration and separation techniques using microextraction methods, particularly beginning in 2010. Because of its toxicity, permissible levels in drinking water were fixed to 40.0 μg/L and 10.0 μg/L by the World Health Organization(WHO) and European commission (EU), respectively [7, 8].

Researchers in the field of analytical chemistry have developed several methods for selenium microextraction. The reviewing of recent descriptions of selenium microextraction will summarize many efforts in one text to help researchers interested in selenium analysis. Briefly, dispersive Liquid-Liquid microextraction (DLLME), ultrasound-assisted emulsification and dispersive liquid-liquid microextraction (USAE-DLLME), head space hollow fiber based on liquid phase microextraction (HS-HF-LPME), dispersive liquid-liquid microextraction-solidification floating organic drop (DLLME-SFOD), ionic liquid dispersive microextraction (ILDME), ultrasound-assisted emulsification and ionic liquid-dispersive liquid-liquid microextraction (USA-IL-DLLME), hollow fiber-liquid phase microextraction (HF-LPME), ultrasound-assisted-hollow fiber-liquid phase microextraction (USA-HF-LPME), solidification floating organic drop-microextraction-ultrasound-assisted-back extraction (SFOD-ME-USABE), head space-solid phase microextraction (HS-SPME), direct immersion-solid phasemicroextraction (DI-SPME), dispersive solid phase microextraction (DSPME), electrochemically controlled in-tube solid phase microextraction (EC-in-tube-SPME), and Dual silica monolithic capillary microextraction (CME) have been developed for analyzing selenium in the environment and in biological samples, and these techniques have been combined with different instrumental analyses such as HPLC, GC-MS, ET-AAS, ICP-OES, IMS, GC-AED, HG-AAS, X-FS, GC-ECD, HG-AFS, GF-AAS, ETV-ICP-MS, and EDXFS. Microextraction techniques connected with several instruments can be summarized as follows.

## 4.1.1 Selenium LPME coupled with Atomic Absorption Spectroscopy (AAS)

In consideration of all published studies (Fig. 3), it has become clear that atomic absorption and fluorescence spectroscopy are the most favorable techniques for the determination of selenium using microextraction techniques. AAS is the most suitable technique for analysis of trace metals and nonmetals like selenium. Also, most of the well-known previously developed methods for the determination of selenium and other metalloids or nonmetals used atomic absorption and fluorescence techniques, because they offer low interference and high sensitivity, and they have been proven effective for inorganic selenium. These techniques are relatively inexpensive, and the equipment is found in many laboratories around the world [9]. In the same regard, AAS is considered to be the best technique for selenium determination and has been implemented for a variety of environmental samples [10].

Figure 2

LPME and SPME, methods developed for preconcentration and separation selenium.

Figure 3

Represents the number of publications from 2008 to 2016 that describe the use of different instruments for selenium analysis after diverse microextraction techniques.

In addition, multiple studies have applied LPME techniques to the preconcentration and separation of selenium from matrices (Fig. 2) that are different from those for which SPME techniques are used. Among the liquid phase microextraction techniques (Fig. 1), the dispersive liquid-liquid microextraction technique was a favorable technique for preconcentration and separation of selenium because it offers low solvent consumption [4], good extraction efficiency [11], a short time to reach the equilibrium state [11], the option to perform in batch mode [12], high recovery rates, relatively high enrichment, and environmental compatibility [13].

Selenium can enter the environment through different mechanisms, including mining, agriculture, petroleum refineries, metal industries, and food and drugs industries[14 - 16]. Selenium is present in aqueous matrices in the environment and in some biological matrices.

Ghasemi and co-workers [17] successfully developed a new method for preconcentration, separation, and speciation of selenium (Table 1) from various water and soil samples using hollow fiber - liquid phase microextraction(HF-LPME) coupled with electrothermal atomic absorption spectroscopy (ET-AAS). The limit of detection of 5.0ng/L was obtained based on 3s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope) for five blank solution measurements. The linear dynamic range, with a correlation coefficient of 0.995, was 0.05-35.0 μg/L, a range lower than those established by previous work [18 - 20]. The method has been validated, and an enrichment factor of 480 was calculated from the slope ratio of the calibration curve of the preconcentrated sample and the untreated sample. Validation was assessed using a reference material that contained a known trace amount of selenium. Researchers were able to achieve a 99.70% recovery of the selenium, demonstrating no significant difference between the certified and measured values. Good accuracy was obtained using a reference material of water containing trace amounts of selenium, which resulted in no significant difference between the certified and measured values, and the recovery of 99.70% agreed with previous work [18, 19]. In addition to good recovery, a suitable precision of 3.1% relative standard deviation was obtained, which was a lower RSD value than was established by previous researchers [18]. The method developed by Ghasemi and co-workers thus demonstrated low cost, simplicity, sensitivity, reproducibility, and efficiency, confirming the usefulness of this method for the analysis of selenium in environmental samples. Carryover and cross-contamination of the hollow fiber remain disadvantages of this method [17].

Table 1

Selenium LPME techniques combined with AAS

Similarly, Serafimoviska and co-workers developed another method for semi-microextraction, preconcentration, and speciation of antimony. A precision, expressed as RSD, of less than 9.0% was obtained, which agreed with previous work [18], and the accuracy, tested with river water reference material SLRS-5, was high, suggested by recoveries of 98% and 108%, which also agreed with previous work [18 , 19]. Later, the researchers found that this method can be successfully applied for preconcentration, separation, and speciation of selenium as well as antimony [22]. In this method (Table 1), they used the liquid phase semi microextraction technique combined with electrothermal atomic absorption spectroscopy (ET-AAS). Similar to the previous method ammonium pyrrolidine dithiocarbamate (APDC) was used as a chelating agent. Xylene was used as the extraction solvent. The sample volume and the extraction time were 20.0 ml, and 10 min, respectively. The total dissolved inorganic selenium ranged from 15.0-75.0 µg/L. It should be noted that the authors have not yet validated the antimony method for selenium [22]. Assuming that the method will be validated, it offers advantages of simplicity, accuracy, and rapidity of the analytical process. The key disadvantage is that the sample must be injected manually into the graphite furnace.

Selenite, selenate and total selenium were analyzed by an online Ionic liquid dispersive microextraction technique (ILDME) (Table 1) connected to electrothermal atomic absorption spectroscopy (ET-AAS) [23]. The parameters that affect preconcentration and separation were carefully studied. This method has been successfully applied to water and garlic samples (Table 1). The detection limit, enrichment factor (EF), and sample volume were 15ng/L (calculated from the signal intercept), 20 (calculated from the calibration curve ratio before and after extraction), and 4ml, respectively. The relative standard deviation was 5.1% in good agreement with previous work [18] as was the accuracy of 96% and 104% achieved for selenite and selenate [18 , 19] and the linearity of the assay (correlation coefficient of 0.9993) [19 , 20]. Several reagents and organic solvents worked well in the assay, and no clear disadvantage was mentioned.

Cordoba and others [9] successfully developed a method for determination and speciation of selenate and selenite in different brands of edible oils (Table 1) after solid phase extraction directly from oil using dispersive liquid-liquid microextraction (DLLME), ultrasound assisted-dispersive liquid-liquid microextraction (USA-DLLME), and ultrasound assisted-ionic liquid-dispersive liquid-liquid microextraction (USA-IL-DLLME) combined with electrothermal atomic absorption spectroscopy (ET-AAS). After passing the sample through a hydrophobic-lipophilic balance (HLB) solid phase cartridge, selenite and selenate were eluted by 3% HNO3 [9]. In USA-DLLME direct extraction from the oil samples, a mixture of (4:1) isopropyl alcohol and 3% HNO3 was used as dispenser solvent. A linear dynamic range of 0.05-1.20 μg/kg was obtained; however, the authors did not report the calibration curve correlation coefficient, leading the reader to question validation. The detection limit was 0.03 ng/g for DLLME using an aqueous phase [9]. On the other hand, in USA-IL-DLLME, [C12min][Tf2N] was used as a dispersive solvent. The detection limit and linear dynamic range of 0.04 ng/g, and 0.0-1.10 µg/kg were obtained, respectively, but again no calibration curve correlation coefficient was reported. The precisions of DLLME and ILDLLME were less than 4.40 and 5.10%, respectively, meaning that they were satisfatory according to reported standards [18]. The method was tested with a spiking method, and the accuracy of 96-101% with oil samples and 94-103% with fish liver and fish oil agreed with published standards [18 , 19]. This method is a comparatively simple method.

A new method, ultrasound assisted-Hollow fiber-single drop microextraction (UA-HF-SDME) (Table 1) coupled with graphite furnace-atomic absorption spectroscopy (GF-AAS), has been developed for selenium analysis [24]. It was applied to the determination of selenium in vegetable and fruit samples. N-octylacetamide in toluene was used as the extraction solvent. The extraction time, sample volume, enrichment factor (EF), limit of detection (LOD), and linear dynamic range were 15.0 min, 2.0 ml, 35.0, 0.08 ng/L (calculated from signal to noise ratio), and 0.2 - 5 ng/ml with correlation coefficient of 0.988 (less than the value established in previous reports [19 , 20]), respectively. Suitable accuracies of 91.5 to 96.7% recovery were obtained, which agreed with established standards [18 , 19]. A suitable precision of 3.8% RSD (associated with [18]) was obtained. The speed, simplicity, and sensitivity were reported as advantages of this method.

A new combination of 1-phenylthiosemicarbazide and 1-hexyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide [C6MIM][Tf2N] for the separation and preconcentration of selenium species in some food and beverage samples using USA-IL-DLLME (Table 1) coupled with graphite furnace-atomic absorption spectroscopy GF-AAS has been developed [25]. The volume of ionic liquid, pH, the concentration of the complexing agent, and the extraction time were optimized. The limit of detection was 12.0 ng/L calculated from 3 s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope) of the blank solution, and an enrichment factor of 150 was calculated from the slope ratio before and after the preconcentration. The sample volume was 10.0 ml and the linear dynamic range was 0.40-3.0 μg/L; however, the correlation coefficient value was not reported. A good precision of 4.2% RSD, in agreement with published standards [18], was obtained. The accuracies of selenite and selenate, calculated using a spiking technique, were both reported as 96-101%, in agreement with published standards [19]. The accuracy was also tested using certified reference materials of hard drinking water and tomato leaves; in these experiments, the relative error was less than 3.0%, which indicated good agreement with the certified concentration [25]. The advantages of this method were high enrichment factor, efficiency, simplicity, reliability, and fulfillment of the principles of green chemistry.

Wang and co-workers [26] have developed a new method for determination of selenite and selenate (Table 1) using magnetic effervescent tablet-assisted ionic liquid dispersive liquid-liquid microextraction (MEA-IL-DLLME)coupled with graphite furnace atomic absorption spectrometry (GFAAS). Ammoniumpyrrolidine dithiocarbamate (APDC)and 1-butyl-3-methylimidazolium hexafluorophosphate [C4MIM] [PF6] were used as a complexing agent and an extraction solvent, respectively. Briefly, sodium carbonate and sodium dihydrogen phosphate was used as a carbon dioxide source, and 1-butyl-3-methylimidazolium hexafluorophosphate [C4MIM][PF6], and magnetic nanoparticles (Fe3O4) were ground well and condensed into the magnetic effervescent tablet. After the Se-APDC complex formed, the tablet was added to the sample solution and immediately carbon dioxide was released, resulting in increased extraction efficiency through the effective distribution of 1-butyl-3-methylimidazolium hexafluorophosphate [C4MIM][PF6] into the sample solution in a short time. The limit of detection was 0.021 μg/L, as defined as the three times the standard deviation of the blank solutions divided by the calibration curve slope. The linear dynamic range was 0.05-5.0 μg/L, with a strong correlation coefficient of 0.9992, in agreement with published standards [19 , 20]. The precision, as RSD of repeated samples, was 2.90%, in good agreement with published work [18]. Accuracy in good agreement with standards [19] was established using five certified reference materials of food and beverages. The authors did not report the method’s enrichment factor, which would be necessary to complete method validation [26]. Reported advantages include speed, reliability, and reproducibility.

## 4.1.2 Selenium LPME Coupled with Gas Chromatography (GC)

Generally, analysis using gas chromatography requires volatile compounds with low boiling points. In the case of inorganic selenium, derivatization prior to microextraction is mandatory. In addition, this technique will be suitable for seleno-amino acids and other selenium-containing organic compounds. Assadi and co workers [14] have developed methods for preconcentration and determination of inorganic selenite (Table 2) using dispersive liquid-liquid microextraction (DLLME) prior to Gas Chromatography-electron capture detector (GC-ECD) analysis.

Table 2

Selenium LPME techniques combined with GC.

In a test case of this method, 4-nitro-o-phenylenediamine was used as a derivatization agent. Ethanol and chlorobenzene were used as disperser and extraction solvent, respectively [14]. The limit of detection, linear dynamic range, and enrichment factor were 0.005 μg/L, calculated from the signal to noise ratio, 0.015-10.0 μg/L with a correlation coefficient of 0.998 [18], and 122, calculated based on the calibration curve slope ratio before after the extraction, respectively. This method was applied for the determination of selenite in various water samples. The precision of 4.1% RSD (in agreement with published standards [18]) was obtained. The accuracy, favorably compared to published standards [20], was found using a certified reference material. A recovery of 95.2% between the observed and certified results was obtained [14]. This method offers simplicity, a small organic solvent volume, and short extraction times as advantages.

A new method for the determination and preconcentration of seleno-amino acids from different biological samples was developed (Table 2) using gas chromatograph-inductively coupled plasma-mass spectrometry (GC-ICP-MS) after hollow fiber-liquid phase microextraction (HF-LPME) [27]. The method is based on a modification of seleno-amino acids by ethyl chloroformate (ECF) with an ethanol-pyridine mixture as catalyst. Following the reaction, 3:1 v/v toluene: chloroform was used as an extraction solvent. This method was applied for the determination of seleno-amino acids in garlic, cabbage, and mushroom. The limits of detection were 11.0, 15.0, and 23.0 ng/L (calculated from the three times standard deviation divided by calibration curve slope) for selenoethionine, selenomethionine, Selenium-methyl-selenocysteine, respectively [27]. Good linearity for the selenomethylselenocysteine and selenomethionine of 0.9983 and 0.9986 (associated with [18]) were obtained, respectively. The precision was satisfactory, according to published standards [18], for selenomethionine and selenomethylselenocysteine (3.3 and 8.6%, respectively). The accuracy, determined using certified reference material for the selenomethionine and selenomethylselenocysteine, were suitable, with recoveries of 96.2 and 98.9% that were in good agreement with published standards [19]. This method was simple, fast, and inexpensive, and it used very small volumes of organic solvent.

Dimethyl selenium (DMSe), and dimethyl diselenide (DMDSe) were successfully analyzed from different environmental and biological samples using a newly developed preconcentration and separation method that employs head space-hollow fiber-liquid phase microextraction (HS-HF-LPME) (Table 2) combined with gas chromatography-mass spectrometry (GC-MS) [28]. The authors compared different extraction solutions to select the best organic solvent, and N-decanol showed the best extraction efficiency. The limits of detection were 65.0 ng/L, and 57.0 ng/L (calculated based on the three times the standard deviation of the blank and the slope of the calibration curve) for DMSe and DMDSe, respectively. The dynamic linear ranges were 0.50-590.0 μg/L (R2 = 0.98) and 0.40-480.0 μg/L (R2 = 0.99) (compare to reported standards [18]) for DMSe and DMDSe, respectively. Suitable precision [18] as RSD for DMSe and DMDSe were 4.8 and 3.9% respectively. A spiking technique and recovery were used to determine the accuracy using different standard solutions. Recoveries of 88.0-108.6% and 88.7-107.0% associated with [19] for DMSe and DMDSe were obtained, respectively [28]. This method is reported to offer simplicity, speed, environmental safety, and efficiency, but a disadvantage is the possible carryover and contamination of the hollow fiber.

Selenite and selenate found in environmental waters were determined (Table 2) using ultrasound-assisted emulsification microextraction (USAEME) and dispersive liquid-liquid microextraction (DLLME) coupled with gas chromatography-flame ionization detection (GC-FID) [29]. The low-density solvent toluene was used as an extraction solvent in the two techniques. This method was applied for the determination of selenite, selenate, and total selenium in tap water, river water, sea water, waste water, and drinking water. The limits of detection were 0.05 and 0.11 ng/ml (calculated based on the signal to noise ratio) for USAEME and DLLME, respectively. The linear dynamic ranges were 0.20-35.0 ng/ml (correlation coefficients of 0.9988, in good agreement with reported standards[18]) and 0.45-75.0 ng/ml (correlation coefficient of 0.9986) for USAEME and DLLME, respectively. Satisfactory precisions, defined as RSD, were 5.32% and 4.57% for USAEME and DLLME, respectively. The accuracies, which were in agreement with published standards [19], for USAEME and DLLME as recovery using certified reference material were 97.66 and 96.57 %, respectively [29]. This method is fast, has a low cost, is reliable, and uses a very small volume of organic solvent. Potential difficulties are automation difficulty, inadequate application to the complexes matrices, and organic phase recovery difficulty.

## 4.1.3 Selenium LPME Coupled with Inductively Coupled Plasma (ICP)

The investigation of published articles confirms that inductively coupled plasma is suitable for inorganic selenium species (selenite and selenate) using chemical modification by different types of organic chelating. Hu et al. [12] have successfully developed a method for speciation and determination of inorganic selenium species (Table 3) using dispersive liquid-liquid microextraction (DLLME) combined with electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS). In this method, 5-mercapto-3-phenyl-1,3,4-thiodiazole-2(3H)-thionepotassium salt (Bismuthiol II), ethanol, and chloroform were used as a chelating agent, disperser solvent, and an extraction solvent, respectively. The detection limit (calculated as the three times standard deviation divided by the calibration curve slope) and linear dynamic range (correlation coefficient = 0.9974, in agreement with published standards [19]) were 47ng/L and 500.0-100000.0 ng/L, respectively. The precision, defined as RSD [8], was 7.2 %. The consistent accuracy [18-20] defined as recovery was 103.5% [12]. This method offered simplicity, speed, sensitivity, low cost, and high selectivity as advantages.

Table 3

Selenium LPME techniques combined with ICP.

Subsequently, the same author and others [30] developed an improved method for determination and speciation of inorganic selenium species (Table 3) using the same techniques (dispersive liquid-liquid microextraction (DLLME) combined with electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS)). Under optimum conditions, the limit of detection and linear dynamic range were 8.60 ng/L (as the three times standard deviation divided by the calibration curve slope) and 40.0-10000.0 ng/L, with a correlation coefficient that was greater than 0.9978 [18], respectively. The satisfactory precision of 9.2% RSD agreed with published standards [18]. An accuracy of 98% was reliable [19] as calculated using certified reference material [30].The advantages of this method were speed, low cost, simplicity, reliability, and the possibility of using batch mode.

Solidified floating organic drop microextraction (SFODME) coupled with electrothermal vaporization-inductively coupled plasma-mass spectrometry (ETV-ICP-MS) has been developed for the determination and speciation of inorganic and organic selenium (Table 3) in tea leaves and tea infusion [31]. In this method, ammonium pyrrolidine dithiocarbamate (APDC) dissolved in 1-undecanol (extraction solvent) was used as a chemical modifier (chelating agent). The detection limits based on the three times standard deviation divided by the calibration curve slope were 0.19 ng/L and 0.26 ng/L for selenite and selenate, respectively. The linear dynamic range was 0.01 to 10.0 ng/ml with a correlation coefficient of greater than0.9957 [31], which disagreed with published standards [19 , 20] and agreed with [26]. Suitable precision calculated based on RSD [18] was 5.4%. The reliablility of the good accuracy was evaluated using certified reference material, and recoveries between 91.0 to 105% were obtained, which agreed with published standards [18 , 19]. Simplicity, accurate and precise results, low detection limit, and high enrichment factors were reported as the advantages for this method.

## 4.1.4 Selenium LPME Coupled with High Pressure Liquid Chromatography (HPLC)

In this section, as described for ICP above, all published articles in this realm measured selenium speciation after chemical derivatization using different organic chelating compounds. Yamini and others [32] have developed a new method for the determination of selenite (Table 4) using hollow fiber-liquid phase microextraction (HF-LPME) prior to analysis via high pressure liquid chromatography - ultraviolet-visible detector (HPLC-UV). This method was applied for the determination of selenite in urine, plasma, and water samples. After complexation with o-phenylenediamine, extraction was performed using 1-octanol. Under optimum conditions, the limit of detection based on the signal to noise ratio was 0.10 μg/L, and the linear dynamic range was 0.20-100.0 μg/L with a correlation coefficient of larger than 0.995 [34], which disagreed with several reports [19 , 20] but agreed with another [18]. The precision (RSD) was less than 3.7% and would be considered good according to published standards [18]. The reproducibility of the accuracy for this method was not reported, unfortunately, because it is considered a very important issue to validate a method, according to several reports [18 - 20]. The simplicity, high reproducibility, high repeatability, and low cost were reported as advantages for this method.

Table 4

Selenium LPME techniques combined with HPLC.

Speciation and determination of selenium using hollow fiber-liquid phase microextraction (HF-LPME) coupled with high pressure liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) have been reported by Barrera [33] (Table 4). Bromobenzene was used as the extraction solvent after chemical modification using o-phenylenediamine. The limit of detection and the linear dynamic range were 131 ng/L and 131.0-50000.0 ng/L (correlation coefficient greater than 0.994 [18]), respectively. Satisfactory precision, according to published standards [18], of 10 17% were attained. Reported accuracies (71 to 99%) were relatively low but agreed to some extent with published standards [18 , 19]. This method was applied to the determination of selenium in plasma and water samples [33]. The advantages reported from this method were simplicity, low cost, and applicability to routine work.

In recent microextraction work using HPLC, Zhou and others [32] established a new method for determination and speciation of selenium in tea (Table 4) using dispersive liquid-liquid microextraction (DLLME) combined with high pressure liquid chromatography-variable wavelength detector (HPLC-VWD). The chemical modification was done using 2,3-diaminonaphthalene. Chlorobenzene and acetonitrile were used as extraction solvent and disperser, respectively. The limit of detection based on the three times signal to noise ratio and the linear dynamic range were 0.11 μg/L and 0.5-40.0 μg/L with a correlation coefficient of 0.9990 [19,20], respectively [34]. The precision of 2.3% (RSD) was suitable according to published standards [18]. Suitable accuracy measured with spiking and recovery was in the range of 91.3-100.0%, in agreement with published reports [18 , 19]. The method was found to be relatively simple, inexpensive, and reliable, and it utilizes very small volumes of organic solvents.

## 4.1.5 Selenium LPME Coupled with Spectrophotometry

The use of spectrophotometry coupled with microextraction is based on the production of colored organoselenium compounds using organic chelating agents that absorb in the visible region. A novel method for the determination and speciation of selenium (Table 5) using indirect-spectrophotometric techniques combined with dispersive liquid-liquid microextraction-solidified floating organic drop (DLLME-SFOD) has been developed [35]. Briefly, the method is based on a redox reaction in which iodide ions are oxidized by dissolved selenite to form iodine. Excess iodide reacts with iodine to produce tri-iodide ion in acidic solution. Tri-iodide forms an ion pair with cetyltrimethylammonium cation (CTAB), which is extracted to a 1-undecanol phase prior to photodiode array spectrophotometer detection. The detection limit (as 3 s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope)), limit of quantification (10s/m), and linear dynamic range with a correlation coefficient of 0.9998 [19 , 20] were 16.0 μg/L, 53.3 μg/L, and 40.0-1000.0 μg/L, respectively [35]. The precision (RSD) of 2.1% was found to agree with established standards [18]. The accuracy, using spiking and recovery, was determined to lie within the range 97.2-99.7% and was found to be satisfactory according to published standards [19]. This method was found to be simple and inexpensive and to work with high accuracy, enrichment factor, and precision, all while utilizing small volumes of organic solvents [35].

Table 5

Selenium LPME techniques combined with Spectrophotometry.

The same group published another, similar, method for speciation and determination of selenium (Table 5) using DLLME-SFOD prior to spectrophotometry detection [36].The derivatization of selenite was achieved using 3, 3-diaminobenzidine (DAB) prior to its extraction by 1-undecanol mixed with ethanol as disperser solvent. The method limit of detection (as3s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope)), limit of quantification(as 10 s/m), and linear dynamic range were 1.6 μg/Land 5.0-600.0 μg/L, respectively. The correlation coefficient for the linear range was not reported. The precision, as RSD, of 2.1% was appropriate [18]. The accuracy, 95 to 101% using spiking and recovery[38],was appropriate as well [19]. This method revealed advantages of low cost, simplicity, reliability, and high enrichment factor.

## 4.1.6 Selenium LPME Coupled With Other Instruments

Atomic absorption spectrometry (AAS) and gas chromatography (GC) have been found to be the most useful devices for selenium analyses using microextraction techniques(ME). Two recent studies have investigated the use of two different devices for this method. Yang and co-workers[37] used APDC as a chelating agent for selenium and extracted to 1-undecanol (Table 6) and then re-extracted with nitric acid to develop a new method using solidified floating organic drop microextraction-ultrasound assisted back extraction (SFODME-USABE) coupled with hydride generation-atomic fluorescence spectrometry (HG-AFS). The limit of detection was 7 ng/L based on the three times signal to noise ratio, and the linear dynamic range was 0.01-5.0 μg/L with a correlation coefficient of 0.9996, which agreed with published standards [19 , 20]. The precision of 2.1% (RSD) also agreed with published standards [18]. Accuracies, between 90.91 and 103.5%, were measured using certified reference materials and recovery [37], and these numbers were in accord with previous reports [19]. The high sensitivity of HG-AFS, the use of very small volumes of organic solvents, and the presence of fewer interferences were reported as advantages.

Table 6

Selenium LPME techniques combined with other Instruments.

A new method for determination of selenite in river water was developed by Kocot and others [36] (Table 6) using dispersive liquid-liquid microextraction (DLLME) with dried spot technique combined with energy dispersive X-ray fluorescence spectrometry (EDXFS). A sample volume of 5.0 ml was used, and diethyldithiocarbamate (DDTC) was used as the complexing agent. The extraction and disperser solvents were carbon tetrachloride and methanol, respectively. The detection limit was 2.0 ng/ml, and the linear dynamic range was 0.0-0.40 μg/ml, though the correlation coefficient was not reported [38]. This method suffers from many problems in selenium applications, and the authors did not conduct an evaluation of the linearity correlation coefficient, accuracy, or precision according to standards [18 - 20]. This method offered a very low detection limit and the ability of to analyze multiple elements, and it fulfills the parameters of green chemistry.

Selenite in soil has been measured (Table 6) using a method developed by Margui [39]. It consists of dispersive liquid-liquid microextraction (DLLME) prior to total reflection X-ray spectrometry (TXRF) analysis. The extraction and disperser solvent were carbon tetrachloride and ethanol, respectively. The detection limit was 1.1 μg/L and the accuracy of 94.3% [39] agreed with published standards [19]. This method is relatively inexpensive, rapid, and sensitive, and it allows for multielement analyses.

## 4.2 Solid Phase Microextraction techniques (SPME)

Preconcentration and separation of selenium using solid phase microextraction techniques were mainly applied to organic selenium (dimethyl selenium and dimethyl diselenium). In some methods, preconcentration and separation of inorganic selenium species occurs by the production of volatile selenium hydrides. In other methods, selenium was preconcentrated and separated after chemical modification using different organic chelating agents.

## 4.2.1 Selenium SPME Coupled with Atomic Absorption Spectroscopy (AAS)

There are two published methods for selenium preconcentration and separation using solid phase microextraction techniques. Firstly, a new method for preconcentration and separation of total selenium (selenite and selenate)uses switchable hydrophobic-hydrophilic transition dispersive solid-liquid microextraction (SHT-DSLME) combined with graphite furnace-atomic absorption spectroscopy (GF-AAS) [40]. This method was applied for the determination of total selenium in various water samples (Table 7). The detection limit (based on the three times standard deviation divided by the calibration curve slope) and the linear dynamic range were 0.015 μg/L and 0.50-10.0 μg/L, respectively [40]. The correlation coefficient of 0.995 disagreed with published standards [19, 20] and associated with [18]. Good reproducibility of 4.5% (RSD) [18] was obtained. The recovery of 99.8% associated with [19] using certified reference material was established. Tetraethylenepentamine (TEPA) modified with multi-walled carbon nanotubes (MWCNTs) was used as a solid adsorbent [40]. This solvent-free method revealed the benefits of novelty, simplicity, and high reproducibility, while no clear disadvantages were apparent.

Table 7

Selenium SPME techniques combined with AAS.

Asiabi and co-workers [41] have developed for the first time a method for the determination of total inorganic selenium (Table 7) using electrochemically controlled in-tube solid phase microextraction (EC-in-tube SPME) coupled with hydride generation-atomic absorption spectrometry (HG-AAS). In this method, the nanostructured composite of polypyrrole (PPy) doped with ethylene glycol dimethacrylate (EGDMA) was used as solid phase. The extraction and deposition of inorganic selenium were achieved by applying negative and positive voltages, respectively. The detection limit (based on the three times signal to noise ratio) and the linear dynamic range were 0.004 μg/L and 0.012-200 μg/L, respectively, and the correlation coefficient of 0.9996 agreed with published standards [19 , 20]. The reproducibility (RSD) was in the range of 2.0-2.5%, in agreement with published standards [18]. The accuracy, determined by spiking and recovery, was 90-110% [19]. This method was successfully applied to the determination of inorganic selenium in tap, waste, and river waters [41] This method is amenable to automation, and it has additional advantages of novelty, simplicity, high sensitivity, high selectivity, speed, and excellent analyte distribution using the voltage control.

## 4.2.2 Selenium SPME Coupled with Gas Chromatography (GC)

As with LPME, most published articles concerning SPME with GC were applied to organic volatile selenium compounds, save for one study. In that study, Zachariadis et al. [42] successfully established a new method for determination of selenite (Table 8) using head space-solid phase microextraction (HS-SPME) combined with gas chromatography-mass spectrophotometry (GC-MS). In this method, polydimethylsiloxane (PDMS) was used as an adsorbent. The detection limit was 0.05 μg/L and the dynamic linear range was 0.08-20.0 μg/L with a correlation coefficient of 0.9997, which matched published standards [19 , 20]. Good precision (RSD) of 4.2% was found [18]. Suitable accuracy as recovery of 96.7% using certified reference material was obtained [19]. This method was applied for the determination of selenite in water and urine samples [42]. This method is believed to be simple, with good sensitivity, high reproducibility, and the ability of screening and monitoring in routine work.

Table 8

Selenium SPME techniques combined with GC.

A novel method for determination of dimethylselenium (DMSe) and dimethyldiselenium (DMDSe) compounds (Table 8) using solid phase microextraction (SPME) coupled with gas chromatography-atomic emission detection (GC-AED) have been developed [43]. It was successfully implemented for determination of DMSe and DMDSe in milk and milk by-products. Carboxen-polydimethylsiloxane (CAR-PDMS) was used as an adsorbent. The detection limits based on the signal to noise ratio for DMSe and DMDSe were 70 pg/ml and 170 pg/ml for liquid and 40.0 pg/g and 95.0 pg/g for yogurt samples, respectively. The linear dynamic ranges (with correlation coefficients greater than 0.990, disagreeing with published standards [19 , 20] and associated with [18]) for DMSe and DMDSe were 0.12-10.0 ng/ml and 0.20-25.0 ng/ml for liquid and 0.10-10.0 ng/g and 0.15-15.0 ng/g for yogurt samples, respectively. The precision values, which agreed with published standards [18], for DMSe were 2.85 and 3.45% for the liquid and yogurt samples, respectively, while the precision values for DMDSe were 3.03 and 5.72% for the liquid and yogurt samples, respectively.The accuracy agreed with standards [19], as the average recovery for this method using spiking was 89.5%. This method is considered as a good procedure for determination of volatile selenium compounds in milk and milk by-products with a good repeatability and accuracy [43]. This method is considered to be simple with a short preconcentration time that offers reduction of undissolved particles and volatile compounds.

Antonio and co-workers [44] have published a new method for the determination of seleno-amino acids (Table 8) using direct immerse-solid phase microextraction (DI-SPME) prior to gas chromatography-triple quadrupole mass spectrometry (GC-QqQ MS).

In this method, selenomethionine (SeMet) and selenomethylselenocysteine (SeMeSeCys) were treated with propylchloroformate and then a solid phase microextraction step using divinylbenzene-carboxen-polydimethylsiloxane (DVB-CAR-PDMS) as an adsorbent. This method was applied for the determination of SeMet and SeMeSeCys in two brands of potatoes. The detection limits, calculated based on the three times standard deviation multiplied by the blank signal, were 0.007 μg/L and 0.010 μg/L for SeMet and SeMeSeCys, respectively. The linear dynamic range for both compounds was 0.10-5.0 μg/L with a correlation coefficient of 0.9960 and 0.9956 for the SeMet and SeMeSeCys, respectively, and these values which were unassociated with published standards [18 - 20]. The limits of quantification of the SeMet and SeMeSeCys were 0.0146 and 0.0294 μg/L, respectively [44]. The accuracies, which were defined as recoveries and agreed with standards [19], for the SeMet and SeMeSeCys were 82.3 - 112.1% and 99.0 - 116.3%, respectively.Precisions agreed to some extent with published standards [18] for the SeMet and SeMeSeCys. These precisions, as RSD, were 9.6 - 11.9%and 8.5 - 13.1%, respectively. This method is feasible for the determination and quantification of selenium-containing amino acids in complex matrices with a satisfactory linearity, good precision, and good accuracy [44]. This method showed simplicity and rapidity and was good for selenium analysis and monitoring in food matrices.

Another head space-solid phase microextraction (HS-SPME) method combined with gas chromatography-mass spectrometry (GC-MS) has been developed (Table 8) for determination of dimethylselenium (DMSe) and dimethyldiselenium (DMDSe) [45]. These authors used nano coated lead dioxide (PbO2) as an adsorbent through an electrodeposition step. In this method, the obtained detection limits defined as 3s (blank)/m (calibrationcurve) were 16.0 ng/L and 11.0 ng/L for DMSe and DMDSe, respectively. Precisions (RSD) were 4.3 and 4.6% for DMSe and DMDSe, respectively, and they agreed with published standards [19]. The accuracy also agreed with published standards [26], as the accuracies calculated by spiking and recovery for DMSe and DMDSe were 86.8-106% and 92.5-102.7%, respectively. The linear dynamic ranges for DMSe and DMDSe were 0.10-600.0 μg/L and 0.08-510 μg/L, but the correlation coefficients of 0.991 and 0.980 [45] did not agree with published standards [19 , 20] and associated with [18]. This method revealed benefits of simplicity, rapidity, and reliability, and it used very low volumes of organic solvents and was suitable for complex matrixes.

In the same regard, another new method for determination of DMSe and DMDSe, this one using direct immerse-solid phase microextraction (DI-SPME) (Table 8) coupled with capillary gas chromatography-mass spectrometry (GC-MS), was developed [46]. In this method, carboxen-polydimethylsiloxane (CAR-PDMS) was used as an adsorbent. The detection limits were 24.0 ng/L and 27.0 ng/L for DMSe and DMDSe, respectively. The linear dynamic ranges obtained were 0.075-100.0 μg/L and 0.036-100 μg/L for DMSe and DMDSe, respectively. The accuracies of 83-92% and 92-103% for DMSe and DMDse, respectively, agreed with standards [19]and were calculated based on spiking and recovery for different sludge and water. The developed method is suitable for quantification of organic selenium in environmental samples with satisfactory accuracy and precision [46]. The advantages of this method include simplicity, high sensitivity, and high reproducibilitiy as well as applicability to all volatile selenium species.

## 4.2.3 Selenium SPME Coupled with Inductively Coupled Plasma (ICP)

A new technique using a dual modified silica monolithic capillary with 3-mercaptopropyltrimethoxysilane (MPTS) and N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS) as solid adsorbents has been developed to the separation and preconcentration of selenite and selenate (Table 9) consecutively prior to their online analysis by inductively coupled plasma-mass spectrometry (ICP-MS) [47]. In this method, a 5.0 ml sample volume was used. The detection limits based on 3 s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope) and the linear dynamic ranges were 31.1, 11.6 ng/L and 0.1-16.0 with a correlation coefficient of 0.9947, which disagreed with standards [18 - [20], 0.05-16.0 μg/L with a correlation coefficient of 0.9924, which disagreed with standards [19 , 20] and associated with [18],for selenite and selenate, respectively. The precision was appropriate as it agreed with standards [18]. The precisions (RSD were 3.3 and 6.5% for selenite and selenate, respectively. The accuracy, using certified reference material, was found to be 106.30% for total selenium, which agreed with published standards [19]. The enrichment factors were 37.5 and 39.0, respectively, which were considered as efficient for the method’s purpose. This method was successfully applied for determination and speciation of selenium in natural waters (tap, rain, lake, and river) [47]. This method was found to be inexpensive, simple, rapid, organic solvent free, reliable, and suitable for complex matrices.

Table 9

Selenium SPME techniques combined with ICP.

Jankowski and others [48] have successfully developed a method for determination of total selenium by producing selenium hydride (Table 9) using sodium borohydride prior to head space-solid phase microextraction (HS-SPME) combined with inductively coupled plasma-optical emission spectrometry (ICP-OES) [48]. Polydimethyl siloxane/carboxine (PDMS/carboxine) was used as a solid adsorbent. The method was used in acidic and alkaline medium and 5 ml was used as the sample volume. Suitable precision (3.4% RSD) agreed with published standards [18]. The accuracy, using certified reference material, was within the range 96-103% and agreed with standards [19]. The detection limit calculated from 3 s/m (defined as the three times standard deviation of the blank solutions divided by the calibration curve slope) was 0.8 ng/mL, and the linear dynamic range was 0.004-25.0 μg/ml with a correlation coefficient of 0.9992 [48], in agreeance with published standards [19 , 20].This method demonstrated benefits such as simplicity, high reproducibility, high accuracy, and suitability for the determination of total selenium from various matrices.

Hu et al. [49] have developed a novel method for the determination and speciation of selenium using SPE (Table 9) with a micro-extracting column filled with N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS) modified with multi-walled carbon nanotubes (MWCNTs) prior to inductively coupled plasma-mass spectrometry (ICP-MS) analysis. In this method, organic solvents were not used. The detection limit and the linear dynamic range were 16.0 ng/L and 0.050-100 ng/ml, respectively. Satisfactory precision was 6.2%, and it agreed with published standards [18]. The accuracy also agreed with published standards [19], and it was calculated by spiking and recovery. The results from these analyses yielded accuracies of 88.9-104.2% [49]. This method is simple, highly selective, fast, and suitable for routine selenium analysis, and it does not require any organic solvents.

## 4.2.4 Selenium SPME Coupled with X-ray Fluorescence and Absorption Spectrometry (XFS) and (XAS)

Along with the instruments mentioned above, there were only a few published research articles demonstrating the use of x-ray spectrometry with microextraction techniques. One such report involves a novel method for determination of selenite (Table 10) after chemical modification by ammonium pyrrolidine dithiocarbamate (APDC) and using dispersive solid phase microextraction (DSPME) prior to X-ray fluorescence spectrometry analysis [50]. Oxidized multi-walled carbon nanotubes (MWCNTs) were used as an adsorbent. After optimization of all parameters, a good precision, which agreed with published standards [18], defined as RSD was 3.2%. A reliable accuracy, which also agreed with published standards [19] and was calculated by recovery, was 97%. The detection limit calculated based on the sensitivity, background count rate, and continuing time was 0.06-0.20 ng/ml [50]. This method is fast, has high selectivity, and is reliable, and it possesses the possibility of sample multi-measurement

Table 10

Selenium SPME techniques combined with XFS and XAS.

Kocot and others [51] have developed a new method for the determination and speciation of selenium (Table 10) using dispersive micro solid phase extraction (DMSPE) with graphene as adsorbent coupled with energy dispersive X-ray fluorescence spectrometry (ED-XFS). In this method, ammonium pyrolidine dithiocarbamate (APDC) and triton-X-100 were used as chemical modifier and disperser, respectively. The detection limit and the linear dynamic range were 0.032 ng/ml and 0.032-500 ng/ml with a correlation coefficient of 0.9995 [19 , 20], respectively. The precision of 5.1% (RSD) agreed with published standards [18]. Appropriate accuracies of 97.7 and 99.2% for selenite and selenate, respectively, also agreed with standards [19]. In addition, this method is suitable for sea water analysis which is a significant advantage compared to other methods [51]. This method revealed additional advantages of simplicity, speed, high precision, high accuracy, the possibility of analysis without sample degradation, and high enrichment factor.

## 4.2.5 Selenium SPME Coupled with Ion Mobility Spectrometry (IMS)

One study has been published using IMS coupled with microextraction techniques. This study describes a new method for determination of selenite (Table 11) after chemical modification by 1,2-diaminobenzene using head space-solid phase microextraction combined with ion mobility spectrometry (IMS) [52]. Polypyrrole was used as an adsorbent. Good precision [18] of less than 6.0% RSD was obtained. The accuracy, defined by spiking and recovery, was within the range 97-107%, in agreement with published standards [19]. The detection limit based on the signal to noise ratio and the linear dynamic range were 12.0 ng/L and 20.0 -320.0 ng/L, respectively [52]. This method is reliable, simple, sensitive, repeatable, and suitable for selenium analysis in complex matrices.

Table 11

Selenium SPME techniques combined with Ion mobility Spectrometry (IMS).

## 5 Remarks, Limitations and Future Trends of Selenium Microextraction Techniques

A wide variety of LPME and SPME methods have been developed for preconcentration and separation of selenium in different matrices. In this regard, various modes of LPME techniques were coupled with AAS. Ethanol was the main disperser agent used in DLLME. Importantly, ethanol is less toxic compared with methanol, carbon tetrachloride, and other organic solvents commonly used in extraction. Shrivas and co-workers [24] attained relatively good detection limits but poor enrichment factors and precisions in applying this method. In contrast, other researchers [17] attained good detection limits, precisions, and enrichment factors. HF-LPME methods revealed the lowest detection limits among LPME coupled with AAS. It is therefore recommended that the method developed by Ghasemi et al. [17] be considered first when selecting a method for preconcentration and separation of selenium. Other methods can be used but accuracy and precision limitations must be carefully considered. GC combined with different LPME modes has also been employed. In this realm, Bidari and co-workers reported the lowest DL, highest precision, shortest extraction time, and highest EF compared with other LPME-GC methods [14]. When considering ICP coupled with LPME, Chen and co-workers [31] attained lowest DL, best precision, and highest EF, but their method was time-consuming. In HPLC connected with LPME, all of the published work had very similar detection limits, except for the method reported by Moreno and co-workers. Their method, though, resulted in relatively good precision. The methods described by Saleh et al. [34] and Zhou et al. [34] can be considered suitable for selenium preconcentration and separation. Spectrophotometric methods coupled with DLLME showed low detection limits, high enrichment factors, and favorable precisions and are thus highly recommended for selenium preconcentration and separation.

Recently, regarding SPME, multi-walled carbon nanotubes modified by different organic compounds were used as the fiber. For the first time, Asiabi and co-workers [41] developed electrothermal-in-tube by using ppy-EGDMA. In the head space-solid phase microextraction and direct immersed-solid phase microextraction, the repeatability, and reproducibility were low compared with the other SPME modes. Zheng [47] for the first time used a modified silica monolithic capillary as an adsorbent fiber. They obtained satisfactory DL, precision, and EF. Another recent report used AAPTS-MWCNTs as the adsorbent fiber, and satisfactory detection limits and precisions were attained [49]. Kocot [40] used for the first time the oxidized multi-walled carbon nanotubes during the dispersive solid phase microextraction (DSPME), and they attained relatively low detection limits. Also, for the first time, graphene and triton-x-100 were used as an adsorbent fiber and disperser, respectively [51]. In one study, ion mobility spectrometry was used coupled with HS-SPME, which resulted in a low detection limit. Overall, we found that solid phase microextraction is time consuming compared with liquid phase microextraction. In the future, the development of new nanostructured fibers should be pursued for preconcentration and separation of selenium because there are many compounds that can be used and mixed to give different properties. We recommend the use of microextraction techniques with electro-spectrometry and voltammetry techniques if suitable electrochemical cells and micro-electrodes can be developed.

## 6 Conclusion

This review provides an illustration of microextraction techniques covering the recently developed solid phase microextraction (SPME) and liquid phase microextraction (LPME techniques) for selenium analysis. SPME and LPME techniques, theoretical backgrounds, and factors controlling the extraction process were discussed. Among all microextraction techniques, LPME were more favorable for selenium preconcentration compared with SPME. Dispersive liquid-liquid microextraction (DLLME) was a favorable technique compared with the other techniques developed for selenium preconcentration. Application of microextraction in selenium analysis has received great attention over the last 10 years due to its low cost and simplicity, high enrichment factors and good recovery, and the limited amount of organic solvents necessary. Most of the developed microextraction methods were developed for the determination of selenium and its species. In this review, among all devices that have been used with microextraction techniques, atomic absorption spectrometry was optimal. This review also highlights the recent and best practice for analysis of selenium species in different matrices.

## Acknowledgement:

The authors are grateful to the Deanship of Graduate Studies (DGS) for providing facilities, and Department of Marine Chemistry, Faculty of Marine Sciences, King Abdulaziz University for their continued support.

## Abbreviations

ME

microextraction techniques;

ME-CSV

microextraction-cathodic stripping voltammetry;

LPME

liquid phase microextraction;

SPME

solid phase microextraction;

PDMS

polydimethylsiloxane;

SDME

Single Drop Microextraction;

DLLME

Dispersive Liquid-Liquid Microextraction;

HF-LPME

Hollow Fiber based on Liquid Phase Microextraction;

WHO

World Health Organization and EU European Commission;

EF

enrichment factor;

LOD

limit of detection;

HF-LPME

hollow fiber - liquid phase microextraction;

ET-AAS

electrothermal atomic absorption spectroscopy;

ILDME

Ionic liquid dispersive microextraction technique;

USA-DLLME

ultrasound assisted-dispersive liquid-liquid microextraction;

USA-IL-DLLME

ultrasound assisted-ionic liquid-dispersive liquid-liquid microextraction;

APDC

ammonium pyrrolidine dithiocarbamate;

UA-HF-SDME

ultrasound assisted-Hollow fiber-single drop microextraction;

GF-AAS

graphite furnace-atomic absorption spectroscopy;

[C6MIM][Tf2N]

l-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide;

MEA-IL-DLLME

magnetic effervescent tablet-assisted ionic liquid dispersive liquid-liquid microextraction;

[C4MIM][PF6]

1-butyl-3-methylimidazoliumhexafluor ophosphate;

GC

Gas Chromatography;

GC-ECD

Gas Chromatography-electron capture detector;

GC-ICP-MS

gas chromatography-inductively coupled plasma-mass spectrometry;

ECF

ethyl chloroformate;

DMSe

Dimethyl selenium;

DMDSe

dimethyl diselenide;

HS-HF-LPME

GC-MS

gas chromatography-mass spectrometry;

USAEME

ultrasound-assisted emulsification microextraction;

GC-FID

gas chromatography-flame ionization detector;

ICP

Inductively Coupled Plasma;

ETV-ICP-MS

electrothermal vaporization-inductively coupled plasma-mass spectrometry;

SFODME

Solidified floating organic drop microextraction;

HPLC

High pressure Liquid Chromatography;

HPLC-UV

high pressure liquid chromatography-ultraviolet-visible detector;

HPLC-ICP-MS

high pressure liquid chromatography-inductively coupled plasma-mass spectrometry;

HPLC-VWD

high pressure liquid chromatography-variable wavelength detector;

DLLME-SFOD

dispersive liquid-liquid microextraction-solidified floating organic drop;

CTAB

cetyltrimethylammonium cation;

DAB

3,3-diaminobenzidine;

AAS

Atomic absorption spectrometry;

SFODME-USABE

solidified floating organic drop microextraction-ultrasound assisted back extraction;

HG-AFS

hydride generation-atomic fluorescence spectrometry;

EDXFS

dispersive x-ray fluorescence spectrometry;

DDTC

diethyldithiocarbamate;

TXRF

total reflection x-ray spectrometry;

SHT-DSLME

switchable hydrophobic-hydrophilic transition dispersive solid-liquid microextraction;

TEPA

tetraethylenepentamine;

MWCNTs

multi-walled carbon nanotubes;

EC-in-tube SPME

electrochemically controlled in-tube solid phase microextraction;

HG-AAS

hydride generation-atomic absorption spectrometry;

PPy

polypyrrole;

EGDMA

ethylene glycol dimethacrylate;

HS-SPME

CAR-PDMS

carboxen-polydimethylsiloxane;

DI-SPME

direct immerse-solid phase microextraction;

GC-QqQ MS

SeMet

selenomethionine;

SeMeSeCys

selenomethylselenocysteine;

DVB-CAR-PDMS

Divinylbenzene-carboxen-polydimethylsiloxane;

PbO2

MPTS

3-mercaptopropyltrimethoxysilane;

AAPTS

N-(2-aminoethyl)-3-aminopropyltrimethoxysilane;

ICP-MS

inductively coupled plasma-mass spectrometry;

ICP-OES

inductively coupled plasma-optical emission spectrometry;

IMS

ion mobility spectrometry.

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## Footnotes

Current Address: Marine Chemistry Department, Faculty of Marine Sciences, King Abdulaziz University, and P.O.Box. 80207, Jeddah, Kingdom of Saudi Arabia. Permanent Address: Higher Council of Environment, Urban and Rural Promotion, Ministry of Environment, Natural Recourses and Physical Development, Khartoum, Republic of Sudan. E-mail:

Accepted: 2017-02-15

Published Online: 2017-05-25

Citation Information: Open Chemistry, Volume 15, Issue 1, Pages 103–122, ISSN (Online) 2391-5420,

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