In the present study we report the construction of a simple atomic emission spectrophotometer and we describe its computer interface and its calibration procedure. The final instrument is employed for the detection of the presence of metal ions in commercially available drugs. The cost of the whole construction is low and it can be readily applied within the secondary education chemistry curricula in order to strengthen students’ engagement and understanding of a modern analytical technique that is based on the well known flame tests.
One of the first methods developed in order to achieve the detection of metals in various samples along with their quantitative determination was atomic emission spectrophotometry (AES) (Skoog, West, Holler, & Crouch, 2014; Strobel & Heineman, 1989). This method is based on the fact that the wavelengths of the radiation emitted by an excited sample are characteristic of the atoms present therein. Furthermore, the study of the structure of our galaxy and the study of the expansion of our Universe is made possible through an analogous technique (Freedman & Madore, 2010). In the case of quantitative analysis, the intensity of the radiation emitted is related to the amount of the analyte present in the sample.
Unfortunately, this fundamental technique and its underlying science is usually absent from the secondary education chemistry curricula. The discussion about metals’ emission spectra in most of the cases stops at the performance in the school lab of the well known flame tests (Shakhashiri, 2011). Nevertheless, recently, a company named Theremino (Theremino Group, https://www.theremino.com/en) freely distributed a software that analyzes the intensity of the incident light on a camera and provides an intensity vs. wavelength diagram. Of course, analogous software are already available in expensive cameras. Moreover, some “smart” phone’ apps recognize intensity of certain colors as well (such as Color Grab or ColorMeter for Android phones). The advantage of the aforementioned software over the other solutions is that it can easily interpret the signal from a low cost USB camera providing an excellent opportunity for the students to construct themselves their own analytical device (vide infra). The theremino documentation (available at http://bit.ly/theremino) provides a procedure for constructing a spectrometer (the heart of the AES), while some youtubers have already performed relative tasks (Electrical Projects, 2018; YouTube account Plenum 88, 2017; Wesley, 2016). But none of them discusses the possibility of the construction of a simple DIY device like this one within the framework of the upper secondary chemistry curricula. Such an instrument could provide the opportunity of the introduction of emission spectroscopy in the school lab.
Thus, in our report, firstly, we describe all the necessary steps in order to construct and calibrate a DIY flame emission spectrophotometer starting from very cheap materials and then we discuss the application of our device in the detection of metal ions in commercially available drug formulations.
The theoretical background
Emission spectrophotometry methods rely on the emission of light from excited atoms. For example, sodium atoms, in their ground state own the 11Na: 1s2 2s2 2p6 3s1 configuration. But when they are in the flame, they get excited. As a result, the electron from the 3s orbital can be found in 3p, 4p or 5p orbitals. However, the produced excited states are not energy stable. The excited electron of the sodium atoms will return to their fundamental energy state emitting a radiation having energy hν equal to the difference in the energy of two levels involved in the transition. Being ν = c/λ it is possible to calculate the wavelengths of the emitted radiations that result to be equal to: 590 nm, 330 nm and 285 nm for the transitions: 3p → 3s; 4p → 3s and 5p → 3s, respectively. The whole procedure is summarized in Figure 1. The sum of a substance’s emitted wavelengths constitute its emission spectrum. In the case of atoms the spectrum consists of specific frequencies and is therefore called linear. Each element’s atomic linear spectrum is characteristic. In other words, it is its “fingerprint”. The linear spectra of lithium and potassium are also provided in Figure 2.
The analytical instruments that exploit the atomic emission consist of the following main parts: (a) The flame (or an oven in other similar techniques). The use of a flame accomplishes (i) the evaporation of the sample, which is usually in the form of aqueous solution, (ii) the atomization of the sample and (iii) the thermal excitation of the atoms present in the sample (Figure 1). Various fuel-oxidant combinations are used to generate the flame. The most easily accessible combination in a school lab is the methane or propane/atmospheric oxygen mixture, which is blended into a Bunsen burner. The maximum temperature achieved by this combination is 2,200 K. (b) The monochromator, an optical device whereby the incident light is separated into the individual radiations it consists of. In general, this separation is accomplished in two ways (i) using a prism, which separates light based on the diffraction effect and (ii) using a diffraction grating. A diffraction grating is a surface, which consists of a very large number of parallel gaps of the same thickness. This surface achieves, through interference and diffraction, the separation of light into its individual frequencies (colors) (Skoog et al., 2014; Strobel & Heineman, 1989). Diffraction grating operation is depicted in Figure 3. (c) A device detecting, amplifying and recording the signal analysed by the monochromator.
Atomic emission spectrophotometry techniques are widely used for the determination of metals and metalloids in a variety of samples. The most common analyses have to do with the detection and the quantitative determination of alkali and alkaline earths in biological fluids, soils, plant materials, food, cements, glasses, natural waters, wastes, etc.
Particular attention should be drawn to the flame included in the experimental course. All experiments with drugs should be performed inside a fume hood.
The housing of our instrument was made with a white cardboard, covered with black paper in order to avoid internal reflections of light. The camera used was the Logitech HD Webcam C270. The 1,000 line/mm diffraction grating was purchased from Arbor Scientific (https://www.arborsci.com/). A common Bunsen burner was employed for the excitation of the samples. Finally, the necessary software for the interface was downloaded from Theremino website free of charge (available at http://bit.ly/thereminosoft).
The construction of the instrument
Figure 4 shows a schematic representation of the housing we have constructed. Firstly, using a cardboard, which was properly coated with a black paper, we constructed a box in which the optical parts of the instrument were placed. The box was rectangular shaped, with a large edge of 20 cm in size and a small edge of 7 cm. In one of the two small faces a suitable thin slit was opened. The slit allows only a parallel beam of light to enter through it inside the instrument. Particular attention was paid to the size of the slit, which was relatively small (∼2 mm wide). Inside the box we placed the camera, after covering its LED and then we glue the diffraction grating in front of its lens. The camera was positioned opposite the slit and at an angle. Its final position was determined after connecting it to the computer (vide infra), in order to obtain the optimum picture of the 1st order emitted beam (Figure 2). On one of the large sides of the box, a suitable exit for camera’s cable was opened. Figure 5 shows the position of the camera. Finally, the box was closed and covered with black paper. The computer interface of our instrument was achieved through the USB cable of the camera and the theremino software. The whole procedure is described in a YouTube video (Evangeliki-ChemClub, 2019).
Calibration and applications
The theremino software provides a two-point calibration. Firstly, the two points chosen were the characteristic yellow line in the sodium spectrum and one of the characteristic lines of the mercury emission spectrum. For the first point, λ1 = 590 nm, a sample of NaCl was directly fed into the flame (Figure 6). We should point out that in order to receive the spectrum we turned off the lab lights. For the second point, we turned our device towards the fluorescent light bulbs of our laboratory and we selected the line at λ2 = 436 nm. The obtained spectra in both cases are shown in Figure 7.
Trying to explore the capabilities of our instrument, we employed it for determining the amount of sodium in a commercially available formulation of ibuprofen. The latter is a widely prescribed non-steroidal anti-inflammatory analgesic drug (2-[4-(2-methylpropyl)-phenyl]-propanoic acid). In our experiments we used neurofen express, in which the active ingredient is ibuprofen’s sodium salt.
In order to detect the sodium present, three pills were taken out of the blister pack and pulverized into a mortar. Subsequently, the powder was placed directly into the flame and the emission spectrum was measured. The whole process took place in the fume hood. The spectrum obtained is shown at Figure 8.
Next we focused on the detection of lithium in lithiofor, a formulation of anhydrous lithium sulfate (it also contains a magnesium salt, but the main persistent line of this metal lies at 285 nm and does not interfere with the rest of the spectrum). Since the one expected lithium persistent line at 671 nm (2p → 2s) (Liu, Sun, & Wang, 2014; Sansonetti & Martin, 2005) lies at a part of the spectrum outside of the calibration area, we recalibrated our spectrophotometer. Thus, instead of the mercury line at 436 nm used previously, we employed the potassium characteristic doublet at 767 nm and 770 nm. These lines are depicted as one wide band in our instrument. We set the center of this band at 768 mm. Then we performed the procedure followed for neurofen express again for lithiofor and the derived spectrum is illustrated at Figure 9. In this figure the anticipated line that is responsible for the red coloring of the flame is shown at 673 nm, very close to the expected value of 671 nm. In the same spectrum a yellow line centered at 592 nm is also present, raised mainly from the incomplete combustion of the organic/inorganic part of the drug formulation (sulphate ions, hydroxypropylmethylcellulose, ammonio methacrylate copolymer and stearate anions).
In the present study we presented the necessary steps in order to construct a low-cost atomic emission spectrophotometry instrument. Furthermore, we described its calibration procedure. Consequently we employed our device in order to confirm the presence of metal ions in two commercially available drugs. Since, our instrument’s response was very good, we propose its adoption to the secondary school teaching curriculum. DIY devices like the one described could enhance students’ perception regarding modern day applications of spectroscopic techniques.
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