Arc Atomic Emission Spectroscopy

Abstract: in this article we describe the construction of a simple apparatus for spectroscopic analysis of atomic light emission from electric arc. The apparatus is based on a high voltage generator that produces an electric arc whose light emission is analyzed by a fiber optic spectrometer.

Introduction

Atomic Emission Spectroscopy (AES) is a chemical analysis method that uses the intensity of light emitted by a flame, plasma, arc, or spark at a particular wavelength to determine the amount of an element in a sample. The wavelength of the atomic spectral line in the emission spectrum provides the identity of the element while the intensity of the light emitted is proportional to the quantity of the element.

Spark or arc atomic emission spectroscopy is used for the analysis of metallic elements in solid samples. For non-conductive materials, the sample is ground with graphite powder to make it conductive. In traditional arc spectroscopy methods, a sample of the solid is commonly ground and destroyed during analysis. An electric arc or spark is passed through the sample, heating it to a high temperature to excite the atoms within it. The excited atoms emit light at characteristic wavelengths which are analyzed by a spectrometer. Modern spark and arc sources with controlled discharges can also be used for quantitative analyzes, but with our DIY apparatus we will be satisfied with qualitative analyzes.

Experimental Setup

The experimental setup consists of a microscope stand (from China) made of aluminum and adjustable in height, it is quite robust and above all cheap, it is easily found on eCommerce sites by searching for “microscope stand”. On the base we fixed a Plexiglas plate and on the height-adjustable support we placed a cylinder, also in Plexiglas. We need these materials to electrically isolate our “stand” from the electrodes that will be polarized with the high voltage necessary to create the spark or the electric arc.
On the Plexiglas base we placed an aluminum plate, connected to the high voltage GND cable, while on the Plexiglas cylinder we inserted a graphite electrode connected to the active high voltage cable. This setup is shown in Figure 1.


Fig.1 – Apparatus for generating the arc, with the holder, electrodes and fiber of the spectrometer

In figure 2 we show the complete apparatus with the electrode support, the positioning system for the optical fiber and the HV generator made with a flyback transformer driven by a royer oscillator. Our HV generator produces a high frequency alternating voltage so the electrodes are alternatively cathode and anode. It is obviously possible, and perhaps even preferable, to opt for a continuous HV voltage, in this last case the electric arc will have a constant polarization.


Fig.2 – Complete experimental setup

The plasma produces light emission due to the ionization of the gases and materials present in the arc production area. Generally speaking, emissions due to nitrogen and oxygen present in the air will always be present, we will also have emissions due to the material of the electrodes, in our case of graphite (carbon), there may also be light emissions due to contaminants, such as the ubiquitous sodium line at 589 nm. The material whose light emission is to be analyzed is positioned on the aluminum plate below the graphite electrode. If the material is a conductor, for example a metal, its vaporization by the plasma effect is usually sufficient to produce the characteristic light emission, especially if the metal has a low melting point. Alternatively, it is also possible to place on the aluminum plate some salts, in granular form or dissolved in aqueous solution. Also in this case the heat of the plasma vaporizes the sample which is ionized and produces the characteristic spectrum.

Measures of Spectra

The first spectrum acquired was that of the arc produced between two graphite electrodes, shown in figure 3. The peaks in this spectrum correspond to the emissions of oxygen and nitrogen gases and the carbon of the electrodes, there is also the sodium line, present as a contaminant. This spectrum will be considered as a reference for subsequent analyzes.


Fig.3 – Arc emission spectrum with graphite – graphite electrodes 

In figure 4 we report the spectrum of the arc obtained with a solution of strontium salts. The main emission line of strontium is clearly seen, together with the sodium contamination line.


Fig.4 – Arc emission spectrum with strontium salts

In figure 5 we report the arc spectrum obtained with a solution of indium salts. The main emission line of the indium is clearly seen.


Fig.5 – Arc emission spectrum with indium salts

In figure 6 we report the arc spectrum obtained with a graphite and a lead electrode. Lead has a low melting point so it produces a fairly evident emission around 400 nm.


Fig.6 – Arc emission spectrum with lead electrode

In figure 7 we report the arc spectrum obtained with a graphite and a magnesium electrode. Several characteristic emission lines are obtained.


Fig.7 – Arc emission spectrum with magnesium electrode

For the copper emission analysis we used a copper sulphate solution, shown in the image below. The solution is poured onto the aluminum plate and the action of the electric arc produces its vaporization, bringing copper ions into the plasma that produce the emission lines shown in the spectrum in figure 8.



Fig.8 – Arc emission spectrum with copper salts

Conclusions

Our apparatus for the analysis of atomic light emission spectra generated by a plasma state produced by an electric arc or spark has allowed us to acquire some interesting spectra with the evidence of the characteristic emission lines. The quality of the spectrum obtained depends greatly on the degree of contamination of the samples and electrodes. Better results could be obtained in an inert atmosphere, for example with a flow of argon gas on the sample being analyzed. We have also noticed that the correct positioning of the optical fiber is very important, as it must “point” to the area of ​​the electric arc closest to the sample under examination. With a little patience and with the optimal choice of the sample, our apparatus also allows us to obtain good quality spectra that can be used for the qualitative analysis of the sample.

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