Atomic Emission Spectroscopy Lecture 20

Applications of Plasma Sources 1. Since plasma sources result in a very large number of emission lines, these sources can be used for both qualitative and quantitative analysis. 2. The signal obtained from plasma sources is stable, has a low noise and background, as well as freedom from interferences. 3. Requires sample preparation similar to AAS

4. Plasma sources are usually best suited for operation in the ultraviolet region, therefore, elements having emission lines below 180 nm (like B, P, S, N, and C) can only be analyzed under vacuum since air components absorb under 180 nm. Also, alkali metals are difficult to analyze since their best lines under plasma conditions occur in the visible or near infrared. 5. An analytical emission line can easily be located but will depend on the other elements present since spectral line interferences are encountered in plasma sources due to the very high temperatures used.

6. Linear calibration plots are usually obtained but departure from linearity is observed at high concentrations; due to self absorption as well as other instrumental reasons. An internal standard is often used in emission methods to correct for fluctuations in temperature as well as other factors. The calibration plot in this case is a plot between the concentration of analyte and the ratio of the analyte to internal standard signal. The internal standard is a substance that is added in a constant amount to all samples, blanks, and standards; therefore it must be absent from initial sample matrix. The internal standard should have very close characteristics (both chemically and physically) to analyte.

Elements by ICP-AES Different elements have different emission intensities. Alkalis (Na, K, Rb, Cs) are weakly emitting. Alkaline Earths (Be, Mg, Ca, Sr, Ba ) are strongly emitting.

INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (ICP-MS) - Very sensitive and good for trace analysis - Plasma produces analyte ions - Ions are directed to a mass spectrometer - Ions are separated on the basis of their mass-to-charge ratio - A very sensitive detector measures ions - Very low detection limits

Introduction of Solid Samples A variety of techniques were used to introduce solid samples into atomizers. These include: 1. Conductive Samples If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. Upon drying, this solid composite can be used as an electrode. The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are swept into the plasma by argon flow.

Laser Ablation Sufficient energy from a focused intense laser will interact with the surface of samples (in a similar manner like arcs and sparks) resulting in ablation. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation. Laser ablation is becoming increasingly used since it is applicable to conductive and nonconductive samples.

The Glow Discharge Technique The technique is used for sample introduction and atomization as well. The electrodes are kept at a 250 to 1000 V DC. This high potential is sufficient to cause ionization of argon, which will be accelerated to the cathode where the sample is introduced. Collision of the fast moving energetic argon ions with the sample (cathode) causes atomization by a process called sputtering. Samples should thus be conductive to use the technique of glow discharge. The vapors of molecules and atoms are swept into the plasma source for complete atomization and excitation by flowing argon. However, nonconductive samples were reported to be atomized by this technique where they were mixed with a conductor material like graphite or powdered copper.

Emission Spectroscopy Based on Arcs and Sparks Samples are excited in the gap between a pair of electrodes connected to a high potential power supply (200 VDC or 2200- 4400 VAC). The high potential applied forces a discharge between the two electrodes to occur where current passes between the two separated electrodes (temperature rises due to very high resistance).

The very high temperature (4000-5000 oC) realized in the vicinity between the two electrodes provide enough energy for atomization and excitation of the samples in this region or when the sample is, or a part of, one of the electrodes. Arc and spark methods are mainly used as qualitative techniques and can also be used as semiquantitative techniques.

Sample Handling and Preparation If the sample is conductive and is of a shape that can be directly used as an electrode (like a piece of metal or coin), that would be the choice for sample introduction in arc and spark techniques. Otherwise, powdered solid samples are mixed with fine graphite and made into a paste. Upon drying, this solid composite can be used as an electrode. The discharge caused by arcs and sparks interacts with the surface of the solid sample creating a plume of very fine particulates and atoms that are excited and emission is collected. The figure below shows some common shapes of graphite electrodes used in arc and spark sources.

Carbon electrodes Sample pressed into electrode or mixed with Cu powder and pressed - Briquetting (pelleting) Cyanogen bands (CN) 350-420 nm occur with C electrodes in air -He, Ar atmosphere Arc/spark unstable each line measured >20 s needs multichannel detection

Instruments for Arcs and Sparks In most cases, emission from atoms in an arc or spark is directed to a monochromator with a long focal length and the diffracted beams are allowed to hit a photographic film. This typical instrument is called a spectrograph since it uses a photographic film as the detector.

Spectrograph Beginning 1930s photographic film detector Cheap Long integration times Difficult to develop/analyze Non-linearity of line "darkness“

Potential Source Photographic Film Graphite Electrodes