; Overview of Atomic Spectroscopy
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Overview of Atomic Spectroscopy


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									Overview of Atomic Spectroscopy

Atomic spectroscopy has experienced remarkable growth and diversity in the past few years, making it more difficult for analysts to keep up with developments in the field. Atomic spectroscopy is not one but three techniques: I) atomic absorption, 2) atomic emission, and 3) atomic fluorescence. The first two are the most common and widely used techniques. The linear relationship between the amount of light absorbed or emitted and the amount of species of interest is called the Beer-Lambert Law. It can be used to find unknown concentrations by measuring the light emitted or absorbed.

1. Atomic Absorption is the process where vaporized atoms absorbs light and is measured. The basic instrument for atomic absorption requires a light source, an atom source, a monochrometer to isolate the specific wavelength of light, a detector, some electronics to treat the signal, and a data display. The light source is usually a hollow cathode lamp. 2. Atomic Emission is a process in which the light emitted by excited atoms or ions is measured. The basic instrument for atomic emission is similar to atomic absorption except that it has no primary light source. The critical component in emission is the atomization source because it must provide all the energy to excite as well as atomize the atoms. Previously many sources were tried but the ICP eliminates most all problems associated with past emission sources. This has revolutionized the utility of atomic emission spectroscopy. The ICP is an argon plasma maintained by the interaction of an RF field and ionized argon gas. The ICP can reach temperatures around 10,000 K with sample temperatures between 5,000 and 8,000 K. These temperatures allow complete atomization of elements thus minimizing chemical interference effects. In ICP-MS, the function of the Mass Spectrometer is similar to that of the monochrometer in Atomic Absorption or ICP Emission systems. In ICP-MS, rather than separating light into wavelengths, the mass analysis separates the ions from the ICP according to their mass/charge ratio. The ICP-MS combines the multielement capabilities and broad linear range of ICP with the exceptional detection limits of graphite furnace AA.

Selecting the Proper Atomic Spectroscopy Technique

There are four techniques normally Suited for analytical determinations and they are:

1) Flame Atomic Absorption 2) Graphite Furnace Atomic Absorption 3) Inductively Coupled Plasma Emission 4) Inductively Coupled Plasmal Mass Spectrometry A clear understanding of the analytical problems and the capabilities provided by the different techniques is necessary. Some important criteria for selecting a particular technique include 1) detection limits, 2) analytical working range, 3) sample throughput, 4) cost, and 5) ease of use.

Detection Limits Without adequate detection limit capabilities, lengthy preparation may be required prior to analysis. Typical detection limits for the major atomic spectroscopic techniques are shown in Table 1. Generally, the best detection limits are attained using ICP-MS and Graphite Furnace Atomic Absorption as shown below.

Table 1. Atomic Spectroscopy Detection Limits (micrograms/liter)

All detection limits are given in micrograms per liter and were determined using elemental standards in dilute aqueous solution. All detection limits are based on a 98% confidence level (3 standard deviations). Atomic absorption and ICP emission detection limits were determined using instrumental parameters optimized for the individual element. ICP emission detection limits obtained during multielement analyses will typically be within a factor of 2 the values shown. Cold vapor mercury detection limits were determined with a FIAS-200 flow injections system with amalgamation accessory. Hydride detection limits were determined using a MHS-10 Mercury/Hydride system. Fumace AA (Model 5100 Pc with 5100 ZL Zeman furnace Module or Model 4100 ZL) detection limits were determined using STPF conditions and are all based on 20 microliterL sample volumes and use of a L'vov platform. ICPMS detection limits were determined using an ELAN 5000. Letters following an ICP-MS detection limit value refer to the use of a less abundant mass for the

determination as follows:a-C 13 b-Ca 44 c-Fe 54, d-Ni 60, e-S 34, f-Se 82

Sample Throughput Sample throughput is the number of samples which can be analyzed or elements to be determined per unit time. Analyses near the limit of detection or where absolute precision is required are more time consuming than other less demanding analyses. GFAA is basically a single element technique because of the need ~ thermally program the system to remove solvent and matrix components prior to atomization. The GFAA has a relatively low Hample throughput where a typical determination (single burn) requires 2-3 minutes per sample per element. ICP Emission is a true multielement technique with exceptional sample throughput. IC P emission systems typically can determine l~80 elements per minute in individual samples. For few elements the ICP is limited by the time needed to equilibrate the plasma which is typically 15-30 seconds (0.25-0.5 minutes). ICP-MS has the same multielement capabilities and time requirements as IC? but can get much better detection limits like those in GFAA. To run one sample by GFAA it takes 4 minutes to precondition the graphite tube, 8 minutes to calibrate using a four point calibration method. Another 8 minutes are taken up by internal analysis checks and blanks. that is 22 minutes just for calibration without any complications. To analyze the unknown Hample takes 2-3 minutes providing it is in the right working range or it will need to be diluted which takes still more time. And to do eight elements it will take approximately 4 hours (240 minutes). On the ICP it takes 10 minutes to calibrate, in a typical case 23 elements, and then l minute per burn but all 23 elements are determined simultaneously. That is a total of 11-12 minutes. That is 20 times faster and there is three times more information.

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