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Atomic spectrometry

Atomic spectrometry A branch of chemical analysis that seeks to deter­mine the composition of a sample in terms of which chemical elements are present and their quantities or concentrations. Unlike other methods of elemental analysis, however, the sample is decomposed into its constituent atoms which are then probed spectroscopically.

In routine atomic spectrometry, a device called the atom source or atom cell is responsible for producing atoms from the sample; there are many different kinds of atom sources. After atomization of the sample, any of several techniques can determine which atoms are present and in what amounts, but the most common are atomic absorption, atomic emission, atomic fluorescence (the least used of these four alternatives), and mass spectrometry.

Most atomic spectrometric measurements (all those just mentioned except mass spectrometry) exploit the narrow-line spectra characteristic of gas-phase atoms. Be­cause the atom source yields atomic species in the vapor phase, chemical bonds are disrupted, so valence electronic transitions are unperturbed by bonding effects. As a result, transitions among atomic energy levels yield narrow spectral lines, with spectral bandwidths commonly in the 1-5-picometer wavelength range. Moreover, because each atom possesses its unique set of energy levels, these narrow-band transitions can be measured individually, with little mutual interference. Thus, sodium, potassium, and scandium can all be monitored simultaneously and with minimal spectral influence on each other. This lack of spectral overlap remains one of the most attractive features of atomic spectrometry.

In atomic absorption spectrometry, light from a primary source is directed through the atom cell, where a fraction of the light is absorbed by atoms from the sample.

The two most common kinds of atom cells employed in atomic absorption spectrom­etry are chemical flames and electrical furnaces. Chemical flames are usually simple to use, but furnaces offer higher sensitivity.

The most common primary light source employed in atomic absorption spectrometry is the hollow-cathode lamp. Conveniently, the hollow-cathode lamp emits an extremely narrow line spectrum of one, two, or three elements of interest. As a result, the atomic absorption spectrometry measurement is automatically tuned to the particular spectral lines of interest.

In atomic emission spectrometry, atomic species are measured by their emission spectra. For such spectra to be produced, the atoms must first be excited by thermal or nonthermal means. Therefore, the atom sources employed in atomic emission spec­trometry are hotter or more energetic than those commonly used in atomic absorption spectrometry. Although several such sources are in common use, the dominant one is the inductively coupled plasma. From the simplest standpoint, the inductively coupled plasma is a flowing stream of hot, partially ionized (positively charged) argon. Power is coupled into the plasma by means of an induction coil.

There are two common modes for observing emission spectra from an inductively coupled plasma. The less expensive and more flexible approach employs a so-called slew-scan spectrometer, which accesses spectral lines in rapid sequence, so that a num­ber of chemical elements can be measured rapidly, one after the other. Moreover, because each viewed elemental spectral line can be scanned completely, it is possible to subtract spectral emission background independently for each element. The alter­native approach is to view all spectral lines simultaneously, either with a number of individual photo-detectors keyed to particular spectral lines or with a truly multichan­nel electronic detector driven by a computer. This approach enables samples to be analyzed more rapidly and permits transient atom signals (as from a furnace-based atomizer) to be recorded.

Elemental mass spectrometry has been practiced for many years in the form of spark­source mass spectrometry and, more recently, glow-discharge-lamp mass spectrometry. However, a hybrid technique that combines the inductively coupled plasma with a mass spectrometer has assumed a prominent place.

At the high temperatures present in an inductively coupled plasma, many atomic species occur in an ionic form. These ions can be readily extracted into a mass spec­trometer.

The advantages of the combination of inductively coupled plasma and mass spec­trometry are substantial. The system is capable of some of the best detection limits in atomic spectrometry, typically 10-3 to 10-2 ng/ml for most elements. Also, virtually all elements in the periodic table can be determined during a single scan. The method is also capable of providing isotopic information, unavailable by any other atomic spectrometric method for such a broad range of elements.

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عن Akram Amir El Ali

استاذ الكيمياء التحليلية ومصمم غرافيك

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