Detector, atomic spectrometer specific

The association of a spectrometer with a liquid chromatograph is usually to aid in structure elucidation or the confirmation of substance identity. The association of an atomic absorption spectrometer with the liquid chromatograph, however, is usually to detect specific metal and semi-metallic compounds at high sensitivity. The AAS is highly element-specific, more so than the electrochemical detector however, a flame atomic absorption spectrometer is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed, then multi-element detection is possible as already discussed. Such devices, used as a LC detector, are normally very expensive. It follows that most LC/AAS combinations involve the use of a flame atomic absorption spectrometer or an atomic spectrometer fitted with a graphite furnace. In addition in most applications, the spectrometer is set to monitor one element only, throughout the total chromatographic separation. 

Transient signals are typically obtained in atomic spectrometry when samples are introduced by flow injection techniques or when the spectrometer is used as an element-specific detector in hyphenated techniques. Inductively coupled plasma mass spectrometry has nowadays become the detection technique of choice for multielement-specific detection in speciation as it allows multielemental... 

HPLC units have been interfaced with a wide range of detection techniques (e.g. spectrophotometry, fluorimetry, refractive index measurement, voltammetry and conductance) but most of them only provide elution rate information. As with other forms of chromatography, for component identification, the retention parameters have to be compared with the behaviour of known chemical species. For organo-metallic species element-specific detectors (such as spectrometers which measure atomic absorption, atomic emission and atomic fluorescence) have proved quite useful. The state-of-the-art HPLC detection system is an inductively coupled plasma/MS unit. HPLC applications (in speciation studies) include determination of metal alkyls and aryls in oils, separation of soluble species of higher molecular weight, and separation of As111, Asv, mono-, di- and trimethyl arsonic acids. There are also procedures for separating mixtures of oxyanions of N, S or P. 

The working principle of a helium-specific mass spectrometer leak detector is as follows [7], The mass spectrometer mainly consists of three parts an ion source, a deflection system and an ion collector. When the atoms pass the ionisation chamber of the detector, they are ionised to become positive ions. When these ions are then accelerated in the ionisation chamber, they obtain sufficient energy and enter into the magnetic field of the detector. These positive ions travel in a specific mbit in the magnetic field. As different ions have specific orbit radii only ions with the same orbit radius as that of the spectrometer can pass a narrow gap, as indicated by K in Figure 2.13, and reach the ion collector K to form an ion stream. 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

Fig. 4.7 Schematic drawing of an atomic absorption spectrometer Light of a particular wavelength, absorbed by a specific element, is focused upon an atomized sample and the amount of that light that is absorbed is measured by the detector. The amount of light missing is proportional to the amount of a specific element in the atomized sample...

D. Chakraborli, D.C.J. Hillman, K.J. Irgolic, and R.A. Zingaro, Hitachi Zeeman graphite furnace atomic absorption spectrometer as a selenium-specific detector for ion chromatography. Separation and determination of selenite and selenate, j. Chromatogr., 249,81,1982. 

The gas chromatograph may be interfaced with atomic spectroscopic instruments for specific element detection. This powerful combination is useful for speci-ation of different forms of toxic elements in the environment. For example, a helium microwave induced plasma atomic emission detector (AED) has been used to detect volatile methyl and ethyl derivatives of mercury in fish, separated by GC. Also, gas chromatographs are interfaced to inductively coupled plasma-mass spectrometers (ICP-MS) in which atomic isotopic species from the plasma are introduced into a mass spectrometer (see Section 20.10 for a description of mass spectrometry), for very sensitive simultaneous detection of species of several elements. 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

Bye, R. and Paus, P. E. (1979) Determination of alkylmercury compounds in fish tissue with an atomic absorption spectrometer used as a specific gas chromatographic detector. Anal. Chim. Acta, 107,169-175. 

Table 3.4 compares detection limits with secondary fluorescers to the results with the RMF method and 15-kV broadband excitation [16,17]. Four different fluorescence analyzers were tested (units A, B, C, and D), and the results were corrected for differences in performance for the energy-dispersive spectrometers employed on each unit. Unit A used a chromium anode tube, and unit B used a tungsten anode tube. Unit A was a commercial, general-purpose instrument. Unit B was specifically designed for atmospheric aerosol analysis, where closer coupling between the tube, fluorescer, sample, and detector could be employed with some sacrifice of insensitivity to specimen-positioning errors. Table 3.5 lists the x-ray tube operating conditions required for Table 3.4. For medium- to high-atomic-number elements, the secondary fluorescer method provides detection limits equivalent to the RMF element, but requires much higher x-ray tube power. For light elements.

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