Chromatographic detectors, atomic absorption

Chlorophenols, determination of 102-104, 285, 350 Chlorophylls, determination of 104-106,203-205, 248-260 Chromatographic detectors, atomic absorption 32-33 fluorescence 29-31 infrared 31, 32 inductively coupled plasma atomic emission 33-35 Raman 31, 32 visible 29 Chromate, determination of 60-65 Chromium, determination of 166, 234, 235,477-481... 

AgN03 = silver nitrate CICN = cyanogen chloride CN" = cyanide ion CNATC = cyanides not amenable to chlorination (Rosentreter and Skogerboe 1992) AAS = atomic absorption spectroscopy EPA = Environmental Protection Agency FIA = flow injection analysis GC/ECD = gas chromatograph/electron capture detector HCN = hydrogen cyanide NaOH = sodium hydroxide NIOSH = National Institute for Occupational Safety and Health... 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

Investigations of lead speciation in various environmental samples have relied upon gas and liquid chromatographic separations coupled to mass spectrometric and atomic absorption spectrometric detectors. The combination of atomic absorption spectrometry with gas chromatography (GC-AAS) has proved to be the most widely applied technique. Sample types have included air, surface water, air particulates, sediments, grass, and clinical materials such as blood. A review of speciation analyses of organolead compounds by GC-AAS, with emphasis on environmental materials, was published (Lobinski et al., 1994). 

Multielement analysis will become more important in industrial hygiene analysis as the number of elements per sample and the numbers of samples increases. Additional requirements that will push development of atomic absorption techniques and may encourage the use of new techniques are lower detction and sample speciation. Sample speciation will probably require the use of a chromatographic technique coupled to the spectroscopic instrumentation as an elemental detector. This type of instrumental marriage will not be seen in routine analysis. The use of Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) (17), Zeeman-effect atomic absorption spectroscopy (ZAA) (18), and X-ray fluorescence (XRF) (19) will increase in industrial hygiene laboratories because they each offer advantages or detection that AAS does not. 

Before the development of efficient chromatographic separation techniques and selective and sensitive detectors, analytical methods for the determination of specific analytes in environmental samples were very limited. Those methods depended on highly selective chemical reactions that are relatively rare and difficult to discover, or on very selective physical measurements such as atomic absorption or emission techniques for elemental analytes. Therefore only a relatively few analytical methods for the most common and amenable organic and inorganic compounds or... 

Brinkman, F.E., Blair, W.R., Jewett, K.L., and Iverson, W.P. Application of a liquid chromatograph coupled with a flameless atomic absorption detector for speciation of trace organometallic compounds, J. Chrom. Sci. 15, 493-503 (1977). 

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. 

The gas chromatograph is equipped with an electron capture detector or with an atomic absorption detection system using samples of 1-50 ml. 

As noted earlier, the most widely used piece of equipment after the headspace module is a gas chromatograph, which is in turn connected to a suitable (flame ionization, electron capture, mass spectrometric, atomic absorption, atomic emission) detector. Some high-resolution detectors including mass spectrometers have been directly connected to the HS module. 

A widely used photometer used as a high-pressure liquid chromatographic (HPLC) detector uses the intense 254-nm resonance line produced by a mercury arc lamp (see Chapter 6). Others employ a miniature hollow cathode lamp as a very-narrow-wavelength intense source. For example, a zinc hollow cathode lamp gives a line at 214nm that is adequately close to the maximum wavelength of peptide bond absorption (206 nm) so that it can be used to measure peptides and proteins. Details on the hollow cathode lamp are found in the section on Atomic Absorption Spectrophotometry. The hollow cathode lamp also has a long, useful Hfetime if a lower-current, nonpulsed power supply is used. j... 

Besides the fractionation of metalloids through SEPs, a series of methods has been proposed for the determination of individual species in the various oxidation states (Gong et al., 2002 Kahakachchi et al., 2004). The most popular detectors for metalloid speciation are inductively coupled plasma-mass spectrometry (ICP-MS) and atomic fluorescence spectrometry (AES), especially after liquid chromatographic separation and hydride formation, which are increasingly replacing atomic absorption spectrometry (AAS). Speciation analysis of pollutants in terrestrial environments is, however, beyond our scope in this chapter. 

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. 

Currently, fewer ihan ten companies worldwide manufacture CH in.struments. Some two dozen companies offer supplies and accessories for CF. The initial cost of equipment and the expense of maintenance for CF are generally significantly lower than those for ion chromatographic and atomic spcctro.scopic instruments. I hus, commercial CF instruments with standard absorption or fluorescence detectors cost 10,(KK) to 65.(XK), Addition of mass spectromctric detection can raise the cost significantly. 

After this brief review of theory, let us turn our attention to existing practice, as exemplified In environmental methods of analysis. Environmental methods of analysis employ many of the common analytical Instruments In analyzing a wide spectrum of chemicals In a variety of matrices. Instruments commonly used Include spectrophotometers (atomic absorption, visible. Inductively coupled plasma), gas chromatographs (with a variety of detectors. Including the mass spectrometer), and automatic analyzers. 

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