Atomic absorption spectroscopy detectors

Detector Detection in FIA may be accomplished using many of the electrochemical and optical detectors used in ITPLC. These detectors were discussed in Chapter 12 and are not considered further in this section. In addition, FIA detectors also have been designed around the use of ion-selective electrodes and atomic absorption spectroscopy. 

Atomic Absorption Spectroscopy. Mercury, separated from a measured sample, may be passed as vapor iato a closed system between an ultraviolet lamp and a photocell detector or iato the light path of an atomic absorption spectrometer. Ground-state atoms ia the vapor attenuate the light decreasiag the current output of the photocell ia an amount proportional to the concentration of the mercury. The light absorption can be measured at 253.7 nm and compared to estabUshed caUbrated standards (21). A mercury concentration of 0.1 ppb can be measured by atomic absorption. 

A schematic diagram showing the disposition of these essential components for the different techniques is given in Fig. 21.3. The components included within the frame drawn in broken lines represent the apparatus required for flame emission spectroscopy. For atomic absorption spectroscopy and for atomic fluorescence spectroscopy there is the additional requirement of a resonance line source, In atomic absorption spectroscopy this source is placed in line with the detector, but in atomic fluorescence spectroscopy it is placed in a position at right angles to the detector as shown in the diagram. The essential components of the apparatus required for flame spectrophotometric techniques will be considered in detail in the following sections. 

Tin compounds are converted to the corresponding volatile hydride (SnH4, CH3 SnH3, (CH3 )2 SnH2, and (CH3 >3 SnH) by reaction with sodium borohydride at pH 6.5 followed by separation of the hydrides and then atomic absorption spectroscopy using a hydrogen-rich hydrogen-air flame emission type detector (Sn-H band). 

Essentially the same spectrometer as is used in atomic absorption spectroscopy can also be used to record atomic emission data, simply by omitting the hollow cathode lamp as the source of the radiation. The excited atoms in the flame will then radiate, rather than absorb, and the intensity of the emission is measured via the monochromator and the photomultiplier detector. At the temperature achieved in the flame, however, very few of the atoms are in the excited state ( 10% for Cs, 0.1% for Ca), so the sample atoms are not normally sufficiently excited to give adequate emission intensity, except for the alkali metals (which are often equally well determined by emission as by absorption). Nevertheless, it can be useful in cases where elements are required for which no lamp is available, although some elements exhibit virtually no emission characteristics at these temperatures. 

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... 

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. 

In 1955, A. Walsh recognized this and showed how the absorption from the great preponderance of unexcited molecules could be exploited analytically.Thus, in atomic absorption spectroscopy (AAS) the light from a (usually modulated) somce emitting the spectrum of the desired analyte element is passed through a sample atomization cell (such as a flame or graphite tube furnace), a monochromator (to isolate the desired somce emission line) and finally into a detector to allow measmement of the change in somce line... 

Following the work of Lundegardh in the twenties, emission flame spectroscopy became established as an analytical tool in almost every branch of science. Although hollow cathode tubes were first studied by Paschen (P2) in 1916, and although atomic absorption spectroscopy had found occasional application, notably in the mercury vapor detector W20), it remained for Walsh (W2) in Australia in 1955 to recognize the essential advantages inherent in absorption over emission methods and revive general interest in this technique. Shortly thereafter but apparently independently, Alkemade and Milatz (A2, A3) in Holland devised instruments and applied atomic absorption spectroscopy in their laboratory. Walsh and his co-workers have since contributed a remarkable volume of work on instrumentation and application, and patents are held by Walsh on his method in Australia, Europe, and America. 

Olsen et al. (48, 20) have described an interesting method for the determination of lead in polluted seawater using FIA and flame atomic absorption spectroscopy. The system incorporates a Chelex-100 column for on-line preconcentration of the sample. The preconcentration and elution step improves the detection limit for lead by a factor of four (50 nM). Further increases in sensitivity are easily possible. The combination of this preconcentration step with a more sensitive detector, such as anodic stripping voltammetry, may make possible the determination of trace metals in seawater on a routine basis. 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

Detectors used in atomic absorption spectroscopy are usually photometric detectors. 

For pesticides, a combination of GC-MS and LC-MS techniques is used to analyze quantities in the ppb range. Special detector systems such as ECD (electron capture detector) and AAS (atomic absorption spectroscopy) are used for detection and quantification of halogen and heavy metal content. 

Volatile complexes of organic compounds of metals are, however, used more widely in GC analysis [45, 46, 211]. The main adwmti e of the GC analysis of volatile compounds of metals is the possibility of analysing trace amounts of metals with the use of ECDs and microwave emission detectors. When detectors of this type were used, GC methods were compared with such methods as neutron-activation analysis and atomic-absorption spectroscopy. The field of apphcation for this method is indicated in Table 1.5, illustrating the analysis of trace amounts of elements in the form of volatile complexes and compiled from data pubUshed in the literature. 

I) Analytical technique or method, occasionally unfeasible with the Involvement of an operator —this book abounds In Illustrative examples of this kind. Thus, electrothermal vaporization atomic absorption spectroscopy demands the automation of the sample thermal treatment In the graphite tube via a microprocessor programming the different heating stages involved (automation of methodology). Likewise, the use of Image detectors In spectroscopy calls for computerized data acquisition, impossible with manual operators. 

FIGURE 6-13. Resonance detector for atomic absorption spectroscopy. [From A. Walsh, Physical Aspects of Atomic Absorption, ASTM STP 443 (1968). Used by permission of the American Society for Testing Materials.]... 

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