Atomic absorption detectors

Emphasis was therefore put on analytical procedures able to determine many elements in parallel and/or requiring almost no previous separation. procedures preferred were X-ray fluorescence using a Am source and Si(Li)-detector, atomic absorption spectrophotometry, gamma spectrometry using tracer isotopes and Ge(Li)-detector and acid-base titrations with recording of the pH-volume derivative. Table 2 summarises the use of these methods for the different elements, and it also gives a rough indication of interferences, sensitivity and accuracy obtained. 

Atomic-absorption and atomic-emission spectrophotometers both require an atomizer, a monochromator, and a detector. Atomic absorption requires, in addition, a radiation source. 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

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... 

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. 

Figure 24, The basic principle used in atomic absorption. The sample is sprayed into the flame, and the calcium and magnesium emission from the lamp is absorbed. The extent of absorption is measured on the detector arm translated in terms of concentration.

Table 8.80 shows the present status of speciation methodology. For trace-metal speciation, atomic absorption detectors feature a relatively high absolute detection limit (10 pg level), as compared to the 0.1 to 1 pg sensitivity level for molecular ion MS techniques as well as for MIP-AES. The detection limit of LEI-ToFMS is in the attogram range. Speciation has been reviewed [550]. Various monographs deal with speciation analysis [542,551,552]. 

Andreae [712] used four different detectors in his investigations the electron capture detector (for the methylarsines), the quartz cuvette atomic absorption detector (for arsenic and antimony species), the graphite furnace atomic... 

The electron-capture detector was originally found to be a sensitive detector for the methylarsines [716]. After improvements of the atomic absorption detectors had been made (especially concerning adsorptive losses and peak shapes of the methylarsines), it was found that this detector could be used to replace the electron-capture detector, which because of its lack of specificity and its sensitivity to contamination and changes in operating conditions was very inconvenient to work with. 

The simplest analytical method is direct measurement of arsenic in volatile methylated arsenicals by atomic absorption [ 11 ]. A slightly more complicated system, but one that permits differentiation of the various forms of arsenic, uses reduction of the arsenic compounds to their respective arsines by treatment with sodium borohydride. The arsines are collected in a cold trap (liquid nitrogen), then vaporised separately by slow warming, and the arsenic is measured by monitoring the intensity of an arsenic spectral line, as produced by a direct current electrical discharge [1,12,13]. Essentially the same method was proposed by Talmi and Bostick [10] except that they collected the arsines in cold toluene (-5 °C), separated them on a gas chromatography column, and used a mass spectrometer as the detector. Their method had a sensitivity of 0.25 xg/l for water samples. 

Another variation on the method [4] with slightly higher sensitivity (several ng/1) used the liquid nitrogen cold trap and gas chromatography separation, but used the standard gas chromatography detectors or atomic absorption for the final measurement. These workers found four arsenic species in natural waters. 

Atomic absorption spectrometry used either by direct aspiration (to determine total mercury) or as an element-specific detector for gas chromatography (to determine organically bound mercury) are now discussed. 

The techniques used for the investigation of organotin compounds in seawater are atomic absorption spectrometry, gas chromatography, or gas chromatography using AAS as detector. 

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). 

A problem encountered with atomic absorption is that emission from the flame may fall on the detector and be registered as negative absorption. This can be eliminated by modulating the light source, either mechanically or electronically, and using an a.c. detector tuned to the frequency of modulation of the source. D. C. radiation, such as emission from the flame, will then not be detected. A high intensity of emission, however, may overload the detector, causing noise fluctuations. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Atomic fluorescence spectrometry has a number of potential advantages when compared to atomic absorption. The most important is the relative case with which several elements can be determined simultaneously. This arises from the non-directional nature of fluorescence emission, which enables separate hollow-cathode lamps or a continuum source providing suitable primary radiation to be grouped around a circular burner with one or more detectors. 

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