Atomic absorption spectrometric detectors

A limitation of this technique is its lack of sensitivity compared to that avculable by other techniques (eg inductively coupled plasma atomic emission spectrometry). 

Suitable instrumentation is supplied by Thermoelectron, Perkin-Elmer, Varian Associates, GBC Scientific and Shimazu. All of these suppliers supply equipment with autoseunplers and mercury and hydride attachments. 

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Conversion to tetra-alkyl lead compounds using nBu MgCI then gas chromatography with atomic absorption spectrometric detector Petroleum ether extraction, glc... 

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). 

The performance of the flame atomic absorption spectrometric detector is enhanced substantially by the FI mode of sample introduction. Besides the decrease in sample volume already mentioned, additional contributions which may be of interest in separation and preconcentration systems include ... 

In addition to the thermal conductivity detector (TCD), flame photometric (FPD) and atomic absorption spectrometric (AAS) detectors, which offer high sensitivity and the advantage of selectivity, are also suitable for use with inorganic substances (Table 1.2). By-products from chemical reactions of the analytical process will not be detected. 

The association of a spectrometer with the liquid chromatograph is usually for the purpose of structure elucidation of the eluted solute, a procedure that will be discussed in a later chapter. The association of tui atomic spectrometer with the liquid chromatograph, in contrast, is almost exclusively for the specific detection of the metalic and semi-metalic elements. The atomic spectrometer is a highly specific detector, and for element detection perhaps more so than the electrochemical detector. However, in general, a flame atomic absorption spectrometric (AAS) system is not as sensitive. If an atomic emission spectrometer or an atomic fluorescence spectrometer is employed then multi-element detection is possible. The inductively coupled plasma spectrometer can also, under some circumstances, provide multi-element detection but all three instruments are extremely expensive particularly in terms of an LC detector. It follows that most LC/AAS combinations employ a flame atomic absorption spectrometer or occasionally an atomic spectrometer fitted with a graphite furnace. Furthermore the spectrometer is usually set to monitor one element only, throughout the development of any given separation. 

Pb(C2Hs)4 in air or other gas samples was determined, after appropriate sampling [428, 482] by gas chromatography using flame ionization [331, 357, 531, 565], electron capture detectors [239, 250, 306, 334], a photoionization detector [369], mass spectrometric detection [264, 531, 565, 574], or detection by atomic absorption spectrometric procedures [322, 328,... 

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. 

The contribution of flow analysis to improving the performance of atomic spectrometry is especially interesting in the field of standardisation. FIA can provide a faster and reliable method to relate the absorbance, emission or counts (at a specific mass number) to the concentration of the elements to be determined. In fact, flow analysis presents specific advantages to solving problems related to the sometimes short dynamic concentration ranges in atomic absorption spectrometry, by means of on-line dilution. The coupling of FI techniques to atomic spectrometric detectors also offers tremendous possibilities to carry out standard additions or internal standardisation. 

Atomic spectrometric methods Here, the entire sample is atomized or ionized either by flame or inductively coupled plasma and transferred into the detector. The most common techniques in this class are flame atomic absorption spectrometry (FAAS) and inductively coupled plasma mass spectrometry (ICPMS). A general characteristic of these methods is the determination of the total concentration of the analyte without the direct possibility of distinguishing its specific forms in the sample. 

AAS = atomic absorption spectrometry GC/FID = gas chromatography/f1ame ignition detector GC/FPD = gas chromatography/f1ame photometric detector ICP/AES = inductively coupled plasma atomic emission spectroscopy ICP/MS = inductively coupled plasma with mass spectrometric detection... 

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. 

In-line filtration without a filtering element is also feasible. To this end, a three-dimensional reactor [299], also called a knitted or knotted reactor (see 6.2.3.4), can be used, as emphasised in the landmark article reporting the flow injection determination of lead in blood and bovine liver by flame atomic absorption spectrometry [300]. The analyte was co-precipitated the complex formed was retained on the inner walls of a knitted reactor and then released by isobutyl methyl ketone and transported to the detector. Interference from iron(III) at high concentrations was circumvented, sensitivity was markedly improved and precise results were obtained. This innovation was recently exploited to remove organic selenium and determine the speciation of inorganic selenium in a flow-injection system with atomic fluorescence spectrometric detection [301]. 

The flame atomic absorption spectrometer is inherently a flow-through detector, with which the sample solutions are continuously fed into the nebulizer-burner system through suction. Despite the relatively large volume of the spray chamber (usually about 1(X) ml) in comparison to the spectrometric flow-cell, the detector was shown to have very little contribution to the dispersion of the injected sample in comparison to other components of the FI system [11]. With careful optimization, as little as 50-80 //I sample may be injected to achieve 80-95% of the steady state signal obtained by conventional sample introduction (see Fig. 2.14). 

Luminescence molecular detectors have also been used for online monitoring of dissolution tests and the characterization of toxic residues using bioluminescence assays. Atomic (atomic absorption spectroscopy, inductively coupled plasma-atomic emission spectroscopy (ICP-AES)) detectors have been coupled to robotic stations either through a continuous system acting as interface or by direct aspiration into an instrument from a sample vial following treatment by the robot. Mass spectrometric and nuclear magnetic resonance (NMR) detectors... 

A simple gas chromatographic method determines total arsenic in urine as arsine with a PN detector. The detection limit is 50 ng. The gas sample is separated on a 1.2 m glass column filled with Chromosorb 103 80-100 mesh at a furnace temperature of 30 C [153]. MMAA and DMAA are separated as thioglycolic acid methyl esters on a glass column (1.8 m, 2 mm ID) packed with Chromosorb G AW-DMCS coated widi 2.5% XE-60 for flame ionization detection. The detection limit is 10 ng [154]. Triphenylarsine formation is a more time-consuming method. But the combination of a gas chromatograph with a microwave emission spectrometric detector reaches a detection limit of SO ng/Iiter. The method is also applied for the determination of alkylarsenic acids [132,155], Atomic absorption spectrometers [134] and mass spectrometers [135] were also used as detectors of gas chromatographs. 

The utility of laser diodes for spectroscopic applications has been demonstrated in molecular absorption spectrometry, molecular fluorescence spectrometry, atomic absorption spectrometry, and as light sources for detectors in various chromatographic methods. Recent advances in laser diode technology fueled by consumer demand for high-speed, high-capacity DVD players have resulted in the availability of blue laser diodes with output powers up to 50 mW at 473 nm. These light sources are appearing routinely in commercial spectrometric systems. 

In fact, flow analysis presents specific advantages for solving problems related to the sometimes short dynamic concentration ranges in atomie absorption spectrometry, by means of on-line dilution. The eoupling of FI techniques to atomic spectrometric detectors also offers tremendous possibilities to carry out standard additions or internal standardisation. 

However, I is the emission intensity emitted over the whole space angle. Here, the % of space angle with which the radiation is collected into the optical system is to be considered, together with the transmittance of the spectrometer [according to Eq. (156) and the characteristics of the radiation detector, Eqs. (186-189)]. Evidently, in all the equations cited many constants are not exactly known and all types of atomic spectrometry are relative methods nevertheless, the intensities measured can be traced back to the concentrations of the analyte in the sample in a stringent way. This also applies to the other atomic emission, atomic absorption, atomic fluorescence, and mass spectrometric methods discussed here. 

Common gas chromatographic detectors that are not element- or metal-specific, atomic absorption and atomic emission detectors that are element-specific, and mass spectrometric detectors have all been used with the hydride systems. Flame atomic absorption and emission spectrometers do not have sufficiently low detection limits to be useful for trace element work. Atomic fluorescence [37] and molecular flame emission [38-40] were used by a few investigators only. The most frequently employed detectors are based on microwave-induced plasma emission, helium glow discharges, and quartz tube atomizers with atomic absorption spectrometers. A review of such systems as applied to the determination of arsenic, associated with an extensive bibliography, is available in the literature [36]. In addition, a continuous hydride generation system was coupled to a direct-current plasma emission spectrometer for the determination of arsenite, arsenate, and total arsenic in water and tuna fish samples [41]. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

Concerning the requirements of the detector, it is important to stress that interfacing a detector with an FIA system yields transient signals. Therefore, desirable detector characteristics include fast response, small dead volume and low memory effects. FI methods have been developed for UV and visible absorption spectrophotometry, molecular luminescence and a variety of electrochemical techniques and also for the most used atomic spectrometric techniques. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 274). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in table 8.1. Sources of radiation physically separated... 

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