Detectors atomic absorption spectrometry

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. 

These vitamers are UV absorbers, but their detection is complicated by the low level present in foods and the low sensitivity of this detector. Other detectors, like flame atomic absorption spectrometry and inductively coupled plasma (1CP)-MS, may be applied, but without much increase in sensitivity. 

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. 

In atomic absorption spectrometry (AA) the sample is vaporized and the element of interest atomized at high temperatures. The element concentration is determined based on the attenuation or absorption by the analyte atoms, of a characteristic wavelength emitted from a light source. The light source is typically a hollow cathode lamp containing the element to be measured. Separate lamps are needed for each element. The detector is usually a photomultiplier tube. A monochromator is used to separate the element line and the light source is modulated to reduce the amount of unwanted radiation reaching the detector. 

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... 

AMS = accelerated mass spectroscopy FAAS = flame atomic absorption spectrometry GC/ED = gas chromatography/electron capture detector GFAAS graphite furnace atomic absorption spectrometry ICP-AES = inductively couples plasma-atomic absorption spectrometry NA = not applicable NAA = neutron activation analysis... 

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). 

J. M. Harnly, The future of atomic absorption spectrometry a continuum source with a charge coupled array detector, J. Anal. Atom. Spectrom., 14 (1999), 137. 

A. F. Silva, D. L. G. Borges, B. Welz, M. G. R. Vale, M. M. Silva, A. Klassen, U. Heitmann, Method development for the determination of thallium in coal using solid sampling graphite furnace atomic absorption spectrometry with continuum source, high-resolution monochromator and CCD array detector, Spectrochim. Acta, 59B (2004), 841. 

Further developments are expected due to the nonspecificity of the ECD detector. The possibility of coelution with other compounds has triggered the interest in interfacing chromatography systems to Hg-specific detectors. The volatility of zerovalent Hg and the ease of thermal decomposition of alkyl-Hg compounds have allowed for a wide variety of solutions that use cold vapor (CV) atomic absorption spectrometry (AAS) in a fused silica quartz cell. Significant improvements were observed when the atomization and detection were performed directly in the fused silica furnace at 780°C using an 02 flow and after considerable reduction of the transfer lines [26],... 

The second major environmental application of FFF has been the use of an element-specific detector, usually in series with a UV detector, to provide elemental composition data along with the PSD. Graphite-furnace atomic absorption spectrometry has been used off-line on fractions collected from the FFF run. However, the multi-element detection, low detection limits and capability to function as an online detector have made inductively coupled plasma mass spectrometry (ICP-MS) the ideal detector for FFE85-86 The sample introduction system of the ICP-MS is able to efficiently transport micron-sized particles into the high-temperature plasma,... 

Radiation absorbed by atoms under conditions used in atomic absorption spectrometry may be re-emitted as fluorescence. The fluorescent radiation is characteristic of the atoms which have absorbed the primary radiation and is emitted 1n all directions. It may be monitored in any direction other than in a direct line with radiation from the hollow-cathode lamp which ensures that tha detector will not respond to the primury absorption process nor to unabsorbed radiation from the lamp. The intensity of fluorescent emission is directly proportional to the concentration of the absorbing atoms but it is diminished by collisions between excited atoms and other species within the flame, a process known as quenching. Nitrogen and hydrocarbons enhance quenching, and flames incorporating either should be avoided or their effect modified by dilution with argon. 

Shuvaeva, O.V., Koscheeva, O.S., Beisel, N.F. Arsenic speciation in waters using HPLC with graphite furnace atomic absorption spectrometry as detector. Anal. Sci. 17, 179-181 (2001)... 

The application of atomic absorption spectrometry to quantitative analysis is illustrated in Figure 2.2. The incident radiation at resonance wavelength with intensity /q is focused on the flame containing the atoms in their fundamental state and is transmitted with a reduced intensity I determined by the concentration of the atoms in the flame. The radiation is directed to the detector where the intensity is measured. The quantity of absorbed light is determined by comparing / to /q. 

Early colorimetric methods for arsenic analysis used the reaction of arsine gas with either mercuric bromide captured on filter paper to produce a yellow-brown stain (Gutzeit method) or with silver diethyl dithiocarbamate (SDDC) to produce a red dye. The SDDC method is still widely used in developing countries. The molybdate blue spectrophotometric method that is widely used for phosphate determination can be used for As(V), but the correction for P interference is difficult. Methods based on atomic absorption spectrometry (AAS) linked to hydride generation (HG) or a graphite furnace (GF) have become widely used. Other sensitive and specihc arsenic detectors (e.g., AFS, ICP-MS, and ICP-AES) are becoming increasingly available. HG-AES, in particular, is now widely used for routine arsenic determinations because of its sensitivity, reliability, and relatively low capital cost. 

Mercury, the only metallic element with significant volatility at room temperature, has been conventionally determined for many years by atomic absorption spectrometry, as the mercury vapor detector (W20) is based on this principle. Lindstrom (L7) used a flame to volatilize the mercury in the liquid sample, but determined its concentration in the exhaust gases with the mercury vapor meter after cooling and purification in a filter that removed particulate matter. The method is said to be capable of detecting 0.1 pg % of mercury in the original liquid sample... 

Persson and Irgum determined sub-p.p.m. concentrations of DMAA in seawater by electrothermal atomic absorption spectrometry. Graphite-furnace atomic absorption spectrometry was used as a sensitive and specific detector for arsenic. The technique allowed DMAA to be determined in a sample (20 ml) containing a 10 -fold excess of inorganic arsenic with a detection limit of 0.02ng As ml ... 

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