Flame Microsampling

Most FAAS determinations are performed using a continuous flow of solution taken up by the types of nebulizers described in Section 6.2.2.I. Uptake rates in these types of nebulizers can be as high as 5 mL/min, so a significant amount of sample solution is needed for a measurement. 

The Shimadzu Scientific Instruments AA-7000 has a flame microsampling option that uses a sampling port, which allows 50-100 pL of sample solution to be injected into the flame for a measurement. This microsampling option can be used when sample is in limited supply. In addition, it minimizes salt buildup on the burner and minimizes waste. The use of such small volumes allows autocalibration using one standard and also permits autodilution of the sample, simply by injecting less volume into the flame. More details are available at www. shimadzu.com. 

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Scale of Operation Atomic absorption spectroscopy is ideally suited for the analysis of trace and ultratrace analytes, particularly when using electrothermal atomization. By diluting samples, atomic absorption also can be applied to minor and major analytes. Most analyses use macro or meso samples. The small volume requirement for electrothermal atomization or flame microsampling, however, allows the use of micro, or even ultramicro samples. 

A microsampling system known as the Delve s cup is a hybrid of flameless and flame techniques. The sample is placed in a small crucible, which is held in the flame by means of a wire loop. The sample is ashed in a cooler part of the flame and then moved to the hotter part in order to cause the rapid vaporization of the element. The cup is held beneath an opening in a nickel or aluminium tube which is in the light path of the instrument. The atomic vapour... 

J.R. Sarbeck, P.A. St. John, J.D. Winefordner, Measurement of microsamples in atomic emission and atomic fluorescence flame spectrometry, Mikrochim. Acta 1 (1972) 55. 

The successful application of the CFD method in combination with subsequent gas chromatographic separation and the use of an BCD has resulted in the extensive development of this technique. However, other CFD methods aimed at obtaining derivatives that can be selectively detected by other selective detectors (e.g., sodium thermionic, flame photometric) have not been developed adequately, despite their obvious promise. It seems that the high selectivity of the method should be used for the elaboration of selective methods of functional group analysis in order to identify compounds at the picogram level. This is especially pertinent to the analysis of microsamples on capillary columns. 

In AAS, FIA has been applied to hydride generation and cold vapour techniques, microsampling for flame atomic absorption, analysis of concentrated solutions, addition of buffers and matrix modifiers, dilution by mixing or dispersion, calibration methods, online separation of the matrix and analyte enrichment, and indirect AAS determinations. 

Although most samples are commonly presented as liquids for atomic emission spectroscopy, direct solid sample analysis has the advantage that no major pretreatment or dissolution steps are required [44]. This minimises dilution errors or contamination from reagents and reduces the reagent and manpower cost per sample. In addition, improved detection Hmits may be obtained if microsamples or microanalysis are possible without any further dilution. However, the analyst has to ensure that the solid material sampled is representative of the bulk material. ICP-AES has generally a remarkable tolerance for total dissolved sohds compared to ICP-MS or flame AAS so that, depending on the overall matrix, between 2 and 25 % suspended sohds can be coped with. Therefore, most of the sohd sample introduction devices described below are dedicated for ICP-AES. 

Gaspar A, Berndt H. Thermospray flame furnace atomic absorption spectrometry (TS-FF-AAS)—a simple method for trace element determination with microsamples in the pg/1 concentration range. Spectrochim Acta B. 2000 55 587-97. 

In pneumatic nebulization for ICP-OES, continuous sample feeding requires a sample aspiration time of about 30 s so as to attain a stationary signal, a measurement time of around 5-10 s, and a rinsing time again of 30 s at minimum. However, discrete sampling is also possible with injection systems known from flame AAS [140, 141] and by flow injection. Work with sample aliquots of down to 10 xL then becomes possible, which is particularly useful, for example, in work with microsamples [156] or for the analysis of solutions containing high salt contents [443]. 

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