Metal analysis hydride generation methods

Maintaining the quality of food is a far more complex problem than the quality assurance of non-food products. Analytical methods are an indispensable monitoring tool for controlling levels of substances essential for health and also of toxic substances, including heavy metals. The usual techniques for detecting elements in food are flame atomic absorption spectroscopy (FAAS), graphite furnace atomic absorption spectrometry (GF AAS), hydride generation atomic absorption spectrometry (HG AAS), cold vapour atomic absorption spectrometry (CV AAS), inductively coupled plasma atomic emission spectrometry (ICP AES), inductively coupled plasma mass spectrometry (ICP MS) and neutron activation analysis (NAA). 

The analysis of petroleum products for the presence of toxic metals is an environmental requirement and results of analysis must be made known to all concerned bodies including the public. In order to establish the true concentrations present, a reliable method must be employed in order to be certain of the results. A sensitive method must be available and the use of an ICP coupled with an ultrasonic nebuliser, hydride generation or cold vapour method for Hg offer the most sensitive and reliable methods available. If analysing for lower levels of these metals, a mass spectrograph with hyphenated attachments may be required. 

Detection of extremely low levels of metals may be possible by the use of hyphenated techniques such as hydride generation, ICP-OES/graphite furnace, ultrasonic nebuliser and cold vapour trap for Hg, and by utilising the axial viewing mode of the ICP-OES could achieve results close to ICPMS levels. Table 7.14 shows a brief list of metals and methods that are commonly considered for food analysis. 

The most complicated speciation analysis is in plants and biological samples. Besides the compounds and ions already mentioned metals in living systems participate in a lot more complicated species. They may be bound to amino acids, proteins, peptides and the separation methods may influence their original distribution. For the determination of organo-arsenic, selenium, lead and tin hydride generation combined with different preconcentration steps may be used (Dedina and Tsalev, 1995). 

Hydride generation increases the power of detection of atomic spectrometric methods for the determination of certain elements, and allows their matrix-free determination. However, the technique is prone to a number of systematic errors. First, the hydride-forming elements must be present as inorganic compounds in a well-defined valence state. This may require sample decom x>sition prior to analysis. In water analysis, treatment with H2SO4/H2O2 may be effective [106]. Traces of heavy metals such as Cu" may have a catalytic influence on the formation and dissociation of the hydrides, as investigated by Welz et al. [107] in atomic absorption with quartz cuvettes. These in-terferents can be masked by complexation with tartaric acid or coprecipitated with La(OH>3. Calibration by standard addition is advisable. 

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