Matrix destruction, atomic spectroscopy

In most cases the determination of organometallic complexes by atomic spectroscopy techniques is the only acceptable method because the analysis is selective, accurate and precise. Analysis of these complex salts may only involve a simple dilution in a solvent or destruction methods depending on the matrix it is formulated into. The presence of some sample matrices containing organometallic complexes can be severely restricted by the matrix material to achieve accurate detection and quantification of these salts. 

Decomposition involves the liberation of the analyte (metal) of interest from an interfering matrix by using a reagent (mineral/oxidizing acids or fusion flux) and/or heat. The utilization of reagents (acids) and external heat sources can in itself cause problems. In elemental analysis, these problems are particularly focused on the risk of contamination and loss of analytes. It should be borne in mind that complete digestion may not always be required as atomic spectroscopy frequently uses a hot source, e.g. flame or inductively coupled plasma, which provides a secondary method of sample destruction. Therefore, methods that allow sample dissolution may equally be as useful. 

A number of methods exist for the determination of parts-per-billion (ng/g) levels of chromium in aqueous media (Table 8.1). These are repeatedly reviewed as new techniques are introduced (4,5,6). Potentially all these techniques could be applied to petroleum samples after matrix destruction, but in practice, only a few have been utilized. After wet oxidation of a large sample (> 100 g), 10 to 50 fig of chromium may be determined by a colorimetric procedure with 1,5-diphenylcarbohydrazide after iron, copper, molybdenum, and vanadium are extracted as the cup-ferrates (3). In survey analyses, Cr levels as low as 5 ng/g have been measured by optical emission spectroscopy after ashing (2,3) or directly by neutron activation with extended irradiation and counting times (1). Concentrations of chromium above 100 ng/g in used lubricating oils have been measured directly by flame atomic absorption (8) for lower concentrations, heated vaporization atomic absorption (HVAA) has been utilized (9). In the Trace Metals Project, two procedures using this latter technique were evaluated for the determination of 10 ng Cr/g in a variety of petroleum matrices. 

Various measurement techniques have been used after matrix destruction. Small amounts of mercury are generally determined by conversion from an ionic species in aqueous solution to the elemental vapor, which is measured spectroscopically by atomic fluorescence, ultraviolet, or atomic absorption techniques (1,5, 6,9,10,11,12,13,14,15,16). Review articles covering the determination of small amounts of mercury in organic and inorganic samples (17) and the determination of mercury by nonflame atomic absorption and fluorescence spectroscopy (18) have recently appeared. In certain instances detection limits of 1 ng/g have been possible. 

To further xmderstand the mechanisms of complexations. Palmer et al. [95] went on to use a non-destructive spectroscopic technique. X-ray photoelectron spectroscopy (XPS), to investigate the interactions of the drug (P-HCl)-polymer (Na CMC) complex. This technique provides a quantitative determination of the elemental surface composition of the tablet matrix. The peaks fits of C (Is), O (Is), N (Is) and Cl (2p) are shown in Figure 2.7. For the analyzed APENa CMC PEO hydrated tablet, C (Is) peak at 288.48 eV is a strong indication of an amine group attached to the carbon atom. 

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