Microwave trace analysis

Raw foods were freeze-dried and analyzed for carbon isotopes using mass spectrometry. Cooked foods were prepared following historic recipes, then were freeze-dried prior to analysis. For the trace element analysis, foods (both raw and cooked) were wet ashed using nitric acid in Teflon lined pressure vessels and digested in a CEM Microwave oven. Analysis of Sr, Zn, Fe, Ca and Mg was performed using Atomic Absorption Spectrometry in the Department of Geology, University of Calgary. 

HPLC-QFAAS is also problematical. Most development of atomic plasma emission in HPLC detection has been with the ICP and to some extent the DCP, in contrast with the dominance of the microwave-induced plasmas as element-selective GC detectors. An integrated GC-MIP system has been introduced commercially. Significant polymer/additive analysis applications are not abundant for GC and SFC hyphenations. Wider adoption of plasma spectral chromatographic detection for trace analysis and elemental speciation will depend on the introduction of standardised commercial instrumentation to permit interlaboratory comparison of data and the development of standard methods of analysis which can be widely used. 

Nearly every area of measurement science can boast of progress in measuring ever-smaller quantities of chemicals, but several stand out in their stunning trace-analysis capabilities. Trace-metal analysis has come to be dominated by methods that volatilize the sample and then either measure its spectroscopic emission or absorption, or measure the masses of the gaseous metal ions using mass spectrometry. Volatilization is accomplished by various thermal means that include flames, furnaces, and inductively coupled or microwave plasmas. The com-... 

Table 6.5 Selected applications of microwave-assisted extraction in trace analysis...

The concentration of metals that are detrimental to catalysts added can vary between 20.0 ppm for Fe to 100 ppm for Ni and lOOOppm for V. The presence of these metals necessitates the need for analysis of these metals to determine their concentrations prior to the cracking process. The best method to analyse these oil samples needs to be rapid and accurate. Careful selection of the method either from experience or by trial and error may be applied depending on the metal and the concentration. Sample dissolution in a solvent or solvent mixture is considered the easiest but may not be suitable for low limits of detection. Destructive sample preparation methods, i.e. oxygen bomb combustion, microwave acid digestion followed by pre-concentrating may be required for trace analysis and/or with the aid of a hyphenated system, e.g. ultrasonic nebuliser. Samples prepared by destmctive methods are dissolved in aqueous solutions that have very low matrix and spectral interferences. 

Many investigations have been carried ont of procednres for improving the analytical qnality of GC methods by changing the matrix, increasing the concentration of the pertinent analytes and redncing the interference of other componnds present in the sample. Preconcentration by LLE, before or after derivatization, is most freqnently apphed in GC trace analysis however, other techniqnes, snch as SPE, sample stacking (see Section V.A.l) and some of their modifications, snch as simnltaneons distillation and extraction (SDE) and SPME, are also mentioned. Application of microwave-assisted processes (MAP) dnring sample preparation seems to improve recoveries. 

Decomposition of these stone formations using nitric acid and perchloric acid, followed by a few drops of hydrofluoric acid in a microwave oven should be the method of choice for trace analysis of these stone formations using most analytical methods requiring the sample in solution. 

Fecher P.A. A. Nagengast. (1994). Trace analysis in high matrix aqueous-solutions using helium microwave-induced plasma-mass spectrometry. /. Anal. At. Spectrom., Vol.9, ppl021-1027, ISSN 1364-5544... 

Sample Prep Web contains information and advice regarding analytical sample preparation, speciated analysis, trace analysis and microwave chemistry... 

For trace analysis, the main ceramic elements of interest are Zn, Pb, Cu, Bi, Sb, Sn, Ag, As, Mn, Cr, Se, and Hg. Many of these are environmentally important. In certain cases the detection limits of flame AAS are inadequate, so that hydride generation for antimony, selenium, arsenic and bismuth, cold vapor for mercury, and graphite furnace AAS for lead and cadmium are required. A variation of AAS is atomic fluorescence, and this is used to achieve the detection limits needed for Hg and Se in environmental samples. Microwave digestion techniques for sample preparation are becoming more common, where, unlike fusion, there is no risk of loss of volatile elements from unfired samples and fewer reagents are... 

ICP-AES is normally preferred for trace analysis because of its generally better detection limits, freedom from chemical matrix effects, and multi-element capability. When available, it is used instead of AAS. Like AAS, hydride and mercury vapor attachments can be used to enhance the detection limits of the elements listed above. As an alternative to normal sample preparation or microwave procedures, laser ablation is creeping in as an alternative approach. The same pressed discs used for XRE are ablated by an IR or UV laser in a chamber that replaces the spray chamber used normally. The sample preparation takes minutes rather than days or hours, eliminates reagents (even pure reagents are less pure than ceramic powders), and is less susceptible to external contamination. 

An AA spectrometer is also available with a graphite furnace and vapor generation accessories for the trace analysis of lead, antimony, arsenic, and mercury at parts-per-billion levels. AA is used for quantitative analysis of these metals in polymers as well as finished formulations. It has been used to determine the elemental composition of catalysts and plastic additives, polymer formulations, and composite materials. Samples may be rapidly acid digested prior to analysis using a microwave oven or similar techniques. Microwave furnaces are also available for dry ashing. 

Contaminated water samples are prepared for electrochemical heavy metal trace analysis either by UV photolysis or by oxidative microwave digestion. 

Microwave spectroscopy is used for studyiag free radicals and ia gas analysis (30). Much laboratory work has been devoted to molecules of astrophysical iaterest (31). The technique is highly sensitive 10 mole may suffice for a spectmm. At microwave resolution, frequencies are so specific that a single line can unambiguously identify a component of a gas mixture. Tabulations of microwave transitions are available (32,33). Remote atmospheric sensing (34) is illustrated by the analysis of trace CIO, O, HO2, HCN, and N2O at the part per trillion level ia the stratosphere, usiag a ground-based millimeter-wave superheterodyne receiver at 260—280 GH2 (35). 

Vol. 51 Trace Element Analysis of Geological Materials. By Roger D. Reeves and Robert R. Brooks Vol. 52 Chemical Analysis by Microwave Rotational Spectroscopy. By Ravi Varma and Lawrence... 

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