Trace metals analytical losses

Widening interest in the quaHty of the environment has led to increased demand for information on a wide range of trace-metal contents of foodstuffs. Trace metals in foodstuffs are normally determined by spectroscopic techniques after complete destruction of the organic matrix. Destruction is achieved either by wet oxidation or by dry ashing additional treatment is normally required in order to obtain the metals of interest in a form suitable for analysis. Both methods of destruction are time consuming and tedious this is particularly true of the wet-oxidation procedure, which has the additional disadvantage of being potentially hazardous the methods require considerable analytical skill and experience. Both methods are prone to produce erroneous results either by the loss of an element of interest or by adventitious contamination from the component parts... 

Flame atomic absorption spectrometry can be used to determine trace levels of analyte in a wide range of sample types, with the proviso that the sample is first brought into solution. The methods described in Section 1.6 are all applicable to FAAS. Chemical interferences and ionization suppression cause the greatest problems, and steps must be taken to reduce these (e.g. the analysis of sea-water, refractory geological samples or metals). The analysis of oils and organic solvents is relatively easy since these samples actually provide fuel for the flame however, build-up of carbon in the burner slot must be avoided. Most biological samples can be analysed with ease provided that an appropriate digestion method is used which avoids analyte losses. 

Trace analysis has its special hazards for the unwary. The most important of these are loss of material in the analytical process and contamination by outside sources. Everyone realizes that trace constituents can be lost from samples, but few are aware of the many ways in which this can occur. For example, phosphate has been observed to disappear mysteriously from water samples in polyethylene bottles (10). Nitric acid, used to clean plastic vials, has been observed to convert these surfaces to ion exchangers, which readily take up as much as 10 12 moles per sq. cm. of trace metals (16). Lead nitrate solutions unless made distinctly acidic, plate out much of the lead on the walls of glass bottles. While everyone realizes that formation of a precipitate is liable to carry out trace constituents either by adsorption or occlusion, it is not as well-known that vanishingly small amounts of precipitates—amounts likely to be overlooked on casual observation—may also do this. The fly-ash and soot, which seem to be inescapable components of city air,... 

Trace metal concentrations in seawater are so low that contamination of the sample and loss of metal to container walls are critical problems in any analytical technique. These problems are particularly severe when the water sample must undergo extensive chemical treatment prior to the determination step. Most available techniques require such a chemical step or steps, because of their inadequate sensitivity and/or inability to determine the metal in the presence of the other sea water salts. Even those neutron activation procedures established for analysis of elements in seawater usually require a preactivation concentration step (2). 

The Trace Metals Project conducted a study to identify the type of container which would provide minimum losses of arsenic and mercury by precipitation, volatilization, adsorption, or diffusion. Solutions of organomercury and organoarsenic compounds added to petroleum feedstock were used. Because of the relative ease with which mercury and arsenic can be determined at sub-parts-per-million levels in a hydrocarbon matrix by instrumental neutron activation analysis (INAA), this technique was used for the analytical measurements. The solutions were stored in five different types of glass and/or plastic containers and sampled periodically over eight months (12). The results of the study are summarized in Tables 2.III and 2.IV. 

One of important aspects of the Trace Metals Projects was the cross-check program of proposed analytical methods. For example, evaluation of the wet digestion flame atomic absorption method for cadmium determination (13) involved the preparation of petroleum samples spiked with cadmium cyclohexanebutyrate. Before these samples were shipped to cooperating laboratories, a check of the cadmium content showed unexpectedly low recoveries. To confirm this apparent loss, portions of gasoline were spiked at the 30 ng/g level with cadmium cyclohexanebutyrate and cadmium sulfonate. The samples were stored in Teflon bottles and were analyzed over a month. The results are shown... 

Sample Collection and Storage. This step in an analytical study offers many opportunities for loss of integrity of samples, and must be described in full detail. Precisely what tools will be used to acquire the samples Disposable scalpels, for example, are coated with an oil that can contaminate tissues. Metal tools obviously should not be used to take samples for trace metal determination. Good judgement can not be assumed details must be provided. 

Losses of analyte elements may of course also violate sample integrity. In biomedical research, however, losses of trace metals are much less to be feared than additions of exogenous material. 

A particular problem associated with samples having very low (trace and ultra-trace) levels of analytes in solution is the possibility of losses by adsorption onto the walls of the container or contamination by substances being leached from the container by the sample solvent. Trace metals may be depleted by adsorption or ion-exchange processes if stored in glass containers, whilst sodium, fxrtassium, boron and silicates can be leached from the glass into the sample solution. Plastic containers should always be used for such samples. 

N1 and Zn from a graphite rod were significantly lower than from a tantalum filament, suggesting that these free metal atoms can be liberated by chemical reduction of their respective oxides, rather than by direct thermal dissociation. Findlay et al (19) emphasized the hazards of preatomlzatlon losses of trace met s In electrothermal atomic absorption spectrometry, when the ashing temperature Is permitted to exceed the minimum temperature for vaporization of the analyte. 

Usually, samples are presented for analysis as liquids. Thus, solid samples must be dissolved. Analytical or ultra-high-purity grade reagents must be used for dissolution to prevent contamination at trace levels. Certain volatile metals (e.g. cadmium, lead and zinc) may be lost when dry ashing, and volatile chlorides (e.g. arsenic and chromium) lost upon wet digestion. It is particularly easy to lose mercury during sample preparation. Appropriate steps must be taken in the choice of method of dissolution, acids and conditions (e.g. whether to use reflux conditions) to prevent such losses. 

The collection and preparation of water samples requires individual approaches for different analytical tasks. If heavy metals or long-lived radionuclides at the trace and ultratrace concentration range are to be determined in water samples by ICP-MS, especially careful sampling is necessary to avoid possible contamination (using clean bottles and containers washed and cleaned before use, for example, with 2 % nitric acid and high purity water to stabilize traces in the samples), and the loss of analyte by adsorption effects or precipitation should be also considered. 

Extensive analytical efforts to fully characterize the oil shales are underway at Exxon Research and Engineering Company s Baytown Laboratories. No significant losses of any metals of concern are observed during high temperature ashing. An alternate means of rapid ash determination uses a Parr combustion bomb. The ash can be dissolved by alkaline fusion in a Claisse fluxer or by acid dissolution in a Parr bomb. The solutions thus prepared are analyzed by atomic absorption or by inductively coupled plasma emission spectrometry for major (Al, Ca, Fe, K, Mg, Na, Si, Ti) and trace elements (As, B, Ba, Be, Cd, Co, Cr, Cu, Li, Mn, Mo, Ni, P, Sr, U, V, Zn). Kerogen enriched shales need to be ashed before the dissolution, otherwise low recoveries are obtained. Overall accuracy and precision of metals determination is within... 

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