Contamination analytical signals

The optimal analytical GDMS instrument for bulk trace element analysis is the one providing the largest analytical signal with the lowest background signal, the fewest problems with isobaric interferences in the mass spectrum (e.g., the interference of with Fe ), and the least contamination from instrument com-... 

Total metal concentrations are often very low in natural systems and in performing speciation analysis it is necessary to measure even lower concentrations on attempting to resolve component species. Therefore, very sensitive methods are needed and there is a high risk of contamination, alteration and/or adsorption losses. The ideal speciation method would be sufficiently sensitive and selective to be used directly on natural water samples, would involve minimal perturbation of the sample, and would furnish an analytical signal directly dependent on the (chemical) reactivity of the element of interest (Buffle, 1981a) (Fig. 8.1). 

The active ingredients are commonly divided into chemical classes, such as pyrethroids, chloroorganic, and organophosporus. Coextracted substances may affect the analyte signal. To avoid this matrix effect, standard solutions should be prepared by using an extract from a non-contaminated sample, or a calibration curve calculated by the standard addition method. 

Atomic spectrometric tests these tests, taken at the 589.00/589.59 nm doublet, may be too sensitive for qualitative analysis, as contaminations from reagents can cause significantly high analytical signals. The test should be repeated at the 330.23/330.30 nm doublet, where sensitivities are lower. Colder air/acetylene flames are sufficient. 

Evaluation of contamination is usually through the use of blank samples, i.e., samples that do not contain any analyte. As analytical techniques become more sensitive, and lower and lower concentrations are measured, blank samples are very important. The blank samples aid an interpretation of the result and add confidence. This is important where comparative methods of analysis are used such as chromatography or ultraviolet spectrophotometry. In these instances, the absence of an analyte signal in the blank sample allows an analyst to infer that the signal in the samples and standards is, in fact, due to the analyte. 

The generation of the analytical signal through a nuclear reaction makes the determinations virtually independent of the chemical form of the elements and their chemical environment. The determinations are, to a certain degree, independent of the sample matrix, and in most cases, can be made free of any blank. The blank-free attribute is aided by the fact that the methods require much less sample manipulation than other techniques and they are not as susceptible to reagent and laboratory contamination. The reduction or elimination of blank contributions includes radiochemical processing performed after irradiation (see Sect. 30.5), where inadvertent chemical contamination would add only nonactivated analyte that would not contribute to the measured signal. 

The DESI solvent, and solvent and gas flow rates, should be optimized for the specific tissue and analyte. Under optimal conditions, only the most energetic part of the DESI spray plume should be able to extract the analyte out of the tissue, otherwise the washing effect (15) will result in cross contamination and signal and resolution loss. Also, if the gas and/or the solvent flow rates are high, it can extensively damage the tissue section. It is advised to sacrifice a tissue section to optimize these parameters for maximum ion signal and resolution. 

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