Adsorption interference with analysis

A potentially more sensitive and selective approach involves reaction of formic acid with a reagent to form a chromophore or fluorophore, followed by chromatographic analysis. A wide variety of alkylating and silylating reagents have been used for this purpose. Two serious drawbacks to this approach are that inorganic salts and/or water interfere with the derivatisation reaction, and these reactions are generally not specific for formic acid or other carboxylic acids. These techniques are prone to errors from adsorption losses, contamination, and decomposition of the components of interest. Enzymic techniques, in contrast, are ideal for the analysis of non-saline water samples, since they are compatible with aqueous media and involve little or no chemical or physical alterations of the sample (e.g., pH, temperature). 

In starting a residue analysis in foods, the choice of proper vials for sample preparation is very important. Available vials are made of either glass or polymeric materials such as polyethylene, polypropylene, or polytetrafluoroethylene. The choice of the proper material depends strongly on the physicochemical properties of the analyte. For a number of compounds that have the tendency to irreversible adsorption onto glass surfaces, the polymer-based vials are obviously the best choice. However, the surface of the polymer-based vials may contain phthalates or plasticizers that can dissolve in certain solvents and may interfere with the identification of analytes. When using dichloromethane, for example, phthalates may be the reason for the appearance of a series of unexpected peaks in the mass spectra of the samples. Plasticizers, on the other hand, fluoresce and may interfere with the detection of fluorescence analytes. Thus, for handling of troublesome analytes, use of vials made of polytetrafluoroethylene is recommended. This material does not contain any plasticizers or organic acids, can withstand temperatures up to 500 K, and lacks active sites that could adsorb polar compounds on its surface. 

Artifacts from the resin can interfere with the chromatographic analysis of the XAD resin extract. For example, the artifact may be a pollutant being studied, or coelution of the resin artifacts and compounds of interest may occur during capillary gas chromatographic (GC) analysis. Artifacts can also take part in competitive adsorption during sampling. This situation can cause sample breakthrough because certain compounds are preferentially collected. 

A strong adsorption of dyes observed in IC columns was not encountered in CE when analyzing chloride, sulfate and phosphate in sulfonated dyes [52]. Proteins need not be removed prior to CE analysis and do not interfere with low-molecular-mass ions as is the case in IC [53]. 

Similarly, the calibration graph may be non-linear, particularly if peak heights are used as a quantitative parameter, and during manipulations with low concentrations of the solute when its adsorption on the surface of the support, column walls, etc., occurs to a significant extent. Fig. 1.2 illustrates the improvement that was obtained by the conversion of the sample compound. In the direct determination of morphine by GC, the dependence of the ratio of the peak height of morphine to that of squalene on the amount of compound injected is non-linear and therefore quantitative evaluation is difficult. An analogous calibration graph for the TMS derivative, in contrast, is linear. Hence, if a suitable derivative is used a drawback that could interfere with the GC analysis itself can be overcome [2]. 

The preferred sample preparation method for residual solvent analysis of pharmaceuticals is direct injection of the dissolved sample (11,60). With this technique, the recovery is most reliable because there is no opportunity for recovery loss due to adsorption or entrapment. The other techniques involve a separation of the volatiles before the GC injection and there is a risk that the volatile will be trapped. Typical solvents for this analysis are water, dimethyl sulfoxide, benzyl alcohol, and dimethylformamide (11,12,61). The three latter solvents are chosen because they are higher-boiling than commonly used pharmaceutical solvents and thus elute after them and do not interfere with the analysis. Water offers the advantage that it contributes little interference with a flame ionization detector. 

A number of studies have investigated interferences with the LAL assay and have attempted to optimize the assay (Douwes et al., 1995 Hollander et al., 1993 Thorne et al., 1997). These studies have demonstrated that results may vary depending upon the sample matrix, the extraction method and the assay method. Other constituents present in the sample may interfere with the LAL assay and cause inhibition or enhancement of the test or aggregation and adsorption of endotoxins, resulting in under- or over-estimation of the concentration. Techniques such as spiking with known quantities of purified endotoxin and analysis of dilution series of the same sample have been described to deal with these interferences (Hollander et al., 1993 Milton et al., 1990 Milton et al., 1992 Whitakker, 1988). Studies in the laboratories of the authors of this chapter have demonstrated within-Iaboratory coefficients of variation between 15 % and 20 % in routine assay work. When extra care is taken to optimize precision this can be reduced to under 5 % in the endpoint chromogenic assay (Thorne, unpublished data). Several interlaboratory comparison studies have been performed and demonstrate much greater variability. One in-depth comparison of two laboratories experienced in the LAL assay. 

In addition to spectroscopic techniques, microcalorimetry, which measures transition enthalpies (AH) and transition temperatures (To.) as proteins undergo cooperative unfolding, has also been applied to the study of conformational changes resulting from protein adsorption. Since PS latex particles show no thermal transitions over the temperature range of interest for protein structural analysis, then-presence does not interfere with this technique. HSA, a three-domain protein whose... 

Nonlinearity of the Langmuir adsorption isotherms is observed even in non-competitive chromatographic processes. Individual adsorption isotherms can be found experimentally using frontal analysis at overload conditions however, the adsorption isotherms in the separation of mixtures are different because of the interference with other compounds. In PHPLC method development efforts to optimize preparative chromatography using software... 

If a quick analysis is not possible, samples should be preserved with the addition of ultrapure HNO3 (up to pH less than 2), which prevents precipitation of metal hydroxides or adsorption of metal ions on the walls of the container. In addition, samples should be cooled down to 4°C in order to minimize microbial activity. An additional advantage of refrigeration as a preservation method is that it neither affects the sample composition nor interferes with any analytical method. Furthermore, refrigeration helps to retain in solution several elements (i.e., Hg, As, Se, Cd, and Zn), which may be lost due to volatilization at increasing temperatures. In the particular case of Hg analysis, the addition of 10% K2Cr207 is recommended to conserve it for a few days. 

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