Potential Interferences

The use of the above assays to evaluate the toxicity of SPMD extracts is not without potential interferences. Sabaliunas et al. (1999, 2000) examined the potential role of oleic acid and elemental sulfur as contributors to the toxicity of 

Using a preemptive approach, Lebo et al. (2004) have shown that oleic acid and methyl oleate can be removed from triolein prior to use of the triolein in SPMDs (see Chapters 4 and 5). Dialysates from SPMDs prepared using triolein purified by the Lebo et al. (2004) method exhibited lower acute toxicity (Microtox assay) than SPMDs prepared with unpurified triolein. Also, the YES assay demonstrated that the purification method had removed all background estrogenic activity from SPMD extracts. Eor these reasons, the use of triolein purified by the method of Lebo et al. (2004) is standard for all SPMD studies conducted at CERC, USGS. Also, SPMDs with triolein purified by the Lebo et al. (2004) method are available from the commercial vendor upon request. 

Ankley, G.T. Tillitt, D.E. Giesy, J.P. Jones, P.D. Verbrugge, D.A. 1991, Bioassay-derived 2,3,7,8-tetrahlorodibenzo-/ -dioxin equivalents in the flesh and eggs of Lake Michigan Chinook Salmon and possible implications for reproduction. Can. J. Fish. Aquat. Set 48 1685—1690. 

Bailey, R.E. 1957, The effect of estradiol on serum calcium, phosphoms, and protein of goldfish. J. Exp. Zool 136 455 69. 

Beauvais, S.J. 1997, Eactors Affecting Cholinesterase in Aquatic Animals. Ph.D. Thesis, Iowa State University, Ames Iowa. 

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Were there any potential interferences that had to be eliminated If so, how were they treated ... 

Barnett and colleagues developed a new method for determining the concentration of codeine during its extraction from poppy plants. As part of their study they determined the method s response to codeine relative to that for several potential interferents. For example, the authors found that the method s signal for 6-methoxycodeine was 6 (arbitrary units) when that for an equimolar solution of codeine was 40. 

The accuracy of a method depends on its selectivity for the analyte. Even the best methods, however, may not be free from interferents that contribute to the measured signal. Potential interferents may be present in the sample itself or the reagents used during the analysis. In this section we will briefly look at how to minimize these two sources of interference. 

The quantitative analysis for reduced glutathione in blood is complicated by the presence of many potential interferents. 

When the analytical method s selectivity is insufficient, it may be necessary to separate the analyte from potential interferents. Such separations can take advantage of physical properties, such as size, mass or density, or chemical properties. Important examples of chemical separations include masking, distillation, and extractions. 

In some situations the rate at which a precipitate forms can be used to separate an analyte from a potential interferent. For example, due to similarities in their chemistry, a gravimetric analysis for Ca + may be adversely affected by the presence of Mg +. Precipitates of Ca(01T)2, however, form more rapidly than precipitates of Mg(01T)2. If Ca(01T)2 is filtered before Mg(01T)2 begins to precipitate, then a quantitative analysis for Ca + is feasible. 

Many samples containing silicon also contain aluminum and iron. After dehydration, these metals are present as AI2O3 and Fe203. These oxides are potential interferents since they also are capable of forming volatile fluorides. 

In Chapter 7 we examined several methods for separating an analyte from potential interferents. For example, in a liquid-liquid extraction the analyte and interferent are initially present in a single liquid phase. A second, immiscible liquid phase is introduced, and the two phases are thoroughly mixed by shaking. During this process the analyte and interferents partition themselves between the two phases to different extents, affecting their separation. Despite the power of these separation techniques, there are some significant limitations. 

Selectivity in FIA is often better than that for conventional methods of analysis. In many cases this is due to the kinetic nature of the measurement process, in which potential interferents may react more slowly than the analyte. Contamination from external sources also is less of a problem since reagents are stored in closed reservoirs and are pumped through a system of transport tubing that, except for waste lines, is closed to the environment. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Methods for iodine deterrnination in foods using colorimetry (95,96), ion-selective electrodes (94,97), micro acid digestion methods (98), and gas chromatography (99) suffer some limitations such as potential interferences, possibHity of contamination, and loss during analysis. More recendy neutron activation analysis, which is probably the most sensitive analytical technique for determining iodine, has also been used (100—102). 

Few of the assumed conditions are fully satisfied in practice. The last three items relate to potential interference between separated phases. Such interference can occur and leads to poor sedimentation performance if an excessive volume of the sedimented phase is retained in the centrifuge. 

The principal route of macroHde excretion is by way of the Hver. Effects of macrohdes on hepatic metaboHc enzymes, particularly cytochrome P-450, have been studied in order to identify and reduce potential interference with metaboHsm of other dmgs (21—23,444—447). Several macrohdes are initially... 

Each interference type may require a different elimination strategy, the simplest of which is to remove the potential interference before beginning the assay. Removal is commonly done in manual analysis, usually by deproteini2ation, eg, by addition of some denaturing agent such as trichloroacetic acid. 

Describe the potential interferences (a) in the nondispersive infrared (NDIR) method for measuring CO and (b) in the chemiluminescent method for measuring NO2. 

Take photographs, as appropriate, and detailed notes concerning visible airborne contaminants, work practices, potential interferences, movements, and other conditions to assist in determining appropriate engineering controls. 

Alternatively, LC is used for the separation and quantification of PAHs using both UV and fluorescence detection. The analytes are identified based on their relative retention times and UV and/or fluorescence emission spectra. For UV detection an efficient cleanup is a prerequisite since this detection method is not very selective (almost universal for PAHs), and hence it also responds to many coeluting compounds. Due to the high specificity of fluorescence detection for most PAHs, this LC detection method is less susceptible to potential interferences. As in the case of GC the apphcation of internal standard(s) is mandatory since solvents have to be evaporated during the cleanup, which may result in partial losses of some of the more volatile analytes. 

Once the sample has been processed in such a way as to maintain residue stability, to prevent cross-contamination, and to ensure homogeneity, strategies to extract the drug from the tissue and to isolate the drug residue from potential interferences must be evaluated. The following sections will review these two concepts separately. 

Either because of potential interference with other functional groups present in the molecule or because of special structural features, the following reactions require careful selection of reagents and reaction conditions. Identify the special requirements in each reactant and suggest appropriate reagents and reaction conditions for each transformation. 

Solvent extraction is defined as the process of separating one constituent from a mixture by dissolving it into a solvent in which it is soluble but in which the other constituents of the mixture are not, or are at least less soluble. The three main reasons for using solvent extraction are (i) to isolate a component or analyte of interest (ii) to remove potential interferents from a matrix and (iii) to preconcentrate an analyte prior to measurement. 

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