Residues procedure

The best way to test the practicability of the multi-residue approach is to start with the GC determination step. Most often the inability to vaporize the intact compound means that it is not possible to include a new pesticide in the multi-residue scheme. In the case of common moiety methods, a decomposition step is needed to produce the common analyte. Often for that step, modification of the reaction conditions (such as pH and temperature) are necessary, which would lead to a significant deviation from standard multi-residue procedures. 

Calibration data (e.g., linearity or sensitivity) are not discussed in detail between laboratories, but a typical calibration starts with 50% of the lowest fortification level and requires at least three additional calibration levels. Another point of calibration is the use of appropriate standards. In 1999 a collaborative study tested the effect of matrix residues in final extracts on the GC response of several pesticides.Five sample extracts (prepared for all participants in one laboratory using the German multi-residue procedure) and pure ethyl acetate were fortified with several pesticides. The GC response of all pesticides in all extracts was determined and compared with the response in the pure solvent. In total, 20 laboratories using 47 GC instruments... 

The determination, evaluation and calculation of residues procedures are described in Sections 2.2.5-2.2.1. 

Lee and Stokker [161] have developed a multi-residue procedure for the quantitative determination of 11 triazines in non saline waters by a gas chromatographic method using a nitrogen-phosphorus detector. Ametryne, Atraton, Atrazine, Cyanazine, Prometron, Prometryne, Propazine, Simazine, Simetone and Simetryne were used. All of them could be successfully quantified on both the Ultrabond 20m and 3% OV-1 columns. Extraction was by methylene chloride and clean-up on Florasil. Recoveries of triazines at 10, 1.0... 

Parallel, and infrequently duplicative, efforts of the type described here were judged by the authors to be justifiable in light of the recent worldwide interest in the herbicide glyphosate in addition to the fact that the only previously published residue procedure for the herbicide (17) has generally become to be regarded as lengthy, cumbersome and subject to low recoveries. 

Some practical applications of the concepts discussed earlier have recently been encountered in our laboratory. During the initial stages of the development of a multi-residue procedure for chlorinated phenols (14), several problems were encountered. 

Many of the analytical procedures for pentachlorophenol in the recent literature do not call for a hydrolysis step prior to extraction of a urine sample. During the course of our research in the development of a reliable multi-residue procedure for chlorinated phenols, we found that much more pentachlorophenol could be extracted from the urine if the sample was hydrolyzed with hydrochloric acid (23). 

As mentioned previously, the formation of the methyl derivative is similar to several methods that already have been developed for organotin pesticides. The utilization of this derivative in conventional gas chromatographic residue procedures presents many challenges. Electron capture detection is sensitive enough but requires extensive clean-up while with FID and FPD the sensitivity is lower than desired. The development of a routine GC/MS procedure (single or multiple ion monitoring) was chosen then as the only possible alternative. A SIM (selective ion monitoring) method is specific for retention time and as well as for characteristic ions (m/e). 

In analyses of extract from freeze-dried fish [89] gel chromatography on Bio-Beads S-X3 with cyclohexane/ethyl acetate (1/1) was used for removal of neutral fat. Then adsorption column chromatography on silica gel (1.5% water) according to the multi-residue procedure of Specht and Tillkes [93] was applied. Elution with n-hexane separated TCBTs and PCBs from the main portion of organochlorine pesticides and other more polar compounds. The hexane extract was evaporated to 0.5 ml for GC-MS determination [89,90]. 

When incorporated into regulatory residue procedures, these methods work satisfactorily with various plant and animal extracts down to levels of 0.02-0.1 ppm, depending on the pesticide concerned (Table I). 

Following this way, the standard weighted residuals procedure together with finite element approximation of both fields of the hydrostatic stress ct and the hydrogen concentration C [6] may be adopted to develop corresponding procedure for diffusion modelhng coupled with the stress analysis. 

O Keeffe MJ, O Keeffe M, Glennon JD. Supercritical fluid extraction (SFE) as a multi-residue procedure for beta-antagonists in bovine liver tissue. Analyst 1999 124 1355-1360. 

Bioassays for BT toxin are sensitive because the test animals function as a concentrator and a detector simultaneously. When exposed to very low concentrations, the toxin is accumulated as the animal feeds over a period of days. Compared to immunoassays, bioassays for the BT toxins are generally 50-100 times more sensitive (Table II). As with other residue procedures, sample work-up can be employed to extend the detection limits of an enzyme immunoassay and to remove interfering materials at the same time. Such work up will be essential if immunoassays are to be as sensitive as bioassays using filter feeding organisms. 

It is seen from Table 8.1 that Galerkin appears to be the most accurate method for this specific problem. However, when more terms are used in the trial solution, the Galerkin method presents more analytical difficulties than the collocation method. As a matter of fact, all the weighted residual procedures... 

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