Solvent exchange aqueous

Sample preparation techniques vary depending on the analyte and the matrix. An advantage of immunoassays is that less sample preparation is often needed prior to analysis. Because the ELISA is conducted in an aqueous system, aqueous samples such as groundwater may be analyzed directly in the immunoassay or following dilution in a buffer solution. For soil, plant material or complex water samples (e.g., sewage effluent), the analyte must be extracted from the matrix. The extraction method must meet performance criteria such as recovery, reproducibility and ruggedness, and ultimately the analyte must be in a solution that is aqueous or in a water-miscible solvent. For chemical analytes such as pesticides, a simple extraction with methanol may be suitable. At the other extreme, multiple extractions, column cleanup and finally solvent exchange may be necessary to extract the analyte into a solution that is free of matrix interference. 

For pesticide residue immunoassays, matrices may include surface or groundwater, soil, sediment and plant or animal tissue or fluids. Aqueous samples may not require preparation prior to analysis, other than concentration. For other matrices, extractions or other cleanup steps are needed and these steps require the integration of the extracting solvent with the immunoassay. When solvent extraction is required, solvent effects on the assay are determined during assay optimization. Another option is to extract in the desired solvent, then conduct a solvent exchange into a more miscible solvent. Immunoassays perform best with water-miscible solvents when solvent concentrations are below 20%. Our experience has been that nearly every matrix requires a complete validation. Various soil types and even urine samples from different animals within a species may cause enough variation that validation in only a few samples is not sufficient. 

Labile species are usually main group metal ions with the exception of Cr2+ and Cu2+, whose lability can be ascribed to Jahn-Teller effects. Transition metals of classes II and III are species with small ligand field stabilization energies, whereas the inert species have high ligand field stabilization energies (LFSE). Examples include Cr3+ (3d3) and Co3+ (3d6). Jahn-Teller effects and LFSE are discussed in Section 1.6. Table 1.9 reports rate constant values for some aqueous solvent exchange reactions.8... 

The increase in ionic radius from Be2+ to Mg2+, which is accompanied by an increase in coordination number from 4 to 6, is responsible for a substantial increase in lability (Table III, (37-43)). The two activation volumes measured are positive as well as all the activation entropies. The rate laws determined for non-aqueous solvents in inert diluent are first order, showing a limiting D mechanism for all solvent exchange reactions on [MgS6]2+. 

Solvent exchange on Pd2+ and Pt2+ complexes shows a variation in lability of about 16 orders of magnitude and is generally characterized by either negative or near zero AV values. The exchange of non-aqueous solvents has been studied in inert diluents and was found to have a... 

Tetrahydrofuran (THF) is another important process solvent often used to solvate reactions involving strong bases. The workup of strong base reactions often includes an aqueous extraction, creating the problem of contaminated aqueous waste because THF is completely water-miscible. Typically, elaborate steps such as solvent exchange by distillation are taken to avoid THF contact with water. This is an energy-intensive process, and a more economical solution is desirable. Hatton s group has examined the... 

DNPH is often susceptible to formaldehyde or acetone contamination. It should, therefore, be crystallized with acetonitrile to remove any impurities. Repeated crystallization may further be performed to achieve the desired level of purity for DNPH. A 100-mL aliquot of aqueous sample is buffered with a citrate buffer and pH adjusted to 3 0.1 with HC1 or NaOH. The acidified sample is then treated with DNPH reagent and heated at 40°C for an hour under gentle swirling. The DNPH derivatives of aldehydes and ketones formed according to the above reaction are extracted with methylene chloride using liquid-liquid extraction. The extract is then solvent exchanged to acetonitrile for HPLC determination. 

A 200-mL aliquot of an aqueous sample was pH buffered and derivatized with DNPH. The DNPH derivatives were extracted with methylene chloride and the extract was solvent exchanged to 50 mL acetonitrile. Analysis by HPLC-UV showed the presence of methyl ethyl ketone (MEK) derivative which was quantitated as 2.7 mg/L in the extract, using the calibration standards prepared from the solid derivative. Determine the concentration of the MEK in the sample. 

Aqueous samples are extracted with methylene chloride using a separatory funnel or a continuous liquid-liquid extractor. Solid samples are extracted with methylene chloride-acetone mixture (1 1) by either sonication or Soxhlett extraction. The methylene chloride extract should be finally exchanged to hexane or iso-octane or methyl tert-butyl ether. The latter solvents should be mixed with acetone during solvent exchange. The extracts should then be cleaned up by Florisil. Often Florisil cleanup reduces the percent recovery of analyte to less than 85%. A preliminary screening of the extract should, therefore, be done to determine the presence of interference and the necessity of florisil cleanup. Gel permeation cleanup also lowers the analyte recovery and thus is not recommended. If a FPD is used in the GC analysis, the presence of elemental sulfur can mask the analyte peaks. In such a case, sulfur cleanup should be performed. Sample extraction and cleanup procedures are described in Chapter 1.5. 

Aqueous samples repeatedly extracted with methylene chloride extracts combined and concentrated by evaporation of methylene chloride solvent exchanged to hexane florisil cleanup (for removal of interferences) extract analyzed on GC-ECD or GC/MS. 

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