Freeze-drying procedures, analytic

Before the extraction procedure may commence, the sample must be prepared in such a way that it is in a condition for extraction of the analyte(s). For analyzing sulfonamide residues in liquid samples such as milk, a pretreatment dilution step with water prior to direct fluorometric detection may be required (207). Dilution of milk with aqueous buffer (208) or sodium chloride solution (209) prior to sample cleanup has also been reported. For the analysis of honey a simple dissolution of the sample in water (210, 211) or aqueous buffer (212) is generally required. Semisolid samples such as muscle, kidney, and liver, require, however, more intensive sample pretreatment. The analyte(s) must be exposed to extracting solvents to ensure maximum extraction. The most popular approach for tissue break-up is through use of a mincing and/or homogenizing apparatus. Lyophilization (freeze-drying) of swine kidney has been carried out prior to supercritical-fluid extraction of trimethoprim residues (213). 

The main problem in carrying out total multielemental determinations in milk (as in other biological samples) is the nature of the matrix, which may interfere with the analytical technique employed for the measurement. In this sense, pretreatment of the samples becomes necessary so as to minimize matrix effects as much as possible (e.g., by destroying the organic matrix). An alternative to such destructive acid attacks is the direct analysis in milk whey samples by simply diluting the sample previously obtained by centrifugation. The main preparation procedures for milk samples (whole, skimmed, or freeze-dried) can be classified as follows (a) use of diluted solutions in order to minimize matrix and molecular... 

Accuracy and Analytical Quality Control Aspects The analysis of certified reference materials (CRMs) following the same analytical procedures was performed for assessment of the accuracy of the procedure and for quality control (QC). Yet, the available CRMs are mostly freeze-dried, not fresh or deep-frozen. In Table 22.3 an overview of CRMs in a seafood matrix with respect to organic Hg is given. It is certainly beneficial that more and more CRMs are becoming commercially available. Recently, a new CRM for trace elements in a matrix of lyophilized tuna fish (IMEP-20) has been produced [44]. Apart from total Hg (4.32 mg kg-1 dry mass) and Me-Hg (4.24 mg kg-1 dry mass), this material is also certified for other elements such as As, Pb, and Se. 

Freeze-dried serum certified for the determination of trace elements is used to check the accuracy and precision of analytical procedures for quantifying low levels of trace... 

Available procedures for removing aqueous matrices, which entails concentrating the analytes to be isolated, are quite varied in nature the best choice in each case depends on the properties of the analytes and interferents. Such procedures include freeze-drying [155], the use of sorbents (e.g. anhydrous sodium sulphate [156] or diatomaceous earth... 

The determination of these piesticides in freeze-dried water requires the use of complex analytical procedures, sample extraction, clean-up, including numerous sources of possible systematic errors. Different methods used in laboratories working independently and which had previously proven their ability to perform the request task were... 

The procedures used by different laboratories to extract, isolate, and analyze the biomarker hydrocarbon contents of sediments are rarely identical, but they differ largely only in their details. The general analytical scheme in which the fundamental elements common to the different procedures has been nicely summarized by Rullkotter (2000). For their analysis, hydrocarbons first need to be separated from sediments by extraction into an organic solvent. The most common solvent that is used for dried sediment samples is dichloromethane, although other solvents or mixtures of solvents are also employed. A risk involved with the common practice of first drying the sediments is that some organic compounds may become irreversibly associated with minerals and will not be released by subsequent solvent extraction. Moreover, freeze-drying of sediment samples can sometimes create a more serious problem — contamination of the natural hydrocarbon composition with pump oil vapors (Barwise et al., 1996) — unless suitable filters are employed. For these reasons, extraction of wet sediment samples is sometimes preferred. In these cases, solvents like methanol or acetone that can remove the sediment porewater are used first and then followed by mixtures of these hydrophilic solvents with non-polar solvents such as dichloromethane. 

The most reliable method of determining the polymer content of a bacterial cell culture or partially purified digest is to freeze-dry the sample and extract gravimetrically the PHB into chloroform solution. Indeed this is the technique used to calibrate all other methods but it is rather time-consuming and several more rapid analytical procedures have been developed. 

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