Sample preparation, generally method performance

Considering the numerous applications, heart-cut LC-LC has convincingly proven its value. Nevertheless, in LC-LC specific method development is generally needed for each analyte. Moreover, heart-cut procedures require accurate timing and, therefore, the performance of the first analytical column in particular should be highly stable to thus yield reproducible retention times. This often means that in LC-LC some kind of sample preparation remains necessary (see Table 11.1) in order to protect the first column from proteins and particulate matter, and to guarantee its lifetime. 

Validation of methods for quantitative determination of impurities includes precision studies. Repeatability is generally assessed by analysis of the same sample or samples prepared by the same analyst in replicate assays within a short duration of time. Repeatability should be assessed using (i) a minimum of nine determinations covering the specified range for the procedure (e.g., three concentrations/three replicates each) or (ii) a minimum of six determinations at 100% of the test concentration. Repeatability is evaluated by averaging the mean results from replicate assays and calculating the standard deviation (SD) and RSD. Repeatability of the method can be stated as either SD or RSD values. If an instrument is required for assay performance, then the same instrument should be used for the replicate assays. 

Validation is needed to demonstrate that the analytical method complies with established criteria for different performance characteristics [82]. When these different characteristics are being evaluated individually, this is generally done for the analytical method as such—where the input is the purified or isolated analyte and the output is the analytical result. However, MU covers the whole analytical procedure, starting from the original sample lot. The assessment of MU (see Section 8.2.2) is in line with the so-called modular validation approach. Modular validation refers to the modularity of an analytical procedure divided up into several sequential steps needed to analyze the material. These may be sample preparation, analyte extraction, and analyte determination (Figure 7). Each step in the procedure can be seen as an analytical system and can thus be validated separately and combined... 

Limits of detectability for the desired elemental analyses vary depending upon the matrix, elements, methods of sample preparation, and quality of instrumentation applied. Generally, these are on the order of 1 to 100 parts per million. The limit of detectability, however, is only one criterion in evaluating methods of analysis. The liiue of analysis is important, particularly in production and process control laboratories, in multi element spectrometers, it is possible to perform as many as 30 simultaneous elemental determinations in from 20 to 120 seconds, depending upon the material being analyzed. 

Because preparation involves specialized procedures and instruments, most of analytical laboratories have Sample Preparation Sections, such as Organic Extraction and Metal Digestion shown in Figure 4.2. (The General Chemistry Section does not have a separate preparation group, as sample preparation is usually part of the analytical procedure). Because laboratory accuracy and precision strongly depend on the individual s technique, sample preparation personnel must be trained in each procedure, and their proficiency be documented. The laboratory must have a set of SOPs for preparation methods performed and must ensure that the Sample Preparation Group personnel are trained to follow them to the letter. 

In a modern laboratory, automated computer software for data acquisition and processing performs most of data reduction. Raw data for organic compound and trace element analyses comprise standardized calibration and quantitation reports from various instruments, mass spectra, and chromatograms. Laboratory data reduction for these instrumental analytical methods is computerized. Contrary to instrumental analyses, most general chemistry analyses and sample preparation methods are not sufficiently automated, and their data are recorded and reduced manually in laboratory notebooks and bench sheets. The SOP for every analytical method performed by the laboratory should contain a section that details calculations used in the method s data reduction. 

Catalyst manufacturers are continually seeking new catalyst markets. Small catalyst samples for testing are generally available at no or moderate cost. Sometimes these require confidentiality agreements as discussed earlier or a nonanalysis agreement. This pledges the recipient of the catalyst not to try to determine the catalyst composition or method of preparation. If catalyst performance in a process is the objective, these restrictions are no problem. 

This book intends to supply the basic information necessary to apply the methods of vibrational spectroscopy, to design experimental procedures, to perform and evaluate experiments. It does not intend to provide a market survey of the instruments which are available at present, because such information would very soon be outdated. However, the general principles of the instruments and their accessories, which remain valid, are discussed. Details concerning sample preparation and the recording of the spectra, which is the subject of introductory courses, are assumed to be known. Special procedures which are described in monographs, such as Fourier transformation or chemometric methods, are also not exhaustively described. This book has been written for graduate students as well as for experienced scientists who intend to update their knowledge. 

With the exception of immunoaffinity extraction, which is a specialized and elaborate sample-preparation approach [27,28], solid-phase extraction generally provides the cleanest extract of all sample-preparation techniques in terms of selectivity. The price paid for this performance is that method development is generally the most complex and time consuming [29—31]. Generic conditions for automated 96-well solid-phase greatly reduced the need for extraction method development and work for about 85% of the small organic molecule analytes typically encountered in drug discovery [32]. These approaches are described later. 

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