Comparing sample preparation techniques

LLE, SPE, SPME, and SBSE applications for the extraction of semivolatile organics from liquids were discussed. Others [134,153,154] have compared sample preparation techniques. When examined collectively for perspective, the sample processing techniques can be perceived as variations on a single theme as practiced by today s analysts (Figure 2.55). 

MAP makes use of physical phenomena that are fundamentally different compared to those applied in current sample preparation techniques. Previously, application of microwave energy as a heat source, as opposed to a resistive source of heating, was based upon the ability to heat selectively an extractant over a matrix. The fundamental principle behind MAP is just the opposite. It is based upon the fact that different chemical substances absorb microwave energy... 

It is apparent from Chapter 3 that new sample preparation technologies generally are faster, more efficient and cost effective more easily automated and safer use smaller amounts of sample and less organic solvent provide better recovery and meet or exceed precision and accuracy compared to traditional sample preparation techniques. Conventional methods of the analysis of additives in polymers are mostly based on the separation of the polymer matrix and additives by means of extraction. Many extraction principles are... 

A comparative study of extraction methods from formalin-fixed mouse liver by Jiang et al.11 highlights the importance of sample preparation technique. Formalin-fixed mouse tissue was extracted using five different protocols (1) 6M guanidine-HCl without heating (2) 6M guanidine-HCl... 

The good recoveries reported above by some earlier workers in SFE clearly demonstrate the feasibility and comparative accuracy of SFE as a sample preparation technique for a variety of analytes and matrices. However, the question of the "robustness" of SFE as an analytical tool easily used on a routine basis by chemists throughout the analytical community remains to be answered. What is the expected variability of results for any given application and how much user interaction is required to maintain functional equipment sample after sample Such information - precision and mean-time-between-failures (or mean-time-between-maintenance) - has not been routinely reported in published literature since the emphasis has heretofore been on initial feasibility experiments. 

TABLE 1.4. Advances in Sample Preparation Techniques Compared with Advances in Detection Technologies... 

The purpose of sample preparation is to create a processed sample that leads to better analytical results compared with the initial sample. The prepared sample should be an aliquot relatively free of interferences that is compatible with the HPLC method and that will not damage the column. The whole advanced analytical process can be wasted if an unsuitable preparation method has been employed before the sample reaches the chromatograph. Specifically, analytical work with samples from fermentation processes require a sample pre-treatment that eliminates the fermentation broth before the analytes can be injected into the chromatographic columns. This is primarily to remove macromolecular sample constituents, which easily clog the columns. Complex matrices often require a more selective sample preparation than for instance pharmaceutical solutions. In practice the choice of sample-preparation procedure is dependent on both the nature and size of the sample and on the selectivity of the separation and detection systems employed. Sample pre-treatment may includes a large number of methodologies. Ideally, sample preparation techniques should be fast, easy to use and inexpensive. In papers I and II careful sample pre-treatment was performed before all injections. 

The methods available in most literature are not necessarily suitable for all samples and often do not include the most modern sample preparation techniques. Most methods often require proof of their reliability by including recovery studies to support confidence in the analysis. Analysis of an unknown plastic material with unknown levels of metals often requires extreme care in all stages of analysis when compared with a known plastic and using a well tried and developed method that has background information. In some cases where measurements of extremely low levels of metals are required, entirely different and more sensitive methods are often used taking special precautions in avoiding contamination and interference. 

An interesting paper comparing several sample preparation techniques was published by Jennings and Filsoof (1 2). In this work, equal amounts of ten aroma chemicals (Table II) were combined and then subjected to various isolation techniques. 

Similarly, the sample preparation must be complementary to the instrumental method. Chen and Pollack (1) compared four sample preparation techniques for a capillary electrophoresis assay of a peptide in human plasma. Table 3 is a... 

Coupling a microfluidic or microfabricated system to MS appears to be fruitful for both the detection on the chip (microchip point of view) and for the MS analysis (MS point of view). On the one hand, MS is a powerful technique for on-chip detection due to its sensitivity and the amount of information it provides on the sample on the other hand, by using microfluidics prior to the MS analysis, new opportunities for the field of MS are created as it provides better MS capabilities compared to conventional sample preparation techniques. 

Dixon EM, Gardner MJ and Hudson R (1997) The comparability of sample preparation techniques for the determination of metals in sediments. Che-mosphere 35 2225-2236. 

The first publication that reported the use of LC—MS for quantification of IsoPs in urine used reversed-phase LC coupled with ESI/MS. The method used only 1 mL urine and the clean-up procedure using solid phase extraction (SPE) columns gave quantitative recovery of the IsoP. The chromatographic runs and SPE purification methods are short, and this results in a very easy and user-friendly procedure (Li et al., 1999). A comprehensive review by Tsikas et al. describes sample preparation techniques and compares GC—MS methods with the most recent LC—MS/MS methods (Tsikas et al., 2003). The focus is primarily on 8-f5o-PGF2ci and highlights the difficulty to detect only one IsoP isomer without immunoaffinity chromatography preparation. The large concentration differences for the various IsoP classes in urine are also addressed. 

Attempts to develop automated methods from first principles require that sufficient time, hiunan skills, and resource are available and that these are not critical elements in the method development equation. Therefore, before beginning any work, it is important that sufficient information be assembled to enable a risk and technical feasibility assessment to be carried out on the proposed automated sample preparation method. Thus, it is important that the method selected comprises sample preparation processes that are amenable to automation. The techniques that have the greatest potential for automation are solid-phase extraction and high-performance liquid chromatography. Other sample preparation techniques such as liquid-liquid extraction, protein precipitation, and ultrafiltration are either difficult to automate or may not be cost-efficient to do so compared with alternative approaches. 

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