Extraction in sample preparation

The role of extraction in sample preparation consists in analyte concentration, clean-up and change of physical form. Extraction procedures, though time-consuming, have these advantages ... 

SOLVENT EXTRACTION IN SAMPLE PREPARATION AND PRETREATMENT STEPS... 

Toivo, J., Piironen, V., Kalo, P., and Varo, P. 1998. Gas chromatographic determination of major sterols in edible oils and fats using solid-phase extraction in sample preparation. Chroma-tographia 48 745-750. 

One of the main purposes of membrane extraction in sample preparation is to enrich the analyte, i.e., to increase the concentration of the analyte to permit determination of low concentrations. Plotting the concentration of analyte in the acceptor (Ca) either directly as determined by analysis of the acceptor phase or as a concentration enrichment factor Ee (Ca/Cs— where Cs is the initial concentration in the sample) versus time will typically produce a curve which initially raises approximately linearly and asymptotically eventually reaches a steady equilibrium value. See Figure 12.4 [46]. 

During the last few years, miniaturization has become a dominant trend in the analysis of low-level contaminants in food and environmental samples. This has resulted in a significant reduction in the volume of hazardous and expensive solvents. Typical examples of miniaturization in sample preparation techniques are micro liquid/liquid extractions (in-vial) and solvent-free techniques such as solid-phase microextraction (SPME). Combined with state-of-the-art analytical instrumentation, this trend has resulted in faster analyses, higher sample throughputs and lower solvent consumption, whilst maintaining or even increasing assay sensitivity. 

Miniaturisation of scientific instruments, following on from size reduction of electronic devices, has recently been hyped up in analytical chemistry (Tables 10.19 and 10.20). Typical examples of miniaturisation in sample preparation techniques are micro liquid-liquid extraction (in-vial extraction), ambient static headspace and disc cartridge SPE, solid-phase microextraction (SPME) and stir bar sorptive extraction (SBSE). A main driving force for miniaturisation is the possibility to use MS detection. Also, standard laboratory instrumentation such as GC, HPLC [88] and MS is being miniaturised. Miniaturisation of the LC system is compulsory, because the pressure to decrease solvent usage continues. Quite obviously, compact detectors, such as ECD, LIF, UV (and preferably also MS), are welcome. 

It is generally difficult to identify developments with high potential where interferences do not preclude general application. To ensure the relevance of a method, its application to real sample analysis must be demonstrated. The accuracy of an analytical method should be confirmed by an independent method, or by the analysis of certified reference materials. Detailed comparative studies of the method developed with other well-established methods for polymer/additive analysis are not frequent in the analytical literature. Nevertheless, some examples may be found in Section 3.6. Improvements in analytical techniques are reasonably sought in sample preparation and in hyphenated chromatographic techniques. However, greatest efficiency is often gained from the use of databases rather than accelerated extraction or hyphenation. 

Organic extraction and sample preparation Separatory funnel liquid-liquid extraction Continuous liquid-liquid extraction Solid-phase extraction (SPE) (3535A in update IVB)... 

The tissue of interest should be removed from the animals using equipment that has been treated to ensure that it is RNase-free. The surface hair of the animals can be soaked in 70% ethanol to minimize the inclusion of hair or dander in the isolated tissues. A sterile disposable Petri plate, placed on top of a bed of crushed ice, is a suitable RNase-free surface for microdissection. Immediately after isolation from the animal, individual samples of tissue can be flash frozen by immersion in liquid nitrogen and stored in cryovials in liquid nitrogen or at -70°C until all samples from an experimental set have been obtained. RNA extraction and preparation of single-stranded cDNA can then be performed simultaneously on all samples. This will minimize differences between RNA populations that result from differences in sample preparation. 

Poole, C. E, Principles and Practice of Solid-phase Extraction, In Sampling and Sample Preparation for Field and Laboratory Fundamentals and Hew Directions in Sample Preparation, Pawliszyn, J., Ed., Vol. XXXVII, Elsevier Science, Amsterdam, Netherlands, pp. 341-387, 2002. 

Determination Of MBOCA in Human Urine. MBOCA is commercially important as a curing agent for polyurethanes and epoxy resin systems. Since MBOCA was found to be carcinogenic in animals and is a suspected human carcinogen, it is important to have a reliable method available for the determination of MBOCA in the urine of those workers who are potentially exposed to this compound. In a NIOSH publication a GC method was described for the determination of MBOCA in urine. However, since HPLC does not require derivatization and a lower detection limit was expected, the GC method was modified to be performed by HPLC/ . In order to be able to compare both methods, we used the same extraction procedure. The extraction and sample preparation procedures are as follows ... 

SFE also would appear to have utility in sample preparation methods. Lopez-Avila et al. (1992) applied SFE to the recovery of a variety of analytes, including organophosphorus pesticides, from solid matrices. The unoptimized extraction from sand gave a recovery of 54% for diazinon. Supercritical trifluoromethane has been shown to extract diazinon from glass beads with a recovery of 86% (Hillmann and Bachmann 1995). Organophosphorus pesticides have also been recovered from Tenax-GC, an adsorbent used to collect diazinon during air sampling, and analyzed directly by GC (Raymer and Velez... 

Avoidance of errors in sample preparation (extraction, derivatization) could be minimized by rigorous training of laboratory personnel, including appreciation of the patient behind each anonymous test tube. An environment free of noise and distractions is required to minimize the risk of serial solvent extractions being pooled in the wrong tube redundant labeling of glassware and step-by-step checklists are also critical elements of error prevention and detection. 

Liquid-liquid extractions, although valuable in sample preparation procedures, are often time-consuming, laborious, and costly, necessitating multiple partitioning in order to achieve adequate isolation of the analytes. Thus, whenever distribution problems exist or the partition coefficients of the analytes do not favor their... 

These preparations have one thing in common which can be used advantageously in sample preparation. The body should be able to distinguish the drug from the matrix and extract it. With this in mind, the analyst can decide on an initial approach to sample preparation by first considering the dosage form. Solid dosage... 

Too low optical density Too low concentration in sample Prepare sample boil under reflux with KOH, extract with 1 1 ether/petroleum ether, evaporate organic phase, and dissolve residue in isopropanol... 

The robustness of a sample preparation technique is characterized by the reliability of the instrumentation used and the variability (precision) of the information obtained in the subsequent sample analysis. Thus, variations in controlled parameters and sequences are to be avoided. In sample preparation methods employing supercritical fluids as the extracting solvents, it has been our experience that minimal variations in efficient analyte recoveries are possible using a fully automated extraction system. The extraction solvent operating parameters under automated control are temperature, pressure (thus density), composition and flow rate through the sample. The precision of the technique will be discussed by presenting replicability, repeatability, and reproducibility data for the extraction of various analytes from such matrices as sands and soils, river sediment, and plant and animal tissue. Censored data will be presented as an indicator of instrumental reliability. 

New methods for non-destructive quantitative analysis of additives based on MIR spectra and multivariate calibration have been presented [67, 68], One of the limitations in the determination of additive levels by MIR spectroscopy is encountered in the detection limit of this technique, which is usually above the low concentration of additive present, due to their heavy dilution in the polymer matrix. The samples are thin polymer films with small variations in thickness (due to errors in sample preparation). The differences in thickness cause a shift in spectra and if not eliminated or reduced they may produce non-reliable results. Methods for spectral normalisation become necessary. These methods were reviewed and compared by Karstang et al. [68]. MIR is more specific than UV but the antioxidant content may be too low to give a suitable spectrum [69]. However, this difficulty can be overcome by using an additive-free polymer in the reference beam [67, 68, 69, 70]. On the other hand, UV and MIR have been successfully applied to quantify additives in polymer extracts [71, 72, 66]. 

The extraction is the most important step in sample preparation of biological and environmental matrices. Many devices have been used for the extraction procedures. The most important techniques used before conventional chromatography and capillary electrophoresis analyses include ... 

Sample preparation in NLC and NCE is the most important step in analysis due to the nano nature of these modalities. The sampling should be carried out in such a way as to avoid changes in the chemical composition of the sample. The quantitative values of species depend on the strategy adopted in sample preparation. Extraction recoveries may vary from one species to another and they should, consequently, be assessed independently for each compound as well as for the compounds together. Materials with an integral analyte, that is, bound to the matrix in the same way as the unknown, which is preferably labeled (radioactive labeling) would be necessary, which is called method validation. As discussed above few papers described off- and online sample preparation methods on microfluidic devices. Of course, online methods are superior due to lower risk of contamination and error of methods. Not much work been carried out on online nanosample preparation devices, which need more research. Briefly, to get maximum extraction of analytes, sample preparation should be handled very carefully. 

All fuel methods analyze GRO with a purge and trap sample introduction technique, whereas semi volatile diesel fuel and heavy, non-volatile motor oil (DRO and RRO) are first extracted from soil or water samples, and the extracts are injected into the analytical instrument. This distinction in sample preparation gave rise to the terms of total purgeable petroleum hydrocarbons (TPPH) or total volatile petroleum hydrocarbons (TVPH) and total extractable petroleum hydrocarbons (TEPH). A group of petroleum fuels with the carbon range of C7 to Cig may be analyzed with either technique. Common petroleum fuels and other petroleum products fall into these three categories as shown in Table 2.3. 

These errors with the exception of missed holding time are correctable provided that the sample still exists in a quantity sufficient for re-extraction and reanalysis. The short holding time for organic analysis extraction (7 days for water samples and 14 days for soil samples) is always a limiting factor in sample preparation. (Organic analyses have two different holding times as shown in Appendices 12 and 13, one for extraction and the other one for analysis). 

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