Analytical challenges

GC provides the basis for most analytical approaches reported in the literature, and the use of alkali-catalysed methylation has proven to be the most accurate method for analysis of CLA (Yurawecz et al., 1999). Sodium methoxide is the catalyst used most widely and has the advantage that it does not isomerize conjugated double bonds or form methoxy artifacts. 

Analysis of the CLA content and profile of animal tissues or biological fluids containing a mixture of lipid classes is more difficult. In order for all of the fatty acids to be methylated, a two-stage methylation procedure is recommended. Kramer et al. (1997) evaluated many different combinations of acid/base catalysts and concluded that the best compromise was the use of sodium methoxide followed by a mild acidic methylation, which resulted in the methylation of the majority of the fatty acids with minimal isomerization of the CLA isomers. However, mild boron triflouride or 1% methanolic sulphuric acid with a minimal temperature and reaction time are often used with good success. 

Additional analytical methods are appropriate when a more complete characterization of the CLA isomers in biological samples is required. Most often, a combination of GC and silver ion-HPLC is used and permits excellent separation and identification of positional and geometrical isomers of CLA (see Adlof, 2003, and Kramer et al., 2004, for detailed reviews of this approach). In addition, the use of gas chromatography-mass spectrometry (GC-MS) has become increasingly popular and represents a very powerful technique for identification of the position of double bonds in fatty acids (see Dobson, 2003), and the orientation of those bonds in CLA isomers (Michaud et al., 2003). 

In summary, the analysis of CLA can be simple or extensive. The particular objectives and the anticipated use of the analytical data will determine the extent to which individual CLA isomers need to be separated, identified and quantified (Christie, 2003). Methodology for the analysis of CLA and related fatty acids continues to evolve and it is recommended that the reader consult recent reviews and publications in this area before undertaking such analysis for the first time. We recommend Christie (2003) and Kramer et al. (2004) as excellent practical guides on the analysis of CLA. 

In addition, periodic updates on CLA analysis can be found at wwwdipidlibrary.co.uk. 

In addition to background contamination, other challenges are associated with the measurement of PFCs in environmental samples. In particular, challenges are associated with standards, calibration and quantitation, and matrix effects [93, 94]. 

Until recently, obtaining high-quahty chemical standards for the analysis of PFCs was difficult. Although some standards were available commercially, others were available only from manufacturers, and had variable purity and isomer profiles [94,128]. These impurities and structural isomers were not always well documented and could contribute to inaccurate analytical results. For example, shorter chain PFCA impurities observed in a chemical standard of PFTA could result in a negative bias of unknown proportions in quantitation when mixed standard solutions are used without correction [94]. Today, an increasing 

Obtaining appropriate internal standards and standard reference materials has also proven difficult [94]. As a result, researchers have employed a variety of chemicals for internal standards [93]. For example, Hansen et al. [105] used the 6 2 FTS/THPFOS to quantify PFHxS, PFOS, PFOA and PFOSA in biological matrices. This internal standard was subsequently measured in groundwater samples from military bases [98] and illustrates the importance for researchers to scan samples for any analyte being used as an internal standard. A suite of and O-mass labelled PFCs have become commercially available and it is recommended they be employed as internal standards [93,97,128]. Prior to use, it is suggested that the purity of these labelled standards be confirmed to ensure the absence of native (unlabelled) PFCs. 

To investigate the data quality of PFC measurements, a worldwide interlaboratory study was conducted in 2005 involving 38 laboratories from 13 countries [93]. Each laboratory analysed 13 PFCs in three environmental samples and two human samples. Results indicated approximately 65% agreement for PFOS and PFOA in human blood and plasma samples, but agreement for other PFS As and PFCAs was much lower and most laboratories underestimated the PFC concentrations in fish extracts due to matrix effects. The study concluded that additional work is needed to improve the analytical techniques employed for the analysis of PFCs. 

Several recommendations arose from the interlaboratory smdy to minimize analytical challenges and to ensure data quality. As discussed above, it is recommended that mass labelled PFCs be employed as internal standards [93, 97]. It should be noted, however, that some electrospray ionization suppression may still occur if these internal standards are used at high concentrations [97]. Matrix effects can also be minimized by employing matrix-matched calibration standards in lieu of solvent-based calibration standards [97]. Unfortunately, matrix-matched standards can be impractical when an appropriate clean matrix cannot be found [94]. Other quality assurance and quality control measures, such as spike and recovery analyses of an analyte added to the sample matrix, repetitive analysis of samples to determine precision and comparison of internal standard quantitation to quantitation via standard additions, are also useful in determining data quality [94]. 

The determination of the most appropriate method for a given analytical problem is the first demanding task. Routine polymer/additive analysis, where the nature and approximate concentrations of the components 

One of the greatest challenges in the laboratory is to prove that accurate results have been generated for unknown samples. 

With improving detector performance, the smaller can be the sample size and, consequently, the more rapid the sample pretreatment. However, as shown repeatedly in quantitative analysis, small sample sizes (several mg) face homogeneity problems and set a 

Pressure SFE, PHWE, PFE, HPSE, PLE, PMD, HPLC, OPLC, HPPLC, PD-SEC, (H)PDSC, HP-OIT, HPSEM SIMS, XPS, AES, TEM, SEM 

Speed SFE, SPE, MAE, ASE , HSGC, HSLC, HSTLC, SEC-LS, SPE-TLC, EDXRF, EPMA, EN Soxhlet, D/P 

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Due to their wide range of analytical challenges centralized analytical laboratories are required to adopt a series of QM systems simultaneously. For example, the Competence Center Analytics of BASF AG in Ludwigshafen is certified and accredited to operate under four different QM systems. Undoubtedly, QM systems play a vital role in a modern industrial analytical laboratory. The sale of many products of the chemical industry is not possible without a GLP-certified analytical laboratory. However, in practical tenus the different QM systems can potentially reduce the efficiency of the analytical process and lead to increased costs. 

Measurement of U-series disequilibria in MORB presents a considerable analytical challenge. Typical concentrations of normal MORB (NMORB) are variable but are generally in the 50-150 ppb U range and 100-400 ppb Th range. Some depleted MORB have concentrations as low as 8-20 ppb U and Th. The concentrations of °Th, Pa, and Ra in secular equilibrium with these U contents are exceedingly low. For instance, the atomic ratio of U to Ra in secular equilibrium is 2.5 x 10 with a quick rule of thumb being that 50 ng of U corresponds to 20 fg of Ra and 15 fg of Pa. Thus, dissolution of a gram of MORB still requires measurement of fg quantities of these nuclides by any mass spectrometric techniques. 

The development of the biotechnology industry presents new and novel molecules derived from nature. The utilization of these molecules as pharmaceutical products presents an analytical challenge of a magnitude greater than ever confronted. The drug candidate and closely-related molecules have... 

Obtaining accurate results for sulfur determination at ppm level is illustrative of a problematic element. Trace sulfur analysis is an analytical challenge. Sulfur cannot be determined by conventional AAS, since its... 

The analysis of microorganisms in mixtures provides a continuing analytical challenge, for which a number of solutions have been proposed ... 

It is our wish to encourage the analytical community to discover more about this most exciting analytical technique and to consider it a powerful alternative in the resolution of a variety of analytical challenges. 

Basic soils present a unique analytical challenge. Most of these soils contain calcium carbonate (CaC03) as the primary base. Basic soils also contain magnesium and, to a lesser extent, sodium carbonate. Although soils containing lithium and potassium carbonate are known, they are uncommon. These compounds produce a basic solution when dissolved in water. This means that adding either water as an extractant or water containing small amounts of salt is not effective because the soil already contains salts and solutions immediately become basic when added to these soils. 

HTS plates permit to determine drug permeability across a cell monolayer with a throughput similar to that of the parallel artificial membrane permeation assay (PAMPA), which measures rate of diffusion across a lipid layer.46 As is the case with PAMPA, the tiny surface area of the filters of the 96-well HTS presents an analytical challenge for compounds with low-to-moderate permeability. 

Xenoliths from Siberian continental lithosphere, with Archean model ages, had b Li as low as +0.5 (Eouman et al. 2000). If these values accurately represent the Archean mantle, they suggest the potential for Li isotopic evolution in the Earth, from lighter compositions in the ancient mantle to what is seen in present-day MORE. In spite of the analytical challenges presented by ultramafic rocks, more data from these materials are crucial to an understanding of Li in the mantle, and in resolving questions about the appropriateness of the accepted MORE mantle range. 

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