Instrumental techniques relative precision

NMR has been used comparatively little for quantitative analysis although peak areas are directly proportional to concentration. The principal drawbacks are the expensive instrumentation and a lack of sensitivity. The latter can be improved with the aid of computers to accumulate signals from multiple scans or by using a pulsed (Fourier transform) technique. Relative precision lies in the range 3-8%. 

Electrospray ionization and matrix-assisted desorption ionization were both introduced around the same time, in the late 1980s. In fact matrix-assisted laser desorption ionization (MALDI) was first mentioned in the literature in 1987 (Karas et al., 1987). In the years prior to that, there were limited reports of the application of laser desorption MS. Early developments in MALDI focused primarily on macromolecules, particularly peptides and proteins. Historically, MALDI ion sources have predominantly been coupled to time-of-flight (TOF) instruments. TOF requires precise timed ionization events, and since ions are generated in MALDI by a pulsed desorption, this combination is complementary. Mass spectra generated by MALDI can be relatively simple, containing predominantly singly charged ions. The importance of both ESI and MALDI are well proven in the analysis of biomolecules, and both techniques were awarded the Nobel Prize for chemistry in 2002 (Chapter 1). 

The immediate future is relatively clear. Continued advances in instrumental techniques, particularly in mass spectrometry and NMR, will make it possible to measure increasingly accurate and precise KIEs on increasingly small amounts of material. At the same time, continued growth in computational power and in the methods of KIE interpretation will make TS analysis an increasingly powerful tool. Currently, one major drawback is that it is too time consuming at present for application in the pharmaceutical industry. TS analysis will have to become much faster to see wide application outside of academia. 

ICP-OES is one of the most successful multielement analysis techniques for materials characterization. While precision and interference effects are generally best when solutions are analyzed, a number of techniques allow the direct analysis of solids. The strengths of ICP-OES include speed, relatively small interference effects, low detection limits, and applicability to a wide variety of materials. Improvements are expected in sample-introduction techniques, spectrometers that detect simultaneously the entire ultraviolet—visible spectrum with high resolution, and in the development of intelligent instruments to further improve analysis reliability. ICPMS vigorously competes with ICP-OES, particularly when low detection limits are required. 

Recently a decreased level of CE activity has been noticed with a shift of attention towards other separation techniques such as electrochromatography. CE is apparently not more frequently used partly because of early instrumental problems associated with lower sensitivity, sample injection, and lack of precision and reliability compared with HPLC. CE has slumped in many application areas with relatively few accepted routine methods and few manufacturers in the market place. While the slow acceptance of electrokinetic separations in polymer analysis has been attributed to conservatism [905], it is more likely that as yet no unique information has been generated in this area or eventually only the same information has been gathered in a more efficient manner than by conventional means. The applications of CE have recently been reviewed [949,950] metal ion determination by CE was specifically addressed by Pacakova et al. [951]. 

The relative ability of the two techniques to produce comparable data structure is impacted by a number of factors. Clearly, instrumental sensitivity, precision, and accuracy play a role for certain elements. INAA, for instance, reported nickel concentrations below detection in all but one sample in the dataset, PRW209, for which a concentration of 20.9 ppm was measured. Using LA-ICP-MS, a concentration of 20.8 ppm was measured for this same sample. However, this technique was also able to detect measurable concentrations of Ni in all other samples, ranging from 13 to 23 ppm. For other elements, high background noise to signal ratios limit the sensitivity, precision and accuracy of measurement by LA-ICP-MS. This is particularly a problem for two elements... 

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