Effect of Sampling

Yarmoff J A and McFeely F R 1988 Effect of sample doping level during etching of silicon by fluorine atoms Phys. Rev. B 38 2057-62... 

Vitha, M. F. Carr, P. W. A Laboratory Exercise in Statistical Analysis of Data, /. Chem. Educ. 1997, 74, 998-1000. Students determine the average weight of vitamin E pills using several different methods (one at a time, in sets of ten pills, and in sets of 100 pills). The data collected by the class are pooled together, plotted as histograms, and compared with results predicted by a normal distribution. The histograms and standard deviations for the pooled data also show the effect of sample size on the standard error of the mean. 

The following data were collected as part of a study to determine the effect of sampling variance on the analysis of drug animal-feed formulations.2... 

The following set of experiments introduce students to the important effect of sampling on the quality of analytical results. Each experiment is annotated with a brief description of the principles that it emphasizes. 

Having established that a finite volume of sample causes peak dispersion and that it is highly desirable to limit that dispersion to a level that does not impair the performance of the column, the maximum sample volume that can be tolerated can be evaluated by employing the principle of the summation of variances. Let a volume (Vi) be injected onto a column. This sample volume (Vi) will be dispersed on the front of the column in the form of a rectangular distribution. The eluted peak will have an overall variance that consists of that produced by the column and other parts of the mobile phase conduit system plus that due to the dispersion from the finite sample volume. For convenience, the dispersion contributed by parts of the mobile phase system, other than the column (except for that from the finite sample volume), will be considered negligible. In most well-designed chromatographic systems, this will be true, particularly for well-packed GC and LC columns. However, for open tubular columns in GC, and possibly microbore columns in LC, where peak volumes can be extremely small, this may not necessarily be true, and other extra-column dispersion sources may need to be taken into account. It is now possible to apply the principle of the summation of variances to the effect of sample volume. 

It has also been shown that the effect of sample volume on peak width will be most significant for the early peaks (the most narrow peaks). In addition, the degrading effect of sample volume will progressively decrease as the capacity ratio of the peak becomes larger. However, the resolution of both late and early peaks are equally important and, consequently, the limiting sample volume will be that which restrains the dispersion of the first peak to 5% or less. 

The effect of sample volume on peak width has been considered and treated theoretically in Chapter 6 however, it is of interest to determine the maximum sample volume that can be tolerated with modern columns packed with small particles. The maximum sample volume is defined by the following equation,... 

To minimize the effect of sample volume on dispersion, and ensure that there was minimum dispersion from the valve and valve connections, a 0.2 pi Valeo internal... 

Figure 4.19 demonstrates the effect of sample concentration on the separation of polyethylene oxide (PEO). At a concentration of 1.6 mg/ml of each... 

Koch, P.L., Tuioss, N. and Fogel, M.L. 1997 The effects of sample treatment and diagenesis on the isotopic integrity of carbonate in hiogenic hydroxylapatite. Journal of Archaeological Science 24 417--t29. 

Figure 7. Effect of sample size on apparent molecular weight for soya lecithin phosphatide fractions (conditions same as for Figures 5 and 6 (O) ethanol-soluble fraction (phosphatidyl choline), oligomer GPC (%) ethanol-soluble fraction (phosphatidyl choline), "main column (l ) ethanol-insoluble fraction (other phos-p hat ides), "oligomer GPC (A) ethanol-insoluble fraction (other phosphatides),...

Figure 4, Effect of sample concentration on the distribution of milk proteins on Spherogel TSKSW 2000 column. Instant, nonfat, dry milk was dissolved in mobile phase at the indicated concentrations and 10-fjJL aliquots injected under the conditions outlined in Figure 1.

FIGURE 11.22 Densitograms illustrating the effect of sample dilution with methanol. System silica/T + AcOEt -l- MeOH (83 15 2). Plates triple developed (a) 2 mg of Chelidonium majus extract dissolved in 0.2 ml of methanol (b) 2 mg of the extract dissolved in 1.6 ml of methanol. For abbreviations, see Figure 11.16. (For details, see Waksmundzka-Hajnos, M., Gadzikowska, M., and Hajnos, M.., J. Planar Chromatogr., 15, 289-293, 2002.)... 

Effects of To reduce effects of sampling on Nonintrusive Nonintrusive... 

Fodor, P. and Molnar, E. (1993). Honey as an environmental indicator Effect of sample preparation on trace element determination by ICP-AES. Microchim. Acta 112,113-118. 

Consistent with other analytical methods, immunoassays must be validated to ensure that assay results are accurate. Initial validation involves an evaluation of the sensitivity and specificity of the immunoassay, while later validation includes comparison with a reference method. Because a goal of immunoassays is to minimize sample preparation, validation also includes testing the effects of sample matrices and(or) sample cleanup methods on results. The final steps in validation involve testing a limited number of samples containing incurred residues to determine if the method provides reliable data. 

Several considerations influence the suitability of the immunoassay as a qualitative or quantitative tool for the determination of tissue residues. These include the assay format, the end user (on-farm or laboratory use), effects of sample matrix on the analysis, cross-reactivity considerations, detection levels required of the assay, target tissues to be used in the assay, and the use of incurred or fortified tissues for validation of the immunoassay against accepted instrumental methods. Although these variables are often interrelated, each topic will be discussed in further detail below. 

Czok, M., Katti, A. M., and Guiochon, G., Effect of sample viscosity in high-performance size-exclusion chromatography and its control, /. Chromatogr., 550, 705, 1991. 

When considering libraries of spectra for identification purposes, the effect of sample preparation on spectral characteristics is also important. Two FUR sampling methods have been adopted for IR analysis of TLC eluates in the presence of a stationary phase, namely DRIFTS [741] and PAS [742], of comparable sensitivity. It is to be noted that in situ TLC-PA-FTIR and TLC-DRIFT spectra bear little resemblance to KBr disc or DR spectra [743,744]. This hinders spectral interpretation by fingerprinting. For unambiguous identification, the use of a reference library consisting of TLC-FTIR spectra of adsorbed species is necessary. 

Windig, W. Kistemaker, P. G. Haverkamp, J. Meuzelaar, H. L. C. The effects of sample preparation, pyrolysis and pyrolysate transfer conditions on pyrolysis mass spectra. J. Anal. Appl. Pyrolysis 1979,1, 39-52. 

Murphy, R.E., Schure, M.R., Foley, J.P. (1998). Effect of sampling rate on resolution in comprehensive two-dimensional liquid chromatography. Anal. Chem. 70, 1585-1594. 

Marvin et al. [302] have discussed the effects of sample filtration on the determination of copper in seawater, and concluded that glass filters could seriously affect the reliability of subsequent analysis. 

In CL measurements many factors that influence the intensity of the CL signal should be taken into account. The CL signal may depend on the geometry of the sample. Internal refraction and reflection at the air-solution interfaces are important factors in determining the measured CL intensity, and should be taken into account, for example, when a CL cocktail is placed over a sample. The effect of sample geometry can be evaluated using model systems, such as enzymes... 

Table VI. Effect of Sample Configuration on the Minimum Ignition Temperature (in Oxygen) of Cottonwood Chars...

Schmitt, C.J. and S.E. Finger. 1987. The effects of sample preparation on measured concentrations of eight elements in edible tissues of fish from streams contaminated by lead mining. Arch. Environ. Contam. Toxicol. 16 185-207. 

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