Sampling precision

The sampling precision of the measured data depends on the signal amplitude. The difference between simulated and experimental data can be mainly explained by the low numerical precision of the measured data. 

There are advantages to direct solid sampling. Sample preparation is less time consuming and less prone to contamination, and the analysis of microsamples is more straightforward. However, calibration may be more difficult than with solution samples, requiring standards that are matched more closely to the sample. Precision is typically 5% to 10% because of sample inhomogeneity and variations in the sample vaporization step. 

Comparison of Various FNAA Techniques for Assay of Synthetic Octol Samples Precision of Single-Axis Rotation FNAA for Assay of Octol Plant Samples Fast Neutron Activation Analysis for Nitrogen in Explosives by... 

In practice, chemists rarely try to measure out an exact mass. Instead, they estimate the mass required and spoon out that mass approximately. Then they measure the mass of the sample precisely and convert it into moles (by using Eq. 2, n = m/M) to find the precise amount that they have obtained. 

In most alpha and mass spectrometric methods for which sample preparation is extensive and chemical recoveries can vary considerably from sample to sample, precise elemental concentrations are determined by isotope dilution methods (e.g., Faure 1977). This method is based on the determination of the isotopic composition of an element in a mixture of a known quantity of a tracer with an unknown quantity of the normal element. The tracer is a solution containing a known concentration of a particular element or elements for which isotopic composition has been changed by enrichment of one or more of its isotopes. 

Table 34-7 Individual sample precision and accuracy for combined Methods A and B and Labs 1 and 2 - Production samples...

Bishop [75] determined barium in seawater by direct injection Zeeman-modulated graphite furnace atomic absorption spectrometry. The V203/Si modifier added to undiluted seawater samples promotes injection, sample drying, graphite tube life, and the elimination of most seawater components in a slow char at 1150-1200 °C. Atomisation is at 2600 °C. Detection is at 553.6 nm and calibration is by peak area. Sensitivity is 0.8 absorbance s/ng (Mo = 5.6 pg 0.0044 absorbance s) at an internal argon flow of 60 ml/min. The detection limit is 2.5 pg barium in a 25 ml sample or 0.5 pg using a 135 ml sample. Precision is 1.2% and accuracy is 23% for natural seawater (5.6-28 xg/l). The method works well in organic-rich seawater matrices and sediment porewaters. 

Purge the injector daily and before sample analysis to remove bubbles in the sampling syringe. This is critical for sampling precision. 

The criterion of mean-unbiasedness seems to be occasionally overemphasized. For example, the bias of an MLE may be mentioned in such a way as to suggest that it is an important drawback, without mention of other statistical performance criteria. Particularly for small samples, precision may be a more important consideration than bias, for purposes of an estimate that is likely to be close to the true value. It can happen that an attempt to correct bias results in lowered precision. An insistence that all estimators be UB would conflict with another valuable criterion, namely parameter invariance (Casella and Berger 1990). Consider the estimation of variance. As remarked in Sokal and Rohlf (1995), the familiar sample variance (usually denoted i ) is UB for the population variance (a ). However, the sample standard deviation (s = l is not UB for the corresponding parameter o. That unbiasedness cannot be eliminated for all transformations of a parameter simply results from the fact that the mean of a nonlinearly transformed variable does not generally equal the result of applying the transformation to the mean of the original variable. It seems that it would rarely be reasonable to argue that bias is important in one scale, and unimportant in any other scale. 

A method for the determination of personal exposure to benzidine-based dyes has been developed. This procedure involved the reduction of benzidine-based dye filter samples to free benzidine with neutral buffered sodium hydrosulfite solution. The benzidine-containing reduction solution was then analyzed by high performance liquid chromatography. The reduction was found to be quantitative by visible-spectrum analysis. This reduction and analysis method was evaluated with four benzidine-based dyes over the range from 12 to 300 micrograms per sample. Precision for the reduction and analysis of the four dyes falls within % coefficient of variation. This method can differentiate between benzidine-and benzidine congener-based dyes. Results are reported in terms of free benzidine. 

Specifications How good do the numbers have to be Write specifications Pick methods to meet specifications Consider sampling, precision, accuracy, selectivity, sensitivity, detection limit, robustness, rate of false results Employ blanks, fortification, calibration checks, quality control samples, and control charts to monitor performance Write and follow standard operating procedures... 

Calibrations were carried out for the GC/PID or the GC/MS daily. Calibration standards were prepared based on standard reference materials obtained from Supelco Chromatography products. A check standard was analyzed every ten samples to assure calibration and accuracy. A reagent blank was included in each analytic batch of samples. Blanks were made from reagent or make-up water and matrix similar to the sample. A spiked sample was analyzed every twenty samples. This was done by splitting an appropriate sample into two subsamples and adding a known quantity of TCE to one of the split samples. The purpose of a spiked sample is to determine the extent of matrix bias or interference on TCE recovery and sample to sample precision. Accuracy was assessed by analysis of external reference standards (separate from calibration standards) and by percent recoveries of spiked samples. Precision was assessed by means of replicate sample analysis. It is expressed as relative percent difference (RPD) in the case of duplicates or relative standard deviation (RSD) for triplicate (or more) analyses. Recovery was 96% or more for all spiked samples, and RPD/RSD are less than 7% for all samples. 

After sufficient grinding, buffer should be added so that there is at least a 3 1 buffer to sample ratio.19 It is not necessary to determine the volume or weight of the sample precisely, but try not to exceed a final total volume of 5 ml. The efficiency of the extraction will be improved by increasing the buffer to sample ratio. This is especially true if the tissue used was not thoroughly minced. Transfer the minced tissue plus buffer to 15-ml Corex tube. 

Precision the degree of random variation in repetitive measurements of the concentration of an analyte in a sample. Precision is usually measured by the standard deviation or the relative standard deviation of the measurements. 

The fundamental anisotropy r is a time-independent molecular parameter. Thus, the OACF can be sampled quasi-continuously directly in the time domain, provided one can sample precisely r(t). This properties makes FAD a rather unique tool for discussing the different models for the OACF. (In principle, the transient Kerr Effect is also able to provide such a sampling, but, at the present time, this latter techniques does not seem to reach the same precision). 

The approximate relationship between the constitution heterogeneity (CH) and the statistical variance, discussed in the next section, can be used to determine the optimal sample mass when sampling to determine the relative amount of trace constituents in the lot. Two series of samples of large and small sample masses are collected and analyzed. The statistical coefficients of variation are calculated. These, together with the values of the sample mass, can be used to determine the minimum sample mass needed to obtain the desired sampling precision of the estimated critical content of the trace constituent. For details, see Chapter 20 of Pitard (1993). 

Bilonick, Richard A. (1989). Quantifying Sampling Precision for Coal Ash Using Gy s Discrete Model of the Fundamental Error, Journal of Coal Quality, Vol. 8, pp. 33-39. 

A motorised stage is used to position the sample precisely ( c, y, z) under the probe. Reproduction of sample position must be within one micrometre. 

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