Sample average

The sfe of chlorpyrifos methyl from wheat followed by on-line Ic/gc/ecd has been investigated (93). Extraction profiles were generated to determine the maximum analyte recovery and the minimum extraction time. Using pure CO2, a 65% recovery of chlorpyrifos methyl spiked onto wheat at 50 ppb was reported. When 2% methanol was added to the CO2, the recovery from a one gram sample averaged 97.8% (n = 10, 4.0% RSD). 

The value of n — 1 is used in the denominator because the deviations from the sample average must total zero, or... 

A remarkable property of the normal distribution is that, almost regardless of the distribution of x, sample averages x will approach the gaussian distribution as n gets large. Even for relatively small values of n, of about 10, the approximation in most cases is quite close. For example, sample averages of size 10 from the uniform distribution will have essentially a gaussian distribution. 

The t distribution is also symmetric and centered at zero. It is said to be robust in the sense that even when the individual obseiwations x are not normally distributed, sample averages o x have distributions which tend toward normahty as n gets large. Even for small n of 5 through 10, the approximation is usually relatively accurate. 

Number of averages Typically, a minimum of three samples should be collected for an average. However, the factor that determines the actual number is time. One sample takes three to five seconds, a four-sample average takes 12 to 20 seconds, and a 1000-sample average takes 50 to 80 minutes to acquire. Therefore, the final... 

The mean of several readings (x) will make a more reliable estimate of the true mean (yu) than is given by one observation. The greater the number of measurements (n), the closer will the sample average approach the true mean. The standard error of the mean sx is given by ... 

The means resulting from several (k) repeat determinations on every of (j) samples and/or sample preparations must comply several such sample averages Xmean, would go into the over-all average Xmean,total-In the simplest case oi j = Ijk - 2 (duplicate determinations on each of two sample work-ups), both sample averages xji and Xj2 would have to comply. It could actually come to pass that repeat measurements, if foreseen in the SOP, would in this way not count as individual results this would cut the contribution of the measurement s SD towards the repeatability by v/2, x/3, etc. For true values /x that are less than 2-3 a from the limit, the OOS-risk would be reduced. 

Sample Average size [pm] Speiafic gravity [gem j DVB Content [wt%] Remarks... 

Fig. 6.7. Evolution of the sample averaged (R< ) as a function of MC time. The initial value of e(N) = C = 1.0 was changed to the values indicated after 600 MC steps. The indicated melt value corresponds to a comparable system with explicit chains with repulsive Lennard-Jones interactions and a number density of 0.85 cr-3 (from [45])...

Cocoa butter is the fat obtained from subjecting chocolate liquor to pressure. Since the alkaloids are sparingly soluble in fat, only trace amounts of theobromine and caffeine in cocoa butter have been reported. The theobromine and caffeine content of four cocoa butter samples averaged 0.008% and 0.038%, respectively.33... 

Table VIII. NBS Smoke Chamber Data for EMI Coated Structural Foam Samples Average Non-Flaming Data ...

Sample Average particle sizea (p,m) Morphology3 fiob Conversion0 (%) Selectivity1 (%) Yield" (%)... 

Suspect product qualitatively identical to good sample average carbon chain = 12.6... 

NIOSH (1976) compared the medical records of 326 workers exposed to phosgene with those of 6,288 unexposed workers from the same plant. Personal air sample measurements at this plant (20-min samples) showed phosgene concentrations ranging from undetectable to 0.02 ppm, and there was a 15 sample average of 0.003 ppm. Fixed-position air samples (20-min or 2-h collection) ranged from undectable to 0.13 ppm in 51 of 56 samples, and >0.14 ppm in 5 of 56 samples. There were no differences in pulmonary function or deaths attributable to respiratory disease between the exposed and control populations. 

McIntyre et al. [58, 59] conclude that the extraction of polychlorobiphenyls and organochlorine insecticides is most efficient at a total solids concentration of lg L-l, using the extraction procedure described above. The recovery of p,p -DDE from subsamples was always found to be the lowest of the four determinands considered (61.8%), while recoveries of Aroclor 1260, 7-HCH, and Dieldrin from the diluted sample averaged 96.3, 89.4 and 82.9% respectively at lg L-l total solids. 

Stable labelled isotopes are spiked into samples before extraction and the ratio of unlabelled compound and stable labelled isotope was used to quantitate the unlabelled compound. Analysis is by high-resolution gas chromatography-mass spectrometry. Fifteen standard water samples and ten standard soil samples containing 2,4-D at known concentrations were analysed. Compound concentrations ranged from 100 to lOOOOug per kg for soil samples. Average recoveries were over 84% and method precision, given as relative standard deviation, was better than 19%. 

Recoveries in soil samples, averaged less than 50% even after corrections. This may have been due to degradation of the compounds by soil microorganisms or to chemical and physical interactions with the soil. Mestranol recoveries averaged 26-30% from soils at O.lppm. Recoveries of ethynyloestradiol were even lower, presumably because of its greater chemical reactivity due to the slightly acidic hydrogen in the 3-hydroxy position. 

Petrographic observations were made on polished thin sections, for 313 outcrop samples. Average sample spacing was one sample per 1 km within a radius of 15 km or less from the known porphyry Cu-Mo deposits, and one sample per 2 km outside this area. Five rock types are distinguished in the district andesites, diorites, granodiorites, dacites, rhyolites and sandstones. Rhyolites and dacites dominate in the zone of known porphyry Cu-Mo deposits. Sandstones are much less common and were not studied. 

Toxic pollutants (p,g/L) samples Average Median Maximum samples Average Median Maximum... 

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