Sub-sampling

Regardless of the analytical requirements, sub-sampling is best avoided by collection of duplicate samples on site if possible. The preparation of each duplicate sample may then be tailored to specific analytical parameters. Often, sub-sampling cannot be avoided and it is necessary to take the largest possible aliquot from a well mixed (preferably crushed or milled) sample. If necessary, the sub-sample may be formed by combining a number of smaller samples for added safety. 

Regardless of the type of sub-sampling method used, some loss of fines may be expected, though, with care dust losses and separation effects should be less 

Second, easily flowing granules or powder may be riffled. This is recommended for fertilizers Oohnson, 1990b). Riffle boxes (sample dividers or splitters) are available to BS812 and BS1377 from  

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Roughly speaking, a siimdadon sample can be regarded as a collecdon of sub-samples, each making a... 

The homogeneity determination of the bacteria in the materials is performed by viable count followed by statistical evaluation of the cormts of sub-samples from the same capsule solution and of total counts of different capsules of one batch. An example for the homogeneity determination for a batch of capsules containing Enterococcus faecium is also presented in (Janning et al. 1995). 

The importance of particle size is directly proportional to the sub-sample size recommended by the analytical method. The larger the sub-sample size the larger the acceptable particle size. For sub-sample sizes of ig or greater a soil sieved through a imm screen is generally acceptable. Therefore if the sample is relatively coarse, e.g up to 2mm particles and the matrix CRM is an uniform sub-micron powder, it may be necessary to use a much larger sample from the material under test than for the CRM. 

Define the specimen(s) and the sampling procedure(s) to obtain a representative sub-sample of the materials to be examined. 

Weigh seven or more sub-samples (ground and homogenized) of an untreated control sample of the matrix of interest. The weight of each sub-sample should be the same as that proposed to be used during sample analysis. 

Eortify each of these sub-samples with the analyte(s) of interest such that the concentration of the analyte(s) in the matrix equals the estimated limit of quantification (ELOQ). 

Once the MDL has been calculated, it is important to prove that the chosen analytical procedure is practically capable of detecting the analyte(s) at the MDL. To prove the practicality of the MDL, the analyst should spike triplicate sub-samples of an untreated control sample at the MDL, extract the fortified control samples and analyze them on the instrument. Well defined chromatographic peaks would prove the validity of the calculated MDL. 

Substrate Sub-sample (g) Solvent Technique Cleanup (pgmL- ) ... 

Solid-phase sorbents are also used in a technique known as matrix solid-phase dispersion (MSPD). MSPD is a patented process first reported in 1989 for conducting the simultaneous disruption and extraction of solid and semi-solid samples. The technique is rapid and requires low volumes (ca. 10 mL) of solvents. One problem that has hindered further progress in pesticide residues analysis is the high ratio of sorbent to sample, typically 0.5-2 g of sorbent per 0.5 g of sample. This limits the sample size and creates problems with representative sub-sampling. It permits complete fractionation of the sample matrix components and also the ability to elute selectively a single compound or class of compounds from the same sample. Excellent reviews of the practical and theoretical aspects of MSPD " and applications in food analysis were presented by Barker.Torres et reported the use of MSPD for the... 

Figure 7. age vs. growth band age for three coral sub-samples, all younger than 200 years old (after Edwards 1988 and Edwards et al. 1988). All three points lie on a 1 1 line indicating that the °Th ages are accurate and that initial is negligible, justifying the use of Equation (1) to determine °Th age. 

Initial Pa/ U levels are more difficult to assess, primarily because there is no long-lived or stable isotope of protactinium that can be used as an index isotope. Edwards et al. (1997) analyzed a set of surface coral sub-samples younger than 1000 years by both °Th and Pa techniques. For all samples, Th concentrations were less than 100 p g so that initial °Th/ U values were negligible. Each sub-sample yielded °Th and a ages identical within analytical errors (Fig. 8), indicating that initial Pa/ U was negligible. This suggests that surface corals with typical Th values do not require corrections for initial Pa. Whether or not corals with elevated Th require such corrections is an open question. 

Figure 4. Osmond type II isochron plot for sub-samples from single growth layer in stalagmite SVC-98-3.B from Spring Valley Caverns Morale et al. 2003).

Figure 5. U and Th isotope measurements for 9 coeval sets of sub-samples from GB-89-24-1, Grand Bahama (Beck et al. 2001) are plotted in (A) (230Th/232Th)A-( U/2 Th)A and (B)...

In a detailed study of numerous suites of co-eval sub-samples from speleothems from Soreq Cave, Israel, Kaufman et al. (1998) demonstrate that Th is positively correlated with Si, Fe and Al, which suggest that Th is associated primarily with... 

A unique strength of concordia diagrams is their ability to reveal modes of diagenetic disturbance. If U-Th-Pa isotopic data for numerous co-eval sub-samples that have experienced different degrees of the same diagenetic process are plotted in or vs. space, they should fall on a curve or straight line... 

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