Segregation errors

Lame and Defize [334] showed that the fundamental sampling error for soil only affects the analytical variance when sample sizes are less than 10 g [335]. For larger samples, the variance is determined by the segregation error. A sampling board method for estimation of the segregation error was described. Skalski and Ward [335] showed that a two-way compositing strategy could be used to attribute detected contamination in composited samples directly to constituent samples without further analyses. 

Segregation errors, which are due to segregation within the bulk and can be minimized by suitable mixing and the use of a large number of incremental samples to form a larger test sample. 

Condition (mix) a one-dimensional enclosed liquid or gas stream. Since a cross-streaim sample is impossible to obtain, giving the stream a more homogeneous cross section reduces the grouping and segregation error (GSE) and the impact of incorrect delimitation. It also reduces the impact of taking an incorrect sample. 

Error 2 Grouping and segregation error (GSE) (material variation)... 

The second error, the grouping and segregation error (GE), is also related to the material variation, in this case the distribution heterogeneity. At the level of the small scale where we actually take a sample, particles may segregate by particle size or shape. If we cannot select material totally at random, then our sample will be biased. In addition, we sample groups of particles, not one at a time as in SRS. So in addition to the unavoidable FE, we have an additional component in bulk sampling that contributes to our total sampling error. The GE is not present in classical SRS. 

FIGURE 6 Mixing upstream of the sample collection reduces the segregation error when a correct, cross-stream sample cannot be obtained. 

Verschaeve L, Kirsch-Voldees M and Susanne C (1984) Mercury-induced segregational errors of chromosomes in human lymphocytes and in Indian muntjac cells. Toxicol Lett 21 247-253. 

Grouping and Segregation Error this error arises from nonrandom distribution of particle sizes, usually a result of gravitational effects it can be minimized by selecting samples for analysis from many randomly selected primary samples or by careful homogenization and splitting of the sample. 

Improper sampling can introduce errors originating at the surface (8.7) because x-ray emission spectrography operates only on a surface layer of critical thickness (6.4). Powders are particularly subject to errors that result from segregation and lack of uniformity in general (7.8, 7.15). Nonuniform residues from the evaporation of solutions can be troublesome. 

A subsequent study by the same research group focused on pits in olives. This time the olives travelled at various speeds through a 2 T horizontal bore magnet. Segregating 300 pitted and non-pitted olives gave classification errors of 4.3, 4.7, 2.3 and 4% at belt speeds of 0, 5,15 and 25 cm/s, respectively. 

Sedimentation runs should be conducted at a constant temperature, not only so that Ap and T7 are known, but also to minimize disturbances due to convection. Any sort of disturbance will obviously disrupt the segregation of the particles by size that has occurred as a result of sedimentation. An intrinsic difficulty with the balance method lies in the fact that the liquid below the balance pan is less dense than the liquid with dispersed particles above the pan. Thus, there is a tendency for a counterflow of pure solvent to arise, which would introduce an error in the particle size analysis. 

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