Nugget effects

KEYWORDS Analysis, Nugget effects, ICP-MS, Sub-sampling, Quality Contro... 

The nugget effect causes sub-sampling errors in PGE determinations. Previously, large sub-samples (30 g) of all samples were analyzed to decrease sub-sampling errors. This is not cost-effective. Our new approach is firstly, a 10 g sub-sample is used for the routine analysis of all samples secondly, samples with anomalous values are selected for duplicate or triplicate determinations, and the average value of these determinations is considered trustworthy. The selection of these samples is mainly based on the Pt/Pd ratio, statistics of RD% of coded duplicate analyses and total batch data distributions. 

PGE nugget effect is effectively controlled by duplicate or triplicate determinations of samples with relatively high PGE contents. 

Figure 3.24 Principal variogram illustrating the three key parameters nugget effect [MPE, V(0)h range and sill.

Three parameters are necessary for each model Sill, Range and Nugget Effect. Each one has a specific meaning and has important effect on a Theoretical variograms for used data are as follow ... 

According to Eq. 4-28 the theoretical semivariogram function has the value y = 0 for / = 0. Semivariograms obtained from experimental data often have a positive value of intersection with the y(/)-axis expressed by C0 (see Fig. 4-5). The point of intersection is named nugget effect or nugget variance. This term was coined in the mining industry and indicates an unexplained random variance which characterizes the microinho-... 

Fig. 4-5. Example of an experimental semivariogram with nugget effect Cc, sill C, and range L...

The required distance has to be chosen from the empirical semivariogram or (if the first sampling was done equidistantly) by autocorrelation analysis also (see example for soil sampling in Sections 9.1 and 9.4). Clearly, the required distance depends on the relationship between nugget effect and sill. The length determination is described in detail by YFANTIS et al. [1987],... 

Element Model Nugget effect Sill Range in km... 

Estimation of the nugget effect can provide valuable information about the process. For example, if it is substantially larger than an estimate of the material variation obtained independently, then we know that extraneous variation and/or bias is being introduced through incorrect sample collection, handling, or imacceptably large analytical variation. Extrapolation of the variogram to... 

Figure 4.15 Extrapolation of the variogram to estimate the nugget effect.

Figure 4.19 Variogram based on hourly samples with nugget effect extrapolated from minute samples.

Figure 4.20 Nugget effect is much smaller with a better analytical method.

Suppose we can reduce the analytical variation by more than half by using a more precise method. This reduces the nugget effect substantially, and we perform another variographic experiment. By overlaying all the information, the variogram looks like that in Figure 4.20. 

Suppose a plant operates on 12 h work shifts and that samples are normally taken at the beginning of each shift. For our variographic experiment, we collect a sample every hour for several days and estimate the nugget effect with a sample every minute. Combining this information results in the variogram given in Figure 4.21. 

Fractions are used to allow comparison of the nugget-effect value to the estimated material variation. 

The rate of discovery of pristine nonmare rocks has been limited by the scarcity of large lunar rock samples. Identification of a coarse cumulate or otherwise plutonic texture is difficult without a thin section at least several mm across, and determination of trace siderophile elements becomes increasingly difficult (and prone to the nugget effect) if available sample mass falls below —0.1 g. Lunar geochemists must resist... 

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