Zero-contamination sampling

As the plastic liners are removed from the probe, they are capped on both ends, the appropriate labels affixed, and promptly placed in a freezer (an in-field sectioning technique used for further partitioning of the 0-15-cm core is described later in this section). By convention, red plastic caps are placed on top of the core (i.e., the end that was closest to the soil surface) and black caps are placed on the bottom. Use of the two-color capping system is important when the cores are sectioned at a later time. This approach is referred to as zero-contamination sampling and is the industry standard in field soil dissipation. 

Figure 7 Alternative zero-contamination sampling method for soil...

Figure 6 Diagram of zero-contamination soil sampling procedure...

In the mass titration method, the PZC is determined as the natnral pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667], Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the pH drift method [183], Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670], To this end, a series of artificially contaminated samples was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed mass titration was used in [672], but it was different from the method discussed above. 

The effect of DC bias on a contaminated sample at 100% RH is shown in Figure 5. At bias levels corresponding to threshold and super-threshold levels for electrochemical reactions, the impedance spectrum shows the capacitive loop that intersects the real axis at low frequency (.1 Hz). Zero-DC-bias data, which are not shown, form a similar arc that is large compared to the scale of this plot. This behavior is modelled by a parallel RC circuit, whose resistance decreases from 1 x 10 to 1.6 x 10 and whose capacitance remains constant at approximately 30000 pF, as DC bias is raised from 0 to 3.0 V. The resistances agree with those measured in DC leakage current experiments. The capacitances are 100 times larger than those measured on the clean sample at 100 % RH. 

Coulometry (5) is not usually the technique employed. Even in the absence of kinetics, the several minutes required for the electrolysis seems excessive and destmction of the sample is not a desirable result. Furthermore, coulometric precision can be exceptionally poor at low concentration, and currents almost never decay to zero because of the trace contaminants present. One has to decide when zero current has been obtained. 

Two types of blanks need to be prepared with each batch of samples. To subtract background levels of contamination occurring during instrumental analysis, an instrument blank, which consists of a matrix-matched solution with no internal standard, is made. Generally, an instrument blank is run at the start of the analysis and will be included in the calibration with assigned concentrations of zero for all elements to be measured, so, in effect, this is subtracted from all the subsequent samples. 

FIGURE 2.12 Mossbauer spectra of intermediate X recorded at 4.2K for B = 0. (a) Earlier preparation containing a large fraction of diferric species, outlined by the solid curve. The two minor lines, indicated by the arrows, belong to sites a and b of X. (b) Zero-field spectrum of X, from a later sample, obtained by subtracting two contaminants as described in the text. Dashed line belongs to site a. 

During the five months of operation with the zero rare earth octane catalyst, the effective fresh catalyst addition rate, after correction for catalyst loss from the unit as fines, was about 5 tons/day. Based on a rare earth material balance (Table II) that was used to give the best estimate of pedigree, the equilibrium sample consists of 88% USY octane catalyst. The remaining 12% should be a mixture of the prior two catalysts, the first of which contains a contaminant rare earth level of 0.5 wt% versus 0.1 wt% for the octane catalyst. The balance of this mixture is the rare earth-Y catalyst from the previous changeover which exhibits a rare earth level of 0.85 wt% (Table II). 

System or Instrument Blank. It is a measure of system contamination and is the instrumental response in the absence of any sample. When the background signal is constant and measurable, the usual practice is to consider that level to be the zero setting. It is generally used for analytical instruments but is also applicable for instruments for sample preparation. 

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