Depth of sampling

As we have seen, the electron is the easiest probe to make surface sensitive. For that reason, a number of hybrid teclmiques have been designed that combine the virtues of electrons and of other probes. In particular, electrons and photons (x-rays) have been used together in teclmiques like PD [10] and SEXAFS (or EXAFS, which is the high-energy limit of XAES) [2, Hj. Both of these rely on diffraction by electrons, which have been excited by photons. In the case of PD, the electrons themselves are detected after emission out of the surface, limiting the depth of sampling to that given by the electron mean free path. 

Some major concerns in sampling include required depth of sampling, whether or not sequential samples at different depths will be... 

FIGURE 6.28 Precise control of depth of sampling can be achieved with an autosampler employed in a /tPI.C system. The dimensions indicated correspond to those used during the evaluations described. 

Field measurements included pH, Eh, EC, temperature, depth of water table and depth of sample collection. Separate, field preserved sub-samples were collected for cation (ICP-MS/OES), anion (IC), alkalinity (titration), DOC, P04 and Au and PGE analysis (using carbon sorption). 

The depth of sampling is important. Deeper samples yield increased elemental concentrations. Our results also indicate... 

Solids. Conventional near-infrared reflectance analyzers use a variety of methods to position the sample into the incident collimated beam and collect reproducibly the diffusely reflected radiation to measure the absorption which takes place in the body of the sample traversed. Solid samples are ground with care to achieve reproducible and reasonably uniform granulation for calibration and analysis measurements. The overall scattering characteristics of the sample which shift the reflectance baseline and control the depth of sample penetration and opportunity for absorption become a part of the method and the empirical analytical equation (6,9). Solid sampling is summarized on the following table. 

Factors that affect the rate of low-temperature ashing other than radiofrequency power and oxygen flow rate are the coal particle size and depth of sample bed. Typical conditions for ashing are a particle size of less than 80 mesh, a sample layer density of 30 mg/cm2, oxygen flow rate of 100 cm3/min, chamber pressure of about 2 torr, and a 50-W net radio-frequency power. The total time required is 36 to 72 hours, and specified conditions must be met during the procedure to obtain reproducible results. 

Variations Between Lakes. Results of a study to evaluate sulfide production variation with water depth is given in Table V. In this experiment, samples were taken from five different sediment depths over a two-day period at each lake in early October. At both lakes sulfate reduction exceeded putrefaction by a factor of approximately 2 with overall mean rates of 0.55 and 0.29 mg S L-kH1 respectively. Sulfate reduction exceeded cysteine decomposition in all samples except one collected from Third Sister Lake at 17 m. Results of this study snow a good correlation at Third Sister Lake between percent hydrogen sulfide production attributable to putrefaction and depth of sampling station (r=0.94) and oxidation-reduction potential (r=0.98). This correlation was not observed at Frains Lake. A possible factor m differences observed may be the physical nature of the sediment at Frains which was less dense and more flocculent than thatofTliird Sister. 

Heavy metals concentration generally decreases in urban soils away from the main road network and with increasing depth of sampling. This can be explained by the strong dependence of these contaminants on the use of motor vehicles—leaded fuels for Pb, tire wear for Zn and Cd, brake pads for Sb, converters and exhaust systems for platinum group elements (PGEs). 

Figure 15.7. Stoichiometric correlations among nitrate, phosphate, oxygen, sulfide, and carbon. The correlations can be explained by the stoichiometry of reactions such as equation 3 concentrations are in micromolar, (a) Correlation between nitrate nitrogen and phosphate phosphoms corrected for salt error in waters of the western Atlantic, (b) Correlation between nitrate nitrogen and apparent oxygen utilization in same samples. The points falling off the line are for data from samples above 1000 m (Redfield, 1934, p. 177). (c) Correlation between nitrate nitrogen and carbonate carbon in waters of the western Atlantic, (d) Relation of sulfide sulfur and total carbonate carbon in waters of the Black Sea. Numbers indicate depth of samples. Slope of line corresponds to AS /AC = 0.36. (From data of Skopintsev et al., 1958, as quoted in Redfield et al., 1966.) (e) Correlation of the concentration of nitrogen to phosphate in the Atlantic Ocean (GEOSECS data). The slope through all the data yields an N/P ratio close to 16.

Even where these general problems do not appear to arise, the shallow depth of sampling attained with a probe may fail to provide an appropriate sample, due to the high diffusibility of He, which promotes equilibration of He in near-surface soil gas with the atmosphere. Thus, Butt and Gole (1985) found that He concentrations increased with depth and that samples from less than 3 m had lost most, if not all, of any excess He. Similarly Hinkle (1994) found no significant variations in samples collected at various depths in the... 

Core designation County Depth of sample(m) Mean loss on ignition Mean total carbon content... 

Note the reading on the scale and calculate the reading for the appropriate depth of sampling for the 0.02mm cutolf at the temperature of the suspension (Table 2.1.1). 

Table 2.1.1. Depth of sampling for silt + clay (- 0.02 mm) after 5 minutes, and for clay (- 0.002 mm) at 5 hours 30 minutes ...

This study focuses on the two most important lithostratigraphical units of the Swiss Molasse basin, the Lower Freshwater Molasse and the Upper Marine Molasse. The present-day depth of samples ranges from 15 to 1300 m the sample locations are shown in Fig. 1. 

Each individual sample should originate from a continuous core which has been taken to the predetermined total depth of sampling. Resources permitting, there should also be at least one set of deep cores to characterize the profile to the ground water, if the soil profile has not already been established from past or current well drilling procedures. 

The calibrated half-life for aldicarb is longer than the half-life which was calculated based on field data (Table II). This occurs because runoff loss of aldicarb as well as leaching below the depth of sampling are not accounted for in field-calculated half-lives, which are calculated based only on aldicarb remaining at each sampling date. An additional possible avenue of loss is plant uptake of aldicarb. However, the total amount of uptake was not estimated in the field, nor was it simulated in PRZM. As such, it can be considered that plant uptake loss was "lumped" in the calibrated (and calculated) half-lives. 

Table V summarizes the fate and transport of aldicarb in the calibration scenarios. In North Carolina, the simulations predict that 4.2% of applied aldicarb was lost via runoff and none leached below the depth of sampling. The runoff result cannot be verified since field data of runoff were not taken. However, since the soil was a loam soil, seme water runoff would be expected, and a fraction of the soluble aldicarb present in top zone on the date of runoff would also run off. As a result, the calibrated half-life, 43 days, is higher than the range calculated, 27-39 days.

In the Cameron, Wisconsin simulations, aldicarb did not leach below the depth of sampling for the planting or emergence appli-... 

Nebraska soils, Knox County, Nebraska Collected by a cleaned shovel. Sample 19 was obtained 10 mi west of Crofton by digging into a recently eroded stream bank so that the depth of sample from surface... 

Because of inconsistencies in the depths of sampling and analytical methods, direct comparison of all samples was not possible. (Spain et al, 1983)... 

Table X. Variation of Selenium Content of Seawater with Depth of Sample (33)...

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