Sampling and Subsampling

Contamination Issues During Sampling

Geological

Sampling of geological materials can require hundreds of kilograms of material to be processed for the determination of certain elements present in inclusions. For example, Mo and Nb in granite.

When using tungsten carbide crushing and grinding apparati, the common contaminants are W, Cr, Mn, V ,Co, Fe, Cu, Zr and Zn. Sieves made of nylon are recommended to avoid contamination by all metals.

Soils

Soil is heterogeneous and the composition varies with depth. Essential and hazardous element distributions down to 50 cm are of interest.

Grinding in Alundum ball mills is recommended to avoid contamination from most metals (Al, Si, and Fe are common contaminants from the Alundum). Sieves made of nylon are recommended to avoid contamination by all metals.

Air

Air particulates are collected by passing a known volume of air through a filter or impactors. The analyst is collecting samples on filters which are a common source of contamination due to impurities inherent in the filter itself. There are no significant concerns for the collection of air samples beyond conventional practice.

Water

A typical EPA scenario consists of:

Water samples are taken in polyethylene bottles that are pre-washed with detergent, followed by 10% HNO3 and distilled water.

The pH of the sample is adjusted to 2 with HNO3 to minimize adsorption.

Water samples should be stored at 4°C to minimize changes due to biological activity
(e.g. - redox processes).

The analyst should be concerned about contamination from the collection container as well as adsorption and precipitation of the analyte(s) of interest after collection. Of the elements of environmental interest, Hg is the most difficult to keep in solution. Publications of the collection of water samples should be consulted for specific applications.

Biological

Contamination of biological materials is of considerable concern since the trace metals are typically in the ng/g level. Contamination from the air, sampling devices, and sample storage containers are common. As minimum precautions, a laminar flow clean air bench, plastic sampling devices, and containers that are non-wettable (PTFE preferred) should be employed. In addition the samples are maintained at 4°C during transport and at -18°C for storage.

Some experts maintain that sampling errors have negated most published information on trace-element determinations in biological matrices.¹ Thiers stated, "Unless the complete history of any sample is known with certainty, the analyst is well advised not to spend his time in analyzing it."² Some examples given by Thiers showed a drop in Mn from 2.7 to 0.6 ppb in blood plasma (stainless steel needles contaminate blood with Mn) and a drop in Zn from 1.7 to 0.95 ppb in blood plasma (glass and rubber stoppers contaminate blood with Zn). Other sources of contamination for Al, V, Cr, Mn, Co, Ni, As, Mo, and Cd include plastic tubes, parafilm, wooden applicator sticks, acids, laboratory tissues (Kimwipes and Kleenex), and filter paper.

Contamination Data

The following Tables show contamination potentials from various elements. The data in Tables 1, 2, and 3 was originally printed in X-Ray Spectrometry.³

Table 3.1: Container Materials
(ppm levels of contaminants)


Table 3.2: Environmental
(elemental composition of contaminants in ppm)


Table 3.3: Contamination of Mineral Acids
(by leaching of container wall during evaporation)

Contamination From Speciation Change

Trace metals associated with colloids and particles are considered inert⁴:

1. The sampling scheme should involve separation of these various forms of metal species in situ or shortly after sampling.
2. The quality of a representative water sample can be defined as the degree to which the sample retains its composition and properties after the removal from the original environment.
3. For trace elements, the main factors affecting the quality are contribution of elements due to contamination and loss of species due to sorption and volatilization.

The key point is that contamination can come from the breakdown of "inert" colloids / particles. The following figure illustrates this sample collection contamination issue:

The ability to differentiate between phsico-chemical forms is essential for assessing biological uptake of trace elements...

"Following the introduction of non-contaminating techniques for sampling, sample handling, and analysis, as well as developments within analytical techniques and instruments, the concentration levels of trace elements in unpolluted natural waters have been shown to be a factor of 10 to 1000 lower than previously accepted. Thus, the progress made in our understanding of trace element behavior in natural water systems is closely related to improvements within analytical chemistry."⁴

1. J. Versieck, L. Vanballenberghe, A. De Kese, D. VanRenterghem, Biological Trace Elemental Research 12 (1987): 45-54.
2. R. E. Thiers, Methods of Biochemical Analysis, ed. D. Glick (New York: Interscience, 1957): 274-309.
3. B. Holynska, "Sampling and Sample Preparation in EDXRS," X-Ray Spectrometry 22 (1993): p. 192.
4. B. Salbu, D. Oughton, Trace Elememental Analysis of Natatural Waters (Boca Raton, FL: CRC 1995): 41-69.

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