Contamination From the Analyst and Apparatus

Apparatus Contamination

Another potential source of contamination is casued by the apparatus that comes into contact with the sample. Careful planning is required by the analyst as to the type of apparatus that comes into contact with a particular trace metals analysis. All apparatus should be specified in the procedure along with any cleaning procedures or special precautions. Although the type and degree of contamination that occurs when using certain apparatus can be predicted, the importance of one or more blanks with every preparation should be apparent from the following table.

Table 10.1: Results of Spectrochemical Analysis
(Hydrogen Fluoride, Hydrogen Chloride, and Nitric Acid [all high purity] analyzed after evaportation in Teflon, Platinum, and Quartz dishes)

Elements determined, ng/g

     ND = Not detected
     TR = Trace, not evaluated quantitatively

A Closer Look at Quartz

Quartz is a popular material for apparatuses used in trace elemental analysis. There are two types of quartz: opaque and transparent.

Opaque quartz has the highest trace element concentration and should not be used for trace analysis.

Transparent quartz (types I and II) are made from naturally occurring quartz crystals or sands. Type I is made by electric melting and type II by flame melting. Type II has slightly less impurities than type I (some impurities are volatilized by the flame). Type III quartz is made synthetically by vapor phase hydrolysis of pure silicon compounds such as SiCl₄. This type of quartz is more pure than the natural quartz with the exception of Cl ^- which is ~ 50 ppm. Type IV quartz is synthetically made from SiCl₄ using a process involving electrical fusion of the oxidized staring material. It is as pure as type III, with respect to trace metal content, and contains much more Cl^-. Use synthetic quartz whenever possible.

Table 10.2 shows typical impurities in natural and synthetic quartz.

Table 10.2: Recorded Impurity Levels in Types of Quartz¹

     ND = Not detected

Table 10.3 shows a comparison of impurities in natural quartz to other common laboratory apparatus.

Table 10.3: Trace Element Concentration of Container Materials

Figure 10.4 demonstrates how quartz is the cleanest of all sample container materials that can withstand typical ashing temperatures (T > 400 °C).

Apparatus Tips

• Avoid filtration due to contamination from the filter paper. It is far better to centrifuge the sample using polyethylene tubes (acid-leached).
• If filtration is necessary, use Teflon filters as your first preference. These are followed by cellulose acetate, polypropylene, or polycarbonate filters that have been acid-leached.
• Avoid rubber stoppers and all tubing except that made from Teflon or polyethylene.
• For grinding, use apparatus made from quartz, tungsten carbide, or pure molybdenum since they introduce significant contamination of only a single element (the tungsten carbide does introduce some cobalt).
• When sieving, use nylon sieves.
• Ovens and muffle furnaces are obvious sources of contamination and should be replaced with flames if at all possible.
• Microwave digestion in a closed Teflon (PFA) digestion vessel offers many obvious advantages over other forms of sample preparation.

1. T. Murphy, National Bureau of Standards Special Publication 422, "Accuracy in Trace Analysis: Sampling, Sample Handling, and Analysis", Proceedings of the 7th IMR Symposium.

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