Contamination From the Analyst and Apparatus

Overview

Part 8, Environmental Contamination, discussed the importance of a clean room to reduce air contaminants while performing trace analyses. A clean room is equally important for minimizing contamination from the analyst.

Contamination From the Analyst

The area pictured in Figure 10.1 is the buffer zone between the outside air and the main clean room. Air entering with the analyst is filtered (at the rate of 1.5 times per minute) before the analyst enters the main clean area. A cart contains a HEPA vacuum used for cleaning objects to be taken into the main clean room. A "sticky mat" on the floor just inside the room will remove shoe dust and other contaminants. Another sticky mat (not pictured) is placed just before the entryway to the buffer zone.

The analyst pictured here is wearing disposable "clean room" clothing. There are three separate pieces (head covering, foot covering, and main coverall). The clothing is worn over normal street clothes and should never leave the clean room area.

Common Contaminants

• Sweat contains Ca, Mg, Pb, K, NH₄⁺, SO₄²⁻, PO₄³⁻, and Cd (for those who smoke).
• Cosmetics can contain high concentrations of Al, Be, Ca, Cu, Cr, K, Fe, Mn, Ti, and Zn.
• Some hair dyes contain Pb(OAc)₂.
• Dandruff shampoo can contain significant levels of Se.
• Eye makeup may contain Hg as a preservative.
• Calamine lotion is almost pure ZnO.
• Watches and jewelry contain an assortment of elements and should not be worn in the laboratory.

Analyst Tips

• Keep the volume of reagents to a minimum.
• Keep sample preparation and measurement times to a minimum (use timers and don't let a digestate sit overnight or even during a lunch break).
• Keep a history of beakers and other apparatus as to chemical exposure.
• Exercise care in handling reagents (exposure to atmosphere, or pouring liquid back into the same container or into the wrong container, can result in serious contamination).
• Disposable powder-free gloves prevent contamination from earlier exposures, as opposed to reusable gloves (vinyl is better for difficult manipulations such as pipetting and handling wet glassware, while latex is more rugged).
• Disposable laboratory coats prevent contamination from earlier exposures.
• Disposable foot coverings keep any dirt that the "sticky mat" missed contained.

TIP: Dangerous operations require appropriate laboratory clothing regardless of the potential for contamination.

Apparatus Contamination

Another potential source of contamination is caused 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.

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) is 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 fewer 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 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 starting 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 10.3 shows a comparison of impurities in natural quartz to other common laboratory apparatus.

Other Reagents

The "purity" situation for salts and other reagents is typically not as favorable as it is for water and acids. Typically, the highest purity solid that can be confirmed is 99.999% (5-9's, or TMI 10 ppm). This translates to 100 ppb total impurities for a 100-fold dilution (i.e., 1 g of 5-9's pure reagent into 100 mL of solution). We recommend that you know your supplier's definition of "high purity."

CAUTION: Do NOT use solid reagent-grade materials when preparing samples for trace metals analysis, whenever possible.

If necessary, a blank should be performed to confirm the acceptability of reagent(s) or to identify problem elements/impurities. Purification of unacceptable reagents may be accomplished by mercury cathode electrolysis, extraction with dithizone or cupferron, ion exchange, or crystallization.

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1. Ralph K. Iler, The Chemistry of Silica, John Wiley & Sons: New York, (1979).
2. K. J. Mikkelsen, M. J. Alberg, J. K. Prestidge, Micro, 37 (1995).

Further Reading

• Part 10: Contamination From the Analyst and Apparatus
• Trace Analysis Guide: Table of Contents
• More Guides and Papers