In testing for metals at part-per-billion and part-per-trillion levels, a clean laboratory and clean sampling equipment are absolute necessities. Sample containers that are routinely cleaned in acid baths can easily become contaminated in the laboratory, both from the air and from residual metals in the acid bath solution used for cleaning.

Commonly asked questions are, “how dirty can an acid bath be and still adequately clean sampling equipment?” and “what other sources of contamination might be present in the laboratory?” An awareness of potential sources of contamination is critical to valid test results.

To address this issue, Battelle conducted two experiments. The first compared three methods used to clean laboratory equipment [Environmental Protection Agency (EPA) Methods 1631 and 1638 and a Battelle method] to determine which method is the most effective and at what concentration in dirty acid baths metals cause contamination of sample equipment.

Ten water-quality criteria metals (antimony, cadmium, copper, lead, mercury, nickel, selenium, silver, thallium, and zinc) were spiked into each of five acid baths at various concentrations. Sampling equipment was soaked in each bath according to the methods, rinsed, dried, and filled with 1 percent nitric acid for storage. The storage water was analyzed for each metal.

Battelle scientists drew the following conclusions from this test:

• EPA Method 1631 is recommended for cleaning Teflon containers used to collect samples for mercury but is not recommended for cleaning containers for other metals. Polyethylene bottles are not recommended for collecting mercury samples, regardless of the method used to clean them.
• The Battelle method for cleaning Teflon is equivalent to the EPA method at low levels of contamination and superior at higher levels of contamination. The Battelle method is less labor-intensive and more cost-effective.
• Lead and zinc may be of concern if concentrations reach 100,000 µg/L in routinely used acid baths. At this concentration, they are the limiting metals for Teflon following EPA Method 1638.
• Acid baths can be used longer than six months if monitored and should be checked at least quarterly.

The second test looked at atmospheric deposition of metals in the laboratory. Results showed that any metal areas subject to acid fumes or vapors, such as acid hoods with evidence of corrosion, sinks, and balances, are sources of contamination. Areas covered with Teflon or protected by plastic are virtually metal free. Particulates carried on lab personnel shoes or clothing and finely ground solid-matrix samples can also contribute to airborne contamination.

For more information on ultra-clean sampling techniques and laboratory procedures, contact Linda Bingler at (360) 681-3627, l.bingler@pnl.gov.

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