May, 2003

Can your samples preserve their integrity?

Whether in the field or the lab, samples that lose their integrity through contamination or other changes no longer represent the population or lot from which they were taken. Lab chemists are experts at preserving the integrity of their samples, from the time the lab receives them, for the period of storage, during preparation, and throughout the chemical or physical analysis. "Sample handling" is the common phrase used, but "preserving the integrity of the sample" is more descriptive.

Some sources of contamination are easy to spot, such as using an unclean container to collect samples, shown below.

Other types of contamination require a trained eye to uncover, such as in the following example provided by Lynn Vanatta, a chemist at Air Liquide-BalazsTM Analytical Services.** Water used in the semi-conductor industry is allowed to contain only extremely small amounts (less than 50 parts per trillion) of chloride because of its corrosive nature. Measuring chloride is tricky because several sources of contamination are present.

Problems with contamination in measuring chloride in water:

    1. The chemist herself is a source of contamination since chloride is present at high levels on human skin.
    2. Lab containers can contaminate the water samples.
    3. The laboratory itself can affect the sample.
    4. The internal parts of the analytical instrument can be a problem.

Solutions:

    1. Vinyl gloves must always be worn.
    2. Certain types of polystyrene and high-density polyethylene have been found to be the best materials for sample bottles.
    3. If hydrochloric acid is being used nearby, the fumes most certainly will contaminate an open water bottle.
    4. The use of polyether ether ketone (PEEK) is the best way to avoid this contamination source.

Moral: Even if a correct, random, and representative sample is taken from the field or from your process, poor sample handling can alter its contents and give you misleading results. Bad information leads to bad decisions.

** Lynn Vanatta is a research chemist for the BalazsTM Analytical Group within the Electronics division of Air Liquide. Her division provides ultrapure chemicals and laboratory services to the semiconductor industry. Lynn is in charge of developing analytical methods to test the purity of fab-related chemicals. Since only ppb or ppt levels of contaminants are allowed in most products, it is of utmost importance that samples not be contaminated during any stage of the laboratory-analysis procedure. In validating her methods, Lynn uses statistical techniques in conjunction with chemistry principles to assure reliable data are reported. She publishes much of her research and is co-author of a continuing statistics column in American Laboratory, a trade journal for chemists.

More information about sampling equipment and procedures for solids, liquids, and gases is available in the Short Course and in the short introductory book A Primer for Sampling Solids, Liquids, and Gases.


Patricia L. Smith, Ph.D., Owner
Phone: 806.798.3420
Fax: 281.749.8158
E-mail: alpha@alphastat.com
Web: www.alphastat.com

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