准确测定系列1:Laying the Foundation

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2005/03/30

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1. Laying the Foundation

What is Trace Analysis?
Some analysts refer to trace analysis as a measurement below one ppm (µg/g) while others use the term to describe an analyte concentration low enough to cause difficulty. This difficulty may be caused by the sample size or the matrix (i.e. - the concentration of the analyte of interest relative to the matrix or the sample size causes difficulty for the analyst). Most trace analysts using ICP-OES / ICP-MS prefer the latter definition. Regardless of which definition you prefer, most analyses that require a measurement using ICP-OES or ICP-MS fall within the category of trace analysis.

Stages of a Trace Analysis
Planning - Prepare a plan that considers the objective. Planning should begin with a discussion between the analyst and the initiator during which all possible problems are defined. The analyst is responsible for method selection or development.

Sample Collection and Storage - Ideally the analyst is involved in this stage, but if not, the analyst should be informed of the sampling procedure at the very least. Sample representation and contamination issues must be considered.

Sample Preparation - Contamination issues are a major concern during this stage, but not the only concern.

Sample Measurement - The major concerns during this stage are:

Availability of Certified Reference Materials for method validation, plus stable and accurate calibration standards, interference standards, and quality control standards.

Achieving the required precision. It serves no purpose in acquiring a precision that has been reduced to less than one-third of the sampling error. In situations where the sampling error is small and the highest level of precision is required, the analyst faces a difficult task in acquiring precision equivalent to classical wet chemical techniques.

Obtaining the required sensitivity and determining the detection limit of the measurement.

Overcoming interferences using ICP-OES that include matrix differences between standards and samples; spectral interferences (i.e. - direct spectral overlap, wing overlap, interference with background point); chemical enhancement of atom lines by high matrix element compositions (axial view); and drift due to nebulizer plugging, changes in sample argon, power supply instability, or room temperature changes.

Overcoming interferences using ICP-MS that include matrix differences between standards and samples; mass-discrimination effects; isobaric interferences; detector dead-time; and drift due to nebulizer plugging, changes in sample argon, power supply instability, or room temperature changes.

Calculating and Reporting the Data - Working with error budgets and calculating the uncertainty is the trickiest part of this stage. An article entitled Understanding Error Budgets is available online as an additional reference.

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Training
Trace analysis is extremely difficult. All too often, samples submitted as "routine" actually require highly skilled analytical chemists using complex chemical treatments and expensive state-of-the-art equipment.

Today, many analysts do not have the proper training, nor do they have access to a more experienced colleague that could offer assistance. The availability of sensitive "push-button" instrumentation is ever increasing. Laboratory supervisors should not assume that an analyst is trained to perform trace analysis if the instrument's instruction manual was the only source of training. Education should be provided for the analyst. Furthermore, job experience and training records should be kept and reviewed on an annual basis.

Recommended References
For some applications, a search of the literature is required. Larger companies typically have a library and often a staff of chemists trained to conduct literature searches. For smaller companies without these facilities, the Internet is of great value in finding technical information.

A search of the scientific literature, complete with the ability to download or order specific papers, is available through the CAS Document Service at stneasy@cas.org. Visit our links section for additional links to EPA, AOAC, and other published methods.

The following is a list of highly recommended reference books and resources for the trace analyst:

General References:

CRC Handbook of Chemistry and Physics; Lide, D. R., Ed.; CRC Press: Boca Raton, FL.

Encyclopedia of Analytical Science; Townshend, A., Ed.; Academic Press: New York, 1995, Vols. 1-10.

IV's Analytical Periodic Table; Information on elemental compatibility, stability, sample preparation, preferred lines and spectral interferences for ICP-OES and ICP-MS; Inorganic Ventures / IV Labs: 2001-2002.
Sampling:

Sampling and Sample Preparation; Stoeppler, M., Ed.; Springer Publishing: New York, 1994.

Crosby, N. T.; Patel, I. General Principles of Good Sampling Practice; The Royal Society of Chemistry: Cambridge, U.K., 1995.
Sample Preparation:

Gorsuch, T.T. The Destruction of Organic Matter; Pergamon Press: Elmsford, NY, 1970.

Introduction to Microwave Sample Preparation, Theory and Practice; Kingston, H. M., Jassie, L. B., Eds.; American Chemical Society: Washington D.C., 1988.

A Handbook of Decomposition Methods in Analytical Chemistry; Bock, Rudolf, Ed.; Halsted Press, Div. Wiley & Sons: New York, 1979; translated by Ian L. Marr.

Mizuike, A. Enrichment Techniques for Inorganic Trace Analysis; Springer-Verlag: New York - 1983.
Trace Analysis:

Trace Analysis: A Structured Approach to Obtaining Reliable Results; Prichard, E., MacKay, G. M., Points, J., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1996.

Guidelines for Achieving Quality in Trace Analysis; Sargent, M., Mackay, G., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1995.
Environmental Analysis:

Smith, Roy-Keith Handbook of Environmental Analysis; Genium Publishing: U.S.A., 1994.

Berger, W.; McCarty, H.; Smith, Roy-Keith Environmental Laboratory Data Evaluation; Genium Publishing: U.S.A., 1996.
Quality Assurance & Statistical Technique:

Taylor, J. K., Quality Assurance of Chemical Measurements; Lewis Publishers: Chelsea, MI, 1987.

Mark, H.; Workman, J. Statistics in Spectroscopy; Academia Press: San Diego, CA, 1991.

Taylor, J. K. Statistical Techniques for Data Analysis; Lewis Publishers: Chelsea, MI, 1990.

Swartz, M. E.; Krull, I. S. Analytical Method Development and Validation; Marcel Dekker: New York, 1997.

Accreditation and Quality Assurance in Analytical Chemistry; G黱zler, H., Ed.; Springer-Verlag: New York, 1994.

Good Laboratory Practice Standards; Garner, W.Y., Barge, M. S., Ussary, J. P., Eds.; American Chemical Society: Washington D.C., 1992.
ICP-MS References:

Plasma Source Mass Spectrometry - Developments and Applications; Holland, G., Tanner, S. D., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1997.

Inductively Coupled Plasma Mass Spectrometry; Mantaser, A., Ed.; Wiley-VCH: New York, 1998.

Taylor, H. E. Inductively Coupled Plasma Mass-Spectrometry, Practices and Techniques; Academic Press: New York, 2001.

ICP-OES References:

Inductively Coupled Plasmas in Analytical Atomic Spectrometry; Montaser, A., Golighty, D. W., Eds.; VCH Publishers: New York, 1992.

Thompson, M.; Walsh, J. N. A Handbook of Inductively Coupled Plasma Spectrometry; Blackie: London, U.K., 1983.

Developments in Atomic Plasma Spectrochemical Analysis; Barnes, R. M., Ed.; Heyden: London, U.K., 1981.