Page:NIOSH Manual of Analytical Methods - Chapter C.pdf/5

1. generated samples, the stability of collected samples or possible interferences to accurate use of the method. Methods should be tested for ruggedness so that critical steps in the analysis can be identified. Experimental designs have been published which permit rapid evaluation of a number of factors involved in the analysis (e.g., ^[1]).
2. Standards
   Standard solutions may be for either identification or quantitation of the analyte of interest. The bulk material used for preparation of standards should be of sufficient purity to avoid errors in the identification or quantitation. Where available, Standard Reference Materials (SRMs available from the U.S. National Institute of Standards and Technology) or other well-characterized standards are useful for assuring that results are consistent with other organizations and agencies.^[2] Laboratory reagents and standards should also be properly labelled with contents, and receipt and expiration dates. For quantitation, sufficient numbers of standards should be prepared so that adequate confidence intervals on reported values can be obtained.^[2][3] As a guideline, we recommend that calibration curves be prepared with triplicate points at each of at least five different concentration levels. The standard curve should be prepared so that linearity can be assured over the range of the curve. Also, it is important that the concentration of standard solutions be chosen to bracket the actual samples. In this way, extrapolation outside the range of standards is avoided. The concentration of standard solutions should be chosen to be consistent with the purpose of the sampling. That is, if it is desired to demonstrate whether exposure standards are being met, then the standard curve should be constructed to bracket the concentrations which would be encountered at or near the applicable exposure standard. If it is desired to demonstrate whether a compound is present in the atmosphere being sampled, then the calibration curve should be constructed closer to the limit of detection. During measurement, working standards should be interspersed with field samples. In this way, it should be possible to detect if instrument drift becomes significant. Internal standards are useful for correcting instrument response for the actual amount of sample injected into a chromatograph. The internal standard should be chosen so that its retention time is reasonably close, without peak overlap, to the peak of interest.
3. Blanks (also see page 26)
   A particular analysis may involve several types of "blank" measurements including reagent blanks, media blanks or field blanks. Reagent blanks measure the signal contribution from solvents, acids or other reagents used by the laboratory in preparing samples for analysis. Media blanks measure the signal contribution from the collection media (impinger solution, filter, sorbent tube, etc.) and field blanks measure signal contribution of the media plus any contamination which may have occurred during handling, shipping and storage before analysis. The nature and number of blank measurements will depend on the method and circumstances, but the purpose of all blank measurements is to help prevent errors in identification and quantitation of field samples.^[2]

1. Youden W. J. and E. H. Steiner. Statistical Manual of the Association of Official Analytical Chemists, AOAC, Washington, DC (1975).
2. Taylor, J. K. Quality Assurance of Chemical Measurements, Lewis Publishers, Chelsea, MI (1987).
3. Taylor, J. K. Statistical Techniques for Data Analysis, Lewis Publishers, Chelsea, MI (1990).