IS YOUR ANALYTICAL RESULT ACCURATE?

New analytical instruments and more sophisticated analytical methods have lead the data user to rely unconditionally on the accuracy of the environmental measurement. For example, a company has a permitted concentration of mercury of 150 parts per trillion in their plant's effluent to the local sewer authority. Elemental mercury at the parts per trillion levels can be measured by USEPA method 1631. The effluent sample is analyzed after it has been diluted by a factor of 10. The sample that enters the instrument contains what is then measured at 21 ng/L. After a multiplication by the dilution factor of 10 , the reported concentration is 210 ng/L. This is a permit violation. The laboratory is certified for this method by the state and has performed the analysis according to the method. The sample was diluted, thus reducing the possibilities of interferences causing false positives. Is the reported result accurate?

The calibration curve from 0.5 to 100 ng/L gave a correlation coefficient of 0.999. The instrument responses to low concentrations of mercury in pure water indicate that the method and the laboratory are doing very well.

The Method of Standard Additions (MSA)

In USEPA SW-846 method 7000A, the method of standard additions is described. This technique is best described as a multiple set of spiking experiments using 10, 20, 30, and 40 ng/L spikes of mercury to the sample. When the MSA data are plotted, a correlation coefficient of 0.809 is obtained. These results strongly indicated that the reported value (210 ng/L) was wrong. The MSA experiment shows there is so much positive interference in the method for this sample that the real mercury concentration may be “ND” (non-detected).

Conclusion

All results from any analytical method can be checked for accuracy using the method of standard additions. The MSA is applicable to organic analyses as well as inorganic analyses by any quantitative method.