Trace analysis (ie. where the analyte concentration is of the order of parts per million or less) is particularly challenging. A recent study of mercury in Lake Michigan found levels near 1.6 pM (1.6 x 10-12M), which is two orders of magnitude below concentrations observed in many earlier studies.1 It was concluded that all previous investigators had inadvertently contaminated their samples. Similarly, a study of handling techniques for the analysis of lead in rivers investigated variations in sample collection, sample containers, protection during transportation from the field to the lab, filtration techniques, chemical preservatives, and preconcentration procedures.2 Each individual step that deviated from the best-known practice doubled the apparent concentration of lead in stream water. Clean rooms with filtered air supplies are essential in trace analysis. Even with the best of precautions, the precision of trace analysis becomes poorer as the concentration of analyte decreases.
Your laboratory notebook should always describe how a sample was collected and stored and exactly how it is handled, as well as stating how it was analysed.
Remember, "Unless the complete history of any sample is known with certainty, the analyst is well advised not to spend his [or her] time in analysing it." 3
| 1. | R. P. Mason and K. A. Sullivan, Environ. Sci. Technol., 1997, 31, 942. |
| 2. | G. Benoit, K. S. Hunter and T. F. Rozan, Anal. Chem., 1997, 69, 1006. |
| 3. | R. E. Thiers, Methods of Biochemical Analysis, (D. Glick Ed.), Vol. 5 (New York, Interscience, 1957), p. 274. |