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Environmental ContaminationTrace Analysis Guide: Part 8During the course of a sample preparation, contamination can occur from the environment, the reagents, the apparatus, and even the analyst. In many cases the blank determines the lower limit of detection. The next several parts of this guide deal with contamination. OverviewEnvironmental contamination is caused by particulate and/or gaseous matter in the air. It has been reported that air in an analytical laboratory can contain up to 200 µg/m3 of particulate matter containing Ca, Si, Fe, Na, Mg, K, Tl, Cu, Mn, and lesser amounts of other elements1. In addition, this reference states that normal rural area airborne particle counts have been reported to be 1,400,000/m3 for particles greater than 0.5 µm and that normal metropolitan area particle counts have been reported to be 53,000,000/m3 for particles greater than 0.5 µm. Reducing Environmental ContaminationUse the chemical blank as the performance criteria. Dealing with environmental contamination is expensive. Therefore, reduce the blank to the lowest possible value by eliminating contamination from the sample container, the chemist, and sample preparation apparatus. Achieve this by doing the following:
The effect of the laboratory atmosphere on lead blank levels is shown in Table 8.1.
Hoods, which enhance atmospheric exposure and hence contamination, are necessary for the safety of the analyst. Atmospheric contamination has been reduced significantly through the use of "clean rooms" that use HEPA filtered air. HEPA (High Efficiency Particulate Air) filters are 99.99 % efficient in removing particulates down to 0.3 µm. A Positive Pressure Filtered Air "Clean" Room is shown in the following architectural diagrams. ![]() ![]() ![]() The effectiveness of clean rooms in eliminating environmental contamination is illustrated in Tables 8.2 and 8.3 which show a significant reduction in the blank. (Concentrated in Conventional Chemical and Clean Rooms)
(Concentrated in Conventional Chemical and Clean Rooms)
For budgets that cannot handle the cost associated with clean room facilities there have been several ingenious designs that are effective in dealing with environmental contamination. Diagram 4.4 illustrates some of these designs using equipment that is relatively inexpensive and easily obtained. ![]() Avoiding Environmental Contamination
1. T. Murphy, National Bureau of Standards Special Publication 422, "Accuracy in Trace Analysis: Sampling, Sample Handling, and Analysis", Proceedings of the 7th IMR Symposium. Further Reading |