ANALYTICAL
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Analytical Chemistry in Environmental Science IV. Standard Reference Materials of an analytical method is its performance on actual samples of the type to be analyzed. All indirect procedures, such as testing for the completeness of recovery of a standard addition, are subject to doubt, because the chemical form of the addition is not necessarily identical to the form present in the sample, and its physical distribution may be different. For trace methods, losses and contamination are constant hazards. For many years, standard reference materials have provided a reliable check on analytical methods in certain fields, notably metals and alloys. Over a period of years, such materials have been prepared, analyzed in cooperative programs involving many laboratories, and oertified by the National Bureau of Standards as standard reference materials. Even in such fields, the problem has become increasingly challenging as the compositions of industrial alloys have become more complex and as trace metals have become increasingly important. I n environmental science, an especially difficult problem is emerging because of the wide diversity of materials to be analyzed for trace constituents. I n bhe case of metal pollutants, a notable beginning has been made by the addition of a preparation of orchard leaves to the standard reference materials available from the NBS. This sample has been analyzed for major constituents and certain minor and trace metals by a t least two different analytical procedures and certified for these components. Other constituents have been given a provisional certification through analysis by a single method considered reliable. To make a standard reference material useful, it must be available in relatively large quantities in a stable and uniform condition. Clearly this requirement cannot be met for many materials of environmental concern, and an alternative procedure is necessary. For example, it would be impossible to store and ship reliable reference materials for trace gaseous contaminants such as ozone or sulfur dioxide in air. Here, a method for a known rate of addition of the contaminant to a stream of purified air is carefully standardized, and the airstream is used as the calibration material. When we consider the variety of environmental samples, including atmospheric particulates, soil, plant and animal tissue, water, sediments, etc., and also the variety of contaminants, including organic materials and various specific forms of metals, the enormity of the problem becomes apparent. For a long time to come, analytical chemilsts must exercise their ingenuity to validate their methods in the absence of the ultimate test using a standard reference material.
T
HE ULTIMATE TEST
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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