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( a u ,L. f SECTION V : r GEP ERAL QUALITY CONTROL PRACTICES A. Blanks 1. method Blanks Method blanks are introduced on a daily basis or at a frequency of ane-in-twenty if more than that number of determinations are run in one day. The blanks consist of organic-free or deicnized water which is then carried through the analytical scheme as for real E samples. They serve to locate any contamination asso- ciated with laboratory storage, laboratory instrumen- tation, or equipment coming in contact with the sample during processing. They provide the fundamental base- line against which the sample signal is measured. 2. Field and Tra,rel Blanks These closely related types of blanks find their most extensive application in anal ysis faz volatile organic compounds. For each kind, the blank begins as organic- free reagent water in the laboratory. For a travel blank, a sample vial is filled wit'3 the reagent water at the laboratory and then carried with the '�iample containers out to the field anu back to the laboratory. Field blanks are carried to the sample site in a cepa- rate container, with a vial being filled at the sampling location. In either case, the blank serves to identify and nossibly correct for airborne contami- nation associated with collecting or transporting the .s } sample. 3. Sample Blanks These blanks came into play when native sample char- acteristics--most often color or. turbidity--interfere with a determination. If the method in question relies i on spectrophotometric detection, the sample's original absorbance at the wavelength of interest is measured and subtracted from the absorbance of the final developed color. The value is recorded in the bound bench hook so both the apparent concentration and the corrected value may be calculated. i V-1 i