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SECTION 1 89 Precision for the Oil&Grease example generates RPDave 6.12 and SRpD 5.82. The I ; data quality objective for precision is: RPDave-2SRPD to RPDave+2SRpD,or 6.12 - 2(5.82) to-6.12+2(5.82), or I I_ 0- 17.8 t. A negative value for precision is nonsense,and so zero is always the lower value for t a the range. Although it is possible for the lower value to be some other positive value 4" than zero,in general zero is used because the analyst never wants to eliminate the possibility that he can get exactly the same answer twice in a row as an acceptable result. It is important to realize that DQOs are statistical ranges and that under normal operating conditions around 5%of test results will fall outside these ranges. Data quality { objectives should be updated at least on an annual basis if not more frequently. Within the laboratory,it is possible to calculate DQOs for each analyst for each test,however when providing DQOs to end users of the data,it is probably more representative of the j laboratory's capabilities to report a composite value from all the technicians performing a f test. 3. Instrument Detection Limits In Figure 1-20 the question arises as to whether the signal at point A indicates the { presence of an analyte or is just instrument noise. Instrument detection limit(IDL)is a measure of the normal instrument noise. It provides a guide to what is noise and what is a real signal and is an evaluation of the maximum sensitivity of an analytical instrument to perform an analysis. It is set at three times the standard deviation of the instrument noise level. IDL is determined by the following procedure(EPA 40 CFR Part 136, Appendix B,July 1993): 1. Prepare a calibration curve for the test with standards. 2. Analyze seven(7)laboratory water blanks. 3. Record the response of the test for the blanks. 4. Prepare the mean(%Rave)and standard deviation(S%R)of the results from the blanks as above. g 5. The IDL is three times the S%R on the calibration curve(Figure 1-21). I j Response WOO* 5 Time Figure 1-20. Instrument detection limit problem. Genium Publishing Corporation