Precision - Hach DR/850 Procedures Manual

CHEMICAL ANALYSIS INFORMATION

Precision

Estimating Precision
Note: Occasionally, the calculated MDL may be very different than Hach's estimate of
the detection limit. To test how reasonable the calculated MDL is, repeat the procedure
using a standard near the calculated MDL. The average result calculated for the second
MDL derivation should agree with the initial calculated MDL. Refer to 40 CFR, Part 136,
Appendix B (7-1-94), pages 635-637 for detailed procedures to verify the MDL
determination.
Note: Run a laboratory blank, containing deionized water without analyte, through the
test procedure to confirm that the blank measurement is less than the calculated MDL. If
the blank measurement is near the calculated MDL, repeat the MDL procedure using a
separate blank for analysis for each standard solution portion analyzed. Subtract the
average blank measurement from each standard and use the corrected standard values to
calculate the average and standard deviation used in the MDL.
Every measurement has some degree of uncertainty. Just as a ruler with
markings of 0.1 mm leaves some doubt as to the exact length of a
measurement, chemical measurements also have some degree of
uncertainty. The quality of the entire chemical method determines
the precision.
Uncertainty in chemical measurements may be due to systematic errors
and/or random errors. A systematic error is a mistake that is always the
same for every measurement made. For example, a blank can add to each
measurement for a specific compound, giving consistently high results
(a positive bias). Random errors are different for every test and add either
positive or negative bias. Random errors may be caused by variation in
analytical technique and cause response variation. Hach chemists work
hard to eliminate systematic errors in Hach procedures using Hach
reagents, but response variation occurs in all chemical measurements.
The method performance section in each procedure provides an estimate
of the procedure's precision. The procedures use a "replicate analysis"
estimate, based on real data.
In replicate analysis, a Hach chemist prepares a specific concentration of
the analyte in a deionized water matrix. The standard is then analyzed
seven individual times with the two reagent lots used in the calibration
(14 total samples). A standard deviation of the two sets of seven values is
calculated. The larger value is reported in the method. The reported value
provides an estimate of the "scatter" of results at a particular point in the
calibration curve.
It is important to stress that the estimates are based on a deionized water
matrix. Precision on real samples with varying matrices can be quite
different than these estimates.
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