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Chemical Analysis
Chemical Analysis
Page 38
f. Verify the calibration curve adjustment (Standard Adjust) currently in
use. The factory-stored default calibration should be used initially to
check the standard.
g. Verify that the dilution factor option is correct.
If the instrument setup is correct and the method procedure specifics are being
followed correctly, go to step 3.
3. If the standard solution check does not match the expected results, check the
reagents used in the test and the analytical technique as follows:
a. Determine the age of the reagents used in the test. While most Hach
reagents have a long shelf life, many factors affect this (i.e., storage
temperature, storage conditions, microbial contamination). Replace
suspect reagents and run the standards check again.
b. Run a deionized or distilled water blank through the entire process;
include sampling and storage, digestion, and colorimetric determination.
Some chemicals will add a small amount of color to a test; this is not
considered unusual. However, color development in excess of 10% of the
range of the test may indicate a problem with one of the reagents or the
dilution water.
c. To troubleshoot the procedure, delete the parts one by one. First, using
the standard solution, omit preservation and storage, doing only
digestion and colorimetry. If this analysis is correct, examine the
procedure used to store the sample. Ensure that it is the procedure
prescribed for the chosen parameter. If the sample is acidified for storage,
be sure the correct acid is used and the sample is adjusted to the proper
pH level before testing.
If the standards check is still incorrect, run the standard on just the
colorimetry. If the results are correct, examine the digestion procedure.
Ensure that the amount of reagents used and the pH after the digestion
are correct for the procedure. (See the procedure for the parameter
in question.)
4. If the standard solution gives a correct value, but the results of the sample
measurement are questionable, there may be an interference in the sample. To
check for an interference:
a. Spike the sample. Use a standard addition test instead of a standard
solution test to include any possible interferences.
To a pair of cells containing fresh sample water, add an amount of
standard equal to two times the concentration of the sample. Process
both samples using the same reagents, instruments, and technique. The
spiked sample should show an increase equal to the amount of standard
added. Calculate percent recovery as shown below. Ideally, the results
should be 100%, with results from 90 to 110% considered acceptable.
Refer to the procedure notes for possible interferences and ways to
eliminate them.
b. Run a series of dilutions on the sample. Make sure your sample is within
the range of the test. A sample out of range for the method may give
erroneous results because of under- or over-development of the color,
excess turbidity, or even sample bleaching. Run a series of dilutions to
check for this possibility.
5940022ChemAnal.fm
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