Hach DR/850, Procedures Manual, Page: 43 / 460

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CHEMICAL ANALYSIS INFORMATION
, continued
Branch H
Check operation of the instrument and/or apparatus used to perform the
test. Check glassware used in the procedure and make sure it is extremely
clean. Dirty pipets and graduated cylinders can cause contamination and
will not deliver the correct volume. Check delivery of pipets by using
deionized water and a balance; 0.2 mL = 0.2 grams.
If a defect is found in the instrument and/or apparatus, repeat Branch B
after repair or replacement. If the instrument and apparatus are working,
proceed with Branch I.
Branch I
After determining the procedure, reagents, instrument and/or apparatus
are correct and working properly, you may conclude the only possible
cause for standard additions not functioning correctly in deionized water
is the standard used for performing standard additions. Obtain a new
standard and repeat Branch B.
Branch J
If the standard additions gives the correct result, the analyst must then
determine if an interfering substance(s) is present. If interfering
substances are present, proceed to Branch C. If they are not present, the
analysis is correct.
If you still cannot identify the problem, extra help is available. Please call
our Technical Support Group at 800-227-4224 (U.S.A.) or 970-669-3050.
A representative will be happy to help you.
Method Performance
Estimated Detection Limit
Ranges for chemical measurements have limits. The lower limit is
important because it determines whether a measurement is different from
zero. Many experts disagree about the definition of this detection limit,
and determining it can be difficult. The Code of Federal Regulations (40
CFR, Part 136, Appendix B) provides a procedure to determine the
“Method Detection Limit” or MDL. The MDL is the lowest concentration
that is different from zero with a 99% level of confidence. A measurement
below this MDL may be useful, but there is a greater chance that it is
actually zero.
The MDL is not fixed; it varies for each reagent lot, instrument, analyst,
sample type, etc. Therefore, a published MDL may be a useful guide, but
is only accurate for a specific set of circumstances. Each analyst should
determine a more accurate MDL for each specific sample matrix using the
same equipment, reagents and standards that will routinely be used
for measurements.