Hach DR 2800, User Manual, Page: 57 / 96

57
Advanced Operations
6.2 Standard Additions–Monitoring/Checking Results
Refer to section 6.2.1 on page 58 for instructions on how to perform
a standard addition using the DR 2800.
The accuracy of measured values (their correspondence with the
actual concentration of the parameter in the sample) and the
precision (correspondence of the measurement results obtained
from several samples containing the same concentration of the test
parameter) can be determined or improved using the standard
addition method. This method (also referred to as spiking) serves to
identify sample-specific interference factors, e.g. substances in the
sample that alter the analysis results (sample matrix effect),
a defective measuring instrument or contaminated reagents.
Standard Additions is performed by adding a known amount of a
standard solution to a sample. If results are not close to 100%
recovery, an identifiable problem exists.
If the use of standard additions is appropriate for a test, a Standard
Additions method will be in the procedure under Accuracy Check.
Follow the detailed instructions provided.
If the results are about 100% recovery for each addition, they are
likely correct. Results can be verified by running a standard solution
through the test. Poor recovery indicates that a problem exists.
For example, to test for interference in the sample, repeat the
standard additions using deionized water as the sample to check
the reagents, instrument, and technique. If recovery is now about
100% for each addition, interference is likely present in the sample.
1. For poor recoveries with the deionized water, use the following
list to find the problem:
• Follow the procedure exactly:
• Are the reagents added in the correct order?
• Is enough time allowed for color development?
• Is the correct glassware in use?
• Is the glassware clean?
• Does the test need a specific sample temperature?
• Is the sample pH in the correct range?
2. Consult the procedure in the DR 2800 Procedure Manual to
answer these questions.
3. Follow the instructions in Troubleshooting on page 79 to check
the performance of the instrument.
4. Check the reagents. Repeat the standard additions using fresh
reagents. If the results are now good, the original reagents
were probably bad.
5. If nothing else is wrong, the standard is almost certainly bad.
Repeat the standard additions with a new standard.
6. If the problem persists, contact Technical Support.