Standard: 2003 NELAC
Section: Appendix D.4.4c.3
Link to relevant standard
This section does not specify count times when determining background measurements used for sample subtraction and background measurements used for contamination checks. In addition the background measurement frequency specified for proportional counters is weekly and was changed to daily in the proposed TNI standard V1M6section 1.7.1c.iii.
Typically the count time used for background subtraction is as long as your longest sample count time which for drinking water samples can be counted for 48 hours. Under the current 2003 NELAC Standard performing background measurements for 48 hours on a weekly basis is impractical and would be impossible under the proposed TNI standard.
Request For Interpretation-
What is your interpretation of the count time and frequency for determining background measurements that are used for subtraction and the count time and frequency for determining background measurements that are used for contamination checks? If the laboratory can provide background measurement data that demonstrates consistent background readings over long periods of time, can the lab use this information to justify reducing the frequency and or reducing the count times for taking background measurements?
TNI FINAL RESPONSE:
(Quality Systems Expert Committee and NELAP AC, 3-31-13)
It was the intent of the authors that background measurements for gas-proportional counters were required once a week and the results would be subtracted from the total measured activity in a sample. The counting time on a background measurement should be as long as the counting time of an average sample (although that is not stated in the NELAC Standard).
Background check measurements were required for each day of use and served to check for contamination of the detector. The value obtained is not subtracted from the total measured activity in a sample but is simply a quick check for detector contamination. The counting time for a background check measurement can be relatively short and it certainly does not need to be as long as the counting time of a sample. Again, a required counting time was not specified in the NELAC Standard.
D.4.4 c) 3) requires that background measurements be performed on a weekly basis. Negative controls (such as Method blanks) are discussed in D.4.1 a). Method blanks must be prepared with each preparation batch.
As the Standard is written, there is no allowance for a longer period between background measurements.