Combined Interpretations of the 2003, 2009, and 2016 Standards that apply to Volume 1 of the 2016 TNI Standard
MODULE 6: RADIOCHEMICAL TECHNICAL REQUIREMENTS
Section: 1.5.4 Measurement Uncertainty
Question: We are not sure exactly what this section is requiring of us. What does it mean by "the experimentally observed precision at each testing level"? We are assuming that the simplified version of this section would say that our calculated precision values from our duplicates cannot be greater than the uncertainty of either sample used in the calculation. Is that correct?
TNI Response: Section 1.5.4 states specifically that "the experimentally observed precision at each testing level [of the precision evaluation in section 1.5.3] shall not be statistically greater than the maximum combined standard uncertainty of the measurement results at that level, although it may be somewhat less." Section 1.5.3 establishes different approaches for "reference methods" and for laboratory-developed (or modified) methods. For "reference methods", 1.5.3 a) the standard deviation of the results is calculated for at least four spiked samples as described in Section 1.6. The standard deviation of the four replicate results is compared to the calculated combined standard uncertainty for each of the four results. The CSU is acceptable as long as it is equal to or greater than the experimental standard deviation of the four results. 1.5.3 b) addresses non-grandfathered, non-reference methods. It states that the laboratory shall use a documented procedure to evaluate precision and bias. An acceptable approach will determine the standard deviation for at least three blank samples, and at least three replicate samples at each of three known activities that span the range of activities expected from samples to be analyzed using the method. The calculated CSU for each sample result is compared to the standard deviation of results at that activity level and is considered acceptable as long as it is statistically equivalent, or slightly lower. Analysis of duplicate samples will not meet the minimum requirements of either of these two approaches. This section was extensively revised in the 2016 standard. The SIR is obsolete.
Section: 1.7.1 Instrument Set-up, Calibration, Performance Checks, and Background Measurements
Question: Is factory characterization of gamma detectors using monte carlo simulation software an accepted means of efficiency determination. If yes, which simulation software is accepted (i.e. GEANT4, MCNPX)?
TNI Response: No. Section 1.7.1 a) states that "Instrument calibration shall be performed with reference standards as defined in Section 220.127.116.11.c). The standards shall have the same general characteristics (i.e., geometry, homogeneity, density, etc.) as the associated samples." This section was extensively revised in the 2016 standard. The SIR is obsolete.
Question: 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 V1M6 section 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. 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 Response: 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. The 2009 standard specifies counting times and this section was extensively revised in the 2016 standard. The SIR is obsolete.
Section: 18.104.22.168 Radiation Measurement Batches (RMBs)
Question: Section 22.214.171.124(c) in TNI Standard V1M6 is not clear in regards to counting two Radiation Measurement Batches (RMBs) by sharing the same detector between the batches (“interspersing” samples).
For example, the lab utilizes an internal procedure and log to define the Radiation Measurement Batch (RMB) and relevant “grouping” of samples as required in section 126.96.36.199(c). The two separate RMB batches, which are started on separate dates, last for 14 days from their respective starting dates. One RMB batch consists of NPW (accredited by TNI) with a 1.5-L counting geometry, whereas another RMB batch consists of charcoal (non-accredited by TNI) in a 50-mL counting geometry. The batches have their own, independent Quality Control (QC) systems of 4 QC samples each. The detectors are independently calibrated for these geometries. The detectors are performance tested according to Section 188.8.131.52. Therefore, each RMB has full QC ensuring that the data of accredited samples are not compromised by the non-accredited samples and vice-versa.
The gamma spectrometry counting of these RMBs shares the detectors, for the following reason. Since the samples for each RMB arrive in the lab with time gaps within 14 days, these gaps are filled by counting samples from another batch. Allocating a specific detector to an RMB would result in the detector idling during the time gaps. This lab cannot process all its customers’ samples in a timely fashion with essential equipment idled. TNI V1M6 Section 184.108.40.206(c) does not stipulate that detectors cannot be shared. TNI V1M6 does not require allocation of the detectors to batches, although it does not prohibit it.
The lab questions are as follows: 1) Are samples within an RMB required to be counted on the detector consecutively? 2) Is it acceptable for 2 different RMBs to be counted on the same detector if the samples between 2 different RMB are interspersed, resulting from time gaps in each RMB?
TNI Response: Section 220.127.116.11.c) is part of the overall 1.7.2 section for "Quality Control for Radiochemistry". Specifically 18.104.22.168.c) defines how samples shall be grouped for the purpose of assigning batch QC, allowing for use of Preparation Batch or RMB as outlined in the section. This section does not discuss sample counting/measurement. Note: the Preparation Batch and Radiation Measurements Batch (RMB) are defined in section 1.3 "Terms and Definitions".
There is not a requirement set out in Module 6 for all samples in a Preparation Batch or RMB to be counted in a consecutive order without other samples being interspersed. In fact, unless specifically stated in a promulgated method, samples can be counted across multiple detectors on multiple days, with samples from other batches (Preparation Batches or RMB) counted in-between on the same detector(s). All that is required for any individual sample measurement is that the requirements in section 1.7.1 are met for the detector on which the sample is counted. This would include documented Initial Calibration, Calibration Verification, Instrument Performance Checks, Subtraction Background Measurements, etc., as outlined in the subsections of 1.7.1.
Section: 22.214.171.124 Reporting Results
Question: Section 126.96.36.199.d states that all radiochemical results shall be reported with an estimate of uncertainty, as discussed in Section 1.5.4.
However, section 188.8.131.52.f states that “project- or client-specified reporting requirements can take precedence over the requirements of this Standard”.
Does part f, section 184.108.40.206 supersede section 1.5.4.b and 220.127.116.11.d if a client requests that all types of radiochemistry uncertainty (i.e., Total uncertainty, counting uncertainty, Standard Uncertainty, k-factors) in section 1.5.4.b shall not be included on the report?
TNI Response: Section 1.7.3 applies to Data Evaluation and Reporting, with 18.104.22.168 (Reporting Results) specific to results shown on the final deliverable(s) to the client. 22.214.171.124.f was included in acknowledgement that certain programs (e.g. SDWA) target a level of uncertainty based upon method requirements (e.g. detection limit goal), and that State reporting forms, which may be filled out by the laboratory, often do not include a place to list the uncertainty. If a client does not want to see any uncertainty results on their final report, that is their prerogative (although in the Committee’s opinion the result is not “complete” without some form of uncertainty). However, this does not absolve the lab of the requirements of other sections of Module 6 or the Standard as a whole, including Module 2, Section 5.10 (Reporting the Results). The third paragraph in Module 2, Section 5.10.1 states:
“In the case of tests or calibrations performed for internal customers, or in the case of a written agreement with the customer, the results may be reported in a simplified way. Any information listed in 5.10.2 to 5.10.4 which is not reported to the customer shall be readily available in the laboratory which carried out the tests and/or calibrations.”
Thus, regardless of what is presented on the report, the uncertainty must be determined by the laboratory and readily available. And, if a client requests data be “reported in a simplified way”, the laboratory must maintain the written agreement. Also, note that Module 2, Section 126.96.36.199 requires that the report shall include “a statement of compliance/non-compliance with requirements and/or specifications”.