There were many questions generated by the last newsletter in such a short amount of time, so I felt a supplement might be beneficial to everyone. The two major questions were related. The first was the impact of blanks on the detection limits and the second was how could a reasonable detection limit be developed for the method if there were so many questions and contradictions.
The latter is the easier of the two questions, since the procedure has been used for some time. The use of the standard deviation for a large enough series of samples should give a reasonable method detection limit (MDL). I believe the sample concentration may have to be closer to the detection limit to get sufficient deviation, since the internal audit samples we have give an MDL which is below that of our combined analytical MDL. The composition of the sample may be significant too. The breakdown of the two halves of the sample could be developed separately and the total combined, much as I referenced in our Report Limit (RL) discussion. Another aspect which should be considered is the collection of the samples to be used. The collection in the lab would give the best theoretical limit, while having actual field collection with various crews would give a good practical limit for general field use. The better experienced crews should do better than non-experienced crews, but which would be more reasonable for the regulatory community? That is a question which requires some thought and probably funding.
The other question concerned blanks and their use in the development of the MDL . One thought on an MDL was a multiple of the blank, but there is a concern in that regard. The blank may not be cumulative in these samples. For example, the blank levels for the blanks collected during the same time frame as the internal audits indicated that if the blank had a cumulative effect, the 100 ppmC audits should fail more often than they did given the significant percentage the blank should have been added. Since we collected a zero air blank using the same procedures and sample volumes as the internal audits, the concentration comparisons should have been very close. The problem with the internal audits is the lack of significant failures attributed to blank addition. In fact, the failure rate would probably go up if there had been any type of blank subtraction. This phenomenon was also seen in the U.S. EPA audits administered by ERG in the 50-150 ppmC range. If an average blank were subtracted from each of the results, there was a higher failure rate than with no subtraction. A later set of mostly laboratory audits showed a better pass rate with blank subtraction than without. Thus, the determination of the cumulative aspect of a blank is still in question, especially for the older data sets. The newer data set may be the outlier as well, but only time and more data will tell.
There are also the anecdotal cases of a few field samples which were reported at significantly lower concentrations than the “normal” blanks we had collected. When there are three samples from different crews at different times which indicate a consistent concentration over three runs which is significantly lower than the average blanks being collected, there is the possibility there is some matrix effect. I do not know what it might be about having a sample, which removes some of the response seen with a sample gas rather than zero air with no components other than oxygen, nitrogen, and a few ppmC carbon dioxide. There is insufficient data to draw a firm conclusion, but there is enough anecdotal information that causes me to wonder how these factors interact.
I realize these answers do not cover the question of just what the MDL for Method 25 actually may be. That lack of an answer is because there is really too many conflicting sets of data, which may or may not be correctly applied to this method. If the EPA audit data is correct, there may be no significant impact from the blank. If the anecdotal information from the field samples is correct, there may be no significant impact from the blank. If our internal audit data on the ~100 ppmC range samples is correct, there may be no significant impact from the blanks. If the new round robin audit study data is correct, there may be some impact from the blanks. If the common understanding of what a blank is and how it works is correct, there may be an impact from the Method 25 blanks, but what is truly known is there is a lot of room for discussion and further research on the subject.
Triangle Environmental Services, Inc.