This newsletter covers the analytical limits for Methods 25 and 25-C, both real and theoretical. Depending on how each type of limit is defined, the options are numerous and the implications more so.
Everyone is probably familiar with the old Method Detection Limit (MDL) for Method 25, which set the use guideline at any concentration over 50 ppmC. That is where a lot of the discussions begin, but there are some issues with such a limit since the two fractions being sampled have other limits which come into play. One such recent discussion involves audits, blanks, and their impacts on one another in a proposed alternate method. Working from a blank as a basis is one way to determine some form of limit, but that only works well if the blank is constant and consistent in the effect on samples. The old data from the U.S. EPA audit program showed an average blank subtracted from the audits in the <150 ppm range slightly decreased the pass rate. Newer data from a series of private audits in a round-robin-ish comparison showed an increase in pass rate for field audits and no real change for laboratory audits. These data on audits and blanks seemed to indicate a possible lower limit for quantification from the previous MDL of 50 ppm.
The question then came up as to what the statistical detection limit would be based on a 100 ppmC audit over a period of years. The thought was to essentially do the same type of statistical analysis on these audits as is performed on an analyzer to determine an MDL. As it turned out, we had some 159 audits at ~100 ppmC performed on a regular basis for over a decade and an additional 61 audits at ~150 ppmC performed in the early to mid 1990s. There were more of the audits taken but some of the recoveries were performed to check a system to confirm or deny contamination or in the attempt to remove contamination. Those were excluded because they would clearly bias the results, but it was understood that some of the audits used might also have had some small amount of contamination that was not noted on the copy I was using. There is also the possibility of a typo or transposition since the information was entered into a spreadsheet by hand and I was known to have made an error in that regard once. I believe it was a Tuesday, but I digress.
Since I was unsure whether to add the 150 ppmC audits or not, I did not perform the statistical analysis. If I included the 150 ppmC audits and the results were skewed, I would have a problem with justification for removal and if I did not include them I would have a similar concern. So, I allowed someone else to make that determination on the addition or not.
The data set was pretty tight since most of the audits had passed the 20% requirement of our SOP, but the results of the statistical were unbelievably low.
The standard deviation of the differences of this data indicate that you should be able to tell the difference between zero and a sample as low as about 0.4 ppm and have some confidence in the quantitative results above 1 ppm.
This is actually below the combined analytical MDL for the method. The analytical MDL for just the condensible fraction, which is measured as CO2, of 0.11 ppmC and the condensible fraction, which is measured as propane, of 0.75 ppmC. These analytical MDLs are determined by the analysis of seven replicates of 3.270 ppm CO2 and 2.040 ppm propane rather than the combined analysis of a couple of hundred 100/150 ppmC audits. The results of these data are different from the initial attempts when a higher concentration standard was used. It could be that if the concentration in the audits had been lower, the standard deviation would have also expanded to a more expected range. Thus, the two statistical methods do not seem to agree. The base 0.86 ppm from the combined analytical MDL for both fractions does not include any dilution, which would normally triple or quadruple the MDL when compared to the report limit (RL) after all of the dilution factors were applied. This would give a rough MDL of ~3.5 ppm for the combined analytical RL for both fractions. Even increasing the amount to ~9 ppm to have some confidence in the ability to determine what is sample from noise still leaves a significant difference between the current use limit and potential use limit. If a much lower use level or combined MDL were to be used, thus compressing that differential, we might need to consider another approach with the RL, such as reporting flagged concentrations between the analytical MDL and first calibration point, but that does not seem to be very likely at this point.
There were also concerns that although the audits were collected as they would be in the field and using the lowest possible sample volume to allow for the maximum potential impact from equipment, etc, the benefits of collecting them in the laboratory may have limited the deviation between audits. If this is the case, the problems with the audits being collected in the field may be more numerous and/or severe than initially expected. Methanol is reactive and the standards used were only certified for six months, however the cylinders used in these audits gave consistent concentrations as carbon for years even after the certification had expired. The thought was that if there started to be a pattern of failures <80% of the expected concentration, that would be an indication of a problem associated with the short certification period. This too was surprising in that no impact was ever documented concerning any issue with the expired standard .The potential loss of sample due to the reactivity also should have made the data more scattered rather than less scattered.
As it is, we will continue to report a RL for each compound in each sample based upon the lowest calibration point and the amount of dilution applied to the sample during recovery or in order to get any analyte within our calibration range. Given the use limit had been set at 50 ppmC by the U.S. EPA, the use of the lowest standard normally indicates a RL which is well below that 50 ppmC level. The only exclusion is the Method 25-C results with very high concentrations of both CO2 and CH4, which can result in an NMOC RL of well over 100 ppmC due to the dilution factors applied. As always the RL is listed on the data page for each and every compound for each and every sample.
Triangle Environmental Services, Inc.