How to deal with the uncertain position of ethane with Method 25. Since Method 25 reports Non-Methane Organics, which includes ethane, there is some question as to how to deal with it.
The most simple way to deal with ethane is to allow it to be included in the reported NMOC or TGNMOC concentrations and assume this is a worst case scenario situation. This works better if there is very little to no ethane expected or there have been low levels documented by previous analyses. The real issue is when the ethane concentration is sufficiently high to affect a permit.
There are those who propose to modify the backflush valve switch timing to allow the ethane to elute before the valve switches and only consider that which elutes afterward as NMEOC. This seems like a logical approach, but assumes there is nothing which would be lost in the baseline upset caused by the valve switch and that there is nothing which elutes between CO2 and ethane. The former is possible however unlikely, but the latter is not correct. We have seen instances where a compound has eluted after the CO2 peak and before the ethane peak. We do not know what this compound or compounds may be, but they have been documented. They could be quantified and included in with the compounds in the backflush, but the fact they exist makes the modification of the valve timing more of a concern. To be sure technically there would have to be some specific studies performed and a regulatory review to also ensure it is legally acceptable.
Our current solution is to perform a separate analysis of the sample tank after the Method 25 analysis. This analysis uses the modified valve timing to allow the ethane to elute and generally only the ethane is reported. Since this is the same analytical instrument and only reporting the ethane, this concentration can be directly subtracted from the NMOC concentration reported for the sample tank on a one-to-one basis, leaving only the NMEOC concentration for the tank. The trap fraction is not affected by the ethane so it would not be adjusted and the reported TGNMEOC would only be reduced by the ethane concentration. This is a little more expensive and difficult, but it retains the inherent reporting requirements of Method 25 while allowing an ethane only concentration to be determined. If this reported concentration is not sufficient to justify the added analytical costs, the first option of the worst case scenario may be the better option.
In some cases we have only reported the methane and ethane concentrations with this modified analysis to give a total for methane/ethane for the source stream without also reporting the NMOC for the sample stream. This is more commonly used to determine a subtraction concentration for a field FIA.
Triangle Environmental Services, Inc.
P.O. Box 13294 122 US Hwy 70 E
Research Triangle Park, NC 27709 Hillsborough, NC 27278
(919) 361-2890 (800) 367-4862 Fax: (919) 361-3474