TO-15 ISTDs – What do they mean to you?

During recent discussions with colleagues, it became apparent that my interpretation of the internal standards (ISTDs) used for the U.S. Environmental Protection Agency’s (EPA’s) Method TO-15 may not mean the same thing to everyone. So before I kick off this discussion, let me first breakdown the terminology (as defined by me) to establish some common ground:

• Tuning Standard: Utilized to “tune” or optimize the MS detector parameters to ensure that mass assignments and abundances are correct. TO-15 utilizes 4-bromofluorobenzene (BFB) as a tuning standard.
• Internal Standard: Utilized for calibration and quantification by accounting for instrument variability in response and/or retention time from run to run. TO-15 utilizes bromochloromethane, 1,4-difluorobenzene, and chlorobenzene-d5.
• Surrogate Standard: Utilized to monitor for bias/variability throughout the entire sample extraction process. TO-15 makes no mention of surrogate standards; however, they are generally compounds similar to the target analytes in chemical composition and behavior in the analytical process, but which are not normally found in environmental samples.
• We only need the aforementioned definitions, so I will not be covering any of the following: matrix spikes, blank spikes, field duplicates, laboratory control spikes, matrix spike duplicates, method blanks, and the list goes on infinitum... depending the analyst, laboratory, and/or method.
  

Now that we have our terminology squared away, let us use EPA Method 625 as a benchmark for our discussion. With 625 you add surrogate standards to the sample pre-extraction, which allows you to evaluate your extraction efficiency… and then you add internal standards post-extraction, but pre-analysis, which are utilized for quantification. Of course all of this is injected on an instrument, which has been tuned.

 

Now with that paradigm at the forefront of our minds, it is important to note that most of the commercially available TO-15 preconcentrators add the “ISTDs” (quotation on purpose… hint hint) from a separate, designated canister onto the preconcentrator traps for each sample analysis, prior to the loading of the sample. So therefore the TO-15 “ISTDs” go through almost (I will explain in the next sentence) the entire “sample extraction” process. To be more in line with the 625 paradigm the standard(s) would have to be added to each canister prior to extraction, which is feasible, but just not done because the TO-15 preconcentrators allow end user to cheat this step.

In any event…any variability associated with the extraction process (e.g., breakthrough, relative humidity effects, desorption efficiency, etc…) would be represented by a change in response in the ISTDs. This… to me… sounds more like a surrogate standard/internal standard hybrid. You get the best of both worlds in my opinion and you do not have to do anything but let the software work for you.

But I already know my opinion (and now you do)… so I want to know what others are thinking. So if you care about air (I know… a little lame) please feel free to chime in here? Do you consider the TO-15 ISTDs to truly represent just ISTDs or are they more like surrogate standards… and why?