PAH soil sample test problem

[warrender]

I use six PAH internal standards as surrogate. I got six surrogates recovery increasingly high in some soil samples. For example, the recovery of the first three surrogate were around 80-90%, the forth one went up to 120%, the fifth one were 150%, and eventually, the sixth one went up to 200%, 300% sometimes. We don't do PAH soil and water sample cleanup. We use SIM mode. These surrogate peaks looks in good shape.
This problem does not happen with every samples, so I suppose it's because of matrix interference. Because most time these problem happened with dark sample(few times but still had sometimes happened with clear sample). But, how come interference could have the same RT and same ions with these deuterated compounds? And also under these kind situation, my test results for those later coming out target compounds also become very high.
Can someone give some idea?

[beersnob]

Are you using a single ("recovery") standard to measure the recoveries of your six internal standards and, hence, treating then "as surrogates." Or are you using six internal standards and six surrogates? What are your typical recoveries in samples where recoveries are OK?

I would expect you might see recoveries 30 - 100% for Naphthalene - Benzo(ghi)perylene. The spectrum of recoveries you describe sounds like that multiplied by 3; which you would get by adding 66% too little recovery standard. The fact that you're saying you get the same excessive recoveries for both your "surrogates" and for the native compounds, leads me to believe you are spiking your surrogate up front, processing your sample, and then adding your internal standard prior to analysis. If you do this and, once again, add too little internal standard, it would account for both your erroneous surrogate recoveries and excessive recoveries of native compounds. Or, maybe the surrogates were spike 2 or 3 times instead of just once.

"But, how come interference could have the same RT and same ions with these deuterated compounds?" No. It's not an interference. It's sloppy standard addition or a sloppy standard. That's my first guess.

[sassman]

Because most time these problem happened with dark sample(few times but still had sometimes happened with clear sample).

If it usually only happens with the darker samples then it is likely due to matrix effect. I don't think it is a problem with your IS addition method. How are you calculating recovery of the surrogates (relative to a non-soil blank with only surrogate)?

But, how come interference could have the same RT and same ions with these deuterated compounds? And also under these kind situation, my test results for those later coming out target compounds also become very high.

Since the recoveries become progressively higher with the larger mass compounds, I don't think it is due to mass interferences. Rather, it is likely because of some mass discrimination happening in the injector. Perhaps you can try a different kind of injection liner?

[gpronger]

First, I did not see mention on what's happening with the recovery of the internal standards which will impact the calculated concentrations of the surrogates. With dirtier samples, I have seen the recovery of "heavier" internal standards drop off faster than "lighter" surrogates so that the recovery of the surrogate is biased higher due to the decreasing response of the internal standards. What made this very evident to us was that for the matrix spikes, the analytes with retention times very close to the internal standards had very good recoveries, while those with retention time somewhat earlier had positive recoveries. One of the surrogates we use was terphenyl-d14 which we quantitated versus chrysene-d12; on dirtier samples the recovery of the chrysene-d12 tended to drop to just passing (we used the common +200% to -50% for internal standards) which would cause the terphenyl-d14 to drift high.