Hi everybody!

This is a question about analyzing aromatic hydrocarbons via GC-irMS.

We are currently running saturate and aromatic fractions from oil and rock extracts for CSIA using GC-irMS. This is a common practice in organic and isotope geochemistry, and the methods are well known.

We were able to analyze the saturate fractions without inconvenience. However, the aromatic fractions don’t show up or the signal is very low. Somehow the aromatic compounds in the sample disappear. I’m listing here some things we have tried and information you need to know:

- Both fractions, saturates and aromatics, were analyzed via GCMS first and they both look fine.

- The n-alkane standards also look ok.

- We run standards of naphthalene, phenanthrene and methylphenanthrene, individually and also mixed, and they also look fine. When I say they “look fine” I mean that the intensity of the peaks and the isotope ratios are the expected.

- The same GC conditions were used for GCMS and GC-irMS (same column, same temperature program, etc.)

- We also spiked an aromatic fraction with n-alkanes, and with phenanthrene in another case, and guess what?! The standards make it through but the sample does not. Isn’t it so strange?

- We have also tested several concentrations of aromatics, up to 30 times more concentrated than a saturate fraction, and the outcome is the same: either no signal or very tiny peaks.

- Increasing the volume injected (e.g. from 1 to 4 uL) has not helped either.

- We have tried different solvents (DCM:Isopropanol, DCM, n-hexane) and this did not make a difference.

- We tested aromatic fractions that were prepared by different people as well, and the outcome is the same.

- By the way, we are using a Thermo Trace GC – GC-IsoLink system interfaced to a Thermo MAT 253 mass spectrometer via a Thermo Conflo IV. It is the first time that this instrument is being used for liquids; it's only been used for gas analysis in the past.

I have talked to several colleagues and friends and nobody has seen anything similar. I can only think of a matrix effect going on but why? And which one? We replaced all capillaries, we have new reactor tubes; everything in the instruments seems fine.

Have you seen anything like this before? What do you think could be happening?

Thanks in advance,

Jaime