Chromatography Forum

Hi,

In the last few months, I've been attempting to optimise a GC-MS (SIM) method which is designed to detect the presence of a number of analytes including Dibutyl Phthalate.

I assumed rather stupidly, that DBP wouldn't be anywhere near as problematic as it has been, but in recent weeks I can't get a single blank that is actually truly blank and nothing I have done seems to help address the problem in any way. I initially thought the issue related to some tubing used on my 'homemade' nitrogen blowdown equipment, but having eliminated this along with Gilson pipette tips I am a bit of a loss. My blanks have been poured directly from a solvent bottle into 2ml vials, so there is no contact with any plasticware other than the vial septum (PTFE-silicone) and the GC septum (a standard Agilent BTO septum).

Can anyone make any suggestions that might help please?

Kind Regards

TD

As you know (or have discovered), phthalates are everywhere, and frequently difficult to track down the source in analyses. This paper discusses in detail how to address the presence of dibutyl phthalates in blanks:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5423241/

As a simple start, I'd try vials, caps, and probably even solvent from a different vendor. Then I'd look into cleaning up the solvent to confirm if it were the source (yes, even freshly opened new bottles of 'high-grade' solvents can contain phthalates). Obviously not a solution, but it would help track down the problem.

I went down this road with bis-phenol-A a few years ago. It’s everywhere and you just can’t get a BPA free blank. What I had to do was run enough blanks to get a some stats on what was coming from my reagents/vials/caps/etc. and then the sample had to be X number of standard deviation units bigger than the blank for me to say it was “real”. That was the best I could do.

Some quick tests you might try:

Punch out the septum and run a vial without it, just to see if that makes a difference.

If you evaporate down your solvent by 1/2, does the concentration double?

Run a blank of nothing - no injection.

Excellent advice. Thank you all!

Based on what's been posted, I think the take home message is that you're not going to get blank blanks when working with phthalates (and also BPA). Given that I'm using DBP as one of a number of indicators I think I may choose to avoid phthalates altogether and switch to another analyte which is less common environmentally. Lesson learned!