Method 524.2 Bromoform curve quadratic

[coombs_ryan]

For Method 524.2 Drinking Water:

Does anyone have any ideas why Bromoform could become Quadratic in nature on the curve? I have a Stratum Concentrator with a #9 trap inside. My curve starts at 0.4 and goes to 40 but at around 15ppb on the curve Bromoform starts to become quadratic. Any ideas why this happens?

Changing the specs around like purge times, dry purge flows, desorb flow, temps, minutes, doesn't really change anything as it's still always quadratic in the curve. I've gone to all extremes tweaking the instrument but nothing changes the fact that it still curves upward instead of being Linear!

I guess i'm asking if anyone knows the chemistry behind this or maybe i'm missing something all together. Thanks.

[BCK]

Normally I have seen bromoform be a problem when water management is a problem. If you are doing by the book 524.2 purge methods, I believe you have at least a 2 minute desorb. You a pushing a lot of extra water into the system. I don't know what GCMS you are using but I have had a lot of success using the extended linearity drawout plate on Agilent 5975 and 5973. Bromoform look alot better as well as more stability in your ISTD. If your ISTD is dropping over the 12 hours run time and then the response jumps back up after sitting for a period of time, it further confirms the moisture problem. Hope this helps.

[jymmit]

We use Tekmar and varian 3400 GC and 4D MS. We also see quadratic curve for bromoform, but not the other trihalomethanes. It is also more pronounced in a 60 analyte VOC mix as compared to the 4 component THM mix. Note, our curve gets flatter at higher concs, possibly indicating saturation of the trap or MS. Haven't investigated it as we can live with quadratic.

[coombs_ryan]

We've added a heater to the sparger and it pretty much made Bromoform linear overnight... we still don't get a full 100% recovery for the compound but i'll take 90% with a linear curve right now... Gas saver helps too...

BCK... do you have a part number for that Agilent "extended linearity drawout plate"?

Also have you found any differences in compound recovery or responses over time, especially with Bromoform, with your 5975 vs. 5973?

[mjh32586]

Not sure if this will help but I read recently in an App note by Agilent for techniques for optimizing voc's in water using purge and trap gc/ms that "It is helpful to tune the MSD so as to produce a 174/95 ion ratio that is in the 90-120% range because this improves the signal for bromoform (base peak=173), which purges with poor efficiency."