8260, Water and Chloromethane

[Jonezy_Lothos]

I am looking for options on how to deal with water getting on column. I am having issues getting Chloromethane to come out with a respectable RSD. Any suggestions?

Info: 8260 Method, Atomx Auto sampler, Agilent 7890 GC/240 MS

I have tried:

guard column(s)
lengthening the run
changing out the P&T Trap (#9, #10 and K)

I do not want to lengthen the run time because of less sample through put and the guard columns tends to just move the water to a later run time which interfered with PCE . Has anyone modified the water management system on the purge and trap?

Thanks,

Jones

[Steve Reimer]

What is your split ratio? Can you do a dry purge? Desorb for 1 minute only.
One lab I know of went to a 0.18 mm x 20 m column and a 70:1 split to help with declining sensitivity on a 5975.

[Jonezy_Lothos]

Split is 40:1, I dry purge for 2 min and desorb for 1 min

Column is a 624, .320x30m

I would like to stay with the larger bore column if possible

Jones

[Bigbear]

I think you are shooting yourself in the foot using a 0.32 column. What's your column flow , 2cc/min?
I use a 0.18 column @ 0.6 cc/min and a 150:1 split. I get %RSD's <10 for most of my compounds. I aslo use a Vocarb3000 trap. I did not like the #9 trap.

[Steve Reimer]

What is a respectable RSD? I am looking at data from a large multilab study using LCS data and for chloromethane in water by 8260 the RSD is 15%. Most common target compounds are less than 12%.

[James_Ball]

I am using either the Rtx-502.2 0.18X40m or the Rtx-VMS 0.18X40m columns and using Vocarb3000 traps with Encon/Encon Evolution purge and traps 1-2min dry purge and 2min desorb with 0.6ml/min flows and 40:1-70:1 split ratios depending on the instrument and project sensitivity needs. Oven temp starts at 35C.

We normally have %RSD on calibration curves for Chloromethane around 5-10% and have little trouble passing calibration checks every 12 hours at 20% Drift, usually it will be less than 10%. I usually see more water interference at trichlorofluoromethane but it still is low enough to not cause problems. On one instrument I calibrate from 1.0ppb to 200ppb and chloromethane is at 9.2%RSD using average response factors.

Runtimes for us are about 30minutes per sample.

[Yama001]

Yes, I would be surprised at water/MeOH being the problem with chloromethane, though the .32 column might be why - normally it interferes with bromomethane/chloroethane/trichorofluoroethane on the 624 phase, sometimes a little later if the trap is funky. I stick with a .25mm ID myself, for the older 5890s at least. A VOCARB trap is usually best.

The methanol often can distort response for the peaks it coelutes with; this can be handled if necessary by keeping the methanol volume constant across the calibration levels. This is often not worth the effort, as most techniques will yield a "good enough" RSD. It can make a difference on performance evaluations, though the acceptance limits are so wide that most results are good enough there as well.

If chloromethane was giving me trouble I would first suspect leaks, then look at how the standards were being handled (temperature fluctuations, headspace, poor syringe technique).

[Jonezy_Lothos]

I think you are shooting yourself in the foot using a 0.32 column. What's your column flow , 2cc/min?

Flow is 1.4mL/min. We use the larger bore column to help shorten our runtime (12.5 min) and handle the load we sometime put on column. We tried a .25 and .18 column and had issues with linear range (non-linear calibration curves) and overloading the column.

What is a respectable RSD?

RSD have to be under 15%



Thanks for the input. It is water that is co-eluting with Chloromethane (I have lowered the mass on the MS and water comes out before/after chloromethane). Do we just have to bite the bullet and lengthen our runtimes to deal with the water on column?

Jones

[James_Ball]

I think you are shooting yourself in the foot using a 0.32 column. What's your column flow , 2cc/min?

Flow is 1.4mL/min. We use the larger bore column to help shorten our runtime (12.5 min) and handle the load we sometime put on column. We tried a .25 and .18 column and had issues with linear range (non-linear calibration curves) and overloading the column.

What is a respectable RSD?

RSD have to be under 15%



Thanks for the input. It is water that is co-eluting with Chloromethane (I have lowered the mass on the MS and water comes out before/after chloromethane). Do we just have to bite the bullet and lengthen our runtimes to deal with the water on column?

Jones

The smaller diameter columns can often be run at higher split ratios to solve the problem with linear range. One of my instruments is calibrated at 0.2ppb-200ppb for most analytes running a split of 60:1. If I am only calibrating down to 2ppb then I can usually get linear results up to 400ppb.

Something I discovered after reading an old Agilent application is that loss of linearity is often from having the EM volts set to high instead of overloading the column. Use the absolute lowest EM volts you can get by with while adjusting the low end sensitivity using your split ratio and see how that works.

[Jonezy_Lothos]

Thanks for the input guys.

I have switched to a .25mm x 30m column with a 80:1 spit and that has made the water peak more manageable.