Volatiles by 8260/624 Inconsistent Response Factors

[Jaclyn]

I'm having an issue with the poorly-purging compounds showing inconsistent response factors during calibration (up to 80% RSD), including 2-butanone, propionitrile, acetone, tert-butanol, and tetrahydrofuran. It appears that these compounds have abnormally small peaks in every other calibration standard.

I have cleaned the source, replaced the traps, installed a new column, and cleaned the transfer line and inlet with methanol but am still having the issue. I recently changed the desorb time from 1 minute to 4 minutes to be compliant with 624, and I suspect that might be contributing to the problem. The baseline is higher than before - Could this be due to too much water entering the system?

Here are the parameters I am using:

Agilent 6890 GC & 5973 MSD w/ helium
Vocarb K trap on Encon Evolution
Purge: 11 minutes @ 30C & 40 mL/min, Dry Purge: 1 min & 40 mL/min, Desorb preheat: 250 C, Desorb: 4 min @ 260C, Bake: 7 min @ 260C
Column: DB-624 - 0.18 mm ID, 20 m length, 1 um film
Temperature program: 45C initial for 2 min; rate 15 to 250C
Split ratio 50:1
Flow: 0.8 mL/min
Split flow: 39.6 mL/min
Inlet: 150C & 16.8 psi
MSD Quad: 150 C
MSD Source: 230 C

If anybody has any ideas about this I'd appreciate the help.

[Steve Reimer]

"Could this be due to too much water entering the system?"
Yes.

I know of a system similar to yours where they needed to go to an 80:1 split to have consistent results. ( 5973 with 0.18 column and K trap)

624.1 (still in draft ) does not specify a set desorb time.

[James_Ball]

The 4 minute desorb time was set back when everyone was using charcoal/tenax/silica gel traps and packed or megabore columns with a jet separator. It doesn't work well with modern traps like the Vocarb3000 and using the split interface.

When using the Evolution I normally purge with a sample temp of 40C and desorb 1-1.5 minutes. I can get away with 40:1 split at the inlet but can also run up to 80:1 depending on how much sensitivity I need. One change I have made on my instruments is that I run the Edwards E2M2 rough pumps and have a molecular sieve moisture trap installed right at the inlet to the pump to help prevent so much moisture getting into the pump oil. The elevated temperature helps with the purge efficiency of the ketones.

MORT temperature I use is MORT ready 39C and MORT bake 180C. It seems to keep the MORT cleaner longer with the lower bake temp and it is plenty high enough to remove the residual water.

I am using the Restek Rxi-624SILMS column which you can take up to 300C to bake off any high boilers more efficiently and it seems to work really well when run at normal run temps up to 240C.

[Jaclyn]

Thanks for the information. I will look into the 624SilMS column for the next time I need to replace the column. I think my current column has a max temp of around 250, so a higher max temp would be beneficial. I changed the desorb time back to 1 minute and that fixed the issue, though it seems like it did take several days for the system to remove residual water.

[James_Ball]

Thanks for the information. I will look into the 624SilMS column for the next time I need to replace the column. I think my current column has a max temp of around 250, so a higher max temp would be beneficial. I changed the desorb time back to 1 minute and that fixed the issue, though it seems like it did take several days for the system to remove residual water.

It can take a few days to dry out the system. I have noticed that if I let mine sit idle for the weekend my responses will be higher on first run on Monday than they are throughout the week. If you have the oil diffusion pump instead of the turbo molecular pump it will be even worse as it picks up moisture.

[Bigbear]

It sounds like water. If you can swing it, purchase the Agilent water management kit. Refer to their paper # 5991-0029EN.
Something else that helps that I was doing before we got the kit was to use the gas saver to up the flow to 200 cc/min at 1/2 the desorb time. I desorb for 3 min so I have my saver set for 1.5 min. I do do not run many samples, so I keep my oven at the ramp max for about 10 min. I adjusted this time so even with the extended "bake out", the oven is ready before the P&T gets to desorb.