x-post from 'hyphenated' forum-GCMS vs GC FID retention time

[jdaknis]

I'm attempting to run a residual solvent method that was developed for GC/FID on an HP6890 w/ HP5973 MSD.

The method in question has been run on a 6890 with FID several times before. The analytes are methanol and chloroform. On the FID instrument, methanol typically elutes @ ~2.2 minutes and chloroform @ ~4.2 min.

When I run the same method on the MSD, methanol is eluting @ 1.1 min and chloroform @ 2.2.

I've confirmed that the oven temperature program is the same, and the oven temp is accurate throughout the run. I've also tried more than one column. Retention times remained the same.

The instrument parameters are as follows -

Oven program:

T °C / Hold, min / Ramp, °C/min

50 / 3 / 20
150 / 0 / 30
300 / 10 / NA


Flow: Constant, 2.0 mL/min

Injector:

Temp = 200 °C
Carrier: Helium, split ratio = 20:1
Injection volume: 2 uL

Aux temp: 280 °C (Original method has FID temp set at 300 °C)

Column is a Restek Rtx-5, 30 m, 0.32 mm i.d., 1um film

Run Time: 23 minutes

The sample solvent is DMF. I know this isn't the best solvent to use for MS applications, but my hands are tied at this point. That said, the DMF peak elutes in just under 5 minutes on the MSD but is not typically observed when using FID because the method states that data should only be collected for 6 minutes. . . Another clue that everything is eluting too quickly on the MSD.

My question is this: Has anyone observed such a drastic decrease in retention when switching from FID to MS before? Is this behavior inherent to the technique (due to vacuum pulling at the detector end??), or is some other explanation more feasible?

Thanks for any feedback.

[Consumer Products Guy]

I think you've got the reason: the MSD operates under a vacuum so the actual flow rate is faster. On the Agilent Flow Calculator free utility, there are bullets to select as to whether the column is atmospheric (FID) or vacuum (MSD). See http://www.chem.agilent.com/cag/servsup ... s/GCFC.htm for this free utility download, this is about as computer-literate as I get.

[jdaknis]

I think you've got the reason: the MSD operates under a vacuum so the actual flow rate is faster.

Thank you for the link, and for confirming what I suspected. Your reply is greatly appreciated.

[Ron]

The actual answer is the linear velocity of the carrier gas in the column connected to vacuum is higher than the linear velocity of the column going to atmospheric pressure. Unfortunately with the column you are using you can't reduce the head pressure of the mass spec system to a low enough value to match the retention times on the FID.