difluoroethane (DFE) using HS-GC

[judyjong]

I'm trying to tweak an existing alcohol method that uses t-butanol as the internal standard. I've had to lower the flow to increase the retention of DFE, which means that t-butanol would come out much later than the compound. Are there other internal standards that I could use that are more suitable for tests involving DFE? I've thought of methanol, but it's one of the compounds that is tested regularly in the lab.

Thanks in advance for any suggestions!

[chromatographer1]

tert-pentanol

sec-butanol

n-propanol

n-butanol

etc

[Peter Apps]

Increasing retention by slowing the carrier flow has probably degraded the separation. Have you tried temperature programming to elute the butanol more quickly ?

Peter

[Bigbear]

Try lowering your initial temp.

[judyjong]

Thanks for all the suggestions.

I've tried lowering the initial temperature, but this doesn't help much since the boiling temp of DFE is around -25. I've also tried temperature programming, but because the flow rate is quite slow, t-butanol comes out quite broad and still very late. Flow programming helps a bit more, but the signals are not optimal.

I'm still in the process of searching for a compound that would elute quite quickly on a DB-624 and BAC-2 column to allow a faster run time....

[chromatographer1]

I am afraid your posts are confusing to me. I thought you were doing a test for alcohol (ethanol), using t-butanol as an ISTD.

Are you testing for DFE and Methanol?

What is your matrix?

What are you testing to find and measure?

Methylene dichloride, acetone, or isopentane are other common low boiling point solvents you can use for ISTD.


good luck,

Rod

[Bigbear]

A few more details would be helpful.
what are your column dimentions?
what is your GC temp program
how, and how much sample are you injecting
specifics of your injection port configuration ( liner, split ratio, ect.)

I would advise against using DCM as an internal standard.

[judyjong]

It's a dual column HS-GC-FID the lab is currently using to test alcohol in biological samples. The dimensions for the DB-624 column is 30m x 530um i.d., film thickness of 3 um. The dimensions for the BAC-2 column is 30m x 530um i.d., film thickness of 1 um.

I've actually been able to get quite good separations and sharp peaks with acetonitrile as the internal standard. The initial pressure is 1.5 psi for the first 6 minutes to elute the DFE, and a pressure ramp of 5 psi/min to a max of 15 psi for a total runtime of 9 mins. The retention times I get for DFE and ACN are approx. 5.5 mins and 8 mins respectively.

The main question I have now is whether there's a maximum pressure ramp I can set, since the setting I have now increases the average velocity of 15 cm/sec to 50 cm/sec in a minute. I haven't had any noise issues with the column though, but was wondering if this increase would cause any damages to the machine or column. I'm a student so I don't have a lot of hands-on experience with the machine. I really appreciate your help and feedback. Thanks again!

[chromatographer1]

I would not expect any damage to the instrument or the column as long as you stay within the published limits of the hardware.

best wishes,

Rod