Chromatography Forum

Hi all,
asking some help here. I am using GC/MS to analyze hexane, heptane and isopropyl alcohol mixtures. I have the freedom to select a solvent.
The solvent that used to dissolve the standard was methanol. However, I really having a hard time to separate the three compound from the solvent, maybe due to their b.p.s being too close?
so two questions:
(1) is there any tip to optimize the separation conditions to make the peak show up?

(2)should I switch to an easier solvent? if so, any suggestions?

Thanks so much for your help!!

You haven't told us you current chromatographic conditions. In most cases (G1) Hexane elutes at the solvent front.

Hi, Thanks so much for your reply.
right now I am using mathanol as solvent. I used splitless injection, splitless time of 1 min. the inlet temp. was 30 degree. oven start at 30 degree and with the ramp rate of 10 degree/min. I started to collect signal after 1 min.

Hi, Thanks so much for your reply.
right now I am using mathanol as solvent. I used splitless injection, splitless time of 1 min. the inlet temp. was 30 degree. oven start at 30 degree and with the ramp rate of 10 degree/min. I started to collect signal after 1 min.

This is going to go on forever if you feed us the information in dribs and drabs. We need to know what column you are using; length, diameter, stationary phase and film thickness, your carrier gas flows, how much you are diluting your sample, and what volume you inject. What are the MS settings ?

30C for the inlet temperature is way too low - is it a typo for 300C ?

Are your samples just mixtures of those three compounds, or are you looking for those as components of more complex mixtures ?

Peter

Hello Pete,
Thanks so much for your reply.
Sorry for the confusion.
I am analyzing the mixture of hexane, heptane and isopropyl alcohol in methanol solution. the concentration of these three compound are 50 ng/mL. (would Dichloromethane be an easier solvent in this system?) I want to optimize my analyzing conditions before moving onto monitoring the real sample, which would be residues on the surfaces.

The volume I inject was 1 uL, using splitless injection mode with splitless time of 0.5 min. the inlet temperature was 230 degree. the split flow was 10 mL/min.

The source temp for the Ms was 200 degree, I am collecting signals from 1 ~ 10 mins. monitoring signals between 50 to 200.

The column I used was Restek DB 5, 30 m X 0.25 mm X 0.5 um.

Is there any problem with the conditions?

Thanks so much for your help!!

You need a different column. Under conditions used in 8260 for VOAs those compounds are easy to separate but the column is a much thicker film. My current example is a 20 meter 624 column with a 1 um film thickness. Check out https://www.epa.gov/hw-sw846/validated- ... ectrometry for some conditions.
You won't need the purge and trap but your inlet conditions will be critical. If you can get away with it, you would be better to use a split injection.

Unless you have made another typo you are putting only 0.05 ng per compound onto the column, and so the reason you see no peaks is because you are trying to operate below the limit of detection. Increase your sample concentration by 100 times and follow Steve Reimer's advice on the column.

Peter

I agree on the column. The regular 5 phase columns are difficult to use for anything boiling before pyridine or toluene, and hexane will definitely elute before those.

50-200amu will allow you to detect those analytes without methanol showing up, but if it co-elutes with hexane it will still cause a drop in sensitivity. To get hexane to separate from the methanol solvent without cryo (starting at 35C) you will need the Rtx-624, Rxi-624SilMS or Rtx-505.2 column. They have film thickness adjusted just for volatiles analysis in the proper film to diameter ratio.

Also try with an injection port temperature of 150-200C, it will give smaller solvent expansion volume while still being high enough to evaporate all of the analytes.