Chromatography Forum

I need to do an analysis of several organic solvents....hexanes, isopropanol, ethyl-alcohol, and DMAE. I am using a DB-5MS column and do not have FID or headspace system. I know that these solvents are all soluble in acetone...so i planned to spike the acetone with approx 20uL of each solvent and then running it using a high split ratio and oven ramp temps set to separate each according to individual boiling points. So far I have had little success as was curious if this was ideal for the instrument i have??? Agilent GC (think 5973) with MSD (single quad) EI.

Good luck on separating those solvents from acetone on DB-5 or similar columns. I wonder if cryogenic would even help, those are essentially non-retained.

mlewando1980

You are trying to use a surgical knife to cut up boulders.

You need the proper tool to do a proper job for the task you wish to achieve.

A long THICK film DB-5 might be able to perform the task you wish to do, but with a MSD you probably have a thin film version.

But if you have a lot more time than money, then try it and perhaps you might be able to achieve your goal, but I wouldn't hold my breath.

GOOD LUCK,

Rod

Chromatographer1--- Obviously I am a novice user of GC but I really appreciate your feedback.
I actually have a ZB-5MS which i am assuming is the same as a DB-5MS?? Also according to the literature my column is suitable for residual solvent analysis. I am not exactly doing "residual" solvent analysis but my solvents technically are/could be the same analytes. So why wouldn't it work?

This is a question in itself but why can non-polar and polar columns essentially separate the same analytes....essential oils??? For example, my column has a non-polar stationary phase yet is suitable for the same analysis as a waxy (polar) stationary phase.

It is one thing to have a column which will separate trace amounts of solvents from each other. It is another thing to have a column which will separate the broad and tall carrier solvent from trace solvents. If Hexane and methanol separate from each other, wonderful..... but if trace amounts are in acetone and you inject acetone onto a 5% Phenyl column and the peak of acetone is wide enough to hide both hexane and methanol peaks, what have you accomplished?

Perhaps you can separate pentane and hexane on a non-polar column, and you can separate them on a polar column. BUT, if the separation is 0.04 min on the polar column and 1.5 min on a non-polar column, which is to be preferred?

And if hexane and hexene will separate on a non-polar column by 0.1 min and 0.5 min on a polar column, which is to be preferred?, especially if you can separate cis and trans isomers on a polar column but not on a non-polar column?

Almost ANY column will separate homologs of a series but it depends upon how much separation, and other interferences to determine WHICH column is to be preferred.

best wishes,

Rod

I need to do an analysis of several organic solvents....hexanes, isopropanol, ethyl-alcohol, and DMAE. I am using a DB-5MS column and do not have FID or headspace system. I know that these solvents are all soluble in acetone...so i planned to spike the acetone with approx 20uL of each solvent and then running it using a high split ratio and oven ramp temps set to separate each according to individual boiling points. So far I have had little success as was curious if this was ideal for the instrument i have??? Agilent GC (think 5973) with MSD (single quad) EI.

Hi, first of all you are using the wrong instrument but you already know that, i'm just remarking it.
With a DB-5 it's impossible to separate acetone from 2-propanol, especially if you use acetone as a solvent with a very big peak.
Try using Carbon Disulfide as the solvent and you should be able to reach your goal.

Tinjector: whatever you like as long as it is above 150°C, try 250°C
Oven: 40°C for 1 min
100°C (200+? i don't know your highest boiling compund) @5 °C/min
keep transferline and analyzer at a temperature above the highest oven temperature but not too high, you don't want your compund to leave the detector too fast. If it was an Ion trap i could recommend some temps but since is a quadrupole i suggest you to ask someone more competent for the analyzer temperatures used for solvents determination.
split ratio: as higher as you can (at least 1:50), using GC/MS you should avoid to read the solvent peak, the risk to burn the EI filament after several run is high.

Following the above conditions and with your column you will find the compunds in this order and baseline separed:
methanol, ethanol, 2-propanol, CS2 (solvent), 2-butanone, ethyl acetate, isobutanol, n-butanol, benzene+cyclohexane, methylisobutylketone, toluene etc. I don't really know where hexane will elute, it should be far from CS2 and maybe after butanols.

Anyway if you plan on analyzing more compunds consider purchasing a "putcompanynamehere"-624 column with a thick film and make a favor to your MS and buy a FID, they are very cheap these days.

Good luck.

1) Why would he want to use CS2 with an MSD? With an FID, two enthusiastic thumbs up, but with an MSD?? If you do switch to CS2, remember to turn your filaments off for the solvent peak.

2) The comment about -624 column is very appropriate.

3) Do you have cryo available? If so, this could potentially solve your problems.

4) Have you thought about DMSO as the solvent?

Best regards,

1) Why would he want to use CS2 with an MSD? With an FID, two enthusiastic thumbs up, but with an MSD?? If you do switch to CS2, remember to turn your filaments off for the solvent peak.

I suggested CS2 because i know for sure it elutes right in the middle between 2-propanol and 2-butanone/ethyl acetate, this way he should be able avoid reading solvent peak if he set EI time segments correctly.
I didn't suggest other solvent because i am not sure about retention times with his intrumental conditions and column, but of course he can try many other solvents like DMSO.
Acetone is the worst because it elutes with 2-propanol.

I have done this successfully by diluting with something with a longer retention time than any of the analytes I am looking for. Use a very short initial delay (or none), then turn off the filament again before the long retention time diluent hits the filament. Decane and dodecane come to mind as diluents. I used decane, but there should be lots of useable choices.

Depending on how many solvents elute and their retention times, you may not get complete separation, but with GC-MS you don't necessarily need it. Deconvolution software and unique masses are your friends here. As others have suggested, use a moderate inlet temp, a low initial oven temp, and a slow ramping rate. You will also need to know the retention time of your diluent so you know when to turn off your filament.