Peak tailing due to higher initial temperatue in GCMS analys

[Sandra7]

I want to understand the exact mechanism behind peak tailing (of the first peak) due to higher initial temperature in the temperature program in a GC-MS analysis performed via manual injection. Can someone please explain?

[Peter Apps]

Best guess in the absence of any detail at all, is that the peaks are tailing as they come out of the inlet, and that they are sharpened up by thermal focussing on the relatively cooler column. If the column is hotter (which is what I presume you mean by initial temperature) then the thermal focussing effect is weaker, and you still see the tail on the peak at the end of the column.

Some feedback to the forum on how you are progressing with the other projects that you sought advice on would be nice.

Peter

[Sandra7]

Best guess in the absence of any detail at all, is that the peaks are tailing as they come out of the inlet, and that they are sharpened up by thermal focussing on the relatively cooler column. If the column is hotter (which is what I presume you mean by initial temperature) then the thermal focussing effect is weaker, and you still see the tail on the peak at the end of the column.

Thanks a lot Peter. I think the answer is just what the prof told me told he didn't use the phrase "thermal focussing".

Sorry about not giving the detail. I had too much going on that day with a car accident two days earlier in the evning - I am not making an insurance claim because I hit a curb just at the right angle and hence really hard). Picking up the car today.

It was the first day of this GC analysis of derivatized fatty acids (C12, C14, C16, & C18), namely methyl esters. In the 1st analysis, I pretty much the temperature program the way the prof had shown us as demo, which was initial temperature of 100ºC (hold for 2.5 min), ramping 30ºC/min, final temperature 250 (hold for 3 mins). Total run time 10.5 min. Injection was splitless and inlet heater temp was set at 250ºC; Septum purge flow was 3ml/min. If I remember correctly, professor said that the injection was no direct-on-column approach. We were told to leave the pressure setting alone. I will record it today. last time, I was busy familiarizing myself with the instrument and then rushed to return a rental car in time.

To reduce some time, I raised the initial temperature to 140ºC and peak tailing occurred. Peak tailing for very large for the 1st peak, pretty big for the second and small for the 3rd and 4th peaks.

Some feedback to the forum on how you are progressing with the other projects that you sought advice on would be nice.

Oh, yes, Peter. All lab work (1. Intro to HPLC, 2. Intro to GC, 3. SPE extraction+ HPLC analysis) had gone well. Currently working on Derivatization + GC analysis (lab 4). The professor is very strict abut the report writing though. Lab 3 is due next Monday.

As for the specifics w/ HPLC instrument (during lab 3), on last day which was last Tuesday, the ret time shift happened again, obviously due to eluent composition running through the column. Due to time limit for use of the instrument, I just let it be since the one peak I was analysis at different conc: came out during the analysis.

For the current lab, I do have another question that I didn't get to ask the prof that day whether or not the inlet heater temperature should be the same as initial temperature of the temperature program. I will ask him today.

[Peter Apps]

Hi Sandra

Unless you are doing fancy programmed temperature vaporization injections then inlet temperature of the GC needs to be closer to the final temperature of the column programme than the initial temperature. If you have had the inlet at 100 or 140C then I am not surprised that your peaks tailed - they will have evaporated slowly out of the inlet because of the too low temperature. With the inlet at 100C and the column at 140C the temperature gradiant is opposite to what you want.

ALso, with splitless injections you get more tailing than with split injections. This is usually handled by opening the split 30s or a minute after the injection.

Peter

[Sandra7]

Hi Sandra

Unless you are doing fancy programmed temperature vaporization injections then inlet temperature of the GC needs to be closer to the final temperature of the column programme than the initial temperature. If you have had the inlet at 100 or 140C

Inlet Temp was 250; initial temp was 100 and then in 2nd analysis, raised to 140.

then I am not surprised that your peaks tailed - they will have evaporated slowly out of the inlet because of the too low temperature. With the inlet at 100C and the column at 140C the temperature gradiant is opposite to what you want.

ALso, with splitless injections you get more tailing than with split injections. This is usually handled by opening the split 30s or a minute after the injection.

Thanks for the tips. I will check what it is set now for opening the split.