Chromatography Forum

Hi Friends
we have a GC method and the details are as mentioned below

Column : DB 624 column
(30 m x 0.32mm ID), 1.8 µm film thickness or Equivalent.)
Injector Temperature : 180°C
Detector : FID
Detector Temperature : 250°C
Flow : 1.7ml/min, constant flow
Carrier gas : Helium/ Nitrogen
Oven Temperature : 60°C to 220°C at the rate of 20 °C per minute and hold at 220°C for 3 min.
Injection type : Split less (Purge flow :- 20 ml/min)

Headspace Parameters:

Vial Oven Temperature : 80°C
Transfer line Temperature: 110°C
Loop Temperature : 90°C
GC Cycle Time : 18 minutes
Vial equilibration Time : 60 minutes
Pressurization time : 2.0 minutes
Loop fill time : 0.2 minutes
Loop Equilibration Time : 0.2 minutes
Injection Time : 0.08 minute
Vial Shake : Low
Loop Size : 1.0 ml
Headspace Vial : 20 mL
Headspace Vial Pressure : 14 psi

Preparation of Standard Solution:

Weigh accurately about 534 mg of Toluene into a 50mL volumetric flask containing about 20 mL of Dimethyl formamide. Dissolve and dilute to volume with Dimethyl formamide and mix well. 5.0 mL of the above solution transferred into a 50 mL volumetric flask and then dilute to volume with dimethyl formamide. 5.0 of the above solution dilute to 100 mL volumetric flask with Dimethyl formamide.

Transfer 5.0 mL of the solution into a headspace vial and seal the vial immediately.

Now the question is that the baseline is very straight and good in th first injection and went bad with other injections , , please can anybody explain how can attach chromatograms also .

I would increase the temperatures in the loop and the transfer lines over the boiling points of the solvents. I would make the injection temperature the same as the detector. If this indeed is the problem I would expect to see more peaks than expected not a issue with the base line. You could be injecting to much onto the column in that case you will need to increase to split ratio.

What are you dissolving?

If indeed this is more of a baseline issue, since a loop is involved, a dirty jet could be an issue.

What exactly is happening to the base line, is it becoming noisy? Is it a loss in sensitivity?

Also why are you using He/N2 instead of just He?

Just some thoughts.

Instructions for posting chromatograms are in a sticky at the top of the LC forum.

Peter

Hi

Agree with the previous input with regard to temperatures and boiling points, ie raise valve/loop and transferline temps to above bp of both toluene and DMF (set like 155-170°C).

Secondly you may have to other issues:

1.
Loop Equilibration Time : 0.2 minutes this setting is usually too long only increses risk of interaction with loop wall, for standard HS application with a 1ml loop I have found 0,05min sufficient.

Injection Time : 0.08 minute. This seems low, you have stated a total flow of 1,7ml/min take that times 0,08min=> 0,136ml which is less than the loop volume of 1ml, so it seems like you are not inject the fulll loop volume, which may add to your problems. Usually I tend to make sure that a minimum of two loop volumes pass through the loop during injection, preferly more if possible.

2. You are running splitless HS injection. This does not always work well, depending on instrument and set up. For instance, Agilent HS-GCs with EPCs tend to protest if total flow drops below 10ml/min.

Hi Friends
we have a GC method and the details are as mentioned below

Column : DB 624 column
(30 m x 0.32mm ID), 1.8 µm film thickness or Equivalent.)
Injector Temperature : 180°C
Detector : FID
Detector Temperature : 250°C
Flow : 1.7ml/min, constant flow
Carrier gas : Helium/ Nitrogen
Oven Temperature : 60°C to 220°C at the rate of 20 °C per minute and hold at 220°C for 3 min.
Injection type : Split less (Purge flow :- 20 ml/min)

Headspace Parameters:

Vial Oven Temperature : 80°C
Transfer line Temperature: 110°C
Loop Temperature : 90°C
GC Cycle Time : 18 minutes
Vial equilibration Time : 60 minutes
Pressurization time : 2.0 minutes
Loop fill time : 0.2 minutes
Loop Equilibration Time : 0.2 minutes
Injection Time : 0.08 minute
Vial Shake : Low
Loop Size : 1.0 ml
Headspace Vial : 20 mL
Headspace Vial Pressure : 14 psi

Preparation of Standard Solution:

Weigh accurately about 534 mg of Toluene into a 50mL volumetric flask containing about 20 mL of Dimethyl formamide. Dissolve and dilute to volume with Dimethyl formamide and mix well. 5.0 mL of the above solution transferred into a 50 mL volumetric flask and then dilute to volume with dimethyl formamide. 5.0 of the above solution dilute to 100 mL volumetric flask with Dimethyl formamide.

Transfer 5.0 mL of the solution into a headspace vial and seal the vial immediately.

Now the question is that the baseline is very straight and good in th first injection and went bad with other injections , , please can anybody explain how can attach chromatograms also .

please find chromatograms , First chromatogram is Good Baseline Second injection is Blank with disturance at Toluene Rt and Third Chromatogram is again Standard with disturbance @ Tolune agian , i hope you know my problem now

It looks almost like carryover As a diagnostic test, try the same series of runs, but hold the column at its maximum temperature for another 10 min. I suspect that a late eluting peak will show up.

What hardware is this run on ?

Peter

It looks almost like carryover
Peter

Hi

Agree with Peter, have seen tendances to this "hump" just prior to the DMF peak on 624 columns "disturbing"toluene peaks both at my own place and in this forum in the past but not quite as significant (pA wise).

As stated earlier, you may have headspace settings that cause this carry over (potentially DMF "bleeding" from headspace valve/loop).

Are there any situations where you don't want a high FID temperature? I like to keep it around 300.

Isn't 2 minutes for vial pressurization also a bit excessive?

The above posters are right though, you should adjust your headspace temperatures, adjust your injection time, and then after doing two injections let it run hot/isothermal for awhile and see if you get late eluting peaks.