Chromatography Forum

We have problem with a GC-FID head space method.
In the running sequence from one injection to the next we got irregular peak sheap (e.g. tailing or shoulder), but in the further injection the peak shape becomes ok again without any intervention. As this anomaly is random in the sequence, if we meet it at solution which contains more components, all peaks show more tailing than normal peak shape or the peaks have shoulder.
Has somebody any idea what can cause the problem? We think that the problem comes from the injection part of the method, but we do not know the exact reason. The injection volume is high - 1 ml, and we use 6%cyanopropylphenyl-94%dimethylpolysiloxane, 30 m, 0.53 mm and 3µm column.
Many thanks for your answer.

What samples are you analysing ? What are your flow rates and temperatures ?

Peter

Why are there broad, irregularly-shaped peaks that elute early in my gas chromatogram ifacetimeapp?

Are you injecting in split or splitless mode?

What is the nanogram of analyte on column for the peaks with problems?

At first thank you for the questions.

Flow rate: 2 ml/min
Split ratio: 1:10
Injector temperature: 200 oC
Column temperature: 40oC for 10 minutes and after we heat it up
Carrier Nitrogen
We inject IPA STD solution with concentration of 0.4 mg/ml by HS.

At first thank you for the questions.

Flow rate: 2 ml/min
Split ratio: 1:10
Injector temperature: 200 oC
Column temperature: 40oC for 10 minutes and after we heat it up
Carrier Nitrogen
We inject IPA STD solution with concentration of 0.4 mg/ml by HS.

I put those into the pressure/flow calculator and do notice that the inlet pressure would be only 1.46psi, not sure if such low pressure is causing the problem or not. If the instrument has trouble controlling the pressure when it is that low it could cause some peak shape problems possibly.

Have you tried this with a 0.32ID column which would increase the head pressure to 4psi? Or possibly a pulsed injection where you increase the head pressure/flow at time of injection then return to optimal linear velocity during the rest of the run?

At first thank you for the questions.

Flow rate: 2 ml/min
Split ratio: 1:10
Injector temperature: 200 oC
Column temperature: 40oC for 10 minutes and after we heat it up
Carrier Nitrogen
We inject IPA STD solution with concentration of 0.4 mg/ml by HS.

I put those into the pressure/flow calculator and do notice that the inlet pressure would be only 1.46psi, not sure if such low pressure is causing the problem or not. If the instrument has trouble controlling the pressure when it is that low it could cause some peak shape problems possibly.

Have you tried this with a 0.32ID column which would increase the head pressure to 4psi? Or possibly a pulsed injection where you increase the head pressure/flow at time of injection then return to optimal linear velocity during the rest of the run?

Many thanks for your valuable answer.

Syringe needle or injector septa?

Syringe needle or injector septa?

We had some trial measurement and we could exclude them.

We are still working on the solution, and we have maybe a ray of hope. It is sure that the problem comes from the injection. We increased flush time (now it is 180 s), and septum purge flow (8 ml/min), the injection speed is 500 µl/s and we changed RTX-624 column to DB-624 column. Every other parameters remained the same.
We injected 1000 µl and 500 µl with 2.5 ml needle in the same seqence. The first 30 injection was with method of 1000 µl injection volume, and the second 30 injections were with method of 500 µl injection volume.
The anomaly occurred at 35th and 54th injection (injection volume of 500 µl). The peaks were not splitted, but we got Gaussian curved peak with about double area size.
Have somebody an explanation about above observation?

Many thanks again for all of your help.

P.S.: We also increased the inlet pressure in other sequence, but the anomaly is not solved. However our method needs to be developed further. We have no experience in pulsed split mode, unfortunately.

Dear Wesskla
your process depended selective method for test your compound
my advice for your increase equilibrium time and Loading time same time mean Injection time dep oh manufacture , decrease flow rate and take care with spilt ratio.


Omer Hazim