Chromatography Forum

Dear All,

I HAve a brand new GCMS instrument, a 7890+5975C with a PTV injector from Gerstel without the cooling unit.
My problem is as follows:

In a normal Split injection with this instrument the RSD% between 9 injection of the same sample (FID MDL test sample from Agilent) is around 50%. It's a crazy number, I know.
Have You any idea, why?
No leak, in the injector.

Best Regards,

Attys

You said your Gas chromatograph is brand new?! Yet, the repeatibility is only 50% between 9 consecutive injects of your Standard ? Amazing!?
Please tell us:
1) list the precise area count values for each injection---maybe the first 3 or 4 injections have 0% RSD, but your subsequent injects are accumulating the variance. This would suggest a "carryover contamination" problem--which is easy fix.

2) list for us the analyte name and method run parameters. This would help us to determine if sufficient elution of the query analyte is occuring.

3)

HI,
Thanx for your advice.
I think it is not a carryover issue. Between the first 4 injection it can appear the 50%.
The sample contains 3 compound of C15, C16 and C17 hydrocarbons in hexane or haptane. The area are around 20 million. I will do the exact number soon.
There is one more think: If I injected 0,2 uL than 1 uL of the same sample I got 2 times higher peak. ??


Thanx again.

A.

You said your Gas chromatograph is brand new?! Yet, the repeatibility is only 50% between 9 consecutive injects of your Standard ? Amazing!?
Please tell us:
1) list the precise area count values for each injection---maybe the first 3 or 4 injections have 0% RSD, but your subsequent injects are accumulating the variance. This would suggest a "carryover contamination" problem--which is easy fix.

2) list for us the analyte name and method run parameters. This would help us to determine if sufficient elution of the query analyte is occuring.

3)

Hi, there has been a thread noting this issue,

CTC GC-PAL Autosampler and GC-MS Reproducibility,
viewtopic.php?t=7002&highlight=

in which I wrote that the liners have only 200ul volume for vapour.

so you need to state how much your are injecting, the solvent, what inlet pressure, the type of liner (straight through or with baffles(goose-necks)).

Have you compared it against your rear injection port (a non PTV) ?


Alex

attyska,

What is the precision if you inject multiple 0.2 uL injections? Is this a case of overwhelming the system with too high a concentration? C15, 16, and 17 should work very well on a PTV.

Best regards.

HI. You are right, i have a liner with baffles.
all the time I have injected 1 ul of sample. You suppose that it is too much for the 200 ul liner, as i understand.
Unfortunately this is the only one injector in the GC, i haven't another one.
I have 250°C injector and the solvent was hexane. The exact pressure I will state later.
Thanks for comments.
Attys.

Hi, there has been a thread noting this issue,

CTC GC-PAL Autosampler and GC-MS Reproducibility,
viewtopic.php?t=7002&highlight=

in which I wrote that the liners have only 200ul volume for vapour.

so you need to state how much your are injecting, the solvent, what inlet pressure, the type of liner (straight through or with baffles(goose-necks)).

Have you compared it against your rear injection port (a non PTV) ?


Alex

Hi,

I dont know now, I will do it soon. The concentration of the C15-C17 is not high, they are dissolved in hexane or octane, the conc. is around 25 ug/L.

attyska,

What is the precision if you inject multiple 0.2 uL injections? Is this a case of overwhelming the system with too high a concentration? C15, 16, and 17 should work very well on a PTV.

Best regards.

I put in a liner with Baffles, the volume is 150 ul, with hexane taking 203ul volume @ 250°C, attached a screen shot,

Also if you buy a 5uL Syringe and not use a 10uL, the minimum injection can be halved, N.b. the 5uL have a tendency to bend the plunger, if you don't maintain the tower.

Why is the volume of the liner significant in a split injection ?

Peter

Hi

Just have to ask a basic question first, attyska are you actually doing the test according to the manual?

Reason for asking is at least for the 6890 model, the test is runned in splitless mode, cold injection, not hot split. Actually Agilent provides a warning for liner overflow with hot split injects at 1µl or above with those typical small volume liners at hot injections and states that test intention do not cover other stuff like injection repeatability.
[/u]

Good point Peter ,
I should read threads more carefully

Hopefully Krickos comment will prevail with the solution.
Alex

Hi Chemstation

I suspect that the repeatability problems are just another symptom of the extreme difficulties of repeatably controlling the flash vaporization and subsequent pressure and flow peturbations. The Grobs have published at length on this. The persistance of flash vaporization when alternatives are now available off the shelf is a bit of a puzzle.

Peter

Hi,

First of all: the splitless injection was done and the rsd% was around 2 %.
After it I have started to do splitless injections and found the issue.
I have done now the split/splitless with 0.2 ul injection with 25 psi injector pressure at 250 °C. The results will be here soon.

Why not cold injection? Because no cooling unit with my instrument. So i'm not able to make cold injections.

I am very out of date on the issues of PTV injection.

I thought the only way of doing PTV injection was to ensure the sample was injected as a liquid ie the injector was cold, and then ramped up rapidly. In this case, despite the fast ramp, the sample does not "flash vaporise" due to the time it takes for heat to transfer to the sample.

If you do not have a cooling unit, what temperature is the injector? Is it just at the temperature of the column? I don't understand the conditions of your injection. I am also confused as to the whether you are trying to do split or splitless injection. If you are trying to do a classical splitless injection and the temperature of the injector is such that the liquid is not condensed, the smaller volume of PTV injection liners is going to be a problem.

Sorry if I confused the issue-I'm trying to learn something.

Dear All,

I HAve a brand new GCMS instrument, a 7890+5975C with a PTV injector from Gerstel without the cooling unit.
My problem is as follows:

In a normal Split injection with this instrument the RSD% between 9 injection of the same sample (FID MDL test sample from Agilent) is around 50%. It's a crazy number, I know.
Have You any idea, why?
No leak, in the injector.

Best Regards,

Attys

Almost PTV inlet is contaminated, i already meet this problem. I must replace a new one