Area Count Reproducibility

Chromatography Forum: GC Archives: Area Count Reproducibility
Top of pagePrevious messageNext messageBottom of pageLink to this message  By HP-wax on Tuesday, July 27, 2004 - 01:09 pm:

My area counts are varying by more than 5% RSD.
I am using a polar column, with FID. Split (1:50)
I have checked for leak at the injector, detector ends; changed septum but the problem is persistent.

The retention time and the peak height is also not consistent.

Alcohol is analyzed with an internal standard. 0.5ul -manual injection.

Thanks in advance for any hints to solve the problem.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Tuesday, July 27, 2004 - 04:14 pm:

(1) Try a new syringe
(2) Use more uniform technique
(3) Get boss to buy you autosampler


Top of pagePrevious messageNext messageBottom of pageLink to this message  By anon on Tuesday, July 27, 2004 - 08:08 pm:

Uniform technique...? for injection?


Top of pagePrevious messageNext messageBottom of pageLink to this message  By xxx on Tuesday, July 27, 2004 - 10:53 pm:

HI

The aera counts of the alcohol or of the istd are varying by more than 5%
And the ratio????How much rsd?

Bye


Top of pagePrevious messageNext messageBottom of pageLink to this message  By anonimna on Wednesday, July 28, 2004 - 03:58 am:

i had such a problem several weeks ago.
i had an option to chose between constant flow or constant pressure (automatically controled electronics).with constant flow,we had problems.once we switched to constant pressure instead of cnst. flow,my problems persisted no more.
maybe it works for you as well.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By la rubia on Wednesday, July 28, 2004 - 04:08 am:

maybe your total flow (column +purge+split)is too low.rise it up!


Top of pagePrevious messageNext messageBottom of pageLink to this message  By chromatographer1 on Wednesday, July 28, 2004 - 04:27 am:

check the installation of the column in the injector. Some GCs are very sensititve to column positioning. Do you have reactive analytes you are measuring? Is your injection liner clean and inert? Are you using a Chaney adapter? Is your septum leaking or is there a leak in your injection system?

Good luck


Top of pagePrevious messageNext messageBottom of pageLink to this message  By GC user on Wednesday, July 28, 2004 - 05:40 am:

If the area counts are lower when the retention time is longer the problem is almost certainly a leak. I have found that most users overtighten the septum nut on Agilent GCs, which will lead to cored septa and the types of problem you are having. Never tighten the septum nut with a wrench, only finger tighten using the recommended procedure.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By HP-wax on Wednesday, July 28, 2004 - 07:56 am:

1)Does oven temperature program, like ramp rate , final temperature and hold time have an effect on area count?
2)I have two ramp rates, one at 5 deg/min and then another at 35 deg/min. Is increasing from 5 to 35 deg/min permissible to reduce the analysis time ?
3) Also is it ok to use 10ul syringe to inject 0.5ul?
Thanks for all the responses.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By GC user on Wednesday, July 28, 2004 - 09:21 am:

Ramp rate should not affect total area counts, althought the peak shape may be affected.

Faster ramp rate to reduce analysis time should not cause problems, as long as the faster ramp does not cause peaks to co-elute.

0.5 uL injections using a 10 uL syringe can be a problem. With manual injection usually some or all of the excess solution in the needle is vaporized. Unless you are very careful with the time in the injection port being consistent this can have an adverse effect on precision. To inject such a small volume reproducibly a solvent flush injection would probably be best.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By Anonymous on Wednesday, July 28, 2004 - 10:01 am:

Use a 1ul syringe for volumes that small; like above poster noted, lots (relatively) of volume held up in needle of 10ul syringe.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By HP-wax on Wednesday, July 28, 2004 - 09:07 pm:

Hi

I have given details below of the RSD's. I am sure there is some problem with the GC. Like mentioned before, I have looked at the injector and detector ends for leaks,Changed the septum, Cleaned the liner as best as I could and i have the flow rates around 150ml/ min at the split and around 6 ml/ min at the detector . Oven temperature is 80C.

(a) Area Ratio - RSD = 1.7%
(b) Analyte Area Count - RSD = 22 %
(c)Internal Std Area Count- RSD = 19.9 %
(d)Analyte Retention Time - RSD = 0.22 %
(e)Internal STD Retention Time - RSD = 0.25 %
(f)Analyte Height - RSD = 36 %
(g)ISTD Height - RSD = 28 %

Please post suggestions for any possible solution to these problems.

Thank you
HP-wax


Top of pagePrevious messageNext messageBottom of pageLink to this message  By ab on Wednesday, July 28, 2004 - 11:43 pm:

HP-wax,

Try to analyse your mix without temperature programming. For example, ethanol can be analysed very well at 90 C on polar column.

Your results are pretty for quantitative analysis. Retention times are very good. But yes, I think there is some problem with splitting.

If you are using water as a solvent lessen the injector temperature to 140 C.

Some regulator or filter in the split line it can be cause.

GC user,

Leakage can't be cause of retention time shift, unless a head pressure is not changed.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By ab on Wednesday, July 28, 2004 - 11:50 pm:

sorry,
unless a head pressure is changed.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By ced on Thursday, July 29, 2004 - 02:05 am:

Hi

Your area ratio rsd is correct,your retention time rsd is correct for both analyte and istd.

But the rsd of your istd alone is to high,you do have a not reproductible injection.

1-If your RSD% are with 0.5 ul injection,with a 10 uL syringe,try 1 uL injection,or 0.5 with a 5 uL syringe.

2-Try another liner with restriction like,laminar cup spliter,glass wool,carbofrit...to improve the evaporation process.


Top of pagePrevious messageNext messageBottom of pageLink to this message  By HP-wax on Thursday, July 29, 2004 - 07:15 am:

I am injecting 0.5ul with a 0.5ul syringe. Could this be a problem?
Regards


Top of pagePrevious messageNext messageBottom of pageLink to this message  By HP-wax on Thursday, July 29, 2004 - 07:27 am:

Hi
I am sorry, My area ratio - RSD is acutally 3.3 %. I had it looking at the wrong cell (Excel)..
I guess this throws off the quantitation...

Regards


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