Result difference between two different GC-MS

[rsa01]

Hi all,
I hope all are good.here in our lab we are having 2 GC-MS and we are analyzing some markers in petrochemical samples.our analysis range is 10ppb-1000ppb. But the problem is both the instruments results are different.ex:if we are analyzing one sample in GC-MS-1 it showing 200 ppb the same sample GC-MS-2 showing 260 ppb so almost 30% variation between 2 instruments.
what all are the factors will cause this difference? please give some suggestions to minimize this problem

Regards
Ashok

[Peter Apps]

Are you running a calibration on each instrument ?

The less you tell us about the methods you are using the longer and less useful the list of possible problems becomes.

Peter

[rsa01]

Hi Peter,
Yes i am running the calibration in both the instrument.I am calculating results based on calibration only.please find my method details
GC Parameters:
Oven Temperature Initial 80ºC
Oven Temperature Final 325ºC
Septum Purge 3mL/min
Injection Volume 1µL
Split Ratio 20:1
Aux. Temperature 280ºC
Injector Temperature 250ºC
Post run Temperature 325ºC
Post run flow 0.5725mL/ min
Total run time 19 min.

Oven Program:
Initial Temperature ºC Per minute Final Temperature Minutes
80 50 220 2.20
220 30 280 5.13

MS parameters:
Ion source Temperature 230ºC
Analyser Temperature 150ºC
Solvent Delay 2.5 minds

Detector off 5.0 min
SIM (Selective ions) 187,240,315 m/z
Please tell me the possible problems

Regards
Ashok

[CE Instruments]

I think in this case we need to know more about your calibration procedure and how you treat the data. Does this only affect early peaks or are all the peaks affected ? To what extent ?
Discrimination ?

[Peter Apps]

What is the run to run repeatability on each of the instruments ?

What are the columns, pressures and flow rates ?

Peter

[rsa01]

Hi peter,
We are getting run to run repeatability for QC with in 2 %.We are using HP-1MS column(Non-polar column)in both instruments.constant flow 1 ml/min

Regards
Ashok

[rsa01]

Hi CE Instruments,
We are using external STD calibration.we are analysing only one analyte.
Regards
Ashok

[dlbenach]

Are you injecting the same standard and sample preps into each GC? Want to rule out prep issues and make sure the instruments are the only variable.

[kubowicz.tomasz]

Hello

If you run standards before your samples (and RSD for5-6inj is ok) it shouldn't be an issue (however it is possible).But lets keep hardware away for a moment...
As I understand you're using SIM as main signal and your results/calculations are based on SIM signal. Are you using total SIM or target ion for calibration?
Have you checked integration for target ion? Is it acceptable? Is target ion for sample integrated in the same way as for standard?

Regards

Tomasz Kubowicz

[rsa01]

Hi dlbenach,
We are injecting same sample in both the instrument i hope this problem only in GC not in sample pre.....

Regards
Ashok

[rsa01]

Hi kubowicz.tomasz,
Yes we are using SIM for calibration & result calculation.we are using target ion SIM (only one ion) 240 for our analysis this ion is working very good there is no interference in the matrix.we are using the same target ion for sample & standard result calculation..............

Regards
Ashok

[kubowicz.tomasz]

Hello

If you're using target ion you need to make sure that integration for this specific ion is correct. When you click "integrate" on SIM chromatogram it is integration for total SIM (target ion+ 2 or 3 qualifiers). So in theory you can have perfect integration for total SIM and very poor integration for target ion (it can cause problem with final results).
But if you're sure that integration is correct it could be something with hardware (GC). I would check inlet (pressure test) and run 5-6 std to check RSD.

Regards

Tomasz Kubowicz

[rsa01]

Hi,
I am sure about the integration. OK i will check the inlet.

Regards
Ashok

[James_Ball]

What are the concentrations of your calibration standards?

Do they bracket the concentration of the sample you are testing, such that if you are getting a sample concentration in the 200-260ppb range do you have standards that are both above and below that concentration?

Some people do try to inject a 100ppb standard and still report samples in the 200ppb range, but to be most accurate and to get the best reproducibility between instruments the calibration should bracket the expected range of the samples.