C13 detecting issue on GC-MS Shimadzu QP 2010 Plus NCI

[Irina]

Hi, I am setting the ISO 1948 method for dioxins and furans on GC-MS and I can not see the internal standard compound which have a C13 atom. I can easily separate and quantify the target compounds on GC-MS with negative chemical ionization, but the C13 I can not see any peak either on NCI, EI or pozitive chemical ionization.

Any thoughts?

Regards,
Irina

[JMB]

1. Are these "dioxins and furans" TCDD's and TCDF's ?

2. What are the actual RT values for the analytes and the expected RT for the 13C I.S. ?

[Irina]

1. Are these "dioxins and furans" TCDD's and TCDF's ?

2. What are the actual RT values for the analytes and the expected RT for the 13C I.S. ?

1. yes they are

2. from RT 17 to 29 for analytes and I was expecting appropriate RT for 13C compounds. But even if I inject only 13C labelled compound and look for both scan and their ions I can not see any peak and I do not know why. I have the same column as in the ISO (TG-5MS TraceGold 30m, 0.25 mm, 0.25 um), injection temperature I tried it from 100 to 330 and still no peak. I'm stuck and I can not waste to much of the solution because they where like 200 ul for each calibration point and the cost was about 3000 euro.

[JMB]

I would suspect that your 13C internal standards are possibly bad; do you have a manufacturer certificate of analysis with GC/MS chromatogram and spectrum for each IS ??

[Peter Apps]

Double check the concentrations of your labelled standards, and the dilution (is any) that you used to make your working standards.

Peter

[Irina]

The standard staid a few months opened, but on their certificate expiration date is on 2017. I only have a chromatogram with both labelled and unlabeled standard where it said they are showing up at the same RT as the unlabeled.
I did not use any dilution because it was a set of 5 calibration point, an extraction solution (to add when start extracting the samples) and a sampling one which included only the IS at a concentration of 0.1-0.2 microg/ml.
Irina

[JMB]

Since,

1) your GC/MS system is operating correctly,
2) you do not perform any dilution of the IS solution,
3) but the I.S. is below LOD

I think that we have to assume that the I.S. solution is below the concentration specified by the manufacturer.

I know the I.S. is very expensive, but to resolve the issue you might consider injecting an aliquot that has been concentrated by a factor of at least 10; whatever the result, you would then have some solid evidence to provide to the mfr.

By the way, are using full-scan MS or selected ion monitoring in your method ? SIM would certainly give better sensitivity than full-scan data acquisition.

One last caution, before you challenge the mfr. check ALL of your exptl. details, then re-check ALL of your exptl. details, then have a third party check ALL of your exptl. details.

[JMB]

One more thought, test your GC/MS system to see that it meets the Shimadzu GC/MS sensitivity specification before you consume any additional 13C standards.

If not, apply the usual techniques---have a clean glass liner/new septum/ cut 1-2 cm from head of column/clean the ion source/clean the analyzer etc.

Good Luck !!

JMB

[Peter Apps]

Do you have standards of unlabelled analytes where you can clearly see good peaks ? What is the concentration of those standards and how does it compare to the concentration of the labelled standards. ?

Peter

[Irina]

Peter,
I have good chromatogram for unlabelled compounds and the concentration is from 0.005 ug/ml to 0.2 ug/ml.
JMB,
I cleaned up again the ion source, change the liner and septa and now I am waiting for the vacuum to be stable and I will check again the IS and I will concentrate it also.
Thank you all for reply. I will let you know how it works.

Irina

[Peter Apps]

The concentrations are very low - the maximum that you are putting on column (assuming a 1 ul splitless injection) is 0.2 ng.

Have you directly compared the response for labelled and unlabelled compounds at the same concentration ?

Peter

[Irina]

I think the column was the problem because after I changed it with a new one I can see the peaks for C13 also. I know the concentrations are very low and that's why I am analyzing only in SIM mode and I didn't see the bleeding of the column.

Thank you all for your suggestion