Chromatography Forum

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

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. 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.

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 ??

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

Peter

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

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.

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

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

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

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

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