Benzene-d6 converting to Benzene during analysis?

[James_Ball]

Yesterday I was running an analysis for Benzene using Benzene-d6 as internal standard. Purge and Trap with GC/MS. My first three blanks had Benzene-d6 area counts near 75000 but in the standards the Benzene-d6 area counts dropped to 42000 for the last calibrator. Curve was from 2ppb to 40ppb with Benzene-d6 held at 20ppb. Purging 10ml water in the soil position of an Encon autosampler at 40C. The Benzene-d6 area counts declined over the course of the calibration standards but came back to near 75000 in the final blank. This morning I did notice I was on my cleanout method for the purge and trap which had the valve oven and transfer line set to 200C. Today I reran my calibration curve and it had consistent Benzene-d6 areas at 80000 area counts with the transfer line and valve oven at 130C.

Would the higher temperatures be causing the Benzene-d6 to lose deuterium in the presence of Benzene?

First time I have ever had this happen and I am stumped

Also carrier gas is Hydrogen and purge gas is Helium.

Any ideas out there as to what this could be?

[JMB]

"My first three blanks had Benzene-d6 area counts near 75000 but in the standards the Benzene-d6 area counts dropped to 42000 for the last calibrator. Curve was from 2ppb to 40ppb with Benzene-d6 held at 20ppb."

My guess is that benzene and benzene-d6 partially co-elute, and that at 200 C there is some H/D exchange occurring, especially with the 40 ppb standard.

If this is true, then you might be able to detect an enhanced m/z 79/78 ratio in the spectrum of the 40 ppb standard.

[James_Ball]

"My first three blanks had Benzene-d6 area counts near 75000 but in the standards the Benzene-d6 area counts dropped to 42000 for the last calibrator. Curve was from 2ppb to 40ppb with Benzene-d6 held at 20ppb."

My guess is that benzene and benzene-d6 partially co-elute, and that at 200 C there is some H/D exchange occurring, especially with the 40 ppb standard.

If this is true, then you might be able to detect an enhanced m/z 79/78 ratio in the spectrum of the 40 ppb standard.

I have managed to get them all but baseline resolved which helps on the false positives. Cooling the transfer lines seems to have brought back the Benzene-d6, just though it odd to lose the deuterium, though I guess it could have come from a reaction with the Hydrogen carrier at the elevated temperatures. Been 25 years since I worked with the reactions of substituting hydrogen and deuterium so little foggy on the reaction dynamics now.

[Alp]

GCMS is really not my cup of tea, so excuse me if this is really off base. However, I find this an interesting problem so I'll go ahead and throw some ideas out there, even if they are way off.

Some metals can act as catalysts for this sort of hydrogen exchange on benzene.

http://link.springer.com/article/10.100 ... rue#page-1

http://pubs.acs.org/doi/abs/10.1021/j100851a050

Is there a metal surface that might be at different temps depending on if you are set for 130 deg or 200 deg?

Is there some form of buildup that might deposit on a surface and act as a catalyst ? Is there something in your sample that might act as a catalyst?

Alp

[mckrause]

1. You are running a hydrogen carrier. Huge source of protons.

2. Catalytic activity increases with temp.

3. You can get oxidative exchange of deuterium. Try getting recovery of Toluene-d8 out of chlorinated solutions!

Sounds like you fixed your problem.

[James_Ball]

GCMS is really not my cup of tea, so excuse me if this is really off base. However, I find this an interesting problem so I'll go ahead and throw some ideas out there, even if they are way off.

Some metals can act as catalysts for this sort of hydrogen exchange on benzene.

http://link.springer.com/article/10.100 ... rue#page-1

http://pubs.acs.org/doi/abs/10.1021/j100851a050

Is there a metal surface that might be at different temps depending on if you are set for 130 deg or 200 deg?

Is there some form of buildup that might deposit on a surface and act as a catalyst ? Is there something in your sample that might act as a catalyst?

Alp

The injection port is stainless steel so there is probably some nickle in that, and so is the 8 port valve in the purge and trap. The transfer line is Silcosteel which is a stainless steel with a fused silica type coating. Also there are probably all kinds of junk built up on some of the lines that could be acting as a catalyst.

[James_Ball]

1. You are running a hydrogen carrier. Huge source of protons.

2. Catalytic activity increases with temp.

3. You can get oxidative exchange of deuterium. Try getting recovery of Toluene-d8 out of chlorinated solutions!

Sounds like you fixed your problem.

When doing semivolatile analysis once I encountered a problem where I lost a surrogate that had 12 deuterium on it, found out that was caused by the sample being too acidic. The spectra of the peak was rather neat as you could see masses present for the loss of each deuterium, but so many masses is terrible for sensitivity when looking at only one.

Temperature seems to be the deciding factor of whether or not the loss of deuterium happens at all, but it was funny how the presence of benzene seemed to accelerate the loss. What was also strange was that the benzene response still appeared linear, it didn't seem that the benzene-d6 converted completely to benzene. I imagine I didn't see where it went because was not looking for the intermediate analogs (-d5 -d4 ect).