Cyclohexene in MeCl2 used in GC/MS

[ods-at-pacific]

We were developing a method that involved the GC/MS analysis of 2-pentanone. We were using methylene chloride as the solvent. We found what appeared to be a contaminant at the ~50 ppm level in the methylene chloride. The mass spectrum of this compound matched that for cyclohexene. Using the NIST RI database we found that cyclohexen has RI values in the range of 701 to 657. The RI values for 2-pentanone are in the range of 716 to 645. This corresponded to our results which has these two compounds coeluting.

We did a Web search and found a couple of articles talking about cyclohexene being used as a stablizer in methylene chlorie. There was no indication on our reagent bottle that this was the case for the material purchased. When we checked the Aldrich catalog, we found listings for methylene chloride which is stablized with amylene which has RI values in the range of 425 to 514.

I would like to know if anyone else has had a problem with cyclohexene in methylene chloride and if changing to methylene chloride stablized with amylene solved the problem. Was in necessary to oxidize the cyclohexene and distill the methylene chloride to separate out the oxidized product to get pure methylene chloride.

Thanks for anything you might know about such a situation.

[gpronger]

David,

My first response would be the use of purge and trap and avoid the whole issue of methylene chloride assuming what you're setting up to do is an extraction technique for the 2-pentanone.

There is also unstabilized methylene chloride available from some vendors, but after speaking with the technical staff at a few solvent companies, you have a strong potential for the unstabilized methylene chloride to react with whatever its used with.

We were performing EPA method 625 type extractions with library searches, so what had started us down the path to finding a "cleaner" solvent due to having problems when performing library searches. The early part of the chromatogram had a number of peaks which were either the preservatives or by-products. We were trying to minimize these effects, hence the interest in the unstabilized methylene chloride.

What the solvent manufacturers technical staff explained was that the by-products of amylene and cyclohexene are all predominately very early eluting leaving the rest of the chromatogram free of interferences. Due to the potential reactions between unstabilized methylene chloride and analytes which would give us by-products in the middle of the chromatogram, we ended up using a high grade of stabilized (amylene) methylene chloride and simply not utilizing the very early part of the chromatogram for library searches.

For your purposes, the unstabilized methylene chloride may work.