A co-worker came to my building to start his PhD after I had submitted my thesis. He was sponsored by Coca-Cola to study slip-additives in the HDPE films in of bottle caps. It turns out that these compounds have been leaching out and crystallizing on the surface (as polyhedra). Coca-Cola wish to quantify this leaching and as such the student has to determine a way to extract them without dissolving the HDPE. He approached me and asked for advice on which solvent the compounds would be soluble in. My first question was the molecular structure of the three compounds, which is shown below.
I then wanted to know which solvents would dissolve HDPE. A web search tells me that chlorinated and aromatic solvents will dissolve HDPE. The student tells me that these and other slip-additive compounds have been successfully extracted before using diethyl ether and that they have been run through GC-MS using DCM. Fortunately, the supplier sent a rather large sample bag of each compound. I recommended a solvent survey as laid out below in order to find a solvent in which the compounds have high solubility.
As we had plenty of the compound, I thought it would be good to run GC-MS of the pure samples. We prepared GC-MS vials with 1 mg of compound into 1 mL of DCM. We found that erucamide and oleamide dissolved easily in DCM but behenamide was very difficult to dissolve in DCM. After lots of shaking, we eventually added 10% methanol to these vials and the behenamide then slowly dissolved. Curious that the absence of the double bond could make it so much less soluble than erucamide. I am told that behenamide is the more active of the three compounds as an anti-slip agent (it produces more friction).
The compounds were more difficult to elute through GC-MS than expected. Nothing eluted on a standard method, probably due to the high molecular weight of the three compounds (337, 339 and 281 respectively). A higher temperature method still struggled to elute much of each compound. Only a very high temperature splitless method was successful with sample delivery at 300 °C; oven temperature starting at 200 °C, increasing to 250 °C at 2 °C/min, then 250–300 °C at 4 °C/min (42.5 min total). This method was harsh enough to make most (~80%) of the compound decompose on the injector port. The decomposition products for all three compounds were M-18 and eluted in around two thirds of the time of the intact compound.
It would be desirable to derivatize the free amine (-NH2 group) in order to obtain a species that travels more readily through a GC-MS column. This could be done by an acylation using acetyl chloride or acetic anhydride. Or it could be done by trifluoroacetic acid, which has the advantage of creating a compound that can be observed by 19F-NMR. But it also has the disadvantage of making a higher molecular weight product than a regular CH3 acylation.
Derivatization of Behenamide
Here the derivative is an acyclic methyl imide. The imide group is usually stabilized by being part of a heterocycle, but I see no reason why these methyl imides should not be stable enough to analyze by GC-MS. Imides however, have very acidic NH groups that will often form an alkali salt or an N-halo bond. Thus it is possible that the N-chloro methyl imide is formed if acylation is carried out by acetyl chloride. The trifluoromethyl imide is likely to be a more stable species due to the NH being less electron dense.
However, when we apply these same derivatization techniques to erucamide and oleamide, the use of acetyl chloride will give HCl as a by-product. HCl will readily undergo an alkene addition reaction at the C=C bond. As there are no functional groups nearby to impart steric or electronic effects, we can expect a 50:50 mixture of the two HCl addition products.
This will greatly complicate the mass spec interpretation as it will increase the number of species and also increase the mass of those species on a compound that has already proved hard to elute. The by- product of acetic anhydride is acetic acid and this will not be acidic enough to readily undergo and addition to an alkene. Therefore it should be far simpler to stick with acetic anhydride or trifluoroacetic anhydride for derivatization of all three compounds.