Interaction of Dichloromethane Solvent with n-Alkylamines Analyzed by Electron Ionization GC–MS

Mar 01, 2010

Additional data were needed if an unambiguous structure was to be assigned. The solution was then analyzed using APCI and direct infusion into a mass spectrometer that had sufficient resolving power to produce an accurate mass. A number of peaks representing the protonated stearylamine and protonated acetonitrile adducts were observed, but one peak stood out because it was at an even m/z value greater than that corresponding to the mass of the protonated stearylamine and did not appear to represent an adduct of stearylamine; therefore, possibly representing an ion containing only a single atom of nitrogen. This peak was at a nominal m/z value of 282, which meant that the unknown has a nominal mass 12 Da higher than that of stearylamine (281 Da for the unknown compared to 269 Da for stearylamine). It also meant that the peak at m/z 280 in the mass spectrum represented an [M – H]+ ion.


Figure 6
The accurate mass measurement resulted in an elemental composition of C19H39N for the unknown. A rings-plus-double-bonds calculation for this elemental composition gave a value of one. There are at least two proposed structures that conform to this elemental composition and R + Db results; one is n-C17H35–CH2–N=CH2, (a methylimine) and the other is a substituted pyrrolidine. There are no mass spectra of n-alkyl methylimines in the NIST08 Database; however, the database does contain spectra of several alkyl-substituted pyrrolidines. If the unknown is an n-alkyl-substituted pyrrolidine, based upon the number of carbon atoms present in the unknown and the fact that the unknown is most likely a result of stearylamine, the best candidate would be an n-alkyl methyl pyrrolidine. There are spectra of both 2-methyl-5-n-alkyl pyrrolidines (Figure 6) and 1-methyl-2-n-alkyl pyrrolidines (Figure 7). The mass spectra of 1 methyl-2-n-decyl pyrrolidine and 1-methyl-2-n-dodecyl pyrrolidine in the NIST08 Database had relatively low NIST Numbers (10,695 and 12,300, respectively) indicating that the spectra were from some of the original EPA/NIH data used in the initial formation of the database, meaning that the quality may not be as high as more recently acquired spectra. The NIST Number is a sequential number assigned to every spectrum when it is entered into the NIST Mass Spectral Archive and currently stands at 366,127. The inclusion of these spectra in the database is warranted because there are no other spectra of these compounds, and there are no obvious errors or unexplainable peaks. The compounds are not commercially available, therefore, remeasurement would require synthesis of the compounds. Based upon a lack of synonyms and inclusion of these compounds in other databases, they are not of particular significance and likely to be encountered in analyses of unknowns; therefore, there is no current motivation to remeasure these spectra.


Figure 7
Although there are no spectra in the NIST08 Database of methylimines, the mass spectra n-alkyl nitriles (R–C≡N) show good intensities of peaks representing EE+ ions containing a single atom of nitrogen every 14 m/z units from [M – 15]+ down. This supports the rationale that the peaks in the spectrum of the unknown represent ions that have retained the single atom of nitrogen.


Figure 8
The software was used to determine the accurate mass and elemental composition of the ion represented by the nominal m/z 84, m/z 85, m/z 126 peaks. The m/z 84 ion has an elemental composition of C5H10N and a proposed structure +CH2CH2CH2CH2–N=CH2. The accurate mass determination based on the peak at m/z 85 resulted in an elemental composition of C5H11N, an OE+• ion possibly formed according to the mechanism shown in Figure 8.

The peak at m/z 280 in the mass spectrum of the unknown does represent an EE+ fragment ion formed from the molecular ion; not by the loss of a methyl radical, as had originally been speculated, but by the loss of a hydrogen radical. The molecular ion peak is present in the mass spectrum of the unknown but cannot be distinguished from the carbon-13 isotope peak of the [M – 1]+ peak. Based upon all the data, the impurity is stearyl methylimine. The spectra of the unknown chromatographic peaks associated with the other five n-alkylamines were identical to the spectrum identified as stearyl methylimine with the exception of the m/z value of the [M – H]+ peak.


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