Mass spectrometry of analytical derivatives. 1. Cyanide cations in the spectra of N- alkyl-N-perfluoroacyl- α-amino acids and their methyl esters

Unusual Regularity in GC Retention of Simple Amino Acid Derivatives

BACKGROUND

Application of simple regularities and general principles along with direct use of reference gas chromatography retention index data for reliable structure determination of compounds can be enhanced by determination of new regularities that are specific to certain structural elements.

OBJECTIVE

Revelation and interpretation of an anomaly in the elution order of alkyl esters of alkoxycarbonyl derivatives of glycine and alanine on standard and semi-standard non-polar phases.

METHOD

Preliminary derivatization of amino acids to alkyl esters of N-alkoxycarbonyl analogs and interpretation of their gas chromatographic characteristics.

RESULTS

Alkyl esters of N-alkoxycarbonyl derivatives of alanine (Alkyl = C₂H₅, n- and iso-C₃H₇) elute prior to the same derivatives of glycine, despite the presence of an additional methyl group at C⁽²⁾ in the molecule. Elution order is reversed for methyl esters of N-methoxycarbonyl derivatives.

CONCLUSION

It is established that the peculiar behavior of alkyl esters of N-alkoxycarbonyl derivatives of glycine and alanine agrees with the concepts of gas chromatography and the known retention index regularities of organic compounds. A decrease of retention index values is a result of an introduction of an additional methyl group to a carbon atom connected to two polar fragments in a molecule like CH₂XY. The dependence of the difference of retention index values for homologs of the types of CH₃-CHXY and CH₂XY vs. the total mass of fragments (X + Y) is similar to those for other sub-groups of analytes.

Mass spectrometry of analytical derivatives. 2. "Ortho" and "Para" effects in electron ionization mass spectra of derivatives of hydroxy, mercapto and amino benzoic acids

Derivatives requiring either anhydrous or aqueous reaction conditions were prepared for robust and reliable gas chromatography/mass spectrometry (GC/MS) characterization of hydroxyl, mercapto, and amino benzoic acids Methylation and trialkylsilytation are employed for blocking the acidic function. Alkyl, trimethylsilyl, acetyl, perfluoroacyl and alkoxycarbonyl derivatization groups are introduced to hydroxyl, mercapto and amino functions. The electron ionization induced fragmentation characteristics of corresponding derivatives are explained by comparing the MS¹ spectra of unlabeled compounds to their ²H and ¹³C labeled analogs, and analysis of collision-induced dissociation data from MS² spectra. Competing fragmentation alternatives are identified and specific decomposition processes are detailed that characterize (a) ortho isomers due to interaction or vicinal functional substituents and (b) para isomers prone to forming para quinoid type structures. Skeletal and hydrogen rearrangements typical for methyl benzoates and the blocking groups are considered when discussing diagnostically important ions. Characteristic ions produced as a result of rearrangements in ortho isomers are classified, and skeletal rearrangements required to produce para quinoid type ions specific for para isomers are noted. Key ions for structure elucidation and differentiation of isomers for derivatives of substituted benzoic acids by GC/MS are suggested.