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.

Gas chromatographic-mass spectrometric characterization of thebaol, an opium constituent, and its structural analogs.

A GC-MS method is described for the characterization of thebaol, a component of opium poppy. The method includes preliminary sample derivatization to TMS, TBDMS, TFA, PFP and HFB substituted products. Fragmentation of resulting derivatives is unique under electron ionization, and proceeds via consecutive loss of two radicals that violate the "even-electron rule". Peaks of [M-2CH3]+. and [M-C4H9-CH3]+. ions show maximum intensities in the spectra of trimethyl- and tert-butyldimethylsilyl-thebaols. Elimination of perfluoroalkyl and methyl radicals from M+. is characteristic for TFA, PFP and HFB thebaols. The same fragmentation peculiarity is characteristic for derivatives prepared from related natural compounds containing vicinal 2-methoxyphenol moieties. The unique fragmentation of trialkylsilyl and perfluoroacyl derivatives of thebaol can be successfully used for thebaol determination within complex mixtures. This is part 4 from the series "Analytical derivatives in mass spectrometry", parts 1, 2 and 3 see [1-3].

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.

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

The central mission for the development of the National Institute of Standards and Technology/National Institutes of Health/Environmental Protection Agency Mass Spectral Library is the acquisition of reference gas chromatography-mass spectrometry data for important compounds and their chemical modification products. The addition of reliable reference data of various derivatives of amino acids to The Library, and the study of their behavior under electron ionization conditions may be useful for their identification, structure elucidation and a better understanding of the data obtained when the same derivatives are subjected to other ionization methods. N-Alkyl-N-perfluoroacyl derivatives of amino acids readily produce previously unreported alkylnitrilium cations of composition [HC≡N-alkyl](+). Homologous [HC≡N-aryl](+) cations are typical for corresponding N-aryl analogs. The formation of other ions characteristic for these derivatives involves oxygen rearrangement giving rise to ions [C(n)F(2n+1)-C≡N(+)C(n)H(2n+1)] and [CnF(2n+1)-C≡N(+)-aryl]. The introduction of an N-benzyl substituent in a molecule favors a process producing benzylidene iminium cations. L-Threonine and L-cysteine derivatives exhibit more fragmentation pathways not typical for other α-amino acids; additionally, the N(ω)- amino group in L-lysine directs the dissociation process and provides structural information on the substitution at the amino functions in the molecule.

Electron ionization mass spectra of alkylated sulfabenzamides.

Mono-, di- and trialkyl derivatives of 'sulfabenzamide' (N-4-aminophenylsulfonylbenzamide) have been prepared and their electron ionization (EI) mass spectra examined. It is found that the fragmentation of N-alkylsulfabenzamides (alkyl = CH(3) to n-C(5)H(11)) proceeds via a very specific rearrangement process. The proposed mechanism involves an intermediate formation of distonic molecular ions, and the driving force for this process is the formation of stable N-alkylphenylcyanide cations [R-N(+)≡CC(6)H(5)]. The findings are confirmed by exact mass measurements, tandem mass spectrometry (MS/MS) experiments and deuterium labeling.

Unique para-effect in electron ionization mass spectra of bis(perfluoroacyl) derivatives of bifunctional aminobenzenes.

A new kind of 'para-effect' under electron ionization (EI) conditions has been discovered for a series of bis(perfluoroacyl) derivatives of o-, m- and p-phenylenediamines, -hydroxybenzeneamines and -mercaptobenzeneamines of a common structure RCOX-C(6)H(4)-NHCOR (X = NH, S, O; R = CF(3), C(2)F(5), C(3)F(7)). Only the para-isomers showed successive loss of a radical RCO* and a molecule RCN, leading to very intense peaks in the EI spectra. The composition and the origin of the [M-COR-NCR](+) ions were confirmed by exact mass measurements and linked scan experiments. The proposed mechanism of their formation takes into account likely para-quinoid structures of the precursor ions. A similar rearrangement has not been observed for para-isomers in the series of bis(perfluoroacyl) derivatives of benzenediols, mercaptophenols and dimercaptobenzenes.