Mass Spectrometry of Analytical Derivatives. 1. Cyanide Cations in the Spectra of N-Alkyl-N-Perfluoroacyl-α-Amino Acids and Their Methyl Esters

Abstract

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 [CnF2n+1–C≡N+–CnH2n+1] and [CnF2n+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 Nω-amino group in l-lysine directs the dissociation process and provides structural information on the substitution at the amino functions in the molecule.

Unusual Regularity in GC Retention of Simple Amino Acid Derivatives

“…However, the corresponding ΔRI value is not used for regression and this point is presented only as a comparison. In case of methyl esters of N-methyl-N-trifluoroacetyl-glycine ( IX , RI = 1016) and alanine ( X , RI = 1042) all acidic and amine hydrogen atoms are substituted [2, 18]. These derivatives do not demonstrate anomaly.…”

Stepwise extraction, chemical modification, GC–MS separation, and determination of amino acids in human plasma#

“…This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. 1 Certain commercial materials and instruments are identified in this paper in order to specify the experimental procedure adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the identified materials are necessarily the best available for the purpose.…”

“…For the selection of a derivatization method, reactions leading to various derivatives, such as trialkylsilyl-AA, alkyl esters of N-alkyl, N-acyl, N-perfluoroacyl and N-methyl-Ntrifluoroacetyl AA, have been considered. All six major criteria for chemical modification reactions must be satisfied for successful application of the derivatization of amino acids: (1) the reactions should be fast, efficient and easily performed, (2) reactions should proceed quantitatively, (3) reactions should yield predominantly one product for each compound, (4) the reaction products should be volatile and stable under GC conditions, (5) reaction products should exhibit good GC properties, and (6) the mass spectra of derivatization products should contain characteristic peaks, and be readily distinguishable. Per the requirements, a reaction of AA with chloroformates was preferable mainly because several functional groups are derivatized in a single step, similar to trialkylsilylation, and additionally the reaction is performed in aqueous media (essential for the analysis of biological samples), and mass spectra of the derivatization products contain peaks of diagnostically important ions usable for structure elucidation.…”

“… # This is part 3 from the series “Analytical derivatives in mass spectrometry”, parts 1, and 2 see .…”

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# This is part 3 from the series "Analytical derivatives in mass spectrometry", parts 1, and 2 see [1,2].A method for isolation of enriched fractions of amino acids from human plasma followed by derivatization, gas chromatography separation and mass spectrometry identification is described. The method involves a stepwise extraction of plasma constituents with the use of two solvents: (a) extraction with methanol yields a concentrate of polyols, urea, carbohydrates and most long-chain saturated and unsaturated aliphatic acids, and (b) further solubilization with water produces mainly a concentrate of amino acids.

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