Determination of the atomic composition of isobaric ions in the mass-spectrometric study of the L-asparagine monohydrate

Abstract

Investigation of the mechanisms of structural changes in biomolecules caused by their interaction with electron beams is an important task [1], since these molecules are highly sensitive to ionizing radia tion and this knowledge is of basic significance for solving problems of mutagenesis and radiation protec tion. Amino acids are among the simplest biologically relevant organic molecules and serve as convenient model systems for the investigation of radiation induced damage. L asparagine (L Asn) belongs to the class of the 20 most common amino acids and is stable in both anhydrous and hydrated states under standard conditions. In recent years, the crystalline L aspar agine monohydrate C 4 H 8 N 2 O 3 · H 2 O (LAM) and its complexes have been extensively studied as new potential nonlinear optical materials [2][3][4][5], which implies the need for evaluating the thermal stability and radiation resistance of this compound under working conditions. The data on the dissociative ionization of amino acids are very restricted and rather contradictory, which is mostly related to the fact that these biomole cules undergo fragmentation under experimental con ditions, due to both the interaction of molecules with incident electrons during dissociative ionization and their thermal destruction upon heating, which is nec essary for sample conversion into the gaseous state.This Letter presents the first results of our experi mental investigation of the mass spectrum of LAM ionized by electron impact, with an attempt at exactly determining the atomic composition of the fragment ions within the mass range of m/z = 12-46 amu.The charged products of the fragmentation of the LAM molecules under electron impact were studied by the method of mass spectrometry that is capable of both identifying the interaction products and tracing the dynamics of their formation. The measurements were performed with a commercial magnetic mass spectrometer of the MI 1201 type, which was signifi cantly modified by linking to an IBM PC with a digital system of mass spectrum sweep and data acquisition control [6]. The modified mass spectrometer exhibited high temporal stability detection (zero level drift, 1 mV/h) and high sensitivity of the ion current detection (detection threshold, ~10 -16 A). A digitized scheme of pulsed sweep control ensured scanning of the mass spectrum at a minimum step of Δm 0 .0001 amu. The mass spectra were obtained at a con stant electron energy of 70 eV and recorded using both an analog channel of ion current measurements (spec tral sweeps) and a digital channel with point by point scanning at a 0.004 amu step and the input signal inte gration time of 0.1 s.The typical normalized mass spectrum of LAM in the 10-90 amu range measured at an ion source tem perature of 160°C is presented in the figure. The ion yield (relative abundance) is plotted on a semilogarith mic scale because the peak intensities range within two orders of magnitude. As can be seen, the mass spec trum consists of separate groups of peaks, each occu pying an ...