ISSN 10637850, Technical Physics Letters, 2012, Vol. 38, No. 4, pp. 347–350. © Pleiades Publishing, Ltd., 2012. Original Russian Text © V.S. Vukstich, L.G. Romanova, A.V. Snegursky, 2012, published in Pis’ma v Zhurnal Tekhnicheskoi Fiziki, 2012, Vol. 38, No. 7, pp. 97–103.

Determination of the Atomic Composition of Isobaric Ions in the MassSpectrometric Study of the LAsparagine Monohydrate V. S. Vukstich, L. G. Romanova, and A. V. Snegursky* Institute of Electron Physics, National Academy of Sciences of Ukraine, 88017 Uzhgorod, Ukraine *email: [email protected] Received November 14, 2011

Abstract—The mass spectrum of the Lasparagine monohydrate (C4H8N2O3 · H2O) ionized by the 70eV electron impact has been studied within the m/z = 10–50 amu mass range. A method for determining the most probable atomic composition of the products of fragmentation is suggested. The maximum error of determination of the mass of a fragment ion by this method did not exceed 0.04%. The molecular fragments were analyzed using a commercial MI1201 mass spectrometer linked to an IBM PC with a digital system of mass spectrum sweep and data acquisition control. DOI: 10.1134/S1063785012040153

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. Lasparagine (LAsn) 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 Laspar agine monohydrate C4H8N2O3 · H2O (LAM) and its complexes have been extensively studied as new potential nonlinear optical materials [2–5], 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 MI1201 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 pointbypoint 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 about 8amuwide interval. In the present study, we have determined the exact masses corre sponding to peaks in the intervals of 12–18, 25–32, and 39–46 amu. The NIST database [7] contains no data on the mass spectrum of LAM. In the spectrum of anhydrous LAsn [7], all peaks of significant intensity fall within the 12–90 amu range. A comparison of the mass spec tra of anhydrous LAsn and LAM reveals significant differences in the channels of dissociative ionization of these molecules. In particular, the most intense (100%) peak in the mass spectrum of the anhydrous

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the insufficiently high resolution of the commercial MI1201 mass spectrometer, which did not allow the O O peaks of isobaric ions to be separated. For this reason, H H in order to identify the observed fragments, it was nec H2N essary to ensure the maximum possible accuracy of OH 10 calibration of the mass scale and determination of the O NH2 peak centers. Preliminary calibration of the spectrometer scale in the 10–50 amu mass range was performed with respect 1 to peaks of the following ions with integer masses: + + H2O+ (m/z = 18 amu), N 2 (m/z = 28 amu), O 2 (m/z = 32 amu), and Ar+ (m/z = 40 amu). Then, the 0.1 scale was corrected within narrower intervals with respect to exact masses of the corresponding reference ions. For example, in the 11–19 amu interval, these 0.01 were the C+ (m/z = 12.000 amu) and H2O+ (m/z = 10 20 30 40 50 60 70 80 90 18.010560 amu) ions. The temporal stability of the m/z mass scale upon calibration was checked by recording a series of ten mass spectra with a 20min interval. Normalized mass spectrum of LAM in a 10–90 amu range It was found that the deviations of the positions of the measured at an electron energy of 70 eV and an ion source peak maxima from their average for all seven frag temperature of 160°C. The inset shows the structural for ments in the given mass interval did not exceed mula of the LAM molecule (C4H8N2O3 · H2O, m = ±0.04%. 150 amu). It should be noted that the calibration of the mass scale and the analyzer spectrum sweep were carried LAsn corresponds to m/z = 87 amu, while the relative out only in the digital mode. The lower the masses of intensity of this peak in the mass spectrum of LAM is fragments, the higher the precision at which the frac as small as 0.09%. Significant distinctions are also tional part of their masses can be determined. Digital observed in the interval of 50–70 amu. recording of the mass spectra makes possible both Evidently, the main factor that determines a differ their display and the storage of digitized data on a disk ence in the yields of fragments with the same integer in the F8E4 (X scale) and F7E4 (Y scale) formats with masses from the LAM and LAsn molecules is the the time of each point recording in the hh:mm:ss for presence of a bound water molecule in the LAM struc mat. Thus, the digital system of mass spectrum sweep ture. The absence of a peak at m/z = 87 amu in the and data acquisition control allows the peak positions mass spectrum of LAM gives grounds to suggest that and amplitude to be determined up to within ±1 amu this water molecule forms hydrogen bonds with oxy with the accuracy up to the fourth digit. gen of a carboxy group and hydrogen atoms of the In the analog ion yield recording mode, the data amino group, thus hindering detachment of the car acquisition program retains spectral information only boxyl group. A difference in the temperatures of the for a period of mass spectrum display on the monitor. ion source may be another important factor account The mass spectrum sweep can also be stored on a disk, ing for variations in the peak intensities. but this information will be lost during recording of the The mass spectrum of LAM is characterized by low next spectrum. Thus, one should keep in mind that the selectivity. The presence of six heteroatoms in the ini displayed spectral scans should be processed immedi tial molecule and the existence of numerous fragmen ately upon recording. Separately, in a different graph tation channels complicate identification of the ele ical editor, the online information is only retrieved on mental composition of fragments. In the mass spectra the spectral sweep so that the accuracy of determina of heteroatomic organic molecules, many of the peaks tion of the peak position is lower by an order of mag with integer masses correspond to various possible nitude. In the present study, the mass spectra of LAM were atomic compositions (isobaric ions). In addition, a certain contribution to the observed distribution of recorded and processed in both analog and digital peak intensities may be due to the presence of natural modes in the 11–19, 24.5–32.5, and 38.5–46.5 amu isotopes, which makes it necessary to measure the intervals. Every time, the exact calibration procedure masses of fragment ions with high precision in order to was repeated prior to the next record. Reference was determine their elemental formulas. The knowledge of made with respect to the three pairs of peaks including the exact elemental composition of the fragment ions 12 and 18, 25 and 32, and 39 and 46 amu. In the analog is necessary for determining the channels and mecha mode, the monitor marker was set at the peak top, nisms of the dissociative ionization of molecules. One after which the ion current intensity was measured and difficulty that was encountered in solving this task was the peak position was determined to within ±1 amu Rel. abundance 100

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DETERMINATION OF THE ATOMIC COMPOSITION OF ISOBARIC IONS

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Identification of the atomic composition of the fragment ions in the mass spectrum of LAM (zero value of the mass differ ence corresponds to reference peaks used in the calibration of the mass scale) Integer mass number 12

Fragment ion formula C+ +

13

CH

14

+

N

15

NH+

16

Exact mass, amu

Experimental mass, amu

Mass difference, %

12.0

12.0

0.0

13.0078225

13.0066

+0.009

14.003074

14.0029

+0.002

15.010897

15.0111

–0.001

+

16.0187194

16.0161

+0.016

+

–0.013

H2N

17

H3N

17.0265420

17.0287

18

H2O+ C2H+ + C2 H2 + C2 H3

18.010560

18.010560

0.0

25.007822

25.007822

0.0

26.015645

26.0143

+0.005

27.023467

27.0208

+0.0096

28.018719

28.0146

+0.015

29.026542

29.0196

+0.024

30.034364

30.0243

+0.034

31.98983

31.98983

0.0

39.010897

39.010897

0.0

40.018719

40.0158

+0.008

41.026542

41.0258

+0.002

42.034364

42.0386

–0.010

43.042187

43.0351

+0.023

44.013634

44.0209

–0.017

45.021457

45.0261

–0.010

46.005475

46.005475

25 26 27 28 29 30 32 39 40 41 42 43 44 45 46

CH2N+ CH3N+ CH4N+ + O2 C2HN+ C2H2N+ C2H3N+ C2H4N+ C2H5N+ CH2ON+ CH3ON+ + CH 2 O 2

with the accuracy to the third digit. In the digital mode, the peak intensity and corresponding mass were calculated by centering at the halfheight, which ensured accuracy up to within ±1 amu with the accu racy up to the fourth digit. Since the results of mass determination for a particular peak may depend on the relative contributions of isobaric ions to the total signal and peak shape, the peak contour can be asym metric. For this reason, there always exists some dif ference in the results of processing the digital and analog spectra, whereby the positions of a maximum of an asymmetric contour can deviate from the posi tion of its center. An additional circumstance that facilitated identi fication of fragments in the mass spectrum of LAM with the aid of the digital system of mass spectrum sweep and data acquisition control was that fragments with ion masses within 10–50 amu were represented by three groups of peaks, each occupying an interval about 8 amu wide. This circumstance ensured suffi ciently precise calibration of the mass scale and accu TECHNICAL PHYSICS LETTERS

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rate determination of the centers of peaks. The most probable chemical formula for each particular frag ment ion was selected so as to minimize the difference between the experimental values of masses and the known masses of isobaric ions. The experimental val ues of masses were determined from an analysis of the entire series of spectra recorded in both digital (point by point) and analog (sweep) modes. The results of determination of the elemental com position of the 22 fragment ions of LAM in the 12– 46 amu range are summarized in the table, which indi cates the most probable isobaric ions, the correspond ing exact masses, and the measured values of masses. As can be seen, the difference between the experimen tal values and the calculated masses of isobaric ions reaches 0.034% for a single fragment (m/z = 30 amu) and exceeds 0.02% for only two fragments (m/z = 29 and 43 amu). It can be suggested that these peaks are additive and represent superposition of two or more isobaric ion components.

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It should be noted that the dependence of the yield of the LAM fragment ions on the electron energy (including the appearance thresholds) was not studied in this work. This and the role of the migration of hydrogen atoms, cascade fragmentation, and bound water molecule in the electronimpact and thermal fragmentation will be subjects of subsequent investiga tions.

1. V. S. Vukstich, A. I. Imre, and A. V. Snegursky, Tech. Phys. Lett. 35, 1071 (2009).

3. M. Fleck and A. M. Petrosyan, J. Crystal Growth 312 (15), 2284 (2010). 4. M. Shakir, V. Ganesh, M. A. Wahab, G. Bhagavan narayana, and K. Kishan Rao, Mater. Sci. Eng. 172, 9 (2010). 5. M. Shakir, B. Riscob, K. K. Maurya, V. Ganesh, M. A. Wanab, and G. Bhagavannarayana, J. Crystal Growth 312 (21), 3171 (2010). 6. V. S. Vukstich, A. I. Imre, and A. V. Snegursky, Instr. Experim. Tech. 54, 207 (2011). 7. National Institute of Standards and Technology (NIST). NIST Standard Reference Databases, NIST Chemistry Webbook, http://webbook.nist.gov/chemis try.

2. R. Kripal, I. Mishra, A. K. Gupta, and M. Arora, Spe crochim. Acta A 71 (5), 1969 (2009).

Translated by P. Pozdeev

REFERENCES

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