High Resolution Mass Spectra Analysis with a Programmable Calculator
David K. Holdsworth University of Papua New Guinea Papua New Guinea
The mass ~Dectrometeris used a t low resolution to indicate the molecula; ion (M+) and other fragmentation ions. T h e atomic arrangement ,,fa fragment corresponding to a peak can be determined a t high resolution hy calculating the mass usine accurate atomic masses for 'H. I4N, and I f i 0 based on 12C = 12.00000; that is, 1.00783, 14.00307, and 15.99491, respectively. For any mass numher the calculation of every possihle comhination of C, H, N, and 0 together with the accurate molecular mass is a very tedious exercise. Impossible comhinations of atoms and omissions are the commonest errors. A program has been descrihed recently ( I ) in which a pocket-sized programmable calcr~latoris used to calculate comhinations of C, H, and 0 for a given mass. Programs have now been written for a pocket calculator, such as the ~ ~ ~ l ~ t t ~. pp . ~2 9~instrument ~k ~ ~ d(98 pro. crammable steps), to analyze mass spectra data in a laboratow or classmom where a computer is not available. ~h~ programs are designed to 1) Calculateand display all comhinations of C, H, N,and 0 for agiven mass, immediately displaying the high resolution mass for each formula hawd on accurate atomic masses of I2C, 'H, I4N, and 'Q. 2) Predict whether the formula displayed represents a stahle molecule or molendar ion. If so the numh~rof douhle honds and rings in the molecule are shown to assist assigning molecular structures. :31 Calculate and display the approximate peremtaw isotopic peaks M + 1, M 2, and the ratio M + IIM 2. 41 Analyge metastable peaks in the mass spectrum to elucidate ion decompasifion pathwa.ys. Ion Formulas and Their Accurate Masses T h e first program is structured as follows:
+
+
must he at least
atom in the co,npoond or fragment ion and the maximum number of atoms is the
integer portion of ion massll2. (h) The numlw 01. hydrogen stoms displayed (NH)must be no mare than 2Nc + N N 2, whereNcand N ~ a r thenumherofcarhon e and nitrogen atoms in the formula.
+
(cl NNcan he given a maximum value, if this is known (dl NOdoes not exceed 2Nr + NN+ 2. (el The instrument displaw "Error" t o end the program when all dC,H. N, and 0 with theiraccuratemass p.wsihlen~mhinati~ms
values have heen displayed. The high resolution mass value enahles the correct formula of a molecular or fragment ion to he determined immediately. Occasionally the masses of two ions are too close to be distinguished reliably hy a high resolution instrument. In such a case the ions can he distinguished by examining their isotopic peaks (program 3). Double Bonds and Rings A short program can he written for a calculator to display a numher which represents the numher of douhle bonds and rings in a molecule, equal to ' h ( 2 ~ c NH N N 2). his is not only a useful aid in drawing molecular structures hut also it can serve as a diagnostic test of formulas which ohey the nitrogen rule ( 2 ) and can represent stahle molecules or molecular ions (M+). These formulas display a whole numher while those that represent fragmentation ions only do not.
+
+
Isotopic Abundance Peaks Molecular formulas can also he determined hy isotopic abundance measurement, This is especially useful when twr, have accurate measurements too close to he distinguished reliahly by a high resolution mass spectrometer. Small peaks are f()und in a spectrum a t masses higher than that of the molecular ion M+, since carbon, hydrogen, nitrogen, and oxygen contain small amounts of l:+C,W, liN, liO, and "0 isotopes, respectively. These low intensity peaks are the first and second isotopic peaks (M 1 ) and (M 2). T h e abundance of these heavy isotopic molecule-ions can he calculated from a knowledge of the natural abundance of the atomic isotopes (3).A short calculator program can he written t o display the approximate Percentage values of (M + I ), (M 2) and the ratio (M l)/(M 2), assuming ' N makes a contrihution to ( M + I ) , IHOto (M 2), ':'C and 'H to both.
+
+
+
+
+
+
Volume 57. Number 2. February 7980 I 99
Table 1. formula of ion
accurate
mass value
Data Displayed by Calculator fEntrv:59\ double approximale % of Mt + bonds rings (M 1) (M 2 ) ratio -
+
+
+
M+?
59.0372, it follows that its formula must be CzHsNO with one double bond, possibly acetamide, acetaldoxime or N-methyl formamide. The correct formula could also have been found from the ratio of the second and first isotopic peaks. The structure of the compound could be deduced by analyzing the fragmentation peaks of the spectrum or the nmr spectrum. Metastable Ions
Table 2. mass observed
,
Table 3.
ComDarison of Ion Peaks
mass calculated
composition of ion
Assignment of Observed Metastable Peaks
entrv
m.
dis~lav
ml
+
+
m2
neutral fragment
Metastable peaks m * appear in a spectrum as broad peaks of low intensity a t an integral or non-integral value of mle. They are formed when an ion,of initial mass m l enters the acceleratine field but dissociates into a neutral fraement and ach,~rgedrinnl ma-. m.. aiter full nr, t-lcration and h~.furt, lrc~ny rlrflrrtni In the mwnct1~f ~ d dT h e rcl:itionshir~herwen m 1. m a and mi is approximately m * = (m2)2/ml: The neutral fragment lost is normally a stable molecule or radical. Some common losses are as fi~llows(with integer masses) H (I), H z (21, CH:I (15), H z 0 (la), C O and C2H4 (28), C 2 H i and CHO (29), CRHG( 4 2 , C R H(43) ~ etc. In practice, only the measured value of m * is known. Finding values of m, and mz whichgive the solution of the equation nearest the value is a matter of trial and error. Nomograms have been devised to assist, using logarithmic scales of m l , mz and m * (4). A perspex rule is used to connect many trial values of m l and m z which cut the m * scale near the observed value. A rapid method of linking m t , m 2 and m * has been devised usinp a orouammed calculator. The value of m * for the met&table Gm is stored and the operator keys in the values of strong mass fragmentation peaks observed in a spectrum ( m l ) . The instrument first displays a calculated value of my, then the mass value of the neutral fragment (m2 - m t ) . The urobable fraementation oathwav is indicated when the calculated value of m2 is a near integer corresponding to another strone k the soectrum and the neutral fragment value ..~. e a in corresponds to that of a stable molecule or radical. Calculator analysis of a mass spectrum
An alkaloid virosecurinine. isolated from the leaves of Securinega uiroso ( 5 ) growing in Papua New Guinea, showed maior mass s ~ e c t r a oeaks l a t m l e 217 (M+).. 134.106.84.78. . . . . 77, 56, and 55. Precise masses were found using a high resoTable 4.
Fragmentation Pathways Indicated by Metastable Peak
metast.2ble
fragmentation
83.9 76.1
CsHe02' CsHsi C,H,O+ CsHmNi CrH.nNt
57.5 53.4 37.3
--A
pathway
C,H,O+ CeHst GH,+ CsHIt C.HsNt
+ co f H + co
+ NHS + CaHa
The actual ratio can he ohtained from the mass spectrum and a comparison with the calculator data can be used to decide the correct formula. Table 1 shows the data displayed by the calculator for a mass peak of 59. The time taken to display all the possible formulas and the corresponding accurate mass values was 85 sec. This was found to he quicker than the operator could tabulate the information. For a compound whose mnlecular ion Mt was found to be
100 1
Journal of Chemical Education
lution mass spectrometer and were compared with data obtained from the calculator to obtain the following unequivocal assignments. Metastable peaks were observed a t m l e 83.9,76.1,57.5,53.4, and 37.3. Calculator analysis of metastable peak values with major fragmentation peak values led to the following assignments. Using data from Tables 2 and 3, the fragmentation pathways shown in Table 4 were assigned. Full details of the programs and their operations can he ohtained upon request. Literature Cited
. , ?. .. I:,) Dyke. S. F..Fhyd. A. .I.. Sninstwrg. M.. nnci l'heilhald. R. S , "Orynnir Sperinwupv: An intndurlirm~Penguin Hoskr I.Ld. IHnrmsndiworth. Enyiandl. 1971. pp. 165-
