Harold M. Bell Virginia Polytechnic Institute and State University Blocksburg, 24061
II
Computer Ancdysis of hotope Clusters in Mass Spettrometry
In teaching mass spectrometry, considerable emphasis is placed on finding elemental compositions corresponding to a measured ion mass. Computer programs have been written to list all compositions consistent with an experimentally observed mass value (I), and tables of mass values as a function of elemental composition are available (2). Another subject which is usually treated in some detail is the determination of relative ahundances of the various isotopic species corresponding to a given elemental composition. The elementary probability theory involved in this manipulation has been discussed by a number of authors (3-5). Each of these subjects is important in its own right. However, in the analysis of mass spectra they should not he treated separately. Too often students are advised first to find all possible compositions of the ion in question, using the first peak of the isotope cluster, and then in an entirelv seoarate manipulation check to determine which of these formulas shows the correct intensity (probability) distribution across the entire cluster. While the procedure works, it does have some drawbacks. It can be very time consuming, hoth for inexperienced students confronted with simple systems and for researchers working with systems having several multi-isotope elements. Also, it is somewhat difficult to incorporate the experimental uncertainty in the peak intensity measurements into this pmcedwe. In an effort to avoid these difficulties a computer program has been written which combines the two procedures of formula determination from a single mass and verification by isotope cluster analysis. The program thus calculates formulas corresponding not to a single ion, hut rather to the entire ion cluster. Apart from the obvious saving of time, the program has other decided advantages. Most of the elements found in organic molecules are taken into consideration, and others could easily he added. The current list includes carbon, hydrogen, nitrogen, oxygen, sulfur, and the halogens. Also, the program allows for inclusion of experimental uncertainty in hoth the mass and intensity measurements. It is written hoth in Focal for the PDP-8 family of computers, and in Fortran for an IBM system with time sharing option (TSO) through a remote terminal.' The following data are required: (1) the maximum number of atoms of each element to he allowed in a single compo~ition;~ (2) the measured mass of the first (lowmass) peak in the cluster; (3) the maximum allowable difference between the measured mass and the calculated mass; (4) intensity data for each peak in the cluster, and the maximum allowable difference between the measured intensity and the calculated intensity. A maximum of 20 peaks, spanning 19 amu is allowed. The oroeram first calculates an elemental comnosition . corresponding to the measured mass of the first peak on the cluster. The loeic used is verv similar to that used by Shrader (I). The iSotope cluster Lorresponding to this ele-
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mental composition is then calculated using elementary orohabilitv theorv (3-5) and intensities are compared with those expkmen~al'lydetermined. If the intensity of every ~ e a kin the calculated cluster agrees with the corres~onding experimentally observed peak, within the aliowed error, the composition is listed as successfully meeting the input requirements. Otherwise the composition is listed, hut flagged as being impossible. The program continues through this procedure until all of the comhinations of the elements listed above, within the specified limits, are exhausted. A typical run with the Focal version of the program is shown in the figure. Applications The ability of the program to rapidly perform calculations which at best can he described as tedious and uninteresting should make it of value in problem solving situations, hoth in the teaching and research laboratory. Also, for students who are inexperienced in mass spectrometry the program can he used as an efficient teaching tool. For example, it is easily shown that the number of elemental combinations corresponding to a given mass increases dramatically hoth with the number of allowed elements and with increasing mass. The power of high resolution mass spectrometry can he demonstrated by reducing the uncertainty of the measured mass from about 0.2 amu to 0.001 amu, and observing the decrease in the number of elemental combinations which fit the input data. Lastly, the number of allowed elemental comhinations can be studied as a function of the uncertainty in peak intensity measurements. Literature Cited (11 Shrader. SLeohen R.. ''Infroduelory Mass Soectrometrv." . Allvn . and Bacon. Inc. R o a m 19il. p.233. (2) Loderberg, Joshual. CHEMEDUC..49,613(L9721. 131 Bonyon. J. H.. "Mars Specfromefry i n d Its Applications to Owsnic Chemistry;' Elrevier. NewYork. 1160. p. 291. 141 Margraue,J.L., endPolanrky,R.R.. 1,CHEM.EDUC., 31,335 (19621 151 Hugenfoblar, E.. and Lolipar. J.. J. CHEM. EDUC.. d9.610(19721.
WIX NO OF ATOMS
X120 F20
548
/ Journal of Chemical Education
5:2
0:4
BR:0
110
MASS OF F I R S T PEAK I N CLUSTER :76. MASS OF LAST PEAK IN CLUSTER :79.
ERROR I N THESE READINGS :.2 MASS INTENSITY ERROR 76 166 :2 77 :3 il 78 :3 :I 79 :0 :.5
C
H
6 5 3 2 1 3 2 I
'The program is available upon request. It could be easily modified for use on any computer system supporting Fortran, with or without remote terminal. ZA savings in time is realized if the maxima are kept realistically low.
N:O CL:0
1
0
8
S
0
R 4
0 0 0
4
0
0 8 4 0 0
0 0 0 0 0
0 1 2 3 4
0
I 2
0
0 0 0 0 0 1 1 1 2
F
0 0 0
0 0 0 0 0 0
CL BR I 0 0 0 0 0 0 W 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
WAS5
ERROR
76.031 75.995 76.053
0.0313 -0.0051 0.8525 6.8160 -0.0204 0.0347 -0.0017 -0.0381 -0.0559
76.816
75.980 76.035 75.998 75.962 75.944
CLUSTER OK NO NO
NO NO
NO YES YES
NO NO
Sample of isotope cluster analysis using the Focal program, with CJHsS as the "unknown." Numbers following the colons were provided by the operator: all other numbers were computer printed.
