the computer bulletin board
a
b
Figure 2. Comparison between a single scan spectrum of ethyl acetate (a)and a 64 scan cross-correlatedspectrum of the same product (b) The equation that rules ringing in the time domain has the following form:
where i is the square root of -1,b is the sweep rate in radians per second, and t is the time in seconds. To cross-correlate this function with the spectrum, the latter must be taken to the time domain. This is done by an inverse Fourier transform of the spectrum in the frequency domain. The spectrum in the time domain can be filtered with well-known FT NMR apodization functions, thus improving sensitivity or resolution (16).ARer crosscorrelation (complex multiplication of the two functions in the time domain), data are taken again to the frequency domain by performing a direct Fourier transform. A single scan CW spectrum and a multiple scan cross-correlated spectrum are compared in the Figure 2. Conclusions With a small investment, simple electronic circuits and some programming, an old CW NMR machine was transformed into a powerful instrument for simple routine analysis. Since all the data are in digital form, spectra can be stored in the computer disk to be recalled when needed. Simple program routines make integration, J calculations and ~ e a assimments k vew easv in com~arisonwith the origihal machlhees h he work described here can be easilv adaoted to other CW NMR machines. as well a s d i f f e r e n t " ~ ~ /hoards r j ~ and computer languages other than Turbo Pascal 6.0. 'Author to whom correspondence should be addressed. (Note to editor: this author's address has changed and will change again. A longterm address will be provided at an appropriate later time.) 'Current address: Department of Chemistry,Corneli University. Ithaca, New York 14853 A316
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Acknowledgment The authors wish to acknowledze the s u ~ ~ oofrthe t CEC through Avant C11.0317.U. We &o thank'the invaluable advice of Hector Casal from British Petroleum Research (US).
Interpretation of Mass Spectra: A Direct Learning Approach through Software Thomas ~ e h m a n 'and Grigory vagenin2 Bethel College North Newton, KS 67117 Interpretation of mass spectra is best learned by working with many spectra rather than with a text that offers numerous generalizations illustrated by a limited set of spectra. Software available from the National Institute of Standards and Technology (NIST) makes this possible in an attractive format at no cost. Students can peruse 735 carefully chosen spectra on a demonstration disk that uses the same menu-driven display commands as the library of 62.000 mectra develooed for eeneral scientific use. Both graphical and tabular displays are available for each compound, which is identified on screen by formula and name. The structure also appears for all but the largest molecules. Synonymous names are given for each compound; for aspirin the list exceeds 100 entries because of its commercialization. The instructional utility of the software derives primarily from its flexibility in calling up specific spectra, or groups of spectra for simultaneous display. Asearch can be made by formula, by molecular weight, by reference number, by a particular peak in the spectrum, or by a sequence of specifications of molecular weight, name fragment, allowed elements, numbers of atoms of these elements, and up to ten specified peaks. Up to five spectra can be dis-
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Figure 3. Electron ionization mass spectra of four C,H,,02 isomers. The absence of a molecular ion under these conditions IS a common trait of esters. The ion at masslcharqe ratio 43 is the commonest one in electron ionization spectra. played simultaneously along the same horizontal axis, so that similarities and differences are seen a t once. This is well illustrated by Figure 3, which shows the spectra of four acetic acid esters, all of them C6H1202 isomers. The spectra accomodate varied instructional uses. Amone these are a simple introduction to mass spectra and isotopes for general ;hemistry students, a broad kxamination of spectral features resulting from particular organic fnnctibnal groups, much data chat ph&cal chemist r y s t u d e n t s c a n examine w i t h t h e enereetics of fragmentation in mind, and a substantial set of c~mpounds of current interest in biomedical mass spectrometry. All modern electron ionization mass spectrometers offer a lihrary of spectra as part of the software, so that the student who becomes a competent user of this library gains valuable experience. Questions such a s the following are easily answered by examining spectra or by conducting a simple search: (1) What are the commonest fragment ions? (2) What are the commonest neutral losses?
(3) Which mmpaund groups are Likely to have spectra Lacking
a molecular ion? For example, do the axiranes, with strained rings, survive ionization to produce molecular ions? (4) How useful is mass spectrometry in distinguishing isamers? ( 5 ) What isotope patterns are helpful in determining molecular composition? ( 6 ) Haw do the fragmentions of aliphatic and aromatic rings compare? (7) Does a substituted benzene ring usually survive ionization? (8) What fragmentation pattern indicates a long aliphatic hydrocarbon chain? One of the first steps taken to infer the com~ositionof a substance from its mass spectrum is to look for patterns of the common isotopes. Chlorine is the prime examole: the patterns due to one, two, and three chiorines in the same ion can all be seen in the spectrum of C,&C18. Bromine, with two isotopes of almost equal abundance separated by two mass units, is another easily recognized indicator of (Continuedon nextpage)
Volume 70 Number 12 December 1993
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the computer bulletin board composition. More subtle, but no less important, is the wntribution of 13C a t a mass one unit heavier than the molecular ion. Because hydrogen and oxygen, the other elements most common to mass spectrosco y, have heavy isotopes that are much less wmmon than C, the size of this peak oRen gives a good estimate of the number of carbon atoms
A
resent. The Help screen declares that "Molecular weights are integers calculated using the most abundant isotope of each element." This is correct for low-resolution mass spectrometry but not for chemistry in general. The difference appears, for example, when the mass spectrometrist assigns an integer molecnlar weight of 84 to CHIC^.. In high-resolution mass spectrometrfthe molecular &eight is the sum of the precise masses of the most abundant isotopes. l'hrie features of the software require further comment. The first is that all the spectra were produced as a result of ionization by electrons accelerated to 70 eV. This is longestablished standard practice, but it yields a surprising number of spectra lack&! a molecular con. The exp&meG tal response is to ionize with less energetic electrons, or to produce ions by other means. of course the software user does not have these options. Though many sclentlsts rely on mass spectrometjto obtain molecular weights, the beginning student may not appreciate what the technique can do. A second feature is that all spectra are shown at a resolution of one mass unit. This overlooks oeaks a t half-integral masses due to the double ionization of an ion of odd mass. But such peaks are seldom seen, and never intense, so the loss of information is negligible. Finally, the program amply illustrates why mass spectrometry is such a powerful analytical tool when placed in tandem with separatory techniques. Amass spectrum is a rich source of information that leads to positive identification of a minute amount of a pure substance in the vast majority of cases. This software enables the user to begin to interpret mass spectra and to develop essentially any level of proficiency. The demonstration disk, Demo Version 4.0, can be obtained from Joan Sauerwein. National Institute of Standards and Technology, ~ i a n d a r dReference Data, 221lA320. Gaithersbure. MI) 20899. Hardware reauirements arb a hard disk,-i.3 MByte of storage, an 8b286, 80386, or 80486 PC with VGA, EGA, CGA or Hercules graphics and 640 K of RAM. Literature Cited
m i n i C m i n g s : Wokingham, England, 1990. 6. Wieder. S.Intmduetion to MalhCAD for Seiantiste ~d En-7s:
Maraw-Hill:
..... ....-.... 7. Donnelly, D. MdhCAD for Introduclor, Physics; Addisan-Wesley: Wokingham, End a n d 1992 - ~
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8. Nicholson, R. 5.;Shsin, I.And. Ckem 1884,36,706. 9. Camaeho, L.: Ruiz, J . J. J.Eketmno1. C k m . 1883,158, 341. lo. T60NMR Spedmmtor System Manual; V&sn Inc.: Palo Alto, CaMomia, 1971. 11th ~ ed.; ~ , ~~t~ ti^^ he.: M ~ ~ I ~Mo ~Wo I, ~1991. , 11. m 8 0 1 senesM U ~ U 12. Hornwit& P.: Hill, W. TheA?tofEkefmnCs, k d ed.; Csrnbtidge University Press: Cambridge, 1989. 13. Barjat, H.: Belton, P.S.: Doodfellow, B. J.Anolyaf 1%. 118.73-17. 14. Dadok, J.;Spreeher,R. F J . Mog.Ras 1914.13, %248. 15. Gupta, R.K, Ferretti, J. A: Becker, E. D. J. Mog &s. 1974. IS, 27%290. 16. Hofinan, R. E.; Lwy,G. C. PmgmssNMR Sprrfmscopy 1991,23,211-258.
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