The Determination of the Natural Abundance of the Isotopes of Chlorine An Introductory Experiment in Mass Spectrometry R e b e c c a M. O'Malley The University of South Florida, Tampa, FL 33620 Technical developments over the last twenty years have resulted in Mass Spectrometry becoming an extremely powerful and versatile analytical technique. Currently it finds aoolications in studies involving biomolecules, petrochemicals, "harmacology, geochemistry, forensic chemistry, and the environment ( I ). In spite of the large number of research articles and reviews dealing with the de\,elopments and diverse applications of the technique, little attention has been given to the teachinr of this rooic. oartirularlv the oractical aspects of it. Those a&les t h a t b a i e been published have beenconcerned with calculator oromams (2-5). instrument simulation (6),or spectra interpretat& (7). A rather specialized interoretive exoeriment on Ion Cvclotron Resonance Mass Spectrometry has also heen published (8). This article describes a laboratory experiment which has been developed as an introduction to the basic principles and experimental technique of Mass Spectrometry for fourth year BSc. students. The ultimate objective of the experiment is the determination of the natural isotopic abundance of chlorine. As background information the students are told that the elemrnt chlorine occurs naturally as the two isotope~:'~CI and "CI, with an approximate abundance ratiu of 31. They are instructed first to obtain the mass spectra of four substanres: Dichloromethane. Trichloruethvlene. Tetrachloro....-- .- - ethylene, and ~richlorotrifluoroethaie (1,1,2-trichloro1.2.2-trifluoroethane). As can he seen. each of these substances chlorine atoms. The isotopic ratio is contains a number determined bv analvsis of the intensitiesof clusters of peaks in the mass spectra arising from ions containing different numbers of chlorine atoms. The choice of this type of experimental determination was felt to he anorooriatelv relevant in that Mass Spectrometry is currentli;sed exteniive~yfor the determination of isotopic ratios (9-11). A particularly interesting example of this application of the technique has been the Mass Spectrometric analysis of the isotopic ratios '5N/1WN,40Ar/"Ar, and IZ9Xe/ '32Xe in the atmosphere surrounding Mars in the Viking mission (12). These isotopic ratios were found to be different from terrestrial values, indicating that Mars had lost part of its atmosphere in the past. T h e mass spectra are run on a commercially available Varian E M 600 Mass Spectrometer. This is a single-focusing, 60' sector, magnetically scanned instrument with a heated inlet and a linear mass readout. I t has a mass range from 4 to about 330 amu with a resolution of approximately 200 (MI AM, 10% Valley). After some initial, individual instruction in the operation of the instrument the students are able to run their own spectra, and with the aid of the experimental and data analysis procedures outlined below, they are usually able to complete the experiment without too much difficulty.
of
Experimental Chemicals
All chemicals used in this experiment are analytical grade and are used without further purification. CAUTION: Some of these chemicals are toxic. It is recom-
mended that reagent bottles be stored in the hood and that samples are loadid into capillary tuhes in the hood, just prior t o proceeding to the Mass Spectrometer. ExperimentalPmcedure
The mas%speetra of the four substances are recurded wing a Varian EM 600 Masa Spectrometer in conjunrriun with n Houston Instruments Omniscrilne Series 2000 X-Y Recorder. All the samples are liquid at room temperature and pressure and are introduced into the instrument using the recommended procedure. A small length of glass capillary tubing containing some of the sample is inserted into the sample "wand," which is threaded into the sample introductioninlet. The wand is oushed carefullv throueh a vacuum seal and the samole enters the infetchamher where the&essure islow (about to&). ~~~Herr it vaporizesand isallowed t u ienk i n t o the ~ & s ~ p e c t r o m r t & source. All spectra are recorded using an ioniring energ).of 7OeV. For mass scale calibration purposes it is found convenient to record the mass spectrum of each substance bath in the presence and absence of a standard substance whose spctnun is well known. (Thisis usually nitrobenzene.) It is important to allow a reasonable amount of oumo-down time between samnles . (at . least 15 min) to orevent coninmihion of the subseouent ~~~. samole . bv. the .oreviok one. Also. since the interpretation otthe data inv01ves measuring the intensliles of the peaks in the spectra, the"Y"cuntrul of the recorder should be adjusted earef$I to ensure that all of the peaks in each spectrum are on scale ~~
~
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~
~~~~~~~~
~~
~~~
~~
~
~~~~
~~
Data Analysis Procedure
For each Hpectrum mle values are assigned for all the major peaks. The chlorine containing clusters of peaks are identified and assigned structural formula%.Theseare checked by addinp. up all the possible isoto~ie ~~.combinations. The intensitv data for e&cluster oeaks is collected in talrle form and then the relative intensities within the clusters arp used to calculate the isotopic distrihtion. ~
of
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lntemretatlon and Data Analysis Typical mass spectra for the four suhstances are shown in Figures 1-4. The interpretation the massspectra of this type of compound is relatively simple. Initially a molecular ion is formed by electron impact: M+e-Mtt2e Then this molecular ion decomposes by a simple bond cleavage reaction to produce a fragment ion, Ft+ and a radical R. (which is usually CI.) M?-F,+tR. Sometimes fragmentation reactions occur in which even electron species are eliminated, e.g., Mt
-
Fzt + HCI
Fragment ions are also capable of decomposing to produce further fraement ions in s i m i i reactions. In some comoounds the fragmentation reactions proceed so rapidly that they dominate the reaction scheme and no molecular ion is observable. Examination of Figure 4 shows this to be the case for 1,1,2-trichloro-1,2,2-trifluoroethane; the highest mass cluster of ions, occurring a t Me 151,153, and 155 corresponds to the loss of CI. from the molecular ion. Volume 59
Number 12 December 1962
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Tetrachloroethylene 48
86
51
47
88 4
4
JI
'
mle
Figure 1. Mass spectrum of dichloromehne.
Figure
Figure 3. Mass spechum of tetrachlomelhylene.
mle
i.Mass specbum of lrichloroelbylene.
Figure 4. Mass specbum of hlchlorotrifluoro&hane (1.1,2-lrichlw~l,2.2-lrifluoroelkane).
The appearance of all of these mass spectra is complicated by the presence of the various numbers of chlorine atoms in the ions. The intensities of the peaks arising from the possible combinations of the isotopes in a particular ion can be calculated using the binomial expansion (13): n(n - l)a("-2)b2 (a + b)" = a" + nn("-l)b + 2!
where a and b are the fractional abundance8 of the two isotopes and n is the number of isotopic atoms present in the ion. A simnle wav to look at this is to take the ratio 3TC1/"7CI to be 1.00/;, theithe relative intensities of the peaks are kx 1:2x:x2
1:3r:3rz:x3 1:4x:6x2:4x3:x4
C D E
1 CI Atom
2 CI Atoms
(1 CL atom containing ion) (2 CI atom containing ion) (3 C1atom containing ion) (4 C1 atom containing ion)
Putting in the approximate values of 1.00/0.33 for 3WlP7CI gives the relative intensities for the peaks in the clusters as illustrated in Figure 5. For convenience the intensities of the peaks have been labelled A,B; C,D,E; F,G,H,J, and K,L,M,NQ. By taking different intensity ratios within a particular cluster, values of x can be obtained e. g.,BIA = x, DIC = 2x, EID = xI2, GIF = 3x, HIG = x, LIK = 4x, MIK = 6x2,MIL = 3/2x, etc. The data obtained from the four mass spectra are summarized in Tables 1-4. For each observed mle value structural formulas have been assigned, and possible decomposition 1074
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Journal of Chemical Education
3 CI Atoms
4 CI Atoms
Figure 5. Appmximale relative intensltles for peaks In ion clusten containing 1. 2.3. and 4 chlwine atoms.
procegses leading to their production have been indicated. The clusters of peaks used to calculate values of x have been labelled, according to the type of cluster, using the ABC notation. A value of r is recorded for each ion cluster. This has been
Table 1. Pos~lbleDecompositi~n Reaction
-
+e
CH&b
-
W&12t
+ CI. CHCI?+
CH2Clf
CCI+
CHICh?
--
Assigned Structural Formula
I
+ 2e
CH&l.?
cJ7c~+
Table 9.
C*Clf
Assigned Smctural Formula
E D C
0.294
B
0.448 121.0a A
-
-
Assigned Struclural Formula
mle
I CoH3'Cir?
136
mle
x
Summary ot Trlchloroethylene Data
Measured Intensiiy and Panem Tvoe
Measured intensity and Panern Type
-
Table 4.
Summary 01 Tetrachloroethylene Data
Possible Decomposition Readinn
12.0 85.0 139.0 54.0
48 47
CJSCI+
Pos~lbleDe~~mposition Reaction
-
88 86 84 51 50 49
CH2CI+
+ HCI
Measured lnlenslty and Pattern T y p
mle
CH2*'CI2? CH2J7CPClt CH23SCi2?
Table 2.
C*Cl,f
Summary 01 DlchloromdhaneData
x
Possible DBC~mposltlon Reaction
x
J
Summary ot TrlchlorotrHluorwthaneData Assign& Slructural Famula
mle
Measured lntenslty and Panern Type
x
+ CI.
Conclusion
obtained by taking all the possible combinations of the intensities within a cluster to obtain values of x, and then averaging these values. For example, taking the CDE cluster a t m/e 151,153,155 in the spectrum of 1,1,2-trichloro-1,2,2 trifluoroethane: DIC = 2x eives x = 0.312, EIC = x 2 gives x = 0.333 and E/D = xi2 gives x = 0.356. Averaging thise three values gives x = 0.334 for this cluster as recorded. Some peak intensities were not used in the analysis and their values are therefore not recorded in the tables. The reason for this was either that the peak intensity was too small to be measured with a reasonable decree of confidence, or overlapping ofadjacent peaks made intensity assignments difficult. Averaging the fourteen values of x recorded in Tables 1-4 gives a mean value for x of 0.336 with a standard deviation of 0.038. Considering the level of sophistication of the instrumentation used, this value compares well with the currently accepted value of 0.325 (14).
This has been used successfullv during four ~ - exneriment ~ ~ academic quarters as part of an instrumental methods course. On successful comnletion of the work, the students understand the basic principles of Mass Spectrometry and are able to operate the instrument to obtain their own mass spectra. In addition, they have a real appreciation of the effect that naturallv occurring isotopes have on the appearance of a mass spectrum. This is &aludble later when they come to interpret the mass spectra of totally unknown substances. Finally, in making the isotopic abundance measurement, they have gained experience in analyzing their own, experimentally obtained data; a process which is essential in experimental science. ~
Acknowledgmem The author wishes to thank all the students who worked through this experiment during its development. Thanks are Volume 59 Number 12 December 1982
1075
also due to R. F. Federspiel for instructing the students in the use of the instrument. Literature Cited (1) Ligon, Jr., Waodfin V.,Scienco.MS, 151 (1979). (2) Attard,A. E., h e , H. C., J. CHEM. EDUC., 55,428 (1978). (3) Moore, W.. Nelson, D., Goldsack, R. J., J. CHEM. EDUC., 55,573 (1978). (4) T u s e h k a , T . , J . c ~E~~~u. c . . 5 4 A 1 7 9(1979). (51 Ho1dsvorth.D. K., J. CHEW.EDUC.,57.99 (1980). (6) Finet, D..J. CHEM E~uc.,57.232(1980).
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(7) Gunt, J. E., J. CHEM. EDuc.,58,511 (1981). (8) Burnier, R. C., Frieser, B. S., J. CHEM.EDVC., 56,687 (1979). (9) ~e ~ d e w, E., Mil~chenhorn,G.,rnt. J. M~~~ spaetrom. and ron P ~ Y S . ,58, 91 (1981). (10) Minster, J. F., Ricard, L. Ph.,Inl. J Mom Spectmm, and Ian Phys., 37.259 (1981). (11) Mermeh@ngas. N.,Rosmm, K. J. R.,LkLaetete, J. R., lnt. J. Moss Speclrom. and Ion Phys.,37,1(1981). (121 Orb. J., Nooner, D.." P m d i d M a s Sp"trmtrtly:~Sditor: MiddIediW,, B. S., PIenm Press, New York, 1979, P. 355. (13) Mclafflrty, F W.. "lmrprefation afMas;*Swctrs,"Third Edition,Univenity Sdence Bmka, Mill Valley. California, 1980, P.17. ( 1 0 See Reference 13 Inside front mver.
