J. R. Leech1 and K. E. Daugherlyz.' University of Pittsburgh Pittsburgh, Pennsylvonio 15213
I
I
I
Relative Isotopic Abundances in Halogenated Methane Fragments An undergraduate experiment
Although the mass spectrometer has been widely accepted and used in research for about a quarter of a century, its high cost and difficulty of operation have generally discouraged its use in many college undergraduate laboratories. When students are exposed to this technique, i t is usually only through instructions on the theory of operation and interpretation of the resulting spectra and not by means of actual manipulation and use. However, with the introduction of the Varian EM-600, a low cost, medium resolution, magnetically scanned mass spectrometer is now suitable for undergraduate work. This paper is a description of one possible mass spectrometer experiment for undergraduates a t the sophomore level in analytical chemistry. I t is designed for the determination of the relative isotopic ahundances of halogenated methane fragments and the subsequent comparison with known abundances. Calculations The probability calculations necessary to predict the relative abundances of specific isotopic arrangements within a given charged fragment can be found in most texts on probability theory. The application of this mathematical treatment to mass spectrometry has been discussed in numerous books and articles (1-5). For a given fraement the ~robabilitvof a snecific arrangement is the " prohurt of the mul~iplicityof the arrangement times the relative isotopic abundances for the elements. The multiplicity is given by (n!/q! (n-q)!), where n is the number of atoms having variable isotopic weight, q represents the number of atoms having a specific isotopic weight, and (n-q) is the number of atoms having an isotopic weight different from the isotopic weight of the q atoms. The sum of probabilities for all of the peaks representing possible isotopic arrangements within the fragment must equal one. For example, Table 1 presents the calculations for the ,relative probabilities of the peaks representing the CC13+ species, using the approximate relative abundances of 0.75 and 0.25 for 3JC1 and 37C1, respectively. The average natural abundances are 0.7553 and 0.2447, respectively (6). (Lederer, et al., (7), gives observed variations in natural abundances.) A more sophisticated analysis might additionally consider the relative abundances of I2C and '3C (0.9889 and 0.0111, respectively), and where applicable, any other isotopic species present within the fragment, e.g., 'H, 2H, 3H (0.99985, 0.00015, and less than 0.00001, respectively) (6). For a discussion of the calculations involving two or more isotopic species within a fragment, see Margrave and Polansky ( 5 ) . Although a student should be made aware of these relatively minor isotopic abundances, the authors ~~
~
~~~
'Present address: Hydmn Corporation, New Brunswick, New Jersey. Address correspondenceto this author. The authors have designed and constructed a simple gas injection system for the EM-600. Plans are available by writing to K. E. D.
Table 1. Calculation of Relative Probability of Peaks Representing the CC13+ Species m/e
Fraement
123
Ca7CI~+
ProbaMulti~licitv x Abundance = bilitv
5:
0! (3 - O)!
=1
(.25)(.25)(.25) SUM
1/64 64/64
feel that the undergraduate instructional benefits and increased accuracy resulting from their inclusion in this experiment might not warrant the necessary additional calculation time. This experiment has been simplified to include only those isotopes having major relative abundances, such as 3561 and 37C1, and 19Br and SlBr. Examples Compounds studied in this experiment were spectral grade halogenated methanes, specifically CClr, CHCls, CH~CIZ,CBrC4 and CH3I. All of these compounds are easily ohtainahle, relatively inexpensive, and suitable for handling by undergraduates. Since the EM-600 is not currently available with a gas injection system,3 the chosen compounds are liquids a t room temperature. Solids with high vapor pressures can also be analyzed. Additionally, since fragments of over approximately 330 m / e cannot he seen on the EM-600, this limitation should be taken into consideration when designing an experiment using this spectrometer (8). McLafferty has considered the problems of obtaining exact isotopic abundance measurements (9). The three factors primarily responsible for errors are mass discrimination, memory effects, and background. Memory effects are particularly important for halogens or halogen-containing molecules (10). The authors have found, however, that by waiting 15 min between runs, reproducible results were. obtained for the halogenated compounds examined in this ex~eriment. A and c of the figure show typical spectral results forCC4 and CBrCla. T o a student with a basic understanding of mass spectrometry, it should be obvious that these spectra represent different compounds whose fragments contain. atoms of different isotopic abundances. Although various methods were possible for the analysis and interpretation of these spectra, the method chosen for this work was a comparison of peak heights for individual fragments. This differed from the customary practice of referring all relative abundances to a base peak. However, this direct comparison within a specific fragment yielded more easily interpreted results. In the cases where direct measurements from total spectra were impossible, it was found that scanning selected peaks at increased amplification improved both the accuracy and precision of the Volume 50, Numbera, ~ u g u s1973 f
/
569
Table 2. Results for CCI.
m/e
Varian EM-BOO spectra of A. CCI& 8. CCIa at increased amplitude: and C, CC13Br.
measurements. This technique is illustrated in B of the figure, where the portion of the CCl4 spectrum between m/e 25-55 was chosen for closer scrutiny. T o find the relative abundances of the isotopes under study, the height of the smallest peak in a given fragment was assigned a value of 1, and ratios were determined for all other peaks resulting from isotopic changes within the same fragment. Care was taken to assure that all ratios were for the same species, and not for peaks representing further fragmentation. It was useful to realize that the number of peaks representing a given fragment was the same as the number of possible combinations within the fragment. Table 2 shows the results for CC11, divided according to fragmentation products. Column 2 shows the isotopic arrangement responsible for a specific m l e value. Ratio I resulted from the theoretical calculations, using approximate relative ahundances of 0.75 and 0.25 for 35C1 and 3TI, respectively. Ratio Il was obtained by the same calculations, but using the actual ahundances. Ratios III and IV were from experimental data, with Ratio 111 from McLafferty, et al., (11) and ratio IV resulting from the data obtained by this experiment, presented in the last column. An analysis of Table 2 indicates that the results obtained on the EM-600 were similar to the theoretical ratios. Deviations of the authors' experimental results for CC14 from theoretical ratios ranged from 0.7% to 7.8%. These deviations compared quite favorably with deviations of 1.6% to 5.6% for results reported by McLafferty, et al. (11). The ratio found for I9Br and SIBr in the CBrC13 spectrum was 0.50-0.49 as compared to the theoretical ratio of 0.505-0.495 (6). For the fragment CBrCI+ having peaks a t m l e 126, 128, and 130, the experimental ratios were found to he 3.15:4.05:1. Theoretical ratios for this same fragment are 3.14:4.11:1. The more interested student may wish to calculate ratios for the CBrCIz+ fragment (5). Similarly, analysis of fragments in the CHC13, CH~CIZ, and CBr4 spectra gave good results for C1 and Br abundance ratios. Since the natural abundance of 12'1 is 1.0000, CH31 was useful for demonstrating the spectrum of 570
/ Journal of Chemical Education
Species
Ratios
I
II
III
IV
Heights (mm)
a halogenated methane with no major isotopic variations. Simplified calculations for CHCl3 and CHZC~Z.when compared with CC14, showed that all fragments with the same number of chlorines (e.g., CH*Clz+ and CHClz+) should have the same peak ratios. Comparison of the spectra of these compounds, however, give ratios which were not equal. This was partially because of an increased influence of the previously discussed "minor" isotopic effects. The major source of this discrepancy, however, was from the overalp of fragments differing only in the loss of protons. While the m / e values for 12C37C4+ and *2C1H23~C135C1+are different (85.9318 and 85.9504, respectively), the medium resolution mass spectrometer with a resolution of approximately 150-200 (12), does not have adequate resolution to separate these peaks. A resolution of approximately 4600 is necessary. The molecular ion peak (parent ion peak) for many halogenated methanes such as C C 4 and CBrC13 is not ohserved. The reasons for this are that the hromo and chloro groups either weaken bonds or stabilize the fragments better than the molecular ion, thus facilitating the fragmentation of the molecule. Hence, the probability of seeing a molecular ion peak is small. The stability of the molecular ion can be increased by the presence of T electron systems and cyclic structures (13). Conclusions
The foregoing experiment is one approach to student use of a medium resolution mass spectrometer. Through the use of this instrument with various halogenated metbanes, a student is able to observe natural isotopic ahundance ratios and to compare observed results with theoretical calculations using known abundances. The student should encounter little difficulty in obtaining satisfactory results. The instructor might question the expenditure of additional time necessary to instruct the sophomore chemistry student in the details of the mechanical operation of the equipment. This problem can easily be avoided by the instructor running the various samples in a laboratory demonstration. In view of the small amount of time required for calculations, the accuracy of the experimental method, and the deceptive error in the approximation of the chlorine isotope ratio as 3:l and the bromine isotope ratio as 1:1, students should be encouraged to calculate the experimental isotope ratios and to compare the results with the reported values. The error in the isotope ratio for chlorine calculated from the m l e data for CC18+ in Table 2, is less than 1%, hut the error in the peak ratios looks much larger when experimental results are compared to the theoretical peak ratios calculated from the rounded off isotope ratio. Work is continuing on further uses of this instrument as both an instructional aid as well as for quick, in-lab research analysis.
The authors wish to thank the reviewer
c.. (E~~CDII. " ~ ~ . d b m k of chemiatn and ~ h ~ ~ 149th i ~ E~:I. ~ . TIE " Chemical RubberCa.. 1968. pp. B4toB92. ,,, ,., Ho~lsnder. J. M.. Per~man.I., . ~ ~ t of~aotopes: . i ~ l6thEd.l. J O ~ " w i ~ w a n dsans, IW., N ~ WYWL,1967. (81 "EM-MO M a u Spectrometer. I, General Infmmation." Varisn lnatrument Division, Palo Alto. California. p. 1.1. , I91 McLaffaffaffty, F. W., "Mssa Spcetmmctry of Organic lorn.'' Academic P r e ~ New York. 1963.p. 189. (101 Melton, C. G., Gilpatrick, 0. 0.. Baldock. R.. and Hesly. R.M., Anal. Chem.. 28. 1049 119561. I111 Stenhagen. E.. Abrshamuon, S.. and Mdsffffrty. F. W., "Ath. of Mass Spedrsl Dats." lnterseienee Publishers, Division of John Wiley and Sons. Inc.. New York. 1969, p. 899. (12) FMerence 181. p. 1-3. We have found a resolution ofappmximately I80 with our instrument. I131 Reference ( 3 ) ~51. . (61
Acknowledgment of
this article
for his helpful comments. Literature Cited (1) Beyno", J. H..,'Mass Speefmmetry end Its Applications +aOrganic Chemistry." Elsevier Publishing Ca.. NeuYork. 1960. pp. 295-98. (2) Beyno". J. H.. and Williams. A. E.. "M- and Abundance Tables for Use in Maaa . Spectrometry," Elsevier Publishing Co.. New York. 1 3 6 3 ~VII. Orgsnie Chemistry Applieafiona:' (3) ~ iK.."Mars ~ Spctrnmatry ~ ~ ~ ~ , McGraw-Hill BookCo.. Lne.. NewYark. 1962, pp.223-27. (41 Robinson, R. J., Werner. C. G.. and Gohlke. R. S., J. CHEM. EDUC., 41, (57 119701. (5) Margrave. J. L., andPolanslty, R.B., J. CHEM.EDUC.,39,335(19621.
W D B S ~ . R.
,.
Volume 50, Number 8, August 1973
/
571
