contamination of the heterocompound fraction by hydrocarbons and sulfur compounds is negligible for samples boiling below 850" F., and minor for samples boiling below 1000" F. On the basis of these various checks of the accuracy of the method, the experimental determination of total heterocompounds is be1--2% weight. lieved accurate within For a fuller discussion of method accuracy and scope, see reference ( 3 ) . It was also observed for the samples of Figure 1 that there is an approximate relationship between sample oxygen and nitrogen contents. This fact permits the direct estimation of total heterocompounds when the sample boiling range and nitrogen content are known, as is frequently the case in our Laboratory for samples which are submitted for detailed compositional analysis. The molecular weight of the heterocompounds is first estimated as in Figure 2, and the per cent heterocompounds then obtained
*
from Figure 3. Comparison of experimental and estimated heterocompound values for the samples of Figure 1 shows a standard deviation of =t170 weight. Further comparison of experimental and estimated heterocompound contents with maximum heterocompound d u e s suggests that the accuracy of either estimated or measured heterocompound values is about the same, as long as total heterocompounds do not exceed 20%. Analysis of several shale oil samples (containing 30-45y0 heterocompounds) indicated that estimation is not reliable for such samples. Full size drawings of Figures 2 and 3 are available on request. EXPERIMENTAL
Determination of Total Heterocompounds. A 400-mm. X 12-mm. glass-Teflon column is packed with 4% H20-Al2O3 [see (d)], and 1 gram of sample (any petroleum related distillate) is charged. Elution is begun with 25%
volume benzene-pentane; 50 ml. are collected and discarded. A second 50ml. fraction is collected, eluting with 50yo volume methanol-benzene. The latter fraction is stripped of solvent under nitrogen on a steam bath and weighed. The weight of recovered fraction is assumed equal to the weight of heterocompounds in the original sample charged to the separation. Precision is =k0.37, weight. LITERATURE CITED
(1) Hastings, S. H., Johnson, B. H., Lumpkin, H. E., ANAL. CHEM. 28 1243 (1956). (2) HoM-ard, H. E., Ferguson, W. E., Snyder, L. R., $m. Sac. Testing M a leriabs, E-14 Meeting, St. Louis, 1965. ( 3 k Snyder, L. R., ANAL. CHEW 37, (13 (1965). (4) Snyder, L. R., Zbid., 36, 774 (1964).
Union Research Center Union Oil Co. of California Brea, Calif.
L. R. SNYDER
An Improved Mass Spectrometer Photoionization Source SIR: The use of a windowless photoionization source to produce ions for analysis in the 4EI 1Yf.S.9. Mass Spectrometer has been the subject of a recent studv ( 1 ) . DesDite its usefulness this source "is considered to be limited in its application. First the light beam enters the mass spectrometer through the vacuum lock port in the ion source housing. This prevents siniultaneous
use of a solid sample probe and thus places a serious limitation on the types of compound which may be studied. In addition the possibility that a contribution from Dhotoelectrons was Dresent in the total ionization could n i t be discounted. Apart from electrons produced in the microwave discharge, it seemed possible that the quartz capillary used for transmitting the light from the
discharge to the ion chamber might be acting as a capillary multiplier of unknown gain. EXPERIMENTAL
The photoionization shown in ~i~~~~ has been designed and constructed with & view to eliminating the problems outlined abo.cre. In general operating principle and basic construc-
I
I
2450 M/cs
Pumps gas inlet
Figure 1 .
Photoionization source VOL. 38, NO. 13, DECEMBER 1966
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I
tion it is similar t o the device previously described (1). However the quartz capillary (diameter 0.5 mm.) which connected the source to the ion chamber has been replaced by a short, earthed, brass tube. This tube which just protrudes through the source housing has an internal diameter of 2 mm. identical t o that of the quartz capillary which has been introduced into the upper portion of the light source. With these modifications a more stable discharge as well as a higher light flux in the ion source is obtained. Despite the four-fold increase in capillary diameter the base pressure in the mass spectrometer does not exceed 3 X 10-7 mni . The light source is mounted in the side of a specially modified ion source housing so that the light beam traverses the path normally occupied by the election beam. Compared to the earlier transverse configuration the new arrangement not only permits the study of solids of low volatility (4) via the direct insertion probe but also affords a considerable increase in total ion intensity. The latter niay now be measured on the lower ranges of the unmodified total ion current collector, The electron trap and filament assemblies in the conventional ion source halve been replaced by stainless steel plates containing 3 mm. square apertures for passage of the photon beam. To change operation from electron impact to photoionization the electron beam collimating magnets are removed and the tvpo ion sources interchanged. The complete operation can be performed in 30 minutes. RESULTS A N D DISCUSSION
Comparison of spectra obtained with the earlier source (I) and the improved source (11) have revealed a number of significant points. These observations are illustrated by Figure 2 which compares the relative abundances of the principal ions occurring in the mass spectrum of 2-pentanone. (It should be noted that an error occurs in Table I of Reference ( 1 ) . The electron impact spectrum was recorded at 70 e.v. and not 21 e.v.) It can be seen that the spectrum (c) is very similar to the electron impact spectra (a) and (b). However the spectrum (e) using the new light source shows significant differences, the most important being the large increase in relative abundance of the molecular ion ( X ) . I t must be concluded that the epectrum (c) contains a large contribution from ionization by high energy electrons confirming the speculations outlined above. This further confirms the view that the decompositions observed earlier ( 1 ) are in fact essentially
1942
ANALYTICAL CHEMISTRY
(a) Electron I m p a c t !21 eV) I
II
I
I
1
I
(b) Electron I m p a c t (70 eV) I
I II
/M
I
IM
I
( c ) Light Source 1
I
I
(helium) IM
1
4
I
(d) Light Source I C (argon)
1
(e) Light Source
I
IM
I IM
I
I
4b
I C (helium)
5’0
6’0
1 8’0
7b
I
9’0
Id0
m/e Figure 2. Principal ions in the mass spectrum of 2pentanone (normalized a t m,’e = 43)
thermal and not due to differences in electron impact and photoionization cross sections. It is evident that light source I1 contains a much higher proportion of photons. Spectrum (d) was obtained using argon instead of helium as the excited species in the light source. An approximately two-fold increase in total ionization is obtained a t the expense of a rather higher gas pressure (IO-8 mm.) and an increased heating of the light source. The relatiue increase in parent ion abundance produced by photoionization has been observed in many spectra. The fact that the photoionization spectrum (e) in Figure 2 does not compare with electron impact either a t 70 e.v. (a) or 21 e.v. (b) would suggest that subsequent break up of the molecular ion may be a function of the initial preparation of that ion. This may also explain why Watanabe (5) has been able to measure the molecular ionization potentials of higher alkyl esters by photoionization whereas it has proved impossible by electron impact ($1
’)’
ACKNOWLEDGMENT
The author acknowledges the technical cooperation of ilss&iated Electrical Industries, Manchester, England. Thanks are also due to Professor C. il. McDowell for his continued interest and encouragement. LITERATURE CITED
(1) Brion, C. E., ANAL.CHEM.37, 1706
(1965).
(2) Brion, C. E., Dunning, W. J., Trans. Faraday, SOC.59, 647 (1963). (3) Brion, C. E., Farrell, D. W., unpublished work, 1966. (4)Brion, C. E., Hall, L. D., S.Am. Chem. soc. 88,3661 (1966).
(5) Mottle, J., Nakayama, T., Watanabe, K., “Final Report on Ionization Potentials of Molecules,” University of Hawaii, 1959. C. E. BRION Department of Chemistry University of British Columbia Vancouver 8, B. C., Canada The National Research Council of Canada provided financial assistance in this work.
