The analytical method described has the advantages of rapidity, sensitivity, and giving clean-cut separations, but the nitrogen peak height (or area) is affected by the presence of nitric oxide and carbon monoxide. Because the important variable is composition and not sample size, more extensive calibration is needed than normal. The enhancement of the nitrogen peak height is linear with respect to per cent nitric oxide, but because nitric oxide is not directly determined, its peak height cannot be used to evaluate a suitable correction for the nitrogen content. Up to a t least l5%, carbon monoxide has the same effect as nitric oxide. I n one problem, carbon dioxide and nitrogen were of the same order of magnitude, and carbon monoxide was less than 15% of the sample. The calibration was reasonably simple: the nitrogen peak heights were observed for a series of samples containing known amounts of nitric oxide, nitrogen, and carbon dioxide a t known total pressure in the sample bulb. The samples were made by diluting a mixture of nitrogen and carbon dioxide (in about equal amounts) with varying amounts of nitric oxide. The calibration curve was constructed by plotting as ordinate the factor, F, by which the nitrogen peak height must be multiplied to give the known nitrogen pressure and as abscissa the ratio, R, of total sample bulb pressure in millimeters to nitrogen peak height in millivolts. Factor F varied linearly (1.04 to 0.93) for values of R ranging from 1 to 6, corresponding to samples ranging from pure to 167, nitrogen; for R = 12, F = 0.87. SAMPLING TECHNIQUES
The gas sampling and transfer units are simple. Standard vacuum stopcocks (hollow precision-ground, 2-mm.
B
V
b
Figure 2. Gas sampling and transfer apparatus a. Standard vacuum stopcock with taper joints b. Altered stopcock
oblique bore) with standard taper joints a ) may be used to transfer samples from the reaction system to the chromatography column. The internal volume of each stopcock and the lead by which each stopcock is evacuated are adjusted to constant volume (to 10.01 cc.) by tedious glass blowing in connection with a simple gas buret. The bulb on the bottom of these stopcocks may be altered appreciably without affecting the bearing surface. A given batch of stopcocks is relatively uniform in volume when purchased. The ready interchangeability of sample tubes is well worth the glass blowing effort. To eliminate dead space and nonrepresentative samples from the system to be sampled, a vacuum stopcock may be altered as shown in Figure 2, b. It is sealed into the gas system a t A and B , so that in a flow system the gas enters a t A and leaves by B. The black section of the stopcock is filled, by
ox each lead (Figure 2,
heating, with Apiezon wax W-100 (James G. Biddle Co., Philadelphia, Pa.) to eliminate dead volume. This stopcock is also sealed to a vacuum system a t V . The sample bulb is connected by its standard taper joint, S’,to standard taper joint, S, and evacuated through V. By turning stopcock X through 180°, a representative gas sample flows through S into the sample bulb, The pressure of the gas sample may be measured by a manometer attached to the gas system. Then X and X’ are turned 90” to close the system and the sample bulb. The gas trapped in leads S and 8’is wasted. I n a kinetic study, a series of gas samples may be removed a t recorded time intervals and analyzed at convenience. To introduce the gas sample into the chromatography column in a reproducible manner without introducing foreign gases, the sample bulb is connected to the sampling section of the column. The leads are evacuaFd through V’; Turning stopcock X through 180 permits expansion of the sample into the fixed volume of the sampling section which can be closed off subsequently. The sampie may be swept out and into the column with the helium stream. This method is wasteful of sample in that the sample bulb and leads are left with an appreciable fraction. Variations of this method are possible, depending on the availability of sample and the sensitivity of the sensing and recording units. LITERATURE CITED
(11 Shah. hl. S.. Oza. T. M., J . Chem. Soc. 1931, 32. (2) Smith, R. K , , Lesnini, D., Mooi, J., J . Phys. Chem. 6 0 , 1063 (1956). \-I
,
I
(3) Szulcxewski, D. H., Higuchi, T., ANAL.CHEJI.29, 1541 (1987). (4) Welton, W. M., Drake, N. L., IKD. ENG.CHEIL,AXAI,,ED.1,20 (1929).
RECEIVED for review September 7 , 1957. ;Iccepted March 20, 1958. Work done under Contract K8onr54700 with the Office of Saval Research.
Standardization of Mass Spectra by Means of Total Ion Intensity ARCHIE HOOD’ Houston Research laboratory, Shell Oil Co., Houston, rex.
b A method for standardizing mass spectra of petroleum oils and related pure compounds gives spectra which are standardized with respect to both liquid sample volume and instrumental sensitivity. It is based on the total ion intensity of the spectrum and requires no knowledge of sample size or instrumental sensitivity. Spectral peaks of standardized mass spectra can b e
1218
0
ANALYTICAL CHEMISTRY
compared directly; therefore the method has broad application in correlations of mass spectra with molecular structure, leading to development of analytical methods for petroleum oils.
I
mass spectrometric methods for analyzing heavy petroleum oils, a major difficulty has
been inability t o determine accurately the amount of sample introduced into the inlet system of the mass spectrometer. When the sample size is not known accurately, one spectrum cannot be compared directly with another. No means is available for measuring
N THE DEVELOPMENT of
Present address, Shell Development Co., Houston, Tex.
the whole series, and to draw the best smooth curve through the experimental points. However, such techniques are subject to numerous inaccuracies. II70rk based on total ion intensity (ZZ)-i.e., the summated peak heights of a mass spectrum-has provided a nev approach to overcoming the samplesize problem. Stevenson and Muller (8) showed for C1 to Cp hydrocarbons that ZI/mole is approximately proportional to the sum of the number of valence electrons in the molecule (one valence electron for each hydrogen atom and four for each carbon atom). On this basis they were able to develop a mass spectral method of analyzing gasolines without having to meter either the pure calibration hydrocarbons or the analytical samples. Otvos and Stevenson (6) have recently extended those studies to show that Zl/mole is proportional to the sum of the atomic ionization cross sections of a molecule (ZQ2):
accurately the small sample pressures a t the high inlet temperatures (200" to 350" C.) required to vaporize petroleum oils in the mass spectrometer. Therefore, the usual procedure for high mass work has been to attempt to introduce a known liquid volume of a sample (2, 4, 6, 9) and to compare the resulting mass spectrum with that of an equal volume of a reference standard (such as n-CIG or n-Cr4)which has been run during the same day. Experience Iyith this approach has led the author to expect an error of about * 5 to 10% in the determination of spectral peak sensitivities (peak height per unit of sample). For isolated samples, therefore, it has been necessary to use average results from repeated sample introductions. For the correlation of spectra of a series of related compounds, it is convenient to plot the height of a desired spectral peak against some element of structural variatione.g., carbon atoms per molecule-for
SUESCRIPT= N W E E R OF RINGS
I
0.9 S
25
20
1
-
1
35
30
1
1
1
1
1
40
1
1
1
1
0
ZIimole
= kl. 2Qt
(1)
For hydrocarbons this gives the equation : ZI,'niole =
kp
(H
+ 4.16 C)
(2)
m-here H and C represent the number of hydrogen and carbon atoms per molecule. When they converted Equation 2 to a volume basis, the application of a limited number of molar volume values for hydrocarbons suggested the interesting approximation that for hydrocarbons with six or more carbon atoms per molecule, the total ion intensity is a direct measure of the liquid volume of the sample charged to the mass spectrometer] regardless of hydrocarbon type or molecular weight. METHOD
OF STANDARDIZING MASS SPECTRA
Although the above approximation provides a very simple approach to overcoming sampling problems, better accuracy is usually desired for correlations of mass spectra of pure compounds leading to the development of analytical methods, Therefore] the author has gone back to the fundamental relationship of Equation 1 and applied it in a slightly different manner to develop an accurate and practical method of standardizing high-mass spectra with respect to liquid sample volume and, simultaneously, to instrumental sensitivity. The method is this: The height of any peak or group of peaks in a mass spectrum can be converted to standard conditions of sample volume and instrumental sensitivity by multiplying by the ratio ZIatd/ZIobsd. The term Z I o b s d refers to the total ion intensity observed in any given mass spectrum, and its numerical value is obtained by summating the heights of all of the peaks of that experimental mass spectrum. This value must be determined for each spectrum. The term Z l s t d refers to the total intensity which would be expected in the mass spectrum of a given compound or mixture for a standard liquid volume and standard instrumental sensitivity. Thus EIstd is an ion sum per unit of liquid volume rather than per mole. Its numerical value for a given material is calculated from Equation 3; it is a constant value for that particular material and thus for all spectra of that material. ZIstd
=
++
2.829 x loKX d X ( H 4.16 c 12.8 S 3.84 N 3.29 0 )
+
M
,oooL ; ! I I
Figure 2.
I
,
CARBON IATOMSI IN ALKYL IO SUBSTITUENT, I I I R
1
I
,
,
+ (3)
I
20
where
ZIatd
=
d
=
Parent-peak intensities for pure alkyl benzenes,
total ion intensity for standard liquid volume and instrumental sensitivity density a t 20" C.
VOL. 30, NO. 7, JULY 1958
1219
iM H,C,S,X,O
= =
molecular weight number of hydrogen, carbon, sulfur, nitrogen, and oxygen atoms per molecule [Their coefficients are relative atomic ionization cross sections ( 5 ) . A set of relative atomic ionization cross sections, just published by Lampe, Franklin, and Field (3) disagreed c o n s i d e r a b l y with many of the Otvos and S t e v e n s on v a 1u e s . Hovever, for hydrocarbons and for organic compounds containing mostly hydrogen and carbon atoms, the disagreement has very little effect on the application of any of the three equations of the present paper. Thus the unsettled state of our knowledge of ionization cross sections should not detract from the usefulness of the described method of standardization of mass spectra of petroleum oils and related pure compounds.]
The constant in the equation (2.829 X lo5) gives a value of 1.000 x 105 for z l s t d for n-CZ4. The value of lo5 in the constant was included simply t o keep the ratio x l s t d / z l o b s d at a n order of magnitude of about 1, n hen a liquid sample volume of about 0.0002 to 0.0005 ml. is charged. For many pure compounds the molecular formula and density needed for the calculation of Z s t d from Equation 3 are readily available. For some pure compounds and most petroleum fractions, however, this may not be the case. A more rapid and convenient method of obtaining L’I,td for hydrocarbons in the latter category is provided by Figure 1, which is a plot of Z s t d as a function of hydrocarbon type and of the number of carbon atoms per molecule. The families of curves were drawn from calibrations of z z s t d for most of the pure hydrocarbons that have been synthesized by Smerican Petroleum Institute Research Project 42. T o use Figure 1 for obtaining z l s t d for a mixture, it is necessary to h a r e a knowledge of the average carbon number and of the distribution of hydrocarbon types. This information can usually be obtained conveniently from the parent-peak region of the mass spectrum of the material in question. The circular points in Figure 1 are not experimental points but are included for convenience in locating the correct hydrocarbontype curve for a given number of saturate (S) rings and aromatic (A) rings per molecule. 1220
ANALYTICAL CHEMISTRY
The most tedious step, and probably the weakest point, in calculating the ratio of z I s t d / z l o h s d is the determination of Z I o b s d . First of all, it requires the digitization and summation of all peak heights of the spectrum. As it is desirable to be able to standardize published spectra or other spectra that do not include the CI and Cz region, the observed total ion intensity has been defined arbitrarily as the sum of the peak heights for all masses above m/e 38. In addition, as most heavy hydrocarbons require scans at more than one magnet current to record all peaks above m/e 38, some arbitrary correction is needed to convert the scans to a common basis. For two scans starting a t about m / e 36 and 66, respectively, it has been found convenient to use the high-mass scan for masses greater than m/e 76 and to use the low-mass scan for m/e 39 through m/e 76. The peak heights on. the loa-mass scan are corrected arbitrarily to the basis of the high-mass scan by multiplying by the high-masslow-mass ratio for the summated peaks from m/e 77 through m/e 85. The problems inherent in this method of determining Z Z o b s d should be overcome by the development of a satisfactory means of recording instrumentally the total signal. Such a n instrumental modification should make the standardization both rapid and accurate. Experimental data have shown that for heavy hydrocarbons total signal or total ion intensity is essentially independent of the temperature of the ionization chamber over a fairly wide operating range-Le., for a n-butane cracking pattern (m/e 43/58) of 8.5 to 11.9 in a modified Consolidated Model 21-103 mass spectrometer. This is an important consideration in the standardization of mass spectra from different sources, such as the spectra in the American Petroleum Institute Research Project 44 Catalog of Mass Spectra. For the standardization of mass spectra of compounds other than heavy hydrocarbons, the possibility that a n effect of ionization chamber temperature on total ion intensity does exist should be kept in mind-for example, such an effect is indicated in the work of Stevenson ( 7 ) on the ionization of argon. APPLICATIONS
The standardization of mass spectra by means of total ion intensity has been applied with excellent results to the correlation of mass spectra with niolecular structure. The improvement over the method of charging directly a known liquid volume is shown in Figure 2, which is a graph of the effect of molecular weight (or carbon number) on the parent-peak intensity per unit liquid volume for a series of mono-n-alkylbenzenes. The small open circles represent Z.Jlrd values, and the solid line has
been drawn through these data. The smoothness of the curve reflects the precision of the corrections and is most gratifying. The solid black circles represent parent-peak intensities based on the assumption of accurate charging volumes of the individual hydrocarbons, and the scatter of points indicates the unreliability of that assumption. The crosses represent relative parent-peak intensities per unit liquid volume for a synthetic blend containing known volumes of four pure alkyl benzenes; although the experimental results appear to be reasonably good, the available amounts of most pure high-mass hydrocarbons are too small to make the use of blends generally practical for determining relative spectral peak sensitivities. The method of standardizing spectra has also been applied to the study of accuracy of the mass spectral method of Clerc, Hood, and O’Xeal ( I ) for analyzing lubricating oil saturate fractions. The approximate sensitivities of the C4 to regions reported for the various hydrocarbon types were reasonably well confirmed, as seen in the tabulation. Relative Sensitivity Hydrocarbon Type Alkanes Soncondensed cycloalkanes Condensedcycloalkanes hlonoaromatics
2I
(1)
method
1 0
1 0
1 2 10
1s
. .
1 1 0 0
These are simply specific examples of many types of applications in which the total-ion-intensity method of standardizing spectra has been used and is espected t o be used. ACKNOWLEDGMENT
The author wishes t o thank E. G. Carlson for his assistance in this work. LITERATURE CITED
(1) Clerc, R. J , Hood, A,, O’Xeal, 11.J., A s . 4 ~ CHEJI. . 27, 868 (1955) (2) Friedel, R. A , , Sharkey, A . G., Jr., Humbert, C. R., I b z d . , 21, 1572
(1949). (3) Lampe, F. W.,Franklin, J. L., Field, F. H., J . -4m.Chena. SOC.79, 6129 ( 1957). (4) O’Seal, 31. J , Wier, T. P., A \ ~ L . CHEN.23, 830 (1951). (5) Otvos, J. IT., Stevenson, D. P., J . Am. Chem. Sac. 78, 546 (1956). ( 6 ) Purdy, K. M , Harris, R J., A\.zL. CHEX 22, 1337 (1950). (7) Stevenson, D. P., J . Chem. Phys 17, 101 (1949). (8) Stevenson, D. P , Jluller, C. E., private communications (9) Taylor, R. C., Young, W. 8 , Isu. EKG. CHEV., A s . 4 ~ . ED. 17, 811 (1945). RECEIVEDfor revieiv July 1, 1957. ACcepted March 10, 1958. Committee E-14 on Mass Spectrometry, American Society for Testing Materials, Xew Tork, 1Iay 20-24, 1957.
