Table I. Isotopic Content of COz from sec-Butyl Phthalate Column Sample a Mass 46/(44 45) ratio Enriched standard CO, 0.0285956 Normal standard CO, O.Oo4047 Enriched standard Cot 0.028614 Enriched standard CO, at normal flow rate 0.028555 Two samples of normal standard CO, ... Enriched standard CO, 0.028556 Samples listed in the order passed through the column. Ratio before treatment on column: enriched standard = 0.028595; normal standard = 0.004031.
+
With a helium flow rate of 30 cc per minute and the column at room temperature, the retention times relative to N2 as 0 were: COZ,0.5 minutes; ( C W , 4 minutes; S a , 9 minutes. Thus the COzis well separated from these impurities and it is quite easy to trap CO, free from SO2 and (C-. These examples suggest that in general the retention time of the gas on the phthalate ester columns depends on the boiling point of the gas, and may serve as a guide for applications involving the removal of other nonvolatile impurities from carbon dioxide. The impurity HCI was not encountered in any of our samples either as a peak in the gas chromatogram or in the mass spectrum. It is not known if it was absent in the pyrolyzed samples or whether it may have been removed'by reaction with the column materials--e.g., copper tubing. However, if HCl is a major impurity in other samples, it might be difficult to remove from COZon the sec-butyl phthalate column. The HCI could be conveniently removed by passing the sample over zinc amalgam as originally suggested by Anbar and Guttman (2).
using C-22 firebrick solid phase and a silicone oil liquid phase, employed a similar method of C02 purification but report no details on the accuracy of the results. Heating oxygen-containing compounds with Hg(CN)1 is a common method of converting the oxygen to COz (2). It was found that the cyanogen impurity produced by this method could be readily separated from the desired COZ product. Also SO2 and HzS produced from the pyrolysis of benzidine sulfate with HgClrHg(CN)9, can be readily separated from the COz by chromatography on the secbutyl phthalate column.
The authors gratefully acknowledge the helpful discussions and encouragement of Henry Taube. The gas chromatographic unit was made available by the Center for Materials Research of Stanford University.
(2) M. Anbar and S. Guttman, J. Appl. Rad. Isofopes, 5, 223 (1959).
RECEIVED for review November 14, 1966. Accepted March 9,1967. This work was supported by the A. E. C. under contract No. AT(04-3)-236, Stanford University.
ACKNOWLEDGEMENT
Analysis of Complex Mixtures of Aromatic Compounds by High-Resolution Mass Spectrometry at Low-lonizi'ngVoltages B. H. Johnson and Thomas Aczel Esso Research and Ehgineering Co., Baytown Chemicals Research Laboratory, Bay town, Texas
THECONTINUOUS EXPANSION and progress of petroleum technology requires more and more sophisticated means of characterizing the composition of petroleum products. Mass spectrometry has played a dominant role in this effort since the early nineteen-fifties. The introduction of commercially available high-resolution mass spectrometers in recent years h a s further extended the scope of the mass spectrometric technique. The use of low-ionizing voltages restricts the mass spectra of aromatic and olefinic materials to the molecular ions (2-3). The spectra are thus greatly simplified and interferences among ihe components of the mixture are reduced to a minimum. This permits the determination of the carbon number distribution within each compound type and, in general, makes the applicability of the method less dependent on sample origin and history than in the case of high voltage methods. The use of high-resolution instruments eliminates one .najor limitation of low-resolution low-voltage techniques,
that is, the inability to distinguish more than seven compound types. At a resolution of about 10,OOO, pairs of compound types such as alkylbenzenes and benzothiophenes, naphthenobenzenes and pyrenes, naphthenonaphthalenes and dibenzofurans, etc. are resolved and can be analyzed separately. The potential of the combination of high resolution lowvoltage technique was recognized by Lumpkin (4) and Reid and coworkers (5). The peak matching mass measurement (6) techniques applied by these authors is, however, extremely lengthy when applied to very complex mixtures, and is not applicable to the determination of minor and trace components. This paper describes a method for making mass measurements directly from the recorded m a s s spectrum which greatly reduces the time required as compared to the
(4) H. E. Lumpkin, ibid.,p. 2399. Reid, W. L. Mead, and K. M. Bowen, paper presented
( 5 ) W. K.
Field and S. H. Hastings, ANAL.CHEM., 28, 1948 (1956). (2) H. E. Lumpkin, Ibid.,30, 321 (1958). (3) H. E. Lumpkin and Thomas Aczel, Ibid., 36, 181 (1964).
(1) F. H.
682 *
ANALYTICAL CHEMISTRY
at the Institute of Petroleum/ASTM Spectrometry Symposium, Paris, September 1964. (6) K. S . Quisenberry, T.T. Scolman, and A. 0. Nier, Phys. Reo., 102, 1071 f 1956).
instrumental peak matching techniques and which permits the identification of trace components. As a consequence, this technique is now iised to characterize petroleum materials on a routine basis. EXPERIMENTAL
The apparatus used in this work is an Associated Electrical Industries Ltd., MS-9 high-resolution instrument. This is a double focusing apparatus, of the Nier-Johnson geometry. The output of the instrument is recorded with a MinneapolisHoneywell UV visicorder. The MS-9 is essentially a qualitative instrument, and its use for quantitative purposes requires a careful standardization of the operating conditions. The main variables are the position of the magnet, the electron beam and repeller voliages, ion beam fociising, and the widths of the source and collector slits. lnterpretation of charts requires a repeatable spectrum which is adequately resolved for the whole mass interval investigated, usually from mje 600 to m/e 100. This can best be accomplished by optimizing the position of the magnet at the high end of the mass interval to be studied. This is necessary because the resolution decreases rapidly above the mass at which the magnet is optimized, but only slowly below. In addition, one obviously needs the highest resolution at the high mass end of the spectrum. The effective electi-on beam energy is kept constant at 12.0 electron volts which essentially eliminates fragments from the types of compounds likely to be present in petroleum fractions. This effe,:tive energy is due both to the electron beam voltage and the repeller voltage, according to the expression : Effective V
E,lectron beam V
+
repeller V
(1)
Any deviation of the repeller voltage from zero, desired sometimes for increasing sensitivity and repeatability has to be compensated by a corresponding change in the electron beam voltage . The ion beam focusing is optimized at the effective electron beam voltage used in each experiment. A set of typical instrumental conditions is: source slit width, 0.0018 inch; collector slit width, 0.0016 inch; effective electron voltage, 12.13electron volts; multiplier gain, 4400; scan speed, 8.7 minutesjoctave; chart speed, 0.4 inchj second; and sample size, 3-10 ma. A resolution of about 10,000 is obtained at these conditions. The maximum mje ratios, at which the doublets usually encountered in petroleum can be completely separated with a resolving power of about 10,OOO are listed. Maximum m/e at which doublet
is resolved AM
with 10,ooO resolving power
0.0939
939 905
ca-3 4SH,
0.0905 0.0364 0.0165
'3CH-N C r *SH,
0.0081 0.0034.
Doublet c-12w GH?,-a*S CHI-0
364 165 81
34
In effect, this resolution allows the identification of the first three doublets at masses higher than listed because partially resolved peaks can also be used. On the other hand, going to much higher resolving powers would reduce the intensity of the peaks without significantly improvirig the resolution of the last two doublets. The total ionizatio?, expressed as the sum of the monoisotopic peak heights of all parent peaks is about IO to 20
13C12C1gHls
e
I
L 2
Figure 1. Portion of a high-resolution low-voltage spectrum of a petroleuni mixture
thousand millimeters. Base line noise is about 2 to 3 mm, and thus individual components can be detected in amounts of 0.05% or less of the total ionization. A portion of a typical spectrum obtained under these conditions is illustrated in Figure 1. DISCUSSION
Chart Reading Technique. Mass measurement from the recorded spectra is based on the relation between the masses and the position of the corresponding peaks on the record. This relationship is expressed by the following equation:
where Mx = Ma, Mb = ib - Ia = I, - iz = Ib - I, =
unknown mass known masses distance between Ma and M a distance between Ma and M , distance between Mb and M,
The relationship assumes a uniform logarithmic scanning rate and a constant chart speed. This was shown experimentally to be valid only for mass intervals of less than four. Thus for a low voltage spectrum covering several hundred integral mass numbers, a large number of individual external standards would be required. However, this difficulty can be circumvented in petroleum and similar materials which consist of homologous series of compounds. In these cases throughout the spectrum there are generally one or more parent peaks at every even mass number followed by their 13C isotopes. The distance between the pairs corresponds to the known mass difference AM('3C - IZC)= 1.0034 and therefore provides internal mass-distance standards at every second mass interval. Over these short intervals a linear relationship is assumed between mass and distance. It is furthermore assumed that M ( H ) = AM(13C - l2C) = AM(**CH2- 13C). The error introduced by the latter assurnp tion, (of the order of 0.004 amu), is partially compensate
