Figure 2. Amplifier interface between mass spectrometer and computer Resistors are %-W carbon. Capacitor values are microfarads. SI, SPDT gain switch. 1 = X1, 2 = X50. Sz,SPDT mode switch. 1 = normal, 2 = bilinear (gain XSO, 0 to -6 V; gain X2, -6 V to -10 V). P25AU, P45ALU are supplied by Philbrick/ Nexus, Dedham, Mass. 02026
below m/e 70 can be tolerated since high-resolution data in that mass range are required rarely and manual peak-matching techniques are, of course, still usable. The original output amplifier of the mass spectrometer was replaced with that shown in Figure 2. It consists of a current amplifier (P25AU) and an inverting voltage amplifier (P45ALU) which provide the required 0- to - 10-V signal (3). In the X50 gain, bilinear mode, the output voltage is 50X the input voltage over the range of 0 to -6 V. From -6 to - 10 V, the amplifier gain is reduced to approximately 2X by the action of diode 1N629. This simple form of signal compression ( 4 ) permits the scanning of ion signals of low intensity without overloading the computer input when signals of high intensity occur. Quantitative peak-area data, also pro-
s. Lewis, Tennessee Eastman Company, Kingsport,Tenn., private communication, 1969. (4) C. N. Reilley, University of North Carolina, Chapel Hill, N.C., private communication,1970.
vided by DS-30, are distorted by this compression, but this has not been a serious disadvantage for applications in structure characterization. This system has been in use for over one year and is capable of providing the mass measuring accuracy shown in Table I. Ten consecutive scans of a sample consisting of 0.8 p1 of perfluorotributylamine and 0.2 p1 of hexachlorobutadiene mixed with 1 pl of high-boiling perfluorokerosene (the mass reference compound) were obtained. The following instrumental conditions were used: electron beam, 70 V, at 100 pA, 8 kV ion energy, electron multiplier 140 V/stage. The static resolution was adjusted to approximately 7,000. An all-glass inlet system at 175OC was used to contain the sample. The beam monitor indication was 2 X A throughout the scans. The average error of each of the scans lies between 3.1 and 5.7 ppm; the individual errors range between - 12.1 and +13.5 ppm. This compares favorably with the AEI specification of 5-ppm root-mean-square error for peaks in a given scan when DS-30 is used with the mass spectrometer for which it was designed (AEI MS-902). The ions presented in Table I were chosen to avoid the inclusion of unresolved doublets. In routine operation, where less care in instrument or peak-shape adjustment is exercised, the errors are approximately 20% greater than those shown. We have found that the mass accuracy remains constant when the 21-llOB is operated within the resolution range of 7,000 to 15,000 (10% valley). It is important to reduce the ripple on the electric sector voltage to the lowest possible value and to be certain that the output transistors of the magnet power supply are operating well within their suggested limits. Failure to observe these precautions results in reduced mass measuring accuracy.
(3) P. E. Morrisett and J.
RECEIVED for review January 19, 1972. Accepted March 29, 1972.
Determination of Mass Spectrometric Sensitivity Data for Hydroaromatic Compounds J. L. Shultz and A. G . Sharkey, Jr. Pittsburgh Energy Research Center, Bureau of Mines, US. Department of the Interior, Pittsburgh, Pa.
R. A. Brown Esso Research and Engineering Co., Linden, N.J. CURRENTINTEREST in utilizing coal to meet the projected energy requirements for the nation has stimulated interest in coal gasification and liquefaction processes. Products from coal hydrogenation have been studied at the Pittsburgh Energy Research Center since mid-1940. Other investigators have also contributed to the literature of coal-derived compounds, as indicated in the extensive bibliography published by Wiley and Anderson ( I ) . Many of these studies depend heavily upon low-voltage mass spectrometric analysis of products. In the low-voltage method only the sensitivity of the molecular ion is required for analysis of mixtures. Aczel et al. (1) J. L. Wiley and H. C. Anderson, US. Bur. Mines Bull., No. 485, (1950). 1486
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
(2) and Kessler et al. (3) both report on use of the method for the analysis of hydroaromatics. Both methods are limited in application because of a lack of reference compounds, including hydroaromatic hydrocarbons. The work reported here indicates that the sensitivities of these hydroaromatic compounds can be predicted from the sensitivity of the corresponding aromatic ring compound. This observation is based on the behavior of 15 hydroaromatics listed in Table I. (2) Thomas Aczel, J. Q. Foster, and H. H. Karchmer, Reprints, 157th Nat'l Meet. Amer. Ckem. SOC.,Diu. Fuel Chem., 31, NO. 1, Minneapolis, Minn., April 1969, p 8. (3) T. Kessler, R. Raymond, and A. G. Sharkey, Jr., Fuel, 48, 179, (1969).
100
Table I. Hydroaromatic Compounds Investigated
Compound Tetrahydronaphthalene Octahydronaphthalene Decahydronaphthalene 9,lO-Dihydroanthracene
Source BuMines BuMines BuMines
80 60
$f
2
9,lO-Dihydrophenanthrene
Octahydrophenanthrene
BuMines BuMines
6 ; 5
Esso Esso Esso
ig
Tetradecahydrophenanthrene
Dihydropyrene Tetrahydropyrene Hexahydropyrene Hexahydropyrene Dodecahydropyrene Hexadecahydropyrene Octahydrocoronene Docosahydrocoronene Tetracosah y drocoronene
BuMines BuMines BuMines BuMines BuMines BuMines
40
,%
Esso Esso
5 5
H.
20
$ % 10
logl0y
I177
- 386.2~
8 6
0
.0001
.OW2
,0003
,0004
,0005
.OOC6
,0007
Decrease in reciprocal malecular weight of hydraaramatlc compound relative la aromatic ring system,
EXPERIMENTAL DATA AND RESULTS
Bureau of Mines (BuMines) data were obtained at a metered voltage of 7.5 eV and inlet system temperature of 300 OC on a CEC model 21-103C mass spectrometer for 11 hydroaromatic compounds. Data for four additional hydrocarbons (hexahydropyrene data were obtained by both laboratories) were obtained at Esso Research and Engineering Co. (Esso) on a similar mass spectrometer at an inlet temperature of 205 "C. The purity of the BuMines samples was estimated at >90% based on low-ionizing voltage data. First and second approximations were calculated to obtain sensitivity values representing the pure compounds. At 300 OC, the extent of dehydrogenation of the hydroaromatic structure in the mass spectrometer is believed to vary with each aromatic ring system as well as with the condition of the ion source ( 4 ) . Since complete vaporization of the oils in which the hydroaromatics occur generally requires an inlet temperature of approximately 300 "C, no attempt was made to obtain sensitivities for these samples at the optimum temperature to minimize dehydrogenation. Compounds examined at Esso included dihydroanthracene which is known to decompose extensively at 300 "C ( 4 ) . For this reason, an inlet temperature of 205 O C was employed for all their calibration data. Both gas chromatography and mass spectrometry measurements were used by Esso to correct observed sensitivities to a pure compound basis. Figure 1 illustrates the relationship between the sensitivities of hydroaromatic compounds and their parent, aromatic ring systems. The decrease in reciprocal molecular weight of a hydroaromatic (or perhydroaromatic) compound relative to the aromatic ring system is determined (x) and the sensitivity of the hydroaromatic read from the curve or calculated from the equation in terms of per cent of the sensitivity of the parent aromatic ring system ( y ) . (4) J. L. Shultz, Spectrosc. Lett. l ( 8 and 9), 345 (1968).
Figure 1. Equations relating hydroaromatic compound sensitivities to sensitivity of aromatic ring system a. Hydroaromatic compounds b. Perhydroaromatic compounds
Equation a for hydroaromatic compounds except perhydro, (log,,y = 1.98 - 736.6~)calculated from the BuMines experimental data, should have its y intercept at 2.00 since the aromatic sensitivity is arbitrarily designated as 100% (shown by the dotted line). Equation b (logloy = 1.177 - 386.2~)for perhydro compounds, determined from only 4 points, will probably be more sensitive than equation a to different mass spectrometers and operating parameters. Deviation of the dotted line (b) representing the best fit visually from the solid line representing the equation is slight. The hydroaromatic sensitivity data obtained at 205 "C were not used in calculating Equation a. Four out of the five values are below the line of the equation, while three out of eleven values obtained at 300 OC are low. Since no attempt was made to standardize operating parameters such as ionizing voltage, electron current, source temperature, etc., the authors believe the influence of inlet temperature on relative sensitivity behavior is not critical. Data obtained using these calculated sensitivity factors should be comparable to the + l o % of the amount present usually estimated for low-ionizing voltage analyses. CONCLUSIONS
Sensitivity data calculated from these equations should be comparable to low-ionizing voltage sensitivity data for other classes of compounds and permit semi-quantitative analyses accurate to +IO% of the amount present. RECEIVED for review January 24, 1971. Accepted March 30, 1972. Work by Esso Research is a contribution from CRCAPRAC Project CAPE 12-68.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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