High Mass Spectrometry Propylene Polymer, Alkylated Benzene, and Wax Analysis R. A. BROWN, D. J. SKAHAN, V. A. CIRILLO, and F. W. MELPOLDER The Atlantic Refining Co., Philadelphia, Pa.
b During the past decade the high mass spectrometer has assumed an outstanding position for analyzing mixtures of organic compounds having low vapor pressure. Progress is described relating to the improvement of instrumental and operating techniques which provide greater reliability and versatility than previously attainable. Quantitative methods of analysis are presented for propylene polymers, alkylated benzenes, and petroleum wa%es. The wax calculation procedure is designed for handling on a digital computer asshown. Calibration data are included which should enable other laboratories to use these methods directly. laboratories employ -trometers A modifiedfor ofcommercial mass specthe analysis of heavy m h m m
organic compounds. These spectrometers are equipped n-ith heated sample inlet systems (250" to 350" C.) which permit spectra to be obtained of hydrocarbons containing as many as 50 carbon atoms per molecule. The resolution required in this work is such as to permit quantitati1.e measurement of ions over the mass range 2 to 600. O'Seal and Wier first described a mass spectrometer of this type (16). Other similar mass spectrometers also are discussed in the literature (12, 15). During the past fmv years these instruments have been demonstrated t o be readily adapted t o hydrocarbon-type analysis-Le., kerosine and heating oil distillate ( 2 ) , gas oil (10, 12), and lubricating oil (6, 14). I n addition, they are ideally suited to detecting impurities in relatively pure compounds ( I S , 16). Many of the applications t o date have been in hydrocarbon analysis, but other types of compounds also have been successfully analyzed. In nonhydrocarbon analysis the work of McLafferty (IS) has been outstanding. Others have identified and quantitatively determined nitrogen compounds in heating oil distillate and shale oil (8, 18), sulfur compounds in gas oil (la), and aliphatic alcohols in mixtures having a CI8 to Cas molecular weight range (4). Operating techniques have been fairly
standard. Very simply, a heated sample bottle is used with an ion source which is like that used in low molecular weight analysis. Samples are intrcduced through a gallium-sealed heated sintered disk or a Teflon plug system (5) and are heated until entry into the ion source. I n practice, although these techniques enable work to be accomplished, considerably more trouble is encountered as compared with systems operated a t room temperature which deal with relatively volatile compounds. These difficulties include slow sample introduction, local hot or cold spots, inconvenient pump-out valves, excessive carbon formation in ion sources, sample decomposition, unstable mass spectra, poor resolution, and lack of calibration compounds. These problems have been studied in this laboratory and numerous improvements in apparatus and techniques have been instituted. Based on these efforts the high mass spectrometer in this laboratory has become a highly reliable instrument operated by skilled routine workers. Coincident with the effort to improve operating conditions, a number of methods were developed which are of interest to the petroleum and chemical industries. Of interest in the detergent field, for instance, are methods for the analysis of Ce-CI5propylene polymers and CI8-Ca4alkylbenzenes. These procedures provide for the determinaGLASS KNOB INDIUM SINTERED DISK
E>
Figure 1.
HEAT I NG WIRE
Heated sintered disk
tion of hydrocarbon types and a molecular weight distribution of the principal components. Considerable compositional information concerning paraffin waxes has been published (9, 20). A detailed method for analyzing such samples has been adapted to digital computation. APPARATUS A N D TECHNIQUES
The spectrometer used in this work is a modified Consolidated 21-103A instrument which is now able to handle compounds with a volatility as low as that of a Csohydrocarbon. Sample Inlet System. The conventional sample introduction system consists of a heated sintered disk sealed with gallium. Because of the exposure of this metal t o air, partial oxidation occurs t o form oxides which do not thoroughly wet glass. As a result air leakage paths develop. -4 new design for a sintered disk as s h o w in Figure 1 eliminated this trouble by making such an air path highly unlikely. It also resulted in better heat distribution around the disk and a greatly accelerated rate of sample vaporization into the inlet bottle. The glass knobs which are shown are used to anchor the heater wire. It was also found that high purity indium (99.99%) could be used in place of the much more expensive gallium. Indium oxidizes to a degree similar to that of gallium. The sample in a 2-liter glass bottle is placed in an insulated oven kept at 300' C. Glass inlet and connecting lines are also maintained a t 300' C. Glass lines are difficult to keep a t an evenly distributed temperature because of bends and the difficulty of getting a uniform heat distribution from wires. Experience has shown that this can be done but that liberal use of thermocouples is required for proper control of heat. Thermocouples tend to oxidize and, to minimize this oxidation, couples are placed in oil-filled wells which are bent t o conform to the outside circumference of the line whose temperature is to be measured. NICKEL T U B E
GOLD T U B E
Figure 2. Cover plate internal heater
VOL. 31, NO. 9, SEPTEMBER 1959
1531
To provide heat between the sample leak and the ion source an internal heater assembly was designed (Figure 2). The heater is comprised of several concentric tubes sealed together to allow a heater coil to be wound over the entire length in a region of atmospheric pressure. Radiation losses are reduced by the 0.5-mil platinum foil shield placed as shown. The heater wire is insulated by a glass braid and wrapped bifilarly so as to bring both wire leads out of the tube together. The connector between the internal heater and the precision tubing of the ion source is a gold tube held in position by a glass spring. A 1-mm. glass wafer provides electrical insulation between the gold tube and ion source. This heater is energized with direct current from a direct current power supply to minimize 60-cycle vibration inside the magnet gap. Temperatures of 300" and 210" C. are maintained on the high and low sample pressure sides of the leak, respectively. An integral part of a sample inlet system is the pump-out valve. Valves described previously were cutoffs in which molten gallium (f.2, 16) provided the seal. These valves are objectionable because of spillage and the desirability of eliminating metal whereever possible. Caldecourt has described a valve consisting of a metal bellows on the vacuum side and a Teflon seat (6). Because of the thermal instability of Teflon a t 300' C., this particular valve could not be used, but a similar design has been utilized as shown in Figure 3. The valve seat consists of a polished 18/9 ball and socket joint which is champfered to eliminate seizure, The stainless steel bellows has an Q/16 inch travel. A typical pump-out rate of this system is 98% of a CSOhydrocarbon in 3 minutes.
Ion Source. During the early years of operation of this spectrometer spect r a of Cl&60 hydrocarbons were not highly reproducible, even though a constant ion source temperature was apparently maintained. This variability was of constant concern in the applications dependent upon quantitative measurements. A study showed that these spectral fluctuations could be minimized by varying the ion source temperature on a daily basis. To do this conveniently, a circuit was installed by which the current to the ion source heater could be readily adjusted manually. Very simply, the circuit includes a rheostat, an ammeter to indicate heater current, and a potentiometer for temperature measurement. I n practice, the ion source temperature is varied so as to maintain the n-hexadecane 127/226 peak ratio a t 1.40 =t0.02. Stability was dependent upon the usual butane-butene treatment of the tungsten filament. I n addition, some nonlinearity occurred in ion intensities over a sample pressure range of 15 to 100 microns. To eliminate such errors 1532
ANALYTICAL CHEMISTRY
Q -WAX
SEAL
TO
DlFF 1819 BALL Et
- SOCKET PYREX JOINT TO M.S.
Figure 3.
Pump-out valve
all mixtures and calibrations are normally run a t 15 to 30 microns. I n regard to the problem of a stable filament, Sharkey and coworkers (19) have carried out a more systematic study of filament conditioning. Based on x-ray examination of filament surfaces, they found that a stable filament seems to be associated with the formation of ditungsten carbide (W2C). An over-conditioned filament, on the other hand, is characterized by tungsten carbide (WC) and can be expected to give fluctuating spectra. It was determined, however, that such a filament could be stabilized by treatment with oxygen. This work led to a trial use of a rhenium filament by Robinson and Sharkey (17). The performance of rhenium was highly encouraging, in that stable patterns and relatively long life were achieved. An additional advantage for rhenium is that it does not react with carbon or oxygen as does tungsten to change its emissivity. A number of other laboratories have now recorded similar success with rhenium filaments ( 7 ) . Performance of Spectrometer. The mode of operation described previously has been adhered t o for more than 3 years, during which a large number of samples have been analyzed, This work has coincided with repetitive calibration data of numerous compounds. n-Hexadecane, for instance, is run once or twice daily a8 a reference compound. Its spectrum 0.02 for the is held constant a t 1.40 127/226 peak ratio by adjustment of the ion source temperature. The temperature is adjusted only a t the beginning of each day and is held constant from then on. Spectra of Czp and lighter compounds are relatively constant. This is not true of heavier compounds, however. n-Triacontane, for instance, varies considerably more, as 1.0 obshown by the range of 5.6
*
*
served for the peak ratio 197/430. Over-all, however, spectra of calibrating compounds are generally stable enough to reduce greatly the frequency of calibration runs. A very gratifying observation of spectral behavior has been the stability of compounds in the sample inlet system. Earlier experience with an operating temperature of 300' C. had shown some serious problems of thermal decomposition. As the inlet system is now constituted, little, if any, hydrocarbon decomposition is observed. This is attributed to the maintenance of evenly distributed heat and an almost complete lack of metallic parts in the sampling apparatus. Other organics, such as sulfur and nitrogenous compounds, are also stable, although oxygenated compounds frequently are not. Even such compounds, however, can be handled successfully, if, as is usually the case, the spectrum of the compound(s) is reproducible (4). APPLICATIONS
A number of applications of high mass spectrometry have been described. Many of these are compound-type analyses which are not so dependent upon reproducible spectra as are component analyses. To place component analyses on a reliable and routine basis, it is absolutely essential to have reproducible spectra. Relatively pure calibration materials aJe also needed. After the improvements discussed above had been accomplished, an effort was made to study reproducibility of spectra, procure suitable calibration standards, and establish methods of calculation for polymer, alkylated benzenes, and waxes. PROPYLENE POLYMER ANALYSIS
Propylene polymer of low molecular weight (Cg-C15)is a major petrochemical which is manufactured and/or used by a number of chemical and petroleum companies. To satisfy the need for complete compositional information on such samples, calibration material and a calculation procedure have been devised to determine molecular weight distribution of mono-olefins and hydrocarbon types such as paraffins, mono-, di-, and/or cyclo- and tri- and/or dicyclo-olefins, alkylbenzenes , and "alkenyl" benzenes (empirical formula, CJL-s). Calibration standards of mono-olefins of single carbon number were prepared using high efficiency distillation and silica gel percolation on selected stocks. Calibrations for other compound types were deduced from pure compounds obtained from API Projects 6 and 42. Data in Table I present
experimental conditions, summation (or sigma) peaks, and volume sensitivity coefficients according to molecular weight. The principle of summation peaks for type analysis was first discussed by Brown (1). Four simultaneous equations consisting of calibration coefficients as shown in Table I and mixture peak sums are solved to determine each of four hydrocarbon types. This calculation provides a total olefin concentration, which is then subdivided into the empirical types CnHln,CnHPn--, and CnH2n--4by using the ratios based on the 2125, 2123, and 2121 matrices. I n practice, inverse solutions are set up for repetitive calculations. In order to select calibrations of the proper molecular weight to fit a given mixture, a molecular weight distribution of mono-olefins must first be calculated. An average molecular weight can then be determined. Coefficients to carry out this step are listed in Table 11, which also includes relative volume sensitivities of parent ions. These coefficients apply to monoisotopic peak values. Low voltage techniques can be used to determine molecular weight distribution with greater accuracy, but they provide no information regarding nonolefinic constituents (11). As a matter of interest data in Table I11 show a c16 polymer analysis using a 70-volt mass spectrum. Repeatability observed for samples, in general, is *0.2’%. Absolute accuracy of the hydrocarbontype determination has been estimated to be + 2 to 3%.
I.
Matrices for Hydrocarbon-Type Analysis of Propylene Polymers
Olefins
(Volume sensitivity coefficients) Alkyl- AlkenylbenbenParaffins zenes“ zenesb Carbon S o . 9.0 47.8 4.4 6.2 2121 145 0.6 2.1 2123 296 82.5 2125 0.9 11.8 206 0.1 Carbon No. 10.0 63.5 13.0 2121 12.9 192 7.0 2123 1.2 1.2 287 80 2125 12.4 200 0.2 Carbon No. 11.0 20.6 19.3 73.6 2121 223 2.5 11.2 2123 1.3 275 79.1 2125 0.2 13 2 193
2 41 2 43 2 77
176 34.3 4.0 0.2
2 41 2 43 2 77
177 42.5 5.1 0.2
2 41 2 43 2 77
2103
178 53.5 5.5 0.2
2 41 2 43 2 77 2 103
179 68.0 5.9 0.1
83.5 245 1.5 0.2
2 41 2 43 2 77
179 82.3 6.3 0.2
87.5 257 1.6 0.3
2 41 2 43 2 77
179 98.4 7.0 0.2
90.1 265
179 112 7.3 2103 0.2 CnHzn-~.
91.5 269 1.6 0.3
2103
2103
2103
2103
1.6
0.3
2 41 2 43 2 77
Carbon No. 2.0 26.0 23.6 2121 5.7 14.6 2123 2125 260 76.5 14.4 182 Carbon No. 3.0 30.6 28.0 2121 11.8 18.1 2123 245 70.5 2125 16.0 172 Carbon No. 4.0 34.2 30.4 2121 2123 20.3 20.2 228 64.9 2125 17.6 158 Carbon No. 5.0 2121 35.6 31.6 37.6 21.3 2123 209 60.5 2125 19.2 144
Olefin Typec C,H*, CnHzn- 2 0.1
0.2 20
0.2
1.9
20
CnH2n- 4
7.2 60
70 0 0
0
70 0.6
8.1
60 0.5
0
0.3 3.5 20
60
9.3
70
0.9
0
0.4 5.0 20
10.2 60.0 1.2
70 1.4
0.6 6.5 20
11.2 60
70
1.5
1.8 0
0.8
8.0 20
12.3 60 1.6
70 1.9
0.9 9.4 20
13.2 60
70 2.1
1.1
0
0
0
1.8
‘ Relative volume sensitivity coefficients. CAn-8.
ALKYLATED BENZENE ANALYSIS
5
Detergent alkylates are essentially monosubstituted alkylbenzenes manufactured by alkylating benzene with a Clz propylene polymer. I n practice a fairly wide molecular weight range may be encountered along with impurities, such as saturates and “alkenyl” benzenes (empirical formula C,H2n-8).
Table II.
Table
Ionizing voltage, 70. 2 41 = 41 55 2 43 = 43 57 2 77 = 77 78 148, etc. 2103 = 103 104 2121 = 121 122 2123 = 123 124 125 126 2125
Mass spectrometer opersting conditions. Magnet current, 0.9 ampere. 69 83. 71 85. 79 91 92 105 106 + 119 120 + 133 134 147
++ ++ ++ + + + + + + + ++ ++ 117 + 118 + 131 + 132 + 145 + 146, etc. 135 + 136 + 149 + 150, etc. + + 138 + 151 + 152, etc. + ++ 137 139 + 140 + 153 + 154, etc.
L.
+
+
+
580 220 200
700 260 215 102 95 28
Summary of Monoisotopic Peak Contributions in Propylene Polymers
Per Cent
84 98 112 126 140 154
44 4
0 100
76 40 5 0 100
168
182 196 210 224 238 252 Rel. vol. sensitivity of parentpeak 19.7
100 56
18 5 0
100
190 95 40 11 1
0 100
200 100 50 17 7 6 0
100
340 200 75 25 6 6
365 220 125 64 30 22 10
100
0
50
0
1
100
15.7
12.8
10.5
8.51
6.83
5.66
4.73
3.94 VOL. 31,
3.48
400 220 130 80 39 22 15 4 10
0 100
3.01
80
69 22
18
21 23 6 0
100 2.64
NO. 9, SEPTEMBER 1959
31 38
36 12 6 0
100 2.31 1533
Table 111.
Analysis of a Propylene Polymer Distillate
Component Paraffins Mono-olefins
VOl. 7 0 4
c 1 3
1
c 1 4
1
Vol. 7 0 4 23 9
13 3 100
20 22
CIS CIS
Table IV.
Component
Matrices for Hydrocarbon Type Analysis of Alkylated Benzenes
(Volume sensitivity coefficients) 1?araffins
Cycloparaffins
Alkenylbenzenes (CnHzn- s)
Alkylbenzenes
Carbon KO,15 (C, Benzene) 2 71 2 67 2 77
78.0 18.7 0.3
2 71 2 67 2 77
82.0 21.3 0.3
Carbon No. 16 ( Clo Benzene) 10.7 8.1 178. 7.7 0 350. 0 32.9
6.2 10.8 77.0 308.
2 71 2 67 2 77
83.0 24.1 0.3 0
Carbon No. 17 (Cll Benzene) 14.9 9.1 186. 9.8 0 350. 0 34.3
6.2 10.8 77.0 308.
2 71 2 67 2 77
2103
84.0 26.9 0.3 0
Carbon No. 18 (C12Benzene) 17.3 10.5 192, 11.9 0 350. 0 35.7
6.2 10.8 77.0 308.
2 71 2 67 2 77
84.0 29.4 0.3
Carbon No. 19 (CISBenzene) 21.6 12.6 196. 12.6 0 350. 0 37.1
6.2 10.8 123. 308.
Z 71 2 67 2 77 2103
84.0 31.1 0.3
Carbon No. 20 (Cld Benzene) 23.9 14.7 199. 14.0 0 350. 0 38.2
6.2 10.8 123. 308.
2 71 2 67 2 77
84.0 32.8 0.3
Carbon KO.21 (CISBenzene) 25.5 16.8 196. 14.7 0 350. 0 39.2
6.2 10.8 123. 308.
2 71 2 67 2 77
83. 34.9 0.3
2103
2103
2103
2103
2103
6.2 10.8 77.0 308.
0
0
0
0
0
Carbon No. 22 ( C16 Benzene)
2103
2 71
67 z 77 2103
W A X ANALYSIS
6.2 10.8
123. 308.
0
Carbon No. 23 ( C17 Benzene) 28.4 22.4 189. 17.5 0 350. 0 41.3
82.0 36.1 0.3 0
6.2 10.8 123. 308.
Carbon Na 24 (CISBenzene) 71 81.0 27.6 25.2 6.2 2 67 37.3 184. 19.3 10.8 z 77 0.3 0 350. 123. 0 2103 0 42.4 308. Mass spectrometer operating conditions. Ionizing voltage, 70. Magnet current, 1.3 amperes. 2 71 = 71 85. 2 67 = 67 68 69 81 82 83 96 97. Z 77 = 77 + 78 79 91 92 105 106 119 120 133 134 147 + 148, etc 2103 = 103 104 117 118 131 + 132 145 146, etc 2
+ + + + + + ++ ++ + + + + + + + + + + +
--
1534
--I
0
-
ANALYTICAL CHEMISTRY
--_
+
- -.
+
-
+
-
To develop a method of analysis, calibration fractions were first collected using highly refined separation and distillation techniques. In all, monosubstituted benzenes were obtained consisting of Cp, Clf, C,a, and CIS side chains. I n addition, a didodecylbenzene fraction was prepared. A study.of spectra of these fractions and those of pure paraffins and cycloparaffins resulted in a hydrocarbon-type analysis including alkylbenzenes, paraffins, cycloparaffins, and the “alkenyl” benzenes. A few alkenylbenzenes provided the basis of calibration for this class of compounds. This approach is not likely to introduce significant errors, as these compounds are present in minor quantities. Instrumental conditions, volume sensitivity coefficients, and the sigma peaks involved are tabulated in Table IV. Matrix solutions are used in the same manner as for the propylene polymers. Normally, alkylbenzenes are first determined as a group and then resolved according to molecular weight. To do this, monoisotopic parent peaks are calculated and then corrected for contributions of heavier compounds. These contributions and the relative volume sensitivities of respective parent peaks are shown in Table V. This method of analysis has been applied to hundreds of samples and has played an important role in both laboratory and plant processing studies. I n one study a statistical survey of the analytical data showed that the reproducibility was good enough to n-arrant reporting analyses to 0.01%. Reproducibility over a span of several years is within 10.20/,. The absolute accuracy is difficult to determine, but comparison of analyses of total samples with the composite of silica gel percolate fractions indicates each hydrocarbon type is accurate within *l%. An analysis of a crude alkylated benzene distillate is shown in Table VI.
-_
.----I
I n the petroleum industry the ability of a mass spectrometer to analyze wax has been of considerable value in research and process studies. This was first discussed by O’Neal and Wier (16) and since then others have made extensive studies to relate composition with wax properties. In this laboratory an effort was made to place the method on a reliable, routine basis. Trouble was encountered because normal para& spectra varied enough SO that frequent recalibration m a s necessary. T o allow for this range of variation, five sets of calibrations were determined so that one set could be selected to analyze any given sample. In practice, calibrations were selected which seemed to fit a samde
Table V.
Summary of Monoisotopic Peak Contributions in Alkylated Benzenes
Parent Peak
162
176
190
204
162 176 190 204 218 232 246 260 274 288 302 316 330
100
...
1 1 100
1 1 1 100
Rel. vol. sensitivity of parent peak
100
23.0 17.0 12.0 8.69
best. This procedure minimized recalibration, but at least 2 hours was still required to obtain a n analysis using a desk calculator. Because a faster calculation was desirable, it was decided to program the calculation for a digital computer. I n the course of developing this program simplifications resulted which also shorten the hand calculation. The computer used in this work is a Marchant Miniac I11 which has a 4096-word memory. However, any computer with a memory of 1500 words or more can be used. The principal problem involved in converting to machine calculation arose from the !arge number of calibration data which had to be considered. In a typical problem which covers the Cl4 to Ca6range and includes normal paraffins, isoparaffins, monocycloparaffins, and alkylbenzenes, coefficients are selected from a total of 6000 values. Actually, however, only about 1400 of these are used for a given sample. For this reason, it was decided first to develop a program which would work with one selected set of 1400 coefficients. I2fforts in this direction resulted in a method of calculation requiring only 736 coefficients; of even more importance, it was found possible to make these coefficients do the work of 6000 by applying, in effect, correction factors to mixture peaks. The calculation is carried out with polyisotopic peaks only and consists of two steps: 1. Calculate partial weight fraction of Clc to C g normal, iso-, and monocycloparaffins and alkylbenzenes. 2. Normalize partial weight units to 100%.
Normal paraffins and alkylbenzenes can be calculated in a straightforward manner, whereas the other compounds are more complex. A. Calculation of Normal Parafis. A 3 X 3 matrix needs to be solved to determine the partial weight fractions
218
232
Per Cent 6 6 1 6 1 1
100
5.80
1 1 1
260
274
288
302
316
330
6 6 6
6 6 6 3
1 100
6 6 6 6 2 2
7 10 6 4 2 2 2
8 15 8
1 1 1 100
6 6 6 4 2
9 20 10 2 2 2 2 2 2
1 1
100
4.60
246
3.83
3.35
of each normal paraffin. The system is: Normal
Is0
Cyclo
all
a12 a22 a32
ala
all a31
a23 a33
Peak Pm pm-1
p m -2
where all, azl, as1 = weight sensitivities of normal paraffins at peaks P m , P m - 1 ,
prn-2 alz, a32,a13, a23, as3 = weight sensitivities of iso- and cycloparaffins, re-
spectively, a t P m , etc. m = corresponds to the molecular weight mass of the normal paraffin being calculated m - 1 = molecular weight mass minus l m - 2 = molecular weight mass minus 2 P = mixture peak a t m, m - 1, m - 2 I n practice an inverse solution is cal-
Table
VII.
19 20 21 22 23 24 25 26 27 28 29 30 31
32 33 34 35 36
1 1 1 100
100
3.00
2.70
Table VI.
1 1 1
100
2.43
1 1 1
1 1 1
100
2.21
100
2.00
Analysis of an Alkylated Benzene Distillate
Vol. % 0
Component Paraffins Cycloparaffinsand/or
9
mono-olefins hlkylbenzenes CIS
17
CI 9 C?O C?, C2?
11
12 19 9 6 4 3
C?.
1 1
8 100
culated for each normal paraffin which appears as pn = Kipm K2Pm-' K3Pm-2 (1) K1, KP, and K3 are positive or negative
+
+
Inverse Equations for Calculating Partial Weight Fractions of Normal Paraffins (Equation 1)
Carbon No. of n-Paraffin 14 15 16 17 18
1 1 1
2 2 2 2 2
KI 0.247 0.297 0.348 0.401 0.462 0.537 0.608 0.744 0,881 1.044 1,255 1.520 1.760 2.090 2.447 2.870 3.346 3.721 4.020 4.374 4.587 4.803 5.031
rn 198 212 220 240 254 268 282 296 3 10 324 338 352 366 380 394 408 422 436 350 464 478 492 506
K2 -0.037 -0.048 -0,060 -0.074 -0.089 -0.110 -0.131 -0,168 -0.209 -0.258 -0.324 -0.409 -0.494 -0.609 -0.737 -0.895 -1.078 -1.241 -1.382 -1.548 -1.670 -1.800 -1.947
m-1 197 211 225 239 253 267 281 295 309 323 337 351 365 379 393 407 42 1 435 449 463 477 49 1 505
K1
0.003 0.004
0,005
0.007 0.009 0.012 0.015 0.019 0.025 0.032 0,044 0.058 0.072 0.092 0.115 0.143 0.180 0.215 0.248 0.279 0.309 0.343 0.385
VOL. 31, NO. 9, SEPTEMBER 1959
m-2 196 210 224 238 252 266 280 294 308 322 336 350 364 378 392 406 420 434 448 462 476 490 504 1535
1536
ANALYTICAL CHEMISTRY
iso-, and monocyc b a r a f f i n s Jrespectively
constants which are valid for all mixtures As it is desired to be able to determine 23 different compounds of this type, 23 equations are needed, which means that 23 X 3 or 69 constants are stored in the computer. Values of K1, K z , and K3 are tabulated in Table VII. I n Equation 1p n represents the partial weight fraction of each normal p a r a f h as represented by n,which indicates the number of carbon atoms in its molecule. Thus, n varies from 14 to 36 corresponding with C1, to B. Calculation of Isoparaffins and Cycloparaffins. Isoparaffins are calculated using relatively intense ions which correspond to the isoparaffin molecular weight minus 43 mass unitsi.e., a Cs isoparaffin of molecular weight 394 would be calculated using the 351 peak. Peaks of this mass series are selected because it is felt that an average isoparaffin will dissociate so as to lose 43 mass units. All normal paraffins whose molecular weights exceed the mass of that peak being used for a given isoparafh contribute to that peak and consequently all of these contributions must be considered. Contributions of cycloparaffins to isoparaffin peaks consist only of a peak due to heavy isotopes of carbon or hydrogen. Thus, the contribution of cycloparaffins to a given isoparaffinic peak is confined only to cycloparaffins of one molecular weight species. In the case of cycloparaffins, parent ions are used for analysis. Such peaks coincide with the (molecular weight minus 44) peak of a given isoparaffin as ell as with fragment peaks from any normal paraffin having a molecular weight greater than that of the cycloparaffin. Equations which represent partial weight fractions ( f i ) of iso- and monocycloparaffins are given below. ( p - -~pc
[;
x
Sem-l
SLm-2
Som-l/Scm-2
=
Zcm-l
pc
= KI" P m - 2
+ K2"
pi
Isoparaffins can be expressed by Equation 4,
n = 36 Z K3" pn
= isotopic con-
tribution of monocycloparaffins to P m - l peaks. This is a constant for any given molecular weight. Sim-2/S~m-1 = 0.35 = a constant ratio which has been measured for isoparaffins (16) and confirmed in this laboratory.
n=c
(3')
This is the working equation for cycloparaffins; the values of K1", Kz", and K3" are tabulated in Table IX. C. Calculation of Alkylbenzenes. Benzenes are easily determined b y Equation 5, Pb
or
Irb
=
p m -8 -~ Sa" -8
(5)
K"'pm-8
where = partial weight fraction of
pb
To simplify the working equations, Equation 4 may be written as p i = K1' p m - 1 + K2' p m - 2 n = 36 ZK3' ~n (40 n = c = i - 3 Values of Kl', Kt', and K3' are tabulated
Table X. Inverse Coefficients for Calculating Partial Weight Fractions of Alkylbenzenes (Equation 5)
Carbon No. of Alkylbenzene 14 15
16
0.077 0.077 0.077 0.077 0.083 0.089 0.095 0.100 0.102 0.105 0,108 0.111 0.114 0.118 0.121 0.125 0.129 0.134 0.138 0.143
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33
(2)
-
K1"' 0.077 0.077 0.077
17 18
-
y y ] ) / S % m - l
u, x
To facilitate the final calculation of iso- and monocycloparaffins, the value of pc from Equation 3 is substituted in Equation 2 and solved for p i . This can be further simplified by recognizing that
in Table VIII. These constants are applied to the appropriate mixture peaks and partial weight fractions of normal p a r a h s in order to calculate partial weight fractions of isoparaffins. Equation 3 is the basis of the cycloparaffins calculation and may be represented as
34
35 36
m-8 190 204 218 232 246 260 274 288 302 316 330 344 358 372 386 400 414 428 442 456 470 484 498
alkylbenzene mixture peak a t mass m - 8 = weight sensitivity coefficient of alkylbenxene a t m - 8 mass K"' values are shown in Table X.
Pm-*
=
D. Development of Calculation Procedure. The equations discussed above lend themselves to machine computation and 736 coefficients can now accomplish what 1400 did previously. These equations can also be used in manual computations. Some coefficients are constant and others are variable. The number of coefficients involved, their make-up, and variability are outlined in Table XI. The variable coefficients present a major problem because only a small proportion of samples can be calculated with just one set of coefficients. I n practice, any one of five sets is normally applicable. This might be handled by having 5 X 552 = 2760 (variable) plus 184, giving a total of 2944 coefficients to be stored in the computer. This approach was not attractive, because along with the program and permanent routines the computer capacity was taxed, excessive loading time of the program resulted, and a large number of man-hours were necessary to calculate the coefficients. I n an attempt to handle all samples using one set of coefficients, a study was
where partial weight fraction of normal, 190-, and monocycloparaffins and by definition, i = n+3=c+3 P - l , Pm-2 = mixture peakc at masses m - 1 m . 2 as in normal paraffin calculatior S. S, 8, = weight sensitivity en. efficients of ~ P ~ H
pn,p i , fie =
Table XI.
Summary of Coefficients Needed to Calculate a Paraffinic W a x
No. of Coefficients Needed
Components Normal araffins Isopara#ns Cycloparaffins Alkylbenzenee
~
Fixed 69 46 46
Variable
_23_
c!
184
552
0 276
276
How Calculated Inverse so1;ition Equation 4 Equation 3' Equation 5
I
VOL. 31, NO. 9, SEPTEMBER 1959
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~
C17 - -. C18 c19 c20 c21 c22 C23 C24 C25 C26 C27 C28 C29 C30 C31 C32 c33 c34 c35 C36 c37 C38 c39
Table XII. Analysis of a W a x as Calculated by Miniac Ill Normal Is0 Cyclo Ben no n00.0 00.0 00.0 00.4 00.0 00.0 00.0 00.7 00.0 00.0 00.0 00.0 01.6 00.0 00.0 03.7 01.3 00.2 00.0 04.7 01.4 00.1 00.0 00.0 01.7 00.3 05.3 00.0 01.8 05.4 00.4 02.9 05.5 00.0 01.1 01.7 03.3 05.6 00.0 00.0 02.0 04.0 04.0 03.8 00.0 03.8 02.0 03.3 04.1 00.2 02.5 01.7 00.3 01.6 03.8 00.8 03.3 00.4 01.6 00.3 02.2 00.4 00.9 00.0 00.0 00.2 01.9 00.0 00.0 00.0 00.9 00.0 00.8 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 00.0 46.8 14.4 01.5 37.2
and monocycloparafms are eliminated in similar fashion. Table XI1 presents a typical analysis. Because of the slow input time of a Flexowriter in this case, it takes 6 minutes to enter the mixture peak data, do the calculation, and type out the results. Of this only about 2 minutes are needed to calculate and type out the final analysis. ACKNOWLEDGMENT
The authors acknowledge the suggestions of J. A. Alexander, who constructed the glass apparatus described. They are indebted to P. J. Contino for help in the laboratory and to Frances Galbraith who carried out most of the calculations. Thanks go also to E. J. Levy for several constructive ideas regarding the apparatus. REFERENCES
(1) Brown, R. A., ANAL.CHEW23, 430
made of all normal paraffin calibrations over a 2-year period. The results indicated that a systematic difference in patterns may occur between two ion sources. Because of this uniform fluctuation of fragment ions, it was found possible to incorporate a correction factor to be applied to all fragment ions in a mixture. The correction is built into the computer program and is determined according to the comparison of the observed and calculated mixture peak at mass 197. I n practice it was more convenient to correct the parent ions-Le., the partial pressures of normal paraffins. If the coefficients being used are theoretically correct, this comparison should show a zero difference. During calculation by the computer the following correction schedule is followed : Observed Peak at Mass 197 Minus Calculated Peak +2.3% +4,6% +6.9% +9.2% +11.5%, etc. -2.3% -4.6% -6.9T0, etc.
COMPUTER PROGRAM
As soon as the mathematical solutions and pattern variations had been determined, it became feasible t o program 6
ANALYTICAL CHEMISTRY
Carbon No. Partialpressure
(1951). (2) Brown, R. A,, Doherty, W., Spontak, J., Consolidated Electrodynamics Corp., Pasadena, Calif., Group Rept. 84 (1951). (3) Brown, R. A., Melpolder, F. W., Young, W.S., Petrol. Processing 7, 204 (1952). (4) Brown, R. A., Young, W. S., Nicolaides, Nicholas, ANAL. CHEM. 26, 1653(1954). (5) Caldecourt, V. J., Ibid., 27, 1670 (1955). (6) Clerc, R. J., Hood, A., O'Neal, M. J., Jr., Zbid., 27, 868 (1955). (7) Consolidated Electrodynamics Corp. Clinic, Meeting of ASTM Committee
31 32 33 34 35 26 27 29 30 28 0.10 0.00 0.00 0.00 0.01 1.00 2.20 3.30 2.40 1.20
In this case the 0.1 value of nC2e should be considered to be 0.00, and this can readily be done in manual computations. To do this within the computer, successive values of L ( ~ (partial
Partial List of Correct Ion Factors Applied to Partial Weight Fractions of Normal Paraffins nC14 nC25 n C 7 1.044 1.076 1 . 0000 (1.044)2 (1.076)* 1.OOOO (1.044)8 (1. 076)a 1.0000 (1,044)' (1.076)' 1.0000 1.0000 (1.044y, etc. (1.076y 1.0000 0.956 0.924 1 . 0000 (0.956)a (0.924)* 1.0000 (0.956)8,etc. (0.924)'
Calculations based on these factors were found to agree favorably with manual computations. As a matter of fact, they should be more precise, because a lower limit of &2.3% is now placed on the difference as contrasted with 3=5% formerly permitted in desk calculator results.
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a computer to perform wax calculations. The program is straightforward, although it consists of considerable detail, as indicated by the fact that 768 memory locations are utilized. Other than the intricacies of the calculation itself, only one major problem occurred in devising the program: random errors due to the spectral data. It is possible, for instance, to calculate incongruous molecular weight distributions, such as:
weight fractions of normal paraffins) are searched until a maximum value is located (nCaa = 3.3 in this case), then pn values are checked at successively lower molecular weights until a value
