748
Anal. Chem. 1982, 5 4 , 748-755
Probe Microdistillation/Mass Spectrometry in the Analysis of High-Boiling Petroleum Distillates L. R. Schronk' Department of Chemlstry, Texas A&M Universl& College Station, Texas 77843
R. D. Grlgsby*2 Department of Blochemistry and Biophyslcs, Texas A&M University, College Station, Texas 77843, and U.S. Department of Energy, Battlesvile Energy Technology Center, Bartlesville, Oklahoma 74003
S. E. Scheppele U.S.Department of Energy, Bartlesville Energy Technology Center, Bartlesville, Oklahoma 74003
Results for a monoaromatics fractlon from the 535-675 O C distillate of a Wllmlngton, CA, crude oil demonstrate the efflcacy of probe microdistliiation/mass spectrometry for the quailtative and quantitatlve analysis of mlxtures containing relatively nonvolatile substances. The probe temperature was controlled by a programmer either linearly or to maintain a constant ion current by feedback of the voltage from the ion current monltor to the programmer. Ions were produced by 70-eV and 10-eV electrons and by field ionlratlon. Quailtatively, ail three methods of ionization produced the same spectral patterns. Detectable compounds are distrlbuted in a molecular weight range from 400 to 776 and In -Z(H) series from 8 to 30. About 90% of the fractlon is comprised of compounds having -Z(H) numbers from 12 to 24. Volatility decreases wlth Increasing molecular weight for compounds of a given hydrocarbon type but Is effectively independent of -Z(H) for compounds having the same carbon number.
Mass spectrometric analysis of complex mixtures, such as those derived from coal and petroleum, depends on sample introduction by an expansion-volume inlet, direct probe, or chromatographic interface. A probe is commonly used for low-volatility samples that cannot be introduced by the other devices. Although spectra generated by probe introduction are widely used in qualitative analysis, their quantitative value has received only limited acceptance. In the past, relatively poor accuracy in measurement and control of sample temperature has hindered the development of reliable quantitative probe methods. The current study was undertaken to evaluate recent advances in probe-introduction technique and data reduction (1-7) for analyzing samples derived from high-boiling petroleum distillates. Results are presented on the analysis of a fraction separated from 535-675 "C distillate of a Wilmington, CA, crude oil. This sample was obtained from extensive fractionation of the original crude through the technique developed for the American Petroleum Institute Research Project 60 (8-12). EXPERIMENTAL SECTION History of the Sample. The details of the API 60 method are well documented in the literature and will not be repeated here. Instead, only a brief description of the separation will be given to identify the origin of the sample. The 535-675 "C disPresent address: Cities Service Technology Center, P.O. Box 3908, Tulsa, OK 74102. 2Presentaddress: U.S. Department of Energy, Bartlesville Energy Technology Center, P.O. Box 1398, Bartlesville, OK 74003. 0003-2700/82/0354-0748$01.25/0
tillate was obtained by vacuum distillation of the crude at a temperature of 371 OC and a pressure of 5 mtorr in a Rota-Film apparatus (Arthur F. Smith Co.). Acids and bases were removed by ion-exchange chromatography on Amberlyst A-29 and A-15, respectively (Rohm & Haas Co.). Complexation chromatography on ferric chloride separated neutral-nitrogen compounds. The remaining fraction, which presumably contained hydrocarbons and some 0, N, and S compounds, was separated into saturates, monoaromatics, diaromatics, and polyaromatic polars by adsorption chromatography on silica/alumina. Each of these fractions was further separated into 25-40 fractions by gel-permeation chromatography (GPC) on Styragel (Waters Associates, Inc.). A total of seven samples, including saturates, monoaromatics, and diaromatic concentrates, and four GPC fractions were analyzed by mass spectrometry. Monoaromatic GPC fraction 19 of separation 209-76 was collected near the midpoint of the chromatographic peak. The results of ita analysis are presented below. Instrumentation and Operating Conditions. Two mass spectrometers were used: a DuPont/CEC 21-llOB located at Texas A&M University, College Station, TX, and a Kratos/AEI MS-30 located at the Bartlesville Energy Technology Center, Bartlesville, OK. Spectra were recorded oscillographicallyon the CEC 21-llOB at 3000 resolution and by Kratos DS-55SM data system on the MS-30 at resolutions of 3000 and 12000. The latter instrument was equipped for double-beam operation. A resolution of 3000 was selected to maximize ion intensities for quantitative analysis on the MS-30 and still allow elemental compositions to be obtained for those peaks not consisting of multiplets. Compositions obtained at 3000 and 12 000 resolution were consistent, but not all of the ions visible in the lower-resolution spectra were seen in the spectra recorded at the higher resolution. Spectra were generated by electron impact (EI) in both instruments, but ionization by 70-eV electrons was used exclusively with the CEC 21-llOB. Both 70-eV and 10-eV electrons were used for ionization with the MS-30. In addition, the MS-30 was fitted with a prototype field ionization (FI) source (13),and several spectra were generated with this mode of ionization. The FI source was developed jointly between Kratos ScientificInstruments, Ltd., and the Bartlesville Energy Technology Center. At the time the distillation experiments were being conducted,extensive recording of FI spectra for the sample was not feasible. Enough recordings were made, however, to allow a comparison between FI spectra and those produced by electron impact. The performance of the FI source will be discussed in a later publication. Quartz direct introduction probes (Masspec, Inc.) were used with both instruments. Power for the probes and measurement of sample temperature were provided by a programmer (Masspec, Inc.). Spectra were recorded under conditions of constant ion current by feedback of the voltage from the ion current monitor to the programmer. Thus, the probe temperature was controlled to maintain a constant ion current throughout the distillation. Samples in a pentane solution were placed directly in the probe, and the solvent was evaporated with a stream of air. The CEC 21-llOB required sample loads of -0.7 mg to produce spectra 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
1
-0h
720
a Y
749
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5
360 , ,
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f
O
< r
250
50
100
150
200
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,
~I
,
~"
XI
Y
',.,,,.,,
250
,,, ,"
300
:i
0
350
500
600
550
650
700
750
Figure 2. Spectrum recu&sd at cnnstant bn current on a KratosIAEI MS-30 with ionization by 10 eV electrons. Probe temperature: 193
z
-
450
m/r
v)
Kz
400
400
200-
OC.
toor t 0
150-
544
I
100
518
~
80
50
I
5p
558
,574
~
0
350
390
430
470
510 m/z
550
590
630
670
F l g w 1. Specbun recordedat cmstant ion curenton a hpont1CEC 21-1106 wim bnizatbn by 70 eV electrons. Robe tanmature: 203 OC.
with intense molecular ion peakp. Loads of -0.2 mg were found to be sufficient for the MS-30. Linear temperature programming at 7 OC/min was used to produce spectra with the MS-30, and significant ions arising from the sample were recorded over a range of 149256 OC. The mass spectrometer was calihrated for double-beam operation by recording Spectra for triscperfluoroheptyl,-r-triazine (PCRResearch Chemicals, Inc.). Spectra for the reference compound and the sample were recorded from the upper limit of the mass range ( - m , z 1200) u) - m , 2 2fi0 at a scanning rate of 100 stdecade. For ionization at 10 eV, the supreasion of fragment ion formation was tested hy introducing ethylbenzene into the expansion-volume inlet and measuring the intensities of the m / z 106 and 91 ions. An I,,/l,, value of 1 0 1 was ronsidered adequate for running the sample. During the distillation, ethylbenzene was replaced hy hexahromobenzene in the expansion volume, and molecular ion intensities of CRRre were recorded t n monitor the variation of aenaitivity with total-ion current. This step in the procedure was not required when spectra were recorded at constant ion current.
RESULTS AND DISCUSSION Spectra Recorded et Constant Ion Current. A problem wcurring with sample introduction by direct probe is maintaining a constant total ion current while a spectrum is being recorded. For the analysis of a relatively pure sample, the probe temperature can be i n c r d manually until a sufficient ion current is ohtained tn permit recording of the spectra. Then, only slight increases in temperature are required until the recording is completed. Tbii p d u r e i9 less satisfactory for mixture analysis hecause the composition of the sample may change significantly during the scan by the depletion of the more volatile componenta. Particularly with high-resclution instruments, which may require more than 1 min for spectral recording, the ion current may decrease considerably from the start of the scan to the finish ifthe probe temperature is held constant. The resulting spectrum is thus not representative of the compoaition of the vapor distilling from the sample. Although the temperature may be inrreaaed manually, better reproducibility in spectra is obtained if the ion
m/z
Flgure 3. FIeH lonlzatlon spectrum recorded at constant ion current on a KratasIAEI MS-30. Probe temperature: 220 OC.
current itself is used to control the temperature. This mode of programming is achieved by feedback of the voltage from the ion-current monitor to the device controlling the temperature. Thus, temperature is automatically increased to maintain a constant ion current. The monoaromatics GPC fraction was distilled under conditions of constant ion current on the CEC 21-llOB and the MS-30. Figure 1 shows the 70-eV spectrum recorded on the CEC 21-llOB at a probe temperature (initial) of 203 "C. Molecular ions are distributed from about m / z 430 to 670, and m/z 540 is near the center of the distribution. By comparison with the 170 "C spectrum (not shown), the distribution in Figure 1 is shifted upward about 30 masa units. Increasing the probe temperature to 234 "C caused a further upward shift of about 70 units in the distribution. The l o w - m ion series 27,41,55, ...,and 29,43,57, ...,show the presence of aliphatic groups in the molecules, and the series 77,91,105, ... indicates the existence of aromaties. Ions in the series 117, 131, 145, ..., arise from hydroaromatics. The distribution of molecular-ion intensities observed for low voltage electron impact (LV/EI) and for field ionization are reproduced in Figures 2 and 3,respectively. As discussed below, the compostion of the sample is dominated by -8 Z(H) through -30 Z(H) hydrocarbons. The relative cross sections and mole sensitivities for LV/EI and FI of at least the lower molecular weight homologues in the -8 Z(H) tbrough -22 Z(H) series have been correlated (14). Furthermore, the available data for LV/EI (15, 16) and for FI (14)indicate that the relative mole sensitivities for compounds in a given hydrocarbon series become essentially independent of carbon number with the initial dependence on carbon number di-
750
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table I. Molecular-Ion Intensities of Hexabromobenzene and Sensitivity Correction Factors for Spectra Recorded during the Probe Microdistillation of Monoaromatics GPC Fraction 19 a intensity of mlz scan probe 549 no. 547 551 553 555 temp, "C ZmIm Ft 1
2 3 4 5 6 7 8
9 10 11
12 13 14 15 16 17 18 19 20 21 22 23 24 25
90 102 106 113 121 131
140 149 158 166 175 184 193 201 209 21 7 225 233 241 24 8 256 264 271 279 287
27 8 289 27 7 261 225 183 126 111
123 111
86 52 75 60 122 41 120 130 125 129 89 214 237 225 221
730 752 730 621 513 413 346 271 24 5 24 6 24 5 185 203 226 220 208 191 21 8 27 6 238 195 497 552 581 550
977 1018 871 841 684 546 421 332 3 24 335 24 2 289 321 310 336 321 3 24 290 283 305 250 687 858 774 668
702 686 653 602 508 395 298 224 230 229 179 171 188 240 227 273 298 190 282 218 187 482 610 507 522
tx mIm 25
=
a
237 265 251 243 201 116 119 119 94 58 109 94 83 146 136 172 187 120 145 120
2924 3010 2782 2568 2131 1653 1310 1057 1016 979 861 791 870 982 1041 1015 1120 948
0.533 0.518 0.560 0.607 0.732 0.943 1.19 1.48 1.53 1.59 1.81
1.97 1.79 1.59 1.50 1.54 1.39 1.65 1.40 1.54 1.89 0.751 0.625 0.687 0.730
1111 1010
825 2076 2493 2268 2136
104
196 236 181 175
1559
Resolution: 3000; electron energy: 1 0 eV.
minishing as the number of condensed rings in the parent compound rises, i.e., as the specific Z(H) value becomes more negative. Since the ion intensity for compounds of a given carbon number in a specific -Z(H) series is proportional to the number of those species in the gaseous mixture, the close resemblance in the LV/EI and FI molecular-ion intensity distributions is expected. The results obtained from the LV/EI/MS and FI/MS analyses of the asphaltene aromatic neutrals from a coal liquid reinforce this conclusion (17). Comparison of Figure 1 with Figures 2 and 3 reveals a remarkable and unexpected similarity in the distribution of molecular-ion intensities produced by 70-eV electrons with those obtained from LV/EI and FI. Other factors being equal, the molecular ion intensity distributions for a given ionization technique are determined by the dependence on molecular structure of the ionization cross section and the contribution of fragment ions to the cross section. For aromatic compounds of interest in fossil-fuel mass spectrometry, fragment ions account for a considerably greater fraction of the cross sections for ionization by 70-eV electrons than for ionization by either ca. 10-eV electrons (15,16) or high-electric fields (14).Since the use of 70-eV rather than 10-eV electrons would increase the dynamic range of an analysis, especially as the resolution is increased, both the origin and generality of the phenomenon observed by comparing Figure 1with Figures 2 and 3 deserve further investigation. Spectra Recorded w i t h Linear Temperature Programming. Increasing the probe temperature linearly with time often resulta in a change in the total-ion current by more than a factor of 10 during a distillation. As the ion current increases, the sensitivity of the mass spectrometer decreases because increasing positive charge in the ion source defocuses the ion beam. Significant errors will occur in quantitative results unless the sensitivity changes are corrected. Variations can be corrected by monitoring ion intensities of a substance introduced through an expansion volume simultaneously as the sample is distilled. A change in sensitivity is then cor-
w
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0
3 v)
5
I 10
SC A
.
0
.'.. . I I5
20
II I
25
U U M BE R
Figure 4. Decrease in mass spectrometer sensitivity with increasing
total-ion current.
rected from the change in peak height in the spectrum of the reference compound. The decrease in ion intensity in the reference spectrum caused by effusion from the expansion volume during the time of the distillation (-20 min) is negligible. Hexabromobenzene was introduced through the expansion volume during the distillation of the monoaromatics GPC fraction. By summing the molecular-ion intensities for each scan and plotting these sums against scan number, the decrease in sensitivity occurring during the distillation is made clearly visible as shown in Figure 4. Correction factors, F,, are calculated for each scan from eq 1,where 2,Jm is the s u m
of the molecular-ion intensities for the reference compound at the temperature of the scan, CtCmIm is the sum of CmIm over all the scan temperatures, and N is the number of scans (2). When multiplied by the correction factors, the intensities of ions originating from the sample are corrected to their proper values. The calculations are performed with a com-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
751
--
Table 11. Maximum Heights and Areas of Elimination Curves Ordered by Increasing Temperature of Maximum a
a
m Iz
-Z(H)
T(IMAX), K
IMAX
AREA
624.0 628.0 648.0 646.0 632.0 638.0 622.0 642.0 630.0 636.0 658.0 644.0 654.0 640.0 650.0 656.0 662.0 660.0 652.0 666.0 676.0 668.0 672.0 670.0 692.0 688.0 664.0 680.0 674.0 696.0 698.0
20 16 24 26 26 20 22 16 14 22 28 14 18 18 22 16 24 26 20 20 24 18 28 16 22 26 22 20 26 18 16
508.2 i: 1.7 509.3 f 2.0 509.6 f 1.2 51!0.8 i 1.6 511.0 f 3.5 511.4 f 1.8 511.4 f 3.2 511.5 f 1.8 511.6 i 3.5 512.0 i 2.3 53.2.8 i 1.7 513.1 f 2.3 513.1 -1- 2.0 51.3.2 f 2.3 53.3.3 t 1.9 513.3 i 3.1 513.7 f 2.2 513.9 i: 2.3 514.6 f 2.1 515.2 f 1.8 515.5 f 2.2 515.5 f 1.4 515.6 f 1.7 515.9 f 2.2 515.9 f 1.9 516.1 ?: 2.3 516.3 f 2.7 516.3 f 1.3 516.3 f 2.6 516.3 f 1.3 516.6 i 2.0
0.86E+ 03 i- 0.9E+ 02 0.773+03 i: 0.1Et03 0.63Et 03 f 0.6E+02 0.52Et 0 3 f 0 . 6 E t 02 0.60Et03 f 0.2Et03 0.75E+03 f 0 . 9 E t 0 2 0.81Et03 i 0.2E-i-03 0.60Et 03 f 0 . 7 E t 02 0.62Et03 f 0.2Et03 0.733+03 f 0 . 1 E t 0 3 0.49E+ 03 f 0.7Et 0% 0.59Et03 f 0.1Et03 0.63E+ 0 3 ?: 0.1E+03 0.75Et03 i: 0 . 1 E t 0 3 0.65Et 03 f 0.9E+ 02 0.59Et03 f 0.2Et03 0.523+03 i 0.1E+03 0.50E+ 03 f 0.1E+03 0.693+03 i 0 . 1 E t 0 3 0.56Et 03 f 0.8E+02 0.45Et 03 i 0.7Et 02 0.50E+03 i 0.5Et02 0.42E+03 f 0.5E+02 0.45Et 03 i: 0.7Et 02 0.43E+ 03 f 0.6E+02 0.36E+03 ?: 0 . 6 E t 0 2 0.56Et03 f 0.8E+02 0.46E+03 f 0 . 5 E t 0 2 0.39Et03 f 0 . 6 E t 0 2 0.373+03 % 0.4E+02 0.31E+03 i 0 . 5 E t 0 2
0.41Et05 f 0.4Et04 0 . 3 3 E t 0 5 f 0.4Et04 0 . 2 6 E t 0 5 i 0.2E+04 0 . 2 3 E t 0 5 f 0.2E+04 0 . 2 4 E t 0 5 f 0.6E+04 0.343+05 f 0.3E+04 0.33Et05 f 0 . 7 E t 0 4 0.27Et05 f 0.3E+04 0.26E+ 05 f 0.6E+ 04 0.34E+ 05 i 0.4E+ 04 0.19Et 05 f 0.2Et 04 0.23Et05 f 0 . 3 E t 0 4 0 . 2 3 E t 0 5 f 0.4E+04 0.30E+ 05 f 0.4Et 04 0.27Et05 * 0.3Et04 0.18E+05 f 0 . 6 E t 0 4 0.20Et05 I 0.3E+04 0.19Et05 0.3Et04 0.29Et05 i: 0 . 4 E t 0 4 0.233+05 f 0.3Et 04 0.20Et05 f 0.2E+04 0.21Et 05 f 0.2E+ 04 0 . 1 6 E t 0 5 f 0.2E+04 0.19Et05 i 0.2E+04 0.15Et05 f 0.2Ei-04 0.15E+05 f 0.2E+04 0.26Et05f 0.3Et04 0 . 1 8 E t 0 5 f 0.2E+04 0.18E+05f 0.2E+04 0.13E+05 f 0.1E+04 O.llE+05 f 0.2E+04
Partial output from siecond computer program. Column of -Z(H) numbers has been added.
puter program that allows up to seven intensities to be entered for the reference compound. Table I presents the results for the data shown in Figure 4. A second computer program uses the corrected intensities from program 1 to calculate parameters for eq 2, which exe-BIT
=
+ eC(T-D)
(2)
presses sample-ion intensity as a function of absolute temperature, T (18,19). Parameters A, B, C, and D are estimated by nonlinear regression. 'The numerator in eq 2 expresses the increase in ion intensity as a function of T and is derived from theory. Parameter B is d a t e d to the heat of vaporization of a compound (20-22). The decrease in ion intensity caused by compound depletion ici expressed empirically as a function of T by the denominator in eq 2. A plot of I vs. T gives elimination curves analogous to those obtained for ordinary molecular distillation (23). See Figure 5. Program 2 calculates three other parameters for the curves: IMAX, the maximum height of the curve; T(IMAX), the temperature of the maximum; and AREA, the area under the curve. All parameters are given with 90% confidence intervals. As seen in the figure, T(1MAX) increases with increasing carbon number fop members of a homologous series. Curves having the same T(1MAX) value within experimental error may correspond to ions arising from a single compound. If this is true, the relative rtbundances of the ions are proportional to the areas under the curves (18). Table I1 shows a partial listing of the output from the second program. The parameters for the curves have been ordered according to T(1MAX) to show the relative volatility of the components. Members of a homologous series show a decrease in volatility with increasing molecular mass. For example, molecular ions in the -16 Z(H) series ( m / z 628,642, 656, ...) have T(1MAX) values differing by about 2 degrees. The trend is evident even though the difference is of the order
of the experimental error in the calculation of T(1MAX). The explanation of this observation lies in a relationship between T(IMAX), enthalpy of vaporization, and the amount of the component being distilled. A brief discussion of the theory has been given previoudy (20) and will be expanded in a future paper. Components having the same number of carbons but differing by degree of unsaturation have the same T(1MAX) values within experimental error over a narrow molecular weight range. For example, molecular ions containing 48 carbons ( m / z646,648,650, ..., 658) have an average T(IMAX) of 512.5 f 1.7 "C. Those containing 47 carbons (m/z 632,634, 636, ..., 644) have an average value of 511.3 f 2.1 "C. Therefore, components having the same nominal molecular mass but differing by one carbon and 12 hydrogens would not produce distinguishable elimination curves unless sufficient resolution were used to resolve the two molecular ions. As an illustration, the precise mass measurements of m / z 644 were midway between the expected values for C47Hs0 (644.6260) and C48H68 (644.5321), showing the presence of an unresolved doublet. Although two maxima in the elimination curve might have been expected, only one was observed consistent with the above argument. The possibility of distinguishing between molecular ions of the same nominal mass but belonging to different compound types (e.g., hydrocarbons and ketones) by deconvolution of their elimination c m e s has not been investigated. Elemental compositions for molecular ions recorded during the probe microdistillation were determined from precise measurements a t a resolution of 3000. Although the MS-30 has a maximum resolving power of 16000 the lower value was selected to maximize sensitivity for spectra produced with 10-eV electrons. As long as multiplets were not present in the peaks, the resolution was usually sufficient to limit the elemental compositions to one possibility for each peak based on the assumption that only hydrocarbons were present.
752
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
Table 111. Precise Mass Measurements for Ions in the -18 Z( H) Series Recorded during the Probe Microdistillation of Monoaromatics GPC Fraction 19 a
1000-
pro be temp, "C
400w z
c 2001
390
410
430
470
450 T,
490
510
530
K
Flgure 1. Three elimination curves for molecular ions in the -16 Z(H)
series.
To confirm the assignment of elemental compositions, spectra were also recorded on the MS-30 at a resolving power of 12000 and an ionizing energy of 70 eV. The ion masses provided no evidence for the presence of compounds containing oxygen or nitrogen, in agreement with the data acquired at a resolution of 3000 and the value of 0.018% N by weight determined by elemental analysis on the monoaromatics concentrate (24). No percentage was given for oxygen, but a value of 1.99% by weight was reported for sulfur (24). Thus, the possibility exists that a significant number of sulfur-containing compounds are present in the monoaromatics GPC fraction. Assuming that these compounds are uniformly distributed throughout the GPC fractions, approximately one-third of the molecules in the fraction should contain sulfur. This value is based on an average molecular weight of 550 and the assumption that a molecule contains no more than one sulfur atom. The presence of sulfur-containing compounds in the monoaromatics GPC fraction should be readily determinable by examination of the ion masses in the spectra recorded at loo00 resolving power. Distinction between elemental compositions differing by C4 and SH16(AM = 97.3 mmu) and by C2H8 and S (AM = 90.5 mmu) is easily accomplished. The only problem arises in distinguishing between compositions differing by C3 and SH, (AM = 3.4 mmu). Doublets consisting of ions differing in elemental composition by these combinations are not resolved above e m / z 100 except at higher resolution (25,26). However, if the mass of an unresolved doublet can be measured with sufficient accuracy, the value will represent the weighted mean of the masses of the hydrocarbon and sulfur-containing ions. Elemental compositions differing by C3 and SH4 can be distinguished in singlets if the mass measurement error is less than =3 mmu. To test for the presence of sulfur-containing compounds in the monoaromatics GPC fraction, the experimental masses of fragment ions in the range from m / z 49 (the lower limit of the spectra recorded at 12000 resolution) to m / z 293 were compared to the theoretical values for CH and CHS combinations. The experimental masses were obtained by relating the time moments of the peaks (calculated from Kratos' DS55SM software) to the masses through regression analysis on the equation In M = A
+ Bet + ... + I.ts
(3) where M and t are mass and time, respectively, and A , B, ..., I are the parameters. The details of the calculations will be described in a later publication. Combinations of C, H, and S ranging from C,H2,+1S through C,H2,-13S were tested against the possible hydrocarbon combinations. The better of two possible compositions differing by C4 and SHls or by
exptl
rnlz
intens, arb expected elemental error, units mlz comp mmu
193 402.3282 81 402.3286 193 416.3487 61 416.3443 193 430.3659 64 430.3599 193 444.3828 52 444.3756 193 458.3817 76 458.3912 193 472.4074 120 472.4069 193 486.4249 216 486.4225 209 500.4474 334 500.4382 209 514.4625 497 514.4538 209 528.4756 666 528.4695 209 542.4966 891 542.4851 209 556.5050 895 556.5008 225 570.5287 859 570.5164 233 584.5404 878 584.5321 233 598.5542 940 598.5477 233 612.5686 813 612.5634 233 626.5828 773 626.5790 241 640.6012 657 640.5947 241 654.6110 546 654.6103 241 668.6289 498 668.6260 241 682.6287 398 682.6416 241 696.6405 327 696.6573 241 710.6530 259 710.6729 241 724.6646 183 724.6886 248 738.6875 206 738.7042 248 752.6855 155 752.7199 248 766.6919 1 1 7 766.7355 a Resolution: 3000; electron energy:
C,,H,, C,,H,, C,,II,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, C,,H, C,,H,, C,,H,, C,,H,, C,,H,, C,,H,, 1 0 eV.
-0.4 4.4 6.0 7.2 -9.5 0.5 2.4 9.2 8.7 6.1 11.5 4.2 12.3 8.3 6.5 5.2 3.8 6.5 0.7 2.9 -12.9 -16.8 -19.9 -24.0 -16.7 -34.4 -43.6
CzHs and S was easily selected, and only hydrocarbon combinations were found. For example, the experimental mass 111.1167 was sufficiently close to the theoretical mass of CsHI6 (111.1174) to allow this possibility to be selected over C6H7S (111.0268). Differentiation between C3 and SH4combinations was difficult. However, the mass measurement accuracy was sufficient for most peaks to allow the hydrocarbon possibility to be selected. As an illustration, C13H9 (theoretical mass: 165.0704) was selected over C10H13S (theoretical mass: 165.0738) for the ion having an experimental mass of 165.0696. For 48 ions examined to which C3or SH, combinations could be assigned, the experimental values of the masses consistently agreed more closely with the masses of the hydrocarbon possibilities. Therefore, from the results of all the precise mass measurements, no evidence of sulfur could be found. A possible explanation for the absence of sulfur-containing compounds in the monoaromatics GPC fraction is the nonuniform distribution of these compounds in the GPC eluate. Although data are not available to correlate molecular structure with elution volume for the Wilmington 535-675 "C distillate, a study of this kind has been made for the 370-535 "C distillate (12). On the basis of the identification of model compounds in GPC fractions by low-resolution, low-voltage mass spectrometry, sulfur-containing compounds were found to be eluted in the earlier fractions from the monoaromatics concentrate. Therefore, if the same correlations between structure and elution volume apply to the chromatography of the higher-boiling distillate, significant number of sulfurcontaining compounds should not be present in the fraction used in this study. Table I11 gives the results from the precise mass measurement a t 3000 resolution for ions in the -18 Z(H) series. The experimental m/z values were recorded at the given probe temperature and are typical of those obtained near the maximum in the corresponding elimination curve. Most of the measurements are sufficiently accurate to justify the el-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
70'
*
4
30c
w
l
402 430 458 486
514 542
..
*
570 598 626 654 682 m/z
710 738 766
Figure 6, Distribution of homologues in the -18 Z(H) series.
emental compositions shown. Possible combinations in the -4 and -32 Z(H) series are eliminated for all but the highest masses because their expected m/z values differ by 93.9 mmu from the expected values given in the table (12[M~]- Mc = 12[1.0078246]- 1.2 = 0.0939). The two highest masses, m/z 752 and 766, may contain significant contributions from components in the -32 Z(H) series. However, the abundances of these masses tire quite small as noted below. Abundances of components in the major homologous series from -14 to -22 Z(H) are normally distributed. Figure 6 shows the areas under the elimination curves plotted against molecular mass for components in the -18 Z(H) series. The most abundant homologues are distributed around m / z 570. Excessive data scatter prevents extending the boundaries below m / z 402 and beyond m / z 766. Components in -Z(H) series below 14 and above 22 also appear to be distributed normally. However, the presence of unresolved doublets in the spectra prevents a clear distinction to be seen between normally distributed abundances and other possibilities. Table IV show!, the carbon number distribution for components in the sample. Mole fractions X104 are given for molecular ions distributed from m / z 400 to m / z 776. In calculation of mole fractiions, the area under the elimination curve for each molecular ion is divided by the sum of all the areas, and the assumption is made that molar sensitivities are equal for all molecular ions detected in the 10-eV spectra (14). Isotropic contributions from components having two atoms were subtracted from the areas before the mole fractions were calculated. Agreement between experimental and expected intensities of isotopic molecular ions containing one 13C was tested. Most of the deviations were less than &lo%.Nominal masses have been divided between two -Z(H) series where unresolved doublets occur in the peaks. For example, the masses 406,420,434, ..., 770 are divided between the -141-28 Z(H) series, depending on the precise mass measurements. Masses below m/z 560 clearly belong in the -14 Z(H) series. However, the precise masses of peaks near m / z 672 are intermediate between the two expected values. Those masses having precise measurements greater than the mean of the two expected values have been placed in the -14 Z(H) series and those having precise measurements less than the mean are classified in the -28 Z(H) series. The abundances of peaks having precise mass determinations near the mean of the expected values could be in error by as much as a factor o f 2. Assuming that an unresolved doublet consists of triangular peaks of equal height and width corresponding to the two elemental compositions, the height of the composite peak would equal the sum of the heights of the individual peaks if complete overlap occurs. Because of compensating errors in abundapces, the presence of unresolved doublets in the spectra contributes a relatively small error to the sum of the abundances for a series. As an illustration, assume that !X% of the abundance of m / z 618
* 753
arises from a -12 Z(H) ion and the other 50% from an ion in the -26 Z(H) series. Also assume that 94% of the abundances of m / z 464 and 772 arise from ions in the -12 and -26 Z(H) series, respectively. Both assumptions are supported by precise mass determinations. By linear interpolation of the abundances of masses between 464 and 772, inclusively, the sums for the -12 and -26 Z(H) series are found to be, respectively, 821 and 475 instead of 912 and 384. Thus, the -12 Z(H) series contributes about 8% instead of 9% to the sample, and the contribution from the -26 Z(H) series is about 5% instead of 4%. Similar errors of about 1% are found in the sums of the abundances for the other series containing unresolved doublets.
CONCLUSION Probe microdistillation of mixtures at constant ion current rather than constant temperature produces ion intensities that do not change appreciably over the period of time required to record a spectrum. This technique is especially useful with high-resolution mass spectrometers, which may require well over 1 min for spectral recording. Seventy-electronvolt spectra of the monoaromatics GPC fraction used in this study are remarkably similar to those produced by low-voltage electron impact and by field ionization. This phenomenon deserves further investigation to establish the classes of compounds for which it is a characteristic. If molecular-lion sensitivities for 70-eV ionization are predictable for homologues in various classes, as they are for low voltage and field ionization, then quantitative analyses on mixtures of these homologues can be performed with lower determination limits a5 compared to analyses performed with ion formation by the latter two methods. Linear programming of probe temperature causes wide variation in total ion current during the distillation of a sample. Because mass spectrometer sensitivity is affected by this variation, significant errors will be introduced into quantitative results unless the change in sensitivity is corrected. Factors needed to correct the sample ion intensities can be calculated from the variation in ion intensities of a substance introduced through an expansion-volume inlet simultaneously as the sample is distilled from the probe. Components of a given homologous series in the monoaromatics GPC fraction produced elimination curves with maxima appearing at temperatures that increase with increasing carbon number. Molecular ions corresponding to compounds in two overlapping homologous series gave unresolved doublets in the mass spectra recorded at a resolution of 3000. Although these ions led to unresolved elimination curves, their abundances could be estimated by proportioning the areas under the curves between the two ions with factors calculated by linear interpolation of the precise masses of the peaks between the expected masses of the ions. The presence of sulfur-containing compounds in a mixture with hydrocarbons can seriously affect analyses performed by mass spectrometry unless sufficient resolution is used to resolve multiplets corresponding to C/CHS ions. By examination of the precise masses of low-mass ions recorded at a resolution of 12 000, the extent to which sulfur-containing compounds are present can be estimated. By use of this approach, only hydrocarbons could be detected in the monoaromatics GPC fraction although it was isolated from a crude containing 2% sulfur by weight. The probe microdistillation performed on the monoaromatics GPC fraction provided sufficient data to permit a quantitative distribution to be calculated. Components were detected having molecular masses in a range from 400 to 776 and -Z(H) numbers from 8 to 30. About 90% of the components have -Z(H) numbers in a range from 12 to 24. Components in -Z(H) series from 14 to 22 are normally dis-
754
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4, APRIL 1982
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tributed. Those detected in other series may also be distributed normally although the data were insufficient to rule out other possibilities. ACKNOWLEDGMENT We thank Q.G. Grind&iff and G. P. Sturm, Jr., Bartlesville Energy Technology Center, for technical assistance in acquiring some of the mass spectra used in this work. P. E. Pulley, Data Processing Center, Texas A&M University, programmed the nonlinear regression and provided other programming assistance. Appreciation is expressed to K. J. Irgolic, Department of Chemistry, Texas A&M University, for acting as graduate advisor to L.R.S. in the absence of R.D.G. LITERATURE CITED Richardson, J. S. Ph.D. Dissertation, Texas A&M Unlversity, College Station, TX, 19'78. Norman, E. J. Ph.D. Dlssertatlon, Texas A&M Unlversity, College Station, TX, 1977. Schronk, L. R. M.S. Thesis, Texas A&M University. College Station, TX, 1978. Schronk, L. R.; Grlgsby, R. D.; Hanks, A. R. Presented at the 27th Annual Conference on Mass Spectrometry and Allled Toplcs, Seattle, WA, June 3-8, 1979; paper No. MAMOAI. Grlndstaff, Q. G.; Hwang, C. S.; Marriott, T. D.; Benson, P. A.: Scheppele, S. E. Presented at the 27th Annual Conference on Mass Spectrometry and Allied Topics, Seattle, WA, June 3-8, 1979; paper No. MAMOA2. Grlgsby, R. D.; Schronk, L. R.; Grindstaff, Q. G.; Scheppele, S. E. Presented at tho 27th Annual Conference on Mass Spectrometry and Allled Topics, Seattle, WA, June 3-8, 1979; paper No. MAMOA3. Grlgsby, R. D.; Schronk. L. R.; Hanks, A. R. Presented at the Texas A&M University Lignite Symposium, College Station, TX, April 17-18, 1980. Hlrsch, D. E.; Hopkins, R. L.; Coleman, H. J.; Cotton, F. 0.: Thompson, C. J. Anal. Chem. 1972, 44, 915-919. Jewell, D. M.; Weber, J. 14.; Bunger. J. W.; Planchet', H.: Latham. D. R. Anal. Chem. 1972, 44, 1391-1395. Coleman, H. J.; Dooley, J. E.; Hlrsch, D. E.; Thompson, C. J. Anal. Chem. 1973, 45, 1724-1737. Dooley, J. E.; Hlrsch. D. E.; Thompson, C. J.; Ward, C. C. Hydrocarbon Process. 1974, 53 (ll), 187-194, and references clted therein. Hirsch, D. E.; Dooley, J. E.; Coleman, H. J.; Thompson, C. J. Rep. Invest.---US.. Bur. Mlnes 1974, No. 7893.
755
(13) Scheppele, S. E.; Grindstaff, Q. G.; Pavelka, E. A,; Evans, S.; Banner, E. A.; Tudge, H. U.S.Department of Energy, Bartlesville, OK, and Kratos Sclentiflc Instruments, Ltd., Manchester, Enaland. unoublished result~,1980-1981. (14) Scheppele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Marriott, T. D.: Perreira, N. 0. Anal. Chem. 1976, 48, 2105-2113. (15) Kearns, G. L.; Maranowski, N. C.; Crable, G. F. Anal. Chem. 1959, 31. 1646-1651. (18) Lumpkin,H.E.; Aczel, T. Anal. Chem. 1964, 36, 181-184. (17) Scheppele, S. E.; Benson, P. A.; Greenwood, G. J.; Grindstaff, Q. G.; Aczel, T.; Beier, B. F. A&. Chem. Ser. 1961, No. 195. (18) Grlgsby, R. D.; Hansen, C. 0.; Mannerlng, D. G.; Fox, W. G.; Cole, H. H. Anal. Chem. 1971, 43, 1135-1137. (19) Grlgsby, R. D.; Norman, E. J.; Pulley, P. E. Presented at the 23rd Annual Conference on Mass Spectrometry and Allied Topics, Houston, TX, May 25-30, 1975; paper No. U-9. (20) Grlgsby, R. D. Presented at the 20th Annual Conference on Mass Spectrometry and Allied Topics, Dallas, TX, June 4-9, 1972; paper No. D11. (21) Beynon, J. H. "Mass Spectrometry and its Applications to Organic Chemistry"; Elsevier: New York, 1980; pp 474-479. (22) Thomas, R. H. P. Ph.D. Dissertation, Texas A&M Universltv, Colleae Station, TX, 1978. (23) Burrows, G. "Molecular Dlstillatlon"; Oxford Unlverslty Press: London, 1960. (24) "Characterlzatlon of the Heavy Ends of Petroleum", Annual Report No. 14, Amerlcan Petroleum Institute Research Project 60, 1973, p 19. (25) Lumpkln, H. E.; Elliott, R. M.; Evans, S.; Hazelby, C.; Wolstenholme, W. A. Presented at the 23rd Annual Conference on Mass Spectrometry and Allied Topics, Houston, TX, May 25-30. 1975; Paper No. H-17. (28) Scheppele, S. E. "Characterization of Coal-Derived Liquids and Other Fossil Fuel Related Materials Employing Mass Spectrometry"; US. Department of Energy, Contract No. EX-78-S-01-2537, Quarterly Report, March 30June 29, 1978, pp 119-123.
RECEIVED for review September 17,1981. Accepted December 28, 1981. This work was supported by U S . Department of Energy Contract No. DE-AC19-80BC10171,project 1672 of the Texas Agricultural Experiment Station, and funds from the Center for Energy and Mineral Resources, Texas A&M University. Taken in part from the Ph.D. dissertation of L. R. Schronk and presented in part at the Second Chemical Congress of the North American Continent, Las Vegas, NV, Aug 24-29, 1980.
Minimization of Errors in Fixed-Time Reaction Rate Methods by Optimization of Measurement Time F. J. Holler," R. K. Calhoun, and S. F. McClanahan Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506
A theoretical development of the effect of random and systematic fiuctuatlon in rate constants on the relative precision of reaction rates for first-order and pseudo-first-order reactions is presented. By the use of propagation of error theory, it Is demonstrated that at t = l / k = 7 reaction rates are essentially independent of small fluctuations In the rate constant. Simulated data support the theoretical predictions. The mlnlmization of rate error at a time other than t = 0 leads to a comparison of the relative precision of inltiai concentration of analyte as determlned from lnltlal rates and from rates measured at 7. The comparison, drawn through the use of propagation of error theory and by the generalization of simulated data, shows that much better preclslon may be obtalned when the rate is measured at T . Experimental data collected on the oxidation of iodlde by hydrogen peroxide are in excellent agreement wlth the theory. The relative standard deviation of the rate determined from these data at t = 7 is a factor of 11.3 smaller than that determined for t = 0. 0003-2700/82/0354-0755$01.25/0
Over the past several years there has been steady growth in the literature of kinetic analysis. However, relatively few papers have appeared that explore the fundamental limitations of kinetic analysis or that present comprehensive studies of the methodology of kinetics measurements. Ingle and Crouch ( I , 2) have discussed the relative merits of fixed-time and variable-time met hods and have developed the signalto-noise ratio theory for these methods, particularly with regard to spectrophotometricdetection. Wilson and Ingle have presented a similar treatment in conjunction with the development of a fluorometric reaction rate instrument (3). In a recent work Carr (4)has investigated the effects of random variations in rate constants on the precision of kinetic methods. Because of difficulties inherent in variable-time methods, only fixed-time methods were treated. By assuming that rate measurements are carried out by measuring a change in the concentration of the monitored species, ACM,over the fixed time of the experiment and by applying propagation of Q 1982 Amerlcan Chemical Soclety
