z 121 mass chromatogram of Paraho shale oil

Anal. Chem. 1984, 56,701-708

70 1

Fine Structure in the m / z 121 Mass Chromatogram of Paraho Shale Oil E. J. Gallegos Chevron Research Company, Richmond, California 94802

Capillary GWhigh-resolution MS was used in a detailed analysis of the Ion moieties giving rise to several m / z 121 mass chromatographic peaks. At hlgh MS resolution, three mass chromatograms are uncovered. These are due to C8HsO+,C,H,,N+, and C,H,,+. C,H,O’s are fragments from both dl- and trimethyiphenois. C,H,,N Is assigned tentatively to several homologous serles of n -alkyl-substituted pyridines. C,H,,+ Is due to a dicyclic sesquitepene, a tricyclic dlterpene, and the pentacycllc triterpenes, hopenes, and hopanes. Potentlal biological procursors to the n -alkylpyridine homologues are discussed.

An important aspect of synfuel process engineering is the removal of certain nitrogen and oxygen components that are present in the feed. These components, particularly those containing nitrogen, are catalyst poisons; they also contribute to gum formation in fuels, pistons, valves, and rings of internal combustion engines. The energy industry is beginning to refine heavier fractions of petroleum. They are also beginning to seriously consider processing coals and oil from shale. These feedstocks introduce higher levels of heteroatom components. A detailed knowledge of the heteroatom environment is needed to optimize the process design for their removal. High-resolution gas chromatography/high-resolutionmass spectrometry/computer (HRGC/HRMS/C) techniques are reported here in the provisional identification of several homologous series of alkylpyridines, phenols, terpenes, and terpanes analytically isolated from Paraho shale oil. HRGC/HRMS/C techniques were also used to follow the hydrodenitrification, HDN, procedure for the removal of the heterocompounds, as well as to monitor the effect of hydrogenation on alkenes. Diagenetic considerations of the precursors to the several homologous pyridines provisionally identified in this work may also be useful as “biomarkers” to the organic geochemist interested in fossil fuel prospecting.

EXPERIMENTAL SECTION These experiments were done on a shale oil from the Green River formation which has undergone a Paraho retort process, therefore, called Paraho shale oil. This means that both the oil from the kerogen and the bitumen were retorted. Five samples were analyzed: the raw Paraho shale oil; lo%, 70%, and 95% hydrodenitrifed (HDN) samples; and a polar cut from the Paraho shale oil. The polar cut was obtained by elution with 50:50 chloroform/methanol over alumina after flushing with benzene to remove the saturate and aromatic hydrocarbons. This fraction was used to get hydrocarbon interference free GC/MS mass spectra of the homologous pyridines cited in this study. HDN treatment was achieved over silica-alumina group 8 plus group 6 catalysts. Elemental analysis gave 2.26% N, 1.24% 0, 0.68% S, and 11.42% H for the raw Paraho shale oil and 0.11% N, 0.009% 0, 0.0006% S,and 13.63% H for the 95% HDN sample. GC/MS/C data were obtained with a 60-m, fused silica, Durabond-1 capillary column from J & W Scientific in a 5700A Hewlett-Packard gas chromatograph coupled directly to the ion 0003-2700/84/0356-0701$01.50/0

source of a VG 7070H double focusing mass spectrometer. An INCOS Finnigan MAT data system was used for instrument control and data acquisition, Full scan, low MS resolution GC/MS/C data were acquired at 3-9 intervals from mass 10 to 600. Multiple ion detection (MID) data were taken at a MS resolution of -9600 using accelerating voltage scans at 1.5-9 intervals. Full scan 9600 resolution data were also obtained at 3-9 intervals from mass 100-500. The full scan data were taken with a VG ZAB double focusing mass spectrometer.

RESULTS This work is part of an ongoing effort to identify and quantitate the many heteroatom-containing compounds present in Paraho shale oil. This effort also has the intent of making these identifications without resorting to prior separation of the sample into saturate, aromatic, and polar fractions. m / z 121 Profile. This study was prompted by the high visibility of the m/z 121 mass chromatogram compared to all other odd mass chromatograms in the sample. Figure 1is the reconstructed ion chromatogram ( R E ) and the m/z 121 mass chromatogram from the GC/MS/C analysis of raw Paraho shale oil. The carbon numbers of the normal alkanes are marked. The crosshatched peaks in the m/z 121 mass chromatograms are due to C8H90 fragments from phenols. The CQH13fragments are due to di- and tricyclic alkenes as identified, whereas the shaded peaks are due to fragments from the pentacyclic hopanes and hopenes. The remaining peaks are due to C8H1,N fragments mainly from alkylpyridines. Figure 2 shows the results of a high-resolution MID analysis a t m/z 121 of raw Paraho shale oil. Shown are the C8HQ0 (phenols) mass chromatogram, the C8H11N(n-alkylpyridines) mass chromatogram, and the CgH13 (cyclic alkene) mass chromatogram. Figure 3 shows a close-up of the m / z 121 CsHllN mass chromatogram. In this figure, four series of nitrogen-containing compounds were identified. The “D” series is composed of three components. See inset of Figure 3. All these series are considered homologous; Le., within a series they differ in the length of a normal alkyl side chain. This is based on the regularity of retention times with increased parent peak mass and the fact that they track exactly with the normal paraffin retention times. A partial mass chromatographic MAP showing the relationship of fragment m/z 121 to parent ions M+ (331 to 261) and M+ - 1 (330 to 260) and the relative retention times of some of the B, C, and D series are shown in Figure 4. These data were generated from a separate low MS resolution GC/MS run. CION, C15N, CZoN,and C30N refer to CIOHISN,Cl5H,,N, C ~ O H ~and ~ NC30Hs5N, , the molecular formulas, respectively, of the parent ions all with base fragment ions at m/z 121 C8H1,N. Their general molecular formula is C,H,,-,N. Hydrogenation. This section deals with the effect of hydrodenitrification (HDN) on alkenes in general and on the CsH90, C8H1,N, and C9H13profiles specifically. The denitrification results are specific only to this sample and are not necessarily typical of all shale oils. 0 1984 American Chemlcal Society

702

ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1984

'""'1

I7f6

25508B C

ma

DISCUSSION Phenols, C8H90. The interpretation of the alkylphenol mass chromatogram is straightforward. Coinjection with diand trimethylphenols confirmed the identify of every peak in the C8H90 trace, as labeled in Figure 2. Cyclic Alkenes, CgH13.The first group of peaks on the CgHISmass chromatogram (Figure 2), -scan 300, is due to the 13Cpeak of m/z 120 from alkylbenzenes. The next intense peak, scan 1100, is due to a dicyclic sesquiterpene with a molecular weight of 206. Its mass spectrum is shown in Figure 8, along with the mass spectrum of the corresponding hydrogenated molecule with a molecular weight of 208. The hydrogenated ClSHg6is reminiscent of those reported earlier (1). This compound differs from either of those reported in that the base peak is mlz 123, followed in intensity by m / z 193 and then the molecular ion, m / z 208. Possible structures of the two dicyclic components are the following:

5Ce"

2380

75 l5BB 00

58 lBB?80

pletely converted to the hopane. The 70% HDN sample shows that the only components left which contribute to the mlz 121 chromatogram are the hopanes. Figure 7 is an exploded view of the mlz 191 mass chromatogram. Shown are the hopenes, shaded peaks in the raw shale oil (top), and their conversion to corresponding hopanes, bottom trace, in the 95% HDN oil. The m/z 217 sterane mass chromatogram shows little effect due to HDN treatment.

~ini

82

R e t e n t i o n Time-

Flgure 1. Gas chromatogram (bottom) and m l r 121 mass chromatogram (top) of raw Paraho shale 011.

Figure 5 compares the 10- to 50-min portion of the total ion current GC trace for the raw shale oil, the 10% HDN oil, and the 70% HDN oil, respectively. Note that at the 10% HDN treatment level all of the acyclic alkenes, shaded peaks in the top trace, have been removed by hydrogenation. Little difference can be seen between the 10% HDN oil and the 70% HDN oil. The acyclic alkenes are presumably converted to their alkane counterpart. Figure 6 gives the low MS resolution composite mass chromatogram at mlz 121 for the raw shale oil, the 10% HDN oil, and the 70% HDN oil-top to bottom. The mass chromatographic data were normalized to the total ionization which served as the internal standard. The 10% HDN sample shows a reduction in concentration of the di- and tricyclic terpanes by about 50%, whereas the CZ7hopene was com-

123

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Figure 2. m l r 121 mass chromatograms Paraho shale oil.


Flgure 3. m l r 121 CBH,,N mass chromatogram Paraho shale 011.

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Figure 4. Mass chromatographic map of the B, C, and D series and the relationship of fragments to parent ions and relathre retention times.

Figure 8. m l z 121 mass chromatograms of the raw and 10% and 70% HDN treated Paraho shale 011, respectively, top to bottom.

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Figure 5. Trace of the 10-50 min gas chromatogram from the raw and 10% and 70% HDN treated Paraho shale oil, respectively, to bottom.

The next large peak occurring, -scan 2300, in the C9H13mass chromatogram is due to a tricyclic alkene. The mass spectrum of this component is shown in Figure 9, along with the corresponding hydrogenated species. The likely structures of the before and after hydrogenation tricyclics are the following:

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isB

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Retention T i m e

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ston 11%

Figure 7. m l r 191 mass chromatogram of the raw and 95% HDN treated Paraho shale oil showing the hopenes, hopanes, and the m l r 217 mass chromatogram for steranes.

CS0N (see Figure lo), whereas the C series shows -10% molecular ions starting at m / z 149 and following through to m / z 289 (see Figure 11). The D series is composed of three to four component multiplets starting at approximately m / z

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259

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m / z 276

Nitrogen Components, C8HllN. The peaks uncovered in the C8HllN mass chromatogram reveal from six to nine alkyl homologous series. Homologous series A, B, C, and D are shown in Figure 3. The striking and common feature in the mass spectra of the homologues in series A, B, and C is a base peak at m / z 121 followed by important fragments at m/z 134 and 148. The A series gives no parent ion information. The B series shows about a 1% to 5% M+ intensity from CIONto

289 The mass spectra of these components not only show important peaks at m/z 121,134, and 148 and at the molecular ion but also at -15, -29, -43, and -57 below M+. Typical spectra are shown in Figure 12. These regularly spaced components are almost surely nalkyl-substituted components containing a C8HllN rearrangement moiety. This is to say that the interval in retention time between mass chromatographic peaks within each of the A through D homologues is regularly spaced and tracks exactly those intervals of the n-alkanes in the same sample. These results recall an earlier report that discussed identification of several substituted n-alkylbenzene homologue serues found

704

ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984 11792

i Y#

M

MIZ

Flgure 8. Mass spectra of the dicyclic sesquiterpene and sesquiterpane in Paraho shale oil. Sam

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F

I MIZ

111).

I

@c3

-

Figure 8. Mass spectra of tricyclic diterpene and ditgrpane in Paraho shale oil.

in the pyrolysis oil from some bituminous United States coals (2).

Possible structures of the m / z 121, C8HlIN, fragment ion are the following:

+ H

111

m/z 93 VI

V

This compound shows a 31% peak at 106 and a 12% peak at m / z 120 with a 2% parent ion at m/z 135. This same compound with an ethyl or two methyl groups on the pyridine ring should give a base peak at m / z 121 and important ions at m/z 134 and 148. All of this evidence suggests that most, if not all, of the CsHllN mass chromatographic peaks of the B, C, and D series are due to 2-n-alkylpyridines with dimethyl or ethyl substitution on the pyridine ring. An alternative to C2H5 substitution on the ring is methyl substitution on the ring with a methyl branch on the side chain giving the following two possibilities for the CBNllN moiety:

A

I I1

IV

Structures I, 11, and I11 could, through a McLafferty-type rearrangement, produce an ion at m / z 121 and peaks at mlz 134 and 148. The monocyclic pyrrole (11), however, through the saturate ring cleavage, would probably have significant MS peaks at other than those in the CnHzn-5,-4 window.

+ H

VII

I

121

121

121

121

I

N-n-Butyl- and N-n-pentylpyrroles are known to undergo proton rearrangement to the pyrrole ring giving rise to a base peak at m/z 81 (3). Presumably, an N-n-alkyl alkene pyrrole (111)or a N-n-alkyl cyclic pyrrole (11)would give a base peak at m/z 121. It is reasonable to suggst that the alkene or cyclic pyrroles result from the degradation, either pyrolytically or geochemically, from porphyrins. It was shown that at 10% HDN the A series of C8Hl,N homologues disappear, along with all other acyclic alkenes. The cyclic alkenes are only about 50% removed, whereas the phenols and remaining C8HI1Ncomponents are only 6% reduced. This suggests that the A homologues may well be due to alkene pyrroles. Anilines (IV) undergo the usual &cleavage of the side chain as the dominant process. In the case of structure IV, the base peak would be m / z 120. If this is a general rule, the anilines may be discounted as members of any of the series reported here. Alkylpyridines are known to undergo the McLafferty rearrangement. This mode of cleavage is most important for pyridines substituted in position 2. 2-n-Pentyl-, 2-n-hexyl-, and 2-n-propylpyridine all exhibit the McLafferty rearrangement to give a base peak a t m / z 93 ( 4 , 5). All show significant peaks at m/2 106 and 120. Addition of two methyl groups to the aromatic ring of these alkylpyridines would give structure I. These components on electron impact should give a base peak at m/z 121 with significant peaks at m / z 134 and 148, along with low-intensity molecular ions. For example, Budzikiewicz and Besler (5) have shown that 2-n-butylpyridine via a McLafferty rearrangement will give a base peak at m / z 93 thus:

VI11

VI1 and VI11 are the most likely structures for the m / z 121 moiety. The A, B, and C series of C8Hl,N homologues are singlets. The D series is composed of groups of at least three peaks. The A series extends from approximately CIONto approximately C2& The B series shows a trinodal distribution which roughly follows from CloN to CI5N to CzoNto CJON,whereas the C series starts at about CloN and ends almost abruptly at CZoN. The D series begins at about CZoN,extending to approximately CmN. All of this evidence suggests an isoprene connection of some sort. Likely mechanisms to account for the observed mass spectra are (1)diagenetic n-alkylation of isoprenoidal-type pyridines


705

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and/or (2) diagenetic degradation of some higher molecular weight biogenic nitrogen-containingprecursor. This argument parallels that used to account for high molecular weight >CS2

I

I

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I

porphyrins found on the geospheres, i.e., diagenetic alkylation of a chlorophyll precursor to the geoporphyrins or degradation of some more highly substituted porphyrin pigment, e.g.,

706


134

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IN

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M+ 219

I

1

L

1

1

I

I

C15N

M+ 233 L

M+ 261 1

200

M / Z 120

300

Mi2

Figure 11. Mass spectra of “C” series of homologous n-alkylpyridines.

chlorobium-chlorophyll (6). The D series, which begins at approximately m/z 289, shows distinctly different mass spectra from the others. They also show a considerably shorter retention time relative to carbon number. (See Figure 4.) This suggests a highly branched alkyl group attached to the pyridine ring and a short C3 to C5 n-alkyl attachment somewhere near the pyridine side of the alkyl group. As might be expected, the mass chromatograms of m/z 135

and to some extent 149 also show homologous n-alkylpyridine series with their mass spectra showing base peaks at m / z 135 and 149, respectively. Exact masses were confirmed from the full scan GC/high-resolution MS data. Their importance with respect to concentration is considerably less than that of the m / z 121. These undoubtedly represent additional n-alkylpyridines. These alkylpyridines represent approximately 10% of the total GC effluent and account for -0.7% nitrogen at an


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247

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1.

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h

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120

300

200 M/Z

Flgure 12.

Mass spectra of some of the “D” serles homologous of n-alkylpyrldlne.

average molecular weight of -200.

CONCLUSIONS Exact mass deconvolution of m / z 121 of Paraho shale oil reveals three main fragment types: CBH90’sdue exclusively to di- and trimethylphenols; C8HllN’s due mainly to homo-

logues of alkylated pyridines, and possibly some alkene pyrroles, and, finally, di-, tri-, and pentacyclic terpenes. Hydrogenation experiments show that the easiest components to remove in this sample are the acyclic alkenes and the C2, pentacyclic hopenes, followed by the di- and tricyclic terpenes and the Ceg hopenes. Next, apparently are the

708

Anal. Chem. 1984, 56,708-713

phenols and last the n-alkylpyridines. These provisionally identified components are most certainly affected differently depending on temperature, pressure, type of catalyst, etc., and as such may serve as very sensitive monitors of retort and processing conditions. Measuring the presence and distribution of such components may be the best way of tracking the processing environment of feeds. Registry No. 2,4-Dimethylphenol, 105-67-9; 3,5-dimethylphenol, 108-68-9;2,4,6-trimethylphenol,527-60-6; 3,4-dimethylphenol, 95-65-8; 2,3,6-trimethylphenol, 2416-94-6; 2,3,4-&methylphenol, 526-85-2; 2,3,5-trimethylphenol, 697-82-5; 3,4,5trimethylphenol, 527-54-8.

LITERATURE CITED (1) Richardson, J. S.; Mlller, D. E. Anal. Chem. 1982, 5 4 , 765-768. (2) Gallegos, E. J. J . Chromatogr. Sci. 1981, 79, 177-182. (3) Budzlklewlcz, H.; Djerasiz, C.; Williams, D. H. "Interpretation of Mass Spectra of Organlc Compounds"; Holden Day: San Francisco, CA, 1964; p 242. (4) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. "Atlas of Mass Spectral Data"; Interscience: New York, 1969; Vol. 2. (5) Budzlklewlcz, H.; Besler, U. Org. Mass Spectrom. 1978, 7 7 , 398-405. (6) Baker, E. W.; Yen, T. F.; Dlckle, J. P.; Rhodes, R. E.;Clark, L. F. J . Am. Chem. SOC. 1987, 89, 3631.

RECEIVED for review June 27, 1983. Accepted January 10, 1984.

Determination of Sulfur as Arsenic Monosulfide Ion by Isotope Dilution Thermal Ionization Mass Spectrometry P. J. Paulsen and W. R. Kelly* Center for Analytical Chemistry, National Measurement Laboratory, National Bureau of Standards, Washington, D.C. 20234

A new procedure has been developed for the determlnatlon of mlcrogram quantltles of sulfur In metals by isotope dliutlon thermal lonlzatlon mass spectrometry. Typically 1% metal samples are splked with 34Senrlchedsplke and dlssolved In a closed system to prevent loss of volatile S compounds uslng a mixture of HCVHNO, acids whlch oxidizes all S to sulfate. The S Is reduced to H2Sand the sulfide precipitated as As#,. The As2S3Is dlssolved In an ammoniacal As3+ solullon to yleld an As/S atom ratlo of two. A small portlon of thls solution, equlvalent to 1.5 pg of S, Is placed on a Re-flat fllament with silica gel and the 32S/34Sratlo Is measured at 950 OC as the thermally produced "Asa2S+ and " A S ~ ~ Smolecular ' Ions. The ionization efficlency Is about 0.1 % and the precision of the a2S/34Sratio measurement is about 0.1 % (1s). Thls procedure has been applled to the determlnatlon of S In 11 Cu base and Fe base alloys ranging In S concentratlon from 2.8 f 0.2 to 81 f 1 pg of S/g (fs, 95% confidence Interval). The chemlcal blank Is the major source of uncertainty at these levels.

The sulfur content of materials has an important effect on their physical properties. One of the most important properties of low alloy steel is toughness, particularly in subfreezing environments. This property can be increased by lowering the S content since toughness increases exponentially below 100 pg of S/g (1). To meet the toughness specification for the Trans-Alaskan pipeline, the Japanese steel companies reduced the S concentrations in their pipe steel to 20-130 pg of S/g. In copper, S concentrations above 25 pg of S/g cause casting problems in static molding. Conductivity and mechanical properties are adversely affected by S at concentrations of a few parts per million (2, 3). Therefore, the accurate determination of S below 100 pg of S/g is of considerable industrial importance. The most common methods of measuring S are gravimetry and combustion using iodate titration or infrared detection. Gravimetric determination of S by BaSOl precipitation is unreliable below 50 pg of S/g ( 4 ) and the accuracy of combustion techniques depends on This artlcle not subject to

accurate standards for instrument calibration. Watanabe (5-7)has developed an isotope dilution procedure to determine S in steels using conventional SO2 gas mass spectrometry. He has reported values lower than the certified values on a number of international standards. Following these reports, Paulsen et al. (8)and Burke et al. (9) developed an isotope dilution procedure for sulfur determination by using a spark source mass spectrometer (ID-SSMS). In this procedure, samples are dissolved and oxidized in a sealed tube to prevent loss of volatile sulfur compounds and ensure complete equilibration of S isotopes from sample and spike. At high levels the ID-SSMS data were in good agreement with the existing certified values; however, at lower levels large differences were observed. For example, SRM 342a, Nodular Cast Iron, was certified at 60 pg of S/g but was found to be 23 f 2 pg of S/g by ID-SSMS (8). Determinations by IDSSMS yielded blanks of less than 1kg of S and measured the altered 32S/34Sratio with a precision of f 3 % (Is). Thus with a 1-g sample, concentrations of a few to several hundred parts per million can be determined with an accuracy comparable to the measurement precision of the spark source mass spectrometer. Recently, Kelly et al. (10) observed an isobaric interference in the Ag mass region while measuring Ag isotopic ratios, using silica gel as an emitter. This interference was positively identified as 7sAs32S+and 7sAs34S+which occur at the two Ag masses with a 107/109 ratio of about 22. Based on this discovery that S can be thermally ionized as the ASS+ molecular ion, we have developed a procedure to measure sulfur concentrations by isotope dilution thermal ionization mass spectrometry (ID-TIMS) in a wide variety of materials by measuring the ASS+molecular ion. In our procedure the sulfur in the sample is oxidized to sulfate in a sealed tube. The sulfate is reduced to H2Swhich is trapped in an As3+-NH3solution and precipitated as AszS3. The As2SBis dissolved in NH3, an aliquot containing 1.5 pg of S is loaded onto a Re-flat filament with silica gel, and the 32S/34Sratio is measured at 950 O C as 75As32S+ and 75A~34S+ with a precision of 0.1%. Since As is mononuclidic, the ion currents at mass 107 and 109 are proportional to the 32Sand 34Sabundances. This procedure makes it possible for all

US. Copyright. Published 1984 by the Amerlcan Chemlcal Soclety

z 121 mass chromatogram of Paraho shale oil

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