1088
Anal. Chem. 1986, 58,1088-1091
€valuations of Heavy Constituents in Fractions of Petroleum Residues Using Gel Permeation and Field Desorption Mass Spectrometry B. S. Larsen' and C. C. Fenselau*
Johns Hopkins School of Medicine, Middle Atlantic Mass Spectrometry Laboratory, 725 North Wolfe Street, Baltimore Maryland 21205
D. D. Whitehurst and M. M. Angelini2 Mobil Research and Development Corporation, P.O. Box 1025, Central Research Division, Princeton, New Jersey 08540
The molecular weight range and dlstrlbutions of deasphalted oil, resin, pltch, and furfural rafflnate from a deasphaited residue have been examined by field dearptlon mass spectrometry and gel permeation chromatography. Masses above 3000 were detected In the rafflnate and above 5000 In the deasphalted oil and resin. Number averages and dlstributlon envelopes from the two technlques as well as vapor-phase osmometry are compared for various samples.
In the development of new or improved methods to upgrade heavy petroleum, it is important to have detailed compositional information to determine intrinsic chemical limitations of a given process or to allow mechanistic interpretation of reactions that occur in a given process. Obtaining such information on the composition of the nondistillate portion of petroleum-derived oils has been limited in the past by the availability of methods capable of handling large polar molecules. Average molecular weights of polar petroleum fractions obtained by vapor-phase or membrane osmometry are questionable because of the problem of molecular association. Molecular weight distributions have been obtained by gel permeation chromatography (GPC), but quantitation of such information has been hampered by the availability of detectors having linear response to molecules of diverse chemical composition and size. Conventional mass spectrometry is not applicable for large molecules as fragmentation and pyrolysis are excessive. In this paper, we describe two new approaches to molecular weight analysis of large molecules that show promise in overcoming these past limitations. Field ionization mass spectrometry (FIMS) has been used to characterize coal-derived liquids ( I ) . The ionization process results in less electronic excitation than conventional electron impact, giving rise to more abundant molecular ion species and less fragmentation (2). Field desorption mass spectrometry (FDMS) has been a method of choice to determine the molecular weight of compounds that are nonvolatile and of .high molecular weight ( 2 , 3). FDMS has been used to determine accurate molecular weight averages of polystyrene (4) and polyglycol oligomers (5). Specialty products are currently produced from petroleum residues by means of solvent fractionation. Typically, residues are deasphalted with a light hydrocarbon stream such as propane or butane to provide asphalt and deasphalted oil. One newly developed process, the Rose process of Kerr-McGee Refining Corp. allows a finer degree of fractionation using 'Present address: E. I. du Pont de Nemours & Company, Central Research & Development Department, Experimental Station, Wilmington, DE 19898. Present address: Stuart Pharmaceuticals, Wilmington, DE 19898.
Table I. Number Averages for Molecular Species
sample
GPC"
FDb
VPOc
raffinate deasphalted oil resin pitch
1119 (1240)
1328 1797
1853
1162 (1504) 1377 (2212) 1188 (2527)
1999
1209
1024 921
" Gel permeation chromatography. Values in parentheses are mass averages. Field desorption average obtained by averaging the summed intensities. Vapor-phase osmometry data obtained from Galbraith Laboratories, Inc., using THF solvent. butane near supercritical conditions (6). This process provides three commercial streams, deasphalted oil (DAO), resin, and pitch. Another large-scale fractionation currently of importance is the separation of aromatic and nonaromatic hydrocarbons from DAO. This is accomplished by the extraction of DAO with a polar solvent such as furfural, phenol, or sulfolane. This nonaromatic insoluble raffiniate is used for the manufacture of lubrication oil. These materials represent a nice series of commercially important heavy petroleum streams of increasing polarity that test the limits of compositional analysis by newly developing techniques.
EXPERIMENTAL SECTION The mass spectrometric measurements were obtained with a Kratos MS 50 instrument equipped with a 23-kG magnet, fitted with 8-kV postacceleration. A combination fast atom bombardment/field desorption (FD) source, which is described elsewhere (7), was used. The spectra were acquired at 6-kV accelerating voltage with a scan rate of 100 sjdecade and a static resolution of 5000. The data system was calibrated from 450 to 5000 amu in the FAB mode using cesium lodide. Spectra were recorded, mass assigned off-line,and averaged by use of the Kratos DS-55 software. The FD emitters used for these experiments were acquired through Vacumetrics, Inc. The sample was loaded onto the emitter by dissolving it in a suitable solvent, and a syringe was used to add a droplet evenly over the emitter surface. The emitter current was manually increased 0.25 mA every two scans until a final current of 32 mA was attained. The nominal mass profiles resulted in a large scatter in the ion intensities by adding the averaged ion intensities for every 100 mass units together with the mass distribution for each sample was more meaningful. The gel permeation chromatography was performed with three columns in series packed with micro styragel (Waters Associates), one 500-A column and two 100-A pore size columns. A 0.1% sample in toluene was introduced into a six-port injector fitted with a 1OO-wLsample loop and injected into the 1mL/min flowing system. The eluting sample was detected by a Tracor 945 LC flame ionization detector in which the sample is separated from the solvent prior to detection by means of a rotating quartz belt (8). The total signal was integrated at 10-s intervals using in-house software. The DAO, resin, and pitch samples kindly provided by R. M. Newcomer of Pester Refining Co., El Dorado, KS, were obtained
0003-2700/86/0358-1088$01.50/00 1986 American Chemical Society
.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1089
u***r*,
"*****I
t
5 =
c
,.....*
we***
I
'i
. i . 1 0 1000 2000 3000 4000 5000 500 1000 1500 2000 2500 3000 M/z
M/Z
'1
01 500
I
9
I
I
t
1000
1500
2000
2500
3000
m
Figure 1. Molecular weight profile for the raffinate sample (A) field desorption and (B) gel permeation chromatography results.
from Mid Continental U.S.Crude. The sample of raffinate was from Arabian Light Crude provided by Mobil Oil.
RESULTS AND DISCUSSION Number averages for the distribution of molecular species in the four samples in order of increasing aromatic and heteroatom content are shown in Table I. The agreement between the three techniques is mixed. Figure l illustrates the gel permeation chromatography and field desorption experiments on the raffinate sample. There is a fairly good agreement in the profiles including the tail toward the higher masses. Profiles or envelopes of the relative abundances of components in various mass ranges can be determined by the mass spectrometric techniques and by use of the flame ionization detector with gel permeation chromatography. These profiles are shown for each of the four samples in Figures 1-4. The structure in the FD profiles is in part a problem of ion statistics but for the most part is reproducible. Vapor-phase
0 :
0
I
1000
I
I
2000~3000
I
4000
I SO00
Figure 2. Molecular weight profile for the deasphalted oil sample field desorption and (B) gel permeation chromatography results.
osmometry (VPO) cannot provide distribution envelopes, but only average values. Although not parallel in details, the overall shapes and dimensions of the envelopes measured by the two techniques are similar. Perhaps the most significant difference is in the height of the high-mass tail from pitch. Number averages for deasphalted oil determined by FD and VPO agree well. The GPC determination is lower, although the envelopes shown in Figure 2 are similar. Values for the resin sample cover a 2-fold range, with the highest averages provided by FD and GPC. The envelopes in Figure 3 suggest that the number and mass averages should lie between those of deasphalted oil and of pitch. Number-averages for pitch are reasonably close for the three techniques. The low-mass portion of the curves is disproportionately high, which causes the mass average to be low. The mass average calculated from the GPC experiment may more appropriately reflect the contribution of high-mass components, as may be independently evaluated from the envelopes shown in Figure 4.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986
1000
m*ww-
*a****
1
1#11***.
111***.
i
E ***ma*. E
a
.11**1.
* * 1 500 1000 WOO 2000 2500 , 3dOO i 1000 2000 3000 4000 SO00 6000 I
I
I
M/Z e-
M 5-
4-
s
8
E
E
3-
2-
I-
0
1000
2000
wz
3000
4000
SO00
Flgure 3. Molecular weight profile for the resin sample (A) field desorption and (B) gel permeation chromatography results.
Desorption efficiency in the mass spectrometer may decrease for these high-mass, possibly polar components. Fast atom bombardment MS, a soft-ionization technique suitable experiments were performed for analysis of polar samples (9), on these samples in order to compare to the FD results. The petroleum residues are too complex a mixture to observe intact species desorption from the surface. A t the low end of the mass range, which is more important in the raffinate sample, care must be taken not to lose the more volatile lower molecular weight compounds during field desorption analysis. Low masses may also be preferentially lost during the solvent evaporation on the Tracor detector. Very polar components may associate and give an artificially high molecular weight in the GPC analysis. High temperatures or more disassociative solvents can be used t o minimize these effects.
CONCLUSION These studies characterize compounds with molecular weights in the range 3000-5000 amu in all four of the residual
500
1000
1
I
I
SO0
1000
a00
I
3000
M/z Flgure 4. Molecular weight profile for the pitch sample (A) field desorption and (B) gel permeation chromatography results.
fractions examined. Both gel permeation chromatography and field desorption mass spectrometry can be used to characterize these heavier components, if the limitations of each at both ends of the mass range are kept in mind. In addition, field desorption mass spectrometry offers the potential for more detailed analysis of chemical types, analogous to field ionization. This capability is presently being explored.
LITERATURE CITED (1) Whitehurst, D. D.; Butrill, S. E.; Derbyshore, F. J.; Farcaslu, M.; Odoerfer, G. A,; Rodnick, L R. Fuel 1982, 61, 944-1005. (2) Beckey, H. D. “Princlples of Fieid Ionlzatlon & Fieid Desorptlon Mass Spectrometry”; Pergamon: New York, 1977. (3) Schulten, H. R. Inf, J . Mass Specfrom. Ion Fhys. 1979, 32, 97-283. (4) Lattimer, R. P.; Harmon, D. J.; Hanson, G. E. Anal. Chem. 1980, 52, 1808. (5) Lattimer, R. P.; Hanson, 0.E. Macromolecules 1880, 1 4 , 1776-80. (6) Newcomer, R. M.; Soltow, R. C. National Petroleum Refiners Association Annual Meeting, 1962, Paper 47. (7) Hanson, G.; Heller, D.; Yergey, J.; Cotter, R. J.; Fenselau, C. Chem. Biomed. Environ. Instrum. 1882, 12, 275-288.
Anal. Chem. 1986, 58, 1091-1097 (8) Dixon, J. B. chimi8 1984, 38, 82-86. (9) Barber, M.; Bordoli, R. s.; Elliott, G. J.; Sedgwick, R. D.;Tyler, A. N. Anal. Chem. 1982, 5 4 , 645A-654A.
RECEIVD for review September 26,1985. Accepted December
1091
4,1985. This research was supported by a grant, PCM 8200954, from the National science ~ ~ ~ ~FAB d spectra ~ t i were obtained a t the Middle Atlantic Mass Spectrometry Facility, an NSF shared instrumentation facility.
Mass Spectrometric Study of Ion Adsorption on Poly(ethy1ene terephthalate) and Polypropylene Surfaces Ronald
D.Macfarlane,* Catherine J. McNeal, and Charles R. Martin
Department of Chemistry, Texas A&M University, College Station, Texas 77843
The adsorption of ions on polypropylene and poly(ethylene terephthalate) surfaces from polar solutions has been studied by use of 252Cfplasma desorption mass spectrometry. The solutes were Li and Cs salts and Rhodamine 6-6 hydrochiorlde in aqueous and ethanol soiutlons. Adsorption was studied as a function of solute concentration and pH and mixed solute solutions were used to ascertain competitive adsorption behavior. Polypropylene and poiy(ethyiene terephthalate) behave Identically functioning as cation exchange surfaces with preference for the adsorption of organic cations. Ion intensities deduced from mass spectra were found to be a function of solution concentration reaching a maximum value at full monolayer coverage. The adsorption sites appear to be small chemlsorbed strongly basic anions present as impurities and the hydroxyl ion, which is in equilibrium wlth water in the mobile phase.
In a recent study, we investigated the possibility that polymer surfaces with ion exchange properties might be useful for preparing samples for solid-state mass spectrometry (1). The concept was devised with the objective of developing a general scheme utilizing the principles of liquid chromatography and solute partitioning a t a liquid/solid interface to selectively adsorb an analyte onto a compatible surface in the presence of other components in the solution. The adsorbed layer is directly analyzed by mass spectrometry. In the first study, we demonstrated that a Mylar polyester film surface coated with a cation exchange polymer, Nafion, had a remarkable affinity for polarizable cations and that the cationic species adsorbed on the Nafion could be detected with high sensitivity by 262Cfplasma desorption mass spectrometry. It has been known for some time that all polymers acquire a surface charge when exposed to an aqueous medium (2). This property is used in coloring synthetic fibers using ionic dye molecules (3) and is also responsible for the coagulation of blood proteins on the surface of polymeric materials used in medical applications (4). The reason why polymers acquire a charge can sometimes be related to particular acidic or basic functional groups in the polymer structure, but even polymers that are pure hydrocarbons such as polystyrene and polyethylene acquire a significant negative surface charge when exposed to a polar solvent. I t has been suggested that the origin of these surface charges may be molecular remnants of the polymerization process containing negatively charged functional groups that lie on the surface of the polymer (5). In this study we investigated the ion adsorption properties of polypropylene and poly(ethy1ene terephthalate) (Mylar)
using solid-state mass spectrometry (262Cf-plasmadesorption) as a probe to elucidate the mechanism of the ion adsorption properties of these polymers. In addition we developed further the concept of the use of polymer films as a substrate and analyte ions as adsorbate as a medium for solid-state mass spectrometry and as a means for the preparation of a more well-defined and controlled matrix for the study of the dynamics of the emission of molecular ions from surfaces. EXPERIMENTAL SECTION Apparatus. Mass Spectrometer. Mass measurements were made with a z62Cfplasma desorption (=‘Cf-PDMS) time-of-flight (TOF) mass spectrometer (6). The system was operated at an acceleration voltage of f 1 0 kV with a 4-mm target-grid spacing and a 55 cm long field-free region. A 10-pCi262Cfsource (Isotope Products Corp., Burbank, CA) was positioned behind the sample on a linear motion feedthrough to within 4 mm of the sample. Details of the internal elements and geometries are given elsewhere (7). Time-of-flight measurements were made with a multistop time interval digitizer with a 78-ps resolution and a 850-11s dead time (8). The secondary ion detector at the end of the flight tube consisted of two microchannel plates in a chevron configuration coupled to a 5 0 4 impedence matched anode (Galileo Electrooptics, Sturbridge, MA, Model FTD 2003). Photoelectron Spectrometer. Photoelectron spectra of electrosprayed and adsorbed CsI were obtained from a photoelectron spectrometer (Kratos, Manchester, U.K., Model XSAM-800) operating with 1453.6-eV photons (A1 Ka) at an incident angle of 30’. Spinner. A Model AHT3A-T-54 photoresist spinner (Headway Research, Inc., Garland, TX) operating at 10 000 rpm was used to remove the liquid phase from the polymer surface after adsorption. Materials. Polypropylene and poly(ethy1ene terephthalate) (Mylar) films, 4 and 1.5 pm thick, respectively, and aluminized on one side, were obtained in a 3.7 cm wide roll from Atlan-To1 Industries, West Warwick, RI. Rhodamine 6-G (laser grade) was obtained from Eastman Kodak Co., Rochester, NY. Procedures. Multilayer Sample Preparation. Multilayer deposits were prepared by the electrospray method (9). Ethanol solutions of solute (5 pg/pL) were sprayed onto an area 1 cm2on the A1 side of the polymer film giving a multilayer approximately 20 pg/cm2 in thickness. Adsorbed Layer Sample Preparation. The polymer film (with the A1 side down) was stretched and mounted in the specimen measurement. holder that serves as the “target” for a 262Cf-PDMS This was then mounted on the head of the photoresist spinner and a 200-pL aliquot of solution was placed on the surface of the polymer with the spinner in the rest position. After a 2-min period, the liquid was rapidly spun off the surface by rapid acceleration to 10000 rpm, which was maintained for a 1-min period. The sample was then transferred to the mass spectrometer for mass analysis.
0003-2700/86/0358-1091$01.50/00 1986 American Chemical Society
~
~
