1238
Anal. Chsm. 1982, 5 4 , 1238-1240
Temperature Programmed Fractionation Inlet System for Mass Spectrometers Kit-ha C. Chan, R. S. Tse," and S. C. Wong Department of Chemistty, University of Hong Kong, Pokfulam Road, Hong Kong
Mass spectra of involatile samples are commonly obtained by pyrolysis-mass spectrometry (Py-MS). Pyrolysis usually takes place either in a batch system outside the mass spectrometer ( I , 2) or at the tip of a direct inlet probe in the ion source of the mass spectrometer (3-5), in both cases usually under high vacuum. When a pyrolyzer outside the mass spectrometer is used, usually only a small portion of the sample is bled to the ion source. When a pyrolyzer inside the ion source is used, the sample normally has to be dissolved and coated on the pyrolyzer, which is usually a filament. Such a pyrolyzer can accept only relatively clean samples, for fear of contaminating the ion source. Temperature is commonly raised by either pulse heating (6) or ballistic heating ( 4 ) ,but more information can be obtained by temperature programmed heating. A low cost temperature programmed fractionator (TPF)has been constructed that can be interfaced with the ion source of any mass spectrometer through a conventional jet separator. As the temperature is raised, the fractionation process includes desorption, evaporation, distillation, and finally pyrolysis. This is carried out under atmospheric or subatmospheric pressure in a stream of helium, thus avoiding some of the possible complications of reactions among the pyrolyzates themselves in a batch system. The T P F inlet system is suitable for handling involatile, untreated, and messy samples, because it can easily be dismantled for cleaning without stopping the mass spectrometer. The authors are aware of two reports of similar devices that use interfaces, each used for a specific purpose (7, 8). In addition to the mass spectra obtained, the total ion signal vs. temperature curve is equivalent to a differential form of a thermogram, providing information in the area of thermal analysis. The TPF also can serve as an isothermal or temperature programmed flow reactor for homogeneous, catalytic, or solid decomposition kinetic studies. The construction of the TPF inlet system is shown in Figure 1. The sample pan and the furnace cup are made of quartz and the rest of the device is made of Pyrex. No grease is used in any of the ground glass joints. The furnace is made up from 21 turns of SWG2O nichrome wire on an 8 mm diameter form and has an overall resistance of 1.2 Q. I t is powered through a step-down transformer by a commercial temperature programmer (a combination of Eurotherm modules 127-03-27625-03-19-00,017-001-03-025-01-19-00,and 031-080-06),capable of programming the furnace temperature from 0.1 to 990 "C/min up to a maximum of about 950 "C. Tungsten wires are used as feedthroughs for electrical connections. The fractionator and the jet separator are mounted in a two-compartment heated box, each compartment having its own isothermal control up to about 200 "C which is also the maximum temperature of the mass spectrometer ion source. The short lead to the mass spectrometer ion source is wrapped with heating tape. The TPF is commonly operated at 200-300 torr with a helium flow rate of -50 cm3/min. The mass spectrometer (a Hitachi RMS-4) main vacuum pressure is usually at -5 x IO4 torr when the TPF inlet system is connected to it. In this mass spectrometer, the total ion monitor is an electrode which intercepts a portion of the ion beam emerging from the ion source. The sensitivity of the TPF inlet system depends on the yield
THERMOCOUPLE
STAINLESS STEEL
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SHIELD FURNACE
2 9 / 3 2 CONE & S O C K E T
TEMP
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ii
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Flgure 1. The construction details of the fractionator.
of the jet separator. Initially the overall sensitivity was found to be of that obtained through the conventional direct inlet probe; but with an improved jet separator, this factor can hopefully be raised to 1/3. In practice, however, usual sample sizes are large enough to make the ultimate sensitivity irrelevant. The jet separator also imposes a mass discrimination effect in favor of M1/2,where M is the molecular weight of the substance going through the jet separator. Figures 2 and 3 show the fractionograms (total ion signal vs. temperature plots, obtained in a y-t recorder) and the corresponding mass spectra of a commercial polystyrene chip and of a viscous sample of polymerizing styrene, each sample 10 mg. The fractionograms and mass spectra were obtained after placing the sample directly in the sample pan in the TPF. If a direct inlet probe pyrolyzer is used, the polymer would have to be dissolved and coated on the pyrolyzer. Moreover, one would normally not place a viscous polymerizing sample on such a pyrolyzer. In Figure 2, the commerical sample shows one single peak while the polymerizing sample shows two, a detail probably not obtainable with pulse or ballistic heating. The first peak is probably due to volatilization and the second due to thermal degradation. The mass spectra in Figure 3 showing major peaks at m/z 104,103,78,and 51 are in general agreement with previous observations (3-5). The fractionograms of a sample (-20 mg) of commercial peanut oil and a small bit (-300 mg) of an untreated peanut were very similar, each showing a single peak at 380 "C and
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0003-2700/82/0354-1238$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982 C O M M E R C I A L
1239
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P O L Y S T Y R L N E
: A
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100
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150
250
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450
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Flgure 2. Fractionogram (total ion signal vs. temperature plot) of a commercial polystyrene sample and a sample of polymerizing styrene.
COMMERCIAL
POLYSTYRFNE
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100
.
COMMERCIAL
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Figure 3. Mass spectra of (i) the commercial polystyrene sample at
375 OC, (ii) polymerlzing styrene at 375 OC, peak B in Figure 2, and (ili) polymerizing styrene at 130 "C, peak A In Figure 2, obtained with a temperature programming rate of 50 OC/min.
MIE
Flgure 4. Mass spectra of (i) a sample of commercial peanut oll at 380 "C and (ii) a blt of untreated peanut at 380 "C, obtalned with a
temperatue programming rate of 50 "C/mln.
a full width at half-maximum between 350 and 450 "C. Mass spectra obtained at two different temperatures are shown in Figures 4 and 5 and provide fingerprinting identification. An interesting point about the mass spectra is that they are those
of free fatty acids, with the characteristic peak at m/z 60 (9). Commonly mass spectra of fatty acids in edible oils cannot be obtained directly. A t most the mass spectra of the methyl
1240
Anal. Chem. 1982, 5 4 , 1240-1243 COMMERCIAL
PEANUT
OIL
AT
ester are commonly acquired (10). By the use of the T P F inlet system, the mass spectra of fatty acids in oil-bearing nuts can be obtained directly, at least for fingerprinting identification purposes.
5OO~C
' I
ACKNOWLEDGMENT
i
The authors thank S. Lai, the univeristy glass-blower, for fabricating the fractionator, C. P. Luk for optimizing the linear temperature programmer, and L. P. Ng for typing the manuscript. They also thank United Oversea Enterprises, Ltd., Phillips Petroleum International, Inc., and Dow Chemical (Hong Kong), Ltd., Tsing Yi Plant, for providing the commerical polymer samples.
LITERATURE CITED -
-
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Flgure 5. Mass spectra of (i) a sample of commercial peanut oil at 500 OC and (ii) a bit of untreated peanut at 500 O C , obtained at a temperature programming rate of 50 'C/min.
(1) Hu, J. C. Anal. Chem. 1981, 53, 942. (2) Schulten, H. R.; Dilssel, H. J. J. Anal. Appl. Pyro/ysis 1980/81, 2 , 293. (3) Futrell, J. H.; Wells, G.; Voorhees, K. J. Rev. Scl. Instrum. 1981, 52, 735. (4) Shlmizu, Y.; Munson, E. J. Po/ym. Sci., Polym. Chem. Ed. 1979, 17, 1991. (5) Lattlmer, R. P.; Harmon, D. J.; Welch, K. R . Anal. Chem. 1979, 51, 1293. (6) Udseth, H. R.; Friedman, L. Anal. Chem. 1981, 53,29. (7) Jane, I., 28th Annual Conference on Mass Spectrometry and Allied Topics, 1980;paper RPMOA6, p 540. (8) Yuen, H. K.; Mappes, G. W.; Grote, W. A. 1l t h North Amerlcan Thermal Analysis Society Conference, 1981; paper 54; Thermochlm.Acta 1982, 52, 143. (9) Heller, S. R.; Mllne, G. W. A. "EPA/NIH Mass Spectral Data Base"; U.S. Government Printing Office: Washington, DC, 1978; Natl. Stand. Ref. Data Ser. (U.S., Natl. Bur. Stand.), NSRDS-NBS 63. (10) Ryhage, R.; Stenhagen, E. J. Lpld Res. 1960, I , 361.
RECEIVED for review January 18,1982. Accepted February 18, 1982.
Determination of Trace Elements in 13 Organic Solvents by Instrumental Neutron Activation Analysis F. S. Jacobs, V. Ekambaram, and R. H. Filby* Nuclear Radiation Center and Department of Chemistry, Washington State University, Pullman, Washington 99 164
The reagent blank in many trace element analysis methods is an important source of error and is often the limiting factor in reducing elemental detection limits. Trace element contents have been measured for high-purity water (1-3), nitric (1,4), sulfuric and hydrofluoric (1,5) acids, and for other inorganic reagents (1, 4 , 5 ) . Methods have been developed for the preparation of ultra-pure reagents using such techniques as subboiling distillation (6). The study of trace element distributions in fossil fuels, for example, petroleum, oil sand bitumen, and coal-derived liquids, requires solvent extraction, size exclusion chromatography, and/or liquid chromatography (analytical and preparative HPLC) in which relatively large volumes of organic solvents are used in relation to sample size. Examples of such separations in trace element studies are the extraction and separation of oil sand bitumen (7), liquid chromatographic separation of solvent refined coal ( 8 , 9 ) ,size exclusion chromatography of coal liquids (10,11),and solvent extraction of solvent refined coal (12). Some studies of this type have been carried out with little regard for the trace element blank from solvents used in separation or extraction procedures. The assumption often made is that organic 0003-2700/82/0354-1240$0 1.25/0
solvents have extremely low trace element contents relative to those of the sample. Determinations of trace elements in organic solvents have been reported by Russian authors. Ovrutskii et al. (13)determined 18 trace metals in organic solvents by emission spectroscopy following evaporation of solvents on a carbon matrix. Detection limits obtained ranged between 0.3 ng 8-l and 10 ng g-l. A similar procedure was used by Kuz'min et al. (14) to determine 14 trace elements in isopropyl alcohol, rn-xylene, dioxane, isoamyl alcohol, and chloroform with detection limits between 0.1 ng 8-l and 10 ng g-l. In later work Kucherova et al. (15)measured 10 trace elements in carbon tetrachloride, chloroform, and methylene chloride and obtained results comparable to those of Ovrutskii et al. (13) and Kuz'min et al. (14). Recently Manoliu et al. (16)used atomic absorption spectrometry to analyze solvent evaporation residues and reported concentration values for Cu, Ni, Fe, Zn, Mn, Mg, Na, K, Ca, and Ag in methanol and isopropyl alcohol to be within the range 3-78 ng 8-l. Most organic solvents manufactured and used in the United States are not routinely analyzed for trace metals, except for 0 1982 Amerlcan Chemical Society
