absorbance in low C1- as in high C1- medium. Thus, if one were to base the calibration data and analysis on the change in absorbance upon adding a high C1- concentration, many potential interferences could be eliminated. Metals that form UV-absorbing chloro anions (e.g., FelI1) still could interfere. Although other methods exist for determining Hg" in MMC [e.g., spontaneous Hg" polarographic wave ( 5 ) ;differential reduction to HgO followed by flameless AA] or vice versa [extraction of MMC into benzene, followed by gas chromatography with electron-capture detector ( 6 ) ] , the UV spectrophotometric method may recommend itself for its simplicity, especially for following changes in the HgI1 concentration as in acid cleavage or photochemical experiments.
LITERATURE CITED
WAVELENGTH, n m
Figure 1. UV method for Hg" in the presence of methylmercury, using 6 M HCI medium (a) Pure MMC, ( b ) MMC irradiated in 6 M HCi at 254 nm for 72 hr, (c) pure HgCI2, (4 MMC irradiated in 12 M HCI at 254 nm for 72 hr. All at 0.05 m M total Hg
present. However, many of these absorbers (e.g., organic compounds, NOS- ion) would have approximately the same
(1) F. Ley and R . Fisher, Z.Anorg. Chem., 82, 329 (1913). (2) C. R. Crymble, J. Cbem. Soc., 105, 658 (1914). (3) Ya-Chi Yuan, "Development of an Ultraviolet Spectrophotometric Method for Determining Atmospheric Sulfur Dioxide", M.A. Thesis, State University of New York at Binghamton, Binghamton, N.Y., 1972. (4) G. Schwarzenbach and M. Schellenberg, Helv. Chim. Acta, 48, 28 (1965). (5) R. 0.Arah, "Voltammetric and Other Properties of Methylmercury Solutions and Distribution of Methylmercury in Environmental Samples". Ph.D. Dissertation, State University of New York at Binghamton. Binghamton, N.Y., 1975. (6) G. Westoo, Acta Cbem. Scand., 20, 2131 (1966).
RECEIVED for review April 7, 1975. Accepted September 24, 1975. The authors gratefully acknowledge support from The Rockefeller Foundation and from the Research Foundation of State University of New York under Grant 407287A.
Analysis of Saturated Hydrocarbons by Field Ionization Mass Spectrometry Medislav Kuras Laboratory of Synthetic Fuels, institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia
Miroslav Ryska Institute of Macromolecular Chemistry, Czechoslovak Academy of Sciences, 162 06 Prague 6, Czechoslovakia
Ji;i Mostecky' Department of Petroleum Technology and Petrochemistry, institute of Chemical Technology, 166 28 Prague 6, Czechoslovakia
The dependence of sensitivity coefficients of Cs to C12 paraffins and Cs to CI4 cycloparaffins on molecular weight and structure, and the influence of these coefficients by possible adsorption on the emitter, have been Investigated. By adding aromatic hydrocarbons of differing types, It is possible to equalize the sensitivity coefficients so that their values within the various hydrocarbon groups lie in the experimental error range regardless of their molecular weight and structure.
Field ionization extends the scope of mass spectrometry for analyzing petroleum fractions. Field ionization gives rise virtually to molecular ions only, even if paraffins and cycloparaffins are used as starting compounds; for saturat196
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
ed hydrocarbon mixtures, it provides information similar to that obtained by low voltage methods for aromatics and olefins. Mead (i ) applied field ionization in the analysis of petroleum waxes. Assuming the sensitivity coefficients to be independent of the molecular weight of the C ~ O - C hydro~O carbons, he obtained good agreement with results provided by the cooperative matrix method. On the other hand, Hippe and Beckey ( 2 ) ascertained for low molecular weight hydrocarbons a considerable dependence of the sensitivity coefficients on molecular weight, structure, and mixture composition. This accounts for their proposal of the iterative procedure for analyzing gasolines; however, this procedure is too involved and time-consuming to make it suitable for serial analyses.
Table I. Mean Values of Sensitivity Coefficients (Ere]) and Their Standard Deviations (std dev) for Hydrocarbons C,-C,, in Different Groups Hydrocarbon group
n-Paraffins Isoparaffins Monocycloparaffins Dicycloparaffins condensed Tricycloparaffinsb
Erep 0.706 0.532 0.616 0.798 1.210
Ere1 S t d dev (EtB)Q Std dev (EtB)
0.960 0.704 0.768 0.756 0.731
source. A Wilkinson razor blade without any exterior coverage was used as the emitter. The blade was activated in acetone for 4 hr while the emitter potential was gradually raised to +8 kV and that of the counter electrode to -2.8 kV. The samples were introduced into the mass spectrometer via an all-glass inlet system, heated up to a temperature of 260 "C. The ion source temperature was 120 OC.
0.328 0.032 0.360 0.016 0.439 0.157 0.312 0.039 0.268 0.099
RESULTS AND DISCUSSION
It became apparent from t h e measured values E,,I(EtB) that the addition of ethylbenzene (or other suitable aromatic hydrocarbon) equalizes the sensitivity coefficients in a Ere1 n - d e c a n e and E r e l ( E t B ) n . d e c a n e = 1.000.b Values the individual hydrocarbon groups regardless of the molecf o r hydrocarbons C, o-Cl 4 . ular weight of the constituents present. Table I summarizes the mean values of sensitivity coefficients and standard de_ _ _ ~ _ _ _ _ _ _ _ _ ~ _ _ _ _ _ ~ viations of these values for various hydrocarbon groups. Table 11. Composition of Saturated Fraction 200-260 C T h e values measured for all hydrocarbons used were emfrom Romashkino Petroleum ployed for the calculation, except paraffins containing a Composition in weight per cent quaternary carbon. T h e Ereland E,,I(EtB) of the latter hydrocarbons are several times lower, but they are without Number o f MonocycloDicycloTricycloc atoms Paraffins paraffins paraffins paraffins any practical significance for petroleum fractions. T h e mean E,,l(EtB) values listed in Table I were applied for an0 2.6 4.5 0 11 alyzing petroleum fractions. 1.6 6.8 8.2 2.0 12 2.5 10.9 11.2 2.7 13 An example of t h e analysis of the petroleum fraction 2.9 8.8 7.8 2.9 14 200-260 "C via the suggested method is shown in Table 11. 2.8 5.2 5.1 2.0 15 T o verify this method, the overall results for various groups 1.7 2.4 2.0 1.0 16 were compared with those obtained via group analysis. For 0.7 0.8 0.6 0.3 17 fractions 140-200 OC, a very good agreement with the Total 12.2 37.5 39.4 10.9 ASTM method ( 4 ) was found; whereas, for fractions 200260 "C, the differences between the method described and that of Snyder e t al. ( 5 ) were somewhat greater, especially In our previous paper ( 3 ) , we referred t o the possibility in the case of a high content of condensed cycloparaffins of suppressing the considerable dependence of the sensitiv(Table 111). ity coefficients of saturated hydrocarbons on structure and T h e analytical method suggested here makes it possible molecular weight by adding an aromatic hydrocarbon. We to determine the molecular weight distribution for differfound with mixtures of n-paraffins c6-cll that the addie n t groups of saturated hydrocarbons, which is difficult if tion of 33% v/v of ethylbenzene equalizes the differences in electron impact methods are used. I t enables only one sent h e sensitivity coefficients for the emitter applied t o such a sitivity coefficient value t o be applied for all members of degree that they lie in the range f10% re1 at the utmost. different groups regardless of their molecular weight, supSince we found a similar effect of aromatic hydrocarbons presses adsorption of the saturated Clz and higher hydrowith other saturated c6-c14 hydrocarbons having different carbons ( 3 ) , and at the same time increases the sensitivity structures, we employed this effect for the analysis of the of measurement (the intensity of molecular ions is insaturated portion of petroleum fractions. creased several times). EXPERIMENTAL All hydrocarbon mixtures and petroleum fractions were measured under identical conditions and on the same emitOf the total number of 90 applied hydrocarbons C ~ - - C L of~varying structure occurring as a rule in petroleum fractions (51 parafter. T h e repeatability of our results lies in a range of f 1 0 % fins, 21 monocycloparaffins, 8 condensed bicycloparaffins, 10 trire1 a t the utmost, even with measurements performed on cycloparaffins), 25 mixtures were prepared by their various combithe same emitter more than 1year later. nations. The relative sensitivity coefficients E,,I were determined It is obvious that the sensitivity coefficients determined for the individual hydrocarbons via the method described before here need not be in complete quantitative agreement with (3); 33% v/v of ethylbenzene was added to the mixtures, and the the results obtained on other emitters. Also, the concentraE,,I(EtB) were determined in the presence of an aromatic hydrocarbon. The purity of the hydrocarbons used was higher than 95%; tion of a n aromatic hydrocarbon (33% v/v) for equalizing in the majority of cases, higher than 99%. The analyzed fractions the sensitivity coefficients need not be optimal for other were prepared from a kerosene fraction of the Romashkino petroemitters (6). In these cases, however, it will be sufficient t o leum by separating aromatics on silica gel, n-paraffins by urea, and find out the optimal concentration and thus also the sensiby subsequent separation via thermal diffusion. tivity coefficients with a suitable calibrating mixture. For The mass spectra of hydrocarbon mixtures were measured on an the emitter involved, it is unnecessary to carry out meaAEI MS 902 mass spectrometer equipped with a combined EI-FI ~~~~
~
~
Table 111. Comparison of Results of Analyses of Petroleum Fractions 140-200 with Adding of Ethylbenzene (FI) and E1 Matrix Methods
C and 200-260
O
C by Proposed Method
Concentration, Fraction 1 Hydrocarbon group
FI
ASTM
Fraction 2 FI
ASThl
Fraction 3 FI
Paraffins 52.1 55.5 81.9 83.3 44.0 Monocycloparaffins 41.5 40.6 18.1 16.3 39.2 Dicycloparaffins 6.4 3.9 0 0.4 14.9) Tricycloparaffins 0 0 0 0 1.9 a Results of FI method and Snyder's method are in wt %, ASTM method
Fraction 4
Fraction 5
Snyder
FI
Snyder
FI
Snyder
37.6 45.8
63.8 31.9 :.3}
59.3 34.0
12.2 37.5
6.7
39'4} 10.9
8.4 47.0 44.6
16.6
in vol %.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
197
surements of the sensitivity coefficients of a whole number of pure hydrocarbons of various groups. The suggested procedure for analyses of saturated petroleum fractions is far less time- and labor-consuming than the iterative procedure, particularly for larger series of analyses and in all those cases where an accuracy of &IO% re1 proves to be satisfactory. For a reliable determination of paraffins, it is desirable to know, a t least approximately, the normal to isoparaffins ratio, the mean sensitivity coefficients of which differ rather considerably (by 0.960 and 0.704, respectively). If this ratio is not known, it is better to employ the mean value of the sensitivity coefficients of both these groups. Preliminary results have shown that the above procedure may be advantageously applied to mixtures of saturated hydrocarbons up to C15-CI6, where the presence of aromat-
ics sufficiently suppresses the adsorption effects of higher molecular weight saturated hydrocarbons.
LITERATURE CITED (1) W. L. Mead, Anal. Chem., 40, 743 (1968). (2) K. G. Hippe and H. D.Beckey, ErdoelKohle, Erdgas, Petrochem., 24, 620 (1971). (3) M. Ryska, M. Kuras, and J. Mostecky. lnt. J. Mass Spectrom. /on Phys., 16, 257 (1975). (4) ASTM D 2789-71, American Society for Testina and Materials. Philadelphia, Pa., 1971. (5) L. R. Snyder, H. E. Howard, and W. C. Ferguson. Anal. Chem., 35, 1676 (19631. (6) W.L. ‘Mead, British Petroleum Research Center, Sunbury on Thames, England, 1974, private communication.
RECEIVEDfor review March 27, 1975. Accepted September 19, 1975.
Field Ionization-Field Desorption Source for Nonfragmenting Mass Spectrometry Michael Anbar” and Gilbert A. St. John Mass Spectrometry Research Center, Stanford Research Institute, Menlo Park, Calif. 94025
A field ionization and desorption source has been deveioped, comprising a rough metal surface at the end of a short, thin metal rod, and a heatable structure which allows its reproducible placement within 50 pm from a counter electrode. The field desorbing rod is extremely simple to prepare and a new one may be used for each sample. This source has been successfully operated at ambient temperature to 350 ‘C. A number of examples of field ionization and field desorption spectra of inorganic and organic compounds obtained by this source are presented. Different ionization processes which take place under field desorption conditions are discussed. Most important among these is the ionizatlon of materials dissolved in a polar organic poiymeric matrix.
Field ionization has drawn considerable interest in recent years because of its ability to produce primarily unfragmented mass spectra (1-3). The analytical applications of this technique have been described before (1-3). The gist of this technique is an efficient ionization source. A number of types of field ionizers have been developedthe Beckey carbonaceous dendrite tungsten wire source ( I ) , the nickel dendrite source ( 4 , 5 ) , the Robertson razor blade edge source ( 6 , 7 ) ,and the SRI porous multipoint source (2).The latter has a fairly high overall ionization efficiency because of the effective sample feed through the porous backing. A large fraction of the energetic ions produced are, however, lost because of their divergence, and only a small fraction can be effectively mass analyzed. Still, reasonable overall ionization X transmittance efficiencies are attained with the porous sources; ranging from 3 X 101j ions/mol in a 90’ sector magnet with a resolution of 500, to 1x ions/mol (at mass 126) using a Yg-inch quadrupole analyzer with the same resolution. These efficiencies are sufficient to allow a large variety of analytical applications (2, 8). 198
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
The usefulness of the nonfragmenting nature of field ionization can be extended to nonvolatile substrates if these are placed directly on the highly curved surfaces which are necessary for field ionization. This mode of ionization has been developed by Beckey et al. and named “field desorption” (9). By placing the material directly on the active surface, one also overcomes the shortcoming of low sample-feed efficiencies of nonporous field ionization sources, and overall ionization X transmittance efficiencies, allowing the detection of lo-” g of material, have been achieved for magnetic sector mass spectrometers with the carbonaceous dendrite sources (9). The SRI multipoint structures are much too expensive to be used as field desorption sources. Whereas they have been used for hundreds of hours in the field ionizing mode, their use is limited to very few analyses in the desorption mode. Also, the carbonaceous dendrite sources are difficult to produce, though their fabrication is not as difficult as that of the SRI multipoint structures. In any case, we looked for as simple a structure as possible which would facilitate field desorption, so that it can be discarded after each sample. This would alleviate cleanup and memory problems. We have found that rough metal surfaces formed by breakage of hard brittle metals, like tungsten or titanium, are effective field ionizers (or field desorbers). Also, iron surfaces may be roughened by acid etching to allow field ionization, We then modified the SRI field ionization source structure, substituting the field ionizing multipoint pedestal (Figure 1) by a rough-ended metal rod (Figure 2). This arrangement allows the rapid interchange of the two types of ionizers and necessitates minimal readjustments of the ion optics system. In this exploratory study, we have used a low resolution 45’ sector magnet which was manually scanned. Obviously, higher quality mass spectra might be obtained using a more advanced mass analyzer. We shall now describe in greater detail the new ionization source and illustrate its performance with a number of examples.
