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Anal. Chem. 1982, 54, 1881-1883
Californium-2512 Plasma Desorption Mass Spectrometry of Solid Coal Sir: The direct analysis of the chemical structure of coal is a subject of considerable interest today. Most techniques require that the coal be dissolved or otherwise liquefied before the analysis can be performed. Relating these analyses to the structure of the original coal is at best difficult because of the poorly understood changes which occur during the dissolution or liquefaction processes. We have recently applied the technique of 252Cfplasma desorption mass spectrometry (252Cf-PDMS)to the direct analysis of several samples of solid coal and to one which had the mineral matter removed by acid treatment. The 252CfPDMS is one of several imass spectrometric techniques developed which can be used to analyze compounds which cannot be readily volatilized thermally without decomposition. Examples of the utility of 252Cf-PDMShave been published (1-3) in which fragile biological compoundri were observed in the mass spectrum in their unfragmentecl condition. This process takes advantage of the very rapid energy transfer which occurs when the high-energy recoil fragments (lo8 eV) of 252Cfimpact a molecule. This energy transfer results in sputtering where momentum is transferred to a molecule and it is knocked out of its lattice in a time less than that required for decomposition. Decomposition products are always observed, but ions with a niolecular weight (mlz)equal to or within a few mass units of the original imolecule are also generally observed. In applying 25zCf-PDM5Zto solid coal samples, one is dealing with a very complex mixture of molecules with widely varying molecular weights. Thus, it is very difficult to unambiguously distinguish between a molecular ion and the fragmentation patterns that may occur. Further work will be necessary using well characterized coallike model compounds before the real potential of this technique for measuring molecular weights and studying the molecular structure of coal can be established. Other important applications of 252Cf-PDMSare envisioned, such as coal to cioal comparisons and determination of the effect of coal treatments on spectra. For example, we have compared the 262Cf-PDMSspectrum of Illinois No. 6 coal with that of an acid demineralized sample of the same coal (Table I). The resultant data were carefully analyzed to determine the extent of observable changes in the molecular structure of the organic components as a result of the acid treatment. We have also made coal to coal comparisons of spectra, but details are not provided in this paper. EXPERIMENTAL SECTION One coal sample was demineralized using HCl and HF by a method similar to the one reported by Radmacher and Mohrhauer (4, 5) and evaluated by others (6, 7) except a steam bath temperature (100 OC) was used instead of 55-60 "C as recommended. Earlier investigators reported significant loss of carbon from acid treatment a$ higher temperatures ( 4 9 ) but later showed that loss of carbon is less extensive at 60 O C . Even at 60 "C COzwas evolved but the COz was assumed to be a product of metal carbonate decomposition rather than decomposition of' organic material. Samples were prepared Eor 252Cf-PDMSanalysis by electrospraying coal, pulverized to less than lo+ m diameter and dispersed in methanol, onto a thin ahminized Mylar film. Samples were then placed in the flight tube of a time of flight mass spectrometer adjacent to a small 262Cfsource. The energetic fission recoil fragments from decay of the 252Cfstrike the film producing a cascade of energy through the film to the coal, resulting in the sputtering of neutral and i~onspecies from coal into the mass spectrometer accelerator. [ons are then accelerated and travel down the flight tube and are mass analyzed. A detailed description of the process and equipment has been reported earlier (1-3,10). 0003-2700/82/0354-1881$0 1.25/0
Table I. Ultimatea and Mineral Mattera Analysis of Monterey County (A) Illinois No. 6 Seam Coal and (B) Illinois No. 6 Seam Acid Demineralized Coal A
B
%MAF~
C H N S
'78.1 5.2 1.8
3.4
76.7 5.2 1.8 3.5
% of Sample (Moisture Included)
ash Si AI Fe F V Ti
5.4
1.6
2.1
0.15
0.63
0.19 0.45 0.46 209 ppm 209 ppm
0.80
49 PPm 240 ppm 240 ppm
a Analysis done by Schwarzkoph Microanalytical Laboratory, 56-19 37th Av., Woodside, N Y 11377. Moisture and ash free basis.
RESULTS AND DISCUSSION The positive ion spectra of Illinois No. 6 coal is shown in Figures 1A-3A, and that of the acid demineralized sample is shown in Figures 1B-3B. A number of peaks are very prominent in both spectra. Although some or even most of these peaks in the spectra undoubtedly result from fragmentation of the molecules present in coal, the results from previous studies on biological and polymeric systems lead us to believe that many of the mass spectral peaks are molecular ions. These ions result from direct sputtering of molecules from the coal surface and, thus, give some indication of the molecular structure in coal itself. However, it must be realized that the experimental procedure has included exposure of the coal samples to both air and methanol and, therefore, the molecular structure could have been affected. The interpretation of these spectra will be strengthened by future work using solid coallike model compounds with well-defined and characterized molecular structures. The 252Cf-PDMSwork on coal we have done so far has led to some very important conclusions concerning the organic components of coal. These conclusions are reached largely by noting the differences between mass spectra of the virgin coal sample and the demineralized sample without having a direct knowledge of whether peaks are molecular ions or fragments. One observation is a clear indication of organic structural changes in coal during the demineralization process as shown in Figures 1-3. For instance, the series of peaks with mlz from 507 to 860 shown in Figure 3A have completely disappeared in the acid demineralized coal shown in Figure 3B. The masses and regularity of these peaks are such that they almost certainly result from organic structure rather than mineral molecules or even clusters of mineral molecules. These peaks and their respective satellites, which are greater by one and two mass units, strongly suggest the expected occurrence of and 13C isotopes in compounds of mostly carbon. This series of peaks is particularly interesting because of the regular pattern with a mass difference of 44 mass units, and one is tempted to suggest that the difference is due to carboxyl groups. However, carboxyl groups are not generally considered to be a major component in coal and to have many such groups on a single molecule in coal would not seem possible. We, 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 l0W0
50W0 40W0
I165
0
9000 80OC
3W0
1wW
ZUW
Y
z
1wU 3W0
52030
7" 60
"
I
100
2W
150 mrz
Figure 1. Positive ion spectra of m / z (57-230) for Monterey County, Ill. No. 6 seam coal analyzed by 252Cf-PDMS: (A) as received; (B) acld demineralized.
I 470
,
'
1
,261
Figure 2. Posttlve ion spectra of m/z (225-510) for Monterey County, IIi. No. 6 seam coal analyzed by *'*Cf-PDMS: (A) as received; (B) acid demineralized.
500 250
* Y
3
8
1000
750 100
25:
550
6W
550
7W
750
8W
850
9W
T Z
Figure 3. Positive ion spectra of m l z (506-905) for Monterey County, Iii. No. 6 seam coal analyzed by 252Cf-PDMS: (A) as received; (B) acid demineralized.
therefore, suggest that this series is more likely due to a number of molecules with structures of the type R(-CH,CH(OH)-),R' R(-CH&H20-),R' 1 2 The peaks would then be a result of a series of compounds with various values of n. Although we do not fully understand the interaction of the molecules with the acid which acts to eliminate the peaks from the mass spectrum of the demineralized sample, we suggest that structures 1 or 2 are especially susceptible to degradation, condensation, or rearrangement in the acid environment. In any case, it is clear that these structures are altered in some major way during the demineralization process. I t may be
that the increase in the intensity of the m / z 470 and 81 peaks relative to a peak at m / z 523, which is thought to result from a laboratory contaminate, is related to the removal of the series from 551 to 860. However, this explanation is speculative at present. The increase in the intensity of the m / z 470 and 81 peaks when coal is acid treated may also be used to suggest molecular weights of 470 and 81 for R and R' in structures 1 and 2. Such molecular weights are compatible with several possible organic structures in coal (i.e., cyclohexenyl with m/z = ai). Significantly, the total molecular weights of this series of compounds, present in solid coal, is in the range from 450 to 900 mass units. Also, the observed spectrum is significantly void of any peaks above 900 mass units, although high molecular weights up to 12 000 have been observed with 252CfPDMS in biological and polymeric solids. Another series of peaks of considerable interest is that with an m / z from 228 to 268 (Figure 2A). These four peaks have a difference of 12-14 m / z and may be extended to mass 372 with a much smaller intensity. The mass difference in this series suggests a group of molecules differing by a single C, CH, or CH2 group and again would appear to clearly result from the organic structure of the coal. In the demineralized coal sample (Figure 2B), the peaks are still present but with significantly decreased amplitudes. Apparently the acid treatment has resulted in dehydrogenation, hydrogenation, or related processes yielding the much more complicated mass spectrum. In the lower mass range (Figure 1) peaks with m / z of 129, 147, and 165 are present in both samples but altered by the acid treatment. These masses also appear to be prevalent in many coals since they were also observed in other samples investigated in this study. Several peaks in the lower mass range were significantly larger in the demineralized coal than in the virgin sample, i.e., 65,73,81, 83,97, 107, 113, 139, 141. Again, this appears to be evidence of attack of the organic structure of the coal by the demineralization process.
CONCLUSIONS These results, though preliminary in nature, show the potential for a significant contribution by 252Cf-PDMSfor the analysis of coal structure and, perhaps more explicitly, for analyzing changes in coal structure after various treatments. With this technique, new information may be gained on molecular weights, functional group, cross-linkingof structures, and stability of structures in coal. In this brief study, we have quite clearly shown that the acid demineralization process produces substantial changes in the organic structure of the coal and should be used with caution in preparing samples for structural analysis studies. We have also shown molecular weights in the range from 450 to 900 mass units in Illinois No. 6 coal and suggested possible structural types to explain these molecular weights. We have found a significant lack of peaks to indicate masses above 900. Further work on coallike model compounds and on a variety of coal samples using 252Cf-PDMSshould be undertaken to further determine and evaluate the potential of this system. Furthermore, several other related mass spectral techniques have potential and should be considered. These include secondary ion mass spectroscopy (SIMS) (II), fast atom bombardment (FAE!),(12),laser induction ionization, (13)and others. The successful application of these and other novel techniques appears to us to hold the best hope of gaining a much needed understanding of the chemical structure of solid coal.
LITERATURE CITED (1) Macfarlane, R. D.; Torgerson, D. F. Int. J . Mass Spectrom. Ion Phys. 1978, 21, 81-92. (2) Macfarlane, R. D.; Torgerson, D. F. Science 1976, 791, 920-925.
Anal. Chern. 1982, 54, 1883-1885 (3) Macfarlane, R. D. NBS Spec. Pub/. 1979, SP 579, 673-677. (4) Radmacher, W.; Mohrhauer, P. Brennst.-Chem. 1955, 36, 236-239. (5) Radmacher, W.; Mohrhauer, P. Bremsf.-Chem. 1956, 3 7 , 26. (6) Bishop, M.; Ward, D. L. Fuel 1958, 3 7 , 191-200. (7) Tarpley, E. C.; Ode, W. ti. R e p . Invest.-US., Bur. Mines 1958, RI 5470. (8) Fleldner, A. C.; Selvig; VV. A.; Taylor, G. B. Tech. Progr. R e p . - U . S . , Bur. Mines 1919, TP 272. (9) Turner, H. G. Trans. A m . Inst. Min., Mefall. Pet. €ng. 1930, 88, 639. (10) McNeal, C. J.; Macfarlane; R. D.; Thurston, E. L. Anal. Chem. 1979, 57,2036-2039. (11) Tingey, G. L.; Lytle; J. MI.; Baer; D. R.; Thomas, M. T. PNL-3650; Pacific Northwest Laboratory, Richland, WA, 1900. (12) Taylor. L. C. E. Ind. Res./Dev. 1981, 23(9), ‘124-128. (13) Joy, W. K.; Ladner; W. FL; Pritchard, E. fuel 1970, 49, 26-38.
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R. D. Macfarlane is a professor In the Chemistry Department at Texas A&M University, College Station, TX.
J. M. Lytle* G. L. Tingey R. D. Macfarlane’ Battelle, Pacific Northwest Laboratory Richland Washington 99352 RECEIVED for review March 11, 1982. Accepted June 7, 1982. Research sponsored by the United States Department of Energy under Contract DC-AC06-76RLO-1830.
Capillary Column Supercritical Fluid Chromatography/Mass Spectrometry Sir: The analysis of complex mixtures is often limited by the molecular weight range and selectivity or present analytical techniques. Gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS) are ultimately limited by the volatility of the isample components to compounds having molecular weights less than approximately 300-400. High-performance liquid chromatography (IIPLC) can be used to separate higher molecular weight and nonvolatile compounds, but the chromatographic efficiency is much less than that of modern capillary column GC and detector sensitivity and selectivity are often inadequate. Modern HPLC using packed columns is rapidly approaching practical limits on the number of theoretical plates available; recent results suggest, however, that improvements in chromatographic efficiency may be obtained with microbore or capillary columns. The ideal detector for chromatography is the mass spectrometer due to its inherent sensitivity and selectivity. However, interfacing HPLC to mass spectrometry is a difficult task because of the fundamental incompatabilitiee in required liquid flow rates and solvent evaporation or removal (1-3). Thus, there is a need for an alternative approach t o HPLC for the analysis of complex mixtures of nonvolatile or thermally labile compounds which providies increased chromatographic efficiency as well as greater calmpatabilitywith mass spectrometry. Supercritical fluid chromatography (SFC) has potential advantages in many appliications relative to both GC and LC which have been demonstrated by a number of workers over the past 15 years (4-10). The direct interfacing of SFC with mass spectrometry may have significant advantages relative to LC/MS and has attracted some interest (11-14). Randall and Wahrhaftig have previously reported on the construction of a supercritical fluid (dense gas) chromatograph/mass spectrometer interface using conventional packed columns and supersonic rnolecular beam techniques (11-13). This approach, however, suffers from the lower chromatographic efficiency relative to that possible using capillary columns and the complexity of four stages of differential pumping required by large mobile phase flow rates (13). Gouw et al. have also presented results using direct introduction with electron impact ionization but this approach seems to be limited by compound volatility and poor sensitivity ( 1 4 ) . The use of capillary column SFC technology (15-17) can obviate difficulties associ,ated with previous SFC/MS interfaces and allows a simple interface readily adapted to existing GC/MS systems. The combination of SFC with mass spectrometry offers the followiiig potential advantages relative to 0003-2700/62/0354-1883$0 1.25/0
GC/MS or LC/MS methods: (a) High molecular weight, polymeric, heterofunctional, and thermally labile compounds can be separated as well as the more volatile species. (b) Capillary SFC columns can provide greatly enhanced chromatographic efficiency relative to HPLC due to solute diffusivities which are about lo2greater in the supercritical fluid than in the corresponding liquid phase and viscosities similar to the gas phase ( 4 , 5 , 9 ) . (c) Solvating power of the mobile phase can be readily controlled with pressure programming (5). Mixed mobile phases ( 4 ) , gradient, and temperature programming are also feasible. (d) SFC using capillary columns provides low mobile phase flow rates which, coupled with high mobile phase volatility, allows optimum interfacing of SFC and mass spectrometry. Capillary column SFC/MS instrumentation has been developed in our laboratory to investigate and apply SFC methods; preliminary results are presented in this report. EXPERIMENTAL SECTION The SFC/MS instrument incorporates a capillary column SFC, a direct fluid injection interface, and a tandem quadrupole mass spectrometer equipped with a dual electron impact-chemical ionization ion source. Figure 1 gives an overall schematic illustration of the instrumentation. The supercritical fluid chromatograph utilizes a Varian 8500 high-pressure syringe pump (8000 psi maximum pressure) and a constant-temperature oven and transfer line. Short lengths of
