Anal. Chem. 1983, 55, 527-532
system was used; Le., the current with .LE = 0 was obtained and subtracted from the measured difference current, and the corrected net pulse current was used as data (circles in Figure 8) for the simplex optimization. Eighteen points were used between -0.100 and -0.440 V for the curve fitting. The optimum values obtained were K' = 4.6 X 10 s-l M-I and El$ = -0.258 V with standard deviation in the residuals of 0.2 after 24 iterations and a CPU time of 41 s. Although there is some error near the apex of the peak, the fit as a whole is still good. It should be also noted that the curve is asymmetric for ",OH, whereas it is symmetric for a system with large k (NaClOJ. The values of the rate constant, k', found with the present fitting method were a little higher than those found by Koryta et al. (dc method) and :Smith (ac method) for both NaClO, and ",OH (Table I), and this may be partially due to the different composition of the background electrolyte. The present method has an advantage of determining k'and El$ simultaneously. The good agreement between the experimental DPP djfference current-potentiial curves and the theoretical D P P difference curren-potential curves for a first-order catalytic process, with calculated values of catalytic rate constants and reversible half-wave potentials that agree very well with literature values, indicates that the theoretical treatment we have used is quite valid. Furthermore, because the capacity current contribution to the total1 current is significantly less in D P P than in normal or dc polarography, the DPP method should
!527
give more accurate values of the catalytic rate constant than these other methods especially for small values of h. Registry No. NaClO,, 7775-09-9;",OH, 7803-49-8;TiCl,, 7550-45-0;(COOH),, 144-62-7;H2SO4, 7664-93-9.
LITERATURE CITED (1) Birke, R. L.; Kim, M.-H.; Strassfeld, M. Anal. Chem. 1981, 5 3 , 852. (2) Ferrier, D. R.;Schroeder, R. R. J . flectroanal. Chem. 1973, 45, 343. (3) Birke, R. L. Anal. Chem. 1978, 40, 1489. (4) Koryta, J.; Tenygi, J. Collect. Czech. Chem. Commun. 1954, 19, 839. (5) Smith, D. E. Anal. Chem. 1963, 35, 610. ( 6 ) Blazek, A.; Koryta, J. Collect. Czech. Chem. Commun. 1953, 18, 326. (7) Koryta, J. Chem. .Zvesti 1954, 8 , 723. (8) Heyrovsky, J.; Kuta, J. "Principle of Polarography": Academic Press: New York, 1966;pp 386-388. (9) Deiahay, P.; Stelhl, G. L. J . Am. Chem. SOC. 1952, 74, 3500. (IO) Miller, S.L. J . Am. Chem. Soc. 1952, 74, 4130. (11) Posplsil, Z.Collect. Czech. Chem. Commun. 1947, 12, 39. (12) McIntyre, J. C. E. J . Phys. Chem. 1967, 71, 1196. (13) Booman, G.L.; Peince, D. T. Anal. Chem. 1965, 37, 1366. (14) MacDonaid, D. D. "Transient Techniques in Electrochemistry"; Pienurn Press: New York, 1977;p 111. (15) Christie, J. H.; Osteryoung, R. A. J . Elecfroanal. Chem. 1974, 49, 301. (16) Youssefi, M.; Birke, R. L. Anal. Chem. 1977, 49, 1380. (17) Nelder, J. A.; Mead, R. Compuf, J . 1965, 7 , 308.
RECEIVED for review September 20,1982. Accepted Novemb'er 22,1982. This research was supported in part by the National Institutes of Health under HEW-PHS Grant 5 ROl-AM184,40 and by City University of New York under Faculty Researfch Award Grant 13699.
Laser Microprobe Mass Analysis Studies on Coal and Shale Samples Nicholas
E. Vanderborgh" and C. E.
Roland Jones
Earth and Space Sciences ,Division, Los Alamos National Laboratory, Los Alamos, New Mexico (97545
Laser-induced desorptlori results are described for coal and shale samples. These solid carbon mlnerals were analyzed with a low-angle-of-incidence photon beam. Results emphasize metallic catlons arid cation adducts In the positive spectra and chlorine adducts In the negatlve. These are contrasted with data obtained from technical polymers and suggest that expected aromatic coal features are masked by desorption of volatile compounds from both shale and coal.
Pulsed, laser-induced idesorption (LID) coupled to timeof-flight mass spectrometry represents an innovative approach to materials characterization (1,2). This analysis technique describes surface layers rather than volumes ( 3 )and has application in the characterization of aerosols ( 4 ) ,the study of biological samples (51, anid, among others, in technical polymers (6). Initial results using the laser microprobe mass analysis (LAMMA) technique were unexpected (7). Pulsed desorption produced large molecules typified by quasi-molecular ions (3). For instance, macromolecular systems show major peaks at (B + H)+ and (B + H)-, where B represents the molecular ion. Other molecular adducts apparently form with alkali metals (7). These "cationization" peaks such as (B Na)+ are often the most intense features in the LAMMA spectrum. Exknsive
+
fragmentation, whichi might be expected to result from such rapid heating, is not apparent. Processes leading to product formation during the LAMU4 experiment are still not well-defined (3, 7). Two product generation mechanisms are described: gas-phase neutral-ion reactions and surface sitripping. Each mechanism is prompted by energy argument9 separated by a somewhat arbitrary boundary between low- and high-energy deposition rates set at lo9W cm-, (3). Conditions during the LAMMA experiment are easily varied to traverse this boundary. The low-energy process assumes that neutrals are theimalized from the surface, and then these fragments become charged through proton transfer or cation addition in the gas phase. There is evidence for the validity of this model, at least under low-energy conditions (7). Surface stripping, on the other hand, assumes that surface potentials result through the rapid evolution of material, leaving fragments in a charged state. Arguments for this mechanism are also credible (3,8). Of course, both mechanisms can operate concurrently. Coals and shales offer unusual compounds for LAMMA investigation. Unlike technical polymers, the nature of bonding in these materials is still under investigation (9-12). Moreover, it is now becoming understood that these natural products are mixtures of stable organic compounds, such ab substituted aromatic hydrocarbons, occluded within a poly meric matrix. Pyrolysis is normally necessary to generate
0003-2700/83/0355-0527$01.50/0@ 1983 American Chemical Society
528
ANALYTICAL CHEMISTRY, VOL. 55, NO. 3, MARCH 1983 He-Ne U S E R
+MMA
RESULTS FOR SUBBITUMINOUS COAL IFRUITMNDI
FREQUENCY MULTIPLIER POSITIVE SPECTRUM
Nd YAG USER
ABSORBER
R
CRO
ION REFLEl
TO VACUUM PUMPS
ELECTRON
IV
PENRECOROER
ULTIPLIER
Figure 1. Schematic of the LAMMA instrument. Coal and shale samples (as small particles) were suspended onto the sample carrier (specimen), and then L I D was accomplished wlth the pulsed Nd-YAG laser. Ion fragments are focused and analyzed with the time-of-flight mass spectrometer.
sufficient volatile material for characterizing the nature of these occluded compounds. Heating rates influence the ratio of devolatilization rates to char-forming rates and thus influence the types and amounts of volatilized products. The LAMMA technique results in intense thermal input directed into surface molecules (3). These natural samples incorporate mineral phases in higher concentration than exists in technical polymers (the residual sensitizers) (13). Considerable work has been reported by using mass spectrometry with coal samples. This has been recently reviewed (14). Coals and shales lack sufficient volatility for normal mass spectral investigation; consequently these earlier studies concentrated on coal volatiles, and coal extracts, and on coal-derived products, especially following liquefaction processing. More recently, mass spectrometry on coals has been coupled to modern analytical pyrolysis methodologies, such as the Curie point technique (15). Like these newer pyrolysis techniques, the LAMMA method is appropriate for whole samples. Such samples also afford another opportunity to explore the LAMMA technique. Initial LAMMA results on two low rank coal samples and a sample of Eocene oil shale are reported.
EXPERIMENTAL SECTION The LAMMA instrument is shown schematically in Figure 1. A Q-switched, frequency quadrupled, Nd-YAG pulsed laser deposits precise energy increments onto selected sections of samples. Coincident with that optical path is a He-Ne, low-power alignment laser. This alignment laser permits selection of a particular sampling site before the Nd-YAG high-power system is fired. The s duration at 265 nm conlaser-desorption pulse of 15 X centrates IO7 to 10" W/cm2 onto a spot as small as 0.5 pm diameter. This is the laser energy that enters the system, not necessarily that which is absorbed by the target volume (16). Product fragments drift into the entrance optics of a time-of-flight mass spectrometer. Desorbed, thermalized ionic species are then selectively accelerated through the spectrometer. Switching polarity on the acceleration optics permits observation of either negative or positive ions. This instrumental arrangement affords no possibility for electron bombardment, and consequently, only charged fragments are observed, The ion beam is reflected through an angle of approximately 150° and then impinges on the electron multiplier. This ion reflector focuses ions of the same mass but slightly divergent energy to the same point and adjusts for the rather large spread within the thermal energies of ions generated during the laser pulse. The geometry is chosen to permit
MASS SCALE lamu'
Figure 2. LAMMA spectra for subbituminous coal. Data were taken on the edge of a coal partlcie. This spectrum was chosen to be representative of coal results: upper trace shows positive ion results: lower trace shows negative ion results.
prompt removal of the large majority of neutral fragments and particulate matter that results from the laser events and still maintain a suitable sensitivity for charged fragment detection. The technique removes a microsample from a larger segment. It is thus ideally suited for sampling heterogeneous materials such as coals and shales. However, geologic specimens are not sufficiently transparent to use in the normal transmission mode, nor is it readily possible to prepare sections of the required thinness to ensure that the laser beam will penetrate. Although it may be possible to cut a thin enough section, existing techniques require the addition of other chemical components that must confuse the mass spectral analyses. Alternatively, the sample used for LAMMA investigation can be reduced to a fine particulate suspended on a metallic grid, which is then introduced into the instrument. Although this method is suited for particulate analysis, it is less useful on coals and shales. Grinding samples sufficiently results in segregation, in part, from differences in hardness between coal components. Then, too, severe grinding requires intense energy input and localized heating, which may well alter the nature of the organic constituents. Alternatively, one can insert small chips into the instrument and focus on the edge of one chip. This technique was used. Samples were ground to pass -1/8, in. mesh, and several pieces were positioned on the sample carrier with metallic adhesive tape so that an individual sample chip projected into the optical field. This particular chip was moved into the beam path so that one edge made a small angle of incidence with the laser beam. On energizing, the laser beam grazed the sample surface, and the escape of desorbed fragments into the entrance optics was not impeded by the bulk of the sample. This technique proved successful for geologic samples. For the investigation reported here, two different geological types were selected. A subbituminous coal sample from the Fruitland Seam at the Sage Pit of Western Coal Company's operation in Farmington, NM, and a similar coal from the Hanna formation (Wyoming)were analyzed. Proximate analyses of these materials have been reported (I1,IZ).Second, a Western oil shale, which originated in the Green River formation at the mine associated with the Colony operations near Rifle, CO, was selected (17).
RESULTS AND DISCUSSION Figures 2 and 3 show positive ion results chosen to represent typical coal spectra. These samples are highly heterogeneous, on the level of sampling set by the LAMMA technique. Consideration of an average spectrum removes many unique features found in one result. Sample heterogeneity is well illustrated with positive spectra by tracking along the surface of a particular coal particle. Resulting data are a series of spectra, generally similar, that exhibit substantial differences
ANALYTICAL CHEMISTRY, VOL. 55, 7 L A M M A RliSULTS FOR WYOMING COAL IHANNAI I
'
POSITIVE SPECTRUM
. 83
711
.. 5c
,00
11c
120
'30
I,
NEGATIVE SPECTkUM
y'
Q-L'~--A~
30
43
'-'_
I
-
JLdJJI~-~cn~LL --------i-i 50 60
60
50
I I
100
110
120 13C 14C 150
-,
I
'60
70 18C
50
Figure 3. LAMMA spectra for bituminous coal: upper trace shows positive ion results; lower trace shows negative ion results.
Table I. Assignment for Suspected Positive Ion Fragments from Coals and Shale
m/z assignment m/z assignment m/z 23
Na+
451
%OH+
65
24 25
Mg+
46
Mn'
48
66 67
26
Mg'
51
27 28
Al+ Si+
54 55
Ti+ Ti+ C, HNa C,H,Na+ CONa+ Fe+ Mn+
CO' CH,O"
56
assignment C,H,+ CU'
72
Zn' C0,Na' COK' FeO'
73 83
FeOH+ C0,K'
88
Sr
+
30
P' CH,OH' CH,Na+
57
41
K' Cat K'
44
SiO'
59 60 63 64
31
37 39 40
&OH+
58
Fe' CaO+ CaOH' MgO,' Ni'
co+ Nit
Cu' TiO' Zn
89
+
FeS+ Y'
98 120 138 144
Fed+ C,H,OH' Mo FeS,+ Ba FeS,'
176
Fe,S,'
94
+
+
+
CO,'
in major peaks. Relative intensities of atomic fragments, especially cations, vary over a wide range. Although callibration will be tedious, it may be possible to use LAMMA for quantitative assessments, rather like microprobe studies of geological samples (13). Identities of suggested atomic and molecular species resulting from these investigations are shown in Table I. Positive spectra from coals contain intense peaks from cations. Because these coals contain about 70% carbon, thought to be largely in aromatic compounds, the low level of carbon-hydrogen fragments was a t first striking. Aromatic fragments are apparent as C5H5+(mlz 65), which perhaps originate from degradation of conjugated aromatic systems. Pendant aromatic groups, visualized as C6H5+( m / z 77) or the tropyllium ion (m/z 91) are not evident. Phenolic moieties should bo appairent by a mass fragment a t m/z 93. That aromatic fragment is not apparent either. These positive coal spectra depict specific elements. For example, the quintet at na/z 46,47, 48,49, and 50 represents the normal isotopic abundances of titanium. Although this interrogation must be on a titanium-rich region, perhaps B rutile mineral, other metallic ions are readily visualized as well. Even a t the relatively lour mass resolution of the instrument
NO.3, MARCH 1983 529 08
(about 300), sufficient sensitivity is evident to discern alterations in normal isotopic distributions. These data also show noninteger peaks so that molecules are apparent, but higher resolution is required before quantification will be possible. Other positive patterns permit definition of the m / z stries of 56, 88, 120, 144, and 176, which corresponds to the pyrite mineral inclusion (Fe+, FeS+, FeSzf, FezS+, and Fe2S2+). Additional positive fragments such as A10+, SiO+,CaO+, ?'io+ (mlz 43, 44, 45, and 64) are apparent. It is obvious that LAMMA complements existing scanning electron microscope/X-ray diffraction (SEM/XDF) techniques for micromineralogic determinations (13),and these data could yield specific information of migration within mineral matrices. Previous LAMMA results show that inorganic salts generate spectra best explained as ionic clusters (8). For instance, molecules of general formula M2X+are the rule for univalent ionic compounds. Generally these fragments suggest multiples found in predicted unit cells with an added cation or anion. Other spectral features are interpreted as the result of cationization steps, i.e., compound formation between ions, especially between E,odium and neutral species. These charged compounds are then visualized with mass spectral techniques. Cationization is especially effective with polysaccharides and less so for polynuclear aromatics (7). Obviously, the results from ionic salts may be thought of as cationization reactions, addition of a cation to a neutral molecule, for example. The negative coal spectra show one striking featurepatterns of peaks that concentrate around multiples of nalz of 12. At first these data were interpreted as carbon oligomers, beginning with the carbon dimer and extending to a valuc?of 16 carbon atoms (6). Such molecular fragments are hydrogen deficient (compounds such as C5H2+ are required to explain the peak at m / z 62) and have no known antecedents either in pyrolysis of these coals (18) or in technical polymers ( 6 ) . The LAMMA instrument has sufficient range to display larger masses than the CI6fragment; no such higher mass fragmeints are detectable under these conditions. There are few fragments in these negative spectra that suggest aromatic character. The carbonate and bicarbonate ions at m / z 60 and 61, if present, overlap the dominant multiples-of-twelve pattern. The small peak at m / z 59 could be ascribed to the acetrite ion. Data suggest elements other than carbon and carbon coimpounds in the negative records. For instance, indications of S-,C1-, PO-, and POz- (mlz of 32, 35-37, 47, and 63, respectively) appear, and SOL or S2-(641, PO, (79), SO,- (80), HSOB- (81), SO, (916),etc., could be indicated by discrete peaks or insinuated by shoulders on peaks of the appropriate unit mass record. A representative positive spectrum of Green River oil shale is shown in Figure 4. Some detail in this record is similar to traces found with coals. One significant difference is the indication of mineral carbon together with oxygenated fragments at m / z 30, 31, and 44. These represent fragments CH20+, CH20H+,and COz+,sensible products considering the nature of this marlstone sample (17). The negative data show the now familiar pattern around multiples of 12, apparently truncated near m / z of 120. Other species that suggest ox,ygenated fragments are apparent. For instance, the series C2HO-, C2Hz0-, CzH30-, and CH3C02- is suggested. The pronounced shoulder at m / z 60 and doubling at mlz 61 give additional indication of COB-and HC03-. To better understand these experimental observations, it is pertinent to review earlier results obtained for laser microprobe mass analyses of technical homopolymers (6). Previous studies investigated thin films of polyethylene, poly(tetrafluoroethylene), poly(viny1 chloride), poly(pheny1 methacrylate), and poly(benzy1 methacrylate).
530
ANALYTICAL CHEMISTRY, VOL.
55, NO. 3,
MARCH 1983
LAVMA RESULTS FOR GREEN RIVER OIL SHALE
30
CC
52
60
70
Bo
30
1M
1’0
120
30
MASS SCALE lamb1
Figure 4. LAMMA spectra for Eocene oil shale. Data were taken from the edge of a shale particle: upper trace shows positive ion results;
lower trace shows negative ion results. Results for polyethylene show positive spectra dominated by Na+ and Kf and compounds that center around multiples of 12 that have been tentatively identified as carbon oligomers (6). Data show regular patterns decreasing in intensity up to an apparent C12 molecule. Positive spectra are of the general formula [n x 121, [(nx 12) + 11, and [ ( n x 12) + 21, suggesting compounds such as Cn+,C,H+, and C,H2+. Previous studies of polyethylene pyrolysis show that the monomer ethylene is one expected minor product (18). LAMMA degradation should also produce that compound and others known to result from thermal degradation of that system. Another explanation for these results is that the sodium ion ( m / z 23) joins with normal neutral fragments to produce cationization products. Thus the peak at mlz 37 could also represent the compound CH2Na+. It is more satisfactory to accept this explanation perhaps than to propose new mechanistic steps for production of these hydrogen-deficient compounds, for instance, C6H+. Cation addition to the monomer ethylene produces C2H4Na+,a proposed assignment for the peak found a t m / z 51. Alternatively, this may represent C4H3+. The negative spectrum for polyethylene is simpler than the positive and less intense. In this case, the only major peaks are those that appear around multiples of 12, decreasing as the mass increases. Peaks a t m / z 35 and 37 may be assigned to chlorine, alternatively they represent C1- and C3H-. Details of the data for multiples-of-12 differ significantly between the positive and negative spectra of polyethylene. (We know that the laser event and resulting product formation is repeated during each pulse; the only difference between the two spectra is the polarity of the accelerating optics.) The negative spectra show a decreased “C,H-” fragment compared to the “C,-“ and “C,H2-” compounds. These data also would result from a compound containing an atom with two prevalent isotopes separated by 2 m / z units, such as chlorine. These chlorine compounds are formed through an analogous “anionization” process. Known fragments such as C2Hzthen appear as m / z 61 and 63, tagged as CI-C2H2-. Such anionization adducts then repeat the data suggested by the positive spectrum and are predicted by previous thermal degradative results. These earlier studies (6) explored another set of vinyl polymers, substituted methacrylates. Conventional pyrolysis of these compounds results in only a limited amount of monomer. There is some evidence for monomer in these spectra-the small peak a t m / z 161 suggests a phenyl methacrylate ion, C6H6--o-COC3H4+.Moreover, the positive spectra of both methacrylates show aromatic species (especially CsHsf
and C7Hs+)and oxygenated aromatic compounds. These apparent fragments result from pendant groups not from degradation of the ethylenic polymer backbone. These two methacrylates show striking differences when the two negative spectra are compared. The benzyl methacrylate negative spectrum is similar to that found for polyethylene. The phenyl methacrylate spectrum is sharply different, with the record dominated by oxygenated fragments including CH3CH2CC02+and CH,CHZCO2CH2”in the positive trace and CfiH60-in the negative. These intense spectral features mask usual spectral features, for instance, the ( n X 12) pattern. Results for the homopolymer of vinyl chloride show chlorine-containing fragments. Halogenated fragments are readily discernible as CC1- (m/z 47 and 49) and perhaps C3HzCl-(m/z 73 and 75). Of course chlorine ( m / z 35 and 37) is the dominant feature in the negative spectrum. And again, the negative spectrum repeats the familiar pattern. These earlier results need to be considered prior to describing the response to coal and shale to LAMMA interrogation. Two somewhat competitive factors occur during the LAMMA technique: volatile production (which generates mainly neutral fragments) and charged fragment production. The LAMMA technique discriminates in that only charged products are observed even though they may be in minor yield. Thus although polymer degradation yields predominantly neutral fragments, only charged fragments are recorded (7). We suspect generally similar product formation mechanism with coals and shales (18). Fragments that exhibit charge stabilization are readily discernible (metal cations). Predominant neutrals add to ionic species to generate the observed spectra (3). These coal shale results are explained in the main as the result of cation and anion additions to usual, high concentration, thermal degradation fragments. Of course direct production of charged species of ions occurs, e.g., Na+, to yield intense spectral features. These previously published spectra taken with homopolymers generally show truncation of the LAMMA record near a mlz of 150, apparently showing no compounds larger than CI1 (6). This result parallels earlier laser pyrolysis studies (18)that suggest that higher molecular weight fragments are not produced with intense, short-lived photon pulses. Ionization probabilities as well as cationization “cross sections” remain essentially constant for hydrocarbons heavier than C7. The duration of the thermal event is set by the laser while the total energy deposition depends upon the absorptivity of the sample and the sample geometry (14). We suspect that a significant fraction of the total energy adsorbed is transferred into vibronic and translational energy of the fragments. Samples are not subjected to extended heat soaking. Thus, volatilization is limited to those surface features ejected rapidly during the heating pulse: LAMMA is a surface degradation process (3,16). Clearly, polyethylene contains larger molecular units than Clz;these remain behind. Visualization of neutral fragments depends upon successful compound formation with a charged fragment, with highest probabilities being the relatively high concentration sodium or chlorine ions. In the charge production step, the bimolecular reaction requires that both components are present at high concentration in a region directly above the sample (7).Movement of fragments away from the sample is a dilution process; thus, homogeneous bimolecular processes are probable close to the sample surface, where concentrations are highest. The interpretation of LAMMA results is complicated by the relatively low dynamic range of the instrument. (This limitation is shared by other mass spectral techniques.) For instance, cation peaks like those found in the positive coal and shale results mask much of the rest of the record. Low in-
ANALYTICAL CHEIMISTRY, VOL.
tensity peaks are not discernible when high concentrations of readily ionizable atomic or molecular species are present. With these observations, we consider data obtained with coal q d shale samples. Laser-induced pyrolysis of coals shows a series of low molecular weight compounds dominated by CO, COz, and CzHz(18). Another important product, elemental carbon, is predicted blut not observed during earlier experiments with gas chromatographic separation. The positive LAMMA spectra are dominated by cations, especially Na+, Kr, and perhaps Fe+. Other peaks are best explained as cation adducts. Oxygenated fragments appear as COzNa+ (mlz 67) and C02Kt ( m / z 83). Lesser quantities of neutral CO are observed as small peaks at m / z 51 and m / z 67. Carbon fragments, especially the predominant CzH, appear a t m / z 48 (C2HNa+),and the corresponding potassium compound appears a t mlz 64. The negative coal spectra are explained in part as the result of anion compound formation, mainly with chlorine, readily discernible at m / z 35 and 37. Adducts between elemental carbon are expected and found at m / z 47,48,49, and 50 as compounds C - U and CH-Cl-. The major Cz fragment is seen as CzCI- and C211C1- in the group of peaks around m / z 59-62. Evidence of COzC1- and COC1- is suggested at m/z 79 and 63, respectively. Alternately, one could assume that compounds such as C,H- (rrzlz 49) result in these data; however, intense peaks such as m / z 47 are not readily explained with a carbon polymer model. Results with oil shale show similar positively charged compounds. The small fragment a t m / z 65 suggests aromatic structures. Again, CzNa+and CO2Na+are apparent at m / z 47 and 67, respectively. Cations (certainly Na+, Kt, Fe’, and perhaps Al+) are major spectral features. Peaks such as m / z 94 might be ascribed l,o either the phenolate ion or, more likely, FeK+. The higher mineral content of shale compared to coal tends to obscure all but the larger carbon peaks. This carbonate-rich compound generates peaks at m / z 47,48, and 49 in the negative spectrum. Certainly the m / z peak can be assigned to C,H, but we again assume that CC1- best explains the m / z 47 trace. Fragments that appear to result from aliphatic compounds are also evident. For instance, the cluster of peaks m / z 84 through 92 are tentatively described as C4H1&-, sensible volatile fragments from this sample, Data for shale truncate the repeating multiples-of-twelve pattern at lower imass values than observed for coal. Solvent extraction of both samlple types yields quantities of hydrocarbons of higher molecular weight than C16 (11, 12). Thus both coals and shales are known to contain sorbed neutral hydrocarbons of intermediate molecular weight. This apparent difference in upper molecular weight ranges (between these coals and shale) must reflect differences in the energy required to exhaust hydrocarbons from these two matrices under these conditions. We suspect that only a fraction of the available thermal energy is used for volatile hydrocarbon production. The high mineral carbon content of the marlstone (oil shale) absorbs an appreciable fraction of the total pulse for mineral degradation processes. Thus more energy is required to generate these Clo through C16 compounds from shale than from coal surfaces. (Other arguments to explain this difference revolve around ionization efficiencies or cross sections for the charge adduct processes. These remain difficult to quantify until rates of molecular production from surfaces are better known.) Both the coal and shale results are explicable assuming a structural model that holds potentially volatile hydrocarbon molecules sorbed within a polymeric matrix (11,12,19). Rapid surface heating either during these LAMMA experiments or more generally with laser pyrolysis (7,18) selectively separates surface species, such volatiles degrade mainly to neutral
55, NO. 3,
MARCH 1983
. I
531
species. These coal and shale data, as well as earlier LAMMA results on technicdl homopolymers, show that a major product mechanism involves neutral reaction with ionic species to yield the requisite charged adducts for mass spectral detect ion. Except for a few peaks, for instance the C6H6+fragment, carbon compounds result from such cationization steps. These data are consistent with a model that views coal as a chemical mixture of “guest” hydrocarbons, especially substituted aromatics, firmly sorbed into a polymeric matrix (11, 12). Rapid heatin,g directs energy selectively into the guest fraction because possibilities for fast thermal transfer are decreased between guest molecules compared to the possibility for intramolecular transfer within the macromolecular “host” structure. Although these LAMMA results parallel other high-temperature degradation processes (18),data suggest that additional primary fragments are discernible with LAMMA compared to techniques with slower detection response. For example, thermodynamic analyses (18)predict that elemental carbon is a major product from laser degradation of coals. These LAMMA data are explained assuming carbon is one product. Althougki calculations are still incomplete, the reaction residence time during which fragments are at sufficllent pressure for effective collisions (between ions and neutrals) must be short. This LAMMA method represents a significant tool for trapping primary thermal degradation products after short, high-temperature excursions. The mechanism for LAMMA spectrum production apptms to result from two processes. Some products are exhausted directly as charged species, but the majority of the molecular fragments are neutral compounds. These neutrals combine with available ions t o form detectable charged molecules. Both cation and anion adducts are involved (7). Clearly, the inorganic components of these coals and shale both interfere with the organic results and define those results. Charged products now result from the addition of available ions generated from random mineral constituents. It should be possible to add specific ions to alter these statistics; for instance, samples might be surface coated with controlled quantities of NaF. Cationization products, sodium compounds in the main, will be as discernible as those presented h u e ; however, the probability for compound formation with other ions, such as naturally occurring potassium, will be decreased. And of course, monoisotopic fluorine (mlz 19) will result in a anion spectrum siimpler than that for chlorine. LAMMA experiments do give a visualization of the primary fragmeints that result from intense, short pyrolysis of coals and shales and show that the primary degradation process gives quite similar results to those found with earlier laser pyrolysis techniques (18).
ACKNOWLEDGMENT The authors thank Clarence J. Karr, Jr. (U.S. Department of Energy, Morgantown Energy Technology Center), and G. R. B. Elliott (Los Altunos National Laboratory) for thoughthl discussions of these data. Likewise, the authors acknowledge H. J. Heinen (Leyhold-Heraeus gmbh) for assistance with the practical parts of this investigation.
LITERATURE CITED Hlllenkamp. F.; Unsold. F.; Kaufmann, R.; Nltsche, R. Appl. Phys. 1979,8,341. Wechsung, R.; Hilletnkamp, F . ; Kaufmann, R.; Nltsche, R.; Unsold, E.; Vogt, H. Mlcrosc. .4cta, Suppl. 1978,No. 2 , 281. Schueler, B.; Krueger, R.,,R. Org. Mass Spectrom. 1980, 15, 295. Kaufmann, R.; Wleser, P. Particle Analysis”; Helnrid, E., Ed.; Natioiial Bureau of Standards: Washington, DC, 1979; NBS Spec. Publ. (U.S.), No. 533. Kaufmann, R.; Hlllnnkamp, R.; Wechsung, R. Med. Prog. Techno/. 1979, 6 , 109. Gardella, J. A.; Hercules, D. M.; Helnen, H. J. Spectrosc. Lett. 19tI0, 13, 347. van der Peyl, G. J. (2.: Isa, K.; Haverkamp, J.; Klstemaker, P. G. Org. Mass Spectrom. 1981, 76,416.
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Anal. Chem. 1983, 55,532-535
(8) Schueler, B.; Krueger, F. R. Org. Mass Spectrom. 1979, 14, 439. (9) Vanderborgh, N. E.; Fletcher, M. A,; Jones, C. E. R. J . Appl. Anal. fyrol. 1979, 1 . 177. (IO) Jones, C. E. R.; Vanderborgh, N. E. J . Chromafogr. 1979, 186, 831. ( 1 1 ) Vanderborgh, N. E.; Jones, C. E. R.; Verzino, W. J.; Haverkamp, J. J . Appl. Anal. fyrol., in press. (12) Verzino, W. J.; Roofer-DePoorter, C. K.; Hermes, R. E, Catalytlc Coal Conversion Support, LA-9269-PR, Los Alamos National Laboratory: Los Alamos, NM, March 1982. (13) Raymond, R., Jr.; Gooley, R. "Analytical Methods for Coal and Coal Products"; Karr, C. J., Jr., Ed.; Academic Press: New York, 1979; Vol. 111, Chapter 48, p 337. (14) Sharkey, A. G.; McCartney, J. T. "Chemistry of Coal Utilization"; Elliott, M. A., Ed.; Wiley-lnterscience: New York, 1981; Chapter 4, especlally pp 204-214. (15) Meuzelaar, H. L. C.; McClennen, W. H.; Metcalf, G. S.; Hill, G. H. 29th Annual Conference on Mass Spectroscopy and Allled Topics, Minneapolis, MN, May 1981; 673.
(16) Helnen, H. J.; Meier, S.; Vogt, H.; Wechsung, R. "Advances in Mass Spectrometry"; Quayle, A., Ed.; Heyden: London, 1980; Vol. 8A, p 942. (17) Vandegrift, G. F.; Winans, R. E.; Scott, R. G.; Horwitz, E. P. Fuel 1980, 59, 627. (18) Hanson, R. L.; Vanderborgh, N. E. "Analytical Methods for Coal and Coal Products"; Karr. C. J., Jr., Ed.; Academic Press: New York, 1979; Vol. 111, Chapters 40 and 73. (19) Silbernagel, B. G.; Ebert, L. B.; Schlosberg, R . H.;R. B. "Long Coal Structure"; Gorbaty, M. L., Ouchi, K., Eds.: American Chemical Society: Washington, DC, 1981, Adv Chem. Ser. No. 192, p 23.
RECEIVED for review January 19, 1982. Resubmitted September 27,1982. Accepted November 18, 1982. This work was supported by the U S . Department of Energy under Contract No. DE-AC21-79MC11530.
Automated Multicomponent Analysis with Corrections for Interferences and Matrix Effects J. H. Kalivas and B. R. Kowalskl" Laboratory for Chemometrics, Department of Chemistry B E IO, University of Washington, Seattle, Washington 98 195
The generalized standard additlon method (GSAM), a multianalyte generalization of the method of standard addltlons, can simultaneously correct for matrix effects and spectral interferences in a multicomponent analysis. Slnce the GSAM requires the standard addltions of all analytes be made, a completely automated instrument was deslgned to Implement the GSAM under computer control. Additionally, the GSAM has been adapted to a new experimental deslgn of maklng standard additlons by weight rather than the usual procedure of additions by volume. This new design is combined with the usual advantages of automation to yield a step toward Intelligent analytical instrumentatlon.
Many recently designed analytical instruments include microprocessor computers. These microprocessors have primarily been used for data acquisition, transformation, storage, and retrieval. Lately, new ways in which microprocessors can be used in chemical analysis are being investigated by analytical chemists ( I ) , namely, utilization of the "intelligence" of computers so they can be used to optimize analyses and detect and correct for problems that may render an analysis invalid. One example is a computer-controlled photon counting spectrometer that makes decisions to reduce wasteful time scanning spectral regions containing no information (2). A truly intelligent instrument would identify sample constituents, select optimal operating parameters, correct for all types of interferences (chemical, physical, spectral) and matrix effects, and accurately estimate the concentrations of all analytes. In the present study, the generalized standard addition method (GSAM) (3) is used in an automated mode, integrated with a microcomputer-controlled analytical instrument, representing a step toward a fully automated intelligent instrument. The GSAM is an experimental design and a calculation procedure for multicomponent analysis based on the method of standard additions ( 3 , 4 ) . The GSAM represents instrument responses as functions of the chemical species present in the sample undergoing analysis and enables one to detect 0003-2700/83/0355-0532$01.50/0
and correct for matrix effects and all forms of interferences. It requires that when there are r analyte concentrations to be determined, the responses from p sensors (electrodes, wavelengths, etc.) be recorded (p 2 r ) before and after n standard additions are made ( n 2 r). The linear model is T
m=l,
..., n
1 = 1 , ..., p
where rm,lis the response of the lth sensor after the mth addition of analyte s, cm,sis the total concentration of the sth component after addition m, and ks,l is the linear response constant for the 2th sensor to the sth component. In matrix notation
R = CK where R is the n X p matrix of measured responses, C is the n X r concentration matrix, and K is the r X p matrix of linear response constants. For further details on solving for C and K in the above equation, the reader is referred to ref 3 and 4. Recently, the model for the GSAM has been extended to include instrument responses as functions of nonchemical parameters, such as time (5). Expressing responses as a function of time allows for the detection and correction of drift that may be occurring during an analysis. The GSAM has also been extended to the case where the number of sensors is greater than the number of analytes ( p > r ) , allowing for the detection and correction of potential interferents (5). Using p > r also makes possible the combination of several relatively unsensitive sensors for a given analyte to form, in effect, a more sensitive sensor. The model for the GSAM with the inclusion of time is 111
rm,l =
C cm,&L + 2 tikti,L
s=l
(3)
i=l
where w represents the polynomial order of the drift process model. Further details are given in ref 5. The GSAM has been applied to inductively coupled plasma atomic emission spectrometry (6),anodic stripping voltametry
a 1983 American Chemical Society
