38
Anal. Chem. 1983, 55, 38-41 (0.7 C
I
1
I E
I1
I
I
I
1
0
5
IO Days
15
20
LITERATURE CITED (1) Wroblewski, F.;La Due, J. S.Proc. SOC.Exp. Biol. Med. 1955, 90, 2 10-213. (2) Henry, R. J.; Chiamori, N.; Goiub, 0. J.; Berkman, S. Am. J . Clin. Pathol. 1980, 34, 381-398. (3) Cabaud, P. G.; Wroblewski, F. Am. J. Clln. Pathol. 1958, 30, 234-236. (4) Morgenstern, S.;Flor, R.; Kesseler, G.; Klein, B. Anal. Blochem. 1985, 13, 149-161. (5) Zimmerman, R. L., Jr.; Guilbault, G. G. Anal. Chim. Acta 1972, 58, 75-81. (6) Smith, M. D.; Olson, C. L. Anal. Chem. 1974, 46, 1544-1547. (7) Guilbault, G. G. "Handbook of Enzymatic Methods of Analysis"; Marcel Dekker: New York, 1976; pp 460-510.
10.5
Flgure 7. Long-term stability of the enzyme electrode. The current decrease (in steady-state) for L-lactate (0.2 mM) and the rate of current decrease for LDH (28 I.U./L) in a solution are sequenttally measured as shown in Figure 6. Each average value for 10 successive measurements per day is plotted agalnst days after the preparatlon of the enzyme electrode.
ACKNOWLEDGMENT The authors are grateful to Toyo Jozo Co. for supplying lactate oxidase. Registry No. LDH, 9001-60-9;L-lactate, 79-33-4;lactate oxidase, 9028-72-2.
(8) Carr, P. W.; Browers, L. D. "Immobilized Enzyme in Analytical and Cllnical Chemlstry"; Wiley: New York, 1980; pp 197-310. (9) Karube, I.; Matsunaga, T.; Teraoka, N.; Suzukl, S.Anal. Chim. Acta 1980. 119, 271-275. Suzuki, S.; Matsumoto, K. Anal. (10) Mlzutani, F.; Tsuda, K.; Karube, I.; Chim. Acta 1980, 118, 65-71. (11) Mizutani, F.;Tsuda, K. Anal. Chlm. Acta 1982, 139, 359-362. (12) Noil, F. Blochem. 2. 1986, 346, 41-49. (13) Meii, D. L.; Maloy, J. T. Anal. Chem. 1975, 47, 299-307. (14) Bergmeyer, H. U.; Bernt, E. I n "Method of Enzymatic Analysis", 4th ed.; Bergmeyer, H. U., Ed.; Verlag Chemie-Academic Press: New York and London, 1974; Vol. 2, pp 574-579.
RECEIVED for review July 21,1982. Accepted October 4,1982.
Determination of Molecular Weight Distribution of Aromatic Components in Petroleum Products by Chemical Ionization Mass Spectrometry with Chlorobenzene as Reagent Gas L. Wayne Sieck Chemical Thermodynarnlcs Division, Center for Chemical Physics, National Bureau of Standards, Washington, D.C. 20234
A chemlcal lonlzation mass spectrometric technlque for dlrect determination of the molecular weight dlstrlbutlons of the major aromatlc components In liquid fuels and other petroproducts Is dlscussed. The basic mechanism Involves selective charge exchange reactions between chlorobenzene cations and the substituted benzenes and naphthalenes present in the sample. Chlorobenzene also serves as the solvent for the fuel, and screenlng of successive samples can be carried out wlth a 3mln turn-around tlme. Depending upon condltions, the paraffinlc components present In the fuel are absent In the resultlng mass spectrum.
appropriate solvent(s) for the untreated sample as the source of the reagent ions for the measurement.
EXPERIMENTAL SECTION
All measurementswere carried out with the NBS high-pressure photoionization mass spectrometer, which was mdified for this study by incorporatinga 0-5-kV electron gun to provide electron impact ionization under typical analytical CI conditions. Unless otherwise indicated, all spectra were recorded by using this auxiliary gun at source temperatures of 150-200 "C (423-473K). Samplesfor study were syringe-injectedand vaporized into a 3-L evacuated Pyrex reservoir (450 K) which was interfaced to the mass spectrometer via a micrometeringvalve. The other essential features of the system have been described previously ( 4 ) .
RESULTS AND DISCUSSION In spite of the vigorous growth in the application of chemical ionization (CI) mass spectrometry for analytical purposes ( I ) ,
most of the attention has focused on the utilization of GC/MS techniques. With respect to the specific problem of oil and petroproduct analysis by CI, studies which involve the introduction of unfractionated bulk samples directly into the mass spectrometer (other than GC/MS) have been restricted to the OH- screening of aromatics (2) and the screening of organ0 sulfur compounds using triple quadrupole MS (3). The present study was undertaken to develop rapid CI methods, requiring no prior sample treatment, which could provide quantitative information concerning aromatic components in liquid fuels and petroproducts. The basic method utilizes an This article not subject to
Solvent Selection. The selection of solvents for use as
sources of reagent ions was based on the satisfaction of the following criteria: (1) It must be a reasonably volatile liquid in which the important components of the sample are soluble. (2) It must not fragment extensively following electron impact ionization under CI conditions. (3) The ion(s) produced initially from the solvent either must be unreactive toward the solvent vapor or must react in a well-specificed manner giving a relatively simple, wellcharacterized, and reproducible reagent ion spectrum. (4) The solvents must be relatively free of those impurities which are likely to be significant components in the samples
US. Copyrlght. Published 1982 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55,
NO. 1, JANUARY 1983
39
pressures in excess of 35-40 mtorr of pure chlorobenzene at 150-200 OC, the following mass spectrum is obtained (normalized to m / z 112, C6H635C1+= 100): m/Z 112,100;m / z 113, 11.1; m/z 114, C6H,3'C1+, 32.5; m / z 115,5.0; m / z 128, Cl0H8+, 6.0; m/z 152, C12H8+,4.1; m/z 153, C12H9+,11.6; and m / z 154, CI2Hl0+,5.6. The higher mass peaks result from the following reactions involving fragment ions:
-
+ CloH8++ C1 C&4+ + C G H ~ C ~C12HB+ + HC1 C6H5+ + C6H,C1- C12Hg' + HC1 C4H3+ C6H5C1
+
'+
g
0
C & 5 + + C6H,&1-
1
NAPHTHALENES '
0
0
c
100
MOL.ECULAR
180
140
220
WEIGHT (AM-UJ
Figure 1. Ionization potentials of certain classes of organic molecules
as a function of their molecular weight.
Some specific moiecules are
included for reference.
to be screened and be readily available and in a condition to be used essentially as supplied (no elaborate purification required). (5) The products of tjhe CI interactions must be unreactive toward the bulk solvent, vapor. (6) The reagent ions must react with each sample component to give, ideally, product ions which are characteristic of that particular component and are easily related to the neutrals from which they were derived. In order to simplify the interpretation, we decided to restrict the solvent(s) to those which give reagent ions which react to yield the corresponding molecular ion (M+) of the component via a charge transfer mechanism:
S+ (from sdvent)
+M
-
M+ + S
(1)
Figure 1 displays the ionization energies or ionization potentials (IP's) of several classes of organic molecules as a function of their molecular weight. Some specific molecules are identified for reference. The important feature of Figure 1is the gap which exists between the IP's (5) of aliphatic and aromatic hydrocarbons. With the exception of benzene (IP = 9.24 eV), the IP's of aromatic hydrocarbons are all 18.82 eV (toluene). On the other hand, the lowest reliable IP which has been determined for a saturated hydrocarbon is that of trans-decalin, 9.14 eV. 'Therefore a careful search was made for solvents which have IP's I 9.1-9.2 eV, since, to a first approximation, the resulting molecular ions should react only with the aromatic components in liquid fuels. Benzene, although it has been successfully used as a reagent gas in GC/MS applications (6, 7), as well as toluene and the xylenes (IP's 8.48-8.58, eV) were all found to be unsuitable due to relatively high impurity levels (>1part in lo4)of higher molecular weight alkyl-substituted benzenes, rendering them useless for the screening of lighter fuels. Heterocyclics, such as substituted pyridines, which have appropriate IP's and are relatively stable toward ionic fragmentation following electron impact ionization, undeirgo protonation and cannot participate in reaction 1. Some ollefins, such as cyclohexene (IP = 8.95 eV) were also investigated but were found to fragment excessively and produce condensation ions which were totally unreactive. The primary solvent eventually chosen was reagent grade chlorobenzene. It has an appropriate IP (9.04 eV), is free from impurities which would mask M+ ions from aromatics, and is an excellent solvent for fuels and petroproducts. At CI
(2)
(3) (4)
+ c1
(5) With the exception of the primary product ions at m / z 128 (same as naphthalene) and m / z 154 (same as ethanonaphthalenes), there are no peaks in the reagent ion spectrum which would mask signals from major aromatic components in petroproducts. Approximately 85% of the total ionization is in the molecular ion, CBH5C1+( m / z 112-115). Sensitivity Measurements. The utility of C6H5C1+(CB') as a reagent ion depends critically upon its reactivity toward various classes of organic compounds. Therefore an extensive evaluation of the process CGH5Cl"
+M
+
C12H10+
M+ + C & j C l
(6)
was carried out to define the relative sensitivities for different types of M (the relative sensitivity for molecules Mi and Mj is defined as the ratio of the intensities or peak heights of the corresponding molecular ions, Mi+ and Mj+, obtained in equimolar mixtures of Mi and Mj in chlorobenzene)under CI conditions. These measurements were completed as follows: (1)mixtures of known composition of three or four compounds (minimum stated purity, 96%) having different molecular weights were prepared and diluted to approximately 1 part in 104-105 of CB. (2) A 100-pL portion of this mixture was then injected into the sample reservoir and the leak rate adjusted to maintain a total pressure of 0.1 torr in the CI chamber. (3) The composite spectrum was then recorded (usually three or four scans) and inspected to verify that the expected M+ ions were the only products of the CI reaction. The chamber pressure was then decreased by a factor of 2 (three more scans) and then increased by a factor of 4 (three more scans), and the relative M+ peak heights compared to ensure that no secondary reactions were occurring in the ion source. (4) The inlet reservoir was then evacuated, followed by the injection of a different mixture containing one component in common with the prior sample. The scanning sequence was repeated, and a network of relative sensitivies assembled by cross-comparison of many such mixtures containing different additives. Conditions were always adjusted to provide a composite mass spectrum in which the sum of the intensities of the M+ ions was 510% of the intensity of CB+ at the time of the determination. This was also the case during the screening of fuel and petroproduct samples. Higher conversions (>lo%) to M+ ions sometimes gave a distorted spectrum due to consecutive charge exchange reactions involving M+ ions and M-type neutrals present in the mixtures. The CB CI sensitivities for a variety of substituted benzenes and naphthalenes, as well as some substituted indans and tetralins, plotted as a function of the molecular weight of M, are shown in Figure 2 (see legend for class identification). All values are normalized to m / z 142 from 1-methylnaphthalene = 1.00. The pseudomonotonic decrease in apparent sensitivity VS. molecular weight, which is clearly evident in the alkylbenzene data, is ascribed to detector response characteristics rather than an actual reduced reactivity of CB+ toward the
40
ANALYTICAL CHEMISTRY, VOL. 5 5 , NO. 1, JANUARY 1983 I
I
5
w
0.1
1
I
I
I
I
ALKANES
LL
0.01
0 0 -
I
100
0 -
I
I
I40
0
1
0
0
0
I
1
I80
0
1
220
MOLECULAR WEIGHT
(A.M.U.)
Flgure 2. Relative sensitivities in chlorobenzene CI toward aromatic and aliphatic hydrocarbons: (0)alkylbenzenes, Including lndans and tetralins; (0)phenylolefins; (0)substituted naphthalenes. higher molecular weight members of given families of hydrocarbons. This is assumed to be true since the electron multiplier, which is of the Cu-Be dynode-type, exhibits a response which is approximately proportional to the inverse of the square root of the mass of the impacting ion for hydrocarbon ions. There are two very important features of Figure 2. Firstly, it would appear that different families of substituted aromatics do not show significantly different relative sensitivities as a function of molecular weight when CB+ is used as the reagent ion. This implies that charge exchange to give M+ is taking place a t every encounter, and since the relative collision coefficients between any given ion and any neutral can be predicted (8) to within a few percent (assuming a prior knowledge of the polarizability and dipole moment of the neutral), it would appear that the relative sensitivity for any given fuel component could be calculated directly without any experimental information. However, this assumes that the only product of the reaction is M+ derived from the neutral M. When the charge transfer reaction becomes highly exothermic, it is possible that the resulting ion will retain sufficient internal energy to undergo dissociation (M+* A+ + B), causing a reduced sensitivity at the m / z value associated with M+ and a more complex CI spectrum. Although dissociative charge transfer was not observed under CI conditions for any of the molecules investigated, it may be manifested in the more exothermic reactions involving higher molecular weight substituted aromatics, which are expected to have still lower IP's. In this context it is appropriate to mention that clustering reactions involving ionized product ions (M+) and the solvent (S), to yield MS-type ions, were undetectable. This WBS to be expected since (i) we use relatively low partial pressures of S (the conversion to MS+ involves third-order kinetics) and (ii) it is well-established (9) that the three-body rate coefficients for production of association ions involving two molecular species of significantly different IP's are generally orders of magnitude lower than the corrssponding resonance case
-
M+ + M-
M
(M),+
+M
The other important feature of Figure 2 is the fact that no signals were detected a t any m / z values from any acyclic alkane or substituted cyclohexane investigated. These included linear and branched alkanes up to heptadecane, and alkylcyclohexanes up to bicyclohexyl. Again, this was anticipated since the I P of chlorobenzene (9.04 eV) is expected to
I.~.~JRIMETHYLBENZEVE
l I03
,
I I 13
WZ
Flgure 3. (A) Charge exchange CI spectrum of a no. 4 fuel oil using chlorobenzene. Substituted benzenes are Indicated by broken lines. (B) Charge exchange CI spectrum of the same no. 4 fuel oil using cyclo-C,H,,+ as the reagent Ion. See text for meaning of composite peaks. (C) Charge exchange CI spectrum of the same no. 4 fuel oil using (1,2,4-trimethylbenzene)+ as the reagent ion. See text for meaning of enlarged portions of spectra 3 A and 3C. be below that of the majority of the saturated hydrocarbons expected to be present in liquid fuels. Sample Screening. The major features in the CI spectra of a no. 4 fuel oil using different solvents are given in Figure 3. Figure 3A gives the net pattern (ions from CB have been subtracted) obtained with CB. The measurement conditions were 0.5% (v/v) of the oil in CB, 100 1L of the mixture injected into the inlet reservoir, CI chamber pressure of 50 mtorr, and 150 "C. Peaks having the appropriate masses for substituted benzenes and naphthalenes are indicated by broken and solid lines, respectively. The spectrum of this same oil using cyclo-C6H12+as the reagent ion is given in Figure 3B. This measurement was made by photoionization ( 4 ) ,instead of electron impact, and is presented here only for comparison. The I P of cyclo-C6H12is 9.88 eV, which lies above those expected for higher molecular weight aliphatics (see Figure 1). Therefore, the cyclo-C6H12+CI spectrum contains composite peaks including contributions from substituted naphthalenes and aliphatics (indicated by brackets in Figure 3B) having the same nominal m / z values. Note that those lower molecular weight aliphatic components with molecular weights between 100 and -160 amu's, although also having IP's < 9.88 eV, do not contribute measurably to the composite spectrum. This due to the fact (10) that rate coefficients for charge exchange decrease when the exothermicity of the overall reaction is low (difference in IP's). Reference to Figure 1 indicates that this is the case for lower molecular weight aliphatics, resulting in a drastically reduced sensitivity for these molecules. To a first approximation, the chlorobenzene CI spectrum may be taken as the actual molecular weight profile of the
ANALYTICAL CHEMISTRY, VOL. 55, NO. 1, JANUARY 1983
major aromatic components in the particular fuel (after correcting for relative semsitivities). This is certainly true for the substituted benzenes. However, as mentioned earlier, the IP’s of the great majorihy of aliphatic hydrocarbons have not been measured, and ewentially no information is available concerning dicyclic molecules, including spiro compounds and terpenes. Even when IP’s have been reported, we have found in some cases that rearrangement ions are produced in the CB CI of bridged hydrocarbons even though the overall reaction is highly endothermic as written. For example, the reported IP of norbornane (C7H12) is 9.80 eV, indicating that electron exchange involving CB+ would be endothermic by at least 0.7 eV. However, we have found (11)that the reaction
+
C6H5C1+ C7H,,(norbornane)
-+
C7H12++ C6H5C1 (7)
is relatively efficient. This is due to the fact that isomerization of C7H12 occurs in the ion-molecule collision complex to yield one or more diolefinic lions having IP’s much less than the IP of CB, 9.04 eV. Consequently,the reaction goes to completion in spite of the apparent endothermicity. Obviously, one cannot rule out similar rearrangement processeia in higher molecular weight cyclics, including cyclohexanes, particularly at the elevated temperatures associated with the screening of heavier fuels. Taken together, these reactions would generate ions in the CI spectrum having the same nominal m / z values as alkano- and alkenonaphthalenes. Furthermore, although processed fuels derived from petroleum are usually low in olefins, this class of colmpounds, having IP’s in the range of 8.5-9.0 eV, would also appear at these same nominal m / z values and distort the composite spectra associated with bridged naphthalenes. In order to assess such contributions, we have utilized an addition solvent, 1,:!,4-trimethylbenzene (1,2,4-TMB),which has an IP of 8.27 eV, placing it far below those anticipated for amy olefin irrespective of the degree of unsaturation. This value also places it above all naphthalenes (see Figure 1). Figure 3C gives the CI spectrum of the same no. 4 fuel oil using 1.2,4-TMB+ as the reagent ion. This measurement was made by taking the original solution of the fuel oil in chlorobenzene,adding 5% (v/v) of 1,2,4-TMB,and scanning the spectrum under the same conditions as those given for Figure 3A. The addition of 1,2,4-TMBserves a 2-fold purpose. The molecullar ion, 1,2,4-TMB+,assumes the role of major reagent ion via interception of CB+ C&Cl+
+ 1,2,4-TMB
-*
1,2,4-’I’MB++C&&l
(8)
and the neutral 1,2,4-TMBpresent in the mixture reads with any ions having IP’s >*8.27eV which might have been generated initially by the interactions of CB+ with fuel components. Comparison of Figure 3A and Figure 3C reveals a complete elimination of‘d signals from lower molecular weight benzenes using 1,2,4-’1’MB,which is consistent with the IP data of Figure 1. The relative intensities of the higher molecular weight benzene derivatives are also somewhat reduced. Although the general profiles of the naphthalene manifolds are similar (solid lines), close inspection of the relative intensities of “naphthalenic” peaks having the same carbon number reveals a significant reduction in the signals at those m / z values which might include contributions from rearrangement (olefinic)ions. For example, one of the groupings, indicated by the arrows and enlargements on 3A and 3C,
41
includes m / z 178, 180, 182, and the base peak, 184, which is uniquely associated with butyl-, methylpropyl-, etc., naphthalenes having the empirical formula CI4Hl6in both spectra. The pure naphthalene profile within this manifold (3C) has the following distribution for m / z 178, 180, 182, and 184 (taking 184 = 1.00); 0.05, 0.08, 0.64, and 1.00. The corresponding distribution in 3A is 0.11, 0.33, 0.82, and 1.00. Consequentlyrearrangement and/or olefinic ions increase the relative signals at those m / z values associated with bridged naphthalenes by more than 60% in this particular manifold. In view of this, we suggest that two separate CI spectra of the same sample are required to define a molecular weight profile for aromatic components in fuels and petroproducts (both CB and 1,2,4-TMB). For qualitative screening, as in forensic applications, CB CI is sufficient. We should mention that spectra such as 3A and 3C are routinely recorded here with a 3-min turn-around time between injection of successive samples. Assuming that the mass spectrometer is preset to scan the m / z range of interest several times within approximately 1 min, the limiting factor is only the time required to evacuate the inlet reservoir and inject a new sample diluted in the proper solvent(s). We have also found that perdeuterionaphthalene, CIODs, can be used as an internal standard to define the actual concentrations of classes of aromatic components in petroproducts. The molecular ion, C l & $ +occurs , at m/z 136, which is clean in the pure solvent spectra. Therefore, a solution of Cl& in CB of known concentration (usually one part in lo4) can be used as the medium for preparing samples for analysis in which the volume dilution of the unknown is accurately controlled. The resulting signal at m/z 136,which is negligible in neat fuels, can be compared with any other features associated with substituted benzenes or naphthalenes which appear in the resultant spectrum. Following any corrections for relative response (Figure 2) and assuming that the entire mass range has been scanned, one can directly calculate the total aromatic content of the unknown. Registry NO. CB’, 55450-32-3; PhCl, 108-90-7; CsH12+, 34473-67-1; C6Hl2, 110-82-7; 1,2,4-Me3C6H3,95-63-6; 1,2,4Me3C6H3+,65018-34-0;CI0D8,1146-65-2.
LITERATURE CITED (1) Symposium on Positive and Negative Chemical Ionization, 29th Annual Conference on Mass Spectrometry, Minneapolis, MN, May 1981, Paper No. FAMOBl through FAMOB15. (2) Sieck, L. W.; Jennings, K. R.; Burke, P. D. Anal. Chem. 1979, 51, 13. (3) Hunt, D. F.; Shabanowltz, J. Presented at the 29th Annual Conference on Mass Spectrometry, Minneapolis, MN, May 1981; p 655,paper no. RPC15. (4) Sieck, L. W. Anal. Chem. 1979, 5 1 , 128. (5) Levin, R. D.; Lias, S.G.; “Ionization Potential and Appearance Potential Measurements, 1971-1981;” NSRDS-NBS Series, No. 71, 1982. (6) Hatch, F.; Munson, M. S. B. Anal. Chem. 1977, 4 9 , 731. (7) Subba Rao, S.C.; Fenselau, C. Anal. Chem. 1978, 5 0 , 511. (8) Chesnavich, W. J.; Su, T.; Bowers, M. T. “Kinetics of Ion-Molecule Reactions”; Ausioos, P., Ed.; Plenum Press: New York, 1979; p 165. (9) Meot-Ner (Mautner), M.; Hamlet, P.; Hunter, E. P.; Field, F. H. J . Am. Chem. SOC. 1978, 100, 5466. (10) Lias, S. G.; Ausioos, P.; Horvath, 2. Int. J . Chem. Klnet. 1976, 8 , 725. (11) Meot-ner (Mautner), M., NBS, unpublished results.
RECEIVED for review July 14, 1982. Accepted September 16, 1982. This work was supported by the Office of Basic Energy Sciences, U.S. Department of Energy.
