Field-Ionization Relative Sensitivities for Analysis of Coal-Derived Liquids Determined as a Function of Ion-Source Temperature and Binary-Mixture Composition S. E. Scheppele," G. J. Greenwood,' and P. A. Benson Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma
The field-ionization sensitivity for heptane relative to benzene has been investigated as a function of both ion-source temperature and binarymixture composition. For an ion-source temperature of 100 O C and for binary mixtures containing in excess of 60 wt YO heptane, the relative gram sensitivlty decreases as the percentage of heptane in the mixture rises. As the ion-source temperature is increased from 100 to 270 'C, the relative gram sensitivity (1) decreases for any given binary mixture and (2) exhibits a decreasing dependence on binarymixture composilion. An ion-source temperature of 270 OC reduces the dependence of the relative sensitivity on mixture composition to within the % std dev (6.3 f 3.0) in the values determined for the individual mixtures and yields a relative gram sensitivity of (23.8 f 2.0) X
Field-ionization mass spectrometry, FI/MS ( I , 21, which produces virtually fragment-ion-free mass spectra ( 2 , 3) is ideally suited to the quantitative analysis of the aromatic components of coal-derived liquids ( 4 - 6 ) . Relative mole and gram sensitivities determined with an ion-source temperature of 270 "C for 61 aromatic hydrocarbons and aromatic compounds containing heteroatoms were found to be independent of mixture composition within the limits of data precision ( 4 ) . Furthermore, the variation in relative sensitivities with change in molecular structure is considerably smaller for field ionization compared to ionization by low-voltage electrons ( 4 ) . Field-ionization mass spectrometry should be ideally suited to group-type quantitative analyses of saturates. In this regard, sensitivities have been reported and metastable spectra investigated for field ionization of a series of saturates using an ion-source temperature of 185 "C (7). The utility of these data is not clear since F I sensitivities of saturates relative to low-molecular weight aromatic hydrocarbons determined using mixtures of the two compound classes exhibit a dependence on mixture composition ( 2 , 8 ,9). Consequently, quantitative analysis of saturate plus aromatic mixtures would appear to require either calibration using mixtures of known composition (8) or addition of an alkyl aromatic such as ethylbenzene to t h e unknown mixture ( I O ) . It is important to note that composition dependent FI sensitivities for saturates relative to low-molecular weight aromatic hydrocarbons were obtained using ion-source temperatures 5125 "C (2,8,9) whereas 260-270 "Cion-source temperatures yielded composition independent relative F I sensitivities for aromatic compounds ( 4 ) . Consequently, a systematic investigation of saturate FI sensitivities as a function of mixture composition, emitter temperature, the average thermal energy of the neutral molecules, ion-source pressures, and emitter potential has been initiated. For binary mixtures of heptane and benzene, we wish to report that both Present address, Plastics and Analysis Division, Research and Development Division, Phillips Petroleum Company, Bartlesville, Okla. 74004.
74074
the magnitude of the gram sensitivity of heptane relative to the value for benzene and the dependence of the relative gram sensitivity on mixture composition is markedly dependent on ion-source temperature.
EXPERIMENTAL Instruments. Field-ionization mass spectra were acquired ( 4 ) using a CEC 21-llOB mass spectrometer equipped with a modified ( 1 1 , 12) combination FI/EI ion source (13). Emitters were conditioned and spectra were obtained with an emitter (ionaccelerating) potential of 5.8 k V and a counter-electrode potential of +500 to -500 V. The change in emitter potential from ca. 6 8 k V ( 4 ) to ca. 5.8 kV reflects substitution of two 0-3000 V Fluke 415 power supplies for the f 7 5 0 V ESA power supply present in our CEC 21-llOB. Samples were introduced into the ion source via the all-glass inlet system (270-300 "C). Mass spectra were acquired for ion-source temperatures of 100, 150, 200, and 270 "C. Compounds and Mixture Preparation. Heptane was used without further purification; benzene was flash distilled. Samples of both compounds were >99% pure by both gas chromatographic and field-ionizationmass spectrometric analysis. Binary mixtures were prepared using weighed quantities of the standard samples. The sealed mixtures were stored at 0 "C.
RESULTS AND DISCUSSION Field ionization of benzene and heptane yielded greater than 99% of CsH6+.and >95% of CiHl6+,a t ion-source temperatures of 100, 150, 200, and 270 "C.Gram sensitivities for field ionization of heptane relative to the gram sensitivities for field ionization of benzene, sH(gj/sB(g), were calculated by multiplying the ratio of the m / e 100 to the m / e 78 ion abundances by the ratio of the weight of benzene to the weight of heptane. A single determination of sH(g)/se(g) at a specified ion-source temperature corresponds to the average value obtained from three spectra recorded for a given amount of a binary mixture introduced into the all-glass inlet system. Samples were introduced into the batch-inlet system using a 10-pL Hamilton syringe because the present inlet system does not permit introduction of precisely known volumes (quantities) of highly volatile materials. Consequently, F I / M S were recorded as a function of ion-source pressure as monitored by the Bayard-Alpert ion-source pressure gauge in studies of the possible dependence of sH(g)/sB(g)on the quantity of a given mixture injected. During the course of these investigations, the response of the ion-source pressure gauge was monitored by injecting, as reproducibly as possible, volumes of a given binary mixture; the dead-volume of the syringe precludes precise knowledge of the volumes actually injected. Table I presents s&)/sB(g) values determined as a function of the weight percent heptane in the mixture and using ion-source temperatures 5110 "C. The dependence of the m / e 78 and m / e 100 ion abundances on the percent partial pressures of benzene and heptane in the gas reservoir reported by Hippe and Beckey in Figure 6 (8) was used to calculate the relative gram sensitivities in column 2. I t is important to note that both the data in Figure 6 and the sensitivity factors for heptane and benzene reported in Table I1 of ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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Table 1. Dependence of FI Sensitivity for Heptane Relative t o Benzene on Binary Mixture Composition at Ion- Source Temperatures < 110 C [sH(g)/sB(g)l x
Table 111. Dependence of FI Sensitivity for n-Heptane Relative to Benzene on Mixture Composition at an Ion-Source Temperature of 270 C
lo2
Wt % heptane
Set l a
Set 2b
25 28 51 60 74 80 90 97
54 55 50 48 41 37 27 20
67
Wt % heptane 25 51 60 74 80 90 97
63 63 64 62 55 39
a
Calculated from curves in Figure 6 of reference 8. Ion-source temperature was either room temperature o r 70-110 C. Present work. Ion-source temperature was 1 0 0 ' C.
Relative gram sensitivity ( x 1 0 2 ) a Day 1 Day 6 20.2 t 24.0 i 21.4 t 23.5 r 24.1 f 23.2 i 21.0i
0.7 1.8 0.1
1.2 0.7 3.0 2.1
24.2 t 24.6 i 23.4 i 26.8 i 26.7 i 25.1 i 26.0 ?;
1.8
1.5 0.8 1.2 1.1
1.6 1.6
Deviations are standard deviations.
a
Table 11. Dependence of FI Sensitivity for n-Heptane Relative to Benzene o n Mixture Composition at Various Ion-Source Temperatures Wt % heptane
25 51 60 74 80
90 97 a
Relative gram sensitivities ( X 102)a at an ion-source temperature of 100 ' C
67.0 i 1.6 62.9 i 5 . 6 63.4 i 5.0 64.0 i 3.4 62.3 ~r 5.1 55.1 i 5.2 39.0 i 5.0
150 'C 47.6 i 47.9 i 46.1 i48.5 f 50.1 i 43.1 * 35.8 i
4.2 2.4 2.1 3.2 2.3 3.0
4.1
200 " C 24.5 1.4 27.1 ? 0.5 29.6 i 0.8 34.4 ? 1.3 30.4 I1.3 30.4 I 2.6 25.7 I 2.5 3
Deviations are standard deviations.
reference 8 for a binary mixture composed of 23.3 and 76.7 mol % heptane and benzene, respectively, yield relative gram sensitivities of 0.55 for a binary mixture containing 28 wt % heptane. Column 3 presents sH(g)/sB(g)values determined in the present investigation. The experimental sensitivity data were obtained using ion-source temperatures of (a) either room temperature or 70-110 "C by Hippe and Beckey (8) and (b) 100 "C in this study. T h e agreement between the two sets of relative gram sensitivity data in Table I is reasonable although not quantitative. In this regard, the sH(g)/sg(g) values in column 2 for binary mixtures containing weight percents of heptane ranging from greater than 79 to less than 100 could reflect the possibility that the m l e 100 and m l e 78 ion abundance curves in Figure 6 (8)are based upon interpolations rather than upon experimental points. However, the data in both columns 2 and 3 show that the gram sensitivity for FI of heptane relative to the gram sensitivity for F I of benzene determined a t ion-source temperatures of ca. 100 "C decreases as the weight percent heptane in the binary mixture increases. The decrease in sH(g)/sB(g) is most dramatic for mixtures containing in excess of 80 wt % heptane. Relative field-ionization gram sensitivities determined as a function of the weight percent heptane in the binary mixture are presented in Table I1 for ion-source temperatures of 100, 150, and 200 "C and in Table I11 for an ion-source temperature of 270 "c. The s&)/sB(g) values are based upon 1 to 6 determinations for gaseous-binary mixture pressures in the ion source ranging from ca. 1.1 X Torr to ca. 9.0 X lo-' Torr. I t should be noted that pressures up to ca. 5.0 x Torr were used in the majority of relative sensitivity determinations. T h e standard deviation in any given average relative sensitivity is based upon the individual sH(g)/sB(g) values calculated from 3 to 18 FI/MS. The percent standard deviations for the 35 average sH(g)/sB(g) values produces an 1848
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
average percentage uncertainty of 6.3 f 3.0. Relative sensitivities were determined as a function of the pressure of the gaseous binary mixture in t h e ion source (pressure range specified above) for mixtures containing the following weight percents of heptane at the indicated ion-source temperatures: 25% at 200 and 270 "C, 51% at 150 and 270 "C, 74% a t 150 "C, 80% at 100 "C, 90% a t 100,150,200, and 270 "C, and 97% at 100,150,200,and 270 "C. The average percent standard deviation for the 15 average relative sensitivities measured as a function of ion-source pressure is 8.3 f 2.5. The average percent standard deviation for the remaining 20 average sH(g)/sB(g) values is 4.8 f 2.5. Although the average percent standard deviation in the relative sensitivities determined over a range in ion-source pressures is on the average slightly greater than the corresponding value calculated for the relative sensitivities measured at essentially a single ion-source pressure, the difference is clearly within the limits of data precision. In fact, the former and latter values of 8.3 f 2.5 and 4.8 f 2.5, respectively, are not statistically different from the average percentage uncertainty of 6.3 f 3.0 obtained for all determinations. The statistical uncertainty in the present data is commensurate with that obtained for the relative sensitivities for field ionization of aromatic compounds ( 4 ) . For a given binary-mixture composition, the data in Table I1 show that the FI sensitivity of heptane relative to the F I sensitivity of benzene decreases as the ion-source temperature is increased from 100 to 200 "C. Furthermore, the variation in sH(g)/sB(g) with change in the weight percent heptane in the binary mixture is seen to decrease as the ion-source temperature is increased from 100 to 200 "C. The data in column 4 of Table I1 suggest that a n ion-source temperature of 200 "C is not quite high enough to reduce the dependence of sH(g)/s&) on mixture composition to within the limits of data precision. The decrease in the relative sensitivities upon increasing the ion-source temperature from 100 to 200 "C appears to be greatest for binary mixtures containing either less than ca. 50% or greater than ca. 90% heptane. The data in Table I11 reveal that increasing the ion-source temperature to 270 "C results in sH(g)/sB(g)values which are independent of binary mixture composition within the limits of experimental precision. I t is encouraging to note that the relative sensitivities for each binary mixture measured over a six-day interval agree in all cases within 20. Averaging the 14 sH(g)/s&) values without regard to the standard deviation in each value leads to an average relative gram sensitivity of Comparison of the data in Tables I1 and (23.8 f 2.0) x I11 suggests that increasing the ion-source temperature from 200 to 270 "C has the greatest effect on the relative sensitivities for mixtures containing between ca. 50 to ca. 90 wt 70heptane. Clearly, medium- to high-resolution F I / M S can be used as a rapid and routine technique for obtaining group-type analyses of unseparated mixtures of saturates and aromatics subject to the demonstration of essentially composition independent FI relative sensitivities for the former in t h e
reflects in part the reproducibility of sample injection. T h e imprecision in sample injection was estimated to be 5510% based upon the characteristics of the 10-pL syringe and the observed ion-source pressures. As seen in Figures l(1) through l ( 4 ) the intensity of the C&&ion, m/e 78, increases linearly as the mixture composition is varied from 3 to 79 mol % benzene at the various ion-source temperatures within the limits of data precision. In contrast to the linear variation in the m l e 78 intensity shown for an ion-source temperature of 100 "C in Figure 1(1),Hippe and Beckey (8)observed that the increase in the CsH6 ion intensity somewhat exceeded linearity as the mole percent benzene in the gaseous binary mixture was increased from 0 to 100 for an ion-source temperature of either room temperature or 70-110 "C. I t should be noted that ion abundance data were obtained by Hippe and Beckey (8) using gaseous binarymixture pressures in the ion source in the low lo-' Torr range whereas the data in Figures l(1)through l(4) were acquired using ion-source pressures of ca. 2 x 10 Torr. Consequently, the apparent discrepancy in the functional dependence of the CsHs ion intensity on the mole percent benzene in the mixture may reflect differences in ion-source pressure and the imprecision associated with both experiments. Figure l(1)shows that the C7HI6ion intensity rises linearly with increasing mole percent heptane for gaseous mixtures containing up to ca. 0.6-0.7 mol fraction of heptane within the limits of data precision. However, at higher mole fractions of heptane, the mole response of the m / e 100 ion intensity exhibits a negative deviation from linearity. The functional dependence of the m / e 100 ion abundance on mixture composition in Figure l(1) is qualitatively similar to that observed by Hippe and Beckey (8). The extent of deviation from linearity in the mole response of the C7HI6ion intensity is seen in Figures l(2) and l ( 3 ) to decrease with increasing ion-source temperature. For an ion-source temperature of 270 "C Figure l(4) reveals that the heptane molecular ion intensity increases, within the limits of experimental precision, linearly as the mole percent heptane in the mixture is increased from 2 1 to 97. Both the decreasing dependence of sH(g)/sB(g)on binary-mixture composition and the decrease in relative sensitivity for a given binary mixture with increasing ion-source temperature reflects a temperature effect on either the C6H6+-ion yield per mole of benzene or the C-Hls+. ion yield per mole of heptane in the gaseous mixture or both. Unfortunately, the imprecision associated with sample injection in the present experiments precludes drawing quantitative conclusions concerning the temperature effect. Consequently, the inlet system is being modified to permit introduction of precisely known quantities of both highly volatile substances and mixtures prepared from them. The ion abundance data in Figures l(1)through l ( 4 ) provide tentative insight into the temperature effect. First, the mole sensitivity of benzene appears to (1) be essentially independent of the moles of benzene in the gaseous mixture a t a given ion-source temperature and ( 2 ) rise as the ion-source temperature is increased. Second, the data suggest an increase in the mole sensitivity for heptane as the ion-source temperature is decreased for all binary mixtures; as the temperature is decreased, the increase appears to become larger as the mole percent heptane increases. Unfortunately the present data are insufficient to formulate a molecular interpretation of the effect of temperature on F I sensitivities equivalent in sophistication to that which exists for temperature effects on electron-impact-induced ionization sensitivities (14) in terms of thermal effects on ionization cross sections (15) and on the unimolecular kinetics ( 1 6 ) of ionic fragmentation reactions (15, 17, 18).
'
I
"31C
P P r C e - :
ae7:.ne
Figure 1. Dependence of the
and C7HI6 ion abundances on the mole percents of benzene and heptane in the binary mixture at different ion-source temperatures. Points designated by (0)and ( 0 )correspond t o C,H, and C,H,, ion intensities, respectively. Figures 1(1), (2), (3), and (4) present data obtained for ion-source temperatures of 100, 150, 200, and 270 OC, respectively
presence of the latter as a general phenomenon for ion-source temperatures 1270 "C. Consequently, sensitivities for F I of a variety of saturates are being determined relative to the sensitivities for F I of aromatics. These relative sensitivities will be used in conjunction with those previously determined ( 4 ) to analyze both synthetic blends and fractions obtained from the separation of coal-derived liquids. Figure 1 presents plots of the variation in the ion abundances for the heptane and benzene molecular ions, mle 100 and 78, respectively, as a function of the mole percent benzene in the mixture for ion-source temperatures of 100, 150, 200, and 270 "C and an all-glass inlet system temperature of 300 "C. Interpretation of the data in Figures l(1) through l(4)
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1849
ACKNOWLEDGMENT We thank H. E. Lumpkin and H. D. Beckey for stimulating comments and discussion.
LITERATURE CITED (1) M. G. Inghram and R. Gorner, J . Chem. Phys., 22, 1279 (1954). (2) H. D. Beckey, “Field Ionization Mass Spectrometry”, Pergamon Press, Oxford, England, 1971. (3) H.D. Beckey in “Biochemical Applications of Mass Spectrometry”, G. R . Waller, Ed., Wiley-Interscience, New York, N.Y.. 1972, Chapter 30. (4) S. E. Scheppele, P. L. Grizzle, G. J. Greenwood, T. D. Marriott, and N B. Perreira, Anal. Chem., 48, 2105 (1976). (5) S. E. Scheppele, G. J. Greenwood, P. L. Grizzle, T.D. Marriott, and N. B. Perreira, 24th Annual Conference on Mass Spectrometry and Allied Topics, 1976, p 481. (6) S. E. Scheppele, G. J. Greenwood, P. L. Grizzle, T. D. Marriott, C. S. Hsu, N. B. Perreira, P. A. Benson, K. N. Detwiler, and G. M. Stewart, Prepr.. Div. Pet. Chem., Am. Chem. SOC.,22(2). 665 (1977). (7) G. G. Wanless and G. A. Glock, Jr., Anal. Chem., 39, 2 (1967). (8) H. G. Hippe and H. D. Beckey, Erdoel, Kohle, Erdgas, Petrochem., 2 4 , 620 (1971). (9) M. Ryska, M. Kuras, and J. Mostecky, Int. J . Mass Spectrom. Ion Phys., 16, 257 (1975). (10) M. Kuras, M. Ryska, and J. Mostecky, Anal. Chem., 48, 196 (1976). (11) Internal Report of the ContinentalOil Company, Research and Development Department, Ponca City, Okla., Report No. 1007-4-1-73, authored by H. M. Curtis.
(12) G. J. Greenwood, P h D Thesis, Oklahoma State University, Stillwater, Okla., 1977. (13) E. M. Chait, T. W. Shannon, W. 0. Perry, G. E. van Lear, and F. W. McLafferty, I n t . J . Mass Spectrom. Ion Phys., 2, 141 (1969). (14) C. E. Berry, J . Chem. Phys., 17, 1164 (1949). (15) W. A. Chupka, J . Chem. Phys.. 54, 1936 (1971). (16) H. M. Rosenstock, M. B. Wallenstein, A. L. Wahrhaftig, and H. Eyring. R o c . Natl. Acad. Scl., U . S . A . , 38. 667 (1952). (17) W. A. Chupka and J. Berkowitz, J . Chem. Phys., 47, 2921 (1967). (18) M. Vestal in “Fundamental Processes in Radiation Chemistry”. P. Ausloos, Ed., Wiley-Interscience, New York, N.Y.. 1968, Chapter 2.
RECEIVED for review April 12, 1977. Accepted July 25, 1977. Results presented in part a t 20th Annual Conference on Analytical Chemistry in Energy and Environmental Technology, Gatlinburg, Tennessee, October, 1976 and in the symposium “Analytical Chemistry of Tar Sands and Oil Shale” (Comparison with Coal and Petroleum) sponsored jointly by the Divisions of Analytical and Petroleum Chemistry, American Chemical Society meeting, New Orleans, March 1977. We wish to acknowledge the Energy Research and Development Administration, contract numbers EX76-S-01-2537, EX-76-C-01-2011, and EY-76-S-02-0020, for generous support of this research.
Determination of Absolute Fluorescence Quantum Efficiency of Quinine Bisulfate in Aqueous Medium by Optoacoustic Spectrometry M. J. Adams, J. G. Highfield, and G. F. Kirkbright” Department
of Chemistry,
Imperial College of Science and Technology, South Kensington, London S W7 2A Y, United Kingdom
A method utilizing optoacoustic spectrometry is descrlbed to permit the determination of the absolute quantum efficiencies of fluorescence for compounds in solid and liquid samples. The method is demonstrated in the determination of the fluorescence quantum efficiency of quinine bisulfate in aqueous solution utilizing quenching of the fluorescence by halide ions.
In recent years there has been a rapid rise of interest in the optoacoustic effect and its application in the examination of solid and liquid samples. Harshbarger and Robin ( I ) , Rosencwaig ( 2 ) ,and Adams et al. ( 3 , 4 )have demonstrated the technique of analytical optoacoustic spectrometry (OAS) in studies of a variety of sample types. In OAS, intensity-modulated electromagnetic radiation is incident upon the sample material enclosed in a cell of constant volume. If the sample absorbs a t the wavelength of the incident radiation, on subsequent de-excitation the absorbed energy may appear as heat and cause a periodic pressure rise in the gas surrounding the sample. This change in pressure may be detected and monitored by a sensitive microphone transducer enclosed within the cell. The resulting electrical signal is selectively amplified using a tuned-amplifier and phase-sensitive detector and presented as a dc potential to a potentiometric chart recorder or other voltage display system. Clearly, optoacoustic spectrometry relies on the radiationless conversion of absorbed energy for the production of an acoustic signal and, hence, is complementary to the conventional techniques of luminescence spectrometry. As a 1850
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
pressure transducer is employed to monitor, indirectly, the temperature of the absorbing species, optoacoustic spectrometry may be considered as a calorimetric method for the detection and study of excited states. Rosencwaig ( 2 ) suggested that the technique might be useful for these purposes but presented no data relating to liquid samples. Callis ( 5 ) has reviewed some modern calorimetric techniques for examining radiationless deactivation in solid and liquid samples employing both microphones and piezoelectric crystals as heat-flow transducers. More recently, Lahmann and Ludewig (6) have employed the optoacoustic technique using a piezoelectric transducer immersed in the sample solution to determine the absolute fluorescence quantum efficiency of rhodamine 6G in aqueous solution. Such calorimetric methods of studying excited states are reported to be rapid, simple, and sensitive. In the work described here the single-beam optoacoustic spectrometer described earlier ( 3 , 4 ) has been employed for the determination of the absolute quantum efficiency of quinine bisulfate in aqueous solution and examination of the quenching effect on this species observed upon the addition of chloride ions to the sample solution.
EXPERIMENTAL The single-beam optoacoustic spectrometer employed for the work discussed here has been described elsewhere (3, 4 ) . Radiation from a 1-kW high-pressure, xenon short-arc illuminator was focused through a rotating sector onto the entrance-slit of an //4 monochromator. The optoacoustic cell ( 4 ) , containing the sample, was positioned at the exit slit of the monochromator. The signal from the sensitive capacitor microphone in the cell, was led t o a lock-in amplifier unit and Apparatus.
