Fluorescence Detector for Analysis of Polynuclear Arenes by Gas Chromatography H. P. Burchfield, R. J. Wheeler, and J. B. Bernos Gurf South Research Institute, Atchafalaya Basin Laboratories, P.O.Box 1177, New Iberia, La. 70S60 A gas phase fluorescence detector has been developed for the analysis of mixtures of polynuclear arenes by gas chromatography. In general, gas phase measurements are more convenient to make and less susceptible to light scattering by the solvent than liquid phase measurements but fluorescence intensity i s lower. Sensitivity might be enhanced through use of an ellipsoidal condensing mirror and removal of the carrier gas by a molecular separator before the sample enters the fluorometer cell. Measurement of fluorescence i s more sensitive and specific than electron capture detection, and, in addition, permits the analysis of mixtures of compounds which cannot be separated on present chromatographic columns.
MANYPOLYNUCLEAR ARENES are present in the combustion products of fossil fuels, tobacco smoke, and smoked foods: some of these are known to be carcinogenic. Gas chromatography has great potential for analyzing complex mixtures of these compounds which has never been fully realized. This is in part due t o the fact that the electron capture detector (ECD), which has been used most frequently for the analysis of these compounds, is nonspecific and is not amenable t o temperature programming because of column bleed. Also, a number of refractive pairs of polynuclear arenes exist which cannot be resolved on any known column packing. By using a liquid phase fluorometer ( I ) o r a gas phase fluorometer as chromatographic detectors, it is possible to overcome many of these difficulties. This paper describes the basic principles of operation of a gas phase fluorescence detector and its potential for the analysis of polynuclear arenes. EXPERIMENTAL
Gas Chromatography. A Micro-Tek Model MT-160 gas chromatograph (Tracor, Inc., Austin, Texas), equipped with a 13.5 mC 03Ni electron capture detector was connected in series to a n Aminco-Bowman spectrophotofluorometer (American Instrument Company, Silver Spring, Md.) as shown in Figure 1. Polynuclear arenes were separated on the G L C column B a n d measured by the electron capture detector C. The column effluent then passed from the ECD through a heated transfer line into the microflow cell D where the compounds were measured by fluorometry. The signals from the ECD and the SPFD were recorded by the dual-pen recorder G . Glass columns (6-ft X l/a-in. i.d.) packed with 10% Dexsil 300 coated on 8OjlOO mesh Chromosorb W (Tek-Lab, Baton Rouge, La., Catalog No. 40009) were used for compound separation. Column temperatures were varied between 240 and 325 “C using the isothermal mode of operation. The injection port was maintained at 290 “C and the EC detector at 325 “C. The optimum carrier gas was N Pat a flow rate of Electrometer sensitivity was in the range of 90 ml!min. 3.2 X 10-9 to 8.0 X 10-lo AFS. Spectrophotofluorometer. An Aminco-Bowman spectrophotofluorometer (SPF) equipped with a 150-watt, 7.5-A, 17-23 volt dc Hanovia 901C1 Xenon lamp, and IP21 photomultiplier tube was used. A 3-mm i.d. X 5 mm 0.d. X 20(1) M. C. Bowman and M. Beroza, ANAL.CHEM., 40, 535 (1968).
1976
0
mm quartz flow-through cell with 75 mm x 4-mm 0.d. extensions of each end was custom-made by American Instrument Company for this work. The cell compartment was heated by placing two 150-watt cartridge heaters in */,-in. X S/4-in. X 1 l/z-in. brass blocks mounted o n the base plate of the cell compartment. Care was taken t o ensure that the brass blocks maintained maximum contact with the cell adapter inside the cell compartment. In this way, cell temperatures up to 325 “C were easily reached. The heater output was controlled by a Variac. Temperatures were monitored with a thermocouple placed within the cell adapter and connected to the pyrometer of the chromatograph. Slits were not used and all adjustable light apertures were set t o allow for maximum excitation and fluorescence energy. The photomultiplier microphotometer was operated at either one-half or full sensitivity and the meter multiplier (MM) settings varied up t o 0.03. M M settings of 0.01, 0.003, and 0.001 could not be used because of high base-line noise. Transfer Lines. A l/s-in. 0.d. x 14-in. stainless steel tube heated by two 100-watt flexible heating tapes and insulated with asbestos tape served as the transfer line from the EC detector to the spectrophotofluorometer. Swagelok unions (l/s-in,) were used t o connect both ends of the transfer line. At the SPF end, the union nut was soldered t o the cell compartment base plate and both union and nut were reamed out t o accept the extension tube of the quartz flow cell. The cell was sealed into the union using two l/s-in. i.d. silicone “0” rings. Line temperatures were controlled by a Variac, were maintained slightly higher than column temperature, and were monitored with a thermocouple connected to the readout of the instrument pyrometer, Effluent from the SPF was conducted into either the blower exhaust tube of the SPF lamp or a sample collector through a l / l d n . Teflon (Du Pont) tube joined to the exit extension tube of the quartz cell with a Swagelok ’/*-in. X 1/16-in.reducing union and silicone “0” rings. Recorder. A Westronics MT22, dual-pen recorder with 1-mV full-scale span was used to simultaneously monitor the outputs from the EC detector and the SPF. A voltage divider between the photomultiplier microphotometer and the recorder could be adjusted to give full-scale recorder deflections for a given value of 2 to 100% transmission as shown on the microphotometer. A 100-MF capacitor was placed across the recorder input terminals to filter out rapid electrical noise. The voltage divider was normally set so that 2 0 z transmission o n the microphotometer gave full-scale recorder deflection. This converted the recorder from 1 mV fullscale t o 10 mV full-scale. Chemicals. Chemicals used in this work were obtained from a number of commercial sources. Compounds which were measured fluorometrically include phenanthrene, fluorene, anthracene, triphenylene, benzo(a)pyrene, 1,2,5,6-dibenzanthracene, perylene, chrysene, pyrene, fluoranthrene, and benzo(g,h,i)perylene. The compounds were dissolved in benzene at concentcations ranging from 5-100 pg per ml. Excitation and emission wavelengths were adjusted t o predetermined values and 1-5 pl aliquots of the sample were injected. Responses of both the electron capture detector and the fluorometer were ob-
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
Chromatographic columns 6 ft, 3 % JXR on 60/80 Gas Chrom Q 6 ft, 3% OV-1 on 80/100 Chromosorb W
Table I. Separations of Refractory Pairs Achieved on Best Columns Evaluated Carrier Retention times, mins Oven flow, temp, "C ml/min I I1 111 IV 160 160 200 250
90 90 90 90
200 180 190 200
120 100 100 100
4.25
5.25 6.00
5.00 5.80
6.25 13.00 9.00 6.75
6.25 12.75 8.75 6.50
V
VI
11.o
11.o
46.7
46.7
4.0
3.7
4.5
6 ft, 10% DC 200 on 80,' 100 Gas Chrom Q 6 ft, 10% Dexsil 300 on 80/100 Chromosorb W 5 % QF-1 on 80/100 Gas 210 12.0 12.0 90 Chrom Q I Benzo(a)pyrene, I1 Perylene, 111 Anthracene, 1V Phenanthrene, V Chrysene, V I Triphenylene.
Table 11. Wavelength Maxima in Liquid Phase of Polynuclear Arenes Excitation, Emission, Compound nm nm Solvent Phenanthrene 25 l a 356 EtOH 273 356 EtOH 295 356 EtOH Fluorene 215 315 EtOH 265 315 FtOH 29 1 315 EtOH 30 1 315 EtOH Anthracene 248 394 EtOH Triphenylene 25 1 36 1 EtOH 258 361 EtOH 270 36 1 EtOH 282 36 1 EtOH Benzo(a)pyrene 269 406 EtOH 289 406 EtOH 300 406 EtOH 1,2,5,6-Dibenzanthracene 292 405 EtOAc 300 405 EtOAc Perylene 258 440 EtOAc Chrysene 270 37 1 EtOAc Pyrene 244 388 EtOH 277 388 EtOH Fluoranthrene 244 466 EtOH 296 466 EtOH __ Benzo(g,/i,i)perylene 297 422 EtOAc 308 a
422
EtOAc
Principal wavelengths are underlined.
tained o n a dual-pen recorder. The peak arising from the electron capture detector appeared first since the sample reached it first and there was a time lapse before it reached the fluorometer. The method used for measuring gas phase spectra is of some interest. Initially, attempts were made t o scan the spectra by stopping flow of carrier gas while the compound was in the microcell. However, the chromatographic peak decayed too rapidly t o permit good scans. This could have been caused by diffusion of the vapor out of the cell o r possibly by diffusion of oxygen, which quenches fluorescence, into the system. In any event, it was necessary to obtain the spectra by repeated injections of aliquots of standard solutions of the compounds with the wavelength set at different values and measuring peak areas. Peak areas were then plotted against excitation and emission wavelengths. RESULTS
Compound Resolution. A number of solid and liquid stationary phases have been evaluated for the resolution of
polynuclear arenes but none of them are completely satisfactory (2-5). Solid phases such as mixtures of inorganic salts have low column bleed and thus can be temperature programmed using the electron capture detector (5). However, peak profiles and resolution are poor. The most satisfactory liquid phases evaluated in these laboratories include JXR (a methyl silicone), OV-1 (a methyl silicone), DC-200 (silicone oil), and Dexsil 300. The latter is a borane-silicone copolymer which is reported to be stable at temperatures up t o 450 "C (6). It is the most stable liquid phase so far evaluated for the separation of the polynuclear arenes and was used in most of this work. However, it could not be temperature programmed using the electron capture detector, and like other liquid phases is not capable of resolving certain refractory pairs of compounds which occur in each molecular weight group. Thus, it will separate fluorene from phenanthrene and anthracene but will not resolve the latter two compounds (Table I). Other refractive pairs include benzo(a)pyrene-perylene, and chrysene-triphenylene. Of these pairs, benzo(a)pyrene-perylene is the more important since the former is a known carcinogen. The liquid phase fluorometric detector described by Bowman and Beroza ( I ) was fabricated in these laboratories to determine whether optical specificity superimposed on partial chromatographic resolution could be used to obtain a complete analysis of mixtures of polynuclear arenes. All compounds could be analyzed for independently, using a combination of the two methods. F o r example, benzo(a)pyrene has a n excitation maximum at 300 nm and a n emission maximum at 406 nm while perylene has a n excitation maximum at 258 nm and a n emission maximum at 440 nm (Table 11). Hence, by appropriate selection of wavelength settings, it is possible to measure these compounds independently (Figure 2). In addition t o improved specificity, the liquid phase fluorescence detector is more sensitive than the ECD to polynuclear arenes by a factor of 2 t o 10, depending o n the compound. Cas Phase Measurements. The excitation and emission spectra of polynuclear arenes in the gas phase were very similar to those found in the liquid phase. I n the case of anthracene, the excitation maximum was shifted to the violet (2) A. Zane, Tobucco Sci., 12, 18-28 (1968). (3) A. Zane, J. Chromarogr., 38, 130 (1968). (4) S. Katsuya, M. Matsui, and N. Iekawa, Bauriuseki Kuguku, 17, 639 (1968). (5) B. H. Gump, J . Chromarogr. Sci., 7, 755 (1969). (6) R. W. Finch, Alidabs Research Notes, 10, 3 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
1977
YtNT
I
C
A
C
E
Figure 1. Block diagram of SPFD gas flow system A . Injection block. B. Chromatographic column. C. ECD. D. SPFD. E. Microphotometer. F. Voltage divider. G. Dual-pen recorder
by 12 nm and the emission maximum shifted to the violet by 2 nm on going from the liquid to the gas phase (Figure 3). However, relative response in the liquid phase was 180 compared to 32 in the gas phase. Similar results were obtained with other compounds. Losses in sensitivity were somewhat greater than anticipated, and possible reasons for this will be discussed later. Since fluorescence in solution depends o n the nature of the solvent, it seemed probable that the carrier gas used for chromatography could influence emission intensity. This was found t o be the case for all polynuclear arenes studied, although to a different degree. Thus fluorescence intensity of fluorene decreased in the order Nz > He > COz > Hz at a cell temperature of 150 "C and gas flow rates ranging from 90 t o 190 ml/min (Figure 4). The same general order was found for anthrancene except that no significant difference was observed between helium and carbon dioxide. Based o n these observations, a decision was made to use nitrogen for all future chromatography with the reservation that hydrogen would be used if removal of carrier gas by a molecular separator placed between the exit of the gas chromatograph and the inlet of the fluorescence cell was found to increase sensitivity. Probably the observed differences are real although there
Figure 3. Comparison of liquid and gas phase excitation and emission spectra of anthracene A . Liquid phase excitation. B. Gas phase excitation. Liquid phase emission. D. Gas phase emission
C.
is a n outside chance that quenching of fluorescence could be caused by oxygen. Oxygen has a triplet state and its introduction into the carrier gas causes pronounced quenching (Figure 5 ) . However, a fairly large amount of this gas would have t o be present t o account for the results illustrated by Figure 5 , and this seems precluded by the use of high quality cylinder gases. It should be noted also that fluorescence intensity decreases with increased flow rate of the carrier gas (Figure 4). This is true for all carrier gases and all arenes. However, below a flow rate of 90 ml per minute, the peaks begin to tail; so this rate was adopted as standard for all subsequent work. In general, a n increase in detector cell temperature decreases fluorescence intensity. This is true for all carrier With some compounds, the curve relating response t o 60-
50
IA
-
\
"\
40-
$
2 30
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-
-
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Figure 2. A . Response of SPFD to perylene (163 ng) at its wavelength maximum. B . Response of SPFD to benzo(a)pyrene (20 ng) at its wavelength maximum. C . Response of SPFD to perylene at benzo(a)pyrene wavelength maxima. E . Responses of compounds to ECD 1978
20
1
100
1
110
I20
l
130
piow
IAX
0-H.
1
~
140 150 160 @L/MIN)
170
1
Q IO
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160
Figure 4. Effect of carrier gas on detector response to Ruorene at various flow rates
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
= Nn
0 = He
0 =
COZ
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HZ
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90 ml/min 150 degree, C
CELL FLOW: CELL IEMP:
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Figure 7. Concentration of anthracene us. response in gas phase
temperature is linear; in others, it is not. F o r example, in the case of pyrene, a minimum occurs at about 250 "C at a flow rate of 120 o r 140 ml/min (Figure 6). With 1,2-benzofluorene, a change in slope approximating a plateau occurs between 220 and 250 "C. This can be accentuated by increasing the flow rate to 160 ml/min, so there is no doubt that this is real. The decrease in sensitivity of the SPFD t o pyrene on increasing the temperature from 200 to 330 "C is about 47 %. Thus, temperature is a n important but not dominant factor in governing sensitivity. The temperature should be kept as
425
DISCUSSION
40C
The use of a fluorescence detector coupled with gas chromatography makes it possible t o analyze for individual compounds in complex mixtures of polynuclear arenes by combining the great resolving power of the two methods. The fluorometric detectors are generally superior to the ECD with respect t o selectivity and sensitivity. The gas phase detector is superior t o the liquid phase detector in all aspects except sensitivity. The liquid flow system used for scrubbing the compounds from the gas phase can be eliminated, and replenishment of the reservoir of liquid with photometric grade solvent is not required. The need for high boiling liquids for use in high temperature chromatography is bypassed. Omission of the liquid phase should eliminate Raman and Rayleigh scattering of light by the solvent, the residual scattering being due t o the cell walls. Both the liquid and gas phase systems should be amenable to temperature programming, providing that stationary phases are chosen which are not fluorescent and d o not yield fluorescent decomposition products. Sensitivity. Reduction in sensitivity o n going from the liquid to the gas phase was greater than anticipated. This could result from a number of factors including temperature, presence of carrier gas a t atmospheric pressure, or leakage of oxygen into the system. The latter is improbable in kiew of the relatively large amount of oxygen required to cause a major decrease in fluorescence intensity (Figure 5 ) . Quantum yields in the gas phase can increase, decrease, or remain constant with increasing temperature, depending on
302
rn Y
0
& Y
g
low as possible without risking condensation of the compound in the cell. Response of the SPFD to polynuclear arenes is linear if plotted on a log/log scale (Figure 7). The sensitivity of the gas phase detector for several polynuclear arenes is as follows: phenanthrene, 2.6 X g/sec; anthracene, 1.1 X 10-10 g/sec, and fluorene, 1.3 X 1O-Io g/sec. In general, the gas phase detector is equal or superior in sensitivity t o the ECD detector but less sensitive than the liquid phase SPFD under present operating conditions.
200
u Y
IW
11,
2&
2;o
210
2:o
2:o
2o; 2;o A 0 2$, CELL TEMPERATURE C '
2ko
j;rk--&-z~>
Figure 6. Effect of cell temperature and flow rate on gas phase detector response to pyrene (I) and 1,2-benzofluorene (11) A . I at 140 ml/min. B . I at 120 mI/min. C. 11 at 120 ml/min. D . I1 at 140 ml/min
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
1979
the nature of the excited molecules. When quantum yield is very low, there is a tendency for it to increase with temperature. Quantum efficiency is given by the equation
+ g)
(11 where p is the probability for radiative transitions, and g is the total probability for different types of nonradiative transitions. Decreases in quantum yield of vapors of complex molecules with temperature are usually explained by the temperature dependence of g in cases where p is only weakly dependent on T. However, if the probability p increases with T, quantum efficiency will also increase. This was shown t o be true in the case of 9-methylanthracene where 77 increased from 0.027 t o 0.039 as the temperature was increased from 118 t o 350 "C (7). However, in the case of 9-diacetylaminoanthracene, efficiency decreased from 0.060 t o 0.048 within the temperature span of 160 t o 300 "C. Likewise, the fluorescence of P-naphthylamine is quenched with a n increase in temperature due t o a n increase in the probability of nonradiative transitions (8), and with some phthalimide derivatives, 9 passes through a maximum. In the case of 3,6-tetramethyldiaminophthalimide,fluorescence decreases in the temperature range of 100 t o 250 "Ct o about 25 %of its orginal value. The quantum efficiency for the fluorescence of perylene at 240 "C and a n excitation wavelength of 4047 A is about 0.36 compared t o about 0.5 for a solution of this compound in ethanol at ambient temperatures (9). This decrease was only 28 % compared t o 200 found on comparing the liquid and gas phase SPFD detectors, indicating that factors other than temperature are involved. Therefore, the temperature dependence of fluorescence of polyatomic molecules in the vapor phase at temperatures up to 300 "C should not make analytical measurements impractical. In general, increasing temperature causes broadening of the bands, wavelength shifts, and loss of fine structure, but these effects are not sufficiently pronounced t o impair resolution. During this investigation, all gas phase SPF measurements were made at atmospheric pressure of the carrier gas. However, most measurements of the fluorescence of complex molecules in the gas phase made for theoretical purposes have been carried at pressures of the order of Torr. Higher sensitivity is attainable at low pressures through reduction in the frequencies of molecular collision. The population of the lowest excited singlet state of a n organic molecule can be depleted by the emission of a photon (fluorescence), conversion t o the triplet state (intersystem crossing), and radiationless conversion t o the ground state (internal convenion) (IO). Recent measurements of this system crossing in the benzene vapor have shown that in general the three processes are of the same order of magnitude. The interaction of two colliding triplets in the vapor phase may result in the following products (11). 77 = PAP
A >A* + 'A delayed fluorescence --*
'A
4- 'A
+ heat (2)
( 7 ) V. P. Klochkov, Bull. Acad. Sci., USSR, Phys. Ser., 27, 566 (1963). (8) N. A. Borisevich and V. A. Tolkachev, ibid.. 24, 527 (1960). (9) V. V. Gruzinskir, ibid.. 27, 576 (1963). (10) G. L. Powell, J . Cliern. Phys., 47, 95 (1967). ( 1 1 ) G. Finger and A. B. Zahlan, ibid., 50, 25 (1969). 1980
The fluorescence quantum yield of benzene vapor approaches a limiting value of 0.2 at high pressures, and a value between 0.35 t o 0.4 at low pressures (12). The fact that the quantum yield is less than unity at low pressures indicates that even in the absence of a significant number of collisions of the excited benzene molecules, nonradiative deactivation of the excited state can occur. However, some workers have claimed that quantum yield may actually approach unity at very low pressures (13). In addition t o collision between triplets, there is a n overall loss of vibration of excited molecules on collisions with other neutral molecules (14). This has been shown by measurements made on the fluorescence spectrum of benzene at 0.12 Torr, both alone and in the presence of foreign gases at pressures of up to 500 Torr. Energy is lost by two processes. Process A is the departure, due to collisions, from the initial levels responsible for the 2617 A resonance doublet into any of the vibrational states below or not far above this initial level. The rate of Process A thus measures the probability that some energy, however small, is lost as a result of one collision. For Process A, collision numbers range from 37 for the least efficient gas (helium) t o 5 for isopentane. Efficiencies increase in the order of He H? < Nz < COz < 0 2 (quench) < isopentane. Process B is a measure of average amount of energy lost per collision. For Process B, the relative efficiencies in removing energy increase in the order of He < N2 < H z < COn < ether, the range being 1 for helium to 37 for ether. Thus it is evident that sensitivity of the gas phase fluorescence method will depend upon the nature of the carrier gas as well as the total pressure. Generally, reduction of total pressure below 2 Torr results in an increase in fluorescence intensity. This will vary from compound t o compound and in some cases quantum efficiency may approach unity. Reduction of pressure in the fluorescence detector could be achieved by use of one of the interfaces employed to connect gas chromatographs t o mass spectrometers. In this application, pressure must be reduced from atmospheric t o about 10-5 to 10-6 Torr. Several types of separators have been used for this purpose. In one of these (15), the heavy organic molecules pass through two holes of small diameter which are very close together, while the helium gas diffuses t o the side of the separator and is pumped away. This technique increases the ratio of sample t o carrier gas about 100 times when two separators are employed in series. An improved version of the apparatus has now been developed which yields a transfer efficiency of approximately 50 (16). Watson and Biemann (17, 18) have developed a separator which consists of a vacuum chamber containing an ultrafine porosity fritted tube with constrictions at each end. Separation is based upon differences in diffusion rates of the carrier gas and organic molecules across the porous wall. Llewellyn and Littlejohn (19) use a silicone rubber diaphragm t o separate carrier gas from sample components by diffusion. The carrier gas is unable to penetrate the barrier because of its N
(12) M. Nishikawaand P. K. Ludwig, J. Chem. Phys., 52, 107 (1970). (13) G. W. Robinson, ibid.,47, 1967 (1967). (14) L. Logan, M. I. Buduls, and J. G. Ross, "Molecular Luminescence International Conference 1968," E. C. Lim, Ed., W. A. Benjamin, New York, N.Y., 1969. (15) R. Ryhage, ANAL.CHEM.,36, 759 (1964). (16) R. Ryhage, 17th Pittsburgh Confeience on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1966. (17) J. T. Watson, and K. Biemann, ANAL.CHEM.,36, 1135 (1964). (18) Ibid., 37, 844 (1965). (19) P. M. Llewellyn and D. P. Littlejohn, 17th Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1966.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
low solubility in the rubber. A two-stage system is required t o remove all of the helium. More recently, a separator of the Llewellyn type which is of all-glass construction and has only one stage has been developed. A more promising procedure for use in this application appears to be the use of hydrogen as a carrier gas and removal of it by passing it through a palladium silver alloy heated to a temperature of 250 “C(20). Hydrogen diffuses through the palladium and is oxidized at its surface by atmospheric oxygen t o water. At a n outlet pressure of 760 Torr and a n input of hydrogen at a rate of 3.4 atm/ml/sec-l, the pressure in the mass spectrometer increased to 0.1 Torr. Higher outlet pressures could be employed by use of a platinum electrolytic separator (21). This apparatus consists of a heated inner tube connected to the exit line of the gas chromatograph. It is surrounded by a palladium cylinder which serves as a cathode. The space between the two tubes is filled with an electrolyte. Hydrogen gas diffuses through the wall of the inner tube and is oxidized immediately to hydrogen ion. The hydrogen ion is then reduced at the counter electrode. This electrochemical cell provides maximum hydrogen diffusion potential across the separator wall by reducing the effective external hydrogen pressure to zero under all operating and environmental conditions. Thus practically all of the carrier gas is removed without loss of compound. However, some compounds are reduced catalytically at the palladium wall. Thus acrolein yields propionaldehyde in 96.7 yield, and methyl acrylate yields methyl propionate in 97.5 yield. Thus even though compounds may be reduced, quantitative measurements can still be obtained. Benzene was not reduced under similar conditions, indicating that reduction products of the polynuclear arenes may not be formed.
Sensitivity could also be increased through use of a n offaxis ellipsoidal mirror condensing system. This system fits in place of the regular lamp of the Aminco-Bowman spectrophotofluorometer without modification and employs a xenon light source and two mirrors; one is an ellipsoidal mirror and the other is a flat mirror that directs the condensed light through a n adjustable slit into the SPFD. The increase in sensitivity, which occurs in both the excitation and emission spectra, results from the fact that the ellipsoidal mirror gathers the xenon arc light from a large solid angle and concentrates it into a smaller solid angle. The solid angle used is the same as that employed in the standard unit except that it contains more photons of electromagnetic radiations. The increased concentration of photons in turn excites a proportionately greater number of sample molecules and causes increased fluorescence from the same sample under these conditions. It is claimed that sensitivity of the standard SPFD can be increased by a factor of 9 to 12. Automation. The fact that each polynuclear arene has its own characteristic excitation and fluorescence maxima lends specificity to the method. However, this is also inconvenient since these wavelengths must be adjusted for each compound between elution of chromatographic peaks. A program could be developed so that the excitation radiation wavelengths would automatically adjust to the optimum value each time a compound of interest emerged from the gas chromatograph. The emission wavelength and base line would also be adjusted automatically. The basic unit would be a n Automatic Function Selector (Tracor, Inc.), which contains a time-sequencing function control, automatic base-line correction, and graph readout. The Selector would provide the signal and power t o activate servomotors which would position the excitation and emission monochromators at predetermined wavelengths prior to the elution of each peak.
(20) J. E. Lovelock, P. G. Simmonds, G . R. Shoemake, and S. Rich, “Advances in Gas Chromatography, 1970,” A. Zlatkis, Ed., University of Houston Press, 1970, pp 1-7. (21) D. P. Lucero, J. C/zvornatogr.Sci., 9, 105 (1971).
RECEIVED for review July 14,1971. Accepted August 2,1971. Work supported by Grant No. AP00814-03 from the Office of Air Pollution, Environmental Protection Agency.
x
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
0
1981
