Anal. Chem. 1985, 57, 2989-2991
rearrangement does not occur for the perfluoroalkanesulfonates discussed above.
CONCLUSION The fragmentations observed for all of the perfluoroalkanesulfonates involve losses of radicals (CflFZn+J followed by the elimination of C,F2, (eq 4). The negative charge is C F ~ ( C F Z ) ~ - C F ~ C F ~ C F , ( C F ~ )CFdCFz),' ~ S O ~ C ~+ 'CF&FzCFz(CFz)mS03--+
CFz=CFz t 'CFz(CFz)mS03-
(4)
probably localized on the SO3 moiety and not involved in these fragmentations. This chemistry is analogous to the proposed mechanism for thermal decomposition of long chain fluorocarbon polymers. For poly(tetrafluoroethylene),an initial C-C bond rupture occurs to form two radical end groups followed by an unzipping of the polymer chain, with sequential losses of CF2,C2F4,and C,F, groups. It has been estimated that the barrier to thermal depolymerization of the radical fragment is only about 44 kcal/mol. Errede suggests that the actual fragmentation occurs by sequential losses of difluorocarbene, which undergoes rapid recombination to form tetrafluoroethylene in the gas phase (17). The formation of a radical and a distonic radical anion is the analogous process but does not occur for the hydrocarbon analogues of the perfluoroalkanesulfonates because the hydrocarbon products are not stable. Losses of the elements of CnF2fl+2, although observed for collisionally activated perfluoroalkanesulfonates, are much more modest processes than for the carboxylates or alkylsulfates. If the mechanism is similar, then CF bond breaking is required, which is a higher energy process than the corresponding cleavage of a CH bond with the hydrocarbon types. Replacement of only a single fluorine with a hydrogen has a profound effect on the fragmentation. Loss of HF becomes dominant and the other fragmentations exhibited by the perfluoroalkanesulfonatescannot compete. The bond strength of CH is about 25 kcal/mol less than for the CF bond (18). In addition, 135 kcal/mol is released in the production of HF compared to only 37 kcal for Fz. Thus, formation of HF from a hydrogen-containingfluorochemical is favored by about 110 kcal/mol compared to loss of F2 from a fully fluorinated homologue. This would explain the significant differences seen in the CAD spectra of these compounds.
ACKNOWLEDGMENT The authors are indebted to Richard Guenthner and Fred Behr of the 3M Commercial Chemicals Division and George
2989
Tiers of 3M Central Research for their helpful discussions and advice and for providing samples of the commercial fluorochemicals used in this study. The continued assistance and support of Bill Stebbings throughout the various stages of this study is gratefully acknowledged. Registry No. (CF3S03)zBa, 2794-60-7;C2F,S03K,2837-92-5; CdH9SOSK, 29420-49-3; CbH11SO3K, 3872-25-1; CeF1&303K, 3871-99-6; CBF17S03K, 2795-39-3; CloF,iSO3K, 2806-16-8; CizFZ5SO3K, 85187-17-3;CF2HCF2S03Na, 377-30-0;CF3CFHCFzS03Na, 3916-24-3; CSHl1CFHCFzSO3Na, 377-56-0.
LITERATURE CITED Love, L. J. C.; Habarta, J. G.; Dorsey, J. G. Anal. Chem. 1984, 56, 1132A-1146A. Heller, D. N.; Fenselau, C.; Yergey, J.; Cotter, R. J. Anal. Chem. 1984, 56,2274-2277. Roberts, G. D.; White, E. V. Biomed. Mass. Spectrom. 1984, 1 1 , 273-275. Lyon, P. A.; Stebbings, W. L.; Crow, F. W.; Tomer, K. B.; Lippstreu, D. L.; Gross, M. L. Anal. Chem. 1984, 56,8-13. Lyon, P. A.; Crow, F. W.; Tomer, K. B.; Gross, M. L. Anal. Chem. 1984, 56 2278-2284. Gross, M. L.; Chess, E. K.; Lyon, P. A.; Crow, F. W.; Evans, S.; Tudge, H. Int. J. Mass Spectrom. Ion Phys. 1982, 42, 243-245. Crow, F. W.; Lapp, R. L. Presented at the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, MN, 1981. Nagase, F. I n "Fluorine Chemistry Reviews"; Tarrant, P., Ed.; Marcel Dekker: New York, 1967; Vol. 1, pp 77. Stacey, M.; Tatlow, J. C.; Sharpe, A. G. "Advances in Fluorine Chemistry"; Butterworth: Washington, DC, 1960; Vol. 1. Stull, D. R.; Prophet H. "JANAF Thermochemical Tables", 2nd ad.; US. Government Printing Office: Washington, DC, 1971; NSRDS-NBS 37. Mandolini, L. J. Am. Chem. SOC. 1978, 100, 550-554. Capon, B.; McMannus, S. P. "Nelghboring Group Participation"; Pienum Press: New York, 1976; Vol. 1. Jensen, N. J.; Tomer, K. B.; Gross, M. L. J. Am. Chem. Soc. 1985, 107, 1863-1868. Jensen, N. J.; Tomer, K. B.; Gross, M. L.; Lyon, P. A,; "Desorption Mass SDectrometry"; American Chemical Society; Washington, DC, 1985; pip 194-208: Tomer, K. B.; Crow, F. W.; Gross, M. L. J . Am. Chem. SOC. 1983, 105, 5487-5488. Bambaglotti, M.; Coran, S. A.; Giannellini, V.; Vincieri, F. F.; Daollo, S.; Traldi, P. Org. Mass. Spectrom. 1984, 11 577-580. Errede, L. A. J. Org. Chem. 1982, 27, 3425-3430. Darwent, B. deB. "Bond Dlssoclation Energies in Slmple Molecules"; National Bureau of Standards: Washington, DC, 1970. I
I
RECEIVED for review May 13,1985. Accepted August 8,1985. This work was supported by 3M and the Midwest Center for Mass Spectrometry, a National Science Foundation Regional Instrumentation Facility at the University of NebraskaLincoln (Grant CHE8211164).
CORRESPONDENCE Trace Detection Based on Chemical Amplification of the Optoacoustic Effect Sir: The gas-phase optoacoustic effect is most frequently produced by irradiating an infrared-active gas with modulated radiation from a laser. The sensitivity of the optoacoustic effect, when used in conjunction with a high-power infrared laser, has proven to be so high that there has been wide application of this effect to pollution monitoring (I-3), trace detection (4),and high-resolution spectrometry (5-7). In contrast to excitation in the infrared, generation of the optoacoustic effect in the visible or ultraviolet region of the
spectrum opens up the possibility of initiation of chemical reactions such that the energy released by the reactions far exceeds the amount of energy absorbed from the light beam. For instance, in mixtures of H2and Clz irradiated with 476-nm radiation from an Ar ion laser (8),although the optoacoustic effect is initiated by a simple photodissociation involving the absorption of a single quantum (hv)of radiation
Clz
+ hv
0003-2700/85/0357-2989$01.50/00 1985 American Chemlcal Society
+
2C1
(1)
2990
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
(a 1
ACOUSTO-
- 70
CHROMATOGRAPH
'7 F] '
CH4
1
- 50 ->
STRIPCHART
E
Y
-I
P
SIGNAL
-12
v)
OSCILLATOR
Flgure 1. Apparatus for detecting trace species elutlng from a gas chromatograph. For detection based on amplification of the CI, signal, N, is used as the carrier gas; for detection through quenching of the acoustic signal, the carrier gas is switched to H., LASER OFF-
the deposition of heat in the spectrophone cell is governed by repetition of the exothermic chain reaction sequence
-
C1+ H2 HC1+ H
H + C1,
-+
(2)
HC1+ C1
The gain in acoustic signal amplitude of several thousand over what is obtained in pure Clz suggests the use of the photochemically generated optoacoustic effect as a trace detection method for compounds capable of undergoing chain reactions. A further property of chain reactions is that they are extremely sensitive to the presence of small quantities of substances that react with the radicals responsible for chain propagation. With the Hz + Cl, reaction, for example, any species that acts to scavenge either H or C1 radicals terminates the reaction sequence (2), drastically reducing the chain length (9,lO). Since the magnitude of the optoacoustic effect is proportional to the amount of heat liberated by the chain reaction, the effect of the addition of a radical scavenger is to quench the chemical liberation of heat in the spectrophone and hence to decrease the acoustic signal amplitude. We report here a new highly sensitive method of trace detection using both chemical amplification and quenching of the optoacoustic effect. In the first scheme (see Figure l),Cl, and the effluent from a gas chromatograph (using N, as the carrier gas) are flowed into a small cylindrical Teflon cell. (Although energy transfer between vibrationally excited HC1 and Nz is expected, the vibrational relaxation time of either gas is sufficiently short at 1 atm that the heat release into translational motion of the gas should closely follow the production of HC1 by the chemical reaction.) An Ar ion laser operated a t 488 nm is amplitude-modulated with a modulation depth of 1using an acoustooptic light modulator (Coherent, Inc., Model 304). The dimensions of the cell are such that the frequency of the first acoustic resonance is much higher than the modulation frequency of the laser. The background signal is caused by photodissociation (11) of Cl,, thus, there is little advantage in modulating the laser a t the frequency of a cell resonance as this would simply increase the amplitude of the background and the desired signal together. (This follows only in cases where there is a high signal-to-noise ratio in the background signal; otherwise amplification of both the signal and the noise by an acoustic resonance is beneficial.) Acoustic signals are detected with an electret microphone having a built-in preamplifier (Radio Shack, Inc., Model 270-092A) whose output is fed to a lock-in amplifier. The magnitude of the lock-in signal is then displayed on a stripchart recorder.
~ A S E ROFF-
I
0
I
2 4 TIME(min) -+
0
I
2
, o 3
TIME (min) + (a) Chromatogram of 8 nmol of CH, injected onto the chromatograph to give an amplified signal. The chromatography column is a 2 m long X 3.1 mm diameter carbon molecular sieve (Spherocarb). The laser modulation frequency is 25 Hz. (b) Chromatogram of 4 nmol of NO showing quenching of the amplified optoacoustic signal. The trace marked H, + Ci, is the lock-in amplifier signal resulting from chemical amplification in the H2 + CI, mixture at 100 mW of laser radiation. The column is a 2 m long X 3.1 mm diameter aluminosilicate molecular sieve 5A. The laser is modulated at 40 Hz. In each chromatogram a base line is recorded with the laser off, which shows the noise originating from turbulence in the detection cell and from the microphone preamplifier to be negligible on the gain scales used here. Modulation frequencieswere chosen to be low in order to increase the magnitude of the acoustic signal but not so low as to degrade response time. Flgure 2.
Table I. Detection Limitsa compound
detection limit
sensitivity
Amplification hydrogen methane ethane propane ethylene acetylene ethylene oxide* vinyl chloride*
2.0 nmol 610 pmol 410 pmol 370 pmol 330 pmol 290 pmol 2.0 nmol 820 pmol
67 pmol/s 20 pmol/s 14 pmol/s 12 pmol/s 11 pmol/s 9.7 pmol/s 40 pmol/s 33 pmol/s
Quenching nitric oxide oxygen propylene
2.0 pmol 57 pmol 21 pmol
80 fmol/s 2.9 pmol/s 530 fmol/s
The detection limits are taken where the signal-to-noise ratio is 2 with a 1-s time constant on the lock-in amplifier. The incident laser power is 10 mW. The sensitivity is computed by dividing the detection limit by the elution time. An asterisk indicates that the comoound is a carcinogen.
A typical response of the detector to injection of CH4 is shown in Figure 2a. With C1, and Nz eluting from the chromatograph into the cell, the optoacoustic signal amplitude is small. When CH, enters the spectrophone, the exothermic chain reactions induced by the laser radiation give greatly enhanced signals. In general, the magnitude of the optoacoustic signal is dependent on both the amount and nature
ANALYTICAL CHEMISTRY, VOL. 57, NO. 14, DECEMBER 1985
f io' 0
100
io5
lo-'
10-2
IO0
L A S E R POWER (W)
Flgure 3. Acoustic gain as a function of incident laser power for several compounds. The gain is defined as the signal amplitude above the base line divided by the CI, base line. The CI, and N, flow rates a r e 10 and 20 cm3/min, respectively. For each of the compounds, 200 nmol Is injected onto the chromatography column. The laser modulation frequency is 25 Hz.
of the compound injected onto the column. The detection limits for several hydrocarbons and Hzare shown in Table I. Signals are found to be proportional to the quantity injected onto the chromatograph over a range of approximately 3 orders of magnitude. The dependence of the gain in signal amplitude on laser power, on the contrary, is not linear. As Figure 3 shows, the optoacoustic gain in the cell decreases as the laser power is increased. This follows as a result of the termination mechanism for chain reactions 2C1 M C12 M (3)
+
-
+
where M is a third species (either Hz, Clz,Nz, or a hydrocarbon in the case of this experiment). As the laser power is increased the concentration of C1 also increases thereby decreasing the mean C1 radical lifetime in the cell. This results in a reduced chain length (121, and consequently, the highest detection sensitivity is found at low laser power. The reactions in eq 2 describe a simple case of a two-center chain reaction. For hydrocarbons, sequential chain reactions are possible where the parent molecule becomes more highly chlorinated in successive chain reactions with atomic chlorine. Since the optoacoustic signal is proportional to the product of the photochemical chain length and the energy liberated per chain cycle (121, it is not surprising that all the hydrocarbons listed in Table I give larger optoacoustic signals than H2 (compared on a per-mole basis). For alkanes, the signal amplitudes are found to increase in the order CH4 < CzH6< C3H8. larger signals are also found in progressing from single to double to triple bonds in the compounds CzH6,CzH4,and (32%
For detection based on quenching, a large chemically amplified optoacoustic signal is produced by using H2 as the carrier gas in the chromatograph. As shown in Figure 2b, injection of NO onto the column causes a marked decrease in the acoustic signal amplitude. Quenching takes place as a result of the scavenging of C1 radicals (13-15)through the rapid reaction
-
C1+ NO NOCl followed by the termination reaction C1+ NOCl NO C12
+
2991
The first of these taken alone, or followed by the second reaction, drastically shortens the photochemical chain length. Of note is the slight increase in gain in the system as NO is flushed from the cell. This is tentatively ascribed to a small perturbation in the C12-to-Hzratio in the cell, which changes the acoustic signal amplitude. The mechanism for quenching by O2 and propylene has been attributed (16-22) to scavenging of H atoms, thus eliminating further chain propagation. Of further note is that the acoustic signal amplitude is insensitive to gaseous CO, NzO, and COz (in accord with their low reactivities (14)),thus rendering this detection scheme highly selective. In fact, the detection sensitivities obtained here are in several cases superior to those found for flame ionization, thermal conductivity, and electron capture detectors frequently used in gas chromatography. The high sensitivity of the optoacoustic effect combined with its capability for rapid monitoring of the progress of a chain reaction suggests its application not only to similar trace detection schemes based on different chemical systems but also to a more direct study of basic kinetic mechanisms of chain reactions by direct measurement of the influence of radiation intensity, chemical composition, and temperature on the optoacoustic phase lag and amplitude.
ACKNOWLEDGMENT The support of this work by the U.S. Department of Health and Human Services under Grants CA29912 and ES03810 is gratefully acknowledged. The use of the Ar ion laser supplied by the Office of Basic Energy Studies of the US.Department of Energy is also acknowledged. Registry No. Hz,1333-74-0;CH4,74-82-8;C2Hs,74-84-0;C3H8, 74-98-6;CzH.4, 74-85-1;C2H2,74-86-2;NO, 10102-43-9;ethylene oxide, 75-21-8;vinyl chloride, 75-01-4; oxygen, 7782-44-7;propylene, 115-07-1.
LITERATURE CITED (1) Kreuzer, L. B.; Kenyon, N.; Patei, C. K. N. Science 1972, 777, 347. (2) Patei, C. K. N. Science 1978, 202, 157. (3) West, G. A.; Barrett, J.; Siebert D.; Reddy K. Rev. Sci. Instrum. 1983, 54, 797. (4) Kreuzer, L. B. Anal. Chem. 1978, 50, 597A. (5) Marinero, E. E.; Stuke, M. Opt. Commun. 1979, 30, 349. (6) Inguscio, M.; Moretti, A.; Strumia, F. Opt. Commun. 1979, 30, 355. (7) Di Lieto, A.; Minguzzi, P.; Tonelll, M. Opt. Commun. 1979, 37, 25. (8) O'Connor, M. T.; Dieboid, G. J. Nature (London) 1983, 307, 321. (9) Benson, S. "The Foundations of Chemical Kinetics"; McGraw-Hill: New York, 1980; Chapter X I I I . (10) Laidler, K. J. "Chemical Kinetics"; McGraw-Hili: New York, 1965; Chapter 8. (11) O'Connor, M. T.; Dlebold, G. J. J . Chem. Phys. 1984, 87, 812. (12) Diebold, G. J.; Hayden, J. S. Chem. Phys. 1980, 49, 429. (13) Hippler, H.; Troe, J. Int. J. Chem. Klnet. 1976, 8, 501. (14) Fettis, G. C.; Knox, J. H. Prog. React. Klnet. 1964, 2 , 1. (15) Ashmore, P. G.; Chanmugam, J. Trans. Faraday SOC. 1953, 49, 254, 265, 270. (16) Kaufman, F. Frog. React. Kinet. 1961, 7 , 1. (17) Benson, S. The Foundations of Chemical Kinetics"; McGraw-Hiii: New York, 1960; p 105. (18) Burns, W. G.; Dainton, F. S. Trans. Faraday SOC. 1952, 48, 39. (19) Hikida, T.; Eyre, J.; Dorfman, L. J. Chem. Phys. 1971, 54, 3422. (20) Kaufman, F.; Gerri, N.; Pascale, D. J . Chem. fhys. 1958, 24, 32. (21) Trotman, A. F.-Dickenson, Milne, G. S., Eds. "Tables of Blmolecular Gas Reactions"; NSRDS-NBS 9, US. Department of Commerce. (22) Thrush, B. A. Prog. React. Kinet. 1985, 3, 63.
Joong-Gill Choi Gerald J. Diebold*
(4)
Brown University Department of Chemistry Providence, Rhode Island 02912
(5)
RECEIVED for review May 20, 1985. Accepted July 31, 1985.
