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Anal. Chem. 1087. 59. 1475-1478
reported by Peerce and Bard (5). Addition of 40% TATM to the film increased the useful lifetime of the electrode up to 24 h if immersed in the solvent continuously and 2 days if removed after a 7-h period and stored dry in air. PVFc/Fc+ films in aqueous solutions have been observed to oxidize even after deaeration (5). This oxidation resulted in a large shift (150 mV overnight) in the potential of the ferrocene/ferricenium couple. By cross-linking the polymer, oxidation is retarded, and the potential remains stable within 4 mV for a t least 6 h. In solvents where ferrocene is highly soluble, such as dimethylformamide (DMF), PVFc/Fc+ alone shows a significant drift (580 mV) in potential within 15 min (5). The addition of 40% TATM stabilized the potential for at least 30 min. In this particular case, increasing the amount of TATM to 90% resulted in electrodes that were stable to better than f5 mV for up to 3 h. For this solvent, the stability of the films increased with the addition of more than 40% TATM cross-linking agent to the film. Comparison of the weight difference of polymer film electrodes before and after immersion in DMF for 20 min showed that a larger percentage of the polymer remained intact with increasing amounts of cross-linking material. Electrodes were air dried overnight, dried 30 min under vacuum, and air-dried another 6 h before the weight of the remaining polymer was determined. Although the films appeared most stable to DMF at very high concentrations of TATM, it could be shown that these highly cross-linked films were far less permeable to counterions in solution and consequently exhibited higher uncompensated resistance and more diffusion-controlled behavior. Cyclic voltammetric measurements of films containing varying amounts of TATM showed that the separation between anodic and cathodic peak potentials increased as a function of the degree of cross-linking. Normally, the difference in peak potentials for a surface-bound species, as is the case for the PVFc/Fc+ redox couple, should approach zero. However, with 90% TATM/benzoyl peroxide added to the film the peak separation is increased to 126 mV. Of course, since negligible
current is passed through the electrode when used as a reference, this decreased permeability has little effect on its performance. Although the stabilization procedure described here is a significant improvement over previous attempts to provide a reliable reference electrode for nonaqueous electrochemistry, further investigation is necessary in order to determine whether this procedure has been fully optimized. For example, recent preliminary studies in this laboratory have shown that other cross-linking agents, such as glycerol propoxytriacrylate, stabilize polymers toward solvent degradation more efficiently than does the TATM. While the emphasis here has been placed on the nonaqueous applications of these electrodes, it has been suggested (8)that, because small wires can be modified equally well by these thin polymer films, such reference electrodes could also be used in microsensor applications where their small size would be of value. Registry No. PVFc, 34801-99-5;(VFc)-(TATM)(copolymer), 107327-59-3;Pt, 7440-06-4; K2Ru(bpy),,63950-81-2;ferrocene, 102-54-5;potassium ferricyanide, 13746-66-2.
LITERATURE CITED (1) Ives, D.; Janz, G. Reference Electrodes; Academic: New York, 1969; pp 110-111, 333-335. (2) Sawyer, D.; Roberts, J. L. Experimental Nectrochemistry for Chemists; Wiley: New York, 1974; pp 53-60. (3) Mann, C. K.; Barnes, K. K. Electrochemical Reactions in Nonaqueous Solvents; Marcel Dekker: New York, 1970; pp 17-23. (4) Grltzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461-466. (5) Peerce, P.; Bard, A. J. J . Nectroanal. Chem. 1960, 108, 121-125. (6) Biubaugh, E. A.; Bushong, W. C.; Shupack, S. I.; Durst, R. A. Anal. Left. 1986, 19, 1777-1785. (7) Chen, Y. H.; Fernandez-Rofojo, M.; Cassidy, H. G. J . Polym. Sci. 1959, 4 0 , 433. (8) Bard, A. J. University of Texas, Austin, personal communication, 1986.
RECEIVED for review August 28,1986. Accepted January 27, 1987. Certain commercial products are identified in order to adequately specify the experimental procedure. This does not imply endorsement or recommendation by the National Bureau of Standards.
Sample Introduction for Capillary Gas Chromatography with Laser Desorption and Optical Fibers Janusz Pawliszyn* and Shi Liu Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322 Capillary gas chromatography is able to analyze very complex mixtures due to the high separation efficiency of capillary columns (I). However, the resolution which is achieved can be limited by the sample introduction technique (2). In order to prevent this apparent peak broadening, various new injection schemes have been developed such as split injection, programmed temperature vaporization and on-column injection (3). However, high-speed gas chromatographic methods using short and small-diametercolumns require even more rapid techniques of sample introduction ( 4 , 5 , 2 4 ) . In addition, recently introduced high-temperature gas chromatographic capillary columns and associated techniques require methods which can rapidly vaporize low-volatility analytical material without significant decomposition (20). In this paper we describe rapid laser desorption as a sample introduction scheme for gas chromatographic analysis. A pulsed laser beam focused onto a surface is able to desorb even such nonvolatiIe sampIes as graphite (6). The degree of fragmentation and ionization of organic matter desorbed from 0003-2700/87/0359-1475$01.50/0
the surface is directly proportional to the power density of the laser pulse. Above lo9 W/cm2, the sample is mostly atomized and ionized. However, below lo6 W/cm2 the laser pulse is able to effectively desorb intact large nonvolatile compounds (7,8). Below lo6 W/cm2 the amount of ionized species with respect to the total weight of the desorbed material is smaller than The exact mechanism of laser desorption is unknown, but it is generally assumed to be a combination of two effects. First, the laser beam striking a thin target creates a small hole by removing the molecules in its way. The energy loss of the beam is used to oblate the target material. This process is very fast and lasts only during the laser pulse (9). Second, a significant fraction of molecules are desorbed through the thermal process, if the thickness of the target is too large for laser beam penetration (IO). In this process the period during which the molecules are emitted approaches a fraction of a millisecond (11). In real experimental configurations both effects occur simultaneously and the dominant one is related 0 1987 American Chemical Society
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to the exact design of the experimental system. The high-energy laser pyrolysislpacked column gas chromatography has already been investigated as a tool in analysis of nonvolatile organic compounds (12),polymers (1.3,and coal (14). This method provides simple and easy to interpret fragmentation patterns with the increase presence of higher molecular weight components with laser intensity decrease (15). However, rapid laser desorption has not yet been investigated as a sample introduction method for gas chromatography analysis. In our experimental arrangement the sample is desorbed with the help of a flexible optical fiber. Optical fibers have already been used to transmit high-energy laser beam pulses to the target in medical applications for destroying kidney stones (16)and for evaporating cholesterol deposits in veins and in the field of optical fiber sensors (17).
EXPERIMENTAL SECTION The experimental design consisted of a flashlamp pumped dye laser Model DL-1200V (Phase-R Corporation,New Durham, NH) operating at about 500 nm and using Coumarin 498 dye (Exciton, Dayton, OH). The laser light was coupled to about 5 m of 100-pm core UV transmitting optical fiber (Spectran Corporation, Sturbridge,MA) by using a precision fiber coupler (Model F-915T, NRC, Fountain Valley, CA). The optical fiber transmitted light into the HP 5890 gas chromatograph equipped with a flame ionization detector (Hewlett-Packard, Palo Alto, CA). In most of the experiments the end face of the optical fiber was etched in order to increase wettability of the surface. The etching was accomplished either by using solutions of NaOH or HF in water. The end of fiber was then carefully rinsed in chloroform,acetone, and methanol and dried at 300 OC for a few minutes. In polymer pyrolysis experiments, the end of optical fiber was additionally immersed in n-octyltrichlorosilane (Petrarch Systems, Bristol, PA), washed again in organic solvents, and dried. Such a prepared optical fiber was inserted into a 1.50-m length of 200-pm-i.d. high-temperature capillarycolumn (bonded methylsicone, 0.1-pm film, Quadrex, New Haven, CT). In some experiments the duck bill valve septum and on-column injection liner (18) were used to allow convenient placement of the fiber into the capillary column. In all experiments the injector heating was turned off. In the initial experiments a short piece of a 20-cm fused-silica tubing was used to connect the injector and the detector ports. The optical fiber with the sample was then inserted approximately 5 mm below the detector. Three compounds varying in their volatility were used as samples: triethylene glycol (Matheson, Coleman and Bell, Norwood, OH); poly(ethy1eneglycol) average molecular weight 400 (Fisher Scientific Company, Fair = 1540 (Carbowax)(WilkensInstrument and Lawn, NJ) and Research, Inc., Walnut Creek, CAI. The samples were transferred onto the fiber by careful immersion of about 1 mm of the fiber tip into a 10% solution of the compound in chloroform. The excess of the sample was then wiped off the fiber surface using a Kimwipe tissue and the solvent was completely removed by blow drying at 50 "C. Power density of the laser pulses was wtimated from the laser operational characteristics and by using the photodiode supplied with the optical filters and a precision electronic integrator. The laser power densities at the end of optical fiber in the desorption experiments varied between lo6 to lo6 W/cm2. For the pyrolysis experiments the power density was about 3 orders of magnitude higher. In chromatographic analysis of the laser-desorbedsample, the poly(ethy1ene glycol) 400 was desorbed into the capillary column from the optical fiber which had been carefully inserted into the column. This fiber was then removed before temperature-programmed analysis. Similarly, a fraction of a nanoliter of the sample was "on-column" injected by using 50-pm4.d. fused-silica tubing inserted into the capillary column and the 1-pL syringe. The "wet-needle" injection was performed by inserting the optical fiber into the capillary column followed by temperature programming without a laser desorption pulse. The amount of sample spilled into the stationary phase during the insertion and withdrawal of the fiber was a very small fraction of the desorbed
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Flgure 1. Experimental configurations: (A) laser desorption of the sample from the end of optical fiber; (B) laser desorption of the sample
from the stationary phase. sample. If the sample was adsorbed on the fiber surface from the diluted solution, the quantity of the spilled sample was below the detection limit of the instrument. If on the other hand concentrated solutions had been used, a very broad low-intensity unresolved peak would have appeared in the chromatogram as a base-line drift. The amount of sample desorbed by a single laser pulse was in the range 10-0.5 ng. In these experiments temperature programming of 60 deg/min was used and the flow rate of the carrier gas, hydrogen, was at 1.5 mL/min. In the experiments involving the desorption of sample components from the stationary phase, the optical fiber was inserted into the capillary column and was located about 2 cm from the injection port. The constant concentration of the headspace sample in hydrogen was supplied in a similar fashion as described in the ref 23. In these experiments the oven temperature was kept isothermally at 200 "C and the H,carrier gas velocity was 60 cm/s. In some experimentsan IBM-compatiblecomputer was used to trigger the laser and collect the data using a DT 2800 data acquisition and control board (Data Translation, Marlboro, MA). The appropriateprogram was written by using a scientific software "ASYST" (Macmillan Software, New York, NY).
RESULTS AND DISCUSSION Even a moderate power density laser pulse (below lo8 W/cm2) is able to desorb several micrograms of sample from a surface (7). High sensitivity of common chromatographic detectors such as a flame ionization detector (FID) allows quantitization of sample components in the picogram range. Therefore, laser desorption, with its very attractive features (fast and nondestractive vaporization of nonvolatile samples) discussed in the introduction, should be a useful tool as a sample introduction scheme in rapid high-efficiency capillary chromatography. The experiments reported in this paper confirm these expectations and describe some basic experimental procedures required in practical application of this technique. Our experience indicates that desorption from the optical fiber is a simple and convenient method for laser beam sample desorption into a chromatographic system. The sample can be placed directly at the end of the fiber as a thin f ilm (Figure lA), or the light coming out of the fiber can be pointed at targeta such as a stationary phase (Figure 1B). These schemes of rapid laser desorption generate very narrow injection "plugs". The first step of the investigations was to confirm the practical feasibility of this sample introduction approach. The three samples were used, varying in their volatility: triethylene glycol and poly(ethy1ene glycol) 400 and 1540. They were desorbed from the end of the fiber directly into the FID detector. In all cases, the outcome was identical: a sharp, high-intensity peak with a half-width corresponding to the time response of the detector (about 300 ms). This experiment confirms our expectations regarding the rapid nature of the
ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987
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Fmre 3. Chromatogram of the volatile species produced during laser pulse pyrolysis of poly(n-octyltrichlorosilane) chemically bonded to the “tip” of optical fiber. Column 1.5 m X 0.2 mm, 0.1 pm film, bonded methylsilicone.
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Figure 2. Chromatogram of the poly(ethy1ene glycol) average molecular weight 400. Sample Introduced by (a) laser pulse desorption from the optical fiber and (b) “oncdumn” InJection. Chromatographic conditions are identkal for both experknents: column 1.5 m X 0.2 mm, 0.1-pm film, bonded methylslllcone; temperature programming, 50 deg/min from 200 to 400 OC.
sample vaporation process already investigated carefully by the researchers using mass spectroscopic detection (IO, 11,19). However, there was an interesting difference between these samples.Although both triethylene glycol and poly(ethy1ene glycol) MW = 400 were desorbed quantitatively from the optical fiber in one laser shot, the poly(ethy1ene glycol) MW -1540 was still present when a second pulse was fired. These results indicate that quantitative removal of the sample from the “tip” of optical fiber is possible for relatively low-volatility compounds which are of interest to gas chromatographers. However, if the molecular weight is very large, the removal of the sample is not complete. In the next part of our investigation we studied chromatograms of the desorbed sample in order to further examine nondiscriminating properties of this method toward lowvolatility components of the sample. Figure 2a shows the chromatogram of poly(ethy1ene glycol) 400 obtained with the laser desorption injection. The sample was placed at the end of the optical fiber and it was inserted deep into the column, above 2 cm from the injection port in the chromatographic oven. In this experiment a short piece of Quadrex high-temperature column (1.5 m) was used with the fastest possible temperature programming (60 deg/min). The total analysis time was only about 4 min, but the peaks were well separated. This can be attributed to the very narrow plug obtained in the laser vaporization technique. Quadrex columns are known to separate such nonvolatile samples, but the total time of
analysis of the poly(ethy1ene glycol) 400 is then about 15 min and it requires a 20-m column when the programmable-temperature vaporizer (PTV) is used (20). The rapid separation shown in Figure 2a using fast temperature programming and a short column, with an optimum carrier gas flow (21),is an example of a new approach to high-speed gas chromatography. This differs from the usual use of high flow velocities of the carrier (422). Figure 2b shows the chromatogram corresponding to “on-column”injection of a few nanoliters of glycol 400 performed from 50-pm-i.d. fused silica tubing. The chromatographicconditions were kept the same as those given in Figure 2a. Clearly, the peaks are not well resolved. We associate this result with the fact that although the droplets of the sample were injected within a very short piece of the column, the evaporation process for the respective sample components was not uniform due to the droplets size, which lead to a broad chromatographic band. On the other hand, after the laser pulse the sample’s nonvolatile componentswere transferred into the stationary phase as a narrow uniformly distributed plug. In order to reduce this problem, a wet-needle-typeinjection was performed. The optical fiber with uniformly adsorbed sample was inserted into the chromatographic column in the same manner as in the laser desorption experiment. However, no laser pulse was fired and the fiber was left in the column throughout the experiment. This type of injection produced a chromatogram similar in resolution and peak intensity distribution to that shown in Figure 2a. The variation in the sample size injected by laser desorption did not have an affect on the ratio of the peak areas. Therefore, the laser desorption technique appears to be a nondiscriminatinginjection method. Figure 2a corresponding to laser desorption injection does not show any peaks associated with the pyrolysis of the sample. However, pyrolysis experiments can also be performed on the face of an optical fiber by increasing the laser pulse density power. Figure 3 shows the isothermal chromatogram corresponding to volatile pyrolysis products produced during the high-density pulse desorption of a polymer chemically bonded to the silica surface [poly(n-octyltrichlorosilane)].The chromatogram indicates the highly cross-linked nature of this polymer. The peaks are not well separated but this situation can be improved by using fast temperature programming. An interesting feature of this chromatogram is the sharp peaks and short separation time (10 s). It is clear that the
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 10, MAY 15, 1987 i
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- .. Flgure 4. Chromatogram of the headspace mixture (decane,aniline,
octanol, N,Ndimethylaniline, camphor, naphthalene, and menthol) desorbed from the stationary phase. laser pulse introduced a very sharp band at the front of the column (about 1mm width). This fact can be used to desorb the head-space sample from the sorbent trap surface. Recently, it was shown that the stationary phase can be used effectively as a trap (23). In these experiments, the sample components were desorbed from the stationary phase by the heating pulse produced by resistive heating. In our experiments we replaced resistive heating with laser pulse desorption of the sample components from the column's stationary phase. The fiber which supplied the laser light was located inside the capillary column (Figure 1B). Figure 4 shows a chromatogram of a seven-component headspace test mixture. The good separation of this multicomponent mixture was obtained in about 10 s. Similar to the chromatogram in Figure 3, the peaks are narrow. In fact, it is clear that the limiting factor in resolution of the f i s t six peaks in this chromatogram is the time response of the detector (about 300 ms). For example, the theoretical width of the first peak (Figure 3) is only about 50 ms according to the Golay equation. An important distinction should be made between laser and resistive heat desorption of the sample from the stationary phase. In thermal desorption, a relatively long period (>lo0 ms) of the front column heating generates the sample plug due to a "zero-retention gap". The laser pulse, on the other hand, produces a rapid (
