On-Line Laser Pyrolysis Cell for Chromatography J. P. Biscar Department of Physics, University of Wyoming, Laramie, Wyo. 82070
USE OF PULSED LASERS for analytical pyrolysis is rapidly increasing. Despite their higher price, this is due to the fact that lasers present remarkable advantages over hot wires, heated tubes, and cups or even high voltage electrical discharges. The output energy of a laser in conventional mode can be made stable within 1% and so the sampling output energies are available from a few joules to hundreds of joules if needed. The energy can be concentrated on any desired area of the sample providing a flexible power density and temperature control. Temperature rise times are relatively fast, typically of the order of one-half millisecond. While lower temperatures are easy to achieve, the laser can produce much higher temperatures than other conventional means. In Q-switched mode, it can break down, through multiphoton ionization, even transparent materials such as sapphire, quartz, etc. We have chosen a water-cooled ruby laser for our experiments. The ruby crystal (4 in. X 9/16 in. of Superior Internal Quality from Union Carbide) is optically pumped by a helical flash lamp surrounded by ceramic reflectors. A quartz roof prism and a sapphire etalon completed the optical cavity. Up to 5000 joules under 5 kV are available from a voltage regulated power supply. Although the ruby could produce 30 joules output, energies in the range of 5 to 10 joules were found to be sufficient for this investigation. The laser shots were separated by exactly six minutes, the time needed to analyze the produced gases. The coupling to the gas chromatograph is such that a new sample was ready by the time the separation was achieved by the column. In earlier works, the degradation was done in a vessel from which sampleswere taken with a syringe and injected into a gas chromatograph ( I ) . A modified syringe (2), or a quartz or Vycor tube have also been connected at the inlet of a chromatograph (3). A system which contains an optically flat surface for the laser entry has also been described (4). The main problem with these devices is that the glass or quartz surfaces through which the laser is focused into the cell are too close to the sample. These surfaces can be damaged by high laser power densities. Besides, as soon as the sample starts to evaporate, the window fogging renders the laser sampling not reproducible enough for quantitative analysis without mentioning the cleaning of the cell after each shot and the frequently required cell replacement. The necessity to confine the pyrolysis products to a small volume in order to obtain sharp peaks is the origin of these difficulties. We have solved these problems with an on-line pyrolysis cell equipped with an electromagnetic valve. A sketch of the cell is shown on the insert of Figure 1. The cell we have built and the system described on Figure 1 present several advantages: (1) R. H. Wiley and P. Veeravgu, J. Phys. Chem., 72, 2417 (1968). (2) 0. F. Folmer and L. V. Azarraga, “Advances in Chromatography,” A. Zlatkis, Ed., Preston Technical Abstracts, Evanston, Ill., 1969. (3) W. T. Ristau and N. E. Vanderborgh, ANAL.CHEM.,42, 1848 (1970). (4) B. T. Guran, R. J. O’Brien, and D. H. Anderson, ibid., p 115. 982
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The cell is all metal with the exception of one quartz window. It is reliable with easy maintenance and is very simple to use. The quartz window, which is six inches from the sample, never becomes contaminated by pyrolysis products or deteriorated by the laser beam. An electromagnetic valve closes within a few milliseconds after the laser pulse preventing the pyrolysis products from expanding into the total volume of the cell, thus providing sharp peaks through the chromatograph. Because the laser pulse energy is constant it is likely that the volume sampled at each shot is the same as well as the amount of gases expanding toward the window when the electromagnetic valve is closing. These gases trapped between the valve and the window are not analyzed to avoid a large tailing of the peaks. They are removed by pumping for the next cycle. As soon as the pyrolysis products have reached the entrance of the column, the six-way valve is turned to position 2, placing the cell out of the He gas carrier circuit, without interrupting the chromatogram in progress. The sample is then replaced, the cell evacuated and reconnected to the He flow by turning the six-way valve to the position 1. The system is ready for a new laser shot as soon as the last peak is recorded. The sample replacement can be achieved mechanically because the sample holder is designed to be an hydraulic piston. With all the valves being electromagnetic, the entire system can be programmed to run automatically a large number of samples, When powder samples are to be analyzed, the laser bench is set vertically. The pyrolysis cell was connected to a Perkin-Elmer gas chromatograph Model 800. It was mainly used to rapidly determine the hydrocarbon content of oil shales. The sample can be a cut of shale or powdered shale. Often the powder was pressed into a pellet of lis inch in diameter and inch thickness appropriate for mechanical sample replacement. A laser beam of approximately six joules, in conventional mode, was focused by a quartz lens of ten inches of focal length on the surface of the sample placed at 9.75 inches from the lens. Figure 2 shows a typical chromatogram obtained from oil shale pellets. The largest peak represents acetylene preceded by ethylene and methane. Natural gas and retort gases contain mainly methane and small quantities of ethane and propane. The acetylene output is an indication of high temperatures achieved under present laser pyrolysis conditions. The fast closing of the electromagnetic valve provides sharp and reproducible peaks through the chromatograph. The data of Figure 2 are obtained with a column three feet long and ’/* inch in diameter packed with silica gel and maintained at 80 “C. A hydrogen flame detector was used with an attenuation of 500. As soon as the valve is closed, the dead volume is formed by the inner volume of the aluminum tube placed between the front end of the valve and the pellet sample. In our cell design, this volume can be varied by replacing this tube with another having a different bore and length. The bore size can be chosen from lis of an inch to half inch.
~ E L E TROMAGNE C TIC
COLUMN DE TEC T O R
t
Figure 1. Diagram of laser pyrolysis system The electromagnetic valve closing after the laser shot provides sharp and reproducible peaks
The data of Figure 2 were taken with a cylindrical chamber *is inch in diameter and ‘14 inch long. The leading edge of the methane peak on Figure 2 is quite sharp while the leading edges of the next two peaks have been broadened. It is clear from Figure 2 that the spreading of the peaks is due to the retention of the column and not to the dead volume of the cell. The dead volume has to be reduced only if it is the major cause of line spreading. In Figure 1 a sketch of the triggering and synchronization system is shown. Near the sign “Trigger,” the wiring of a DPDT relay is indicated. When the relay is in rest pcsition, the upper contacts are shorted; the electromagnetic valve activated by a dc power supply remains open. When the relay is activated, the circuit of the electromagnet is opened and the valve starts closing under the action of the spring (and gravity if in vertical position). The laser is triggered as soon as the relay reaches the work position where it remains for the time of injection of pyrolysis gases into the column. An SCR and ECG series transformer are used to ionize the flash lamp. Because of the inertia of the steel rod constituting the moving part of the valve, the laser beam goes through before the valve is closed. We have not found any improvement by replacing the relay with electronic systems. With the present cell, the reproducibility of the data depends on the homogeneity of the samples and the stability of the laser from shot to shot. The first problem was solved by making pellets from powdered samples. In our experiments, the laser stability was such that data were reproduced within 5 % ; no perturbation has been introduced by the cell. To reduce the laser fluctuations down to 1%, we have to regulate the temperature of the laser head within 1/60 of a degree centigrade and place the whole optical cavity under dry atmosphere. These precautions will be useless if the sample is placed in a commercial glass tube of poor
Figure 2. oi 1 shales
Typical chromatogram of laser pyrolyzed
The largest peak is acetylene, preceded by ethylene and methane. Each division of the horizontal scale represents 1 min
optical quality which furthermore becomes fogged by the pyrolysis products and is often damaged by the laser beam itself. Using this cell we have found that the acetylene output is proportional to the hydrocarbon content of oil-shales and can be used for their rapid characterization (5).
RECEIVED for review December 29, 1970. Accepted February 17, 1971. ( 5 ) J. P. Biscar, unpublished data, 1970.
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