ples were collected at 11 different sites throughout the Sacramento, San Joaquin, and Colorado River Basins. For the sites listed in Table 11, samples from sites 1-6 were collected from penstock or pump intakes, and those from 7-1 1 represent grab samples from the water surface. The samples were collected in four-liter polyethylene bottles and stored for periods of up to one month in dark containers. Since the aim of this study was to observe the behavior of dissolved vanadium in river water and not that associated with suspended particulate matter, the samples were not acidified. A summary of the concentration data from these sites is presented in Table 11, with the range of concentrations observed at each site
as well as the mean concentration value for the samples analyzed. The vanadium concentrations in these natural waters ranged from a low value of 0.2 pg/l to a high value of 49.2 c(g/l. None of the samples analyzed had a concentration below the 0.1 pg/l sensitivity limit of the method for 1-liter samples.
RECEIVED for review September 2, 1969. Accepted October 30,1969. Work carried out under U. S. Public Health Service Research Grant EF-00858. Presented at the 1968 International Conference, Modern Trends in Activation Analysis, Washington, D. C., October 1968.
Design, Construction, and Use of a Laser Fragmentation Source for Gas Chromatography Bohdan T. Guran, Robert J. O’Brien, and Don H. Anderson Eastman Kodak Company, Industrial Laboratory, Rochester, N . Y. 14650
PYROLYSIS TECHNIQUES have extended the use of gas chromatography (GC) in studies of materials with low volatility. Early in the development of GC, Davidson et al. ( 1 ) studied a number of polymers by collecting their pyrolysis products in a cold trap and subsequently transferring them onto a G C column for analysis. Later, more sophisticated thermal pyrolysis units were developed and coupled directly to gas chromatographs ( 2 , 3 ) . They allowed samples to be pyrolyzed directly in the carrier gas stream just before its entrance into the column. Such techniques usually employed hot wires, heated tubes and cups, or heated chambers. Many publications on these techniques are cited in recent review articles (4-6).
Although such thermal pyrolysis techniques allow the gas chromatographer to study nonvolatile compounds, they have several disadvantages which make resul?s difficult to interpret and reproduce. One of the major sources of error in these techniques is the difficulty of obtaining reproducible pyrolysis temperature of the sample and maintaining small temperature gradients in the pyrolysis chamber. Temperature differences at sample position between the carrier gas and the walls, as high as 100 OC have been reported (7). Other common sources of error are incomplete removal of solvents used in coating samples onto the filament wire, and catalytic effects of hot metallic surfaces on the decomposition of samples. A constant problem also is the formation of tars which accumulate in the pyrolysis chamber, in the tubing, and on the column packing. To overcome the problems associated with thermal degradation, Sternberg and Litle (8) developed a pyrolysis technique using high voltage electric discharge. It eliminated some (1) W. H. T. Davidson, A. L. Wragg, and S. Stanley, Chem. Ind., (London), 1954,1356. (2) E. A. Radell and H. C. Strutz, ANAL.CHEM., 31, 1890 (1959). (3) D. F. Nelson and P. L. Kirk, ibid., 34, 899 (1962). (4) R. W. McKinney, J. Gas Chromatogr. 2,432 (1964). ( 5 ) R. L. Levy, ibid., 5, 107 (1967). 40, 33R (1967). (6) R. S. Juvet and S. DalNogare, ANAL.CHEM., (7) R. A. Prosser, J. T. Stapler, and W . E. C.-Yelland, ibid., 39, 694 (1967). (8) J . C. Sternberg and R. L. Litle, ibid., 38, 321 (1966).
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Figure 1. Laser fragmentation source I-Laser rod 2-Flash tube 3-Mirror 4-Objective 5-Eyepiece 6-Fragmentation cell effects resulting from temperature variations but required long pyrolysis periods and large sampling volumes. The positioning of samples within the discharge cell was also critical in obtaining reproducible results. Many of the existing problems in pyrolysis could be eliminated if samples were fragmented more rapidly. This type of fragmentation should be obtained with laser radiation and has been reported by Wiley and Veeravagu (9). They successfully used a laser to fragment samples and analyzed the resulting fragments by gas chromatography. Recently, Folmer and Azarraga (10) reported using a ruby laser to obtain fragmentation of samples directly in the gas chromatographic stream. This paper describes the development of a neodymiumdoped glass laser as a fragmentation source for gas chromatography. The design, construction, and use of a sample fragmentation cell which allows on-line gas-chromatographic (9) R. H. Wiley and P. Veeravagu, J. Phys. Chem., 72,2417 (1968). (10) D. F. Folmer, Jr. and L. V. Azarraga, “Advances in Chromatography-1969,’’ A. Zlatkis, Ed., Preston Technical Abstracts Co., Evanston, Ill., 1969, pp 216-221.
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EXPERIMENTAL
Laser. Commercially available lasers were investigated but were found to be expensive and not easily adaptable to a GC system. For these reasons, a laser fragmentation source was designed and built in the laboratory to be used in analytical techniques requiring rapid volatilization of samples. The laser fragmentation source (Figure 1) consisted of a laser rod, 1; flash tube incorporated in a cylindrical reflector, 2 ; flat mirror, 3; objective, 4 ; eyepiece, 5; and the fragmentation cell, 6. The entire optical assembly was mounted on a 3/4-inch diameter, adjustable rod which allowed the laser to be placed at any convenient elevation. The heart of the fragmentation source was a 6-inch long, 3/s-inch diameter, silicate-base optical glass laser rod with 3 z by weight of neodymium oxide as the doping agent. The laser emitted radiation at 1.06 microns, with a fluorescence line width approaching 0.03 micron. It had an efficiency of approximately 0.5% with energy output of up to 3 joules/cc of glass rod, Beam divergence was low (2 milliradians), for this design geometry. The laser rod and the linear flash tube (PEK XE 16-6) were located in a cylindrical glass reflector 13/4inch diameter and 6.5 inches long (Figure 2). A glass tube, liZ inch diameter and 6 inches long, surrounded the rod and protected it from scratches and thermal shock. The cylindrical reflector (silver coated) caused the majority of the energy flux to impinge on the laser resulting in an efficient system. The output end of the rod had a low reflection evaporated coating (approximately 40 % reflectivity) while a corner-cube mirror provided total energy reflection at the opposite end of the rod, Laser rod, linear flash tube, and the cylindrical reflector were mounted in a small aluminum housing and attached to the optical assembly (Figure 1). An exit port allowed the laser output to pass into the microscope section of the assembly where an optically flat mirror directed the laser radiation to the objective for focusing onto the sample. The sample was viewed before actual firing by a simple rotation of the mirror. The entire assembly was mounted on a microscope rod equipped with vernier adjustments which allowed the assembly to move vertically for focusing and laterally for positioning. The power supply for the flash tube was a capacitor discharge type similar to those used for many laser systems. It had a maximum output capability of 2400 joules and was completely interlocked for safety. During use, the maximum input to the system was set at 900 joules. The energy output of the laser was approximately 4 joules. Fragmentation Cell. In designing a fragmentation cell, the following characteristics were considered necessary : the cell should have a small internal volume to give good gaschromatographic resolution of eluting components ; the cell should have an optically flat window which would allow the 116
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Figure 3. Fragmentation cell passage of 1.06 p radiation of the laser without divergence; and it must be easily disconnected and reconnected to permit rapid sample changes. A cell which meets these requirements was built and is shown in Figure 3. It was constructed of 6-mm 0.d. glass tubing shaped into a “U” tube 4 cm long with the legs separated by 3 cm. A flat quartz disk was fused into thecurved portion (opposite the open end) of one leg. The disk should retain optical flatness after it is fused into the cell. During analysis, the cell was attached to a gas-chromatographic switching valve by the use of ‘14 inch swagelok nuts and inch rubber “0” rings. A stainless steel rod 3 cm long and 4 mm in diameter was used to support the sample pellet in the cell cavity and to reduce dead volume in the cell. Two adjoining ports of a four-port Perkin-Elmer switching valve, with low internal volume, were connected between the flow regulator and the column. The remaining two ports were fitted with inch stainless steel tubing approximately 10 cm long. These stainless steel tubes were inserted into l / 4 inch swagelok caps through inch drilled holes and silver soldered to the caps. The two legs of the fragmentation cell were placed into the caps and were made pressure tight by the “0”rings and nuts. Gas Chromatographic Conditions. The fragmentation cell and valve were attached to a Microtek Model DSS-172/ DPF gas chromatograph equipped with a thermal conductivity and a flame ionization detector. Separation of fragments was obtained on an l/s-inch stainless-steel column 2 feet long and packed with 10% silicon gum rubber, SE-30 on 60/80 mesh Anakrom ABS. The column was kept at 60 “C and flushed with helium carrier gas at 40 cc/min. Chromatograms were recorded on a 1-mV Honeywell strip chart recorder. Sample Preparation. To increase the absorption of laser radiation, samples were prepared by grinding the polymers with approximately 10% powdered carbon. A drop of solvent (methylene chloride) was added during grinding to ensure intimate mixing of carbon with the sample. The mixture was dried thoroughly to remove all traces of solvent. Approximately 20 mg of the sample mixture were placed in a die, with a circular plunger 3 mm in diameter, and pressed into a pellet. The sample pellet was then placed into the fragmentation cell through the leg with the optically flat window on the opposite end, The stainless steel rod was inserted into the
ANALYTICAL CHEMISTRY, VOL. 42, NO. 1, JANUARY 1970
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Figure 4. Fragmentation chromatograms of polystyrene and poly(styrene-butadiene) polymers same leg, taking care not to crush the sample pellet. A swagelok nut and a inch rubber “0” ring were placed on the end of each leg of the cell. The entire cell assembly was then inverted so that the pellet rested on the metal rod, and was placed into the swagelok caps of the switching valve assembly. After tightening the cell assembly onto the swagelok caps, the valve was switched to allow carrier gas to flow through the cell. The objective lens of the laser was positioned above the cell window and the microscope verniers adjusted to bring the sample pellet surface into focus. The flat mirror was rotated 90 degrees to direct the laser radiation onto the sample surface. The laser was fired and the chart recorder started. (Safety glasses with an optical density greater than 8 at 1.06 p should be worn during laser firing to protect the eyes.) Fifteen seconds after firing the valve was switched in the opposite direction to allow the carrier gas to flow directly onto the GC column by-passing the cell. The cell was then disassembled and charged with a new sample. RESULTS AND DISCUSSION
Fragmentation of samples by laser radiation gave chromatograms that were characteristic of the polymer type and were less complex than those obtained by thermal pyrolysis. They show predominantly the low molecular weight fragments with significant peaks due to the monomeric units of a polymer although this is dependent to a large extent on the energy concentration of the laser radiation striking a unit area of the sample surface. Figure 4 shows laser fragmentation chromatograms obtained from polystyrene and poly(styrene-butadiene) polymer. In each case, a large styrene monomer peak eluting at 6.7 minutes is seen. Although there was no evidence of butadiene monomer, the poly(styrene-butadiene) gave a large toluene peak at 3.3 minutes while the polystyrene polymer showed only traces of toluene. The fragmentation chromatograms of all polymers studied were greatly influenced by the diameter of the impinging beam. A beam focused to a fine point gave chromatograms with an abundance of very small fragments while a slightly defocused beam, having a lower energy per surface area, produced less bond breakage and more of the larger fragments. Figure 5 shows the chromatograms of poly(styrene-butadiene) polymer obtained with finely focused and defocused beams. It
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should be noted that no styrene monomer peak was present (at 6.7 minutes) when a focused beam was used but instead a large unidentified peak at 2.7 minutes was seen. A benzene peak appeared at 2.2 minutes and a trace of toluene was seen at 3.5 minutes. When a defocused beam was used, large styrene monomer and benzene peaks were obtained while the peak at 2.7 minutes was substantially reduced in size. This behavior was seen with most polymer samples studied but it could easily be controlled by controlling diameter of the laser beam to obtain reproducible chromatograms. The laser-gas chromatographic facility should be very useful in obtaining qualitative and quantitative information on polymer compositions such as copolymers and mixed polymers. The technique should be especially applicable to polymer samples that are not easily soluble or contain inorganic additives or pigments. RECEIVED for review July 28, 1969. Accepted October 30, 1969.
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