I AIDS FOR ANALYTICAL CHEMISTS Flow Control in Gas Chromatography by Continuously Monitored Pneumatic Switching J. P. Mieure Monsanto Industrial Chemicals Co., 800 N . Lindbergh, St. Louis,
Mo. 63166
Controlling flow through a gas chromatograph (GC) by means of applied pressure differentials (pneumatic switching) is an established technique. Flow through a precolumn in series with the analytical column can be stopped or reversed to aid in achieving the desired separation. Selected portions of effluent can also be diverted from the chromatographic train. The procedure was introduced by Deans in 1965 ( I ) and has been used considerably in Europe (2, 3). However, it has not yet gained widespread acceptance in the United States. Typical applications include backflushing, heart cutting, stripping, and solvent venting. Using the instrumental configurations described in the above references, the timing sequence for controlling these operations is established only by trial and error. A new sequence must be developed each time a different analysis is carried out. Trial and error adjustment may be satisfactory for routine analysis or process control applications, where the lengthy initial set-up time is balanced against a large number of analyses performed without changing conditions. In many research laboratories, however, different analyses are performed on the same instrument, often on a onetime basis. Optimization of switching by trial and error is too time consuming to be considered practical. This paper describes a configuration developed in our laboratory in which effluent from the precolumn is continuously monitored. This signal is used to indicate the proper time t o activate the desired switching operation, thus eliminating the element of trial and error. An important application of this procedure is venting effluent from a combined GC/mass spectrometer (GC/ MS) system. The importance of selective GC/MS effluent venting has been described recently ( 4 , 5 ) .
PRINCIPLE Remotely actuated multiport valves are widely used for changing modes of GC operation in process control instruments. These valves have an upper temperature limitation of 200-250 "C imposed by the materials of construction. The function of these mechanical valves can be performed by pneumatic switching, with no valves required inside the oven. Only columns, connecting lines, tees, and fittings are in the heated zone. Thus, the maximum operating temperature is limited only by the column packing. A potential source of maintenance problems is also eliminated. Pressures at the inlets of a precolumn and the analytical column are preset and regulated. Alternate paths of flow out of the oven are provided. Flow through these alternate routes and, consequently, flow through the chro(1) D. R. Deans, J. Chrornatogr., 18,477 (1965). (2) D. R. Deans, Chrornatographia, 1/2, 18 (1968). (3) M . P. T. Bradley, 10th National Meeting of the Society for Applied
Spectroscopy,St. Louis, Mo., October 1971. (4) J. P. Mieure, J. G . Converse, M . W. Dietrich, and Lewis Fowler, Anal. Chern., 44, 1332 (1972). (5) R. L. Levy and M . A. Grayson. 163rd National Meeting, American Chemical Society, Boston, Mass., April 1972.
matographic system, is controlled by manipulation of inexpensive on-off valves outside of the heated zone. The addition of a thermal conductivity (TC) detector at the exit of the precolumn allows the operator to continuously monitor precolumn effluent. Thus, the correct timing of the switching operations can be accomplished the first time a sample is chromatographed. Prior optimization of the sequence is not required.
EXPERIMENTAL Flow systems and general details of setup and operation of chromatographs with pneumatic switching have been described previously (1-3). Only the modification required to utilize precolumn monitoring will be described in this paper. A configuration for cutting or solvent venting is shown in block diagram form in Figure 1. Effluent from the precolumn passes through a thermal conductivity detector and into the switching region. Each component is detected a few seconds before it reaches the switching zone. Desired components are routed into the analytical column. Unwanted components, as indicated by the TC detector output, are cut and vented from the chromatograph through the vent valve. By observing the TC detector signal, the operator can decide which components to vent. Solvent and/or major components, which would overload the analytical column, can be excluded without affecting the efficiency of the analytical column for separating minor components. Switching is activated by opening an on-off toggle valve, and terminated by closing the valve. Pressures within the chromatograph are adjusted as follows. The vent valve is closed so that flow is through both columns. Pressure regulator 1 and the chromatograph carrier flow valve are adjusted t o give the desired flow into detector 2. When the system has equilibrated, the pressure indicated on pressure gauge 2 is noted. Pressure regulator 2 is set to give a pressure reading 1-2 Ib. higher than before, again read on gauge 2. If temperature programming is to be used, the above adjustments should be carried out with the column oven a t the maximum operating temperature. Next, the vent valve is opened and the needle valve adjusted so that flow through the vent is 1-1% times the flow through both columns. Under these conditions, solvent injected into the chromatograph should not appear a t detector 2. If some solvent does reach detector 2, the needle valve should be opened further until all solvent is excluded. With proper adjustment, sample injections of a t least 100 ~1 can be vented. The precolumn is typically 1/4-inch o.d., 6-36 inches in length, and packed with the same packing used in the analytical column. The packings need not be the same. Packed inserts or separate ovens can also be used if it is desirable to operate the precolumn and analytical column at different temperatures. If the precolumn packing bleeds significantly, this may be reflected as a base-line shift a t both detectors when effluent is vented. The GC used in this work is a Hewlett-Packard F&M 5754B equipped with dual flame/dual thermal conductivity detectors. Precolumn effluent is passed through one side of the TC detector (Detector 1). The exit of the detector is outside the oven, so inch 0.d. X 0.027-inch i.d. stainless steel tubing is used to transfer the stream back inside through the heated port integral to the detector. The portion of the tubing outside the oven is wrapped with resistance wire and heated from a n auxiliary heater power supply in the chromatograph. Helium is purged through the reference side of the detector in the reverse direction a t a flow rate of about 20 ml/min. The detector is normally operated about 20 "C. above the maximum GC oven temperature.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
1981
VEllT VALVE
NEEDLE VALVE
1
DETECTOR 7
Figure 1. Block diagram of modified gas chromatograph
, 8 12 (MIN,) Figure 3. TCD (above) and FID (below) chromatograms of 20% solution of 1,3,5-trIchlorobenzene containing a trace of rn-di0
4
chlorobenzene. The solvent and trichlorobenzene were excluded from the analytical column
7
0
4
8
12
(MIN,) Figure 2. Chromatogram of CI5-C2o a-olefin mixture Top, FID trace of mixture: Middle, FID trace with second and fourth cornponents vented, and bottom, TCD trace of mixture showing portion vented
~~~~
Table I . Chromatographic Conditions inch 0.d. X 9-inch stainless steel Precolumn packed with 5 % Dexsil300 GC (Olin Corporation, New Haven, Conn.) on 80-100 mesh H.P. Chromosorb W (Johns-ManvilleProducts Corporation, Manville, N.J.) '/B-inch 0.d. X 0.085-inch i d . X 6-foot Analytical column stainless steel packed with 10% SE-52 (General Electric Company, Waterford, N . Y . ) on 80-100 mesh H.P. Chromosorb W 30 mi helium/min Flow rate Temperatures, "C programmed 170-250, 8/min. Oven 250 Injector FID TCD TC filament
300 260 120 mA
current
The lines connecting detector 1 to the needle and vent valves and pressure regulator 2 are YE-inch 0.d. X 0.085-inch i.d. stainless steel. Ordinary Swagelok fittings (Crawford Fitting Co., Solon, Ohio) are used. A 4-inch length of YE-inch 0.d. X 0.027-inch i.d. tubing is used to connect the two tee pieces. This capillary 1982
provides a necessary pressure drop between the two columns. The tee pieces are YE-inch Swagelok. No special precautions are taken to reduce internal volumes since the volume of the detector is on the order of 1 ml. In practice, this dead volume is not significant except when attempting low flow separations with capillary columns. Good separations have been achieved using 100 feet of 0.020-inch i.d. support coated open tubular column a t a flow rate of 10 ml/min. The vent and needle valves are Whitey OGS2 (Whitey Research Tool Company, Emeryville, Calif.) and Nupro 2M (Nupro Company, Cleveland, Ohio), respectively. Similar valves could be substituted. Virtually any column satisfactory for conventional GC is also satisfactory for the analytical column. Columms from capillary to Yd-inch 0.d. have been used in this laboratory. Detector 2 can be any type of detector normally used in GC, such as flame ionization, thermal conductivity, or electron capture. The fact that detector 1 may be less sensitive than detector 2 is actually not a serious drawback. In practice, any component large enough to require venting is also easily detectable with the TC detector.
RESULTS The high selectivity possible within this monitored pneumatic switching system is illustrated by cutting selected components from a mixture of C W - C ~ a-olefins. ~ The top trace in Figure 2 shows a chromatogram of 2 p1 of a 10% solution of the olefin mixture recorded by the flame ionization detector (FID) at the exit of the analytical column. The middle trace is a chromatogram of the same mixture with the second and fourth components vented. The TC detector trace, monitoring the separation achieved by the precolumn, is shown in the bottom trace. The portion of the precolumn effluent which was vented is indicated by the arrows. The chromatographic conditions used for this separation are given in Table I. An application of the use of this technique for trace analyses is illustrated in Figure 3. This shows the TC (above) and FID chromatograms (monitoring the precolumn and analytical column, respectively) for a 1OO-gl injection of a 20% solution of 1,3,5-trichlorobenzene. A low level impurity was observed a t 7.2 minutes in the FID chromatogram. Using this very large sample injection and venting solvent and trichlorobenzene, the impurity was identified as m-dichlorobenzene by GC/MS. Two skewed peaks also appear in this chromatogram at 3.5 and 11.0 minutes. These are the minor portions of the solvent and trichlorobenzene peak tails still emerging from the precolumn a t the conclusion of venting.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 11, SEPTEMBER 1973
The intervals when precolumn effluent was vented are indicated by arrows on the TC trace. Terminating venting a t a TC signal closer to the base line would have minimized tailing into the analytical column. Chromatographic conditions were the same as in Table I except that the oven temperature was programmed from 70 to 230 "C at 8 "C/min. In addition to GC/MS and general packed column chromatography, pneumatic switching offers a unique oppor-
Spinning Dropping Mercury Electrode-A
tunity for trace analysis using capillary columns. By venting solvent and/or major components, large injections can be used without the sample loss encountered with conventional inlet splitting. Thus, both high resolution and high sensitivity can be achieved. Received for review November 30, 1972. Accepted April 2,1973.
Practical Analytical Tool
Henry J. Mortko and Richard E. Cover D e p a r t m e n t of Chemistry, St. John's University, Jamaica,
N.Y.
11439
Many efforts have been made to adapt the dropping mercury electrode (DME) to the continuous analysis of stirred or flowing systems. A variety of cells (1) for use with the DME have been constructed to minimize the erratic effects arising from convection. A successful approach to this problem which permits direct immersion of the electrode in the agitated solution involves the controlled detachment of the mercury drops by the vibration or periodic mechanical shock of the capillary (2). This vibrating dropping mercury electrode (VDME) has been demonstrated to be superior to the DME in many respects, both as an analytical detector and for theoretical purposes when drop times are sufficiently small (2-7). As an analytical device, however, the VDME has some serious disadvantages. To obtain millisecond drop times, large mercury pressures are required and the electrode vibrator must be mechanically complex (8). At such short drop times, the charging current increases to the microampere range decreasing VDME sensitivity to below DME levels ( 5 ) .Finally, the vibration of the capillary precludes direct sealing of the electrode into flowing streams. The work reported here demonstrates that it is possible to retain all the analytical virtues of the VDME and eliminate its disadvantages by the simple expedient of spinning the capillary. The spinning dropping mercury electrode (SDME) discussed here differs mechanically from that of Kolthoff and coworkers (9) in two ways. Kolthoffs capillaries were U shaped with the orifice pointing upward while ours are straight barometer-tubing capillaries pointing downward. In addition, Kolthoffs capillaries were of relatively large bore (10) compared with the commercial capillaries employed here. The practical effects of the simple differences are striking. The geometry of Kolthoffs electrode prevents its use in small-diameter flowing streams. The large bore of his capillaries with the attendant large mercury flow rate results in omnipresent maxima of the second kind; the ad(1) (2) (3) (4) (5) (6) (7) (8)
(9) (10)
2. P. Zagorski, Progr. Polarogr., 1962, 549. R. E. Cover, Rev. Anal. Chem., I,141 (1972). R. E. Cover and J. G. Connery, Anal. Chem., 41, 918 (1969). J. G. Connery and R. E. Cover, Anal. Chem., 41,1191 (1969). R. E. Cover and J. G . Connery. Anal. Chem., 41, 1797 (1969). R. E. Cover and J. T. Folliard. J. Electroanal..Chem., 30, 143 (1971). J. T. Foiliard and R. E. Cover, J. Electroanal. Chem., 33, 463 (1971). J. G. Connery, Ph.D. Thesis, St. John's University, New York. N . Y . , 1970. I. M . Kolthoff and Y. Okinaka, Progr. Polarogr., 1962, 357. Y. Okinaka and I . M . Kolthoff, J. Arner. Chem. Soc., 79, 3326 (1957).
dition of surface-active materials to suppress these maxima is necessary if reliable analytical data are to be obtained (11). In addition, apparently because of the large bore, treatment of the capillary with water-repellent materials was found to be essential (12). EXPERIMENTAL All capillaries were obtained from Sargent-Welch Scientific Co., (S-29419)and cut to a length of 15 cm. A variable-speed motor with tachometer feedback was used to power a pulley system with a n output/input ratio of 2 : l . The capillary was rotated in a ball-bearing housing and constrained to rotate around its axis. The speed of rotation was continuously variable and precisely controllable over the range 0-7000 rpm. The rotating mercury reservoir (0.d. 7 mm, 40 cm long) was supported on ball bearings and connected directly to the capillary with Tygon tubing (see Figure 1). Reproducible, trouble-free operation with a single electrode was found over three months of intensive work. Vibration-free mounting and sufficiently high mercury pressures are essential to such performance. All other apparatus and reagents have been previously described ( 3 , 4 ) .
RESULTS AND DISCUSSION The responses of the SDME to various phenomena were studied under a variety of conditions at rotational speeds in the range 0-7000 rpm with mercury pressures as high as 51 cm. The systems studied and the results obtained are very similar to those previously reported for the VDME (3, 4 ) ; increasing rotational speed of the SDME results in behaviors parallel to those found with increasing frequency a t the VDME. Optimal analytical response is obtained with both electrodes at the highest agitational rates. Polarograms obtained with the SDME are essentially identical in appearance with those obtained with the VDME (3, 4 ) . The short drop life at both the SDME and the VDME makes it possible to record polarograms a t high speeds. This property may, therefore, make these electrodes useful with techniques other than DC polarography. Mass-Transfer Controlled Currents. The limiting current-concentration response of the SDME at 7000 rpm and 50 cm of pressure to cadmium in unstirred 0.1M KN03 was found to be linear over the range 0.01-20.0mM. The lower limits of detection and the magnitude of the residual currents were essentially identical a t the SDME and the DME. (11) I. M. Kolthoff, Y. Okinaka, and T. Fujinaga, Anal. Chim. Acta., 18, 295 (1958). (12) I. M . Kolthoff and Y. Okinaka. Anal. Chim. Acta, 18, 83 (1958).
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SEPTEMBER 1973
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