liquid Metering Device for Process-Stream Gas Chromatographs and Small Pumps F. A. Keidel and C. D. Lewis, Experimental Station, E. 1. du Pont de Nernours & Co., lm., Wilmington, Del.
o appears to have been deN scribed in the literature for direct automatic injection of liquid samples SYSTEM
as small as 0.006 ml. into gas chromatographic columns. Liquid sampling for gas chromatography generally employs total vaporization of a liquid sample stream a t a substantial flow rate, followed by cyclic injection of samples of the hot vapor into the chromatograph. Total vaporization is unsatisfactory for samples containing components which are unstable, prone to polymerize, or relatively nonvolatile. A sampling device (Figure 1) has been developed which esscntially combines the features of B motorized hypodermic needle and a dosing stopcock to deliver a minute, precisely metered sample on demand. A section of hypodermic tubing isolates the desired volume of sample or reagent from a supply flowing through a chamber with inlet, b, and outlet, c, ports. To deliver the trapped volume, the metering tube, a, is raised briefly t o a new position in the chamber, so that connection is made to two alternate ports. One of the latter, d, is a source of inert ejecting gas under pressure, and the other port, e, opens to the analyzer or chromatograph, BO that the liquid contained in the sample volume is forced into the delivery line. After about 1 second is allowed for the volume to empty, the metering tube is returned to the onstream position, for rapid refilling and flushing with fresh sample. The most critical factor in design of the metering device is sealing for operation under pressure differentials of up to 50 p.s.i. This is accomplished by a combination of plastic or elastomeric sealing rings, appropriately spaced between ports by fiber or metal washers, with tension on the assembly adjusted by a packing screw, f. Response of the analyzer to a change in concentration in the sample will not be retarded by dead volume in the pump if the spacing washers around the ports are carcfully designed and machined to minimize such holdup. The fittings for “sample in” and “metered sample” should have fillers of Teflon TFE-fluorocarbon resin for the pipe connections to rest on, to avoid dead volumes therein. Also, the delivery line from the pump to the chromatograph should be capillary in internal dimension, to minimize the area of surfaces upon which traces of liquid sample might remain. The parts of the pump which come in contact with the liquid sample can be made of any machineable alloye.g., Type 316 stainless steel. Joints 1456
ANALYTICAL CHEMISTRY
are Heliarc-welded or, if sample conditions permit, silver-soldered. The only other materials with which the samples come in contact are the hypodermic tubing (stainless steel), fiber washers, and the, sealing rings. The latter have usually been made of Viton A fluorinated elastomer, although Teflon TFE-fluorocarbon resin also offered complete resistance to attack by organic liquids. The outer surface of the metering tube is lapped with crocus cloth to remove any burrs formed in drilling the ports. Although such pumps have been operated by solenoid plungers attached to the sample volume, more positive action is obtained by coupling a pneumatic operator to the metering tube. For this purpose, it is convenient to adapt a constant differential flow controller (such as a Model 63SD, Moore Products Co., Philadelphia, Pa.) fitted with a neoprene diaphragm. The diaphragm operator is then actuated by compressed air or other gas from a 20-p.s.i. supply, controlled a t the desired instant by external solenoid valves. The inert gas used for displacing the sample from the sampling volume may conveniently be the carrier gas used in the analyzer or chromatograph. The particular analysis may require this to avoid an interference. The pressure of inert gas should be a t least 5 p.s.i. greater than the pressure existing a t the delivery port for proper operation. In chromatographic work, the d
b-
f -
pump has delivered successfully against column pressures as high as SO p.s.i. A small volume of incrt ejecting gas is inevitably transported with the sample. This gas volume can be kept to a minimum by adjustment of the difference in pressures between the gas supply and the downstream side of the the pump, and by using the pump operator as a valve to isolate the pump from the gas supply a t the instant of sampling. In the latter case, a gas volume as small as five times that of the metered sample will be ejected with the sample. Volumes from O.OOG to 0.08 ml. have been metered successfully by using tubing of appropriate diameter; larger or smaller volumes seem feasible. A precision of delivery indicated by a relative standard dcviation of 1.0% can be expected in application of the metering pump to automatic gas-liquid partition chromatography. This reproducibility was observed in the delivery of 0.006ml. samples of pure cyclohexane a t intervals of 10 minutes to a chromatograph operated a t i o ” C., by measurement of the height of the peak. This pump also has been used for metering small liquid flows of samples or reagents to continuous-flow analyzers. Many devices employing displacement of flexible plastic tubing or diaphragms have been used for similar applications, but they tend to suffer from lack of durability of the plastic material when in contact with organic liquids having solvent properties. Continuous pumping is achieved by rcpeated actuation of this pump a t an appropriate frequency. Metered volumes up to about 1 ml. and actuation rates up to about 20 strokes per minute can be obtained without major modification of the design. Such pumps have been used successfully for long periods of time in delivering liquid hydrocarbon samples a t 1 ml. per minute (0.08 ml. per stroke, 12 strokes per minute) to electrolytic oxygen and water analyzers of the types described by F. A. Keidel [ANAL. CHEM. 31, 2043 (1959); Ind. Eng. Chem. 52,490 (1960) 1. By locating the pump close to the analyzer and using a rapid flow of sample or reagent stream during the stand-by (filling) part of the cycle, the pump will add only a few seconds-Le., the time between cycles, to the response lag of the annlyzer. Liquid metering devices of this design are now commercially available (Lockwood & MacLorie, Inc., Hatboro, Pa.). ACKNOWLEDGMENT
Figure 1. Construction of liquid metering device
Design features suggested by R. G. Jackson of Du Pont and J. h4. Copeland of Lockwood & MacLorie are gratefully acknowledged.
