A High-Temperature Inlet Manifold for Coupling a Gas Chroimatograph to the Time-of-Flight Mass Spectrometer DWIGHT 0. MILLER Research and Development Division, Organic Chemicals Department, Jackson Laboratory, E. 1. du Pont de Nemours & Co., Wilmington, Del.
b The usefulness of the gas chromatograph time-of-flight mass spectrometer i s to a very great extent a function of the inlet system used to couple the two units. This paper describes an inlet system which was designed for applications to higher boiling materials, capable of sustained operation at 200' C. and intermittent operation Internal volume of up to 250' C. the manifold has been reduced to a minimum to provide r'esolution in the time-of-flight (TOF) mimifold equivalent to the gas chromatograph (GC) resolution. Construction materials are of stainless steel and quick-acting toggle valves are employed. The valves are of bolted bonnet design and any part may be repaired or replaced without removing the valve body. The entire manifold is enclosed in an oven with an adjustable temperature control. The system hcis been used successfully for materials boiling from -128' to 314' C. and with concentrations in the p.p.m. range.
T
COMBINATION of the Bendix time-of-flight mass spectrometer with the gas chromatograph has proved to be an extremely useful and versatile qualitative tool. Two general schemes have been used to couple the two together-that is to take the GC effluent a t or slightly above atmospheric pressure and introduce it into the TOF mass spectrometer which is under a pressure of 1 0 4 to 10-6 mm. of Hg. The first method proposed by Gohlke (9) in 1959 made use of a single leak which passed a fraction of the GC effluent continuously ,into 'the mass spectrometer and photographic recording of the spectra as they appeared on the oscilloscope screen. Ebert (1) used the more sophisticated analog output system for presentation of spectrrt and trapped sections of the GC effluent and then allowed the material to leak into the mass spectrometer. We have used the second approach since the trapping technique allows one to obtain a higher quality spectrum and for very small components in a mixture, enhances the sensitivity. However, HE
work with the Ebert manifold (1) demonstrated several design deficiencies. The large internal volume seriously degraded the resolution of the GC and lowered the sensitivity and resolution of the unit for the detection and identification of trace materials. The solenoid valves could not be maintained in a leak-free condition. The use of heating tape for temperature control resulted in local overheating and generally poor temperature regulation. The use of copper and brass as materials of construction resulted in corrosion. Accessibility of the constituent parts of the manifold for repair or replacement was poor. The maximum permissible temperature was approximately 150° C. Because of the limited usefulness of the manifold described by Ebert ( I ) , an inlet system for the GC-TOF unit which avoids the objections raised above was designed. Design Considerations. Components constituting 0.1% or more of the sample can be passed directly into the mass spectrometer through a leak valve, but for components in smaller concentrations, the sample is so diluted with the helium carrier gas that a usable spectrum can be obtained only a t the expense of the vacuum system. To circumvent this problem, the sample can be condensed with liquid nitrogen and the helium removed with an auxiliary vacuum system. On the basis of the above considerations, a trapping system must contain a minimum of two traps to be efficient, a direct-run trap and a cold trap. Because of the desirability of obtaining a spectrum of every component as it emerges from the GC and thus avoiding time-consuming repeat runs, the addition of more traps seems advisable; however, there are several factors which completely negate any advantage of additional traps, Spectra can be obtained with the direct-run trap in less than one minute; consequently, more than one directrrun trap is of very limited usefulness and introduces larger maintenance problems. Furthermore, aa a manifold increases in size and complexity, diffusion of the GC effluent
markedly decreases the resolution available at the TOF mass spectrometer. The cold trap which is used for minor constituents must be located near the GC outlet as any diffusion of the small peaks results in loss of resolution. If a second cold trap is added, the sample must pass through two three-way valves before entering the trap, and as a result this trap is useful only for clearly resolved components. I n work with higher boiling materials, the limiting factor with respect to the speed with which spectra can be obtained is the pumping speed of the mass spectrometer vacuum system and the over-all speed is not materially increased with more than two traps. Maximum retention of the GC resolution is of paramount importance in the construction of an inlet manifold system; this requires connections of minimum internal volume. A second advantage of minimizing the internal volume of the manifold is the reduction of clean-up time of a trap after a sample has been trapped and its spectrum obtained. The speed with which components emerge from the GC dictates the use of quick-acting valves which, in turn, must be capable of operation in an environment of a t least 200' C. The mass spectrometer leak valves must also be of quick-acting design, and those d e scribed by Ebert (1) are satisfactory. The applicability of mass spectroscopy is most severely limited by compounds having low vapor pressure as it is necessary to introduce the sample into the mass spectrometer as a vapor. Heating the inlet manifold will extend the range of the system; consequently, an oven was designed to maintain the inlet manifold at the desired temperature. The valves do not touch the oven, but are heated by radiation and convection. The materials of construction were principally stainless steel and Teflon (E. I. du Pont de Nemours & Co.) TFE-fluorocarbon resin. The Teflon was confined to valve seats and gaskets. The area of Teflon fluorocarbon exposed directly to the mass spectrometer vacuum waa extremely small (in the VOL. 35, NO. 13, DECEMBER 1963
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TO VACUUM
Figure
I.
Schematic showing layout of inlet manifold
order of 2 to 3 square mm.), and no decomposition products were noted in the mass spectrometer background. Copper and brass were rigorously excluded except for use in the exhaust line. The cold trap must be heated after the helium is removed to increase the vapor pressure of the trapped constituent. Direct resistive heating was found to be rapid and convenient. Sample Input Systefn. The sample input system is designed so that a portion of the GC effluent can be isolated and then passed through a leak valve into the T O F mass spectrometer where its spectrum can be btained. The leak valves are identical t o those described by Ebert (1). The three-way valves are George W. Dah1 Demi valves, code number SBT, constructed of stainless steel and with Teflon fluorocarbon seats for vacuum service. The valve ports wore modified by the supplier so that the outlets were 180° and 90" from the inlet. The twoway valves were also supplied by the Dah1 Company and are code number H1B. These are block values with an internal "tee." All of these valves are equipped with toggle actuation and are fabricated for panel monntiug. The valves contained '/,inch female NPT connections, and plugs were machmed to completely iili the connections. The plugs were drilled with a 0.065-inch hole and connections were made hetween valves with 1I8-inch stainless steel tubiog of the same i.d. The joints were silver soldered. The block valve, which controls the vacuum to the cold trap, is a modification of a standard Hoke high-temperature solenoid valve (series 90, normally closed). The valve body was drilled up through the seat so that a '/,inch a d . stainless steel tube could be inserted and silver soldered in place. A new valve seat was formed from the projecting tip of the '/8-inch tubing. What had been the original outlet side of the valve body was milled off and plugged. This modification resulted in it significant reduction of the cold trap volume. The pressure differential across this valve is small enough so that leakage 2034
ANALYTICAL CHEMISTRY
Figure 2.
presents no problem. The actuating coil was placed outside the oven to protect it from overheating. The entire manifold was mounted on a Transite board with the valve handles protruding through the front. An aluminum box was fabricated to enclose the valve bodies and two 250watt strip heaters were attached to the box. The output of these heaters is controlled by a Feuwal thermoregulator, Number 17002. The leak valve adiustmen& protrude through the hack o? the oven for convenience of adjustment, and access to the Fenwal temperature adjustment is provided. At 200' C., the temperature of the manifold was monitored a t several points aud a maximum temperature variation of 10' C. was observed. To reduce the time required for cold trapping, the trap was constructed of thin-wall stainless steel tubing, the inner tube was 15 gauge with an i.d. of 0.054inch and the outer tube was inch 0.d. Means were devised to heat the trap by direct resistive heating. The output of a low-voltage, highcurrent transformer was attached to the ends of the trap and a toggle switch (spring-loaded in the off position) was installed in the 115-volt line to the transformer primary. The secondary leads are 12-gauge copper wire; however, these were attached to the trap with short lengths of 20-gauge copper wire. The 20-gauge connections prevent the electrical leads from acting as heat sinks after the trap has been heated. This heating system allows a complete cold trapping cycle (trap evacuated, condense sample, pump off helium, warm trap, obtain spectrum, and pump out trap) to be completed in approximately 2 minutes for volatile material. The trap can be heated from -190' to 50' C. in 10 seconds with a GOampere current a t 2.5 volts. Figure 1 is a schematic of the manifold viewed from the back. The GC effluententers the manifold at the threeway valve A by means of a 15-gauge 0.054inch i.d. staiuless steel tube. From A , the flow can be directed through the cold trap and out through C to exhaust. Alternately, the flow
Photograph of entire GC-TOF system
can be direct to A', where it can be routed through the direct run trap or directly to exhaust. The leak valves ( B and B') are modified so that they are opcn to flow-through regardless of the position of tbe valve stem. In operation, the cold trap is normally kept closed and evacuated; when it is desirable to trap a small fraction, the trap is cooled with liquid nitrogen, valve A is positioned so that flow is directed into the trap just as the peak emerges from the GC detector, as indicated by the GC recorder. After a slight delay, valve Cis opened and the effluent is allowed to flow through the cold trap to exhaust. When sufficient material has been collected, the trap is closed by means of valves A and C, the helium is pumped off through D,D is closed, the trap is heated, and the sample is passed into the mass spectrometer through the leak valve B. The delay in opening C allows a small positive pressure to build up in the trap and thus prevents material from being pulled from the exhaust line hack into the trap. Cleanup is accomplished by heat and vacuum. For large fractions, the effluent is routed from A to A' and thence to C' and then to exhaust. As the peak reaches a maximum, A' and C' are closed and the sample admitted directly through B' to the mass spectrometer. After a satisfactorv s-aectrum has been obtained, the trap is deaned by purging with the GC effluent. . Figure 2 shows the entire GC-TOF unit with accessory equipment and the inlet manifold in place. The connection from the GC detector is visible at the lower left of the inlet manifold. CONCLUSION
A high-temperature inlet manifold has been described for coupling a gas chromatograph to the time-of-6ght mass spectrometer. The system bas been in daily use for over a year and has been used for the identification of unknown materials on a routine hasis during this time. Applications have covered materials boiling from -128"
to over 300' C. With hig 1-temperature operation, thermal deconiposition is a problem; however, compounds which have sufficient thermal stability to permit separation by GC do not generally decompose in the inlet system. Spectra of volatile material at the 5 to 10 p.p.m. concentration tan readily be obtained using the conventional 20- to 50-pl. GC sample. The minimum volume design of the manifold gives
resolution in the manifold comparable to that indicated by the GC detector. Constant usage has demonstrated that the inlet system is free of maintenance problems and is applicable to a large number of organic materials.
James Farley and thanks John Kraus for the excellent job in constructing the manifold. LITERATURE CITED
ACKNOWLEDGMENT
Ebert, A. 8.)ANAL.CHEM.33, 1863 (1961). (2) Gohlke, R. S., Ibid., 31, 535 (19813).
The author acknowledges the helpful suggestions of Eugene W. Bassett and
~~~~~v~~ for review ~~l~ 12, 3963. Accepted September 4,1963.
(1)
Gas Chromatography of Aqueous Solutions. Determination of Hydrocarbons and Halocarbons EMMETT S. JACOBS Jackson Laboratory, E. 1. du Pont de Nernours and The interference from water in the gas chromatographic analysis of aqueous solutions of hydrocarbons or halocarbons can b e avoided by use of a precolumn which completely removes the water without absorbing these organic compounds. A column containing a mixture of riine parts of phosphorus pentoxide and one part of Desicote- and Siliclad-treated firebrick can b e used to analyze over 50 successive 10-pl. samples,
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AS
CHROUTOGRAPFIC
ANALYSES
of aqueous organic solutions are usually hampered seriously by the large tailing of the water peak. This situation can be overcome by wing a detector such as the hydrogen flame which is relatively insensitive to the presence of water vapor (3). HoweJer, there are times when this type of equipment is not available or cannot Ee used, either because of the potential hazard or because it is necessary to determine other components such as CO, COZ, Nz, 02, and NzO which are not detected by the hydrogen flame detector. Aqueous organic solut ons can best be analyzed by gas chromatography when the water is removed by absorption or by reaction with a mrtterial to produce a more volatile and easily eluted gas. Kung, Whitney, and Cavagnol ( 2 ) have used a precolumn of calcium carbide to coniert the water vapor t o acetylene. The disadvantage of this technique lies in the pclssibility that the acetylene will mask other components in the solution. Drying agents normallj used in analytical chemistry have r o t been employed for the absorption of water injected into a gas chromatograph, probably because most of them are not efficient enough to remove all the water.
Co., P . 0. Box 525, Wilrnington 99, Del.
Phosphorus pentoxide, however, is an excellent drying agent and experiments in this laboratory have shown that a precolumn of this material is capable of absorbing all the water contained in microliter samples injected into a gas chromatograph. In the preparation of the absorption column described, the essential feature is the use of a special silicone-coated firebrick as an inert granular support for the powdered phosphorus pentoxide. Using a precolumn of this mixture to absorb water before it enters the partition column extends the life of the partition column, reveals components masked by the water peak, shortens the analysis time, and allows use of more specific partition columns for resolving the dissolved components. EXPERIMENTAL
Apparatus. An F & M Scientific Corp., Model 119C, gas chromatograph, equipped with a 100,000ohm thermistor detector and a stainless steel preheater was used in all analyses. The detector signal was supplied to a 1-mv. Brown recorder. All columns were prepared from 0.25inch 0.d. copper tubing and the connections were made with Swvagelok fittings (Crawford Fitting Co., 8811 East 140th St., Cleveland 10, Ohio). Gas Chromatograph Conditions. The gas chromatograph columns were composed of two parts: A , water absorption column; and B , partition column. The two columns were connected in series in the order A-B with the preheater attached to the water absorption column and the partition column attached to the detector. The preheater was niaintained a t 200' C. while the columns and detector cell were operated at 40" C. The flow of carrier gas, helium, was maintained a t a rate of 40 ml. per minute.
Preparation of Phosphorus Pentoxide Absorption Column. The phosphorus pentoxide absorption column was prepared by packing a 16-inch length of copper tubing with a mixture of nine parts of phosphorus pentoxide powder and one part of 60- to 80-mesh firebrick which had been coated with Desicote (Beckman Instruments, Inc., Catalog KO. 18772) and Siliclad (ClayAdams Inc., Catalog KO.5-600). The firebrick used in the water absorption column was prepared by treating 200 grams of 60- to 80-mesh firebrick (F&M Scientific Corp.) with 50 ml. of Desicote. After this mixture was rolled for 2 hours, it was transferred to a Buchner funnel and washed with 200-ml. portions of acetone until the acetone filtrate was colorless. This treated firebrick was air-dried by suction on the Buchner funnel for one-half hour and then it was dried overnight in an oven at 100' C. The Desicote-treated firebrick was mixed with 600 ml. of water containing 40 ml. of Siliclad solution in a similar manner with the exception that the treated firebrick was washed twice with water in place of acetone. One gram of the Desicote- and Siliclad-treated firebrick was mixed with 9 grams of phosphorus pentoxide powder in a tightly capped 20-ml. capacity vial. A polyethylene funnel was attached by Tygon tubing to one end of a 16-inch length of copper tubing; the other end of the copper tubing was plugged with borosilicate glass wool and capped with a rubber septum. The vial was shaken for about 3 minutes, or until the firebrick and phosphorus pentoxide powder were thoroughly mised. It was then uncapped and quickly inverted in the funnel. While the vial was held tightly in the fuiuiel, t,he colurnn was vibrated lvith a sindl vibrstiiig tool. The flow of material from the funnel into the column was observed through the clear Tygon tubing. The packing of a 16-inch length of VOL. 35, NO. 13, DECEMBER 1963
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