Direct Steady State Calibration of a Flame Ionization Detector Grant F. Carruth1 and Riki Kobayashi Depurtment of Chemical Engineering, Rice Unioersity, Houston, Texas 77001
QUANTITATIVE ACCURACY with any analytical detector requires careful calibration of the detector. Several more common techniques are the following: The direct injection technique in which the calibration sample is injected directly into the chromatographic system with a syringe or sample valve ( I , 2). Problems are wall adsorption and injection of the full sample. The exponential dilution flask (3). Problems are wall adsorption and knowledge of the initial concentration injected. Diffusion of gas from sealed Teflon tubes ( 4 ) . The filled tube is immersed in the diluting gas and the amount diffused is determined by differential weighing. Diffusion of a vapor out of a capillary tube into a gas stream ( 5 ) , for very low concentrations. Major problems are the degree of temperature and pressure control required to maintain a steady diffusional rate and the determination of the amount of material diffused, since very long periods of time are required to obtain data at very low concentrations. Altshuller and Cohen (6) discuss difficulties and precautions. Preparation of gas mixtures in sample bombs (7, 8), limited t o the higher concentrations. Sequential dilution can have adsorption errors. A flow calibration device (9), employing a motor and reduction gear which drives a hypodermic syringe at a constant flow rate. Concentration changes are made rapidly over wide concentration ranges, wall adsorption can be neglected, and large mixture quantities can be made. King and DuPre (IO) describe a flow dilution technique which saturates a carrier gas with a known vapor and subsequently dilutes the saturated vapor t o provide wide concentration ranges. Early in this study some samples were made up in sample bombs (approximately 730 cm3, tare 1000 grams, and rated t o 1000 psi maximum). Reliable samples could be made down t o a mole fraction of about Below this concentration, results indicated that wall adsorption was becoming important. The diffusion-dilution technique was scouted cursorily but was extremely sensitive to temperature and pressure variations Present address, E. I. du Pont de Nemours & Co., Research and Development Laboratory, Old Hickory, Tenn. 37138 ( I ) H. M. McNair and E. J. Bonelli, “Basic Gas Chromatography,”
Varian Aerograph, Walnut Creek, Calif., 1967. ( 2 ) L. Mikkelson, J . Gns Chrotnarogr., 5, 601 (1967). (3) J. E. Lovelock, ANAL.CHEM.,33, 162 (1961). (4) A. E. O’Keefe and G. C. Ortrnan, ibid., 38, 760 (1966). ( 5 ) D. H. Desty, C. J. Geach, and A. Goldup, “Gas Chroma-
tography 1960,” Butterworths, Washington, 1960, p 46.
(6) A. P. Altshuller and I. R. Cohen, ANAL.CHEM., 32, 802 (1960). (7) H. L. Chang, Ph.D. Thesis, Rice University, Houston, Texas,
1966.
10-11
10-10
10-9
10-8
10-7
MOLAR FLOW RATE OF HYDROCARBON (moleslsec) Figure 1. Calibration curves for flame ionization detector NOTE: Size of original drawing 25“ X 25”
and had uncertainties in measuring the differential height of the liquid hydrocarbon column for very low concentrations. The exponential dilution technique provided qualitative indication on the wide range linearity of the FID. However, the bulk of the calibration data shown as Figure 1 was obtained with the micropump metering system. STEADY STATE FLOW METHOD
Responses over wide concentration ranges of n-paraffin hydrocarbons ethane through n-decane in helium carrier gas were determined by a steady state flow method. Calibration mixtures were prepared by metering known flow rates of the hydrocarbon into a helium carrier gas. Accurate metering was accomplished with an 8-cm3 positive displacement plunger pump (rated to 5000 psi) designed by W. E. A. Ruska of this laboratory and constructed by the Rice Mechanical Engineering Machine Shop. A schematic diagram of the system is shown in Figure 2. Desired discharge rates were obtained by use of a specially designed positive gear transmission with 224 fixed speeds and an overall gear ratio of 4.5 million t o 1, in an approximately geometric progression. Maximum/minimum flow rates were 0.36 cm3/minand 8 X 10-8 cm3/min,respectively. Mechanical details of the pump are described elsewhere (11). The temperature of the pump and injection system was controlled within 10.1 “C. The internal pressure of the pump was measured to *0.004 psi by a minature pressure transducer. The metered flow was injected into the carrier gas by
(8) I. Wichterle, Monograph, Rice University, Houston, Texas,
1970. (9) G. 0 . Nelson and K. S. Griggs, Rec. Sci. Imtrum., 39, 927 (1968). (10) W. H. King, Jr., and G . D. DuPre, ANAL.CHEM.,41, 1936 (1969).
(11) W. E. A. Ruska, G. F. Carruth, and R. Kobayashi, “Micropump-An Apparatus for Steady State Synthesis of Gas Mixtures at Very Dilute Concentrations,” submitted for publication Rec. Sci. Instrum., 1972. ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
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choosing conditions that would optimize the “linear” range; therefore, more nonlinearity at high sample flow rates was accepted. The signal from the FID was measured with a BarberColman electrometer capable of measuring from to A full scale. The standing current of the F I D with an extinguished flame was 6-8 X 10-14 A ; and, with the flame ignited, the standing current was 1.0-1.4 X lo-’* A. With pure helium Zero gas flowing, the signal was typically 1.9-2.5 X 10- l 2 A for flow rates of 12 cm3/min. The helium base line was very stable for a given run and was not significantly affected by small variations in the helium flow rate. Noise with pure helium carrier was about 5 X loux5A.
HELIUM
CARRIER
EXPERIMENTAL
Ethane, propane, and butane were metered as vapors and the other hydrocarbons were metered as liquids. For liquid metering, the hydrocarbon was charged to the pump and all air bubbles were expelled from the pump. The temperature control system for the pump was activated, after which several hours were allowed for the pump and its contents to come to thermal equilibrium. The pressure inside the pump was monitored as the temperature was increasing from room temperature to operating temperature (approximately 90 “F) and the discharge valve periodically opened to prevent a large pressure increase due to the expansion of the liquid. After thermal equilibrium was attained, the carrier gas flow rate was set. (The absolute value of the carrier gas flow rate was relatively unimportant during calibration, as the main interest was in determining the molar flow rate of hydrocarbon and its corresponding signal.) The pump discharge valve was opened and the pump set at the desired speed. The pressure in the pump was monitored with the pressure transducer and the micrometer valve adjusted so that a constant pressure was attained. Sometimes this procedure, especially at low flow rates, required several hours time. After pressure and signal were essentially constant, this value was recorded as the equilibrium value and then another flow rate was chosen in order to span the desired calibration range. An additional procedure based on a higher concentration calibration evolved which conserved time in the medium to very low concentration ranges. This procedure was used on pentane, hexane, and heptane. The pump was filled with a vapor mixture of helium and hydrocarbon such that the hydrocarbon concentration was somewhat less than its saturated concentration at the prevailing temperature and pressure. The mixture then was partially expelled from the pump and pure helium was drawn into the pump to dilute the contents. Approximately a n hour was sufficient to ensure that the composition in the pump was homogeneous. Then the mixture was metered at various volumetric flow rates and the signals were recorded. One of the flow rates with its corresponding signal was chosen as a reference. The reference signal was referred to a section of the calibration curve which had previously been determined by metering pure hydrocarbon so as to obtain the corresponding hydrocarbon flow rate. The remaining molar flow rates of the helium-hydrocarbon mixture were readily calculated by taking volume flow rate ratios with respect to the reference flow rate. This method proved to be somewhat faster at the low concentrations and gave results that agreed with results obtained by the more timeconsuming procedure of pumping liquid at very low flow rates. N o systematic differences were observed between the two methods of making mixtures as long as the lowest flow rate did not differ from the reference flow rate by more than two orders of magnitude. Adsorption problems are relatively unimportant, since an absolute value was not given to the concentration of the vapor mixture. The possibility that dilution errors are accumulative was considered. However, this was not observed, as values laboriously obtained by Liquids.
Figure 2. hlicropump and auxiliary equipment for calibration of detector 1. Pump 2. Constant temp. enclosure 3. Mercury contact thermometer 4. Relay 5. Incandescent lamp 6. Blower 7. Charging vessel 8. Precision thermometer 9. Pressure transducer 10. Cross pattern metering valve 11. Worm gear 12. Worm 13. Micrometer dial 14. Synchronous motor 15. Transmission 16. Motor reversing switch 17. Variable voltage control
pumping against a slight flow restriction provided by a variable orifice. A major problem was generation of a steady concentration of hydrocarbon because concentration variations, due to surface effects, relaxation phenomena, erc., were encountered initially for metered flow rates less than about 0.1 pl/min. Satisfactory performance was attained with a Nupro Cross Pattern Metering Valve as a variable orifice restriction. A mechanical linkage helped secure a reproducible setting of the variable orifice. The amount of hydrocarbon passing the detector per unit time was determined with a Barber-Colman Hydrogen Flame Ionization Detector (FID), with a 1-cm diameter platinum ring as a n ion collector. The ring was positioned 1 cm above the stainless steel burner tip and operated at +240 volts dc with respect t o the grounded burner tip. The F I D was operated a t 116 + 1 “C. Hydrogen Zero gas (
