I/EC
Instrumentation
Ionization Detectors for Gas Chromatography Today's equipment features ultrahigh sensitivity and linearity over wide concentration ranges by P. H. Stirling and Henry Ho, Canadian Industries Ltd.
VU A s CHROMATOGRAPHY separates
complex mixtures into a series of dilute binary mixtures with the carrier gas. The usual differential detectors, such as the thermal con ductivity cell or the gas density balance, sense the small changes in a bulk property of the carrier gas due to the sample components and their sensitivity is limited by the over-all stability of this property. Ioniza tion detectors respond directly to the sample molecules in the chro matographic column effluent and are thus relatively independent of the carrier gas properties. Thermionic emission, gaseous elec tric discharge, thermal exitation, radioactivity, and collision with atoms have all been employed as ionizing processes in ionization de tectors. The most efficient tech nique appears to be collision of the sample molecules with metastable noble gas atoms. These detectors are not equally sensitive to all compounds because of the ionizing process and their deficiencies in this respect can be exceedingly advantageous in some respects. The hydrogen flame ioni zation detector is especially suitable for measuring air pollution or traces of organic materials in aqueous ex tracts because of its insensitivity to carbon dioxide, carbon monoxide, oxides of nitrogen, sulfur dioxide, ammonia, and water vapor. The response of ionization detectors to composition changes is generally extremely rapid and the ionization detectors with small effective vol umes (about 10 cu. mm.) measure the rate of emergence of sample molecules and not the integral con centration of the eluted peak. Argon Ionization Detector. The most efficient ionization detector is the argon ionization detector of
Lovelock which uses the collision of metastable argon atoms with the sample molecules to effect ioniza tion. The metastable atoms are produced by the electron bombard ment of argon gas at atmospheric pressure. The electrons are ob tained from a radioactive source and are accelerated in a strong electric
Available Ionization Detectors Argon ionization gage W. G. Pye & Co., England Perkin-Elmer Co. Barber-Colman Co. Research Specialties Inc. Hydrogen flame ionization detectors Beckman Instruments Ltd. Perkin-Elmer Co. F. & M. Scientific Co. Wilkins Instrument & Research Inc. Loe Engineering Co. Shandon Scientific Co. Ltd. (England) High vacuum detector Burrell Co. Radio frequency glow discharge detector American Instrument Co.
field. Ionizing efficiency, which is many "orders of magnitude" higher than the simple 0-ray gage, and the background noise, which is associ ated with the radioactive source, is correspondingly reduced, thus al lowing high sensitivities to be achieved. The argon ionization gage is really a whole class of detectors. There are many versions with dif fering geometries, electrode arrange ments, and varying means of ex citing the argon into the metastable state. Tesla coil discharges or radio frequency energy and ultraviolet light have been used as alternatives to the radioactive source. The ar gon may be used as the carrier gas or it can be mixed with the chro matographic column effluent.
Lovelock has described three ver sions of the argon ionization de tector, all of which use a central anode inside a cylindrical cathode chamber lined with the ionizing radioactive source. The first or "large" model uses a solid anode approximately 2 to 3 mm. in diameter carried on a Teflon insulator and has an effective volume of approximately 6 cc. The "small" model, which is in tended for use with capillary columns, uses the rounded off end of the column itself as the anode and this is recessed into a small cylindrical cavity in the top of the insulator. The triode detector is based on the "small" detector and uses an extra collector ring electrode placed just inside the ionization chamber to measure the positive carriers. The minimum quantity of propane de tectable in argon by the triode detector was 0.03 part per billion as compared to the 20 parts per billion for the large detector (20 ml. per min. argon). The small and triode detectors are linear over a wider range of response than the simple version and can be used for concentrations up to 0.2%. They monitor the rate of emergence of molecules from a column and their efficiency is independent of flow rate over a wide range whereas the efficiency of the simple detector varies linearly with flow rate because of diffusion effects. They all use a 10- to 50-millicurie source of a /3-emitter or 20 to 50 microcuries of an α-emitter as the ionizing source. By using "tritium foil" in the small argon detector, responses to the "awkward" gases [H2, N 2 , 0 2 , C 0 2 , CO, (CN)2, CH 4 , C2H2, C2H4, H 2 S, N 0 2 , N 2 0 , CH3CI] whose ionization potential lies above the 11.7 electron-volts stored in the metastable argon atoms VOL. 52, NO. 11 ·
NOVEMBER 1960
61 A
INSTRUMENTATION have been obtained. By bleeding a small trace of hydrocarbon such as acetylene or ethylene into the argon detector, an alternative mode of detection of these awkward gases can be utilized. Detection limits of 0.05 p.p.m. for oxygen and 0.15 p.p.m. for nitrogen have been achieved this way. A pulsed form of the argon ionization detector with planar geometry is also capable of determining the permanent gases in concentrations of 10 to 100 parts per billion. Another version of the argon ionization detector uses a sphere and plane geometry and this conveniently allows the electrode spacing to be altered. T h e geometry of these detectors is critical and as yet there is little specific information enabling one to choose between them. Difficulties are sometimes experienced if water at high concentrations passes into the argon detector. This desensitizes it to hydrocarbons and it takes a while for it to recover. Traces of water vapor below 30 p.p.m. have no effect on the detector. Attempts to eliminate the water using calcium carbide plugs and thus converting it to acetylene have been unsuccessful. Backflushing or bypassing techniques should be used if water is present in the sample and the long tails of water peaks on many columns present some difficulties. Hydrogen Flame Ionization Detector. T h e simple detector uses a hypodermic needle as a j e t a n d one electrode and measures the electrical resistance of the flame between this and an adjacent electrode. T h e hydrogen flame detector is linear over a wider range of concentration than the argon gage and may be used successfully up to concentrations of 1 to 5 % , and down to parts per billion. A linearity range of 106 to 108 and a detector limit of 0.1 part per billion is claimed for this detector. T h e linearity is dependent on flame and the sensitivity increases with flame temperature. T h e addition of nitrogen, argon, or oxygen to the flame is advantageous since they either reduce noise or increase the signal. T h e number of ions formed from a sample is smaller than the argon detector but is far larger than would be expected from con62 A
sideration of the flame temperature only. T h e hydrogen flame detector is easy to construct and operate at moderate sensitivities and its insensitivity to water vapor can be readily turned to advantage in analyzing biological materials. T h e flame ionization detector is especially suitable for high temperature gas chromatography, temperature programmed, or moving gradient gas chromatography whereas the argon detector is limited by the breakdown of the tritiatcd titanium hydride at temperatures below 200° C. T h e flame detector is sensitive to fluorocarbons and in this field is superior to all other detectors. T h e extremely small effective volume (1 to 5 cu. mm.) makes it particularly suitable for use with capillary columns. Gas Discharge Detector. This, as first described by Harley & Pretorius and later by Pitkethly, was extremely inconvenient because of the rapid poisoning of the electrodes and the necessity of working at reduced pressure. If the electrodes are constructed of platinum in the form of a short coaxial rod and cylinder they may be subjected to a strong cleaning discharge at the end of each analytical cycle and reproducible results obtained. T h e detector is operated at about 4-mm. pressure and is capable of extreme sensitivity, of the order of parts per billion or better. This detector is sensitive to both permanent and hydrocarbon gases. Radio frequency or Tesla coil gas discharge detectors have been constructed which operate at atmospheric pressure, and various methods of measuring the response of these have been used. O n e method uses a photocell to measure the emitted light intensity and this also gives useful qualitative information from spectroscopic monitoring. Usually probe electrodes are inserted into the discharge a n d the resistance is measured. T h e most successful version of the gas discharge detector developed by K a r m a n and Bowman uses a short 0.02-inch wire inside an Veto 3 / 1 6 -inch metallic cylinder with R F power at 27 meter counts per second applied across the assembly. A sensitivity of some several hundred
INDUSTRIAL AND ENGINEERING CHEMISTRY
times that of a katharometer is possible and good linearity is achieved. T h e glow discharge is sensitive to the sample flow rate because of the small effective volume, which is, however, several times larger than the "small" Lovelock argon or the hydrogen flame ionization detectors and is thus intermediate in behavior. This makes it unsuitable for use with capillary columns unless its dimensions are reduced. Carrier gas temperature and pressure affect it and slow electrode contamination occurs. A cleaning discharge may be used but requires noble metal electrodes for reproducibility. High Vacuum Ionization Detectors. Here thermionic emission from a heated filament is used as the ionization source. A modified electronic vacuum gage is used. Sensitivities a hundred-fold greater than possible with a katharometer were obtained by Bryce and Ryce. Later development by Hinkle and coworkers increased the sensitivity another two decades by collimating the electron beam and stabilizing the emission current. This elegant but complex detector responds to all vapors, is insensitive to flow rate and carrier gas temperatures, and, as it requires a very small sample, it is ideal for use with capillary columns. A Penney vacuum ionization gage which uses a magnet to obtain more efficient ionization can also be employed in a similar m a n n e r to the Bryce a n d Ryce gage. Filaments for these gages are best constructed of rhenium and it is advantageous to use two terminal loop electrodes for the grids and anode so that degassing and flashing-off of contaminants is easily performed. -Ray Ionization Gages
Ionization is accomplished by the electrons emitted from the radioactive source. These detectors are usually used differentially since they effectively measure changes in the collision cross section of the carrier gas. While they are relatively insensitive compared to the other ionization detectors, they can be used at high concentrations of sample gas without "blowing-out" the flame, or creating an arc discharge, or saturat{Continued on page 64 A)
ROLLAND SIMPLIFIES WATER QUALITY CONTROL
View of the exterior of the new Rolland Paper Company water treating plant, designed to purify 4,000,000 gallons of water a day, and housing a Hardinge Automatic Backwash Sand Filter.
T h e Rolland Paper Company, Limited, of St. Jerome, Quebec, is located in the Laurentian district of Canada, a vacation land visited b y m a n y who enjoy the natural beauties of t h e country, t h e excellent cuisine, and good sports. As a manufacturer of fine papers, including rag, rag content a n d sul phite bond, ledger, writing, and spe cialties, the Rolland Company is par ticularly interested in maintaining and improving the high quality of their papers. New techniques and ma chines are continuously being added. One of the most recent acquisitions h a s been a Hardinge Automatic Backwash Sand Filter, plus Flocculation Units, all combined to give t h e m a complete water t r e a t m e n t plant for controlling the quality of their process water.
W i t h good water continuously available as a basic ingredient for their process, product quality control is simplified. T h e vagaries of nature, bringing sudden storms t o muddy-up r a w water sources, are once again only a natural phenomenon, a joy to the farmer, and a prelude to rainbows.
Driving mechanism for one of two Hardinge Flocculating Units which precede the ABW Filter.
General view of the Rolland Paper Company water treatment plant interior showing the 12J^' χ 76'."Hardinge Automatic Backwash Sand Filter. The backwash mechanism travels from end to end of the filter tank along tracks on the sidewalls.
HARDINGE COMPANY, INCORPORATED Moin Office and Works
·
240 Arch St., York, Po.
"Hardinge Equipment—Built Better to Last Longer." Circle No. 32 on Readers' Service Card 64 A
INDUSTRIAL A N D ENGINEERING CHEMISTRY
NEW YORK TORONTO CHICAGO HIBBING SALT LAKE CITY SAN FRANCISCO HOUSTON LAKELAND BIRMINGHAM
INSTRUMENTATION ing as occurs with the more sensitive ionization detectors. Sensitivities of ten to a hundred-fold times that of a katharometer are achieved. T h e chief disadvantage of these detectors is the necessity of using strong ra dioactive sources to obtain good ionization. Ion C u r r e n t M e a s u r e m e n t . Most of these detectors give currents of the order of millionths or billionths of a microampere and require the use of electrometer techniques. T h e glow discharge detectors, however, yield currents of the order of milliamperes for medium sample con centrations a n d are very useful for routine laboratory work. T h e hy drogen flame detector using a large flame can be operated with a simple analog computing amplifier for mod erate concentrations. Electrometer stability is a prime consideration in using these detectors and the use of a vibrating reed impedance con version stage is preferable to the use of electrometer triodes for direct current measurement. Suitable vi brating reeds are available from the Stevens-Arnolds Co. Usage. Hydrogen flame ioniza tion detectors have been in field service for several years as have the low pressure glow discharge and argon ionization gages. As yet, only the argon ionization gage is available in a commercial process chromatograph but by the end of the year it is expected that there will be several process chromatographs available with flame ionization de tectors. Laboratory units using hy drogen flame, radio frequency dis charge, high-vacuum and the argonionization gages are all commercially available. T h e two detectors which promise most for process usage are the a.c.-operated hydrogen flame and the "small" argon-ionization gages and these will form the basis of many future analytical instruments. A portable version of the hydrogen flame detector is available from the Perkin-Elmer Co. as a parts-permillion hydrocarbon detector.
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