by R. F. Wall Monsanto Chemical Co.
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Parts per Million Analyses by Ionization Detector A new ionization chamber analyzer may solve parts per billion analysis problems for air pollution and other applications
I HE development of modern civilization has been accompanied by some disadvantages, among which are growing problems in atmospheric pollution. Studies to determine causative factors and determine methods of abatement have become of increasing concern. A major part of this work is the difficult problem of determining contaminants in the parts per billion range. In addition to air pollution in cities, industrial plant atmospheres often present problems from the standpoint of both health and safety. Many process materials are toxic for long exposures in the low parts per million range. Air fractionation plants for oxygen production are unsafe in the presence of hydrocarbons, particularly acetylene, as has been amply proved by several disastrous explosions. Much oxygen is used for processing organic materials, as in acetylene production, and the proximity of associated plants makes the hazard inevitable and monitoring of trace hydrocarbons important. Materials at the part per million level are significant in many process streams, as catalyst poisons, inhibitor, etc. Oxygen and water vapor have long been effective in very low concentrations in some processes. For a great many reasons analyses down to parts per billion concentrations have become extremely important in a broad spectrum of applications, emphasizing the need for instrumental methods for fast reliable analyses to be obtained with a minimum of skill and maintenance. A new and interesting ionization chamber trace analyzer was recently developed by Mine Safety Appli-
ances and is now on the market. This analyzer offers sensitivities in the parts per billion range for many analyses and in the parts per million range for others. The instrumentation is very simple. The analytical element, a cylindrical ionization chamber, is approximately 1 cm. in diameter by 10 cm. long with a concentric collector electrode. The chamber operates at atmospheric pressure, and there are connections for sample inlet and outlet and for a reagent gas to be admitted near the inlet. Ionization is provided by a small radium source or the equivalent. A potential of from about 22.5 to 45 volts is used to collect the very small ionization current, which is measured by a simple electrometer amplifier. The extreme sensitivity of the device is derived from the marked suppression of the ion current that normally flows between the electrodes by infinitesimally small quantities of particulate matter—smoke or fog—present in the sample. With a clean sample—for example, air—• flowing through the cell, the ion current will be constant, and will show small changes if a few tenths
per cent of contaminant gases are introduced in the sample stream. The small change is completely inadequate for parts per million analyses. The trick that Mine Safety Appliances uses to make the analyzer extremely sensitive to small traces of gaseous contaminants is to make the contaminant react with a reagent gas to form a smoke— for example, hydrogen chlorate vapor with ammonia reagent to cause a significant change in ion current for an extremely small amount of hydrogen chloride present. If the reagent is selective for a particular contaminant in a sample, the analysis will be specific. The practical form of the analyzer uses two detector cells in parallel in a bridge arrangement to obtain maximum stability and independence of variation in sample background and environmental conditions. The present instrument is intended for laboratory or exploratory work, and is not yet available in an explosion-proof model. Although basically simple, the device is extremely sensitive to leakage currents, and deposition of moisture or other material that could lower the
The contaminant is converted into smoke or f o g , to which the instrument is very sensitive I/EC
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Two detector cells a r e used in p a r a l l e l in a new ionization chamber trace a n a l y z e r A potential of 2 2 . 5 to 4 5 volts collects the small ionization current
resistance across the electrode insulator must be avoided ; equivalent resistance of each chamber is of the order of 100 megohms. Although the instrument itself is simple, the ionization phenomena that take place in the detector are not, and just how the smoke particles produce the marked reduction of ion current is not well understood. The ionization current is dependent on the ionization source, composition and pressure of the gas sample, applied electric field for collection of ions, ion mobilities, recombination coefficient, etc., so that the ion current itself is a result of a complex relationship of several factors. Mine Safety Appliances have established that the smoke does not appear to depress the ion current by absorbing the ionizing radiation, but seems to act through ion capture or promoting recombination of ions or both, and possibly through decreasing the mobility of the chamber atmosphere. The sensitivity of the analyzer is rather independent of flow; a flow rate of about 4 to 5 liters per minute is convenient and provides rather fast response. The smoke-developing reagent is used in large excess to obtain a maximum yield of particulate matter from the trace quantities of contaminants present. The excess is not so large as to dilute the sample significantly. All analytical methods that arc used for analyses approaching parts per billion use some technique to obtain an extremely high sensitivity for the component of interest. The mercury vapor detector uses the extreme sensitivity of absorption 72 A
of the mercury resonance line to obtain high sensitivity. Colorimetric procedures for oxygen and dissolved silica in boiler water develop intensely colored dyes. Concentration techniques, such as trapping the contaminants from a large volume of sample in a liquid nitrogen trap, obtain a high multiplying factor for many analytical methods, as the use of the gas chromatograph. With this ionization detector the contaminant of interest is converted selectively into a smoke or fog to which the instrument is extremely sensitive. The ionization analyzer has been mostly used in studies of atmospheric contaminants. All halogen acids have been determined using ammonia as a smoke-developing reagent. Conversely, ammonia has been determined with hydrochloric or acetic acid reagent. Acetic acid avoids problems of fog formation by hydrogen chloride combining with moisture contained in a sample. Nitrous oxide has been determined with ammonia reagent, sulfur dioxide with ammonia, and halogcnated hydrocarbons through pyrolysis followed by development of an ammonium chloride smoke with hydrogen chloride reagent. Hydrogen cyanide can be determined with an acid developing reagent. The problem is to devise a system that will produce a smoke or fog specifically for the trace contaminant of interest, and the examples listed are just a beginning. Determinations of trace hydrocarbons are an obvious need, and some success in this area is believed to have been obtained. The ionization detector analyzer
INDUSTRIAL AND ENGINEERING CHEMISTRY
is so new that relatively little experience has been reported, but it seems to offer certain advantages as a trace detector. The outstanding advantage is a direct analysis of extreme sensitivity. It is very simple, easy to operate, and should require relatively little skill for use. A reagent gas which may present something of a nuisance is required, but the need for solutions is avoided. The response is very fast, and it is extremely sensitive, particularly for favorable cases. The limitation is whether or not a suitable selective smoke-developing sample processing technique can be devised. The extreme sensitivity implies that particular care be taken with the sample system, as is usual for all instruments operating on very low trace analyses. Problems of adsorption and desorption of trace contaminants can be very serious. Filtering, and other sample treatment, must be accomplished without an effect on the trace component of interest. This may introduce serious limitations on the sample treatment possible, as washing the sample or condensing contained moisture. The results of user experience will be very interesting.
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