A Portable Device for Real-time Gas Analysis Using Ion- Selective Electrodes

Portable instrumentation has been developed to measure halogen acid gases inside and around armored vehicles during fire suppression tests at Aberdeen Proving Ground. This technology is used to troubleshoot for leaks, conduct area sampling, and measure occupational exposure levels of toxic gases in air. A continuous denuder extracts the gases into an aqueous medium containing the appropriate ionic strength adjustor. Small air and liquid pumps move the air sample and liquid trapping solution through the analyzer. Flow-through ion-selective and reference electrodes detect the conjugate base of the acid gas and generate a Nernstian voltage. The amount of voltage generated provides concentration information via a direct readout in mV or air concentration in parts per million. An onboard microprocessor provides control of the pump speed, logs the cell voltage along with a date and time stamp, and converts the cell voltage readings to a direct concentration read-out. Under laboratory conditions the results obtained with this instrument compare favorably with values obtained using a midget impinger and ion chromatography from a few ppm to several thousand ppm. The device is totally portable and calibration can be performed quickly and easily under field conditions. Its primary advantages over other hand-held instruments are its wide detection range, ability to detect aerosols, and rapid response time. In addition to the halogen acid gases, this technology has the potential for performing real-time analysis of other gases such as hydrogen sulfide, hydrogen cyanide, and ammonia with the appropriate ionselective electrode. INTRODUCTION During combat and training activities, the soldier may be exposed to a number of toxic gases including acid gases such as hydrogen fluoride (HF) and hydrogen chloride (HCl). Hydrogen fluoride is typically related to the thermal degradation of fluorinated fire suppression agents (such as heptafluoropropane), whereas hydrogen chloride may be produced from weapons systems that use perchlorate based propellants (such as the STINGER and MLRS). In order to measure soldier exposure levels to these acid gases, the Army has developed an analyzer that measures gases and aerosols of hydrogen fluoride, hydrogen chloride, and hydrogen cyanide in real time. The original analyzer is suitcase-size and fairly portable; however, for some testing situations it is too big to be placed inside of an armored vehicle and lacks sufficient portability to be used as an area monitor around the outside of a test vehicle. This study describes an attempt to significantly reduce the size of the acid gas analyzer and to make it totally portable. EXPERIMENTAL Ammo boxes are considered typical stowage and usually fit very easily inside most armored vehicles; therefore, it was decided to fit the components of the suitcase-size acid gas analyzer into a 100-round 50-cal. ammo box. For the first reduction in size we eliminated some of the onboard fluids. The three 125-mL bottles of aqueous standards used for calibration were completely eliminated. Calibration is now conducted before and after each test using external calibration standards. Two 1-liter bottles are normally used in the full size analyzer, one for a vacuum trap and one to contain a pH-buffered trapping solution. Reducing the onboard vacuum trap and buffered trapping solution by one-half was necessary to close the lid on the ammo box. However, the reduction in trapping solution resulted in a run time that was cut in half. Most fire suppression tests last for 30 min. to 1 hr., so the reduced run time does not pose a serious problem. A 3 x 5-in. plastic board covered on both sides with velcro serves as a mounting board with the electronics components on one side and the chemistry components on the other side. The chemistry board consists of a potentiometric ion-selective electrode and a reference electrode (Figure 1). The potentiometric electrode detects the acid gas of interest and generates a voltage in proportion to the concentration of the acid gas being measuring (similar to