Introduction and Response Targets

There are a myriad of applications for chemical sensors, and one important area is the detection of toxic gases and vapors. Toxic industrial chemicals and materials (referred to as TICs and TIMs) represent one class of materials for which chemical sensors are designed and applied. Chemical warfare agents (CWAs) are another set of materials for which chemical sensors are used, and they represent one of the most challenging groups of analytes due to their extreme toxicity, which translates to a very low required detection limit. Detection of explosives additionally requires high sensitivities due to the low vapor pressures of the explosive materials (order of magnitude 6 x 10-6 Torr for TNT and 5 x 10 9 Torr for RDX). 

As mentioned above, the organophosphonate nerve agents are some of the most toxic vapors known to man, and as such they require extraordinary sensitivities from chemical sensors. As an example, the Defense Threat Reduction Agency (DTRA) set the targets shown in Table 7.1. 

As aggressive as these numbers appear, they represent concentrations of agents that are potentially lethal, as opposed to a safe level. Thus, perhaps the most aggressive target for materials of this type is to detect at the allowable exposure level for VX, termed the threshold limit value (TLV), which is given as 0.00001 mg rrC3 ( 1 ppt). 

A critical attribute of a successful chemical sensor is its ability to reject false alarms from background interferent vapors. Sensors that false alarm frequently, whether it be daily, weekly, or even monthly, tend to be ignored with time as the personnel monitoring the devices assume that each new alarm is a false alarm. Sensors which false alarm less frequently (perhaps only once a year or less) tend to be taken more seriously. 

Chemical name Vapor pressure at 20° C Volatility 1% of volatility 

---