Detecting energetic materials with the zNose - Electronic Sensor ...

http://www.estcal.com/TechPapers/Security/TEP_Testing.doc 

Trace Detection of Chemical Warfare 

Agent Simulants with the zNose ® 

Edward J. Staples, Electronic Sensor Technology 

Electronic Nose 

A new type of electronic nose, called the zNose®, based upon ultra-fast gas chromatography, 

simulates an almost unlimited number of specific virtual chemical sensors, 

and produces olfactory images based entirely upon aroma chemistry. In this paper a virtual 

sensor for triethyl phosphate (TEP) was used to quantify the concentration of TEP in 

radiated and unradiated cloth samples. The zNose® is able to perform analytical measurements 

of volatile organic vapors and odors in near real time with part per-trillionsensitivity, 

precision, and accuracy. Satisfying the need for speed, separation and quantification 

of the individual chemicals within an odor is performed in seconds. Using a patented 

solid-state mass-sensitive detector, picogram sensitivity, universal non-polar selectivity, 

and electronically variable sensitivity is achieved. An integrated vapor preconcentrator 

coupled with the electronically variable detector, allow the instrument to measure 

vapor concentrations spanning 6+ orders of magnitude. A portable zNose®, shown in 

Figure 1, is a useful tool for assessing the quality of aromatic products and monitoring a 

wide variety of chemical and biological processes. In addition it can detect and quantify 

the concentration of chemical warfare agents and simulants with part per trillion sensitivity. 

Figure 1- Portable zNose® technology incorporated into a handheld instrument. 

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How the zNose Quantifies the Chemistry of Organic Vapors 

A simplified diagram of the zNose® system shown in Figure 2 consists of two 

parts. One section uses helium gas, a capillary tube (GC column) and a solid-state detector. 

The other section consists of a heated inlet and pump, which samples ambient air. 

Linking the two sections is a “loop” trap, which acts as a preconcentrator when placed in 

the air section (sample position) and as an injector when placed in the helium section (inject 

position). Operation is a two step 

process. Ambient air (vapor) is first sampled 

and organic compounds collected 

(preconcentrated) in the loop-trap. After 

sampling the trap is switched into the 

helium section where the collected organic 

compounds are injected into the helium gas. 

The organic compounds pass through a 

capillary column with different velocities 

and thus individual chemicals exit the 

column at characteristic times. As they exit 

the column they are detected and quantified 

by a solid state detector. 

An internal high speed gate array microprocessor 

controls the taking of sensor 

data which is transferred to a user interface 

or computer using an RS-232 or USB connection. 

Vapor chemistry, shown in Figure 

3, can be displayed as a sensor spectrum or 

a polar olfactory image of odor intensity vs 

Figure 2- Simplified diagram of the zNose 

showing an air section on the right and a helium 

section on the left. A loop trap preconcentrates 

organics from ambient air in the 

sample position and injects them into the helium 

section when in the inject position. 

retention time. Calibration is accomplished using a single n-alkane vapor standard. A 

library of retention times of known chemicals indexed to the n-alkane response (Kovats 

indices) allows for machine independent measurement and compound identification. 

Figure 3- Sensor response to n-alkane vapor standard, here C6-C14, can be 

displayed as sensor output vs time or its polar equivalent olfactory image. 

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Chemical Analysis (Chromatography) 

The time derivative of the sensor 

spectrum (Figure 3) yields the 

spectrum of column flux, commonly 

referred to as a chromatogram. The 

chromatogram response (Figure 4) of 

n-alkane vapors (C6 to C14) provides 

an accurate measure of retention 

times. Graphically defined regions 

shown as red bands calibrate 

the system and provides a reference 

time base against which subsequent 

chemical responses are compared or 

indexed. As an example, a response 

midway between C10 and C11 would 

have an retention time index of 1050. 

Triethyl Phosphate 

Figure 4- Chromatogram of n-alkane vapors C6 to C14). 

Triethyl phosphate is a clear, colorless liquid with a light odor. It has a density of 

1.07, a molecular weight of 182.16, a boiling point of 216 o C, and a vapor density of 6.3. 

The molecular formula is (C2H5O)3PO and its chemical structure is shown in Figure 5. 

Triethyl phosphate (TEP) is often used as a simulant for G agents such as TABUN, 

SARIN, SOMAN, and GF and to define lethality scaling between the simulants and the 

actual agents. TEP can also be used as a chemical warfare agent simulant to evaluate the 

penetration characteristics of clothing, gloves, boots and items of personal equipment. 

Figure 5- Chemical structure of 

triethyl phosphate. 

Figure 6- Testing for clothing contaminated with chemical agent 

simulants during a training exercise . 

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ZNose ® Calibration and Sensitivity 

The zNose ® utilizes ultra-fast gas chromatography 

to speciate and quantify vapor 

concentrations of TEP in just 10 seconds 

(Figure 7). A solid-state SAW detector with 

electronically variable sensitivity allows the 

instrument to operate over a wide range of 

TEP vapor concentrations. 

The instrument was calibrated by directly 

injecting 0.2 µl of methanol containing 

1072 picograms of TEP The sample was 

concentrated, injected, and chromatographed 

using a db5 column ramped from 40 o C to 

180 o C at 10 o C/second. Calibration results 

using SAW detector temperatures of 20 o C, 

40 o C, and 60 o C are shown in Figure 8. With 

a 20 o C detector, the minimum detectable 

amount of TEP was approximately 10 picograms. Sampling ambient air for just 10 seconds, 

the zNose ® Figure 7- Vertically offset TEP chromatograms 

achieved a minimum detection level for TEP of 1 part per billion (8 

nanograms/liter). Lower detector temperatures and/or longer sampling times can be 

used to achieve part per trillion sensitivity. 

Figure 8- Sensitivity vs SAW detector temperature. 

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Sample Preparation 

Simulating exposure to CW agents, five cloth samples were inoculated with 25 µL 

of TEP and then irradiated in a chamber for 1,3,5,7 and 9 minutes. Four control samples 

(C0,C1,C5, and C9) were inoculated, placed in chamber for 0,1,5, and 9 minutes without 

radiation. All samples were stored in sealed plastic ampoules prior to being tested for 

TEP. 

Test Method 

Each cloth sample was removed and placed in a 40 mL septa sealed vial containing 

10 mL of methanol to extract TEP from the sample (Figure 9). A small fraction (


Test Results 

Cloth samples containing TEP showed decreasing amounts of TEP with increasing 

radiation time as shown in the chromatograms of Figure 11. This is confirmed by a plot 

Figure 11- Vertically offset chromatograms from radiated cloth samples showing decrease 

in TEP concentration with radiation time. 

of TEP concentration vs radiation time shown in Figure 12. The concentration of TEP in 

the control samples and spiked methanol solution are shown for comparison. 

Figure 12- TEP concentration vs radiation time. 

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Summary 

The zNose ® answers the need for real-time-speed, precision, accuracy and portability 

in the detection and chemical analysis of chemical warfare agents. Performance of the 

zNose ® was testing using triethyl phosphate, a simulant for G agents TABUN, SARIN, 

and SOMAN. Part per billion sensitivity was easily achieved with 10 second analysis 

time. Extended sampling times and lower detector temperatures allow the system to 

achieve even lower , part per trillion, detection limits. Although the zNose ® contains 

only a single physical sensor, the ability to perform high-speed chromatography enables 

it to functionally perform as if it were an array of specific chemical sensors. Literally 

hundreds of virtual chemical sensors can be created covering a wide array of chemical 

warfare agents. 

An investigation of a chemical warfare agent remediation process using cloth samples 

inoculated with TEP was carried out. Radiated and unradiated control samples were 

tested and results showed nearly complete removal of TEP could be achieved in less than 

10 minutes. 

The ability to perform hundreds of chromatographic analyses per day provides a 

cost-effective solution to sample screening and process development in laboratory and 

field environments alike. This was demonstrated by the testing of all radiated and nonradiated 

samples containing TEP in less than 1 hour. Whereas such projects can often 

take weeks using a conventional instruments, with the zNose® it is possible to complete 

project investigations in less than a day. It is not difficult to see how the zNose® pays 

for itself in less than 30 days.. 

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Detecting energetic materials with the zNose - Electronic Sensor ...

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