A Sensitive Electronic Nose

Gas chromatograph device speeds up VOC identification in air, water, soil samples

• By Edward J. Staples
• Nov 01, 2006

A new type of electronic nose, based on ultra-fast gas chromatography, can perform analytical measurements of volatile organic vapors in near real-time with part-per-trillion sensitivity. This configuration allows results to be directly compared and validated with independent laboratory test results, saving environmental engineers time and money.

How Does It Work?
The zNose;® system uses helium gas, a capillary tube (gas chromatograph column) and a solid-state detector in one section and a heated sample inlet and pump on the other. A "loop" trap links the two sections and acts as a collector, or preconcentrator, when placed in the air section (sample position) and as an injector when placed in the helium section (inject position).

The operation is a two-step process. The inlet and pump section samples ambient air (aroma) and existing organic vapors are preconcentrated on the trap. After sampling, the trap is switched into the helium section where the organic vapors are injected into the gas. The vapors pass through a capillary column at different velocities and thus, individual chemicals exit the column at characteristic times. A solid-detector identifies and quantifies the chemicals.

An internal, high-speed gate array microprocessor records sensor data, which is transferred to a user interface or computer using a wireless connection. 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 the machine to measure and identify compounds independently. The time derivative of the sensor spectrum yields the spectrum of column flux, commonly referred to as a chromatogram.

Real World Applications
This technology has been used in monitoring chemical odors at a foundry, in a river contaminated by a chemical spill of nitrobenzene and aniline, and at a municipal and gas utility site.

Most foundry emissions come from two operations in the casting process: core making and sand handling. In the core making process, new sand is mixed with resins -- phenolic, phenolic-urethane, and others -- and cured to form resin-bound sand forms (cores). The cores are used to create the open spaces in the molds that result in the ability to make hollow castings. The resins generate odors during the core making, core curing, and metal casting processes.

Using the electronic nose, operators measured chemical odors from within a commercial-scale foundry and around the adjacent community. A mixture of amines and phenolic were identified within the plant. Foundry odor chemistry can be seen in the chromatogram. The most prominent compound was phenol, varying in concentration between 40 ppbv and 5 ppmv. Amines, such as triethylamine, were detected at parts per million levels. Concentrations of phenolic compounds in the surrounding neighborhood ranged from 10 pptv to 250 pptv while amine concentrations ranged from 1 ppbv to 16 ppbv.

Real-time odor measurements allow foundry operators to react quickly to changing emission levels and enable environmental regulators to quantitatively and objectively assess the impact of these odors in the community.

Following an explosion at a petrochemical plant in northeastern China, operators monitored the concentrations of aniline and nitrobenzene in the Songhua River. The speed and economics of on-site measurements had direct financial benefits.
Operators collected water samples from the river in vials and immediately measured headspace vapors with the electronic nose. The measurements were calibrated using headspace vapors from water standards. The minimum water concentration detection level was about 70 ppb for aniline and 3 ppb for nitrobenzene. The results also were in agreement with the reported Henry's constants for these compounds.

Because the contamination plume was being carried by the river current, measurements needed to be performed quickly to track progress of the contamination. The plot of nitrobenzene concentration at a water intake near Harbin City illustrates the need for speed. Measurements had to be taken in realtime to accurately assess the size and extent of the contamination.

One environmental priority for today's utility companies is to remediate soil that was contaminated by coal-fired power generators dating back to the mid-1800s. In this process, contaminated soil is excavated and removed to a remote location where hydrocarbons are removed, and the clean soil returned for use as landfill. As a result of onsite excavation, hydrocarbons from coal tars are released into the air. Some are toxic and the U.S. Environmental Protection Agency regulates their concentration. Other hydrocarbons give off noxious odors. Because of the negative impact of these emissions on the surrounding community, site managers often monitor volatile organic compounds and odors on-site and in realtime.

With the electronic nose, a site manager measured soil gas samples from contaminated soil and quickly determined the odor signature. In total, 27 compounds were separated, however, the major hydrocarbons and their concentrations were benzene (9.5 ppm), toluene (5.7 ppm), m,p-xylene (12.6 ppm), naphthalene (17 ppm) and methyl-naphthalene (2.5 ppm).

Although vapor concentrations in close proximity to contaminated soil (less than 1 foot) were in the parts-per-million range, odor concentrations at downwind locations near the site (approximately 200 feet from active excavation) were in the 10 ppb to 50 ppb range. Upwind odor concentrations were much lower, typically in the parts per trillion range. Replicate odor samples (30 second) taken at 80-second intervals showed considerable short term variability, for example 43-percent standard deviation for 35 samples. Morning levels of naphthalene were slightly below odor threshold levels (27 ppbv) while afternoon levels were substantially higher, typically 60 ppbv downwind adjacent to the site. Real-time monitoring of odors surrounding the site allowed the site managers to assess and respond quickly to minimize the impact on the surrounding community.

This article originally appeared in the 11/01/2006 issue of Environmental Protection.