Traditional electronic noses, also called eNoses, use a series of dissimilar, non-specific chemical sensors to generate a recognizable response pattern.
Developers of artificial intelligence algorithms and neural networks have shown interest on eNoses for some time, but the performance of physical sensors is limited by physical instability and overlapping responses. Also, the chemistry of aromas cannot be distinguished or quantified by eNoses.
zNose® is a new kind of eNose based on ultra-fast gas chromatography, simulating an almost infinite number of specific virtual chemical sensors and producing olfactory images based on aroma chemistry. It can perform near-real-time analytical measurements of volatile organic odors and vapors with part-per-trillion sensitivity.
It separates and quantifies the individual chemicals present in an odor within seconds. With a patented solid-state mass-sensitive detector, the zNose® achieves electronically variable sensitivity, universal non-polar selectivity, and picogram sensitivity.
The combination of an electronically variable detector and an integrated vapor preconcentrator enables the device to quantify vapor concentrations ranging 6+ orders of magnitude. This article discusses the application of a portable zNose® (Figure 1) to quantify the concentration of Geosmin and 2-Methylisoborneol present in pond water samples.
Figure 1. Portable zNose® technology incorporated into a handheld instrument.
How the zNose® quantifies the chemistry of aromas
Figure 2 shows a simplified diagram of the zNose® system consisting of two sections. One part uses a solid-state detector, a capillary tube (GC column), and helium gas. The other section is equipped with a heated inlet and pump for sampling ambient air.
A “loop” trap links the two sections and serves as an injector when located in the helium section (inject position) and as an preconcentrator when positioned in the air section (sample position).
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.
The zNose® system operates in a two-step process. The first step is sampling of the ambient air (aroma), followed by collection of organic vapors (preconcentrated) on the trap. Once the sampling process is over, the trap is moved to the helium section where the helium gas is injected with the collected organic compounds.
Since the organic compounds traverse at different velocities through a capillary column, individual chemicals leave the column at characteristic times. When the chemicals leave the column, a solid state detector detects and quantifies them.
The acquisition of sensor data is controlled by an internal high-speed gate array microprocessor. An RS-232 or USB connection is used to transfer the data to a user interface or computer. Aroma chemistry can be presented as a polar olfactory image of odor intensity versus retention time or a sensor spectrum. A single n-alkane vapor standard is used for calibration.
A library of retention times of known chemicals that are indexed to the n-alkane response (Kovats indices) enables machine independent measurement and compound identification.
Chemical analysis (chromatography)
The spectrum of column flux, the so-called chromatogram, is obtained from the time derivative of the sensor spectrum (Figure 3). Accurate measure of retention times can be obtained from the chromatogram response (Figure 4) of n-alkane vapors (C6 to C14).
The system is calibrated by the graphically defined regions (red bands in Figure 4), providing a reference time base. The comparison and indexing of subsequent chemical responses is then carried out against this reference time base. For instance, the retention time index of a response in the middle of C10 and C11 is 1050.
Figure 3. Sensor response to n-alkane vapors standard, here C6 to C14, can be displayed as sensor output vs time or its polar equivalent olfactory image.
Figure 4. Chromatogram of n-alkane vapors (C6 to C14).
Properties of MIB and Geosmin
Figure 5 shows the physical properties of MIB and Geosmin. Although these compounds are relatively volatile, they are also water soluble, which gives the reason for the low Henry’s constant at room temperature. The Henry’s constant increases with increasing temperature as depicted in Figure 6.
Figure 5. Physical properties of MIB and Geosmin.
Figure 6. Henry's Constant vs Temperature
MIB and Geosmin standards
Stock solutions of Geosmin and MIB in methanol (100 µg/mL) procured from Sigma were diluted to produce 2 ng/uL solutions for calibration of the zNose® response.
Calibration by direct injections
The calibration of the zNose® was carried out by directly injecting a prescribed amount of the diluted solution of MIB and Geosmin into the inlet of the device while sampling ambient air (Figure 7). Figure 9 shows the system’s calibrated response to injection of 400 picogram of MIB.
The zNose® showed a linear response (10 counts/picogram) with increasing concentrations of MIB (Figure 10). For MIB, the minimum detection limit of the system was approximately 10 picograms.
Figure 7. Calibration standards
Figure 9. Response to direct injection of 400 picograms of MIB.
Figure 10. Linearity of MIB response
zNose® response to headspace vapors from 20 mL MIB and Geosmin-containing water samples
As shown in Figure 11, a side-ported sample needle coupled to the inlet of the zNose® was used to sample headspace vapors directly from water samples (20 mL water in a 40 mL vial). At room temperature, MIB and Geosmin had a low concentration in headspace vapors from water because they are water soluble.
However, a significant increase in the headspace concentration was observed (Figure 6) when water was heated to 40°C using a two-zone, top and bottom, vial heater (Figure 14).
Figure 12 shows replicate headspace measurements of 20 mL water samples in 40 mL septa sealed vials (offset in x-direction) holding 50 ppm MIB and Geosmin. At room temperature, the amount detected in the 20 mL headspace was 3 ng.
However, the amount detected was increased to 8.8 ng when the temperature of water was increased to 40°C. At room temperature (590 counts/ppm), the minimum detectable amount of MIB and Geosmin in water using direct sampling of headspace vapors was approximately 0.250 ppm (0.25 mg/L).
The minimum detectable amount of MIB and Geosmin was extended to approximately 100 ppb (0.1 mg/L) when the water temperature was increased to 40°C.
Figure 11. Direct headspace sampling of heated water using SlickStick®.
Figure 12. Direct sampling of headspace vapors from water at two different temperatures.
Detection of MIB and Geosmin in pondwater samples
Direct sampling method
Figure 13. Sampling headspace vapors using SlickStick®.
Second stage preconcentration method
The combination of a second stage of preconcentration and water heating can reduce the detection limit for phenol in water to the low ppb range.
The external preconcentrator accessory (SlickStick®) is a tube holding tenax absorbent and is attached between the side-ported sample needle and the inlet of the zNose® (Figure 15). The internal pump of the zNose® allows sampling of headspace vapors at a flow rate higher than direct sampling (100 cm) and for extended times without breakthrough.
Once sampling is over, the internal trap of the zNose® desorbs and collects the vapors concentrated in the tenax of the SlickStick® (Figure 16). The heater (shown in black) is connected to the zNose® and the zNose® software program allows choosing the desorption temperature up to 200°C.
As can be seen in Figure 17, the amount of phenol detected in the headspace vapors from above 40°C water containing 74 ppb of phenol was found to be 774 pg. With a response factor of 10 counts/ppb, the system showed a minimum detection level of 10 ppb (0.01 mg/L) for phenol in water.
Figure 14. Desorbing into inlet sampler of zNose®.
Field portable on-site method for testing pond water
It is a challenging task to perform the previously explained testing methods accurately in the field due to the necessity to heat relatively small amount of water samples. Due to small sample volume, it is possible to collect only picogram quantities of MIB and Geosmin.
Therefore, extreme care needs to be exercised to prevent the introduction of any contaminants from septas, vials, and even ambient air. Since a non-selective detector is used by the zNose®, the MIB and Geosmin signals can be interfered or masked by these compounds.
The advent of a field portable method that uses unheated 2-5 liter water samples can overcome these problems. After collecting the water sample, it is allowed to equilibrate in a large jar or bucket equipped with a lid containing a small hole for 10 minutes.
A sample pump is then used to preconcentrate approximately 1-2 liters of headspace vapor on 200 mg of tenax in a stainless tube. Now, the tenax tube is transferred to the zNose® inlet, where the contents of the tenax tube are desorbed and the amount of MIB and Geosmin is measured. The use of a large volume of water and headspace greatly reduces the effects of contaminants. Moreover, performing the test outdoors prevents the relatively high levels of contaminants from ambient air.
The small sample volume allows collecting only picogram quantities of MIB and Geosmin. Therefore, the analysis must be carried out with extreme care to prevent the introduction of any contaminants from septas, vials, and even from ambient air. The use of a non-selective detector by the zNose® allows these compounds to mask or interfere the MIB and Geosmin signals.
These problems can be overcome by a newly developed field portable method using 2-5 liter of unheated water samples. After collecting the water, it is allowed to equilibrate in a large jar or bucket fitted with a lid containing a small hole for 10 minutes.
A sample pump is then used to preconcentrate approximately 1-2 liters of headspace vapor on 200 mg of tenax in a stainless tube. Then, the tenax tube is transferred to the zNose® inlet, where the tube contents are desorbed and MIB and Geosmin concentrations are measured. The use of a large volume of water and headspace considerably mitigates the effects of contaminants. Moreover, performing the test outdoors avoids the relatively high levels of contaminants from ambient air.
Produced from materials originally authored by Edward J. Staples, Electronic Sensor Technology, CA, USA.
About Electronic Sensor Technology
Electronic Sensor Technology, Inc has developed and patented a breakthrough chemical vapor analysis process. This process applies gas chromatography calculations and technology toward a wide variety of industries, including Homeland Security, Life Sciences, Chemical and Petrochemical, Food & Beverage and Environmental.
How does gas chromatography work? With rapid, accurate analysis of chemical odors and vapors, this patented technology helps to provide real-time analysis for quick response solutions.
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