A recognizable response pattern is produced by conventional electronic noses (eNoses) using a wide range of dissimilar but not specific chemical sensors. For some time, electronic noses gained popularity with developers of artificial intelligence algorithms and neural networks. However, physical sensors have limited performance due to physical instability and overlapping responses. The chemistry of aromas cannot be quantified or separated by eNoses.
zNose®, a new type of electronic nose, is based upon ultra-fast gas chromatography and simulates an almost unlimited number of specific virtual chemical sensors. This new electronic nose produces olfactory images based upon aroma chemistry.
The zNose® is capable of performing analytical measurements of volatile organic vapors and odors in near real time with part-per-trillion sensitivity. It takes only seconds to separate and quantify the individual chemicals within an odor. Electronically variable sensitivity, universal non-polar selectivity, and picogram sensitivity are achieved using a patented solid-state mass-sensitive detector. The instrument measures vapor concentrations spanning 6+ orders of magnitude with the help of an integrated vapor preconcentrator coupled with the electronically variable detector.
In this article, a portable zNose® (Figure 1) is shown to be a useful environmental tool for quantifying the concentration of phenol in air and 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 parts. One section uses a solid-state detector, a capillary tube (GC column) and helium gas, while the other section has a heated inlet and pump, which helps in sampling ambient air.
Connecting the two sections is a “loop” trap, which acts as an injector when placed in the helium section (inject position) and as a preconcentrator when placed in the air section (sample position). The operation involves two steps. First, ambient air (aroma) is sampled and then organic vapors are collected (preconcentrated). After sampling, the trap is transferred into the helium section, where the collected organic compounds are helium gas.
These organic compounds then pass a capillary column with varying velocities and hence individual chemicals exit the column at characteristic times. These chemicals are quantified by a solid state detector as they leave the compound.
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 collection of sensor data is controlled by an internal high-speed gate array microprocessor, which is a computer or user interface using a USB or RS-232 connection.
Aroma chemistry is displayed as a sensor spectrum or a polar of odor intensity vs retention time. This is accomplished by using a single n-alkane vapor library of retention times of known indexed to the n-alkane response (Kovats for machine independent measurement and identification.
on the trap. helium section injected into the through a and thus characteristic detected and coprocessor transferred to a or USB con- Figure 3, can be olfactory image Calibration is standard. A chemicals indices) allows compound
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.
Chemical Analysis (Chromatography)
The time derivative of the sensor spectrum (Figure 3) yields the spectrum of column flux, generally referred to as a chromatogram. An accurate measure of retention times is provided by the chromatogram response (Figure 4) of n-alkane vapors (C6 to C14).
Graphically defined regions, shown as red bands, calibrate the system and provide a reference time base against which subsequent chemical responses are indexed or compared. For instance, a response midway between C10 and C11 would have a retention time index of 1050.
Figure 4. Chromatogram of n-alkane vapors (C6 to C14)
Properties of Phenol
Figure 5 displays the physical properties of phenol. Although it is relatively volatile, it is also extremely soluble in water which accounts for the low Henry’s constant. At room temperature, only 0.00136% will partition into the air from water as headspace vapor.
Figure 5. Physical properties of Phenol
A stock solution of phenol in methanol, produced by dissolving 177 mg phenol crystals in 3 mL of methanol (59 mg/mL), was used as a master calibration mixture. Two additional standards at 147.5 nanograms/microliter (ng/mL) and 14.75 ng/mL were produced by serial dilutions.
Figure 6. Calibration Standards
Calibration by Direct Injection
The zNose was calibrated by injecting a known amount of phenol directly into the inlet of the instrument while sampling ambient air (Figure 7).
Figure 8 shows the calibrated response of the system to an 11.8-nanogram injection of phenol with an indexed retention time of 1085. The response of the zNose®, 1017 counts/nanogram, was linear with increasing amounts of phenol as shown in Figure 9. 10 picograms was the minimum detection limit for phenol.
Figure 7. Calibration by directly injecting a known mass of phenol
Figure 8. Response to direct injection of 11.8 nanograms of phenol
Figure 9. Linearity of Phenol response
Vapor Calibration Standard
A phenol vapor standard (590 picograms/milliliter) was developed by injecting 10 µL of standard solution (147.5 ng) into a 250 mL bottle. A side-ported sampling needle attached to the inlet of the zNose® was used to extract and quantify 3.17 mL vapor samples. Figure 11 shows off-set replicate samples. The vapor concentration in the bottle following the injection is shown in an insert plot.
A maximum of 750 pg per sample indicated 40% recovery due to absorption of phenol on the walls of the bottle. The minimum vapor detection limit for phenol was about 800 parts per trillion (ppt) or 3.25 nanograms/Liter.
Figure 10. Phenol Vapor Standard
Figure 11. Replicate samples taken on phenol vapors in bottle
Direct Detection of Phenol in Water
Headspace vapors from water samples (20 mL water in a 40 mL vial) were sampled directly using a side-ported sample needle fixed to the inlet of the zNose® (Figure 13).
The concentration of phenol in headspace vapors from water at room temperature is low because phenol is highly soluble in water; however, heating the water using a two-zone, top and bottom, vial heater accessory shown in Figure 14 can increase it.
Figure 12. Direct headspace sampling of heated water
Figure 13. Direct headspace sampling of heated water
Replicate direct sample measurements (offset in x-direction) are shown in Figure 15 for water containing 5.9-ppm phenol.
At room temperature 3 ng in the 20 mL headspace was detected while raising the water temperature to 40 °C increased the amount detected to 8.8 ng. The minimum detectable amount of phenol in water at room temperature (590 counts/ ppm) was 0.250 ppm when direct sampling of water headspace vapors was used. The minimum detectable amount of phenol in water at 40 °C was approximately 100 ppb.
Figure 14. Direct Sampling of Headspace vapors from water at two different temperatures
Detection of Phenol in Water Using SlickStick® Accessory
It is possible to reduce the detection limit for phenol in water to the low ppb range by use of a 2nd stage of preconcentration mixed with water heating. The external preconcentrator accessory (SlickStick®) is a tube containing tenax absorbent and attached between the inlet of the zNose® and the side-ported sample needle as shown in Figure 15.
Using the internal pump of the zNose®, headspace vapors are sampled at a higher flow rate than direct sampling (100 ccm) and for a longer time period without breakthrough.
Figure 15. Sampling headspace vapors using SlickStick®
After sampling, the vapors concentrated in the tenax of the SlickStick® are desorbed and collected by the internal trap of the zNose® as shown in Figure 16. The heater, shown in black, connects to the zNose® and the desorbtion temperature can be selected up to 200 °C by the zNose® software program.
Figure 16. Desorbing into inlet sampler of zNose®
As shown in Figure 17, 774 pg of phenol was detected in the headspace vapors above 40 °C water containing 74 ppb of phenol. The minimum detection level for phenol in water was 10 ppb with a response factor of 10 counts/ppb.
Figure 17. Detecting Phenol in water containing 74 ppb phenol
The zNose® electronic nose or portable gas chromatograph allows fast and easy detection of phenol in water and air. Air concentrations into the part-per-trillion range are possible because phenol is relatively volatile chemical.
Headspace measurements are best performed with water samples elevated to at least 40 °C since phenol is highly soluble in water. Sampling headspace vapors with the zNose can reach minimum detectable levels of 10 ppb phenol in water when a 2nd stage preconcentrator accessory sampling is used. A summary chart of phenol MDL amounts is shown in Table I.
Table I. Phenol Minimum Detection Levels
||Minimum Detectable Level
|Direct Sampling in Air
|Direct Sampling Water
At 22.5 °C
At 40.0 °C
This information has been sourced, reviewed and adapted from materials provided by Electronic Sensor Technology.
For more information on this source, please visit Electronic Sensor Technology.