Conventional electronic noses or eNoses employ an array 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 their physical instability and overlapping responses. Also, the chemistry of aromas cannot be separated or quantified by eNoses.
zNose® is a new type of ultra-fast gas chromatography-based eNose capable of simulating an almost infinite number of specific virtual chemical sensors and producing aroma chemistry-based olfactory images.
It can perform near-real-time analytical measurements of volatile organic odors and vapors with a sensitivity down to one part per trillion levels. It separates and quantifies the individual chemicals present in an odor within seconds.
The use of a patented solid-state mass-sensitive detector provides electronically variable sensitivity, universal non-polar selectivity, and picogram sensitivity to the zNose®. 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 effectiveness of a portable zNose® (Figure 1) for measuring the quantity of chemicals in aromas produced when using antiseptic soaps and cleaners. Facilities such as food preparation and production units and hospitals require test procedures to verify the use of antiseptic soaps and cleaners by employees. Such procedures play a key role in stopping the spread of infectious diseases.
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. A solid-state detector, a capillary tube (GC column), and helium gas are present in one section, and a heated inlet and pump is present in the other section for ambient air sampling.
A “loop” trap connecting the two sections plays a dual role by serving as an injector when positioned in the helium section (inject position) and as a preconcentrator when positioned in the air section or 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 system operates in a two-step process, of which the first step is ambient air (aroma) sampling and collection of organic vapors (preconcentrated) on the trap. Once sampling is done, the trap is transferred into the helium section to inject the collected organic compounds into the helium gas.
Since the organic compounds move across a capillary column at different velocities, individual chemicals leave the column at characteristic times, and are detected and measured by the solid state detector.
The acquisition of sensor data is controlled by an internal high-speed gate array microprocessor and an RS-232 or USB connection is used to transfer the sensor data to a user interface or computer. As illustrated in Figure 3, aroma chemistry can be presented as a sensor spectrum or a polar olfactory image of odor intensity vs retention time.
A single n-alkane vapor standard is used for calibration. Machine independent measurement and compound identification can be achieved using a library of retention times of known chemicals indexed to the n-alkane response (Kovats indices).
Chemical analysis (chromatography)
The spectrum of column flux or chromatogram can be obtained from the time derivative of the sensor spectrum (Figure 3). The chromatogram response (Figure 4) of n-alkane vapors (C6 to C14) yields an accurate measure of retention times.
Graphically defined regions (red bands) are used for the calibration of the system and provide a reference time base. The comparison and indexing of subsequent chemical responses are then carried out against the reference time base. For example, the retention time index of a response in the middle of C10 and C11 is 1050.
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
Figure 4.Chromatogram of n-alkane vapors C6 to C14
Antiseptic soap samples
Antiseptic soap and cleaning lotion samples were purchased from Nanozak Corporation and produced by U.S. Chemical. The antiseptic soap is composed of the active ingredient chloroxylenol (0.5%) and other ingredients such as sodium chloride tetrasodium EDTA, phenoxyethanol, cocamide DEA, potassium cocoate, sodium lauryl sulfate, potassium tallate, and proprietary fragrance chemicals.
The cleaning lotion consisted of the same active ingredient as well as other ingredients such as phenoxyethanol, tetrasodium EDTA, styrene acrylates copolymer, cocamide DEA, potassium cocoate, sodium lauryl suflate, potassium tallate, and peach fragrance producing chemicals.
Determining chemical signatures for soap and lotion
To test the odor chemistry of headspace vapors from antiseptic soap and cleaning lotion, approximately 10g of each sample was placed in a septa-sealed 40 mL vials. Headspace vapors were collected and analyzed by piercing the septa using a needle connected to the inlet of the zNose® analyzer as illustrated in Figure 5.
Internal temperatures of the analyzer were set to 165°C and 200°C for the inlet. The chromatographic analysis was performed using a short 10-second sample time (5 mL vapor sample) and db- 5 column temperature that was programmed to ramp from 40°C to 180°C at a rate of 10°C/second. Using this setup, the analysis of headspace vapors was completed within 10 seconds.
Figure 5. Analyzing the chemical signature of headspace vapors from soap and lotion
Figure 6 shows the chemistry of antiseptic soap headspace vapors. The olfactory diagram or Vaporprint® is shown in the top right of Figure 6. The chromatogram displaying 7 major peaks relative to the 7 major chemical components of the headspace vapors is shown in the left side of Figure 6.
The concentration (in counts) and retention indices for those major chemical components are shown in the right bottom of Figure 6. Virtual chemical sensors are defined by placing hatched red bands or windows over each of the peaks. Each virtual sensor is named (e.g. nzk 1057) and the group creates an olfactory signature or a target array of virtual sensors.
Figure 6. Chemistry of antiseptic soap headspace vapors (10 second sample)
Figure 7 shows the olfactory signature and chemistry of the antiseptic cleansing lotion. The results were completely different compared to those obtained for the antiseptic soap. One major peak or compound dominated the lotion aroma, with a concentration of approximately 20 times more than that of the soap and an index of 1044.
Figure 7. Chemistry of antiseptic cleaning lotion headspace vapors (2 second sample)
To improve the resolving power of the chromatograms, the ramping rate of the column temperature was decreased to 5°C/second and the headspace vapors from antiseptic soap and cleaning lotion were measured again. Figure 9 and 10 show the 20 second chromatogram results. As reported earlier, a single compound with an index of approximately 1032 dominated the lotion chromatogram.
Figure 8. Chemistry of antiseptic soap headspace vapors (10 second sample) and 5oC/second column ramping
Figure 9. Chemistry of antiseptic cleaning lotion headspace vapors (1 second sample) and 5°C/second column ramping
Testing chemical vapors from use of antiseptic soap
Using antiseptic soaps and cleansers provides a distinct fragrance or aroma. In this study, this aroma was evaluated as a measure of confirming their use for hygienic purposes. After applying the soap or lotion to wash the hands, the zNose® was used to measure the aroma imparted by sampling the residual vapors (Figure 10).
Figure 10. Using zNose® to test chemical vapors associated with use of antiseptic soaps and lotions
Four replicate chromatograms that are vertically offset for comparison are shown in Figure 11. The top chromatogram was obtained for hands before washing, showing no fragrance or aroma.
The next three chromatograms were obtained sequentially subsequent to washing the hands with the antiseptic soap. The chromatograms showed the compounds defined by red bands (soap headspace vapors), confirming that the antiseptic soap had been applied.
Figure 11. Fragrance from washing hands with antiseptic soap
Figure 12 shows the chemistry of fragrance from hands washed with the cleansing lotion. Surprisingly, a compound with an index of 1215 dominated the aroma, not 1032 as expected based on the earlier results obtained for lotion headspace vapors. Clearly, the more volatile compound rapidly evaporates from the hands.
Figure 12. Cleansing lotion on hands
Figure 13 shows a chromatogram of the aroma obtained from hands washed with the lotion and subsequently rinsed with water. From the results, it is evident that the fragrance was not completely removed from the hands even after rinsing with water.
Figure 13. Chemical vapors after cleansing lotion and rinsing with water
Figure 14 illustrates the persistence of aroma over time, showing replicate chromatograms obtained 90 seconds apart. The results reveal that the fragrance is still approximately 50% of its initial concentration after 8 minutes.
Figure 14. Lotion persistence vs. time
The zNose® ultra high speed gas chromatograph was used to measure the chemical profiling of fragrance from antiseptic soap and cleaning lotion. Using this system, these distinctive aromas can be measured rapidly and efficiently. This approach may be helpful in confirming the presence of hygienic conditions in facilities requiring sterile conditions such as food preparation units or hospitals.
Indexing retention times for compounds of interest using an n-alkane standard provides a simple chemical identification method that does not require multiple chemical standards and enables instrument independent chemical analysis. The results revealed that each soap or lotion sample contained unique odor signatures and marker compounds, which could be used for identification purposes.
The zNose® provides real time measurements, helping facilities to insure the adherence of employees to hygienic standards. Thanks to its high sensitivity, speed, precision, and accuracy, this portable instrument is suitable for cost-effective quality control measurements. Since these measurements are based on proven chromatographic methods, they can easily be assessed by independent laboratory testing.
Near-real-time quantification of a ‘good’ aroma as established by measurements is now possible to ensure the adherence of hygienic procedures. After defining a ‘good’ chemical signature, objective and quantitative quality control testing can be performed with other zNose® analyzers incorporated into the production process.
Produced from materials originally authored by Edward J. Staples, Electronic Sensor Technology, and Art Jones, Nanozak Corporation.
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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