We all know that it is essential for life. However, probably because of its abundance and simple chemical composition, we often regard this tasteless and odorless substance as being important, but quite simple and ordinary. Scientifically, it is the exact opposite. It appears to show extremely complex and unusual behavior. It is the most studied substance on Earth. Yet, scientists are still puzzled over its strange properties. Even the best computers we have today cannot simulate all of the different properties of water.

Let us look at an example of the surprising properties of water. The strangeness of water starts with the fact that it exists on Earth. Water, being composed of two fairly light atoms (hydrogen and oxygen), should be in the gas phase at the usual temperature ranges that exist in our world. In fact, all compounds that are close to it (i.e. H2S, H2Se, and H2Te) are found mostly in the gas phase. But, compared to similar substances, it melts about 100 degrees above the expected melting point and it boils about 150 degrees above the expected boiling point (see Figure 1). The result is that it is the only material that exists naturally in all three forms (i.e. as ice, liquid, and vapor) on Earth.

In addition to the example given in the previous paragraph, water has at least 40 different surprising properties (See for example the “Forty-one anomalies of water” section in Ref 1). But, what is even more astonishing is the fact that most of these anomalous properties of water are absolutely crucial for life. Simply stated, life on Earth depends on these extraordinary aspects of water. Below we will discuss some of the anomalous properties of water and their importance for life. At the end we will briefly try to explain why water behaves so differently.

1-Water Has an Unusually High Heat Capacity

Heat capacity is a measure of the ability to store heat. Formally, it is defined as the amount of heat required to raise the unit mass of a substance by one degree of temperature. If the heat capacity of a substance is high, it will store heat well, i.e. its temperature will not rise much for a given amount of heat. Water has the highest heat capacity among common substances. This has a crucial impact on our life.

It is thanks to this fact that living organisms, which are mostly composed of water, can regulate their body temperature easily. For example, the human body needs to keep its temperature between 36.1 and 37.8 °C. This is only possible because it is composed mostly of water. Since the heat capacity of water is unusually high, even if the temperature of the environment changes greatly, the heat exchange between the body and the environment does not cause a great change in body temperature.

Another consequence is the moderation of the climate near large masses of water. The heat capacity of land is much less than that of water. This is why the temperatures of oceans tend to vary much less than that of land. The temperatures in the oceans vary between -2 0C and 35 0C. On land, temperatures may vary anywhere from -70 0C to 57 0C. Compare also the Moon, which has no water. Temperatures on the Moon range from -155 0C to 135 0C.

In addition to having a great heat capacity, water conducts heat more easily than any other liquid, except mercury. This makes the temperature quite uniform in living organisms. Also, the vertical temperature profile in oceans and lakes is essentially uniform due to this fact.

2- Water Has an Unusually High Heat of Evaporation

When a liquid evaporates, it absorbs heat from the environment. This energy is used to transform molecules into gas form. The heat of evaporation is defined as the amount of heat required to convert a unit mass of liquid into gas. Water has an unusually high heat of evaporation compared to most other common substances. It is so great that you need to supply about five times the amount of energy to evaporate water that is needed to heat it from 0 to 100 0C.

This fact is crucial for the evaporative cooling system of the human body and animals. When we sweat, the sweat absorbs heat from the body in order to evaporate. Since water has a very high heat of evaporation, effectively a large amount of heat is removed from the body through sweating. That is why when we engage in physical activity we sweat. Excess heat in the muscles is easily removed through the evaporation of sweat thanks to the high heat of the evaporation of water.

The high heat of evaporation also prevents dehydration. If it were low, water would then evaporate easily from the body and we would quickly dehydrate.

3- The Density of Water Behaves Unusually as a Function of Temperature

The density of almost all other liquids decreases with increasing temperature. Water is an exception to this. Starting from 0 °C, the density of water increases, and reaches a maximum at 4 °C, decreasing afterwards. Also most other liquids become denser when they condense, but water is an exception to this as well. The density of ice is less than the density of water, which is why ice can float on water.

Both of these exceptions turn out to be extremely important for underwater life. When the weather gets cold near a lake, first the temperature of the lake’s surface starts to decrease. As the temperature of the surface cools to around 4°C, it becomes denser and can move downwards, letting the warmer water reach the surface. Therefore, before the lake can start freezing almost all of the water in it needs to be cooled to approximately 0 °C. If there was not an anomaly at 4 °C then water would begin to freeze from the surface before the entire lake cools to 0 °C. Since water has an enormous heat capacity, the necessity for the entire lake to cool to approximately 0 °C before any freezing can occur delays the freezing considerably. It is also crucial that the density maximum in water is near freezing point, not at any other point.

When the temperature finally gets to 0 °C and the water start to freeze, it will start to freeze on the surface. Since ice is less dense than water, it will float on the surface and will not sink to the bottom. And once a surface layer of ice is formed, it will protect the rest of the lake from the environment and no further freezing will occur. There would not be any underwater life if ice formed on the bottom. It would also take forever for the ice to melt if it was formed on the bottom rather than on the surface.

The fact that water expands upon freezing is also important for the formation of soils. When the water freezes inside a rock, it can easily crack it into pieces, just like a soda placed in the freezer explodes upon freezing. Therefore, one of the most important steps of soil formation is dependent on this exceptional characteristic of water.

4- The Absorption Coefficient Anomaly in the Visible Region

Another interesting property of water lies in its absorption of light. Every

substance has a characteristic absorption spectrum that shows how much light at a particular wavelength is absorbed. If the absorption coefficient is high at a particular wavelength then the material will look opaque at that wavelength. If it is low, light at that particular wavelength will be transmitted and the material will appear transparent.

In Figure 2, the absorption coefficient of water is plotted as a function of wavelength (red line). The first thing you notice is that water has a very high absorption coefficient, except for a very narrow region around 500nm. In this small region of wavelength, the absorption coefficient is ten million times smaller than the neighboring regions. What is more interesting than this enormous drop in the absorption coefficient is that this dip happens exactly at the visible part of the spectrum. The human eye can only see wavelengths between 400-700 nm. This visible part of the spectrum is indicated by a rainbow colored strip in the graph. It is amazing that this exactly coincides with the region where water is transparent. Adding to this pleasant surprise is the fact that the amount of light emitted by the sun peaks around this dip as well.

Everything is conveniently adjusted for the habitants of this blue planet. The maximum intensity of emitted sunlight happens to be in the narrow range of the spectrum that we can see. And water on the atmosphere lets this part of the spectrum through thanks to the strange dip in the water absorption spectrum. Worried about the dangerous UV radiation from the sun? This is taken care of too. Just below the visible region, the absorption coefficient of water is ten million times higher. So water vapor in the atmosphere very effectively removes most of the dangerous UV light and shields us.

The spectrum of the light from the sun, the absorption spectra of water and the visible region of the spectrum that we can see are all physically independent phenomena. Yet, it is worth noting that each of these phenomena behaves in such a way that it seems they should have a precise knowledge of each other. If you think this is too much of a coincidence, there is even more. Water is also designed to maximize our visual pleasure. You are probably astonished by the lovely color match between the blue sky and the blue sea. Most people assume that this is because the sky is blue and the sea appears to be blue because it reflects the sky. In fact this is wrong. Water is blue since its absorption coefficient is higher in red; therefore it absorbs more red and reflects the blue part of the spectrum. This can be seen in Figure 2, in which the absorption coefficient in the red colored segment of the rainbow strip is more than 100 times greater than the blue part. The sky is blue for an entirely different reason (since it is blue light that is scattered the most by the nitrogen in the atmosphere). Again two very different, independent physical phenomena are at work here, but the result is a pleasant view for us.

Why is Water so Strange?

Most of these unusual properties of water are the result of the collective behavior of water molecules. That means that one cannot understand them just by thinking about a single H2O molecule. A single H2O molecule is a polar molecule: the two H atoms are slightly positive and the O atom is slightly negative. So when you put these molecules close to each other, the positively charged H atoms are attracted to the negatively charged O atoms of the neighboring water molecules. This is called “hydrogen bonding.” Because of this, molecules tend to order themselves rather than moving randomly, even in liquid water.

Hydrogen bonding is thought to be responsible for most of water’s strange properties. This is why, for example, water has a high boiling point and a high heat of evaporation. Extra energy needs to be supplied to break the hydrogen bonds before boiling and evaporation can occur. Another example is the high heat capacity. As water absorbs heat, it stores this as potential energy by breaking hydrogen bonds without considerably increasing its kinetic energy. This leads to a small temperature increase, therefore a high heat capacity is created for a given amount of heat.

But not all of the anomalous properties of water are that simple. Some of them are not even understood today, despite a considerable amount of current research. For example, water seems to play a crucial role in protein folding. Protein folding is the process by which each protein acquires a unique three-dimensional shape. And it can only function effectively in this particular shape. Despite millions of different possible folding configurations, a certain protein will always fold into its unique structure within milliseconds. But how can a protein always find its way to the same configuration? Water comes into play at this point. It is thought that the hydrophobic (water repelling) interactions between water and protein molecules and the hydrogen bonding interactions in water are major driving forces in protein folding. The exact details of this are still not known. Understanding them is the key for understanding many diseases and developing drugs.

In summary, water is central to our lives. It accounts for a large proportion of our bodies, we drink it, fish in it, and wash and swim in it. But we usually are unaware of how remarkable it is. Here, we have given only couple of examples of the anomalous properties of water that are also crucial for life. Scientifically, our understanding of water is far from being complete. It seems, as the research continues, that the already long list of mysterious aspects of this miraculous substance will get even longer as we learn more about it. On the philosophical side, it is interesting to note that a very simple molecule (H2O) has been selected as a means to do all of these vital, complicated, and unrelated jobs, all at the same time through its unexpected and surprising properties. 

References 

• “Water Structure and Behavior,” Martin Chaplin, www.martin.chaplin.btinternet.co.uk
• Segelstein, D., 1981: “The Complex Refractive Index of Water”, M.S. Thesis, University of Missouri-Kansas City.
• www.biology.arizona.edu/biochemistry/tutorials/chemi stry/page3.html
• www.hyperphysics.phy-astr.gsu.edu/hbase/chemical/water.html
• “Water – The Marvellous Molecule”, BBC TV program.
• www.bbc.co.uk/worldservice/programmes/archive/030430_molecule.shtml

In today’s cellular communication systems, cellular phones operate in several frequency bands. European systems use the Global System for Mobile Communications (GSM) at around 900 MHz and 1800 MHz; American systems use 850 MHz and 1900 MHz, frequencies that fall between the operating frequency ranges of televisions and microwaves. This frequency range is called non-ionizing, for the wave’s energy does not release electrons from atoms in living tissue. For instance, an X-ray is an ionizing wave that, to a degree, damages exposed biological material. Therefore, most concerns deal with RF energy’s heating effect rather than with ionization.

Technical Motivation

The electromagnetic spectrum extends from DC (direct current) to ionizing radiation. Scientists divide this spectrum into subregions. Cellular phones fall into the ultra-high frequency (UHF) regime, specifically from 300 MHz to 3000 MHz. By itself, a continuous UHF wave carries no information and does not enhance communication. It only becomes useful when modulation, defined as means carrying the information on a high frequency carrier, like UHF, is applied. The most common modulation techniques are amplitude modulation (AM) and frequency modulation (FM).

The capacity of the spectrum’s given section to carry information is limited by the Shannon Theorem. According to this theorem, channel capacity can be increased by increasing the system’s signal-to-noise ratio. In wired communications, channel capacity can be increased by adding more parallel optical fibers. Channel capacity in wireless communications can be increased by transmitting weak signals that attenuate rapidly near the transmitter and thus provide a given portion of the electromagnetic spectrum to be utilized many times. How a given spectrum is allocated among users affects the channel capacity. Therefore, there are several coding techniques, the most common of which are Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), and Code Division Multiple Access (CDMA).

Neglecting some small details, an electromagnetic (EM) wave’s energy is expressed in terms of power density (W/m2) across a surface. Power density measures an incident EM wave’s strength. Easily measured, it is a very preferable metric to UHF fields. For uncontrolled environments, the American National Standards Institute and the Institute of Electrical and Electronics Engineers (ANSI/IEEE C95.1) recommend a 2 to 20 W/m2 for an average external exposure to UHF. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) has similar power density recommendations for limiting the general public’s exposure to RF energy so that people will not be overheated by RF energy. As a comparison, for example, summer sunshine peaks around 1000 W/m2.

However, as power density is not a good indicator inside a living organism, scientists have defined a Specific Absorption Rate: SAR (in W/kg). For uncontrolled environments, ANSI/IEEE limits the spatial-average SAR to 0.08 W/kg whole body and to 1.6 W/kg averaged over any 1 gram of tissue. Also, 1998 ICNIRP restrictions are similar to ANSI/IEEE’s. The SAR can be estimated in three ways.

– Micro-antenna: Small antennas can determine a tissue’s electric field as well as its SAR. But it is difficult to place the antenna, and the tissue’s properties may not be known.

– Miniature thermal probes: Since RF energy heats the tissue, this technique detects the heat and the SAR in the neighborhood of the temperature cell, which then can be computed accordingly. However, this method also seems very difficult technologically.

– Numerical modeling: The numerical modeling of macroscopic bodies enables a numerical simulation, known as the Finite Difference Time Domain (FDTD), that can estimate the SAR. However, this process can be time-consuming and expensive.

Possible Health Issues

An EM wave can effect a biological change in living tissue in two ways: Depositing enough energy while passing through the biological material to alter some structures, or depositing packets of energy larger than the bond energy. Yet neither way seems to be possible, for the photon energy within the UHF zone is far less than the bond energy or the energy required to alter a living tissue’s structures. Therefore, many scientists now argue that UHF radiation at subthermal power levels can cause some biological damage.

Due to relatively low exposure levels, relatively small populations, and a lack of reliable dose estimates, proving or disproving the existence of RF exposure’s biological hazards remains an issue for epidemiology (e.g., statistical analysis of health records and animal studies).

Epidemiological Studies

Epidemiological studies were conducted among people who worked in a high frequency environment, such as radar stations. Search criteria were not limited to cellular and personnel communication system (PCS) frequencies. Due to the nature of radar and other military equipment, broader frequency ranges were covered. The epidemiology of cancer and RF radiation includes studies of cancer mortality rates among those exposed to RF energy.

Throughout these studies, people’s records were searched to determine if their cancer was due to RF exposure. These studies were made in various institutions, including the Radar Laboratory of the Massachusetts Institute of Technology, the U.S. Navy and Air Force, and the Polish military. There was no conclusive evidence that RF exposure increases the risk of cancer. Also, due to the lack of comparisons with total cancer, it was suggested that RF exposure does not have a strong effect on cancer.

Since brain cancer takes a long time to develop and epidemiological studies tell nothing about future risks, these studies have not proved or disproved that RF exposure increases the risk of cancer.

Animal Studies

Animals are the other source of information that potentially may answer people’s concerns. Experiments have studied rats exposed to certain power levels of RF energy. However, these studies found no link between cell phones and cancer.

In 1999, a Motorola-funded research program concluded that exposing rats to pulse-modulated 837 MHz RF energy, very close to that radiated by a digital cell phone, does not cause or develop brain cancer. A study in April 2000 reported that this conclusion is valid for continuous-wave RF (analog cell phones). But a 1995 study at the University of Washington (Seattle) reported that exposing rats to RF radiation at an average whole-body exposure of 1 W/kg of body weight caused breaks in their brain cells’ DNA, which is an indication of cancer. No other study has confirmed this finding.

Other studies have focused on different aspects of RF radiation rather than brain cancer. They searched animals for certain diseases and noticed an increase in disease rate. However, despite such research findings, animal studies seem to be far removed from human health.

Conclusion

Epidemiological findings and animal studies have neither proved nor disproved the health hazards of mobile phones. A February 2000 essay by the U.S. Food and Drug Administration (FDA) stated that: There is currently insufficient scientific basis for concluding either that wireless communication technologies are safe or that they pose a health risk to millions of users. Research activity continues. For example, France’s International Agency for Research on Cancer has received a research project of 8 million euros from the European Commission for a 3-year, wide epidemiological study. Also, the FDA and the Cellular Telephone Industry Association have undertaken a $1 million research project to clarify the health risks of mobile phones.

Meanwhile, some researchers are trying to find the head’s SAR by using electromagnetic simulations (FDTD). So far, they have discovered that it is strongly affected by the cell phone’s position as well as the head’s shape and properties. Therefore phone-makers are trying to design handset designs to reduce the SAR. However, it seems that the debate will remain until scientific proof is confirmed and made available.

• References
• http://www.fda.gov/cdrh/ocd/mobilphone.html.
• IEEE Spectrum. Are Mobile Phones Safe? (August 2000): 23-28.
• Moulder et. al. Cell Phones and Cancer: What is the Evidence for a Connection? Radiation Research Society, 151 (1999): 513-31.

New Findings in Biology

Olfaction was long considered a uniquely mammalian trait. Scientists have disproven this by showing that many intertebrates can smell. For example, birds were thought to be unable to smell, although they have nostrils in their bills. John Audubon, a famous nineteenth-century bird artist, reached this mistaken conclusion by observing vultures confronted with a covered and an uncovered animal corpse, he concluded that they could not smell. The minute weight of the birds’ olfactory bulb consolidated this widespread misconception. Recent research shows that birds use smell when finding and distinguishing food, choosing proper nesting sites and mates, and following avian navigation routes. Ken Stager, an orinthogist at Los Angeles County Natural History Museum, used turkey vultures to disprove Audubon’s vulture experiment. Marine biologist Betsy Bang, who measured the olfactory bulbs and tissues in the brains of 151 bird species, calculated the olfactory bulb’s mass as being between 3% to 37% of the brain’s entire mass. This shows that the ratio, and not the weight, determines a bird’s ability to smell.

Other examples are as follows:

•Pigeons perceive small amounts of odorants. If their olfactory bulbs are blocked, they become lost.

•Certain seabirds (e.g., white chinned petrels) are sensitive to the chemical emitted by their main food (plankton), and so follow an olfactory path over the sea.

•European starlings smell the best region for their nesting site.

•Chickens detect inedible bugs (e.g., bright-colored bad-tasting caterpillars) through smell and sight.

•Salmon return to their hatching sites years later by using the unique olfactory memory of these sites left in their brains.

Smelling is far more developed in mammals, especially dogs and cats, which can sense parts per billion or trillion and can identify millions of different odorants. Science would benefit greatly if such abilities could be reproduced in sensors. But first, how does the human nose smell?

Perceiving Odors

Scientists divide human olfaction into steps. First, a potential odorant emits an odor’s basic elements: volatile organic compounds (VOCs). We perceive an odor when molecules are transformed into an odor by binding the receptor proteins.(1) After binding with certain types of VOCs, these receptor proteins cause depolarization. The electrical charges produce unique signals, which the epithelium’s sensory cells transmit to our neural network (axons).

These signals then are carried to a cluster of neural networks in the brain (glomeruli).(2) Ultimately, the impulse reaches the hypothalamus and describes the scent through a process of classification and identification.

Quantifying Scents

Human odor panels or gas chromatography and mass spectroscopy (GC/MS) are used to identify odors. Such quantification is problematic, however, because it is hard to quantify the VOC’s perception in the nose as a unit of odor. Quantifying mass, volume, temperature, light intensity, and the molecular concentration of a soluble substance in a solution are reasonably objective and can be measured as a multiple of a standard unit.

But a standard olfactory measure does not exist, for it varies according to time and environment. Odor concentration is expressed as a multiple of a threshold: 50% of human “sniffers” must detect-not necessarily identify-it. This threshold is defined by the American Society for Testing and Materials (ASTM), and is accepted as the absolute threshold of odor perception. It takes 5 or 10 odor units for the human panel to identify the odor. GC/MS also can identify the odor’s chemical composition.

Electronic Nose

After developments in electronic sight and hearing, scientists sought similar progress in odor perception. Research began at the University of Warwick (Coventry, England) in the 1980s. Its participants coined the term “electronic nose,” now commonly known as “e-nose.”(3) Their progress made it a commercial commodity with many applications.

E-noses have moved from being metal oxide devices, to conducting polymers, and now to laptop-size or pocket-size odor sensors. The Swiss Federal Institute of Technology (Zurich) has made one the size of a wrist-watch. However, current e-nose use is largely restricted to labs and military applications. Scientists are trying to match or surpass the human sense of smell’s accuracy and sensitivity, after which they will work on surpassing that of the canine species.

Uses and Advantages

E-noses have a wide application in agriculture. Since they can detect minute differences, an e-nose using polymer materials can determine whether a tomato is sun-ripened, picked green, or internally damaged, and whether apple juice comes from a concentrate or is authentic but pasteurized.

Volunteers often test such products. But who wants to determine if corn oil is rancid or canola oil is oxidized? E-noses, having no such “qualms,” detect changed odors in oil samples and provide far more accurate reports.

In animal science and poultry, e-noses provide detailed reports about spoiled food. Judy Arnold, a microbiologist in Athens, GA, researches food quality for the Agricultural Research Service (ARS).

In 1998, researchers discovered that e-noses can detect gases produced by spoiled poultry products. They claim that an e-nose can determine freshness, period of time in a refrigerator, and the amount of fat in white meat. Such an objective evaluation benefits poultry farmers and producers by eliminating returns of “funny-smelling” poultry. E-noses also can detect meat’s decay rate and bacteria, overall quality and freshness, the composition of mixed meat-part products (e.g., processed meat), and how long a ham has been dry-cured. Given this, the e-nose’s ability to examine a bundle of scents makes it very useful. It can perform hundreds of preliminary assessments that would occupy a chemist for months.

The military uses e-noses to detect land mines and traces of chemical-biological weapons. This is important, for over 100 million land mines litter 62 war-torn countries. Although dog-sniffers are useful, this practice is inhumane (dogs are often injured) and impractical (they need lots of training).

E-noses also are better than metal detectors and ground-penetrating radar and infrared imaging-the former detects even tiny pieces of metal, whereas the latter often images pebbles. As e-noses can identify traces of TNT or similar explosives to the 100 parts per quadrillion level, their detection rate is far more accurate and efficient. Nomadics, a Still-water, OK-based company, produces a cigar-box-sized e-nose for this purpose. Tufts University produces an optical e-nose that is designed and functions much like a mammalian nose.

Environmentalists use e-noses to analyze air. For instance, e-noses can report the chemical makeup of odors emitted by a farm’s store of manure, detect the compounds causing that odor, help minimize leaks, and determine a new diet that will decrease such odors. With their ability to detect toxic VOCs and compounds leaking from a factory’s or waste site’s storage areas, e-noses will help environmentalists force industry to change its practices. The major difficulty here remains sampling, as concentrations vary with time and place.

Caltech has used Department of Defense funding to develop a device that identifies odors in seconds. Its 32 components swell like sponges when exposed to a particular vapor, and its resistance (hence conductivity) changes accordingly. As it can detect any type of odor, doctors at the Children’s Hospital in Los Angeles are studying medical applications. Currently, it is applied to patients’ breath to help diagnose upper respiratory infections.

The major advantages of e-noses over human noses in these areas are objectivity; ability to measure odors over long real-time periods; and immunity to fatigue, infection, mental state, hazardous material, and adaptation (gradual loss of sensitivity).

How E-noses Work

E-noses have three functional components: a sample handler, a gas sensor array, and a signal processing system. Its output identifies the odorant, estimates its concentration, and relates its characteristic properties. A sensor recognizes different types and concentrations of odors through its arrays, each of which has a different sensitivity. The resulting combination provides the response pattern that enables the e-nose to identify odorants.

In a typical e-nose, a vacuum pump pulls the first air sample into the tube housing the electronic sensor arrays. The air sampling unit exposes the odorant to the sensor, after which VOCs interact with the surface and the sensor’s active material until reaching a steady state. The sensor’s response is recorded and transmitted to the signal-processing unit. When completed, a washing gas cleanses the sensor. After the reference gas is applied to the unit, the sensor is ready to measure again.

E-nose Technologies

The sensor is the e-nose’s key element, and the sensor type is its defining characteristic. There are 5 types of e-nose sensors, as follows:

Optical sensors: Optical fiber sensors work through fluorescence and chemoluminescence. The tube’s glass fibers contain a thin encoated active material in their sides and at both ends. As VOCs interact with the organic matrix’s chemical dyes, the dye’s fluorescent emission changes the spectrum. These changes then are measured and recorded for different odorous particles.

Fiber arrays with different dye mixtures can be used as sensors. These are fabricated by dipcoating (binding a plastic solution to a substrate), micro electromechanical system (MEMS), and precision machining. The main advantage is that this adjustable tool can filter out noise. Also, since many dye forms are available in biological research, sensors are cheap and easy to fabricate. But the instrumentation control systems are complex, which adds to the cost, and have a limited lifetime due to photo bleaching (the sensing process slowly consumes the fluorescent dyes).

Optical sensors are sensitive and can measure low ppb (parts per billion); however, they are still in the researach stage of development.

Spectrometry-based Sensors: This group consists of a molecular spectrum-based gas chromatography (GC), an atomic mass spectrum-based mass spectrometry (MS), and a transmitted light spectrum-based light spectrum (LS). The first two can analyze the odor’s components accurately, which is a plus. However, their use of a vapor trap to increase concentration can alter the odor’s characteristics. LS devices do not consume the sample, but do require tunable quantum-well devices. GC and MS devices are commercially available, while LS devices are only at the research stage. All spectrometry-based sensors are fabricated by MEMS and precision machining, and can measure odors to a low ppb level.

The GC tube decomposes the odorant into its molecular constituents, and MS forms a mass spectrum for each peak. The spectra then is compared to a large precompiled database of spectral peaks to classify and identify odorants.

MOSFET (Metal-oxide-silicon field-effect-transistor): The basic principle here is capacitive charge coupling. In other words, VOCs react with the catalytic metal and thereby alter the device’s electrical properties. The device’s selectivity and sensitivity can be fine-tuned by varying the metal catalyst’s thickness and composition. MOSFETs are micro-fabricated and commercially available, but can measure only parts per million. They can be manufactured by electronic interface circuits, which minimizes batch-to-batch variation. However, the gas produced by the VOC-metal reaction must penetrate the MOSFET’s gate.

Conductivity Sensors: The sensor types used here are metal oxide or conducting polymer. Both operate on the principle of conductivity, for their resistance changes as they interact with VOCs. Metal oxide sensors are common, commercially available, inexpensive, and easy to produce (they are micro-fabricated). Their sensitivity ranges from 5-500 ppm. However, they only operate at high temperatures (200°C to 400°C).

In conducting polymer sensors, VOCs bond with the polymer backbone and change the polymer’s conductivity (resistance). They are micro-fabricated together with electroplating and screen printing, are commercially available, and can measure from .1 to 100 ppm. They operate at room temperature, yet are very sensitive to humidity. Moreover, it is hard to electropolymerize the active material, which makes batch-to-batch variation inevitable. Sometimes VOCs penetrate the polymer chain, which means that the sensor must be returned to its neutral and reference state-a very time-consuming process.

Piezoelectric Sensors: These devices, which measure any change in mass, come in two varieties: quartz crystal microbalance (QCM) and surface acoustic wave (SAW) devices.

QCM sensors have a resonating disk and metal electrodes on each side. While applying the gas sample to the resonator’s surface, the polymer surface absorbs VOCs from the environment. Thus its mass increases, which increases resonance frequency. As the U.S. Navy has long used QCMs, this technology is familiar, developed, and commercially available. A QCM sensor is fabricated by screen-printing, wire bonding, and MEMS. Althoug it can measure a 1.0 Ng mass change, its MEMS fabrication and interface electronics is a major disadvantages. QCM sensors are quite linear in mass changes, their sensitivity to temperature can be adjusted, and their response to water can vary for the material used.

MEMS techniques should be handled carefully, for the surface-to-volume ratio increases drastically as dimensions approach the micrometer levels. Measurement accuracy is lost when the increasing surface-to-volume ratio begins to degrade the signal-to-noise ratio. This problem occurs in most micro-fabricated devices. SAW devices have much higher frequencies. Since 3-D MEMS processing is unnecessary, SAW devices are cheaper. As with QCM devices, many polymer coatings are available. The differential devices can be quite sensitive. However, interface electronics require more complex electronics than those of conductivity sensors for both QCM and SAW sensors. Also, as the active membrane ages, resonance frequencies can drift and so must be detected for frequency by time. SAW devices are commercially available and sensitive to mass changes at the 1.0 pg level.

Pattern Recognition

Any e-nose’s primary task is to identify an odorant and perhaps measure its concentration. After the signal processing step comes the crucial step of pattern recognition: preprocessing, feature extraction, classification, and decision-making. A database of odors must be formed for comparison purposes.

Preprocessing accounts for sensor drifts and reduces sample-to-sample variation. This can be done by normalizing sensor response ranges, manipulating sensor baselines, and compressing sensor transients.

Feature extraction involves dimensionality reduction, a crucial step for statistical data analysis, since the database’s examples usually are subject to financial constraints. The higher dimensionality caused by sensor arrays is reduced to relevant pattern-recognition information and thus extracts only significant data. As most dimensions are correlated and dependent, it is better to reduce dimensionality to a few informative axes.

Feature extraction usually is accomplished by classical principal component analysis (PGA) or linear discriminant analysis (LDA). PCA is a linear transformation that finds the maximum variance projections and the most widely used technique for feature extraction. But as PCA ignores class labels, it is not an optimal technique for odor recognition.

LDA seeks to maximize the distance between class label examples and minimize the within distance, and thus is a more appropriate approach. LDA is also a linear transformation. For instance, LDA might better discriminate subtle but crucial odor projections, whereas PCA can remove the high variance random noise in a projection.(4)

The classification stage identifies odors. Classical classification techniques are KNN (k nearest neighbors), Bayesian classifiers, and ANN (artificial neural networks]. KNN with, say, 5 nearest points will find the 5 closest matches from the precompiled database. The closest match will be assigned as the tested material’s odorant class.

Bayesian classifiers first assign a posterior probability to the classes in the lower dimension and then pick the class that maximizes the predetermined probability distribution. ANN is closer to biological odor recognition. After being trained by the odor database, it is exposed to the unknown odorant in order to recognize the largest applicable response odorant class. The classifier estimates the class and places a confidence level on it.

In decision-making, risks and application-specific knowledge are considered in order to modify the classification. All decisions are reported-even a nonmatch.

Conclusion

As this article indicates, we can expect great progress in this area. And with each step forward, science and technology will continue to point toward the Greatest Artist’s most subtle designs and allow us to appreciate them better.

Footnotes

1. There are over 100 million receptor proteins of about 1,000 different types.
2. A human olfactory bulb contains approximately 2,000 glomeruli.
3. The terms “electronic nose” and “e-nose” are incorrect, for these devices cannot be considered “real” noses. The correct terminology should be “electronic arrays for chemical sensory and identification.” However, “e-nose” has gained wide acceptance in the literature since it first appeared during a 1991 NATO workshop in Reykjavik, Iceland.
4. Such nonlinear transformations as Sammon nonlinear maps and Kohonen self organizing maps also are used in feature extraction. These preserve the distance between pairs of examples when reducing dimensionality to 2 or 3.

References

• Baltes, Henry, Dirk Lange, and Andreas Koll. “The Electronic Nose in Lilliput.” IEEE Spectrum (Sept. 1998): 35-38.
• Barinaga, Marcia. “Salmon Follow Watery Odor Home.” Science 286 (22 Oct. 1999): 705-6.
• http://csmt.jpl.nasa.gov/enose.html.
• http://faculty.washington.edu/chudler/nosek.html.
• Malakoff, David. “Following the Scent of Avian Olfaction.” Science 286 (22 Oct. 1999): 704-5.
• Mamberts, Peter. “Seven-Transmembrane Proteins as Odorant and Chemosensory Receptors.” Science 286 (22 Oct. 1999): 707-10.
• Perkins, Sid. “Eau, Brother! Electronic Noses Provide a New Sense of the Future.” Science News 157 (19 Feb. 2000): 125-27.
• Schiffmann, Susan, and H. Troy Nagle. “The How and Why of Electronic Noses.” IEEE Spectrum (Sept. 1998): 22-32.
• Stern, Peter and Jean Marx. “Making Sense of Scents.” Science 286 (22 Oct. 1999): 703.
• Wolfgang, Gopel, and Tilo Weiss. “Design for Smelling.” IEEE Spectrum (Sept. 1998): 32-34.
• www.planetee.com/planetee/servlet/DisplayDocument?ArticleID=6899.
• www.sfn.org/briefings/smell.html.