Field of the Invention
The disclosure relates to measuring and testing, particularly for gas analysis, for example breath analysis of organisms and subclasses of breath analysis inside the body. The disclosure most closely relates to technical fields including chemistry, electronics, physics, medicine and others, and can be applied to a variety of technical fields such as analytical and immunological testing.
Description of Related Art
Currently, a number of marker molecules have been identified in breath that could be used to identify disease, disease progression, or to monitor therapeutic intervention and this list is expected increase dramatically since the analysis of breath is ideally suited for population-based studies in the developed and underdeveloped world.
The concept that blood, urine, and other body fluids and tissues can be collected and analyzed to yield information for diagnosis of disease states or to monitor disease progression and/or therapy is the foundation of modern medicine.
However, the use of breath as a collectable sample has not received comparable clinical use, as conducted studies have only been possible so far as a result of enhanced separation of gaseous molecules by gas chromatography, increased selectivity of mass or optical spectrometers and improved limits of detection from high parts-per-million to parts-per-billion.
Breath measurement has enormous potential, in part because of its inherent safety. The only requirement to collect a breath sample is that the subject must be breathing (spontaneously or mechanically supported). Breath analysis can be used to detect disease, monitor disease progression, or monitor therapy.
Recent advances in instrumentation may enable more of this potential to be realized. In particular, the wider availability of real-time, portable monitors would be a breakthrough.
It was discovered decades ago that atoms and molecules interacting with semiconductor surfaces influence surface properties of semiconductors, such as conductivity and surface potential. Seiyama (1962) and Taguchi (1970) first applied the discovery to gas detection by producing the first chemo-resistive, semiconductor gas sensors. Since then, semiconductor gas sensors have been widely used as domestic and industrial gas detectors for gas-leak alarms, process control, pollution control, etc.
Recent years have seen the introduction of solid-state sensors for the detection of different gases, which are based on metal oxide semiconductors. As with catalytic devices, which rely on the absorption of a gas on to a heated oxide surface, the absorption and/or subsequent reaction of a gas on the surface of the oxide produces an electrical conduction change in the metal-oxide itself on the account of the electronic processes involved in the reaction on its surface.
These conductivity changes relate to the amount of gas absorbed on the surface of the oxide and hence to its concentration in the surrounding atmosphere.
The metal-oxide semiconductor sensor is comprised of a tin oxide that is sintered on a small ceramic tube or surface. A coiled wire is placed through the center of the ceramic tube to act as the sensor's heater. Metal wires provide electrical contact between the tin oxide and the rest of the electronics.
The metal-oxide sensor requires between 300 mW and 600 mW of power to operate the sensor at elevated temperature between 300 and 450 degrees Celsius.
The combination of the sensor's operating temperature and composition of the metal-oxide yields different responses to various gases.
When a metal-oxide crystal, such as ZnO2, is heated at a certain high temperature in the air, oxygen is adsorbed on the crystal surface with a negative charge. Then, the donor electrons in the crystal surface are transferred to the adsorbed oxygen, resulting in a removal of positive charges in a space charge layer. This surface potential is formed to serve as a potential barrier against electron flow.
Inside the sensor, electric current flows through the conjunction part (drain boundary) of ZnO2 micro-crystals. At drain boundaries, adsorbed oxygen forms a potential barrier, which prevents carriers from moving freely.
The electrical resistance is attributed to this potential barrier. In the presence of a deoxidizing gas, the surface density of the negatively charged oxygen decreases, thus the barrier height in the drain boundary is reduced. The reduced barrier height decreases the sensor's resistance.
The relationship between the resistance of the sensor and the concentration of the deoxidizing gas can be expressed by the following equation over a certain range of gas concentration:Rs=A*[C]*(−x)  (Equation 1)
Where Rs=electrical resistance of the sensor, A=constant, [C]=gas concentration, and (−x)=slope of the Rs curve. According to Equation 1, the relationship of the sensor's resistance to gas concentration is generally linear on a logarithmic scale within a practical range, determined by current market data and depending from the particular gas, to be from approximately a hundred ppm (parts per million) to several thousand ppm of gas concentration.
Modern metal-oxide methods and the method of preparing a sensitive surface with a laser may resemble the method disclosed herein. For example, a semiconductor oxide gas sensor was introduced through a research team led by Dr. S. Kawi from the department of chemical engineering of the National University of Singapore (NUS). However, researchers indicated that further and extensive experimentation was necessary to understand the nature of the involved processes and to explain the achieved results.
Another method to improve the quality and sensitivity of the ZnO2 layer to tens of ppm is the method of using a laser to scan the sensor's surface. By using a laser, it is possible to change the density of the electrical charge on the sensor's sensitive layer.
The above-described methods and techniques have several disadvantages in common and cannot be used for investigations of gaseous mixtures with low concentration levels.
The use of Equation 1 can be limiting and becomes invalid for small concentrations of a gas because at low concentration levels, changes in the resistivity also occur under the influence of internal factors, such as diffusion and recombination, which are not taken into account by the formula.
The dependency on a logarithmic relationship derived from Equation 1 does not allow for selectively analyzing the effect of similar gaseous components on the semiconductor's sensing layer.
In the presence of destabilizing factors, such as a change in the temperature or a change in the flow of gas, the formula can no longer be applied. Consequently, the destabilizing factors are prevalent at small concentrations.