Many well-known instruments are used to measure the presence of trace chemical vapors in the atmosphere. Examples of such instruments include, but are not limited to, gas chromatographs (GC), ion mobility spectrometers (IMS), mass spectrometers (MS), microsensors based on gas adsorption onto a mass-sensitive surface, electron capture detectors, sensors that use optical stimulation to provide a characteristic emission wavelength for detection, and microsensors based on changes in semiconductor properties when gas adsorbs onto their surfaces. Such instruments may operate in real time by causing one or more flowing streams of gas to enter the instrument through one or more orifices, and the gas may exit through one or more different orifices. At least one of the flowing gas streams entering the instrument includes gas that has been sampled (the “sample gas”) from the surrounding atmosphere or other source of vapor to be analyzed. Alternately, the sample gas may first be allowed to interact with a temporary adsorbing surface in order to concentrate the vapor sample. An example of such an adsorbing surface used with a GC is called a solid phase microextraction fiber (SPME), and special chemical coatings are used to enhance the adsorption of sample vapor. Many other types of vapor concentrators are well known in the art. At some later time a second step is to heat the adsorbing surface by various means in order to desorb the concentrated sample gas into the instrument for measuring trace chemical vapors. The adsorbent surface may also be optionally moved to a separate location prior to desorption. This two step process is operationally equivalent to the real time sample gas acquisition process as a method of acquiring a vapor sample for the vapor detection instrument.
In some cases, the process of taking a sample begins with an operator rubbing an absorbent substance, such as chemical filter paper, onto the surface to be tested. Particles of the chemical of interest may then be transferred and concentrated on the absorber. This intermediate absorber is then brought to the vicinity of the sampling orifice of the IMS. The method of concentrating using an absorbent substance tends to be relatively slow to implement and is subject to variations in the skill of the operator. Additionally, while the absorber is relatively low in cost, the process of taking a great many samples becomes expensive in that the absorber generally should only be used once to ensure consistent results.
The quantity of particles of the target substance on the target surface is usually very small, often corresponding to only nanograms or even picograms of particles per square centimeter. The detection instrument may need to be very sensitive to identify a positive signal from evaporated target molecules when the initial concentration and surface area of target particles is small.
A sampling method that is employed is to provide a gas pump, which draws the sample gas into the detection instrument through a tube. For example, the pump may be disposed to provide a partial vacuum at the exit of an ion source that is a component of the instrument. The partial vacuum is transmitted through the confines of the ion source and appears at the entrance orifice of the ion source. A further tubulation may be provided as an extension to a more conveniently disposed sampling orifice external to the instrument. The operator places a sample in the near vicinity of this external sampling orifice, and the ambient vapor is drawn into the gas flow moving towards the ion source.
The instrument may provide a signal that is approximately proportional to the concentration of target molecule vapor. This concentration is further dependent on the equilibrium vapor pressure of the target molecule, the temperature of the target molecule where it is emitting the vapor, the total flow rate of non-target gas that dilutes the target vapor, and possible adsorption losses on surfaces of the gas sampling system. Existing systems that utilize an adsorbent or particle-collecting surface concentrator sometimes employ an oven to greatly warm the adsorbent material, often up to 200 degrees Centigrade, and thereby increase the target vapor concentration.
In some circumstances, it is desirable for instruments to be able to sample vapors at a distance from the external sampling orifice. Examples may include, but not be limited to, sampling of vapor from complex surfaces that contain many holes, crevices, or deep depressions, textured materials such as cloth, people and animals that prefer not to be rubbed by absorbent material, large three dimensional objects, surfaces that must be sampled in a short time, and surfaces in which surface rubbing by human operators is inconvenient or expensive. In addition, it has been observed that the sampling orifice may become contaminated with vapor-emitting particles if the sample inadvertently contacts the orifice. Such contamination is particularly difficult to remove in a short period of time, thus preventing continuous operation of the instrument. Such contamination could be avoided if vapors are sampled at a distance from the sampling orifice.
The distance where vapors may be sampled beyond the sampling orifice may be increased by increasing the sample gas flow rate, i.e., increasing the pumping speed. However, besides the interference with the performance of the instrument for measuring trace vapors caused by high velocity flow, this method dilutes the concentration of the desired sample vapor by mixing in a much larger volume of ambient gas. Therefore, the sensitivity of the instrument may decline if the sample gas flow rate is increased excessively.
Warming surfaces at a distance using an oven is generally not very efficient. While warmed gas can be blown onto a distant surface, for example with a “heat gun”, when the target surface is a living person or animal, this may not be an acceptable option. Additionally, many surfaces cannot tolerate excessive heating and may be damaged.