This invention relates to apparatus for the selective detection of the presence of a specific target gas in ambient air.
There is a need to detect and measure the presence of chemical compounds in air at variable concentration. Many such compounds are classed as volatile organic compounds, VOC's, which may be harmful to humans, animals and plant life, or may present a risk of fire or may be of other interest.
It is frequently of interest to search for the presence of gaseous species collectively, as unpolluted air contains very low concentrations of VOC's, and therefore the indication of any VOC's in air can often be attributed to a specific source. For example, a source of VOC leakage could be a spill of chemicals, a screened soil sample, a leak site in a chemical tank, or an accelerant in an arson attack. In such scenarios it is preferable for sensors and detectors of target analytes in air to provide a fast and quantitative measurement of their concentration, so that their provenance, in real time, can be ascertained. Sensors and detectors engaging photo-ionisation detection (PID), flame ionisation detection (FID), thermal conductivity detection (TCD) and amperometry are suitable for this purpose.
Specific VOCs, such as benzene, present a serious risk to health, making fast measurement of their concentration in ambient air particularly desirable. Such compounds may present themselves as mixtures with other VOC's which do not present such a danger to health. In detection of a specific VOC of concern, such as benzene, hereinafter for convenience called a target gas, it is well known that gas chromatography (GC) provides an effective method of pre-separation of the target gas from other detectable compounds in a test gas sample.
In gas chromatography, a small sample of gas to be analysed for the presence of a target gas is caused to enter a column that contains a medium, known as a stationary phase, onto which the gas is occasionally adsorbed and desorbed. For an extended time after introduction of the sample, a carrier gas devoid of the target gas is caused to flow through the GC column. By virtue of variable gas absorbency on the stationary phase, each gas constituent in the original sample gas emerges from the column at a characteristic time referred to as the gas elution time. By causing the gas exiting the chromatography column to be presented to a sensor, it is possible to detect the target gas from a prior knowledge of its elution time.
The combination of a GC column and a sensor, usually a PID sensor, has for many decades been used in analytical apparatus used in the laboratory. However, the technique, known as GC-PID, has hitherto only been used in bulky stationary apparatus.
It would be desirable to use the technique in a portable apparatus, that is to say a device sufficiently light and compact to be carried by an individual, so that personnel at risk may be warned when breathing air having a high concentration of the target gas. However, some of the processes commonly used in conventional GC-PID apparatus are incompatible with rendering the technology portable. In particular:
From the above, it will be appreciated that, for it to be practicable, a portable apparatus requires a short column (for shorter elution times), incorporating a stationary phase that is stable on continuous exposure to air (to avoid the need for a supply of an inert carrier gas) and operable at a temperature that in only modestly above commonly encountered ambient temperatures (to reduce the power burden).
A GC column which is operable in air and meets the above requirements can be constructed from readily available materials and components. By way of an example, the column may comprise a metal tube having a length of 5 to 50 cm and a diameter of 1.2 mm diameter and filled with a diatomaceous earth support, such as Chromosorb® 120, which has been previously coated with a suitable stationary phase for separating a target VOC, such as benzene. Bis-cyanopropyl phases tend to be suitable for this purpose.
A further difficulty encountered in making a detecting apparatus portable does not relate to the performance of the GC column nor to the sensor, be it PID FID or TCD, but to the gas handling pneumatic circuit.
For successful operation of a GC-PID apparatus, a regular flow of gas is required through the GC column to set the background signal before introduction of the gas sample to be analysed. This conventionally calls for a pressure cylinder containing the carrier gas or a first pump that operates constantly. To introduce the gas sample into the GC column, a second pump, operating at higher pressure, is required along with various valves, conduits and connectors.
Components such as pumps and electrically controlled valves present several problems when designing a portable apparatus. In particular:
According to a first aspect of the invention, there is provided an apparatus for detecting a target gas in ambient air, the apparatus comprising a GC column, a sensor located downstream of the GC column, a pump, a gas storage chamber and a pneumatic circuit that is operative in a first state to connect the pump to the gas storage chamber in order to store ambient air under pressure within the chamber, while trapping a sample of ambient air within the pneumatic circuit, and in a second state to connect the gas storage chamber to the GC column to cause pressurised air drawn from the storage chamber to act as a carrier gas to advance the trapped sample through the GC column and the sensor, wherein a filter is provided to filter out any target gas present in the air entering into, or the air drawn from, the storage chamber, so as to avoid the presence of any target gas in the carrier gas.
While air may be filtered to remove from it any target gas before it is pumped into the storage chamber, in a preferred embodiment the filter is positioned to filter air flowing from the storage chamber to the GC column.
In some embodiments, the gas storage chamber is a variable volume working chamber.
A variable volume working chamber may conveniently be formed by a bellows, but it is alternatively possible for the chamber to have a movable wall formed by a piston or a rolling diaphragm.
As well as removing target gas from the carrier gas, it is also desirable to filter out water vapour, by the use of a desiccant, in order to avoid problems caused by water condensation.
The design of the gas storage chamber should ensure that the carrier gas pressure, and the gas flow rate through the GB column, should be as constant and uniform as possible, at least for the period of time during which analysis of a sample by the GC column and the sensor is taking place.
The pneumatic circuit may suitably comprise a valve and conduits designed such that ambient air is supplied to the gas storage chamber through a conduit through which the filtered gas flows in the opposite direction towards the GC column, so that the ambient air trapped within the latter section of conduit may serve as the gas sample.
Conventional ambient air sampling systems, as described for example in WO2016/054585 rely on helium, or a gas other than ambient air, to serve as the carrier gas. Such systems employ a six port valve having a stator with six ports disposed in the same plane and spaced apart by 60°, and a rotor that has two positions. In one position, each of three ports is connected to the next port in the clockwise direction and in the second position each of the same three ports is connected to the next port in the counter-clockwise direction.
In some embodiments of the present invention, the pneumatic circuit employs a rotary 4-port two-position changeover valve, the rotor being formed with a conduit that connects two of the four ports in one position of the valve and the other two ports in the other position of the valve, the conduit being operative to trap a volume of gas to serve as the sample to be analysed.
A PID detector is suitable for use as a sensor but the type of sensor employed is not of critical importance to the invention, so long as it is capable of producing an electrical signal when the target gas exits the GC column.
The invention will now be described further, by way of example, with reference to accompanying drawings, in which:
A conventional GC-PID apparatus 10 is shown in
In a first position of the valve 20, shown in
To introduce the sample into the GC column, the valve 20 is rotated to the position shown in
Such an apparatus is difficult to miniaturise for several reasons explained above. If the gas supply 18 is a pressure cylinder it would be cumbersome and heavy to permit the apparatus to operate continuously for an acceptable length of time. If it comprises a pump, then the need for both this pump and the pump 16 to operate continuously would place a heavy burden on the electrical power supply. The size of the sample reservoir and of the GC column result in long elution times, while in a portable apparatus it is desired to minimise the detection time.
A further disadvantage is that ambient air is sucked into the reservoir 22 and the sample resides in the reservoir 22 at sub-atmospheric pressure. If the sample remains under sub-ambient pressure on reaching the PID sensor 14, it creates a risk of ambient air being drawn into the sensor, if the sealing of the sensor is not perfect.
Existing valves of the type used in the pneumatic circuit as shown in
Port 122 is connected to receive the ambient atmosphere 128 that is to be analysed. In the position of the valve 120 shown in
The port 123 of the valve 120 is connected to a GC column 118. Gas discharged from the GC column flows through a PID sensor 126 before being discharged to exhaust. In the position of the valve 120 shown in
To commence sample analysis, the rotor of the valve 120 is turned to the position shown in
The gas sample now flows through the GC column 118 and its constituents leave the column 118 after different elution times. The target gas, if present, will reach the PID sensor 126 at a known time following the changeover of the position of the valve 120 and the strength of the output signal of the PID sensor 126 at this time will be indicative of the concentration of the target gas.
It will be appreciated that the carbon filter may be positioned between the output of the pump 110 and the input of the storage chamber 114, to remove target gas from the ambient air before it enters the storage chamber 114 instead cleaning the air after it has left the storage chamber, to allow it to serve as the carrier gas.
As well as filtering out the target gas, or VOC's generally, the filter 116, or a separate filter containing a desiccant, may be used to reduce the moisture content of the carrier gas to avoid condensation.
While it would be possible to use a fixed volume storage chamber 114, one having a variable volume is desirable as it helps keep to a minimum the volume of air that has to be pumped and filtered. If using a variable volume working chamber, a rolling diaphragm has been found to be the most efficient manner of achieving a movable wall.
In operation, the apparatus starts in the position shown in
The valve shown in
Despite the many advantages of the described and illustrated 4-port valve, it should be stressed that it does not form an essential part of the invention and may be replaced, for example, by electrostatic valves. Indeed, the entire pneumatic circuit using a 4-port valve is only given as an exemplary implementation of the invention.
There are several advantages presented by the disclosed embodiment of invention as compared with GC-PID apparatus provided by prior art. In particular:
Number | Date | Country | Kind |
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1700019.1 | Jan 2017 | GB | national |
1710891.1 | Jul 2017 | GB | national |
Filing Document | Filing Date | Country | Kind |
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PCT/GB2017/053905 | 12/29/2017 | WO | 00 |