Proceso de prueba de las hipótesis

There are many different technologies with the potential for sensing the effluent gases of drug production and approaches must be evaluated from a technical, economic and legal standpoint. A number of methods are discussed below, along with their advantages and disadvantages.

1.3.1. Sniffer Dogs

The gold standard for gas sensing has long been the use of sniffer dogs, whose highly trained noses can detect odours at part-per-trillion (ppt) concentrations24_.

Sniffer dogs are most widely utilised for drug detection at ports of entry, sports events and music festivals, however, canines have been trained for a number of other applications, including the detection of cancerous tumours25_{, endangered insect}

species26 _{and hypoglycaemia emergencies in diabetic patients}27_{(Figure 1.6).}

Although trained canines offer a very efficient means of drug detection, they cannot comment on the vapour they are detecting, or give detailed information about concentration. In addition to this, the use of dogs in clandestine laboratory locations has other ethical implications; it is morally dubious to utilise live animals in potentially harmful chemical environments where their lives may be put at risk. In addition to these concerns, training a sniffer canine costs around $6000, and specially trained dogs cost around $2000 per annum to maintain. Therefore, the use of non-canine chemical sensors that can be easily and cost effectively produced and implemented in many different and potentially hazardous environments is of great interest to law enforcement agencies.

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Figure 1.6. Police sniffer dog on patrol, assessing luggage for the presence of a controlled substance28_.

1.3.2. Ionisation Sensors

Ionisation sensors can be divided into two different technologies, ion mobility spectrometry (IMS) and photoionisation detection (PID). IMS is a highly sensitive technique that uses either a chemical or radioactive ionisation source29 _{to analyse the}

composition of gas mixtures. Ionisation sensors are able to detect a wide range of gases at low concentrations; however, challenges include competitive ion/molecule reactions with matrix molecules, and problems with low temperature operation30_{. Sensing units}

are very expensive ($10,000-$50,000)31_{. Despite this high cost, IMS is well suited to}

detecting relative concentrations of target gases in stationary detection units.

The greatest strength of IMS analysis is the speed of response, which is typically in the order of milliseconds. The speed of response, combined with its high sensitivity, compact design and ease of use has led to IMS instruments use in airports for detection of explosives32_{, weapons}33 _{and drugs}34_.

Photoionisation detection (PID) uses UV light to ionize a gas between two electrodes. When gases are ionised, free electrons are collected at the devices electrodes, resulting in a change in current flow proportional to the concentration of the gas. PID devices have been known to detect volatile species at sub part per billion (ppb) level35_{. The}

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ionisation source and the detector are selected so that they do not ionize the main components in air (O2 & N2). PID is generally non-selective, however selective detection

has been demonstrated using chemically selective filter materials36_.

Portable PID devices are available commercially for a number of applications, including: industrial hygiene37_{, arson investigation}38_{, air quality monitoring}39 _{and cleanroom}

maintenance40_{. Such devices can cost between $500 and $6000, depending on the}

application, mobility and hardware used in the device. 1.3.3. Optical Sensors

Optical sensors are detectors that convert a change in light intensity into an electrical signal. As optical sensors can operate in gaseous or liquid phases, they have been utilised in a variety of technologies, including chemiluminescent, colourimetric and fluorescence sensors. Optical sensors have therefore been implemented in a wide range of applications, including environmental41_{, medical}42 _{and pharmaceutical}43 _{analysis. The}

wide availability of miniature photo-detectors and light sources and the broad usage of optical fibres make optical chemical sensors very attractive for applications requiring portable and compact sensing solutions44_.

Optical sensors have excellent selectivity and are often able to identify multiple components of a chemical mixture in one measurement. Optical sensors rely on a specific chemical reaction to induce an optical change; as a result, sensors are selective to one type or class of analytes. Colorimetric response is, however, binary, with the ability to tell only the presence or absence of a compound45_{. Further disadvantages of}

optical sensors include a limited lifetime and interfering stray light46_.

1.3.4. Conductance-Based Sensors

Electrical conductance-based sensors have been implemented in multiple different environments, with a variety of functions. Examples of conductance-based sensors include chemically-sensitive field effect transistors (chemFETs)47,48_{, thermal sensors}49,50

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alterations in the space charge region (SCR) that are induced by an interacting target gas53_{. Determination of concentration and classification of the interacting gas is often}

possible and conductance based sensors are often cheap due to the simple and low-cost materials required54_.

Conductance-based sensors can detect a large number of target gases to a sub-ppb level in a reasonable amount of time (less than two minutes); disadvantages of this type of sensor include; susceptibility to catalyst poisoning55 _{and sensor inhibitors (such as}

halogens)56_{. One class of conductance based sensors; metal oxide semiconductor gas}

sensors will be discussed in more detail below (Section 1.4).