Nitric Oxides (NOx)

The nitric oxide (NO) molecule is very reactive and unstable. In air, it reacts with oxygen to form the poisonous nitrogen dioxide (NO2). Human activity has drastically increased the production of nitric oxide in combustion chambers, e. g. car engines and power plants.

Health and Environmental Effects

Nitric oxide has a multitude of effects, primarily in the lung but also in other organs, such as the spleen and the liver. In blood it can lead to the creation of methaemoglobin, which cannot transport oxygen. Nitric oxide in the air may later convert to nitric acid in acid rain. Furthermore, both NO and NO2 participate in the ozone layer reduction.

Monitoring Nitric Oxides

The concentration of nitric oxides is measured by using chemiluminescence. Nitric oxides in the sample gas react with ozone and this reaction results in electrically excited molecules. These molecules release their excess energy by emitting photons, which are measured by a photomultiplier tube. The Thermo Fisher Model 42i series monitors measure NO, NOx and NO2 using this principle and offer detection limits down to 1 part per billion in air. Special trace versions measure down to 50 parts per trillion

Monitoring Ammonia

By adding a special converter to the Model 42i ammonia can also be detected.

Sulphur Dioxide (SO2)

Sulphur dioxide is an acid tasting, colourless, foul smelling, toxic gas. Major sources are heat and power generating facilities that use poor quality oil or coal containing sulfur.

Health and Environmental Effects

The effects observed include reductions in pulmonary volume, increases in breathing resistance and symptoms such as wheezing, chest tightness or shortness of breath. It also could lead to headache and nausea. Sulphur dioxide is one of the major precursors of acid rain, which accelerates corrosion of buildings and monuments as well as it acidifies soils, lakes and streams. Furthermore, it leads to reduced visibility.

Monitoring Sulphur Dioxide

The concentration of sulphur dioxide is measured by Ultraviolet Fluorescence. The sample gas is lighted with an UV lamp, which causes the sulphur dioxide part of the gas to absorb energy. This absorbed energy is emitted as a light pulse (photon) shortly afterwards, which is measured with a photo multiplier tube.

The Thermo Fisher Model 43i measures SO2 in the above manner and can detect concentrations lower than 1 part per billionand a special trace version can detect to 50 perts per trillion. 

Hydrogen Sulphide (H2S)

Hydrogen Sulphide is a highly toxic and corrosive gas with an unpleasant smell. H2S is a product of biogenous digestion. It can be found in refineries, blast furnaces, pulp and paper industry, gasworks, cokeries, wastewater and biogas plants.

Health and Environmental Effects

H2S destroys haemoglobin and paralyses the intracellular respiration. In contact with humidity at mucous membranes it converts to alkaline sulfides causing heavy irritation in eyes, nose, throat and lung. H2S is neurotoxic. The bad smell of H2S is unbearable. The lower limit of detection by humans may be as low as 2 ppb. Because of its acidity H2S is a very corrosive gas. It damages switchpanels and other electronics in the plants.

H2S/SO2 Converter, Ultraviolet Fluorescence

SO2 is scrubbed from the sample gas. H2S is thermally converted to SO2 and measured by UV fluorescence. For details about SO2 measurement, see SO2 leaflet. Equipped with an H2S module the Thermo Model 43i can measure H2S only, SO2
only, or both by cycling between the two modes.

Carbon Monoxide (CO)

Carbon monoxide is a highly toxic and flammable gas, which is a major product of incomplete combustion of carbon and carbon-containing compounds. Smoking in residential buildings, offices, vehicles and restaurants can raise the carbon monoxide average as well.

Health and Environmental Effects

A sufficiently high carbon monoxide concentration can reduce the amount of oxygen taken up by the brain to the point that the person becomes unconscious and can suffer brain damage from shortness of oxygen. Carbon monoxide may contribute to the greenhouse effect and global warming.

Monitoring Carbon Monoxide

Non-Dispersive Infrared (NDIR) technology. A beam from an infrared source is directed through a chamber filled with sample gas. Carbon monoxide absorbs light and this decrease is measured by a photodetector.

The Thermo Fisher Model 48i measures using a special version of NDIR which features Gas Filter Correllation (GFC) to further enhance specificity. The system is highly selective for CO.

Carbon Dioxide (CO2)

Carbon dioxide  is a gas composed of two oxygen atoms covalently bonded to a single carbon atom. Carbon dioxide is used by plants during photosynthesisrespiration or used as the raw material to produce other organic compounds needed for plant growth and development. It is produced during respiration by plants, and by all animals, fungi and microorganisms that depend either directly or indirectly on plants for food. It is thus a major component of the carbon cycle. Carbon dioxide is generated as a by-product of the combustion of fossil fuels or the burning of vegetable matter, among other chemical processes. Large amounts of carbon dioxide are emitted from volcanoes and other geothermal hot springs  and by the dissolution of carbonates in crustal rocks.

Monitoring CO2

Carbon Dioxide can be monitored by infra red spectrocopy as CO above. The Thermo Fisher Model 45i is similar to the Model 48i for Carbon Monoxide but operates at a different wavelegth specific to CO2.

In addition the GSS miniature CO2 sensors provide a compact NDIR monitor capability in a compact package for use in embedded applications.

C20 Sensor for CO2

C20 – This is a high performance, general purpose CO₂ sensor covering a broad measurement range to 0–100%. It is used in a wide range of applications – processing industries, agriculture, re-breathers, laboratory and incubators, education, landfill monitoring, portable equipment, personal gas sensors and many more.

• Real time sensing
• Wide range of applications
• Low power consumption - typically 100mW (Spec dependant)
• Low cost
  
• High accuracy
• High poison resistance & long term stability
• Good immunity to other gases & humidity
• Various voltages available from 3.3v to 5.5v
• Low power consumption, suitable for battery and portable applications
• 20mm package
  
• Measurement Range 0%-20%, 0%-65%, 0%-100%

C100 Sensor (Low Concentration - Low Power) 

C100 – A higher sensitivity sensor designed for low concentration markets such as HVAC and IAQ. The standard ranges are 0-2000ppm and 0-2%

• Range 0-2%
• Temperature Range -25C to + 55C
• Accuracy +/-3% reading +/- 40ppm
  
• Power Consumption 220mA
• Response to Gas (T90) 60sWarm-up (T90) 3 minutes Warmup (recommended) 30 mins
• Output (Digital) Serial 9600 , 8,n,1
• Supply 5V, 3.3V

Ozone (O3)

Ozone is a highly poisonous, corrosive substance and a common pollutant. Ozone is formed in the atmosphere by reaction of nitrogen oxides, hydrocarbons, and sunlight. Some kinds of electrical equipment, e.g. television sets, photocopiers and electric motors (which use brushes), generate such an amount of ozone that a person can easily smell.

Health and Environmental Effects

Acute effects include respiratory symptoms, changes in pulmonary function, increased respiratory sensitivity and respiratory inflammation. Ozone damages the leaves of trees and other plants (photooxidation), deteriorating the appearance of cities, national parks, and recreation areas.

Monitoring Ozone

The concentration of ozone is measured by Ultraviolet Photometry. From a high energy UV lamp a beam is directed through a tube, which is filled with the sample gas. The decrease of the light's density, resulting from the presence of ozone, is measured with a detector at the end of the tube.

The Thermo Fisher Model 49i uses UV Photometry to measure ozone to 1 ppb detection.

Miniature Ozone Monitor

The 2Btech monitor is similar to the Thermo Fisher but operates on battery power and is fully portable. The performance specification is similar but due to its smaller size and weight it can be applied to many different applications.

Click the image to see size comparison.

The Model 202 Ozone Monitor, is designed to enable accurate and precise (±1.5 ppb) measurements of ozone ranging from a few ppb to 100,000 ppb (0-100 ppm) based on the well established technique of UV absorbance at 254 nm. The Ozone Monitor, is light weight (2.1 kg.) and has a power consumption of only 4 watts.

The Model 202 Ozone Monitor can be used in almost any application where much larger ozone monitors have been used in the past and in many environments where measurements using traditional ozone monitors are impossible.  The U.S. Environmental Protection Agency (EPA) has assigned the method number "901-O3 Model 202" to the Model 202 Ozone Monitor and includes the National Forest Service Network, which makes use of the the Model 202 in their database. EPA approval as a Federal Equivalent Method is pending.

Volatile Organic Compounds (VOC)

VOC (Volatile Organic Compounds) are not a specific pollutant but a class of chemical compounds which are present almost everywhere (e.g. acetone, benzene, toluene, cyclohexane, n-hexane, formaldehyde, styrene, chlorinated solvents, and several other organic solvents). Some of them are non-toxic and many are noxious. You can observe high pollution levels especially near (petro)chemical industry and whenever fuel is not completely burned. Almost all VOC can be detected by a PID sensor. In ambient air, the reading of a PID is not to be understood as current concentration since its response is specific to each VOC compound. Nevertheless, the reading gives a good idea of the VOC pollution run. The VOC module is ideally suited for the detection of fugitive emissions and fenceline monitoring and suited even for the relatively low concentrations in ambient air.

Health and Environmental Effects

The ability of organic chemicals to cause health effects varies greatly from those that are highly toxic to those without known health effects. As with other pollutants, the extent and nature of the health effect will depend on many factors including level of exposure and length of time exposed. Eye and respiratory tract irritation, headaches, dizziness, impaired vision, and memory impairment are among the immediate symptoms that some people have experienced soon after exposure to some organics.

Measuring VOC's

When measuring VOC's several techniques can be used. If you are measuring a specific VOC and no others are likely to interfere then a PID or FID detector can be used. If you wish to identify a specific VOC in a mixture then Gas Chromatography or Mass Spectroscopy or a combination of both may be required. 

The AMA GC System is a fully automated network ready GC system capbale of seperating specific VOC's such as Benzene, Toluene, Xylenes etc in abient air to sub ppb levels.