INFRARED ANALYSERS

Infrared (IR) analyzers are useful for measuring a broad range of inorganic and organic chemicals in air. Depending upon the chemical, the sensitivity of IR analyzers can be sufficient for industrial hygiene purposes. Because most chemicals absorb IR light, an infrared analyzer may not be selective unless the chemical of interest can be measured at a wavelength which is unique for that chemical in the air sample, or the industrial hygienist is able to determine that other interfering chemicals are not present in the work environment. Some of the routine applications for IR analyzers include measuring carbon dioxide in indoor air quality (IAQ) assessments; anesthetic gases, including, nitrous oxide, halothane, enflurane, penthrane, and isoflurane; ethylene oxide; and fumigants, including ethylene dibromide, chloropicrin, and methyl bromide.

IR analyzers emit an infrared light which is generated from a heated metal source. The infrared portion of the electromagnetic spectrum typically used in infrared analysis ranges from the far infrared region at 400 cm-1 (25 micrometers) to the near infrared region 4000 cm-1 (2.5 micrometers). The amount of infrared light that a chemical absorbs varies with the particular wavelength of light to which it is exposed. For example, acetone, which is a ketone, has a strong, broad absorption band around 1720 cm-1, whereas alcohols have a strong, sharp absorbance band at approximately 3610–3670 cm-1. Because the absorbance of infrared light by a chemical changes with the wavelength of incident light, an absorption pattern, or unique spectrum for the chemical, can be produced by measuring the absorbance of the incident light for a chemical over the 400–4000 cm-1 infrared range. This infrared absorbance spectrum can be compared to a library of known chemicals for identification purposes.

For measuring the amount of a chemical in air, a wavelength is selected for which the chemical of interest absorbs the light. The amount of light absorbed by the air sample at this wavelength would be proportional to the amount of the chemical in the sample if there is no other chemical present in the air which absorbs at that same wavelength. In some instances, a weaker absorbance band at a different wavelength is chosen to measure a chemical in air, if that alternate wavelength is uniquely absorbed by that chemical of interest. The selected wavelength for analysis of a chemical is chosen both because the chemical of interest has sufficient absorbance at that wavelength and sufficient specificity to exclude the absorbance of other chemicals. For example, acetone in air absorbs IR at both 8.4 and 11.0 microns. If methyl acrylate was also known to be present in the air, the 11.0 micron IR wavelength would be selected because methyl acrylate absorbs at 8.4 microns. 

The sensitivity of IR detection can also be varied by changing the path length through which the light source passes. This is accomplished by internally mounted mirrors within the analyzer which can vary the path length for the light source for the Miran 205B analyzer from 0.5 meters to 12.5 meters.

Calibration:

Most analyzer are pre-calibrated for a list of chemicals which are stored in the instrument library. A sampling loop kit which recirculates a known volume of air is available for the instrument which allows the injection of a known amount of a volatile liquid or gas into the IR sampling cell. In this fashion, the instrument's pre-calibration can be verified prior to and after its use. Instrument zeroing is performed by using a charcoal filter attachment to remove chemicals from the air.


Special Considerations:

Infrared analyzers may not be specific for the chemical of interest because other chemicals present in the work environment air may also absorb at the same wavelength. Cell window degradation will occur if the analyzer is used in the presence of ammonia and many alkyl amines, such as methyl amine.

Maintenance:

Field maintenance is limited to replacement of the zeroing filter after 30 uses and replacement of the particulate filter in situations where adsorbed particulates or non-volatile liquids may have contaminated the filter surface. Field calibration is conducted according to the manufacturer's recommendations.