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Gases for analysis

Reliable values

Should a car be allowed on the road or not? This vital question is answered at regular intervals by the official exhaust emissions test. The relevant certificate is only renewed if the amount of pollutants emitted remains within the permissible limits. Whether that is the case is usually determined by means of infra-red spectroscopy, where the concentration of pollutants is indicated by the effect of the exhaust gas on infrared light. To determine their proportions, however, it is necessary to have reference values, and these are provided by zero-grade and calibration gases. They thus play a decisive role in protecting the air we breathe, among other things.

This is how the test works in a service garage: a beam of infra-red light is passed through a chamber in the measuring instrument containing the exhaust gases, these being a mixture of different gases. The gases absorb light of particular wavelengths, which differs from gas to gas. The change in the intensity of the light can be measured. The absorption values thus obtained for the different wavelengths indicate the quantities of various vapours, of unburnt fuel, carbon monoxide (CO) and carbon dioxide (CO2), contained in the sample of exhaust gas. This does, however, require conversion of the measurements that relate to light absorption.

From light absorption to concentration

Two reference points are needed to convert the readings into gas proportions. The first one is provided by a zero-grade gas, a gas of high purity which effectively defines zero for the measurement. The second one comes from a calibration gas, a mixture of gases with a precisely defined composition. “It involves comparing how much light is absorbed when it travels through a pure gas and, for example, through a CO2 mixture with a known composition,” explains Dr. Hermann Grabhorn, who is responsible for specialty gases and analysis at Messer. “These two measuring points can then be used to derive a scale that facilitates the conversion of absorption measurements into concentration values.

”Lambda-Mix from Messer is one example of a calibration gas for exhaust emissions analysis. It consists mainly of nitrogen – like the ambient air – and contains precisely specified fractions of CO, CO2 and propane, which serves as a reference for unburnt fuel. To guarantee the reliability of measurements, instruments used in Germany need to be calibrated once a year. This involves checking their readings against a calibration gas and making any necessary adjustments.

Parts per million and per billion

As a rule, such instruments can measure in the ppm range. This is sufficient for measuring the relevant pollutant levels in the exhaust gas as well as for many other measurements. When it comes to ambient air measurements, i.e. the analysis of those substances that are present in rarefied form in the ambient air but to which humans are still exposed, the concentrations – fortunately – are usually much lower. In these cases, the measurements are often in the ppb range, in other words it involves recording and determining parts per billion, for example to obtain ambient air quality values with regard to the presence of toxic substances such as benzene or toluene in the air we breathe. This requires measuring methods that are much more sensitive than the exhaust gas analysers used in garages. Often this involves the use of gas chromatography, which consists of two steps. First, the sample is broken down into its constituents, the concentrations of which can then be determined very precisely with various detection methods. Heat conductivity, flame ionisation and electron capture detectors, for example, are all used for this. Such methods are utilised in order to analyse air samples from environmental monitoring stations. But gas chromatography is also used in the chemical or pharmaceutical industries, when the accuracy of compositions plays an important role.

Controlled quality

The various methods have two things in common. Firstly, like infra-red spectroscopy, all of them are comparative methods, which need to be calibrated. Secondly, the lower the concentration to be measured and the higher the required measurement accuracy, the greater the requirements for the zero and calibration gases. The production of process and calibration gases requires complex precautions, extreme care in all production processes and continuous quality assurance. This again involves comparisons. The benchmark for this is provided either by gas mixtures which are produced by the relevant national metrological institutes, such as Germany’s Federal Institute for Materials Research and Testing, or by certified mass standards. “Rigorous quality management is absolutely essential in order to be able to guarantee that the calibration gases being produced are always of the highest quality,” explains Hermann Grabhorn. In Belgium, France, Serbia, Hungary and Switzerland, Messer has ISO/IEC 17025 accreditation. “This accreditation is official confirmation that our laboratories have the competence to produce and test calibration gases.”

Gas chromatography

Gas chromatography is a widely used and very accurate analytical method for separating mixtures of substances into individual chemical compounds. The sample – a quantity of less than a millionth of a litre can be sufficient – must be gaseous, or, if required, it is vaporised. A carrier gas, high-purity nitrogen for example, then transports it through an extremely thin, long tube (capillary), which is coated on the inside or contains a viscous liquid. The coating and liquid have a resistance effect on the movement of the carrier gas and sample: depending on their physical properties, different substances take different lengths of time to overcome this resistance and reach the other end of the tube. The individual substances can be distinguished on the basis of the passthrough times. Connected detectors can determine the quantities with great accuracy using a variety of measuring methods.

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