Gas measurement techniques

GC-MS is a gas measurement technique that allows a mixture of unknown substances to be separated into individual components (GC) that can be detected and identified (MS). Separation of the components occurs at the boiling point and the interaction with the stationary phase of the GC column. A GC column is a thin capillary tube of approximately 30 meters in length. The stationary phase is located on the inside of the tube. The polarity of the stationary phase is as close as possible to the polarity of the components, so that separation of the mixture is possible. The column is placed in the oven of the gas chromatograph. A constant amount of an inert gas (helium) flows through the column. The component mixture (if necessary dissolved in a suitable solvent) is applied to the beginning of the column. After the solvent has evaporated, the oven is gradually heated.

When the temperature of the oven is high enough that the boiling point of one of the components in the mixture is reached, this component evaporates and is guided by the gas flow through the column to the detector, the mass spectrometer. The mass spectrometer ionizes and fragments the component into loose mass fragments of different weight and intensity. The total of these mass fragments is called mass spectrum. The standardized settings of the mass spectrometer allows these mass spectra to be reproduced and components to be identified.

In the field of measuring volatile organic components, the GC-MS measuring technique is known as the most sensitive gas measuring technique.

SIFT-MS (Syft-ms) is a form of direct mass spectrometry. This technology, SIFT, was developed in the 1970s (Prof. D. Smith) to study ion / molecule reactions that occur in the atmosphere. Since then, the technique has been further developed into a high-quality analytical measurement technique, among other things through better vacuum technology and better electronics. The customs of Australia and Canada, among others, use this technique for container measurements in their ports.

The measuring principle of the SIFT-MS is based on a mild chemical ionization of molecules with precursor ions under vacuum conditions, in a so-called flow tube. These precursor ions (H3O +, NO + and O2 +) are generated by the system itself and the product ions are accurately measured with the built-in quadrupole mass spectrometer. Productions and interferences are separated from each other using this quadrupole mass spectrometer. Full kinetic control of the reactions in the flow tube allows the samples to be quantified. The SIFT-MS technique derives its selectivity from the fact that each chemical compound reacts in a unique way with the different precursors ions. This reaction produces known product ions for each combination of molecule and precursor ion. The result is a selective and universal detection method for all volatile organic components. The mild chemical ionization provides simplified spectra, resulting in less interference and more precise results.

The quality is guaranteed by the daily performance of various validation rings, from sampling to analysis, and the annual calibration of all devices.

The absorption of infrared radiation from a material at different frequencies gives a unique “spectral fingerprint”. This fingerprint is based on the frequencies at which that material absorbs infrared, but also on the intensities of this absorption. The spectral scan (absorption or transmission) obtained is usually specific to a group of materials. Compounds can be identified by the result on IR spectrum.

All devices are calibrated annually.

Different gases can be detected using these types of sensors. This includes oxygen, carbon monoxide, carbon dioxide and phosphine. A chemical reaction takes place in the electrochemical sensors, producing an electrical signal. By measuring this electrical signal, the gas concentration is determined and displayed. Due to the limited service life of this type of sensors, EWS maintains, calibrates and replaces the sensors itself. For this, EWS has set up its own “Technical and Research Centre”.

The PID meter indicates the amount of the different VOCs (Volatile Organic Components). After this, the Gas Measuring Expert’s quest to explain this value begins.

In this technique, a gas mixture is passed past a UV lamp that bombard the gas with photons with high energy. Gases with an ionization potential (the necessary energy to be ionized) that is low enough are ionized to form free ions (they temporarily lose an electron). These ions are guided past a polarizing detector electrode. The PID meter translates this ion current into an electrical voltage that is proportional to the amount of ionized gas in the sample presented. The signal is then amplified and digitally displayed in the display of the device.

This equipment is periodically checked internally for reliability.

A gas detection tube or gas measurement tube is a glass tube filled with silica gel which is impregnated with substance specific chemicals. These added chemicals cause a colour reaction with the gas to be detected.

Measuring tubes are not very accurate and may have a large deviation. A correction must also be calculated for the temperature. Deviations of up to 30 percent between the measured and actual value occur regularly. It is therefore important that the value measured with the tube is well below the limit value. Good knowledge of the various measuring tubes, thorough training, knowledge of the cargo and the necessary experience are all required to reach a good conclusion regarding the gas composition in the sea container.

Thanks to years of experience, extensive research, strict internal training requirements and procedures, all our gas measurement experts are able to handle all measuring tubes professionally.

The user has a major influence on the result for every applied measurement technique. Our knowledge remains at a high level by continuously training and retraining employees (internal and external). Only by always being open to new techniques and developments can reliability and quality be guaranteed.