In process analysis, quadrupole mass spectrometers are primarily used for the analysis of gaseous or easily vaporizable substances. Liquid or solid samples are hardly accessible for online mass spectrometry.
The first important task of the measuring process is performed by the inlet system. The whole process (ionization of the sample, separation and detection of the ions) takes place in high-vacuum at 0.00001 to 0.000001 mbar and therefore the sample has to enter the ion source under continuously reduced pressure.
Depending on the pressure at the sampling point, various arrangements of apertures and capillaries with bores and inner diameters in the micrometer range are used. Therefore the gas has to be cleaned and filtered thoroughly.
Mass spectrometry in general is characterized by very low consumption of samples. Under normal condition a volume flow of only a few ml/min is sufficient for the measuring process. A significantly greater volume flow is necessary for the rapid transfer of the gas from the extraction point to the analysis system in order to allow a very quick response time. Much depends on the right dimensions of the sample pipeline and on the way the gas is conditioned. Dead times of only a few seconds and response times of less than one second are easily possible even with pipes that are several meters long.
Mass spectrometry is a molecular analysis method. The mixture of gases is ionized at the ion source under vacuum conditions. The quadrupole analyser is able to sort the generated ions (charged atoms, molecules and molecule fragments) according to their mass. Within fractions of seconds the detector can determine the components.
For example, the quantitative determination of a gas mixture with six different components takes no longer than one second for each measuring cycle to figure out the concentration. This particularly fast response is a specific strength of quadrupole mass spectrometers.
The method can be applied universally because the ion source can ionize almost all components of a gas mixture. For example, the inert gases like nitrogen, argon and helium can be easily identified. There is hardly another method to determine their concentration.
The ionisation of molecules takes place under high vacuum, so that mutual interference can be disregarded. As a result, the main components (up to 100 percent) and the secondary components (to the sub-ppm range) can be determined simultaneously. Fast detector electronics allow the precise measuring of ion signals over a wide range (eight or nine orders of magnitude).
The method is only constrained when a complex gas mixture contains unfavorable similarities of molecule and fragment masses. Under those circumstances, the available detection range can be limited, or particular components are lumped together and determined as a group.