Mass Spectrometry

Interesting facts about mass spectrometry

Mass Spectrometry - An Analytical Tool

Mass Spectrum of Air


Mass spectrometers are analytical systems that analyse electronically charged atoms and molecules on the basis of their particle mass. The universal ability to analyse almost all substances that can be ionised has made this technology very popular.

In the past decades, mass spectrometry has found its way into many analytical laboratories due to the development of the coupling technologies GC/MS and LC/MS, which use mass spectrometers as online detectors for chromatographic separation technologies.

At the same time, the use of mass spectrometers as an online analysis tool for the fast and continuous analysis of substance contents has made rapid progress in the research area as well as in industrial laboratories.

The following steps characterise the analytical process of mass spectrometers in general:

  • The substance - preferably in a gaseous state - is transferred into a vacuum system
  • The atoms and/or molecules of the substance are ionized
  • The ions are separated in a mass analyzer according to their particle mass
  • Each particle mass to charge ratio is detected individually so that its quantity can be determined
  • A computer system controls the entire process as well as the data acquisition and processing

Accordingly, a mass spectrometer consists of five essential parts:

  1. Gas inlet system
  2. Ion source
  3. Mass analyzer
  4. Detector
  5. Control and data system

The ion source, the mass analyzer and the detector are located in high vacuum.

Online Analysis

In modern, production-related analytics, online processes are becoming more important than traditional laboratory methods. The latter, that is still dominant particularly in the chemical industry, allows a complete analysis of all selected parameters. However, analysis tools are not directly connected with the production process. This method is therefore called off-line analysis.

Off-line technology is characterized by the fact that samples are taken discontinuously. Analysis time in the laboratory may take hours before having the results.

Laboratory analysis provides greater flexibility in the choice of methods than online analysis. Another advantage is the fact that samples with unknown components can be analysed successfully.

Negative effects result from the greater expenditure of human labour and from the delay between sampling and results, which can last between hours and days. Further disadvantages are the discontinuous equipment utilization and the high cost of the required laboratory tools.

In contrast, online process systems are directly and continuously connected with the production process. The entire analytical process takes place automatically without interruption. Measuring parameters are obtained within seconds or minutes.

This constant and immediate monitoring of relevant process parameters permit fast reactions to changes in the production process and increases the safety of workers, the plants and the environment. It also supports process optimisation and helps to enhance product quality. Thereby it contributes significantly to the increase of efficiency and to the reduction of costs.

Schematic comparison between online and off-line techniques

Quadrupole Mass Spectrometry

Mass spectrometer systems can be divided into a variety of technological categories. Quadrupole mass spectrometers are the most widespread systems because they perform very well. They also stand out due to their compact and robust construction.

For mass separation, quadrupole systems use a high-frequency alternating electrical field. A Faraday cup and a secondary electron multiplier (SEM, Channeltron) serve as detectors.

Quadrupole mass spectrometers allow fast measurements within milliseconds and offer a high dynamic range from the ppb to the percentage level. These systems can be optimised for specific application purposes, therefore providing an attractive price/performance ratio.

InProcess Instruments has specialized in a modular concept of quadrupole mass spectrometers for online gas analysis. We offer a wide variety of individual equipment combinations that are optimised for the customers needs and applications.

Operating Principle of a Quadrupole Mass Spectrometer

Online Mass Spectrometry

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.

Comparison of time scale for the availability of results in the process control system

Practical Solutions

Fully developed and proven solutions are already available for many processes. This includes applications in the chemical industry, e.g. the monitoring of ethylene oxide - ammonia synthesis and the optimisation of complex chemical reactions such as ketene production. Online mass spectrometers are also successfully utilised to monitor emissions and immissions of industrial plants.

Various metallurgical processes (blast furnace, steel, secondary metallurgy) have also successfully been running under mass spectrometry control for years. Additional examples are fermentation control and biogas production.

Further applications can be found in other branches of industry, primarily in the areas of quality control and production. This includes lamp production, glass production and the inspection of special electronic components with protective gas fillings.

However, next to these already available applications, process mass spectrometry is characterized by its problem-solving ability and its special potential for the development of new applications. Together with process development, where the methods flexibility constitutes an advantage, the applications for the actual process control can be developed at the same time. We work closely with process developers, plant constructors and operators to meet their requirements.

Online mass spectrometers on the basis of quadrupole technology have outstanding advantages regarding operation and maintenance costs. Fully automatic operation, long-term stability, and easy handling under industrial conditions are no problem anymore today.

One prerequisite for successful applications under industrial conditions is the ideal interaction between hardware and software. The system has to be integrated into the production environment and into the customer process control system.