Mass Spectrometer Detectors

Mass Spectrometer Detectors www.sge.com Ion Detectors for Virtually all Mass Spectrometry Applications Electron multipliers are used in a wide range of applications to Figure 2. Secondary Electron Emission detect and measure small signals of ions, electrons or photons. One of the main applications for an electron multiplier is for 6 the detection of ions in a mass spectrometer. ACTIVE FILM Multipliers manufactured by ETP ( a division of SGE Group 5 of Companies) are the result of 15 years experience in 4 supplying high performance electron multipliers for use in mass spectrometry applications. The performance of this 3 unique type of ion detector has seen it become widely used in almost all fields of mass spectrometry (including ICP-MS, 2 GC-MS, LC-MS, TOF and SIMS). 1 An electron multiplier is used to detect the presence of ion Secondary Electron Emissions signals emerging from the mass analyser of a mass 0 spectrometer. It is essentially the "eyes" of the instrument (see 0 100 200 300 400 500 Figure 1). The task of the electron multiplier is to detect Incident Electron Energy (eV) every ion of the selected mass passed by the mass filter. How The average number of secondary electrons emitted from the efficiently the electron multiplier carries out this task surface of an ETP electron multiplier plotted against the represents a potentially limiting factor on the overall system energy of the incident primary electron. sensitivity. Consequently the performance of the electron multiplier can have a major influence on the overall (often referred to as a channel electron multiplier or CEM). performance of the mass spectrometer. The principal difference between these two types of multipliers is the TECHNICAL ARTICLE Principles of Multiplier Operation structure used to harness the secondary electron emission process. In The basic physical process that allows an electron multiplier to the case of the discrete-dynode multiplier electron, amplification is operate is called secondary electron emission. When a carried out using an array of electron multiplying electrodes, called charged particle, an ion or an electron, strikes a surface it can dynodes. Ions striking the first dynode cause secondary electrons to be cause electrons associated with the outer layers of atoms to be emitted from the surface. The optics of the dynodes focuses these released. The number of secondary electrons released depends secondary electrons onto the next dynode of the array as shown in on the type of incident primary particle, its energy and Figure 3. These in turn emit even more secondary electrons from the characteristic of the incident surface (see Figure 2). surface of the second dynode. In this way a cascade of electrons is In general there are two basic forms of electron multipliers that developed between successive dynodes, each dynode increasing the are commonly used in mass spectrometry; the discrete-dynode number of electrons in the cascade by a factor of 2 to 3, until the electron multiplier, and the continuous-dynode multiplier cascade of electrons reaches the output electrode where the signal is extracted. Figure 1. Components of Mass Spectrometry In the case of the continuous-dynode multiplier, electron amplification Ion Sorting is carried out using a single continuous channel made from extruded Gas Phase Ion Detection lead-silicate glass. The electron gain occurs by the ion striking the Electron Source Analyser channel wall and causing secondary electrons to be emitted. The Multiplier secondary electrons are then drawn down the channel, by an electrostatic field, until they again impact with the channel wall, emitting even more secondary electrons. This process is illustrated in Vacuum Figure 4. Inlet Data A typical discrete-dynode electron multiplier has between 12 and 24 dynodes and is used with an operating gain of between 104 and 108, Sample Introduction depending on the application. In GC-MS applications, for example, the Data Output: Mass Spectrum electron multiplier is typically operated in analog mode with a gain of The general layout of a mass spectrometer consists of the following around 105. For a new electron multiplier this gain is typically achieved elements; Sample introduction and separation system, Ion source, with an applied high voltage of ~1400 volts (Figure 5). Mass analyser, Ion detection system, Data processing. Noise generated within an ETP electron multiplier is negligible, typically Figure 3. Discrete Dynode Detector the output equivalent of just a few ions per minute. In applications such as ION GC-MS the noise on the output signal from the multiplier is influenced by three factors: First 1. Noise already existing on the ion signal (produced in the Dynode separation column and the ion source). 2. The ability of the input optics of the detector to screen out unwanted radiation (not ions). 3. The statistical noise associated with the number of ions in a measurement (especially for low level ion signals). Ion-optics of an ETP discrete-dynode electron multiplier As a multiplier is used, the surfaces of the dynodes slowly become covered showing the electron gain at each successive dynode. This electron cascading process results in gains up to with contaminants from the vacuum system, causing their secondary 108 being achieved with ~21 dynodes. electron emission to be reduced and the gain of the electron multiplier to decrease. To compensate for this process, the operating high voltage applied to the multiplier must be periodically increased to maintain the required multiplier gain. Figure 4. Continuous Dynode Detector Figure 6 shows the change in the applied high voltage required to High Voltage maintain the gain of a typical electron multiplier for GC-MS over the course of its operating life. The electron multiplier power supply in most GC-MS systems is limited to 3000 volts. When the applied high voltage necessary to achieve the required operating gain begins to approach the limit of the power supply, it is necessary to replace the multiplier with a new one. A typical electron multiplier for GC-MS applications lasts 1 to 3 years. Ion-optics of a continuous-dynode, or channel, electron multiplier. The electron gain occurs as each electron Features of ETP Electron Multipliers strikes the walls of the channel causing secondary The electron multipliers manufactured by ETP are called ACTIVE FILM electrons to be emitted. Multipliers. Their name comes from the specialized surface materials used on the dynodes. This proprietary dynode material has a number of properties that make it very suitable for use in an electron multiplier. It Figure 5. Gain vs Voltage has very high secondary electron emission, which allows exceptional gain to be achieved from each dynode. This material is also very stable 109 in air, in fact an ETP multiplier can be stored for years before being used. As a direct result of the high stability of the active materials used 108 in ETP multipliers they come with a 2 year shelf life warranty. Many testing laboratories take advantage of this long shelf life by keeping a 107 replacement ETP multiplier on hand, ready for immediate installation. This keeps the instrument down time to a minimum. 106 Typical Gain Typical A major advantage of the discrete dynode design of ACTIVE FILM Multipliers is increased operating life. This is a consequence of the 105 mechanism of gain loss in an electron multiplier. 104 One of the main mechanisms of multiplier gain loss is the deposition of 1200 1400 1600 1800 2000 2200 2400 2600 contamination on the surface of the dynodes. The rate of contamination Applied Voltage build-up on the dynode surfaces is influenced by two factors; A gain curve for an ETP electron multiplier to suit a Hewlett-Packard 5972 MSD. Typical operating gain in this instrument is ~2x105 which is achieved with an applied high voltage of ~1400 volts. Figure 6. Applied Voltage vs Operating Life The active dynode surfaces of an Active Film Multiplier are composed of stable-in-air materials and can be repeatedly exposed 3500 to the air with no loss in performance. The original packaging is designed for long term storage. The multiplier is delivered in two 3000 sealed plastic bags, the outer bag containing silica gel to absorb any 2500 moisture. If the multiplier is to be stored for long periods it is best left in its original packaging until required. In its original 2000 packaging, the shelf life of the Active Film Multiplier is guaranteed 1500 for two years from ETP's shipping date. If it is necessary to store 1000 the multiplier without its original packaging, it should be kept in a Applied High Voltage dust free, dry environment. Ideally it should be stored in a glass 500 desiccator containing silica gel. 0 0 0.1 0.2 0.3 0.4 0.50.6 0.7 0.8 0.9 1.0 The multiplier should be handled using normal high vacuum Fraction of Operating Life methods, keeping the multiplier clean and free of contamination. Powder-free gloves should be used to prevent finger-oils from Change in the applied high voltage required to maintain the gain of an electron multiplier over the course of its operating life (for a typical contaminating the multiplier via direct contact with skin. All tools, GC-MS). The applied high voltage must be periodically increased to mountings and equipment should be cleaned before coming into maintain the operating gain of the multiplier. contact with the multiplier. Care should be taken to minimize the time that the multiplier is exposed to airborne particles of dust or lint which would be expected in a typical laboratory environment. Dust particles within the multiplier can cause increased background 1. Partial pressure of contaminants in the residual gas of the noise.

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