www.sge.com TECHNICAL ARTICLE performance of the .mass the of performance overall the on influence major a have can multiplier the of performance the sensitivity. Consequently system overall the on factor limiting potentially a represents task this out carries multiplier electron the efficiently filter.How mass the by passed mass selected the of ion every 1). Figure the essentially is spectrometer.It mass a of analyser mass the from emerging signals ion of presence the detect to used is multiplier electron An SIMS). and TOFLC-MS, GC-MS, ICP-MS, (including spectrometry mass of fields all almost in used widely become it seen has detector ion of type unique this of performance The applications. spectrometry mass in use for multipliers electron performance high supplying in experience years 15 of result the are Companies) of Group SGE of division a ( ETP by manufactured Multipliers spectrometer. FILM mass ACTIVE a in ions of detection the for is multiplier electron an for applications main the of One photons. or ions, of signals small measure and detect to applications of range wide a in used are multipliers Electron electron multiplier, and the multiplier,and electron the spectrometry; mass in used commonly are that multipliers electron of forms basic two are there general In surface incident the of characteristic and energy its particle, primary incident of type the on depends released electrons secondary of number The released. be to atoms of layers outer the with associated electrons cause can it surface a strikes electron, an or ion an particle, charged called is operate to multiplier electron an allows that process physical basic The Operation Multiplier of Principles Ion Detectors for Virtually all Applications allMassSpectrometry Ion DetectorsforVirtually Mass SpectrometerDetectors Figure 1. Figure Mass analyser, Ion detection system, Data processing. Data system, detection analyser,Ion Mass source, Ion system, separation and introduction Sample elements; following the of consists spectrometer mass a of layout general The Introduction Gas Phase Gas Source Sample Inlet The task of the electron multiplier is to detect to is multiplier electron the of task The Components of Mass Spectrometry Mass of Components secondary electron emission. electron secondary continuous- Analyser Ion Sorting Ion Vacuum "eyes" (see Figure 2). 2). Figure (see Data Output: Mass Output: Data of the instrument the of discrete-dynode When a When multiplier Ion Detection Ion Multiplier Electron Data (see around 10 around of gain a with mode analog in operated typically is multiplier electron the example, for applications, GC-MS In application. the on depending with an applied high voltage of ~1400 volts ~1400 of voltage high applied an with and is used with an operating gain of between 10 between of gain operating an with used is and dynodes 24 and 12 between has multiplier electron discrete-dynode typical A 4. Figure in illustrated is process This electrons. secondary more even emitting wall, channel the with impact again they until field, electrostatic an by channel, the down drawn then are electrons secondary The emitted. be to electrons secondary causing and wall channel the striking ion the by occurs gain electron The glass. lead-silicate extruded from made channel continuous single a using out carried is amplification electron multiplier, continuous-dynode the of case the In extracted. is signal the where electrode output the reaches electrons of cascade the until 3, to 2 of factor a by cascade the in electrons of number the increasing dynode each dynodes, successive between developed is electrons of cascade a way this In dynode. second the of surface 3. Figure in shown as array the of dynode next the onto electrons secondary these focuses dynodes the of optics The surface. the from emitted dynodes. called electrodes, multiplying electron of array an using out carried is amplification electron, multiplier discrete-dynode the of case the In process. emission electron secondary the harness to used structure the is multipliers of types two these between difference principal The CEM). or multiplier electron channel a as to referred (often Figure 2. 2. Figure energy of the incident primary electron. primary incident the of energy the against plotted multiplier electron ETP an of surface the from emitted electrons secondary of number average The

Secondary Electron Emissions 5 Ions striking the first dynode cause secondary electrons to be to electrons secondary cause dynode first the striking Ions . For a new electron multiplier this gain is typically achieved typically is gain this multiplier electron new a For . These in turn emit even more secondary electrons from the from electrons secondary more even emit turn in These 0 1 2 3 4 5 6 Secondary Electron Emission Electron Secondary 0 0 0 0 500 400 300 200 100 0 Incident Electron Energy (eV) Energy Electron Incident (Figure 5). (Figure 4 and 10 and 8 , 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 ).

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. vacuum chamber. This means that a cleaner vacuum environment will allow the multiplier to achieve a longer Exposure of the multiplier to a high humidity environment should operating life time. be avoided as it can cause noisy operation. In the event of this situation occurring, the multiplier can be restored by baking for 3 2. The electron density incident on the surface of the dynode. Because ACTIVE FILM Multipliers use large area dynodes in hours in vacuum at 150ºC, or simply by leaving it in vacuum for a their construction, the amount of surface contamination per unit day or more. After baking, the multiplier should be allowed to cool dynode area is decreased, resulting in the multiplier having in the vacuum chamber to avoid the possibility of damage due to increased operating life and improved gain stability. thermal shock.

The rugged design of the Active Film Multiplier greatly reduces the For a typical ACTIVE FILM Multiplier for GC-MS, the total chance of damage through rough or careless handling. 2 active dynode surface area is ~1000mm . This can be compared to Nonetheless, an electron multiplier is a precision instrument and all a standard (chrome) continuous-dynode multiplier that has a total reasonable care should be taken when handling. channel surface area of only around 160mm2 (for a channel with 1mm diameter and 50mm length). This increased surface area spreads out the "work-load" of the electron multiplication process over a larger area, effectively slowing the aging process and improving operating life and gain stability.

The electron-optics used in ACTIVE FILM Multipliers were designed using highly specialized CAD software developed by ETP specifically for this task. This has resulted in a performance- optimized design that efficiently focuses all the ions that enter the input aperture of the multiplier onto the first dynode. This very high ion detection efficiency results in an improved signal to noise ratio on the output spectra. Installation, Storage and Handling Active Film Multipliers are individually designed for each specific mass spectrometer and require no modifications to the instrument. They are designed to be fully compatible with the instrument for which they were designed and can be easily installed.

An Active Film Multiplier requires no preconditioning. However, it is recommended that the applied high voltage be limited to 2200 volts for the first day of operation.

Any multiplier leads should be positioned to have a minimum clearance of 3 millimeters between the lead and any metal parts of the multiplier mounting or mass spectrometer system. This clearance prevents the risk of noise due to arcing or electrical breakdown.