7 Mass Spectrometry Real Time mass spectrometry - the whole package.


The ioniser, mass fi lter and detector are packaged as a fl ange mounted assembly, often referred to as the ‘analyser gauge head’.


To make the analyser gauge head useful for gas analysis it is supplied as a complete gas analyser with the following:


A sample inlet, an ultra-high vacuum system, a safe sample exhaust connection and a PC running software with advanced graphical user interface for control, data acquisition and data display.


The design and confi guration of the sample inlet are critical to the performance of the real time mass spectrometer for real world applications.


The sample inlet provides the interface from the application to the quadrupole mass spectrometer and is probably the most important part of the system.


Figure 4: Diagram of Faraday cup detector.


The Faraday Cup: Detection limit can be evaluated for looking at an example for detecting nitrogen:


Ion current for N2 produced in the EI source and mass selected is of the order of 10-4 amps / hPa


At 10-8 hPa of N2, 10-8 * 10-4 = 10-12 amps At 10-11 hPa of N2 = 10-15 amps


10-15 amps is the typical detection limit for a high sensitivity electrometer, giving the detection limit for Faraday cup detection of the order of 10-11 hPa.


2) The Secondary Electron Multiplier / Single Channel Electron Multiplier (SEM / SCEM): The SEM/SCEM is a device operated at high voltage (typically 1 kV) with a surface engineered to have a strong secondary electron yield. When an ion impacts it, a shower of electrons is released. Each of these electrons undergoes further collisions with the surface, again releasing more electrons, until the cascade reaches the anode, providing a measurable current. The voltage applied gives a gain of typically 103 and it is this gain that increases the analyser performance by lowering the detection limit from 10-11 hPa achieved with a Faraday detector to 10-14 hPa for the SEM/SCEM.


When used with advanced ion counting electronics, each ion event arriving at the SEM/ SCEM detector is detected as a pulse of electrons, coupled with the low noise of pulse ion counting detection leads to fast and sensitive detection, typically a factor of 10 greater sensitivity is achieved compared to a conventional analogue SEM/SCEM.


The combination of EI ion source, mass fi lter and combination of Faraday cup /SEM/SCEM detectors that gives the real time QMS the high dynamic range, of ~ 10 decades.


Figure 7: A complete bench top real time mass spectrometer with heated capillary sample inlet.


Sample inlets for real time mass spectrometry near atmospheric pressure include:


For gas and vapour analysis:


A capillary inlet made from silica or stainless steel. The capillary internal diameter, length and gas fl ow are specifi ed to provide viscous fl ow. The assembly is heated and pumped to provide fast gas and vapour transport to the mass spectrometer. The length of the capillary is typically one or two metres. Response times to changes in gas/vapour composition of < 150 milliseconds are achievable with a capillary inlet.


For dissolved gas analysis in soils, sludges and liquids


Figure 8: Dissolved species probe inlet shown in fl ow through housing with integral thermocouple.


A membrane inlet using permeable silicone or polymer membrane for dissolved gas analysis. The membrane provides the interface from sample to the mass spectrometer. The membrane inlet can be configured to have sample flow past the membrane close coupled to the mass


spectrometer or designed as a probe to immerse in sample with typical length ~500 mm. The response time of the membrane inlet to changes in dissolved gas composition is typically less than one minute.


Figure 5: Close up of a Faraday cup and SEM/SCEM dual detector assembly.


Figure 6: Flange mounted analyser gauge head with ioniser (EI) mass fi lter, detectors Faraday and SEM/SCEM.


Many applications for dissolved gas analysis are away from the laboratory in the fi eld, near rivers, lakes or at sea. For these applications portability and low power consumption are vital to enable researchers’ access to real time data directly from the environment. The system shown below is powered from 12 V DC


Figure 9: Portable Real Time Mass Spectrometer.


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