Quadrupole Mass Spectrometers (Q-MS)
A Mass Spectrometer is an Instrument that produces a “Mass Spectrum”. This means a graph of Mass (molecular weight) verses mass intensity.
The graph below shows a typical Mass Spectrum from such an instrument.
Gas Component: In simple terms the Gas components and concentrations are represented by the features on the graph spectrum. Each peak on the graph represents a particular gas and the height of the peak represents the concentration.
Mass Number: This number is the molecular weight of the particular gas component. This mass is produced by “Ionising” all the gases. Both the background gases within the vacuum system as well as the sample gas that has been introduced into the Instrument.
Graph Peak Intensity: The more intense the peak (greater the peak height) the more of that particular mass/gas there is. The software can compare these peak intensities to calculate the gas composition and concentrations of the sample.
Cracking Pattern: Many gas species fragment into a “cracking pattern”. This is a predicable number of masses and intensities and are seen on the mass spectrum graph. This cracking pattern can be found in publications and is used to identify gas species.
Production of Ions
The Mass Spectrometer requires the sample gas to be Ionised. Ions may be produced in a number of ways. These include, Electron Bombardment (EI) for gases. Chemical produced Ions (CI), Plasma Ion production (ICP) usually for liquids. Laser oblation usually for solids. We are most interested in EI (Electron Ionisation)!
A hot filament within the analyser Ioniser produces electrons of typically -56ev. These electrons are focussed towards the sample gas molecules and “react” with orbiting electrons around the sample molecules.
The sample gas Ions are now re-focused into the mass spectrometer where they are separated by mass to produce the Mass Spectrum.
The Ionisation pattern is predicable and is an exact representation of the type of sample gas.
Filtering and analysing mass
The Quadrupole Analyser again has 3 major parts; Ioniser, Mass Filter and Detector.
Recent improvements in both mechanical design and compact stable electronics have made the QMS suitable for many diverse applications.
Ions produced by the Ioniser travel through the Quadrupole Mass Filter assembly this assembly consists of 4 accurately ground and precisely aligned metal rods.
Both an RF frequency of about 2mHz and a DC component are applied to each opposing 2 rod set and an electrostatic field is manipulated to separate sample Ions to produce a Mass Spectrum. These in turn strike the detector and again the number of Ions striking the detector represents the amount of sample gas of interest.
With its compact and robust construction and ever more powerful software users have become more familiar with this type of multi component analyser.
Mass Spec Analyser Principles
Sample gas is continuously injected into a the vacuum housing held at approximately 1 x 10-6 mbar.
When the sample gas molecule comes in contact with an electron emitted by a hot filament it becomes positively charged and forms an Ion or a series of Ions (cracking pattern) in predicted ratios.
These ions are focused from the Ion Source via transfer optics towards the mass filter (quadrupole rod system). The Rod System consists of 4 accurately ground stainless Steel rods mounted very accurately into a set of ceramic carries.
Once the ion enters the mass filter they are separated into their individual ion weights by the influence of both RF and DC voltages that are applied to apposing rods to create an electrostatic field.
Ions that leave the Mass Filter Rod Assembly strike a detector and produce an electric current.
It is an important principle that the Filtered Ion Current produced at the detector is directly proportional to the concentration of each individual gas species.
This whole Ionisation of the gas sample, transmission through the mass filter, detection and plotting on a computers Y axis takes far less than 1ms making a Mass Spectrometer extremely fast.
Vacuum System Operation:
To achieve a vacuum of better than 1 x 10-6mbar. Requires 2 pumps. The first “backing pump” provides a pressure of 2mbar that allows the ”Turbo Molecular” pump to become effective.
This then takes a further 10 minutes to bring the vacuum housing to 10-6 mbar. This is the operating pressure for the Quadrupole Analyser. All pressures are measured by the full range Vacuum Gauge.
An additional heater helps remove water and background contamination from the clean vacuum housing. Sample gas is injected into the vacuum housing via a sample interface to be analysed.
Production of portable MS Instrument
Hundreds of gas's measured with one instrument
Multiple applications in one design
Large dynamic range of measurements (9 decades)
Fast analysis speeds. (ms).
Very little Sample preparation.
Compact, light weight robust design
Low cost capital expense and cost of ownership
PC based, Powerful, Comprehensive software
Little specialised knowledge required