Mass Spectrometry


Mass Spectrometry in Real Time Peter Hatton & Dr David Lundie, Hiden Analytical Ltd


Mass spectrometry as a technique has grown signifi cantly in recent decades, and this is an understatement. Many analytical requirements are addressed by mass spectrometry in some form or other.


The techniques of ionisation, mass separation and detection are varied, and the process is now used in analytical sectors ranging from relatively simple vacuum and vacuum process monitoring to the more complex life sciences and biomedical applications.


In this article we address the little known but powerful technique of real time mass spectrometry by quadrupole mass spectrometry applied to gas analysis and dissolved gas analysis in environmental studies, catalysis, electrochemistry and thermo-gravimetric mass spectrometry, TG-MS.


The benefi ts of real time analysis are clear; you get instant results! Real time data is available for following reactions, and can be used for diagnostics, and monitoring. Less well known is the incredibly high dynamic range, and user selectivity for the species to be analysed. Up to 10 orders of magnitude can be achieved, and routinely the technique provides for analysis of species from sub PPM concentration and up to 100% concentration within a few seconds.


The versatile nature of the quadrupole mass spectrometer provides for fast scanning and selectivity of any species that has spectral lines within the mass range of the instrument.


Software with a typical modern sophisticated graphical user interface allows users to automatically acquire quantitative data from multiple species where the concentration may be dynamically changing from PPB to 100%.


Real time mass spectrometry applies to applications that operate over a wide range of sample pressure from <1e-6 mbar UHV to high pressure > 30 bar.


This article will describe how the technology works, and how it is applied to several key applications at near atmospheric pressure with both portable and laboratory devices.


How does real time mass spectrometry work?


The term real time mass spectrometry applies to mass spectrometry where there is no sample preparation, no preconditioning and no separation of analytes before analysis. The time from sampling to receiving data points is of the order of a few seconds.


The techniques described here use electron impact ionisation (EI), quadrupole mass fi lter separation and ion detection by Faraday cup and electron multiplier detector.


EI- electron impact ionisation spectrometry due to its speed for scanning, and the high dynamic range.


A quadrupole is 2 pairs of parallel, equidistant metal rods (poles) biased at equal, but opposite potentials. These twin potentials contain fi xed direct current, DC and alternating radio frequency, RF components. By varying the RF component, the resultant fi eld produced by the rods may be varied.


Any ions entering the quadrupole fi eld experience potential differences defl ecting them from their original trajectory. The extent of defl ection of any ion entering the fi eld is related to its mass: charge (m/e or m/z) ratio.


At each interval on the RF scan only one m/e ratio resonates with the fi eld allowing the ion to pass along the z-axis. All other species are defl ected and neutralised by impact upon the rods of the quadrupole and play no further role.


Figure 3: Quadrupole mass fi lters shown, from 20mm pole diameter to 6mm pole diameter offer mass range options up to 5000u. 200u mass range is typically used for most real time gas analysis applications.


Figure 1: Electron Impact ioniser cut away.


Figure 2: Diagram illustrating electron impact ion source operation.


In electron impact ionisation the gas molecules are ionised by electrons emitted from a hot fi lament. Here a fi lament creating electrons at 70 eV is illustrated.


Together with ionisation the ion source is equipped with extraction optics that extract the ions into the mass fi lter.


The mass fi lter:


The mass fi lter differentiates the ions produced and selects species for detection. A common form of mass fi lter is the quadrupole. Other types include time of fl ight, TOF and the classic magnetic sector.


The quadrupole is often recognised as the best choice of mass fi lter for real time mass The detector(s)


Filtered ions that strike the detector result in an ion current which is measured by a sensitive amplifi er or counted as pulses.


Two types of detector are commonly used:


1) The Faraday Cup - an earthed passive conducting surface with a suppressor electrode to avoid false measurement.


Fast moving ions strike the cup cause a ‘shower’ of ‘secondary’ electrons. The use of the ‘cup’ rather than a plate, allows all electrons to be collected.


Any electrons that do escape the geometric shape of the cup are reflected by the negative potential of the suppressor plate back into the cup. Thus the only current measured is that of the arriving ion.


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