Ion formation at the boundary between a fast flow glow discharge ion source and a quadrupole mass spectrometer

Abstract

An ion source is described for use in low power glow discharge mass spectrometry, using fast flowing argon as the discharge gas, and attached to a quadrupole analyser. The flow carries the plasma downstream onto an ion exit sampling cone which can be variably biased relative to the anode, enabling the physical and chemical properties of the plasma to be studied independently of the main ionisation region, the cathode fall. The parts of the plasma studied were characterised by fixed probe potential measurements. Cone bias effects are consistent with a plasma which contains large quantities of high n Rydberg atoms of both the discharge gas and the sputtered cathode. The ionisable atom flux passing through the sampling aperture was measured directly, giving densities inside the plasma of 10 10 -10 11 cm 23 . The densities of lower n Rydbergs are expected to be very much higher. H 2 is shown to quench these states and promote excitation of the cathode atoms to their Rydberg states. Field-ionisation or Auger deexcitation of the Rydberg gas inside the cell gives a high intensity cathode ion beam, which is relatively uncontaminated by the higher ionisation potential species, such as are formed from the discharge gas.

Organic mass spectrometry and control of fragmentation using a fast flow glow discharge ion source

“…The use of an applied voltage to aid ion extraction is common in many different types of source. However, in the FFGD source the application of the cone voltage appears to cause a ‘secondary discharge’ which is independent of I d 28…”

“…Also in this work the ion exit is relatively remote from the cathode fall, negative glow and anode regions of the discharge. When a bias is applied to the ion exit cone, which is negative with respect to the anode, the field at the cone surface is dissipated over a distance of less than or equal to 3 mm and current flows between the ion exit and the anode (1–6% of the discharge current depending on the plasma residence time) 28. There was no change in the discharge current at the cathode, which is 40 mm away from the ion exit and the ion exit bias did not significantly affect the plasma potential in the flow tube (relative change in the field ≤0.05 V cm −1 ).…”

“…4. The discharge gas related ([H 2 O] + , [Ar] + , [ArH] + , [Ar 2 ] + ) ions and the cathode ions ([V] + ) have been omitted for clarity (a typical background mass spectrum has been given elsewhere) 28. However, it should be noted that the application of a positive voltage to the ion exit in general suppressed the background ion signals.…”

“…The ion source and mass spectrometer are described in detail elsewhere 28. The instrument used was a VG Plasma Quad II ICP quadrupole mass spectrometer (Thermo Elemental Ltd, formerly VG Elemental Ltd., Winsford, UK) converted for FFGD studies.…”

Glow Discharge Mass Spectrometry

“…[53][54][55][56][57] Moreover, due to the low residence time of the different species in the plasma region, this configuration was denominated ''fast flow'' source. Additional theoretical modelling studies supported the hypothesis that improved transport efficiency is achieved using the fast flow Grimm-type GD source, resulting in enhanced sensitivity and reduced number of polyatomic species.…”