| AVS 60th International Symposium and Exhibition | |
| Applied Surface Science | Wednesday Sessions |
| Session AS+BI+IS-WeM |
| Session: | Ambient Ionization Mass Spectrometry |
| Presenter: | J.W. Bradley, University of Liverpool, UK |
| Authors: | J.W. Bradley, University of Liverpool, UK K. McKay, University of Liverpool, UK A. Bowfield, University of Liverpool, UK T.L. Salter, National Physical Laboratory, UK J.W. Walsh, University of Liverpool, UK M.R. Alexander, University of Nottingham, UK D.A. Barrett, University of Nottingham, UK I.S. Gilmore, National Physical Laboratory, UK |
| Correspondent: | Click to Email |
The potential benefits of using atmospheric pressure “cold” plasma sources as a means of both desorption and ionisation of material in ambient mass spectrometric analysis of surfaces has been recognized by a number of researchers worldwide. Plasma sources include dielectric barrier jets (PADI [1] and LTP [2]) operating at mid-frequency and also RF corona needle discharges.
Here we develop further strategies to pulse modulate such sources (jet and needle) and investigate the influence of duty cycle on the production of chemical species in the source plasma and the composition of the ionised desorbed species from a range of test surfaces (polymeric, pharmaceutical and biological). For the plasma plume study we use a time resolved (5 µs) Hiden Analytical molecular beam MS (HPR-60) unit, while detailed information on the spatial distribution of surface species from the chosen material samples was done using a Thermo Velos LTQ Orbitrap MS.
The influence of source-sample and sample-instrument distances on the signal intensities is investigated for both sources. We observe that as the duty cycle of the pulse is decreased the positive ion yield shifted towards higher mass clusters, due to a decrease in gas temperature enabling increased hydration reactions. The negative ions also display similar trends with the yield of larger negative ions increasing with shorter duty cycles. From time-resolved ion intensities it is clearly seen that positive ions are produced in the on-phase of the discharge, and decay in the off-phase. Negative ions, in contrast, are produced mainly in the off-phase of the discharge and decay during the on-phase. With increased distance between source and instrument we observe that the yield of large positive ions increases, at the expense of smaller positive ions which are lost before they reach the orifice. Negative ions, on the other hand, show an increase in yield as distance is increased.
Preliminary analysis of common pharmaceutical products suggest that a decreased duty cycle gives improved identification of negative ion surface compounds however, where surface compounds undergo ionisation to form positive ions, larger duty cycles allowed for better identification. The gas-phase plasma chemistry for different arrangments will be discussed.
1] Ratcliffe LV, Rutten FJ, Barrett DA, Whitmore T, Seymour D, Greenwood C, Aranda-Gonzalvo Y, Robinson S, McCoustra M, Anal Chem. (2007) 79 (16) 6094
2] Zhang, JI, Costa, AB,Tao, WA, Cooks, RG, Analyst, (2011), 136, 15, 3091