Ion funnel

I he addition of 2,4-pentanedione to liquid ammonia is a Itigldy ( xothermic ])rocess. Also, ammonia vapor reacts with the ji dil 4on< (o produce an insohd)lo ammonium salt, which tends to clog t he t ip ol t li(> addit ion funnel. Cooling the reaction... 

In summary, the advantages of low-pressure IMS are that ion trap injection can replace ion gates to improve sensitivity, and ion-focusing devices such as the ion funnel can be employed at low pressure to offset ion diffusion. Due to the complexity and size of the instrument, however, no commercially available low-pressure IMSs with static homogeneous electric fields are interfaced to MSs. Nevertheless, their performance in research laboratories has clearly demonstrated many advantages of coupling IMS to MS. 

In an alternative approach to the use of DMS as a filter for ESI before a high-end MS, a low-resolution QMS was fitted with a DMS for real-time chemical analysis in the field. The instrument had a mass resolution of 140 with two stages of differential pumping and an electrodynamic ion funnel to transport the ion beam from ambient pressure to the MS. This prototype DMS-MS detected approximately 1 ppb of dimethyl methyl phosphonate (DMMP) as a simulent for chemical warfare agents. 

Tang, K., Shvartsburg, A.A., Lee, H.N., Prior, D.C., Buschbach, M.A., Li, F., Tolmachev, A.V., Anderson, G.A., Smith, R.D., High-sensitivity ion mobility spectro-metry/mass spectrometry using electrodynamic ion funnel interfaces. Anal. Chem. 2005, 77, 3330. 

Kelly RT, Tolmachev AV, Page JS, Tang K, Smith RD. The ion funnel theory, implementations, and applieations. Mass Speetrom Rev. 2010 29 294-312. 

In the same year, our group in Lausanne published first results from a similar instrument which was equipped with an electrospray ion source for producing closed-shell biomolecular ions, the first demonstrations of which were the measurement of the UV spectra of cold, protmiated aromatic amino acids, tryptophan [46], tyrosine [46, 122], and phenylalanine [122]. Spectroscopic detection is achieved by measuring the small percentage of parent ions that fragment subsequent to UV absorption. The internal temperature of the ions was estimated to be 11-16 K from an analysis of the intensity of hot band transitions of low frequency vibrational modes. If the temperatures achieved in buffer-gas cooled ion traps are low enough and the spectra sufficiently simple, one can often resolve UV absorption spectra for different stable cOTiformers of the molecule [122]. In this case, one can use the IR-UV double resonance techniques so profitably employed in supersonic molecular beam studies [91,123-128] to measure conformer-specific infrared spectra, and this was applied by Steams et al. to both individual amino acids [129] as well as peptides with up to 12 amino acid residues [130]. Subsequent improvements to the Lausanne machine (Fig. 7) included the addition of an ion funnel to... 

The resolution of an IMS device depends in practice not only on the spread of ions At due to diffusion in the drift region relative to the drift time t, but also on the drift tnbe-independent spread of the ion cloud, A/ , brought about by the initial width of the ion pulse entering the drift region and the spread of ions after the drift region (e.g., in the ion funnel). Following Equation (1.4), the resolution t/At is affected only by the two parameters temperature T and drift voltage V. 

A final function of the IMS entrance section is to restrict the flow of buffer gas out of the drift cell. This is accomplished by using a small entrance aperture. If ions enter the drift cell from vacunm then electrostatic lenses can be used to steer the ions and focus them onto the apertnre. Otherwise, ion funnels are used to guide and focns them. 

Shaffer, S. A. Tang, K. Q. Anderson, G. A. Prior, D. C. Udseth, H. R. Smith, R. D., A novel ion funnel for focusing ions at elevated pressure using electrospray ionization mass spectrometry . Rapid Commun Mass Spectrom 1997,11, 1813-1817. 

FIG U RE 3.2 (a) Adiabatically expanding gas jets are formed in the ion funnel system due to the pressure differences between pumping stages, (b) If an electric field barrier is established in ion funnel 2, it is possible to discriminate mobilities of the ions. The drag force by collisions with the atoms or molecules of the gas jet drives the ions and the electric force of the barrier tries to retard them, (c) The classic method of performing ion mobility spectrometry in a drift tube, in which the electric force drives the ions, and the frictional force due to collisions with the stationary gas tries to retard them. 

As the ion funnels are made of stacked ring electrodes, in both of the funnels efficient pumping is expected between the electrodes. Due to the adiabatic expansion through the orifice between the two pressure stages the expanded gas cools down, and this gas jet tends to conserve its integrity for a relatively long time (and distance) in the second ion funnel. It does not easily dissipate—despite the efficient pumping. This cold gas jet enables the measurement of ion mobility in the ion funnel with a sufficient resolution. 

---