The use of the Quadrupole Ion Trap (QIT) as a means of trapping and storing charged particles was first described in 1953 by W. Paul and H. Steinwedel, Zeitschrift fur Naturforschung, 8A; 1953, p 448 and U.S. Pat. No. 2,939,952. The technology continued to develop, and the QIT was first used as a Mass Spectrometer in 1959, as described in E. Fischer, Zeitschrift f. Physik 156, 1959 p 1-26. Since then, the development of the QIT for ion storage and mass analysis has progressed steadily. This progress is reviewed in “Quadrupole Ion Trap Mass Spectrometry”, Raymond E. March and John F. Todd.
More recently however, attention has been focussed on 2D ion traps, which are also referred to as Linear Ion Traps (LIT) and Digital Ion Traps (DIT) as described in “Ion Motion in the Rectangular Wave Quadrupole Field and Digital Operation Mode of a Quadrupole Ion Trap Mass Spectrometer”, L. Ding et al. Vacuum Science and Technology, V.21, No. 3, 2001, p 176-181. These alternative ion traps have greatly advanced the capabilities of ion traps in the field of Mass Spectrometry.
The possibility of using ion traps to store charged particles irrespective of polarity and for the stored particles to then be manipulated has long been recognised. However, until more recently, this aspect of the use of ion traps has been less successful than the utility of the Ion Trap as a Mass Spectrometer (ITMS).
An advantage of an ion trap acting as an ion storage facility came with the discovery and development of the resonant ejection process. Using the resonant ejection process it became possible to retain a specific ion/group of ions (according to their mass/charge ratio) in the ion trap, whilst simultaneously ejecting the other ions from the ion trap. The retained ions are termed the precursor or analyte ions. Once the precursor ions are isolated in the ion trap they are subject to resonant excitation and a collision gas is introduced into the ion trap. This leads to the precursor ions undergoing a fragmentation process. This fragmentation allows component parts of the precursor ions to be identified. From the identification of the masses of the individual fragments and their relative contribution to the mass spectrum, it is possible to elucidate the structure of the precursor ions.
It is also well known that the ion trap can simultaneously retain ions of different polarities (anions and cations). However, the introduction, ejection and detection of both anions and cations stored simultaneously in the ion trap is difficult to achieve in a typical ion trap configuration due to the unipolar nature of the ion optics related to the ion introduction, ejection and detection.
“Anion Effects on Storage and Resonance Ejection of High Mass-to-charge Cations in Quadrupole Ion Trap Mass Spectrometry”, J. L. Stephenson, Jr. and S. A. McLuckey Anal. Chem., 69 (1997) p 3760-66 describes studies performed on the interactions between ions of different polarities within an ion trap.
A number of different experimental approaches have been devised to address the problem of introducing and storing different ions in the ion trap.
One approach used is to provide an additional entrance aperture in the ring electrode of the ion trap, to allow the introduction of the alternative ions into the ion trap. However, this approach has limited viability due to the requirement of using two sets of introduction electrodes, one for analyte ions and the other for reagent ions. Also, the additional entrance aperture gives rise to undesirable field distortion within the ion trap. The basic instrument set up is described by Dearth et al. in their paper entitled “Nitric Oxide Chemical Ionization/Ion Trap Mass Spectrometry for the Determination of Hydrocarbons in Engine Exhaust” Anal. Chem 69 1997 p 5121-5129. This is a very expensive option and there are currently no commercial available instruments like this.
An alternative geometry is described in “Dueling ESI: Instrumentation to study ion/ion reactions of electrospray-generated cations and anions”. Wells J. M. et al. J. Am. Sol, Mass Spectrometry 2002 Jun. 13 (6), p 614-622. This apparatus has two separate ion sources, each with an associated set of transmission optics. The two sets of transmission optics have opposite polarities and are arranged to direct the generated anions and cations into the ion trap through a single entrance aperture.
Electron Capture Dissociation (ECD) is a recently developed technique used in Fourier Transform Ion Cyclotron Resonance (FTICR) that has provided improved and highly desired fragmentation capabilities. In this technique, electrons with appropriate thermal energy are kept in close proximity to an ionised molecule of interest e.g. a protein or peptide. One or more electrons are captured by the molecule of interest which subsequently undergoes fragmentation. ECD seems to be very attractive for fragmentation in ion traps and attempts have been made to adapt the technique but, the optimum conditions for ECD can only be achieved using a couple of specific ion trap designs.
A related technique, known as Electron Transfer Dissociation (ETD) can be used in an ion trap. This technique uses an ion (typically an anion) with a low electron affinity, which acts to transfer an electron in a similar manner to ECD. This technique has been used in the fragmentation of proteins/peptides and appears to be effective in achieving a more complete or preferred cleavage of a protein/peptide backbone. This improved fragmentation is useful in determining the structure and/or other properties of the protein/peptide.
ETD is an example of an ion-ion reaction.
Clearly, to efficiently use this ETD technique it is necessary to introduce ETD anions into the ion trap, to allow ETD anions to interact with the ions to be studied. Recently, Syka et al. in “Peptide and Protein Sequence Analysis by Electron Transfer Dissociation Mass Spectrometry.” John E. P. Syka et al. PNAS, Jun. 29, 2004. Vol. 101 No. 26, pp 9528-9533 have described an apparatus in which analyte ions in the form of protein/peptide cations are introduced in the normal fashion through the entrance aperture of the LIT, whilst the reagent ions in the form of anthracene anions (acting as the ETD anions) are introduced into the LIT at the opposite end of the LIT to the entrance aperture.
As can be seen from the above discussion, the ETD technique has obvious advantages. However, this technique is still not generally applicable to the most common configurations of ion traps without significant mechanical modifications to the ion trap.
To make this ETD technique a truly general purpose technique with widespread applications it is preferable to use a standard ion trap mass spectrometer which requires minimal mechanical modifications.