The invention relates to a method of transmitting ions in a mass spectrometer maintained in a sub-atmospheric pressure regime. The invention also relates to a mass spectrometer, preferably coupled to a gas chromatograph. Mass spectrometers coupled with gas chromatographs (GC-MS) usually employ vacuum ion sources, that is, ion sources maintained at a substantially sub-atmospheric pressure level. One standard form of ionization in GC-MS systems is electron ionization (EI). Therein, the analyte molecules being entrained in a continuous gas-flow of the gas chromatograph enter the source region of the mass spectrometer. They are irradiated with free electrons usually emitted from a filament. By this exposure, besides of being ionized, the analyte molecules are also fragmented in a characteristic manner. EI is a “hard ionization” technique and results in the creation of many fragments of low mass to charge ratio m/z and only a few, if any, molecular ions. The molecular fragmentation pattern depends on the energy imparted to the electrons, typically on the order of 70 electron volts (eV).
Ion sources employed in GC-MS can alternatively apply chemical ionization (CI). In chemical ionization a reagent gas, typically methane or ammonia, is introduced in excess into the source region of the mass spectrometer and ionized by bombardment with high energetic free electrons. The resultant primary reagent ions then react further with remaining molecules in collisions to become stable secondary ions. These secondary ions then cause ionization of the analyte molecules of interest. The process may involve transfer of electrons, protons or other charged species between the reagents. In general, CI as a “soft ionization” technique dissociates the analyte molecules to a lower degree than the hard ionization of EI. Chemical ionization, therefore, is mainly employed when mass fragments closely corresponding to the molecular weight of the analyte molecules of interest are desired.
The analyte ions generated in the ion source volume are accelerated and transmitted on an ion path leading from the ion source to a mass analyzer by application of extraction voltages to ion optical lenses, located for example at the ion exit of the ion source. However, since analyte ions generated in different sub-volumes of the ion source volume traverse different acceleration distances before passing the ion exit, and also the potential gradients created by the extraction voltages within the ion source volume are generally spatially inhomogeneous, the kinetic energy distribution (the kinetic energy is linked to the velocity by Ekin=½*m*v2) of the analyte ions, in particular in the direction of the ion path, is usually relatively wide, for example of the order of one to five electron volts (at full width at half maximum, FWHM). For the sake of conciseness, in the following, the direction of the ion path, along which the analyte ions propagate, is frequently referred to as the axial direction, while summarizing the directions perpendicular thereto as the radial direction.
The wide energy distribution complicates extraction and transmission of analyte ions from the ion source to the mass analyzer, especially when intending to maximize the number of extracted ions using large extraction fields or large extraction apertures. Most mass analyzers used in conjunction with EI or CI ion sources, and quadrupole mass analyzers in particular, show best performance when the initial ion energy distribution and, moreover, the spatial spread of the ions is low. In order to reduce the width of the energy distribution, the ion exit could be configured as an aperture having a limited passable diameter, so that just analyte ions generated in a limited number of sub-volumes of the ion source volume are transmitted to the mass analyzer and analyte ions from the remaining sub-volumes are masked out. This gain in narrow energy distribution width, however, entails a loss of sensitivity as many analyte ions present in the ion source volume and potentially available for the mass analysis are removed and thus not considered in the analysis process.
On the other hand, increasing the number of extracted ions effects a wider initial energy distribution, in particular in the axial direction, and a wider spatial spread so that the mass resolution and/or the transmission efficiency degrade. Therefore, the efficiency of most prior art GC-MS instruments is limited either because they are operated with less than optimal ion extraction from the source in order to minimize the initial ion energy spread or, if the number of extracted ions is increased, the performance of the mass analyzer in terms of resolution and sensitivity suffers.
In the past, there have been attempts for different reasons to condition ion beams by colliding the ions with neutral gas molecules. Such collisional conditioning has been suggested in different mass spectrometric applications, for example, by Douglas et al. (U.S. Pat. No. 4,963,736 A) for focusing of ions generated in an atmospheric pressure electrospray ion source, by Whitehouse et al. (U.S. 2002/0100870 A1) in the pulsing region of an orthogonal time-of-flight mass spectrometer, by Park (U.S. 2003/0042412 A1) in a surface induced dissociation technique for a time-of-flight instrument, or by Baranov et al. (U.S. 2003/0080290 A1) for de-exciting internally excited and hence potentially metastable ions generated in a matrix-assisted laser desorption/ionization ion source. None of these disclosures, however, provide a way of extending the efficiency of an EI or CI source by first performing efficient ion extraction and creating an ion beam of wide energy and spatial spread and then further remediating beam quality through collisional conditioning in an ion guide.
Thus, the need arises to optimize or maximize the transmission efficiency of the ions through the mass analyzer, while also optimizing or maximizing the number of ions extracted from the ion source.