Mass spectrometry (MS) is an analytical methodology used for quantitative and qualitative analysis of organic samples. Molecules in a sample are ionized and separated by a mass filter based on their respective masses. The separated analyte ions are then detected and a mass spectrum of the sample is produced. The mass spectrum provides information about the masses and the quantities of the various analyte compounds that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on a fragmentation pattern.
Analyte ions for analysis by mass spectrometry may be produced by any of a variety of ionization systems. For example, Atmospheric Pressure Matrix Assisted Laser Desorption Ionization (AP-MALDI), Atmospheric Pressure Photoionization (APPI), Electrospray Ionization (ESI), Atmospheric Pressure Chemical Ionization (APCI) and Inductively Coupled Plasma (ICP) systems may be employed to produce ions in a mass spectrometry system. Many of these systems generate ions at or near atmospheric pressure (760 Torr). Once generated, the analyte ions must be introduced or sampled into a mass spectrometer. Typically, the analyzer section of a mass spectrometer is maintained at high vacuum levels from 10−4 Torr to 10−8 Torr. In practice, sampling the ions includes transporting the analyte ions in the form of a narrowly confined ion beam from the ion source to the high vacuum mass spectrometer chamber by way of one or more intermediate vacuum chambers. Each of the intermediate vacuum chambers is maintained at a vacuum level between that of the proceeding and following chambers. Therefore, the ion beam transports the analyte ions through transitions in a stepwise manner from the pressure levels associated with ion formation to those of the mass spectrometer. In most applications, it is desirable to transport ions through each of the various chambers of a mass spectrometer system without significant ion loss. Often an ion guide is used to move ions in a defined direction to in the MS system.
Ion guides typically utilize electromagnetic fields to confine the ions radially while allowing or promoting ion transport axially. One type of ion guide generates a multipole field by application of a time-dependent voltage, which is often in the radio frequency (RF) spectrum. These so-called RF multipole ion guides have found a variety of applications in transferring ions between parts of MS systems, as well as components of ion traps. When operated in presence of a buffer gas, RF guides are capable of reducing the ion energy (velocity) of ions in both axial and radial directions. This reduction in ion energy in the axial and radial directions is known as “thermalizing” or “cooling” the ion populations due to multiple collisions of ions with low energy neutral molecules of the buffer gas. Often, ion guides implemented to “cool” ion populations are referred to as collision cells. Thermalized beams that are compressed in the radial direction are useful in improving ion transmission through orifices of the MS system and reducing radial velocity spread in time-of-flight (TOF) instruments. RF multipole ion guides create a pseudo potential well, which confines ions inside the ion guide. In other applications, principally triple quad LC-MS, the collision cells are used to fragment high energy ions in order to provide additional information regarding their molecular structure.
In constant cross section multipoles, this pseudo potential is constant along the length and therefore does not create axial forces other than at the entrances and exits. This end effect may be overcome at the entrance and exit of the multipole ion guide with a lens or by other techniques. The lenses shield the ions from the RF fields on the poles and may impart to the ions sufficient energy to enter or exit the multipole. Known multipole ion guides normally include a comparatively large diameter entrance, which is useful for accepting ions. However, having an exit of the same large diameter is not desirable for delivering a small diameter beam from the exit. However, known ion guides not having a substantially constant cross section create a variable pseudo potential barrier along the axis of transmission that can create axial forces, which can retard or even reflect ions. Finally, the buffer gas useful in ion cooling can also cause ion stalling in the ion guide. These stalling and ion retarding forces can be overcome or reversed by the addition of a DC gradient along the resistive rods in the multipole assembly. This DC gradient, usually in the order of approximately 2 V to approximately 10 V, generates an accelerating voltage field compelling the ions to move along the axis of the collision cell assay.
One of the drawbacks of known mass spectrometer systems containing collision cells is size. With the increasing desire to provide smaller, more compact instruments, there is a need to reduce the size (“footprint”) of components in the mass spectrometer.
What is needed, therefore, is an apparatus, which guides ions through a mass spectrometry system and that overcomes at least the shortcomings of known apparatuses described above.