US-20260128266-A1 - Interference Suppression in Mass Spectrometers

US20260128266A1US 20260128266 A1US20260128266 A1US 20260128266A1US-20260128266-A1

Abstract

A method of operating a collision cell ( 10 ) in a mass spectrometer is disclosed. The collision cell comprises an entrance aperture ( 116 ), an exit aperture ( 117 ) and electrodes ( 113, 114 ) for producing electric fields. The method comprises feeding ions in a forward axial direction (LD) through the entrance aperture into the collision cell, producing a first electric field to trap ions, and subsequently producing a second electric field to accelerate trapped ions in the forward axial direction. The method further comprises producing a gas flow (G 1 ) which is, at least at the entrance aperture ( 116 ) of the collision cell, contrary to the forward axial direction (LD), so as to reduce the kinetic energy of ions in dependence on their collisional cross sections. A collision cell arranged for carrying out the method is also disclosed, as well as a mass spectrometer comprising such a collision cell.

Inventors

Mikhail Belov
Lothar Rottman

Assignees

THERMO FISHER SCIENTIFIC (BREMEN) GMBH

Dates

Publication Date: 20260507
Application Date: 20241029
Priority Date: 20190326

Claims (20)

1 . A method of operating a collision cell in a mass spectrometer, wherein the collision cell comprises an entrance aperture, an exit aperture, at least one DC exit electrode, and at least one DC axial electrode, the method comprising: feeding input ions in a forward axial direction through the entrance aperture into the collision cell; producing in the collision cell a gas flow that is directed contrary to the forward axial direction in at least a portion of the collision cell; producing, using the at least one DC axial electrode, an axial electric field within the collision cell; spatially separating ions of interest from interference ions within the collision cell with one or both of the gas flow and the axial electric field; during a first time period, trapping a collection of trapped ions comprising the ions of interest within a trap region located near the exit aperture, wherein the trapping the collection of trapped ions comprises producing a first exit potential with the at least one DC exit electrode to restrict the trapped ions from reaching the exit aperture; and during a second time period: releasing the trapped ions in the forward axial direction through the exit aperture by producing a second exit potential with the at least one DC exit electrode; and rejecting the interference ions from the collision cell such that the interference ions do not reach the exit aperture.
2 . The method of claim 1 , wherein the trap region is positioned nearer to the exit aperture than to the entrance aperture.
3 . The method of claim 1 , wherein the rejecting the interference ions comprises purging the interference ions out of the collision cell in an upstream direction.
4 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for decreasing a kinetic energy of the input ions entering the collision cell.
5 . The method of claim 1 , wherein, during the first time period, the axial electric field is arranged for exerting a force on the trapped ions contrary to the forward axial direction across a full axial extent of the trap region.
6 . The method of claim 1 , wherein the trapping the collection of trapped ions comprises exerting a force on the trapped ions in the forward axial direction with the gas flow and against a force of the axial electric field.
7 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for increasing a kinetic energy of the input ions entering the collision cell.
8 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for exerting a force on the input ions toward the forward axial direction and against the gas flow in at least a portion of the collision cell.
9 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for exerting a force on the input ions in the same direction as the gas flow in a first region of the collision cell, and the axial electric field is arranged for exerting a force on the input ions in an opposite direction as the gas flow in a second region of the collision cell that is downstream of the first region.
10 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for exerting a force on the input ions in the same direction as the gas flow in a first region of the collision cell, and the axial electric field is arranged for exerting a force on the input ions in an opposite direction as the gas flow in a second region of the collision cell that is upstream of the first region.
11 . The method of claim 1 , wherein, during at least the first time period, the axial electric field is arranged for exerting a force on the input ions in the same direction as the gas flow over a full length of the collision cell upstream of the trap region.
12 . The method of claim 1 , wherein, during the first time period, the axial electric field has a first axial field magnitude and the first exit potential has a first potential magnitude, and wherein, during the second time period, the axial electric field has a second axial field magnitude that is greater than the first axial field magnitude and the second exit potential has a second potential magnitude that is less than the first potential magnitude.
13 . The method of claim 1 , wherein the first time period is between 2 and 30 times longer than the second time period.
14 . The method of claim 1 , wherein the first time period has a duration of approximately 2 milliseconds (ms) and the second time period has a duration of approximately 0.1 ms.
15 . A method of operating a collision cell in a mass spectrometer, wherein the collision cell comprises an entrance aperture, an exit aperture, and at least one DC exit electrode, the method comprising: feeding input ions in a forward axial direction through the entrance aperture into the collision cell; producing a gas flow in the collision cell that is directed contrary to the forward axial direction in at least an upstream portion of the collision cell; spatially separating ions of interest from interference ions within the collision cell with the gas flow; during a first time period, producing a first exit potential with the at least one DC exit electrode to trap a collection of trapped ions comprising the ions of interest within a trap region located nearer to the exit aperture than to the entrance aperture; and during a second time period: producing a second exit potential with the at least one DC exit electrode to release the trapped ions through the exit aperture; and purging the interference ions out of the collision cell in an upstream direction away from the exit aperture.
16 . The method of claim 15 , wherein the collision cell further comprises at least one DC axial electrode, and wherein the method further comprises, during the first time period, producing an axial field with the at least one DC axial electrode such that the axial field is arranged for exerting a force on the input ions contrary to the forward axial direction in at least an upstream portion of the collision cell when a flow rate of the gas flow is lower than a threshold value.
17 . The method of claim 15 , wherein the collision cell further comprises at least one DC axial electrode, and wherein the method further comprises, during the first time period, producing an axial field with the at least one DC axial electrode such that the axial field is arranged for exerting a force on the input ions in the forward axial direction in at least the upstream portion of the collision cell when the flow rate of the gas flow is greater than a threshold value.
18 . The method of claim 15 , wherein the collision cell further comprises at least one DC axial electrode, and wherein the purging the interference ions out of the collision cell comprises producing, with the at least one DC axial electrode, an ejection axial field that is arranged for exerting a force on the interference ions contrary to the forward axial direction.
19 . A collision cell for use in a mass spectrometer, the collision cell comprising: an entrance aperture for receiving input ions in a forward axial direction; an exit aperture for emitting ions in the forward axial direction; at least one DC exit electrode for producing, during a first time period, a first DC exit potential to trap ions and for producing, during a second time period, a second DC exit potential to release trapped ions in the forward axial direction towards the exit aperture, at least one gas inlet port for receiving a gas flow such that the gas flow is contrary to the forward axial direction in at least a portion of the collision cell, so as to separate ions of interest from interference ions in dependence on their collisional cross sections; and at least one DC axial electrode for producing an axial electric field to exert an axial force on ions within the collision cell; wherein the collision cell is configured such that: during the first time period, the gas flow and the first DC exit potential confine the ions of interest to a trap region located near the exit aperture; and the interference ions are restricted from reaching the exit aperture during each of the first time period and the second time period by one or both of the gas flow and the axial electric field.
20 . The collision cell according to claim 19 , wherein the at least one gas inlet port is arranged such that the gas flow is directed in the forward axial direction in a region of the collision cell downstream of the at least one gas inlet port and upstream of the exit aperture.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of prior U.S. patent application Ser. No. 17/441,857, filed on Sep. 22, 2021, which is a U.S. national stage application of PCT Application No. PCT/EP2020/058615, filed on Mar. 26, 2020, which claims the benefit of GB Patent Application No. 1904135.9, filed Mar. 26, 2019, all of which are incorporated by reference herein in their entirety. TECHNICAL FIELD OF THE DISCLOSURE The present invention relates to the suppression of interferences when performing analyses with a mass spectrometer. In particular, the present invention may be used for, but is not limited to, suppressing polyatomic interferences in trace elemental analysis carried out with a mass spectrometer. More in particular, the present invention relates to a method of operating a collision cell in a mass spectrometer, a collision cell for use in a mass spectrometer, and a mass spectrometer provided with such a collision cell. BACKGROUND OF THE INVENTION Inductively Coupled Plasma (ICP) mass spectrometry (MS) has been extensively used in a variety of applications, including geological, environmental, food and safety, and biomedical studies. In typical ICP-MS analysis, a sample is nebulized into a spray chamber along with a carrier gas. The latter is used to assist in sample ionization and ion transport from the atmospheric pressure region to the downstream elements of a mass spectrometer operating at a reduced pressure. It is well-known that the analyte species are vaporized, atomized, ionized and transported along with other substances, collectively referred to as a matrix, or the matrix ions in the ionized form. In commercial ICP-MS instruments, a typical carrier gas is argon (Ar), which forms a high-temperature (>8,000 K) argon plasma. If a weak-concentration (2%) nitric acid (HNO3) aqueous solution containing analytes in the concentration range of several ppm (parts per million) down to several ppt (parts per trillion) is introduced into the argon plasma, a variety of different matrix ions are formed. These include Ar2+, ArO+, ArH+ and many others. Given, for example, a weak concentration (0.5%) of hydrochloric acid (HCl) in the analytical solution, additional matrix ions, such as ClO+ are also formed. All these ionized matrix species are polyatomic interferences in chemical analysis applications and drastically affect the detection limit of the isobaric monoatomic analytes. Moreover, the interferences exhibit very strong signals, often exceeding the analytical signals by several (two or more) orders of magnitude, thus impeding trace elemental analysis of the isobaric species. For example, molecular ArO+ can interfere drastically with the detection of the major isotope of iron, 56Fe. Several different approaches were developed to address these ICP interference problems. One approach, referred to as Kinetic Energy Discrimination (KED), makes use of a different degree of the kinetic energy loss by the interferences (usually molecular ions) and the analytes when passing through a collision/reaction cell (CRC) filled with inert gas. In the KED mode, ion species are introduced into the CRC and experience multiple collisions with the collision/reaction gas (typically He). Upon exiting the CRC, all ion species are decelerated by the entrance to the analytical quadrupole positioned downstream of the CRC. Deceleration is e.g. achieved by biasing the analytical quadrupole rods several volts (approx. 2 to 3 V) higher than the CRC rods. The analyte species, which exhibit a lower degree of kinetic energy loss, are more readily transmitted through this energy barrier and into the analytical quadrupole and then further toward the MS detector. While widely adopted in the field, the KED approach is characterized by quite drastic losses of the analyte signals, typically up to an order of magnitude in the higher m/z (mass-to-charge) range for ICP-generated ions (m/z 100-m/z 200) and more than three orders of magnitude loss for lower m/z ions (below m/z 50-60). Moreover, if ion selection is employed prior to the CRC operating in the KED mode, the analytical losses are even greater, as the space charge component intrinsically assisting transmission through the pressurized cell is removed. Another approach is described in U.S. Pat. No. 6,259,091, which employs a 3D collision cell filled with a reactive reagent gas, such as H2, to selectively remove argon (Ar) based interferences. The reactive gas introduces two types of beneficial reactions in elemental chemical analysis: it neutralizes the more intense argon-based interference species and shifts the interference and the analyte ions from each other in the m/z domain. Though shown to be efficient for removing Ar+ and lower m/z interferences, the approach is deficient of broader applicability to different types of interferences. It works best in the charge-exchange reactions with the analytes, whose ionization potential is lower than that of t