High-Resolution Ion Trap Mass Spectrometer

Patent metadata

Field	Value
Publication number	US-2017133215-A1
Application number	US-201514933961-A
Country	US
Kind code	A1
Filing date	Nov 5, 2015
Priority date	Nov 5, 2015
Publication date	May 11, 2017
Grant date	—

Abstract

Techniques can increase the resolution and accuracy of mass spectra obtained using ion traps through the use of the actual shape of the ion trap peaks, which is a series of smaller ion ejection events. The peak shapes are identified as changing over a common period of the trapping signal and the excitation signal, at which point the peak shapes repeat. Peak shapes can be characterized over the common period to create N basis functions, each for a different fractional mass for a given scan rate. The N basis functions over the common period can be duplicated (e.g., shifted by the common period) to obtain a set of mass functions that characterize fractional masses over the full scan range. The mass spectrum can be obtained by fitting the set of mass functions to the measured data to obtain a best fit contribution of each mass function to the measured data.

First claim

1 . A method of determining a mass spectrum of a sample, the method comprising:
defining a trap volume with a trapping electric field for trapping ions within a predetermined range of mass-to-charge ratio, the trapping electric field generated by applying a trapping signal to an ion trap device, the trapping signal having a trapping amplitude and a trapping frequency; trapping, in the trap volume, a plurality of ions having the predetermined mass-to-charge ratio range, the plurality of ions corresponding to the sample; generating an excitation electric field superimposed on the trapping electric field, the excitation electric field generated by applying an excitation signal to the ion trap device, the excitation signal having an excitation frequency, wherein K times a trapping period P Ω of the trapping signal equals M times an excitation period P ω of the excitation signal, wherein K and M are integers and wherein K is greater than M, and wherein K times the trapping period P Ω defines a common period of the trapping signal and the excitation signal; changing the trapping amplitude at a ramp rate to sequentially eject sets of ions from the trap volume, each set of ions corresponding to a particular mass-to-charge ratio, wherein a set of ions is ejected when the set of ions are in resonance with the excitation frequency; detecting the sets of ions that are ejected from the trap volume to generate a plurality of measurement points at a data acquisition frequency, each measurement point including an intensity value and a time value, the intensity value corresponding to an amount of ions detected at the time value, wherein the plurality of measurement points are obtained over a time range, and wherein the intensity values form a detection vector y; obtaining a first set of mass functions corresponding to the time range and to a first plurality of mass-to-charge ratios over a first range of mass-to-charge ratios, each mass function corresponding to a different mass-to-charge ratio, wherein the first set of mass functions comprises J subsets of N basis functions, wherein each subset of N basis functions is shifted from another subset of N basis functions by a multiple of the common period; computing optimal values of a composition vector s that provides the detection vector y as a linear combination of the first set of mass functions, wherein the composition vector s specifies amounts of ions having the first plurality of mass-to-charge ratios that were detected; and determining the mass spectrum based on the composition vector s.

2 . The method of claim 1 , wherein K divided by M is an integer.

3 . The method of claim 1 , wherein the plurality of measurement points is greater than the first set of mass functions.

4 . The method of claim 1 , wherein each basis function of a subset of N basis functions correspond to a different time offset within the common period.

5 . The method of claim 4 , wherein N, the ramp rate, and the common period provide at least a desired resolution for the mass spectrum.

6 . The method of claim 4 , wherein a first mass function corresponds to a first mass-to-charge ratio and corresponds to a first time offset within the common period, and wherein changing the trapping amplitude at the ramp rate to sequentially eject sets of ions from the trap volume is performed such that any ions of the plurality of ions having the first mass-to-charge ratio are ejected at the first time offset.

7 . The method of claim 6 , the first time offset is achieved by starting to change the trapping amplitude at the ramp rate from an initial value at a specified offset in the common period.

8 . The method of claim 4 , further comprising creating an initial subset of N basis functions by:
for each Ith time offset of N time offsets comprising the common period:
trapping, in the trap volume, calibration ions having a calibration mass-to-charge ratio, the calibration mass-to-charge ratio being known;
generating the excitation electric field superimposed on the trapping electric field;
changing the trapping amplitude at the ramp rate to eject the calibration ions at the Ith time offset; and
detecting the calibration ions that are ejected from the trap volume to generate a Ith basis function of the initial subset of N basis functions.

9 . The method of claim 8 , wherein changing the trapping amplitude at the ramp rate to sequentially eject sets of ions from the trap volume ejects ions over a common subrange of mass-to-charge ratios during the common period, and wherein each Ith time offset of the N time offsets corresponds to a different mass-to-charge ratio in the common subrange.

10 . The method of claim 8 , further comprising:
specifying the Ith time offsets using one or more of: delaying when changing the trapping amplitude begins and varying an initial trapping amplitude for beginning the changing at a ramp rate.

11 . The method of claim 8 , wherein the calibration mass-to-charge ratio corresponds to a first time offset of the N time offsets, and wherein changing the trapping amplitude at the ramp rate to sequentially eject sets of ions from the trap volume is performed such that any ions of the plurality of ions having the calibration mass-to-charge ratio are ejected at the first time offset.

12 . The method of claim 1 , wherein computing the optimal values of the composition vector s includes:
forming a matrix A of the first set of basis functions defined at the time values of the plurality of measurement points; and solving A t As=A t y for s.

13 . The method of claim 12 , wherein solving A t As=A t y is performed by determining an inverse of (A t A) or by using an optimization technique.

14 . The method of claim 1 , further comprising:
applying another excitation electric field having a different excitation frequency such that different sets of ions of different mass-to-charge ratios are ejected at a same time; obtaining a second set of mass functions corresponding to the time range and to a second plurality of mass-to-charge ratios over a second range of mass-to-charge ratios; computing optimal values of the composition vector s to provide the detection vector y as a linear combination of the first set of mass functions and the second set of mass functions.

15 . The method of claim 1 , wherein the plurality of ions are formed within or injected into the trap volume.

16 . The method of claim 1 , wherein the trapping signal includes DC and AC components.

17 . The method of claim 1 , wherein the data acquisition rate is at least four times the trapping frequency.

18 . A mass spectrometer comprising:
an ion trap device; a trapping generator configured to apply a trapping signal to the ion trap device to define a trap volume with the trapping electric field for trapping ions within a predetermined range of mass-to-charge ratio, the trapping signal having a trapping amplitude and a trapping frequency; an excitation generate configured to apply an excitation signal to the ion trap device to generate an excitation electric field superimposed on the trapping electric field, the excitation signal having an excitation frequency, wherein K times a trapping period P Ω of the trapping signal equals M times an excitation period P ω of the excitation signal, wherein K and M are integers and wherein K is greater than M, and wherein K times the trapping period P Ω defines a common period of the trapping signal and the excitation signal; a controller configured to change the trapping amplitude at a ramp rate to sequentially ej

Assignees

• Thermo Finnigan Llc

Inventors

• Remes Philip M
• Schwartz Jae C

Classifications

• H01J49/429 Primary

  Scanning an electric parameter, e.g. voltage amplitude or frequency · CPC title

• H01J49/0031

  Step by step routines describing the use of the apparatus (H01J49/0081 takes precedence) · CPC title

• H01J49/0036

  Step by step routines describing the handling of the data generated during a measurement · CPC title