Adaptive and targeted control of ion populations to improve the effective dynamic range of mass analyser

Abstract

A method of mass spectrometry is disclosed wherein one or more relatively abundant or intense species of ions in a first population of ions are selectively attenuated so as to form a second population of ions. The total ion current of the second population of ions is then adjusted so that the ion current corresponding to ions which are onwardly transmitted to a mass analyser comprising an ion detector is within the dynamic range of the ion detector.

Claims

1. A method of mass spectrometry, comprising: selectively attenuating one or more relatively abundant or intense species of ions and adjusting or optimising a total ion current so that an ion signal is within a dynamic range of a downstream device; wherein: said adjusting or optimizing the total ion current comprises attenuating all species of ions, and said one or more relatively abundant or intense species of ions are attenuated to a greater degree; the steps of selectively attenuating one or more relatively abundant or intense species and adjusting or optimising the total ion current are achieved by controlling the operation of a mass filter or ion trap; and the steps of selectively attenuating one or more relatively abundant or intense species of ions in a population of ions and adjusting or optimising the total ion current of said population of ions are performed substantially simultaneously.

2. A method as claimed in claim 1, wherein the downstream device comprises a downstream analyser.

3. A method as claimed in claim 1, wherein the downstream device comprises a downstream mass analyser.

4. A method as claimed in claim 1, wherein the step of selectively attenuating one or more relatively abundant or intense species of ions comprises: (i) depleting one or more species of ions or completely removing one or more species of ions; and/or (ii) attenuating one or more species of ions by at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, 95% or 100%.

5. A method as claimed in claim 1, wherein the step of selectively attenuating one or more relatively abundant or intense species of ions comprises either: (i) increasing the number of relatively abundant or intense species of ions which are attenuated so as to allow for the detection of progressively less abundant or less intense species of ions; or (ii) decreasing the number of relatively abundant or intense species of ions which are attenuated so as to allow for the detection of progressively more abundant or more intense species of ions.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawing in which:

(2) FIG. 1 illustrates simulated ion species distributions from an LC separation of a complex mixture before and after the removal of the most abundant ion species present.

(3) FIG. 2 schematically illustrates a mass spectrometer 200 according to an embodiment of the invention.

(4) FIG. 3 schematically illustrates a mass spectrometer 300 according to another embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) A preferred embodiment of the present invention will now be described. According to the preferred embodiment a mass spectrometer is provided comprising a targeted attenuation device which is provided upstream of a mass analyser comprising an ion detector. The targeted attenuation device is preferably arranged and adapted to attenuate the most abundant ion species relative to other less abundant ion species before the ions are passed to the mass analyser. The total ion current is preferably re-optimised prior to the ions being passed to the mass analyser. The targeted attenuation device therefore preferably attenuates the most abundant ion species prior to the introduction of ions into a mass analyser thereby improving the in-spectrum dynamic range.

(6) According to the preferred embodiment the total ion current of ions supplied to the mass analyser is preferably controlled or altered so as to optimise or maximize the number of ion species which can be detected by the mass analyser. At the same time, it is preferably ensured that the mass analyser operates in a linear regime for all ion species being analysed.

(7) According to an embodiment, instead of controlling the total ion current of the ion population, the detector response may be controlled. In this embodiment, the gain of the ion detector may be controlled or adjusted so that the detected signal is within the dynamic range of the ion detector. This may be done when using, for example, photo-multiplier or electron multiplier detectors.

(8) According to the preferred embodiment the observed signal for all ion species is preferably kept within the dynamic range of the ion detector by controlling the total response of the mass spectrometer. Control of the total response may be achieved in a number of ways.

(9) According to an embodiment, the total ion current of ions supplied to the mass analyser may be controlled or adjusted by altering the amount or efficiency of ion production in the ion source. For Electrospray Ionisation (ESI) or Atmospheric Pressure Chemical Ionisation (APCI) sources this may be achieved by adjusting the needle voltage.

(10) According to another embodiment, the total ion current of ions supplied to the mass analyser may be controlled or adjusted using an attenuation device (including those described below) operating in a non-targeted or non-selective mode of operation. According to this embodiment, all of the species of ions are attenuated substantially equally.

(11) According to another embodiment, a single attenuation device may be used for both the targeted attenuation and the total response control or total ion current control. In this embodiment all of the ion species are preferably attenuated, but the targeted or selected ion species are preferably attenuated to a greater degree.

(12) The composition of a sample being supplied to the mass analyser may according to an embodiment be frequently monitored in order to identify one or more highly abundant or intense ion species. For example, N highly abundant ion species may be identified.

(13) The targeted attenuation device is preferably used to deplete in concentration (or completely remove) the N most abundant species of ions which have been previously identified. The N most abundant species of ions are preferably attenuated relative to the other remaining ion species. The N most abundant species of ions are preferably attenuated prior to injection into a mass analyser.

(14) According to the preferred embodiment the total ion current or ion current may be re-optimised prior to injecting the ions into the mass analyser and/or the gain of the ion detector may be re-optimised.

(15) In a particularly preferred embodiment, the approach according to the preferred embodiment as described above may be iterated over a sufficiently short timescale so that more of the most abundant species of ions are attenuated from successive spectra. In this way, ions having relatively high intensities or abundances may be successively attenuated from ions supplied to the mass analyser. For example, the five most abundant species of ions may be attenuated at first, followed by the ten most abundant species, followed by the fifteen most abundant species, and so on. After each successive step of attenuating different numbers of ion species, the total ion current or ion current may be re-optimised and/or the gain of the ion detector may be re-optimised.

(16) The timescale for this iteration may be chosen so as to be compatible with the elution of components from an LC chromatography source. For example, the iteration may be operated over a timescale of the order of a few seconds or less. This embodiment allows for the detection of progressively less abundant ion species.

(17) The degree to which each ion species has been attenuated will in general be known. Thus, according to the preferred embodiment, once a mass spectrum has been recorded, the attenuated components are scaled up in the data by the appropriate factor. In this way, an accurate mass spectrum may be produced.

(18) According to an embodiment, the data produced from a number of iterations over, for example, an LC peak may be combined with the appropriate scaling to produce a mass spectrum for the LC peak with an increased effective dynamic range.

(19) The number of attenuated ion species N, and the method of selecting ion species for attenuation may vary from sample to sample and from spectrum to spectrum, as desired. The specificity of the attenuation will depend on the characteristics of the attenuation device. It is possible that some ion species close in mass or mass to charge ratio (or some other physico-chemical characteristic such as ion mobility) to the target species may sometimes be attenuated to some extent. Nevertheless, the preferred embodiment will result in a higher proportion of the ion current being carried by lower abundance ion species.

(20) A simulation was implemented to illustrate various aspects of the preferred embodiment. The simulation generated ion species with initial abundances sampled from a log-normal distribution. The width of the distribution was chosen to yield approximately 5000 species per decade of dynamic range of abundance. This particular choice of distribution is a reasonable approximation to the observed abundances of peptide species in an analysis of a proteolytic digest of a complex protein mixture.

(21) The species were then subjected to a simulated LC separation of length 100 minutes during which time each species eluted at a randomly chosen retention time with a chromatographic full width half maximum of 12 seconds.

(22) The total ion current was adjusted to keep the ion current for the most abundant species present at a roughly constant value. Since the total number of ions present is dominated by the most abundant species, this also corresponds to keeping the total ion current approximately constant.

(23) While the specific values utilised in the above described simulation may be somewhat sensitive to the details of the assigned abundance distributions and simulated LC conditions, it will nonetheless be appreciated that the general conclusions still apply to a wide range of operating conditions.

(24) FIG. 1 illustrates the results of the simulation wherein the most abundant species of ions in a single simulated spectrum from an LC separation of a complex mixture were removed in accordance with a preferred embodiment of the present invention.

(25) The observed distribution in abundance over a is period is shown in FIG. 1 as the un-attenuated curve.

(26) The ions have been sorted in FIG. 1 in decreasing order of abundance and the vertical axis shows the base 10 logarithm of the ion current for each species. Assuming that the ion detector has a dynamic range of 4.5 decades in abundance or sufficient charge capacity to hold about 1106 ions, then the number of ion species that can be reliably measured at this retention time is just over 40.

(27) When the top five species are completely removed in accordance with an embodiment of the present invention and the total ion current is adjusted to compensate, this number increases to just over 50 (i.e. an increase of 25% is observed in the number of species above the limit of dynamic range). The final experiment involved removing the top 20 most abundant species and again adjusting the total ion current to compensate. This yielded over 70 species within the dynamic range of the ion detector. This represents an increase of around 70% in the number of species above the limit of dynamic range over the case with no attenuation.

(28) It is apparent, therefore, that the present invention represents a significant advance in the art.

(29) The selective attenuation device 202, 303 may take a number of different forms. For example, according to an embodiment the selective attenuation 202, 303 device may utilise resonance ejection of selected mass or mass to charge ratio ranges of ions from an ion trap. According to another embodiment the selective attenuation device 202, 303 may utilise resonance ejection of ions from a continuous ion beam using a quadrupole rod set mass filter. According to another embodiment the selective attenuation device 202, 303 may trap ions, separate the ions according to their ion mobility and then attenuate ions in a time dependent manner so as to attenuate a particular mobility range of ions.

(30) Yet further embodiments are contemplated. For example, the selective attenuation device 202, 303 may involve trapping ions, followed by separating ions axially using a time of flight region to separate the ions released from the ion trap. Ions may then be attenuated in a time dependent manner.

(31) According to another embodiment the selective attenuation device 202, 303 may utilise multiple fills of an ion trap following a filtering device (such as a quadrupole rod set mass filter) operating with non-overlapping specificity in different spectra. According to another embodiment the selective attenuation device 202, 303 may utilise scanning or stepping a mass filter, such as a quadrupole mass filter, over the mass or mass to charge ratio range at a speed or with a dwell time that is linked to mass or mass to charge ratio. According to this embodiment, the speed of the scanning or stepping of the dwell time is preferably faster (or slower) over undesired or unselected mass or mass to charge ratio ranges, and slower (or faster) over desired or selected mass or mass to charge ratio ranges. According to this embodiment, a high resolution quadrupole mass filter may be utilised to attenuate with a mass or mass to charge ratio specificity better than 1 Da.

(32) According to other embodiments combinations of the above described embodiments may be utilised including attenuation of ions having different mass or mass to charge ratio ranges or ion mobility ranges by several devices operating in series.

(33) Time dependent attenuation may be achieved through a reduction in duty cycle using one or more known Dynamic Range Enhancement (DRE) lenses or ion gates.

(34) Various other attenuation methods are also possible.

(35) The mass analyser preferably comprises a Time of Flight (ToF) mass analyser and in particular a Time of Flight mass analyser having an ion detector which displays a non-linear behavior at high ion arrival rates due to the particular ion detection mechanism or due to the process of digitizing the signal.

(36) According to an aspect of the present invention as illustrated in FIG. 2 there is provided a mass spectrometer 200 comprising;

(37) a device 201 arranged and adapted to provide a first population of ions;

(38) a selective attenuation device 202 arranged and adapted to selectively attenuate one or more relatively abundant or intense species of ions in the first population of ions; so as to form a second population of ions; and

(39) a device 203 arranged and adapted to adjust or optimise a gain of an ion detector so that a detected ion signal corresponding to ions received by the ion detector 204 is within a dynamic range of the ion detector 204.

(40) According to an aspect of the present invention as illustrated in FIG. 3 there is provided a mass spectrometer comprising:

(41) a device 301 arranged and adapted to provide a first population of ions;

(42) a device 302 arranged and adapted to adjust or optimise a gain of an ion detector; and

(43) a selective attenuation device 303 arranged and adapted to selectively attenuate one or more relatively abundant or intense species of ions in the first population of ions so as to form a second population of ions so that a detected ion signal corresponding to ions received by the ion detector 304 is within a dynamic range of the ion detector 304.

(44) Alternatively, the mass analyser may comprise an ion trap mass analyser and in particular an ion trap mass analyser for which the charge capacity of the ion trap determines the linear dynamic range of the instrument. Such mass analysers include an Orbitrap mass analyser for which the charge capacity of the C-trap determines the number of ions that can be measured simultaneously.

(45) For ion trap based detector systems the fill time may be adjusted to keep the total charge in the ion trap approximately constant.

(46) The general principle described herein is also applicable to other modes of operation involving a population of ions and an ion detector with a limited dynamic range.

(47) Although the present invention has been described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as set forth in the accompanying claims.