TECHNICAL FIELD

The present disclosure relates to mass spectrometers and mass spectrometry. In particular, the present disclosure relates to tandem mass spectrometry and mass spectrometers that comprise one or more ion storage devices and that are configured to perform tandem mass spectrometry.

BACKGROUND

Sensitivity, selectivity, and analytical throughput are the main figures of merit for mass spectrometers, and perhaps analytical instrumentation in general. Any advance in these areas is immediately exploited by the analytical community, often in compounded beneficial ways. For example, an improvement in sensitivity not only allows the analysis of lower concentration species, but in some cases can be used to eliminate time-consuming sample preparation steps.

The generation of a tandem mass spectrum can be decomposed into a set of steps. For example, in a first step, devices in the mass spectrometer such as power supply voltages are set to values suitable for transmitting and detecting a precursor-ion species of a particular mass-to-charge (m/z) value or a set of precursor-ion species comprising respective m/z values. In a second composite step, these precursor ions are transmitted into or through a portion of the instrument at which they are isolated and fragmented, and the fragment ions are stored in an intermediate location, possibly together with residual precursor ions. In a final step, the fragments of the precursors are analyzed by a mass analyzer (Table 1).

TABLE 1Example of conventional steps forgenerating a mass spectrumStep NumberProcess1Set all devices forprecursor ions2Isolate / Fragment /Store precursor ions3Analyze fragmentsof precursor ions
The traditional and most intuitive way to operate a mass spectrometer is by sequentially performing all of the required steps, from first step to last step, with respect to a first set of precursor ion species and, after the final step has been executed, repeating all of the steps with respect to a next set of ion species that are different than the prior set of ion species. An example of this type of scheme is shown generally at10inFIG. 1in which boxes1aand1brepresent setup steps, boxes2aand2brepresent ion injection steps, boxes3aand3brepresent ion transfer steps and boxes4aand4brepresent mass analysis steps. The time period5encompasses the steps that produce mass spectral data of a first batch of analyte-ion species and the time period6encompasses the steps that produce mass spectral data of a second, different batch of analyte-ion species. The sequence and timing of steps may be controlled by electronic controller or computer31via control of the specific mass spectrometer hardware components (not shown inFIG. 1) that are used to perform the various steps.

The required setup time (e.g., widths of boxes1aand1bofFIG. 1) is generally controlled by the amount of time required to configure a quadrupole mass filter (QMF) so as to transmit precursor ions within a particular desired m/z range. This setup time depends upon the magnitude of the m/z range over which the QMF must slew to achieve a new configuration after previously having been in a configuration to preferentially transmit ions within a different m/z range. The injection time (e.g., widths of boxes2aand2b) is the time required to accumulate and fragment the selected precursor ions in a fragmentation cell, such as a collision-induced dissociation (CID) cell, of the type that temporarily stores the fragment ions. Other types of fragmentation cells include those within which fragmentation occurs by electron capture dissociation (ECD), electron transfer dissociation (ETD), surface-induced dissociation (SID), photo-induced dissociation, etc. This accumulation and fragmentation process generally requires a few tens of milliseconds. Although the fragmentation cell requires its own setup prior to receiving precursor ions, there is no need to consider this fragmentation-cell setup time because it is significantly shorter than the QMF setup time. Thus, the time required, if any, to setup and prepare the fragmentation cell to receive ions may be conveniently absorbed into the setup time indicated by boxes1aand1b. The time required to transfer ions from a CID cell to a mass analyzer is represented by the widths of boxes3aand3band the time required for mass analysis is represented by the widths of boxes4aand4b. While the approach represented inFIG. 1has been very successful due to the high selectivity of the combined processes of isolation and fragment analysis, it tends to discard a great majority of the ion beam.

One of the new frontiers in sensitivity improvement lies in the utilization of all the ions admitted to the instrument in order to take advantage of the fact that the ion stream comprises, at any given time, ions of all compounds of the sample that has been ionized. Senko (Analytical Chemistry, 2013, 85 (24), 11710-11714) described how a general directional ion pathway through a mass spectrometer, from ion source to mass analyzer, may be favorably analogized as a segmented “pipeline” in which each segment executes one of the various process steps. In this context, an “analyte-derived ion species” (either a precursor ion generated from an analyte molecule at an ion source or a fragment of such a molecule generated in a fragmentation cell) may be conceptualized as an object that passes through each segment, is acted upon by the processing stage that corresponds to that segment, and is passed to the next stage. The most important aspect of the pipeline analogy is that different batches of analyte-derived ion species may simultaneously occupy different mass spectrometer pathway segments, each segment corresponding to a different respective step of an overall process. Accordingly, different process steps performed may be executed simultaneously, where each process step may be specifically configured to operate in accordance with the particular analyte-derived ion species that occupy the path segment at which that step is executed.

According to the pipeline concept taught by Senko, a mass spectrometer system may effectively perform processing steps relating to two or more batches of analyte-ion species at the same time. An example a two-stage parallel processing scheme, as taught by Senko, is depicted generally at20inFIG. 2. In this context, an “analyte-ion species” is either a precursor-ion species generated by ionization of a chemical analyte or else a fragment-ion species generated by one or more instances of fragmentation of such a precursor-ion species. In this context, a “batch of analyte-ion species” is a plurality of analyte-ion species, as defined above, that are generated by simultaneous ionization of one or more chemical analytes and that are processed together in a mass spectrometer system. In this context, a “stage” is either a single process step or a set of one or more consecutive process steps that are applied to an individual batch of analyte-ion species, as defined above. Some process steps may be simple ion storage steps. The number of stages of any particular mass spectrometer parallel processing scheme is the maximum number of batches of ion species that that may be present in the general ion pathway (the pipeline) of the mass spectrometer at any given time.

InFIG. 2, boxes1a,1band1crepresent mass spectrometer setup steps that pertain to first, second and third batches of analyte-ion species, respectively. Likewise, boxes2a,2band2cofFIG. 2represent ion injection steps that pertain, respectively, to the first, second and third batches of analyte-ion species. Similarly, boxes3a,3band3cofFIG. 2represent ion transfer steps that pertain, respectively, to the first, second and third batches of analyte-ion species. Finally, boxes4a,4band4cofFIG. 2represent mass analysis steps that pertain, respectively, to the first, second and third batches of analyte-ion species. Each “inject stage” comprises a single pair of setup and injection steps such as, for example, the pair of steps represented by boxes1aand2a. Each “analysis stage” comprises a single pair of transfer and analysis steps, such as, for example, the pair of steps represented by boxes3aand4a.

It can be appreciated that parts of the mass spectrometer system that perform the Setup and Inject steps may partially overlap with parts of the instrument that perform the Transfer and Analyze steps. For instance, it is possible that Setup and Inject steps cannot start until after a storage cell has been emptied of a prior batch of ions. In the example ofFIG. 2, this is illustrated as a lack of overlap, along the time axis, between box3aand box1band between box3band box1c. Thus, while there may be partial overlap between stages (thus improving duty cycle), each stage may not necessarily be totally independent of the other stages. The question of whether or not certain portions of stages can (or cannot) overlap will be apparatus dependent. The present inventors have realized that multi-stage pipeline operation, where the number of stages is three or greater, may improve efficiency still further.

SUMMARY

Against the above background, a method of operating a mass spectrometer that allows for high-speed operation is disclosed. The method consists in separating the various steps needed to produce a mass scan into three or more conceptual stages in a pipeline, such that the instrument is performing steps to process more than two precursor-ion species simultaneously. In general, the number of stages in the pipeline should be at least one more and, preferably, at least two more than the number of storage devices in the instrument, where the storage devices act to “buffer” the flow of ions between mass spectrometer components in a fashion that is analogous to the way computer memory segments are used to buffer the transfer of bytes of information between computer hardware devices. The presently-taught methods and apparatus allow for nearly 100% duty cycle of ion accumulation for precursors of interest.

According to a first aspect, the present teachings provide a mass spectrometry method, comprising: (i) preparing a mass filtering device of a mass spectrometer system to transmit therethrough ion species comprising a pre-determined range of mass-to-charge ratio (m/z) values; (ii) transmitting a first batch of ion species to the mass filtering device and transmitting a subset of the first batch of ion species from the mass filtering device to an ion storage device of the mass spectrometer system and accumulating the subset of the first batch of ion species within the ion storage device, the transmitted subset of the first batch of ion species comprising m/z values within the predetermined range; (iii) transferring the accumulated subset of the first batch of ion species to a mass analyzer of the mass spectrometer system and mass analyzing the accumulated subset of the first batch of ion species using the mass analyzer; (iv) concurrently with at least a portion of either the transferring of the accumulated subset of the first batch of ion species to the mass analyzer or the mass analyzing of the accumulated subset of the first batch of ion species, preparing the mass filtering device to transmit therethrough ion species comprising a second pre-determined range of m/z values; (v) concurrently with at least a portion of the mass analyzing of the accumulated subset of the first batch of ion species, transmitting a second batch of ion species to the mass filtering device and transmitting a subset of the second batch of ion species from the mass filtering device to the ion storage device and accumulating the subset of the second batch of ion species within the ion storage device, the transmitted subset of the second batch of ion species comprising m/z values within the second predetermined range of m/z values; and (vi) concurrently with at least a portion of the mass analyzing of the accumulated subset of the first batch of ion species, preparing the mass filtering device to transmit therethrough ion species comprising a third pre-determined range of m/z values.

According to some embodiments, the method may further comprise (vii) transferring the accumulated subset of the second batch of ion species to the mass analyzer and mass analyzing the accumulated subset of the second batch of ion species using the mass analyzer; and (viii) concurrently with at least a portion of either the transferring or the mass analyzing of the accumulated subset of the second batch of ion species, transmitting a third batch of ion species to the mass filtering device and transmitting a subset of the third batch of ion species from the mass filtering device to the ion storage device and accumulating the subset of the third batch of ion species within the ion storage device, the transmitted subset of the third batch of ion species comprising m/z values within the third predetermined range of m/z values.

According to a second aspect, the present teachings provide a mass spectrometry method, comprising: (i) preparing a mass filtering device of a mass spectrometer system to transmit therethrough ion species comprising a pre-determined range of mass-to-charge ratio (m/z) values; (ii) transmitting a first batch of ion species to the mass filtering device and transmitting a subset of the first batch of ion species from the mass filtering device to an ion storage device of the mass spectrometer system and accumulating the subset of the first batch of ion species within the ion storage device, the transmitted subset of the first batch of ion species comprising m/z values within the predetermined range; (iii) transferring the accumulated subset of the first batch of ion species to a fragmentation cell of the mass spectrometer system, the fragmentation cell generating a first set of fragment-ion species from the subset of the first batch of ion species; (iv) transferring the first set of fragment-ion species to a mass analyzer of the mass spectrometer system and mass analyzing the first set of fragment-ion species using the mass analyzer; (v) concurrently with at least a portion of either the generating of the first set of fragment-ion species or the mass analyzing of the first set of fragment-ion species, preparing the mass filtering device to transmit therethrough ion species comprising a second pre-determined range of m/z values; (vi) concurrently with at least a portion of either the generating of the first set of fragment-ion species or the mass analyzing of the first set of fragment-ion species, transmitting a second batch of ion species to the mass filtering device and transmitting a subset of the second batch of ion species from the mass filtering device to the ion storage device and accumulating the subset of the second batch of ion species within the ion storage device, the transmitted subset of the second batch of ion species comprising m/z values within the second predetermined range of m/z values; and (vii) concurrently with at least a portion of the mass analyzing of the first set of fragment-ion species, preparing the mass filtering device to transmit therethrough ion species comprising a third pre-determined range of m/z values.

According to a third aspect, the present teachings provide a mass spectrometer system, comprising: (a) an ion source; (b) a mass filtering device configured to receive ions from the ion source; (c) an ion storage device configured to receive and accumulate mass-filtered ions received from the mass filtering device; (d) a mass analyzer configured to receive ions from the ion storage device; and (e) a computer or electronic controller electrically coupled to the mass filtering device, the ion storage device and the mass analyzer, the computer or electronic controller comprising computer-readable program instructions operable to: (1) prepare the mass filtering device to transmit therethrough ion species comprising a pre-determined range of mass-to-charge ratio (m/z) values; (2) cause the ion source to transmit a first batch of ion species to the mass filtering device, cause the mass filtering device to transmit a subset of the first batch of ion species to the ion storage device, and cause the ion storage device to accumulate the subset of the first batch of ion species therein, whereby the transmitted subset of the first batch of ion species comprises m/z values within the predetermined range; (3) cause the ion storage device to transfer the accumulated subset of the first batch of ion species to the mass analyzer, whereby the ion species of the accumulated subset of the first batch of ion species are mass analyzed; (4) prepare the mass filtering device to transmit therethrough ion species comprising a second pre-determined range of m/z values, wherein said preparing is concurrent with at least a portion of either the transferring of the subset of ion species or the mass analyzing of the accumulated subset of ion species; (5) cause the ion source to transmit a second batch of ion species to the mass filtering device, cause the mass filtering device to transmit a subset of the second batch of ion species and cause the ion storage device to accumulate the subset of the second batch of ion species therein, whereby the transmitted subset of the second batch of ion species comprises m/z values within the second predetermined range, wherein said transmitting and accumulating are concurrent with at least a portion of the mass analyzing of the accumulated subset of ion species; and (6) prepare the mass filtering device to transmit therethrough ion species comprising a third pre-determined m/z range, wherein said preparing is concurrent with at least a portion of the mass analyzing of the accumulated subset of the first batch of ion species.

According to a fourth aspect, the present teachings provide a mass spectrometer system, comprising: (a) an ion source; (b) a mass filtering device configured to receive ions from the ion source; (c) an ion storage device configured to receive and accumulate mass-filtered ions received from the mass filtering device; (d) a fragmentation cell configured to receive accumulated mass-filtered ions from the ion storage device; (e) a mass analyzer configured to receive fragment ions from the fragmentation cell; and (f) a computer or electronic controller electrically coupled to the mass filtering device, the ion storage device, the fragmentation cell and the mass analyzer, the computer or electronic controller comprising computer-readable program instructions operable to: (1) prepare the mass filtering device to transmit therethrough ion species comprising a pre-determined range of mass-to-charge ratio (m/z) values; (2) cause the ion source to transmit a first batch of ion species to the mass filtering device, cause the mass filtering device to transmit a subset of the first batch of ion species to the ion storage device, and cause the ion storage device to accumulate the subset of the first batch of ion species therein, whereby the transmitted subset of the first batch of ion species comprises m/z values within the predetermined range; (3) cause the ion storage device to transfer the accumulated subset of the first batch of ion species to the fragmentation cell, whereby the fragmentation cell generates a first set of fragment-ion species from the subset of the first batch of ion species; (4) cause the fragmentation cell to transfer the first set of fragment-ion species to the mass analyzer, whereby the first set of fragment-ion species are mass analyzed; (5) prepare the mass filtering device to transmit therethrough ion species comprising a second pre-determined range of m/z values, wherein said preparing is concurrent with at least a portion of either the generating of the first set of fragment-ion species or the mass analyzing of the first set of fragment-ion species; (6) cause the ion source to transmit a second batch of ion species to the mass filtering device, cause the mass filtering device to transmit, to the ion storage device, a subset of the second batch of ion species comprising m/z values within the second predetermined range and cause the ion storage device to accumulate the subset of the second batch of ion species therein, wherein the transmitting and accumulating is concurrent with at least a portion of either the generating of the first set of fragment-ion species or the mass analyzing of the first set of fragment-ion species; and (7) prepare the mass filtering device to transmit therethrough ion species comprising a third pre-determined range of m/z values, wherein said preparing is concurrent with at least a portion of the mass analyzing of the first set of fragment-ion species.

In accordance with another aspect, the present teachings provide a mass spectrometry method comprising: mass analyzing a first discrete batch of ions in a mass analyzer of a mass spectrometer system; storing a second discrete batch of ions within an ion storage device or a fragmentation cell of the mass spectrometer system, wherein the storage of the second discrete batch of ions is concurrent with at least a portion of the mass analyzing of the first discrete batch of ions; and preparing an additional component of the mass spectrometer system to receive a third batch of ions from an ion source of the mass spectrometer system, wherein the preparing of the additional component is concurrent with at least a portion of the storage of the second discrete batch of ions within the ion storage device or fragmentation cell.

