CHARACTERISTIC IONS

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from United Kingdom Patent Application No.

1204723.9 filed on 19 March 2012. The entire content of this application is incorporated herein by reference.

BACKGROUND TO THE PRESENT INVENTION

The present invention relates to a method of mass spectrometry and a mass spectrometer.

Historically, Multiple Reaction Monitoring ("MRM") experiments to select and quantitate specific ions in a complex sample have been carried out using tandem quadrupole instruments wherein a first analytical quadrupole mass filter is arranged to select or isolate specific parent or precursor ions of interest. A fragmentation device is located downstream of the first analytical quadrupole mass filter and is arranged to fragment the parent or precursor ions of interest to form fragment ions. A second analytical quadrupole mass analyser located downstream of the fragmentation device is then arranged to mass analyse the characteristic fragment ions.

Desired parent or precursor ion to fragment ion transitions are determined via a method development stage wherein the transitions are arranged as a function of elution time from a chromatographic device such as a Liquid Chromatography ("LC"), Gas

Chromatography ("GC") or Capillary Electrophoresis ("CE") device. In some applications multiple transitions per parent or precursor ion may be arranged in order to give an added level of confidence that the measured component is actually the correct one.

Recently, it has become apparent that the specificity or selectivity of tandem quadrupole MRM experiments, particularly in relation to complex mixtures such as those seen in proteomics, is insufficient in some circumstances.

In order to address this problem it is known to perform high resolution MRM experiments using a mass spectrometer wherein the second analytical quadrupole mass analyser is replaced with a higher resolution mass analyser such as an orbitrap (RTM) mass analyser or an orthogonal acceleration Time of Flight mass analyser.

Fig. 1 shows a current state of the art mass spectrometer that may be utilised to perform high resolution MRM experiments.

It is known to provide a quadrupole mass filter 2 in conjunction with an orthogonal acceleration Time of Flight mass analyser 4 as shown in Fig. 1 wherein parent or precursor ions are isolated or selected by the quadrupole rod set mass filter 2 and are then subsequently fragmented in a gas cell 3. The resulting characteristic fragment ions are then mass analysed using the high resolution orthogonal acceleration Time of Flight mass analyser 4. Using a high resolution orthogonal acceleration Time of Flight mass analyser 4 has several advantages compared with using a resolving analytical quadrupole mass analyser.

Firstly, the higher resolution orthogonal acceleration Time of Flight mass analyser 4 reduces the likelihood of an interference effecting the quantitative measurement of the characteristic fragment ions.

Secondly, the orthogonal acceleration Time of Flight mass analyser 4 inherently has a high mass measurement accuracy of the order of 1 -3 ppm RMS. This mass accuracy can be used to improve the specificity of the transition.

Thirdly, the orthogonal acceleration Time of Flight mass analyser 4, by virtue of the fact that it is a mass spectrometer as opposed to a mass filter, analyses multiple ions

(characteristic fragment ions in this case) simultaneously and with a high duty cycle. The resulting full mass spectral data contains multiple characteristic fragment ions for the same parent or precursor ions again improving the specificity. Multiple isotopes are also included in the full mass spectrum again improving specificity.

Despite these benefits, current state of the art mass spectrometers similar to the arrangement shown in Fig. 1 nonetheless suffer from some certain problems.

One problem with current state of the art mass spectrometers is that they suffer from some loss in duty cycle as a result of mass analysing ions using an orthogonal acceleration Time of Flight mass analyser 4.

It is known to seek partially to compensate for this by either operating the Time of

Flight mass analyser 4 in an Enhanced Duty Cycle ("EDC") mode of operation wherein the mass range of ions analysed at any point in time is reduced or alternatively by operating the Time of Flight mass analyser 4 in a High Duty Cycle ("HDC") mode of operation or a scanwave/zeno lens mode of operation wherein the dynamic range is reduced.

Typically, the dynamic range of the ion detection system used in conjunction with an orthogonal acceleration Time of Flight mass analyser 4 is inferior to that of a quadrupole rod set mass analyser due to the high digitisation rate requirements of an orthogonal acceleration Time of Flight mass analyser 4.

Current state of the art Time of Flight mass analysers employing Analogue to Digital Converters ("ADC") exhibit significant improvements compared with previous mass spectrometers that used Time to Digital Convertors ("TDC"). Future developments in digitations rates and/or multiple gain stage ADCs promise further improvements.

It is known to seek to improve the dynamic range of an orthogonal acceleration Time of Flight mass analyser by means of Programmable Dynamic Range Enhancements ("pDRE") and Automatic Gain Control ("AGC"). However, these approaches usually involve a loss in sensitivity and/or duty cycle.

State of the art instruments that employ quantitative acquisitions use the same ion or ions for quantification irrespective of the nature of the data.

It is known to calibrate a mass spectrometer by injecting a first calibration sample having a known (low) concentration of calibrant and then measuring the low intensity signal response. Second and further calibration samples having progressively higher calibrant concentrations are then sequentially injected or otherwise introduced into the mass spectrometer and increasingly higher signal intensities or responses are observed.

At a certain point the ion detector will start to saturate and the detector response will not increase further. This gives an indication of the upper limit of quantitation and provides an upper limit to the effective dynamic range of the mass spectrometer.

It is desired to provide a mass spectrometer and a method of mass spectrometry having an improved dynamic range.

SUMMARY OF THE PRESENT INVENTION

According to an aspect of the present invention there is provided a method of mass spectrometry comprising:

determining the intensity of (or quantitating) an analyte by determining the intensity of first characteristic ions when the intensity of the first ions is within a first range; and

determining the intensity of (or quantitating) the analyte by determining the intensity of second different characteristic ions when the intensity of the first ions is outside of the first range.

It should be understood that the first and second characteristic ions relate to e.g. different species of fragment ions and/or different isotopes of the analyte and that the first and second characteristic ions should not therefore be construed as merely relating to two populations of the same analyte ions at different intensities. Accordingly, the mass to charge ratio and/or chemical structure and/or number of neutrons and/or other physico- chemical property of the first and second characteristic ions is preferably different.

The first characteristic ions preferably comprise fragment, product or adduct ions derived from the analyte.

According to another embodiment the first characteristic ions may comprise one or more first isotopes of the analyte.

The second characteristic ions preferably comprise fragment, product or adduct ions derived from the analyte.

According to another embodiment the second characteristic ions may comprise one or more second isotopes of the analyte.

The first range (e.g. an ion detector response or intensity range) preferably substantially corresponds with the detection or unsaturated range of an ion detector.

When the intensity of the first characteristic ions is outside of the first range, the intensity of the second different characteristic ions is preferably still substantially within the detection or unsaturated range of the ion detector.

The method preferably further comprises determining one or more isotope ratios of the analyte in order to confirm the identity of the analyte and/or to identify the analyte.

The step of determining one or more isotope ratios of the analyte preferably comprises:

determining one or more first isotope ratios by analysing a first sample comprising a first concentration of the analyte; and determining one or more second different isotope ratios by analysing a second different sample comprising a second different concentration of the analyte.

The method preferably further comprises controlling an instrument parameter of a mass spectrometer based upon a determination of the intensity or other property of the first characteristic ions and/or the intensity or other property of the second characteristic ions.

The instrument parameter preferably comprises: (i) a collision or fragmentation energy; (ii) an ionisation efficiency; (iii) an ion transmission efficiency; or (iv) an ion detector gain.

The method preferably further comprises separating parent or fragment ions according to a physico-chemical property.

The physico-chemical property preferably comprises ion mobility, differential ion mobility, mass, mass to charge ratio or time of flight.

The method preferably comprises a method of Multiple Reaction Monitoring

("MRM").

According to the preferred embodiment parent analyte ions are preferably selected or isolated by a mass filter. The mass filter preferably comprises a quadrupole rod set mass filter.

Parent analyte ions selected or isolated by the mass filter are preferably fragmented or reacted to form the first characteristic ions and/or the second characteristic ions.

The step of determining the intensity of the first characteristic ions preferably comprises mass analysing the first characteristic ions.

The step of determining the intensity of the second characteristic ions preferably comprises mass analysing the second characteristic ions.

The step of mass analysing the first and/or second characteristic ions preferably comprises mass analysing the first and/or second characteristic ions using an axial acceleration or orthogonal acceleration Time of Flight mass analyser.

The first and second characteristic ions preferably have different masses and/or different mass to charge ratios and/or different chemical structures and/or different number of neutrons and/or one more different physico-chemical properties.

According to another aspect of the present invention there is provided a mass spectrometer comprising:

a control system arranged and adapted:

(i) to determine the intensity of an analyte by determining the intensity of first characteristic ions when the intensity of the first characteristic ions is within a first range; and

(ii) to determine the intensity of the analyte by determining the intensity of second different characteristic ions when the intensity of the first characteristic ions is outside of the first range.

The mass spectrometer preferably further comprises a separator for separating parent or fragment ions according to a physico-chemical property.

The separator preferably comprises an ion mobility, differential ion mobility, mass, mass to charge ratio or time of flight separator. The control system is preferably arranged and adapted to perform a Multiple Reaction Monitoring ("MRM") analysis.

The mass spectrometer preferably further comprises a mass filter for selecting or isolating parent analyte ions.

The mass spectrometer preferably further comprises a fragmentation or reaction device wherein the parent analyte ions selected or isolated by the mass filter are preferably fragmented or reacted, in use, within the fragmentation or reaction device to form the first characteristic ions and/or the second characteristic ions.

The mass spectrometer preferably further comprises a mass analyser for mass analysing the first and/or second characteristic ions and determining the intensity of the first and/or second characteristic ions.

The mass analyser preferably comprises an axial acceleration or orthogonal acceleration Time of Flight mass analyser.

The first and second characteristic ions preferably have different masses and/or different mass to charge ratios and/or different chemical structures and/or different number of neutrons and/or one more different physico-chemical properties.

According to another aspect of the present invention there is provided a method of mass spectrometry comprising:

determining one or more first isotope ratios by analysing a first sample comprising a first concentration of an analyte; and

determining one or more second different isotope ratios by analysing a second different sample comprising a second different concentration of the analyte.

The one or more first isotope ratios and/or the one or more second isotope ratios are preferably used to confirm the identity of the analyte and/or to identify the analyte.

The step of determining the one or more first and/or second isotope ratios is preferably performed using an axial acceleration or an orthogonal acceleration Time of Flight mass analyser.

According to another aspect of the present invention there is provided a mass spectrometer comprising:

a control system arranged and adapted:

(i) to determine one or more first isotope ratios by analysing a first sample comprising a first concentration of an analyte; and

(ii) to determine one or more second different isotope ratios by analysing a second different sample comprising a second different concentration of the analyte.

The preferred embodiment relates to improving the dynamic range of a mass spectrometer and is particularly useful for high peak capacity tandem instruments such as those including both an Ion Mobility Separator ("IMS") device and a Time of Flight mass analyser.

Improved Time of Flight quantitation according to an embodiment of the present invention by using alternative characteristic fragment ions is a new mode of operation for existing instrument geometries and future novel instrument geometries. The preferred embodiment provides the capability to improve the dynamic range of Time of Flight instruments operating in quantitative modes by intelligently determining which characteristic ions to use for quantitation.

The preferred approach is particularly useful in geometries employing additional separations such as ion mobility.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention together with an arrangement given for illustrative purposes only, will now be described, by way of example only and with reference to the accompanying drawings in which:

Fig. 1 shows a known quadrupole-Time of Flight mass analyser which may be used to perform high resolution MRM analyses;

Fig. 2 shows a IMS-quadrupole-Time of Flight mass analyser according to an embodiment of the present invention;

Fig. 3 shows three transitions and illustrates the dynamic range for the most intense species of characteristic fragment ion and Fig. 3A shows a response versus concentration curve for an ADC based detection system and shows the dynamic range for the most intense species of characteristic fragment ions;

Fig. 4 illustrates the dynamic range for the weakest intensity species of

characteristic fragment ion and Fig. 4A shows a response versus concentration curve for an ADC based detection system and shows the dynamic range for the weakest intensity species of characteristic fragment ions;

Fig. 5 illustrates an embodiment of the present invention wherein a x10 increase in the dynamic range is obtained by using the most intense species of characteristic fragment ions at low and moderate analyte sample concentrations to quantitate an analyte and using the weakest intensity species of characteristic fragment ions at high analyte sample concentrations to quantitate the analyte and Fig. 5A shows a response versus

concentration curve for an ADC based detection system and shows how the dynamic range may be extended according to an embodiment of the present invention;

Fig. 6 shows detail of a mass spectrum of a target compound of interest having ten isotopes; and

Fig. 7 shows a table wherein the 45 unique isotopic ratios corresponding to the ten isotopes as shown in Fig. 6 are ranked in order of their detection probability.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

A preferred embodiment of the present invention will now be described in more detail with reference to Fig. 2. The preferred embodiment of the present invention seeks to improve the dynamic range of a detector system of a mass spectrometer. The mass spectrometer preferably comprises an ion mobility separator 5, a quadrupole mass filter 2, a gas cell 3 and an orthogonal acceleration Time of Flight mass analyser 4. The benefits of improving the dynamic range of an orthogonal acceleration Time of Flight mass analyser 4 according to the preferred embodiment will become apparent when considering instrument geometries which employ multiple separation devices such as an IMS-Quadrupole-Time of Flight mass spectrometer as shown in Fig. 2. In this geometry the duty cycle and specificity of MRM experiments can be significantly improved compared with known mass spectrometers as shown in Fig. 1 .

The present invention also extends to other geometries wherein, for example, the order of the IMS device 5 and the quadrupole mass filter 2 may be reversed from that shown in Fig. 2. For example, according to a less preferred embodiment a Quadrupole- IMS-Time of Flight mass spectrometer may be provided and can also provide improved duty cycle and specificity of fragment ions.

According to the preferred embodiment duty cycle and specificity improvements are achieved by separation and compression of ion signals in time. This compression places higher demands on the dynamic range of the acquisition systems of orthogonal acceleration Time of Flight mass spectrometers 4.

The ability to be able to improve the dynamic range of a Time of Flight mass spectrometer 4 according to the preferred embodiment is particularly advantageous.

The preferred embodiment of the present invention seeks to improve the dynamic range of MRM experiments by monitoring and measuring a wide range of characteristic fragment, product or other ions for each parent or precursor ion during a method development stage.

Calibration curves of response versus sample concentration are determined for the range of characteristic fragment, product or other ions. Choice of which calibration curve to use is based on the analyte data acquired.

Multiple calibration curves may be used or combined within a single experiment for a single component if the analyte data indicates a benefit. The choice to switch calibration curves may be done in real time or as part of a post acquisition approach.

Figs. 3-5 illustrate in more detail how this approach may be utilised to improve the dynamic range of a mass spectrometer according to an embodiment of the present invention.

The data shown in Fig. 3 represents three transitions i.e. three different

characteristic fragment ions all of which result from the fragmentation of the same species of parent or precursor ion. The data is acquired using a known Quadrupole-Time of Flight mass spectrometer as shown in Fig. 1 .

In this example the most intense species of characteristic fragment ions has an intensity which is x10 greater than that of the weakest intensity species of characteristic fragment ions. This difference appears as an offset in the log-log plot shown in Fig. 3.

If an arbitrary minimum number of ions (e.g. 100) is required as being necessary for quantification and an arbitrary number of ions (e.g. 10,000) ions is chosen as being the upper limit above which ion detector saturation will occur, then the dynamic range may be defined as 100:1 or two orders of magnitude for this example. The dynamic range for the most intense species of fragment ions is indicated by arrows in Fig. 3. Considering an ADC based detection system then the response versus

concentration curve for Fig. 3 would be as shown in Fig. 3A.

If a response of 100 arbitrary units is defined as the lower limit for quantification, mass accuracy or as the lower detection limit, then the response is linear from that point over two orders of magnitude in concentration until the response starts to saturate at high concentrations.

Applying the same approach to the weakest intensity species of characteristic fragment ions is shown in Fig. 4 and results in the same dynamic range of 100:1 but the concentration range is shifted by an order of magnitude to higher concentrations.

Fig. 4A shows a corresponding response versus concentration curve for an ADC based detection system. For the weakest intensity species of fragment ion the linear range is still two orders of magnitude but is now shifted to a higher concentration range.

Fig. 5 illustrates a method of calibrating or quantitating an analyte sample according to a preferred embodiment. In particular, Fig. 5 shows the benefit of using a combination of two different species of characteristic fragment ions which are preferably present at different concentrations in order to calibrate the mass spectrometer or to quantitate an analyte sample.

With reference to Fig. 5, the most intense species of characteristic fragment ions may be used to calibrate or to quantitate an analyte of interest for responses in the range of 100 to 10,000 ions striking the ion detector per chromatographic ion peak whereas the weakest intensity species of characteristic fragment ions may be used to extend the dynamic range and to quantitate the analyte of interest when the response of the most intense species of characteristic fragment ions is greater than 10,000.

As a result, a x10 fold increase in dynamic range may be obtained according to the preferred embodiment.

Fig. 5A shows a response versus concentration curve for an ADC based detection system and indicates how the ion detector saturates when more than 10,000 ions strike the ion detector per chromatographic peak. It is apparent from Fig. 5A that according to the preferred embodiment a sample can now be quantitated across three orders of magnitude of concentration (in contrast to the conventional approach which is only able to quantitate across two orders of magnitude).

It is apparent that choosing which characteristic fragment ions to use in order to quantitate an analyte of interest depending on which characteristic fragment ions are in the linear response region can lead to an order of magnitude increase in the dynamic range.

The present invention therefore results in a significant improvement in dynamic range compared with conventional approaches to quantitating an analyte sample.

In addition to choosing different characteristic fragment ion based detection limits or saturation limits it is recognised that other criteria may be used when determining the choice of characteristic fragment ion such as interferences, number of ions required to achieve a certain mass precision and which fragment ions to sum to give better detection limits. According to an important further embodiment different isotope peaks of the same species of parent ions may be used as reference points for quantitation rather than using characteristic fragment ions. A related embodiment will be described in more detail below with reference to Figs. 6 and 7.

According to an embodiment the approach as described above is also applicable to other experiments such as Selected Ion Recording ("SIR"). In these experiments a mass filter is not required prior to a mass analyser. Quantitation is performed using precursor ions or fragment ions created within the ion source or during the ionization process.

Alternatively, a fragmentation device may be provided between the ion source and the mass analyser to produce characteristic fragment or product ions.

According to an embodiment selection of which characteristic ions to use in order to provide quantitation of an analyte sample may be implemented as part of a post acquisition or post processing routine. Alternatively, the choice of which characteristic ions to use in order to provide quantitation be implemented in real time on a spectra by spectra basis including on a push by push basis.

According to an embodiment the preferred method of quantitation may be implemented on other mass analysers such as an orbitrap (RTM) mass spectrometer, a FT-ICR mass spectrometer and a quadrupole mass analyser.

According to an embodiment the preferred approach may be used in conjunction with Time of Flight modes such as Enhanced Duty Cycle ("EDC") and High Duty Cycle ("HDC").

According to an embodiment the preferred approach may be used with different detection schemes such as Analogue to Digital Converters ("ADCs") including multistage and non linear ADCs and Time to Digital Converters ("TDCs").

According to an embodiment the preferred approach may be used to compensate for detector saturation as well as ADC or TDC saturation.

According to an embodiment only data associated with the range of characteristic fragment ions is preferably stored.

According to an embodiment mass spectral data may not be stored mimicking the traditional data format of intensity versus time associated with MRM or SIR.

According to an embodiment the chosen or characteristic fragment or other ions may also be used for non-quantitative reasons such as determining isotope or fragment ion ratios and may be used for confirmatory purposes.

According to an embodiment the chosen or characteristic ions may be used for quantitative reasons with an additional confirmatory check of isotope or fragment ion ratios.

In another embodiment the objective of the analysis may be to quantify the amount of a target compound and then obtain confirmatory isotopic ratios. An example of a mass spectrum of a target compound containing 10 isotopes is shown in Fig. 6. The isotopes have nominal masses of 290, 291 , 292, 293, 294, 295, 296, 297, 298 and 299. The most intense isotope has a nominal mass of 292, the second most intense isotope has a nominal mass of 290, the third most intense isotope has a nominal mass of 294 and the weakest intensity isotope has a nominal mass of 299. A compound containing ten isotopes \ l ₂ , l ₃ , l _4j l ₅ , l _6i l ₇ , l _8i l ₉ , l ₁₀ will have 45 different possible isotopic ratios (i.e. Ii :l ₂ li :l ₃ li :l ₄ etc.). The 45 different isotopic ratios act like a unique fingerprint and determination of the various isotopic ratios enables accurate confirmation of the presence of a target compound of interest.

The 45 different possible isotopic ratios for the compound of interest which was analysed and shown in Fig. 6 were ranked by order of their detection probability and are shown in Fig. 7. The acceptability of each of the 45 isotopic ratios are shown for three samples (A, B and C) of increasing concentration.

The weakest analyte sample (A) only has acceptable ratios for the first three isotopic ratios (l ₃ :li l ₃ :l ₅ li :l ₅ ) because only the three most abundant isotopes (having nominal masses of 290, 292 and 294) were above the lower detection limit.

The second analyte sample (B) having a medium or intermediate concentration shows acceptable ratios for all 45 possible isotopic ratios.

The most concentrated or highest concentration analyte sample (C) results in detector saturation for many of the isotopic ratios and hence the ratios based on the most abundant ions will be in error. However, the weaker isotopes are within the dynamic range of the mass spectrometer and overall 15 isotopic ratios are found to be acceptable.

It is therefore possible to extend the dynamic range of the analysis according to a preferred embodiment of the present invention by selecting isotopes which are within the dynamic range of the Time of Flight mass spectrometer when compared to the selection of any one isotopic ratio.

Various further embodiments are contemplated. According to an alternative less preferred embodiment a Time of Flight mass spectrometer is not essential to the present invention. The approach to quantitation as described above may also be utilised with other types of mass spectrometers and separation devices such as ion mobility spectrometers ("IMS"), differential mobility spectrometers ("DMS") and Field Asymmetric Ion Mobility Spectrometry ("FAIMS") devices.

The information gathered from the characteristic fragment ions can also be used to control an instrument parameter such as collision energy or ionization efficiency, ion transmission or detector gain so as to improve the dynamic range.

Although the present invention has been described with reference to 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.