Ion source

Abstract

A method of ionising a sample is provided, comprising providing a fluid sample, wherein the fluid sample contains an analyte, applying one or more pulses of acoustic energy to the fluid sample to cause a spray of the fluid sample to eject from the surface of the fluid sample, and applying an AC, RF or alternating voltage to the fluid sample using an electrode.

Claims

1. A method of ionising a sample, comprising: providing a fluid sample, wherein said fluid sample contains an analyte; applying one or more pulses of acoustic energy to said fluid sample to cause a drop of said fluid sample to protrude or eject from the surface of said fluid sample; and applying energy to said drop such that said drop is caused to fragment into a number of smaller drops, wherein said step of applying said energy to said drop forms a spray; applying a voltage to said fluid sample so as to cause analyte molecules in said spray to ionise and/or polarise; (a) holding said fluid sample at a relatively high potential, and holding an ion inlet device at a relatively low or ground potential, such that the volume between said fluid sample and said ion inlet device forms an electrolytic capacitor; and/or (b) holding an ion inlet device at a relatively high potential, and holding said fluid sample at a relatively low or ground potential, such that the volume between said fluid sample and said ion inlet device forms an electrolytic capacitor; and switching or repeatedly switching between (a) and (b) in a mode of operation.

2. The method of claim 1, wherein said spray comprises a mist and/or atomised particles.

3. The method of claim 1, wherein said step of applying said energy to said drop comprises applying at least one of acoustic, laser and heat energy to said drop.

4. The method of claim 1, further comprising ionising said drops to form ionised particles.

5. The method of claim 4, further comprising transporting said ionised particles into an inlet of a mass spectrometer.

6. The method of claim 1, wherein said voltage is a DC, AC, RF or alternating voltage.

7. The method of claim 1, wherein said voltage is applied to said fluid sample by an electrode.

8. The method of claim 7, wherein said electrode is positioned within said sample.

9. The method of claim 1, wherein said voltage is applied to said fluid sample via a sample holder.

10. The method of claim 9, wherein said sample holder is conductive or made from a conductive material.

11. The method of claim 9, further comprising: (a) holding said sample holder at a relatively high potential, and holding an ion inlet device at a relatively low or ground potential, such that the volume between said sample holder and said ion inlet device forms an electrolytic capacitor; and/or (b) holding an ion inlet device at a relatively high potential, and holding said sample holder at a relatively low or ground potential, such that the volume between said sample holder and said ion inlet device forms an electrolytic capacitor.

12. The method of claim 11, further comprising switching or repeatedly switching between (a) and (b) in a mode of operation.

13. The method of claim 1, further comprising maintaining a constant distance between an inlet orifice and a surface of said fluid sample in response to changes in the level or volume of said fluid sample.

14. The method of claim 1, wherein said fluid sample forms an electrolyte in an electrolytic capacitor.

15. A method of mass and/or ion mobility spectrometry comprising the method of claim 1.

16. An ion source comprising: a sample holder and an acoustic transducer, wherein said sample holder is for containing a fluid sample, and said acoustic transducer is arranged and adapted to apply one or more pulses of acoustic energy to said fluid sample to cause a drop of said fluid sample to protrude or eject from the surface of said fluid sample; a device arranged and adapted to apply energy to said drop such that said drop is caused to fragment into a number of smaller droplets droplets so as to form a spray; and one or more devices configured to: apply a voltage to said fluid sample so as to cause analyte molecules in said spray to ionise and/or polarise; (a) hold said fluid sample at a relatively high potential, and hold an ion inlet device at a relatively low or ground potential, such that the volume between said fluid sample and said ion inlet device forms an electrolytic capacitor; and/or b) hold an ion inlet device at a relatively high potential, and hold said fluid sample at a relatively low or ground potential, such that the volume between said fluid sample and said ion inlet device forms an electrolytic capacitor; and switch or repeatedly switch between (a) and (b) in a mode of operation.

17. The ion source of claim 16, wherein said device comprises at least one of an acoustic transducer, a laser and a heater.

18. A method of ionising a sample, comprising: providing a fluid sample, wherein said fluid sample contains an analyte; applying one or more pulses of acoustic energy to said fluid sample to cause a drop of said fluid sample to protrude or eject from the surface of said fluid sample; applying energy to said drop such that said drop is caused to fragment into a number of smaller drops, wherein said step of applying said energy to said drop forms a spray; applying a voltage to said fluid sample so as to cause analyte molecules in said spray to ionise and/or polarise; and maintaining a constant potential difference between said fluid sample and an ion inlet device.

19. An ion source comprising: a sample holder and an acoustic transducer, wherein said sample holder is for containing a fluid sample, and said acoustic transducer is arranged and adapted to apply one or more pulses of acoustic energy to said fluid sample to cause a drop of said fluid sample to protrude or eject from the surface of said fluid sample; a device arranged and adapted to apply energy to said drop such that said drop is caused to fragment into a number of smaller droplets so as to form a spray; and one or more devices configured to: apply a voltage to said fluid sample so as to cause analyte molecules in said spray to ionise and/or polarise; and maintain a constant potential difference between said fluid sample and an ion inlet device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 shows a schematic of an embodiment of the present disclosure;

(3) FIG. 2 shows droplet ejection in accordance with a prior art configuration;

(4) FIGS. 3A and 3B illustrate droplet ejection under modified conditions;

(5) FIG. 4 shows the [M+H].sup.+ response to the ejection of caffeine;

(6) FIG. 5 shows the [M+2H].sup.2+ response to the ejection of Glu-fibrino peptide;

(7) FIGS. 6A and 6B show two mass spectra obtained from Wafarin;

(8) FIG. 7 shows the effect of liquid surface to sampling nozzle distance;

(9) FIG. 8 shows a schematic of an embodiment;

(10) FIG. 9 shows a schematic of an embodiment in which a controller may be used to maintain a constant distance between a fluid sample and an inlet device;

(11) FIGS. 10A, 10B and 10C show a comparison of drop and spray or mist modes of operation;

(12) FIG. 11 shows the mass spectrometer signal in a mode of operation; and

(13) FIG. 12A shows a schematic of an embodiment in which an electrode may surround and/or form part of a sample holder and FIG. 12B shows a schematic of an embodiment in which an electrode may be placed at least partially between a sample holder and an acoustic transducer.

DETAILED DESCRIPTION

(14) Various embodiments of the present disclosure will now be described.

(15) An ion source in accordance with an embodiment is shown in FIG. 1.

(16) An electrode 50 is optionally inserted into a vial 20, optionally containing a sample of analyte solution. A sampling tube 10 is optionally connected to a mass spectrometer and may be positioned over the vial 10. A pulse of acoustic energy may be produced by a transducer 30. The pulse is optionally focused onto to the surface of the sample or analyte solution, which optionally causes a stream or spray of droplets to be emitted.

(17) An electrode 50 is optionally placed inside the analyte vial 20 so that it is able to apply a voltage directly to the sample or analyte solution. As the droplets leave the sample they may polarise and/or desolvate, optionally forming protonated or deprotonated ions depending upon the voltages applied. These ions are then optionally analysed using the mass spectrometer.

(18) For the sake of simplicity, only one vial 20 is shown in FIG. 1. However, it is understood in practice that the sample reservoir or holder may also be or comprise a collection of reservoirs, for example in the form of racked tubes or microtiter plates. The sample reservoir or holder could also be an individual tube or vial.

(19) In order for the system to produce ions the acoustic set up is modified from conventional conditions used for acoustic liquid transfer, which may be configured to provide a single droplet of known volume, typically of the order 2.5 nL in volume, and/or having a diameter of approximately 170 m, as shown in FIG. 2.

(20) In accordance with the various embodiments, these conventional conditions may be altered to form a stream or spray of smaller droplets, for example having a volume less than 1 L, optionally less than 100 fL, and/or a diameter of less than 15 m. FIG. 3A is a photograph showing a stream of droplets emitted, using droplet ejection under modified conditions. FIG. 3B shows a typical droplet diameter distribution. A typical sonic frequency to produce a stream or spray of smaller droplets may be greater than 10 MHz, and optionally 10-12 MHz or 11 MHz.

(21) Lower frequency and/or longer wavelength pulses may produce larger droplets, e.g. droplets having a large or larger diameter. Higher frequency and/or shorter wavelength pulses may produce smaller droplets, e.g. droplets having a small or smaller diameter. Droplet volume may be controlled and/or reproducible. The production rate of droplets, or the amount of droplets in the spray, may be greater than 200 droplets per second, optionally 200-1000 droplets per second.

(22) In accordance with various embodiments, the application of a voltage to the sample or analyte solution optionally results in the formation of an electrical circuit, wherein the air gap and/or analyte between the sampling tube and vial (or a counter electrode) becomes the dielectric of an electrolytic capacitor. The sample or analyte solution optionally forms the electrolyte of the electrolytic capacitor. The droplets are optionally polarised as they align opposite to the electric field, and are optionally ionised in an electro-spray like process as they leave the surface. The protonation of the sample may be driven by the voltage applied to the sample or analyte solution. It should be noted that the solvents generally used in mass spectrometry, for example methanol (33.1), water (80.4), may have quite high relative permittivity Cr.

(23) FIG. 4 shows the [M+H].sup.+ response of the mass spectrometer to the ejection of caffeine, with approximately 250 nL ejected from a 10 g/mL solution in water, containing 0.1% formic acid. Note that the intensity scale in FIG. 4 is logarithmic, and that the signal drops to the background level quickly on the cessation of the acoustic energy. The voltage applied to the analyte can be greater than 1 kV, and optionally greater than or substantially equal to 2 kV. The droplet ejection rate may be greater than or equal to 500 Hz.

(24) FIG. 5 shows the [M+2H].sup.2+ response of the mass spectrometer to the ejection of Glu-fibrino peptide (63 mM in water and 0.1% formic acid). Again, the intensity scale is logarithmic and drops immediately to the background level on the cessation of the acoustic energy. This optionally shows the formation of multiply charged positive ions.

(25) FIGS. 6A and 6B show mass spectra obtained from Wafarin (50 mM). FIG. 6A is a first mass spectrum using positive ion mode (+2.2 kV applied to the liquid), and showing the [M+H].sup.+ ion at 309 Da. FIG. 6B is a second mass spectrum using negative ion mode, and showing the [MH].sup. ion at 307 Da.

(26) The effect of the spacing of the sampling tube 10 (or electrode) from the surface of the sample or analyte solution on the intensity of the MS signal has been investigated and shown in FIG. 7.

(27) The distance between the sampling tube 10 to the surface of the sample or analyte solution may be an important parameter in the reproducibility and efficiency of this mass spectrometer. In various embodiments, this distance is closely controlled. The surface position may be already measured using acoustic methods, and optionally during auto set up of the acoustic solvent delivery system, and so this may be used as a closed loop feedback parameter. The surface position, or the distance between the sampling tube 10 to the surface of the sample or analyte solution, may be measured using a laser, for example laser range finding, or using capacitance changes, etc.

(28) A laser or hot probe may be used to generate the droplets of a correct size and/or volume distribution.

(29) Different geometries for applying the field are envisaged, for example a more practicable approach may be to apply the high voltage to the sampling nozzle as shown in FIG. 8.

(30) Conductive sample plates or analyte vials could be used. This would enable the grounding to be provided through the solid portions of the containers to each of the fluid samples in the reservoirs.

(31) FIG. 9 shows a further modification that optionally maintains a consistent gap or distance from the sampling tube 10 to the surface of the sample or analyte solution, optionally based on measurement of the fluid height.

(32) The use of sonar and acoustic impedance measurements has been described previously (see, for example, U.S. Pat. No. 8,453,507 to Labcyte, Inc.) in order to calculate the fluid depth. Such a measurement can be made prior to generating drops from each well and optionally periodically to find if the well has changed. Reasons for the change could be fluid transfer, evaporation or an increase in fluid from absorption from the atmosphere. The fluid depth information for each well can then provide motion instructions to a positioning means 62, which then optionally adjusts the distance between the sampling tube 10 and the surface of the sample or analyte solution, to optionally ensure that this distance or gap remains consistent and/or constant.

(33) A predetermined distance between the sampling tube 10 and the surface of the sample or analyte solution may be measured and/or recorded, and the positioning means 62 may adjust the distance between the sampling tube 10 and the surface of the sample or analyte solution to maintain it at the predetermined distance.

(34) Maintaining a constant voltage and/or distance between the sampling tube 10 and the surface of the sample or analyte solution, may provide a consistent field strength between the sample and sampling tube 10.

(35) Alternatively, it may be possible to maintain the field constant by measuring the distance between the sampling tube 10 and the surface of the sample or analyte solution and altering the applied voltage.

(36) Optionally, for some fluids and analytes, improved signal quality for the analyte of interest in the mass spectrometer may be achieved when the sampling tube 10 or inlet orifice is positioned within the sample reservoir. Hence, the outer diameter of the inlet orifice may be sufficiently small to facilitate entry into the reservoir and to produce adequate field strength, optionally without arcing to the reservoir wall. Reducing the gap distance to the fluid may allow for absolute voltage reduction to minimize this potential and increase the robustness of sample loading and signal quality.

(37) Droplet sizes, flow rates and droplet size distribution requirements may vary by analytical instrument and/or interface. Various embodiments create droplets in the form of a spray or mist, and such instrument modes optionally remain compatible with existing acoustic microplates. FIGS. 10A-10C show the difference between a drop instrument mode and a spray or mist instrument mode.

(38) In a drop instrument mode the acoustic transducer 30 may apply a pulse of acoustic energy to the surface of the sample that can cause a single drop to emerge from the surface of the sample. This single drop may then be ionised and may be transported into the sampling tube 10 due to e.g. vacuum pumping.

(39) In a spray or mist instrument mode the acoustic transducer 30 may apply a pulse of acoustic energy to the surface of the sample that can cause a spray or mist to emerge from the surface of the sample. Analyte molecules in this spray or mist may then be ionised and may be transported into the sampling tube 10 due to e.g. vacuum pumping.

(40) In a mode of operation the polarity of the voltage applied to the sample and/or electrode may be switched between positive and negative polarities. The voltage applied in such a case may be an AC, RF or alternating voltage. Alternatively, a voltage device may be arranged and adapted to switch between voltage polarities in use. Application of a positive voltage optionally causes production of negative ions to form from the droplet, stream or spray. Application of a negative voltage optionally causes production of positive ions to form from the droplet, stream or spray. The mass spectrometer may be arranged to detect positive and/or negative ions.

(41) These modes of operation can reduce charging instabilities in the fluid sample, or sample holder. For example, switching polarities may dissipate charge that builds up in the fluid sample, or sample holder.

(42) An example of this mode of operation is shown in FIG. 11, in which it can be seen that switching between positive and negative voltage polarities optionally results in the alternating production of negative and positive ions. The mass spectrometer may be arranged and adapted, or configured to detect positive ions, as shown in FIG. 11. This means that negative ions may not be detected. In various embodiments, the mass spectrometer can be arranged and adapted to switch between detecting positive and negative ions in synchronisation with the switching between positive and negative voltage polarities as described herein.

(43) Alternatively, the mass spectrometer may be arranged and adapted, or configured to switch between detection of positive and negative ions at the same switching frequency as the AC, RF or alternating voltage. In this manner, all ions would be detected by the mass spectrometer.

(44) The voltage applied in these modes of operation may be between 5-10 kV, and optionally 8-10 kV. The switching frequency may be provided to match the rate of drop, droplet, stream or spray ejection, or may be triggered by ejection of a drop, droplet, stream or spray from the fluid sample. The switching frequency may be a multiple of the rate of drop, droplet, stream or spray ejection, optionally so that the polarity is switched more than once per ejection cycle. The switching frequency may be <1 HZ, <2 Hz, <5 Hz or <10 Hz, and is optionally between 0.5-5 Hz.

(45) FIG. 12A shows an ion source in accordance with an embodiment in which a sample holder 20 may be used to retain the sample or analyte solution. The sample holder 20 may be resistive, non-conductive, semi-conductive or dielectric. An electrode 50 may at least partially surround the sample holder 20 but optionally does not contact the sample or analyte solution. In various embodiments, the electrode 50 may be built into the sample holder 20 whilst still not contacting the sample or analyte solution itself.

(46) FIG. 12B shows a similar arrangement in which a plate, mesh or grid electrode may be located beneath the sample holder 20, and optionally between the sample holder 20 and the acoustic transducer 30.

(47) The other parts of the ion source of the embodiments as shown in FIGS. 12A and 12B, with like reference numerals, may be the same as discussed above.

(48) In the embodiments as shown in FIGS. 12A and 12B, a voltage, for example a DC, AC, RF or alternating voltage may be applied to the electrode 50 and the sampling tube 10 may be held at a ground potential. Alternatively, the electrode 50 may be held at a ground potential, and a DC, AC, RF or alternating voltage may be applied to the sampling tube 10. The embodiments as shown in FIGS. 12A and 12B may be used with any of the modes of operation discussed above, including the modes of operation in which the polarity of the voltage applied to the sampling tube 10 and/or electrode 50 may be switched between positive and negative polarities.

(49) Although the present invention has been described with reference to various 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.