Abstract
A secondary ultrasonic nebulisation device is disclosed comprising: a liquid sample delivery capillary; a sample receiving surface arranged for receiving a liquid sample from the capillary; and an ultrasonic transducer configured for oscillating the surface so as to nebulise the liquid sample received thereon, wherein the device is configured such that the oscillations of the surface by the ultrasonic transducer cause charged droplets and/or gas phase ions to be generated from the sample.
Claims
1. A mass or ion mobility spectrometer comprising: a secondary ultrasonic nebulisation device comprising: a liquid sample delivery capillary; a sample receiving surface arranged for receiving a liquid sample from the capillary; and an ultrasonic transducer configured for oscillating the surface so as to nebulise the liquid sample received thereon, wherein the device is configured such that the oscillations of the surface by the ultrasonic transducer cause charged droplets and/or gas phase ions to be generated from the sample; a vacuum chamber; an ion inlet orifice arranged between the sample receiving surface of the nebulisation device and the vacuum chamber, for receiving said charged droplets and/or ions; a structural component arranged in a flow path of the nebulised droplets and/or ion; and a heater configured to heat the structural component to 50 C., wherein the heater heats the structural component such that, in use, the droplets and/or ions are heated by the structural component.
2. The mass or ion mobility spectrometer of claim 1, wherein the structural component is arranged in the vacuum chamber.
3. The mass or ion mobility spectrometer of claim 2, wherein the structural component is arranged in the flow path of the charged droplets and/or ions such that, in use, the droplets and/or ions impact onto the structural component.
4. The mass or ion mobility spectrometer of claim 2, wherein the structural component comprises a heated bead.
5. The mass or ion mobility spectrometer of claim 1, wherein the structural component is the ion inlet orifice.
6. The mass or ion mobility spectrometer of claim 1, wherein, in use, a gas flow is provided from the secondary ultrasonic nebulisation device to the vacuum chamber.
7. The mass or ion mobility spectrometer of claim 1, wherein the heater is configured to heat the structural component, in use, to a temperature selected from the group consisting of: 60 C.; 70 C.; 80 C.; 90 C.; 100 C.; 120 C.; 140 C.; 160 C.; 180 C.; 200 C.; 250 C.; 300 C.; 400 C.; 500 C.; 600 C.; 700 C.; 800 C.; 900 C.; and 1000 C.
8. The mass or ion mobility spectrometer of claim 1, comprising a mass analyser or ion mobility analyser downstream of said structural component.
9. The mass or ion mobility spectrometer of claim 1, comprising a sample separation device for separating a liquid sample; wherein said liquid sample delivery capillary forms part of, or is arranged to receive the liquid sample from, the sample separation device.
10. The mass or ion mobility spectrometer of claim 9, wherein the sample separation device is a liquid chromatography separation device.
11. The mass or ion mobility spectrometer of claim 9, further comprising a fluid injection system for injecting a fluid into the liquid sample downstream of the sample separation device for increasing the flow rate out of an outlet of the liquid sample delivery capillary.
12. A mass or ion mobility spectrometer comprising: a secondary ultrasonic nebulisation device comprising: a liquid sample delivery capillary; a sample receiving surface arranged for receiving a liquid sample from the capillary; and an ultrasonic transducer configured for oscillating the surface so as to nebulise the liquid sample received thereon, wherein the device is configured such that the oscillations of the surface by the ultrasonic transducer cause charged droplets and/or gas phase ions to be generated from the sample; a vacuum chamber; an ion inlet orifice arranged between the sample receiving surface of the nebulisation device and the vacuum chamber, for receiving said charged droplets and/or ions; a structural component arranged in a flow path of the nebulised droplets and/or ion; and a heater configured to heat the structural component, wherein the sample receiving surface is located in a nebulising region that is, in use, maintained at about room temperature.
13. A method of nebulising a liquid sample using a secondary ultrasonic nebulisation device comprising: delivering a liquid sample to a sample receiving surface; ultrasonically oscillating the surface so as to nebulise the liquid sample, wherein said ultrasonically oscillating causes charged droplets and/or gas phase ions to be generated from the sample; urging said charged droplets and/or gas phase ions through an ion inlet orifice, into a vacuum chamber and onto a heated structural component, wherein a heater heats the structural component such that, in use, the droplets and/or ions are heated by the structural component.
14. The method of claim 13, wherein urging said charged droplets and/or gas phase ions into said vacuum chamber and onto said structural component comprises providing a gas flow from said sample receiving surface to said vacuum chamber.
15. A method of mass spectrometry or ion mobility spectrometry comprising: performing a method of nebulising a liquid sample as claimed in claim 13; and mass analysing or ion mobility analysing said gas phase ions, ions derived from said gas phase ions, or ions derived from said charged droplets.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Various embodiments will now be described, by way of example only, and with reference to the accompanying drawings in which:
(2) FIG. 1 shows a schematic of a prior art primary ultrasonic nebuliser;
(3) FIG. 2 shows a schematic of a secondary ultrasonic nebuliser according to an embodiment of the present invention wherein a jet or column of liquid sample is ejected from a capillary onto a sample receiving surface;
(4) FIGS. 3A-3B show schematics of a secondary ultrasonic nebuliser according to other embodiments of the present invention wherein a liquid junction is formed between the capillary and sample receiving surface;
(5) FIG. 4A shows chromatograms for five analytes obtained using a secondary nebuliser according to an embodiment of the present invention, and FIG. 4B shows chromatograms for the same five analytes obtained using a conventional ESI source;
(6) FIGS. 5A and 5B show chromatograms obtained for 6-tocopherol using a secondary nebuliser according to an embodiment of the present invention and a conventional ESI source, respectively;
(7) FIG. 6 shows a schematic of the liquid junction between the capillary and sample receiving surface in the embodiment of FIG. 3A;
(8) FIG. 7 shows the relationship between the amount of liquid added to the liquid junction of FIG. 6 and the ultrasonic transducer frequency that is required for nebulising this liquid, for various liquid flow rates through the capillary; and
(9) FIGS. 8A and 8B show schematics of secondary ultrasonic nebulisers according to further embodiments of the present invention, wherein the axes of the ultrasonic horn and the inlet orifice are perpendicular.
DETAILED DESCRIPTION
(10) Atmospheric Pressure Ionisation/mass spectrometry (API/MS) sources have previously utilised ultrasonic nebulisers that convert a flow of liquid from a liquid chromatography (LC) column into a charged aerosol. These are primary ultrasonic nebulisers where the liquid flow passes directly through an electrospray nozzle which is ultrasonically agitated via a piezoelectric transducer.
(11) In the secondary ultrasonic nebuliser described herein, the liquid flow is deposited onto a surface which, in turn, is ultrasonically agitated. In order to produce a stable aerosol from a secondary ultrasonic nebuliser, it is necessary to produce a wetted zone (finite volume reservoir, or film) on the transducer surface and to deliver a continuous, unperturbed liquid flow to the wetted zone. Liquid from a LC column can be fed to the transducer by positioning a capillary tube at some distance from the transducer and aiming the unbroken liquid jet to create a wetted zone on the vibrating surface. For example, for a 1:1 mixture of acetonitrile/water and a 130 m internal diameter stainless steel tube at room temperature, a primary jet is formed at a flow rate of approximately 0.8 mL/min where the linear velocity of the liquid column is of the order 1 m/s. For liquid flow rates below this value, an intermittent, dripping flow may result which limits the practical flow rate range to wide bore analytical LC and UPLC columns. The onset of a stable liquid jet will depend on factors such as the internal diameter of the capillary, capillary material, liquid surface tension and temperature. In addition, the stability and directionality of the jet can be further influenced by factors such as capillary tip surface finish, cleanliness and induced charging from electric fields in the API source volume. All these factors impose limits on the amenability of secondary ultrasonic nebulisation to a wide range of chromatographic conditions.
(12) An API source is disclosed herein that comprises a capillary tube that is very close to, or in contact with, a transducer surface such that it forms a stable liquid junction with the surface over a wide range of flow rates and mobile phase compositions.
(13) FIG. 1 shows a schematic of a prior art primary ultrasonic nebuliser that is configured as an electrospray probe for an API source that provides ions for analysis by a mass spectrometer. Here, a liquid column 1 derived from the flow from a LC column is passed through a capillary 2 which is surrounded by an ultrasonic nozzle 3 that is in mechanical communication with the capillary 2. The nozzle 3 transmits ultrasonic energy from a transducer 4 to the capillary 2, which agitates the liquid column 1 to form a nebulised spray plume 6. In order to form charged droplets of predominantly one polarity, it is conventional to connect the capillary 2 to a high voltage power supply 5 which promotes double layer charge formation at the high electric field region where the liquid column 1 emerges from the capillary 2. Ions and charged droplets enter the mass spectrometer for analysis via an ion inlet orifice 7, which forms a boundary between the atmospheric pressure of the nebulised spray volume and the first vacuum region 8 of the mass spectrometer. A cone gas nozzle 9 is also shown that serves as a conduit for a flow of nitrogen gas that runs counter-current to the spray direction for aiding desolvation of charged droplets and reducing contamination of the inlet orifice 7.
(14) FIG. 2 is a schematic of an embodiment of the present invention which shows a secondary ultrasonic nebuliser that is configured as an API source for a mass spectrometer. Here, a flow of liquid from a LC column passes through a capillary 2 and forms a liquid column 1. The liquid column 1 is directed to impact on an ultrasonically agitated surface 11, which receives ultrasonic energy from a transducer 4 via a cylindrical transmitting horn 10. Experimental observations have revealed that the optimum conditions for spray stability and ion detection sensitivity occur when a wetted region or surface film 12 forms on the agitated surface 11. The size of the film 12 is exaggerated in FIG. 2 and is typically a few times greater than the width of the liquid column 1. The agitated surface 11 can receive a periodic displacement in either an axial or transverse direction, depending on the design of the ultrasonic system. The arrangement shown in FIG. 2 is best suited to oscillations along the longitudinal axis of the horn 10, where the force on the liquid reservoir 12 is normal to the agitated surface 11, thus leading to a nebulised spray plume 6 which has a significant velocity component in the X-direction towards the ion inlet orifice 7. Conversely, a transverse displacement (into and out of the page) would tend to disperse the liquid film 12 across the surface 11 resulting in the absence of a significant spray plume.
(15) As shown and described in the arrangement of FIG. 1, it is possible to raise the electrical potential of the horn 10, and hence agitated surface 11, via connection to a high voltage power supply. A positive potential, for positive ion mass spectral analysis, can lead to ion signal enhancements. However, this effect is believed to be primarily due to a liquid column 1 steering effect in which a negatively induced voltage on the liquid column 1 results in its attraction towards the agitated surface 11 and more favourable conditions for a stable spray plume. The application of ultrasonic power is believed to be more central to the process of generating charged droplets than an applied voltage. Furthermore, it is important to position the agitated surface 11 such that the distance x between the film 12 on the agitated surface and the inlet orifice 7 is relatively small, e.g. x5 mm. When distance x is relatively small, ions and charged droplets that are generated are accelerated towards the ion inlet orifice 7 under the influence of the high velocity gas flow that exists between the atmospheric pressure of the source and the lower pressure of the first vacuum region 8.
(16) The embodiment of FIG. 2 has been used as an API source for MS analysis at flow rates of, for example, 0.9 mL/min. Here, a stainless steel capillary 2 was used with an internal diameter of 130 m and an outer diameter of 220 m. The capillary 2 was positioned approximately 10 mm above surface 11. Surface 11 was formed by cutting a 45 chamfer on the end of a stainless steel rod having a length of 30 mm and a diameter of 1.6 mm. This rod was screwed into the cylindrical ultrasonic horn 10, which had a length of 90 mm and a diameter of 5 mm. The horn was driven by a 40 kHz transducer 4, which produced an axial displacement of 18 m at the end of the horn 10.
(17) When the LC flow rate is reduced to 0.3 mL/min, the liquid column 1 becomes unstable and exhibits a dripping flow which results in an intermittent MS signal. The onset of dripping is determined by the linear velocity of the liquid column 1 as it emerges at the end of the capillary 2. It can be shown that:
(18)
where d is the internal diameter of the capillary 2, F is the liquid flow rate and v is the velocity of the liquid column 1 at the end of the capillary 2.
(19) As described above, it is found that typical LC solvents such as water, acetonitrile and methanol at room temperature will form a continuous jet for liquid column velocities in excess of around 1 m/s. According to equation (i) above, this condition is satisfied for a capillary 2 having an internal diameter d of 130 m when the liquid flow rate F is 0.9 mL/min, but is not satisfied at a liquid flow rate F of 0.3 mL/min. In practice, a flow rate of 0.3 mL/min could be accommodated by the embodiment shown in FIG. 2, whilst forming a continuous jet, by reducing the internal diameter of the capillary 2 to around 80 m. Alternatively, a flow rate of 0.3 mL/min could be accommodated by post-column addition (PCA) of an additional solvent flow between the LC column and the capillary 2 so as to boost the total liquid flow to 0.9 mL/min (for the capillary internal diameter of 130 m). Both of these methods have been applied successfully at LC flow rates of 0.3 mL/min using the apparatus described in this paragraph. In fact, PCA of carefully chosen solvents can benefit MS sensitivity according to the particular chemistries of certain analytes.
(20) From the above, it is apparent that a variation of this embodiment that utilises a dynamically adjustable capillary bore would provide an API source that could span a typical range of LC flow rates. The need to reduce the internal diameter of the capillary 2 in FIG. 2 poses some practical limitations at low flow rates (e.g. 0.1 mL/min) since small diameter capillaries are prone to blocking with real life samples and can form asymmetric jets due to poor handling or even modest tip contamination.
(21) FIG. 3A is a schematic of another embodiment of the present invention. This embodiment uses the same components and operating parameters as that described in FIG. 2, but differs in that the capillary 2 is located in very close proximity to the ultrasonically agitated surface 11 such that it creates a liquid junction 13 between these two components. For example, the distance between the end of the capillary 2 and the surface 11 may be 0.1-0.5 mm. Depending on the orientation of the capillary 2 relative to the surface 11, care may need to be exercised to avoid contact between the end of the capillary 2 and the surface 11, which could mechanically deform the end of the capillary 2 and seal the bore of capillary 2 during agitation of the surface 11. Accordingly, it may be desired that the capillary 2 does not contact the surface 11 at all during agitation of the surface 11. This may be particularly desired when the angle between the axis of the capillary 2 and the surface 11 is greater than a certain value, e.g. when is 20. However, in other embodiments in which the angle is relatively low, e.g <200, the end of the capillary 2 is less likely to be mechanically deformed to the extent that the bore will be sealed if the capillary 2 contacts with the surface 11. In these embodiments the end of the capillary may contact the surface 11 during agitation of the surface 11, and may even remain in contact with the surface 11 during agitation. Such an embodiment is shown in FIG. 3B. As in FIG. 3A, it can be seen that the embodiment of FIG. 3B also forms a liquid junction 13 between the capillary 2 and surface 11. Although the capillary blocking effect has been described to occur when the capillary 2 contacts the surface 11 with an angle 200, it will be understood that the effect may still occur (to a different extent) at other angles.
(22) The embodiments having a liquid junction 13 eliminate the flow rate dependency issues highlighted above. This liquid junction technique has been demonstrated to operate routinely as an API/MS source with a 40 kHz transducer, a capillary 2 having an internal diameter of 90 m and for flow rates in the range from 0.1 to 0.8 mL/min. The peak-to-peak displacement of the ultrasonically oscillated surface 11 was approximately 18 m.
(23) The ultrasonic apparatus described in FIG. 3A has been compared to a conventional nitrogen-assisted ESI probe for the UPLC/MS analysis of a mixture of six analytes by multiple reaction monitoring (MRM) on a triple quadrupole mass spectrometer. In the ESI experiment, a cold nitrogen gas was used to aid nebulisation and a hot nitrogen gas flow was used to aid droplet desolvation. In the ultrasonic nebulisation experiment, a hot nitrogen gas flow (typically 250 C., not shown in FIGS. 3A-3B) was used to assist desolvation of the nebulised droplets. The following six analyte amounts were eluted into the API sources using a water/acetonitrile gradient at a flow rate of 0.6 mL/min: hydroxyprogesterone (500 pg), sulphadimethoxine (10 pg), tolbutamide (1 ng), j-estradiol (1 ng), caffeine (20 pg) and -tocopherol (1 ng).
(24) FIG. 4A shows the UPLC/MS chromatograms obtained for five of the above six test analytes using the liquid-junction ultrasonic source, and FIG. 4B show the UPLC/MS chromatograms obtained for the same five test analytes using the conventional ESI source. From a comparison of the data, it is apparent that both sources produce equivalent peak half widths, which range from typically 1.0 to 2.2 seconds under these chromatographic conditions. The number in the upper right hand corner of each chromatogram represents the MS signal intensity obtained from each analyte in arbitrary units. These intensities suggest that the ESI source produces greater ion signals by factors of 2 to 144. However, if the signal-to-noise ratios for both source types are analysed, it is found that the performance is more comparable. Both sources give equivalent signal-to-noise ratios for caffeine and -estradiol, whilst the signal-to-noise ratio obtained with the liquid-junction ultrasonic source can exceed that of ESI in the case of -tocopherol. Improvements in signal-to-noise are primarily due to a reduction in chemical noise background with the ultrasonic source, which could be advantageous for single quadrupole MS or other scanning MS systems.
(25) FIGS. 5A and 5B illustrate an additional advantage of the ultrasonic source over the ESI source. FIGS. 5A and 5B compare the chromatograms obtained for the least polar analyte of the above six analytes, -tocopherol, which elutes under very high organic (acetonitrile) conditions. The analyte -tocopherol is believed to produce a main peak (B) at 10.46 mins, and a second peak (A) at 9.35 mins which may be related or may be a contamination peak. As shown in FIG. 5A, the ultrasonic source produces strong peaks for both peaks A and B, whereas the ESI source shows only a very weak response for the main peak B. The weak ESI response can be correlated to a collapse in the total ion signal that occurs at approximately 10 mins in FIG. 5B, at which point the mobile phase composition reaches 100% acetonitrile. In comparison, as shown in FIG. 5A, the ultrasonic source is stable and produces a relatively flat baseline with no signal collapse under identical chromatographic conditions.
(26) As described above, additional experiments have shown that further enhancement of the ultrasonic source performance can be obtained by PCA of appropriate solvents to the solution eluting from the chromatography column. For example, the analysis of -tocopherol may be enhanced by the PCA of water, whilst -estradiol and tolbutamide may be enhanced by the PCA of acetonitrile.
(27) The use of a hot nitrogen gas flow for intersecting the spray plume is critically important for the ESI source sensitivity at the chromatographic flow rates described herein. On the other hand, the ultrasonic source signal intensity is found to be very weakly dependent on the temperature of the gas and can operate satisfactorily at room temperature. However, the ultrasonic source is found to be strongly dependent on the temperature of the ion inlet system, i.e. the internal and external surfaces that constitute the source components 7, 8 and 9 in FIGS. 1-3. This temperature dependence is different for each analyte and can, for example, result in two orders of magnitude signal loss for acetaminophen as the inlet temperature is decreased from 150 C. to 80 C. This would suggest that gas phase ions can be formed downstream of the point of nebulisation. Sensitivity enhancements may therefore be achieved by heating a structural component of the spectrometer that is downstream of the agitated surface 11 so as to assist in ionising the nebulised analyte. For example, a heated bead or other obstruction may be arranged in the first vacuum region 8 such that droplets from the spray plume 6 impact onto the obstruction downstream of the ion inlet orifice 7. The obstruction may be heated to a temperature in the range 100-1000 C.
(28) In the embodiments shown in FIGS. 2 and 3 it is important to create a stable surface film 12 or liquid junction 13 from which a spray plume 6 is ejected during the application of ultrasonic power. In order to maintain equilibrium, the volume of liquid entering the film 12 or junction 13 per periodic displacement cycle must be equal to the total volume of liquid ejected in the form of droplets from the film 12 or junction 13 per cycle.
(29) FIG. 6 shows a close-up schematic of the liquid junction 13 of FIG. 3A, formed between the ultrasonically agitated surface 11 and the liquid capillary 2. It is arbitrarily assumed that the liquid junction 13 is in the form of a cylinder (exaggerated in size in the drawing) of radius r and maximum height h. The volume of liquid that is added to the liquid junction 13 prior to each ejection cycle is represented by a cylinder of radius r and height h. Thus, in order to preserve equilibrium, greater liquid flow rates from the capillary 2 would require greater ultrasonic frequencies for the efficient ejection of liquid into plume 6. From the equilibrium conditions described above, it can be shown that the transducer frequency, f, will be given by:
(30)
where F is the liquid flow rate through the capillary 2, r and h are the radius and height respectively of the liquid cylinder added to the liquid junction 13 prior to each ejection cycle, and n is the number of ejections per cycle.
(31) For axial ultrasonic oscillations, as described in the embodiments of FIGS. 2 and 3, n will be equal to one and liquid ejection will occur when the agitated surface 11 is at its closest approach to the inlet orifice 7.
(32) FIG. 7 shows the relationship between the height h of the liquid cylinder added to the liquid junction 13 prior to each ejection cycle and the transducer frequency f, for various liquid flow rates that are typically used in LC/MS applications. Here, the capillary tube bore diameter is 90 m and it is assumed that the radius r of the liquid cylinder is 150 m, i.e. approximately 3 times larger than the capillary tube bore diameter. By analogy with the impact of high Weber number water droplets on a metal surface, it is known that the initial droplet from the capillary 2 spreads along the surface 11 to form a thin film that is typically a few microns thick. At this thickness, the film becomes unstable and disintegrates to produce a number of secondary droplets, where the number of droplets is proportional to the droplet Weber number. If a similar thickness of instability is assumed for the upper portion of the liquid junction 13, i.e. say h=2 m, then FIG. 7 would suggest optimum transducer frequencies of 5000 Hz, 25000 Hz and 115000 Hz for LC flow rates of 0.05 mL/min, 0.2 mL/min and 1.0 mL/min, respectively. According to this model, it seems reasonable to assume that the 40 kHz fixed frequency used in this work was appropriate for the 0.6 mL/min flow rates of FIGS. 4 and 5. Furthermore, the ultrasonic source according to the present invention may incorporate a variable frequency transducer to optimise sensitivity and stability across the full spectrum of LC flow rates.
(33) FIGS. 8A and 8B show plan views of embodiments of the present invention where the axes of the ultrasonic horn 10 and the inlet orifice 7 are perpendicular. Such arrangements are useful, for example, in API/MS sources in which it is not possible to align the axis of the ultrasonic horn 10 in the direction of the inlet orifice 7 due to access restraints. According to the embodiments of FIGS. 8A and 8B, the liquid capillary 2 (not shown) delivers an analyte solution to the surface 11 in the same way as described in the other embodiments so as to create a liquid film 12 or a liquid junction 13. The axis of the liquid capillary may be orthogonal to both the longitudinal axis of the transducer 10 and the axis through the ion inlet orifice 7 (e.g. the capillary 2 may be above and perpendicular to the page). The ultrasonically agitated surface 11 may be chamfered at 450 to the axis of the longitudinal horn 10 and the transducer 4 may oscillate the horn 10 along its longitudinal axis.
(34) In the embodiment of FIG. 8A, the plane of the chamfered surface 11 is parallel to the axis through the ion inlet 7. As such, the axial ultrasonic displacement of the surface 11 results in a spray plume with main velocity components that are perpendicular to the axis through the ion inlet 7 (e.g. portion 14 of the plume). This will generally lead to relatively low ion signals, although it will still function as an ion source, particularly when the surface 11 is relatively close to the ion inlet 7 (e.g. within a distance x5 mm).
(35) In the embodiment of FIG. 8B, the plane of the chamfered surface 11 partially faces towards the ion inlet 7 (i.e. the horn 10 is rotated 450 about its longitudinal axis relative to FIG. 8A). The axial ultrasonic displacement of the surface 11 results in a displacement force that is normal to the surface 11. This arrangement therefore results in a spray plume with greater velocity components directed towards the ion inlet 7 (e.g. portion 15 of the plume) and which significantly improves ion signal and hence source sensitivity.
(36) It is known in the field of API sources that highly volatile additives, such as those used in Matrix-Assisted Inlet Ionisation (MAII) sources, can be added to LC solvents to promote ionisation. These additives typically undergo sublimation and display triboluminescence effects such as 3-nitrobenzonitrile. These additives may be used in embodiments of the present invention to enhance ionisation, for example, in regions of high shock such as that which exists at the ultrasonically agitated surface 11.
(37) 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.
