Guiding spray droplets into an inlet capillary of a mass spectrometer
Charged droplets are guided along a defined path from a droplet source to a droplet sink. A focusing pseudopotential distribution generated by audio frequencies on electrodes of a guiding device guide the charged droplets from the droplet source to the droplet sink with low loss. The droplets can be driven along the droplet guide by a gas flow, an axial electric field or a combination of both. For example, charged droplets from a spray capillary of an electrospray ion source at atmospheric pressure may be introduced into the inlet capillary leading to the vacuum system of ion analyzers, a procedure similar to that used up to now in nanoelectrospraying, but with substantially higher flow rates. In the guiding device, the droplets can be manipulated in different ways, for example evaporated down to a desired size. The introduction of small droplets into gas-aspirating capillaries is of interest because it is possible to keep the droplets on axis by Bernoulli focusing and to guide them in large quantities and with low loss through the capillary. The ability to guide the droplets makes it also possible to install a segmented inlet capillary with intermediate pumping, which allows pumping capacity to be saved. Advantageously, the sensitivity of ion analyzers such as mass spectrometers or ion mobility spectrometers by at least one order of magnitude.
1. A method for transferring charged droplets along a path, comprising:
providing a guiding device including a plurality of electrodes configured as one of a (i) multipole rod system, (ii) a stack of apertured diaphragms and (iii) a pattern of electrodes disposed around the path;
generating a radial pseudopotential distribution by alternating voltages on the electrodes such that the charged droplets are kept radially on the path, wherein the alternating voltages are from the audio-frequency range between approximately 20 and 20,000 hertz, and have an amplitude of between approximately 20 and 3,000 volts; and
transferring the charged droplets along the path from a droplet source through the guiding device to a droplet sink, the droplet sink being a gas-aspirating inlet capillary leading to an ion analyzer operating in a vacuum, wherein the charged droplets are introduced into the inlet capillary, and wherein substantially no ions enter the inlet capillary since most of them are drawn to the electrodes of the guiding device and discharged.
2. The method of claim 1 , further comprising generating a gas flow profile and an electric field profile along the path in the guiding device, and selecting a transfer velocity dependent on mobility of the charged droplets.
3. The method of claim 2 , where direction and the profile of the electric field and direction and the profile of the gas flow in the guiding device are configured such that larger droplets with higher mobility remain longer in the guiding device than smaller droplets.
4. The method of claim 3 , further comprising:
heating the gas flow such that the charged droplets evaporate down to a desired size within the gas flow without evaporating them completely; and
providing charged droplets with substantially equal diameters from the guiding device.
5. The method of claim 1 , further comprising generating a two-dimensional quadrupole alternating field with the electrodes.
6. The method of claim 1 , further comprising selecting frequency and amplitude of the alternating voltage at the guiding device such that the charged droplets are subject to a shaking motion that causes larger droplets to vaporize and disintegrate down to a desired size faster than smaller droplets without evaporating them completely.
7. The method of claim 1 , wherein the droplets in the droplet guide are irradiated with at least one of infrared light, ultrasound and microwaves.
8. The method of claim 1 , further comprising generating the charged droplets with an electrospray ion source.
9. An apparatus for transferring charged droplets along a path, comprising:
a droplet source;
a electrospray ion droplet guide including a plurality of electrodes that generate a two-dimensional multipole field on an axis of the droplet guide; and
a gas-aspirating inlet capillary leading to an ion analyzer in a vacuum;
an audio AC voltage generator supplying the electrodes of the droplet guide with AC voltages having a frequency of between approximately 20 and 20,000 hertz and an amplitude of between approximately 20 and 3,000 volts so that the droplets generated by the droplet source can be transferred through the droplet guide to the inlet capillary where they enter the inlet capillary, wherein substantially no ions enter the inlet capillary since most of them are drawn to the electrodes of the droplet guide and discharged.
10. The apparatus of claim 9 , wherein the two-dimensional multipole field generated by the electrodes is a two-dimensional quadrupole field.
11. The apparatus of claim 9 , where the droplet guide includes an insulating tube, and where the electrodes are disposed on an inner surface of the insulating tube.
12. The apparatus of claim 11 , wherein the electrodes in the insulating tube form a pattern that generates both a radial two-dimensional quadrupole alternating field and an axial DC field profile.
13. The apparatus of claim 9 , further comprising a gas supply to supply gas to the droplet guide.
14. The apparatus of claim 9 , where the droplets are generated by a spray capillary of an electrospray device.
15. The apparatus of claim 9 , wherein the droplets exit a first segment of a segmented inlet capillary, and are transferred by the droplet guide to a next segment of the segmented inlet capillary.