Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
1999-09-16
2002-10-08
Anderson, Bruce (Department: 2881)
Radiant energy
Ionic separation or analysis
With sample supply means
C250S281000, C250S282000
Reexamination Certificate
active
06462336
ABSTRACT:
The invention relates to an ion source for a mass spectrometer and to a method of providing a source of ions for analysis. Mass spectrometers normally operate at low pressure and the present invention is particularly concerned with an ion source which operates at atmospheric pressure. Such ion sources include electrospray ion sources and atmospheric pressure chemical ionisation (APCI) ion sources.
Mass spectrometers have been used to analyse a wide range of materials, including organic substances, such as pharmaceutical compounds, environmental compounds and biomolecules. For mass analysis, it is necessary to produce ions of such sample compounds and biomolecules. Of particular use in the study of biological substances are mass spectrometers which have ion sources for creating ions of the sample compounds, where such ion sources operate at atmospheric pressure.
One such ion source is the electrospray ionisation (ESI) source which typically consists of a small tube or capillary through which a sample liquid is flowed. The sample liquid comprises the sample compounds and molecules to be analysed contained in a solvent. The capillary is maintained at a high potential difference relative to an adjacent surface. The liquid emerges from the tube and disperses into fine ionised droplets as a consequence of the high electric field at the tip of the capillary. The droplets are then desolvated by heating them to evaporate the solvent. Eventually, the ionised droplets become so small that they are unstable, whereupon they vaporise to form gaseous sample ions.
Another form of atmospheric pressure ion source is the atmospheric pressure chemical ionisation (APCI) ion source which uses a heated nebulizer to convert droplets of sample solution into the gaseous phase before ionisation. A corona discharge electrode is located adjacent to the nebulizer outlet. This ionises the surrounding gas and the nebulized solvent molecules. Since sample molecules generally have greater proton affinity than solvent molecules, collisions between them result in preferential ionisation of the sample molecules. In this way, gaseous sample ions are produced. ESI and APCI are complementary techniques, in that ESI is limited to charged or polar compounds, whereas APCI can be used for less polar compounds.
One problem with any technique involving droplets of sample solution is that despite the use of desolvation techniques, undesolvated droplets, dust and neutrals can enter the spectrometer producing a noise signal at the detector. Such particles can be prevented from entering the vacuum system of the mass spectrometer by, for example, using an opposing flow of dry gas ( e.g. nitrogen). However this solution is cumbersome and complex and involves the provision of a gas flow system and a supply of expensive pure gas.
Another approach is shown in U.S. Pat. No. 5,171,990, which shows an electrospray ion source in which the spray is directed off axis so that undesolvated ions do not enter the vacuum system. Similarly, U.S. Pat. No. 4,861,988 shows an electrospray ion source wherein the axis of the spraying capillary is offset from the axis of the sampling orifice to prevent sampling of large cluster ions.
U.S. Pat. No. 5,495,108 shows an electrospray/APCI mass spectrometer with orthogonal sampling to reduce vapour in the vacuum system and resultant noise. The spray is directed transversely across the sampling orifice, desolvated ions being electrostatically attracted into the mass spectrometer while solvent vapour and undesolvated ions to not enter the spectrometer region.
Van der Hoeven et al in J. Chromatog. A Vol. 712 (1995) pp. 211-218 discuss an electrospray interface adapted from a thermospray source, in which the longitudinal axis of the electrospray needle assembly, the entrance to the electrospray interface and the outlet to the vacuum pump which evacuates the interface are disposed generally along a first axis, the longitudinal axis of the mass spectrometer forming a second axis which is disposed transversely to the first axis, an electrostatic repeller electrode also being disposed along the second axis and directly opposite the entrance to the mass spectrometer. Neutrals and undesolvated ions should therefore tend to be evacuated directly by the pump, only required desolvated ions tending to be repelled into the mass spectrometer.
A disadvantage of any ion source system (such as U.S. Pat. Nos. 5,171,990 and 4,861,988) in which the sprayed sample is directed generally towards the spectrometer entrance, and there is a line of sight path between the location of ion generation and the entrance, is that some undesired particles can still enter the spectrometer. A further problem associated with line of sight sources is that of streaming. This is a consequence of the fluid dynamics of the system. When a gas flows through an aperture from an area of high pressure into an area of low pressure, a so called “Zone of silence” forms around and downstream of the aperture. Inside this zone the gas molecules acquire a high velocity, the molecules following straight streamlines with the highest intensity being along the aperture axis. The closer the spectrometer entrance is to this axis, such as in a line of sight source, the more gas will stream through directly into the spectrometer, increasing the load on the vacuum system within the spectrometer.
On the other hand, in those systems, (e.g. Van der Hoeven et al) in which the sample ions are directly generally transversely to the spectrometer entrance, an electrostatic repeller is required to deflect desolvated ions into the Sass spectrometer.
In one aspect, the present invention provides an ion source for a mass spectrometer which operates at a low pressure comprising an atmospheric pressure sample ioniser operative at atmospheric pressure to provide a sample flow containing desired sample ions entrained with undesired gas and droplets, an interface chamber having an evacuation port, and a vacuum pump connected to the evacuator port to hold the interface chamber at a pressure intermediate atmospheric pressure and the operating pressure of the mass spectrometer, the interface chamber having an entrance orifice located to collect desired sample ions with entrained gas and droplets into the interface clamber from said sample flow of said sample ioniser said entrance orifice having a flow axis and forming a stream of gas into said interface chamber along said flow axis, and an exit orifice for sample ions to exit the interface chamber to the mass spectrometer, wherein the interface chamber is arranged to disrupt said stream of gas to provide a dead region within said chamber of no net gas flow direction and said exit orifice is located in said dead region.
Preferably, the interface chamber has a flow disrupting surface intersecting the flow axis of the entrance orifice. Then the interface channel may form a flow channel between the entrance orifice and the evacuation port and said flow disrupting surface is provided by a flow disrupting member in said flow channel.
The invention also provides an ion source for a mass spectrometer which operates at low pressure comprising an atmospheric pressure sample ioniser operative at atmospheric pressure to provide a sample flow containing desired sample ions entrained with undesired gas and droplets, an interface chamber having an evacuation port, and a vacuum pump connected to the evacuation port to hold the interface chamber at a pressure intermediate atmospheric pressure and the operating pressure of the mass spectrometer, the interface chamber having an entrance orifice located to collect desired sample ions with entrained gas and droplets into the interface chamber from said sample flow of said sample ioniser and an exit orifice for sample ions to exit the interface chamber to the mass spectrometer, wherein there is no line of sight path in the interface chamber between the entrance orifice and the exit orifice. Alternatively, the exit orifice may be in line of sight with said entrance orifice, wherein the line of sight is at leas
Anderson Bruce
Haynes and Boone LLP
Masslab Limited
Wells Nikita
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