Radiant energy – Ionic separation or analysis – With sample supply means
Reexamination Certificate
2002-01-02
2004-03-02
Lee, John R. (Department: 2878)
Radiant energy
Ionic separation or analysis
With sample supply means
Reexamination Certificate
active
06700119
ABSTRACT:
BACKGROUND
The present invention relates to an ion source for a mass analyser, and particularly to an ion source which operates at atmospheric pressure.
Mass spectrometers normally operate at low pressure, for analysing materials such as organic substances. To permit mass analysis, ions of the material under investigation must be generated. It is particularly desirable for biological substances that the ion source operates at atmospheric pressure.
The first stage in this type of material analysis is typically to pass the material through a chromatograph. Depending upon the application, it is possible to use either gas chromatography (GC) or liquid chromatography (LC). The present invention is particularly concerned with LC.
The next stage in the analysis is to generate a source of ions from the LC eluent.
Several atmospheric pressure ion sources for doing this are known. The electro-spray ionization (ESI) source typically consists of a small tube or capillary through which a sample liquid consisting of the LC eluent 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 vaporize 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; in both cases an aerosol is generated in the atmospheric pressure region.
Common to all atmospheric pressure ionisation (API) sources for mass spectrometers is an ion inlet orifice that forms an interface between the API region and the low pressure region of the source or mass analyser. This orifice is generally of necessity small (typically less than 0.5 mm in diameter) to allow the vacuum system attached to the mass analyser to maintain a satisfactory vacuum (1 mpa or less) therein at a finite pumping speed.
In recent years, there has been a tendency for the API source of commercial LC mass spectrometers to be arranged orthogonally of the ion inlet orifice. This is because of the improved tolerance to involatile components in the LC eluent with this geometry.
One particular problem with known API sources is their relative inefficiency. Even a very good known LC mass spectrometer has an efficiency of only about 10
−6
, when considering the total ion signal theoretically available from the analyte in the liquid phase compared with the eventual ion signal received at the detector of the mass spectrometer. The reasons for this are believed to include incomplete ionisation of the analyte, incomplete desolvation (wherein some ions remain in the liquid phase within the aerosol generated by the API source) and transmission losses through the ion source and mass analyser.
U.S. Pat. No. 5,756,994 shows one particular implementation of an LC source. As seen in FIG. 1 of that patent, the LC source consists of an ion block having an entrance chamber and an evacuation port connected by a smaller diameter extraction chamber. Ions in the atmospheric pressure region pass into the entrance chamber through the entrance cone and are carried by a high velocity viscous jet from the entrance chamber through the extraction chamber and into the evacuation port. A second, exit cone is located within a conical recess in the ion block so that its apex lies flush with the core of the extraction chamber. The exit cone is electrically insulated from the ion block by means of an insulating ring. A voltage is applied between the exit cone and ion block and as a result a proportion of ions are extracted from the jet in the transfer lens.
This arrangement suffers from a number of drawbacks. Firstly, due to the rapid expansion of the incoming gas, the jet undergoes considerable cooling and in an attempt to combat this problem a considerable heat input must be applied to the ion block to promote desolvation and prevent the formation of solvent cluster ions. The heater in turn introduces considerable cost to the API source assembly as a result not only of the heater itself, but also the thermocouple, necessary electrical connections, associated power supplies and control electronics. In addition, to prevent excessive thermal losses from the ion block due to conduction, the ion block must be mounted on an insulating filled PTFE block such as PEEK which is also expensive and, moreover, is not totally compatible with API sources.
Another orthogonal API source has been proposed in GB-A-2,324,906. The device described therein requires no electrostatic field for ion extraction as the entrance cone, ion block and exit cone are held at the same potential. As seen in
FIG. 1
of this document, the incoming expanding jet impinges directly onto a disrupter pin, which increases the turbulence of the flow. This also serves to increase the internal energy of the gas stream and in doing so promotes desolvation and prevents solvent cluster formation. Thus the disrupter pin performs the same function as the ion block heater employed in the device of U.S. Pat. No. 5,756,994, but without the associated hardware costs. Additionally, the internal geometry of the ion block in GB-A-2,324,906 is designed such that the apex of the exit cone resides within an eddy of the viscous gas flow path (see
FIG. 1
thereof). Ions then have an increased probability of passing through the exit cone. Thus the arrangement of GB-A-2,423,906 provides a similar overall ion transmission efficiency to the arrangement described in U.S. Pat. No. 5,756,994. Furthermore, because the probe described in GB-A-2,324,906 may be orientated orthogonally to the optical axis of the instrument in an horizontal plane, a neater and more compact source design is possible.
However, the arrangement shown in GB-A-2,324,906 requires the source region to be operated with a relatively high pressure inside the ion volume, typically of order 1.5 kPa (15 mbar), for efficient operation. This is an important consideration as the increased source pressure results in an associated higher gas throughput into the intermediate and analyser vacuum regions. For a given pumping system this results in correspondingly higher pressures in the two regions. High analyser pressures may result in ion signal loss and higher background noise levels. Thus a pump with higher pumping speed and thus higher cost must be employed to gain the required vacuum.
It is an object of the present invention to address these and other problems associated with the prior art.
SUMMARY
According to the present invention, there is provided an ion source for a mass spectrometer which operates at a low pressure comprising:
an atmospheric pressure sample ioniser operable at atmospheric pressure to provide a sample flow containing desired sample ions;
an interface chamber having an entrance aperture, an exit aperture and an exhaust port, the entrance aperture being arranged to receive sample ions provided by the atmospheric pressure sample ioniser entrained in a gas flow, and the exit aperture being arranged for sample ions to exit the interface chamber to the mass spectrometer; and
a vacuum pump in communication with the exhaust port of the in
Gurzo Paul M.
Haynes and Boone, LLP.
Lee John R.
Thermo Finnigan LLC
LandOfFree
Ion source for mass analyzer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ion source for mass analyzer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ion source for mass analyzer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3213501