GB2146170A - Ion source for mass spectrometer - Google Patents

Abstract

An ion source of chemical ionization type for a mass spectrometer or the like has an ionization chamber which receives a reactant gas via openings together with a sample gas via opening 4. The reactant gas which is irradiated with an electron beam reacts with the sample gas to ionize the sample gas. The ionization chamber has a separate exhaust opening 28 so that the chamber may be evacuated independently of the electron beam irradiation and the take out of ions. This permits supply of a large quantity of reactant gas and facilitates controlling the pressure inside the ionization chamber. Combinations of ion source, liquid chromatograph and mass spectrometer are detailed (eg Fig. 5). The pressure may be automatically controlled (Fig. 6). Chemical and electron impact ionization are specified. Valve 9 conductance changes due to vibration or temperature variations are countered. <IMAGE>

Description

SPECIFICATION lon source for mass spectrometer or the like The present invention relates to an improved ion source adapted to be used in a mass spectrometer and, more particularly, to an improved ion source utilizing chemical ionization.

Reference will now be made to Figs. 1 to 3 of the accompanying drawings, of which: Figure 1 is a schematic diagram of a conventional ion source of chemical ionization type; Figure 2 is a schematic diagram of the main portions of a conventional instrument in which a liquid chromatograph and a mass spectrometer are combined in one unit; and Figures 3(a) and 3(b) are graphs showing the manner in which the peak intensity of the background of mass spectra obtained by the instrument of Fig. 2 changes as the pressure inside the ionization chamber varies.

Referring to Fig. 1 there is shown a conventional ion source of the chemical ionization (Cl) type, where the ion source is surrounded by a housing 1. The inside of the ion source is evacuated by an oil diffusion pump 2, and an ionization chamber 3 is placed within the source. The side wall of the chamber 3 is provided with a sample inlet opening 4, a reactant gas inlet opening 5, an electron inlet opening 6, and an ion exit opening 7. Also mounted in the ion source are a sample inlet tube 8, a needle valve 9, a reactant gas supply tube 10, a filament 11 for producing electrons, and electrodes 1 2 for focusing ions.

In this structure, the reactant gas which has been introduced in the ionization chamber 3 through the inlet opening 5 is ionized by the electrons that are admitted through the inlet opening 6. Then, a sample is inserted into the chamber through the inlet opening 4 to cause proton transfer reaction with the reactant gas ions, thus ionizing the sample.

In the ion source of the chemical ionization type, the optimum pressure inside the ionization chamber is considerably strictly determined by the kind of the reactant gas. If the pressure actually obtained deviates from the optimum pressure at all, the efficiency of ionization deteriorates greatly, decrease the quantity of ions available. Accordingly, the pressure inside the ionization chamber must be very accurately adjusted and set.For this reason, in the conventional instrument described above, the quantity of leakage of the reactant gas from the electron inlet opening 6 and from the ion exit opening 7 is reduced to a minimum by setting the diameter of these openings 6 and 7 at 500 ym or so, for example, in order to keep the pressure inside the ionization chamber 3 at a value best suited to the ionization, for example 1 Torr, and to maintain the inside of the housing of the ion source in a condition of a sufficiently high vacuum. Thus, the quantity of reactant gas supplied into the chamber 3 is made as small as 0.1 to 0.4 ml/min. This requires that the needle valve 9 be accurate enough to control the minute quantity of gas. Hence, a slight change in the conductance of the valve 9 results in a great change in the pressure inside the ionization chamber 3.Accordingly, the human operator has been required to adjust the valve with meticulous care.

Referring next to Fig. 2, a conventional instrument in which a liquid chromatograph and a mass spectrometer are combined in a unit is schematically shown. This combination instrument includes the high performance liquid chromatograph 1 3 and an interface 14 connecting the ion source of the mass spectrometer to the chromatograph. The housing of the mass spectrometer is indicated by reference numeral 1. The interior of the interface is evacuated by a vacuum pump 1 5. The effluent, i.e., the liquid which emerges from the chromatograph 13, is supplied into a nozzle 1 9 mounted in the interface via pipes 16 and 17 and a splitter 18. The liquid is then emitted from the nozzle 1 9 as a jet of droplets toward a receiver nozzle 20 disposed opposite to the nozzle.The droplets travelling through the receiver nozzle 20 are vaporised since they are heated by a heater 21. Then, the resultant vapor is carried into an ionization chamber 23 formed in an ion source block. A filament 25 emits an electron beam which is introduced into the ionization chamber 23 through an electron inlet opening 24. The generated ions are sent to the mass analyser not shown) through an ion exit opening 26.

The ion source described just above is an ion source of the chemical ionization type that operates on the same principle as the ion source already described in connection with Fig. 1, and it uses the solvent included in the effluent from the liquid chromatograph 1 3 as a reactant gas to cause ionization. Specifically, the effluent containing the solvent is transported into the ionization chamber through an inlet opening 27, and then the reactant gas, or the solvent, is ionized by the electrons which have been introduced through the inlet opening 24 from a filament 25. Thereafter, proton transfer reaction takes place between the resultant ions and the effluent in a gaseous state, thus ionizing the effluent. Then, the produced ions are moved through the exit opening 26 into the mass analyser (not shown) for analysing purposes.The prior art instrument constructed as described above has the disadvantage that the pressure within the ionization chamber inevitably varies over a range considerably greater than the optimum range on account of changes in the flow of effluent supplied from the chromatograph to the ionization chamber or for other cause.

Figs. 3(a) and 3(b) show the peak intensity of the background of each mass spectogram against the pressure inside the ionization chamber. The measurements were carried out by the present inventors using the instrument shown in Fig. 2, where the liquid chromatograph is connected to the ion source of the mass spectrometer. As can be seen from the graphs of Figs. 3(a) and 3(b), as the pressure changes, the efficiency of ionization alters, resulting in changes in the quantity of ions produced. This creates changes in the peak intensity, leading to variations in the base line drawn on the chromatogram or splitting of the spectral patterns of the mass spectra obtained.

Thus, difficulties have been encountered in analysing the data which is obtained by means of the mass spectrometry.

It is a main object of the present invention to provide a mass spectrometer having an ionization chamber which permits its inner pressure to be controlled with ease.

It is another object of the invention to provide a mass spectrometer having an ionization chamber in which changes in the pressure inside the chamber are kept within a narrow range.

It is a further object of the invention to provide a mass spectrometer having an ionization chamber which allows a large quantity of sample to be supplied into it.

According to the invention there is provided an ion source for mass spectrometry or the like and having an ionization chamber, means for introducing ionizable material into the ionization chamber, means for causing an electron beam to enter the ionization chamber, and means for taking the ions produced in the ionization chamber out of the chamber, wherein there is an exhaust opening in the ionization chamber coupled to means for evacuating the ionization chamber.

The invention will further be described with reference to Figs. 4 to 6 of the accompanying drawing, of which: Figure 4 is a schematic diagram of an ion source of chemical ionization type according to the present invention; Figure 5 is a schematic diagram of an instrument, in which a liquid chromatograph and a mass spectrometer are combined in one unit, embodying the present invention; and Figure 6 is a diagram similar to Fig. 5 but showing another example of the instrument according to the invention.

Refeering to Fig. 4, the structure of an ion source of chemical ionization type embodying the concept of the present invention is schematically shown, and in which the same components as those of Fig. 1 are given the same reference numerals as in Fig. 1. The novel structure is similar to the ion source of Fig. 1 except that the side wall of the ionization chamber 3 is provided with a novel exhaust opening 28 exhibiting a relatively large conductance and that this opening 28 is connected to a vacuum pump 30, such as a rotary pump, via an exhaust pipe 29.This structure has the advantage that the pressure inside the ionization chamber 3 can be kept at a value best suited to ionization even if the quantity of reactant gas supplied into the chamber 3 through the needle valve 9 Is increased by a factor of ten or more. because the interior of the chamber 3 is evacuated by the pump 30 through the exhaust opening 28. Further. the inside of the envelope of the ion source can be maintained at a sufficiently high vacuum. Therefore, the flow of reactant gas can be changed by the needle valve 9 over a broader range. whereby the ion source can be operated quite efficiently. In addition, by increasing the flow of reactant gas in this way, the effect of changes in the conductance of the needle valve 9 due to vibration or temperature variations upon the pressure inside the ionization chamber 3 is minimized.

Thus, changes in the quantity of ions produced, which arise from the changes in the pressure and will cause variations in the peak intensity obtained from the mass spectrometer, can be prevented. The instrument of Fig. 4 can further be equipped with a flow control means, such as a needle valve, in the exhaust pipe 29 so that the pressure in the ionization chamber can be controlled by this flow control means.

Referring next to Fig. 5, there is shown an instrument according to the invention in which a liquid chromatograph and a mass spectrometer are combined in one unit. In Fig 5, the same components as those in Fig. 2 are indicated by the same reference numerals as in Fig. 2. This instrument is characterised in that the ionization chamber 23 is provided with a novel exhaust opening 31 which is connected to a vacuum pump 33, such as an oil rotary pump, via an exhaust pipe 32.

Although the sample inlet opening 27 also serves to permit transfer of the reactant gas into the ionization chamber in the same way as the instrument of Fig. 2, the greater part of the gas is rapidly exhausted by the vacuum pump. Consequently, it is possible to keep the inside of the ionization chamber at a pressure optimal for ionization even if a large quantity of sample gas and reactant gas is supplied than conventional. Additionally, the gas introduced into the ionization chamber will not remain in it for long, thus improving the response of the chromatograph.

In the instrument of Fig. 5, the reactant gas necessary for the ion source of chemical ionization type is produced by vaporising the solvent contained in the solution emerging from the high performance liquid chromatograph. Obviously, in case where the solvent in the chromatograph cannot be employed as the reactant gas, the ionization chamber must be equipped with a means to introduce the reactant gas. It is also possible to control the pressure inside the ionization chamber by providing a flow control means in the exhaust pipe 32 and adjusting this control means.

Referring next to Fig. 6, there are shown the main portions of another example of the invention, and in which the interface and the liquid chromatograph are omitted. Inserted in the exhaust pipe 32, which connects the ionization chamber 23 to the vacuum pipe 33, and a fixed flow-impeder 34 having a fixed conductance and a flow-controlling solenoid valve 35 for controlling the conductance of the exhaust pipe 32. A vacuum gauge 36 is connected to the ionization chamber 23 via a communication pipe 37. The output delivered by the gauge is fed to the solenoid valve 35 via a differential amplifier 38. A reference power supply 39 applies a reference voltage to the other input terminal of the differential amplifier 38.

It is now assumed that the qptimum value of the pressure inside the ionization chamber in the structure described above is 1 torr. The value of the reference voltage is set corresponding to this value of 1 torr. As long as the pressure inside the ionization chamber is kept at this value, the conductance of the solenoid valve 35 is maintained substantially constant. If the effluent from the liquid chromatograph increases in flow thus to increase the inner pressure to 1.5 torr, for example, then the output from the vacuum gauge 36 changes accordingly. This causes the input applied to the solenoid valve 35 from the differential amplifier 38 to vary accordingly.

Then, the valve 35 increases its conductance, resulting in an increase in the rate at which the gas is evacuated from the chamber. As a result, the pressure inside the ionization chamber decreases back to the original value of 1 torr. Inversely, if the pressure decreases, the conductance of the valve 35 is reduced thereby to reduce the rate at which the gas is evacuated, then the pressure is increased up to the original value. Thus, the pressure is invariably controlled by the feed back control system. Hence, the pressure inside the ionization chamber is always held constant independently of changes in the flow of the effluent from the chromatograph. Accordingly, the output from the mass spectrometer will not be adversely affected by the changes in the flow.

Further, it is possible to set the pressure inside the ionization chamber equal to the optimum value depending on the kind of reactant gas by appropriately setting the reference voltage and arbitrarily setting the conductance of the solenoid valve 35.

In the instrument of Fig. 6, the pressure inside the ionization chamber is directly detected by the vacuum gauge. Usually, a liquid chromatograph is equipped with a sensor to detect the pressure of liquid transported. In case where the output from the sensor is judged to correspond to the pressure inside the ionization chamber, the output from the sensor may be supplied to the differential amplifier 38. In this case, the vacuum gauge can be dispensed with, contributing greatly to a reduction in the cost to manufacture the instrument.

As thus far described, the present invention prevents the pressure inside the ionization chamber of the combined liquid chromatograph/mass spectrometer from varying to always maintain it in a range optimum for ionization. Consequently, it is possible to prevent undesirable phenomena including variations in the base line of chromatograph and of mass spectrum and introduction of noise into the peaks.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that changes and variations may be made without departing from the spirit of the invention. For example, the invention is not limited to the ion source utilizing only chemical ionization as mentioned above, but rather it can also be applied to an ion source which can make use of both chemical ionization and electron impact ionization. Further, although the exhaust opening 28 or 31 that opens into the ionization chamber is disposed opposite to the reactant gas inlet opening 5 (or 27 in the embodiment of Fig. 5) to provide the maximum efficiency of exhaust, if this arrangement imposes some restrictions, they may be disposed at a suitable angle to each other.

Claims (9)

1. An ion source for mass spectrometry or the like and having an ionization chamber, means for introducing ionizable material into the ionization chamber, means for causing an electron beam to enter the ionization chamber, and means for taking the ions produced in the ionization chamber out of the chamber, wherein there is an exhaust opening in the ionization chamber coupled to means for evacuating the ionization chamber.

2. An ion source as claimed in claim 1 wherein the means for introducing ionizable material comprises means for introducing a sample into the ionization chamber and means for introducing a reactant gas into the ionization chamber through a flow control means.

3. An ion source as claimed in claim 1 wherein the means for introducing ionizable material comprises means for vaporising the effluent from a liquid chromatograph and for introducing the resultant vapor into the ionization chamber.

4. An ion source as claimed in any of the preceding claims wherein there is provided means for varying the rate of the evacuation made by the evacuating means.

5. An ion source as claimed in claim 4 and pressure-detecting means for delivering an output corresponding to the pressure inside the ionization chamber, and means for controlling the evacuation rate varying means according to the output from the pressuredetecting means.

6. An ion source as set forth in claim 5, wherein the evacuation rate varying means consists of a solenoid valve.

7. An ion source as set forth in claim 5, wherein the pressure-detecting means consists of a vacuum gauge connected to the ionization chamber.

8. An ion source as set forth in claim 5, wherein the pressure-detecting means consists of a sensor for detecting the pressure of liquid transported by the liquid chromatograph.

9. An ion source substantially as hereinbefore described with reference to Fig. 4 or Fig.

5 or Fig. 6 of the accompanying drawings.