Electron multiplier for mass spectrometer

Abstract

A secondary electron multiplier includes: a conversion dynode for emitting a secondary electron in response to an incident ion; a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; and a first voltage applying device for applying a first negative voltage to the conversion dynode and sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, wherein the secondary electron multiplier is configured to sequentially multiply the emitted secondary electron by the second-stage and subsequent dynodes. In the secondary electron multiplier, any of the second-stage and subsequent dynodes have a second voltage applying device for applying a second negative voltage. The secondary electron multiplier has an improved ion detection efficiency without a large reduction of a usable period thereof, thereby enhancing the sensitivity of a mass spectrometer.

Claims

1. A method for increasing an ion detection efficiency of a secondary electron multiplier, the method comprising the steps of: applying a first negative voltage from a first voltage applying device to a conversion dynode of the secondary electron multiplier to set an amplification gain of the electron multiplier, the conversion dynode configured for emitting a secondary electron in response to an incident ion, wherein the secondary electron multiplier comprises a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, wherein the secondary electron multiplier is configured for sequentially multiplying the emitted secondary electron by the second-stage and subsequent dynodes; applying a second negative voltage from a second voltage applying device separate from the first voltage applying device to independently bias a second negative voltage-applied dynode, wherein the second negative voltage-applied dynode is any of the second-stage and the subsequent dynodes; and subsequent to the applying the first negative voltage and the applying the second negative voltage, changing the first negative voltage applied to the same conversion dynode to increase an ion/electron conversion yield, by increasing an absolute value of the first negative voltage applied to the same conversion dynode; and changing the second negative voltage in a controllable manner that increases a secondary electron emission efficiency at the second negative voltage-applied dynode and a dynode subsequent thereto, and recovers from a reduction of the amplification gain caused by deterioration of the electron multiplier, by increasing an absolute value of the second negative voltage.

2. The method according to claim 1, wherein the second negative voltage-applied dynode is any of second- to fifth-stage dynodes of the plurality of dynodes.

3. The method according to claim 1, wherein the second negative voltage-applied dynode is a third-stage dynode of the plurality of dynodes.

4. A secondary electron multiplier, comprising: a conversion dynode for emitting a secondary electron in response to an incident ion; a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; a first voltage applying device configured for applying a first negative voltage to the conversion dynode and sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, the secondary electron multiplier being configured to sequentially multiply the emitted secondary electron by the second-stage and subsequent dynodes; and a second voltage applying device separate from the first voltage applying device and configured for: applying a second negative voltage to independently bias a second negative voltage-applied dynode, wherein the second negative voltage-applied dynode is any of the second-stage and subsequent dynodes; and changing the second negative voltage to increase a secondary electron emission efficiency at the second negative voltage-applied dynode and a dynode subsequent thereto, and recover from a reduction of the amplification gain caused by deterioration of the electron multiplier, by increasing an absolute value of the second negative voltage.

5. The secondary electron multiplier according to claim 4, wherein the second negative voltage-applied dynode is any of second- to fifth-stage dynodes of the plurality of dynodes.

6. The secondary electron multiplier according to claim 4, wherein the second negative voltage-applied dynode is a third-stage dynode of the plurality of dynodes.

7. The secondary electron multiplier according to claim 4, wherein the first voltage applying device is configured for changing the first negative voltage to increase an ion/electron conversion yield, by increasing an absolute value of the first negative voltage.

8. A secondary electron multiplier, comprising: a conversion dynode for emitting a secondary electron in response to an incident ion; a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; a first voltage applying device configured for: applying a first negative voltage to the conversion dynode and sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, the secondary electron multiplier being configured to sequentially multiply the emitted secondary electron by the second-stage and subsequent dynodes; and changing the first negative voltage to increase an ion/electron conversion yield, by increasing an absolute value of the first negative voltage; and a second voltage applying device separate from the first voltage applying device and configured for: applying a second negative voltage to independently bias a second negative voltage-applied dynode, wherein the second negative voltage-applied dynode is any of the second-stage and subsequent dynodes; and changing the second negative voltage to increase a secondary electron emission efficiency at the second negative voltage-applied dynode and a dynode subsequent thereto, and recover from a reduction of the amplification gain caused by deterioration of the electron multiplier, by increasing an absolute value of the second negative voltage.

9. The secondary electron multiplier according to claim 8, wherein the second negative voltage-applied dynode is any of second- to fifth-stage dynodes of the plurality of dynodes.

10. The secondary electron multiplier according to claim 8, wherein the second negative voltage-applied dynode is a third-stage dynode of the plurality of dynodes.

11. The method according to claim 1, wherein the second negative voltage-applied dynode is a fourth-stage dynode of the plurality of dynodes.

12. The method according to claim 1, wherein the second negative voltage-applied dynode is a fifth-stage dynode of the plurality of dynodes.

13. The method according to claim 1, wherein a difference between the first negative voltage and the second negative voltage controls the electron emission efficiency at the second negative voltage-applied dynode.

14. The secondary electron multiplier according to claim 4, wherein the second negative voltage-applied dynode is a fourth-stage dynode of the plurality of dynodes.

15. The secondary electron multiplier according to claim 4, wherein the second negative voltage-applied dynode is a fifth-stage dynode of the plurality of dynodes.

16. The secondary electron multiplier according to claim 4, wherein a difference between the first negative voltage and the second negative voltage controls the electron emission efficiency at the second negative voltage-applied dynode.

17. The secondary electron multiplier according to claim 8, wherein the second negative voltage-applied dynode is a fourth-stage dynode of the plurality of dynodes.

18. The secondary electron multiplier according to claim 8, wherein the second negative voltage-applied dynode is a fifth-stage dynode of the plurality of dynodes.

19. The secondary electron multiplier according to claim 8, wherein a difference between the first negative voltage and the second negative voltage controls the electron emission efficiency at the second negative voltage-applied dynode.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views.

(2) FIG. 1 is a schematic view showing an exemplary construction of a secondary electron multiplier according to the present invention.

(3) FIG. 2 is a graph indicating voltages of each dynode of the secondary electron multiplier constructed according to the present invention.

(4) FIG. 3 is a graph indicating voltages of each dynode of the secondary electron multiplier of high energy dynode type, to which the present invention is applied.

(5) FIG. 4 is a schematic view showing a basic concept of an exemplary inductively-coupled plasma mass spectrometry.

(6) FIG. 5 is a schematic view of a construction of an exemplary secondary electron multiplier of conventional art.

(7) FIG. 6 is a graph indicating the relationship between the electron energy and the secondary electron emission of a dynode.

(8) FIG. 7 is a graph indicating the relationship between the voltage applied to a conversion dynode and the ion detection sensitivity.

(9) FIG. 8 is a drawing wherein voltages of each dynode of a standard secondary electron multiplier of conventional art are plotted; and

(10) FIG. 9 is a drawing wherein voltages of each dynode are plotted when a negative voltage to be applied to a first dynode is lower (larger in the absolute value of voltage) than an ordinary voltage.

DETAILED DESCRIPTION

(11) Embodiments of the present invention are explained in detail by referring to the accompanying drawings. FIG. 1 schematically shows an exemplary construction of a secondary electron multiplier 100 according to the present invention. A significant difference between this secondary electron multiplier 100 and the above-described conventional secondary electron multiplier 10 (FIG. 5) is that the secondary electron multiplier 100 includes a first voltage applying device and a second voltage applying device. The first voltage applying device may be configured for applying a first negative voltage to the conversion dynode and sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes. The first voltage applying device may include a first power source 85, and a resistance for serially and sequentially connecting each dynode. The second voltage applying device may be configured for applying a second negative voltage to any of the second-stage and subsequent dynodes. The second voltage applying device may include a second power source 110. The constituent elements as in above-mentioned FIG. 5 are denoted by the same reference numerals to omit explanations of the same constituent elements.

(12) In FIG. 1, the secondary electron multiplier 100 of the present invention has the second power source 110 separately from the first power source 85. The second power source 110 can be controlled by a control signal from a controller (not illustrated) in the same manner as the first power source 85, and a voltage output from the second power source 110 may be variable. For example, the second power source 110 may output a voltage in the range from about 500 V to about 3000 V, preferably from about 800 V to about 2500 V. A (second) negative voltage V output from the second power source 110 may be applied to a third dynode dy3 of the secondary electron multiplier 100 independently of divided voltages from the first power source 85. Alternatively, the negative voltage output from the second power source 110 may be applied to not the third dynode dy3 of the secondary electron multiplier but a second dynode dy2, a fourth dynode dy4, or a dynode adjacent thereto (e.g., dy5). Hereafter, explanation will be made on the assumption that the negative voltage V from the second power source 110 is applied to the third dynode dy3.

(13) As described above, the negative voltage V from the second power source 110 in the secondary electron multiplier 100 of the present invention is applied to the third dynode dy3 independent of the first negative voltage V applied to the first dynode dy1. When the negative voltage V is applied to the third dynode dy3, the voltage of dynode dy3 is changed and the voltages of second and subsequent dynodes can be changed. Therefore, in the secondary electron multiplier 100, the negative voltage V applied to the first dynode dy1 controls the ion/electron conversion efficiency, the difference between the negative voltages V and V controls the electron emission efficiency at the second and third dynodes dy2 and dy3, and the negative voltage V controls the electron emission efficiency at fourth and subsequent dynodes, eventually enabling an increase of amplification gain of the secondary electron multiplier 100. For example, when the voltage of the dynode dy3 is decreased (becomes larger in the absolute value), the secondary electron emissions of fourth and subsequent dynodes are increased, and eventually the amplification gain of the secondary electron multiplier 100 is increased. This signifies that a reduction of the negative voltage V can recover the amplification gain that has been decreased by the deterioration of the secondary electron multiplier 100.

(14) As described in the BACKGROUND, there has been a drawback of shortening a usable period of a secondary electron multiplier as a consequence when a negative voltage to be applied to the first dynode dy1 is more reduced (increased in terms of the absolute value of voltage) than an ordinary voltage in order to enhance the ion detection efficiency of the secondary electron multiplier. However, according to the present invention, most of amplification gain of the secondary electron multiplier 100 can be controlled, as described above, by the voltage V of the third dynode dy3 independently of the voltage V of the first dynode. Therefore, even when the secondary electron multiplier 100 of the present invention is used at a negative voltage V applied to the first dynode dy1 lower than the ordinary voltage since an early stage of its use, a reduction of amplification gain caused by the deterioration of the secondary electron multiplier 100 can be recovered by decreasing the voltage V of the third dynode dy3. Accordingly, the secondary electron multiplier 100 of the present invention can increase the ion detection efficiency without a large decrease of usable period. When this secondary electron multiplier 100 of the present invention is used for an ion detector of a mass spectrometer, the sensitivity of the mass spectrometer can be enhanced as a result.

(15) For example, FIG. 2 is a graph, in which voltages of each dynode are plotted when a usable period of a standard secondary electron multiplier, to which the present invention is applied, is divided into three stages of initial, middle and final. Like FIGS. 8 and 9, FIG. 2 is also drawn on the assumption that the secondary electron multiplier 100 has 10 stages of dynodes. In FIG. 2, an initial or middle negative voltage applied to the first dynode dy1 is reduced by about 400 V to about 750 V in comparison with a case for a conventional standard secondary electron multiplier (e.g., FIG. 8). That is, the ion detection efficiency of the secondary electron multiplier 100 of the present invention is more increased than that of a conventional one. At the same time, application of the negative voltage V to the third dynode dy3 can largely change voltages between the dynodes subsequent to the dy3 from the initial to the final stages, and enables a long-term recovery of amplification gain, which has been decreased by the deterioration of the secondary electron multiplier 100.

(16) Thus, in one non-limiting embodiment, a method for increasing an ion detection efficiency of a secondary electron multiplier includes: applying a first negative voltage to a conversion dynode of the secondary electron multiplier, the conversion dynode configured for emitting a secondary electron in response to an incident ion, wherein the secondary electron multiplier comprises a plurality of dynodes configured to have multi-stages from second to final stages for receiving the secondary electron; sequentially dividing the first negative voltage to apply to each of the second-stage and subsequent dynodes, wherein the secondary electron multiplier is configured for sequentially multiplying the emitted secondary electron by the second-stage and subsequent dynodes; reducing the first negative voltage to increase an ion/electron conversion yield; and applying a second negative voltage to any of the second-stage and the subsequent dynodes in a controllable manner to increase or decrease a secondary electron emission efficiency at mid-stage and subsequent dynodes.

(17) The present invention may be applied to a secondary electron multiplier of high-energy dynode type, for example, wherein a voltage of about 10 kV may be applied to a first dynode. In this case, a second negative voltage may be applied to a fourth dynode dy4. Alternatively, the second negative voltage to be applied may be applied to not the fourth dynode dy4 but a third dynode dy3 or a fifth dynode dy5; or it may be applied to 10th dynode when such secondary electron multiplier has about 20 stages of dynodes.

(18) FIG. 3 is a graph, in which voltages of each dynode are plotted in the same manner as in FIG. 2, pertaining to a secondary electron multiplier of high-energy dynode type, to which the present invention is applied. The figure is drawn on the assumption that such secondary electron multiplier has 10 stages of dynodes. In this case, the second negative voltage is applied to a fourth dynode dy4. The ion/electron conversion efficiency at a first dynode is extremely high due to the above-described advantage of a secondary electron multiplier of high-energy dynode type. Then, the first signal amplification section with an extremely low applied voltage (extremely large in the absolute value) and fourth and subsequent signal amplification sections are connected via three stages of dynodes. Thus, the electron accelerating voltage therebetween is dispersed and the secondary electron emission efficiency or the electron amplification efficiency from second to fourth dynodes is not as much reduced as that of a conventional secondary electron multiplier of high-energy dynode type. Therefore, the ion detection efficiency can be enhanced. This advantageous effect is also expected on an element having a low mass number, on which it has been difficult to enhance the sensitivity even in comparison with a standard secondary electron multiplier. Further, the negative voltage V applied to the fourth stage may be used to increase or decrease an amplification gain of the secondary electron multiplier in the same manner as the applied voltage to the second stage in a conventional secondary electron multiplier of high-energy dynode type.

DESCRIPTION OF REFERENCE NUMERALS

(19) 10, 100 Secondary electron multiplier 11 Mass spectrometry 82 Ion detector 85, 110 Power source 90 Signal processing section

(20) It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitationthe invention being defined by the claims.