GB2133924A

GB2133924A - Mass spectrometry

Info

Publication number: GB2133924A
Application number: GB08301188A
Authority: GB
Inventors: Motohiro Naito
Original assignee: Jeol Ltd
Current assignee: Jeol Ltd
Priority date: 1983-01-17
Filing date: 1983-01-17
Publication date: 1984-08-01
Also published as: GB2133924B; GB8301188D0

Abstract

A mass spectrometer comprises an ion source 1, a first field eg. a double focussing arrangement 2, 3 as shown, which selects ions according to their mass-to-charge ratio, a field- free region 5 in which daughter ions are formed from parent ions by ion dissociation or ion fragmentation and superimposed electric and magnetic fields 6, 8 at right angles forming a second field, the arrangement being such that the intensity of said magnetic field is changed from a first value during a first stage to a second value during a second stage, while the intensity of said electric field is swept during each stage. Both the energy and mass of the daughter ions can be measured by this mass spectrometer arrangement and a detailed mathematical analysis of the procedure is given. The field-free region 5 may comprise a helium-filled collision chamber. <IMAGE>

Description

SPECIFICATION Mass spectrometer and method of mass spectrometry This invention relates to a mass spectrometer and method of mass spectrometry for measuring both energy and mass of daughter ions formed by dissociation or fragmentation of parent ions.

It is known that the measurement of ion dissociations or ion fragmentations is effective for analyzing constitutive properties of organic compounds or organic mixtures.

In the conventional apparatus for measuring ion dissociations or ion fragmentations, ions produced and accelerated by an ion source are selected according to their mass-to-charge ratio by a first field such as a magnetic field or doublefocussing field. The mass-selected parent ions, which have a desired m/e ratio, are directed to a collision chamber in a field-free region and collide with inert gas, such as helium gas, supplied in the chamber, so that some of the parent ions form daughter ions. Daughter ions are also formed in the field-free region without the collision chamber by the self ion fragmentation reaction of the parent ions. The daughter ions are directed to a second field such as an electric field or doublefocussing field. In the case of using an electric field as the second field, the daughter ions can be selected according to their kinetic energy precisely.However, the spectrum width of the kinetic energy of any daughter ion is broad, since a part of the internal energy of the parent ion is released at the dissociation. Therefore, the mass of the daughter ion can not be measured with high resolution.

In the case of using the double-focussing field as the second field, the mass spectrum of the daughter ions can be obtained with high resolution, since the daughter ions are doublefocussed by the field. However, daughter ions having the same mass but various kinetic energies are focussed on an ion collector at the same time under certain field conditions.

Therefore, the kinetic energy spectrum of the daughter ions can not be obtained in this case.

The object of the present invention is to provide a mass spectrometer and method of mass spectrometry for measuring both the kinetic energy and mass of daughter ions.

How the aforegoing object is attained will become apparent by reading through the following systematic exposition. According to one aspect of the invention there is provided a mass spectrometer comprising an ion source, means for selecting ions produced and accelerated by said ion source according to their mass-to-charge ratio, a field-free region in which some of the mass selected parent ions form daughter ions, means for creating superimposed electric and magnetic fields at right angles through which the ions from said field-free region pass, means for establishing two stages during which the intensity of said magnetic field is set at a first value and then at a second value respectively, and means for sweeping the intensity of said electric field during each stage.

According to another aspect of the invention there is provided a method of analysis by mass spectroscopy, the method consisting in generating ions in an ion source; selecting ions from those generated by virtue of their mass-tocharge ratio: passing the selected ions as parent ions through a field-free region; forming daughter ions from some of the parent ions; passing the parent and daughter ions through superimposed electric and magnetic fields at right-angles to each other; controlling the electric and magnetic fields in two stages: (a) setting the magnetic field to a first value while sweeping the electric field: and (b) setting the magnetic field to a second value while sweeping the electric field; and deriving signals from the ions according to their deflection by the fields.

The invention will further be described with reference to the accompanying drawings, of which: Figure 1 shows one embodiment of this invention; Figure 2 shows a cross-sectional view of the embodiment shown in Figure 1 through A-A; and Figure 3 shows the relation between Vd and M.

Referring to Figure 1 and Figure 2,1 is an ion source, for producing and accelerating sample ions. The accelerating ions are selected according to their mass-to-charge ratio by a doublefocussing mass spectrometer comprising an electric field 2 and a magnetic field 3. The massselected parent ions are directed to a collision chamber 5 in a field-free region, to which helium gas is supplied. Some of the parent ions collide with the helium gas and form daughter ions by the collision-induced dissociation. The mixed ions including the remaining parent ions and the daughter ions are directed to a mass spectrometer having superimposed electric and magnetic fields. The details of this type of mass spectrometer are described in British Patent No.

1,549,219. This mass spectrometer comprises magnetic pole pieces 6 and 6' which create the magnetic fields, electrodes 8 and 8' which create the toroidal electric field almost at right angles to said magnetic field, and two flat auxiliary electrodes 10 and 10' known as Matsuda plates which are arranged between said magnetic pole pieces 6 and 6' and interpose said electrodes 8 and 8'. The intensity of said magnetic field created between said pole pieces is changed by a power source 7. The intensity of electric field created by said electrodes is swept by a variable power source 9. A compensating voltage is fed into said auxiliary electrodes 10 and 10' from a power source 11, the compensating voltage being varied in proportion to the square of the sweep signal voltage from said sweep power source 9.

There is a slit in a baffle 12 beyond which an ion collector 1 3 is arranged. The ion collector output signal is fed into a computer 14 via an A-D converter 15. The computer 14 controls said power source 7 and 9 and carries out data processing based on the ion collector output signals. The results of the data processing are fed into a recorder 16.

In the above described arrangement, the mixed ions, after being passed through the field-free region, are introduced into said superimposed field created by the pole pieces 6 and 6' and the electrodes 8 and 8'. The voltage fed into said electrodes from said variable power source 9 is swept with the magnetic field being set at a first fixed intensity. At that time, the compensating voltage proportional to the square of the sweep signal voltage from the power source 9 is applied to the auxiliary electrodes 10 and 10' from the power source 11. Thus, there is no focal length aberration over the entire superimposed field, and at each sweep value of the electric field, ions introduced into the superimposed field are deflected in accordance with the m/e ratio, pass through the slit 12 and are detected by the ion collector 13.The detected signal is fed into the computer 14 via the A-D converter 15 and is memorized in accordance with the sweep voltage.

After the first sweeping of the electric field voltage, the computer controls the power source 7 so as to change the intensity of the magnetic field created by the pole pieces 6 and 6'. Then the electric field voltage is swept again and the detected signals are fed into the computer 14 from the ion collector 1 3. The signals recorded by the recorder 16 allow values for both the energy and the mass of the daughter ions to be obtained.

The present invention is characterised by equipping a mass spectrometer with a superimposed field, and makes it possible to determine both the mass and energy of the daughter ions by varying the strength of the magnetic field constituting the superimposed field in two steps and sweeping the m/e ratio by sweeping the voltage of the electric field constituting the superimposed field at both steps.

Details should be made clear by the following description.

The basic relationship for a mass spectrometer with a superimposed field is M=MJ1Vd/Vo)2 (1) where M=the m/e ratio of the ions measured, Vd=the electric field voltage applied between electrodes 8 and 8', M=the m/e ratio of the ions measured at Vd=O and V=the electric field voltage for measuring the ions whose m/e is oo.

Figure 3 shows the relationship between Vd and M. Accordingly, by determining the relationship shown in Figure 3 (in other words, determining the values of V0 and Mo in equation (1)) as a mass scale with reference to an appropriate standard sample, the m/e ratio corresponding to the value of Vd can be determined according to the mass scale.

However, it is a significant matter that even if an accurate mass scale has been determined, when the accelerating voltage for the ion beam or the strength of the magnetic field is varied afterwards, errors will appear from the mass scale.

Now, assume that parent ions m0+ whose m/e ratio is m0 are selected by the double focussing mass spectrometer consisting of electric field 2 and magnetic field 3; a considerable part of the parent ions are dissociated into daughter ions ml+ and neutral particles having mass number m0-m1@ and the daughter ions m1 + and the parent ions m0+ not dissociated and introduced into the mass spectrometer employing a superimposed field.

We investigate the m/e ratio of the detected ions when the voltage of the electric field of the superimposed field is swept under two cases, case 1: accelerating voltage is Va and the strength of the magnetic field constituting the superimposed field is H0' case 2: accelerating voltage is Va and the strength of the magnetic field is H0,.

In case 1, m0 the m/e ratio of the present ions, is given according to the equation (1) as follows: M0=M00/(1-Vd0/V00)2 (2) where Moo and V00 are the coefficients to give the mass scale under the conditions (Va, H,), and Vd0 is the electric field voltage at which the parent ions are detected.

Similarly, the m/e ratio of the daughter ions according to the mass scale of case 1 (Mo=MooS V0=Voo) is given as follows: m1=M00/(1-Vd1/V00)2 (3) where Vd1 is the electric field voltage at which the daughter ions are detected.

However, since the daughter ions have the kinetic energy eVa' as if they are accelerated by a different accelerating voltage Va' by means of energy fragmentation at the time of dissociation, the above calculated m1 according to equation (3) is merely a virtual mass. The real m/e ratio of the daughter ions m1+ is given as follows: m1=M01/(1-Vd1/V01)2 (4) where M01 and VOR are the coefficients to give the mass scale under the condition (Va', H,).

Here, if K is assumed K=Va'/Va=m,/mO (5) M01 and V0, can be expressed as follows: Mo1=MoJK (6) Vo1=KVoo (7) However, since the value of kinetic energy fragmented at the time of dissociation is unknown, M01 and VO, cannot be determined.

Accordingly, by eliminating M01 and VO, from equations (4), (5), (6) and (7), the following equation can be obtained.

m @/K=m0=M00/K2(1 -Vd 1/KV00)2 (8) Further, since equation (8) is equal to equation (2), following relationship can be obtained.

Vd1/V00=(K-1)+Vd0/V00 (9) On the other hand, in case 2 under the condition that the accelerating voltage is Va and H0 is H0'. the m/e ratio of the parent ions is given similar to case 1 as follows: m0=M00'/(1-Vd0'/V00')2 (10) where M00' and V00, are the coefficients to give the mass scale under the condition (Va, H0') and Vd0' is the electric field voltage at which the parent ions m0+ are detected.And assuming that H0,/H0=A, M00' and V00, are given as follows: M00,=A2M00 (11) V00'=V00 (12) Accordingly, by substituting equations (11) and (12) for equation (10) in order to eliminate M00' and V00'. equation (10) can be rewritten as follows: m0=A2M00/(1-Vd0'/V00)2 (10)' Further, according to the mass scale under the condition (Va', H0'), the real m/e ratio of the daughter ions m1+is given as follows: m1=M01'/(1-Vd1'/V01')2 (13) where M01 and V01' are the coefficients to give the mass scale under the condition (Va', H0') and Vd,' is the electric field voltage at which the daughter ions are detected in case 2.

Here, M01', and V01' are expressed similar to equations (6) and (7) as follows: Mo1'=Moo'/K (14) V01,=KV00 (15) Accordingly, by eliminating M01, and VO1 from equations (5), (13), (14) and (15), the following equation which corresponds to equation (8) can be obtained.

m1/K=m0=A2M00/K2(1-Vd1'/KV00)2 (16) Since equation (16) is equal to equation (10)', following equation which corresponds to equation (a) in case 1 can be obtained.

Vd1'/V00=(K-1)+Vd0'/V00 (17) Here, since m is unchangeable in either two cases, equations (2) and (10)' are equivalent.

Accordingly, the following equation can be obtained.

Vd0'/V00=A(Vd0/V00-1)+1 (18) Further, by substituting equations (9) and (18) for equation (17) for eliminating Vd0'/V00@ the following equation is obtained.

Vd1'/V00=K(1-A)+AVd1/V00 (19) Now assuming that Mx, is the virtual m/e ratio of the daughter ions detected in case 2, determined according to the mass scale of case 1, Mx, is expressed as follows: Mx1=M00/(1-Vd1'/V00)2 (20) Then, by substituting equation (19) for equation (20), the following equation is obtained.

Mx1=M00/[1-(1-A)K-AVd1/V00]2 (21) As will be understood from the above equation, Vd1, is eliminated in this equation. This means -that the virtual m/e ratio of the daughter ions detected in case 2 can be given according to the mass scale established in case 1.

In equation (21), Mx, and Vd, are obtained by practical measurement, Moo and VOO are determined in case 1. Accordingly, if A can be determined, then K is determined according to equation (21), next, m, can be calculated according to equation (8).

It will be understood from the following description that it is possible to determine the value of A according to the mass scale established in case 1.

Now, assuming that Mx0 is the virtual m/e ratio of the parent ions detected in case 2, determined according to the mass scale of case 1 (M00@ V00)@ Mx0 is expressed as follows: Mx0=M00/(1-Vd0'/V00)2 (22) Then, the following equation is obtained by dividing equations (10)' by equation (22) as follows: mJMxO=A2 (23) In equation (23), M can be determined accurately according to equation (2) at case 1, and Mx0 can be obtained by practical measurement at case 2.

Therefore, the value of A can be determined according to equation (23), subsequently, the value of K can be determined according to equation (21) by substituting the determined value of A. As a result, the value of m can be determined according to equation (8) by substituting the determined value of K.

Furthermore, the value of energy which the daughter ions possess also can be calculated on the basis of the value of K(=aVa'/eVa).

To summarize, the exact m/e ratio of the daughter ions can be determined according to the following procedure: (a) establishing the mass scale at case 1 (b) in case 1, determining the m/e ratio m0 of the parent ions according to said mass scale, (c) in case 1, obtaining the electric field voltage Vd at which the daughter ions are detected, (d) in case 2, determining the virtual m/e ratio of the parent ions Mxo according to said mass scale, (e) in case 2, determining the virtual m/e ratio of the daughter ions Mx, according to said mass scale, (f) determining the value of A according to equation (23), (g) determining the value of K according to equation (21), (h) determining the value of m according to equation (8), and (i) determining the value of energy of the daughter ions on the basis of the value of K(=eVa'/eVa) Numerous variations on the above-described invention will occur to one skilled in the art. For example, in case 2, the value of Hot may be chosen at zero. By so doing, since the value of A becomes zero, calculation can be made easy.

Regarding the mass spectrometer arranged in front of the collision chamber 5, any type of mass spectrometers may be adoptable.

In the above described embodiment, in order to compensate for aberration of the focal point pursuant to the electric field voltage sweep, a proper voltage is applied to the auxiliary electrodes called Matsuda plates along with the electrical field voltage sweep. However, it is possible to use a lens for compensating for the focal distance without using auxiliary electrodes.

In that case, a quadrupole lens is arranged outside the superimposed field. Further, in the above described embodiment, the ions dispersed by the superimposed field, which is created by the pole pieces 6 and 6' and the electrodes 8 and 8', are detected by the ion collector 13. However, it is possible to add an electric field or a superimposed field arranged in tandem with said superimposed field for correcting the increase in energy dispersion when measuring ions having high m/e ratio. The details of these arrangements are described in British Patent No. 1,549,219.

Furthermore, in the measurement of the daughter ions formed by the self ion fragmentation reaction of the parent ions, the collision chamber is unnecessary.

Claims

1. A mass spectrometer comprising an ion source, means for selecting ions produced and accelerated by said ion source according to their mass-to-charge ratio, a field-free region in which some of the mass-selected parent ions for daughter ions, means for creating superimposed electric and magnetic fields at right angles through which the ions from said field-free region pass, means for establishing two stages during which the intensity of said magnetic field is set at a first value and then at a second value respectively, and means for sweeping the intensity of said electric field during each stage.

2. A method of analysis by mass spectroscopy, the method consisting in generating ions in an ion source; selecting ions from those generated by virtue of the mass-to-charge ratio; passing the selected ions as parent ions through a field-free region; forming daughter ions from some of the parent ions; passing the parent and daughter ions through superimposed electric and magnetic fields at right-angles to each other; controlling the electric and magnetic fields in two stages; (a) setting the magnetic field to a first value while sweeping the electric field: and (b) setting the magnetic field to a second value while sweeping the electric field; and deriving signals from the ions according to their deflection by the fields.

3. A mass spectrometer or method according to claim 1 or claim 2 wherein, during either one of said two stages, the magnetic field intensity is zero.

4. A mass spectrometer or method according to any of the preceding claims wherein a collision chamber is arranged in said field-free region, inert gas being fed into said collision chamber.

5. A mass spectrometer or method according to any of the preceding claims wherein the electric and magnetic fields are automatically controlled by a microprocessor unit.