Time-of-flight Mass Spectrometer Having Both Linear And Curved Drift Regions Whose Energy Dispersions With Time Are Mutually Compensatory

Abstract

A mass spectrometer is disclosed which is based on the linear time-of-flight principle. Initial ion energies are compensated for by combining a curved drift region with a linear drift region so that ions of the same mass but differing energies reach the collector at the same time.

Description

BACKGROUND OF THE INVENTION

This invention pertains to mass spectrometers and more particularly to an energy focusing time-of-flight mass spectrometer.

There are many different types of mass spectrometers. Magnetic mass spectrometers separate ions according to their mass-to-charge by noting the angular deflection while passing the beam through a magnetic field. Radio frequency mass spectrometers utilize a combination of electrostatic and radio frequency field in such a way that ions of only one mass-to-charge ratio may pass through. The present invention is particularly concerned with linear time-of-flight mass spectrometers.

In its simplest embodiment, the known type of time-of-flight mass spectrometer comprises an ion source and a collector disposed at opposite ends of an evacuated field-free drift tube. Upon the introduction of gas molecules into the ionization region of the spectrometer source, ions are formed, usually be electron bombardment, which are periodically pulsed out of the source by source grids toward the collector by either one or several electric fields established between appropriately spaced grids. Since the velocity of the ions in the field-free drift tube region is a function of the mass-to-charge ratio, ion separation occurs. The amount of separation depends strongly on the length of the tube. Therefore, when the ions reach the collector they have separated into ion bunches with the lightest group reaching the collector first. As such, proper electrical circuitry connected to the collector will show a complete mass spectrum (in time) of the gas molecules ionized in the source.

In previous linear time-of-flight mass spectrometers, all ions are accelerated through a field to the same energy or to the same momentum. They are then allowed to drift in a field-free region. Since they all have the same energy but have different masses, they will separate into bunches according to their mass. These bunches are then collected at a fixed point in the field-free region and their variations in arrival time are measured as a measure of atomic mass or more properly mass-to-charge ratio.

One serious problem has always existed in this type of instrument. This problem is that all ions are not at rest before the initial accelerating pulse is applied and therefore all ions of the same mass-to-charge ratio may have different energies. Thus, ions of the same mass will not reach the collector at the same time and the ion bunches may overlap one another. The ions have initial ion energies which arise from several sources. The most easily understood source of energy spread is that associated with thermal energy. Certain classes of molecules exhibit energies greater than thermal. This energy is imparted to them during the ionizing process. While this mechanism is not completely understood, it has been experienced by many investigators. Variations in starting positions within the initial region also contributes to variations in energy.

This invention provides a time-of-flight mass spectrometer which compensates for slight variations in initial energy so that ions of equal mass-to-charge ratio but with slightly different initial energies will arrive at the collector or detector simultaneously. This invention also provides a means for limiting the amount of energy spread of ions which are transmitted through the spectrometer ion flight path.

SUMMARY OF THE INVENTION

A time-of-flight mass spectrometer having energy focusing means is provided which compensates for ions of equal mass having differing energies so that all ions of the same mass reach a collector simultaneously. The ion path comprises a curved region and a linear drift region. The present invention relies on the variation in angular velocity with ion energy to give a time dispersion in a curved region which is opposite the time dispersion in the linear drift region. In this way, one region compensates for the adverse effect of energy spread in the other.

DESCRIPTION OF THE DRAWINGS

An illustrative embodiment of the present invention is shown in the following drawings, in which:

FIG. 1 is a schematic illustration of a standard prior art linear time-of-flight mass spectrometer,

FIG. 2 is a schematic illustration of a time-of-flight mass spectrometer according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The equipment schematically illustrated in FIG. 1 is a typical prior art time-of-flight mass spectrometer 10. The mass spectrometer 10 consists of four basic elements which are: an ion source 12 to produce ions of the molecular species entering the ionization region 13, a grid system 14--15 that draws the ions from the ion source and accelerates the ions down the drift tube, a field free linear drift tube or region 16 for confining the ion "packet" and supplying the distance required to separate ions according to mass-to-charge ratio, and an ion collector 18 to collect the ions and measure their intensity.

Region 13 designates the region of ion formation typically accomplished by passing an electron beam 20 from a cathode 22 through a gas to be analyzed which is introduced into the region 13. A power supply 24 is shown supplying a heater current to the cathode 22. Cathode 22 has a negative potential bias supplied by source 24 to provide ionizing electrons with energy sufficient to ionize the gas in region 13.

Upon application of a voltage pulse to grid 14 from the pulse source 26, the heterogeneous group of ion masses formed in region 13 are accelerated, each ion acquiring substantially the same kinetic energy but differing in exit velocity through grid 15 in proportion to the square root of its mass-to-charge ratio, the heavier ions having lower velocities and therefore becoming separated from the lighter ions. As a result, discrete ion bunches of different mass-to-charge ratios pass through grid 15 and either drift or accelerate, depending on the specific potentials through drift region 16 to collector 18. If only ions of the same energies are present, the lightest group reaches the collector 18 first, followed by groups of successively heavier mass. However, as previously described, all ions of equal mass will not have equal energies and thus ions of equal mass and differing energies will not reach the collector simultaneously and the groups or bunches of ions may overlap one another.

An illustrative embodiment of the novel construction of the present invention is shown in FIG. 2. In this embodiment, an electrostatic sector 28 is combined with a field free linear drift tube or region 30 to form the ion path between an ion source region 32 and an ion collector 34. As previously described, ions are created by electron bombardment in the source and are accelerated towards the electrostatic sector 28 by means of applying a pulse to one of the accelerating grids within the source region 32.

The electrostatic sector 28 has a curved construction and has ion-deflecting means, such as electrostatic plates or electrodes 36 and 38, therein which produce a uniform electrostatic field within the curved sector or region. The electrodes are typically of cylindrical shape and have an angle .theta.; however, other shapes such as toroidal, spherical, etc., may be utilized.

It is the property of an electrostatic sector of this type that ions of different energy or velocity passing therethrough take different paths. The faster ions take longer and therefore ions which enter sector 28 with high velocities leave later than ions which enter at the same time with lower velocities.

The electrostatic field existing within the sector makes the ions follow a curved path. Hence, when two ions of the same mass but with slightly different energies or velocities are made to follow a circular path by a uniform electrostatic field, they will assume different radii. The faster ion will take a radius such that its angular velocity about the center of curvature of the field will be less than that of the slower ion. The path of the high energy ion and the low energy ion are represented by 40 and 42 respectively. Therefore, if a high energy and a low energy ion of equal mass enter the electrostatic field at the same time, the slower ion will exit first.

At the end of the sector 28 is placed an energy slit 44 which limits the lowest and highest energy ions and passes only the chosen energy span. At this point ions of equal mass but slightly different velocities are dispersed along the ion path with the high velocity ions trailing behind the lower velocity ions. From here the ions continue through the field free linear drift region 30 with the faster ions again overtaking the slower ions, just as both ions reach the detector.

The motion of ions traveling in the radial electrostatic field and the field free linear drift region may be mathematically explained as follows, in which the following symbols will be used:

m = mass of ion

F = centripetal force of ion

v = rectilinear velocity of ion

r = radius of ion path

K = kinetic energy of ion

w = angular velocity of ion

.theta. = angle of electrostatic sector, radians

t.sub.1 = time of flight through electrostatic sector

t.sub.2 = time of flight through linear region

T= t.sub.1 +t.sub.2

E.sub.(r) = field intensity in the electrostatic sector

Vf = voltage applied to electrostatic sector

R.sub.1 = radius of inner sector plate

R.sub.2 = radius of outer sector plate

d = length of linear region

A body undergoing uniform circular motion follows a path of radius r, given by

The kinetic energy of the body is given by

2K= mv.sup.2 (2)

the rectilinear velocity by

the angular velocity by

when equations (1), (2), and (3) are combined.

The time t.sub.1 is expressed by

indicating t.sub.1 is proportional to the square root of the kinetic energy.

The time t.sub.2 is expressed by

indicating t.sub.2 is inversely proportional to the square root of the kinetic energy.

When the total time

T= t.sub.1 +t.sub.2 (7)

is expressed as

it becomes apparent that changes in time of flight caused by variations in

kinetic energy tend to cancel one another.

The centripetal force F is generated by an electrostatic field E (r), described by the equation

It is also seen from the above calculations that other structural arrangements are possible. For instance, the linear drift region 30 can be placed in front of the electrostatic sector 28 or the linear drift region 30 can be divided and one part thereof placed on each end of the electrostatic sector 28. It is also possible to split the electrostatic sector 28 and place one part thereof on each end of the linear drift region 30. Thus, the total effective length of the ion path resulting from the combination of a field free linear drift region and a curved electrostatic region provides equal times-of-flight between the source and collect or for ions of equal mass and differing energy.

The present invention relies on the variations in angular velocity with ion energy to give a time dispersion which is opposite the time dispersion in the linear drift region. In this way, one section of the instrument compensates for the adverse effect of energy spread in the other, so that the two together do what neither can do alone.

Therefore, a linear time-of-flight mass spectrometer is now provided which limits the energy spread of transmitted ions and compensates for the energy spread of the ions which are transmitted.

While the form of apparatus herein described constitutes a preferred embodiment of the invention, it is to be understood that the invention is not limited to this precise form of apparatus.

Claims

We claim:

1. In combination with a time flight mass spectrometer of the type having an ion source, an ion collector, an energy focusing ion path between the ion source and collector, and means for periodically accelerating ions from the ion source to the energy focusing ion path, the improvement wherein said energy focusing ion path comprises:

a linear drift region providing a first ion time dispersion according to the mass-to-charge ratio; and

a curved drift region providing a second ion time dispersion depending on the variation in ion angular velocity with ion energy, said second time dispersion being opposite said first time dispersion wherein one drift region compensates for the adverse effect of energy spread in the other drift region whereby all ions of the same mass and differing energy that leave the ion source at the same time reach the collector at the same time.

2. A time-of-flight mass spectrometer comprising,

an ion source,

means for accelerating the ions from said source in a specified direction,

an ion collector,

an ion path between said source and said collector, said ion path comprising,

a curved ion drift region including means providing a uniform electrostatic field deflection therein, wherein the time of flight of an ion through said curved drift region is proportional to the square root of the kinetic energy of the ion,

a linear ion drift region wherein the time of flight of an ion through said linear drift region is inversely proportional to the square root of the kinetic energy of the ion whereby changes in time of flight caused by variations in ion kinetic energy cancel one another and thereby providing equal times of flight for ions of equal mass and differing energy between said source and collector.

3. A mass spectrometer as set forth in claim 2 further comprising,

electrostatic plates providing said uniform electrostatic field, and

means at the exit end of said curved drift region for limiting the acceptable ion energy spread of transmitted ions.

4. A mass spectrometer as set forth in claim 2 in which said curved ion drift region precedes said linear ion drift region.

5. A mass spectrometer as set forth in claim 2 in which said linear ion drift region precedes said curved ion drift region.

6. A mass spectrometer as set forth in claim 2 in which said linear ion drift region is divided whereby one part precedes and the other part follows said curved ion drift region.

7. A mass spectrometer as set forth in claim 2 in which said curved ion drift region is divided whereby one part precedes and the other part follows 8In an energy focusing time of flight mass spectrometer of the type having an ion source, an ion accelerating means, an ion collector, and ion path between said ion source and said ion collector, the improvement in energy focusing means wherein said ion path comprises:

a curved ion drift region;

ion-deflecting means providing a uniform ion deflection in said curved ion drift region wherein ions of equal mass and different energies are made to follow different paths therein whereby variations in ion angular velocity depend on ion energy to provide an ion time dispersion; and

a linear ion region providing an ion time dispersion opposite the time dispersion created by said curved ion drift region whereby ions of equal mass and differing energies leaving said ion source at the same time will travel different paths and have equal times of travel and thereby reach

said ion collector at the same time. 9. A mass spectrometer as set forth in claim 8 in which said ion deflecting means comprise,

electrostatic plates providing a uniform electrostatic deflection field in said curved drift region wherein said ions assume different radii whereby ions of high energy have an angular velocity about the center of curvature of said curved drift region less than that of ions of lower energy and thereby ions of lower energies leave said curved drift region before ions of higher energy that enter said curved drift region with said ions of low energy, and

means at the exit end of said curved ion drift region for limiting the acceptable ion energy spread of transmitted ions.