JPS6245664B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method and apparatus for analyzing trace ions using an analyzer placed in a vacuum chamber. More particularly, the present invention relates to a method and apparatus for improved ion focusing. Another aspect of the invention relates to a method and apparatus for reducing clustering of neutral molecules around trace ions. The method and apparatus according to the invention can also be used to perform ion fragmentation.

High pressure or atmospheric pressure ion sources are well known for trace component analysis. For example, atmospheric pressure chemical ionization sources have been used for mass spectrometry. It is well known that when using such sources, binding of neutral molecules around the ions occurs in the region where ion formation takes place. Additionally, the ions must enter the vacuum chamber through an orifice through which the high pressure gas enters the vacuum chamber and expands. When this gas expands, it rapidly cools down and thus binding of gas molecules around the ions occurs during this expansion.

The bonding of atoms and molecules around the ions to be analyzed adds yet another spectral peak to the observed mass spectrum, resulting in undesirable measurement results. This complicates the interpretation of the results of the analysis. That is, when the mixture is analyzed, the signal corresponding to the unbound ions to be measured is thereby reduced or eliminated. The possibility of this bonding is higher in gases that condense easily, such as water vapor, but the presence of gases such as nitrogen and argon is more likely to cause bonding.

A conventional drawback is that when ions entering the vacuum chamber are converged or focused by a mass spectrometer, a large kinetic energy spread, or variation in energy values, is imparted to the ions by this focusing process. There is something called. This spread of energy causes a significant deterioration in the resolving power of the mass spectrometer. To prevent this energy spread, energy filters or low focusing voltages are used, but both techniques result in significant attenuation of the ion signal.

Accordingly, one object of the present invention is to provide improved ion focusing with reduced energy spread. This is accomplished by applying a focusing electric field to the ions in specific regions of the free jet expanse of the sampling gas into the vacuum chamber. In particular, an electric field is provided to focus the ions in a selected region of the freeget sufficiently close to the orifice, such that the ions cannot undergo a kinetic energy spread beyond a predetermined maximum for a given electric field. It becomes like this. This maximum value is determined by the resolution required of the mass spectrometer. In a high-performance quadrapole mass spectrometer, an energy spread of more than about 2 ev degrades the ability to resolve peaks 1 a.mu apart. In selected regions of the freeget, the gas is so dense that collisions between ions and gas molecules often occur, limiting the kinetic energy experienced by the ions and thus preventing kinetic energy spread. The electric field applied downstream of this selected region is controlled to limit the spread of energy subsequently applied to the ions.

In another aspect of the invention, the electric field applied to the ions in the above-mentioned region is of sufficient strength to impart internal energy to the ions such that the cluster bonds or physical bonds are broken without breaking the chemical bonds. are doing. As an example, the electric field may provide an internal energy in the range of 0.1 to 1.5 ev. The electric field applied downstream of the selected region is then controlled to limit the spread of energy imparted to the ions.

If a large energy spread is allowed, the electric field in the selected region can increase to a magnitude sufficient to break chemical bonds as well as cluster bonds, thereby causing ion fragmentation. can. This produces a spectrum similar to the electron impact spectrum.

The focusing and unclustering functions obtained by using several appropriately placed electrostatic lenses or, in the preferred embodiment, a single lens, provide signals of non-cluster bound ions to the mass spectrometer. increases significantly.

The gas that enters the vacuum chamber when the freeget expands is
It is preferably cryogenically pumpable and pumped by cooling the surface of the vacuum chamber.
This allows for large freegets.
The large freejet facilitates positioning and configuration of the electric field generating means used to focus and decluster ions. Also, a large ion signal will enter the vacuum chamber.

Hereinafter, the present invention will be explained in detail based on embodiments shown in the accompanying drawings.

1 to 4 show an embodiment of a mass spectrometer used in the present invention. Figures 1 to 4
In the configuration shown in the figure, the apparatus has an ion reaction chamber 2, a gas curtain chamber 4, and a vacuum chamber (analysis chamber) 6. The reaction chamber 2 and the gas curtain chamber 4 are connected by an interface plate 8, and the gas curtain chamber 4 and the vacuum chamber 6 are connected to each other by an orifice plate 1.
Connected by 0.

The reaction chamber 2 has a bell-shaped inlet 12 and a plenum 1.
Cylindrical duct 14 connected at 16 to 8
have. The plenum 18 is connected by a duct 20 to a synchronous fan, not shown, which operates to draw air or other gas to be analyzed into the duct 14 through the bell inlet 12. A stabilizing screen (not shown) may be provided in front of the inlet 12 to prevent swirling and aid in the formation of laminar flow.

Inside the duct 14, there is a central electrode 2 elongated in the axial direction.
2 is provided. As shown in FIG. 2, this electrode 22 is held from three sides by an insulating material such as nylon 24 at a position that coincides with the axis of the duct 14. Another wire, not shown, is arranged to provide the necessary voltage to the electrode 22. The part of the duct 14 surrounding the electrode 22 is insulated from the bell-shaped inlet 12 by an insulating joint, indicated schematically at 26, and is therefore insulated from the duct 14 downstream of this joint 26.
The wall portion of forms the second or outer electrode 28 . A ring-shaped ionization device 30, such as a tritium foil, is placed between the two electrodes 22,28.

In operation, air or other carrier gas containing the gas to be analyzed is drawn into the cylindrical duct 14 by a synchronous fan. The aspiration speed is then high enough to create laminar flow but to reduce sample diffusion. As the gas passes through the ionizer 30, ions are formed in the region 32 resulting in a mixture of positive and negative ions within the gas. The ion generation region is annular, and its cross section is as shown in FIG. 1A. In this figure, distance from device 30 is plotted on the y-axis (with the device at zero point) and the relative number of ions produced is plotted on the x-axis.

During this ion production, the beta radiation from the tritium foil ionizes the air or other carrier gas components to form primary reactant ions (after a series of reactions in the air or other carrier gas). Some of the primary reactant ions further react with molecules of the trace gas to form product ions from the trace gas. This forms a mixture of product and reactant ions. Of these mixtures, the product ions should be preferentially selected and analyzed.

An electric field created by appropriate potentials applied to electrodes 22 and 28, interface plate 8 and orifice plate 10 is superimposed on the fluid flow. Ions flow with partial velocity Ve = Vf + KE. where Ve is the partial flow velocity, E is the partial electric field vector and K is the mobility of the object in question. The potential applied to the device and the configuration of the device (described in more detail below) are such that the ions to be measured, whose polarity and mobility are selected, are located in a central region forward and downstream of the central electrode 22 from the reaction region 32; It is arranged so that it converges in an area that coincides with the axis of . Therefore, the ions to be measured are transmitted through the central opening 34 of the interface plate 8.
They are moved in concentrated bundles towards. A portion of the ions from the sample flowing through reaction region 32 pass through central opening 34 in the interface plate.

Transfer of the collected ions to the vacuum chamber is carried out by gas curtain chamber 4 and cryogenic pumping in the vacuum chamber. In particular, the gas curtain chamber 4 is supplied with a suitable gas (such as CO ₂ or argon) which has low reactivity with the ions to be analyzed and which can be easily cryopumped in the vacuum chamber 6. something is chosen. Curtain gas is
It is chosen to have a vapor pressure much less than atmospheric pressure (less than 10 ^-4 Torr) at a temperature that allows the walls of the vacuum chamber to undergo adequate cooling. The curtain gas acts as a curtain or membrane and is placed between the reaction chamber 2 and the vacuum chamber 6. This curtain gas is passed through the inlet duct 40.
The duct 40 is arranged to form a substantially circular flow pattern with a circumferential component but mostly directed radially inwardly into the gas curtain chamber 4. (Figure 3). The curtain gas is suitable for collection into the vacuum chamber 6 and is provided through an opening 34 in the interface plate 8.
The flow rate is sufficient to be excessive so as to slowly flow from the air into the duct 14 being sucked by the fan, and sufficient to prevent carrier gas from entering the space between the interface plate 8 and the orifice plate 10. given the flow rate. However, the concentrated ion flux is attracted by the appropriate attraction potential of the orifice plate 10 against this quiet outflow of curtain gas.
It is captured by the curtain gas portion passing through the orifice 42 of the orifice plate and carried into the vacuum chamber 6.

The vacuum chamber 6 (see also FIG. 4) has fins 4
6 and includes a liquid cooling tank 44 with 6. Preferably, tank 44 contains liquid nitrogen 48 . As the curtain gas containing the desired ions diffuses out of the orifice 42, the _CO2 molecules strike the fins 46 where they solidify, lowering the pressure within the vacuum chamber. The fins 46 have a suitable trapping surface configuration to maximize trapping and coagulation of _CO2 molecules (as is well known in the cryopump art). This provides a high equivalent pumping rate and an operating vacuum of 10 ^-5 to 10 ^-6 Torr (suitable for mass spectrometry) is obtained by using an orifice 42 with a diameter of about 0.033 cm. This diameter is larger than the commonly used orifice diameter of up to 0.002 cm or less, and therefore the cross-sectional area of the ion flux entering the vacuum chamber is, according to the present invention,
The opening area ratio is increased by more than several hundred times.

Once a vacuum is established (initially a moderate vacuum is created by suitable means such as a small mechanical pump and then the vacuum is increased by a cryo-pump), the curtain gas entering the vacuum chamber is released as a free jet 50. spread. Ions entering the vacuum chamber also spread out within the freeget 50, but are focused by an electrostatic ion lens member into a mass spectrometer, such as a quadrupole mass spectrometer (described in detail below). Mass spectrometer 52 analyzes ions by charge-to-mass ratio and performs quantification by ion counting or other suitable techniques. A small auxiliary vacuum pump 54 may be provided to remove non-condensable impurities (such as nitrogen) from the vacuum chamber.
A Getter ion pump is suitable as the pump 54.

The conventional focusing method is
An electrostatic lens is placed sufficiently downstream of the orifice 42, and a relatively high voltage is applied to the electrostatic lens compared to the orifice plate to focus the ions into the mass spectrometer. When a suitable vacuum (10 ^-3 Torr or less) is maintained within the vacuum chamber, such focusing results in a large energy spread of the ions introduced into the mass analyzer, which can degrade the resolution of the analyzer. This leads to a decrease in detection ability.

The present inventors have discovered that by applying a predetermined electric field to a specific region of the freeget 50, good focusing of ions can be achieved, and at the same time, a small energy spread that does not cause problems is given to the ions to be analyzed. discovered. It has also been found that by applying an electric field downstream of the specific region of the free jet 50, it is possible to perform cluster decomposition to a desired degree while limiting the spread of energy given to the ions.

In more detail with reference to FIG. 5, the expanding curtain gas jet 50 has three regions 60, 62 and 64. In region 60, which may be referred to as the continuum region, the gas jet is expanding rapidly but is still relatively dense. In this region, an electric field is applied to alter the ion's trajectory without imparting significant kinetic energy to the ion.

In the region 62, which can be called the transition region, the concentration of the gas jet becomes very low, and the electric field is adjusted so that an appropriate E/n value is obtained, where E is the electric field (volt/cm) and n is the concentration number of the gas. When ions are given a large amount of energy, they can collect a large amount of energy. At the same time, there are enough gas molecules in this region to cause collisions with neutral gas molecules in the freeget. Generally speaking, region 62 may be defined as a region where both cluster decomposition, ie, physical bond breaking, and ionic fragmentation, ie, chemical bond breaking, can occur.

The third region 64 is called the vacuum orbit or free molecular flow region. In this region the gas molecules are very diluted and the possibility of collisions between ions and these gas molecules is extremely low. In this area,
A suitable focusing screen and an electrostatic lens including a suitable vacuum ion lens are used. The average ion free path in region 64 is
It has dimensions of the order of cm (2 cm or more).

Taking advantage of the presence of these regions as shown in FIG. 6, suitable focusing elements are placed in regions 60. In region 60, the curtain gas (containing ions) expands into a wide cone, but the ion flux is distributed according to the cosine square law so that the ions are contained within a total cone angle of 90 degrees. An electrostatic lens is therefore provided which is formed by two conically inclined rings 70, 72. Rings 70, 72 are supported by thin wires (not shown) and directed toward orifice 42 to prevent interference with the expanding gas. During actual expansion of the curtain gas, the apex of the cone is slightly offset outward from the orifice plate 10, so that the rings 70, 72
The cone opposite will be slightly displaced outwardly from the orifice plate 10 at its apex 73 (more than one orifice diameter).

When a suitable electrical potential is applied to the rings 70, 72 with respect to the orifice plate 10, a saddle-shaped electric field is created as shown by the electric field lines 74 (a two-dimensional field is of course shown in the figure). Representative trajectories of ions are shown at 76. Ions whose orbits are initially located outside the rings 70, 72 are lost, but the number of such ions is extremely small. Ions whose initial trajectory has an angle of less than 45° with respect to the axis of the device are
is deflected inwardly to pass through the tip 78 of 2. At this position the electric field created by the voltage applied to the ring loses its influence on the ions.
The ions enter region 62 and enter expanded gas jet 8
Take the direction of motion of 0. This causes ions that initially had a total cone angle of 90° to be focused to a smaller total cone angle, approximately 30-45°, and many ions are focused within region 64 and enter the mass spectrometer. become. Since the energy that the ions can receive within the region 60 is very small, the spread of kinetic energy given to the ions by focusing within the region 60 is very small.

Another advantage is derived from the above-described arrangement based on the fact that the large separation of ions from the gas stream is carried out under relatively high pressure. That is,
A pressure stage can be utilized to expel most of the gas flow from the conical section where most of the ions are focused. An example of this pressure stage is shown in FIG. 7, in which a second ring 72 is extended and attached to a separator plate 82 to define a space 84 in region 60 from which gas is discharged at relatively high pressure. Ru. The gas enters the space 8 via a duct 88
4 to a pump 86. This relieves the load on the vacuum pump (usually a cryo-pump) used for the rest of the vacuum chamber.
This allows the use of a large orifice for a given size of cryo-pump, or the use of a small cryo-pump for a given size of orifice.

By judiciously choosing the voltage applied to region 60, the cluster decomposition reaction can be controlled to a certain extent. In region 60, the ions cannot receive huge energy, but they do receive some energy. By controlling the magnitude of this energy, ions and molecules stick to each other when they collide (cluster bonding).
It is possible to suppress the tendency, or it is possible to separate weakly bound substances by making very low-energy bonds. The shape (spacing, dimensions and angles) of rings 70, 72 or other lenses in region 60 is varied according to the application of the device.

A screen, such as screen 90 (shown in Figure 6), is used to facilitate ion interaction. By appropriately choosing the voltage V2 of the screen 90 with respect to the voltage V2 of the ring 72, the area 6
Cluster binding in 2 can be controlled. When V1=V2, there is no voltage difference between ring 72 and screen 90, so ring 7
2 and the screen 90 does not occur. Alternatively, the voltage V2 could be significantly different from the voltage V1 so that sufficient energy breaks relatively weak cluster bonds or causes clusters or ions with stronger energy bonds to separate. can.

In the embodiments of the invention described above, early focusing in a relatively dense region of the freeget 50 significantly increases the available ion signal in the mass spectrometer compared to focusing at a greater distance from the orifice. At the same time, since the energy that ions can receive in this region is limited, it is clear that the spread of kinetic energy given to ions is also limited. Also, by placing the screen 90 at a suitable location in the freeget region 90 and applying a suitable electric field, it is possible to effect at least some decomposition of the cluster bonds and still limit the spread of the energy of the ions. It is also clear that it is possible. Of course, the further away the screen 90 is located in the region 62, the lower the concentration of gas and therefore the longer the ions will travel during a collision and have a larger kinetic energy spread. become. How far screen 90 is located within region 62 depends on the spectrum being analyzed. For simple spectra with only a few widely separated peaks, a larger kinetic energy spread is allowed than for svectors with many closely spaced peaks.

Another embodiment of the present invention will be described with reference to FIGS. 8 and 9. The dashed reference numbers in FIGS. 8 and 9 indicate parts that correspond to the parts shown with the same reference numbers in FIGS. 1 to 7.

As illustrated in FIG.
via the plate 8' and the wall 98 into the reaction area 32' in a cylindrical chamber. The discharge needle 22' is mounted in the tube 14' by an insulating support 100 and a thin support arm 24' aligned with the axis of the tube. A suitable potential V3 is applied to needle 22' in conjunction with potential V4 applied to intermediate face plate 8' to produce a corona or other discharge condition from the tip of needle 22'. Typically,
The sample gas contains chemically reactive gases, i.e. water vapor, isobutane, benzene, methylene chloride or other suitable reactants, which are reacted by an electrical discharge between the tip of the needle and the intermediate face plate 8'. Ionized. The reactant ions react with the trace molecules at atmospheric pressure in the reaction region 32', producing trace ions. It has been found that the ionizing discharge at the tip of needle 22' is stabilized by passing the flow of sample gas through the needle. This stabilization is improved by the tapered or inwardly converging apex 102 of the tube or sheath 14', which tip 102 causes the sample gas to pass through the tip of the needle 22'. , to converge the streamlines of this sample gas.

The ions generated in the reaction region 32' are attracted into the gas curtain chamber 4' through the openings 34' in the intermediate plate 8' by means of a suitable attraction potential on the intermediate plate 8'. Gas curtain chamber 4' is separated from vacuum chamber 6' by orifice plate 10', which includes orifice 42'. The orifice plate 10' has on its downstream side a diverging conical surface 103 which expands the gas in the free jet 50', thus moving the gas in the direction most desired for the pumping action.

Prior to this, a suitable curtain gas, such as argon gas or nitrogen gas, is supplied into the gas curtain chamber 4' via the inlet 40' at a pressure sufficient to close the gas curtain chamber against flow of sample gas. be done. Gas flowing into the gas curtain chamber 4' causes a slight excess of gas to be vented through opening 34' into reaction chamber 32' where this excess gas and excess sample gas are vented via duct 20'. The amount is sufficient to ensure that The remainder of the curtain gas, which is directed into the gas curtain chamber along with trace ions, flows into the vacuum chamber through orifice 42'. If the sample gas is a gas that can be cryo-pumped, then a curtain gas is not needed or the gas flow is
The flow of sample gas into the vacuum chamber is controlled so that it is not completely closed off. A suitable electrical potential is applied to orifice plate 10' to assist the ions in passing through orifice 42'.

If the axis of the needle aligns aperture 34' and orifice 42', photons resulting from ultraviolet radiation from needle 22' will enter vacuum chamber 6' and pass through mass spectrometer 52' (FIG. 9). ) is known to produce "noise" due to the emission of photoelectrons from the ion detection device 104a (FIG. 9). This noise causes the shaft 105a of the needle 22' to
Opening 34', orifice 42' and through hole 104b
A large reduction can be achieved by slightly offsetting the axis 105b which extends to the detection device 104a via (FIG. 9). This deviation is the diameter of a normal orifice (usually 0.002 to 0.004 inch).
It has been found that this slight deviation has a negligible effect on the movement of ions into the vacuum chamber. The diameter of the through hole 34' is usually
Since the diameter is about 1/8 inch, the orifice 42' and the through hole 104b (the diameter may be 0.25 inch, but the diameter downstream of the orifice is usually 10 inches).
The optical line-of-sight shift between the detection device 104a
provides the main restraint on the movement of photons to . Prior to this, the cooling surfaces or fins 46' of the vacuum chamber 6 are cooled by a suitable cooling liquid or mechanical cooling method to a temperature sufficiently low to solidify the gas entering the chamber 6.

A tapered frusto-conical lens element 72' is disposed within the area 62' of the vacuum chamber. Before that, the cone of the lens element is inserted into the orifice 4.
2', specifically lens element 72' directed at a point slightly downstream of orifice 42'.
consists of three thin arms 10 spaced apart from each other.
6 so as to cause as little disturbance to the gas flow as possible.

In FIG. 8, the lines of equal electric field strength developed between the upstream end or inlet 108 of lens element 72' and orifice plate 10' are indicated at 74', and the ion trajectories are indicated at 76'. .
(The ion trajectories do not converge, but instead diverge radially outward, similar to free jets.) Since ions tend to move in trajectories perpendicular to the iso-field strength lines, convergence is circular. Area 6 occurring in area 60' and extending slightly into area 62'
Since no special lens is placed in 0', lens element 72' placed in area 62' performs the function of both lens elements 70 and 72 shown in FIG. This condition is the same as that which occurs when, instead of applying a slightly repulsive voltage to lens element 70, an attractive potential (the voltage portion between the voltage on plate 10 and the voltage on lens element 72) is applied. be. At this intermediate voltage, lens element 7
The function 0,72 is the single lens element 7 in FIG.
2'.

As previously stated, the position of lens element 72' in transition zone 62' of free jet 50' is determined by various conditions. If the entry into lens element 72' is too downstream, the spread of the kinetic energies of the ions when sufficient potential is applied to break up the clusters may become too large to permit analysis of complex spectra. Dew. However, for simple spectral analysis, it is acceptable to place the lens element 72' downstream within the transition region 62'. As previously mentioned, the entrance 108 to the lens element 72' is located in the area 6
If placed too far upstream within 2', it is difficult to impart sufficient energy to the ions to cause the desired degree of cluster disintegration.

In a conventional quadrupole mass spectrometer, 1a.mu
It was found that in order to clearly separate mass peaks separated by atomic mass units, the kinetic energy spread of the ions must be smaller than 2ev. Therefore, the entrance 108 of lens element 72'
is positioned close enough to the orifice 42' that this energy spread is not exceeded under the action of the electric field applied between the lens element and the orifice plate. Entrance 108 is a continuous area 60
This requirement can be met while achieving early convergence.

However, for cluster decomposition, the ions must be endowed with an internal energy of 0.1 to 1.5 ev. (Of course, dissipation can occur even with internal energies greater than 1.5ev, but in this case they are used with a kinetic energy greater than that normally required.) To impart this large amount of energy to the ion, , entrance 10
8 is in the transition region 62' rather than the normal region 60', and the gas density is low enough for the ions to obtain the desired internal energy through ion collisions, and the kinetic energy obtained by the ions is low enough to obtain the desired internal energy. high enough to limit spread (preferably
2ev or less).

To meet the above conditions, the following parameters may be applied. First of all, the electric field created between orifice plate 10' and lens 72' must not be so great that a discharge occurs between them. Because lens 72' is conical and close to the orifice plate with substantially pre-ionized gas between it and orifice plate 10', this can limit the maximum voltage applied. At the same time, the electric field must be sufficient to achieve the above objectives.
Next, for a source gas having a density of 2.7×10 ¹⁹ moles/cm ³ (i.e., the gas in the gas curtain chamber in which the gas curtain is used), orifice 42' and lens element 72' along axis 50b are The spacing Xl between the inlets 108 is best given by:

Xl=50D ^+40D _-25D where D is the diameter of orifice 42'. At this location of the inlet 108, the gas density is large enough that the ions often collide with molecules so that they do not acquire unduly high energy, and that the ions impart enough energy to cause substantial cluster dissolution. is small enough to be acquired.

If the density of the source gas increases, e.g. if the pressure of the curtain gas becomes higher than atmospheric pressure or if the curtain gas is at a low temperature, the preferred placement of the inlet 108 of the lens element 72' is further downstream. If the density of the curtain gas is reduced, the preferred placement of the inlet 108 to the lens element 72' becomes closer to the orifice. Any distance X from orifice 42'
The density n of the gas in the free jet located at is n=ns・0.161・cos ² (1.151θ)/
(X/D) ² , where ns is the density of the source gas, θ
is the angle from axis 105b, and D is the diameter of the orifice.

From this relationship, it can be seen that if the density of the source gas varies by the factor m (multiple m), the distance Xl will differ by √. In other words, in this case, it can be expressed as Xl=[50D ^+40D _-25D ]√.

The tolerance of ^+40D _-25D is due to the allowable energy intensity in the particular application of the invention. If you have to analyze a complex spectrum,
Very low ion energy intensities are desired, with the entrance 108 of lens element 72' being located no more than 50D from the orifice, and as little as 25D from the orifice.
Large energy intensities are acceptable when few ions are analyzed.

In the application of the invention, a converging or dissipating electric field is always applied within the free jet 50. As is well known in the field of fluid mechanics, free jet 50 has a length y along its axis given by:

Here, D is the diameter of the orifice, P ₀ is the pressure of the source gas, and P is the back pressure of the vacuum chamber. The end of the free jet becomes a shock wave, behind which the pressure actually increases. When P ₀ = 760 torr (1 atmosphere) and P = 10 ^-3 torr, y = 610D (which is much smaller than the preferred distance of the entrance 108 of lens element 72' from the orifice, Xl = 50D ^{+ 40D} _{- 25D} . The range of the jet is determined for effective pumping; if the pumping is insufficient, the free jet is terminated by a shock wave before the most desirable area for locating the inlet 108. Thus, a vacuum is always maintained sufficient (typically at a distance of at least 2Xe from the orifice) for the shock wave defining the end of the free jet to be sustained downstream of the entrance 108 of the lens element 72'. If the pumping action is sufficient, the free jet 50 can actually extend into the region of the vacuum stage, where the jet 50 is present and
In this case, no shock wave is generated. In addition, the cone of the lens element causes only minimal disturbance to the flow, for example this cone usually has a sharp tip,
It disturbs the minimum annular solid angle of the flow and thus this cone itself does not form a shock wave.

Free molecular flow region 64' where ions are free jets
After entering, additional converging lenses are used if necessary. In the free molecular flow region 64', the gas is of low density such that collisions between ions and molecules only occur infrequently and the ions cannot gain sufficient additional energy intensity. FIG. 9 shows the element 120,1
A three-element Ainchel lens 118 having 22, 124 is shown as an example, and this lens 118 collects ions into the mass spectrometer 52'. In a preferred embodiment of the invention, the first element 120 of the einchel lens is placed at a distance approximately equal to 500D from the orifice. This is Freedom Jet 5
0' and specifically within the free molecular region 64' of the jet 50'. The first element of the Einchel lens is conically shaped and deflects the gas from the ion path. The conical deflection device constituted by the element 120 also includes another conical deflection device 1.
38 (insulated from deflection device 124) follows, which deflection device 130 extends from the rear end of element 120 to a position proximate cooling fins 46'. These deflection devices 120 ^, 130 are located at the separating wall 82 in FIG. form a similar separation wall. In addition, the conical element 120 in the free jet 50' prevents the formation of shock waves at the end of the free jet 50' in the passage of the ion beam 136, thus reducing ion beam turbulence. As a result, signal loss is reduced. On the other hand, the above-described apparatus guides the ion beam 136 out of the free jet 50' without disturbing the focused beam by the shock waves that normally occur at the ends of the free jet.

In one mode of operation of the invention, orifice plate 10' is held between +5 and +60 volts, lens element 72' is grounded, and first element 120 of inchiel lens 118 is also grounded. Therefore, there is no electric field between the first element 120 of the Einchel lens and the lens element 72.
Since lens element 72' is grounded and the cold shell of the vacuum chamber is also grounded, the area between the first element 120 of the cold shell lens and the entrance 108 of lens element 72' is a substantially free field. be. In reality, electric field lines from the einchel lens needle element 122 (the needle element is maintained at approximately 20 volts) are present around the first element 120, but the electric field thus generated is weak. (about 1/2 volt/cm) and the electric field is so localized near the aperture of the Einchiel lens that it does not extend into regions where collisions between ions and neutral molecules often occur.

The present invention also allows ion fragmentation to be generated under conditions in which the ion fragmentation energy intensity is reduced and all ions are transmitted, thereby improving sensitivity.

If the electric field between lens element 72' and plate 10' of FIG. 8 is increased, the internal energy required of the ions for collision will be sufficient to break the chemical bonds in the ions. becomes large (eg, 2.0 to 6.0ev), thus causing ion fragmentation. The abundance of these splittings can be closely approximated to the parent electron impact splitting patterns used for formal identification in conventional electron impact ion sources in mass spectrometers. While shock-induced fragmentation has been observed and occurs upon collision with the atmosphere, the present invention reduces fragmentation that occurs within the free jet itself. The combined ion energy intensity is greater than that resulting from the relatively weak electric field used for dissipation (thus reducing the quality of the ion mass spectrometry results)
On the other hand, splitting is very useful for formative identification in the analysis of single ion mixtures. Typically, a voltage of 50 to 200 volts is applied between lens 72' and orifice plate 10' for splitting.

In summary, the present invention provides convergence and dissipation electric fields, and these fields are applied to selected regions of the free jet 50 such that the spread of energy trapped in the ions is
Maintained at acceptable levels (depending on application). In regions where the ions can acquire an energy spread that is unacceptable for the application, no electric field or a very weak electric field is applied. Behind this region, for example in the free molecular flow region, another convergence electric field can be applied if required.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of the configuration of a mass spectrometer to which this invention is applied, Figure 1A is a graph of the ion formation region of the apparatus in Figure 1, and Figure 2 is a view taken along line 2-2 in Figure 1. 3 is a sectional view taken along line 3-3 in FIG. 1, FIG. 4 is a sectional view of the cryogenic pump shown in FIG. 1, and FIG. 5 is a sectional view of the vacuum chamber of the device shown in FIG. 6 is a sectional view showing the arrangement of the focusing member in the area shown in FIG. 5; FIG. 7 is a sectional view showing a modification of the configuration shown in FIG. 6;
FIG. 8 is a cross-sectional view of another embodiment of the present invention, and FIG. 9 is a cross-sectional view of another portion of the structure shown in FIG. 2...Reaction chamber, 4...Gas curtain chamber, 6...
Vacuum chamber, 8...Interface plate, 10
...Orifice plate, 34...Opening, 42...
...Orifice, 50... Fringe, 52...
Mass spectrometer, 60, 62, 64... area, 7
0,72... Electrostatic lens.

Claims (1)

[Scope of Claims] 1. A method for focusing ions moving from a gas region 4 through a vacuum chamber 6 to an analyzer 52 disposed within the vacuum chamber, comprising: (a) introducing a gas from the gas region into the gas region; (b) creating a vacuum in the vacuum chamber such that the gas enters the vacuum chamber through the orifices and forms a freeget 50, 50'; and maintain the gas in the gas region at a high pressure, where the concentration of the gas is reduced by the vacuum chamber away from the orifice where free molecular flow occurs where collisions between ions and gas molecules are rare. (c) transporting said ions from said gas region into said vacuum chamber through said orifice; (d) a region 60 where collisions between ions and gas molecules occur frequently; While the ions are in the free jet at 62', a focusing electric field is applied to the ions, and the kinetic energy given to the ions by this electric field is averaged by the transfer of kinetic energy between the ions caused by the collision. an electrostatic field arranged such that the focusing electric field causes focusing of ions only in the regions 60, 62' where the collisions occur frequently; In the path of ions provided by lens elements 70, 72, 72' in front of said vacuum region 64 and behind said region 60, 62' where said collisions are frequent, said electrostatic lenses 70, 7
2, 72' so that no electric field is applied, so that the ions are focused toward the analyzer by the focusing electric field, and the spread of the kinetic energy of the ions is suppressed. How to focus on ions. 2. The collision-prone region 60 is set sufficiently close to the orifice 42 so that ions under the electric field do not experience a kinetic energy range of more than about 2 ev. Method described. 3. The collision-prone region 60 is such that the ions therein experience an internal energy between 0.1 and 1.5 ev and do not experience a kinetic energy width of more than about 2 ev in the presence of an electric field. The method described in Scope No. 1. 4 An orifice along said axis such that Xl = (50D ^{+ 40D} _{- 25D} )√, where D is the diameter of the orifice and m is a factor where the gas concentration number differs from 2.7 x ¹⁰¹⁹ molecules/ ^cm3 . 2. The method of claim 1, wherein the entrance end 108' of the electrostatic lens 72' is at a distance Xl from . 5. The gas is a gas that has a vapor pressure significantly lower than atmospheric pressure at a predetermined temperature, and the gas is solidified in the vacuum chamber by cooling a part of the vacuum chamber to a predetermined temperature or lower, and is used as a cryogenic pump. 2. A method according to claim 1, wherein the method is characterized in that: 6 An apparatus for analyzing trace ions comprising: (a) a vacuum chamber 6 having an orifice plate 10 with an orifice 42; (b) means for applying trace ions to a source gas near the orifice plate; and (c) an axis of the orifice. When gas expands from the orifice in the vacuum chamber so as to form a freeget approximately centered at
(d) means for transporting ions along with a gas into the vacuum chamber from an orifice along said axis; and (e) means spaced apart from said orifice into said vacuum chamber. (f) a trapezoidal cone-shaped electrostatic lens 70, 72, 72' having a sharp tip and disposed in the vacuum chamber; (g) a connection between the entrance of the electrostatic lens and the orifice; distance between
When Xl is the diameter of the orifice and D is the factor where the gas concentration number differs from 2.7 × ^{10 19} molecules/ _cm ³ , then and means for attaching an electrostatic lens so as to face each other.