Time-of-flight mass spectrometer

Details Time-of-flight mass spectrometer Time-of-flight mass spectrometer

2002-09-23

2004-10-12

Lee, John R. (Department: 2881)

Radiant energy

Ionic separation or analysis

Ion beam pulsing means with detector synchronizing means

C250S281000, C250S288000, C250S3960ML

Reexamination Certificate

active

06803564

ABSTRACT:

The present invention relates to a time-of-flight mass spectrometer. More particularly, the present invention relates to a time-of-flight mass spectrometer having an ion reflector.
BACKGROUND OF THE INVENTION
Time-of-flight mass spectrometers analyze the mass numbers (more exactly, mass-to-charge ratios) of ions by measuring the flight times, i.e. the times that the ions take to travel from the ion source to the ion detector. To improve the accuracy of the analysis of the mass numbers, an ion reflector is used to temporally converge the ions so that the flight times of ions with the same mass number become as equal as possible.
By a known construction of a time-of-flight mass spectrometer, ions created in an ion source are introduced into a field-free drift space and are then reflected by an ion reflector. The ion reflector is composed of a series of parallel plate electrodes, which generates an electric field for reflecting ions back into the field-free drift space. The ions reflected by the ion reflector are detected by an ion detector.
To improve the accuracy of the measurement of flight times, the time deviations of ions due to the initial position should be far smaller than their flight times. Therefore, the ions are often pulsed, or otherwise bunched in time downstream of the ion source. In the initial state, however, the ions have different kinetic energies and hence are diversified in velocity, which causes an undesirable spread of flight times.
The ion reflector is used to compensate for this spread of flight times. Ions with larger initial velocities penetrate deeper into the ion reflector due to their great kinetic energies, where they spend more time before being reflected back into the field-free drift space. In the field-free drift space, on the other hand, the ions spend less time because they have greater velocities. Thus, the increase and the decrease in the flight time cancel each other out. The electric field strength is determined so that the above-described compensation of flight time effectively works over a wide range of initial velocities.
An ion reflector having a uniform (or linear) electric field is called a single-stage reflector. This type of ion reflector can compensate for a spread of flight times only up to the first derivative of ion energy: it can effectively converge the flight times only for a relatively small range of ion energy. Thus, while having been successfully used in many applications, single-stage reflectors are still limited in respect to their ability to compensate for flight times.
To provide a wider range of ion energy compensation, another type of ion reflector, called a dual-stage reflector, uses two stages separated by a fine grid mesh, each stage having a uniform electric field. In the dual-stage reflector, the first stage, which is short in length and has a relatively strong electric field, reduces the energy of ion by more than two thirds. The decelerated ions with their energies being one third or less of the initial energies are reflected in the second stage having a weak electric field. The ions reflected thereby pass through the first stage again, being accelerated there, and return to the field-free drift space. The two stages, working as described above, compensate for the spread of flight times up to the second derivative of ion energy.
The dual-stage reflector was first developed by Mamyrin et al. (B. A. Mamyrin, V. I. Karataev, D. V. Shmikk and V. A. Zagulin, Zh. Eksp. Teor, Fiz. 64 (1973) 82-89; Sov. Phys. JETP., 37 (1973) 45-48). This type of reflector provides the best resolution when the first stage is very short and has an electric field strength much greater than that of the second stage, i.e. when the ratio of the electric field strength of the low-field second stage to that of the high-field first stage is small.
Typically, the first stage is designed to have a length of about 1% of the total length of the reflector. This design is theoretically supported by the fact that the resolution derived from the condition for second order compensation is proportional to the ratio of the ion energy at the boundary of the two stages to the initial ion energy at the front of the reflector.
The maximum value of this ratio is theoretically one third. This value, however, is practically unattainable because it requires the first stage to be infinitely short and the electric field strength to be infinitely great. Therefore, the length of the first stage is chosen as short as possible within a range where no practical problem arises in respect of electric discharge, mesh size effect, etc.
In practice, the amount of energy reduction at the boundary of the two stages is set to be less than about 0.7 of the initial ion energy, which is slightly greater than two thirds, and the aforementioned ratio of the electric fields in the two stages is less than 0.25.
A concise explanation of the dual-stage reflector is available in Mass Analysis, Vol. 35, No. 4 (1987) pp. 186-200. With the average kinetic energy of ions denoted by U
0
and the spread of the kinetic energy denoted by ±&Dgr;U/2, the resolution R under the condition for second order convergence is given by the following approximate equation, which is the third
R
=
32
3
⁢
(
U
0
Δ
⁢
 
⁢
U
)
3
×
(
1
-
4
⁢
 
⁢
l
1
L
⁢
(
1
+
E
1
2
⁢
E
s
)
)
derivative of the ion energy:
where L is the length of the field-free drift space, l
1
is the length of the first stage, E
1
is the electric field strength of the first stage and E
s
is the electric field strength in the accelerating region of the ion source. E
s
is determined as great as possible to reduce the turn-around time. Therefore, the final term E
1
/(2E
s
) can be usually ignored.
Dual-stage reflectors have excellent mass resolutions and are effectively applicable to most high-resolution applications currently used. The dual-stage reflector, however, is accompanied by a problem resulting from the use of the mesh or grid, which is necessary to separate the two stages or to separate the reflector from the field-free drift space in order to generate a uniform electric field in each of two stages. That is, the ions need to go through the mesh or grid four times, where they suffer scattering and deflection. This deteriorates the ion detection sensitivity of the apparatus.
U.S. Pat. No. 4,731,532 discloses an ion reflector designed without a grid or a mesh, as shown in
FIG. 1
, to alleviate the deterioration of the sensitivity.
In this ion reflector, however, the electric field in the first stage is so strong that it penetrates into the second stage or into the field-free drift space, which causes the equipotential surfaces to be bent on both sides of the first stage. This bending of the equipotential surfaces deflects the ions and, as a result, causes a shift of the flight times of the ions.
These effects are corrected by additional electrodes, called the focusing electrodes, attached to the front of the first stage to prevent the ion dispersion.
Another type of grid-less reflector corrects the flight times over a wider range of energy. The ion reflector, disclosed in the U.S. Pat. No. 4,625,112, uses a quadratic electric field to reflect the ions, which, in theory, provides the perfect temporal correction. This ion reflector, however, is very difficult to design because it has no field-free electric field and hence the electric field should be exactly the same as theoretically specified throughout the entire flight path of the ions from the ion source to the ion detector. Furthermore, even when the electric field is quadratic at around the electrodes, the electric field at around the central axis of the reflector is deviated from that field, which makes it difficult to obtain the desired performance. Another ion reflector disclosed in the U.S. Pat. No. 5,464,985 uses a curved electric field.
Each of the two patents embodies a method of determining the electric field strength that is zero or close to zero at the front of the reflector and gradually increases as it goes deeper into

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