Radiant energy – Ionic separation or analysis – Ion beam pulsing means with detector synchronizing means
Reexamination Certificate
2000-04-27
2004-01-06
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
Ion beam pulsing means with detector synchronizing means
C250S283000, C250S286000, C250S282000, C250S288000, C250S397000
Reexamination Certificate
active
06674068
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a time-of-flight (TOF) mass spectrometer and, more particularly, to a TOF mass spectrometer having an ion detector that is prevented from saturating.
2. Description of the Related Art
An orthogonal acceleration time-of-flight mass spectrometer (OA/TOF-MS) is an instrument for performing a mass analysis by producing ions continuously by an ion source, introducing the ion beam emitted from the ion source into an ion reservoir, accelerating the ions in the ion reservoir in a pulsed manner in a direction orthogonal to the direction of introduction of the ions, and measuring the flight time from the instant when the ions are accelerated to the instants when the accelerated ion pulses are detected by a final ion detector.
FIG. 1
schematically shows the configuration of an OA/TOF-MS employing an electrostatic reflecting mirror. Now let us consider the instrument as shown in
FIG. 1
that is polarity positive. It has an external ion source
1
for producing positive ions continuously by electron impact (EI), chemical ionization (CI), fast atom bombardment (FAB), electrospray ionization (ESI), atmospheric-pressure chemical ionization (APCI), inductively coupled plasma-mass spectrometry (ICP-MS), or other ionization techniques.
An ion beam emitted from the external ion source
1
at a positive accelerating potential V
1
is focused in the z direction by a focusing lens
2
to which a positive potential V
F
is applied. Then, the beam is admitted into an ion reservoir
3
having an effective length y
0
. The ion reservoir
3
is equipped with a push-out plate
4
. The reservoir
3
is also provided with an ion extraction grid
5
and an exit grid
6
that are located opposite to the push-out plate
4
. The extraction grid
5
is at ground potential. The exit grid
6
is at a negative potential. Thus, an electric field is developed to push out ions in a direction (z direction) orthogonal to the direction of introduction of the ion beam (y direction).
If a push-out pulse
7
consisting of a positive voltage of 2V
s
is applied to the push-out plate
4
, a field gradient is momentarily produced in a region
8
that extends from the push-out plate
4
to the exit grid
6
across the ion extraction grid
5
. This region
8
is known as the two-stage accelerating region. As a result, the ions in the ion reservoir
3
are simultaneously accelerated in the z direction and expelled as ion pulses. The ions are reflected by a mirror portion
9
mounted at an opposite position. Then, the ions travel toward a final ion detector
10
consisting of microchannel plates (MCPs) or the like.
Strictly, the ions have y-direction velocity components given when they are introduced into the ion reservoir
3
. Therefore, if the ions undergo z-direction forces by the electric fields produced in the two-stage accelerating region
8
, i.e., between the push-out plate
4
and the ion extraction grid
5
and between the grid
5
and the exit grid
6
, the direction of travel is shifted to the y direction slightly from the z direction.
When the ions undergo the above-described acceleration, a given energy corresponding to the potential difference between the push-out plate
4
and the exit grid
6
is uniformly imparted to the ions and so ions of smaller masses have greater velocities and ions of greater masses have smaller velocities when the acceleration ends. Because of the velocity variations as described above, the ions are mass-dispersed while they are traveling through a reflectron TOF-MS spectrometer portion
12
placed at a negative potential V
2
by a mass spectrometer portion power supply
11
. Consequently, the ions are dispersed into ion pulses according to mass. As the ions having smaller mass-to-charge ratios (m/z; m: mass, z: valence number) reach the final ion detector
10
sooner, mass dispersion occurs. Thus, the ions can be observed as a mass spectrum.
A tandem MCP (a couple of MCPS), usually employed as an ion detector for TOF-MS to maintain an appropriate secondary electron multiple gain ranging 10
4
-10
6
, is made of millions of very thin capillary tubes of conductive glass bundled together, each having a diameter of 10 to 25 &mgr;m and a length of 0.24 to 1.0 mm. Each tube acts as a secondary electron multiplier. Since secondary electrons travel only less than 1.0 mm in the microchannel plate, the plate can respond at a high speed of 1 nanosecond (ns) to applied pulsed, charged particles. On the other hand, where a photomultiplier tube or secondary electron multiplier tube where secondary electrons travel about several centimeters is used, a response time of about 5 ns is necessary.
Generally, the mass resolution of a time-of-flight mass spectrometer is given by:
R
=
M
Δ
M
=
t
TOF
2
·
Δ
t
(
1
)
where M is mass in dalton (Da), &Dgr;M is a mass difference, t
TOF
is the flight time of ion M
+
, and &Dgr;t is the width of an ion pulse. The ion pulse width &Dgr;t is independent of the location on the Z-axis where a measurement is made. As the width in the final ion detector becomes narrowest, the mass resolution R is optimum. Accordingly, the width of the incident ion pulse is ideally equal to the width of the output signal from the secondary electron multiplier in the final ion detector. In practice, however, it is inevitable that the final ion detector itself will produce time spread, adding to the pulse width &Dgr;t in the denominator of Eq. (1).
Normally, in a high-resolution time-of-flight mass spectrometer, the pulse width &Dgr;t is about 5 ns at the entrance of the final ion detector. Since the time spread is roughly 5 ns as mentioned above where a photomultiplier or a secondary electron multiplier is used, the mass resolution R of the high-resolution TOF mass spectrometer is greatly affected. For example, when ions impinge on the final ion detector, consider the pulse width (t=5 ns) When leaving the final ion detector, the pulse width is temporally spread out to be (t=5+5=10 ns) Consequently, the mass resolution of the TOF mass spectrometer drops to ½. For this reason, microchannel plates (MCPs) capable of responding in less than 1 ns are often used, especially in high-resolution TOF mass spectrometers.
In this case, however, the problem with the use of microchannel plates (MCPs) is that the linear range of output/input is limited in principle. In particular, the linearity of a microchannel plate is determined by a strip current value intrinsic in the microchannel plate. The linear range of output/input is narrower and indicated by three digits; in the case of a secondary electron multiplier, the range is wider and indicated by 5 digits. The strip current also acts to neutralize the electric charge of secondary electrons produced by the microchannel plate. It is known that the microchannel plate starts to saturate when the average output current of the microchannel plate is 5% to 6% of the strip current.
Of course, where the gain of the microchannel plate is set high, secondary electron-saturation of the microchannel plate tends to occur. Once such saturation takes place, the time taken to neutralize secondary electrons by the strip current is on the order of microseconds (&mgr;s). If more secondary electrons are produced, the time is increased. The microchannel plate is insensitive, i.e., in a dead-time state, until the neutralization is completed. The outputs indicative of peaks of ions impinging during this insensitive period are zero. Peaks indicating these ions are absent from the mass spectrum. If the microchannel plate saturates repeatedly, deterioration of the microchannel plate is accelerated, thus shortening the lifetime.
As an example, it is assumed that a dead time of 1 &mgr;s occurs. We now discuss what mass spectral range is absent from the produced ion spectrum. Where a monovalent ion having a mass of M Da (dalton) is accelerated by V volts and travels L cm through a free space, the flight time
Hashmi Zia R.
Jeol Ltd.
Lee John R.
Webb Ziesenheim & Logsdon Orkin & Hanson, P.C.
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