Electric lamp and discharge devices – Photosensitive – Secondary emitter type
Reexamination Certificate
2000-04-03
2003-09-09
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Photosensitive
Secondary emitter type
C313S1050CM, C313S533000
Reexamination Certificate
active
06617768
ABSTRACT:
TECHNICAL FIELD
This invention relates to ion detectors for mass spectrometry. In particular, the invention relates to a hybrid electron multiplier detector for time of flight mass spectrometry.
BACKGROUND ART
Mass spectrometry is an analytical methodology often used for quantitative elemental analysis of materials and mixtures of materials. In mass spectrometry, a sample of a material to be analyzed called an analyte is broken into particles of its constituent parts. The particles are typically molecular in size. Once produced, the analyte particles (ions) are separated by the spectrometer based on their respective masses. The separated particles are then detected and a “mass spectrum” of the material is produced. The mass spectrum is analogous to a fingerprint of the sample material being analyzed. The mass spectrum provides information about the masses and in some cases quantities of the various analyte particles that make up the sample. In particular, mass spectrometry can be used to determine the molecular weights of molecules and molecular fragments within an analyte. Additionally, mass spectrometry can identify components within the analyte based on the fragmentation pattern when the material is broken into particles (fragments). Mass spectrometry has proven to be a very powerful analytical tool in material science, chemistry and biology along with a number of other related fields.
A specific type of mass spectrometer is the time-of-flight (TOF) mass spectrometer. The TOF mass spectrometer (TOFMS) uses the differences in the time of flight or transit time through the spectrometer to separate and identify the analyte constituent parts. In the basic TOF mass spectrometer, particles of the analyte are produced and ionized by an ion source. The analyte ions are then introduced into an ion accelerator that subjects the ions to an electric field. The electric field accelerates the analyte ions and launches them into a drift tube or drift region. After being accelerated, the analyte ions are allowed to drift in the absence of the accelerating electric field until they strike an ion detector at the end of the drift region. The drift velocity of a given analyte ion is a function of both the mass and the charge of the ion. Therefore, if the analyte ions are produced having the same charge, ions of different masses will have different drift velocities upon exiting the accelerator and, in turn, will arrive at the detector at different points in time. The differential transit time or differential ‘time-of-flight’ separates the analyte ions by mass and enables the detection of the individual analyte particle types present in the sample.
When an analyte ion strikes the detector, the detector generates a signal. The time at which the signal is generated by the detector can be used to determine the mass of the particle striking it. In addition, for many detector types, the strength of the signal produced by the detector is proportional to the quantity of the ions striking it at a given point in time. Therefore, for these detector types, the quantity of particles of a given mass often can be determined as well as the time of arrival. With this information pertaining to particle mass and quantity, a mass spectrum can be computed and the composition of the analyte can be inferred.
Of significant importance to the performance of a TOF mass spectrometer is the design and performance of the ion detector. Ideally, the detector should have high sensitivity, low noise and high dynamic range. In addition, the detector should provide good temporal resolution. Sensitivity is a measure of the ability of the detector to register the presence of particles arriving individually. An ideal detector would be able to register the arrival of a single ion of any mass and arbitrary energy. However, in practice, detectors often require a number of ions arriving simultaneously to produce a measurable response or signal. High sensitivity refers to the ability of a detector to produce a measurable signal from the impact of a single or very small number of ions. Dynamic range, on the other hand, is a measure of the ability of the detector to produce a signal that is proportional to the number of particles striking the detector at a given point in time. High dynamic range refers to the situation when there are a very large number of particles striking the detector and the detector is still able to produce a signal that is proportional to the number of particles. Temporal resolution refers to the ability of a detector to distinguish between particles based on time of arrival. The arrival of a particle at a detector is often referred to as an “event”. If two events occur at times that are less than the time resolution of the detector, the particles will be indistinguishable and will be registered by the detector as having the same mass. Therefore, time resolution afforded by a detector determines the mass resolution of the TOF mass spectrometer.
A number of different detector types are used in TOF mass spectrometers. Among these are the channeltron, Daly detector, electron multiplier, Faraday cup, and microchannel plate (MCP). The channeltron is a horn-shaped continuous dynode. The inside of the channeltron is coated with an electron emissive material such that when an ion strikes the channeltron it creates secondary electrons. These secondary electrons create more electrons in an avalanche effect and are ultimately detected as a current pulse at the output of the channeltron. The Daly detector is made up of a metal knob that produces secondary electron emissions when struck by an ion. The secondary electrons are accelerated in the Daly detector and, in turn, strike a scintillator that produces photons. The photons are detected as light by a photomultiplier tube (PMT) that then produces the output signal of the detector indicating the presence of an ion impact. An electron multiplier (EM) is similar to a photomultiplier and consists of a series of biased dynodes that emit secondary electrons when the first dynode is struck by an ion. A Faraday cup is a metal cup placed in the path of the ion beam. The cup is connected to an electrometer that measures the ion-current of the beam. The microchannel plate (MCP) is an array of glass capillaries the inside surfaces of which are coated with an electron-emissive material. The capillaries, which typically have an inner diameter of 10-25 um, are biased at high voltage so that when an ion strikes the electron-emissive coating, an avalanche of secondary electrons is produced. The secondary electron avalanche cascade effect creates a gain of between 10
3
and 10
4
and ultimately produces an output current pulse corresponding to the initial ion impact event.
FIG. 1
illustrates a typical MCP
10
detector configuration along with an expanded close-up cross-section
18
of a single channel within the MCP. The MCP
10
is positioned in front of an anode plate
11
such that the analyte ions
12
strike the MCP
10
instead of the anode plate
11
. An analyte ion
12
that enters a channel
14
eventually strikes the sidewall
15
of the channel
14
within the MCP
10
. The sidewall
15
is coated with an electron emissive material. The impact of the analyte ion
12
on the electron-emissive material coating the sidewall
15
causes the emission of secondary electrons
16
. The secondary electrons
16
created by the impact of the analyte ion
12
radiate from their point of creation and often impact the sidewalls
15
of the channel
14
, for example, as illustrated in FIG.
1
. Each impact of secondary electrons
16
with a sidewall
15
can result in the creation of more secondary electrons
16
. The end result is that one analyte ion
12
results in the creation of a large number of secondary electrons
16
that ultimately exit the MCP
10
and strike the anode plate
11
, often a Faraday cup, where they can be detected as a current pulse. The total number of secondary electrons exiting the MCP and striking the anode plate
11
that are produced by the impact of a single analy
Agilent Technologie,s Inc.
Patel Ashok
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