Note: Descriptions are shown in the official language in which they were submitted.
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PHOTOEMISSION INDUCED ELECTRON IONIZATION
Cross Reference to Related Application
This application claims priority to Application No.
61/326,524 filed on April 21, 2010.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of detection
apparatuses used to screen for the presence of explosives
and other chemical entities.
2. Background Information
Mass spectrometry (MS) is one of the most important
analytical methods for analyzing samples and materials for
chemical composition. A main component of a MS instrument
is the ionization source. Ionization sources are also used
for other analytical instrumentation such as ion mobility
spectrometry (IMS). There are many types of ionization
sources used in these instruments depending on the analysis
applications, and also the methods and types of molecules
that are being analyzed.
There has been new developments in ionization sources
for MS instruments that are based on atmospheric pressure
ionization (API). These type of sources are typically
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T 1
employed for MS analyses that use liquid chromatography
(LC) for sample preparation and separation. The most
common ionizers are electrospray ionization (ESI),
atmospheric pressure chemical ionization (APCI) and
atmospheric pressure photoionization (APPI). Recent
developments have included developing multimode sources
including some combination of these sources.
Whereas significant development of ionization sources
has been made in recent years for LC-MS, very little new
development has been made for the related analytical
instrumentation of gas-chromatography (GC) MS (GC-MS). The
ionizers for GC-MS typically occur at less than atmospheric
pressure. The method of chemical ionization (CI) is based
on introducing some external source of gaseous molecules
(e.g., methane, methanol, ethanol, etc.) into a chamber in
which a discharge is formed. These ions then transfer
their charge to molecules that might be eluting from a GC
column. The sample eluting from the GC column is vaporized
due to the heat of the GC column and is therefore in
gaseous form when entering the ionization volume, which for
CI is at about 1 torr to optimize the discharge and charge
transfer process.
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The more common method of ionization for GC-MS is
electron ionization (EI), which is based on creating a
stream of electrons that are accelerated to some energy by
electrical fields and crossing it with a sample that elutes
from a GC column. The typical source of the electrons is a
heated filament that when heated by passing a current
through the wire causes thermionic emission of electrons.
This process is essentially the same as what occurs in an
incandescent light bulb. In order for the filament to have
reasonable lifetime against burning out due to gaseous
bombardment and oxidation, it must be in a vacuum region,
typically less than lO-5 torr. The filaments can be coated
with various materials (e.g., thorium) that increase the
efficiency of thermionic emission of electrons allowing the
filament to be operated at somewhat reduced temperatures,
however, the tolerance to reduced vacuum conditions (i.e.,
higher pressures) is not significant. There are various
types of devices that are referred to as cold cathodes that
can improve on the tolerance of electron emitters to
reduced vacuum conditions; however, again to achieve
reasonable lifetimes, the pressures are on the order of 10-4
torr or less.
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Because of the limitation of standard EI sources with
pressure there have been some ion source designs that
attempt to generate electrons at high vacuum and then focus
them through slits into a tube or other chamber where the
sample is maintained at higher pressure. These closed ion
sources achieve a higher density of electrons and gaseous
sample; however, the intensity of the electron beam is
greatly reduced due to inefficiencies in the transmission
through the slit.
The process of photoemission of electrons off of
surfaces is generally known in the art and the explanation
of the process was first revealed by Einstein in his Nobel
Prize winning work on the photoelectron effect. Einstein
showed that materials such as metals have work functions
for electron detachment and that the energy of the electron
E detaching from a material of work function W due to the
impingement of a photon of energy by is given by the simple
equation
E = by - W
The condition for emitting an electron is therefore by W.
The typical value of w for metals and other photocathode
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materials is in the 4-5 eV range. The upper range
corresponds to photons of wavelength about 250 nm.
The physical process of photoemission of electrons is
used routinely in devices such as photodiodes and
photomultiplier tubes. These devices measure the intensity
of light by converting photons that hit the photocathode
surface into electrons that can be measured by current
measuring devices.
The development of photoemission as an ionization
source; however, is not common. A few publications discuss
methods based on directing a W laser at a metal surface in
vacuum to generate electrons that are then accelerated
through a molecular beam to achieve ionization (Rohwer et
al. 1988; Syage et al. 1989; Boyle et al. 1991; Boesl et
al. 1994). However lasers are very complex and expensive
and therefore not practical for a general purpose chemical
analysis device. 'Furthermore the UV lasers are invariably
pulsed lasers and therefore do not allow a continuous beam
of electrons, which is desirable for most common methods of
GC-MS analysis and direct sampling MS analysis.
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There are very few developments of photoemission of
electrons as an ionization source for MS, IMS, or any
analytical method. U.S. Patent No. 4,574,004 issued to
Schmidt-Ott et al discloses a method for charging particles
by irradiating. with light that causes photoemission of
electrons from the particle, but this does not use the
resulting electrons as an ionization source. U.S. Patent
5,461,280 issued to Kane discloses a method to enhance a
cold-cathode emitter by irradiating the surface with
photons thereby allowing electron emission with a lower
applied potential electric field. However, this method
does not address total electron flux, or use as an
ionization source.
U.S. Patent No. 4,713,548 issued to Kim discloses a
photoelectron emitter comprising a UV light source. and
photocathode to create negative ions; however, this
invention does not disclose or teach how to achieve
positive ionization or a combination of ionizations. U.S.
Patent No. 7,196,325 issued to Syage,discloses a photon
source and photocathode for ionization in combination with
a glow discharge source. However, this patent, nor any of
the others referenced disclose or teach how to achieve a
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narrow energy electron beam, convenient replacement without
disrupting the vacuum, integration with a GC sampling
system, nor the combination of low and medium pressure
operation along with the combination of PE-EI and PI.
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SUMMARY OF THE INVENTION
A monitor that can detect at least one molecule. The
monitor includes a housing with a passage that can receive
a sample, and a photocathode that is located within the
housing. The monitor also includes a first ultraviolet
light source that can direct ultraviolet light onto the
photocathode to create electrons that ionize molecules
within the sample, and a detector that is coupled to.the
housing to detect at least one ionized molecule.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1A and 1B are illustrations showing a prior art
monitor that ionizes molecules of a sample;
Figures 2A and 2B are illustrations showing different
arrangements for a monitor of the present invention;
Figures 3A and 3B are illustrations of an embodiment
where photoemitted electrons are generated on the opposite
side of an ionization volume;
Figures 4A and 4B are illustrations of an embodiment in
which the light source is mounted outside of a vacuum
chamber;
Figures 5A-C are illustrations showing different
arrangements of an embodiment of a monitor that operates at
higher pressure;
Figures 6A and 6B are illustrations of embodiments that
can achieve negative ionization of photoemitted electrons;
Figures 7A-C are illustrations of embodiments that can
achieve both photoemission El and photoionization;
Figures 8A and 8B are illustrations of embodiments
showing monitors operating at different sample pressures;
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Figure 9 is an illustration of an embodiment in which
the photoemission light source impinges on an electron
amplifier device; and,
Figure 10 is a graph showing mass spectra of toluene,
acetylnitrile, and methanol in air recorded for a device
that can operate in both PE-EI and in PI mode.
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DETAILED DESCRIPTION
Disclosed is a monitor that can detect at least one
molecule. The monitor includes a housing with a passage
that can receive a sample, and a photocathode, that is
located within the housing. The monitor also includes a
first ultraviolet light source that can direct ultraviolet
light onto the photocathode to create electrons that ionize
molecules within the sample, and a detector that is coupled
to the housing to detect at least one ionized molecule.
The monitor enables electron ionization (EI) of a sample
for chemical analysis without the disadvantages of current
methods that use a hot filament or other thermal cathode
devices. The ionization device is especially useful for
GC-MS instrumentation since it can provide highly
fragmented ions, which are useful for structure analysis
and for identifying molecules against an EI data base.
However, it can also provide a high yield of unfragmented
ions, which are important for the emerging application of
GC-MS/MS; wherein instead of fragmenting in the initial
ionization step, it is desirable to form parent ions that
are then fragmented in a controlled manner through mass-
selected ion collision in a buffer gas chamber.
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What is described is a technology for EI that is much
more tolerant of higher pressure conditions than
conventional EI sources and can therefore operate at much
higher sampling densities leading to much higher detection
sensitivity. Furthermore, the light source can be mounted
external to the vacuum system enabling replacement without
venting the vacuum. Venting the vacuum of a MS or other
analyzer requires a complete shutdown of pumps and
electronics and the cycle time for venting, servicing, and
pumping down to restore analysis conditions can be very
long (i.e., several hours). The replacement of the light
source in the disclosed monitor can be accomplished in just
a few minutes. This is particularly desirable for in-field
analysis and for real-time analysis, where down time can
not be tolerated.
The disclosed monitor is advantageous over the prior
art because; it can operate at much lower temperatures and
can operate at much higher pressures leading to greater
detection sensitivity; is not susceptible to oxidation so
it can be used in analyzers that use direct air sampling;
the energy distribution of the electrons from the
photocathode surface is very narrower due to low
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temperature and a narrow photon energy bandwidth (<0.1
electron volts); allows for a large electron emitter
surface that greatly reduces space charge effects that can
limit electron flux at low emission energies; has a longer-
life than typical filament light sources of the prior art;
is easy to maintain because the light source can be mounted
exterior to the vacuum; and can be turned on and off
rapidly and evenly and can therefore be operated in either
continuous or pulsed mode as well as modulated to extend
the life by turning it on precisely when needed without
requiring a warm up time.
The monitor may also combine photoemission electron
ionization (PE-EI) with direct photoionization (PI), which
have novel benefits for GC-MS and direct MS analysis. These
benefits include; a capability to ionize a wider range of
molecules; an option to use PE-EI to ionize and fragment
molecules similar to conventional EI in order to obtain
structure analysis and/or use PI for ionizing molecules
with minimum fragmentation which is desirable for many MS
analyses and particularly for hybrid MS/MS type analyses;
and a capability to generate both positive and negative
ions with the advantages stated above.
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Referring to the drawings more particularly by
reference numbers, Figure 1A shows the basic configuration
of a conventional EI source 10 that uses a hot filament to
generate electrons by thermionic emission. The source 10
consists of a filament 21, a housing 22 maintained at some
voltage, an electrostatic gate lens 23 that is at a
switched voltage to allow electrons to exit or not, an
electrostatic lens 24 at some voltage. The filament 21 is
normally set to the same or similar voltage as the housing
22 so that the electron energy exiting lens 24 has an
energy given approximately by the difference in the voltage
applied to the housing 22 and lens 24. The electrons pass
through the ionization region 30 and impact a lens plate
32. The electrons cross through a sample that exits from
the sample inlet 14. Electrons can impact molecules to
create ions such as M+, which are then typically propelled
by electrostatic lenses such as 31 and 33 to a detector 34.
Figure 1B illustrates the basic embodiment of monitor
with a photoemission electron ionization-(PE-EI) source 100
that includes an ultraviolet (UV) light source 101 and a
photocathode surface 102. The light source 101 emits
ultraviolet light that hits the photocathode 102 to create
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electrons. The photoemission electrons can then be
accelerated into the ionization volume 30 by the same means
described above for the prior art source 10.
Figures 2A-C illustrate a variety of configurations and
operation of a PE-EI source. Figure 2A shows a source 200
that is similar to the source 100 in Figure lB except that
the gate and acceleration electrostatic lenses 223 and 224,
respectively are curved allowing the electrons to be
focused to the center of the ionization volume 30. This
may be desirable because depending on the divergence of the
light for the W source chosen and the distance from the
photocathode surface, the photoemitted electrons may be
spatially dispersed. For some applications it is desirable
to spatially disperse the photoemitted electrons in order
to maximize the total flux of electrons coming off the
surface. Figure 2B shows an ionization source 240 that
uses a slanted surface 103 for the emission of electrons.
Figures 3A and 3B illustrate an embodiment in which the
light source 101 irradiates a photocathode surface 301 on
the opposite side of an ionization volume 330. Figure 3A
shows an arrangement in which the photocathode surface 301
and the electrostatic lens 302 are planar. The electron
CA 02738053 2011-04-21
energy is given by the voltage difference between 302 and
301. Figure 3B shows a similar arrangement except that the
photocathode surface 311 and electrostatic lens 312 are
curved in order to achieve a focusing of the photoemitted
electrons to the center of the ionization volume 330.
Figure 4 illustrates an embodiment in which the light
source can be mounted outside the vacuum chamber. This
embodiment is particularly useful if the detection system
is based on mass spectrometry or another chemical analyzer
that requires vacuum. With this embodiment the ionization
source can be replaced if necessary without venting vacuum,
which enables operation to be restored in a matter of
minutes rather than hours as discussed above. Figure 4A
shows the configuration in which the light source 101 is
mounted orthogonal to the path of the sample inlet 14, the
ionization volume 30 defined by the electric field gradient
across electrodes 31 and 32 and the final path of the ions
M+ to the detector. Figure 4B shows a view of this
embodiment rotated 90 degrees. The light source 101 is
shown mounted exterior to the electron source region 420
and the UV light travels through a UV transmission window
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410. This window material can be any of several standard
materials such as.quartz silica or sapphire.
One of the main attributes of the disclosed
photoemission EI source is the capability to operate the
electron source at pressures much higher than is typical
for standard filament or cold cathode EI devices. This
enables operation at sample pressures more than a few
orders of magnitude higher and this can lead to a
proportionately higher yield of ions for detection. As
discussed above, the typical pressure under which
conventional EI sources operate is about 10-4 to 10-6 torr
due to the deterioration of these sources in the presence
of atmospheric gases and particularly oxygen. The
photoemission light sources that are disclosed here do not
have such pressure limitation, which enables embodiments
that can achieve very high sensitivities not achievable by
conventional EI sources.
Figures 5A-C illustrate a variety of embodiments for
higher pressure PE-EI. Figure 5A shows a simple ionization
source 500 in which the sample inlet 514 fills an
ionization region 520. The light source 101 emits light
that impinges on the electrostatic lens 533, that also
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serves as the photocathode material. Further electric
fields are achieved by the housing lens 532 and the back
lens 531. By applying the appropriate electric field
gradient across these lenses so that the voltage gradient
declines from lens 531 to 533, one can effect the
acceleration of electrons from lens 533 to 531 creating El
of sample eluting from inlet 514. The positively charged
molecules formed, such as M+ will be accelerated in the
opposite direction from the electrons in the direction of
lens 533 and exit the aperture 540 and toward an analyzer
such as a mass spectrometer.
Figure 5B shows an embodiment of an ionization source
510 that has two stages of ion acceleration. The utility
of this source is the capability to contain the ions within
the ion source and optimize the EI process in the first
region and provide for a second region of ion acceleration
defined by electrodes 534 and 535 that can be optimized to
direct molecule ions out of the ion source aperture 520 and
toward an analyzer. Figure 5C shows a variation of this
embodiment in which the source 520 has slanted electrodes
536 to provide a focusing effect of the photoemitted
electrons toward the centerline of the ion source to
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achieve better ionization efficiency and better
transmission efficiency of the formed ions out of the ion
source aperture 540.
Figures 6A and 6B show embodiments of the higher
pressure PE-EI source configured for negative ionization.
The ionization source 600 in Figure 6A is very similar to
the ionization source 500 in Figure 5A except that the
voltages applied to electrostatic lenses 531 and 533 will
be such that the electric field gradient will be lower in
order to reduce the electron energies and maximize the
process of electron attachment to form negative ions. The
lower electric field will also allow negative ions to exit
aperture 540. Due to the higher pressure in ionization
region 520 compared to the analyzer region 620, negative
ions may stream out of the aperture 540 even if the
electric field gradient is negative from lens 531 to lens
533. In this case both positive and negative ions may
stream out together.
Figure 6B shows a configuration in which photoemitted
electrons are created on the surface of electrode 531 The
advantages of this configuration is that the flow of
electrons and negative ions is in the same direction and
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follows the desired electric field gradient across
electrodes 531 and 533. With this embodiment it is not
necessary to rely on the pressure flow through aperture 540
to carry ions out. This configuration also has the
advantage of impeding the flow of positive ions through
aperture 540 since they will be drawn to electrode 531 due
to the lower voltage on 531 versus 533.
Figure 7A illustrates a multimode source 700 that
contains provisions of PE-El using light source 101 and for
direct PI using light source 701. These light sources can
generate different types of ions and therefore may offer
selectivity in the types of molecules that are detected.
For example the PI light source 701 will only ionize
molecules whose ionization potential is below the photon
energy of source 701. By way of example light source 701
could be a low pressure krypton discharge source that emits
photons at 10.0 and 10.6 eV. Only molecules that have
ionization potentials below 10.6 will be ionized and
observed. This is beneficial for air sampling by avoiding
ionizing of the most common air molecules (e.g., N21 02, H2O,
CO2, CO, Ar, etc.) .
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Figure 7B shows an embodiment of an ion source 720 that
can have a similar capability to ion source 700. In this
embodiment the vacuum UV (VUV) light source 701 can achieve
both direct PT as well as PE-EI. However, the total number
of available photons for light source 701 may be
significantly lower than for light source 101, so that the
PE-ET efficiency will not be as high with the single light
source configuration 720. However, the ion source may be
more simple than ion source 700, which could be desirable
for certain applications and where economy of the
instrument is important.
Figure 7C shows an embodiment of an ion source 740 that
has both the UV light source 101 and 7W light source 701,
but mounted in different regions of the ion source.
The two ionization mechanisms PE-El and PI that can.be
achieved in ion sources 700, 720, and 740 can be operated
simultaneously to maximize the number and types of
molecules that can be detected; or it can operate in one or
the other mode depending on the application; or it can
operate in rapidly switching mode. Other modes of
operation can also be performed, particularly including
detection of positive and negative ions.
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Figures 8A and 8B shows other multimode embodiments.
Figure 8A shows a combination ion source 800 that basically
merges ion source 510 in Figure 53 with ion source 240 in
Figure 2B. The source 800 is shown by way of example.
Other combinations of higher pressure sources such as 500,
510, 520, 600, and 620 can be combined with lower pressure
ion sources 100, 200, 240, 300, 340, and 400.
Figure 8B shows an embodiment of a combination
multimode ion source 850 that combines the higher pressure
PI source 750 in Figure 7B using light source 701 with the
lower pressure ion source 240. This combination is
complementary because direct PI ions are formed in
ionization region 520 and PE-EI type ions are formed in
ionization region 30. The PE-EI ions can be primarily
highly fragmented, which is desirable for GC-MS analysis
whereas the PI ions can be primarily parent ions with
minimum ion fragmentation, which is desirable for GC-MS/MS
experiments.
As discussed above for the multimode sources 700,
720, and 740 in Figures 7A-7C,. the multimode sources 800
and 850 in Figure 8 can operate in simultaneous mode, in
one or the other mode,.or in rapid switching mode. Other
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modes of operation can also be performed, particularly
including detection of positive and negative ions.
Figure 9 shows an embodiment 900 in which the light
source 101 impinges onto a device that photoemits electrons
and then amplifies the electrons through a charge
amplification process. This can be a glass or similar
material with multiple perforation channels and an electric
field applied across it to accelerate electrons into the
channels to generate additional electrons. Such a device
is often called a microchannel plate, but other devices can
also be used including devices with chevron structures.
The use of this amplified photoinduced electron emission
source can be used for the previously described
embodiments.
Figure 10 shows plots of mass spectra resulting from a
higher pressure ion source similar to 750 in Figure 7B.
The PI lamp is a VLN Kr discharge light source. Direct PI
does not require any electric fields on electrodes L1, L2,
and L3 other than to provide a focusing field to
concentrate the ions through the aperture in L3. However,
at the pressure of the PI source of about 1 torr, the flow
of molecules generally sweeps the ions out of the aperture
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without the need for strong electric fields. For direct PI
L1, L2, and L3 were all set to OV. A sample was introduced
that contained low level vapors of toluene, acetonitrile,
and methanol in air. Only toluene has an ionization
potential below the VW photon energies of 10.0 and 10.6
eV. As seen in the top graph of Figure 10, only toluene
ion is seen consisting of the molecular ion T+ and a smaller
signal due to loss of one hydrogen [T-1]'. This is a
typical signal and ion distribution for PI.
The bottom graph in Figure 10 shows the results when L3
is set to 400 V. This gives a spectrum that looks like an
EI spectrum. Acetonitrile and methanol ions are now seen
(as protonated molecular ions) as are N2+ and 02+. Since
these molecules do not ionize by PI, the mechanism
occurring here is PE-EI. At L3 = 400 V, photoemitted
electrons are now accelerated to energies that are
sufficiently high to achieve El. It is further noted that
the ion distribution for toluene now shows that the signal
intensity is greater for [T-1]+ than for T+ as is observed
in EI spectra.
we now consider properties of the light source and
photocathode material that can be used to construct a PE-EI
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source for the various embodiments described above. For
photocathodes with work functions of 4-5 eV, photon
energies around 250-300 nm may be desired. There are
several options for light sources in this region, the most
common being 266 nm lasers (Nd:YAG, 4th harmonic) and 254 nm
mercury lamps (hot-cathode plasma discharge). However,
lasers are expensive and complex and the mercury lamps are
bulky and cannot provide directed UV light. Although some
mercury lamps are equipped with light guides allowing
directed W light with outputs on the order of ?l W/cm2.
Another option is recently commercialized deep UV light
emitting diodes (LEDs). The advantage of LEDs are small
size, robustness, digital control, instant on/off, low
voltage, and no hazardous waste.
By way of example we will consider as a reference
source a commercially available LED that emits at 260 nm
.(Gaussian distribution from 255 to 264 nm) and provides
directed UV output of 300 pW for continuous operation and
mW in pulsed mode. A ball lens can be installed to
focus the light to a 1.5-2 mm spot if desired. Other
options include near LTV LEDs, such as at 365 nm ( 5 nm),
which provide radiant outputs in the range of 0.1 - 10
CA 02738053 2011-04-21
W/cm2. However, this wavelength corresponds to energy of
3.4 eV and therefore the photocathode material chosen would
need to have a work function less than that value.
Photocathode materials may range from standard metals
to exotic inorganic compositions used in photodiode and
photomultiplier detectors. Metals such as stainless and
copper have quantum efficiency ~ values of about 10-4
electrons/photon. Molybdenum has a ~ value of about 0.05..
Materials such as CsI, CsTe, Ki, and KBr have ~ values in
the UV of about 0.5.
The electron photoemission current that can be achieved
by the disclosed devices can be estimated through the
following analysis. The current is given by:
I = Px4xq/hv
Where I is current in amps, P is radiant power in watts,
is photocathode quantum efficiency in units of electrons
per photon, q is the charge of an electron, and by is the
energy of the photon. The table below shows calculated
photoemission currents for various values of radiant power
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and quantum efficiency and assumes a photon energy of 255
nm, which is 4.86 eV/photon.
I(pA)
P(mW) 0.3 10 .300
0.0001 0.006 0.2 6
0.0010 0.056 1.9 56
0.0100 0.564 18.8 564
0.1000 5.641 188.0 5,641
0.5000 28.204 940.1 28,204
Standard EI sources used in MS systems, such as for GC-
MS analysis typically operate at emission currents of about
1-1000 ptA. The table above shows that comparable to
greater emission current can be achieved by the disclosed
invention.using typical W light sources and photocathode
materials.
. While certain exemplary embodiments have been described
and shown in the accompanying. drawings, it is to be
understood that such embodiments are merely illustrative of
and not restrictive on the broad invention, and that this
invention not be limited to the specific constructions and
arrangements shown and described, since various other
modifications may occur to those ordinarily skilled in the
art.
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