U.S. patent application number 11/414177 was filed with the patent office on 2006-08-31 for probing method using ion trap mass spectrometer and probing device.
Invention is credited to Minoru Sakairi, Yasuaki Takada.
Application Number | 20060192101 11/414177 |
Document ID | / |
Family ID | 11736470 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060192101 |
Kind Code |
A1 |
Takada; Yasuaki ; et
al. |
August 31, 2006 |
Probing method using ion trap mass spectrometer and probing
device
Abstract
An analyzing method which includes ionizing a sample,
dissociating plurality of precursor ions which have different m/z
values, and deciding if fragment ions of a predetermined m/z value
are present.
Inventors: |
Takada; Yasuaki; (Kiyose,
JP) ; Sakairi; Minoru; (Tokorozawa, JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET
SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
11736470 |
Appl. No.: |
11/414177 |
Filed: |
May 1, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11123202 |
May 6, 2005 |
7057169 |
|
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11414177 |
May 1, 2006 |
|
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10311270 |
Dec 13, 2002 |
6894276 |
|
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PCT/JP00/06411 |
Sep 20, 2000 |
|
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|
11123202 |
May 6, 2005 |
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Current U.S.
Class: |
250/282 |
Current CPC
Class: |
H01J 49/004 20130101;
H01J 49/0031 20130101; G01N 33/0057 20130101; H01J 49/424
20130101 |
Class at
Publication: |
250/282 |
International
Class: |
B01D 59/44 20060101
B01D059/44; H01J 49/00 20060101 H01J049/00 |
Claims
1. A method for analyzing, comprising the steps of: ionizing a
sample; dissociating plurality of precursor ions which have
different m/z values; and deciding if fragment ions of a
predetermined m/z value are present.
2. The method for analyzing according to claim 1, further
comprising the step of selecting ions from the fragment ions of the
predetermined m/z value, dissociating the selected fragment ions,
and analyzing the dissociated ions.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of U.S. application Ser.
No. 11/123,202, filed May 6, 2005, which is a continuation of U.S.
application Ser. No. 10/311,270, filed Dec. 13, 2002, now U.S. Pat.
No. 6,894,276, which is a 371 of PCT/JP00/06411, filed Sep. 20,
2000, the contents of which are incorporated by reference
herein.
TECHNICAL FIELD
[0002] The present invention relates to techniques for detecting
explosives and drugs, and particularly to a detecting device using
ion trap mass spectrometer.
BACKGROUND ART
[0003] Detecting devices are demanded for detecting explosives in
order to prevent terrorism and maintain security in the midst of
the aggravated international conflicts. Baggage inspection
apparatus using X-ray transmission are now widely used as detecting
devices chiefly in airports. Since the X-ray detecting device
detects objects as lumps and identifies dangerous materials from
information of shape, it is called bulk detection. The detecting
method based on gas analysis is called trace detection, and used to
identify substances from chemical analysis information. The trace
detection is characterized in that very small amounts of
ingredients attached to a baggage or the like can be detected. A
security scanner of greater precision is desired to produce by
combining bulk detection and trace detection in order to socially
strengthen the security.
[0004] Custom offices also use detecting devices in order to detect
forbidden chemicals sneaked through various routes. Although bulk
detectors and drug-sniffing dogs are chiefly used in custom
offices, a trace analyzer for forbidden chemicals is eagerly
desired to produce in place of the drug-sniffing dogs.
[0005] Various analyzing methods such as ion mobility spectroscopy
and gas chromatography have been tried as trace detection. Research
is being conducted for developing apparatus having high speed, high
sensitivity and high selectivity as important factors in the
detecting device.
[0006] In these situations, a detecting method based on mass
spectrometry is proposed that is fundamentally excellent in speed,
sensitivity and selectivity as, for example, disclosed in
JP-A-7-134970. A conventional detecting device based on mass
spectrometry will be described with reference to FIG. 16.
[0007] An air suction probe 1 is connected through an insulation
pipe 2 to an ion source 3. The ion source 3 is connected through an
exhaust port 4 and an insulation pipe 5 to an air exhaust pump 6.
The ion source 3 has a needle electrode 7, a first aperture
electrode 8, an intermediate pressure portion 9 and a second
aperture electrode 10. The needle electrode 7 is connected to a
power source 11, and the first aperture electrode 8 and second
aperture electrode 10 are connected to an ion accelerating power
source 12. The intermediate pressure portion 9 is connected through
an exhaust port 13 to a vacuum pump. An electrostatic lens 14 is
disposed in the stage succeeding the intermediate pressure portion.
A mass spectrometric portion 15 and a detector 16 are disposed in
the stage succeeding the electrostatic lens 14. A detected signal
from the detector 16 is supplied through an amplifier 17 to a data
processor 18. The data processor 18 detects a plurality of m/z
values (mass number of ion/valence of ion) of particular chemicals,
thus deciding if the detected gas contains a particular
chemical.
[0008] This data processor 18 has a mass determining section 101, a
drug-A determining section 102, a drug-B determining section 103, a
drug-C determining section 104 and an alarm driving section 105. An
alarm display 19 that is driven by the alarm driving section 105
has display portions 106, 107, 108 disposed.
DISCLOSURE OF THE INVENTION
[0009] The above prior art has the following drawbacks.
[0010] The above device detects chemicals by using the m/z values
of ions generated by the ion source. Therefore, when a chemical
substance exists that generates ions of the same m/z values as
those of the detected drug, there is a possibility that an alarm is
issued despite the fact that there is no drug.
[0011] More specifically, there is the problem that while the
device is detecting an awakening drug within baggage, it may
misreport in response to the ingredients of a beauty product stored
in the baggage. This misreport is caused by low selectivity of the
mass spectrometric portion for analyzing ions, or when the device
cannot distinguish the ions of the drug incidentally having the
same m/z values from the ions of cosmetics.
[0012] Tandem mass spectrometry is known as a method of increasing
the selectivity in the mass spectrometer. As devices for
implementing the tandem mass spectrometry, there are triple
quadrupole mass spectrometer and quadrupole ion trap mass
spectrometer. In the tandem mass spectrometry, the following steps
are performed:
[0013] (1) First-stage mass analysis, in which mass analysis is
made to measure the m/z values of ions generated by the ion
source;
[0014] (2) Selection, in which ions having a particular m/z value
are selected from the ions having various m/z values;
[0015] (3) Dissociation, in which the selected ions (precursor
ions) are dissociated by collision with neutral gas to produce
resolvent ions (fragment ions); and
[0016] (4) Second-stage mass analysis, in which mass analysis is
made for fragment ions.
[0017] When the precursor ions are dissociated, the strength of
chemical bond at each place of the molecule determines where the
molecule is cut away. Therefore, analyzing the fragment ions will
make it possible to obtain a mass spectrum containing an extreme
abundance of molecular structure information of precursor ions.
Accordingly, even if the m/z values of ions generated by the ion
source are the same by chance, it can be decided if the objects
being detected are contained in pieces of baggage by examining the
mass spectrum of fragment ions.
[0018] Thus, if the mass spectrometric portion 15 in the
conventional probing device of FIG. 16 is replaced by a triple
quadrupole mass spectrometer or quadrupole ion trap mass
spectrometer to implement the tandem mass spectrometry, the
selectivity can be improved, and the misreport can be reduced.
However, the tandem mass spectrometry takes a longer time than the
normal mass spectrometry, and thus the detecting speed of the
detecting device cannot be improved.
[0019] For the above reasons, the detecting device has been
requested to have high selectivity and high speed.
[0020] Accordingly, it is an object of the invention to provide a
high-speed, less misreport detecting device for explosives and
banned drugs by using fast screening mode and high-selectivity
detailed checking mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a diagram showing the algorithm of one embodiment
of the invention.
[0022] FIG. 2 is a diagram showing one example of the construction
of a detecting device for embodying the invention.
[0023] FIG. 3 is a diagram showing one example of the construction
of the ion source for embodying the invention.
[0024] FIG. 4 is a diagram showing the construction of the
vapor-sampling portion for embodying the invention.
[0025] FIG. 5 is a timing chart of the voltage application in the
embodiment of the invention.
[0026] FIG. 6 is a diagram showing the procedure of MS/MS
spectrometry in the embodiment of the invention.
[0027] FIG. 7 is a diagram showing the procedure of spectrometry in
the embodiment of the invention.
[0028] FIG. 8 is a flowchart of the tandem mass spectrometry.
[0029] FIG. 9 is a diagram showing the algorithm of another
embodiment of the invention.
[0030] FIG. 10 is a timing chart of the high-frequency voltage
applied to the ring electrode and end cap electrode.
[0031] FIG. 11 is a diagram showing the frequency of the voltage
applied to the end cap electrode during the trap and selection time
interval.
[0032] FIG. 12 is a diagram showing the situation in which the ion
trap and dissociation are simultaneously progressed in the
embodiment of the invention.
[0033] FIG. 13 is a diagram showing the frequency of the
high-frequency voltage applied to the end cap electrode when the
ion trap and dissociation are progressed at a time in the
embodiment of the invention.
[0034] FIG. 14 is a diagram showing the algorithm of still another
embodiment of the invention.
[0035] FIG. 15 is a diagram showing the construction of the device
of the embodiment according to the invention.
[0036] FIG. 16 is a diagram showing the construction of the
conventional mass spectrometer used for detecting dangerous
objects.
BEST MODE FOR CARRYING OUT THE INVENTION
[0037] Embodiments of the invention will be described in detail
with reference to the accompanying drawings.
[0038] FIG. 1 is a diagram showing the algorithm of the first
embodiment of the invention.
[0039] The detecting method of this embodiment has a first analysis
step 201 for acquiring mass spectrum, a first decision step 202 for
deciding if there are ions of a first peculiar m/z value, a second
analysis step 203 for tandem mass spectrometry according to the
decision results in the first decision step 202, a second decision
step 204 for deciding if there are ions of a second peculiar m/z
value, and an announcement step 205 for issuing an alarm in
accordance with the decision results in the second decision step
204. The measuring operation by the steps 201 and 202 is called as
the screening mode, and the measuring operation by the steps 203
and 204 as the detailed checking mode.
[0040] To detect, in step 201 the ions generated from the sample
gas are analyzed, and in step 202 it is decided if ions of the m/z
value corresponding to that derived from the detected object are
detected. For example, when amphetamine as a kind of stimulant drug
is ionized in the positive atmospheric pressure chemical ionization
mode, it produces pseudo molecular ions (M+H).sup.+(M is sample
molecule, and H is proton) that is the addition of proton to
amphetamine molecule. Since the m/z value of the false molecular
ions is 136, decision is made of if ions of 136 in m/z are detected
in step 202 (first decision).
[0041] Here, the m/z value to be decided in step 202 is of course
dependent upon the object to be detected. A plurality of different
m/z values may be decided for various different narcotic drugs and
stimulant drugs.
[0042] When the analysis time in the first analysis step 201 is
selected to be 0.1 second, the results from the repetition of step
201 and integration of the measured results may be used for the
decision in step 202. Since the random noise can be averaged by the
integration, the error in step 202 can be reduced.
[0043] When ions of a predetermined first peculiar m/z value are
decided to exist in step 202, the second analysis step 203 for
tandem mass spectrometry (hereafter, MS/MS) is performed. Step 203
includes the processes of precursor-ion selection, precursor-ion
dissociation, and mass spectrometry of fragment ions. In order to
increase the analysis precision, longer time should be taken in
step 203 than in step 201.
[0044] Step 203 can produce a mass spectrum with abundant molecular
structure information. This mass spectrum is decided (second
decision) in step 204 to have a m/z value peculiar to the object
being detected or not. If it is present, the alarm is activated to
send an alarm signal.
[0045] In step 204, if the mass spectrum of the object being
detected by tandem mass spectrometry is previously measured and
stored as database, referring to the database can make high
precision decision.
[0046] The detecting method using the algorithm shown in FIG. 1
will be described below in more detail. During the detecting
operation, the screening mode (namely, measurement in step 201 and
decision in step 202) is repeated. If the decision in step 202 is
made after the accumulation of 10 measurements each of which is
selected to take 0.1 second for step 201, the total time necessary
for the detecting becomes about 1 second. If the object being
detected is doubted, or decided to be present in the baggage in
step 202, the program goes to the high-selectivity detailed
checking mode. If the decision in step 204 is also made after the
accumulation of 10 measurements each of which is selected to take
0.5 second for step 203, the total detecting time including the
screening mode beginning with step 201 becomes about six seconds.
The baggage inspection associated with security gate should usually
be completed in a few seconds including the times of the carrying
in and setting of the baggage in the detecting device, detecting it
and the carrying out of it. Therefore, the possible detecting time
to be actually taken is in the range from one to two seconds.
However, since the object to be detected is supposed not to be
placed in most pieces of baggage, the detecting can be completed in
about one second by the screening mode. Accordingly, use of the
algorithm shown in FIG. 1 makes it possible to suppress the average
detecting time to about 1.about.2 seconds per piece of baggage even
if the device takes the time necessary for the processes up to the
high-selectivity detailed checking mode. Thus, the baggage
inspection can be made without remarkably interfering with the flow
of baggage at the security gate. In addition, since the decision
based on tandem mass spectrometry is finally performed by the
high-selectivity detailed checking mode, the selectivity can be
increased, and misreport can be reduced.
[0047] Since the detailed checking by tandem mass spectrometry
takes much time as described above, an alarm lamp should be turned
on or a signal for the operator to easily detect should be produced
in the stage where the program goes to step 203 after the decision
of step 202, or when the program shifts from the screening mode to
the detailed checking mode.
[0048] FIG. 2 is a diagram showing the construction of the
detecting device for embodying the invention. Here, a quadrupole
ion trap mass spectrometer (thereafter, referred to as the ion trap
mass spectrometer) is used for the mass spectrometry portion. An
ion source 20 is coupled to a gas introduction tube 21, and exhaust
pipes 22a, 22b. The gas from the gas sampling port is sucked in by
a pump connected to the exhaust pipes 22a, 22b, and introduced
through the gas introduction tube 21 to the ion source 20. The
ingredients contained in the gas introduced into the ion source are
partially ionized. The ions generated by the ion source and part of
the gas introduced into the ion source are fed through first,
second and third apertures 25, and taken in a vacuum space 27 that
is evacuated by a vacuum pump 26. These apertures have a diameter
of about 0.3 mm. The, electrodes of these apertures are heated up
to a range from about 100.degree. C. to 300.degree. C. by a heater
(not shown). The remaining gas not fed into the first aperture 23
is exhausted to the outside of the device from the exhaust pipes
22a, 22b through the pump.
[0049] There are differential evacuation spaces 28, 29 between the
aperture electrodes 23, 24, 25, and these spaces are evacuated by a
roughing pump 30. The roughing pump 30 is usually a rotary pump,
scroll pump or mechanical booster pump. A turbo-molecular pump may
be used for the evacuation of these regions. The aperture
electrodes 23, 24, 25 to which voltages are applied thus improve
the ion transmittance and split cluster ions generated due to
adiabatic expansion by the collision with the residual
molecules.
[0050] In the device shown in FIG. 2, a scroll pump of 900 liters
per min is used for the roughing pump 30, and a turbo-molecular
pump of 300 liters/min for the vacuum pump 26 that evacuates the
vacuum space 27. The roughing pump 30 is also used dually as a pump
for evacuating the back pressure side of the turbo-molecular pump.
The pressure between the second and third apertures 24, 25 is about
1 torr. In addition, only the first and third apertures 23, 25 with
the second aperture electrode 24 removed may be used to produce the
differential evacuation space. However, since the gas inflow
increases as compared with the previous case, it is necessary to
increase the evacuation speed of the vacuum pump used and the
distance between the apertures. Also in this case, it is important
to apply a voltage between the apertures.
[0051] The generated ions are converged by a converging lens 31
after passing through the third aperture 25. This converging lens
31 is usually an Einzel lens formed of three sheets of electrode.
The ions further pass through a slit electrode 32. The ions passed
through the third aperture 25 are converged on the opening of the
slit electrode 32 by the converging lens 31. The neutral particles
that are passed but not converged collide with this slit portion,
and thus they are not easy to arrive at the mass spectrometry
portion side. The ions passed through the slit electrode 32 are
deflected and converged by a double cylinder type deflector 35 that
is formed of inner and outer cylinder electrodes 33, 34 having a
large number of apertures. The double cylinder type deflector 35
deflects and converges the ions by using the electric field of the
outer cylinder electrode that is leaked from the apertures of the
inner electrode. The details are already disclosed in
JP-A-7-85834.
[0052] The ions passed through the double cylinder type deflector
35 are introduced into the ion trap mass spectrometer that is
formed of a ring electrode 36 and end cap electrodes 37a, 37b. A
gate electrode 38 is provided to control the timing of the ions
incident to the spectrometer. Flange electrodes 39a, 39b are
provided to prevent the ions from arriving at quartz rings 40a, 40b
that hold the ring electrode 36 and end cap electrodes 37a, 37b,
charging the quartz rings 40a, 40b.
[0053] The inside of the ion trap mass spectrometer is filled with
helium from a helium gas supply tube (not shown) so that it is
maintained at a pressure of about 10.sup.-3 torr. A mass
spectrometer controller (not shown) controls the ion trap mass
spectrometer. The ions introduced into the mass spectrometer
collide with helium gas to lose their energy, and are caught by an
AC electric field. The caught ions are ejected out of the ion trap
mass spectrometer in accordance with the m/z value of ions by
scanning with the high-frequency voltage applied to the ring
electrode 36 and end gap electrodes 37a, 37b, and the ejected ions
are detected by a detector 42 through an ion ejecting lens 41. The
detected signal is amplified by an amplifier 43, and then processed
by a data processor 44.
[0054] Since the ion trap mass spectrometer has an ion trapping
characteristic within the inside (the space surrounded by the ring
electrode 36 and end cap electrodes 37a, 37b), the ions can be
detected by increasing the ion introduction time even if the amount
of ions is small due to low concentration of the substance being
detected. Therefore, even if the sample concentration is low, the
ion trap mass spectrometer can increase the concentration of the
ions at a large magnifying power, and thus pretreatment
(condensation) of sample can be very simplified.
[0055] FIG. 3 is a magnified view of the ion source portion of the
device shown in FIG. 2. The gas introduced through the sample gas
introduction tube 21 is once introduced into an ion drift region
45. This ion drift region 45 is substantially kept at atmospheric
pressure. Part of the sample gas introduced into the ion drift
region is fed to a corona discharge space 46, and the remaining
portion is exhausted out of the ion source through the exhaust pipe
22b. The sample gas introduced into the corona discharge space 46
is ionized when it is brought in a corona discharge region 48
generated at around the tip of a needle electrode 47 by applying a
high voltage to the needle electrode 47. At this time, in the
corona discharge region 48, the sample gas is introduced in the
direction substantially opposite to the flow of ions that drift
from the needle electrode 47 to the opposite electrode 49. The
generated ions are introduced into the ion drift region 45 through
an opening 50 of the opposite electrode 49 by an electric field. At
this time, a voltage is applied between the opposite electrode 49
and the first aperture electrode 23, thereby drifting the ions so
that the ions can be efficiently led to the first aperture 23. The
ions introduced into the first aperture 23 are fed into the vacuum
space 27 through the second and third apertures 24, 25.
[0056] The amount of gas flowing in the corona discharge space 46
is important for high-sensitive and stable detecting. Therefore, a
flow controller 51 should be provided in the exhaust pipe 22a. In
addition, a heater (not shown) should be used to heat the drift
region 45, corona discharge space 46 and gas introduction tube 21
from the standpoint of sample suction prevention. Although the
amount of gas flowing in the gas introduction tube 21 and exhaust
pipe 22b can be determined by the throughput of a suction pump 52
such as a diaphragm pump and the conductance of the pipe
arrangement, such a controller as the flow controller 51 shown in
FIG. 3 may be provided in the gas introduction tube 21 or exhaust
pipe 22b. By providing the suction pump 52 downstream relative to
the ion generating area (namely, the corona discharge space 46) as
viewed from the gas flow, it is possible to reduce the effect of
the contamination (suction of sample) within the inside of the
suction pump 52.
[0057] FIG. 4 is a diagram showing one example of the sample-gas
collecting portion of the device according to the invention. The
detecting device is roughly divided into a main body 53, a gas
suction unit 54, a case 55, and the data processor 44. The gas
suction unit 54 is connected through the gas introduction tube 21
to a probe 56. The operator handles the probe 56 to bring it near a
baggage or the like so that the air around the baggage can be
sucked and detected.
[0058] The operation of the ion trap mass spectrometer in the
embodiment of the invention will be described just for reference.
FIG. 5 and FIG. 6 are timing charts of the voltages applied to the
ring electrode and end cap electrodes. FIG. 5 shows the operation
in step 201 of FIG. 1, and FIG. 6 shows the operation in step
203.
[0059] In step 201, in the ion trapping time interval, 302, a high
frequency voltage is applied to the ring electrodes to generate an
electric field for catching ions within the mass spectrometer. In
addition, the voltage applied to the gate electrode is adjusted to
control the ions to be introduced through the gate electrode into
the mass spectrometer. Then, in the analyzing time interval, 303,
the voltage applied to the gate electrode is adjusted to prevent
the ions from further flowing in. The amplitudes of the
high-frequency voltages applied to the ring electrode and end cap
electrodes are controlled so that ions of different m/z values can
be sequentially ejected out of the spectrometer within which the
ions exist caught by the electric field. The detector then detects
the ejected ions to produce a mass spectrum. In the residual ion
removing time interval, 301, the ions remaining within the mass
spectrometer are removed by turning off the voltage applied to the
ring electrode.
[0060] The ion trapping time interval 302, analyzing time interval
303 and residual ion removing time interval 301 are selected to be
typically 0.04 second, 0.05 second and 0.01 second, respectively.
Thus, the mass spectrum can be produced once in 0.1 second. If the
sample air concentration is so rare to need high sensitivity, the
ion trapping time interval 302 may be extended.
[0061] The operation in step 202 will be described with reference
to FIG. 6. The operations in the ion trapping time interval 302 and
analyzing time interval 303 are the same as in step 201. In the
selection time interval, 304, ions of predetermined m/z values are
selected from the various ions trapped in the ion trapping time
interval 302, and caused to remain, and the other ions are removed.
In this selection time interval 304, the filtered-noise field can
be used that is disclosed in, for example, RAPID COMMUNICATIONS IN
MASS SPECTROMETRY, VOL 7, 1086-1089 (1993). In the dissociation
time interval, 305, the predetermined m/z ions selected in the
selection time interval 304 are energized and forced to collide
with helium gas within the mass spectrometer to produce fragment
ions. In order to energize the ions, it is necessary that a high
frequency voltage be applied between the end cap electrodes to
accelerate the ions within the mass spectrometer. When the
accelerated ions collide with the helium gas, part of the kinetic
energy of the ions is converted to the internal energy of ions.
During the time when the ions repeatedly collide with the helium
gas, the internal energy is accumulated to break the weak chemical
bonds of the ions, thus causing dissociation.
[0062] Since the tandem mass spectrometry has ion loss in the time
intervals of selection and dissociation, it is necessary to trap
sufficient amounts of ions in the ion trapping time interval 302 in
order to acquire satisfactory mass spectra of fragment ions.
Therefore, the ion trapping time interval 302, analyzing time
interval 303, selection time interval 304, dissociation time
interval 305 and residual ion removing time interval 301 are
selected to be typically 0.40 second, 0.05 second, 0.03 second,
0.01 second and 0.01 second, respectively. Thus, the mass spectrum
is acquired once in 0.5 second.
[0063] In the field of analysis, the mass spectrometry is used for
various intended purposes. When the mass spectrometry is used for
the detecting device, however, there are different points from the
normal analysis.
[0064] While the normal analysis treats a very large number of
ingredients, the detecting device detects extremely limited
substances. Selecting and detecting a few main components can find
a bomb, for example, which is produced by mixing various different
detonating explosives. In addition, the normal analysis determines
the value of the concentration of substance, while determination of
only the presence or absence of the object suffices for the
detecting.
[0065] Thus, the method of operating the mass spectrometer
particularly effective in the detecting device will be described
with reference to FIG. 6. When the trapped ions are ejected out of
the ion trap mass spectrometer, the ejection efficiency varies
depending on the mass scanning speed. In other words, in the
analyzing time interval 303 of FIG. 5, when the rate of increase of
the amplitude of the voltage applied to the ring electrode is
increased so that the analyzing time interval 303 can be completed
in a short time, the amount of ions ejected out of the spectrometer
and arriving at the detector increases and thus the sensitivity is
improved. However, when the rate of increase of the voltage
amplitude applied to the ring electrode is raised in the analyzing
time interval 303, the mass resolution is reduced, and it is not
known exactly when the ions are ejected. Thus, the measured m/z
values deviate from the correct values. For this reason, when the
ion trap mass spectrometer is operated, a step 207 for selecting
particular ions (corresponding to the selection time 307) is
provided between a step 206 for ion trap (corresponding to the ion
trapping time interval 302) and a step 208 for ejecting ions
(corresponding to the analyzing time interval 303) as shown in FIG.
7. That is, limiting the m/z of ions remaining within the mass
spectrometer compensates for the reduction of the resolution due to
the fast mass scanning. Specifically, when amphetamine is detected,
the m/z-136 positive ions are first checked. Thus, in step 206 the
ions generated by the ion source are trapped within the mass
spectrometer, and in step 207 the ions other than the m/z-136 ions
are removed and the m/z-136 ions are selectively caused to remain.
Then, in step 208 for ejecting ions, fast mass scanning is made so
that the ions remaining within the mass spectrometer can be
effectively ejected out of the mass spectrometer. Thus, since the
m/z-136 ions definitely arrive at the detector, precise mass
selection is not necessary in the step 208, so that the analyzing
time can be reduced and that high-sensitive detecting can be
performed. This method is effective not only for the screening time
but also for the case of fully examining by tandem mass
spectrometry. A step 210 for selecting the m/z of ions remaining
within the mass spectrometer can be provided between a step 209 for
dissociation and the step 208 for ejecting ions as shown in FIG.
8.
[0066] Moreover, in this method, since the kind of object to be
detected is limited, the database of objects is created on the data
processor, and effectively used for the detecting. More
specifically, in the tandem mass spectrometry, the optimum values
of the length of the dissociation time and the amplitude of the
high frequency voltage applied to the end cap electrodes to
energize the ions at the time of dissociation are dependent upon
the chemical substance to be detected. Therefore, a step 211, as
shown in FIG. 9, is provided to examine the optimum analysis
conditions for the components of each detected object to create a
database, and when the operator feels the suspicion of the presence
of a particular substance in the fast mode, to refer to the
database at the time of shifting to the detailed checking mode so
that the optimum analysis conditions for this substance can be read
in. Thus, a satisfactory mass spectrum of fragment ions can be
obtained, and precise decision can be made. For example, the
optimum analysis conditions for each of various banned chemicals
such as amphetamine and cocaine are examined to create a database,
and the analysis conditions for amphetamine when amphetamine is
suspected or for cocaine when cocaine is suspected are read out
from the database and analyzed.
[0067] While the substance to be detected is ionized and followed
by the screening based on the m/z of ions in the above description,
the screening can be performed without necessarily specifying the
m/z of ions by mass isolation. FIGS. 10 and 11 are diagrams useful
for explaining the second embodiment of the invention in which the
screening is made according to the ion current value. FIG. 10 is a
timing chart of high frequency voltage applied to the ring
electrode and end cap electrodes. Although the ion trapping time
interval 302 and ion selection time interval 304 may be
sequentially provided as in FIG. 6, an ion trapping and selection
time interval 302, 304 for simultaneously trapping and selecting
ions is provided as in FIG. 10. FIG. 11 shows the frequency of the
high frequency voltage applied to the end cap electrodes at the
trapping/selection time interval 302, 304. The ions trapped within
the ion trap mass spectrometer have easy-to-resonate frequencies
according to the m/z. Thus, when a signal including frequency
components other than those (f1, f2, f3) corresponding to the m/z
of the ions of the substance to be detected is applied to the
electrodes, only the ions having the target m/z are trapped at the
trapping/selection time interval 302, 304. Then, an ion ejection
time interval 306 is provided, and the ions remaining within the
mass spectrometer are ejected in the ion ejection time interval,
and detected by the detector. Here, when any signal is obtained, an
alarm is issued or more detailed check mode is brought about. In
this method, no time is taken for the mass spectrometry, and thus
faster screening can be made.
[0068] The third embodiment of the invention that is effective for
detecting explosives will be described with reference to FIGS. 12
and 13. This embodiment is a screening method using ions derived
from nitro groups obtained by dissociation of explosive. Energizing
easily decomposes the nitro compound, and the nitro group tends to
easily dissociate. Therefore, as shown in FIG. 12, a
trap/dissociation time interval 302, 305 for simultaneously
trapping and dissociating ions is provided. FIG. 13 shows the
frequency of the high frequency voltage applied to the end cap
electrodes in the trap/dissociation time interval 302, 305. A weak
signal including the frequencies (f1, f2, f3) corresponding to the
m/z of ions of a substance to be detected, but not including a
frequency (f4) corresponding to the m/z of ions derived from the
nitro group being detected is applied to the end cap electrodes.
When the ions of an explosive being detected are trapped, they
resonate with the high frequency voltage applied to the end cap
electrodes, and as a result energized to collide with helium. At
this time, the nitro group is dissociated, and thus the ions
derived from the dissociated nitro group, for example, of
NO.sub.2.sup.- are detected in the analysis time interval 303, thus
the screening being made. The merit of this method is that even if
the sample also includes nitro compounds of other substance than
that being detected, the dissociated nitro group is detected, and
thus the object can be found.
[0069] In the above method, higher-level mass spectrometry may be
made in order to further increase the selectivity as shown in FIG.
14. In other words, a step 212 (called MS/MS/MS or MS.sup.3) may be
provided that selects ions of particular m/z from the fragment
ions, dissociates them and makes mass spectrometry for the
dissociated ions. These processes of selection, dissociation and
mass spectrometry can be repeated until a satisfactory selectivity
can be obtained (generally this is called MS.sup.n).
[0070] In this invention, the time required for the detecting in
the fast screening mode is different from that in the detailed
check mode. Thus, as shown in FIG. 15, the detecting device
according to the invention should be used together with a baggage
carrying mechanism 57. The carrying mechanism 57, a carrying
mechanism controller 58 and the detecting device body 53 are
connected by signal lines 59a, 59b. In the screening mode, the
carrying mechanism carries pieces of baggage at a constant speed,
but when the main body 53 is switched to the detailed check mode in
which the detecting is made in a few seconds, the speed of the
carrying mechanism 57 is controlled to be slow.
[0071] Thus, according to the invention, the detecting can be made
with high speed and with high selectivity, and thus the presence or
absence of substances to be detected can be examined without
disturbing the flow of baggage and passengers.
* * * * *