U.S. patent application number 17/046890 was filed with the patent office on 2021-05-20 for method and apparatus for operating a vacuum interface of a mass spectrometer.
This patent application is currently assigned to Thermo Fisher Scientific (Bremen) GmbH. The applicant listed for this patent is Thermo Fisher Scientific (Bremen) GmbH. Invention is credited to Marcus MANECKI, Daniel PRECHT, Georgina THYSSEN, Christoph WEHE, Sven WOHLGETHAN.
Application Number | 20210151310 17/046890 |
Document ID | / |
Family ID | 1000005388168 |
Filed Date | 2021-05-20 |
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United States Patent
Application |
20210151310 |
Kind Code |
A1 |
WEHE; Christoph ; et
al. |
May 20, 2021 |
Method and Apparatus for Operating a Vacuum Interface of a Mass
Spectrometer
Abstract
A method is disclosed for operating a mass spectrometer vacuum
interface, the vacuum interface comprising an evacuated expansion
chamber downstream of a plasma ion source wherein the expansion
chamber is pumped by an interface vacuum pump to provide an
interface pressure in the chamber; the method comprising using a
controller to automatically, or according to user input, control
the throughput of the interface vacuum pump to control the
interface pressure dependent on one or more operating modes of the
spectrometer.
Inventors: |
WEHE; Christoph; (Bremen,
DE) ; MANECKI; Marcus; (Bremen, DE) ; THYSSEN;
Georgina; (Bremen, DE) ; WOHLGETHAN; Sven;
(Bremen, DE) ; PRECHT; Daniel; (Bremen,
DE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Thermo Fisher Scientific (Bremen) GmbH |
Bremen |
|
DE |
|
|
Assignee: |
Thermo Fisher Scientific (Bremen)
GmbH
Bremen
DE
|
Family ID: |
1000005388168 |
Appl. No.: |
17/046890 |
Filed: |
April 4, 2019 |
PCT Filed: |
April 4, 2019 |
PCT NO: |
PCT/EP19/58457 |
371 Date: |
October 12, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 49/105 20130101;
H01J 49/067 20130101; H01J 49/24 20130101 |
International
Class: |
H01J 49/10 20060101
H01J049/10; H01J 49/06 20060101 H01J049/06; H01J 49/24 20060101
H01J049/24 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 13, 2018 |
GB |
1806080.6 |
Claims
1. A method of operating a mass spectrometer vacuum interface, the
vacuum interface comprising an evacuated expansion chamber
downstream of a plasma ion source at atmospheric or relatively high
pressure, the expansion chamber having a first aperture that
interfaces with the plasma ion source to form an expanding plasma
downstream of the first aperture and a second aperture downstream
of the first aperture from the plasma for skimming the expanding
plasma to form a skimmed expanding plasma; wherein the expansion
chamber is pumped by an interface vacuum pump to provide an
interface pressure in the chamber; the method comprising using a
controller to automatically control the throughput of the interface
vacuum pump to control the interface pressure dependent on one or
more operating modes of the spectrometer.
2. A method according to claim 1 wherein the throughput of the
interface vacuum pump is controlled depending on one or more
operating conditions of the plasma ion source and/or one or more
elements of interest to be mass analysed by the spectrometer.
3. A method according to claim 2 wherein the plasma ion source is
an inductively coupled plasma (ICP) ion source and the one or more
operating conditions comprises a plasma temperature, plasma torch
position and/or plasma gas flow.
4. A method according to claim 1 wherein the controller comprises a
computer and associated controlling electronics interfaced to the
interface vacuum pump.
5. A method according to claim 1 wherein the interface vacuum pump
is a fore vacuum pump for a high vacuum pump that pumps a high
vacuum region of the mass spectrometer and the interface pressure
in the chamber is controlled to be in a range 0.1-10 mbar.
6. A method according to claim 1 further comprising acquiring data
from the mass spectrometer at the controller and using the data to
adjust the throughput of the vacuum pump in order to optimise the
detection sensitivity of the spectrometer for at least one
element.
7. A method according to claim 1 further comprising changing the
plasma ion source operating conditions from a first operating
condition to a second operating condition, or vice versa, and
respectively automatically controlling the throughput of the
interface vacuum pump from a first throughput when operating the
plasma at the first operating condition to a second throughput when
operating the plasma at the second operating condition, wherein the
first and second throughputs are different to each other.
8. A method according to claim 7 wherein the first operating
condition is a hot plasma condition and the second operating
condition is a cold plasma condition.
9. A method according to claim 7 wherein the first throughput
provides a first interface pressure in the interface vacuum stage
to optimise detection sensitivity for at least one element of the
sample being mass analysed using the first operating condition and
the second throughput provides a second interface pressure to
optimise detection sensitivity for at least one element being mass
analysed using the second operating condition.
10. A method according to claim 9 wherein the at least one element
whose detection sensitivity is optimised under the first operating
condition is different to the at least one element whose detection
sensitivity is optimised under the second operating condition.
11. A method according to claim 9 wherein the first and second
interface pressures are controlled to be substantially the
same.
12. A method according to claim 1 wherein more than two different
operating conditions are provided, each having a respective vacuum
pump throughput set by the controller.
13. A method according to claim 1 further comprising providing a
pressure gauge in the expansion chamber, (i) measuring the
interface pressure using the pressure gauge, (ii) providing signals
representative of the measured pressure to the controller, and
(iii) comparing the measured pressure to a set pressure using the
controller and if the controller determines that there is a
difference between the measured and set pressures, the controller
adjusts the throughput of the vacuum pump to reduce the difference
between the measured pressure and set pressure, wherein the steps
(i)-(iii) are repeated in a feedback loop to maintain the interface
pressure substantially at the set pressure.
14. A method according to claim 1 wherein the controller is
connected to a Visual Display Unit (VDU), such that the initially
automatically controlled throughput of the pump and/or the
interface pressure are displayed to a user, and wherein by means of
a graphical user interface (GUI) displayed on the VDU and an input
device, the user overrides the automatic control and manually sets
a particular throughput of the vacuum pump and/or a particular
interface pressure.
15. An apparatus for operating a mass spectrometer vacuum
interface, comprising: a plasma ion source for generating a plasma
at atmospheric or relatively high pressure; an evacuated expansion
chamber downstream of the plasma ion source, the expansion chamber
having a first aperture that interfaces with the plasma ion source
for forming an expanding plasma downstream of the first aperture
and a second aperture downstream of the first aperture for skimming
the expanding plasma to form a skimmed expanding plasma; wherein
the expansion chamber is pumped by an interface vacuum pump to
provide an interface pressure in the expansion chamber; and a
controller configured to automatically control the throughput of
the vacuum pump dependent on one or more operating modes of the
spectrometer.
16. An apparatus according to claim 15 wherein the controller is
configured to automatically control the throughput of the vacuum
pump dependent on a plasma condition and/or a measurement mode.
17. An apparatus according to claim 15 wherein the plasma ion
source is an inductively coupled plasma (ICP) ion source.
18. An apparatus according to claim 15 wherein the controller
comprises a computer and associated controlling electronics
interfaced to the interface vacuum pump.
19. An apparatus according to claim 15 wherein the throughput of
the vacuum pump can be continuously or quasi-continuously adjusted
by the controller across a range of throughput speeds.
20. An apparatus according to claim 15 wherein the interface vacuum
pump is a fore vacuum pump for a high vacuum pump that pumps a high
vacuum region of the mass spectrometer.
21. An apparatus according to claim 15 wherein the controller is
configured to automatically adjust the throughput of the interface
vacuum pump from a first throughput when operating the plasma ion
source at a first operating condition to a second throughput when
operating the plasma ion source at a second operating condition,
wherein the first and second throughputs are different to each
other.
22. An apparatus according to claim 21 wherein the first operating
condition and the second operating condition differ in the
temperature of the plasma, preferably wherein the first operating
condition is a hot plasma condition and the second operating
condition is a cold plasma condition.
23. An apparatus according to claim 21 wherein the first throughput
provides a first interface pressure in the interface vacuum stage
to optimise detection sensitivity for at least one element of the
sample being mass analysed using the first operating condition and
the second throughput provides a second interface pressure to
optimise detection sensitivity for at least one element being mass
analysed using the second operating condition.
24. An apparatus according to claim 23 wherein the at least one
element whose detection sensitivity is optimised under the first
operating condition is different to the at least one element whose
detection sensitivity is optimised under the second operating
condition.
25. An apparatus according to claim 23 wherein the controller
controls the first and second interface pressures to be
substantially the same.
26. An apparatus according to claim 15 wherein a pressure gauge is
located in the expansion chamber and a feedback loop is provided
between the pressure gauge and the controller so that the
controller can continuously adjust the throughput of the vacuum
pump to maintain the interface pressure at a set pressure.
27. An apparatus according to claim 15 wherein the controller is
connected to a Visual Display Unit (VDU), such that the throughput
of the pump and/or the interface pressure are displayed to a user,
and wherein by means of a graphical user interface (GUI) displayed
on the VDU and an input device, the user can command the controller
to set a particular throughput of the vacuum pump and/or a
particular interface pressure.
28. (canceled)
29. (canceled)
30. (canceled)
31. (canceled)
Description
FIELD
[0001] The invention relates to the field of mass spectrometry and
in particular to a method and apparatus for operating a vacuum
interface, more particularly, but not exclusively, an
atmosphere-to-vacuum interface of a mass spectrometer. The method
and apparatus are suitable for use principally with a plasma ion
source, such as an inductively coupled plasma (ICP),
microwave-induced plasma (MIP), or laser-induced plasma, ion
source. The following description will focus on embodiments using
inductively coupled plasma mass spectrometry (ICP-MS) for
illustration.
BACKGROUND
[0002] The general principles of ICP-MS are well known. ICP-MS
instruments provide robust and highly sensitive elemental analysis
of samples, down to the parts per trillion (ppt) range and beyond.
Typically, the sample is a liquid solution or suspension and is
supplied to the plasma by a nebulizer in the form of an aerosol in
a carrier gas, which is generally argon or sometimes helium. The
nebulized sample passes into a plasma torch, which typically
comprises a number of concentric tubes forming respective channels
and is surrounded towards the downstream end by a helical induction
coil. A plasma gas, typically argon, flows in the outer channel and
an electric discharge is applied to it, to ionize some of the
plasma gas. A radio frequency (RF) electric current is supplied to
the helical torch coil and the resulting alternating magnetic field
causes the free electrons to be accelerated to bring about further
ionization of the plasma gas. This process continues until a steady
plasma state is achieved, at temperatures typically between
5,000K-10,000K. The carrier gas and nebulized sample flow through
the central torch channel and pass into the central region of the
plasma, where the temperature is high enough to cause atomization
and then ionization of the sample. The sample ions in the plasma
next need to be formed into an ion beam, for ion separation and
detection by the mass spectrometer, which may be provided by a
quadrupole mass analyser, a magnetic and/or electric sector mass
analyser, a time-of-flight mass analyser, or an ion trap mass
analyser, among others.
[0003] Thus, in ICP-MS, ions are formed under atmospheric pressure
or relatively high pressure (e.g. over 100 mbar) outside the main
vacuum system of the spectrometer. For most mass analysers, a
vacuum having a pressure of <5.times.10.sup.-5 mbar is required.
An interface region is therefore provided that regulates the
transfer from the atmospheric pressure ion source to the high
vacuum mass analyser (see FIG. 1, which is described below). This
typically involves a number of stages of pressure reduction,
extraction of the ions from the plasma and ion beam formation, and
may include a collision/reaction cell stage for removing
potentially interfering ions from the mass analysis. The first
stage of pressure reduction is achieved by sampling the plasma
through a first aperture in a vacuum interface, typically provided
by a sampling cone having an apertured tip, which typically has an
inner diameter in the range 0.5 to 1.5 mm. The sampling cone is the
typical component which interfaces with the plasma source at
atmospheric, or relatively high (>100 mbar), pressure. The
sampled plasma expands downstream of the first aperture into an
evacuated expansion chamber, wherein the pressure is typically a
few mbar (e.g., 1-10 mbar). The central portion of the expanding
plasma then passes through a second aperture, typically provided by
a skimmer cone, into a second evacuation chamber having a higher
degree of vacuum than the expansion chamber. As the plasma expands
through the skimmer cone, its density reduces sufficiently to allow
extraction of the ions to form an ion beam, using strong electric
fields generated by ion lenses downstream of the skimmer cone. The
resulting ion beam may be deflected and/or guided onwards towards
the mass spectrometer by one or more ion deflectors, ion lenses,
and/or ion guides, which may operate with static or time-varying
fields.
[0004] A collision/reaction cell may be provided upstream of the
mass spectrometer to remove potentially interfering ions from the
ion beam. These are typically argon-based ions (such as Ar+,
Ar.sup.2+, ArO.sup.+), but may include others, such as ionized
hydrocarbons, metal oxides or metal hydroxides. The
collision/reaction cell promotes ion-neutral collisions/reactions,
whereby the unwanted molecular ions (and Ar.sup.+, Ar.sup.2+) are
preferentially neutralized and pumped away along with other neutral
gas components, or dissociated into ions of lower mass-to-charge
ratios (m/z) and rejected in a downstream m/z discriminating (mass
filter) stage. Alternatively, the analyte ions may be
preferentially subjected to mass shift reactions such that the
resultant mass shifted ions can be separated from the interfering
ions in a downstream m/z discriminating stage. U.S. Pat. Nos.
7,230,232 and 7,119,330 provide examples of collision/reaction
cells used in ICP-MS.
[0005] The ICP-MS instrument should preferably satisfy a number of
analytical requirements, including high transmission, high
stability, low influence from the sample matrix (the bulk
composition of the sample, including, for example, water, organic
compounds, acids, dissolved solids, and salts) in the plasma, and
low throughput of oxide ions or doubly charged ions, etc. These
parameters can be highly dependent upon the interface
characteristics.
[0006] The interface characteristics are influenced by different
processes, such as particle and gas dynamics, any secondary
discharge, as well as kinetic energies of the charged species
passing the interface (including also doubly charged, oxides, and
hydroxide species). Changes in the plasma and/or interface, e.g.
through changing sampling components (such as the inner diameter of
the aperture of the sampler and skimmer cones), measurement under
hot or cold plasma conditions, and use of organic solvents, have a
direct impact on the interface characteristics and, therefore, on
the ion transport efficiency.
[0007] Referring to FIG. 1, usually a fore vacuum pump 40, also
known as a roughing pump, is used for the evacuation of the
interface 3 between the sampling and skimmer cones 2 and 12. The
sampling cone 2 is used to sample the atmospheric pressure plasma
flame 4 that extends from the end of the plasma torch 6. The
sampling cone 2 has a central aperture 8 having an inner diameter
of approx. 1 mm, which typically allows a pressure P.sub.1 of
approx. 1-5 mbar in the interface region 3 between the sampling
cone 2 and the skimmer cone 12, e.g. if a pumping speed of 5-15 L/s
is used. These conditions determine the correct position of the
aperture of the skimmer 12 inside the so-called zone of silence of
the extracted plasma shown by the dotted line 14 in FIG. 1. A
typical distance between the tips of the sampling and skimmer cones
can be about 10 mm. The extracted jet beam forms a concentric shock
wave structure which ends in a shock wave front called the Mach
disk 16. The region within this shock wave structure is named the
zone of silence 14 and contains the ions, electrons and neutrals,
of which the ions have to be transferred to the mass analyser. This
means that the skimmer cone aperture should be positioned in the
zone of silence for adequate extraction of analyte ions. In other
words, if the sampler and skimmer cones are static, as they
generally are, the interface pressure in the region 3 must be low
enough to position the Mach disk behind the aperture of the skimmer
[see Inductively Coupled Plasma Mass Spectrometry, Akbar Montaser,
John Wiley & Sons, 1998, ISBN 0471186201, 9780471186205]. The
correlation between Mach disk and interface pressure was described
by Olney et al. [Olney et al., J. Anal. At. Spectrom., 1999, 14,
9-17] as:
x M D o = 0.67 P 0 P 1 ##EQU00001##
where: x.sub.M, is the distance between Sampler and Mach disk
D.sub.o, is the diameter of Sampler orifice P.sub.0, is the Source
pressure (atmosphere pressure) P.sub.1, is the Interface
pressure
[0008] Chiappini et al., Development of a high-sensitivity
inductively coupled plasma mass spectrometer for actinide
measurement in the femtogram range, J. Analytical Atomic
Spectrometry, 1996, 11, 497-503 have shown that reducing the
interface pressure can improve the instrument sensitivity. In that
case, the lower pressure was achieved by adding an additional pump
to the vacuum system.
[0009] Beyond the interface 3, the plasma is subjected to an ion
extraction field by ion extraction optics 20, which draws positive
ions from the plasma into an ion beam, repelling electrons and
allowing neutral components to be pumped away. The ion beam is then
transported downstream by ion optics (not shown) for mass analysis
by a mass analyser 30 (not shown in detail). The ion beam may be
deflected and/or guided from the extraction optics 20 towards the
mass analyser 30 by one or more ion deflectors, ion lenses, and/or
ion guides (not shown), which may operate with static or
time-varying fields. A collision/reaction cell may be located
upstream of the mass analyser, optionally with a mass filter
located upstream of the collision/reaction cell (the latter
configuration may be provided by a triple quadrupole arrangement as
in the Thermo Scientific.TM. iCAP.TM. TQ ICP-MS).
[0010] The above outlined fundamental principles of the zone of
silence and the shock wave description of the plasma do not explain
the impact of the interface pressure completely, however, and there
remains a need to improve instrument sensitivity in ICP-MS, under
both routine and custom research experimental conditions. Against
this background the present invention has been made.
[0011] United States patent U.S. Pat. No. 6,265,717 discloses an
ICP-MS device with an interface for transferring ions from the ICP
to the MS. The interface is provided with a controller for
increasing the pressure in the interface from its normal pressure
in order to selectively reduce interfering ions. A variable valve
in the pump line can be controlled by a system controller connected
to a personal computer.
[0012] Japanese patent application JP H11-185695 discloses an
ICP-MS device provided with a variable valve for pressure
regulation in an interface chamber to cope with both hot and cold
plasma. How the variable valve is controlled is not disclosed.
SUMMARY
[0013] According to an aspect of the invention there is provided a
method of operating a mass spectrometer vacuum interface, the
vacuum interface comprising an evacuated expansion chamber
downstream of a plasma ion source at atmospheric or relatively high
pressure (compared to the pressure in the expansion chamber), the
expansion chamber having a first aperture that interfaces with the
plasma ion source to form an expanding plasma downstream of the
first aperture and a second aperture downstream of the first
aperture from the plasma for skimming the expanding plasma to form
a skimmed expanding plasma; wherein the expansion chamber is pumped
by an interface vacuum pump to provide an interface pressure in the
chamber; the method comprising using a controller to automatically
control or regulate the throughput of the interface vacuum pump to
control the interface pressure. Thereby it is possible to optimise
a detection sensitivity of the spectrometer, i.e. detection limits,
for one or more elements being subject to mass analysis by the mass
spectrometer. The control of the throughput of the vacuum pump is
preferably dependent on one or more operating modes of the
spectrometer, i.e. the method preferably comprises automatically
setting the throughput dependent on one or more operating modes of
the spectrometer. Preferably, the throughput of the vacuum pump is
automatically controlled depending on one or more operating
conditions of the plasma ion source and/or one or more elements of
interest to be mass analysed by the spectrometer. Preferably, the
vacuum pump is controlled directly, for instance by varying its
operating voltage and/or operating current so as to vary its speed,
rather than indirectly as is the case when using a valve.
[0014] According to another aspect of the invention there is
provided an apparatus for operating a mass spectrometer vacuum
interface, comprising:
[0015] a plasma ion source for generating a plasma at atmospheric
or relatively high pressure;
[0016] an evacuated expansion chamber downstream of the plasma ion
source, the expansion chamber having a first aperture that
interfaces with the plasma ion source for forming an expanding
plasma downstream of the first aperture and a second aperture
downstream of the first aperture for skimming the expanding plasma
to form a skimmed expanding plasma; wherein the expansion chamber
is pumped by an interface vacuum pump to provide an interface
pressure in the expansion chamber; and
[0017] a controller configured to automatically control the
throughput of the vacuum pump.
[0018] The controller is preferably configured to automatically
control the throughput of the vacuum pump dependent on one or more
operating modes of the spectrometer. The operating modes can
include a plasma condition and/or a measurement mode (e.g. an
analysis of a specific element). The controller is preferably
configured to automatically control the throughput of the vacuum
pump depending on one or more operating conditions of the plasma
ion source and/or one or more elements of interest to be mass
analysed by the spectrometer (measurement modes).
[0019] The one or more operating conditions of the plasma ion
source preferably comprises the plasma temperature (i.e. as
determined by the power supplied to the plasma ion source), a
plasma torch position and/or a plasma gas flow. The identification
of the one or more elements of interest to be mass analysed by the
spectrometer can be input to the controller by a user such that the
controller can automatically control the throughput of the vacuum
pump to optimise the detection sensitivity for the one or more
elements.
[0020] The invention in this way enables the interface pressure to
be regulated, i.e. controlled, by a controller so as to optimise
the detection sensitivity of the spectrometer for a particular
element or elements being analysed under the given operating
condition mode or set of operating conditions (e.g. the power
supplied to the plasma/plasma temperature). In particular, the
invention can ensure that the optimal interface pressure is
provided by regulation of the interface vacuum pump throughput or
pump speed to give the best instrument sensitivity under different
experimental conditions (e.g. hot/cold plasma). Such regulation of
the interface pressure dependent on the characteristics of
different measurement modes is not provided in commercial ICP-MS
instruments. In addition, directly controlling the interface vacuum
pump is more efficient than regulating the interface vacuum through
a valve or other mechanism.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 shows a schematic view of a vacuum interface
region.
[0022] FIG. 2 shows a schematic view of a mass spectrometer.
[0023] FIG. 3 shows bar diagrams of intensities of selected
elements measured on an ICP-MS instrument using different interface
vacuum pump speed settings for both hot (upper diagram) and cold
(lower diagram) plasma conditions.
DETAILED DESCRIPTION
[0024] In order to enable a more detailed understanding of the
invention, various embodiments will now be described.
[0025] Referring to FIG. 2 there is shown a mass spectrometer 10
according to an embodiment of the present invention. The mass
spectrometer comprises a vacuum interface as generally shown in
FIG. 1. The spectrometer overall comprises three vacuum stages: the
interface vacuum 3, an intermediate vacuum 5, and a high vacuum 7.
The inductively coupled plasma (ICP) torch 6, as described above,
generates a high temperature plasma at atmospheric pressure from a
gas such as argon and receives a sample containing one or more
elements to be mass analysed and ionises the sample in the plasma.
In general, the plasma ion source, can be an ICP, MIP, laser
induced plasma, or other type of plasma ion source. Thus, the
plasma ion source generates elemental ions that can be mass
analysed in the downstream mass analyser. The description of the
operation of a plasma ion source and introduction of a sample
thereto is described above. Although the plasma is generally at
atmospheric pressure, it can be lower than atmospheric pressure but
at least a relatively high pressure (typically at least 100 mbar)
compared to the interface pressure in the expansion chamber. Thus,
the plasma ion source is generally at a pressure higher than 100
mbar, typically at atmospheric pressure. The ICP conditions may be
varied between hot (e.g. 1550 W ICP torch power) and cold (e.g. 550
W) and optionally a warm setting intermediate between the hot and
cold settings.
[0026] The plasma containing ions, gas and electrons is sampled
through a first aperture provided by an aperture sampler cone 2 and
forms an expanding plasma downstream thereof in an expansion
chamber as shown in FIG. 1. The plasma is then skimmed by a second
aperture provided by an apertured skimmer cone 12 and forms a
skimmed or secondary plasma expansion downstream thereof. Between
the sampler cone 2 and the skimmer cone 12 is an expansion chamber
that forms the interface vacuum stage 3, wherein the pressure is
typically desired to be in a range 0.1-10 mbar, preferably 1-10
mbar.
[0027] Beyond the interface vacuum stage 3, the skimmed plasma is
subjected to an ion extraction field by ion extraction optics 20
located in the intermediate vacuum stage 5, which draws positive
ions from the plasma into an ion beam, repelling electrons and
allowing neutral components to be pumped away. The intermediate
vacuum stage 5 is typically pumped to approximately
1.times.10.sup.-5 to 5.times.10.sup.-5 mbar The ion beam is then
transported downstream in the intermediate vacuum stage 5 by 90
degree bending ion optics 22 to a gas-filled collision/reaction
cell 24 comprising a quadrupole for removal of interferences.
Beyond the intermediate vacuum stage 5, the ion beam is finally
guided to a quadrupole mass analyser 30 and an ion detector 32,
such as a SEM, located in the high vacuum stage 7, wherein the
pressure is generally less than in the intermediate vacuum stage 5
and thus typically less than 1-5.times.10.sup.-5 mbar. Optionally,
a mass pre-filter, such as a quadrupole mass filter, can be located
upstream of the collision/reaction cell 24 to enhance removal of
interferences from the ion beam. The latter configuration therefore
provides a triple quadrupole arrangement. It will be appreciated
that other types of mass analysers may be provided as alternative
to the quadrupole mass analyser, such as a magnetic and/or electric
sector mass analyser, a time-of-flight mass analyser, or an ion
trap mass analyser, among others.
[0028] The expansion chamber of the interface vacuum stage 3 is
typically pumped to a pressure in the range 1 to 10 mbar by an
interface vacuum pump 40. The interface vacuum pump is preferably a
fore vacuum pump, also termed a roughing pump. The intermediate and
high vacuum stages 5 and 7 are pumped by a split flow
turbomolecular pump (not shown), the exhaust of which is pumped by
the fore vacuum pump 40. Thus, the fore vacuum pump in some
embodiments is conveniently the fore vacuum pump for a high vacuum
pump that pumps a high vacuum region of the mass spectrometer.
[0029] The regulation of the throughput (or speed) of the vacuum
pump preferably comprises automatically controlling the throughput
of the vacuum pump using a controller 50, which is interfaced to
the pump. The controller may be a computer-based controller. The
controller 50 may comprise a computer and associated controlling
electronics, for example a controlling voltage supply, that are
interfaced to the vacuum pump 40. The throughput of the vacuum pump
40 can be regulated by the controller, for example via digital
ports on the pump that interface to the controller and using
software that is run on the computer of the controller. In this
way, the speed of the pump can be controlled, for example, via an
external voltage to control the settings of the frequency converter
of the pump. The vacuum pump is preferably configured so that its
throughput can be continuously or quasi-continuously (for example,
substantially continuously, and/or at certain intervals) adjusted
by the controller across a range of throughput speeds (measured for
example in pumping speed (L/s), or pumping cycles per second (Hz)),
i.e. the pump has more than just two or three discrete pumping
speeds that can be set but effectively many times that number due
to its continuously adjustable speed.
[0030] The interface vacuum pump is preferably a fore vacuum pump,
such as a rotary vane type, Scroll type, Roots type or diaphragm
type, especially oil-free models of such pumps. Preferably,
therefore, the interface vacuum pump is dry fore vacuum pump. Such
pumps are preferably capable to achieve a pressure in the range 0.1
to 100 mbar. The interface pressure of the expansion chamber is
typically arranged to be in the range 1 to 10 mbar. The throughput
or pumping speed of the interface vacuum pump is preferably
controlled by changing the speed of the pump, for example by
changing the rotation speed of a rotary vane type or Roots type of
pump. Alternatively, or additionally, in some embodiments, the
throughput of the fore vacuum pump can be controlled by controlling
the size of an aperture located on the input side (i.e. upstream)
of the pump. An electromechanically variable aperture, e.g. in a
proportional valve, interfaced to the controller can be used for
this purpose.
[0031] The controller may also, for example via suitable
controlling electronics, control the operation of the mass
spectrometer 10, including any one or more of the plasma ion
source, ion optics, collision cell and mass analyser. The computer
of the controller may also acquire and process data from the mass
spectrometer, in particular signals from the mass analyser and
detector 32, e.g. to generate a mass spectrum from the mass
analysis. In some embodiments, the data acquired from the mass
spectrometer, optionally after processing by the controller, may be
used by the controller to adjust the throughput of the vacuum pump
in order to optimise the detection sensitivity of the spectrometer
for at least one element.
[0032] In some embodiments, the throughput of the vacuum pump 40 is
automatically controlled depending upon the operating conditions of
the plasma ion source 6. For example, it is known to operate a
plasma ion source of a mass spectrometer at either a hot plasma
condition, for example with a power for an ICP ion source of
1300-1700 W (giving a plasma temperature of e.g. 8,000-10,000 K),
or a cold plasma condition for example with a power for an ICP ion
source of 400-600 W (giving a plasma temperature of e.g. about
5,000 K). The choice of hot or cold plasma may depend upon the
particular elements in the sample desired to be mass analysed and
the sample matrix for example. Cold plasma for ICP-MS has
advantages for the ultralow detection of certain trace elements. At
low RF power, the overall ionization efficiency in the ICP is
decreased, limiting the formation of background argon and some
sample matrix based interferences. This reduces the requirement for
operation in additional interference reduction modes, thereby
reducing analysis time and improving sample throughput. With the
ICP ion source operating at a lower RF power (generally <600 W)
in cold mode, the plasma is smaller and any interaction with the
sampling interface and sample introduction system is reduced. A
preferentially higher signal to noise ratio can be obtained in cold
plasma than in hot plasma for low ionization potential elements
(such as Li and Na, etc.), which leads to lower limits of
detection.
[0033] However, changing the plasma conditions may lead to a change
in the interface pressure for a given throughput of the fore vacuum
pump and, consequently, result in interface characteristics that
allow for less than optimum ion transfer and detection
sensitivity.
[0034] In order to address this issue, embodiments of the invention
may comprise changing the plasma ion source operating conditions
from a first operating condition to a second operating condition,
or vice versa, and respectively automatically adjusting the
throughput of the interface vacuum pump from a first throughput
when operating the plasma at the first operating condition to a
second throughput when operating the plasma at the second operating
condition. The first and second throughputs are preferably
different to each other. The controller 50 preferably controls the
operating conditions of the plasma ion source 6, for example in
accordance with a selection of an operating condition (e.g. the
plasma temperature or power provided to the ICP torch), for example
input by a user, and automatically adjusts the throughput of the
interface vacuum pump 40 depending on the conditions, e.g. in
accordance with a computer program (software) run on the computer
of the controller 50. The controller 50 may automatically adjust
the throughput of the interface vacuum pump 40 by varying the
pump's supply voltage and/or current, for example the amplitude of
the supply voltage and/or, in case of an AC (alternating current)
pump, the frequency of the supply voltage.
[0035] In a preferred embodiment, the first operating condition and
the second operating condition differ in the temperature of the
plasma, more preferably the first operating condition may be a hot
plasma condition and the second operating condition may be a cold
plasma condition. As an example, the throughput of the vacuum pump
may be increased when changing from the hot plasma condition to the
cold plasma condition, for example by increasing the speed of the
vacuum pump 40. Other different operating conditions may relate to
standby (plasma off), different plasma torch positions and/or
different plasma gas flows.
[0036] Referring to FIG. 3, there is shown the intensities of
selected elements (.sup.7Li, .sup.59Co, .sup.115In, .sup.209Bi,
.sup.238U) in an iCAP TQ Tune Solution (1 ppb Li, Co, In, Ba, Ce,
Bi, U in 2% HNO.sub.3) measured by a Thermo Scientific.TM. iCAP.TM.
TQ ICP-MS using different fore vacuum pump speed settings, which
results in different interface pressure values as measured by a
Pirani gauge. The fore vacuum pump was a Roots pump manufactured by
Leybold Oerlikon, model ecoDry 65plus. The upper bar diagram in
FIG. 3 shows the results of an experiment using hot plasma
conditions (ICP power 1550 W). The lower bar diagram shows the
results of an experiment using cold plasma conditions (ICP power
550 W). Table 1 shows the pump speeds (in pump cycles or rotations
per second, Hz) and the corresponding pressures obtained in the
vacuum interface under the Hot and Cold conditions.
TABLE-US-00001 TABLE 1 Pump Interface Interface speed pressure
(mbar) pressure (mbar) (Hz) HOT plasma COLD plasma 100 3.323 3.803
120 1.980 2.371 130 1.769 2.118 140 1.654 1.980 160 1.511 1.769 180
1.351 1.654 200 1.320 1.581
[0037] Firstly, it is seen that with the same pump speed, different
interface pressures are obtained under the hot and cold plasma
conditions. For example, with a pump speed of 130 Hz, a Pirani
pressure of 1.769 mbar is achieved under hot plasma conditions
whereas only a vacuum pressure of 2.118 mbar is achieved under cold
plasma conditions. Accordingly, if cold plasma is selected, the
pump speed has to be increased from 130 Hz to 160 Hz to achieve a
similar interface pressure of 1.769 mbar. The benefit of increasing
the pump speed in the cold plasma condition can be seen from the Li
intensities, which increase by a factor of approximately 2 when
increasing the pumping speed from 130 to 160 Hz. Secondly, it is
seen that the instrument's detection sensitivity for each element
has a dependence on the interface pressure and, furthermore, the
behaviour of the sensitivity for different elements does not show
the same pattern of interface pressure dependence for all elements.
In this way, it can be seen that an optimum interface pressure can
be set by the controller by appropriate control of the vacuum pump
throughput that provides an optimum detection sensitivity for a
particular element. Maintaining the interface pressure at that
optimum while performing mass analysis on the spectrometer for that
element can be achieved by means of a pressure gauge signal that
feeds back to the controller as described further below.
[0038] The first and second throughputs of the vacuum pump are
preferably set by the controller 50. In more detail, the first
throughput is preferably set by the controller, preferably
optimised, to provide a first interface pressure in the interface
vacuum stage 3. Preferably, the first interface pressure optimises
detection sensitivity for at least one element of the sample being
mass analysed using the first operating condition. The second
throughput is preferably set by the controller, preferably
optimised, to provide a second interface pressure. Preferably, the
second interface pressure optimises detection sensitivity for at
least one element being mass analysed using the second operating
condition. The at least one element whose detection sensitivity is
optimised under the first operating condition may be the same or
different, but preferably different, to the at least one element
whose detection sensitivity is optimised under the second operating
condition. In some embodiments, it is advantageous that the
controller controls the first and second interface pressures to be
substantially the same (preferably they are the same pressure
within 10%, or more preferably within 5%, i.e. the lower pressure
being within 10% or 5% of the higher pressure of the two). It will
be appreciated, therefore, that in this way the interface pressure
in the expansion chamber is maintained substantially constant by
the adjustment of the throughput of the vacuum pump upon changing
the plasma conditions. It will be appreciated that other operating
conditions than the hot and cold plasma conditions, e.g., sampling
depth, cone orifices or sample matrix (organics, aqueous), could
also be changed to provide the first and second operating
conditions and the throughput of the vacuum pump adjusted
accordingly by the controller, e.g. to maintain the substantially
constant interface pressure in the expansion chamber.
[0039] It will be appreciated that, in some embodiments, more than
two different operating conditions may be provided, e.g. set by the
controller 50, each having a respective vacuum pump throughput set
by the controller. Thus, one or more further operating conditions
and one or more further throughputs may be employed in an analogous
manner in addition to the first and second operating conditions and
throughputs described. As an example, a third operating condition
of the plasma ion source, such as a "warm" plasma condition, which
is intermediate in power, and thus temperature, between the hot and
cold conditions, may be employed with the controller setting a
corresponding third throughput of the vacuum pump to provide a
third interface pressure. The third interface pressure may be
substantially the same or different to the first and second
interface pressures (preferably different). As another example, a
further operating condition of the plasma ion source may be a
switched-off or standby condition (i.e. the plasma is turned off),
with the controller then setting a corresponding further throughput
of the vacuum pump, especially the speed of the pump, to provide a
further interface pressure. Generally, where the operating
condition is a switched-off or standby condition, the corresponding
throughput (speed) of the vacuum pump is lower than it is for the
other operating conditions (e.g. when analysis is being performed
with the spectrometer) thereby to provide a lower vacuum (higher
pressure) for the interface pressures in the expansion chamber. In
this way, by operating with a lower speed of the vacuum pump, a
reduced power consumption or "Eco" mode is provided during standby
conditions.
[0040] In some preferred embodiments, a pressure gauge 60, e.g. a
Pirani gauge, can be located in the expansion chamber of the
interface vacuum stage. In operation, controlling the throughput of
the vacuum pump 40 can comprise providing a set pressure value to
the controller 50, which can be user input (e.g. via a graphical
user interface) or provided in the computer software of the
controller based on the operating conditions, receiving at the
controller 50 pressure signals over time from the pressure gauge 60
indicative of the interface pressure in the expansion chamber over
time and controlling the throughput of the vacuum pump 40 using the
controller 50 so as to maintain the interface pressure at the set
pressure. The set pressure may be a pressure that is desired to be
maintained at a given time, e.g. the first interface pressure
and/or the second interface pressure mentioned above. Thus, a
feedback loop is provided between the pressure gauge 60 and the
controller 50 so that the controller can continuously adjust the
throughput of the vacuum pump 40 to maintain the interface pressure
at the set pressure (e.g. the pressure that is desired to be
maintained at a given time, such as either the first interface
pressure or the second interface pressure). In this way, if the
pressure rises above the set pressure, the pressure signal received
by the controller will cause the controller to adjust the
throughput of the pump so that the pressure is lowered to the set
value. Similarly, if the pressure falls below the set pressure, the
pressure signal received by the controller will cause the
controller to adjust the throughput of the pump so that the
pressure is raised to the set value. This type of control can be
used to maintain a set pressure in the expansion chamber while mass
analysis is performed by the mass spectrometer. The same interface
pressure can thereby be maintained over time across the mass
analysis of a sequence of samples.
[0041] In some of the above embodiments, in which the first and
second throughputs provide first and second interface pressures
corresponding to the first and second operating conditions, the
first and second pressures may be substantially the same pressure
which may be the set pressure that is used by the controller in a
feedback loop with the pressure gauge to continuously adjust the
throughput of the vacuum pump to maintain the interface pressure at
the set pressure under both the first and second operating
conditions. In some of the above embodiments, in which the first
and second throughputs provide first and second interface pressures
corresponding to the first and second operating conditions, the
first and second pressures may be different pressures. In that
case, the first interface pressure may be a first set pressure that
is used by the controller in the feedback loop with the pressure
gauge to continuously adjust the throughput of the vacuum pump to
maintain the interface pressure at the first set pressure under the
first operating condition, and the second interface pressure may be
a second set pressure that is used by the controller in the
feedback loop with the pressure gauge to continuously adjust the
throughput of the vacuum pump to maintain the interface pressure at
the second set pressure under the second operating condition. Thus,
some embodiments of the invention comprise providing a pressure
gauge in the expansion chamber. The use of the pressure gauge can
comprise the steps: (i) measuring the interface pressure using the
gauge and (ii) providing signals representative of the measured
pressure to the controller, wherein (iii) the controller compares
the measured pressure to a set pressure and if there is a
difference between the measured and set pressures, the controller
adjusts the throughput of the vacuum pump to reduce the difference
between the measured pressure and set pressure, wherein the steps
(i)-(iii) are repeated in a feedback loop to maintain the interface
pressure substantially at the set pressure.
[0042] In some embodiments, a user can override the automatic
control and the set interface pressure that may be provided by the
software of the computer-based controller. Thus, at least for a
period of time the automatic regulation of the pump throughput by
the controller can be overridden by a user such that for the period
the throughput is set by the user. Typically, the user will
directly input the pump throughput to the controller. For example,
in some such embodiments, a user can adjust the throughput of the
vacuum pump and thus the interface pressure in order to optimise
the detection sensitivity for one or more elements that the user is
interested in. This enables a user for a period of time to tune the
throughput of the vacuum pump so as to optimise the detection
sensitivity. The user can thereby adjust the vacuum pump throughput
based on observing the effect that such adjustment has on the
detection sensitivity of one or more elements in a mass spectrum
produced on the spectrometer. In such embodiments, the controller
is preferably connected to a visual display or monitor 70 (i.e.
VDU--Visual Display Unit), such that certain parameters are
displayed to the user, for example the throughput of the pump, e.g.
pump speed, and/or the interface pressure. The displayed throughput
of the pump is, at least initially, preferably the automatically
controlled throughput, that is, the throughput as determined by the
automatic control. The VDU 70 may also display other information to
the user, such as one or more of the operating or sampling (plasma)
conditions, a sample identification, mass spectral data etc. In
some such embodiments, an `override` mode can be implemented, by
means of a graphical user interface (GUI) displayed on the VDU and
an input device 72, such as a mouse or keyboard for example, the
user can command the controller to set a particular throughput of
the vacuum pump and/or a particular interface pressure. In an
example, the GUI may provide an Instrument Control Window, which
may contain a slider, which can be moved by the user using an input
device, eg using a mouse, to set the pump throughput (speed). The
slider may be moved to any point on a scale from 0-100% of the
maximum pump speed in the Instrument Control window and thus is
used for regulation of the pump speed and throughput in a defined
range for achieving the interface pressure of interest. It may be
possible using the invention to control the interface pressure
continuously or quasi-continuously from 0.1 to 200 mbar, preferably
0.1 to 100 mbar, and more preferably 0.1 to 10 mbar by regulation
of the throughput of the fore vacuum pump.
[0043] In some embodiments, the GUI can be used by the user to
input a selection of an operating condition (e.g. the plasma
temperature or power provided to the ICP torch), and/or an
identification of one or more elements of interest to be mass
analysed by the spectrometer. In this way, the controller can
automatically control the throughput of the vacuum pump to optimise
the detection sensitivity for one or more elements to be mass
analysed depending on the selection of operating condition.
Furthermore, when the user inputs an identification of one or more
elements of interest to be mass analysed, the controller can
automatically control the throughput of the vacuum pump in a
fine-tune mode to optimise the detection sensitivity for the one or
more elements. In such embodiments, the controller can be
programmed (e.g. by its software) to set different throughputs of
the vacuum pump for different elements, with each element having
its own respective throughput setting that optimises detection
sensitivity for that element under the selected operating (plasma)
conditions, and a throughput of the pump is thereby set by the
controller according to the element or elements of interest
specified by the user.
[0044] In a further aspect, alternative to automatic control of the
throughput of the pump and/or the interface pressure, there is
provided a method of operating a mass spectrometer vacuum
interface, the vacuum interface comprising an evacuated expansion
chamber downstream of a plasma ion source at atmospheric or
relatively high pressure, the expansion chamber having a first
aperture that interfaces with the plasma ion source to form an
expanding plasma downstream of the first aperture and a second
aperture downstream of the first aperture from the plasma for
skimming the expanding plasma to form a skimmed expanding plasma;
wherein the expansion chamber is pumped by an interface vacuum pump
to provide an interface pressure in the chamber; the method
comprising using a controller to control the throughput of the
interface vacuum pump to control the interface pressure, wherein a
user inputs to the controller a particular throughput of the vacuum
pump and/or a particular interface pressure and the controller sets
the throughput of the vacuum pump and/or a particular interface
pressure according to the input, for example to optimise detection
sensitivity for one or more elements being subject to mass analysis
by the mass spectrometer. Preferably, the controller is connected
to a Visual Display Unit (VDU), such that the throughput of the
pump and/or the interface pressure are displayed to the user,
wherein by means of a graphical user interface (GUI) displayed on
the VDU and an input device the user commands the controller to set
a particular throughput of the vacuum pump and/or a particular
interface pressure. The features of the invention described above
are also applicable to the further aspect. For example, when the
user has input the throughput of the pump and/or the interface
pressure to be set by the controller, the controller and the
pressure gauge maintain the set pressure by the feedback method
described.
[0045] Similarly, the further aspect also provides an apparatus for
operating a mass spectrometer vacuum interface, comprising: a
plasma ion source for generating a plasma at atmospheric or
relatively high pressure; an evacuated expansion chamber downstream
of the plasma ion source, the expansion chamber having a first
aperture that interfaces with the plasma ion source for forming an
expanding plasma downstream of the first aperture and a second
aperture downstream of the first aperture for skimming the
expanding plasma to form a skimmed expanding plasma; wherein the
expansion chamber is pumped by an interface vacuum pump to provide
an interface pressure in the expansion chamber; and a controller
configured to control the throughput of the vacuum pump, whereby a
user can input to the controller a particular throughput of the
vacuum pump and/or a particular interface pressure and the
controller sets the throughput of the vacuum pump and/or a
particular interface pressure according to the input, for example
to optimise detection sensitivity for one or more elements being
subject to mass analysis by the mass spectrometer. Preferably, the
controller is connected to a Visual Display Unit (VDU), such that
the throughput of the pump and/or the interface pressure are
displayed to the user, and wherein by means of a graphical user
interface (GUI) displayed on the VDU and an input device, the user
can command the controller to set a particular throughput of the
vacuum pump and/or a particular interface pressure.
[0046] In some embodiments, the data acquired from the mass
spectrometer, optionally after processing by the controller, may be
used by the controller to adjust the throughput of the vacuum pump
in order to optimise the detection sensitivity of the spectrometer
for at least one element. The controller may acquire data from the
mass spectrometer and optionally may process the data to generate a
mass spectrum therefrom. For one or more elements of interest, for
an initial throughput of the interface vacuum pump, the controller
may determine the signal intensity (at the detector) from the data.
The controller may then adjust the throughput of the interface
vacuum pump, re-acquire data from the mass spectrometer and
re-determine the signal intensity. This may be repeated until an
optimum throughput of the interface vacuum pump (and thereby
optimum interface pressure) is found that corresponds to a maximum
signal intensity for the one or more elements. The controller may
then maintain the optimum throughput of the interface vacuum pump
for measurement of the one or more elements. The procedure may be
repeated for one or more different elements to find a respective
optimum throughput of the interface vacuum pump for each
element.
[0047] Advantageously, the invention enables an increase of
sensitivity of the mass spectrometer by an easy-to-use regulation
of the interface pressure for different sampling conditions, e.g.
hot or cold plasma, or different measurement modes. An ICP mass
spectrometer is thus provided in which the regulation of the
throughput of the fore vacuum pump is used for optimization of the
interface pressure for achieving the best instrument sensitivity
under different experimental conditions.
[0048] In view of the above disclosure, it will be appreciated that
embodiments of the invention can be provided in accordance with the
following clauses: [0049] 1) A method of operating a mass
spectrometer vacuum interface, the vacuum interface comprising an
evacuated expansion chamber downstream of a plasma ion source at
atmospheric or relatively high pressure, the expansion chamber
having a first aperture that interfaces with the plasma ion source
to form an expanding plasma downstream of the first aperture and a
second aperture downstream of the first aperture from the plasma
for skimming the expanding plasma to form a skimmed expanding
plasma; wherein the expansion chamber is pumped by an interface
vacuum pump to provide an interface pressure in the chamber; the
method comprising using a controller to automatically control the
throughput of the interface vacuum pump to control the interface
pressure dependent on one or more operating modes of the
spectrometer to optimise detection sensitivity for one or more
elements being subject to mass analysis by the mass spectrometer.
[0050] 2) A method according to clause 1 wherein the throughput of
the interface vacuum pump is controlled depending on one or more
operating conditions of the plasma ion source and/or one or more
elements of interest to be mass analysed by the spectrometer.
[0051] 3) A method according to clause 2 wherein the plasma ion
source is an inductively coupled plasma (ICP) ion source and the
one or more operating conditions comprises a plasma temperature,
plasma torch position and/or plasma gas flow. [0052] 4) A method
according to any preceding clause wherein the controller comprises
a computer and associated controlling electronics interfaced to the
interface vacuum pump. [0053] 5) A method according to any
preceding clause wherein the interface vacuum pump is a fore vacuum
pump for a high vacuum pump that pumps a high vacuum region of the
mass spectrometer and the interface pressure in the chamber is
controlled to be in a range 0.1-10 mbar. [0054] 6) A method
according to any preceding clause further comprising acquiring data
from the mass spectrometer at the controller and using the data to
adjust the throughput of the vacuum pump in order to optimise the
detection sensitivity of the spectrometer for at least one element.
[0055] 7) A method according to any preceding clause further
comprising changing the plasma ion source operating conditions from
a first operating condition to a second operating condition, or
vice versa, and respectively automatically controlling the
throughput of the interface vacuum pump from a first throughput
when operating the plasma at the first operating condition to a
second throughput when operating the plasma at the second operating
condition, wherein the first and second throughputs are different
to each other. [0056] 8) A method according to clause 7 wherein the
first operating condition is a hot plasma condition and the second
operating condition is a cold plasma condition. [0057] 9) A method
according to clause 7 or 8 wherein the first throughput provides a
first interface pressure in the interface vacuum stage to optimise
detection sensitivity for at least one element of the sample being
mass analysed using the first operating condition and the second
throughput provides a second interface pressure to optimise
detection sensitivity for at least one element being mass analysed
using the second operating condition. [0058] 10) A method according
to clause 9 wherein the at least one element whose detection
sensitivity is optimised under the first operating condition is
different to the at least one element whose detection sensitivity
is optimised under the second operating condition. [0059] 11) A
method according to clause 9 or 10 wherein the first and second
interface pressures are controlled to be substantially the same.
[0060] 12) A method according to any preceding clause wherein more
than two different operating conditions are provided, each having a
respective vacuum pump throughput set by the controller. [0061] 13)
A method according to any preceding clause further comprising
providing a pressure gauge in the expansion chamber, (i) measuring
the interface pressure using the pressure gauge, (ii) providing
signals representative of the measured pressure to the controller,
and (iii) comparing the measured pressure to a set pressure using
the controller and if the controller determines that there is a
difference between the measured and set pressures, the controller
adjusts the throughput of the vacuum pump to reduce the difference
between the measured pressure and set pressure, wherein the steps
(i)-(iii) are repeated in a feedback loop to maintain the interface
pressure substantially at the set pressure. [0062] 14) A method
according to any preceding clause wherein the controller is
connected to a Visual Display Unit (VDU), such that the throughput
of the pump, which is preferably initially automatically
controlled, and/or the interface pressure are displayed to a user,
and wherein by means of a graphical user interface (GUI) displayed
on the VDU and an input device, the user overrides the automatic
control and manually sets a particular throughput of the vacuum
pump and/or a particular interface pressure. [0063] 15) An
apparatus for operating a mass spectrometer vacuum interface,
comprising: a plasma ion source for generating a plasma at
atmospheric or relatively high pressure; an evacuated expansion
chamber downstream of the plasma ion source, the expansion chamber
having a first aperture that interfaces with the plasma ion source
for forming an expanding plasma downstream of the first aperture
and a second aperture downstream of the first aperture for skimming
the expanding plasma to form a skimmed expanding plasma; wherein
the expansion chamber is pumped by an interface vacuum pump to
provide an interface pressure in the expansion chamber; and a
controller configured to automatically control the throughput of
the vacuum pump dependent on one or more operating modes of the
spectrometer. [0064] 16) An apparatus according to clause 15
wherein the controller is configured to automatically control the
throughput of the vacuum pump dependent on a plasma condition
and/or a measurement mode. [0065] 17) An apparatus according to
clause 15 or 16 wherein the plasma ion source is an inductively
coupled plasma (ICP) ion source. [0066] 18) An apparatus according
to any of clauses 15 to 17 wherein the controller comprises a
computer and associated controlling electronics interfaced to the
interface vacuum pump. [0067] 19) An apparatus according to any of
clauses 15 to 18 wherein the throughput of the vacuum pump can be
continuously or quasi-continuously adjusted by the controller
across a range of throughput speeds. [0068] 20) An apparatus
according to any of clauses 15 to 19 wherein the interface vacuum
pump is a fore vacuum pump for a high vacuum pump that pumps a high
vacuum region of the mass spectrometer. [0069] 21) An apparatus
according to any of clauses 15 to 20 wherein the controller is
configured to automatically adjust the throughput of the interface
vacuum pump from a first throughput when operating the plasma ion
source at a first operating condition to a second throughput when
operating the plasma ion source at a second operating condition,
wherein the first and second throughputs are different to each
other. [0070] 22) An apparatus according to clause 21 wherein the
first operating condition and the second operating condition differ
in the temperature of the plasma, preferably wherein the first
operating condition is a hot plasma condition and the second
operating condition is a cold plasma condition. [0071] 23) An
apparatus according to clause 21 or 22 wherein the first throughput
provides a first interface pressure in the interface vacuum stage
to optimise detection sensitivity for at least one element of the
sample being mass analysed using the first operating condition and
the second throughput provides a second interface pressure to
optimise detection sensitivity for at least one element being mass
analysed using the second operating condition. [0072] 24) An
apparatus according to clause 23 wherein the at least one element
whose detection sensitivity is optimised under the first operating
condition is different to the at least one element whose detection
sensitivity is optimised under the second operating condition.
[0073] 25) An apparatus according to clause 23 or 24 wherein the
controller controls the first and second interface pressures to be
substantially the same. [0074] 26) An apparatus according to any of
clauses 15 to 25 wherein a pressure gauge is located in the
expansion chamber and a feedback loop is provided between the
pressure gauge and the controller so that the controller can
continuously adjust the throughput of the vacuum pump to maintain
the interface pressure at a set pressure. [0075] 27) An apparatus
according to any of clauses 15 to 26 wherein the controller is
connected to a Visual Display Unit (VDU), such that the throughput
of the pump and/or the interface pressure are displayed to a user,
and wherein by means of a graphical user interface (GUI) displayed
on the VDU and an input device, the user can command the controller
to set a particular throughput of the vacuum pump and/or a
particular interface pressure. [0076] 28) A method of operating a
mass spectrometer vacuum interface, the vacuum interface comprising
an evacuated expansion chamber downstream of a plasma ion source at
atmospheric or relatively high pressure, the expansion chamber
having a first aperture that interfaces with the plasma ion source
to form an expanding plasma downstream of the first aperture and a
second aperture downstream of the first aperture from the plasma
for skimming the expanding plasma to form a skimmed expanding
plasma; wherein the expansion chamber is pumped by an interface
vacuum pump to provide an interface pressure in the chamber; the
method comprising using a controller to control the throughput of
the interface vacuum pump to control the interface pressure,
wherein a user inputs to the controller a particular throughput of
the vacuum pump and/or a particular interface pressure and the
controller sets the throughput of the vacuum pump and/or a
particular interface pressure according to the input. [0077] 29) A
method according to clause 28 wherein the controller is connected
to a Visual Display Unit (VDU), such that the throughput of the
pump and/or the interface pressure are displayed to the user,
wherein by means of a graphical user interface (GUI) displayed on
the VDU and an input device the user commands the controller to set
a particular throughput of the vacuum pump and/or a particular
interface pressure. [0078] 30) A method according to clause 28 or
29 wherein a pressure gauge is located in the expansion chamber and
a feedback loop is provided between the pressure gauge and the
controller so that the controller can maintain the interface
pressure at the pressure set. [0079] 31) An apparatus for operating
a mass spectrometer vacuum interface, comprising: a plasma ion
source for generating a plasma at atmospheric or relatively high
pressure; an evacuated expansion chamber downstream of the plasma
ion source, the expansion chamber having a first aperture that
interfaces with the plasma ion source for forming an expanding
plasma downstream of the first aperture and a second aperture
downstream of the first aperture for skimming the expanding plasma
to form a skimmed expanding plasma; wherein the expansion chamber
is pumped by an interface vacuum pump to provide an interface
pressure in the expansion chamber; and a controller configured to
control the throughput of the vacuum pump, whereby a user can input
to the controller a particular throughput of the vacuum pump and/or
a particular interface pressure and the controller sets the
throughput of the vacuum pump and/or a particular interface
pressure according to the input.
[0080] The use of any and all examples, or exemplary language ("for
instance", "such as", "for example" and like language) provided
herein, is intended merely to better illustrate the invention and
does not indicate a limitation on the scope of the invention unless
otherwise claimed. No language in the specification should be
construed as indicating any non-claimed element as essential to the
practice of the invention.
[0081] As used herein, including in the claims, unless the context
indicates otherwise, singular forms of the terms herein are to be
construed as including the plural form and vice versa. For
instance, unless the context indicates otherwise, a singular
reference herein including in the claims, such as "a" or "an" means
"one or more".
[0082] Throughout the description and claims of this specification,
the words "comprise", "including", "having" and "contain" and
variations of the words, for example "comprising" and "comprises"
etc, mean "including but not limited to", and are not intended to
(and do not) exclude other components.
[0083] The present invention also covers the exact terms, features,
values and ranges etc. in case these terms, features, values and
ranges etc. are used in conjunction with terms such as about,
around, generally, substantially, essentially, at least etc. (e.g.,
"about 3" shall also cover exactly 3, or "substantially constant"
shall also cover exactly constant).
[0084] The term "at least one" should be understood as meaning "one
or more", and therefore includes both embodiments that include one
or multiple components. Furthermore, dependent claims that refer to
independent claims that describe features with "at least one" have
the same meaning, both when the feature is referred to as "the" and
"the at least one".
[0085] Any steps described in this specification may be performed
in any order or simultaneously unless stated or the context
requires otherwise.
[0086] All of the features disclosed in this specification may be
combined in any combination, except combinations where at least
some of such features and/or steps are mutually exclusive. In
particular, the preferred features of the invention are applicable
to all aspects of the invention and may be used in any combination.
Likewise, features described in non-essential combinations may be
used separately (not in combination).
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