U.S. patent number RE35,413 [Application Number 08/220,963] was granted by the patent office on 1996-12-31 for electrospray ion source with reduced neutral noise and method.
This patent grant is currently assigned to Finnigan Corporation. Invention is credited to Mark E. Hail, Iain C. Mylchreest.
United States Patent |
RE35,413 |
Mylchreest , et al. |
December 31, 1996 |
Electrospray ion source with reduced neutral noise and method
Abstract
An electrospray ion source having a capillary tube for directing
ions from an ionizing region to an analyzing region including a
skimmer in which the capillary tube is oriented to cause
undesolvated droplets to strike the skimmer.
Inventors: |
Mylchreest; Iain C. (Milpitas,
CA), Hail; Mark E. (Yardley, PA) |
Assignee: |
Finnigan Corporation (San Jose,
CA)
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Family
ID: |
24825524 |
Appl.
No.: |
08/220,963 |
Filed: |
March 30, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
703471 |
May 17, 1991 |
05171990 |
Dec 15, 1992 |
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Current U.S.
Class: |
250/282;
250/288 |
Current CPC
Class: |
G01N
30/7293 (20130101); G01N 30/7246 (20130101); G01N
30/7266 (20130101); G01N 30/7273 (20130101); G01N
2030/621 (20130101); G01N 30/7293 (20130101); G01N
30/7273 (20130101) |
Current International
Class: |
G01N
30/00 (20060101); G01N 30/72 (20060101); G01N
30/62 (20060101); B01D 059/44 (); H01J
049/00 () |
Field of
Search: |
;250/281,282,288,289 |
References Cited
[Referenced By]
U.S. Patent Documents
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4300044 |
November 1981 |
Iribarne et al. |
4542293 |
September 1985 |
Fenn et al. |
4861988 |
August 1989 |
Henion et al. |
4885076 |
December 1989 |
Smith et al. |
4963735 |
October 1990 |
Okamoto et al. |
4994165 |
February 1991 |
Lee et al. |
5349186 |
September 1994 |
Ikonomou et al. |
|
Other References
Bruins, A. P., et al., "Ion Spray Interface for Combined Liquid
Chromatography/Atmospheric Pressure Ionization Mass Spectrometry",
Analytical Chemistry, 59(22):2642-2646 (1987). No Month..
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Primary Examiner: Anderson; Bruce C.
Attorney, Agent or Firm: Flehr, Hohbach, Test, Albritton
& Herbert
Claims
What is claimed is: .[.
1. An ion source of the type which includes an ionization chamber
and an adjacent low pressure region including a skimmer having an
orifice, a capillary tube having an axial bore communicating
between the ionization chamber and the low pressure region whereby
ions and gases in said ionization chamber flows through said bore
into said low pressure region, directing the axis of the capillary
tube away from the skimmer orifice whereby droplets and/or
particles flowing through the bore are not allowed to pass through
the skimmer, while ions are transmitted into the low pressure
chamber..]..[.2. An electrospray ion source as in claim 1 in which
said ionization chamber includes means for electrospraying a sample
to be analyzed..]..Iadd.3. A method of operating an ion source,
said ion source including a low pressure chamber having a skimmer
with an orifice and a capillary tube having a bore with an axis,
said method comprising the steps of:
desolvating charged droplets in said ion source to produce a flow
of ions and undesolvated droplets;
causing said flow of ions and undesolvated droplets to flow from
said bore along said axis of said bore into said low pressure
chamber; and
offsetting said axis of said bore from said orifice of said skimmer
so that said ions flow through said orifice while said undesolvated
droplets impinge upon said skimmer. .Iaddend..Iadd.4. A method of
operating an ion source, said ion source including a low pressure
chamber having a skimmer with an orifice, and means to communicate
with the low pressure chamber, said method comprising the steps
of:
desolvating charged droplets in said ion source to produce a flow
of ions and undesolvated droplets;
causing said flow of ions and undesolvated droplets to flow from
said communicating means along an axis of said communicating means
into said low pressure chamber; and
offsetting said axis of said communicating means from said orifice
of said skimmer so that said ions flow through said orifice while
said undesolvated droplets impinge upon said skimmer. .Iaddend.
Description
BRIEF DESCRIPTION OF THE INVENTION
This invention relates generally to electrospray ion sources and
more particularly to sources having reduced neutral noise.
BACKGROUND OF THE INVENTION
The electrospray process consists of flowing sample liquid through
a small tube or capillary which is maintained at a high voltage
with respect to a nearby surface. The liquid is dispersed into fine
electrically charged droplets or by the voltage gradient of the tip
of the capillary. The ionization mechanism involves the desorption
at atmospheric pressure of ions from the fine electrically charged
particles. In many cases, a heated gas is flowed in counter-current
to the electrospray to enhance dissolution of the electrospray
droplets. The ions created by the electrospray are then mass
analyzed in a mass analyzer such as a mass spectrometer.
Under the appropriate conditions, the electrospray resembles a
symmetrical cone consisting of a very fine mist (or fog) of
droplets (ca. 1 .mu.m diameter). Excellent sensitivity and ion
current stability can be obtained if the fine mist is produced.
Unfortunately, the electrospray "quality" is highly dependent on
the bulk properties of the solution being analyzed. The most
important of which are surface tension and conductivity. A poor
quality electrospray may contain larger droplets (>10 .mu.m
diameter) or a non-dispersed droplet stream.
The use of sheath liquid and a focusing gas are often used to
insure stable sprays when electrospraying high aqueous content
sample solutions. One type of electrospray interface apparatus
includes an inner needle for transferring a liquid sample to an
ionizing region at one end of the needle, a first outer tube
surrounding and spaced from said needle for flowing a liquid past
the tip of said needle, and a second outer tube surrounding the
first tube to define a second cylindrical space for flowing a gas
past the end of said first tube and needle to focus the
electrospray.
In U.S. Pat. No. 4,542,293 there is described the use of a tube
made of an electrical insulator for conducting ions in the ionizing
electrospray region at atmospheric pressure and a low pressure
region. A glass or quartz capillary is suitable. Ions and gas are
caused to flow from the ionization region through the tube into the
low pressure region where free jet expansion occurs. A conductive
coating is formed on the ends of the insulating tube and a voltage
is applied thereacross to accelerate ions which flow through the
tube. A conducting skimmer is disposed adjacent the end of the tube
and is maintained at a voltage which causes further acceleration of
the ions through and into a lower pressure region including
focusing lenses and analyzing apparatus.
The electrospray process employing a sheath liquid and focusing gas
provides small droplets or particle which are desolvated by the
addition of a counter current drying gas. Occasionally, larger
undesolvated droplets or particles will traverse into the capillary
and will acquire substantial kinetic energy which will allow it to
pass through the skimmer and into the lens region. The droplets or
particles impinge upon surfaces and forms secondary ions. These
ions are random and cause noise to be observed at the analyzer
detector, thereby decreasing the signal to noise levels and
producing noise spikes in the ion chromatogram.
OBJECTS AND SUMMARY OF THE INVENTION
It is a general object of this invention to provide an ion source
and method in which high velocity droplets or particles are
prevented from flowing into the lens region of an associated
analyzer.
It is another object of the invention to provide an ion source
which includes a skimmer and means for directing high velocity
droplets or particles away from the skimmer aperture.
The foregoing and other objects of this invention are achieved in
an electrospray ion source of the type which includes a tube
communicating between the ionizing region and a low pressure region
with a skimmer having an aperture through which ions pass, the
skimmer separating the low pressure region from a lower pressure
region which includes lenses and an analyzer, where analysis is
carried out. The capillary is oriented so that undesolvated
droplets or particles travelling through the capillary are
prevented from passing through the skimmer aperture into the
analysis region.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects of the invention will be more
clearly understood from the description to follow when read in
conjunction with the accompanying drawings of which:
FIG. 1 shows an electrospray ion source coupled to an analyzing
region via a capillary tube;
FIG. 2 is an enlarged view of the region 2--2 of FIG. 1;
FIG. 3 is a sectional view taken along the line 3--3 of FIG. 1
showing the tube support; and
FIG. 4 is an enlarged view of the region 4--4 of FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, an electrospray ion source 11 is schematically
shown as associated with an analyzer chamber 12. The source
includes an input needle 13 into which a liquid sample 14 is
introduced.
Referring particularly to FIG. 2, the needle includes an inner tube
16 in which the liquid sample is introduced. Surrounding the tube
is a second tube 17 which defines with the first tube an annular
region through which a liquid is introduced for mixing with the
sample liquid to reduce the surface tension and form droplets. An
outer tube 18 forms a second annulus through which a focusing gas
is introduced to focus the droplets as they exit the needle towards
a capillary tube 19. As previously explained, the needle, or
capillary, is maintained at a high voltage with respect to the
nearby surfaces forming the ionization chamber 21 and as the liquid
is dispersed, the droplets or particles are charged by the voltage
gradient at the tip of the capillary. The ionization mechanism
involves the desorption at atmospheric pressure of ions from the
fine electrically charged particles. A counter-flow of gas
indicated by the arrow 22 enhances the desorption process. The gas
flows through a chamber 23 past the end of the capillary 19 and
exits the ionization chamber 21 as indicated schematically at
24.
A chamber 26 maintained at a lower pressure than the atmosphere
pressure of the chamber 21 communicates with the ionization chamber
via the capillary tube 19. Due to the differences in pressure, ions
and gas are caused to flow through the capillary 19 into the
chamber 26. A voltage is applied between conductive sleeves 27 and
28 to provide a voltage gradient. The end of the tube 19 is
supported by, for example, three supports 29 disposed at
120.degree. with respect to one another.
The end of the capillary is opposite a skimmer 31 which separates
the low pressure region 26 from a lower pressure region in the
analyzer 12. The skimmer includes a central orifice or aperture 32
which normally is aligned with the axis of the bore of the
capillary. The skimmer is followed by ion optics which may comprise
a second skimmer 33 and lenses 34, which direct ions into the
analyzing chamber and into a suitable analyzer.
As described above, the solvated droplets or particles flow into
the capillary and acquire kinetic energy which allows them to pass
through the skimmer aperture 32 into the lens region including
skimmer 33 and lenses 34. These droplets or particles impact on the
surfaces of the skimmer 33 or the lenses 34 and create secondary
ions. These ions are random and find their way into the detector
and cause noise to be observed at the detector, thereby decreasing
the signal-to-noise level and producing electronic spikes in the
mass spectrum.
In accordance with this invention, the axis of the capillary is
altered or directed as shown in FIG. 4 by adjusting the supports 29
so that the axis is offset from the skimmer orifice or aperture. In
this way, there is no alignment between the bore of the capillary
and the orifice of the skimmer. The tendency is for the large
droplets or particles to move to the center of the flow in the
capillary and travel in a straight line. These droplets or
particles traveling in a straight line strike the skimmer. This is
illustrated in FIG. 4, which shows that large particles 36 travel a
straight line and impinge upon the skimmer 31. The droplets or
particles are thereafter pumped away by the vacuum pump associated
with the chamber 26 as illustrated by the arrow 37. The realignment
of the axis of the capillary tube does not cause any significant
loss in the observed signal intensity of the ions being analyzed.
This is due to the fact that the analyzed ions and gases undergo a
free jet expansion and are deviated from the axis to travel through
the orifice or aperture 32.
Thus, there has been provided a method and apparatus for reducing
neutral noise in an electrospray.
* * * * *