Probe for thermospray mass spectrometry

A thermospray probe assembly which allows for easy replacement or exchange the central capillary tube without requiring replacement of the entire probe assembly when the capillary tube becomes degraded and/or clogged. The replaceable capillary tube is secured at a tip end to a tip of the outer tubular member of the probe assembly by a set screw. The other end of the replaceable capillary tube is sealingly secured by a compression fitting in an adapter connected to the outer tubular member.

TECHNICAL FIELD 
The present invention relates to probe assemblies, and more particularly 
relates to thermospray probe assemblies with replaceable capillary tubes. 
BACKGROUND ART 
Thermospray is a practical ionization method for interfacing liquid 
chromatography with mass spectrometry. Since its introduction, a vast 
number of compounds that would otherwise have been very difficult to 
analyze using mass spectrometry have been successfully identified and 
quantified. Thermospray has been particularly useful in the analysis of 
drugs and metabolites from biological matrices. 
One of the drawbacks associated with thermospray is that nonvolatile salts 
and other nonvolatile materials tend to degrade its performance, 
ultimately clogging the thermospray probe assembly and destroying it. 
Several solutions to this problem have been proposed including the use of 
replaceable ceramic tips and sonication. Such solutions do not prevent the 
capillary tubing of thermospray probe assemblies from degrading and 
clogging. Once the capillary tubes become degraded and clogged, the entire 
probe assembly has to be replaced. 
Since a considerable expense is incurred when the probe has to be replaced, 
there is a need for a thermospray probe assembly which includes a 
replaceable capillary tube. 
DISCLOSURE OF THE INVENTION 
It is according one object of the present invention to provide an interface 
for liquid chromatography and mass spectrometry. 
Another object of the present invention is to provide a thermospray probe 
assembly. 
A further object of the present invention is to provide a thermospray probe 
assembly which has a replaceable capillary tube. 
A still further object of the present invention is to provide a combination 
of a thermospray probe assembly and a removable capillary tube. 
According to these and other objects of the present invention which will 
become apparent as the description thereof proceeds, the present invention 
provides a thermospray probe assembly which includes: 
an outer tubular housing; 
an adapter, the outer tubular member being connected at one end thereof to 
one end of the adapter; 
a capillary tube which extends through the adapter and the outer tubular 
housing, the capillary tube having a tip end which extends to a tip end of 
the outer tubular housing; 
means for releasibly securing the capillary tube in the adapter; and 
means for releasibly securing the tip end of the capillary tube in the tip 
end of the outer tubular housing. 
The present invention also provides for an inprovement over existing 
thermospray probe assemblies which involves means to releasibly secure a 
tip end of the capillary tube to a tip end of the outer housing. 
The present invention further provides for the combination of a thermospray 
probe assembly and a plurality of interchangeable capillary tubes which 
includes: 
an outer tubular housing; 
an adapter, the outer tubular member being connected at one end thereof to 
one end of the adapter; 
means for releasibly securing one of the plurality of capillary tubes in 
the adapter; and 
means for releasibly securing a tip end of one of the plurality of 
capillary tubes in the tip end of the outer tubular housing.

BEST MODE FOR CARRYING OUT THE INVENTION 
The present invention is generally directed to an interface between a 
liquid chromatography column and a mass spectrometer. The interface 
receives components which have been separated by the chromatography 
column. The components, e.g., ions, are contained in a liquid eluent which 
is vaporized as it is passed through the interface. As a result, the 
interface (referred to hereafter as a thermospray probe or "probe"), 
provides a vaporized form of the separated components to the mass 
spectrometer. 
The thermospray probe of the present invention includes a capillary tube 
through which fluids from a chromatography column are transferred to a 
mass spectrometer. The capillary passes through the end of the probe and 
extends to the tip of the probe. Along a portion of the capillary, a 
heating element, e.g., an electrical resistance heating element, is 
provided which surrounds the capillary tube. The heating element is used 
to vaporize eluents or carrier fluids which contain components which are 
to be analyzed by the mass spectrometer. 
The portion of the probe which contains the heating element has an outer 
tubular member. A Teflon tubing is provided in the annular space between 
the outer tubular member and the capillary tube, having the surrounding 
heating element. A metal insert, e.g., stainless steel, is fixed to the 
tip of the outer tubular member. The metal insert receives the tip of the 
capillary tube and includes means to releasibly secure the tip of the 
capillary tube. 
The end of the outer tubular member which is opposite the tip of the probe 
is attached to an adapter which provides means to receive and secure the 
capillary tube therein. That is, one end of the adaptor is attachable to 
the outer tubular member, for example by a threaded connection. The other 
end of the adaptor provides means, for example a compressible fitting, 
which secures the capillary therein. The capillary tube passes through 
both ends of the adapter. 
A particular feature of the probe assemble of the present invention is that 
the capillary tube can be easily replaced. Known probe assemblies do not 
allow for replacement of capillary tubes. Rather, the entire probe 
assembly has to be replaced at a substantial cost. Such replacement is 
needed when the capillary tubes degrade and/or become clogged over time. 
In addition to being able to replace degraded and/or clogged capillary 
tubes, the present invention allows different types of capillary tubes to 
be exchanged, as necessary, when different components are to be analyzed. 
For example, stainless steel, titanium, nickel, special coated or lined, 
e.g., glass-lined stainless steel, and other types of capillary tubes can 
be utilized in a single probe assembly. Prior to the present invention, a 
different probe assembly was required for each type of capillary tube 
utilized. 
FIG. 1 is a cross-sectional view of a conventional probe assembly. The 
probe assembly in FIG. 1 includes a steel capillary tube 1 and an outer 
tubular member 2 which surrounds one end of the steel capillary tube 2. A 
tip thermocouple 3 and a control thermocouple 4 are provided to monitor 
the temperature of the capillary tube 1. The tip of the capillary tube 1 
and the outer tubular member 2 are welded together at point 5 as shown. 
This weld, provides a high vacuum seal between the capillary tube 1 and 
the outer tubular member 2, and prevents the capillary tube 1 from being 
removed from the probe assembly. 
FIG. 2 is a cross-sectional view of probe assembly according to one 
embodiment of the present invention. The probe assembly 10 of the present 
invention includes a capillary tube 11. As shown, the capillary tube 11 
passes through an adapter 12 and through an outer tubular member 13. The 
outer tubular member 13 is connected to one end 14 of the adapter 12 by a 
suitable connection, such as cooperating threads 15 on the outer tubular 
member 13 and an internal threaded bore 16 in the adapter 12. The other 
end 17 of the adapter 12 includes a tapered seat 18 for receiving therein 
a compressible fitting 19, e.g., ferrule. A threaded bore portion 20 is 
provided between the tapered seat 18 and the other end 17 of the adapter 
12 to receive a nut 21 with complementary threads which can be turned so 
as to put pressure on the compressible fitting 19. For convenience, nut 21 
can have a large head 22 with a gripping surface on an outer circumference 
23 thereof. A smooth central bore 24 continues from the tapered seat 18 to 
the threaded bore 16 at end 14 of the adapter 12. 
As shown in FIG. 2, a heating element 25 surrounds the capillary tube 11. 
According to a preferred embodiment, the heating element 25 is an 
electrical resistance heating element According to a more preferred 
embodiment, the capillary tube 11 is used as a heating element. According 
to this embodiment, an electric current is passed through the capillary 
tube 11 along its length in a known manner. A heat resistant material is 
provided between the capillary tube 11 and the outer tubular member 13. In 
a preferred embodiment, an intermediate tubular member 26 is provided 
between the capillary tube 11 and the outer tubular member 13. Such 
intermediate tubular member 26 is preferably made of a heat resistant 
insulating material such as Teflon and insulates the heating element from 
the outer tubular member 13. 
A small bore 27 is provided in the intermediate tubular member 26 through 
which the leads 28 of a thermocouple 29 are be fed. The thermocouple 29 is 
positioned near the tip of the probe assembly, as illustrated, and is 
utilized to monitor the temperature at the tip of the probe assemble. The 
leads 28 of the thermocouple pass through the outer tubular member 26 near 
the adapter 12 through a small hole 30 which is sealed with an epoxy resin 
31. Another thermocouple 32 is positioned adjacent the capillary tube 11 
upstream of the adapter 12. A temperature comparison between the 
thermocouples 32 and 29 is used to control the output of the heating 
element 25 in a known manner. 
An insert 33 is provided in the outer tubular member 13 at the tip of the 
probe assembly. The insert 33 is fixed, e.g., by welding or soldering, to 
the inside of the outer tubular member 13 and includes a threaded bore 34 
in which a set screw 35 can be inserted and tightened against the 
capillary tube 11. 
In use, the capillary tube 11 is inserted through a bore 36 in the nut 21, 
compression fitting 19, smooth bore 24 of the adapter 12 , and the heater 
element 25 in intermediate tubular member 26. The capillary tube 11 is 
positioned so that the tip 37 of the capillary tube is substantially flush 
with the end if the insert member 33. Once the capillary tube 11 is 
positioned, the set screw 35 is inserted into bore 34 and tightened 
against the capillary tube member 11. Next, the compression fitting 19 is 
inserted through bore 20 to tapered seat 18 and nut 21 is tightened in 
bore 20 to cause the compression fitting 19 to compress around and 
sealingly secure the capillary tube 11. 
When used to introduce components into a mass spectrometer, it is essential 
that the probe assembly be sealed against the high vacuum of the mass 
spectrometer. In tests, it has been found that the compressions fitting 19 
utilized in the probe assembly of the present invention provides a 
resealable vacuum seal. 
To remove the capillary tube 11 , set screw 35 and nut 21 are each 
withdrawn to release capillary tube 11, which can then be pulled out of 
the end of the probe assembly. Once removed, a new capillary tube can be 
inserted and secured in the probe assembly in a reverse manner. 
In use, the capillary tube 11 is connected to the outlet of a 
chromatography column and the tip of the probe assembly if connected to 
the sample inlet of a mass spectrometer in a known manner. 
According to a preferred embodiment, the capillary tube 11 has an outside 
diameter of 1/16 inches and an inside diameter of 0.005 inches and is made 
from stainless steel. In this preferred embodiment, the outer tubular 
member 13 has an outside diameter of 1/4 inches and an inner diameter of 
5/32 inches and is also made of stainless steel. These and the relative 
sizes of the other elements of the probe assembly can be easily determined 
on the basis of flow rates both from the chromatography column and to the 
mass spectrometer. 
Nut 21, compression fitting 19, and adapter 12 are preferably made of a 
non-corrosive material such as PEEK (polyetherethylketone), Vespel.RTM. (a 
polyimide from Du Pont), or the like. The other elements of the probe 
assembly should be made of heat-resistant, non-corrosive materials. 
Any type of capillary tube can be utilized in the probe assemble including, 
stainless steel, nickel, titanium, special coated or lined, e.g., 
glass-lined stainless steel, etc. The selection of the type of capillary 
tube is determined on the basis of the inertness of the capillary tube to 
a particular sample/carrier fluid. 
The probe assembly of the present invention is not solely limited for use 
in conjunction with chromatography columns or mass spectrometers. Discrete 
samples could be passed through the probe assembly which might be attached 
to the sample injection port of a mass spectrometer or to the sample 
injection port of other analyzers, such as a flame photometer. 
Although the present invention has been described with reference to 
particular means, materials and embodiments, from the foregoing 
description, one skilled in the art can easily ascertain the essential 
characteristics of the present invention and various changes and 
modifications may be made to adapt the various uses and characteristics 
without departing from the spirit and scope of the present invention as 
described by the claims which follow.