Note: Descriptions are shown in the official language in which they were submitted.
CA 02783028 2012-07-12
GAS CHROMATOGRAPH-MASS SPECTROMETER TRANSFER LINE
BACKGROUND
The invention relates to a heated gas sample transfer line from a gas
chromatograph (GC) to an ion source of a mass spectrometer (MS). Various
combinations for coupling of gas chromatographs (GC) with mass spectrometers
(MS) are known in the art. In the GC, samples are injected onto a GC column
through an injection port and become separated while passing through the GC
column. The effluent of the GC column is conveyed from the GC oven to the ion
source of the MS within a column extension of a transfer line. In the ion
source,
the sample molecules are ionized, for example by electron impact or chemical
ionization, before being analyzed according to their mass-to-charge ratios.
During the transfer of the effluent from the GC column oven to the ion source,
it
is necessary to maintain a uniform temperature along the column extension. If
a
significant temperature gradient exists so that the temperature varies at
different
points along the column extension, cold spots may occur to cause condensation
from the gas phase of the sample so that it will either not be passed through
to
the MS or will exhibit excessive chromatographic peak broadening or peak
tailing. On the other hand, hot spots that appear may cause some compounds to
degrade thermally with a resultant change in their chemical structure. Similar
effects can occur even if the transfer line is at uniform temperature if the
temperature of the transfer line is either too cold or too hot during the
elution of
any given chemical compound. Additionally, excessive temperatures of transfer
line can lead to elevated "chemical noise" and lower signal-to-noise ratio for
any
given analytical results. Temperature variations along the length of the
transfer
line of +1- 10 Celsius are generally acceptable, although variations of less
than
+1- 5 Celsius are required in some applications.
Usually, GC-MS transfer lines are rigidly attached to the housing of the mass
spectrometer and provide a uniform temperature environment on the column
extension when column effluents are conveyed through the walls of the GC oven
and the mass spectrometer into the ion source.
1
CA 02783028 2012-07-12
MS ion sources have to be cleaned in regular periods, or the filaments for
electron generation have to be replaced. For these maintenance operations, the
ion sources have to be taken out of the MS housing. Generally, they are
mounted
with fasteners that are sometimes hard to access, require clean tools and
potentially can be lost inside the instrument. In addition, the column
extension
has to be removed from the transfer line, and the transfer line has to be
disconnected from the ion source, always with a risk to damage the GC column
or the column extension, respectively.
SUMMARY
In a first aspect, the invention provides a transfer line for conveying the
column
effluent of a gas chromatograph to an ion source of a mass spectrometer,
comprising: a transfer line body, means for moving the transfer line body, and
gas seal between the housing of the mass spectrometer and the transfer line
body to prevent a vacuum leak when the transfer line body is moved by the
means. The means can move the transfer line body along its axis between two
end positions (extended/retracted end position). Inside the transfer line
body, the
column effluents are conveyed within an extension of the gas chromatography
column (column extension) into the ionization chamber of the ion source, where
the sample molecules are ionized, for instance, by electron ionization (El) or
by
chemical ionization (Cl).
In a first embodiment, the gas seal comprises bellows fastened to the transfer
line body and to the housing of the mass spectrometer such that the transfer
line
body is movable between two end positions without breaking the vacuum in the
mass spectrometer. The bellows can be structured to exert a force on the
transfer line body, preferably directed towards the inside of the mass
spectrometer. Preferably, the transfer line body comprises threads and the
means for moving comprises a hand wheel engaging the transfer line body via
complementing threads. On the other hand, the means for moving can also
comprise a pump for evacuating the ion source region and generating a force on
the transfer line body which extends from outside the mass spectrometer into
the
2
CA 02783028 2012-07-12
evacuated ion source region. Thus, the transfer line body is move by the
forces
on the bellows generated by the evacuation and venting processes due to the
changing pressure differences.
In a second embodiment, the gas seal comprises at least one sealing ring (0-
ring) positioned between the transfer line body and the housing of the mass
spectrometer. Here, the transfer line body preferably comprises threads and
the
means for moving comprises a hand wheel engaging the transfer line body via
complementing threads.
In a third embodiment, the transfer line body comprises a head piece that
forms a
part of the ion source, when the transfer line is in the extended end
position. The
column effluents are conveyed in the column extension and released at a head
piece into the ionization chamber of the ion source. The head piece can
comprise
an electrically insulated electrode serving as an ion repelling electrode in
the ion
source, when an appropriate voltage is applied to the electrode.
In a fourth embodiment, the transfer line body comprises an inner tube that
contains an extension of the column of the gas chromatograph and that is
fastened to the column of the gas chromatograph, inside the oven, in a vacuum-
tight manner by a ferrule and compression nut. Furthermore, the transfer line
can
comprise a gas inlet between gas chromatograph and mass spectrometer to
convey additional gas to the ion source, for example to provide the ion source
with an appropriate gas for chemical ionization. The additional gas is
preferably
conveyed to the ion source in the tubular space between the inner tube and the
column extension. The transfer line can further comprise a heating cartridge
and
a temperature sensor to heat the inner tube and the column extension inserted
in
the inner tube to a desired temperature.
In a second aspect of the invention, the transfer line and the ion source are
positioned such that the transfer line body presses the ion source, which is
not
rigidly fixed to the MS housing, into a recess seat of the housing of the mass
spectrometer, when the transfer line is moved to the extended end position,
and
aligns the ion source in an operating position. When the transfer line is
retracted
and the mass spectrometer (or at least the ion source region) is vented, the
ion
3
CA 02783028 2012-07-12
source is released from being fixed, so that the ion source can be easily
removed
from the ion source housing, without any further unscrewing or unclamping,
only
by disconnecting some electrical contacts. The ion source is preferably
clamped,
but not rigidly attached to the housing of the mass spectrometer after being
aligned in the operating position.
Unique to the transfer line according to the invention is that it can
simultaneously
perform multiple functionalities: providing an isothermal conduit for the GC
column effluents, closing the ionization chamber, forming an ion repelling
electrode of the ion source, self-aligning and fixing the ion source in place
for
operation. Venting the ion source region and/or moving the transfer line
automatically releases the ion source from the recess seat, allowing easy ion
source removal. Upon reinsertion, the ion source is reliably centered and held
securely by the force supplied by the transfer line. The bellows, as preferred
gas
seal, further provide a lower heat-loss from the transfer line to the housing
of the
mass spectrometer compared to rigidly mounted transfer lines known in the art.
The lower heat loss enables a highly homogeneous temperature inside the
transfer line and in the column extension, respectively. The intimate thermal
contact between the head piece of the transfer line body and the ion source
ensure isothermal conditions up to the ion source if the temperature set
points for
the ion source and the transfer line are equal. The head piece also serves to
protect the end of the column extension when removing the ion source for
cleaning or during cleaning of the inner walls of the ion source housing
without
the need to remove the column extension from the transfer line body.
BRIEF DESCRIPTION OF THE DRAWINGS
Figures 1 and 2 illustrate a schematic cross-section of a first embodiment of
a
movable GC-MS transfer line according to the present invention.
In figure 1, the transfer line is shown in an extended end position after
evacuating
the ion source region (17). The evacuation generates a pressure difference and
causes the bellows (9) to extend. Thereby, the head piece (7) of the transfer
line
body presses against the ion source (12) and forms one side wall of the
4
CA 02783028 2014-03-19
ionization chamber (14). The ion source (12) which is not rigidly fastened to
the
housing (10) is pushed into a recess seat (13) of the MS housing (10) and
aligned in an operating position.
In figure (2), the transfer line is shown in the retracted end position after
venting.
The bellows (9) retract the transfer line body and the ion source (12) is
thereby
released from its recess seat (13). The ion source (12) is free for removal to
being cleaned, repaired or replaced. The transfer line body slides through the
thermally insulating wall (15) of the GC oven when being moved.
Figure 3 illustrates a schematic cross-section of a second movable GC-MS
transfer line in the extended end position. In contrast to the first
embodiment
shown in figures 1 and 2, the transfer line body of the second embodiment
comprises threads (5a) at outside of the outer tube (5b) and is moved by a
hand
wheel (16) attached to the housing (10), engaging the transfer line body via
complementing threads.
Figure 4 illustrates schematically a cross-section of a third movable GC-MS
transfer line with implementation of a heat pipe. The heat pipe comprises an
outer tube (20), an inner tube (21) holding the column extension (3), a fluid
reservoir (22), and a heating element (23) with temperature sensor.
DETAILED DESCRIPTION
The preferred embodiments will now be described with reference to the
drawings. The embodiments shown herein provide movable transfer lines. The
transfer lines are located between a gas chromatograph and a mass
spectrometer. The details of the gas chromatograph and the mass spectrometer
are omitted in the drawings in order to clarify the essential features of the
embodiments, only the walls of the GC oven (15) and the housing (10) of the
mass spectrometer are shown to some extent. The disclosed transfer lines
5
CA 02783028 2012-07-12
enable moving a transfer line body along its axis while maintaining the vacuum
of
the mass spectrometer sealed to the outside atmosphere.
The transfer line body of the preferred embodiments comprises a column
extension (3), a head piece (7), a heater cartridge (4), an inner tube (5a),
an
outer tube (5b), and a plate (8) that connects the column extension (3) and
the
tubes (5a, 5b). Bellows (9), most preferably metallic bellows, are welded on
one
side to the plate (8), and on the other side to the housing (10), forming a
vacuum-
tight connection between the transfer line body and the housing (10), but
allowing
the transfer line body to move along the axis of the column extension (3) by a
few
millimeters to some ten millimeters. The GC column (1) is fastened vacuum
tight
to the inner tube (5a) by a ferrule and compression nut (2) and is extended by
extension column (3) up to the head piece (7).
A first preferred embodiment is illustrated in figures 1 and 2. The transfer
line
body is presses the against the ion source (12) when the ion source region
(17)
is evacuated (figure 1), and is retracted when the ion source region (17) is
vented
(figure 2). The venting and evacuating of the ion source region (17) generate
counteracting forces onto the bellows (9) which can move the transfer line
body
between the two end positions (extended/retracted end position), shown in
figures 1 and 2, when the transfer line body and the bellows (9) are
adequately
designed.
By moving the transfer line body into the extended position shown in figure 1,
the
transfer line body pushes against the ion source (12) which is not rigidly
fastened
to the housing (10) such that the ion source (12) is pressed into a recess
seat
(13) of the housing (10). The ion source (12) is secured in this position by
the
force supplied by the transfer line body and is aligned by the recess seat
(13) in
its operating position. The recess (11) in the housing (10) securely guides
the ion
source (12) between the operating position and the maintenance position, shown
in figure 2. By venting the ion source region (17), the transfer line is
retracted due
to the resilient forces of the bellows (9) and the ion source (12) is released
for
removal. In the maintenance position, the ion source (12) can be easily
removed
for cleaning, without any unscrewing or unclamping, and without dismounting
the
6
CA 02783028 2012-07-12
transfer line assembly or even disconnecting the GC column. Only the electric
contacts have to be disconnected.
In figure 1, the ion source region (17) is evacuated and the head piece (7) of
the
transfer line body forms a side wall of the ion source (12). The head piece
(7)
comprises an electrode (7b) which is electrically insulated from the transfer
line
body and the ion source (12). Therefore, the electrode (7b) serves as an ion
repelling electrode, when an adequate electric potential is applied to it. A
person
skilled in the art knows El and CI ion sources for GC-MS instruments, so it is
not
necessary to explain these ion sources in detail here. An El ion source
usually
comprises: an ionization chamber, elements for heating the walls of the
ionization
chamber, filaments for electron generation, permanent magnets and yokes to
guide accelerated electrons from the filaments into the ionization chamber,
ion
extraction and acceleration lenses, and contacts for the supply of electric
voltages.
The GC column (1) is fastened vacuum tight to the inner tube (5a) by a ferrule
and compression nut (2) and is extended by the extension column (3) up to the
head piece (7). The transfer line body further comprises a gas inlet (6)
between
the oven of the gas chromatograph and the mass spectrometer. The position of
the gas inlet (6) is shown by way of example only. It is equally possible to
choose
another position and/or another orientation along the transfer line body such
as
indicated with the dashed contour (6*). The additional gas is introduced in
the
annular space between the column extension (3) and the inner tube (5a). The
gas supplied by the gas inlet (6) mixes with the GC column effluent as both
enter
the ionization chamber (14). The gas may serve as a medium for chemical
ionization (Cl) of the effluents from the GC column. The transfer line body
preferably comprises an electrical cartridge heater (4) inserted into the
space
between the inner (5a) and the outer tube (5b) to maintain the temperature of
the
inner tube (5a) and thus of the column extension (3) at a desired value. The
temperature is controlled by a feedback loop, which maintains the temperature
of
a sensor integrated in the electrical cartridge heater (4).
7
CA 02783028 2012-07-12
A second preferred embodiment is illustrated in figure 3. Equivalent elements
of
both embodiments share the same reference signs. In this embodiment, bellows
(9) are designed to exert a force towards the inside of the mass spectrometer.
The bellows (9) push the transfer line body against the ion source (12) and
holds
it in the extended end position (operating position) even when the ion source
region (17) is vented, i.e. the ion source (12) is pre-aligned within the
recess seat
(13) in the vented state. The pre-alignment prevents a subsequent misalignment
of the ion source (12) when the ion source region (17) is evacuated and thus
the
pressure forces, being exerted on the ion source (12) by the transfer line
body,
increase. In the vented state, the transfer line body is retracted from the
extended end position by a hand wheel (16) which is attached to the housing
(10). The exterior of the outer tube (5b) of the transfer line body comprises
a
thread (5c). A corresponding thread is provided on hand wheel (16). Thus, when
the wheel (16) is rotated, it moves the transfer line body in the axial
direction up
to several ten millimeters, such that the transfer line can be substantially
moved
between the two end positions. Since the ion source (12) can be held by hand
while the hand wheel (16) is turned to move the transfer line body, a recess
in
the housing (10) is not necessarily required. The bellows (9) maintain the
vacuum seal regardless of the position of the hand wheel (16), such that the
transfer line can be moved towards the ion source or retracted from the ion
source without breaking vacuum.
A third preferred embodiment is illustrated in figure 4. Equivalent elements
of
both embodiments share the same reference signs. In contrast to the first and
second embodiment, the transfer line body comprises a well-known heat pipe to
maintain the temperature of the inner tube (21) and thus of the column
extension
(3) at a desired value. The heat pipe consists of the inner tube (21) holding
the
column extension (3), an outer tube (20), a fluid reservoir (22), and a
heating
element (23) with temperature sensor. The heating device (23) heats the fluid
inside fluid reservoir (22), and the vapor generated condenses at all
positions of
the inner wall of the heat pipe which are at a lower temperature than the
other
parts of the heat pipe. The heat pipe holds the temperature along the inner
tube
8
CA 02783028 2014-03-19
(21) highly constant, as a rule much better than +1- 1 Celsius. The heat pipe
can
even be used to heat the ion source (12). A favorable fluid for the heat pipe
is
water, working well in the temperature region above about 150 Celsius. For
lower temperature regimes, a number of other liquids can be used. The simple
5 heat pipe shown in figure 4 can be improved by further capillary means along
the
internal wall, causing the backflow of the fluid to the heater region. The
heat pipe
can be enclosed by insulating material and an additional tube, so that the
outer
tube (20) does not contact directly the housing wall (10) or the GC oven wall
(15).
The transfer line of the third embodiment does not have an additional gas
inlet,
but a Cl line may well be added inside the GC oven at the end of the transfer
line.
The above description relates to a specific embodiment of the invention;
however, the invention can be implemented using other embodiments to achieve
the same improvements and features. It should be understood that processes
and techniques described herein are not inherently related to any particular
apparatus and may be implemented by any suitable combination of components.
Further, various types of general purpose devices may be used in accordance
with the teachings described herein. It may also prove advantageous to
construct specialized apparatus to perform the method steps described herein.
The present invention has been described in relation to particular examples,
which are intended in all respects to be illustrative rather than restrictive.
Those
skilled in the art will appreciate that many different combinations of
hardware,
software, and firmware will be suitable for practicing the present invention.
Moreover, other implementations of the invention will be apparent to those
skilled
in the art from consideration of the specification and practice of the
invention
disclosed herein.
What is claimed is:
9
