Note: Descriptions are shown in the official language in which they were submitted.
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Liquid Evaporator
The present invention relates to a liquid evaporator and to a method for
evaporating
liquids.
Liquid evaporators are used for converting liquids into the gas phase and are
employed in
various applications.
US7618027B2 discloses, for example, a liquid evaporator for generating a
highly pure gas
with a low vapour pressure, the gas being used in the field of
microelectronics.
W005/016512A1 discloses, for example, a liquid evaporator which can be used in
a
method for removing a volatile compound from a substance mixture.
Liquid evaporators also have widespread use in analysis, where a sample
quantity of a
liquid to be analysed is first converted into the gas phase in order to make
it accessible for
an analysis method. US7309859B2 discloses, for example, a liquid evaporator
which can
be used in an ion source for mass spectrometers.
Particularly in the field of analysis, special requirements are placed on a
liquid evaporator.
In the case of substance mixtures for quantitatively reliable analysis, for
example, it is
important for the concentrations of all components of the substance mixture in
the gas
phase to be identical to the concentrations in the liquid. To this end, it is
necessary to carry
out the evaporation of the sample volume completely. In order to avoid
recondensation, the
gaseous sample then needs to be delivered, for example, to the analysis system
above the
highest evaporation temperature of the components involved.
To date, liquid evaporators have preferably been configured as constantly
heated systems
to which liquid samples are supplied continuously in small quantities. In
order to ensure
the necessary complete, and as far as possible simultaneous, evaporation of
the
components which have different evaporation temperatures, a high heat capacity
of the
evaporator is generally necessary, together with a high thermal mass. This
entails a high
energy demand and - owing to the in general spatially extended structure of
the evaporator
- a correspondingly high power demand and concomitantly sometimes also long
dead times
between sampling and evaporation.
Owing to the increasing performance of modern analysis systems, the sample
quantity
required for an analysis has constantly decreased over the course of time. For
example,
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micro mass spectrometers are known (see for example W008/101669A1) which can
function with minimal sample quantities.
The reduction of the required sample quantity is on the one hand advantageous.
Smaller
sample quantities mean, for example, a shorter evaporation time and therefore
a higher
temporal resolution with which changes in sample composition can be recorded.
On the other hand, the liquid evaporator needs to be adapted to the small
sample quantity
in order to be able to fully exploit the advantages of a smaller sample
quantity.
On the basis of the known prior art, the object therefore arises to provide a
liquid
evaporator and a liquid evaporation method, with which small volumes of
liquids can be
evaporated reliably. The sample volume to be evaporated is in this context
preferably less
than 100 l, particularly preferably less than 10 l, more particularly
preferably less than 1
l.
The sample quantity evaporated should be representative of the liquid from
which it is
taken. The liquid evaporator should be economical to manufacture and operate,
have a low
energy demand for the evaporation and ensure rapid evaporation. The liquid
evaporator
should either have self-cleaning capabilities, so that it is even possible to
evaporate liquids
which contain dissolved substances that may possibly leave a deposit, or be
configured as a
disposable article.
According to the invention, this object is achieved by a method for
evaporating at least a
part of a liquid according to Claim 1, and by a liquid evaporator according to
Claim 4,
which is formed in order to carry out the method. Preferred embodiments may be
found in
the dependent claims.
The present invention therefore firstly provides a method for evaporating at
least a part of a
liquid, characterized in that a liquid is fed through a channel past an
opening, the opening
leading to a vapour chamber which is maintained at a temperature above the
evaporation
temperature of the liquid, and the liquid is heated in the region of the
opening by
electromagnetic radiation so that at least a part of the liquid evaporates and
the vapour
enters the heated vapour chamber.
The present invention secondly provides a liquid evaporator comprising at
least
4
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- a body in which a channel and a chamber, which are connected to one another
through
an opening, are formed, the channel being configured so that a liquid can be
fed past the
opening and an evaporation region adjacent to the opening, and optionally
including the
opening, can be irradiated with electromagnetic radiation,
- means for heating the chamber.
The method according to the invention and the evaporator according to the
invention use
the energy of electromagnetic radiation in order to evaporate a sample
quantity of a liquid
by local heating. To this end, a liquid is passed through a channel which
leads past an
opening into a chamber. The region of the opening is accessible for
electromagnetic
radiation, i.e. it can be irradiated with electromagnetic radiation. In this
region - also
referred to below as the evaporation region - the liquid is locally heated by
means of
electromagnetic radiation to such an extent that a sample quantity evaporates.
The
evaporated sample quantity passes through the opening into a chamber - also
referred to
below as the vapour chamber - which can be heated to a temperature above the
evaporation
temperature of the liquid, so that the vapour does not condense in the
chamber.
The electromagnetic radiation is preferably supplied to the evaporation site
from outside
the liquid evaporator. The evaporation site is therefore preferably provided
with a cover
which is at least partially transparent for the electromagnetic radiation
being used. An at
least partially transparent cover is intended to mean a cover which preferably
transmits a
majority of the electromagnetic radiation and absorbs and/or reflects only a
small part, so
that a majority of the incident energy reaches the evaporation site and is
available there for
heating a sample quantity. A high transparency and therefore low absorption of
the cover
also have the effect that the cover itself is not heated.
Preferably, the evaporation region is configured so that it absorbs a high
proportion of the
electromagnetic radiation and converts it into heat. For example, the inner
walls of the
channel may consist of a material which absorbs a majority of the incident
energy and
converts it into heat. It is likewise conceivable for the inner walls of the
channel to be
coated in the evaporation region with a material which has a high absorption
coefficient for
the radiation being used. In both of these cases, by means of the evaporator
it is even
possible to evaporate liquids which themselves have only a low absorption
coefficient for
the radiation being used; the heating takes place indirectly. The evaporation
region, when it
is adapted for indirect heating of the liquid, will also be referred to below
as the absorber.
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Advantageously, however, the radiation source used is matched to the liquid to
be
evaporated so as to allow direct heating of the liquid. Direct heating has the
advantage that
the environment of the liquid is heated only slightly, and detrimental effects
on the analysis
due to a heated environment (lower temporal resolution, contamination, damage,
etc.) are
thus minimized.
A laser beam is preferably used for the irradiation. A pulsed, focused laser
beam is
particularly preferably used. The laser pulse length is preferably selected so
that the
thermal time constants of the absorber and/or the liquid to be evaporated are
long
compared with this pulse length. This means that even a single pulse is
sufficient to
evaporate a sample quantity.
The quantity of liquid evaporated may be varied by varying the length of the
laser pulse
and by varying its power. For the evaporation of larger quantities, a pulse
sequence may
also be used. Since the instant of the evaporation can be established very
accurately, the
vapour generation can be synchronized with sampling in an analyser so that a
correlation
measurement with increased measurement sensitivity is possible. Furthermore,
this method
allows segments of a sample flow to be evaporated and analysed in a controlled
way, so
that in particular even sample compositions which vary greatly as a function
of time or, for
example, unmixed or undissolved components carried by the liquid (emulsions,
cells) can
be recorded in a defined or selective way.
The laser beam is preferably supplied to the evaporation region through a
cover, which is
at least partially transparent for the laser beam, by means of free-beam or
fibre optics.
The transition from the channel to the vapour chamber is configured so that,
because of the
capillary forces prevailing in the opening, the liquid cannot flow freely into
the chamber.
Suppression of the liquid egress is preferably achieved by a sufficiently
large difference
between the cross section of the chamber dimension at the position of the
opening and the
length of the interface with the channel and its cross section. As an
alternative or in
addition, the inner walls of the channel and the chamber may be provided, at
least in the
opening region, with layers having different surface energies. If the liquid
to be evaporated
is an aqueous solution, for example, the opening region of the channel may be
coated
hydrophilically and that of the chamber may be coated hydrophobically.
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In order to permit sampling from the middle of the channel, particularly in
the case of
small cross sections and therefore non-turbulent laminar flow, the channel
preferably has a
curvature in the region of the evaporation. The curvature leads to different
flow rates in the
inner and outer curve regions. Dean vortices are generated, which lead to a
flow
perpendicular to the flow direction and convey liquid elements from the middle
of the
channel to the channel edge in the opening region.
Since the briefly heated position is constantly flushed with the following
liquid, it is also
possible to achieve the effect that solids possibly precipitated during the
evaporation are
either redissolved or mechanically removed by the liquid flow and transported
away. The
Dean vortices occurring in the curvature region of the channel assist this
self-cleaning
process.
The vapour chamber has a gas outlet, through which the vapour can leave the
liquid
evaporator according to the invention. Besides the gas outlet, the chamber may
be provided
with further connections which, in particular, allow evacuation and optionally
also flushing
of the chamber between the sampling intervals, in order to avoid cross-
contamination of
samples from one evaporation process to the next.
As an alternative, this may also respectively be done in a controlled way
after a plurality of
evaporations, in order to increase the available gas quantity and/or its
pressure for the
injection or to permit averaging over a plurality of sample volumes directly
before the
analysis.
The vapour chamber may be heated by means of a heating element, which is
preferably
operated electrically.
In order to keep the vapour uniformly at a high temperature, preferably
lamellar structures
made of a thermally conductive material may additionally extend through the
vapour
chamber, which preferably is locally heated by surface contact heating, these
structures
likewise being heated by the heating device via thermal conduction.
These structures also facilitate the evaporation of droplets which have been
entrained by
the vapour from the channel into the chamber.
The structures are preferably applied so that they prevent particles from
passing through
the gas outlet (see Fig. 2), i.e. they preferably shield the opening and the
gas outlet from
one another.
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Since the input of energy for the evaporation preferably takes place
optically, i.e.
contactlessly, and the heating of the chamber does not need to be integrated
directly
therein, the evaporation system is simple to produce and in principle can also
be replaced
easily in the event of contamination. In a preferred embodiment, the liquid
evaporator
according to the invention is configured as a disposable article.
The body of the liquid evaporator according to the invention, in which the
channel and
chamber are formed, may be configured in one piece or a plurality of pieces.
It is
preferably configured in one piece.
The liquid evaporator is preferably a microsystem, the structures of which are
produced by
means of microfabrication techniques.
The production of structures in microsystems is known to the person skilled in
the art of
microsystem technology. Microfabrication techniques are, for example,
described and
illustrated in the book "Fundamentals of Microfabrication" by Marc Madou, CRC
Press
Boca Raton FLA 1997 or in the book "Mikrosystemtechnik fUr Ingenieure"
[Microsystem
technology for engineers] by W. Menz. J. Mohr and 0. Paul, Wiley-VCH, Weinheim
2005.
A more detailed description of silicon-on-silicon technology may for example
be found in
Q.-Y. Tong, U. Gosele: Semiconductor Wafer Bonding: Science and Technology;
The
Electrochemical Society Series, Wiley-Verlag, New York (1999). With regard to
glass-on-
glass technology, reference may be made by way of example to the following
publications:
J. Wie et al., Low Temperature Glass-to-Glass Wafer Bonding, IEEE Transactions
on
advanced packaging, Vol. 26, No. 3, 2003, pages 289-294; Duck-Jung Lee et al.,
Glass-to-
Glass Anodic Bonding for High Vacuum Packaging of Microelectronics and its
Stability,
MEMS 2000, The Thirteenth Annual International Conference on Micro Electro
Mechanical Systems, 23-27 January 2000, pages 253-258.
Microsystem technologies are fundamentally based on the structuring of silicon
and/or
glass substrates with a high aspect ratio (for example narrow trenches (- m)
of great depth
(-100 m)) with structuring accuracies in the micrometre range using wet
chemical,
preferably plasma etching processes combined with sodium-containing glass
substrates
adapted in terms of their thermal expansion coefficient (for example Pyrex'),
which are
provided with simple etched structures and preferably connected to one another
with a
hermetic seal directly by so-called anodic bonding, or alternatively with a
thin Au layer
functioning as a solder alloy (AuSi).
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Metal structures with a high aspect ratio can be produced by electrolytic
growth in thick
photoresists (> 100 m) with comparable accuracy (UV-LIGA). By using thin-film
technologies such as high vacuum evaporation and sputtering, PVD processes or
chemical
vapour deposition (CVD processes) preferably in a plasma, in combination with
photolithography and etching techniques, functional layers such as
metallizations,
hydrophobic or hydrophilic surfaces and functional elements such as valve
seals and
diaphragms, heating elements, temperature, pressure and flow sensors can be
integrated on
these substrates in a fully process-compatible technology.
The structures of the liquid evaporator according to the invention are
preferably produced
in a silicon-on-glass technology, silicon being used for the body and glass
being used for
the transparent cover. This combination, preferably connected hermetically by
anodic
bonding, allows highly accurate structuring of the various components of the
system,
particularly in silicon (photoetch technology, DRIE, coating). Silicon is
chemically and
thermally stable like glass, and in contrast to glass it is a good thermal
conductor with low
heat capacity (heated chamber with uniform temperature) and a good optical
absorber for
conventional laser wavelengths. The heat losses by dissipation through the
glass substrate
are low.
The combination of silicon and glass makes it possible to achieve local input
of the optical
energy into the channel edge as well as thermal decoupling of the channel and
the vapour
chamber. For thermal decoupling, the vapour chamber and the liquid sample
channel are
preferably separated by horizontal and vertical incisions in the highly
thermally conductive
body. Mechanical stability with low heat transfer is ensured by a transparent
cover, for
example made of glass or a polymer.
It is also conceivable to produce the liquid evaporator according to the
invention from
polymer materials, for example by means of injection moulding techniques. A
composite
material is preferably used for the body, for example a polymer in which
carbon (carbon
black, carbon nanotubes) is dispersed in order to increase the absorption of
electromagnetic
radiation and the thermal conductivity.
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Owing to the conventional dimensions in microsystem technology, in the range
of a few 10
to 100 m, a system produced in this way is also especially suitable for
analysing or
producing small sample volumes (nl liquid, l in the gas phase).
In order to evaporate such small sample quantities, only low laser energies in
the mWs
range are required even for liquids having a high enthalpy of vaporization,
for example
water, which are preferably delivered by standard semiconductor lasers,
coupled via fibres
or lenses, in the wavelength range of for example 400 nm to 980 nm even with
the
necessary short pulse durations (for example I W for 1 ms, 100 mW for 10 ms).
The sample volume to be evaporated is preferably less than 100 l,
particularly preferably
less than 10 l, more particularly preferably less than I l.
The liquid evaporator according to the invention is preferably suitable as a
sample
evaporator in a microanalysis system. The present invention therefore also
provides the use
of the liquid evaporator according to the invention in a microanalysis system,
particularly
in a micro gas chromatograph or a micro mass spectrometer, as described for
example in
the article "Complex MEMS: A fully integrated TOF micro mass spectrometer"
published
in Sensors and Actuators A: Physical, 138 (1) (2007), pages 22-27.
The liquid evaporator according to the invention and the method according to
the invention
ensure the following points:
= simultaneous evaporation of liquids and liquid mixtures
= reliable evaporation of small volumes (preferably even < 1 l)
= short dead time between the evaporation of two samples
= rapid evaporation, so that a high temporal resolution can be achieved during
the analysis
= evaporation of representative samples
= low energy requirement for the evaporation
= low equipment outlay
= even suitable for media having a high enthalpy of vaporization
= even suitable for media comprising dissolved solids (deposit)
= economical operation (production and replacement)
= low outlay on peripherals
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= sampling synchronizable with the analysis of the sample (lock-in principle)
= doseability of the gas volume
= adjustability of the injection gas pressure
= averaging by means of synchronization and multiple pulses
= statistical selection (time window) of the samples to be averaged
= averaging in the gas space
= self-cleaning
The invention will be explained in more detail below with the aid of figures,
but without
being restricted thereto.
The following references are used in all the figures:
List of References
1 electromagnetic radiation
2 transparent cover
3 liquid
4 opening
channel
6 body
7 vapour stream
8 heating element
9 vapour chamber
curvature
11 transverse flow
12 lamellar structures
13 gas outlet
14 incision
16 connection
Fig. 1 shows a perspective representation of a liquid evaporator according to
the invention
from above. The liquid evaporator comprises a body 6, in which a channel 5 and
a chamber
9 are formed. The channel 5 and the chamber 9 are connected to one another
through an
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opening 4. The channel 5 has a curvature 10 in the region of the opening 4. A
liquid is fed
through the channel 5 past the opening 4.
The transition from the opening 4 to the chamber 9 is configured so that,
because of the
capillary forces prevailing in the opening, the liquid cannot flow into the
directly adjacent
region 9.
In the region of the opening, the liquid is irradiated by means of
electromagnetic radiation
and therefore heated. In the present case, the irradiation takes place from
the direction of
the observer (from above).
Owing to the irradiation, the liquid is heated and a part of it evaporates,
this part entering
the chamber 9 through the opening 4 as a vapour stream. Below the chamber 9,
there is a
heating element by which the chamber can be heated (not shown in Fig. 1; see
Fig. 3).
The vapour chamber 9 and liquid sample channel 5 are thermally decoupled by
horizontal
incisions 14 in the highly thermally conductive body 6.
Fig. 2 shows the liquid evaporator of Fig. 1, in the chamber 9 of which
lamellar structures
12 are introduced. The lamellar structures are preferably heated by the
heating element
below the chamber 9 (not shown in Fig. 2; see Fig. 3) via thermal conduction.
Fig. 3 shows the liquid evaporator according to the invention of Fig. 1 in
cross section
along a straight line from A to B. Besides the components already described
above in
relation to Fig. 1, a transparent cover 2 can be seen in Fig. 3 which extends
over the entire
body 6. A heating element 8 is furthermore installed below the vapour chamber.
The
irradiation of the liquid takes place through the transparent cover,
preferably by means of a
focused laser beam 1.
