DE10007498B4 - electrospray - Google Patents

Abstract

contraption for atomising of liquids with means (1, 3) to a liquid to bring to an electric potential, by a variety of means, with which the liquid brought to potential mechanically breaks up or atomized is where the mechanical dicing or atomizing are such that the exit area of the liquid from the mechanical atomizer by a plurality of conical, tubular or thread-like elements is formed, the surfaces the conical, tubular or filamentary Elements at least in the region of their open ends made of fluoropolymers such as PTFE, FEP or graphite fluoride or pyrocarbon exist.

Description

The The invention relates to a device for atomizing electrically charged Liquids.

Out the prior art is known, a sample solution, ie a liquid to pump through a metal capillary that is at high electrical potential across from a counter electrode is located. Under these conditions, the sample solution becomes electrochemical charged and at the end of the capillary at atmospheric pressure electrohydrodynamic atomized into an aerosol of small charged droplets.

A Such device is for example in electrospray mass spectrometry used. Subsequent decays the charged, atomized Lead drops, under constant Evaporation of the solvent, after all for the release of ions. These ions become a mass analyzer via a pumping system fed.

In Electrospray mass spectroscopy is the inner diameter the capillaries normally at about 100 microns, the applied potential difference at 3-6 kV and flow rates at 1-10 μl / min. in the The difference is in the nano electrospray mass spectroscopy designated method capillaries with an inner diameter of approx. 1 μm used. The applied voltage is below 1 kV and the flow rates are below about 20 nl / min.

The Electrospray mass spectrometry is today, both in research as well as in the analytical application, one of the most widely used mass spectrometric Techniques.

The Electrospray mass spectrometry is of the following reasons of special importance for the detection of thermally labile and high molecular weight substances such as e.g. of biomolecules (Peptides, proteins, nucleic acids, Carbohydrates and Lipids - Richard Cole; "Electrospray Ionization Mass Spectrometry: Fundamentals, Instrumentation &Applications; John Wiley & Sons Inc., New York, 1997).

The transfer of Analyte ions from the liquid Condition in the gas phase is gentle and induced in comparison to other ionization methods little fragmentation.

Of the Mass spectrometric detection of substances is essentially only determined by the solubility in a polar solvent and a sufficient analyte concentration according to the efficiency the atomization and the detection sensitivity of the mass spectrometer. It can However, the mass range of the spectrometer have a limiting effect.

at molecules with many polar groups becomes the mass range of a spectrometer automatically extended by the fact that the analyte ions in an uploaded State desolvated. N-charged molecules appear on her 1 / nth mass. Thus, e.g. Albumin with a molecular weight from 66.430 as yet detected with a quadrupole mass spectrometer become.

in the Traps of nano electrospray mass spectroscopy are the flow rates and consequently the required sample volumes are much lower. It shows that at these small diameters the ionization efficiency is higher.

Special Significance has the electrospray mass spectrometry in recent years as a detector for the liquid chromatography (LC) and capillary electrophoresis (CE), since the separated sample solution directly after leaving the separation medium in the electrospray capillary forwarded, and so can be detected by mass spectrometry. Also for the future is of an increasing diffusion of this method go out. This coupling requires that the flow rate through the electrospray capillary is adapted to the flow rate of the separation column is.

Currently subject however electrospray mass spectrometry has some limitations, that of an even wider application and efficiency conflict with this analytical method. These restrictions will be briefly described below: The flow rates chromatographically to be separated analyte solutions are in liquid chromatography (eg HPLC) in the range of 1000 μl / min. However, the flow rates of electrospray mass spectrometry are not higher than 10 μl / min. To the flow rates of the original Electrospray mass spectroscopy, which are at 2-10 μl / min, meets these requirements to adapt to the HPLC, today is often the pneumatically assisted electrospray also called ion spray, which uses flow rates from 5 to 1000 μl / min allows. The IonSpray will atomise the liquid supported by a nitrogen gas flow along and around the capillary (sheat gas). Indeed can be achieved with the electrospray at high flow rates (1000 ul / min) only about the same ion intensity as at low flow rates (5 μl / min). By the adaptation to the flow rates commonly used in HPLC must be used for electrospray mass spectroscopy a loss The sensitivity can be accepted by about a factor of 100.

The ion current signal in electrospray mass spectrometry grows at low Concentrations are initially proportional to the analyte concentration only up to a certain point, to even decrease at high analyte concentrations.

Become in electrospray mass spectroscopy strongly salt or bufferhaltige solutions Usually, the analyte signal is used at the expense of the electrolyte suppressed. To this undesirable It is often to avoid the effect necessary, the sample solutions to desalt before the measurement. This suppressive effect is in nano-electrospray mass spectroscopy much lower.

in the General are over electrospray mass spectroscopy only relatively polar analyte solutions one Measurement accessible. When using electrolyte-free analyte solutions in electrospray mass spectrometry (e.g., non-polar analytes in nonpolar solvents) are prerequisites for the Generating a sufficient amount of excess charges no longer given since ion formation in electro-spray mass spectroscopy begins with the electrochemical charging of the analyte solution. To bypass This difficulty must be frequent to the APCI (atmospheric pressure chemical ionization) resorted at which the analyte solution sprayed in a corona discharge becomes. However, this is not as mild ionization as electrospray mass spectroscopy possible.

• • Die Nano-Elektrospray-Massenspektroskopie kann auch mit kleinsten Probenmengen betrieben werden.
• • Mit Hilfe der Nano-Elektrospray-Massenspektroskopie werden bei Flussraten von ca. 20 μl/min etwa die gleichen Ionenintensitäten erzielt wie mit der Elektrospray-Massenspektroskopie bei Flussraten von ca. 5 μl/min und der IonSpray mit Flussraten von etwa 1000 μl/min. Die Empfindlichkeit der Nano-Elektrospray-Massenspektroskopie ist demnach ungefähr vier Dekaden höher als die der IonSpray. Diese höhere Empfindlichkeit bei der nano-Elektrospray-Massenspektroskopie wird darauf zurückgeführt, dass zum einen kleinere Tröpfchen entstehen (Tropfendurchmesser bei der IonSpray im Mikrometerbereich, bei der nano Elektrospray-Massenspektroskopie um eine Größenordnung geringer), zum anderen diese entstehenden Tröpfchen eine relativ hohe Oberflächenladung besitzen. Beide Faktoren unterstützen eine effiziente Ionenfreisetzung.
• • Im Vergleich zur IonSpray kann die Nano-Elektrospray-Massenspektroskopie bei einer um eine Größenordnung höheren Elektrolytkonzentrationen (z.B. Salzkontaminationen, Puffer) in der Analytlösung betrieben werden, ohne dass es zu Problemen durch Unterdrückungseffekte kommt. Um bei der IonSpray von den zunächst größeren Tropfen bis zum Punkt der Ionenfreisetzung zu gelangen, muss mehr Lösungsmittel verdampfen als dies bei den kleineren, hochgeladenen. Tropfen in der nano Elektrospray-Massenspektroskopie notwendig ist. Hierbei kommt es zu einer Aufkonzentration der Elektrolyte im Tropfen, wodurch die Freisetzung der Analytionen unterdrückt wird [R. Juraschek, Th. Düicks, M. Karas; "Nanoelectrospray – more than just a minimized-flow electrospray ionization source"; J. Am. Soc. Mass Spectrom.; 10; 300–308; 1999.

Nano-electrospray mass spectroscopy is a variant of electrospray mass spectroscopy in which significantly thinner, eg gold-plated, glass capillaries (about 2 μm) are used (MS Wilm, M. Mann; Electrospray and Taylor-Cone Theory, Dole's Beam of macromolecules at last "; Int. J. Mass Spectrom. Ion Proc. 136; 167-180, 1994; Wolf D. Lehmann;" Mass Spectrometry in Biochemistry "; Spektrum Akademischer Vertag; Heidelberg; 111; 1996). Since these thin capillaries only allow flow rates of about 20 μl / min, coupling to standard liquid chromatography at flow rates of 100-1000 μl / min proves to be very difficult. Furthermore, the handling of these thin capillaries in the routine analysis is problematic because they clog easily. Nevertheless, the use of these thinner capillaries has several advantages that make their use in some areas of interest:

• • Nano electrospray mass spectrometry can also be run with smallest sample volumes.
• • With nano-electrospray mass spectrometry, approximately the same ion intensities are achieved at flow rates of approximately 20 μl / min as with electrospray mass spectroscopy at flow rates of approximately 5 μl / min and ion spray at flow rates of approximately 1000 μl / min , The sensitivity of nano electrospray mass spectrometry is therefore about four decades higher than that of ion spray. This higher sensitivity in nano-electrospray mass spectrometry is attributed to smaller droplets (drop diameter in the ion spray in the micrometer range, nano electrospray mass spectroscopy by an order of magnitude lower), and secondly, these droplets have a relatively high surface charge , Both factors support efficient ion release.
• • Compared to IonSpray, nano-electrospray mass spectrometry can be operated at an order of magnitude higher electrolyte concentration (eg, salt contamination, buffer) in the analyte solution, without any problems due to suppression effects. In order to get from the initially larger droplets to the point of ion release with the IonSpray, more solvent must evaporate than with the smaller, highly charged ones. Drops in nano electrospray mass spectrometry is necessary. This leads to a concentration of electrolytes in the drop, whereby the release of the analyte ions is suppressed [R. Juraschek, Th. Düicks, M. Karas; "Nanoelectrospray - more than just a minimized-flow electrospray ionization source"; J. Am. Soc. Mass Spectrom .; 10; 300-308; 1999th

From the publication US 5,975,426 A It is known to provide a porous ball at the end of a capillary. The porous sphere atomises a liquid that has been brought to potential. The sphere is made of SiO ₂ .

From WO 92/15339 A1 and the GB 729,842 are known devices for atomizing liquid, which have a means for bringing a liquid to an electric potential and a plurality of means, with which the liquid brought to potential is mechanically divided or atomized, and which are such that the exit region the liquid is formed by a plurality of rohroder thread-like elements, which may consist of porous polyethylene foam according to WO 92/15339 A1 and others.

Out US 5655517 A In particular, it is known to use as sputtering agents a multiplicity of fibers, which may consist of, inter alia, polyester or nylon.

The spraying of finely atomized liquids is also always of economic importance when thin layers are to be applied evenly. In such layers can These are, for example, paints.

task The invention is the creation of a device of the aforementioned Kind, with the high flow rates and at the same time, very good atomizations possible are.

The The object of the invention is achieved by a device having the features of the first claim and by a use according to the Anschprüchen 8 and 9 solved. Advantageous embodiments emerge from the subclaims.

The sophisticated device includes means to estimate the electrical potential of a liquid to change; that the liquid hereby (under certain circumstances Next) is electrically charged. The number of positive or negative electric charge carrier is thus increased by the means.

at the means to the electrical potential of the liquid in the aforementioned manner to change For example, it is an electrode to which the liquid is bypassed. Through this electrode and through a counter electrode then becomes a strong electrical by applying a voltage Field generated. Depending on the geometry, a voltage of 800 V may be sufficient. Typically, this is the applied voltage is at least 3 kV.

The liquid is charged by the aforesaid means by electrochemical means. This is at least always the case when a polar liquid was used. Less suitable are non-polar liquids such as benzene, hexane or toluene. Suitable are aqueous or methanolic solutions, such as salt or sugar solutions. An aqueous solution with dissolved in it Proteins represents another example of a polar and therefore good suitable solution represents.

When Electrode can be provided a metal existing pipe, through which the liquid passes becomes. Alternatively, the electrode is in the liquid. For example, it will be lapped by it.

It must be such a contact between the electrode and the liquid be present that a charge exchange can be made to the liquid To bring potential

The sophisticated device Also includes a variety of means by which the potential brought liquid mechanically finely divided or atomized. It can be this to act a variety of capillaries with very thin diameters of a few microns. The surfaces These mechanical dicing or atomizing exist at least in the area of their open ends made of fluoropolymers such as PTFE, FEP or of graphite fluoride or pyrocarbon.

The occurring adhesion forces between the liquid and the mechanical dicing or atomizing agent are preferred low, to prevent the liquid droplets from escaping from the mechanical atomizers reunite.

The mechanical atomizer are such that the exit area of the liquid from the mechanical atomizer through a plurality of conical, tubular or thread-like elements is formed. The liquid is passed through or along the aforementioned elements. The conical, tubular or filamentary Elements finally show open ends, from which the liquid from the mechanical atomizing exit.

One example for a plurality of tubular ones Elements is a plurality of capillaries. These are essentially parallel to each other or brush-shaped arranged. Through the capillary is brought to potential liquid for atomization passed.

One example for conical Elements represents a membrane that has a variety of conical accents towards one side. The liquid brought to potential becomes the cone-shaped one Structures guided. At the tips of the cones, the liquid enters then off.

Especially to prefer is the thread-like Variant. A bundle of threads ends in the form of a brush. The liquid brought to potential gets along the thread bundle passed and exits at the end of the brush. This variant is not only very easy to produce, but it also allows especially good atomizations the liquid.

The conical, tubular or filamentary Elements point between their open ends, from which the liquid brought to potential from the mechanical atomizer exit, gaps on. Thus, the ends of the brush-shaped ending threads are spatially from each other separated. The atomization then succeeds in a further improved way.

To further enhance atomization, the aforementioned open ends, from which the potential liquid emerges from the mechanical sputtering agents, taper sharply. This is especially true of the threads of Advantage.

It enough, if the liquid in the area of the exit from the mechanical atomizers brought to potential.

to Further improvement, the device has in one embodiment to the invention, a heating means through which the liquid in particular, is heated prior to mechanical sputtering. The atomization succeeds even better.

The The invention will be explained in more detail below by means of an example.

1 shows in section the basic structure of a particularly suitable device of the type mentioned above.

A methanolic sugar solution is passed through a thin tube 1 directed. The tube is made of metal and serves as an electrode. The interior of the tube is covered with a variety of threads 2 filled, which consist of polytetrafluoroethylene. The strings 2 are substantially parallel in the tube and exit at one end of the tube brush-shaped. The threads come at the end of the "brush" pointed.

A counter electrode 3 is planned. This is placed so that the liquid passing through the pipe 1 is passed through, exiting at the brush-shaped ends of the atomizing device in the direction of the counter electrode.

Complementary or alternatively you can the threads off metal wires exist in the area of open ends with fluoropolymer like Polytetrafluoroethylene (PTFE) or coated with graphite fluoride are. On the wires Then the voltage is applied to bring the liquid to potential. The wires then serve as an electrode. The open ends are the ends, from which the liquid is the Dipping or atomising agent leaves.

With Help of the invention become high sensitivities in electrospray mass spectroscopy as well as high flow rates. The process can even be combined with liquid chromatography which requires high flow rates.

The Limitations of Electrospray Mass Spectroscopy in Analytical Applications are in the prior art mainly by an insufficient Ionization efficiency at higher Flow rates, i. larger capillary diameters, given. Since a high ionization efficiency of the atomization of the analyte in as possible depends on small and uploaded drops, conditions must be chosen under which the atomization the solution not only over at the end of the capillary a "thick" spray cone but over the Formation of many fine spray contrasts. This will be achieved by the fact that at the end of the capillary analyte on field-enhancing Structures hitting a decomposition of the liquid into many jets cause. However, the areas of high field strength should not to the occurrence of gas discharges (for example, a corona discharge) to lead, as these would counteract the mild electrospray ionization conditions.

The metal capillary according to 1 has, for example, an inner diameter of 600 μm. The outer diameter is for example 900 microns. In the capillary are, for example, 600 threads or fibers, each having a diameter of 21 microns and made of Teflon. The threads protrude from one end by about 1-2 mm out of the capillary and here have open ends on at least 5 of the aforementioned threads should be provided in a capillary to achieve noticeable desired results. For example, 2000 threads are also possible without any problems.

• 1.) Statt einem einzigen „dicken" sogenanntem Taylor Cone entstehen viele feine Taylor Cones an den Faserenden.
• 2.) Ein stabileres Sprayverhalten auch bei wässrigen Lösungen konnte erzielt werden. Bei der Verwendung herkömmlicher Kapillaren zittert der Taylor Cone immer wieder und bricht zum Teil ganz zusammen.
• 3.) Auch bei höheren Flussraten ist ein stabiler Spray möglich. Bei herkömmlichen Kapillaren bricht der Spray bei höheren Flüssen zusammen.
• 4.) Mehr Ionen können aus der gleichen Lösungsmittelmenge erzeugt werden.
• 5.) Eine höhere Nachweisempfindlichkeit kann erzielt werden. Eine Verbesserung um ca. Faktor 100 wurde bisher erreicht.
• 6.) Es wurde bisher keine nachteilhafte Koronaentlandung beobachtet.
• 7.) Eine milde Ionisierung mit wenig Fragmentbildung, wie bei Nano-Elektrospray-Massenspektroskopie, ist möglich.
• 8.) Eine höhere Toleranz gegenüber Salzkontaminationen (z.B. stark pufferhaltige Lösungen) ist in vielen Fällen möglich.
• 9.) Ein stark verringertes neutrales Rauschen im Vergleich zur normalen Elektrospray wurde bei der Spektroskopie beobachtet.

It could be the following effects with in 1 achieved embodiment compared to a conventional electrospray capillary.

• 1.) Instead of a single "thick" so-called Taylor Cone many fine Taylor Cones arise at the fiber ends.
• 2.) A more stable spray behavior even with aqueous solutions could be achieved. When using conventional capillaries, the Taylor Cone trembles again and again breaks down completely.
• 3.) Even at higher flow rates, a stable spray is possible. In conventional capillaries, the spray breaks down at higher flows.
• 4.) More ions can be generated from the same amount of solvent.
• 5.) A higher detection sensitivity can be achieved. An improvement of about a factor of 100 has been achieved so far.
• 6.) There has been no adverse Koronaentlandung observed.
• 7.) A mild ionization with little fragment formation, as in nano electrospray mass spectroscopy, is possible.
• 8.) A higher tolerance to salt contamination (eg solutions containing a lot of buffer) is possible in many cases.
• 9.) A greatly reduced neutral noise compared to normal electrospray was observed in spectroscopy.

The behavior is similar to a variety of nano-electrospray, without the disadvantages of Schwieri To handle, the constipation of the nano-capillaries and the low flow rates have to accept.

for the first time succeeds in realizing a "multi-nano electrospray mass spectroscopy" at essentially the benefits of normal electrospray mass spectrometry associated with those of nano electrospray mass spectrometry.

A Coupling to standard liquid chromatography methods (100-1000 μl / min) is therefore possible, because high flow rates can be used.

• • Feldverstärkende Strukturen (gemäß 1 dünne Faserenden): Bei hohen Feldstärken, die räumlich sehr begrenzt sind (an spitzen Nadeln), können sich besonders effektiv sprühende Spraykoni bilden. Außerdem sind die Bedingungen zur Ausbildung einer Koronaentladung auf einen räumlich so geringen Bereich beschränkt, dass kaum nachteilhafte Koronaentladungen auftreten können.
• • Unpolare und nicht-benetzende Oberfläche der Teflonfasern: Die aus der Kapillare heraustretende Flüssigkeit wird durch die einzelnen Fasern aufgespalten. Normalerweise würde sich die Flüssigkeit wieder durch Kohäsionskräfte zu einem einzigen Flüssigkeitskonus zusammenziehen (kann z. B. geschehen, wenn man reine Metallfaserpinsel oder Glasfaserpinsel verwendet); durch die nicht-benetzende Oberfläche der PTFE- Fasern wird dies größtenteils verhindert. Einzelne kleine Analyttröpfchen wandern in Richtung des angelegten Feldes und versprühen dann am Faserende.
• • Metallkapillare oder irgendeine andere Kontaktierung, an der die Analytlösung vorbeiströmt: Hier wird die vorbeiströmende Analytlösung aufgeladen (auch Glaskapillaren oder Kunststoffkapillaren können verwendet werden, wenn an irgendeiner Stelle die Analytlösung aufgeladen wird.

The following factors are of particular importance according to the current state of knowledge.

• • field-enhancing structures (according to 1 thin fiber ends): At high field strengths, which are very limited in space (on pointed needles), spraying spray can be particularly effective. In addition, the conditions for forming a corona discharge are limited to a spatially so small area that hardly disadvantageous corona discharges can occur.
• • Non-polar and non-wetting surface of Teflon fibers: The liquid emerging from the capillary is split by the individual fibers. Normally, the liquid would again contract by cohesive forces into a single cone of liquid (eg, when using pure metal fiber brush or fiberglass brush); This is largely prevented by the non-wetting surface of the PTFE fibers. Individual small analyte droplets migrate in the direction of the applied field and then spray at the fiber end.
• • Metal capillary or any other contact, past which the analyte solution flows: Here, the flowing analyte solution is charged (also glass capillaries or plastic capillaries can be used, if at any point the analyte solution is charged.

• • Geringes Totvolumen, so dass die chromatographische Auflösung bei der Flüssigkeitschromatographie oder bei elektrophoretischen Trennprozessen nicht beeinträchtigt werden dürfte.
• • Fluorpolymere wie PTFE sind resistent gegen alle gebräuchlichen Lösungsmittel.
• • Fluorpolymere wie PTFE weisen auch beim Messen unterschiedlichster Proben hintereinander keine Memoryeffekt auf.
• • Lange Haltbarkeit auch bei starker Beanspruchung: Die PTFE-Fasern sind gegenüber elektrochemischen Korrosionsprozessen mit der Analytlösung und unter Atmosphärendruck inert und ermöglichen so lange Messzeiten.
• • Gasentladungen treten, wenn überhaupt, nur zwischen der Gegenelektrode und der Metallkapillare (bzw. Kontaktierungsstelle) auf, so dass hiervon die Teflonfasern nicht verändert werden.

Further advantages of this construction:

• • Low dead volume, so that the chromatographic resolution in liquid chromatography or electrophoretic separation processes should not be affected.
• • Fluoropolymers such as PTFE are resistant to all common solvents.
• • Fluoropolymers such as PTFE also have no memory effect when measuring a wide variety of samples in succession.
• • Long-lasting even under heavy use: The PTFE fibers are inert to electrochemical corrosion processes with the analyte solution and under atmospheric pressure, thus enabling long measurement times.
• • Gas discharges occur, if at all, only between the counter electrode and the metal capillary (or contact point), so that the Teflon fibers are not changed thereby.

The The invention is used inter alia for efficient ion formation, for the uniform coating of surfaces or for effective sputtering set. An effective atomization is for example in the fuel injection in engines of Interest.

fibers made of polyethylene, nylon, natural polymers also showed good characteristics. However, PTFE proved to be the best material.

The Use of a capillary, wherein one edge of an opening with threads Fibers or needles, also proved successful. The Needles were made of carbon. The needles were also able to the outlet opening a capillary to a noticeable improvement over the To achieve state of the art. A slightly deteriorated embodiment consisted of providing sharp edges or slots instead of tips at the end of the mechanical atomizer.

The Invention can also be used in combination with "sheat gas".

In 2 For example, the influence of the material M, which forms the surface at the open ends of the mechanical dicing or atomizing agents, is illustrated by the example of various solutions. The greater the signal intensity I, the better the desired result is achieved. PTFE is therefore very suitable. At least iron is suitable.

In 3 Results are shown that have been achieved according to the prior art with a normal electrospray capillary and with the device according to the invention. The signal intensities I as a measure of the desired effects could be substantially increased by the invention. In both cases, the solution used was 10-4 mol / l raffinose in methanol.

Claims (9)

Device for atomizing liquids by means ( 1 . 3 ) to bring a liquid to an electric potential, by a variety of means, with which the liquid brought to potential is mechanically divided or atomized, wherein the mechanical dicing or atomizing means are such that the exit region of the liquid from the mechanical sputtering agent is formed by a plurality of conical, tubular or filamentary elements, the surfaces of the conical, tubular or filamentous elements being at least in their range open ends of fluoropolymers such as PTFE, FEP or graphite fluoride or pyrocarbon exist. Device according to claim 1, which is the means to a liquid to bring to an electrical potential, electrodes used are. Device according to one of the preceding claims, in which one electrode is a capillary ( 1 ). Device according to one of the preceding claims, in which in a tube ( 1 ) a plurality of fibers or threads ( 2 ) are parallel, the fibers or threads ( 2 ) protrude at one end of the tube. Device according to one of the preceding claims, in a voltage source to a serving as an electrode tube and connected to a counter electrode. Device according to one of the preceding claims, wherein the fibers or filaments provided as dewatering or atomizing means ( 2 ) are tapered at at least one end. Device according to one of the preceding claims, wherein the fibers or filaments provided as dewatering or atomizing means ( 2 ) consist of metal wires provided as electrode, which are coated at least at one open end with PTFE, FEP, graphite fluoride or pyrocarbon. Use of the device according to one of the preceding claims in mass spectrometry. Use of the device according to one of the preceding claims 1 to 7 in the surface coating.