EP1713111A1 - Déterminer les caractéristiques voltage-courant d'une source d'électrospray - Google Patents

Déterminer les caractéristiques voltage-courant d'une source d'électrospray Download PDF

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Publication number
EP1713111A1
EP1713111A1 EP05110248A EP05110248A EP1713111A1 EP 1713111 A1 EP1713111 A1 EP 1713111A1 EP 05110248 A EP05110248 A EP 05110248A EP 05110248 A EP05110248 A EP 05110248A EP 1713111 A1 EP1713111 A1 EP 1713111A1
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EP
European Patent Office
Prior art keywords
voltage
electrospray
current
characteristic
unit
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EP05110248A
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German (de)
English (en)
Inventor
Walter Schrenk
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Agilent Technologies Inc
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Agilent Technologies Inc
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Priority to EP05110248A priority Critical patent/EP1713111A1/fr
Publication of EP1713111A1 publication Critical patent/EP1713111A1/fr
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J49/00—Particle spectrometers or separator tubes
    • H01J49/02—Details
    • H01J49/10—Ion sources; Ion guns
    • H01J49/16—Ion sources; Ion guns using surface ionisation, e.g. field-, thermionic- or photo-emission
    • H01J49/165—Electrospray ionisation

Definitions

  • the present invention relates to electrospraying.
  • US patent No. 6,462,337 to Li et al. discloses a method of providing ions to a mass spectrometer having an interface member with an orifice through which ions are received for analysis.
  • a fluid is provided at a capillary tip of an electrospray ionization source and a potential difference is applied between the tip and the interface member so as to direct an ion beam from the tip toward and through the orifice.
  • a field enhancing potential is applied on an auxiliary electrode so as to increase the electric field gradient from the capillary tip at least part way toward the interface member.
  • a focusing potential may be applied to the auxiliary electrode to generate an electric field which decreases beam divergence from at least part way toward the interface member. Both the field enhancing and focussing potential may alternately be applied to the auxiliary electrode.
  • Electrospray Ion Generation with Stimulation relates to an electrospray ionization device comprising a capillary tube arranged in a chamber and adapted for injecting a sample solution into the chamber.
  • the electrospray ionization device further comprises a high voltage source adapted for generating an electric field between the tip of the capillary and an area of the chamber, thereby generating a spray of ion flow of the injected sample solution.
  • a generator is provided, which is adapted for generating an alternating signal with a frequency of at least 10 kHz.
  • the generator is coupled to the capillary tube or the tip for a stimulation of ion droplets generation out of the injected sample solution in response to the generated signals.
  • a method for determining a voltage-current-characteristic of an electrospray unit comprises an electrospray tip and an entrance electrode of a measurement unit.
  • the electrospray tip is adapted for generating a spray ion flow.
  • the method comprises applying a voltage between the electrospray tip and the entrance electrode so as to direct at least parts of the spray ion flow towards the entrance electrode, and varying the voltage applied between the electrospray tip and the entrance electrode.
  • the method further comprises detecting, in dependence on the applied voltage, an ion current between the electrospray tip and the entrance electrode, and at least partially determining a voltage-current-characteristic of the electrospray tip.
  • the voltage applied between the electrospray tip and the entrance electrode ionizes and vaporizes a fluid sample supplied to the electrospray tip.
  • the ionized droplets are accelerated towards the entrance electrode.
  • This spray ion flow gives rise to an ion current between the electrospray tip and the entrance electrode.
  • a voltage-current-characteristic of the electrospray tip is obtained.
  • This voltage-current-characteristic contains a lot of information about the electrospray tip's properties and may therefore be used as a starting point for deriving various different parameters of interest.
  • the voltage-current-characteristic may be used for determining an optimum operation voltage, for tracking wear, for diagnostic purposes, for triggering replacement of the electrospray tip, and for various other purposes.
  • the entrance electrode comprises a sampling orifice that defines the respective measurement unit's entrance path. Those parts of the spray ion flow that pass through the sampling orifice are subjected to further analysis by the measurement unit.
  • the measurement unit is a mass spectrometer. Further preferably, the measurement unit is a time-of-flight (TOF) mass spectrometer.
  • the mass spectrometer is adapted for analyzing the mass distribution of those parts of the spray ion flow that pass through the sampling orifice. After the masses of the ionized species in the spray ion flow have been determined, the sample's various compounds may be identified.
  • the spray ion flow comprises both solvent ions and analyte ions, which are both accelerated towards the entrance electrode.
  • both the analyte ions and the solvent ions contribute to the total ion current between the electrospray tip and the entrance electrode. Because of the solvent ions' contribution to the ion current, a current of considerable magnitude is obtained.
  • the voltage-current-characteristic is used as a starting point for determining parameters that indicate an actual state of the electrospray unit.
  • the method might comprise deriving, from the voltage-current-characteristic, a parameter that specifies an action to be taken. Possible actions might e.g. comprise setting the applied voltage to a preferred value, repositioning the electrospray unit, etc.
  • the voltage-current-characteristic is acquired by performing a voltage sweep of the applied voltage.
  • the voltage may be continuously increased during a predefined period of time, and in dependence on the applied voltage, the corresponding ion current may be determined.
  • a sudden rise of the detected ion current is observed at a certain voltage, which will furtheron be referred to as the "ignition voltage".
  • the ignition voltage which denotes the onset of the spray ion flow, can be derived from the acquired voltage-current-characteristic.
  • a voltage sweep is performed in the downward direction, with the applied voltage being continuously decreased as a function of time, and ion current is determined as a function of the applied voltage.
  • ion current is determined as a function of the applied voltage.
  • a so-called "cut-off voltage” is determined, with the cut-off voltage corresponding to a sudden decline of the measured ion current. As soon as the applied voltage drops below the cut-off voltage, the spray ion flow ceases.
  • voltage sweeps are performed both in the upward and in the downward direction, and both the ignition voltage and the cut-off voltage are determined.
  • the acquired voltage-current-characteristics are used for determining a preferred operation voltage of the electrospray unit.
  • a preferred operation voltage of the electrospray unit By setting the voltage applied between the electrospray tip and the entrance electrode to this operation voltage, a stable and reliable spray ion flow is accomplished. Because of the spray ion flow's improved stability, the accuracy of measurement results determined by the measurement unit is improved as well.
  • a range of preferred operation voltages is determined by analyzing the voltage-current-characteristic and determining a voltage range that is characterized by a linear dependence between ion current and applied voltage. It has been found that in this voltage range of linear operation, a homogeneous vaporization and ionization of sample fluid is accomplished.
  • the operation voltage is set to a value above the ignition voltage. By setting the operation voltage to a value that is greater than the ignition voltage, it is made sure that the spray ion flow generated at the electrospray tip is not disrupted.
  • the voltage-current-characteristic is taken as a starting point for deriving the electrospray tip's actual condition.
  • a parameter indicating wear may be derived.
  • low performance electrospray tips may be identified and replaced before any non-reliable measurement results are acquired.
  • aging of the electrospray tip is monitored by tracking the value of the ignition voltage. For example, due to corrosion, the electrospray tip's ignition voltage might be shifted to a higher value.
  • the electrospray tip's actual condition is determined by analyzing fluctuations of the ion current.
  • the electrospray unit's actual mode of operation might be an irregular mode of operation, like e.g. a burst mode or a spitting mode. Irregular spray patterns indicate that the electrospray tip does not operate properly.
  • the various frequency components of the voltage-current-characteristic are evaluated. For example, fluctuations occurring on different time scales may be analyzed. Preferably, techniques like e.g. Fourier analysis may be used for determining the electrospray unit's mode of operation.
  • the acquired voltage-current-characteristic is used for positioning the electrospray tip relative to the measurement unit's entrance path.
  • the spray ion flow between the electrospray tip and the entrance electrode depends both on the applied voltage and the geometric arrangement of the electrospray tip relative to the measurement unit's entrance path.
  • the voltage-current-characteristic can be used for evaluating whether the electrospray tip is correctly positioned relative to the measurement unit's entrance path. If the voltage-current-characteristic does not have a desired shape, repositioning of the electrospray tip relative to the sampling orifice will be initiated.
  • the voltage-current-characteristic is repeatedly determined while the electrospray tip is being repositioned.
  • the electrospray nozzle is correctly positioned relative to the entrance path.
  • the axial distance between the electrospray tip and the entrance path is varied.
  • the axial distance is varied until an optimum voltage-current-characteristic is obtained.
  • the angle between the electrospray tip and the measurement unit's entrance path is varied. This allows adjusting the spray ion flow's direction relative to the measurement unit's entrance path.
  • a control unit for an electrospray unit is adapted for varying a voltage applied between an electrospray tip and an entrance electrode of a measurement unit, and for detecting, in dependence on the applied voltage, an ion current between the electrospray tip and the entrance electrode.
  • the control unit is further adapted for at least partially determining a voltage-current-characteristic of the electrospray unit.
  • Embodiments of the present invention further relate to an electrospray unit for providing a spray ion flow to an entrance path of a measurement unit.
  • the electrospray unit comprises an electrospray tip adapted for generating a spray ion flow, an entrance electrode of the measurement unit, and a power supply adapted for applying a variable voltage between the electrospray tip and the entrance electrode so as to direct at least parts of the spray ion flow towards the entrance electrode.
  • the electrospray unit comprises a control unit as described above.
  • the electrospray tip which is adapted for vaporizing and ionizing a fluid sample, is implemented as a needle.
  • the electrospray tip is implemented as an electrospray nozzle that is part of a microfluidic device.
  • the microfluidic device might e.g. comprise channels adapted for supplying fluid sample to the electrospray nozzle.
  • the microfluidic device might further comprise one or more electrodes adapted for electrically contacting the electrospray nozzle.
  • microstructuring techniques are used for manufacturing the microfluidic device and the electrospray nozzle.
  • the electrospray unit comprises a positioning unit, whereby the electrospray tip may be attached to the positioning unit, and whereby the positioning unit is adapted for positioning the electrospray tip relative to the entrance path of the measurement unit.
  • the positioning unit might be adapted for varying the axial distance between the electrospray tip and the sampling orifice.
  • the positioning unit might be adapted for varying an angle of the electrospray tip relative to the measurement unit's entrance path.
  • the positioning device might comprise one or more linear drives adapted for adjusting the electrospray tip's position relative to the entrance path.
  • Embodiments of the invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit.
  • Software programs or routines are preferably applied for controlling the voltage applied to the electrospray unit, and for evaluating a corresponding ion current.
  • Fig. 1 shows an electrospray unit together with a measurement unit's entrance path
  • Fig. 2 shows how a voltage-current-characteristic of an electrospray unit is acquired
  • Fig. 3 shows two voltage-current-characteristics of a single electrospray unit that have been acquired by performing voltage sweeps in the upwards and downwards direction, respectively;
  • Fig. 4 illustrates the behaviour of a low-quality electrospray tip
  • Fig. 5 shows the variation of an electrospray unit's voltage-current-characteristic as a function of operation time
  • Fig. 6 shows how ion current depends on the axial distance between an electrospray tip and a measurement unit's entrance path.
  • Fig. 1 shows an electrospray unit 1 together with an entrance path of a mass spectrometer 2.
  • a microfluidic device 3 with an electrospray nozzle 4 is used as an electrospray ionization source.
  • a fluid supplied via a channel 5 is vaporized, in order to generate a spray ion flow 6.
  • a spray electrode 7 connected to ground the fluid supplied via the channel 5 is kept at ground potential.
  • An electrospray ionization source generates ions at atmospheric pressure while a mass spectrometer requires a pressure of generally under 10 -4 Pascal. Therefore, mass analysis requires introducing the ions into a vacuum chamber.
  • This can be achieved by means of a transfer capillary 8 having a small sampling orifice 9 (typically 0.1 to 2 millimeter in diameter), the transfer capillary 8 being located between a low pressure chamber of the mass spectrometer 2 and the atmospheric pressure region.
  • the transfer capillary 8 comprises a metal-coated tip 10, which can either be an outer coating or an inner coating, if the tip comprises a small inner metal coating layer.
  • a ring-shaped electrode 11 is located around the sampling orifice 9.
  • a gold wire 12 provides an electric connection between the metal-coated tip 10 and the ring-shaped electrode 11.
  • a voltage source 13 applies a voltage V ESI between the spray electrode 7 and the ring-shaped electrode 11, whereby the voltage source 13 is controlled by a control unit 14.
  • the voltage V ESI might be in the range between 1 kV and 2.5 kV.
  • the electrospray unit further comprises a counter-electrode 15, with a voltage V CE of approximately -500 V being applied between the ring-shaped electrode 11 and the counter-electrode 15. Hence, the counter-electrode 15 is kept at a potential between 0.5 and 2 kV.
  • the electric field between the electrospray nozzle 4 and the ring-shaped electrode 11 is adapted for accelerating electrospray ionization droplets towards and through the sampling orifice 9.
  • the total spray ion flow comprises a flow 16 of analyte ions as well as a flow 17 of solvent ions, with the intensity of the flow 17 of solvent ions being much larger than the intensity of the analyte ion flow.
  • a part of the flow 16 of analyte ions passes through the sampling orifice 9.
  • the remaining part of the flow 16 of analyte ions as well as the flow 17 of solvent ions hit the ring-shaped electrode 11 and contribute to a total ion current (TIC), which is measured by a current determination unit 18.
  • TIC total ion current
  • the microfluidic device 3 is fixed to a positioning unit 19 that allows positioning the electrospray nozzle 4 in at least one of the x-, y- and z-direction.
  • the positioning unit 19, which might preferably comprise one or more linear drives driven by stepper motors, might further be adapted for adjusting an angle ⁇ between the electrospray nozzle 4 and the mass spectrometer's entrance path.
  • Fig. 2 shows how a voltage-current-characteristic of the electrospray unit 1 of Fig. 1 is determined.
  • the control unit 14 controls the voltage source 13 in a way that the voltage V ESI applied between the spray electrode 7 and the ring-shaped electrode 11 is continuously increased as a function of time.
  • Curve 20 shows this continuous increase of the applied voltage V ESI .
  • the applied voltage V ESI is indicated, and time t is sketched along the abscissa 22.
  • Curve 23 relates to the total ion current (TIC) measured between the electrospray nozzle 4 and the ring-shaped electrode 11, which is also shown as a function of time.
  • TIC total ion current
  • the right ordinate axis 24 depicts the measured ion current (in nA).
  • a characteristic ignition voltage V IGN a sudden rise 25 of the measured ion current is observed.
  • the ignition voltage V IGN corresponds to the onset of the electrospray ion flow.
  • the control unit 14 starts to continuously decrease the applied voltage V ESI , as a function of time, as indicated by curve 26.
  • Curve 27 shows the corresponding ion current determined by current determination unit 17.
  • a characteristic cut-off voltage V CUT a sudden drop 28 of the measured current is observed, and the spray ion flow is stopped.
  • the cut-off voltage V CUT lies below the ignition voltage V IGN .
  • FIG. 3 shows two voltage-current-characteristics 29, 30 that have been acquired for one single electrospray ionization unit.
  • Voltage-current-characteristic 29 corresponds to a voltage sweep in the upwards direction
  • voltage-current-characteristic 30 corresponds to the downwards direction.
  • the electrospray ionization voltage V ESI in volts
  • V ESI in volts
  • Voltage-current-characteristic 29 has been obtained by recording the total ion current as a function of a continuously increasing electrospray ionization voltage V ESI , and therefore, the curve starts at the ignition voltage V IGN .
  • Voltage-current-characteristic 30 has been recorded while continuously decreasing the applied voltage V ESI , and for this reason, the voltage-current-characteristic 30 extends down to the cut-off voltage V CUT . Due to the electrospray unit's hysteresis, the ignition voltage V IGN lies above the cut-off voltage V CUT . In the example of Fig. 3, V IGN ⁇ 2125 volt, and V CUT ⁇ 2040 volt.
  • the voltage-current-characteristics 29, 30 shown in Fig. 3 can be used for determining an optimum range of operation voltages for the electrospray ionization unit. Initially, the voltage-current-characteristics are acquired and analyzed, then, the electrospray ionization unit's operation voltage is set to a preferred value that ensures a stable spray ion flow, and then, a given sample may be subjected to spray ionisation and analysed by the mass spectrometer.
  • the voltage-current-characteristics 29, 30 comprise various different regions that are characterized by different types of behaviour.
  • the measured ion current substantially shows a linear dependency on the applied voltage V ESI .
  • the slope of the voltage-current-characteristics 29, 30 is approximately equal to 15 nA/100 V.
  • a preferred operation voltage V OP should lie within the voltage range 33 of linear operation. Furthermore, the operation voltage V OP should lie above the ignition voltage V IGN , in order to prevent disruption of the spray ion flow.
  • V OP has been set to 2250 volt. The corresponding ion current is approximately equal to 65 nA.
  • Fig. 4 shows two voltage-current-characteristics 34, 35 of a low-quality electrospray nozzle.
  • Voltage-current-characteristic 34 which starts at the ignition voltage V IGN , has been acquired by recording ion current as a function of a continuously increasing voltage.
  • Voltage-current-characteristic 35 has been recorded while continuously decreasing the applied voltage.
  • Voltage-current-characteristic 35 extends down to a cut-off voltage V CUT , which lies below the ignition voltage V IGN .
  • the ion current does not fluctuate significantly.
  • regions 37 fluctuations of the detected ion current are observed, which indicate a non-stable mode of operation of the electrospray source.
  • the current fluctuations in region 37 might indicate a burst mode or a spitting mode of the electrospray ionization source.
  • the fluctuations might e.g. be detected by performing a frequency analysis of the acquired voltage-current-characteristics, whereby fluctuations typically occur on a time scale of about one second. Fourier analysis might be a suitable method for detecting current fluctuations. In the example of Fig. 4, it can be concluded that the respective electrospray nozzle is of minor quality and should better be replaced.
  • Analysis of the acquired voltage-current-characteristics can further be used for monitoring wear of an electrospray tip. For example, by tracking ignition voltage over operation time, it is possible to monitor aging of an electrospray tip. An user may be informed in due time that a replacement of the electrospray tip is necessary.
  • the three-dimensional diagram of Fig. 5 has been obtained by repeatedly acquiring a voltage-current-characteristic of an electrospray tip during the electrospray tip's operation time. 300 different voltage-current-characteristics have been recorded during a time period of 150 hours, which means that every 0.5 h, a voltage-current-characteristic has been acquired. The number of a respective voltage-current-characteristic is indicated along axis 38. Axis 39 shows the applied voltage V ESI (in volts), and axis 40 indicates a measured current (in nA). Fig. 5 shows the time dependence of an electrospray tip's voltage-current-characteristic.
  • a voltage-current-characteristic 41 is obtained, which is characterized by a rather low ignition voltage. Then, during consecutive runs, voltage-current-characteristics 42 are acquired.
  • the reason for the completely different shape of the voltage-current-characteristic 41 is that initially, the electrospray tip is at least partly covered with non-covalently bound materials. After these materials have been vaporized, the shape of the voltage-current-characteristic changes considerably.
  • An acquired voltage-current-characteristic of an electrospray unit might further be used as a starting point for repositioning the electrospray tip relative to the mass spectrometer's entrance electrode.
  • the electrospray tip can be repositioned until a desired voltage-current-characteristic is obtained.
  • the positioning unit 19 might comprise facilities for positioning the electrospray tip in at least one of the x-, y-, z-direction.
  • the angle ⁇ between the electrospray tip and the axis of the mass spectrometer's entrance path might be varied as well.
  • Fig. 6A shows how an electrospray tip 43 is axially positioned relative to a sampling orifice 44 of a mass spectrometer.
  • the spray ion flow generated by the electrospray tip 43 comprises an analyte ion flow 45 and a solvent ion flow 46.
  • Fig. 6B shows the measured ion current as a function of the axial distance between the electrospray tip 43 and the sampling orifice 44. As long as the distance in z-direction is rather small, the solvent ion flow 46 contributes to the total ion current. Accordingly, the total ion current is initially quite large.
  • the optimum operation range can be determined. Generally, the operation range is chosen such that solvent ion flow does not contribute to the total ion current.

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  • Other Investigation Or Analysis Of Materials By Electrical Means (AREA)
EP05110248A 2005-11-02 2005-11-02 Déterminer les caractéristiques voltage-courant d'une source d'électrospray Withdrawn EP1713111A1 (fr)

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Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6337480B1 (en) * 1997-03-15 2002-01-08 Analytica Of Branford, Inc. Disposable microtip probe for low flow electrospray

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6337480B1 (en) * 1997-03-15 2002-01-08 Analytica Of Branford, Inc. Disposable microtip probe for low flow electrospray

Non-Patent Citations (4)

* Cited by examiner, † Cited by third party
Title
CECH N B ET AL: "PRACTICAL IMPLICATIONS OF SOME RECENT STUDIES IN ELECTROSPRAY IONIZATION FUNDAMENTALS", MASS SPECTROMETRY REVIEWS, JOHN WILEY AND SONS, NEW YORK, NY, US, vol. 20, no. 6, November 2001 (2001-11-01), pages 362 - 387, XP001182862, ISSN: 0277-7037 *
JURASCHEK R ET AL: "Pulsation phenomena during electrospray ionization", INTERNATIONAL JOURNAL OF MASS SPECTROMETRY, ELSEVIER SCIENCE PUBLISHERS, AMSTERDAM, NL, vol. 177, no. 1, 3 August 1998 (1998-08-03), pages 1 - 15, XP004134816, ISSN: 1387-3806 *
LOPEZ-HERRERA ET AL: "An experimental study of the electrospraying of water in air at atmospheric pressure", JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY, ELSEVIER SCIENCE INC, US, vol. 15, no. 2, February 2004 (2004-02-01), pages 253 - 259, XP005102388, ISSN: 1044-0305 *
THOMPSON J W ET AL: "Investigation of electrospray ionization and electrostatic focusing devices using a three-dimensional electrospray current density profiler", JOURNAL OF THE AMERICAN SOCIETY FOR MASS SPECTROMETRY, ELSEVIER SCIENCE INC, US, vol. 16, no. 3, March 2005 (2005-03-01), pages 312 - 323, XP004804945, ISSN: 1044-0305 *

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