WO2016163046A1

WO2016163046A1 - Electrical discharge nozzle to be used in electrospray ionization

Info

Publication number: WO2016163046A1
Application number: PCT/JP2015/079979
Authority: WO
Inventors: 正美村田
Original assignee: 旭サナック株式会社
Priority date: 2015-04-09
Filing date: 2015-10-23
Publication date: 2016-10-13

Abstract

An electrical discharge nozzle is configured by being provided with a metal cylindrical capillary tube and a metal needle. The needle is installed in the center of the cylindrical capillary tube, electrically connected to the cylindrical capillary tube. The tip of the needle is made to protrude from the electrical discharge-end opening of the cylindrical capillary tube. The length of protrusion of the needle tip is a dimension that fits within the Taylor cone generated at the tip of the cylindrical capillary tube.

Description

Discharge nozzle for electrospray ionization

The present invention relates to an electrolysis method in which a high voltage is applied between a discharge nozzle having a capillary that is a capillary tube and a counter electrode to form a liquid supplied through the capillary into fine droplets with excess charge and sprayed toward the counter electrode. The present invention relates to a discharge nozzle used in a spray ionization method.

Electrospray ionization is a liquid spray technique that uses the electrospray phenomenon, also called electrostatic spraying. A high voltage is applied between the pointed liquid and the counter electrode to make the liquid into fine droplets with excess charge. This is a technique for spraying toward the counter electrode. In recent years, the application range has expanded to coating organic EL and biopolymers on substrates, formation of nanometer-sized fibers, electrostatic coating, sample introduction in mass spectrometers, and the like.

Although the electrospray phenomenon has many unclear points, it is generally considered as follows. As shown in the principle diagram of FIG. 12, a high voltage of several thousand volts is applied between the metal capillary 50 that is a thin tube and the counter electrode 51, and the sample liquid 52 is caused to flow through the capillary 50. The liquid 52 coming out from the capillary tip forms a liquid pool 52a. A strong electric field toward the counter electrode 51 exists at the capillary tip. When the capillary 50 side has a positive potential, positive ions in the liquid pool are electrophoresed on the liquid surface side, and negative ions are electrophoresed on the inside. The negative ions electrophoresed on the inside reach the surface of the metal capillary 50 and cause an oxidation reaction under a high electric field. The negative ions, that is, electrons, are imparted to the metal capillary 50 to be neutralized. The positive ions electrophoresed on the surface of the liquid pool at the tip receive a strong electromagnetic attraction force toward the counter electrode 51, and the tip of the liquid pool 52a tends to extend in the direction of the counter electrode 51. On the other hand, surface tension acts on the liquid 52 in the liquid pool 52a. Since the surface tension is a force that reduces the surface area of the liquid, it acts in a direction that prevents the tip of the liquid pool 52a from extending. When the electromagnetic attraction force and the force due to surface tension are balanced, the liquid pool 52a at the tip of the capillary has a conical shape with a vertex angle of 90 °. This conical shape is called a Taylor cone. When the electromagnetic attraction force is higher than the voltage at which the Taylor cone is formed, the tip of the Taylor cone extends toward the counter electrode 51 and finally separates into a droplet 53 charged with excess charge. When excess charge is discharged by the separation, the tip of the liquid pool 52a once returns to a rounded shape. Thereafter, a Taylor cone is formed again, and the droplet 53 charged with excess charge is separated and released. The excess positive ions on the droplet 53 that have separated and jumped out repel each other by the Coulomb force and gather on the surface of the droplet 53. Since the volume of the droplet 53 is small and the excess charge is large, a large force acts outward on the surface of the droplet 53. The droplet 53 is reduced in volume due to the evaporation of the solvent during the flight. As the volume decreases, the charge density increases and the outward force on the surface increases. When the limit called the Rayleigh limit is reached, it breaks up into a large number of fine droplets 53a. Since the fine droplets generated by the splitting are also charged with excessive charges, they reach the Rayleigh limit again by solvent evaporation and re-split. By repeating such splitting, gas phase ions are finally generated and sprayed on the counter electrode 51. The generation of gas phase ions by repeating such splitting is also called Coulomb explosion because of Coulomb repulsion.

In order to cause such an electrospray phenomenon, a high voltage is applied between the capillary 50 and the counter electrode 51 to generate a strong electric field at the tip of the capillary, and an electromagnetic attractive force that overcomes the surface tension is generated in the liquid pool 52a. It is necessary to let The required voltage is several thousand volts. A liquid such as water requires a particularly high voltage because of its high surface tension. The applied voltage is preferably lower due to the configuration of the power supply device 54. In addition, when the voltage applied to the capillary 50 is high, corona discharge occurs near the capillary tip, preventing the formation of a stable Taylor cone.

JP 2014-223491 A

The present invention has been made to solve these problems of the prior art, and the problem is that the voltage applied to the discharge nozzle required to cause the electrospray phenomenon can be lowered, and the vicinity of the capillary tip is reduced. An object of the present invention is to provide a discharge nozzle used in an electrospray ionization method that is also effective in suppressing the generation of corona discharge.

The invention according to claim 1 for solving the above problem is a discharge nozzle used in an electrospray ionization method, comprising a metal cylindrical tubule and a metal needle, wherein the needle Is attached to the center of the cylindrical tubule while being electrically connected to the cylindrical tubule, and the tip of the needle protrudes from the discharge side opening of the cylindrical tubule, and the protruding length of the tip of the needle is: A discharge nozzle characterized in that it is dimensioned to fit within a Taylor cone generated at the tip of a cylindrical thin tube.

According to such a configuration, the needle tip protruding from the capillary tip, which is a cylindrical tubule, has a larger curvature and is closer to the counter electrode than the capillary tip, so a stronger electric field is formed than the cylindrical tip of the capillary tip. Is done. As a result, a stronger electric field is generated even when the applied voltage is the same as that of a conventional nozzle without a needle. Therefore, the voltage causing the electrospray phenomenon can be made lower than that of the conventional type, and the burden on the power source can be reduced. In addition, since the voltage applied to the discharge nozzle can be lowered, there is an effect that the generation of corona discharge near the tip of the discharge nozzle is also suppressed.

The invention described in claim 2 is a discharge nozzle used in an electrospray ionization method, comprising a metal cylindrical tubule and a metal needle, and the needle has an outer diameter. Is slightly smaller than the inner diameter of the cylindrical tubule and forms a gap for allowing the sample liquid to pass between the inner surface of the cylindrical tubule, and the tip of the needle protruding from the opening of the cylindrical tubule has a truncated cone shape. A discharge nozzle having a shape.

With such a configuration, the flow rate of the sample liquid to be supplied can be greatly increased.

According to a third aspect of the present invention, in the discharge nozzle according to the first or second aspect, the cylindrical tubule is formed of a nonconductive material instead of a metal, and an applied voltage is supplied to the needle. The discharge nozzle is configured as described above.

Thus, if the capillary, which is a cylindrical thin tube of the discharge nozzle, is made of a non-conductive material, the generation of corona discharge near the capillary tip can be more effectively suppressed.

It is a longitudinal cross-sectional view of the discharge nozzle which concerns on 1st Embodiment. 3 is a view showing a state in which a liquid pool 7 of a sample liquid is formed at the tip of a capillary 2. FIG. It is a figure which shows the state from which the charged droplet 8 protruded from the front-end | tip of the liquid pool 7 used as the Taylor cone shape. It is a figure which shows the electric field generation | occurrence | production condition in the conventional discharge nozzle 10 in which the needle | hook is not provided in the center. It is a figure which shows the electric field generation condition in the discharge nozzle 1 which provided the needle | hook 3 in the center. It is a figure which shows the photograph of the front-end | tip part of the discharge nozzle 1 made as an experiment. It is a figure which shows the photograph of the state in which the Taylor cone was formed in the front-end | tip of the prototype discharge nozzle 1, and the droplet was discharge | released. It is a figure which shows the whole photograph of the state by which the droplet discharge | released from the discharge nozzle 1 raise | generates a Coulomb explosion, becomes an infinite number of fine droplets, and is sprayed on the counter electrode. It is a figure which shows the comparison experiment result of the Taylor cone at the time of changing an applied voltage with a photograph. It is a longitudinal cross-sectional view of the discharge nozzle 10 which concerns on 3rd Embodiment. It is a figure which shows the photograph of the state which is producing the electrospray phenomenon using the discharge nozzle 10 which concerns on 3rd Embodiment. It is explanatory drawing of an electrospray phenomenon.

Hereinafter, a configuration example of a discharge nozzle used in an electrospray ionization method according to the present invention will be described in an embodiment with reference to the drawings.
(First embodiment)
FIG. 1 is a longitudinal sectional view of a discharge nozzle according to the first embodiment. The discharge nozzle 1 includes a capillary 2, a needle 3, and a support block 4. The capillary 2 of this embodiment is a thin metal cylindrical tube, and is attached to and supported by a metal support block 4. The needle 3 is a thin metal needle formed with a sharp tip, and is inserted through the support block 4 into the center of the capillary 2. The tip of the needle 3 protrudes slightly from the opening of the capillary 2. The support block 4 supports the capillary 2 and the needle 3 in an electrically connected state. A flow path 5 through which the sample liquid supplied to the capillary 2 passes is formed inside. A DC high voltage is applied to the support block 4 from the power source 6.

A DC high voltage is applied from the power source 6 between the discharge nozzle 1 and a counter electrode (not shown). In this state, the sample liquid is supplied into the capillary 2 through the flow path 5 formed in the support block 4. The sample liquid forms a liquid pool 7 at the tip of the capillary 2 as shown in FIG. Since a high voltage is applied between the capillary 2 and the needle 3 of the discharge nozzle 1, a high electric field is generated at the tip of the discharge nozzle 1. When the applied voltage to the discharge nozzle 1 is positive, the electric field is directed to the counter electrode. The positive ions present in the liquid pool 7 move to the counter electrode side surface of the liquid pool 7 by this high electric field. On the other hand, negative ions are directed toward the capillary 2 and the needle 3 side.

The negative ions that reach the surfaces of the metal capillary 2 and the needle 3 cause an oxidation reaction under a high electric field, and give electrons to the capillary 2 and the needle 3 to be neutralized. For example, in the case of hydroxide ions, two water molecules, one oxygen molecule, and four electrons are generated by an oxidation reaction of four hydroxide ions. The four electrons moved to the nozzle 1 and moved to the power source 6 and the counter electrode via the wiring cable as the droplets charged with positive ions, which will be described later, flew toward the counter electrode, and were charged with positive ions. The positive charge carried to the counter electrode by the droplet is neutralized.

The positive ions that have moved to the surface of the liquid pool 7 facing the counter electrode have a strong attractive force toward the counter electrode due to a strong electric field. By this strong suction force, the tip of the liquid pool 7 extends toward the counter electrode. On the other hand, surface tension is applied to the liquid pool 7, and the surface tension acts in a direction to prevent the tip of the liquid pool from extending toward the counter electrode. When the two forces are balanced, as shown in FIG. 2, the liquid puddle 7 has a conical shape called a Taylor cone with a vertex angle of 90 °. When the applied voltage is higher than the voltage at which the Taylor cone is formed, the tip of the Taylor cone further extends toward the counter electrode. Finally, the liquid at the tip is separated from the liquid pool 7, and as shown in FIG. 3, it becomes droplets 8 charged with positive ions and jumps toward the counter electrode. It is the start of the electrospray phenomenon. When the charged droplet 8 is ejected, the charge amount of the liquid pool 7 is reduced, so that the suction force is weakened and the tip is rounded. Thereafter, a Taylor cone is formed again, and the droplet 8 charged with positive ions is discharged again toward the counter electrode. Such an operation is repeated.

The feature of the discharge nozzle 1 of the present embodiment is that a metal needle 3 is provided at the center of the capillary 2. The tip of the needle 3 is sharpened and the tip is projected from the opening of the capillary 2. When a positive potential is applied to the discharge nozzle 1, the metal surface of the discharge nozzle 1 is positively charged and an electric field directed to the counter electrode is generated. The electric field on the metal surface becomes stronger as the sharp portion has a larger curvature and the portion closer to the counter electrode at a negative potential. In the conventional discharge nozzle 10 shown in FIG. 4 in which the needle 3 is not provided at the center, a strong electric field indicated by an arrow is generated at the cylindrical end of the capillary 2 tip. In contrast, in the discharge nozzle 1 of the present embodiment, a strong electric field indicated by an arrow is generated at the tip of the needle in addition to the cylindrical end of the capillary as shown in FIG. The tip of the needle 3 has a sharp shape, and is closer to the counter electrode than the tip of the capillary, so that a stronger electric field is formed than the cylindrical end of the capillary tip. For this reason, the positive ions in the liquid pool are likely to concentrate on the tip of the needle 3 where a stronger electric field is generated. That is, in the discharge nozzle 1 of the present embodiment having the needle 3, a stronger electric field is generated at the tip of the needle 3 even if the applied voltage is the same as that of the nozzle without the conventional needle 3.

This means that, if reversed, the voltage causing the electrospray phenomenon is lower than that of the conventional type. If the voltage applied to the discharge nozzle 1 can be lowered, the burden on the power source 6 is reduced. Further, if the voltage applied to the discharge nozzle 1 can be lowered, the occurrence of corona discharge near the tip of the discharge nozzle can be suppressed. Thus, in the discharge nozzle 1 having the needle 3 of the present embodiment, the voltage that causes the electrospray phenomenon can be made lower than that of the conventional type, and the effect of suppressing the occurrence of corona discharge can be achieved.

FIG. 6 is a photograph of the prototype discharge nozzle 1, FIG. 7 is a photograph of a state in which a Taylor cone 8 is formed at the tip of the discharge nozzle 1 and droplets are discharged from the tip, and FIG. It is the whole photograph of the state which the droplet which caused the Coulomb explosion has become the countless fine droplet, and is sprayed on the counter electrode. The metal capillary of the discharge nozzle used has an outer diameter of 0.64 mm and an inner diameter of 0.34 mm. The diameter of the central needle is 0.12 mm. The protruding length of the needle from the capillary tip is 0.5 to 0.6 mm. The distance between the capillary and the counter electrode is 30 mm. And it experimented by flowing isopropyl alcohol as a sample liquid. The voltage at which the stable electrospray phenomenon started under these conditions was 2,900V. On the other hand, when the same capillary was used without a needle, the starting voltage of the electrospray phenomenon was 3,500V. By providing the needle, the applied voltage could be reduced by 600V.

The protruding length of the needle 3 from the opening of the capillary 2 is set to a size that fits inside the Taylor cone generated at the tip of the capillary. If the protruding length of the needle 3 is too long, the Taylor cone is not formed well, and the discharge becomes unstable. On the other hand, if it is too short, the effect of lowering the applied voltage cannot be obtained sufficiently. (1) and (2) in FIG. 9 are comparisons of the state of the Taylor cone when the applied voltage is changed with the protruding length of the needle 3 being the same. The outer diameter of the capillary 2 was 0.71 mm, the protrusion length of the needle 3 was 0.75 mm, and the applied voltage was 3250V for (1) and 3050V for (2). The Taylor cone is longer in (2), and the Taylor cone is longer when the applied voltage is lower. This may be due to the following reasons. That is, when the applied voltage is high, a strong electric field gives a sufficient positive charge density to the sample liquid near the tip of the needle 3, and the sample liquid is separated into droplets at an early stage. On the other hand, when the applied voltage is low, a sufficient positive charge density is not provided near the tip of the needle 3, and the Taylor cone is elongated due to gravity. When the tip of the Taylor cone is pointed, the positive charge density at the tip increases and the electromagnetic attractive force from the counter electrode increases, and at the same time, the surface tension to be pulled up by the taper is weakened and separated.

On the other hand, although not shown, when the applied voltage is the same and the protruding length of the needle 3 is changed, the Taylor cone becomes longer when the needle 3 is lengthened. However, if it is too long, the Taylor cone will not be formed well. For this reason, the protruding length of the needle 3 and the applied voltage are set to dimensions and voltages such that the needle 3 is housed in the Taylor cone and a stable Taylor cone is formed and discharge is stably generated. The optimum set value also varies depending on the type of sample liquid and the distance between the nozzle and the counter electrode.

(Second Embodiment)
FIG. 10 is a longitudinal sectional view of the discharge nozzle 10 according to the third embodiment. This discharge nozzle 10 is different from the discharge nozzle 1 according to the first embodiment shown in FIG. 1 in the shape of a needle 13 that passes through the center of a capillary 2 that is a cylindrical capillary. The needle 13 is a needle that is thicker than the needle 3. The shape of the tip of the needle 13 is different from the shape of the tip of the needle 3. That is, the outer diameter of the needle 13 is made thick so that it is slightly smaller than the inner diameter of the capillary 2. The gap between the outer surface of the needle 13 and the inner surface of the capillary 2 is narrowed to such an extent that the required amount of sample liquid can be passed. The tip portion of the needle 13 protruding from the opening of the capillary 2 is formed in a downward truncated cone shape.

When the sample liquid passes through a narrow gap between the outer surface of the needle 13 and the inner surface of the capillary 2, the sample liquid comes into contact with the wide outer surface of the thick needle 13. The sample liquid that has passed through the gap and reached the open end of the capillary 2 now flows down to the frustoconical tip while contacting the wide side surface of the frustoconical portion of the needle 13. When a high voltage is applied to the needle 13, a small Taylor cone 14 is formed at the tip of the frustoconical shape, causing an electrospray phenomenon, and the sample liquid is ejected as a charged droplet toward the counter electrode. FIG. 11 is a photograph of a state in which discharge is performed by causing an electrospray phenomenon using the discharge nozzle 10 of the present embodiment.

The characteristic of the electrospray phenomenon using the discharge nozzle 10 of the present embodiment is that the sample liquid comes into contact with the needle 13 to which a high voltage is applied before reaching a droplet in a wide area. An electrochemical reaction occurs on the surface of the needle 13. That is, when the applied voltage is positive, the negative ions in the sample liquid give electrons to the needle 13 and are neutralized. Neutral molecules are separated into positive ions and negative ions to give negative ions to the needle 13, and the positive ions go to the tip of the Taylor cone. The amount of this electrochemical reaction is determined by the contact area between the sample liquid and the needle 13. In the discharge nozzle 10 of the present embodiment that is in contact with the needle 13 over a wide area, the amount of electrochemical reaction that occurs until the tail cone 14 at the tip is reached increases. For this reason, when the amount of positive ions per unit flow rate of the droplets reaching the tailor cone 14 at the tip is the same, the first type using a conventional discharge nozzle having no needle or a thin needle 3 is used. Compared to the discharge nozzle 1 according to the embodiment, it becomes very large. Since it is considered that the amount of positive ions carried away by one droplet ejected from the tip of the Taylor cone is almost constant, this means that the discharge nozzle 10 of this embodiment can discharge a large amount of droplets.

When the discharge nozzle 1 according to the first embodiment and the discharge nozzle 10 of the present embodiment were tested and compared, the following results were obtained. In the discharge nozzle 1 of the first embodiment, the inner diameter of the capillary 2 is 1.83 mm, and the outer diameter of the needle 3 is 0.12 mm. On the other hand, in the discharge nozzle 10 of the present embodiment, the inner diameter of the capillary 2 is 1.83 mm, and the outer diameter of the needle 13 is 1.0 mm. The protruding lengths of the needle 3 and the needle 13 were the same, the applied voltage was the same, and isopropyl alcohol was used as the sample liquid. The discharge nozzle 1 and the discharge nozzle 10 were compared with each other in terms of the flow rate of the sample liquid capable of causing normal discharge, and the discharge nozzle 1 according to the first embodiment was 0.02 mmL / min. On the other hand, in the discharge nozzle 10 of this embodiment, it was 0.50 mmL / min, and the flow volume of 25 times compared with the discharge nozzle 1 was able to flow. Thus, the discharge nozzle 10 of this embodiment has a great effect that a large discharge nozzle capable of increasing the flow rate of the sample liquid can be manufactured.

(Third embodiment)
The discharge nozzle according to the third embodiment has the same structure as the discharge nozzle 1 of FIG. 1 according to the first embodiment or the discharge nozzle 10 of FIG. 10 according to the second embodiment, and the material of the capillary 2 Is changed from a metal to a non-conductive material. Although glass is suitable as the non-conductive material, a non-conductive synthetic resin may be used. If the capillary of the discharge nozzle is made of a non-conductive material, the generation of corona discharge near the tip of the capillary can be more effectively suppressed.

In the drawings, 1 and 10 are discharge nozzles, 2 is a cylindrical capillary (capillary), 3 is a needle, 4 is a support block, 5 is a flow path, 6 is a power source, 7 is a liquid pool, 8 is a charged droplet, 14 is Shows Taylor Cone.

Claims

A discharge nozzle for use in an electrospray ionization method, comprising a metal cylindrical tubule and a metal needle,
The needle is attached to the center of the cylindrical tubule in a state of being electrically connected to the cylindrical tubule,
The tip of the needle protrudes from the discharge side opening of the cylindrical tubule,
A discharge nozzle having a protruding length at the tip of the needle that is within a Taylor cone formed at the tip of the cylindrical capillary.
A discharge nozzle for use in an electrospray ionization method, comprising a metal cylindrical tubule and a metal needle,
The needle has an outer diameter slightly smaller than the inner diameter of the cylindrical tubule, and forms a gap for allowing the sample liquid to pass between the inner surface of the cylindrical tubule,
The tip of the needle protruding from the opening of the cylindrical capillary is a frustoconical shape.
3. The discharge nozzle according to claim 1 or 2, wherein the cylindrical tubule is formed of a non-conductive material instead of metal, and an applied voltage is supplied to the needle.