Patent Application
20080054176
a and 6b are sectional views illustrating other examples of the structure of a capillary;
An orifice 11 provided with a miniscule hole 11a is attached to a mass analyzer 10 at the ion introduction port thereof. The miniscule hole 11a serves as the ion introduction port. The interior of the mass analyzer 10 is held in vacuum.
A housing 21 of an ionization apparatus 20 is attached hermetically to the vessel wall of the mass analyzer 10 so as to surround and cover the orifice 11. The space delimited by the housing 21 and orifice 11 is an ionization space 22. The interior of the ionization space 22 is held in vacuum (e.g., 10−3 Torr) by an exhaust device (pump) (not shown).
A capillary (made of silica or alumina) 23 for supplying a liquid sample is provided penetrating the wall of the housing 21. The distal end of the capillary 23 is inside the ionization space 22 (housing 21), and the base end thereof projects outwardly of the housing and is connected to a coupling body 30. Though the details will be described later, a diamond tip 24 is attached to the end of the capillary 23. An infrared laser device 25 is disposed outside the housing 21. An infrared laser beam having a wavelength of 10.6 μm is emitted by the laser device 25 and impinges internally of the housing 21 through a transparent wall portion of the housing 21 or window formed by a transparent body. The laser device 25 is disposed in such a manner that the emitted laser beam will be projected upon the diamond tip 24 at the end of the capillary 23 along the axial direction of the capillary 23.
As illustrated in
The capillary 23, which is a slender tube formed by an electrical insulator such as plastic or silica (glass), is internally provided with a slender cavity 23a extending in the lengthwise direction.
The diamond tip 24 attached to the end of the capillary 23 is conical in shape and is formed to have a small cavity 24a at its center. The diamond tip 24 is bonded and affixed to the end face of the end of the capillary 23 in such a manner that the small cavity 24a of the diamond tip 24 and the slender cavity 23a of the capillary 23 will communicate along a straight line. The capillary 23 is disposed in such a manner that the diamond tip 24 will be situated in the vicinity of the hole 11a in the orifice 11 of the mass analyzer 10.
The coupling body 30 is formed to have passageways 35, 36 in a T-shaped configuration. The passageway 35 passes through the center of the coupling body 30 and is open at both ends. The passageway 36 is formed to be perpendicular to the passageway 35 and the two passageways communicate with each other.
The base end of the capillary 23 is connected to the coupling body 30 to one end of the passageway 35 via a plug 31 so that the slender cavity 23a is communicated with the passageway 35. A plug 33 for maintaining watertightness is provided in the other end of the passageway 35. A conductive wire (e.g., a platinum wire, which is strongly resistant to corrosion) 26 is inserted into the passageway 35 through the plug 33 from outside the plug 33 and reaches the vicinity of the end of the capillary 23 (namely a point 5 to 10 mm short of the diamond tip 24) through the slender cavity 23a. A sample introduction tube 34 is connected to the outer end of the passageway 36 via the plug 32. The liquid sample is supplied from the introduction tube 34 to the capillary 23 through the passageways 36, 35.
A positive (or negative) high voltage is applied to the conductive wire 26. As a result, as shown in
Under these conditions, the liquid sample inside the small cavity 24a of the diamond tip 24 is irradiated with the pulsed infrared laser beam from the laser device 25. The sample is instantaneously heated and vaporized by the laser beam. Since at least the water content of the liquid sample absorbs the infrared laser beam, the heating by the laser beam is performed effectively. Further, since diamond does not absorb infrared light, vaporization is achieved in a state in which the sample is confined, so to speak, in the small cavity 24a.
Positive (or negative) ion molecules or ion atoms thus vaporized are attracted to the negative voltage applied to the orifice 11 and are introduced into the mass analyzer 10 from the hole 11a.
In a case where the mass analyzer has been connected for chromatography or the like, it will suffice for the liquid sample to be supplied continuously to the diamond tip 24 and for the sample to be irradiated with the infrared laser beam, which is generated continuously.
Silicon and germanium, etc., can be used instead of diamond as materials that do not readily absorb infrared light. The capillary itself may be formed by silicon or germanium.
In a case where the capillary has been formed by an electrical conductor such as metal, the conductive wire 26 will be unnecessary and it will suffice if the positive or negative high voltage is applied to the conductive capillary per se.
As mentioned above, the capillary 23 is disposed in such a manner that the diamond tip 24 is situated in close proximity to the outer side of the hole 11a in the orifice 11 of mass analyzer 10. A conductive wire may or may not be inserted into the capillary 23. In this embodiment, the corona-discharge electrode 28 is provided in the vicinity of the end of the capillary 23.
As mentioned above, the diamond tip 24 is irradiated with an infrared laser beam of narrowed focal point and a sample in an aqueous solution inside the small cavity 24a of the diamond tip 24 is vaporized completely. Though there are cases where ions that existed in the liquid are vaporized as is as ions, molecules that have remained neutral, or neutral molecules that have become neutralized by recombination of positive and negative ions, also are generated.
The sample gas that has been completely vaporized is jetted from the end of the diamond tip 24 owing to irradiation with the infrared laser beam. The corona-discharge electrode 28 is disposed very close to the end of the diamond tip 24 from which the gas is jetted. A corona discharge is induced by applying a positive or negative high voltage upon the corona-discharge electrode 28. When the corona discharge is caused by the application of a positive high voltage, a protonated neutral sample [M+H]+ is mainly produced. In a case where a negative high voltage is applied, negative ions [M−H]− obtained by deprotonating neutral sample molecules are mainly produced. Since ionization is performed in a state in which the sample molecules have been concentrated near the end of the diamond tip 24 by the corona discharge, the neutral-molecule ionization efficiency can be improved. Accordingly, neutral-molecule detection efficiency that is obtained is an order-of-magnitude higher than that of the conventional atmospheric-pressure ionization method (a method in which a sample gas is ionized in a state in which the sample molecules have been dispersed over the entirety of the ionization chamber).
Conventionally, the analysis of neutral molecules in a liquid sample entails first converting the liquid sample into droplets by ultrasound or by a nebulizer and subsequently heating the vessel wall to vaporize the liquid sample and achieve atmospheric-pressure ionization. In accordance with the method of this embodiment, it is unnecessary to promote vaporization of the liquid sample by raising the temperature of the vessel wall of the ionization chamber. As a result, soft ionization can be performed without an easily thermally decomposable biological sample being caused to decompose. With infrared-laser irradiation of the diamond tip 24, the diamond tip 24 is not heated. In addition, the energy of the laser beam is expended in severing the hydrogen bonds of the solvent and does not lead to vibrational excitation of the molecules. Accordingly, an advantage obtained is that decomposition of the sample molecules can be almost completely ignored.
The ions that have been generated under atmospheric pressure pass through the hole 11a in the orifice 11 and are sampled and undergo mass analysis in vacuum. Examples of the mass analyzer 10 that can be used are an orthogonal time-of-flight mass spectrometer, a quadrupole mass spectrometer and magnetic-field mass spectrometer.
a illustrates another example of a corona-discharge electrode. The end of the conductive wire (a metal wire or platinum wire) 26 that has been inserted into the capillary 23 is caused to project outside slightly (several millimeters) beyond the end of the diamond tip 2, and the end of the conductive wire 26 is made to serve as a corona-discharge electrode. The end of the conductive wire 26 may be ground to a sharp point in order to facilitate the generation of discharge plasma.
As set forth above, a sample of an aqueous solution is passed through the capillary 23 and the liquid sample that flows out of the diamond tip 24 is irradiated with the laser beam (infrared laser: 10.6 μm) to thereby completely vaporize the sample. Under these conditions, a high voltage (several hundred to several kV) is impressed upon the conductive wire 26 that has been passed through the center of the capillary 23, thereby inducing a corona discharge at the end of the conductive wire 26. Ions are generated in the plasma by this corona discharge. For example, with a sample of an aqueous solution, the solvent is water and therefore a large quantity of hydrated clusters of protons is generated by electrical discharge of water vapor.
Generation of H+(H2O)n cluster ions in water-vapor plasma
H2O+e (electron)→H2O++2e (1):
electron ionization (induced in plasma)
H2O++H2O→H3O++OH (2):
proton migration reaction
H3O++nH2O→H3O+(H2O)n (3):
cluster ring reaction The H3O+and hydrated cluster ions H3O+(H2O)n cause a proton migration reaction with an analyte component B in the sample, thereby generating H+B.
H+(H2O)n+B→H+B+nH2O (4)
Since this reaction occurs in atmospheric pressure, it causes a very large number of collisions between the H+(H2O)n ions and ambient gaseous molecules. Consequently, even if the concentration of the analyte component B is very low, the component B can be detected with satisfactory sensitivity because the reaction (4) takes place in an efficient manner.
As set forth above, the method of this embodiment is a combination of the atmospheric-pressure ionization method and complete vaporization (by the laser spray method) of a liquid sample by irradiation with a laser. In the case of a biological sample, it is preferred that the solvent be water. In the case of a sample in an aqueous solution, water vapor is produced by irradiation with a laser beam. A property of water vapor is that it does not lend itself to generation of a discharge plasma. This problem is mitigated greatly by mixing in a rare gas (argon gas, etc.) as an ambient gas.
As shown in
This method is such that if the molecules are molecules having a proton affinity greater than that of water molecules, all of these can be detected with high sensitivity. Since there are usually many biological molecules having a proton affinity greater than that of water molecules, this method is very effective in analyzing biological samples. Further, by combining this method with liquid chromatography (LC) (where a liquid sample that is output from LC is supplied to the capillary 23), the mixture components are isolated beforehand and it is possible to detect each component separately. With an ordinary LC detector (ultraviolet absorbing detector, etc.), identification of the molecules is difficult. By comparison, the mass analysis method using the above-described ionization method is such that the molecule B undergoes mass analysis as BH+, and therefore the molecular weight of the analyte component is obtained. Further, ions are extracted from the atmospheric-pressure ion source to the side of vacuum and cause collision-induced dissociation, thereby making it possible to obtain molecular structure information as well.
The above-described ionization method vaporizes an aqueous sample momentarily by irradiation with an infrared laser beam and causes the gaseous sample to converge to the center of the diamond tip (i.e., concentrates the sample without allowing it to diverge), in which state the corona discharge is produced at the center. As a result, first reaction ions H3O+ (H2O)n (in a case where the solvent is water) are produced. These reaction ions H3O+ (H2O)n repeatedly collide a large number of times with the ambient gaseous molecules under atmospheric pressure. If there is even a single collision with a molecule of the analyte component, the proton migration reaction (4) will always take place. After collisions a large number of times, therefore, the major part of the protons (H+) of the reaction ions H3O+ (H2O)n eventually shift to the molecules B of the analyte component, the molecules B are ionized (protonated) and electric charge migrates to the molecules B (protonated B molecules, i.e., H+B, are generated). This process can be regarded as a process that utilizes an ion molecule reaction (proton migration reaction) to concentrate the molecules B in the form of ions (H+B). With this ionization method, analysis on the ppb level can be performed with ease. (It is possible to ionize 1/10 components, which corresponds to a concentration efficiency of 109. The reaction ions undergo collisions with ambient molecules at least 109 times.)
In a case where a plurality of types of molecules having different proton affinities are mixed with the sample, ion—molecule reactions (proton migration reactions) take place sequentially and there may be instances where it is difficult to perform identification and analysis of each component. However, by combining this method with LC, the components are isolated beforehand by liquid chromatography and then the components flow out to the diamond tip. Even though the sample is a mixed sample, therefore, the possibility that a plurality of types of samples will be mixed together at the end of the diamond tip need not be taken into account.
In
A skimmer 41 provided with a somewhat large aperture 41a is attached to a mass analyzer 40 at the portion thereof having an ion introduction port. The aperture 41a serves as the ion introduction port. The interior of the mass analyzer 40 is held in vacuum.
A housing 51 of an ionization apparatus 50 is attached hermetically to the vessel wall of the mass analyzer 40 so as to surround and cover the skimmer 41. The space delimited by the housing 51 and skimmer 41 is an ionization space 52. The interior of the ionization space 52 is held in a high vacuum (e.g., 10−6 to 10−7 Torr) by an exhaust device (pump) (not shown).
A sample table 53 is provided in the ionization space 52 inside the housing 51 and is supported by the arm of a cryogenic freezer 54 placed outside the housing 51. The cryogenic freezer 54 has the capability to effect cooling to, e.g., 10 K. Further, grids 55 that guide ions to the aperture 41a of the skimmer 41 are provided inside the housing 51.
As shown in
The sample is, e.g., a biological sample (DNA, protein molecules, etc.) and has been mixed with an inorganic matrix such as water or SF6 having a low molecular weight.
The substrate is not limited to the shape shown in
Thus, the substrate 60 holding the sample that has been mixed with a matrix is attached to the sample table 53 inside the ionization space 52. A positive or negative high voltage is applied to the substrate 60. The sample on the substrate inside housing 51 is irradiated obliquely with an infrared laser beam from an infrared-laser source 56 disposed outside the housing 51. The low-molecular-weight inorganic matrix that includes water absorbs the infrared light in a highly efficient manner and causes a shock wave to be generated in the vicinity of the surface thereof. The shock wave generated is directed toward the substrate 60. Through this process, the matrix and sample are heated rapidly, the sample is desorbed and gaseous-phase positive or negative ions are generated efficiently owing to the high-strength electric field impressed upon the protrusions 61 or the protrusions of porous silicon. These ions head in a direction perpendicular to the surface of the substrate 60 and are guided into the time-of-flight mass analyzer 40 from the aperture 41a of the skimmer 41.
Since the matrix comprises an inorganic material of low molecular weight, the material will not constitute a large noise component even if it is ionized and introduced into the mass analyzer 40.
Since a matrix that includes water absorbs infrared light, the sample is heated rapidly. Because a biological sample also includes a water component and absorbs infrared light, it is heated efficiently.
Since the sample is frozen in the above embodiment, it can be prevented from drying.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP04/04520 | 3/30/2004 | WO | 00 | 9/28/2006 |
