JP2001043826A - Atmospheric chemical ionization method in mass spectrograph - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent deposition of carbide and make analysis accurate by splaying a liquid sample with nebulized gas, and mixing a specified ratio of oxygen to nitrogen as nebulized gas when nebulized sample molecules are ionized by corona discharge from needle electrodes. SOLUTION: An ionization interface has a nozzle 22 through which a liquid sample flows and a nebulized pipe 23 coaxially arranged on the outside of the nozzle 22, and nebulized gas is passed through a gap between the nozzle 22 and the nebulized gas pipe 23. The nebulized gas is compulsorily sprayed on the liquid sample flowing out of the tip of the nozzle 22, a heater 24 is installed in the front of the nozzle 24, supersonically released liquid drops are heated with a heater 24, collide with gas molecules of atmospheric pressure and minutely crushed, and a solvent in the liquid drops is vaporized. By using synthetic air comprising 20-21.5% oxygen gas and the balance nitrogen gas as the nebulized gas, deposition of carbide on needle electrodes is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an atmospheric pressure chemical ionization method (APCI) in a liquid chromatograph mass spectrometer (hereinafter referred to as "LC-MS"), and more particularly to a negative APCI for generating negative ions. .

[0002]

2. Description of the Related Art In LC / MS in which a liquid chromatograph (LC) and a mass spectrometer (MS) are combined, a liquid sample flowing out of an LC column is vaporized, and a target component is ionized while removing a solvent. A dedicated interface is used. Such an interface generates gas ions by atomizing a liquid sample by heating, high-speed gas flow, high electric field, and the like. Atmospheric pressure chemical ionization (APCI) and electrospray ionization (ES)
I) is the most widely used.

In APCI, a liquid sample flowing out of a column is guided to the tip of a nozzle, and a nebulizing gas is ejected through a nebulizing pipe disposed coaxially with the nozzle, thereby forcibly spraying the liquid sample into an atomization chamber. Let it. A needle electrode is arranged in front of the nozzle, and a high voltage is applied to the needle electrode to generate a corona discharge, whereby the vaporized molecules of the sample are ionized. The sample molecule is
There are ones that easily emit positive electrons and become positive ions, and those that easily receive electrons and become negative ions.

[0004]

One of the problems in the so-called negative APCI that generates negative ions is as follows. That is, when a certain organic solvent is used as a mobile phase in LC, carbide is deposited on the needle electrode with the passage of time. This whisker-like precipitate grows with the passage of time, so the position where corona discharge occurs is shifted, or in extreme cases, the precipitate separates from the needle electrode at the moment of discharge, and undesired discharge occurs at another position. May occur. Therefore, when the above-mentioned precipitates are generated, ionization becomes unstable, and analysis cannot be performed accurately.

It is known that the generation of this kind of precipitate largely depends on the type of mobile phase. For example, in a mixed solvent of acetonitrile and water of 50% each, the precipitation is extremely large, and the inventors of the present invention have a problem. According to the experiment described in the above, it grows to a length of about 5 mm by continuous use for 10 minutes.

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and it is an object of the present invention to prevent the precipitation of carbide on a needle electrode in a negative APCI and to perform an accurate analysis. It is an object of the present invention to provide an atmospheric pressure chemical ionization method in a mass spectrometer that can be used.

[0007]

The present inventor conducted an experiment focusing on the relationship between the type of nebulizing gas and the above-mentioned precipitation of carbide on the needle electrode. As a result, it has been found that the above phenomenon can be solved by mixing a predetermined ratio of oxygen gas with nitrogen which is generally used as a nebulizing gas. The mechanism has not been elucidated,
It is considered that oxygen has a higher electronegativity than nitrogen and carbon.

[0008] That is, an atmospheric pressure chemical ionization method according to the present invention comprises a nozzle for ejecting a liquid sample, an auxiliary gas thin tube attached to the nozzle and ejecting a nebulizing gas in the same direction as the liquid sample ejection direction, In a mass spectrometer having a needle electrode disposed in front thereof, a liquid sample is sprayed with a nebulizing gas, and an atmospheric pressure chemical ionization method in which atomized sample molecules are negatively ionized using corona discharge from the needle electrode. Further, a gas in which a predetermined ratio of oxygen is mixed with nitrogen is used as the nebulizing gas.

If the mixing ratio of oxygen to nitrogen is too high, if a low flash point solvent is used as a solvent for the liquid sample, there is a risk of causing ignition or explosion. Therefore, the mixing ratio of oxygen is preferably set to be approximately the same as that in the atmosphere or in the vicinity thereof, and specifically, it is preferable to be approximately 10 to 30%.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the atmospheric pressure chemical ionization method according to the present invention will be described below with reference to the drawings. FIG. 1 shows LC-MS applying this atmospheric pressure chemical ionization method.
FIG. 2 is a detailed configuration diagram of the ionization unit in FIG.

In FIG. 1, a liquid chromatograph (L
C) LC between the part 10 and the mass spectrometer (MS) 30
An ionization interface 20 is provided for vaporizing and ionizing the sample eluted from the column 14 of the section 10, removing the vaporized solvent, and sending the sample to the MS section 30.

In the LC section 10, a liquid sending pump 12 sucks a solvent (mobile phase) stored in a solvent tank 11 and flows the same through a column 14 at a constant flow rate. The sample injecting unit 13 injects a predetermined amount of the sample solution into the solvent. When passing through the column 14, each sample component in the sample solution is temporally separated,
The elutes are sequentially introduced from the four outlets and introduced into the nozzle 22.

In the ionization interface 20, a liquid sample is sprayed from a nozzle 22 into an atomization chamber 21 in an atmospheric pressure atmosphere and ionized. The inside of the analysis chamber 34 of the MS unit 30 is evacuated to a high vacuum state, and two intermediate vacuum chambers 32 and 33 provided between the atomization chamber 21 and the analysis chamber 34.
Are partitioned by partition walls each having a small opening, whereby the degree of vacuum increases stepwise from the atomization chamber 21 to the analysis chamber 34.

As described above, the sample ions generated in the atomization chamber 21 are drawn into the intermediate vacuum chambers 32 and 33 by the differential pressure, and further drawn into the analysis chamber 34, where the sample ions are formed of four rod electrodes. It is sent to the center space of the pole filter 35. A voltage in which an AC voltage and a DC voltage are superimposed is applied to the quadrupole filter 35, and only ions having a specific mass number (mass m / charge z) corresponding to the voltage are applied to the quadrupole filter 35. And arrives at the ion detector 36. In the ion detector 36, a current corresponding to the number of reached ions is extracted.

Referring to FIG. 2, ionization interface 2
Explaining the configuration of No. 0 in more detail, a nebulizing tube 23 is disposed coaxially outside the nozzle 22 through which the liquid sample flows, and a nebulizing gas flows through a gap between the nozzle 22 and the nebulizing tube 23. As shown in FIG. 1, the nebulized gas is supplied by adjusting a predetermined gas stored in a gas cylinder 26 to a predetermined flow rate by a flow controller (or a pressure control valve) 27.

The nebulizing gas forcibly sprays the liquid sample flowing from the tip of the nozzle 22. A heater 24 is provided in front of the nozzle 22, and the droplet ejected at supersonic speed collides with gas molecules at atmospheric pressure heated by the heater 24, and is further finely pulverized. Solvent evaporates. A needle electrode 25 is provided further in front of the nozzle 22, and gas molecules contained in the nebulized gas and molecules of the vaporized solvent are discharged by corona discharge generated by a high voltage (about several kV) applied to the needle electrode 25. Is ionized by The sample molecules chemically react with the buffer ions thus generated, and are negatively ionized. The generated ions jump into the desolvation tube 31 opened in front of the ion and are sent to the intermediate vacuum chamber 32.

In the above configuration, generally, pure dry nitrogen gas is used as a nebulizing gas. On the other hand, in the atmospheric pressure chemical ionization method according to the present embodiment, A-grade synthetic air is used as the nebulizing gas. This synthetic air has an oxygen gas content of 20.
２21.5%, with the balance being nitrogen gas. According to the experiments performed by the inventors of the present application, even when an organic solvent in which acetonitrile and water, each of which causes significant precipitation of the above-mentioned carbides, are mixed by 50%, is used as the solvent of the LC portion 10, the nebulizing gas as described above is used. It has been confirmed that the use can completely suppress the precipitation of carbide on the needle electrode 25.

Of course, the supply source of the nebulizing gas is not limited to a gas cylinder in which a gas prepared in advance is stored. For example, a so-called ZERO-Air generator may be used.

[0019]

As described above, according to the atmospheric pressure chemical ionization method of the present invention, even when a specific organic solvent is used as a mobile phase of a liquid chromatograph, carbide is deposited on a needle electrode. Can be suppressed.
Accordingly, accurate analysis is possible, and an organic solvent, which has conventionally been difficult to use in negative atmospheric pressure chemical ionization, can be used as a mobile phase.

[Brief description of the drawings]

FIG. 1 shows a negative APC according to an embodiment of the present invention.
1 is a configuration diagram of an LC-MS to which I is applied.

FIG. 2 is an enlarged view of the ionization interface in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... LC part 20 ... Ionization interface 21 ... Atomization chamber 22 ... Nozzle 23 ... Nebulize tube 24 ... Heater 25 ... Needle electrode 26 ... Gas cylinder 30 ... MS part 31 ... Desolvation tube 32,33 ... Intermediate vacuum chamber 34 ... Analysis Room

Claims (1)

[Claims] 1. A mass having a nozzle for ejecting a liquid sample, an auxiliary gas thin tube attached to the nozzle and ejecting nebulizing gas in the same direction as the ejection direction of the liquid sample, and a needle electrode disposed in front of the nozzle. In the analyzer, while spraying the liquid sample with nebulizing gas,
An atmospheric pressure chemical ionization method for negatively ionizing atomized sample molecules using corona discharge from a needle electrode, wherein a gas obtained by mixing a predetermined ratio of oxygen with nitrogen is used as the nebulizing gas. Atmospheric pressure chemical ionization method.