EP0174222A1 - Method of mass spectrometry by time-of-flight, and spectrometer therefor - Google Patents

Abstract

A source (11) comprising a solid surface is subjected to the action of an electric field between this surface and an electrode (15), which causes the simultaneous emission of electrons and negative ions released by spontaneous desorption; the electrons and negative ions are received successively by a detector (12) to determine the mass spectrum as a function of the differences between the instants of reception of the electrons and the negative ions.

Description

The present invention relates to the determination of mass spectrum by time of flight.

In a time-of-flight mass spectrometer, ions are extracted from a source containing a product to be analyzed and their mass is determined by measuring their time of flight to a detection device.

The ion source is for example constituted by a solid surface from which the ions are released by desorption. Several techniques are used for this purpose. Thus, the solid surface can be bombarded by primary ions accelerated by cyclotron or be subjected to high energy radiation. It is also well known to use a radioactive source ²⁵² Cf which emits two fragments of fission in opposite directions, one being directed towards the solid surface to release ions and the other towards a metallic sheet to eject electrons whose detection provides the time reference (start signal).

The object of the present invention is to provide a method allowing a determination of the time-of-flight mass spectrum under conditions much simpler than those prevailing with the methods currently known.

This object is achieved by a method according to which, according to the invention, a source comprising a solid surface is subjected to the action of a constant electric field causing the simultaneous emission, on the one hand, of electrons and, on the other hand, negative ions released from said surface by spontaneous desorption, and the mass spectrum is determined from the differences in time of flight of electrons and negative ions between the source and a detection device.

The present invention is based on the finding that, unexpectedly, the application of a constant electric field causes the time correlated emission of electrons and negative ions. The reception of electrons by the detection then supplies the time reference for measuring the times of flight of the negative ions arriving on this detection device. In addition to the simplicity of implementation, the method according to the invention has the advantage of being non-destructive with respect to the ion source.

Spontaneous desorption does not require an electric field of very high intensity. As a guide, a constant intensity field between 1 and 3MV / m may be sufficient.

The present invention also aims to provide a time-of-flight mass spectrometer allowing the implementation of the method defined above.

This object is achieved by means of a spectrometer comprising: an enclosure which can be connected to a vacuum source, means for liberating ions by desorption from a source formed by a solid surface placed in the enclosure, a detection device comprising means for detecting ions from the source, and a measuring device for determining the mass spectrum sought from quantities representative of time of flight of the ions between the source and the detection device, spectrometer in which, in accordance with the means are provided for establishing a constant electric field between the solid surface and a grid-shaped electrode placed in front of this surface in order to cause the simultaneous emission, on the one hand, of electrons and, on the other hand negative ions released from said surface by spontaneous desorption; and the measuring device comprises means for developing quantities representative of the time intervals separating the reception of electrons and the reception of negative ions by the detection device.

• - la figure 1 est une vue très schématique d'un mode de réalisation d'un spectromètre de masse à temps de vol conforme à l'invention, et
• - les figure 2A et 2B sont des spectres obtenus pour un même composé organique respectivement avec le procédé conforme à l'invention et avec un procédé de l'art antérieur.

Other features of the method and of the spectrometer in accordance with the invention will emerge on reading the description given below, by way of indication but not limitation, with reference to the appended drawings in which:

• FIG. 1 is a very schematic view of an embodiment of a time-of-flight mass spectrometer according to the invention, and
• - Figures 2A and 2B are spectra obtained for the same organic compound respectively with the process according to the invention and with a process of the prior art.

The spectrometer shown in Figure 1 essentially comprises a tube 10 connected to a vacuum source (not shown) for establishing a high vacuum, for example ₁₀ ^-6 to 10 ^-7 Torr inside the tube 10.

An ion source 11 is arranged in the vicinity of a first end, or rear end, of the tube 10, inside the latter. In the example illustrated, the source 11 is constituted by a thin metallic sheet 11a, in the form of a flat disc, on the front face of which is deposited a thin uniform layer 11b of a compound to be analyzed in mass, in particular an organic compound. . The sheet 11a is for example an aluminum sheet of thickness equal to 5 microns. The compound to be analyzed is deposited on the sheet 11a, for example by electrostatic spraying, the mass of compound deposited being for example of the order of a few micrograms.

In the vicinity of its second end, or front end, the tube 10 contains a detection device 12 which is formed from micro-channel pancake detectors and which is connected to a measuring device 13 outside the tube.

According to an essential characteristic of the invention, a simultaneous emission of electrons and negative ions from the source 10 is caused by subjecting the latter to the action of a constant electric field. For this purpose, a grid-shaped electrode 15 is placed in front of the sheet 11a, parallel to this sheet and spaced from it, and a potential difference is established between the metal sheet 11a and the grid 15. For example, the grid 15 is brought to the reference potential (ground) while a constant negative voltage V- supplied by a voltage generator 16 is applied to the sheet 11a by means of a conductor passing through the wall of the tube 10.

The electrons and negative ions emitted under the action of the electric field are accelerated by this one and "fly" until detection device 12 passing through the electrode 15 which is preferably formed by a very fine grid with a high transparency rate (for example 90%).

Electrons and ions are emitted simultaneously and received successively in order of increasing mass. For each electron or ion received, the detection device 12 produces an electrical signal sd which is applied to the measurement device 13. Since there is a time correlation between emission of electrons and emission of ions, and the flight time of an electron is known, the reception of an electron by the detection device can be used as a time reference for measuring the times of flight of the negative ions subsequently received.

The measuring device 13 comprises a constant fraction discriminator circuit 21, a time-digital converter 22 and a data acquisition circuit 23.

The circuit 21 transforms each signal sd into a pulse calibrated at a level compatible with the circuits used downstream. Such a constant fraction discriminator circuit is known per se; we can in particular use the circuit marketed under the reference 7174 by the French company ENERTEC (SCHLUMBERGER).

The output of circuit 21 is, on the one hand, connected directly to a start control input 22d of the time-digital converter 22 and, on the other hand, connected by a constant delay circuit 24 to an input 22s of control d 'shutdown of the same converter. For the converter 22, the circuit can be used for example, the principle of which is described by E. Festa and R. Sellem in the publication of the United States of America "Nuclear Instruments and Methods" No. 188 (1981) page 99. After having received a start signal, such a converter can accept, in a predetermined limited time interval (for example 16 or 32 microseconds) several stop signals (for example 32) and provides, in response to each stop signal , a digital word representing the time elapsed between the reception of the start signal and the reception of this stop signal. These words digital signals are recorded via the data acquisition circuit 23 connected to the output of the converter 22.

In almost all cases, an operating cycle of the converter 22 is initiated in response to the reception of an electron. The same signal, delayed by circuit 24 provides a first counting result, which allows, on the one hand, a visualization and an accounting of the electrons received and, on the other hand, to have a precise reference for the measurement of the reception time of negative ions since the corresponding signals are also routed to the stop command input 22s through the delay circuit 24.

The results or accounts obtained at the same relative instants of successive cycles of operation of the converter 22, during an observation period, are cumulated to provide the mass spectrum sought. As an indication, the duration of an observation period is a few minutes.

FIG. 2A shows the mass spectrum of the organic compound valine (of molecular weight 117) obtained by means of a spectrometer such as that of FIG. 1, the tube 10 having a length of 0.3m and a diameter of 0.1m . A voltage of -9kV was applied to the sheet 11a, the grid 15 being spaced therefrom by 5mm.

The first peak in FIG. 2A is produced by the reception of electrons delayed by circuit 24. This first peak provides an offset origin for the measurement of flight times, and its integral gives the total number of electrons which have generated a start signal. A desorption efficiency for a negative ion of mass n can be defined as the ratio between the number of counts in the peak of mass m and the number ne-. For the negative ion valine- (m = 116 ^- ), the spontaneous desorption yield thus calculated is 1% in this example.

For comparison, Figure 2B shows the mass spectrum of the same compound obtained by means of a spectrometer in which a conventional radioactive source of ²⁵² Cf is used to release the ions by desorption. It will be noted that the spectrum obtained with the spectrometer according to the invention differs from that of FIG. 2B by the presence of more marked peaks for the masses corresponding to C7, CH-, O ^- and OH ^- .

Although an example of implementation of the invention has been described above for obtaining a spectrum of an organic compound of relatively low molecular weight, it should be noted that molecular ions of masses ranging from 2000 to 3000 have been observed by this spontaneous desorption technique.

Furthermore, the invention is of course not limited to the determination of mass spectra of organic compounds. Measurements can be carried out, for example, on metallic sources consisting directly of a sheet of the metal or alloy to be examined brought to the desired potential.

The intensity of the electric field to be established to carry out the spontaneous desorption of negative ions simultaneously with the emission of electrons is to a certain extent chosen according to the nature of the molecular deposit 11b and the performance of the measuring device. Indeed, the emission begins when the intensity of the electric field exceeds a certain threshold. In addition, as the emission increases when the intensity of the field increases, the number of electrons can become such that the acquisition capacity of the measuring device is saturated and a part of the events is lost for values of intensity exceeding a certain threshold.

In the case of the example of implementation of the invention described above, with a space of 5 mm between the sheet 11a and the electrode 15, it has been noted that the emission begins when the voltage applied to the sheet lla becomes, in absolute value, greater than 3 or 4 kV. For a value of 10 kV over 5 mm, the number of electrons counted in one second is less than 10,000. At the value of 15 kV over 5 mm, the number of electrons has become such that the acquisition capacity of the measuring device used in this example is saturated.

Generally speaking, a field value between 1 and 3 MV / m seems to be suitable, the adaptation of this value being carried out according to the nature of the molecular deposit. It will be noted that this value remains much lower than the levels which are used to carry out a desorption by a violent field effect in mass spectrometers of magnetic type. In addition, in the case of the present invention, the moderate electric field is applied throughout the duration of the observation and the instant of its application does not constitute a time reference.

Claims (4)

1. Method for determining the mass spectrum by time of flight,
characterized in that a source comprising a solid surface is subjected to the action of a constant electric field causing the simultaneous emission, on the one hand, of electrons and, on the other hand, of negative ions released from said surface by spontaneous desorption, and the mass spectrum is determined from the differences in time of flight of electrons and negative ions between the source and a detection device. 2. Method according to claim 1, characterized in that the electric field is produced by establishing a potential difference between the solid surface and an electrode in the form of a grid parallel to the latter. 3. Method according to any one of claims 1 and 2, characterized in that the intensity of the electric field is between 1 and 3MV / m. 4. Time-of-flight mass spectrometer, comprising an enclosure (10) which can be connected to a vacuum source, means for liberating ions by desorption from a source (11) formed by a solid surface placed in the enclosure , a detection device (12) comprising means for detecting ions from the source, and a measurement device (13) for determining the mass spectrum sought from quantities representative of time of flight of the ions between the source and the detection device,
characterized in that means (16) are provided for establishing a constant electric field between the solid surface and an electrode (15) in the form of a grid placed in front of this surface so as to cause the simultaneous emission of electrons and, on the other hand, negative ions released from said surface by spontaneous desorption; and the measuring device (13) comprises means (21, 22) for developing quantities representative of the time intervals separating the reception of electrons and the reception of negative ions by the detection device (12).

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