EP0200645A1 - Sample introduction method and device for a mass spectrometer - Google Patents

Abstract

The device enables to introduce microsamples in the ionization source of the spectrometre. It comprises a reactor (10) provided with microsample introduction means, microsample heating means, means for connection with a vacuum source and means (38, 40) with adjustable flow-rate for supplying a reactant for the transformation of the microsample into gaseous compounds, a choked and calibrated passage (12) for the gas flow from the reactor, and a sublimation tube (40) connected on the one hand to the passage and on the other hand to the source of ions of the spectrometre through a valve (16), and provided with means enabling to take it to an adjustable cryogenic temperature.

Description

The invention relates to the field of sample analysis by mass spectrometry and it relates more particularly to the methods and devices making it possible to introduce, into the spectrometer, a micro-sample flow rate, the ions of which are subjected to analysis.

It finds a particularly important application in the field of isotopic analysis, which often has to be carried out on small samples, for example because these samples are highly radioactive or particularly valuable.

At present, mass spectrometers use either a source of thermionic ions or a source of ions with electron bombardment of a gas flow.

The first solution has the advantage of allowing to use ^* samples of very low weight, frequently from 0.1 to 10 micrograms. The sample is deposited, usually in liquid form, on a refractory metal tape. By evaporation of the liquid, a solid deposit is obtained. The ribbon is placed in the ion source of the device, then brought to high temperature (2600 ° C for example) by Joule effect. The sample then emits neutral molecules and ions. The latter, accelerated and focused in the form of a particle beam, are subjected to analysis.

If this technique has the advantage of allowing the use of very small quantities of samples, it has in return many disadvantages. The intensity of the ionic currents obtained at the spectrometer collector is low: it implies, in practice, that it is possible to measure currents as low as 10 ^-17 amps, which requires multipliers. The emission of ions by thermo-ionization is poorly understood, its stability and its evolution over time do not are not always perfectly under control. It is necessary to take into account the effects of isotopic fractionation or mass discrimination by corrections, generally established by calibrating the apparatus with known products. Stable and reproducible measurements can only be obtained by precisely controlling the purity of the sample, its method of preparation and deposition, the purity of the refractory constituting the support, degassing, the rate of temperature rise.

All these limitations translate into the fact that one cannot hope to exceed a precision of the order of a thousandth when the isotopic ratios in the sample are of the order of 1/200, which is common in the nuclear field.

In addition, a mass spectrometer using a thermionic source cannot be used to conduct chemical composition analyzes and can very easily be connected online to a separation or treatment line.

Part of the drawbacks listed above of thermionic sources is eliminated in electron bombardment sources, which are of much wider application since they allow chemical as well as isotopic analyzes, but imply that the sample to analyze either gaseous or easily vaporizable.

The usual method consists in introducing the sample from a sealed container through piping and micro-leakage valves allowing a well determined and very low gas flow to pass without altering the very low pressure which must prevail in the analyzer of the spectrometer. The molecules of gas or vapor which pass at very low flow rate are subjected to the action of a beam of electrons of determined energy which ionizes the gas to give rise to ions subjected to analysis.

The intensity of the ion currents obtained is usually of the order of 10-9 A, that is to say much higher than in thermo-ionization spectrometers, which simplifies the measurement.

As it is not necessary to "break the vacuum" in the source to introduce the sample, contrary to what happens in the case of a thermionic source and that the time before stabilization is shorter, the time necessary to obtain a result is generally reduced by a factor of the order of 4, which makes the device usable online.

In return for the advantages mentioned above, electron bombardment sources have drawbacks which make them difficult to use in certain cases.

In particular, it is necessary to have larger samples than in the first case and to handle them. We do not know how to design sample bottles, transfer volumes and valves with internal capacities of less than a few cubic centimeters in total, and the interaction of gaseous molecules with the walls that contain them reveals phenomena. of memory which influence the results of the measurements and oblige to take into account corrective factors, determined from consumable standards.

In other words, measurements made using a spectrometer using an ion source by electron bombardment are generally differential measurements, which guarantee high precision, typically 50 to 100 times higher than with a thermionization source.

It can be seen that each of the known solutions has drawbacks which make it unsuitable for a certain number of applications. In particular, ion sources using ionization of a molecular flow low require large volumes of samples, which represents a serious constraint or even prohibitive in some cases.

The present invention aims to provide a supply method and device for a mass spectrometer, using the ionization technique of a very low flow rate, typically using an electron beam, but responding better than those previously known to the requirements of the practice, in particular in that they authorize the use of samples of very low mass.

To this end, the invention proposes in particular a process for introducing micro-samples in gaseous form into the ionization source of a mass spectrum, process according to which the sample is transformed into gaseous compound by heating in an atmosphere of a reactive gas, a flow of the compound and of the reactive gas in molecular flow regime is organized towards a wall maintained at a temperature low enough to trap the gaseous compound and the reagent and the gaseous compounds are selectively released by temperature control. of said wall.

The invention also proposes a device for introducing micro-samples into the ionization source of a mass spectrometer, comprising a reactor provided with means for introducing the micro-sample, with means for heating the micro-sample, means for connecting to a vacuum source and means for supplying an adjustable flow rate of reagent for transforming the micro-sample into gaseous compounds; a throttled and calibrated passage of gas flow from the reactor; and a sublimation tube connected on the one hand to the passage, on the other hand to the ion source of the spectrometer via a valve, provided with means making it possible to bring it to an adjustable cryogenic temperature.

It can be seen that this device keeps all the advantages of using an electron bombardment source in the apparatus: it makes it possible to work with relatively intense ion beams, which simplifies their measurement; it avoids breaking the vacuum in the source to introduce the sample; It is not necessary to have watertight containers to handle the samples and connect them to the device.

And, compared to the conventional device for electron bombardment spectrometer, the proposed device has many advantages: the size of the samples to be analyzed is reduced to a few micrograms; It is not necessary to have sealed containers for handling the samples and introducing them into the device; the consumption of standards or reference products can be reduced to the order of magnitude of that of the samples, the preparation of which is simple and rapid.

In addition, several devices can easily be placed at the input of the electron bombardment ion source of a spectrometer, which allows easy comparisons of samples with each other or with a standard. In addition, a device allowing the admission of a reference gas contained in a sealed bottle can be provided to compare it with the sample gas from the sublimation tube.

The invention will be better understood on reading the following description of a device which constitutes a particular embodiment thereof given by way of non-limiting example.

• - la figure 1 est un schéma de principe du dispositif,
• - la figure 2 montre un dispositif particulier suivant l'invention, en coupe suivant un plan passant par son axe.

The description refers to the accompanying drawings, in which:

• FIG. 1 is a block diagram of the device,
• - Figure 2 shows a particular device according to the invention, in section along a passing plane by its axis.

The device shown in Figures _t 2 can be viewed as comprising a reactor 10, the essential element of which is a micro-oven with adjustable temperature, a passage 12 sufficiently constricted for the flow to take place therein in the form of molecular flow, and a micro- sublimator 14. The microsublimator is connected, by means of a valve 16, to the ion source 18 of the spectrometer, which can be of any of the types making it possible to ionize a low flow of gas which penetrates it . Typically, this source will perform ionization by electron bombardment.

The reactor 10, the schematic diagram of which is shown in FIG. 1, comprises an enclosure, generally cylindrical, in the axis of which is placed the actual micro-oven 20 consisting of a metal tube capable of withstanding high temperatures, for example nickel , nichrome or "monel". Means are provided for heating the oven by the Joule effect. In Figure 1, these means are shown in the form of an electrical source 22 connected to one end of the tube, the other of which is grounded. Another solution consists in winding an electric heating resistor around the tube 20. This tube can carry a temperature sensor 24 connected to a circuit 26 for regulating the temperature by modulating the electric power supplied by the source 22.

A sample holder 28 is provided to allow the introduction of a very small quantity of samples, in the form of a deposit on a needle or a thread. The head of this sample holder will be provided to seal the microfour.

One end of the tube 20 forming a micro-oven is connected, by a valve 30, to a vacuum source 32 (mechanical primary pump for example) and to a source 34 of reagent, of a nature such that it gives rise with the sample to a gaseous or volatile compound. The sources 32 and 34 are each provided with a shut-off valve 36 and 38. As a general rule, the valves 30 and 38 at least must be made of a material resistant to very corrosive gases, since it will frequently be necessary to use highly reactive chemical species, such as fluorine. The valve 30 must also be strictly sealed.

The constricted passage 12 may have a fixed passage section. In this case, a diaphragm or a capillary duct can be used. It can also be adjustable and formed by a conventional type micro-leakage valve or a piezoelectric valve, the opening of which is caused by the deformation of a piezoelectric crystal under the action of an electric field. But, in all cases, the passage must prohibit any entry of ambient air and it must offer a passage section having a sufficiently small diameter (typically a few microns) so that the gas flow between the reactor 10 and the microsublimation tube maintained at low pressure or in molecular regime. We know that in this regime the free path of the gaseous molecules is greater than the transverse dimensions of the passage.

The microsublimator 14 will generally consist of a tube 40 of small diameter, one end of which is tightly connected to the passage 12 and the other end is connected, by means of the valve 16, to the ion source 18. This tube is provided with cryogenic temperature cooling means. These means are represented in FIG. 1 in the form of an enclosure 42 provided with an inlet and an outlet for fluid at very low temperature. Another solution is to place the tube in the circuit of a cryogenerator. To allow the temperature of the tube 40 to be adjusted, adjustable heating means are associated with it. In the case of Figure 1, these means are constituted by a heating resistor 44 wound around the tube 40 and supplied by an electric generator 46 of adjustable power. A temperature probe may be placed on the tube 40 to regulate, via a circuit similar to circuit 26, the temperature of the tube to an adjustable value. This temperature can also be slaved to a reference value by the intensity of the ion beams received at the collectors of the mass spectrometer.

To do this, a signal is taken from the ion current amplifier. This is constantly compared with a reference representing the chosen temperature, this reference being able to be programmed itself using a computer. A voltage is therefore obtained which is converted into calibrated pulses giving quantities of energy supplying the heating systems of the tube 40.

Each time the measurement signal decreases, a corresponding energy quantity pulse is applied to the heating, thereby regulating the quantity of product introduced to the source of the mass spectrometer. In practice, it will be necessary to bring the tube to a temperature which can vary between a few K and a few hundred K.

A valve 46, in parallel with the valve 16, makes it possible to connect the outlet of the tube 40 to a vacuum pump.

An additional connector provided with a valve 47 (or more) may be provided to connect the ion source to a reference gas supply and / or to another device similar to that which has just been described.

In the particular embodiment shown in FIG. 2, the elements 10, 12 and 14 of FIG. 1 are grouped together to form a one-piece assembly made up of several assembled parts, for example by welding.

In FIG. 2, where the members corresponding to those in FIG. 1 are designated by the same reference number, the reactor is delimited by two nozzles 48 and 50 and a cylindrical shell in the axis of which the tube 20 is placed, a few millimeters in internal diameter, forming the micro-oven. The downstream end of this tube is grounded via the end piece 50. The upstream end, isolated from ground by a pin 52, is connected to the electrical heating source by means of a tab 54 which crosses the vi.ro- tightly. The sample holder 28 comprises a head screwable into the end piece 48, the seal being ensured by a seal 58. A locking screw 60 provided in the head makes it possible to retain a thread or a needle 62 for supporting the dry sample. A channel 64 formed in the end piece 48 makes it possible to connect the tube 20 to a valve 30 for admitting the reagent (gaseous fluorinating agent in general) or to a vacuum pump.

In the end piece 50 is machined a seat 66 for receiving the micro-leakage valve (not re rêsentée ^p) constituting the passage-12 to the micro-enhancer 14. The latter has a very similar constitution to that of the reactor 10, except that the ferrule is provided with fittings 68 and 70 for inlet and outlet of cryogenic fluid. The micro-sublimation tube 40 is connected, via an insulating pin 72, to the end piece 50 and its downstream end is welded to an end piece 74 provided with a seat intended for the valve 46 (not shown) .

Finally, this nozzle comprises a tubular extension 76 intended to be connected to the valve 16.

As a simple example of implementation of the method according to the invention, the introduction of samples for the isotopic analysis of uranium will now be described.

The device used is of the type shown in FIG. 2.

The sample must first be transferred to the sample holder 28. On the wire 62, which for example is 0.8 mm in diameter and 7 cm long, is deposited, using a micro-pipette , a few drops of uranyl nitrate containing a total of for example 10 micrograms of uranium to be analyzed.

To transform this deposit into a solid phase, the wire is laid between two electrical contacts and a heating current is circulated by the Joule effect. The uranyl nitrate is transformed into a deposit of U0 _3, then U ₃₀₈ when the temperature exceeds 350 ^° C.

The wire covered with the deposit is placed in the sample holder 28 and the latter fixed to the end piece 48. The tube 20 is put under vacuum by pumping up to a pressure of the order of 10 _{_} ³ torr. And heating the tube 20, by passing current to a temperature of about 400 ^* C to remove the residual water vapor.

The temperature of the micro-sublimation tube 20 is then brought to that of liquid nitrogen by circulation of this nitrogen around the tube 40, from the connection 68 to the connection 70 by the circulation of a heat transfer gas (helium for example) brought to the temperature of liquid nitrogen.

We can then proceed to the first phase of implementation of the method according to the invention, constituted by the trapping of gaseous products in the micro-sublimator 14.

For this, a calibrated flow of very pure fluorine is sent into the tube 20, through the valves 38 and 30. The products of the reaction (UF ₆ and 0 ₂ ) escape at very low flow rate through the valve 12 and are trapped in the tube 40, distributing according to the solidification temperature.

Once trapping is complete, the valve 12 is closed. The pumping valve 46 is open, and one very gradually heats the tube 40 by passing an electric current. The oxygen sublimes and it is evacuated by the vacuum pump, through the valve 46.

When the temperature increases beyond the sublimation point of oxygen and reaches the sublimation point of hexafluoride, we can switch to the supply of the ion source.

For this, the valve 46 is closed and the valve 16 is opened. The heating is carried out with a temperature programming such that, as soon as the flow rate of hexafluoride reaches a predetermined value (that is to say when one reaches the set value of the intensity of the ion beam in the spectrometer), the temperature is controlled, so that the flow rate, measured by means not shown, remains constant until the mass of hexafluoride trapped uranium. In practice; with a flow rate of 10 ¹³ molecules per second, the depletion time is approximately 30 minutes when 10 micrograms of uranium have been deposited on wire 62.

All this time is available to make isotopic ratio measurements, which can then be compared to those of a standard uranium hexafluoride, admitted to the spectrometer through valve 47.

Another solution consists in using two devices of the kind shown in FIG. 2. One of them receives a wire carrying a deposit whose isotopic ratio is to be measured, the other a deposit of U 0 of known isotopic composition.

It can be seen that, whatever the embodiment used, both the high precision and the speed of measurement of a gas source and the small mass of sample in play of a thermionization spectrometer are obtained. .

This very low mass allows in particular the use of a device according to the invention for the analysis of irradiated fuels containing the isotopes of plutonium, the analysis then being carried out on PuF ₆ formed by fluorination of PuO ₂ .

The invention is not however limited to these particular embodiments. It is applicable whenever a reaction giving a gaseous compound of the sample is available. For example, the method is applicable in the case of carbon, which can be fluorinated to give CF ₄ , which is particularly interesting for the isotopic analysis C ₁₂ / C ₁₄ used in dating.

Claims (7)

1. Process for introducing gaseous micro-samples into the ionization source of a mass spectrum, process according to which the sample is transformed into a gaseous compound by heating in an atmosphere of a reactive gas, a flow of the compound and the reactive gas in molecular flow regime towards a wall (40) maintained at a temperature low enough to trap the gaseous compound and the reagent and the gaseous compound is selectively released by controlling the temperature of said wall. 2. Device for introducing micro-samples into the ionization source of a mass spectrometer, comprising a reactor (10) provided with means for introducing the micro-sample, means for heating the micro-sample, means for connecting to a vacuum source and means (38.40) for supplying an adjustable flow rate of reagent for transforming the micro-sample into gaseous compounds; a throttled and calibrated passage (12) for gas flow from the reactor; and a sublimation tube (40) connected on the one hand to the passage, on the other hand to the ion source of the spectrometer by the intermediate. of a valve (16), provided with means making it possible to bring it to an adjustable cryogenic temperature. 3; Device according to claim 2, characterized in that the reactor comprises a tube (20) intended to receive the micro-sample and provided with heating means programmed by Joule effect. 4. Device according to claim 2, characterized in that the sublimation tube is arranged in a chamber for the circulation of a cryogenic fluid and is provided with heating means regulating the output flow to the ion source. 5. Device according to claim 2, characterized in that the constricted and calibrated passage has dimensions such that the flow therein is molecular. 6. Device according to claim 5, characterized in that the passage consists of a diaphragm pierced with a calibrated orifice, a capillary or a micro-leak valve. 7. Device according to claim 2. characterized in that the reactor and the sublimation tube are provided with temperature probes connected to circuits for regulating the heating means.

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