EP0154590B1 - Time-of-flight mass spectrometer - Google Patents

Description

The present invention relates to a time-of-flight mass spectrometer.

In a time-of-flight mass spectrometer, the ions from a source are accelerated by an electric field and their mass is determined by measuring the time of flight of the ions until they arrive at a detector.

. In conventional direct time of flight spectrometers, the ions are emitted at one end of the spectrometer and are received, after direct flight, at the other end. With these spectrometers, it is possible to analyze in mass all the ions from the source, including molecular ions which decompose in flight, after acceleration, possibly giving rise to neutral species. However, the mass resolution of direct flight spectrometers can sometimes be insufficient.

It is well known to improve the mass resolution by lengthening the path of the ions by reflection by means of an ion mirror receiving the ions from the source and reflecting them towards the detector. The ion mirror is formed by a set of parallel grids, spaced from each other and establishing an electric field suitable for decelerating and reflecting ions. Before their reflection, the ions penetrate more or less deeply into the mirror according to their kinetic energy. By a suitable configuration of the mirror, it is thus possible to compensate for differences in speed between ions of the same mass to ensure that these ions arrive at the same time on the detector after reflection.

A spectrometer of this type is described in an article by R. Igersheim et al. published in "International Journal of Mass Spectrometry and Ion Physics, Vol. 20, No. 1, May 1976, pages 77-88, Elsevier Scientific Publishing Co., Amsterdam (NL). In this known spectrometer, the ions from the source located at one end of a flight tube are directed towards the mirror located at the other end of the tube, following an incident path which forms a small angle compared to normal to the mirror. The ions are reflected towards a detector located close to the source, on the other side of the axis of the tube with respect to it.

But if the use of a mirror brings advantages, it makes impossible the complete analysis of the metastable molecular ions which decompose during the flight by giving neutral species, these latter being obviously not reflected by the mirror.

It has been proposed to remedy this deficiency by using a first detector placed so as to receive the ions reflected by the ion mirror and a second detector placed behind the mirror so as to receive any neutral species. Figure 1 attached shows this configuration as disclosed in an article by H. Davigel et al. published in "International Journal of Mass Spectroscopy and Ions Physics", Vol. 52, Nos 2/3, September 1983, pages 223-240, Elsevier Science Publishers Amsterdam (NL). The mirror M is inclined at 45 ° on the path of the ions coming from the source S to reflect the ions towards a detector D1, in a direction perpendicular to that of emission, while the neutral species and the ions having a sufficient kinetic energy to pass through the mirror are received by a detector D2.

This known construction has drawbacks.

First, it is in practice impossible to use the mirror to compensate for differences in speed of ions of the same mass. In addition, the study of metastable ions would require a mirror capable of reflecting ions of very different masses, ranging from the mass of the non-decomposed ion to the masses of the ionic fractions originating from the decomposition in flight. It would therefore be necessary to have a relatively deep mirror and the reflected trajectories of the ions would be significantly spaced from each other, depending on the depth of penetration into the mirror. To be able to intercept all the reflected ions, it would then be necessary to have a detector D1 of large dimensions, that is to say difficult, even practically impossible to achieve.

The use of a shallow mirror to reflect ions whose kinetic energy lies within a fairly wide range means the establishment in the mirror of an intense electric field causing brutal reflection. The differences in residence time in the mirror are then minimal even for ions of significantly different kinetic energies. It follows that for metastable ions, the difference is very minimal between the time of flight of an undecomposed ion and that of an ionic fraction after decomposition in flight, the complete ion and the ionic fraction arriving on the mirror with the same speed. A precise analysis of metastable ions involving the measurement of this time-of-flight difference cannot therefore be carried out.

The object of the present invention is therefore to provide a time-of-flight spectrometer allowing precise and complete analysis of metastable molecular ions while retaining very good resolution in mass and with a relatively simple and inexpensive structure.

This object is achieved by means of a spectrometer of the type comprising an ion source to be analyzed, an ion mirror receiving ions from the source, a first detector arranged so as to receive the ions reflected by the mirror, and a second detector arranged behind the mirror, so as to be able to obtain both a spectrum of ions reflected by the mirror and received by the first detector and a spectrum of neutral species which may have appeared during the flight and received by the second detector, spectrometer, in which , in accordance with the invention, the source, the ion mirror, the first detector and the second detector are centered on the same axis of symmetry and arranged substantially perpendicular thereto, and the first detector is located between the source and the mirror and is in shape annular to leave a central passage for ions from the source.

The arrangement of the spectrometer elements along the same axis allows a compact realization. In addition, it is possible to give the mirror the desired depth without resulting in a dispersion of the trajectories of the reflected ions as a function of their masses. Consequently, there is no practical obstacle to producing the mirror so as to compensate for variations in the speed of ions of the same mass. As will be seen below, it then becomes possible to carry out a precise study of metastable ions by correlation between the "reflex" spectrum developed from the signals of the first detector and the "neutral" spectrum developed from the signals of the second detector.

The source of ions to be analyzed is for example formed by a solid surface bombarded by particles to release from this surface the ions whose mass analysis is to be carried out. The bombardment can be carried out using primary ions produced by a radioactive source 252Cf, heavy ions accelerated by cyclotron, ions of a few KeV of neutral atoms, or even laser radiation.

• la figure 1, déjà décrite illustre une configuration d'un spectromètre à temps de vol de l'art antérieur;
• la figure 2 est une vue schématique en coupe d',un mode de réalisation d'un spectromètre selon l'invention;
• les figures 3a à 3c illustrent de façon très schématique les trajectoires d'ions issus de la source et les spectres correspondants obtenus;
• la figure 4 est un organigramme d'opérations effectuées pour l'aquisition de données nécessaires à l'élaboration de spectres "neutres", "reflex" et "corrélés", et
• les figures 5a à 5f illustrent des spectres "neutre", "réflex" et "corrélés" obtenus avec une source d'ions particulière.

Other features and advantages of the spectrometer according to 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, already described, illustrates a configuration of a time-of-flight spectrometer of the prior art;
• Figure 2 is a schematic sectional view of, an embodiment of a spectrometer according to the invention;
• Figures 3a to 3c very schematically illustrate the trajectories of ions from the source and the corresponding spectra obtained;
• FIG. 4 is a flowchart of operations carried out for the acquisition of data necessary for the development of "neutral", "reflex" and "correlated" spectra, and
• Figures 5a to 5f illustrate "neutral", "reflex" and "correlated" spectra obtained with a particular ion source.

In FIG. 2, the reference 10 designates a source of molecular ions whose mass analysis is to be carried out. The source 10 is formed by a metal surface 10a on which molecules are deposited.

A source 11 of primary ions is placed at equal distance between the source 10 of secondary ions and a detection device 12 intended to supply the starting signal.

In the example illustrated, the source 11 is a radioactive source of 252Cf. California 252 is a radioactive isotope that decays by emitting two fission fragments in opposite directions.

One of the fragments emitted towards the rear of the spectrometer reaches a metal sheet 12a of the detection device 12 and ejects electrons from it. An electric field is established between the sheet 12a and an electrode 12b to accelerate the ejected electrons backwards. These are received by a detector 12c located at the rear of the spectrometer and which supplies an electrical pulse SO constituting the starting signal.

The other forward fission fragment releases secondary ions from the metal surface 10a by desorption. The secondary ions released are accelerated by an electric field established between the metal surface 10a and an electrode 13 which can, for example, be brought to respective potentials of 10 to 20 kV and 0 kV.

An ion mirror 14 receives the secondary ions emitted and reflects them towards a detector 15.

The mirror 14 is located near the front end of the spectrometer. It comprises a first region delimited by two grids 14a and 14b which are thin and parallel to each other; this first region constitutes a region of deceleration of the ions received, when an electric retarding field is established between the gates 14a and 14b. The mirror then comprises a reflection region delimited by the gate 14b and a gate 14c between which a delay field is also established. For example, the potentials of the gates 14a, 14b and 14c can be respectively equal to 0, 2/3 U and U with U = ± 8 kV or ± 10 kV. Annular electrodes 14d at 14h are arranged at regular intervals between the grids 14b and 14c. The potentials of these electrodes are chosen so as to impose a uniform variation of the potential between the grids 14b and 14c and thus give the mirror the desired properties. In particular, as is known per se, the mirror 14 is designed so as to compensate for speed differences between ions of the same mass so that they arrive at the same time on the detector 15. This compensation results from the fact that at equal masses, the fastest ions penetrate deeper into the mirror before reversing their direction of movement.

The detector 15 is of annular shape and is arranged on the rear side of the spectrometer, but in front of the acceleration space between the surface 10a and the electrode 13. It allows the passage in its center of the secondary ions emitted by the source 10 and coming from this acceleration space. The arrival of ions reflected on the detector causes the emission of a pulse S1 constituting an arrival signal.

A second detector 16 is placed at the front end of the spectrometer, behind the ion mirror 14, in order to receive the species having passed through the mirror without being reflected and to provide, in response, an arrival signal S2.

When the mirror 14 is activated, the species arriving at the detector 16 are the neutral species which appeared as a result of the decomposition during the flight of metastable molecular ions, the non-decomposed ions and the ionic decomposition fractions being reflected by the mirror and received by the detector 15.

When the mirror 14 is not activated, operation of the spectrometer in a conventional manner (direct flight without reflection) is possible. It may for example be advantageous to compare the results obtained, on the one hand, in the form of a "reflex" spectrum of ions and a direct spectrum of neutral species, when the mirror 14 is activated, and of on the other hand, in the form of a direct spectrum of ions and neutral species, when the mirror 14 is not activated.

According to a characteristic of the invention, the assembly constituted by the source 10 of secondary ions, the ion mirror 14, the first detector 15 and the second detector 16 is of axial symmetry with respect to the optical axis of the ions. There is no deflection or return of the ions at a different angle from that of the direct path. The size of the assembly is therefore relatively small, the various constituent elements listed above being housed in a straight tube 17 connected to a vacuum source (not shown).

The mass spectrum "reflex" is worked out from the signals SO, S1 while the mass spectrum of the neutrals is worked out in a similar way from the signals SO, S2.

For the development of the "reflex" mass spectrum, a time-digital converter 18 is connected to the detectors 12 and 15. The converter is triggered in response to the signal S0. Each time an ion arriving at the detector 15 causes the emission of a signal S1, the converter 18 provides digital information representing the time elapsed since its triggering, that is to say the time of flight of the ions. To make the converter 18, the circuit can be used for example, the principle of which is described by E. Festa and R. Sellem in the publication "Nuclear Instruments and Methods" No 188 (1981) p. 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 supplies, in response to each signal d 'stop a digital word representing the time elapsed since the reception of the starting signal. The digital information thus provided after each desorption is recorded in a memory circuit of a processing device 20 to be accumulated with that obtained in response to the other desorptions in order to develop a mass spectrum by plotting the flight time and on the ordinate, the number of events counted during successive disorptions. The mass spectrum has peaks each indicating a repetition of identical flight times, that is to say a repetition of reception of ions of the same mass corresponding to the abscissa of the peak.

A second time-to-digital converter 19 is connected to detectors 12 and 16 to provide the mass spectrum of the neutrals. The elaboration of the mass spectra, as briefly presented above, is carried out by means of the processing circuit 20 in the form of a microprocessor circuit. In short, the digital information supplied by the converter 18 constitutes write addresses in a “reflex” spectrum memory MSR where the events detected by the detector 15 are accumulated. After an analysis time determined by the operator, the content of the MSR memory is read to develop graphical information making it possible to display the spectrum, "reflex" on a cathode ray tube screen 22. Likewise, the digital information supplied by the converter 19 constitute addresses of writing into a spectrum memory of neutral MSNs where the events detected by the detector 16 are cumulated. At the end of the analysis time, the content of the memory MSN is read in order to produce graphical information making it possible to display the spectrum of neutral on the screen of the tube 22. Writing and reading in the memory MSR or MSN, the elaboration of the graphic information and the control of the display on the screen of the tube 22 are controlled by a circuit 21 in a manner well known per se which it is not useful to detail here.

Although consideration has been given to the use of ²⁵² Cf fission fragments for the desorption of secondary ions, this can also be obtained with laser radiation directed onto the surface 10a or by ions with an energy of 10 at 100 KeV monoloaded or multicharged with, in the latter case, a state of charge which can be high (for example up to 30 ⁺ ). Neutral atoms can also be used for desorption by impact on the surface 10a. Finally, ions with an energy of several MeV (for example up to 100 MeV or more), such as those delivered by a particle accelerator (tandem cyclotron, etc.) can also create the desorption of secondary ions.

The spectrometer according to the invention is particularly advantageous in that it allows, with a simple structure, to combine a high resolution in mass, thanks to reflection by the ionic mirror, with the possibility of detection of neutrals which, in certain cases, contribute significantly to the molecular "peak" of the spectrum obtained. As an indication, in use with reflection, a mass resolution of about 2500 can be obtained, while in use with direct flight, this mass resolution reaches only about 600.

The use of the spectrometer for the study of metastable molecular ions will now be described in detail.

FIG. 3a illustrates the trajectory of a metastable molecular ion m ⁺ between the source 10 and the detector 15, it being assumed that the ion does not decompose during flight. The ion m ^* is accelerated to a speed v and enters the mirror to a depth d where a potential Um reigns, this depth d being a function of the kinetic energy of the ion m '. FIG. 3a also shows the contribution of the m ⁺ ions to the reflex spectrum in the form of a line at the time of flight tm ^+.

In the case of FIG. 3b, it is assumed that the metastable ion m ⁺ is decomposed practically at the time of the passage of the primary ion and a extremely brief moment after. To simplify, we also assume that the decomposition gives rise to an ionic fraction m ¹⁺ and to a neutral fraction m0 (m ⁺ → m1 ⁺ + m0). The ion m1 ⁺ is accelerated to a speed V and penetrates the mirror to the depth d.

FIG. 3b also shows the contribution of the ionic fractions m1 ⁺ in the reflex spectrum in the form of a line at the flight time tm1 ⁺ in advance with respect to tm ⁺ .

In the case of FIG. 3c, it is assumed that the decomposition of the metastable ion m ⁺ occurs after the exit from the acceleration space. The ionic fractions mls ⁺ and neutral m0 keep the speed v. The neutral fraction will then reach the detector 16 after a flight time tm0 which corresponds to the flight time tm ⁺ of the non-decomposed metastable ion. The ion fraction m1s ⁺ is reflected by the mirror 14 but its residence time there is less than that of the ion m ⁺ because if their speed is the same, their energies are different. The ion m1s ⁺ penetrates to a depth d1s where a potential Um1s reigns. The mls ⁺ ion fraction then reaches the detector after a flight time tm1s ⁺ between tm1 ⁺ and tm ⁺ . FIG. 3c shows the contribution of the neutral fractions m0 in the form of a line at the flight time tm0 (corresponding to tm ⁺ ) in the spectrum of neutrals and the contribution of the ionic fractions m1s + in the form of a peak at the flight time tm1s ⁺ (between tm ¹ and tm ⁺ ) in the reflex spectrum.

m étant la masse de l'ion m⁺ et K un coefficient qui est déterminé par calibration en utilisant un ion moléculaire métastable dont la réaction de décomposition est bien connue. La valeur de dt est déterminée d'après le spectre "reflex" en mesurant la différence entre les axes des pics aux temps tm⁺ et tm1s⁺. La décomposition de l'ion métastable en vol s'accompagne d'une modification plus ou moins sensible de la trajectoire et de la vitesse des fractions par rapport à la trajectoire et à la vitesse initiale de l'ion; il en résulte, sur le spectre reflex, un élargissement du pic de la fraction ionique par rapport aux pics des ions non décomposés. Aussi, pour obtenir un résultat avec une précision suffisamment élevée, il est important que les deux pics aux temps tm⁺ et tm1s⁺ soient bien distincts l'un de l'autre, donc que la différence entre les temps de séjour dans le miroir soit significative. Il ne peut pas en être ainsi dans le cas d'un miroir ayant très peu de profondeur et présentant un champ électrique d'une grande intensité réfléchissant brutalement et de manière sensiblement uniforme des ions dont les masses sont dans une plage assez étendue.It is important to note that the difference between the flight times tm ⁺ and tm1s ⁺ comes from the difference dt between the residence times in the mirror 14. The mass m1s of the ionic fraction m1s ⁺ is deduced from the measurement of the difference dt. We have indeed:

m being the mass of the ion m ⁺ and K a coefficient which is determined by calibration using a metastable molecular ion whose decomposition reaction is well known. The value of dt is determined from the "reflex" spectrum by measuring the difference between the axes of the peaks at times tm ⁺ and tm1s ⁺ . The decomposition of the metastable ion in flight is accompanied by a more or less noticeable change in the trajectory and speed of the fractions with respect to the trajectory and the initial speed of the ion; this results in a widening of the peak of the ion fraction on the reflex spectrum with respect to the peaks of the non-decomposed ions. Also, to obtain a result with sufficiently high precision, it is important that the two peaks at times tm ⁺ and tm1s ⁺ are clearly distinct from each other, so that the difference between the residence times in the mirror is significant. This cannot be the case in the case of a mirror having very little depth and having an electric field of great intensity suddenly and substantially uniformly reflecting ions whose masses are within a fairly wide range.

The peaks produced in the "reflex" spectrum by ionic fractions originating from the in-flight decomposition of metastable molecular ions can be relatively weak compared to the peaks produced by non-decomposed desorbed ions in flight.

According to a feature of the invention, enhancement of these peaks is carried out by analysis of coincident information. If we refer to Figure 3c, we see that the spectra of neutrals and "reflex" are correlated. Indeed, assuming 100% efficiency in the detection and transmission of the information detected, at each event counted in the neutral spectrum (reception of a neutral fraction) corresponds to at least one event in the "reflex" spectrum ( reception of at least one ionic fraction complementary to the neutral fraction). When a peak appears in the spectrum of neutrals at time tmO, a reflex spectrum correlated with the mass mO is produced by retaining the events detected by means of the detector 15 only in so far as an event is detected by the detector 16 in a time window centered on overm. In this way a relative reinforcement is produced in the correlated reflex spectrum of the peaks of complementary ionic fractions of the neutral fraction m0 since the events which do not coincide with the detection of a neutral fraction are not taken into account.

The development of correlated spectra is carried out in practice as follows.

A spectrum of neutrals is first developed in order to allow the operator to visualize the peaks of neutral fractions and to predetermine time windows centered on each peak axis, for example a window (t1m, t1M) for a first peak. , a window (t2m, t2M) for a second peak, and so on. The predetermined limit values are therefore saved.

The processing circuit 20, in addition to the memories MSR and MSN comprises memories MSR1, MSR2, ... intended for recording the information necessary for the development of the correlated spectra.

This development is carried out under the control of circuit 21 by implementing a program whose flow diagram is represented in FIG. 4. It is assumed that two time windows (t1m, t1M) and (t2m, t2M) have been predetermined by the operator.

• lecture et enregistrement des informations numériques tvR fournies par le convertisseur 18 (temps de vol "reflex") en réponse à chaque signal de départ S0,
• écriture dans la mémoire MSR aux adresses définies par les informations tvR enregistrées,
• lecture et enregistrement des informations numérique tvN fournies par le convertisseur 19 (temps de vol des neutres),
• écriture dans la mémoire MSN aux adresses définies par les informations tvN enregistrées (les opérations de lecture, enregistrement et écriture concernant les temps de vol reflex peuvent être conduites en parallèle avec elles concernant les temps de vol des neutres),
• recherche si une fraction neutre est reçue pendant la première fenêtre de temps prédéterminée, par réalisation d'un test t1 m < tvN < t1 M; si ce test est positif, écriture dans la mémoire MSR1 aux adresses définies par les informations tvR enregistrées,
• recherche si une fraction neutre est reçue pendant la deuxième fenêtre de temps prédéterminée, par réalisation d'un test t2m < tvN < t2M; si ce test est positif, écriture dans la mémoire MSR2 aux adresses définies par les informations tvR enregistrées
• si la fin d'analyse n'est pas demandee, retour en attente de la réception d'un nouveau signal de départ,
• si la fin d'analyse est demandée, retour au programme principal, par exemple pour exécuter une demande d'affichage d'un spectre par conversion sous forme graphique des informations enregistrées dans l'une des mémoires MSR, MSN, MSR1, et MSR2.

From the start of the analysis, the following operations are performed:

• reading and recording of the digital information tvR supplied by the converter 18 (flight time "reflex") in response to each start signal S0,
• writing to the MSR memory at the addresses defined by the recorded tvR information,
• reading and recording of digital tvN information provided by converter 19 (flight time of neutrals),
• writing to MSN memory at the addresses defined by the recorded tvN information (read, record and write operations concerning the reflex flight times can be conducted in parallel with them concerning the neutral flight times),
• research if a neutral fraction is received during the first predetermined time window, by carrying out a test t1 m <tvN <t1 M; if this test is positive, write to the memory MSR1 at the addresses defined by the recorded tvR information,
• research if a neutral fraction is received during the second predetermined time window, by carrying out a test t2m <tvN <t2M; if this test is positive, write to MSR2 memory at the addresses defined by the recorded tvR information
• if the end of analysis is not requested, return awaiting receipt of a new start signal,
• if the end of analysis is requested, return to the main program, for example to execute a request to display a spectrum by converting the information recorded in one of the memories MSR, MSN, MSR1 and MSR2 into a graphical form.

FIGS. 5a, 5b and 5c respectively illustrate a spectrum of neutrals, a complete reflex spectrum and a correlated reflex spectrum obtained by analysis of the organic compound adenosine.

The spectrum of neutrals shows two peaks at times corresponding to masses 136 and 268. The full reflex spectrum also shows two peaks at times corresponding to masses 136 and 268. We thus find in the spectrum of neutrals and in the reflex spectrum contributions of ions 136 and 268 according to whether or not they decomposed during the flight. The peak at time tm corresponding to mass 268 is not visible in FIG. 5b, the time scale being different from that used in FIG. 5a.

The reflex spectrum also has a weak peak widened at time tm1 s. This peak is much more visible in FIG. 5c which represents the reflex spectrum correlated with the mass 268 of the spectrum of neutrals. The enhancement of the ion fraction peak by correlation is particularly clear. It is also noted, as has already been indicated, that the peak of ion fraction is much more spread out in time than the peaks of non-decomposed ions, this being due to the dispersion of speed and trajectory following the decomposition. Measuring the difference between the abscissa tm1s and that abscess tm of the mass 268 makes it possible to determine the mass ml of the ionic fraction. In this example, the decomposition is of the following form: 268 ⁺ → (B + 2H ^* ) + neutral and the mass of ionic fraction is equal to 136.

The spectrum of neutrals in FIG. 5a also shows a peak for the mass 136. FIGS. 5d and 5e show corresponding parts of the normal reflex spectrum and of the "reflex" spectrum correlated with the mass 136. The latter highlights peaks widened at times tm2s, tm3s and tm4s, corresponding to decompositions of the 136 ^* ion into 18 ^* + neutral, 94 ^* + neutral and 119 ⁺ neutral, respectively.

• lecture du contenu N1 de la mémoire MSR à une première adresse,
• lecture du contenu n1 de la mémoire MSR1 à la même adresse,
  • calcul de n'1 = n1 - kN1,
• écriture de n'1 à une première adresse d'une mémoire MSR'1 (non représentée), et
• passage à l'adresse suivante jusqu'à lecture complète des mémoires MSR et MSR1.

An improvement in the enhancement of the peaks of ionic fractions is still possible by eliminating from the correlated "reflex" spectrum of FIG. 5e the events which do not result from decompositions in flight. This is obtained by subtracting from the correlated "reflex" spectrum a fraction of the full "reflex" peak, this fraction being determined by the operator so as to eliminate a clearly recognizable peak which we know is not due to a fraction ionic from decomposition. In the example illustrated, one can base oneself for example on the peak corresponding to the mass 136. The operator determines the ordinate N of this peak on the spectrum "reflex", normal, the ordinate n of the corresponding peak on the correlated reflex spectrum to predetermine a ratio k = n / N. A corrected correlated spectrum of events not due to in-flight decompositions is then developed under the control of circuit 21 by implementing a program comprising the following operations:

• reading of the content N1 of the memory MSR at a first address,
• reading of the content n1 of the memory MSR1 at the same address,
  • calculation of n'1 = n1 - kN1,
• writing of n'1 to a first address of a memory MSR'1 (not shown), and
• go to the next address until MSR and MSR1 memories are read completely.

The information contained in the memory MSR'1, which is a linear combination of the information contained in the memories MSR and MSR1, is read to be converted into graphical form in order to display on the screen the corrected correlated spectrum.

Claims (3)

1. Time-of-flight mass spectrometer comprising a source of ions, to be analyzed (10), an ion mirror (14) receiving ions issued from the source, a first detector (15) disposed so as to receive the ions reflected by the mirror, and a second detector (16) disposed behind the mirror, whereby a spectrum of the ions reflected by the mirror and received by the first detector can be obtained, as well as a spectrum of any neutral species appearing during flight and received by the second detector, characterized in that the first detector (15) is situated between the ion source (10) and the mirror (14) and is of annular shape to form a central passageway for the ions issued from the source (10), and characterized in that the source (10), the ion mirror (14), the first detector (15) and the second detector (16) are centered on the same axis of symmetry and disposed substantially perpendicularly thereto.

2. Spectrometer according to claim 1, characterized in that it further comprises: first and second time-digital conversion means (18, 29) to convert to a digital form the time of flight between the source (10), and, respectively, the first and the second detector (15, 16); first and second storage means (MSR, MSN) for recording the number of events detected respectively by the first and the second detector as a function of the time of flight, in order to work out a spectrum of the ions reflected by the mirror and received by the first detector, and a spectrum of any neutral species which have appeared during the flight; selection means for selecting at least one time window defined by minimum and maximum predetermined time values; supplemental storage means (MSR1, MSR2,...); and correlation means acting in response to the detection of a neutral by the second detector (16) after a time of flight, comprised in a selected time window, to authorize the recording, in a respective supplemental storage means, of the number of events detected by the first detector as a function of the time of flight.

3. Spectrometer according to claim 2, characterized in that means are provided for calculating and recording information obtained by linear combination of the contents of the first storage means (MSR) and of the contents of a supplemental storage means (MSR1, MSR2,...).

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