MULTIPLE TARGETS FOR PRECISE MASS MEASUREMENT

Abstract: The invention is directed to multiple targets used in fast atom bombardment (FAB) mass spectrometry, fast ion bombardment, second ion mass spectrometry, plasma desorption mass spectrometry and similar techniques to facilitate precise mass measurements, increasing resolution.

Description: MULTIPLE TARGETS FOR PRECISE MASS MEASUREMENT The present invention relates to mass spectrometry; in particular fast atom bombardment (FAB) mass spectrometry, fast ion bombardment, secondary ion mass spectrometry (SIMS), plasma desorption mass spectrometry and similar techniques hereinbelow referred to as "FAB". Precise mass measurement is of prime importance in mass spectrometry. ' The usual way to do it is by "peak matching" or by "computerized scanning methods". Both methods, well known to the skilled technician in the mass spectrometry art, require that the sample (for which the precise mass is desired) and a reference compound (with peaks for which accurate masses are known) should produce ions at the same time in the mass spectrometer. Furthermore, high resolution of the mass spectrometer is also required. Oftentimes 10,000 resolution gives satisfactory results, but sometimes higher resolution is necessary. The Kratos MS-50 mass spectrometer, which is one of the commercially available instruments with highest resolution (not counting Fourier transform ones) has an ultimate resolution specified as 150,000 in "low field mode" (up to about 1200 dalton) and 50,000 in "high field mode". A first problem in fast atom bombardment (FAB) is that using conventional targets the resolution is significantly smaller than the value specified for the instrument (using electron impact). This limits the application of mass spectrometry in FAB, especially for precise mass measurements. A further and more complicated problem is that in"FAB" the sample (often in a liquid matrix) is loaded onto a solid support (target), and this can provoke many difficulties mainly caused by interactions in the liquid phase. In fact, when the sample, the reference compound and the matrix, if present, are mixed, they may, and very often do, interfere with each other, hindering the analysis. This drawback could be circumvented by using a split probe, where the target is cut in the middle,' thus maintaining the two sides separated, one side for the sample and one side for the reference compound. However, this solution is not useful because deposition of a liquid sample on a cut target presents practical problems and above all because electromagnetic fields in the mass spectrometry ion source could limit the resolution and sensitivity and affect the two sides in different ways. We ' found that using two targets close to each other having a combination of particular dimensions of the area which originate ions and a particular position in the mass spectrometer, instead of the conventionally single larger target (which has about 2 mm diameter or larger) the resolution in FAB can be significantly improved, and the effect of electromagnetic fields on the two targets may become similar. For example, the ultimate resolution of Kratos MS-50 instrument in "high field mode", FAB, 8 kV accelerating voltage, observing the m/Z 912 peak in Csl was, at a particular occasion, 22,000 using a "conventional" 2.5 mm diameter probe, whereas 73,000 using a 1.0 mm diameter probe according to the present invention (specified resolution in electron impact: 50,000). In other occasions similar differences were found. Therefore, an object of the present invention comprises a twin target for mass spectrometry characterized in that the line connecting the two target areas, from which the ions originate, is parallel with the source slit of the mass spectrometer, said areas being limited in the direction perpendicular to the source slit in a range comprised between about 0.3 mm and 2 mm, and their use thereof. In fact, we found that when the line connecting the two target areas is parallel with the source slit (i.e. it is parallel with the non- focussing or z direction of a sector field mass spectrometer) ions originating from the two targets respond similarly to the effects of the tuning controls and the optimum settings of these controls are similar: therefore it is possible to obtain an instrument setting which gives good sensitivity and resolution for ions originating from both sides. On the contrary, when the line connecting the two target areas is perpendicular to the source slit, the tuning controls affect ions originating from the two target areas in a different (and often opposite) manner and it is not possible to obtain reasonable sensitivity and/or resolution for both targets simultaneously. For example, if sensitivity is tuned for one target, the sensitivity may be reduced on the other target by 1000 times or more, and the peak shape and resolution is also much worse. Furthermore, according to the present invention in order to have high resolution it is necessary also to keep the dimension of each target in the direction perpendicular to the source slit as small as possible, for example ranging from about 0.3 mm to 2 mm, preferably between 0.5 to 1.3 mm. In the invention, regarding each individual target, the most important c thing is not restricting the area of the target but restricting its dimension in the direction perpendicular to the source slit. As used therein the term "source slit" means a slit in which the cross section dimensions can be different. It is clear that 0 when it is stated "in a direction parallel" or "perpendicular" to the source slit said direction concerns the longest dimension of the slit. The shape of the target areas in twin targets is not very critical and they can be optimized similarly 5 to the single conventional targets. Larger target areas give better sensitivity, smaller ones better resolution. Direction of the targets is important, especially in high resolution, as discussed above. The distance of the target areas from the source o slit of the mass spectrometer is an important parameter in FAB spectrometry independently if single or multiple targets are used. If the design of the mass spectrometer permits optimization of this distance easily (e.g. Kratos MS-50 instrument) the height of the targets is not important for carrying out the present invention. On the other hand, when the distance between the target area and the source slit is not possible or difficult to optimize (e.g. the case of Vacuum Generators ZAB mass spectrometer), the 0 support has to be designed in a way to provide the optimum distance of the target area (where the ions originate) from tKe source slit of the mass spectrometer. It is important to emphasize that the particular combination of the targets orientation and of the dimension of target areas which we found permits to solve the problem of high resolution precise mass measurement in FAB spectrometry whereas the problems of dimension and orientation were less important or non-existant at low resolution. If necessary more than two target areas can also be used to have, for example, more than one reference compound in the experiment. There can be other applications, too: for example when a reference compound is used as a standard for quantitative measurements. An example of using three targets is when on one the sample, on the other a reference compound for mass determination, on the third a standard for quantification is put. According to the present invention it is also possible to use multiple targets having the properties described above. The use of multiple targets can solve a different though probably less important problem i.e. how to test the alignment of a fast atom (particle) gun. At present a possible way to solve this problem is to place a small piece of DV sensitive paper on the target area, switch on the equipment and then check the position of the burn or discoloration of the paper. It is a simple and useful check, but it does not give quantitative results, it may be difficult to position the paper properly and not every laboratory has DV paper at hand. Using the multiple targets of the invention we can test the alignment of the FAB or modify the probe position to offset slight misalignments. This can be conveniently accomplished by a probe containing several targets, by one in which the position of the target area can be changed in the plane perpendicular to the probe shaft or by a set of probes having the target area in various off-center positions. Using either approach, abundance of ions originating from various spatial positions can be determined quantitatively, and therefore the alignment of the fast particle beam in FAB can be checked. Use of small targets are expedient to increase spatial resolution. Till now the optimization of target position could be routinely done only in the direction of the probe shaft, while it was not possible in the plane perpendicular to it on most of the commercial instruments. Drawings and characteristics of various multiple targets are given below, together with some applications. Examples, rather than precise design features are given, mainly because these multiple targets have to be adopted to the mass spectrometer in which they are used, and these may vary substantially. Also, the design may vary upon the application, and the optimum will be different for various purposes. However, based on these concepts it will be easy to modify or to design a multiple target for the required job. Most of the present examples will be related to Kratos mass spectrometers and to FAB, though they can be easily applicable to other mass spectrometers and to other techniques (e.g. SIMS, liquid SIMS, plasma desorption, etc.) too. Any material suitable for a "conventional" (single) target can be used for manufacturing the multiple targets of the invention (e.g. copper, stainless steel, etc.). In the drawings: 5 FIGURE 1 - shows a basement with the twin targets of the invention. FIGURE 2 - shows the same of Figure 1 but 90° translated. 10 FIGURE 3 - shows the top view of the targets reported in figures 1 and 2 FIGURE 4 - shows a target with a screw thread to, be inserted into the basement. FIGURE 5 - shows two target areas of different 15 sizes FIGURE 6 - shows four targets symmetrically placed. 20 25 30 I. 35 DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 is a diagramatic representation of the probe of the invention (applied to a Kratos MS-50 instrument) which comprises a single piece formed by a circular basement( ), and two targets(2), placed close to each other. The length of each single target is not important for carrying out the present invention even if it can be conveniently about 4 mm, and the distance between said two targets is preferably not larger than 1 mm. The most important element of the targets is the area, (3), where the ions originate. A preferred dimension in the direction perpendicular to the source slit is about 1 mm while the other dimension can be tailored to anybody's need. In Figure 2 is shown the same piece described in Figure 1 but turned by 90° degrees. It can be noted how the target is bevelled in a similar way of a conventionally used target. The Figure 3 shows the piece described above seen from the top. It can be noted that it has to be positioned parallel to the source slit^). In such a case very high resolution can be obtained on both target areas simultaneously (using the same instrument tuning). A convenient way of tuning is to put Csl on one target and Nal on the other target, focus the m/z 912 peak of Csl on the oscilloscope, set the decade box to show the m/z 922 peak of Nal in high mass and tune the instrument while alternating between the two peak . The Figure 4 shows a further preferred embodiment of the invention which comprises a "base" (T)into which two small rods, the actual targets, are placed (either requiring precise mechanical fit, using a screw thread (s)oτ by other means). In a further preferred embodiment of the invention, as shown in Figure 5 the two target areas, (β), and,^), have different size. It is advantageous to use them when one of the two compounds to be studied (usually the sample) has a significantly lower sensitivity than the other. By placing the less sensitive compound on the larger side (lj compensates for this effect. Note that in order to keep high resolution, the dimension in the direction perpendicular to €he source slit is kept small, preferably about 1 mm. In this way, it is possible to have a tuning good for both sides simultaneously. This figure 5 also illustrates that circular cross section of the target is not an important element. As stated above figure 6 shows four targets symmetrically placed. The distance between the external points of a couple of targets (A and B) is preferably about 3 mm while the empty space between each target is preferably 0.6 mm (β They may be used if more than two compounds are to be studied simultaneously or for measuring the angular distribution of ions produced by the particle beam (i.e. alignment of the particle gun). For studying the particle beam distribution several probes with several targets having different positions can be used. A more general version consists of a base with holes (perhaps with threads) into which small targets (e.g. rods) can be placed. The number of targets, just as the size of the targets may vary depending on the application. For testing a particle gun it may be convenient to use only one target with a reference compound on it and move the target (rod) from one position (hole) to an other position in the base. The important thing is, that the target position can be easily varied in the place perpendicular to the probe shaft. This version, with the holes properly placed, can be used to offset slight misalignment of the particle beam.

Claims: CLAIMS 1. Twin or multiple targets for mass spectrometry characterized in that the line connecting the two target areas, from which the ions originate, is parallel with the source slit, said areas being limited in the direction perpendicular to the source slit in a range comprised between about 0.3 mm and 2 mm. 2. Twin or multiple targets for mass spectrometry according to claim 1 wherein the direction perpendicular to the source of the targets is comprised between 0.5 and 1.3 mm. 3. Twin or multiple targets as in claims 1 and 2 for fast atom bombardment, fast ion bombardment, secondary ion mass spectrometry and plasma desorption mass spectrometry. 4. Use of twin or multiple targets according to claims 1 and 2 in fast atom bombardment, secondary ion bombardment and plasma desorption spectrometry for improving high resolution in precise mass mea¬ surements. 5. Use of twin or. multiple targets according to claims 1, 2 and 3 for testing the alignment of the particle gun in fast atom bombardment, fast ion bombardment, secondary ion mass spectrometry and plasma desorption mass spectrometry.

Inventors: VEKEY KAROLY, VEKEY, KAROLY
Applicants: LEPETIT SPA, GRUPPO LEPETIT S.P.A.
Application year: 1990
Main classification: H01J49
IPC codes: H01J49