Radiant energy – Inspection of solids or liquids by charged particles – Methods
Reexamination Certificate
2000-11-07
2002-09-03
Anderson, Bruce (Department: 2881)
Radiant energy
Inspection of solids or liquids by charged particles
Methods
C250S281000, C250S288000, C250S307000, C250S309000, C250S42300F
Reexamination Certificate
active
06444980
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to apparatus for the production and extraction of charged particles, particularly such apparatus wherein the emission of charged particles is stimulated by light irradiation.
BACKGROUND OF THE INVENTION
The emission of charged particles stimulated by light irradiation is a fundamental physical process used in many modern analytical techniques. One of the main requirements of a system using such a process is to combine the most efficient generation and transmission to the desired destination of the emitted charged particles with the most effective use of the light irradiation.
Light Stimulated Charged Particle Emission.
A fundamental physical process first noted when electrons were seen to be emitted when irradiated by a light source, light stimulated particle emission forms the basis of many materials analysis techniques. Nowadays the full range of the electromagnetic spectrum may be utilised and the charged particle may be an electron, ,positron, anion or cation.
Charged Particle Extraction.
For the most efficient extraction of charged particles in an electric field, it is generally accepted that the closer the initial trajectories are to being parallel with the axis of extraction, the higher the efficiency. In this case the efficiency refers to the number of charged particles that are transferred from the region of their emission to their destination with the desired parameters optimised. For example, the parameter may be energy or time dispersion and the optimisation is to minimise these parameters. One solution to optimising these parameters is to exclude those that do not satisfy the requirements. This is often achieved by physically preventing them being detected. This reduces the number of charged particles analysed and sensitivity problems may arise if the source is of low emissivity. A compromise must then be made between sufficient sensitivity and the optimisation of the required parameters to make the analysis meaningful.
An important factor in the efficient extraction of charged particles is the coincidence or near coincidence of the axes of emission and extraction.
MALDI Analysis
Presently such a system is widely, although not exclusively, used in the Matrix Assisted Laser Desorption and Ionisation (MALDI) technique used for the analysis of biological, biochemical and polymeric materials as described in Protein & Polymer Analyses up to M/Z 100,000 by Laser Ionisation Time-of-Flight Mass Spectrometry, K. Tanaka et al. Rapid Comm. Mass Spectrom. Vol.2, pp 151-153, 1988. Key to this technique are the desorption and ionisation processes which allow intact large molecules to be extracted from the sample, a so-called “soft ionisation” technique. Typically, a substance called the matrix, which is in solution, is combined with the substance to be analysed, the analyte, also in solution, on a sample stub or slide. This combination is allowed to dry and then placed inside an evacuated chamber. Emission and ionisation of intact large molecules is then stimulated by the use of a laser. The ionised molecules produced are then accelerated away from the sample by an electric field and into an analyser. The use of a pulsed laser allows a relatively simple and low cost Time-of-Flight (ToF) mass analyser to be used for obtaining information from the sample, e.g. identifying the molecular weight.
In MALDI, the matrix is chosen for its good absorption of energy from the laser and, additionally, through a photon and/or chemical ionisation process, provides a mechanism that produces a quantity of ions from the analyte for analysis. To obtain the best results from the wide range of analytes which can be analysed by the MALDI technique many different matrices are used, each offering some different characteristics which are dependent on the chemistry of the analyte. Therefore, it is necessary to match the matrix to the analyte to be analysed. However, this condition of matching the matrix and analyte to achieve the best level of information often gives rise to non ideal conditions for optimised ion extraction. These non ideal conditions manifest themselves in the form of surface roughness and inhomogeneities in the combination of matrix and analyte. In the case of poor mixing of matrix and analyte, care in the preparation of the sample can alleviate many of the problems, as described, for example, in “Growing Protein-doped sinapinic acid crystals for Laser Desorption,” Xiang and Beavis, Organic Mass Spectrometry, Vol.28, pp 1424-29, 1993. However, surface roughness can be very difficult to eliminate, as the drying process often leads to unavoidable crystallisation. This crystallisation can give rise to surface roughness of the order of 10 to 50 microns or more as described in “A comparison of matrix/analyte protein surface distributions in MALDI samples by XPS analysis ” by A Smith et al. Proceedings of the 45th ASMS Conference, p1041, 1997.
Another issue with biological and biochemical analysis is efficient sample utilisation. In many cases the amount of a substance that is available for analysis is very limited so effective ionisation and extraction is often an important factor in MALDI analysis. Model of the MALDI Process
The fundamental MALDI process of ejection and ionisation is not currently well understood. In part this is because of the large variety of matrix/analyte combinations. That is to say, what might be understood for one particular matrix/analyte combination may not apply to a different combination. This makes a common model difficult, if not impossible, to define. Another difficulty in performing an analysis of both the physical and chemical processes is that they take place in the order of tens of nanoseconds making measurement extremely difficult.
It is however possible to have a good qualitative model of the MALDI process. In this model the pulsed laser irradiates a region of the sample. Some molecules in this region receive sufficient energy to escape from the sample. This is called laser ablation. The laser is pulsed at a high energy, but with a short duration, of the order of nanoseconds, to remove some of the material from the sample. This material is ejected in a supersonic plume away from the surface of the sample. Either during or shortly after ablation, a number of the sample molecules become ionised. They can then be extracted by the application of a suitable electric field.
Angular Distribution of Ablated Material.
FIG. 1
shows where the pulsed laser beam
1
irradiates an area of the sample
2
which is ablated
3
.
FIG. 1
also shows the angular distribution
6
for a point source which has the largest number of particles ablated along trajectories perpendicular to the surface. Extrapolating this point distribution across the surface gives rise to the distribution shown by the dotted line
7
. In general, the distribution of ablated material has an axis of ejection
5
perpendicular to the surface and only has a weak dependence on the angle of incidence
4
of the irradiating light source
1
. This is for the case of a perfectly flat surface. However, if we compare this to the case of a rough or curved surface then the angular distribution of the ablated material is modified. In
FIG. 2
we see a curved surface which we can treat as a series of straight line segments or tangents
2
joined together as shown which is irradiated by a light source
4
. By treating each of these tangents as a local surface we then can define a local axis
5
which is perpendicular to that local surface
2
. Each local surface
2
therefore has its own local axis
5
. The distribution of ablated material
3
from the local surface about the local axis
5
is the same as for a point source as shown in FIG.
1
. If we now extrapolate these to obtain the distribution
4
over the whole curved surface that is irradiated we see that it is considerably broadened compared to the case of the flat surface. By a logical extension of the argument this broadening of the angular distribution can also be seen to be true for a concave as well a
Kawato Eizo
Smith Alan Joseph
Tanaka Koichi
Anderson Bruce
Hashmi Zia R.
Leydig , Voit & Mayer, Ltd.
Shimazdu Research Laboratory (Europe) Ltd.
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