Radiant energy – Ionic separation or analysis – With sample supply means
Patent
1993-12-21
1999-01-12
Berman, Jack I.
Radiant energy
Ionic separation or analysis
With sample supply means
250281, 250282, H01J 4904, H01J 4916
Patent
active
058594310
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a sample holder to be used in the analysis of a sample by Laser Desorption mass spectrometry (LDMS). In LDMS, ions are sputtered from the surface of a condensed phase sample by photon bombardment and subjected to mass analysis.
There are many embodiments of Laser Desorption mass spectrometers which differ in detail. An important feature of certain embodiments is the use of a matrix material in which the analyte of interest is dispersed. In the procedure described by M. Karas et. al. (Int. J. Mass Spectrom. Ion Processes 78 53 (1987), a large molar excess of a matrix which has a strong absorption at the wavelength of the incident radiation is mixed with the sample to be analysed. For example, they dissolved a sample of a bovine insulin in an aqueous solution containing a thousand-fold molar excess of Nicotinic Acid (59-67-6). A drop of the solution was placed on a metal plate, evaporated to dryness, introduced into the mass spectrometer, and irradiated with 266 nm ultraviolet photons from a frequency quadrupled pulsed Neodymium YAG laser. Desorbed ions were accelerated to an energy of 3 keV and analysed by measuring their time of flight to an electron multiplier detector.
The sensitivity of analysis by a Laser Desorption mass spectrometer depends critically on the detailed sample loading procedure. Ions can only be produced from those regions of the sample deposit which are irradiated by the laser beam. Sample which is not irradiated is wasted. The laser beam is generally focused to a small spot, typically 0.1 mm diameter. In principle, such a laser beam can be rastered over a much larger area. However, it is difficult to design extraction optics to accept ions from a very large area and focus them onto the detector without introducing a time spread which would degrade the mass resolution of the instrument. In addition, the mechanism to achieve controllable rastering over a large area adds cost and complexity to the instrument. A more desirable approach is to restrict the size of the sample deposit to a practical minimum. This raises the difficulty of identifying the precise spot at which the sample should be loaded on a relatively large area sample holder. It is also necessary to constrain the droplet to this spot while it dries. An object of the present invention is to provide a means of constraining the droplet to a predefined area while the solvent evaporates.
Identifying the spot at which the sample is to be loaded is not a trivial matter. The printing of marks using commercially available inks would limit the range of solvent systems which could be used for loading samples. Indented or engraved lines tend to attract the sample away from the desired spot by capillary attraction. For mass analysis by Time-of-Flight, it is important that the area from which ions originate is essentially flat, otherwise the variation in path length will cause a reduction in mass resolution. For this reason, a dished indentation to locate and contain the sample droplet is not feasible. Another object of the present invention is to provide a sample holder in which the optimum location for the sample deposit is clearly identified.
A further critical aspect of the sample loading procedure concerns the uniform drying of the droplet of sample and matrix solution. For reproducible results, it is necessary to achieve a reasonably homogeneous crystalline deposit on the sample target. If, for example, the sample and matrix have a tendency to separate on crystallisation, a slowly drying droplet may deposit the majority of the sample as a peripheral ring which is outside the area to be irradiated. Thus, a further object of the present invention is to provide a sample holder which enables a reasonably homogeneous sample deposit to be achieved.
The present invention provides a sample holder for use in mass spectrometry comprising a plate having a flat, said flat including a first region having a smooth surface surrounding a second region having a rough surface, said second region defining the location for load
REFERENCES:
patent: 4620103 (1986-10-01), Kambara et al.
patent: 4705705 (1987-11-01), Bross
patent: 5083020 (1992-01-01), Takahashi
Cotrell John Stanley
Mock Kuldip Kaur
Berman Jack I.
Finnigan Mat Limited
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