Radiant energy – Ionic separation or analysis – With sample supply means
Patent
1993-12-21
1995-10-24
Anderson, Bruce C.
Radiant energy
Ionic separation or analysis
With sample supply means
250423P, 250282, B01D 5944, H01J 4900
Patent
active
054612357
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a Mass Spectrometry apparatus and a method of Mass Spectrometry and, in particular, to a Mass Spectrometry apparatus and method for use in Laser Desorption. Mass Spectrometry (LDMS). In LDMS, ions are sputtered from the surface of a condensed phase sample by photon bomardment and subjected to mass analysis.
There are many embodiments of LDMS which differ in detail. In the preferred 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, a sample of bovine insulin was dissolved 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.
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. In order to achieve the required power density of 10.sup.67. Watts per square cm from a low power laser, the beam is focused to a small spot, typically 0.l mm diameter. It is not practical to load and confine the sample to such a limited region, so there exists a requirement to raster the laser beam over a larger area.
There are many devices available for deflecting laser beams with the necessary speed and precision, but they are either too complex, too bulky, or too expensive to be incorporated into a simple and inexpensive mass spectrometer. Examples of prior art include mirrors mounted on motorised spindles, mirrors mounted on galvanometer armatures, and mirrors positioned by other types of actuator such as piezo-electric crystals, voice-coils, stepper motors, and the like.
In LDMS, good quality results are only obtained when the laser irradiance is close to an optimum setting. This optimum setting varies according to the nature of the sample and the choice of chemical matrix, so it is also necessary to provide a means of accurately adjusting the laser power density.
The method described by M. Karas et. al. (Int. J. Mass Spectrom. Ion Processes, 78 53 (1987)), involves achieving a coarse adjustment by attenuating the beam power using an absorbant glass filter and achieving fine adjustment by altering the sharpness of the beam focus. Other systems have been described which also rely on manual selection of fixed absorbance filters.
Continuous, remote adjustment of laser power is necessary to achieve the ease of use required for a mass spectrometer to be commercially acceptable. This could be achieved by using a motor actuated gradient density filter. Alternatively, U.S. Pat. No. 4,398,806 describes a variable attenuator using the principle of Fresnel reflection from four counter-rotating glass wedges.
The invention provides a mass spectrometry apparatus including means to irradiate a specimen with a beam of radiation to desorb ions for mass analysis, the apparatus comprising means for deflecting and scanning said beam, and attenuator means arranged to vary the attenuation of said beam such that the attenuation is a function of the beam position in the scan.
For particular embodiments the beam of radiation is monochromatic. Those skilled in the art will appreciate that no source of radiation emits truly monochromatic radiation in the sense of radiation of a single or unique wavelength. However, radiation of a very narrow band of wavelengths can be obtained, for example, from some lasers.
The present invention enables a single scan of a sample by a beam of radiation to produce results for different power densities of radiation incident on the sample. Such results may be used, in LDMS, by distinguishing the optimum results for an op
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Cottrell John S.
Keynes Milton
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