Mass spectrometer with MALDI laser system
The invention relates to a mass spectrometer comprising a laser system for mass-spectrometric analysis with ionization of analyte molecules in a sample by matrix-assisted laser desorption. A mass spectrometer with a pulsed UV laser system produces a spatially distributed spot pattern with peaks of uniform energy density on the sample, increasing thereby the degree of ionization for analyte ions as compared to conventional spot patterns. The spot pattern with peaks of uniform energy density can be produced by homogeneous illumination of a pattern generator, for example a lens array. The homogeneous illumination can be generated by a low-cost beam-shaping element, which does not act on the UV beam but on the original infrared beam, in conjunction with changes to the beam cross-section and beam profile brought about by the nonlinear conversion crystals. This beam shaping not only produces a beam profile which illuminates the pattern generator homogeneously with low losses, but at the same time increases the efficiency of the frequency multiplication and the lifetime of the conversion crystals so that cost savings are achieved because less laser energy is required and the lifetime is increased.
1. A mass spectrometer comprising a laser desorption ion source having a laser system for the ionization of a sample by matrix-assisted laser desorption, comprising:
a pulsed solid-state laser system;
conversion crystals for increasing the frequency, and a pattern generator in the frequency-increased laser beam, wherein
a beam-shaping optical device is located between the solid-state laser system and the conversion crystals, which converts the pulsed laser beam having a Gaussian profile into an approximately rectangular beam with approximately homogeneous energy density.
2. The mass spectrometer of claim 1 , wherein the solid-state laser system generates a pulsed infrared beam and the conversion crystals increase the frequency of the laser light by a factor of three resulting in a UV laser wavelength in the range between about 300 and 450 nanometers.
3. The mass spectrometer of claim 1 , wherein the beam-shaping optical device comprises a lens operating with refraction.
4. The mass spectrometer of claim 1 , wherein the beam-shaping optical device comprises an diffractive optical device.
5. The mass spectrometer of claim 1 , comprising two nonlinear conversion crystals to increase the beam frequency, operating with walk-off compensation.
6. The mass spectrometer of claim 1 , comprising a pattern generator generating a spot pattern with one of 4, 7, 9, 16, 19, and 25 spots.
7. The mass spectrometer of claim 6 , wherein the pattern generator comprises a lens array.
8. The mass spectrometer of claim 6 , comprising a telescope and an optical lens system, which image the spot pattern onto the sample.
9. The mass spectrometer of claim 8 , wherein the spots on the sample have diameters of about 10 micrometers at most.
10. The mass spectrometer of claim 8 , further comprising a rotatable mirror system located between the pattern generator and the telescope, which can be used to control the spot positions on the sample.
11. The mass spectrometer of claim 1 , wherein the laser system is configured to generate a sequence of laser light pulses with a pulse rate in the range of about 1 to 10 kHz.
12. A laser system for the ionization of a sample by matrix-assisted laser desorption in a mass spectrometer, with a pulsed solid-state laser system, conversion crystals to increase the frequency, and a pattern generator in the frequency-increased laser beam, comprising a beam-shaping optical device between the solid-state laser system and the conversion crystals, which converts the pulsed laser beam having a Gaussian profile into an approximately rectangular beam with approximately homogeneous energy density.
13. A method for the ionization of a sample by matrix-assisted laser desorption in a mass spectrometer, comprising:
providing a sample with analyte molecules; and
ionizing the analyte molecules with a laser system for the ionization of a sample by matrix-assisted laser desorption in a mass spectrometer, with a pulsed solid-state laser system, conversion crystals to increase the frequency, and a pattern generator in the frequency-increased laser beam, comprising a beam-shaping optical device between the solid-state laser system and the conversion crystals, which converts the pulsed laser beam having a Gaussian profile into an approximately rectangular beam with approximately homogeneous energy density, and subsequently measuring the ionized analyte molecules mass-spectrometrically.