Blanking out of pulses in pulsed lasers for LDI mass spectrometers
The invention relates to optically pumped and pulsed solid-state lasers which are used in mass spectrometers in particular for ionization by matrix-assisted laser desorption (MALDI) and which operate at pulse frequencies of up to 10 kilohertz or even higher. The invention proposes that, instead of interrupting the clocked sequence of the laser operation, individual light pulses or groups of light pulses are blanked out so that subsequent light pulses do not have a higher energy density, in accordance with the requirements for LDI processes. Methods and devices for the blanking out of light pulses are provided which are, in particular, low cost and considerably less complex than other methods.
1. A method for the partially discontinuous irradiation of a sample support in a mass spectrometer with light pulses for ionization of samples on the sample support by laser desorption, in which continuously uniform light pulses of an initial energy are emitted from a laser crystal, which is located in an optical cavity between a high-reflectivity mirror at one end and a mirror at the other end which is partially transparent for the initial energy and through which light pulses exit the optical cavity, and where a polarization rotator imparts one of a plurality of predefined directions of polarization to photons of the initial energy, and operates in conjunction with a polarization sensitive component external to the optical cavity such that light pulses having a first polarization direction are guided to the sample support, while light pulses having a second polarization direction, different than the first polarization direction, are prevented from reaching the sample support.
2. The method according to claim 1 , wherein, after exiting the optical cavity, the photons of the initial energy are converted into photons of a target energy, which is higher than the initial energy, by multiplying a photon energy in at least one multiplier crystal.
3. The method according to claim 2 , wherein the polarization-sensitive component comprises said at least one multiplier crystal, which operates substantially only with the light pulses having the first polarization direction, and allows light having the second direction of polarization to pass unchanged so that no frequency multiplication takes place.
4. The method according to claim 1 , wherein the polarization rotator comprises a Pockels cell and a λ/4 wave plate.
5. The method according to claim 4 , wherein the Pockels cell is operated with a high voltage generator which controllably supplies both positive and negative high-voltage pulses of reproducible voltage for changing the directions of polarization.
6. The method according to claim 1 , wherein the optical cavity further comprises a polarization filter.
7. The method according to claim 6 , wherein the polarization filter is not positioned immediately adjacent to the partially transparent mirror.
8. The method according to claim 6 , wherein the polarization filter and the polarization rotator are positioned in the optical cavity between the laser crystal and the partially transparent mirror.
9. The method according to claim 1 , wherein matrix-assisted laser desorption (MALDI) is used for the ionization of the samples in the mass spectrometer.
10. The method according to claim 1 , wherein a laser pulse frequency is set to between 1 hertz and 10 kilohertz.
11. An optical cavity with a high-reflectivity mirror at one end and a partially transparent mirror at the other end, between which a laser crystal is positioned for an emission of uniform light pulses of precisely defined photon energy, furthermore comprising a polarization rotator and a polarization filter, wherein the polarization rotator is positioned directly adjacent to the partially transparent mirror and is controllable to switch between a first state in which said light pulses are output from the cavity with a first polarization direction, a second state in which said light pulses are output from the cavity with a second polarization direction different from the first polarization direction, and a third state in which no pulses are output from the cavity.
12. The optical cavity according to claim 11 , wherein the polarization rotator has a Pockels cell being operated by a high voltage generator which is configured to provide both positive and negative high-voltage pulses of reproducible voltage to switch a polarization direction of the light pulses.
13. The optical cavity according to claim 11 , wherein the laser crystal is a birefringent crystal.
14. A time-of-flight mass spectrometer having an ion source which operates with ionization by matrix-assisted laser desorption (MALDI) and comprises an optical cavity according to claim 11 .