The Maldi Process

The main principle of MALDI goes back to the observation that in a laser-irradiated mixture of substances that differ in their absorption efficiency, the less efficient absorbing substance is already desorbed into the gas phase at less laser energy than would be required for this substance alone, if the more efficiently absorbing substance is provided in excess. This enables desorption of substances (e.g., large biomolecules) that would undergo fragmentation and decomposition if desorption is attempted without matrix assistance. In a typical MALDI experiment, the matrix is present in a 103-fold to 104-fold excess.

To conduct the MALDI process, the sample is deposited on a conductive sample support and cocrystal-lized with matrix. Typical matrix substances are small organic molecules (mainly acids) that have an absorption maximum at the laser wavelength and are able to efficiently cocrystallize with the analyte and to induce its ionization in the gas phase. The choice of the appropriate matrix is key to a successful MALDI-TOF

experiment. Some common matrix compounds, their solvents, absorption wavelength, and the compatible analytes are listed in Table 1.

Several techniques have been reported to efficiently cocrystallize the matrix and the sample. The most commonly used is the so-called dried droplet method. Here, the sample is premixed with the matrix and the mixture is allowed to evaporate slowly at ambient temperature. Other techniques include the fast evaporation and sandwich matrix technique. For the fast evaporation method, a small amount of matrix (usually in a water-acetone mixture) is deposited on the sample. The acetone (or a comparable solvent) rapidly evaporates and leaves a homogeneous crystal surface. The sandwich matrix technique starts with a thin layer of matrix that is applied first, followed by the sample solution and an additional layer of matrix. There is a vast variety of variations of these techniques and usually the user will have to determine empirically which one is suited best for a particular application.

To induce desorption, the matrix sample cocrystals are irradiated with a nanosecond laser beam [e.g., from an ultraviolet (UV) laser with a wavelength of 266 or 337 nm]. Lasers used for MALDI-TOF purposes are mainly UV lasers [e.g., frequency-quadrupled neodymium/yttri-um aluminum garnet (Nd/YAG) lasers], but nitrogen and CO2 lasers are also in use. The use of infrared (IR) lasers is suitable for several applications and has been reported to successfully analyze large DNA fragments.[3] The energies introduced by the laser irradiation are in the range of 1 x 107 to 5 x 107 W/cm2. This energy introduction causes a structural decomposition of the crystal and generates a particle cloud (the plume). The process of desorption can be described as a conversion of laser energy to vibrational oscillation of the crystal molecules. This results in the disintegration of the crystal. The plume contains ions that follow a Boltzmann distribution of their initial velocities. Out of the plume, ions are extracted by an electric field.

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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