Info

ACN, water

Oligonucleotides, peptides

aAcetonitrile.

'Ethanol.

"Methanol.

aAcetonitrile.

'Ethanol.

"Methanol.

secondary electron multiplier, or SEM). Ion masses (mass-to-charge ratios, m/z) are typically calculated by measuring their TOF, which is longer for larger molecules than for smaller ones (provided their initial energies are identical). In mathematical terms, the velocity (v) of desorbed ions that are accelerated through a constant electric field is inversely proportional to the square root of their mass-to-charge ratio.

The most prominent detectors that are used to analyze the ion beam resulting from a MALDI experiment are SEMs. The basic concept relies on the emission of so-called secondary electrons from a coated surface resulting from a strike of fast-moving ions. In these detection devices, a series of dynodes is connected together via a resistor chain. A high voltage is applied across the first dynode (conversion dynode) and the last dynode (anode). The impact of ions on the conversion dynode results in the generation of secondary electrons that are accelerated toward the second dynode where more secondary ions are emitted. This process is repeated multiple times and leads to an amplification of secondary ions at each successive step. The achieved gain in signal amplification is in the range of 107. It has to be noted that the sensitivity of these devices decreases with increasing analyte mass because the electron yield depends on the momentum of the incoming particle, not its energy. A commonly used version of the electron multiplier is the channel electron multiplier (CEM). The CEM provides a higher amplification gain and is more compact and less expensive. Another detector device is the channel plate that essentially is a multichannel version of the CEM. It consists of a multitude of fiber optic cables that provides an increased length-to-diameter ratio. In addition, photomultipliers that convert an incoming beam of ions into photons that are amplified by a conventional photo-multiplier are used.

Because the MALDI process predominantly generates single-charged, nonfragmented ions, parent ion masses can easily be determined from the resulting spectrum without the need for complex data processing. The masses are accessible as numerical data for direct processing and subsequent analysis. TOFs measured during a typical MALDI experiment are in the range of microseconds. The MALDI process and instrumentation as described in the above paragraphs are illustrated in Fig. 1.

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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