Spot size on the target

Figure 2.2 Laser focusing optics for the adjustment of spot size on the target.

The spot size of MALDI preparations and thus the amount of sample necessary to yield a useful layer can be further reduced by so-called anchor targets (Bruker Daltonik). Anchor targets exhibit small hydrophilic spots on a hydrophobic surface. As a result, the evaporating drop of matrix-analyte solution is anchored to such a point where it shrinks until the onset of crystallization exactly within this hydrophilic area. [110] The resulting preparation covers an about lOOfold smaller surface than obtained from a freely spreading drop. In addition to improved detection limits, this technique simplifies automated spot finding due to their precisely defined location on the target. 

MALDI is considered a surface analysis technique and a relatively nondestructive ionization technique (Page and Sweedler, 2002). Typically, a sample spot on the target can be assayed multiple times because only a small fraction is vaporized for each laser pulse. However, no further spectra are detected after a completed ablation by the laser in MALDI. The depleted amounts were observed to be associated with the sample identity, sample spot size, and MALDI matrices (Page and Sweedler, 2002). 

The shape of the focal spot on the target irradiated by the electron beam is essentially a projection of the coiled filament, and its size is typically 1 x 10 mm. When the x-rays from such a focal spot exit through the tube window at a take-off angle a of 6°, the beam has a roughly 1 x 1 mm cross section, if the window is along the extension of the long dimension of the focal spot, as in Figure 2.2. The beam, on the other hand, appears to have a 0.1 x 10 mm cross section, if the window is... 

Electrospray sample deposition (ESDEP) is a sample preparation method where matrix and analyte solutions are sprayed on the target surface under the influence of a high-voltage electric field [44,45]. ESDEP is reported to yield much better shot-to-shot and spot-to-spot reproducibility than the dried-droplet method. The improved results are ascribed to the small and evenly sized crystals that are formed, and as a consequence, improved homogeneity of the MALDI sample surface. Hanton et al. [45] analyzed PEG1450, a narrowly distributed poly(ethylene oxide) sample, using an uncommon... 

Applications presented in this chapter have employed a Voyager DE-STR MALDl TOP mass spectrometer and a QStar Pulsar i QqTOF mass spectrometer (Applied Biosystems, Foster City, CA). The Voyager instrument utilizes a 337 mn nitrogen laser with a 2.5 ns pulse and a repetition rate of 2 Hz. The instrument was operated in linear mode under delayed extraction conditions. The laser spot size on target is —50 xm in diameter. The QStar Pulsar instrument is equipped with a MALDl source and a 337 mn nitrogen laser operating at a 20 Hz repetition rate. The laser spot size is —200 p,m in diameter on target. 

Target spot size and working distance The size of the spot on the target to be measured and its distance from the instrument. 

Measurement spatial resolution (IFOVmeas or MFOV) The spatial resolution describing the minimum target spot size on which an accurate temperature measurement can be made. 

Measurement spatial resolution, IFOVmeas or MFOV - The smallest target spot size on which an infrared camera can produce a measurement, expressed in terms of angular subtense (mrad). The slit response function (SRF) test is used to measure IFOVmeas in optomechanically scanned imagers. The hole response fiinction (HRF) test is used to measure IFOVmeas in optomechanically scanned imagers. 

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