Sample anchor plates

Anchor plates for the preparation of multiple samples have small hydrophilic islands, typically of 100-500 pm diameter, placed on a hydrophobic surface [147]. The hydrophobic surface prevents spreading of the sample solution over a larger area, as otherwise observed for dried-droplet preparations. Instead, the hydrophilic solution contracts onto these islands, thereby concentrating the matrix and analyte onto a small defined area upon solvent evaporation. This confinement to a smaller volume is particularly useful for analytes of low concentration in combination with proportionally lowered matrix concentrations, and also facilitates automated analyses of the fixed-location samples. Anchor sample plates are also commercially available as disposable targets prespotted with matrix and calibration spots. 

Matrix Additives and Influence oTthe Sample Plate Surface 

A few other modifications of the sample preparation have also proven useful in specific cases  

Modified surfaces of sample plates can be used for the affinity capture of analytes from crude mixtures directly on target. Such surfaces can significantly enhance detection sensitivity and can be used for simple sample clean-ups. Titania (Ti02)-coated surfaces, particles or sol-gel systems, for example, have been shown to concentrate phosphopeptides very selectively from peptide fingerprint samples [154, 155]. 

Surface-enhanced laser desorption ionization (SELDI) uses so-called protein chips for the detection of peptides and proteins from complex biological fluids such as blood or urine, often for the identification of diagnostic biomarkers for specific carcinomas [156, 157]. These protein chips can contain various media for positive or negative ion exchange or reverse-phase chromatography, as well as specific antibodies or DNA. The functionaUzed surface is immobihzed on a MALDl sample plate for the selective enrichment of constituents of the complex mixture applied, whereas the not bound supernatant is removed by washing. Unfortunately, a large number of unsubstantiated claims for the detection of disease-related biomarkers has discredited this approach, mostly as a result of poor mass spectrometric performance. 

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Among the many modifications and variations of the simple dried-droplet preparation, two alternatives stand out as particularly useful and widespread, namely surface preparation and anchor sample plates. ... 

In the case of MALDl-MS, the nanoHPLC has to be coupled to the mass spectrometer offline. After separation, the sample is transferred to a sample plate (target), e.g. AnchorChip target (Broker Daltonics, Bremen, Germany), equipped with a special coating that provides improved crystallization of M ALDI samples and a 10- to 100-fold sensitivity increase. Small hydrophilic positions/anchors (600 pm diameter) are applied equidistantly on the hydrophobic surface of the AnchorChip target thereby reducing sample spreading to only a small area. 

Figure 8.11 Illustration of Mauguin twisted nematic cell, reported in 1911. Substrates are thin mica plates, which are uniaxial with their optic axis parallel to plane of plates. Apparently, uniaxial crystal stmcture of mica produces strong azimuthal anchoring of nematic LCs of Lehmann, such that director is parallel (or perpendicular) to optic axis of mica sheets at both surfaces. Mauguin showed that method of Poincard could be used to explain optics of system if it was assumed that LC sample created layer of material with uniformly rotating optic axis in twisted cells.

Fig. 2. Schematic diagram of the apparatus. The superconducting magnetic coils create trapping potential that confines atoms near the focus of the 243 nm laser beam. The beam is focused to a 50 pm waist radius and retro-reflected to allow for Doppler-free excitation. After excitation, fluorescence is induced by an applied electric field. A small fraction of the 122 nm fluorescence photons are counted on a microchannel plate detector. Not shown is the trapping cell which surrounds the sample and is thermally anchored to a dilution refrigerator. The actual trap is longer and narrower than indicated in the diagram...

Microfabricated MALDI sample targets were also prepared from PMM A by computer numerical control (CNC) milling. In this case, enzyme functionalized cylindrical posts (360 p,m x 360 p,m) served as individual sample targets on the MALDI plate and were used for on-probe characterization of nucleic acids. The advantages of this device included reduced sample consumption and handling and reduced analysis times. Alternatively, MALDI targets with twin anchors (400 p.m) were prepared from silicon wafers by anisotropic dry etching in an inductively coupled plasma. 

The above discussion of the Fredericks transition can be extended to include (1) pretilt at the boundaries where the boundary conditions (p 0) = (j) L) = 0 are replaced by (p 0) = (p L) = < o (2) the tilted fields where the magnetic field is applied across the sample at a fixed angle (3) weak anchoring where the director is weakly anchored to both boundary plates. A more detailed discussion can be found in Stewart (2004) and Virga (1994). 

The first simple situation to consider is where the prescribed alignment of the director is the same at both plates with there being no net tilt or twist in the sample. In this event, the strong anchoring boundary conditions are... 

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