Matrix crystal size

Spatial resolution for MALDI—MSI experiments is limited by laser spot size, laser step size, matrix crystal size, and analyte migration. Spatial resolution increases with decreasing laser spot size, but MALDI mass spectrometers are usually equipped with N2 (337 nm) or tripled Nd YAG (355 nm) lasers having relatively large spot sizes (about 100 gm diameter). The rate of energy redistribution rapidly increases with smaller laser spot sizes and higher laser... 

There is still much room for growth in this field. Image resolution, for instance, is limited by the laser spot and matrix crystal size. In addition, MSI is primarily a qualitative technique. At best, semiquantitative results are achieved since, among other things, it is difficult to apply an internal standard to tissue. Despite these challenges, mass spectral imaging offers a complementary approach for the detection of drugs and their metabolites in tissue when used with other established methods. 

An important feature of the OCN system is the ability to minimize the amount of solvent that comes into contact with the tissue. This serves to reduce analyte migration and matrix crystal size to minimize the loss of molecular spatial information. 

Nowadays, pulsed, frequency tripled Nd YAG UV lasers (355 nm) are usually employed for MALDI experiments with a repetition rate of 1000 Hz in commercial instruments for sufficient data acquisition. In MALDI-IMS, the resolving power for application strongly depends on the sample preparation step (e.g., matrix crystal size), stepper motor accuracy, and laser spot sizes. To achieve MALDI-IMS to a practical resolution, the laser spot size of 20 pm is usually used. Therefore, the time needed to obtain images from a sample depends on the number of analyzed spots, the repetition rate of the laser (Hz), and the data collecting and processing speed of computers. For example, imaging a whole-body mouse or rat section with current commercially available MALDI mass spectrometers equipped with lasers operating at 1 kHz would take 2-4 h. 

A typical characteristic of many food products is that these are multi-phase products. The arrangement of the different phases leads to a microstructure that determines the properties of the product. Mayonnaise, for example, is an emulsion of about 80% oil in water, stabilized by egg yolk protein. The size of the oil droplets determines the rheology of the mayonnaise, and hence, the mouthfeel and the consumer liking. Ice cream is a product that consists of four phases. Figure 1 shows this structure schematically. Air bubbles are dispersed in a water matrix containing sugar molecules and ice crystals. The air bubbles are stabilized by partial coalesced fat droplets. The mouthfeel of ice cream is determined by a combination of the air bubble size, the fat droplet size and the ice crystal size. 

A quantitative evaluation of the relaxivities as a function of the magnetic field Bo requires extensive numerical calculations because of the presence of two different axes (the anisotropy and the external field axis), resulting in non-zero off-diagonal elements in the Hamiltonian matrix (15). Furthermore, the anisotropy energy has to be included in the thermal equilibrium density matrix. Figures 7 and 8 show the attenuation of the low field dispersion of the calculated NMRD profile when either the crystal size or the anisotropy field increases. 

It is evident that with the discrete cycles of the non-flame atomizers several reactions (desolvation, decomposition, etc.) which occur simultaneously" albeit over rather broad zones in a flame (due to droplet size distributions] are separated in time using a non-flame atomizer. This allows time and temperature optimization for each step and presumably improves atomization efficiencies. Unfortunately, the chemical composition and crystal size at the end of the dry cycle is matrix determined and only minimal control of the composition at the end of the ash cycle is possible, depending on the relative volatilities and reactivities of the matrix and analyte. These poorly controlled parameters can and do lead to changes in atomization efficiencies and hence to matrix interferences. 

Other interferences which may occur in flame AAS are ionization of the analyte, formation of a thermally stable compound e.g., a refractory oxide or spectral overlap (very rare). Non-flame atomizers are subject to formation of refractory oxides or stable carbides, and to physical phenomena such as occlusion of the analyte in the matrix crystals. Depending on the atomizer size and shape, other phenomena such as gas phase reactions and dimerization have been reported. 

Information about the crystallinity level and crystal size in a food can be obtained by submitting the food matrix to X-ray diffraction. For example,... 

Crystal shape, size, and density all affect the physical properties of the final solid fat matrix. Crystal growth, primary nucleation, and secondary nucleation in fat systems are influenced by many factors, including diffusion, molecular compatibility, TAG structure, nuclei composition and surface properties, number of nuclei, and processing conditions (temperamre and/or shear) (38, 39). It is during the crystallization process of fats that the template for the final physical properties of the material is created. 

The freezing temperature also influences crystal size and hence the rate at which sublimation occurs. It is therefore essential to establish the best freezing conditions for each product. The determination of freezing points or eutectic points is probably the most complex task in the freezing protocol. The presence of any liquid constituent other than water in a liquid matrix lowers its freezing point. It is thus very important to pre-freeze the product to below the eutectic temperature prior to freeze-drying proper. In fact, any small pockets of unfrozen material remaining in the product may expand and compromise the structural stability of the freeze-dried product. 

The spatial resolution offered in MALDI-MSI, also termed MALDI imaging, depends on the laser focus diameter and the average crystal size of the matrix. Recent commercial instrumentation offers spatial resolutions down to 20 pm per pixel. 

In experiments conducted to obtain controlled sizes of filler particles formed in a matrix, several polymers were used as the matrix. Copolymers were synthesized from polyethylene oxide (does not interact with CaCOs) and poly(methacrylic acid) (reacts with in situ crystallizing CaCOj). In the presence of polyethylene oxide, crystals grew to similar sizes as without any polymer. The presence of the poly(methacrylic acid) crystal size of CaCOa was reduced by a factor 5 to 10 depending on the concentration of the filler precursor. 

The effect of acid rain on mortars will depend on the particular mortar in consideration. The most susceptible mortars will be the lime-sand ones. The carbonated lime will be particularly attacked due to the small crystal size of the formed calcite (19,20). The resulting calcium sulfate can crystallize as gypsum [CaS0. 2H20] inducing mechanical stresses into the matrix of the mortar. 

Fig. 8. Schematic design of a spherulite which is formed in the course of polymer crystallization. Spherulites contain blocks of crystalline zones, embedded in an amorphous matrix. The size can be in the pm range and larger...

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