Molecular probe technique

The application of gas chromatography (GC) to the study of polymers has been hampered by their negligible volatility. A solution to this problem is the use of inverse gas chromatography (IOC, also called the molecular probe technique), which was developed by Sknids-red and Guillet (1) in 1969. The word "inverse" indicates that the component of interest is the stationary polymer phase, rather than the injected volatile substances. 

Constriction dimensions measured by molecular probe technique. 

This orthorhombic Mo-V oxide was prepared by hydrothermal reaction of (NH4)6Moy024 and VOSO4. Similar to manganese oxide OMS, cations ([NH4]" ") occupy both channels. To form an empty channel, [NH4]" cations are removed by calcination. Ordered microporosity was first confirmed by a single uptake of N2 and Ar adsorption at very low P/Pq (type 1 behaviour), and the pore diameter was estimated by the molecular probe technique to be ca 0.4 nm (Figure 3.3). This value is close to the aperture diameter of the seven-membered channel. The micropore volume calculated by the Dubinin-Astakhov (DA) equation for smaller gases is about 0.025 cm g , which is the calculated pore volume of a seven-membered channel. These results indicate that this seven-membered ring channel is a micropore. 

The molecular probe technique in combination with XPS has seldom been used since the early 1970s and has mainly been applied to zeoKtes [162-164]. These studies were aimed at identifying and quantifying Lewis acidic and basic sites at catalyst surfaces by monitoring the BE shifts of Nls from adsorbed pyridine [162,163] and pyrrole [164], respectively. We shall discuss the application of this approach to molecular and polymeric species. 

XPS has been employed for many years to characterize the surface acid-base properties of catalysts and metal oxides by various methodologies including Fermi level monitoring. We have shown since the early 1990s the potential of XPS in characterizing acid-base properties of conventional polymers using the molecular probe technique. This approach has recently found application in characterizing commercial resins, photoinitiators, and plasma-treated polymers in relation to metallization. 

Orientational order in liquid crystals is most effectively studied using molecular probe techniques such as NMR spectroscopy. However, the study of translational order, that is structure, requires diffraction techniques, usually involving X-rays or neutrons. A disadvantage of diffraction is that it does not distinguish between static and dynamic disorder. However, this can be achieved using quasielastic neutron scattering. 

All the work described above has established XPS firmly as a flexible method for the evaluation of the acid-base properties of homopolymers, but the technologically more important advantage of XPS is its ability to analyze thin modified or segregated layers not amenable to traditional forms of analysis [129]. The molecular probe technique has been used to good effect by Shahidzadeh et al. in the study of the plasma treatment of poly(propylene) film [130-132]. Their work identified the need to select basic probe molecules for the assessment of acidic surfaces, and dimethyl sulfoxide was shown to be a good choice, although the photoelectron cross section for sulfur is low, as it is for the Cl 2p core level used for trichloromethane this puts an effective limit on the detectability of the molecular probe, i.e., the number (but not the strength) of the acid-base pairs detectable. 

Bakeev, K. N. MacKnight, W. J. Fluorescent molecular probe technique for assessing the interaction between sulfonated polystyrene and poly... 

Guo LD, Santschi PH (1996) A critical evaluation of the cross-flow ultrafiltration technique for sampling colloidal organic carbon in seawater. Marine Chem 55 113-127 Guo LD, Wen LS, Tang DG, Santschi PH (2000) Re-examination of cross-flow ultrafiltration for sampling aquatic colloids evidence from molecular probes. Marine Chem 69 75-90 Guo LD, Hunt BJ, Santschi PH (2001) Ultrafiltration behavior of major ions (Na, Ca, Mg, F, Cl, and SO4) in natnral waters. Water Res 35 1500-1508... 

Through the use of pump-probe techniques pioneered by Zewail and coworkers,62 it is becoming possible to identify the detailed mechanisms of reactions at the molecular level and follow the actual course of a reaction. The study of ammonia clusters has provided an example of what can be accomplished using these techniques. 

Abstract Fluorescent molecules have been widely used as biomolecular labels, enzyme substrates, environmental indicators, and cellular stains and thus constitute indispensable tools in chemistry, physics, biology, and medicinal sciences. The large variation in the photophysics of the available fluorophores connected with chemical alterations give fluorescent probe techniques an almost unlimited scope for the detection of specific molecules and the investigation of intermolecular interactions on a molecular scale. 

The distance scale associated within the glass transition is related to the method used. For example, thermal and mechanical techniques provide macroscopic views of the glass transition, whereas spectroscopy techniques yield a molecular-level view. Thus, it is not surprising to find that molecular-level techniques, such as NMR, may result in lower Tg values compared to those obtained using a macroscopic technique, such as DSC. Both Tg values are correct, but not necessarily equal, given the different points of view the two methods are probing. 

The volume of the solid phase Vp is usually measured by a pycnometric technique, which measures the excluded volume of a pycnometric fluid, whose molecules cannot penetrate the solid phase of PS. A simple example of a pycnometric fluid is helium [55], The pycnometric fluid fill in all void space (pores) accessible to it, and presumably do not adsorb on the surface of PS. In the case of microporous PSs, measurement of the volume accessible for guests with various sizes allows the determination of a distribution of micropores volume vs. the characteristic size of guest molecules. This approach lays the basis of the method of molecular probes. The essence of this method is in the following we have a series of probe molecules with different mean sizes (dl>d2>d3>---). The pycnometric measurements of the excluded volume will give a series The difference A V=Vpi-Vpi(i>j) corresponds to the volume of micropores with pycnometric sizes of d in a range dt[Pg.283]

Because the fMet-Leu-Phe receptor is present only at low levels in neutrophils (-12 x 10 15 g of receptor per cell), it has proved difficult to purify and characterise. Researchers have therefore turned to molecular cloning techniques to gain insight into the molecular structure of this receptor. This approach itself has not been easy because, in the absence of an antibody that specifically binds to the receptor, or else without some amino acid sequence data that can be used to synthesise oligonucleotide probes, cDNA libraries cannot be screened to isolate relevant clones. Therefore, experimental systems in which functional fMet-Leu-Phe receptors are expressed on the surfaces of transfected cells have been used. Two main systems have been utilised expression of mRNA injected into Xenopus laevis oocytes and cDNA cloning into the COS-cell expression vector. 

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