Uptake techniques

With gas leaking into an adsorption cell of volume V at a rate L, pumped by the pumps at a rate RP, the rates of adsorption, RA, and desorption, iZD, are related to the pressure change in the cell, dP/dt, by the expression 

At the low pressures required for these experiments ( 10 7 torr), the molecular mean free path is very large compared with the dimensions of the vessel and molecular flow conditions operate. The rate of pumping is thus directly proportional to the pressure and is written as RP = FP, where F is the system pumping speed (conveniently expressed in units molecules torr-1s-1). At steady state, dP/dt — 0 assuming no desorption, so that PD = 0, we have 

The surface coverage N at a given time t after the start of adsorption, expressed in molecules cm-2, is obtained from the integral 

This parameter is usually measured in situ with the adsorbent rendered inert, either by saturating it or by maintaining it at a temperature sufficiently high that the steady state coverage is effectively zero. (The latter cannot be applied where fragmentation of the gas molecules occurs at 

If the inflow of gas to the cell is suddenly terminated (by closing a valve), L = 0 and the pressure should fall exponentially with time, yielding F from the slope. The volume V must therefore be accurately known. 

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Bogdanffy MS, Sarangapani R, Kimbell JS, F rame SR, PlowchalkDR. 1998. Analysis of vinyl acetate metabolism in rat and human nasal tissues by an in vitro gas uptake technique. Toxicol. Sci. 46 235 16... 

The experimental chemiluminescence results obtained for two ABS samples at 150°C and 190°C are presented in Fig. 5. The chemiluminescence experiments at 190°C have been performed in order to be able to compare the data with the DSC and oxygen uptake results since the sensitivity of the DSC and oxygen uptake techniques is not sufficient enough for their application at 150 C. Besides exhibiting the initial burst of emission, the character of the chemiluminescence vs. time curves for the ABS samples was similar to that for PP and has been treated by applying eg. (16) for the 150°C experiment. The chemiluminescence data together with the DSC and oxygen uptake results are shown in Table III. 

The difficulties with uptake techniques are amplified when used to study adsorption rates on metal films early results underestimated sticking probabilities by a factor of about 50 due to inadequacies in system design. In the system described by Hayward et al. [174], the film is deposited on the walls of a spherical bulb and the gas is introduced to the cell through a diffuser placed at the centre of the cell and designed to produce an equal flow towards the film in every direction. The method is described in Sect. 2.2... 

This method is subject to the same problems as the closely related uptake technique described above in particular, exposures are seldom reported to an accuracy better than 50%, with little reliability in derived values of s. Nevertheless, since multiple pulses may be derived from different adsorbate states on the surface, desorption spectra provide the added advantage of providing a means of measuring the relative coverage in each state, in addition to the absolute total coverage, provided that the different pulses can be reliably deconvoluted. However, care must be taken in the interpretation interconversion between states can occur... 

It must be stated, however,- that Ta uptake techniques measure the unsaturated T4 binding in the serum, not the level of thyroid hormone. An overlap is usually found between the values in untreated myxedema and normal subjects, whereas values in thyrotoxic patients are usually separated from normal values. It is largely fortuitous that the T3 uptake reflects thyroid status so well in the absence of abnormalities in T4-bind-ing proteins. It is now customary to consider the result of a T3 uptake in conjunction with the result of a more direct measure of the thyroid hormone, such as a PBI or total T4 assay (see Section 3.4.2). 

The following method, using a B a-requiring diatom and measuring the rate of radioactive carbonate uptake of cells exposed to different concentrations of the vitamin, is probably the most sensitive and rapid procedure yet available. It is derived from an initial radiocarbon uptake technique by Gold as modified by Carlucci and Silbemagel (Canadian J. Microbiol., 12 175, 1966). 

Alkali is usually added in a second stage. However, with low reactivity high affinity dyes it is possible to add the alkah at the beginning of the dyeing process and control the rate of uptake and chemical reaction by temperature control. With high affinity dyes the exhaustion takes place at low temperature rapidly before the chemical reaction becomes significant. If dyes are carefully selected or synthesized to have identical dye uptake it is possible to include all the electrolyte from the beginning and operate an "ah-in" technique. 

Polymer-based, synthetic ion-exchangers known as resins are available commercially in gel type or truly porous forms. Gel-type resins are not porous in the usual sense of the word, since their structure depends upon swelhng in the solvent in which they are immersed. Removal of the solvent usually results in a collapse of the three-dimensional structure, and no significant surface area or pore diameter can be defined by the ordinaiy techniques available for truly porous materials. In their swollen state, gel-type resins approximate a true molecular-scale solution. Thus, we can identify an internal porosity p only in terms of the equilibrium uptake of water or other liquid. When crosslinked polymers are used as the support matrix, the internal porosity so defined varies in inverse proportion to the degree of crosslinkiug, with swelhng and therefore porosity typically being more... 

The main principles of instrument design are summarized in Table 10.23. In filtration, e.g. for gravimetric analysis, selection of filter material (Table 10.22) requires careful consideration in terms of application, strength, collection efficiency, compatibility with pump, water uptake, etc. Humidity-controlled balance rooms, iTiicrobalances and careful handling techniques may be required. 

Electrochemical tests This group includes the various electrochemical tests that have been proposed and used over the last fifty or so years. These tests include a number of techniques ranging from the measurement of potential-time curves, electrical resistance and capacitance to the more complex a.c. impedance methods. The various methods have been reviewed by Walter . As the complexity of the technique increases, i.e. in the above order, the data that are produced will provide more types of information for the metal-paint system. Thus, the impedance techniques can provide information on the water uptake, barrier action, damaged area and delamination of the coating as well as the corrosion rate and corroded area of the metal. However, it must be emphasised that the more comprehensive the technique the greater the difficulties that will arise in interpretation and in reproducibility. In fact, there is a school of thought that holds that d.c. methods are as reliable as a.c. methods. 

A problem with employment of ASON in a larger clinical setting is their poor uptake and inappropriate intracellular compartmentalization, e.g., sequestration in endosomal or lysosomal complexes. In addition, there is a need for a very careful selection of the ASON-mRNA pair sequences that would most efficiently hybridize. To date, several computer programs are used to predict the secondary and tertiary structures of the target mRNA and, in turn, which of the mRNA sequences are most accessible to the ASON. However, even with this sophisticated techniques, the choice of base-pairing partners still usually includes a component of empiricism. Despite these principal limitations, it has become clear that ASON can penetrate into cells and mediate their specific inhibitory effect of the protein synthesis in various circumstances. 

Plutonium uptake on geologic media under various conditions have been studied extensively during the last few years, and reviewed (e.g. in ref. 60). It should be emphasized that data from various experiments are rarely directly comparable, due to differences in experimental techniques, chemical conditions and other parameters of importance. A detailed discussion of sorption data is outside the scope of this paper. 

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