Adsorption losses

The retention of membranes are often measured in stirred cells. A mass balance on the cell integrated over time from an initial retentate volume (V ) and initial concentration (Q,) to a final retentate volume (Vf) and final retentate concentration (Cf) yields the following expression for the retention (R)  

Equation 3 is used to calculate retention solely on the basis of changes in the retentate volume and concentration. Obviously, if adsorption losses are appreciable, the retention calculated from Equation 3 will be too low since CfWill be lower than it would be without adsorption. For this reason, the retention should also be calculated with reference to the solute concentration in the ultrafiltrate (permeate). 

A simple cumulative mass balance on the cell leads to another expression for 

if there are significant adsorption losses, the retention calculated from Equation 4 will be too high since CUf will be lower than it would be without adsorption. 

If the initial and final retentate concentration (C0 and Cf) are measured along with the final ultrafiltrate concentration (CUf), the retentivities calculated from Equations 3 and 4 may be compared. 

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The properties of an organic tracing compound should minimize loss while in transit. There are two main sources of dye loss, non-adsorptive loss and adsorptive loss. Nonadsorptive losses can be due, among other reasons, to photochemical decomposition, chemical decay, pH effects, and biodegradation of the compound by microorganisms. Adsorption of the tracer onto both organic and inorganic substrates is often irreversible and can be a source of much loss. 

Human serum albumin (HSA) may be used as a protectant against adsorptive loss of proteins present at low concentrations. HSA is present at higher concentration than the active substance and is preferentially adsorbed, coating the surface of interest and preventing adsorption of the drug. For example, insulin is subject to adsorptive loss to hydrophobic materials. Addition of 0.1-1.0% HSA has been reported to prevent this adsorptive loss [9],... 

High adsorption loss observed in the present work in both dynamic and static tests indicates a possibility of multilayer adsorption. However, the long times required to achieve adsorption equilibrium may indicate interlayer adsorption in the clay minerals. 

Phase separation of the saturated solution from the excess solid solute is a critical process. If a filter is employed, it must be inert to the solvent, it must not release plasticizers, and its pore size must be small enough to retain the smallest particles of the solid solute. Furthermore, steps must be taken to monitor, minimize, and preferably avoid losses of the dissolved solute by adsorption onto the filter material [27-30] and/or onto the vessels, pipettes, and syringes. Typically, the first small volume of filtrate is discarded until the surfaces of the filter and/or vessels are saturated with the adsorbed solute, to ensure that the filtrate analyzed has not suffered significant adsorption losses. Adsorption can be a serious problem for hydrophobic solutes, for which filtration would not be recommended. 

Low-density polyethylene containers are suitable for storing seawater samples at 4 °C and natural pH, provided that they are thoroughly cleaned (in 2 M hydrochloric acid for at least a week) and adequately conditioned (with prefiltered seawater for at least one to two weeks). Storage can be prolonged for at least three months (or five months for cadmium) without significant concentration changes. For lead and copper, adsorption losses are observed after five months. 

A potentially more sensitive and selective approach involves reaction of formic acid with a reagent to form a chromophore or fluorophore, followed by chromatographic analysis. A wide variety of alkylating and silylating reagents have been used for this purpose. Two serious drawbacks to this approach are that inorganic salts and/or water interfere with the derivatisation reaction, and these reactions are generally not specific for formic acid or other carboxylic acids. These techniques are prone to errors from adsorption losses, contamination, and decomposition of the components of interest. Enzymic techniques, in contrast, are ideal for the analysis of non-saline water samples, since they are compatible with aqueous media and involve little or no chemical or physical alterations of the sample (e.g., pH, temperature). 

The electron-capture detector was originally found to be a sensitive detector for the methylarsines [716]. After improvements of the atomic absorption detectors had been made (especially concerning adsorptive losses and peak shapes of the methylarsines), it was found that this detector could be used to replace the electron-capture detector, which because of its lack of specificity and its sensitivity to contamination and changes in operating conditions was very inconvenient to work with. 

In static method a known amount of contaminant is introduced into a fixed volume of air in devices such as teflon bags, gas sampling bulbs and gas cylinders, etc. Dynamic methods involve continuous introduction of contaminant (at a controlled rate) into a stream of air. Static methods are generally much simpler to construct and use, however, these suffer from a number of problems. Dynamic methods, while more elaborate and relatively more expensive, offer greater flexibility in concentration range, sample volume and are also less affected by adsorption losses. 

After shaking, the vials were centrifuged to remove the fines from the supernatant and an aliquot of the supernatant was counted against a standard of the particular formulated liquid phase. All experiments were performed in triplicate, at constant activity and Hg-carrier concentration (20 pg Hg/ ml) with controls (liquid phase without sorbent) to check for possible adsorption losses on the container. The experiments were carried out in nitrate and chloride media at pH 1-10. 

Table VII. Percent Solute Adsorption Losses (Gains) Experienced During Screening Test...

The adsorption losses (%) shown in Table VII were used to calculate the amount of solute taken up by a freshly flushed system. Field application of the RO concentration method incorporated conditioning periods in which membranes and other system components were exposed to the sample (and its concentrates) to satisfy and minimize adsorptive solute loss. 

Final sample mass (Aff) and initial concentrate mass, adjusted for expected adsorption losses (Aff), were used to calculate total mass recovery (see last column in Tables VIII-X). These values reflect efforts to correct recoveries for adsorption losses. 

Recovery levels for individual organic compounds were, as expected, less than the values predicted by using the membrane solute rejections. Differences in the actual recovery and the theoretical recovery were partially due to adsorption losses. Mass balance analysis still indicated a deficiency in some cases. Rectification of the inconsistencies in these data was complicated by the limited water solubility of compounds chosen for study, the necessity of using a cosolvent in spiking, and, in particular, the limitations of the analytical procedures at these extremely low concentrations. 

The conjugates are stable for several years at 4°C because the NaN3 inhibits microbial growth and the BSA minimizes denaturation and adsorption losses. These conjugates should not be frozen. 

Adsorption Loss of metals due to adsorption to glass surface Loss of oily materials due to adsorption to plastic surface Use of plastic containers, preservation with nitric acid to pH < 2 Use of glass containers, preservation with sulfuric acid to pH < 2... 

Total metal concentrations are often very low in natural systems and in performing speciation analysis it is necessary to measure even lower concentrations on attempting to resolve component species. Therefore, very sensitive methods are needed and there is a high risk of contamination, alteration and/or adsorption losses. The ideal speciation method would be sufficiently sensitive and selective to be used directly on natural water samples, would involve minimal perturbation of the sample, and would furnish an analytical signal directly dependent on the (chemical) reactivity of the element of interest (Buffle, 1981a) (Fig. 8.1). 

The silylation of all glassware that contacts the plant extract has proven to effectively reduce adsorption losses. As diagrammed in Figure 8, the hydroxyl adsorption sites on the silica surface can be coated with dichlordimethyl silane. The unreacted chloride groups are then displaced with methanol in a substitution reaction. A secondary advantage of the silyation process is that water will not adhere to the glass surface. Aqueous residues bead together, which allows more efficient sample transfers. 

Malmberg, E.W. Smith, L. The Adsorption Losses of Surfactants in Tertiary Recovery Systems in Porous Media in Improved Oil Recovery by Surfactant and Polymer Flooding, Shah, D.O. Schechter, R.S. (Eds.), Academic Press New York, 1977, pp. 275-292. 

Prepare 100 mL of an enzyme solution of FJP. lysozyme HC1 in bldistilled water containing approximately 4000 FJJ. nnils/mL. Use siliconized glassware to prevent adsorption losses. Use quartz cuvettes suited for UV measurements end perform a cuvette correction. Keep the stock solution at a constant temperature of 25°Q Lysozyme in aqueous solutions is very stable, however, it is recommended to renew die solution for every assay series. 

Samples are often acidified with a small amount of nitric or hydrochloric acid if to be used for trace metal determinations, and stored in plastic rather than glass vials to minimize the risk of adsorption losses. Storage containers should be acid washed and thoroughly rinsed with deionized water. Acidification may result in precipitation of naturally occurring dissolved organic matter, however. Speciation studies should be completed immediately after collection on fresh samples. 

Potential of non-specific binding (NSB) to filter membrane or plastic devices. Low recovery from either protein-filtrate or buffer indicates adsorptive losses and/or membrane rejection... 

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