Containers adsorption onto walls

Silylation of apparatus makes it repellant to water and hydrophilic materials. It minimises loss of solute by adsorption onto the walls of the container. The glassware is placed in a desiccator containing dichloromethyl silane (ImL) in a small beaker and evacuated for 5min. The vacuum is turned off and air is introduced into the desiccator which allows the silylating agent to coat the glassware uniformly. The desiccator is then evacuated, closed and set aside for 2h. The glassware is removed from the desiccator and baked at 180° for 2h before use. 

Disposable containers should be used whenever possible to reduce die possibility of contamination. Liquid samples (blood, urine, and bile) are best placed in glass containers which e sealed widi a liner that is impervious to the sample (e.g. a FIFE liner). Liners made of rubber and similar materials should be avoided since they may absorb drugs or condibute contaminants (e.g. plasticisers) to die s ple. Glass may need to be silanised when low concentrations (< 10 ng/ml) of drugs are present to avoid adsorption onto the walls. 

The stability of the sample must be considered. Glucose, for example, is unstable, and a preservative such as sodium fluoride is added to blood samples. The preservation must not, of course, interfere in the analysis. Proteins and enzymes tend to denature on standing and should be analyzed without delay. Trace constituents may be lost during storage by adsorption onto the container walls. 

A detailed discussion of adsorption onto mesoporous solids is beyond the scope of this text, but certain features relevant to microporous solids should be described. Firstly, microporous solids can themselves contain mesoporosity. The most important example of this is observed in zeolites such as Y or mordenite that have been treated after synthesis to remove aluminium from the framework (Section 6.2.3). The migration of silica leaves mesopores that are evident from nitrogen adsorption isotherms and directly visible by electron microscopy. The presence of secondary mesopores enhances diffusion and catalytic properties. Conversely, mesoporous solids that are well ordered on the mesoscale can contain disordered micropores in their walls. The mesoporous channels of calcined SBA-15, for example, are connected by micropores that result from removal of block copolymer chains that run between the large channels in the as-synthesised material. This is observed from nitrogen... 

A particular problem associated with samples having very low (trace and ultra-trace) levels of analytes in solution is the possibility of losses by adsorption onto the walls of the container or contamination by substances being leached from the container by the sample solvent. Trace metals may be depleted by adsorption or ion-exchange processes if stored in glass containers, whilst sodium, fxrtassium, boron and silicates can be leached from the glass into the sample solution. Plastic containers should always be used for such samples. 

During the storage of the sample, loss of analyte can occur via vaporisation, degradation, and/or adsorption. Adsorption of trace organic and inorganic species in seawater to container walls can severely affect the accuracy of their determination. The adsorption of dichlorodiphenyltrichloroethylene [67] and hexachlorobiphenyl [68] onto glass containers has been observed. 

Any detectable effect on the reaction or behavior of a particular system by the interior wall of the container or reaction vessel. Because proteins can form high-affinity complexes with glass and plastic surfaces, one must exercise caution in the choice of reaction kinetic conditions. Wall effects can be discerned if one determines catalytic activity under different conditions that minimize or maximize contact of the solution with the container. In principle, an enzyme-catalyzed reaction should proceed at the same rate if placed in a capillary or a culture tube however, contact with the wall is maximized in a capillary, and wall effects should be more prominent. Some investigators add bovine serum albumin to prevent adsorption of their enzyme onto the container s walls. 

Adsorption of peptides and proteins onto the walls of containers has been reported for various formulations.833 835 General solutions to this problem remain to be found, although surfactants appear to be effective in reducing drug binding to surfaces.836-837 Freeze-dried formulations of peptides and proteins may degrade via moisture adsorption from the headspace of the containers and rubber stoppers.838... 

Much less data are available on the adsorption of elements onto surfaces from blood or urine samples. Stoeppler (1980) did not detect any loss of added Ni from urine samples onto polyethene container walls. Concentrations of nickel or chromium in urine samples, spiked with small concentrations of the metals and stored for 6 months at 4°C did not show a decrease (Kiilunen et al.. 1987). The lUPAC reference method for nickel in urine calls for acidification of urine quality control samples with nitric acid and storage in polypropene tubes with a screw-cap at -20°C (Brown et al., 1981). No adsorption of cadmium onto container walls (type not specified) was seen from urine acidified to pH < 2 (Stoeppler and Brandt, 1980). 

The container material has to be carefully chosen to avoid undesirable effects such as adsorption of the analyte onto the vessel walls or contamination of the solutions. Thus adsorption of organotins onto PTFE and glass has been... 

The solutions were stable at — 20°C in all conditions tested (pH 2 and 6, with and without chloride, without addition of acid). At ambient temperature, samples stored at pH 2 in polyethylene containers showed instability of selenite after one month storage whereas selenate remained stable this difference in behaviour was attributed to possible adsorption of selenite onto the container walls. The stability was better at pH 6 but instability of selenite was still observed after two months storage, whereas selenate remained stable. When a FIFE container was used, dramatic losses of selenite were observed at pH 6. 

---