De-gassing

Water from eertain underground strata, due to de-gassing (page 46). 

The eluant must be filtered and de-gassed before use, e.g. using a Millipore filter system. 

To determine the quinine content of tonic water it is first necessary to de-gas the sample either by leaving the bottle open to the atmosphere for a prolonged period or by stirring it vigorously in a beaker for several minutes. Take 12.5 mL of the de-gassed tonic water and make up to 25 mL in a graduated flask with 0.1M sulphuric acid. From this solution prepare other dilutions with 0.05M... 

The mixing can be done by hand, or in low pressure mixer/dispensers, and in reaction injection moulding (RIM) machines. In the latter operation the de-gassing operation is not required. 

The transfer hydrogenation methods described above are sufficient to carry out laboratory-scale studies, but it is unlikely that a direct scale-up of these processes would result in identical yields and selectivities. This is because the reaction mixtures are biphasic liquid, gas. The gas which is distilled off is acetone from the IPA system, and carbon dioxide from the TEAF system. The rate of gas disengagement is related to the superficial surface area. As the process is scaled-up, or the height of the liquid increases, the ratio of surface area to volume decreases. In order to improve de-gassing, parameters such as stirring rates, reactor design and temperature are important, and these will be discussed along with other factors found important in process scale-up. 

Mobile Phase - Prepare a filtered and de-gassed mixture of alcohol-free chloroform, n-hexane, and tetrahydrofuran (about 70 30 1 by volume). The ratio of components and the flow rate may be varied to meet system suitability requirements. 

Solution Phase Studies. Freshly prepared solutions were used for all experiments. All solution phase studies were carried out using samples which had been thoroughly de-gassed using a minimum of three freeze-pump-thaw cycles for each sample, lire final pressure above the sample was in all cases less than 5 x 1(H mBar. Laser energy densities of less than 2.5 mJ cm-2 were used such that sample degradation was kept to a minimum. 

Once suitable ionization conditions have been established, LC separation can be optimized. As with any LC system, attention needs to be paid to column choice, correct tubing diameters, zero dead volume connections, use of guard columns, and mobile phase filtration and de-gassing. Modem LC pumps can deliver reliable gradients at low flow rates but for capillary LC, precolumn flow splitting or specialized pumps may be necessary. Mobile phase composition and pH should be chosen to... 

To de-gas the solution just before use, reduce the pH of the solution to be de-gassed to about 7 (by bubbling CO2 through the solution) and stir magnetically in a stoppered 5-L Buchner flask. 

Connect the flask to a water vacuum pump. The de-gassing only takes about 2-3 min until the bubbles have cleared slightly but not totally. Another method of de-gassing is by filtering the solution through a membrane (particularly useful for de-gassing HPLC solutions). 

Make up two solutions from the stock solution of 2 M TEAB (Protocol 1). Solution A is 0.1 M TEAB (100 mL of stock made up to 2 L with HPLC grade water) and solution B is 0.1 M TEAB with 30% acetonitrile (100 mL of stock plus 600 mL of acetonitrile made up to 2 L with HPLC grade water). These need to be de-gassed or filtered though a millipore 5 tiM filter before they can be used for HPLC. 

Adsorbents were de-gassed at 100°C for 24 hours prior to heat treatment. Target temperatures were attained at a ramping rate of 10 °C per minute and maintained for a "soak" time of 24 hours, after which the ftimace was allowed to cool to ambient conditions. Heat treated samples are coded as either HTIOOO (1000 °C) or HT650 (650 °C). 

For seawater analysis, it is possible to pump the sample directly from the sea to a shipboard analyser. The sample can then be de-gassed in-line and handled in a shipboard flow system. This possibility, and the main practical, physico-chemical and interpretative aspects, were discussed in relation to the in situ spectrophotometric determination of seawater pH [9]. The attractive features and applications of shipboard flow injection spectrophotometric determinations are discussed elsewhere [10,11]. A compact, shipboard flow analysis system incorporating a LTV photoreactor has also been reported for the determination of total phosphorus in estuarine and marine waters [12]. Another way to sample seawater is to use a towed torpedo-shaped "fish," deployed off the crane arm of a hydrographic winch at a distance of 5 m from the ship, connected to a deck mounted pump by appropriate tubing. Filtration can be incorporated in-line and the filtered sample collected or pumped directly to the flow analyser [13]. This approach (Fig. 8.1) minimises contamination and preconditions the sample, as demonstrated in the chemiluminometric determination of iron(II) in surface seawater. 

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