Automatic column head

The apparatus must be adequately insulated and the column jacket provided with the requisite heat compensation (section 7.7.3). The use of an automatic column head (section 7.5.3) allows the distillation to be performed with a minimum of attention. In the present case it would be useful to place a contact thermometer, connected to a bell via a relay (section 8.2.2), in the column head. The thermometer would be set to the boiling point of benzene, and when a signal was heard, the initial reflux ratio of 9 would be increased, say, to 20. In the same way it would be progressively raised to the final value of 50. The load may conveniently be regulated by means of a contact manometer actuated by the pressure differential (section 8.4.2). In accordance with the amount of the charge the still pot may be a pilot-plant flask (section 7.6.1) or a round-bottomed flask of 4 to 10 litres capacity, heated by a flask heater (section 7.7.1). 

The automatic column heads are based on a time cycle by electronic or mechanical means (c/. section 8.4) the device is alternately switched for a definite period into each of two positions the periods in question determine the reflux ratio. This requires... 

Reflux ratio by an automatic column head, actuated by mechanical or electronic means (sections 7.5. and 8.4.1) ... 

It forms a picrate m 149° (from EtOH-pet ether) from which the free base can be recovered using a basic ion exchange resin and can then be distd through a Todd column using an automatic still head which only collects products boiling below 51°/atm. Polymeric materials if present will boil above this temperature. The hydrochloride has m 85-86°. The pKa in 40% EtOH is 5.33. [Roberts and Chambers JACS 73 5030 1951, Jones JOC 9 484 7944 Emmons J A CS 79 6522 1957. ... 

Sample introduction is a major hardware problem for SFC. The sample solvent composition and the injection pressure and temperature can all affect sample introduction. The high solute diffusion and lower viscosity which favor supercritical fluids over liquid mobile phases can cause problems in injection. Back-diffusion can occur, causing broad solvent peaks and poor solute peak shape. There can also be a complex phase behavior as well as a solubility phenomenon taking place due to the fact that one may have combinations of supercritical fluid (neat or mixed with sample solvent), a subcritical liquified gas, sample solvents, and solute present simultaneously in the injector and column head [2]. All of these can contribute individually to reproducibility problems in SFC. Both dynamic and timed split modes are used for sample introduction in capillary SFC. Dynamic split injectors have a microvalve and splitter assembly. The amount of injection is based on the size of a fused silica restrictor. In the timed split mode, the SFC column is directly connected to the injection valve. Highspeed pneumatics and electronics are used along with a standard injection valve and actuator. Rapid actuation of the valve from the load to the inject position and back occurs in milliseconds. In this mode, one can program the time of injection on a computer and thus control the amount of injection. In packed-column SFC, an injector similar to HPLC is used and whole loop is injected on the column. The valve is switched either manually or automatically through a remote injector port. The injection is done under pressure. 

Gagliardi P. and Verga R.G. (1982) Automatic direct head space GC analysis of flavors with capillary column and multidetector systems. In The Quality of Foods and Beverages, Chemistry and Technology, Athens, 20-21.7.81, Charalambous G. and Inglett Eds, Academic Press, New York, pp. 49-72. 

An important point in analytical distillation is to use a stUl head having as low a dead space as possible. By dead space that volume of liquid is denoted which is retained above the column in the stUl head instead of taking part further in the countercurrent process. The automat ic column heads for the partition of vapour or... 

As opposed to all other column heads the automatic forms have the advantage that they do not usually contain taps, so that there can be no contamination with grease. Furthermore they allow the reflux ratio to be set rapidly and reliably at any desired value. The hold-up is verj sraaE. Large reflux ratios, from about 30 1 up to 100 1, may be adjusted accurately, whilst with valves (even of the fine control tyjx ) the adjustment becomes extremely difficult above a ratio of 30 1. 

Krell s automatic Destinorm column head with. swinging funnel... 

Column heads of semi-technical plants are made up of separate condenser and reflux units. The valves are operated mechanically and, more commonly, electro-magnetically. Their operation is similar to that in laboratory column heads (Figs. 306, 247). The valves are controlled automatically by means of electromagnets, electromotors or pneumatic drives (Fig. 248). For the automatic division of liquid, however, the swinging funnel is chiefly used (Fig. 142) which is actuated by an electromagnet placed outside the column wall. With the magnet switched on the distillate is taken off laterally. 

The timers functioning entirely on electronic principles allow take-off and reflux periods of 0.1 sec to 20 min. to be adjusted with an accuracy of 1% [56]. For an alteration of the reflux ratio they require a change to be made in both the take-off and reflux times. A number of electronic devices are commercially available most of which are provided with bubble-point control. The column head is set at infinite reflux ratio as soon as the pre-set temperature of the contact thermometer placed in the column head is reached. When the temperature falls below this threshold the preselected reflux ratio is automatically switched on again. As an example. Fig. 386... 

If the GC is equipped with some type of electronic pressure or flow control system, the flow rate variations can be reduced to negligible values. In such devices the column length and diameter, type of carrier gas and desired average linear velocity are inputted into the GC software. The GC automatically determines the current column temperature during temperature programming (see below) and adjusts the column head pressure so that the desired u gj. is obtained. A more rigorous approach to measuring volume flow rate and linear velocity (Hinshaw 2002) explicitly considers the effects of gas compressibility. 

Perfluorobutyric acid [375-22-4] M 214.0, m -17.5", b 120"/735mm, d 1.651, n 1.295, -0.17. Fractionally distd twice in an Oldershaw column with an automatic vapour-dividing head, the first distn in the presence of cone H2SO4 as a drying agent. 

Fractionally distd twice in an Oldershaw column with an automatic vapour-dividing head, the first distn in the presence of cone H2SO4 as a drying agent. 

Each sample to be analyzed was dissolved in tris(ethylenediamine)cadmium dihydroxide (1 mL) by stirring overnight, and then water (1 mL) was added. A 1-mL aliquot (concentration < 1.0%) was applied to the column, and elution proceeded with a pressure head of 100 cm and flow rate of 10 mL/h. A Turner 111 fluorometer (excitation filter 2A plus 47B and emission filter 8 plus 65A) fitted with a flow-through door allowed for automatic continuous monitoring of carbohydrates as they were eluted. Relative fluorescence was automatically recorded on a linear strip recorder. Fractions of 3 mL were collected on a FC-80K Gilson microfractionator. Typically, each sample was analyzed several times, usually at different concentrations, to ensure the reproducibility and accuracy of the data. A calibration run using the labeled dextrans was performed a minimum of one time per week. 

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