Vacuum outlet

For coluzms with large plate numbers or operated with a vacuum outlet (as might be the case in GC/MS) such that P 1 the optimum separation time is given by equation (1.59)... 

A 250-mL, one-necked, round-bottomed flask is equipped with a magnetic stirrer and a reflux condenser protected by a calcium chloride drying tube. Into the flask are placed 30.0 g (0.14 mol) of di-tert-butyl malonate (Note 1) 8.4 g (0.28 mol) of paraformaldehdye (Note 2), 1.4 g (0.014 mol) of potassium acetate, 1.4 g (0.007 mol) of cupric acetate monohydrate, and 70 mL of glacial acetic acid. The resulting green-white suspension is placed in an oil bath preheated to 90-100°C and stirred for 2 hr (Note 3). The reaction mixture is allowed to cool to room temperature, and the reflux condenser is replaced with a short-path distillation apparatus, the vacuum outlet of which 1s connected in sequence to a trap cooled in acetone-dry ice, a potassium hydroxide trap, another trap cooled in acetone-dry ice, and a vacuum pump. The receiving flask 1s cooled in acetone-dry 1ce, and the system is evacuated over approximately 1 hr to remove acetic acid and other volatile material... 

Several methods may be used to attach an NMR tube to a vacuum system. The O-ring union, illustrated in Fig. 8.5, permits attachment of standard NMR tubes to a vacuum outlet. Once the NMR tube has been filled with an air-sensitive sample on the vacuum system, it is necessary to bring the tube up to atmospheric pressure with inert gas, remove the tube, and quickly stopper it. Recently, a small in-line valve has been introduced which has a provision for an O-ring seal with a specially constructed NMR tube.22 This valve allows the NMR tube to be sealed before it is disconnected from the vacuum line, and the valve s left on the tube during data collection. Owing to its symmetric design, the valve does not interfere with the spinner. 

The foregoing conclusion is subject to another constraint for gas chromatography a substantial pressure drop is sometimes needed to force the gas phase through the column. In this case the mean pressure, unable to approach zero, should at least be reduced to a minimum, which for true optimization will require vacuum outlet conditions [31]. 

The vaporized glycerine stream passes through the entrainment separator [10] and condenses on the internal U-tube condenser [9]. Droplets of material entrained with the vapor stream impinge on the entrainment separator and flow back to the heated wall through centrifugal force of the rotating assembly. Distillate flows out the distillate outlet [11] and noncondensables flow out through the vacuum outlet [13]. 

The optimum linear velocity for a capillary column depends on the pressure in the column because Wopt is proportional to the average diffusion coefficient, which varies inversely with pressure. Operation of a short wide-bore column at vacuum outlet conditions results in a significantly faster analysis than would occur if the same column was used under atmospheric outlet pressures. Mass spectrometry (MS) has made vacuum GC very easy to implement, since the mass spectrometer provides both detection and a source of vacuum. Vacuum GC can be achieved practically by incorporating a restriction at the inlet end of a wide-bore capillary column, and interfacing the terminal end of the column directly into the MS. The function of the restriction is to deliver an optimal helium flow for the mass spectrometer, and it can be as simple as a short section of 20 pm i.d. capillary (or a longer section of 100-150 pm i.d. capillary). An optimal carrier gas velocity of 90-100cms can be expected for a 10 m x 50 pm column with a restriction at the inlet, and a speed gain of a factor of 3-5 times can easily be obtained. 

Mix the solution containing urea, H2O, acrylamide, NP-40, and ampholytes (kept at —20°C in 1-ml aliquots) in a tube containing a vacuum outlet (Fig. lA, g). Swirl the solution gently until the urea is dissolved. The solution should not be heated. Add ammonium persulfate and TEMED, mix gently, and degas using a vacuum pump. Use a clean rubber stopper to control the vacuum. 

FIGURE 5.25 High-speed isothermal separation of a 13-component mixture without stop-flow operation (a), with a 2-s-wide stop-flow pulse to separate components (1 and 2), (b) and (c) with three stop-flow pulses to separate component pairs (1,2), (10,11), and (12,13). Vacuum outlet GC was used with an outlet pressure of 0.5 atm., and atmospheric-pressure air was used as carrier gas. Components are 1, ethyl acetate 2,2-butanone 3, benzene 4, 1-butanol 5, trichloroethylene 6, n-heptane 7, 2,5-dimethylfuran 8, 2,4-dimethylhexane 9, 3-methyl-l-butanol 10, toluene 11, 2-methylheptane 12, butyl-acetate 13, chlorobenzene. 

FIGURE 5.30 Diagram of autonomous microfabricated GC for air monitoring. Vacuum outlet GC with ambient air as carrier gas is used to eliminate the need for compressed gases. A dual-column ensemble consisting of two 3.0-m-long colnmns with independent temperature control and stop-flow operation is used for selectivity enhancement, and a chemiresistor sensor anay is used for vapor identification. 

To a three-necked round-bottomed flask equipped with a mechanical stirrer, condenser, and vacuum outlet are added 208 gm (2.0 moles) of 1,5-pentanediol, 180 gm (2.2 moles) of 37% formaldehyde (formalin), and 1 gm of sulfuric acid. This mixture is heated to 95 C, and vacuum is applied. Heating is continued for 3 hr at 100°C to afford a wax, m.p. 25°-45°C, hydroxyl number 69.5 and mol. wt. 1610. 

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