Gas ‘blow-down

Preparation of calibration standards Seven standard solutions each containing all the target compounds (phenolics plus DIMBOA) were first prepared in HPLC-grade methanol at concentrations of 0.05, 0.1, 0.5, 1.0, 5.0, 10.0, and 20.0 pg/mL respectively. In separate analyses, 1.0 mL from each of the methanolic standards was pipetted into a 2 mL minivial and dried by gentle nitrogen gas blow-down. A 1.0 mL volume of internal standard p-chlorobenzoic acid at a concentration of 1.0 pg/mL in methanol was then pipetted into the dry minivial and dried again by blowdown prior to formation of silylated derivatives. 

This procedure is useful for removing very small volumes of solvents (about 2mL) from solutes by blowing a stream of nitrogen over the surface of the solution, while warming the solution gently. The main application of the gas blow-down is in the isolation of small amounts of solute from rotary evaporation or small-scale liquid-liquid extraction, for further analysis by instrumental techniques, where the sample size may be 20 mg or less for example, infrared spectroscopy (p. 180), NMR spectroscopy (p. 190), gas chromatography (p. 211) or liquid chromatography (p. 218). 

The simplest system for evaporation by gas blow-down is shown in Fig. 17.3. A Pasteur pipette is connected by a flexible tube to a cylinder of nitrogen, which has a gas blow-off safety system (p. 125). The sample is placed in a special tube with a conical base, such as a ReactiVial . Hold the Pasteur pipette and direct a gentle stream of nitrogen towards the side of the tube so that it flows over the surface of the liquid. As the solvent evaporates, the liquid and tube will cool and may condense atmospheric water into the tube. To prevent condensation, clamp the tube in a warm sand bath or above a closed steam bath or in the hole of a purpose-designed aluminium heating... 

To be able to carry out gas blow-down in a safe and controlled manner. 

Pre-concentration is concerned with the reduction of a larger sample into a smaller sample size. It is most commonly carried out by using solvent evaporation procedures after an extraction technique (see, for example, Chapters 7 and 8). The most common approaches for solvent evaporation are rotary evaporation, Kudema-Danish evaporative concentration, the automated evaporative concentration system (EVACS) or gas blow-down . In all cases, the evaporation method is slow, with a high risk of contamination from the solvent, glassware and blow-down gas. 

Figure 10.4 Schematic of a typical gas blow-down system (Turbovap ) system used for the pre-concentration of compounds in organic solvents.

Chapter 9 deals with the extraction of volatile compounds from the atmosphere. Particular emphasis is placed here on the methods of thermal desorption and purge-and-trap. Chapter 10 focuses on the methods used to pre-concentrate samples after extraction. In this situation, particular attention is paid to two common approaches, namely rotary evaporation and gas blow-down , although details of two other methods are also provided. 

During the molten coiium discharge phase, the first mechanism is unlikely. Under extreme conditions such as the sudden exposure of droplets to shock waves, the disintegration Weber number can be as low as 2 or 3. However, the condition around the disintegrated jet before the gas blow down phase does not meet such extreme gas turbulences requirement The second mechanism indicates that if the initial droplet size given by the above correlation far exceeds die spherical stable drop size limit then droplets will further disintegrate to reach this stability limit The spherical limit is ven by 2o ... 

ASTM F327-78(1989)el Standard Practice for Sampling Gas Blow Down Systems and Components for Particulate Contamination by Automatic Particle Monitor Method... 

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