Helium, as a purge gas

The sensitivity of the instrument for diazepam was found to be about 4 x 10 g/ml. The wire samples the eluent at 10 jil/min at the maximum wire speed and thus as the scan speed of the mass spectrometer was 1 scan/sec this sensitivity corresponded to ca. 7 X 10 10 g of diazepam per spectrum. The pressure in the source could be maintained at 1 x 10 mmHg. The use of helium as a purge gas in the interface reduces the noise level of the mass spectrometer by a factor of three with a commensurate increase in sensitivity. 

Better heat transfer is achieved if helium is used as a purge gas. The heat capacity constant is very sensitive to small changes in the flow rate of helium and so helium should only be used as a purge gas if mass flow controllers are used to control the flow rate. 

Differential scanning calorimetery (DSC) was used to measure heat capacity as a function of temperature. The DSC used in this study was a Perkin-Elmer model DSC-2. Liquid nitrogen was used as a heat sink and helium was used as the purge gas. Samples were usually about 30 mg, and a heating rate of 20°C/min was used for measuring Tgs and Tms. 

Procedure (See Chromatography, Appendix IIA) Use a gas chromatograph equipped with an electrolytic conductivity detector operated in the halogen mode and fitted either with a capillary injector operated in the splitless mode or with a purged, packed injector with a glass insert. Use a 30-m x 0.53-mm (id), fused-silica column, or equivalent, coated with l-(xm Supelcowax 10 or an equivalent bonded carbowax column fitted with a 50-cm retention gap of 0.53-mm, deactivated, fused silica, or equivalent. Set the column temperature to 170° for 5 min, raise the temperature at a rate of 5°/min to 250°, and hold it at that temperature for 10 min. Maintain the injector temperature at 225°. Use helium as the carrier gas at a flow rate of 8 mL/min. 

A third sample preparation method is purge and trap, which aims to extract as close to all of the analyte as possible from the solid or liquid sample and is a deviation from headspace sampling. It works by bubbling a purge gas such as helium through the heated sample vial. The gas carries analyte up into an adsorption tube packed with selective stationary phase. After all the analyte has been trapped in the tube, the gas flow is reversed through the tube to remove any residual solvents. The tube is then directed to the injector port and, heated to desorb the analytes, which are then cold-trapped onto the head of the GC column. From there, the concentrated sample is heated for GC separation. 

The telescope is cooled by supercritical helium gas (initially at about 5 K and 5 atm pressure) stored in external tanks. Figure 3 shows a cutaway view of the telescope surrounded by three radiation shields and supported on conical fiberglass standoffs. The cold He gas is circulated through tubes cooling first the telescope walls, optics, and instrument chamber, then the radiation shields, and finally is vented into space through the telescope as a purge to impede the flow of contaminants onto the cold surfaces. At low flow rates the telescope temperature is maintained at about 17 K and 1 to 2 160-kg tanks are... 

In these gas-flow proportional counters, the windows are exceedingly thin, fragile,. and unavoidably leaky. Such a window is satisfactory only if a steady flow of. the filling gas is maintained at minimum pressure differential against the helium atmosphere in the optical path. The purging of impurities from the counter is an incidental benefit derived from the gas flow. Hendee, Fine, and Brown20 look upon the gas-flow proportional counter as a steppingstone on the road toward a windowless counter. 

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