Direct gas displacement pumps

With such a direct gas displacement pump, the delivery of liquid is virtually pulseless. The liquid pressure is controlled by the pressure controller on the gas supply line. The pressure is limited by the strength of the tank and by the maximum pressure of the gas supply (usually 200 atm, sometimes up to 250 atm). Some commercial direct gas displacement pumps, however, have a maximal pressure of about 100 atm, owing to the safety aspects of working with compressed gases at higher pressures. 

These pumps, especially the direct gas displacement pumps, are the cheapest chromatographic pumps. With pneumatic amplifier pumps, it is possible to obtain high pressures (400 or 500 atm) at low cost. No electric power but only a gas supply and a pressure controller are required. 

Two different kinds of direct injection nebulizers are available commercially. The total consumption nebulizer was developed by Greenfield et al. [36] for ICP optical emission spectrometry. The concept for the Cetac direct injection nebulizer (DIN) was developed by Fassel, Houk, and coworkers [35,37]. It has a narrow sample-carrying capillary [30-50 xm inner diameter (i.d.), 0.5 to 1 m long] that extends slightly past the nebulizer gas tube. A second, auxiliary or makeup, nebulizer gas is introduced through another concentric tube outside the nebulizer gas tube. A gas displacement pump (up to 1500 psi) or HPLC pump is used to deliver the sample to the nebulizer through the long, narrow capillary. 

Pneumatic pumps typically use a pressurized gas as the driving force to force the liquid through the column. The gas pressure can be either applied directly to the liquid (gas displacement pumps) or amplified so that the liquid pressure is greater than the gas pressure (pneumatic amplifier pumps). 

Recently, Houk s laboratory [18,19] described a direct injection nebulizer for use in ICP-MS consisting of a stainless steel tube with an inner diameter of 250 p.m held in a ceramic support tube that is inserted directly into the quartz injector tube of the ICP torch. A positive displacement gas pump is used to achieve a sample flow rate of 120 p,L/min. The low flow rates decrease sample consumption and the essentially 100% transfer efficiency improves detection limits by an order of magnitude. However, solvent loading presents a difficulty and the plasma content of solvent-derived polyatomics such as metal oxides was increased up to threefold [18]. 

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