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at pressures from 60 to 100 psi. Hydrogen Generators In the Packard Hydrogen Generator, hydrogen is generated electrolytically from pure deionized water. Unfortunately, the technology used in hydrogen generators is largely proprietary and technical details are not readily available. The electrolysis unit uses a solid polymer electrolyte and thus does not need to be supplied with electrolytes, only the deionized water. The manufacturers claim the device generates 99.999% pure hydrogen with a reservoir capacity of 4 liter, and an output pressure that ranges from 2 to 100 psi. Other units can produce hydrogen flows that range from 0 to 125 ml/min. to 0 to 1200 ml/min. The oxygen, produced simultaneously with hydrogen at half the flow rate, is vented to air

be hydrogen bonded to the hydroxyl groups and multi-layers of water physically adsorbed on top of these. Water can be hydrogen bonded to the silica gel surface in a number of different ways which are depicted in figure 31. None of the above structures has been confirmed in an unambiguous manner but all are reasonably possible. The center and right hand side structures contain a type of double hydrogen bond and would have high energies of formation and, thus, more stable than the simple hydrogen bond depicted on the left. The right hand structure would be particularly stable as it constitutes a four membered hydrogen bonded ring similar to that which might be expected to form in the strong association of water with itself. Figure 31. Different Ways in Which Water May be Hydrogen Bonded to Silica Gel Hydroxyl Groups

a hydrogen, or hydrogen/nitrogen stream, which is then burnt at a small jet and the ions produced measured by an appropriate pair of electrodes. The response of the detector will depend on the mass of solute eluted per unit time from the capillary column and, thus, will be independent of the hydrogen or hydrogen/nitrogen flow (as this will not effect the rate of solute elution, in terms of mass per unit time). In contrast, if the FID was a concentration device (like the katherometer), as the hydrogen or hydrogen/nitrogen flow dilutes the sample, the response will be directly related to the flow rate of the diluting gas. It follows, that for a mass sensitive device, the sensing volume can be extremely mall and, in fact, insignificant. However, for a device to sense a concentration change, the sensor must have a finite (albeit small) volume

14. The sensor of the NPD is a small rubidium or cesium bead contained inside a small heater coil. The helium carrier gas is mixed with hydrogen and passes into the detector through a small jet. The bead, which is heated by passing a current through the coil, is situated above the jet, and the helium-hydrogen mixture (produced by mixing the column carrier gas, helium with a separate stream of hydrogen) passes over it. If the detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen can be used and the mixture burned at the jet. A potential is applied between the bead and the anode. The heated alkali bead emits electrons by thermionic emission which are collected at the anode and thus produce an ion current. When a nitrogen or phosphorus containing solute is eluted, the partially combusted nitrogen and

to a 2 in. length of 1/2 in I.D. thin walled stainless steel tube filled with about 2 g of nickel catalyst and then to the aspirator. The wide tube was closed with a loose plug of quartz wool and . Figure 43 The Modified Moving Wire Detector The nickel catalyst was prepared by absorbing a saturated solution of nickel nitrate onto 20/40 BS mesh brick dust, decomposing the nitrate at 500˚C for 3 hours followed by reduction of the nickel oxide so produced to metal in a stream of hydrogen at 250˚C. The jet/venturi aspirator (supplied commercially as a molecular entrainer) was placed in line with the hydrogen flow to the detector. To improve the aspirating efficiency, the gas used was a mixture of hydrogen and argon, and with this mixture, the jet/venturi pressure drop continuously sucked the combustion gasses into the hydrogen stream. In figure 43 the reduced pressure side of the aspirator is shown connected to the side limb of the oxidation tube and the two

The actual NPD sensor is a rubidium or cesium bead contained inside a small heater coil. The helium carrier gas is mixed with hydrogen and passes into the detector through a small jet. The bead is heated by a current passing through the coil which is situated above the jet, and the helium-hydrogen mixture passes over it. If the detector is to respond to both nitrogen and phosphorus, then a minimum hydrogen flow is employed to ensure that the gas does not ignite at the jet. In contrast, if the detector is to respond to phosphorus only, a large flow of hydrogen can be used and the mixture burned at the jet. A potential is applied between the bead and the anode. The heated alkali bead emits electrons by thermionic emission which are collected at the anode and thus produce an