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there to support this relatively trivial, but critical device. The oven also will contain a temperature sensor and if necessary an appropriate temperature programmer. As the mobile phase is a gas, there are virtually no interactions between the sample components and the mobile phase and thus the elution time can not be controlled by techniques such as solvent programming or gradient elution. The counterpart to gradient elution in gas chromatography is temperature programming. The column temperature is raised continuously during development to elute the more retained peaks in a reasonable time. It is a similar technique to flow programming but decreases the retention exponentially with temperature as opposed to linearly with flow rate. The temperature was originally programmed in a linear manner using electro-mechanical devices but modern temperature programmers contain a dedicated micro processor for the purpose. Sometimes all controls are initiated from a central

system, particularly if the column is being temperature programmed. Raising the temperature of a gas causes the viscosity to increase, and at a constant inlet pressure, the flow rate will fall. The reduction in flow rate will be related to the temperature program limits and to a certain extent on the temperature gradient. To obviate the flow rate change, mass controllers are used which ensure a constant mass of mobile passes through the column in unit time irrespective of the system temperature. A diagram of a mass flow controller is shown in figure 3.   Courtesy of Porter Instrumentation Company Inc.   Figure 3 The Mass Flow Controller The sensing system consists of a bypass tube with a heater situated at the center. Precision temperature sensors are placed equidistant up stream and down stream of the heater. A proprietary set of baffles situated in the main conduit creates a pressure drop that causes a fixed

analysis (TGA). Formally, the silica was heated for known times at known temperatures and the loss in weight noted (21). With the advent of the programmed TGA apparatus this procedure has been simplified a great deal and the basic thermogram can now provide considerable information on the nature of the surface hydroxyl groups and adsorbed water (22).  The Thermogravimetric Analysis of Silica Gel The sample is suspended from the arm of a continuously  recording  micro balance in a temperature controlled furnace and is heated from a defined starting temperature to a specified final temperature at a designated heating rate usually given as temperature change per unit time. The temperature and the sample weight are continuously digitized and the data stored. The results can then be printed out or presented as an appropriate graph relating sample mass to temperature. To help identify the desorption of different species, derivative curves can also be produced. The results

flow rates, adjust individual gas flows and, when and if necessary, program the mobile phase flow rate. The second unit is the sampling unit which contains an automatic injector which is situated inside a thermostatically controlled enclosure. The injector usually has its own oven, but sometimes shares the column oven for temperature control. The injector oven, if separate from the column oven, is serviced by its own temperature controller which both monitors and controls the temperature. There is normally a separate controller, usually a microprocessor, that controls the injector itself. The injector can range in complexity from a simple sample valve, or mechanically actuated syringe to an automatic multi sampler that is microprocessor controlled. It can have a complex transport system (such as a carousel) that can take samples, wash containers, prepare derivatives and, if necessary, carry out a very complex series of sample preparation procedures before

enough to block the adsorption sites it is not active enough to cause sample decomposition. It is seen that the peaks exhibit excellent symmetry for free acids. Teraphthalic acid has also been used for this purpose to deactivate the support. The column was glass, 3 m long and 2 mm in diameter and packed with a silicone polymer, SP-216-PS on 100/120 mesh Supelcoport which is a proprietary support that has already been deactivated and treated with phosphoric acid. The column was temperature programmed from 130oC to 200oC at 15oC/min. Nitrogen was used as the carrier gas at a flow rate of 20 ml/min.. The separation is effective, relatively rapid, and accurate quantitative results should be easily obtainable from the system. This analysis also demonstrates the need for rapid sample preparation techniques as well as rapid separations. Fast chromatography is of little use if the chromatograph is idle for long periods between samples while they are being prepared

stable despite the temperature program, and that the  high sensitivity and wide dynamic range of the FID make it invaluable for quantitative analysis. Anothertype of analysis frequently carried by the hydrocarbonindustryistheparaffin,  isoparaffin,  aromatic,   naphthalene and olefin estimation(the so-called PIANO analysis), an example of which is shown in figure 21. The column was fused silica, 50 m long and 0.5 mm I.D., and the stationary phase was also Petrocol DH 50.2. The column temperature was held at 35oC for 5 minutes and then programmed up to 200oC at 2o/min. The carrier gas was helium and its velocity through the column 20 cm/sec. The Nitrogen Phosphorus Detector (NPD) The nitrogen phosphorus detector (NPD) (sometimes called the thermionic detector) is a very sensitive, specific detector the design of which, is based on the FID. Physically the sensor appears to be very similar to the FID but, in fact, operates on an entirely different principle. A diagram of