Apparatus for SFC

The type and purity of the mobile phase used for SFC depends largely on the application and the detection system in use [7]. Flame-based detectors can be used only with fluids such as SFg, NH3, CO2 or N2O which have a low background signal. Consequently, the purity of these fluids must be carefully controlled to ensure that they are not contaminated. In addition, a high-pressure 2-/im inlet filter is usually placed in-line before the pump inlet to prevent particulate matter entering the pump. 

Pulse-free syringe pumps are generally used for SFC (particularly capillary SFC), because of the low flow rates employed, and the need to deliver the mobile phase at a constant pressure rather than constant flow rate. Since the solvating power of the mobile phase is proportional to the density of the fluid, the pressure is manipulated at constant temperature for density control. This is performed by using a sensitive pressure transducer in-line between the pump outlet and the column and a microprocessor for pump control. The microprocessor is programmed to generate a set pressure profile [8,9]. 

The method of sample introduction into the column is important for preserving the chromatographic efficiency [10]. In packed-column SFC, an HPLC injection valve capable of withstanding high pressures is employed with a 1-, 5- or 10-/xl sample loop. In the case of capillary SFC, a submicrolitre ( O.l-jul) injection valve is used either for time-split injections or together with an inlet splitter for reduction in the sample size. This is necessary because of the low sample capacity of 50 fim I.D. capillary columns, commonly used for 

The only requirement for the oven is that it accurately and precisely controls the column temperature. This is because even slight fluctuations in temperature can cause large changes in mobile-phase density and hence solute retention. 

One of the major advantages of SFC is its compatibility with both GC and HPLC detectors. GC flame detectors, such as the flame ionization detector (FID) [11,12], nitrogen thermionic detector [12,13], and flame photometric detector [14] have all been interfaced with SFC systems using a capillary restrictor which, while maintaining supercritical conditions in the column, also effectively decompresses the fluid to ambient pressure just before it enters the flame tip [10,15]. HPLC detectors such as ultraviolet and fluorescence detectors are employed when pure organic mobile phases or modified mobile phases are used. With these detectors, analytes are detected spectroscopically in a flow-through cell prior to decompression [16]. 

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