Linear restrictors

Figure 6.13 Flow restrictors of different design A, linear B, tapered C, integral and D, frit. On the right side is shown a modified high pressure cell for UV detection using open tubular columns.

Thimble Volumes. For these experiments the thimble volumes and the extraction times were held constant. To accomplish this, the mass flow of the system had to be varied by changing the flow rate at the pump head for each density step. Controlling the mass flow rate allowed the linear/volumetric flow to be consistent throughout the experiments. This is different from the paprika experiments in which the mass flow was held constant and the extraction times were changed, to keep thimble volumes constant for each extraction step. Flow control is one of the major advantages of variable restrictor based SFE units. 

Restrictors are required at the ends of SFC columns to maintain supercritical conditions throughout the column and to limit overall flow. Several options exist, with frit restrictors being the most popular, followed by integral and linear formats. The frit restrictor is... 

Most commercial ELSDs employ a standard or modified HPLC nebulizer (Venturi flow type). It was believed that this nebulizer was not necessary for SFC because nebulization of the SFC mobile phase is accomphshed by gas expansion in a restrictor which controls pressure and mobile-phase flow rates. To counter the cooling effects of CO2 decompression in the linear fused-silica restrictor and improve heat transfer, Nizery et al., using a Cunow Clichy Model DDL 10 detector, placed the restrictor tip into a heated brass ring and applied heat to a small section of tubing between the restrictor and the drift tube... 

Fig. 1 Schematic diagram of SFC-CLND system (1) CO2 tank, (2) SFC pumping system, (3) autosampler, (4) modifier pump, (5) column, (6) UV detector, (7) pyrolysis inlet with linear restrictor, (8) pyrolysis chamber, (9) reaction cell,...

In order to maintain the desired SF mobile-phase conditions, the end of the column is connected to a restrictor. Although several types of restrictor are available [11], the most popular is the linear restrictor, which consists of a small piece (--10 cm) of a fused silica or metal tube of small inner diameter (50 /rm or less). 

A restrictor is required to maintain supercritical fluid conditions along the column and to control the linear velocity of the mobile phase through the column when the column... 

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