Frit restrictor

The first instance of SFC coupled to ICP-MS was reported by Shen et al. [127] for the speciation of tetraalkyltin compounds. Liquid C02 was used as the mobile phase and the SFC column was completely inserted through the transfer line and connected to a frit restrictor (Fig. 11). The restrictor was heated to approximately 200°C by a copper tube inserted into the ICP torch. Tetramethyltin (TMT), tetrabutyltin (TBT), tetraphenyltin (TPT), tributyltin acetate... 

Microwave induced plasma mass spectrometry has also been used as a detector for supercritical fluid chromatography (SFC) [113] for the separation of halogenated hydrocarbons. The design of an SFC-MIP interface must ensure that the frit restrictor temperature remains at a high temperature to prevent condensation of analytes. Stainless steel transfer lines may be used. The frit restrictor should be connected to a length of deactivated fused silica capillary, inserted through the transfer line, and positioned flush with the aluminum MIP torch inset (Fig. 10.21). 

Supercritical fluid extraction coupled to SFC has been used for the extraction, separation and identification of PAHs from coal. The supercritical extract was expanded with the aid of a frit restrictor accommodated in the sample cavity of a cooled micro-injector, the analytes being deposited by condensation while CO, was sent to waste through a vent valve. Subsequently, the loop contents were connected on-line to the mobile phase of the capillary chromatograph. The extracted analytes were detected by off-line FTIR spectroscopy following collection on a KBr disc and evaporation of the solvent [104]. 

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... 

Figure 7.10. Separation of a broad molecular mass poly(ethylene glycol) trimethylsilyl ether sample using a frit restrictor (top) and integral restrictor (bottom). Column 10 m x 50 xm I.D. SB-biphenyl-30, film thickness 0.25 pm connected to the injector by a 1 m x 0.50 pm uncoated retention gap. Conditions Temperature 120°C pressure program 80 atm to 100 atm at 1.3 atm/min then 100 atm to 560 atm at 5 atm/min and held at 560 atm for 80 min mobile phase carbon dioxide. (From ref. [156]. Wiley-VCH).

Supercritical fluid grade carbon dioxide (Scott Specialty Gases, Plums teadvi lie, PA) was used as the carrier fluid. A Lee Scientific Model 501 supercritical fluid chromatograph equipped with a flame ionization detector (FID) and a nitrogen-phosphorus detector (NPD) was the instrument utilized for these studies. Fused silica capillary columns (50 pm i.d.) were employed for all the experiments. Three column types with stationary phases of three different polarities were used SB-Methyl-100, SB-Biphenyl-30 and Carbowax 20M (0.25 pm films). Frit restrictors were used to maintain pressure and proper flow rates in the column. The restrictor was connected to the end of the column via a zero dead-volume union. The end of the restrictor was positioned in the detector at 1 mm below the end of the flame jet. The detector was operated at 325-350 C with nitrogen make-up gas at 25 mL/min. Split injection was used in these experiments with 0.2 pL injection rotor and a split ratio of approximately 10 1. 

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.

Filter Proportional Pressure frits valves sensors Restrictors... 

The supercritical fluid flushes the extraction cells with a volume of 10 ml and is then expanded in the restrictors. In order to prevent the drive out of solid particles, there are frits with 5 pm pore size at the ends of the extraction cell. With fluid expansion, extracted particles precipitate and are kept in the vials which contain an organic solvent for sampling. Measured average carbon dioxide currents at the end of the extractor were 12 liters at a temperature of 28 °C and 1 bar pressure, resulting in a current of 0.81 ml/min carbon dioxide at the above-metioned extraction conditions. 

Figure 2 shows a schematic of the RESS process, which is operated as follows. CO2 is pumped (a) and raised to the desired pressure. When a cosolvent is used, it is pumped in the same way (b) and introduced into the CO2 flow. This flow is then heated to the desired temperature (c) and allowed to enter a tank loaded with the active substance (d) for extraction. In this part of the process, the solvent power is strong because of the high pressure and because of the possible presence of a cosolvent. This mixture is then depressurized in an expansion vessel (e) by means of a capillary or a nozzle, with a typical inner diameter of 50 to 60 pm. The restrictor must be heated to avoid plugging by solid precipitation. The expansion chamber is generally at or near atmospheric pressure. A frit filter is placed at the exit of the expansion chamber to keep the particles formed in the expansion vessel. A cyclone (f) separates the solvent from the CO2, which can be recycled (g). 

There are many variations on fixed restrictors including a carefully defined fritted zone at the end of a capillary tube. Most of these have led to a un-ending number of comments at supercritical fluid extraction conferences such as the biggest hurdle in SFE is limited restrictor availability and technology. Greibrokk [15, 16] has summed up general experience and feeling about fixed restrictors, especially for SFC. 

Sample preparation Sandwich cream or ointment between two layers of 200 mesh silica gel, extract with carbon dioxide MeOH 95 5 at 300 atmospheres at 55° at 2 mL/min for 75 min (restrictor 300°), sonicate the SFE tube, frits, and silica gel with MeOH 100 mM HCl 25 75 containing 0.1% Tween 80 for 15 min, filter (0.2 pm), inject an aliquot of the filtrate. (SFE removes the hydrocarbon base of the cream or ointment leaving behind the insoluble pol5Tn5rxin.)... 

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