Restrictors, types

There are a variety of restrictor types that control the pressure of the fluid during the chromatographic process. Each manufacturer has patented their individual restriction devices. It is beyond the scope of this text to go into the detail about all of these restrictors. However, by way of explanation, most of the mechanical restrictors that are used in commercial equipments operate under the principle of decreasing a volume or a space that the mobile phase must pass through by some mechanical means. This decrease in volume, if metered accurately, increases the pressure in the system in a controlled manner. As a safety precaution, for any equipment where elevated pressures are used, tubing should be rated with a safety factor of at least 1.5 times the maximum pressure the SFC system can achieve. 

Strober, M., R. Freeman, M. De Antonio, C. Lam pert and J. Diamond (1997). Does adjunctive fluoxetine influence the post-hospital course of restrictor-type anorexia nervosa A 24-month prospective, longitudinal followup and comparison with historical controls. Psychophar-macol Bull 33(3) 425-31. 

An on-line supercritical fluid chromatography-capillary gas chromatography (SFC-GC) technique has been demonstrated for the direct transfer of SFC fractions from a packed column SFC system to a GC system. This technique has been applied in the analysis of industrial samples such as aviation fuel (24). This type of coupled technique is sometimes more advantageous than the traditional LC-GC coupled technique since SFC is compatible with GC, because most supercritical fluids decompress into gases at GC conditions and are not detected by flame-ionization detection. The use of solvent evaporation techniques are not necessary. SFC, in the same way as LC, can be used to preseparate a sample into classes of compounds where the individual components can then be analyzed and quantified by GC. The supercritical fluid sample effluent is decompressed through a restrictor directly into a capillary GC injection port. In addition, this technique allows selective or multi-step heart-cutting of various sample peaks as they elute from the supercritical fluid... 

The liquid enters the micro channel device via a large bore that is connected to a micro channel plate via a slit (Figure 5.2). The slit acts as a flow restrictor and serves for equipartition of the many parallel streams [1, 3, 4]. The liquid streams are re-collected via another slit at the end of the micro structured plate and leave the device by a bore. The gas enters a large gas chamber, positioned above the micro channel section, via a bore and a diffuser and leaves via the same type of conduit. 

The chromatographic and mass spectrometric choices facing the analyst in coupling SFC and MS successfully, namely injection method column type of flow restrictor and mass spectrometer ionisation method and type of vacuum system, have been described [398]. In SFC-MS coupling, the restrictor plays a major role, as the expansion behaviour to a large extent determines the overall performance of the SFC-MS system and defines the range of applications. 

The pressure restrictor that controls the pressure in the column is installed before or after the detector, depending on its type (before for an FID, after for a UV detector). 

Some support structures are also included for detachably retaining the various components of the system. Preferably the support structure can be of the assembly board type , which provides prearranged flow channels and connector ports. The desired components of the system can be fastened into these connectors by pins. The flow control system that makes up the ICS system can include pumps, flow channels, manifolds, flow restrictors, valves, etc. These components are equipped with the necessary fittings that allow them to be sealed with the prearranged or selectively located flow channels or connectors. The flow system can also include detachable mixing devices, e.g., static or ultrasonic, or with a chip-like design. The reaction units, whether chip-like or not, can be of thermal, electrochemical, photochemical or pressure type [84]. 

As previously mentioned, the SFE pump should produce a constant pressure of supercritical fluid with a rate controlled by a flow restrictor after the extraction vessel. There are a number of types of flow control devices, including a capillary made from fused silica, a pinched stainless steel tube, or a variable orifice allowing for electronic control of the pressure. 

Collection on a sorbent involves the use of a solid material, either in the line or at the restrictor outlet. A number of materials have been [13] and continue to be investigated [14-16] with a view to optimizing collection of different types of analytes. This collection mode involves an additional step desorbing the analytes from the sorbent by elution with a small volume of solvent for their subsequent determination or, alternatively, thermal desorption and sweeping by the eluent if an on-line coupled extraction-chromatographic system is being used. 

This collection mode entails the use of an appropriate solid, either in the line or at the restrictor outlet, whether packed in a column or as a bed. The material used for this purpose should ensure proper retention with minimal or no analyte losses, and enable the use of the best possible type and volume of eluent to desorb the target analytes. When the extracted analytes differ in polarity, a mixture of sorbents must be used to ensure adequate collection efficiency. 

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... 

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). 

Fig. 25. Spectral-segmented spectrum of a glow discharge [58]. Grimm-type glow discharge lamp with floating restrictor (8 mm diameter) 50 mA, 3.5 torr, 2 kV [42] Plasmaarray spectrometer (LECO) steel sample 217 A (Research Institute, CKD, Prague, Czech Republic).

A modified version of the Grimm-type GDL has been described by Shao and Horlich [594], The source has a floating restrictor and is designed so as to replace an ICP torch in an ICP-MS. Therefore, the anode is slightly positive with respect to the earthed skimmer interface of the MS system. The simultaneous analysis of an unknown sample and a reference material was carried out by means of a system based on two pulsed GD sources housed within the same tube [595], Optimization of the relative position of the two cathodes was achieved by evaluating the signals produced in GD-MS when using the same specimen for each of them. 

Figure 6.3 Schematic of a SFC installation using a HPLC packed column. Carbon dioxide reaches a supercritical state between the pump and the injector. A pressure regulator ( restrictor ), is located after the column and either before or after the detector, depending upon its type. It maintains the mobile phase under supercritical conditions until the outlet of the column A modifier added in small quantity (less than 10 per cent) enables to control the selectivity of the analytes (diagram based upon a document from Vydac).

The discriminator comprises the analytical column (which is housed in an oven) and a restrictor which immediately follows it. The column is usually a capillary GC column, but packed LC columns can also be used. Once the sample is injected into the supercritical stream it is carried into the analytical column. Different types of stationary phases are available with varying compositions and polarities. The ovens used in SFC are generally conventional GC or LC ovens. 

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