Pressure, supercritical fluid chromatograph

In the first section, the mechanisms involved in size exclusion chromatography are discussed this is an area where additional understanding and clarification still are needed. Data treatment with respect to statistical reliability of the data along with corrections for instrumental broadening is still a valid concern. Instrumental advances in the automation of multiple detectors and the developm.ent of a pressure-programmed, controlled-flow supercritical fluid chromatograph are presented. 

Pressure-Programmed Controlled-Flow Supercritical Fluid Chromatograph... 

For the on-line SFE-NMR experiments, the set-up shown in Figure 7.2.17 can be used. A main pump serves an HP supercritical fluid chromatograph (G1205A), with analytical HPLC columns being used as extraction cells. The continuous-flow NMR cell is connected between the column outlet and the back-pressure regulator. 

Morgan, D.G. Harbol, K.L. Kitrinos, RN. Optimization of a Supercritical Fluid Chromatograph-Atmospheric Pressure Chemical Ionization Mass Spectrometer Interface Using an Ion Trap and Two Quadrupole Mass Spectrometers, J. Chromatogr. A 800, 39 9 (1998). 

Supercritical fluid chromatographic pumps must have both a wide range of compensation and use dynamic compressibility compensation to produce accurate and reproducible flow and composition. Whereas water has a compressibility factor of 75 x 10 /bar, methanol is more compressible at 120 X 10 /bar. Carbon dioxide has widely varying compressibility from 95 to 395 X 10 /bar at 5°C, depending on the pump delivery pressure (column head pressure). The viscosity of pure carbon dioxide is 1 /20 the viscosity of pure methanol. During composition programming, the viscosity of the mixed fluid and the column head pressure increases as the modifier concentration increases. Without dynamic compensation, the actual delivery of the carbon dioxide would roll off. The total flow would be less than the set points and the modifier concentration would be more than the set points. 

Strubinger JR, Song H, Parcher JF. High pressure phase distribution isotherms for supercritical fluid chromatographic systems. 2. Binary isotherms of carbon dioxide and methanol. Anal Chem 1991 63 104-108. 

A supercritical fluid chromatograph consists of a gas supply, usually carbon dioxide, a pump, the column in an oven, a restrictor to maintain the high pressure in the column and a detector. In general there are two possible hardware setups ... 

Sometimes it is desirable to operate a supercritical fluid chromatograph with two detectors. If one of these detectors is a low or atmospheric pressure detector, such... 

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. 

Carbon Dioxide (CO2)—Supercritical fluid chromatographic grade, 99.99 % minimum purity, sun>lied pressurized in a cylinder equipped with a dip tube for removal of liquid CO2. (Warning—Liquid at high pressure. Release of pressure resiflts in production of extremely cold solid CO2 and gas which can dilute available atmospheric oxygen.)... 

The physical and chemical properties of a supercritical fluid depend upon temperature and pressure and are of paramount importance when designing a supercritical fluid chromatograph or extractor. Excessively high pressure or temperature require-... 

The collection of eluent fractions from a conventional preparative HPLC is a trivial matter. Provision of automated fraction collectors suitable for HPLC is no commonplace and instrumentation is highly developed and widely commercially available. However, collection of eluent from a preparative SFC is not trivial, particularly if complete collection of a fairly volatile solute is required. In a supercritical fluid chromatograph using carbon dioxide the expansion of the fluid at depressurisation is very large indeed. If carbon dioxide is depressurised from 300 Bar (4500 psi) to atmospheric pressure is expands to 355 times its original volume if its temperature is maintained constant. Hence at a flow rate of lOcm min" of supercritical carbon dioxide at 300 Bar, which corresponds to about 7 g carbon dioxide per minute, expansion after depressurisation yields 3.5 L of carbon dioxide gas per minute at NTP. 

Numerous high pressure Hquid chromatographic techniques have been reported for specific sample forms vegetable oHs (55,56), animal feeds (57,58), seta (59,60), plasma (61,62), foods (63,64), and tissues (63). Some of the methods requite a saponification step to remove fats, to release tocopherols from ceHs, and/or to free tocopherols from their esters. AH requite an extraction step to remove the tocopherols from the sample matrix. The methods include both normal and reverse-phase hplc with either uv absorbance or fluorescence detection. AppHcation of supercritical fluid (qv) chromatography has been reported for analysis of tocopherols in marine oHs (65). 

The use of both sub- and supercritical fluids as eluents yields mobile phases with increased diffusivity and decreased viscosity relative to liquid eluents [23]. These properties enhance chromatographic efficiency and improve resolution. Higher efficiency in SFC shifts the optimum flowrate to higher values so that analysis time can be reduced without compromising resolution [12]. The low viscosity of the eluent also reduces the pressure-drop across the chromatographic column and facilitates the... 

---