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C190-E200 Technical Report: Supercritical Fluid Chromatography

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C190-E200 Technical Report: Supercritical Fluid Chromatography C190-E200 Technical Report Supercritical Fluid Chromatography Hidetoshi Terada1, Takato Uchikata1, Takanari Hattori1, Keiko Matsumoto1, Yoshiyuki Watabe1, Tadayuki Yamaguchi1, Yasuhiro Funada1 Abstract: Advances in column technology have led to a renewed interest in supercritical uid chromatography, which uses a supercritical uid as its mobile phase. Compared to liquid, supercritical uids have low viscosities and high diffusivities. In this report, starting from the basic principles of supercritical uid chromatography, we introduce examples of high-speed, high-resolution analysis and chiral separation. Keywords: supercritical uid chromatography, SFC 1. Supercritical Fluid 25 Water A supercritical fluid is a state of substance wherein the temperature 20 and pressure are both above its critical point (Fig. 1). Supercritical fluids can dissolve substances better than gases and are more diffu- 15 sive and have lower viscosities than liquids (Table 1). Although various substances have particular critical points, the especially low critical point of carbon dioxide (critical temperature: 31.1°C, critical pres- 10 Carbon dioxide Methanol sure: 7.38 MPa) makes it easy to handle. As it is non-flammable, inert, Critical pressure (MPa) Ethylene Propylene Ethanol low-cost, and non-toxic, it has been widely used in industrial process- 5 es, such as for decaffeination of coffee beans and extraction of hops Methane Ethane Propane Acetone extract and flavor compounds (Fig. 2). Supercritical fluids are also 0 used in analytical fields, including as the main mobile phase in super- −100 0 100 200 300 400 critical fluid chromatography (SFC) and the main extracting solvent in Critical temperature (°C) supercritical fluid extraction (SFE). Fig. 2 Critical Points of Various Substances Supercritical 2. Supercritical Fluid Chromatography Solid Liquid uid SFC is a separation technique that uses a supercritical uid as its main Critical pressure mobile phase (often supercritical carbon dioxide). Because of the prop- Critical point 7.38 MPa erties of supercritical uids, which include low viscosities and high dif- fusivities, SFC can be performed at a lower column back pressure than Gas conventional high-performance liquid chromatography (HPLC). Addi- tionally, a high-speed analysis can be performed at high ow rates and Critical temperature a high-resolution analysis can be performed by using a longer column. 31.1°C Also, recent advances in SFC systems and in the packed columns made Fig. 1 Phase Diagram for Carbon Dioxide for SFC allow analyses to be performed with HPLC-like operation. Although supercritical carbon dioxide has a similar hydrophobicity as hexane, this property alone is often insufcient for the elution of Table 1 Properties of Supercritical Fluids target compounds from a column. These target compounds can be Diffusion coefcient Density Viscosity eluted by adding an organic solvent, called a modier, to modify the 2 3 (cm /s) (g/cm ) (g/cm • s) polarity of the mobile phase. Organic solvents that are compatible Liquid 10−6 1 10−2 with carbon dioxide, such as methanol, ethanol, isopropyl alcohol, Supercritical uid 10−3 0.2 to 0.8 10−3 and acetonitrile, are used as modiers. Organic solvents with an Gas 10−1 10−3 10−4 added acid (e.g., formic acid or acetic acid), salt (e.g., ammonium for- mate or ammonium acetate), or base (e.g., diethylamine) are also used as modiers for the analysis of highly polar compounds. 1 Analytical & Measuring Instruments Division 1 3. Nexera UC SFC-UV System The Nexera UC platform can accommodate a wide variety of analyses and pretreatments and includes an (1) SFC system, (2) online SFE-SFC system, and (3) ofine SFE system. A major difference between a Nexera UC system and a conventional HPLC system is the addition of SFC-MS System a back pressure regulator to prevent mobile phase vaporization inside the column and the pump that delivers the carbon dioxide. The Nexera UC platform is based around the Nexera ultra high-perfor- mance liquid chromatograph, with each Nexera UC system cong- ured by adding a newly developed carbon dioxide delivery unit (LC- 30ADSF), a back pressure regulator unit that allows high-precision pressure control (SFC-30A), and an extraction unit used for SFE (SFE- Fig. 3 Nexera UC Systems 30A) (Fig. 3). The autosamplers and other units designed for liquid chromatography can be used in the Nexera UC system. By using the same column packing material for separation in the (1) SFC system Nexera UC systems as that used in normal phase HPLC analysis (e.g., silica gel), normal phase HPLC analyses can be easily transferred to SFC systems include an SFC-UV system that uses a UV (or PDA) de- SFC analyses while improving the resolution and increasing the analy- tector, a UFMS system (SFC-MS) that uses a mass spectrometer sis speed, as shown in Fig. 5. Transferring analyses from normal phase (MS) that is suitable for high-speed analyses by SFC, and a chiral HPLC to SFC can also substantially reduce the volume of organic sol- screening system that automatically switches between multiple col- vents consumed per analysis, as shown in Fig. 6, which also reduces umns and modiers to examine the analytical conditions. analysis costs. SFC is an environment- and user-friendly technique as (2) Online SFE-SFC system it reduces consumption of toxic organic solvents. Online SFE-SFC systems combine SFE and SFC online to automati- cally perform all steps from target compounds extraction from solid Hexane/Isopropanol samples to analysis. = 99:1 (normal phase LC) (3) SFE pretreatment system 12.00 mL Ofine SFE pretreatment systems are specically designed to ex- Methanol 0.70 mL 94.2% reduction tract target compounds from solid samples. (Nexera UC) The characteristic properties of the supercritical uid used in the Fig. 6 Comparison between HPLC and SFC of Organic Solvent Consumption Nexera UC SFC systems, which include high diffusivity and low viscos- ity, allow for low column pressures even at high ow rates, enabling When using a mass spectrometer for SFC, equipment used for LC/MS high-speed analyses while maintaining column efciency. Because of can be used as is. In SFC, a make-up solution is added after column these factors, the Nexera UC SFC systems can shorten analysis times separation to promote ionization. Conventional SFC systems used to between one third and one fth of the time required for HPLC pressure regulators that had a large internal volume. This required the analysis using the same size column. (Fig. 5). £ow path of the column eluate to be split before entering the mass spectrometer to suppress the effect of extra-column dispersion (Fig. CO2 pump Back pressure 7(a)). The Nexera UC systems use a proprietary low-internal volume UV-VIS detector regulator AutosamplerColumn oven design for their back pressure regulator (patent pending). This allows Liquid CO2 cylinder the £ow path to enter the back pressure regulator and mass spec- trometer in series, so all the column eluate enters the mass spectrom- Analytical column eter (Fig. 7(b)). Increasing the volume of eluate introduced to the Solvent delivery pump mass spectrometer in this way enables higher sensitivity analysis, and Modier Fig. 4 SFC Flow Diagram precludes the effects of split ratio variation, etc., resulting in highly re- producible SFC/MS analysis (Fig. 8, Table 2). 3 Back pressure regulator 1. α-Tocopherol 2 (a) Column oven 2. β-Tocopherol 1 4 3. γ-Tocopherol Conventional 4. δ-Tocopherol Rs = 1.328 SFC/MS ow path Rs = 1.703 Normal phase HPLC 1234 Make-up Make-up pump solution SFC Mass spectrometer 024 6810 12 min SFC conditions Normal phase HPLC conditions (b) Column : Shim-pack UC-SIL Column : Shim-pack HRC-SIL Nexera UC (4.6 mm I.D. × 250 mm L. 5 µm) (4.6 mm I.D. × 250 mm L. 5 µm) SFC/MS ow path Modier : MeOH Mobile phase : HEX/IPA 99/1 (V/V) Column oven Modier conc. : 5 % Flow rate : 1 mL/min Flow rate : 3.5 mL/min Temperature : 40°C Back pressure regulator Temperature : 40°C Detection : UV 290 nm Mass spectrometer Detection : UV 290 nm Make-up Make-up pump Back pressure : 10 MPa solution Fig. 5 Comparison between HPLC and SFC Fig. 7 SFC-MS Flow Path 2 (×100,000) Reserpine 3.0 10 µg/L 5 µg/L 0.5 µg/L ŶPeaks 2.0 1. Phosphatidyl Choline 1 2. Lyso Phosphatidyl Choline 2 3. Phosphatidyl Ethanolamine 4. Lyso Phosphatidyl Ethanolamine 1.0 No splitting 3 5. Phosphatidyl Glycerol 4 6. Phosphatidyl Inositol Flow path split 0.0 5 (Split ratio, Vent:MS = 4:1) 6 0.00.5 1.01.5 min 052.51.0 7.5 10.0 2.5 min Column : Shim-pack UC-Diol 4.6 mm I.D. × 150 mm L. Modier : MeOH Column : Shim-pack UC-Diol 4.6 mm I.D. × 150 mm L. 5 µm Modier conc. : 30 % Modier : MeOH with 0.1 % w/v ammonium formate Flow rate : 3.0 mL/min Modier conc. : 10 % (0 min) ĺ 40 % (10–15 min) Temperature : 40°C Flow rate : 3.0 mL/min Back pressure : 10 MPa Temperature : 40°C Injection volume : 1 µL Make-up solution : MeOH with 0.1 % w/v ammonium formate Sample : Reserpine (0.5, 5, 10 μg/L) Make-up ow rate : 0.1 mL/min Detection : TQ mass spectrometer ESI(+) m/z 609.3 > 195.0 Back pressure : 10 MPa Detection : TQ mass spectrometer (ESI) Fig. 8 Sensitivity With and Without Flow Path Splitting Fig. 9 Phospholipid Analysis Table 2 MS Reproducibility With and Without Flow Path Splitting Fig. 10 shows an example analysis of pesticides of a wide range of po- Injection Retention time Area Height volume larities—from hydrophobic to hydrophilic—using the Shim-pack Ave.
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