Supercritical Fluid Chromatography - · PDF fileTheory and Instru mentation of GC...

Theory and Instrumentation of GC

Supercritical Fluid Chromatography

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Aims and Objectives

Aims and Objectives Aims

• Introduce supercritical fluid chromatography as a powerful solution for chromatographic analysis

• Explain fundamental aspects of supercritical fluid chromatography • Present some fundamental hardware typically used in supercritical fluid

chromatography Objectives At the end of this Section you should be able to:

• Understand the benefits and limitations of supercritical fluid chromatography • To identify some instrumentation typically used in supercritical fluid

chromatography

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Content Introduction 3 Supercritical Fluids 4 Modes of Chromatography 6 SFC Applications 7 SFC Instrumentation 12 Packed SFC 14 Capillary SFC 14 The Mobile Phase 15 Exhaust Gases 16 Organic Modifiers 17 Pumping Issues 19 SFC Columns 20 Packed Column SFC Stationary Phases 21 Capillary Column SFC Stationary Phases 22 Detection 24 Pressure Regulators 26 Advantages and Disadvantages of SFC 27 References 27


Introduction Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique whose properties are said to lie between gas and liquid chromatography. It utilizes extreme conditions of temperature and pressure in such a way that the mobile phase remains as a supercritical fluid, which has properties intermediate between a liquid and a gas.[1] In general selectivity terms, SFC can be regarded as a derivative of normal phase chromatography with the added advantage of low viscosity (and high diffusivity) of supercritical fluids, which results in high column efficiencies. Packed SFC columns are of the same type as those currently used in normal phase chromatography including a host of bonded phase column types which we will study later in this module. Different eluent systems have been used to undertake SFC separations; however, carbon dioxide based systems are by far the most successful and widely used of all of them. When considered as a mobile phase for SFC, carbon dioxide often requires the addition of organic modifiers (such as alcohols) for the elution of polar solutes. The term “subcritical chromatography” denotes chromatographic separations using subcritical fluids as the mobile phase.[2,3] Disadvantages of SFC are mainly mobile phase or equipment related and include:[4, 5]

• Limited choice of mobile phases • Limited analyte solubility in the mobile phase • Unwanted reactions with the mobile phase (for example, at super-critical conditions

CO2 forms carbamic acids with primary and secondary amines) • Repeatable and stable formation of a gradient which includes both supercritical CO2

and a polar organic modifier

HPLC separation of antipsychotics drugs.

Column: C18, 250×4.6 mm, 5 μm. Eluent system: tetrabutyl ammonium sulphate (0.01 M) and methanol (1:1 ratio) pH 3.5, with a flow rate of 1.5 mL/min Detection: UV (λ = 218 nm).

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SFC separation of antipsychotics drugs.

Column: C18, 250×4.6 mm, 5 μm. Eluent system: 16.67% methanol (organic modifier) in CO2, at a flow rate of 3.0 mL/min at a temperature of 60°C and 29.4 MPa outlet pressure Detection: UV (λ = 218 nm). Supercritical Fluids Any substance that is maintained above its critical conditions (heated above its critical temperature AND compressed above its critical pressure) is said to be in a supercritical state and substances under these conditions are termed ‘Supercritical Fluids’.[6] Critical Pressure (Pc): is the highest pressure at which a liquid can be converted to a gas by increasing its temperature. Critical Temperature (Tc): is the highest temperature at which a gas can be converted to a liquid by increasing its pressure. Above its critical temperature a gas will not condense into a liquid phase regardless of how much the pressure is increased; in the same way, above its critical pressure a liquid will never exists in the gas phase regardless of how much the temperature is raised.[7] Supercritical fluids are neither liquids nor gases; rather they are compressible fluids with the characteristic dissolving power of liquids, but diffusivities approaching that of gases. That is why we cannot define the supercritical fluid as a liquid or as a gas but also what make them so interesting as a mobile phase for chromatographic separations. Using the correct conditions of temperature and pressure and mobile phase composition are of critical in SFC separations. However for various reasons of instrument design and operating principle, as well as the effect of modifier addition, workers may unwittingly operate with slightly subcritical fluids –however it is the repeatability of the applied conditions which is important, rather the absolutely surety that one is operating supercritically. Variations in the concentration or type of organic modifier will change the supercritical properties of the system.


Pressure-Temperature of a pure substance (where the subscript c denotes the critical

conditions)

Critical Point: Specifies the conditions (temperature, pressure and sometimes composition) at which a phase boundary ceases to exist. For pure carbon dioxide the conditions of temperature and pressure that define the critical point are: Tc = 31.1 oC, Pc = 73.8 Bar Triple Point: The triple point of a pure substance is the temperature and pressure at which three phases (for example, gas, liquid, and solid) of that substance coexist in thermodynamic equilibrium. For pure carbon dioxide the conditions of temperature and pressure that define the triple point are: Ttr = -56.6 oC, Ptr = 5.17 Bar


Modes of Chromatography Compared with liquids, supercritical fluids have higher diffusivity coefficients and less resistance to mass transfer – both of which lead to sharper chromatographic peaks at higher optimum linear velocities, which is demonstrated using the van Deemter curves opposite.[8] As with all van Deemter plots, lower values of Reduced Plate Height indicate higher efficiency. Selected indicative physicochemical properties of liquids, gases and supercritical fluids

Property Liquid Gas Supercritical Fluid Density (kg/m3) 1000 1 200-800

Viscosity (mPa s) 0.5-1.0 0.01 0.05-0.1 Diffusivity (cm2/s) 10-5 0.1 10-4 – 10-3

In general terms, the optimum linear velocity for a supercritical fluid is around three times greater than an eluent in the condensed phase, allowing SFC separations to be carried out more quickly without loss in efficiency (or the resulting potential loss of resolution).

Typical HPLC and SFC Van Deemter curves.

A close comparison between the physicochemical properties of supercritical fluids and gases reveals that the former have:

• Increased densities (100 – 1000 times) • Increased viscosities • Stronger solvating power (as the solubility of analyte is usually related to the

mobile phase density at a given temperature) Unlike GC, low temperatures can be used when performing SFC separations. Working at lower temperature gives the advantage of:

• decreased retention factors • increased potential for enantioselectivity • permit the analysis of thermally labile samples


The preparative chromatographic separation of chiral enantiomers is one of the fastest growing application areas in SFC. SFC Applications Supercritical fluid chromatography has many of the features of both liquid and gas chromatography; therefore, it’s application area occupies an intermediate position between the two of them. SFC is important in situations where neither GC nor HPLC is capable of performing the required analysis. Consider oleo-chemical compounds, they are too heavy for GC but they usually require detection capabilities not easily found with HPLC. The main advantage of SFC over GC comes from the adjustable nature of the elutotropic strength of the mobile phase which can be controlled through the addition of organic modifiers. As a consequence, SFC has extended the applicability range of chromatography in a way never seen before. To list the full range of SFC application areas is prohibitive, its flexibility makes it suitable to a multitude of application types. Examples of some interesting applications are shown below: Analysis of paraffin:[9] Paraffin is the common name for the alkane hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to the solids with 20 ≤ n ≤ 40

SFC–AFD/FID chromatograms from an injection of 15 μg of paraffin wax dissolved in

carbon disulfide. Column: C18, 150mm×1mm I.D, 3μm particles. Conditions: pressure 150atm; temperature is 120◦C. Mobile phase: 16% methanol modified (filled) SC-CO2 Detection: FID, AFD (Acoustic Flame Detection)


Food analysis:[10] Fish oil is oil derived from the tissues of oily fish. It is recommended for a healthy diet because it contains the omega-3 fatty acids.

In-line UV SFC chromatogram of the phenacyl esters of fish oil.

Column: C18, 50mm×1mm I.D, 3.5μm particles. Conditions: pressure 150atm; temperature is 120◦C. Mobile phase: CO2 (phase A) and acetonitrile/Isopropyl alcohol (phase B). The modifier profile was programmed from 1.2% B (2min) to 7.2% B at 0.3%/min. Detection: UV


Petrochemistry:[12] Fuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue.

SFC chromatogram diesel oil

Column: C18, 250mm×4.6mm I.D, 5.0μm particles. Mobile phase: SFC-grade CO2. Pressure programme: 110 atm to 160 atm for 20 min. Detection: FID, UV Pesticides:[13] To analyze pesticides at trace level in water samples, an extraction-concentration step is commonly carried out before the SFC analysis.

SFC analysis of pesticides from a water sample

Column: C18, 200mm×4.6mm I.D, 5.0μm particles. Mobile phase: SFC-grade CO2 plus 5% methanol Pressure programme: 110 atm to 160 atm for 20 min. Detection: ECD


Chiral Separations:[30] Separation of enantiomeric samples is a serious problem in analytical chemistry; however, SFC is becoming a suitable option when dealing with this type of samples.

Column: chiral, 250mm×4.6mm packed with the 3,5-dimethyphenylcarbamate derivative of amylose coated on 10 μm silica-gel support. Mobile phase: SFC-grade CO2 plus 3.0% methanol Conditions: pressure 150 bar; temperature is 80◦C. Detection: UV-Vis λ = 225nm Cosmetics:[11] Beeswax is a wax formed from a mixture of several compounds. The empirical formula for beeswax is C15H31CO2C30H61. Its main components are palmitate, palmitoleate, hydroxypalmitate and oleate esters of long-chain (30-32 carbon atoms)

SFC-FID chromatogram of beeswax (0.81 mg/mL in CHCl3)

Column: C18, 15cm×4.6mm I.D, 5.0μm particles. Mobile phase: SFC-grade CO2 (no modifier). Detection: Flame ionization detector held at 400◦C


Lubricants:[14] The comprehensive analysis of the components of lubricants is a very difficult task using current analytical tools; however, SFC is becoming important to deal with these complex samples.

SFC/FID–UV–MS analysis of Irganox L57 (commercial car lubricant)

Column: C18, 250mm×4.6mm I.D, 5.0μm particles. Mobile phase: SFC-grade CO2 plus 1.0 – 5.0% methanol Conditions: pressure 150 bar; temperature is 80◦C. Detection: APCI(+)-MS, UV, FID


Preparative SFC:[31] Analysis of highly polar pharmaceutical products.

Preparative SFC separation of a mixture of methylphenylalanine isomers

Column: chiral, 250mm×4.6mm I.D, 5.0μm particles. Mobile phase: 80% SFC-grade CO2 plus 20% organic modifier (1:1 methanol/ethanol) Conditions: pressure 100 bar; temperature is 35◦C. Detection: UV-Vis λ=220nm SFC Instrumentation Supercritical fluid chromatography presents many of the features of both liquid and gas chromatography; therefore, it occupies an intermediate position between the two of them. Two major forms of SFC can be found:[17] Capillary SFC: This mode of SFC presents similarities with conventional capillary GC. Packed SFC: This mode of SFC presents similarities with conventional capillary HPLC. The diagram opposite, illustrate a typical packed SFC instrument; however, from a hardware point of view, the representation is equally valid for both capillary and packed SFC systems.


Packed SFC system

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Packed SFC Packed SFC utilizes HPLC modified instrumentation to achieve supercritical separations; nowadays, it is the most widely used of all forms of SFC. There are several key features which distinguish Packed Column SFC instrumentation from that of a traditional HPLC including:

• The obvious need for a tank of CO2 • Modern systems will contain equipment to regenerate the CO2 – i.e. remove the

organic modifier prior to re-circulating or re-depositing in the tank • A back pressure restrictor placed after the analytical column

When performing packed SFC separations:

• Eluent flow rates are in the order of a few millilitres per minute • Injection volumes range from a few to hundreds of microlitre (one aspect which

makes packed column SFC very suitable for preparative work) A typical packed column SFC chromatographic process is depicted opposite, showing mixing of the supercritical CO2 with an organic modifier, which is achieved in a similar way to the mixing of different eluent components in HPLC. Capillary SFC Capillary column SFC shows similarity to conventional capillary GC; capillary SFC uses GC modified instrumentation to achieve supercritical separations. When performing capillary SFC separations:

• Eluent flow rates are in the order of a few microlitre per minute • Injection volumes are in the order of a few hundreds of picolitre (1pL = 10-12L) • In general, SFC columns have smaller internal diameter than their GC

counterparts

Capillary SFC system

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The Mobile Phase As we have seen, carbon dioxide is the mobile phase fluid of choice for supercritical chromatography, the main reasons including:[15]

• Low cost • Modest critical conditions • Available at high purity • Safety • Easy to use • Lack of another viable alternative

As well as carbon dioxide, many other fluids (like ammonia, numerous chlorofluorocarbons and fluorocarbons, nitrous oxide, etc) have been used as mobile phase components for SFC applications; however, up to date only carbon dioxide has succeeded in becoming widely implemented. The success of carbon dioxide is in great extent due to the addition of organic modifiers (such as alcohols) that permit the analysis of polar analytes. The modifier increases the polarity of the mobile phase to compete with the analyte for active sites on the stationary phase, leading to reduced retention of solutes. This is highly analogous to the use of a ‘delocalising’ solvent in Normal Phase HPLC. Critical properties of selected solvents are shown below. Critical properties of selected solvents

Solvent Tc (oC) Pc (Bar) ρc (g/mL) Ammonia 132.5 113.5 0.24 Benzene 289 48.9 0.3 n-Butane 152 38 0.23 Carbon dioxide 31.1 73.8 0.45 CClF3 28.8 39.5 0.58 CCl2F2 111.7 39.9 0.56 Ethane 32.2 48.9 0.2 Ethanol 243.4 63.8 0.28 Ethylene 9.3 50.4 0.22 Isopropanol 235.3 47.6 0.27 Methanol 240.5 79.9 0.27 Nitrous oxide 36.5 72.3 0.46 n-Propane 96.8 42.6 0.22 Propylene 91.9 46.2 0.23 Toluene 318.6 41.1 0.29 Water 374.2 221.2 0.34


Exhaust Gases With the advent of preparative SFC, the consumption of carbon dioxide and organic modifiers is constantly increasing. As expected, manufacturers are experiencing increasing pressure to make SFC even more environmentally friendly while reducing eluent flow rate costs by reusing it. SFC exhaust gas collection systems usually consist of a set of decompression chambers with temperature control. This devices permit carbon dioxide to regain its gaseous state while the remaining components condense and are recovered. As expected, sample components and organic modifiers tend to remain dissolved in the liquid organic modifier. Once separated, sample components and organic modifiers can be disposed of according to correct local disposal protocols. After purified, the flow of carbon dioxide can be compressed and reutilised. Note that as result of decompressing the SFC effluent, solid carbon dioxide (cardice or dry ice) can form and clog the waste stream.

Exhaust gas collection system for SFC


Organic Modifiers The use of modifiers will affect analyte retention and may even alter their elution order.[15] Modifiers are added to the supercritical eluent system for the following reasons:

• To increase the polarity of the mobile phase (thus improving the solubility of polar analytes)

• To facilitate desorption of polar analytes from the column Analyte and organic modifier molecules will actually compete for active binding sites in the stationary phase; therefore, analyte desorption is promoted by organic modifiers. In practical terms only carbon dioxide with methanol or acetonitrile as modifiers are of significant importance in SFC. However, the list of possible organic modifiers is long and include different alcohols (with isopropanol being perhaps the most popular), cyclic ethers, tetrahydrofuran, isopropylamine, hexane, etc. The presence of modifiers in the mobile phase will determine its critical conditions; for example, the addition of short chain alcohols will raise the critical temperature a few hundred degrees Celsius (composition dependent) thus compromising the analysis of thermally labile samples. For a given amount of substance the temperature, volume, and pressure are interdependent variables, the relationship between them (under certain conditions) can be represented by an equation of state.[16] The addition of organic modifiers will preclude the use of detectors that respond to carbon containing compounds (such as FID). A disadvantage with using organic modifiers is that certain detection modes (such as FID), cannot be used, due to their response to carbon containing compounds.

Equation of state: Equations of state can be applied to calculate physical properties not only of pure substances but to mixtures. PVT properties of mixtures of gases can be predicted by using equations of state, one of them (Patel-Teja) is presented below:

)()( bVcbVVa

bVRTP

−++−

−=

∑∑=

i jijji axxa

∑∑=i j

ijji bxxb

∑∑=i j

ijji cxxc

Where P: pressure of the mixture T: absolute temperature V: Volume of the mixture xij: mol fraction of i-component aij, bij, cij: Patel-Teja Parameters (component dependent)


SFC separation of clenbuterol enantiomers using a chiral stationary phase.

Column: Chiral, 250×4.6 mm, 10 μm. Eluent system: Carbon dioxide with Isopropylamine as organic modifier Detection: UV (λ = 218 nm).

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Pumping Issues In SFC the retention of a solute is influenced by the density of the mobile phase and this in turn is highly influenced by the system pressure. Therefore the design of the pumping system, to achieve a constant pressure, is of overriding importance in SFC.

SFC in-series dual pump design

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Without any shadow of doubt, one of the most challenging aspects of SFC is accurately pumping supercritical fluids. Nowadays, most of the available HPLC pumping systems can handle supercritical carbon dioxide based systems; however, the actual amount of mobile phase delivered will vary with temperature, pressure and composition (amount of organic modifier). To overcome this problem, SFC pumping systems implement electronic speed controlling to dynamically change the compressibility of the mobile phase. Pumping systems lacking the ability to dynamically change compressibility will deliver accurate flow at only one (pre-selected) pressure and composition. In applications where the analysis conditions need to be changed (like in gradient operation), then with each small composition change the pump will become less accurate. Standard HPLC pumping systems usually have an inadequate compressibility compensation range; therefore, they are unable to deliver an accurate flow of mobile phase at supercritical conditions. SFC Columns SFC columns (as well as many other types of liquid chromatography columns) consist of a stainless steel tube filled with the stationary phase. The stationary phase may be bare silica or silica coated with a bonded phase. SFC columns use end-fittings that permit their connection to the rest of the chromatographic system, and are identical to those used in HPLC columns. Packed SFC columns are usually made of stainless steel, however, ceramic columns are also available.[32] The stationary phase is retained at each end of the tube by a sieve or frit. SFC fittings and accessories are generally made of stainless steel.


The surface quality of the tube inside wall is of overriding importance as it affects the flow behaviour through the column, uneven, unsmooth, or irregular surfaces will adversely affect the separation of analytes flowing through the column.[32] SFC capillary columns are of similar design to their GC counterparts, with the stationary phase being chemically immobilized onto the inner wall of the capillary, which is typically made from silica coated with a polyimide to give it strength and flexibility. These columns have very narrow external diameters; therefore, chemical means (rendering ceramic frits) rather than external fittings are the preferred choice to immobilise the column bed.

Packed Column SFC Stationary Phases In general terms, SFC can be regarded as a form of normal phase chromatography with the added advantage of low viscosity (and high diffusivity) of supercritical fluids. Therefore, SFC columns are of the same type as those currently used in normal phase chromatography. For practical reasons, only carbon dioxide based systems are useful as SFC mobile phases; therefore, SFC applications typically require polar stationary phases such as silica amino and diol. It is worth nothing that low to non polar stationary phases (such as octadecylsiloxane-bonded) can also be used with SFC. Selected commercially available SFC columns are listed below.[18]


Capillary Column SFC Stationary Phases Although there are not many clear rules for capillary SFC column selection, the polarity of the stationary phase gives an indication of its applicability and is the key parameter to start with. However, very often different stationary phases may need to be scouted before obtaining a proper separation, in a rather familiar way to anyone working with chiral

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phases during the later 1990’s and early 2000’s. In many cases, SFC column selection has to be performed considering previous work.[19] Modern SFC instruments permit rapid scouting of columns of different nature while altering (if required) the mobile phase. These systems are especially suited for column selection under supercritical conditions and will usually contain automated switching valves etc. One positive is the rapid short run times for SFC made possible due to the unique properties of the eluent. The tool opposite lists selected capillary SFC stationary phases and highlights situations where they can be useful.

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Detection As was previously explained, SFC instruments are hybrids of their HPLC and GC counterparts.

• Packed column SFC instrumentation is more like an HPLC system with a reciprocating pump and a pressurized detector

• Capillary SFC systems resemble GC instruments with a syringe pump SFC systems implement pressure restrictors (or regulators) somewhere after the column to keep the eluent system above its critical conditions. The position of the pressure regulator will determine the type of detection system to be used. If the pressure restrictor is placed after the detector, then the detection system operates as in LC but at much higher pressures – requiring the detector flow cells to be specially designed and manufactured to withstand these pressures without cracking the cell windows or leaking.

Schematic representation of the connection of SFC and a high pressure detection system

(in this case a UV/Vis detector). Here the pressure regulator is located AFTER the detector.


If the pressure restrictor is placed before the detector, then the detection system operates as in GC; however, as the depressurized effluent enters the detector, noisy signals can be rendered. After leaving the column, the supercritical mobile phase decompresses into a gas, allowing SFC to be coupled to many GC detection methods;[9, 21, 24, 29] however, a number of LC devices, have also been used.[22, 23, 25, 26, 27, 28]

Schematic representation of the connection of SFC and a low pressure detection system

(in this case a FID detector). Here the pressure regulator is located BEFORE the detector.


Pressure Regulators Acting as pressure regulators, flow restrictors are commonly used to meet pressure requirements of the SFC system. The outlet of the pressure regulator is usually heated to prevent adiabatic cooling of the expanding supercritical fluid. Adiabatic cooling of SFC mobile phases could render dry ice (solid form of carbon dioxide) formation and flow path blocking. A basic variable pressure regulator design includes a sensor that measures the actual system pressure, this reading is sent to the pressure control system to be compared with the required setting, as a result of this comparison a piston (acting as a flow restrictor) is acted and the pressure increased or decreased as needed. Two such devices are shown below.

SFC pressure regulator

SFC variable pressure regulator


Advantages and Disadvantages of SFC Advantages over HPLC Disadvantages over HPLC • Higher resolution and capacity • Less organic solvent waste • Higher throughput • Fast calibration

• Limited choice of mobile phases • Limited analyte solubility in the mobile phase • Unwanted reactions with the mobile phase • Unsuited for water-soluble analytes

Advantages over GC Disadvantages over GC • Better solvation • Extended range of samples • The strength of the mobile phase

which can be controlled through the addition of modifiers

• Suitable for thermally labile samples

• Limited choice of mobile phases • Unwanted reactions with the mobile phase • Hardware complexity • The addition of organic modifiers will

preclude the use of detectors that respond to carbon containing compounds (such as FID)

References 1. Terry A. Berger. “Separation of polar solutes by packed column supercritical fluid chromatography” Journal of Chromatography A, 785 (1997) 3-33 2. Phyllis R. Eckard, Larry T. Taylor, Gregory C. Slack. “Method development for the separation of phospholipids by subcritical fluid chromatography” Journal of Chromatography A, 826 (1998) 241–247. 3. Rodger W. Stringham, Brian R. Krueger, Jonathan Marshall. “Use of elevated flow rates in preparative subcritical fluid chromatography” Journal of Chromatography A, 1175 (2007) 112–116. 4. H. Fischer, O. Gyllenhaal, J. Vessman, K. Albert. “Reaction monitoring of aliphatic amines in supercritical carbon dioxide by proton nuclear magnetic resonance spectroscopy and implications for supercritical fluid chromatography” Analytical Chemistry. 75 (2003) 62–626. 5. L.S. Daintree, A. Kordikowski, P. York. “Separation processes for organic molecules using SCF Technologies” Advanced Drug Delivery Reviews 60 (2008) 351–372. 6. Chih Wu “Thermodynamics and Heat Powered Cycles: a Cognitive Engineering Approach” Chapter 2. New York, Copyright © 2007 by Nova Science Publishers, Inc. 7 Miguel Herrero, Alejandro Cifuentes, Elena Ibañez. “Sub- and supercritical fluid extraction of functional ingredients from different natural sources: Plants, food-by-products, algae and microalgae” Food Chemistry 98 (2006) 136–148 8. Phyllis R. Brown, Eli Grushka, Susan Lunte. “Advances in Chromatography. Vol 46” Marcel Dekker, Pp 215-220, New York 2005 9. Zhongpeng Xia, Kevin B. Thurbide. “Universal acoustic flame detection for modified supercritial fluid chromatography” Journal of Chromatography A, 1105 (2006) 180–185 10. Isabelle François, Pat Sandra. “Comprehensive supercritical fluid chromatography×reversed phase liquid chromatography for the analysis of the fatty acids in fish oil” Journal of Chromatography A, 1216 (2009) 4005–4012 11. Jianjun Li. “Quantitative analysis of cosmetics waxes by using supercritical fluid extraction (SFE)/supercritical fluid chromatography (SFC) and multivariate data analysis” Chemometrics and Intelligent Laboratory Systems 45 (1999) 385–395 12. Akira Nomura, Joseph Yamada, and Takashi Yarita. “Supercritical-Fluid Chromatograms of Fuel Oils on ODS-Silica Gel Column Using Fluorescence, UV-Absorption, and Flame-Ionization Detectors” The Journal of Supercritical Fluids, 1995,8, 329-333


13 J.L. Bernal, J.J. Jiménez, J.M. Rivera, L, Toribio, M. J. del Nozal. “On-line solid-phase extraction coupled to supercritical fluid chromatography with diode array detection for the determination of pesticides in water” Journal of Chromatography A, 754 (1996) 145-157 14. Gwenaelle Lavison-Bompard, Didier Thiébaut, Jean-François Beziau, Bernadette Carrazé, Pascale Valette, Xavier Duteurtre, Jean-Claude Tabet. “Hyphenation of atmospheric pressure chemical ionisation mass spectrometry to supercritical fluid chromatography for polar car lubricant additives analysis” Journal of Chromatography A, 1216 (2009) 837–844 15. Terry A. Berger. “Separation of polar solutes by packed column supercritical fluid chromatography” Journal of Chromatography A, 785 (1997) 3-33 16. Chiehming J. Chang, Chany-Yih Day, Ching-Ming Ko, Kou-Lung Chiu. “Densities and P-x-y diagrams for carbon dioxide dissolution in methanol, ethanol, and acetone mixtures” Fluid Phase Equilibria 131 (1997) 243-258 17. Larry T. Taylor. “Supercritical fluid chromatography for the 21st century” The Journal of Supercritical Fluids 47 (2009) 566–573 18. C.West, E. Lesellier. “A unified classification of stationary phases for packed column supercritical fluid chromatography” Journal of Chromatography A, 1191 (2008) 21–39 19. Mohamed Maftouh, Christine Granier-Loyaux, Evelyne Chavana, Jerome Marini, Antoine Pradines, Yvan Vander Heyden, Claudine Picard. “Screening approach for chiral separation of pharmaceuticals Part III. Supercritical fluid chromatography for analysis and purification in drug discovery” Journal of Chromatography A, 1088 (2005) 67–81 20 Lu Zeng, Rongda Xu, Derek B. Laskar, Daniel B. Kassel. “Parallel supercritical fluid chromatography/mass spectrometry system for high-throughput enantioselective optimization and separation” Journal of Chromatography A, 1169 (2007) 193–204 21. Nohora P. Vela, Joseph A. Caruso. “Element selective detection for supercritical-fluid chromatography” Journal of Biochemical and Biophysical Methods 43 (2000) 45–58 22. Yukio Hirata, Yukinori Kawaguchi, and Yasuhiro Funada. “Refractive Index Detection Using an Ultraviolet Detector with a Capillary Flow Cell in Preparative SFC” Journal of Chromatographic Science, Volume 34, Number 1, January 1996, pp. 58-62 23. Susanne R. Wallenborg, Karin E. Markides, Leif Nyholm. “Oxidative and reductive amperometric detection of phenolic and nitroaromatic compounds in packed capillary column supercritical fluid chromatography” Journal of Chromatography A, 785 (1997) 121-128 24. Daniel G. Morgan, Kevin L. Harbol, Nicholas Peter Kitrinos, Jr. “Optimization of a supercritical fluid chromatograph–atmospheric pressure chemical ionization mass spectrometer interface using an ion trap and two quadrupole mass spectrometers” Journal of Chromatography A, 800 (1998) 39–49 25. H. Shi, J.T.B. Strode III, L.T. Taylor, E.M. Fujinari. “Feasibility of supercritical fluid chromatography-chemiluminescent nitrogen detection with open tubular columns” Journal of Chromatography A, 734 (1996) 303-310 26. Roger M. Smith, Orapin Chienthavorn, Nicholas Danks, Ian D. Wilson. “Fluorescence detection in packed-column supercritical fluid chromatographic separations” Journal of Chromatography A, 798 (1998) 203–206 27. Brian J. Hoffman, Larry T. Taylor, Stephen Rumbelow, Larry Goff, J. David Pinkston. “Separation of derivatized alcohol ethoxylates and propoxylates by low temperature packed column supercritical fluid chromatography using ultraviolet absorbance detection” Journal of Chromatography A, 1034 (2004) 207–212 28. Kayori Takahashi, Shinichi Kinugasa, Masaaki Senda, Koki Kimizuka, Kyoko Fukushima, Tsutomu Matsumoto, Yasuhiro Shibata, John Christensen. “Quantitative comparison of a corona-charged aerosol detector and an evaporative light-scattering detector for the analysis of a synthetic polymer by supercritical fluid chromatography” Journal of Chromatography A, 1193 (2008) 151–155


29. T. A. Berger. “Simple Correction for Variable Post Column Split Ratios using Pure Carbon Dioxide in Packed Column Supercritical Fluid Chromatography with Independent Pressure and Flow Control” Chromatographia 2001, 54, December (No. 11/12), 783-788 30. J.L. Bernal, L. Toribio, M.J. del Nozal, E.M. Nieto, M.I. Montequi. “Separation of antifungal chiral drugs by SFC and HPLC: a comparative study” Journal of Biochemical and Biophysical Methods 54 (2002) 245–254 31. Lisa M. Nogle, Charles W. Mann, William L. Watts Jr., Yingru Zhang. “Preparative separation and identification of derivatized ß-methylphenylalanine enantiomers by chiral SFC, HPLC and NMR for development of new peptide ligand mimetics in drug discovery” Journal of Pharmaceutical and Biomedical Analysis 40 (2006) 901–909 32. Bernd-Walter Hoffman; Guenter Schaeufele. “Separation Column for Chromatography” United States Patent 5651886. July 29, 1997

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