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High-Performance Liquid Chromatography 28 chapter High-Performance Liquid Chromatography Bradley L. Reuhs ∗ and Mary Ann Rounds Department of Food Science, Purdue University, West Lafayette, IN 47907-2009, USA [email protected] 28.1 Introduction 501 28.2.3.2 HPLC Column Packing 28.2 Components of an HPLC System 501 Materials 503 28.2.1 Pump 501 28.2.3.2.1 General 28.2.2 Injector 502 Requirements 503 28.2.3 Column 503 28.2.3.2.2 Silica-Based Column 28.2.3.1 Column Hardware 503 Packings 504 28.2.3.1.1 Precolumns 503 28.2.3.2.3 Porous Polymeric 28.2.3.1.2 Analytical Column Columns 503 Packings 504 S.S. Nielsen, Food Analysis, Food Science Texts Series, DOI 10.1007/978-1-4419-1478-1_28, 499 °c Springer Science+Business Media, LLC 2010 500 Part V • Chromatography 28.2.4 Detector 505 28.3.3 Ion Exchange 509 28.2.4.1 UV-Vis Absorption 28.3.3.1 Stationary and Mobile Detectors 505 Phases 509 28.2.4.2 Fluorescence Detectors 505 28.3.3.2 Applications of Ion-Exchange 28.2.4.3 Refractive Index HPLC 509 Detectors 505 28.3.3.2.1 Ion Chromatogra- 28.2.4.4 Electrochemical Detectors 505 phy 509 28.2.4.5 Other HPLC Detectors 506 28.3.3.2.2 Ion Exchange 28.2.4.6 Coupled Analytical Chromatography of Techniques 506 Carbohydrates and 28.2.4.7 Chemical Reactions 506 Proteins 510 28.2.5 Data Station Systems 507 28.3.4 Size Exclusion 510 28.3 Applications in HPLC 507 28.3.4.1 Column Packings and Mobile 28.3.1 Normal Phase 507 Phases 510 28.3.1.1 Stationary and Mobile 28.3.4.2 Applications of High Phases 507 Performance SEC 511 28.3.1.2 Applications of Normal-Phase 28.3.5 Affinity 511 HPLC 508 28.4 Summary 511 28.3.2 Reversed Phase 508 28.5 Study Questions 512 28.3.2.1 Stationary and Mobile 28.6 Acknowledgments 512 Phases 508 28.7 References 512 28.3.2.2 Applications of Reversed-Phase HPLC 508 Chapter 28 • High-Performance Liquid Chromatography 501 28.1 INTRODUCTION analysis of separated components is needed. Con- necting tubing, tube fittings, and the materials out High-performance liquid chromatography (HPLC) of which components are constructed also influence developed during the 1960s as a direct offshoot system performance and lifetime. References ( 1) and of classic column liquid chromatography through (6–10 ,15 ) include detailed discussions of HPLC equip- improvements in the technology of columns and ment, with the book by Bidlingmeyer ( 1) especially instrumental components (pumps, injection valves, appropriate for beginners. The unique organization of and detectors). Originally, HPLC was the acronym reference ( 8) is intended for those who may need to for high-pressure liquid chromatography , reflecting the learn chromatography quickly in an industrial envi- high operating pressures generated by early columns. ronment. Two useful books on HPLC troubleshooting By the late 1970s, however, high-performance liq- are those written by Gertz ( 11 ) and Dolan and Sny- uid chromatography had become the preferred term, der ( 12 ). In addition, much information on HPLC emphasizing the effective separations achieved. In equipment, hardware, and troubleshooting hints may fact, newer columns and packing materials offer high be found in publications such as LC ·MS, American performance at moderate pressure (although still high Laboratory, Chemical & Engineering News , and simi- pressure relative to gravity-flow liquid chromatog- lar periodicals. Manufacturers are also a source of raphy). HPLC can be applied to the analysis of any practical information on HPLC instrumentation and compound with solubility in a liquid that can be columns/stationary phase material. used as the mobile phase. Although most frequently employed as an analytical technique, HPLC also may 28.2.1 Pump be used in the preparative mode. There are many advantages of HPLC over traditional low pressure The HPLC pump delivers the mobile phase through the column liquid chromatography: system, typically at a flow rate of 0.4–1ml/min, in a controlled, accurate, and precise manner. The majority 1. Speed (many analyses can be accomplished in of pumps currently used in HPLC (>90% ) are recip- 30 min or less) rocating, piston-type pumps. The dual piston pump 2. A wide variety of stationary phases systems with ball check valves are the most efficient 3. Improved resolution pumps available. One disadvantage of reciprocating 4. Greater sensitivity (various detectors can be pumps is that they produce a pulsating flow, requir- employed) ing the addition of pulse dampers to suppress fluctu- 5. Easy sample recovery (less eluent volume to ations. A mechanical pulse damper or dampener con- remove) sists of a device (such as a deformable metal compo- Application of HPLC to the analysis of food began nent or tubing filled with compressible liquid) that can in the late 1960s, and its use increased with the devel- change its volume in response to changes in pressure. opment of column packing materials that would sep- Gradient elution systems for HPLC are used to arate sugars. Using HPLC to analyze sugars was jus- vary the mobile phase concentration during the run, tified economically as a result of sugar price increases by mixing mobile phase from two or more reservoirs. in the mid 1970s, which motivated soft drink manufac- This is accomplished with low-pressure mixing, in turers to substitute high-fructose corn syrup for sugar. which mobile phase components are mixed before Monitoring sweetener content by HPLC assured a entering the high-pressure pump, or high-pressure good quality product. Other early food applications mixing, in which two or more independent, pro- included the analysis of pesticide residues in fruits grammable pumps are used. For low-pressure gra- and vegetables, organic acids, lipids, amino acids, tox- dient systems, a computer-controlled proportioning ins (such as aflatoxins in peanuts), and vitamins ( 1). valve, followed by a mixing chamber at the inlet to HPLC continues to be applied to these, and many the pumps is used, which results in extremely accu- more, food-related analyses today ( 2–5). rate and reproducible gradients. Gradient HPLC is extremely important for the effective elution of all components of a sample and for optimal resolution. 28.2 COMPONENTS OF AN HPLC SYSTEM It is routinely applied to all modes of HPLC except size-exclusion chromatography. A schematic diagram of a basic HPLC system is shown Many commercially available HPLC pumping sys- in Fig. 28-1 . The main components of this system – tems and connecting lines are made of grade ANSI 316 pump, injector, column, detector , and data sys- stainless steel, which can withstand the pressures gen- tem – are discussed briefly in the sections below. erated. Also it is resistant to corrosion by oxidizing Also important are the mobile phase ( eluent ) reser- agents, acids, bases, and organic solvents, although voirs, and a fraction collector, which is used if further mineral acids and halide ions do attack stainless 502 Part V • Chromatography Schematic representation of a system for high-performance liquid chromatography (not drawn to scale). 28-1 Column(s) and detector may be thermostatted, as indicated by the dashed line, for operation at elevated figure temperature. steel. In other systems, all components that come into contact with the eluent are made of sturdy, inert poly- mers, and even employ sapphire pistons, which are resistant to extreme pH and high salt concentration. The latter systems can be used for all applications except normal phase, which uses organic solvents as the mobile phase. The polymer-based systems have facilitated a wider application of ion exchange HPLC. All HPLC pumps contain moving parts such as check valves and pistons, and are quite sensitive to dust and particulate matter in the liquid being pumped. Therefore, it is advisable to filter the mobile phase using 0.45 or 0.22- µm filters prior to use. Degassing HPLC eluents, by the application of a vac- uum or by sparging with helium, also is recommended to prevent the problems caused by air bubbles in a pump or detector. 28.2.2 Injector The role of the injector is to place the sample into the flowing mobile phase for introduction onto the col- umn. Virtually all HPLC systems use valve injectors , which separate sample introduction from the high- pressure eluent system. With the injection valve in the LOAD position (Fig. 28-2 a), the sample is loaded into an external, fixed-volume loop using a syringe. Elu- ent, meanwhile, flows directly from the pump to the column at high pressure. When the valve is rotated Valve-type injector. The valve allows the sam- to the INJECT position (Fig. 28-2 b), the loop becomes 28-2 ple loop to be ( a) isolated from the pump elu- figure part of the eluent flow stream and sample is carried ent stream (LOAD position) or ( b) positioned onto the column. Such injectors are generally trouble in it (INJECT position). [from ( 9), used with permission.] free and afford good precision. Chapter 28 • High-Performance Liquid Chromatography 503 Changing the loop allows different volumes to 28.2.3.1.2 Analytical Columns The most commonly be injected. Although injection volumes of 10–100 µl used analytical HPLC columns are 10, 15, or 25-cm are typical, both larger (e.g., 1–10 ml) and smaller long with an internal diameter of 4.6 or 5mm ( 9). (e.g., ≤2 µl) sample volumes can be loaded by utiliz- Short (3 cm) columns, packed with ≤3 µm particles, ing special hardware. An important advantage of the are gaining popularity for fast separations; for exam- loop valve design is that it is readily adapted to auto- ple, in method development or process monitoring.
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