high performance liquid chromatoghraphy (HPLC)Presentation Transcript
High Performance Liquid Chromatography (HPLC)S What is HPLC?S Types of SeparationsS Columns and Stationary PhasesS Mobile Phases and Their Role in SeparationsS Injection in HPLCS Detection in HPLCS Variations on Traditional HPLC S Ion Chromatography S Size Exclusion Chromatography
What is HPLC?HPLC is really the automation of traditionalliquid chromatography under conditions whichprovide for enhanced separations duringshorter periods of time!Probably the most widely practiced form ofquantitative, analytical chromatographypracticed today due to the wide rangeof molecule types and sizes which canbe separated using HPLC orvariants of HPLC!!
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PartitioningS Separation is based on the analyte’s relative solubility between two liquid phases Mobile Phase Stationary Phase Solvent Bonded Phase
Instrumentation Gradient Controller • Pump Column Detector Injector Mobile Phases
What does the analyst do?S Select the correct type of separation for the analyte(s) of interest, based on the sample type (among other factors).S Select an appropriate column (stationary phase) and mobile phaseS Select an appropriate detector based on whether universal or compound-specific detection is required or availableS Optimize the separation using standard mixturesS Analyze the standards and sample
What does the analyst do?S Select the correct type of separation for the analyte(s) of interest, based on the sample type (among other factors).
HPLC - ModesS Normal Phase. - Polar stationary phase and non-polar solvent.• Reverse Phase. - Non-polar stationary phase and a polar solvent.
Common Reverse Phase SolventsMethanol CH3OHAcetonitrile CH3CNTetrahydrofuranWater H2O
What does the analyst do? Select an appropriate column (stationary phase) and mobile phase
Columns and Stationary PhasesHPLC is largely the domain of packed columns: S some research into microbore/capillary columns is going on. S Molecules move too slowly to be able to reach and therefore “spend time in” the stationary phase of an open tubular column in HPLC. SIn solution, not the gas phase SLarger molecules in HPLC vs. GC (generally)
Columns and Stationary Phases Stationary phases are particles which are usually about 1 to 20 µm in average diameter (often irregularly shaped): S In Adsorption chromatography, there is no additional phase on the stationary phase particles (silica, alumina, Fluorosil). S In Partition chromatography, the stationary phase is coated on to (often bonded) a solid support (silica, alumina, divinylbenzene resin)
ColumnsS Solid Support - Backbone for bonded phases. S Usually 10µ, 5µ or 3µ silica or polymeric particles.S Bonded Phases - Functional groups firmly linked (chemically bound) to the solid support. S Extremely stable S ReproducibleS Guard - Protects the analytical column: S Particles S Interferences S Prolongs the life of the analytical column
Stationary PhasesS Polar (“Normal” Phase): S Silica, alumina S Cyano, amino or diol terminations on the bonded phaseS Non-Polar (“Reversed Phase”) S C18 to about C8 terminations on the bonded phase S Phenyl and cyano terminations on the bonded phaseS Mixtures of functional groups can be used!!S Packed particles in a column require: S Frits at the ends of the column to keep the particles in S Filtering of samples to prevent clogging with debris S High pressure pumps and check-valves S Often a “Guard Column” to protect the analytical column
The Mobile Phase in HPLC...S Must do the following: S solvate the analyte molecules and the solvent they are in S be suitable for the analyte to transfer “back and forth” between during the separation processS Must be: S compatible with the instrument (pumps, seals, fittings, detector, etc) S compatible with the stationary phase S readily available (often use liters/day) S of adequate purity S spectroscopic and trace-composition usually! S Not too compressible (causes pump/flow problems) S Free of gases (which cause compressability problems)
Typical HPLC Pump (runs to 4,000+ psi)
Polarity Index for Mobile Phases…..S The polarity index is a measure of the relative polarity of a solvent. It is used for identifying suitable mobile phase solvents. S The more polar your solvent is, the higher the index. S You want to try to choose a polarity index for your solvent (or solvent mixture) that optimizes the separation of analytes S usually the index is a starting point S the polarity of any mixture of solvents to make a mobile phase can be modeled to give a theoretical chromatogram S Usually, optimization of solvent composition is experimentalS A similar number is the Eluent Strength (Eo]S Increasing eluent strength or polarity index values mean increasing solvent polarity.S Remember, the analyte(s) and samples must be mobile phase and stationary phase compatible!
Optimization of Mobile Phase Polarity…Changing the mobile phase composition alters theseparation.
Isocratic versus Gradient ElutionS Isocratic elution has a constant mobile phase composition S Can often use one pump! S Mix solvents together ahead of time! S Simpler, no mixing chamber required S Limited flexibility, not used much in research S mostly process chemistry or routine analysis.S Gradient elution has a varying mobile phase composition S Uses multiple pumps whose output is mixed together S often 2-4 pumps (binary to quarternary systems) S Changing mobile phase components changes the polarity index S can be used to subsequently elute compounds that were previously (intentionally) “stuck” on the column S Some additional wear on the stationary phase S Column has to re-equiluibrate to original conditions after each run (takes additional time).
Injection in HPLCS Usually 5 to 1000 µL volumes, all directly onto the column S not much worry about capacity since the columns have a large volume (packed).S Injector is the last component before the column(s)S A source of poor precision in HPLC S errors of 2-3 %RSD are due just to injection S other errors are added to this S due to capillary action and the small dimensions/cavities inside the injectorS 6-PORT Rotary Valve is the standard manual injectorS Automatic injectors are availableS Two positions, load and inject in the typical injectorS Injection loop internal volume determines injection volume.
LOAD (the sample loop)Inject (move the sampleloop into the mobilephase flow)
What does the analyst do?S Select an appropriate detector based on whether universal or compound- specific detection is required or available
Detection in HPLCS Numerous Types (some obscure)S Original HPLC Detectors were common laboratory instruments such as spectrophotometers, etc. S you can even use a SPEC 20! S Usually a narrow linear range (1E3, usually)S Must be solvent -compatible, stable, etc.S Universal S respond to all analytesS Analyte Specific S respond to specific properties of analytesS Non-destructive S mostS Destructive S ELSD, MS and a few others.
Standard Absorbance Detector….S Single Beam UV-VIS instrument with a flow-through cell (cuvette)S Can use any UV-VIS with a special flow cell S Extra connections lead to band-broadening if UV-VIS is far from HPLC column exit.S Usually utilize typical UV-VIS lamps and 254 nm default wavelenth S Can be set to other wavelengths (most) S Simple filter detectors no longer widely used S adjustable wavelength units are cost-effectiveS Non-destructive, not-universal S not all compounds absorb light S can pass sample through several cells at several different wavelenghtsS Usually zeroed at the start of each run using an electronic software command. You can have real-time zeroing with a reference cell.
Diode Array Detector (DAD)S The more common tool for research-grade HPLC instruments S quite versatile...S Advances in computer technology since ~1985 or so have lead to the development of Diode Array instrumentsS Non-destructive, non-universalS DAD scans a range of wavelengths every second or few seconds. At each point in the chromatogram one gets a complete UV-VIS spectrum! S Huge volumes of data S Detailed spectra for each peak and each region of each peak
Refractive Index DetectorS One of a very few Universal HPLC detectors. Non-destructiveS Responds to analytes changing the RI of the mobile phase S requires a separate reference flow of mobile phaseS Extremely temperature sensitive, usually heated S sensitive to temp changes of +/- 0.001 °CS No longer really widely used S Absorbance detectors are relatively cheap.S Useful for process work, on-line monitoring, etc.
ELSD (Evaporative Light ScatteringDetector)S Universal, destructiveS Useful for very large molecules, and a wide linear rangeS Analytes are de-solvated in the detectorS Molecules pass through what is essentially a large cuvette for a UV-VIS instrumentS The reduction in light intensity detected (due to scattering by the analytes) is measuredS The larger and more concentrated a particular molecule is, the greater the scattering.
What does the analyst do?S Optimize the separation using standard mixtures and then for samples.
Optimization of Separations in HPLCS Correct choice of column so the above equilibrium has some meaningful (non-infinity, non-zero) equilibrium constants.S Correct choice of mobile phaseS Decision on the type of mobile phase composition S constant composition = isocratic S varying composition = gradient elutionS Determination if flow rate should be constant S usually it isS Decision on heating the column S heating HPLC columns can influence the above equilibrium….
Types of HPLC Separations (partial list) S Normal Phase: Separation of polar analytes by partitioning onto a polar, bonded stationary phase. S Reversed Phase: Separation of non-polar analytes by partitioning onto a non-polar, bonded stationary phase. S Adsorption: In Between Normal and Reversed. Separation of moderately polar analytes using adsorption onto a pure stationary phase (e.g. alumina or silica)
Types of HPLC Separations (partial list) S Ion Chromatography: Separation of organic and inorganic ions by their partitioning onto ionic stationary phases bonded to a solid support. S Size Exclusion Chromatography: Separation of large molecules based in the paths they take through a “maze” of tunnels in the stationary phase.
Advantages of LC compared to GC: 1.) LC can be applied to the separation of any compound that is soluble in a liquid phase. ‚oLC more useful in the separation of biological compounds, synthetic or natural polymers, and inorganic compounds 2.) Liquid mobile phase allows LC to be used at lower temperatures than required by GC ‚ LC better suited than GC for separating compounds that may be thermally labile
3.) Retention of solutes in LC depend on their interaction with both the mobile phase and stationary phase. ‚ GC retention based on volatility and interaction with stationary phase ‚ LC is more flexible in optimizing separations change either stationary or mobile phase 4.) Most LC detectors are non-destructive ‚ most GC detectors are destructive ‚ LC is better suited for preparative or process-scale separationsDisadvantage of LC compared to GC: 1.) LC is subject to greater peak or band-broadening. ‚ much larger diffusion coefficients of solutes in gases vs. liquids