ERT320 BIOSEPARATION ENGINEERING CHROMATOGRAPHY
CHROMATOGRAPHY Week 9-10 Reading Assignment: Chapter 7. Bioseparations Science & Engineering, Harrison, R; Todd, P; Rudge, S.C and Petrides, D,P
CHROMATOGRAPHY Use in separation, purification & identification of compounds before quantitative analysis is taken up. BASIS:Selective distribution of component in a mixture between 2 immiscible phases in intimate contact with each other 1 stationary phase & 1 mobile phase APPLICATION: Separation of biomolecules, fine & specialty chemicals ANALITICAL TOOLS To determine chemical compositions of sample PREPARATIVE TOOLS To PURIFY & COLLECT 1/ more components of sample
SEPARATION PRINCIPLES Solutes in solution/ volatiles in gas are placed in MOBILE PHASE & passed over a selected adsorbent material [stationary phase] The solutes/ volatiles have differential AFFINITY for the adsorbent material & thus, separation occurs. MOBILE PHASE: Continuous flow of a carrier liquid/ gas STATIONARY PHASE: A bed of solids/ immobilized liquid
STATIONARY PHASE: LIQUID CHROMATOGRAPHY SILICA BASED RESINS: Uncoated/ coated silica ION-EXCHANGE RESINS: Cation/ anion exchangers POLYMER-BASED RESINS: Synthetic/ natural polymers
SILICA-BASED RESINS UNCOATED SILICA i. Compatible with water or organic solvent ii. Serves as a good reversible adsorbent for hydrophilic compounds iii. Organic solvent used as mobile phase, and water is added as the chromatography progresses iv. Not typically stable at extremes of Ph v. Available with high surface area and small particle size; being very rigid; does not collapse under high pressures vi. Denature some proteins and irreversibly bind others vii. Used for purification of many commercial biotechnology products COATED SILICA i. Particles coated with longchain alkanes ii. Has a high affinity for hydrophobic molecules, which increases as the chain length of the bonded alkane increases. iii. Many varieties of the same chain length phase polymerized, simple monolayer and end-capped
STYRENE DIVINYLBENZENE: i. Very stable at ph extremes ii. Support for ion exchange chromatography because of its stability and rigidity AGAROSE: i. Can be crosslinked to form a reasonably rigid bead that is capable of tolerating pressures up to 4 bar. POLYMER-BASED RESINS POLYACRYLAMIDE: i. Used less often, not used as a polymer solid but as hydrogel and used as a size exclusion gel ii. The crosslinking in polyacrylamide can be controlled by the amount of bisacrylamide added in suspension mixture DEXTRAN i. Less rigid and used in size exclusion ii. Can be formed with very large pores iii. Capable of including antibody molecules and virus particles NATURAL POLYMERS: i. Used in hydrogel for a low pressure chromatography resins. ii. Naturally hydrophillic iii. Compatible with proteins and other biomaterials
Resins that have been derivatived with an ionic group Most commonly used ionic groups: a. Sulfoxyl (SO3-) - most acidic b. Carboxyl (COO-) c. Diethylaminoethyl (DEAE) (2C2H5N+HC2H5) d. Quaternary ethylamine (QAE) (4CHN+) - most basic ION-EXCHANGE RESINS CATION EXCHANGERS: i. Acidic ion exchanger ii. Carry a negative charge iii. Attract positive counterions ANION EXCHANGERS: i. Basic ion exchangers ii. Carry a positive charge iii. Attract negative counterions
STATIONARY PHASE: GAS CHROMATOGRAPHY SOLID PHASE: i. Most uses for separation of low MW compounds and gases ii. Common SP: silica, alumina, molecular sieves such as zeolites, cabosieves, carbon blacks LIQUID PHASE: i. Over 300 different phases are widely available ii. Grouped liquid phases: Non-polar, polar, intermediate and special phases iii. Polymer liquid phase Non-polar phase i. Primarily separated according to their volatilities ii. Elution order varies as the boiling points of analytes iii. Common phases: dimethylpolysiloxane, dimethylphenylpolysiloxane Polar phase i. Contain polar functional groups ii. Separation based on their volatilities and polar-polar interaction iii. Common phases: polyethylene glycol Intermediate phase i. Common phase: 14% cyanopropyl phenyl polysiloxane
PARTICLE & PRESSURE DROP IN FIXED BEDS Pressure drop is given by the Darcy equation:
From Blake-Kozeny equation, k gives a function of resin particles size and void friction
Darcy equation applies for rigid particles, such as silica. When the stationary phase particle size is decreased, the pressure drop in the column increases as the inverse square. These increases requires pressure additional power in pumping, as well as more specialized requirements for the construction of the columns and its seals
CHROMATOGRAM DESCRIPTION
CHROMATOGRAM Response of a detector vs time, shown when various components come off a column RETENTION TIME, tr The time at which a component elutes from a column
CHROMATOGRAPHY COLUMN DYNAMICS PLATE MODELS HEIGHT OF EQUIVALENT THEORETICAL PLATE (HETP), H: Where L = Length of the column N = Number of plates
From Gaussian peaks: THE PLATE COUNT (N) can be expressed as the squared average retention time divided by the variance of the peak Where w = peak width at the base tr = average retention time
PEAK WIDTH is used in the definition of resolution, Rs measure of the extent of separation of two peaks in chromatography Where tr1, tr2 = average retention time for separands 1 & 2 w1, w2 = peak width (time) for separands 1 & 2
Chromatography column mass balance with negligible dispersion Mass balance for chromatography: ci = concentration of solute i in the mobile phase = [C]i, qi = concentration of solute i in the stationary phase averaged over an adsorbent particle = [CS]i, ε = void fraction (mobile phase volume/total column volume), commonly 0.3 to 0.4 in fixed beds, v = mobile phase superficial velocity (flow rate divided by the empty column cross-sectional area, Q/A), Deff= effective dispersivity of the solute in the column, t = time, x = longitudinal distance in the column; x = 0 at column inlet
Using an equilibrium isotherm relationship in the form qi =f(ci), EQ. (1) becomes: Where Where qi (ci) is the slope of the equilibrium isotherm at concentration ci.
If we let: Then EQ. (2) becomes: Thus, the expression for ui given by EQ. (3) is the effective velocity of component i through the packed column.
1. All process volumes are scaled-up in direct proportion to the sample volumes Process volumes include the column bed, wash, and elution volumes. 2. Column length is held constant Column volume is increased by increasing column diameter or by having a number of columns operating in parallel 3. Linear (or superficial) velocity is held constant Because column length is held constant, volumetric flow rate increases proportionally with sample volume. Total separation time remains roughly constant in scale-up. 4. Sample composition is held constant Critical factors include concentration, viscosity, ph and ionic strength
BASIC DESIGN CALCULATIONS Typically account for changes in bed height & diameter, linear & volumetric flow rate, and particle size. General approach for scale up is to based on keeping the resolution, Rs constant For linear gradient elution ion exchange & hydrophobic interaction chromatography,
To remove the volume term from the expression for Rs,
Thus, for scale-up with constant resolution from scale 1 to scale 2 for the same product and the same column void fraction, the scale-up equation is: Thus, as the particle size increases on scale-up, the flow rate relative to the column volume must decrease and/or the gradient slope must decrease to maintain constant resolution, which seems correct intuitively.
Easy to develop lab scale processes that use the same resin and same gradient for the commercial process scale In practice only the ratio between column volume and flow rate need be addressed When the bed height can be maintained on scale-up, the mobile phase linear velocity remains the same, and the column is simply scaled by diameter