Presentation on theme: "INTRODUCTION TO CHROMATOGRAPY. H ISTORY The Russian botanist Mikhail Tswett coined the term chromatography in 1906 to describe his experiments in separating."— Presentation transcript:
INTRODUCTION TO CHROMATOGRAPY
H ISTORY The Russian botanist Mikhail Tswett coined the term chromatography in 1906 to describe his experiments in separating different colored constituents of leaves by passing an extract of the leaves through a column
Analytic technique to discover chemical components: a method of finding out which components a gaseous or liquid mixture contains that involves passing it through or over something that absorbs the different components at different rates Chromatography Web Dictionary:
Chromatography Carrier Gas Flow Control Injector Port Column Column Oven Detector Recorder
C HROMATOGRAPHY C OLUMNS Packed Column: Typical HPLC columns but some gas chromatography columns also (especially older columns). The columns are packed with tiny particles. Capillary Column: Typical gas chromatography column which consists of a small diameter tube coated on the inside with stationary phase.
C HROMATOGRAPHY T HEORY
P ARTITION C OEFFICIENT K = Co/Cw Co is concentration in the organic phase (solvent) Cw is the concentration in the aqueous phase (water) Remember from the solvent lecture……….
K = Co/Cw Co is concentration in the organic phase (solvent) Cw is the concentration in the aqueous phase (water) P ARTITION C OEFFICIENT molar concentration in stationary phase molar concentration in mobile phase K =
P ARTITION C OEFFICIENT ETC. mass in the stationary phase mass in the mobile phase volume of mobile phase volume of stationary phase concentration in stationary phase concentration in mobile phase K = k = b =
P ARTITION C OEFFICIENT ETC. k = K/b If mass = volume x concentration then:
E XAMPLE : K = 4 b = k = grams in mobile phase = Compound A: mass = 1 mg Vol. Mobile Phase: 1 mL Vol. Stationary Phase: 1 mL Compound A: mass = 1 mg Vol. Mobile Phase: 1 mL Vol. Stationary Phase: 2 mL K = 4 b = k = grams in mobile phase = Mobile Phase Stationary Phase If the mobile phase is moving, in which situation will compound A move faster through the column?
P ARTITIONING IN A M OBILE P HASE 1.0 mg 0.16 mg 0.83 mg 0.14 mg 0.69 mg 0.12 mg 0.58 mg 0.10 mg 0.08 mg0.07 mg 0.06 mg 0.28 mg Theoretical Plates
P ARTITIONING IN A M OBILE P HASE 1.0 mg 0.00 mg 0.83 mg 0.00 mg 0.69 mg 0.00 mg 0.58 mg 0.03 mg 0.04 mg0.07 mg 0.06 mg 0.28 mg 0.17 mg 0.05 mg0.34 mg 0.28 mg0.01 mg0.00 mg Note: These equilibrium steps to do not actually take place in the column, it is a continuous process.
A NALYTE P EAKS IN THE M OBILE P HASE 1.0 mg 0.00 mg 0.83 mg 0.00 mg 0.69 mg 0.00 mg 0.58 mg 0.03 mg 0.04 mg0.07 mg 0.06 mg 0.28 mg 0.17 mg 0.05 mg0.34 mg 0.28 mg0.01 mg0.00 mg How would you make this broad peak more narrow?
A NALYTE P EAKS IN THE M OBILE P HASE
S EPARATION OF P EAKS
R ETENTION k = (t r – t o )/ t o Where t r = the retention time of the compound, and t o = the dead time Higher values of k mean the analyte will stay in the column longer. The longer it stays, the more time there is for the peak will widen.
S ELECTIVITY a = k B /k A the selectivity factor α and is an indication of how well the compounds will separate. Higher α means larger difference in retention time and more separation
E FFICIENCY Efficiency is a factor that is typically used to describe peak width. High Efficiency - narrow peaks
E FFICIENCY The term that is generally used to describe column efficiency is “number of theoretical plates” or N N = L/H Where: L =column length H = plate height (both in the same units)
N IN P RACTICAL T ERMS... Units for t r and t o ….? Units for W 1/2 …..? N can be measured from the peaks on a chromatogram.. N = 5.54 trtr w 1/2 ( ) 2
R ESOLUTION The purpose of chromatography is to separate or resolve compounds. The separation or distance between two peaks is known as their resolution and is a function of the 3 factors discussed previously: retention (the time it takes for the analytes to elute, related to k), selectivity (how different the analytes are from each other and related to α), and efficiency (how good the column is, related to N)
R ESOLUTION R s = ¼ ( -1/ ) (k/k+1) N ½ The effect on R s of: increasing a…? increasing k…? increasing N…? Efficiency Selectivity Retention
R ESOLUTION R s = 2 (t R-B – t R-A )/(w b-A + w b-B ) Where: A and B are the two peaks t R = retention time and w b = the peak width at the base of each peak R s can also be calculated from actual measurements of peak retention times and measured peak widths
R ESOLUTION With a resolution value of 1.0, two peaks that overlap by about 4%. Values less than 1.0 indicate peaks that overlap, while at a resolution of 1.5, the peaks are considered fully separated.
G OING BACK TO N…. N = L/H The value of N is greatly dependent on the value of H. The value of H depends primarily on four factors: 1) the velocity of the mobile phase, 2) eddy diffusion or multipath diffusion, 3) the diffusion of the compound in the mobile phase 4) the transfer of the compound between the stationary phase and the mobile phase.
H - Theoretical Plate Height H = A + B/u + (C s + C m ) u u = the average linear mobile phase velocity A is a term expressing multipath diffusion B/u is the term for longitudinal diffusion C s is the mass transfer term in the stationary phase C m is the mass transfer term in the mobile phase
A Multipath 1 2 Flow Direction Pathways of two molecules during elution. Distance traveled by molecule 1 is longer than that traveled by molecule 2, thus molecule 1 will take longer to elute. The amount of spreading is affected by the nature of the column material and how well the column is packed. This factor is generally proportional to the particle size of the packing material. This factor must be taken into account for packed columns, but for capillary columns, this term is not needed since there are no particles.
B Longitudinal Diffusion Flow Molecules diffuse from areas of high concentration to areas of low concentration. Over time…. At low velocities longitudinal diffusion has a negative effect on resolution, but this effect is negligible at higher velocities. This term is very important in gas chromatography as diffusion coefficients in gasses are orders of magnitude higher than in liquids. In liquid chromatography, this term is typically close to zero relative to the other terms.
Equilibrium between the mobile and stationary phases is never realized Mass Transfer Terms C s & C m It takes time for analytes to move from the mobile phase into the stationary phase. Because no equilibrium is reached, some of the analytes are swept ahead of the of the main band. It also takes time for molecules to move back out of the stationary phase, and some of the analyte molecules will be left behind by the rapidly moving mobile phase.
Mass Transfer Terms C s & C m The faster the mobile phase moves, the less time there is for equilibrium between the phases and the mass transfer effect on peak broadening is directly related to mobile phase velocity.
VAN D EEMTER P LOT Linear Velocity, u Plate Height, H Multipath Term, A Mass Transfer (both), Cu Longitudinal diffusion, B/u A + B/u + Cu