1	Chapter 28: High-Performance Liquid Chromatography (HPLC) Scope Instrumentation – eluants, injectors, columns Modes of HPLC Partition chromatography Adsorption chromatography Ion chromatography Size exclusion chromatography
2	"HPLC" HPLC Most widely used separation technique Broad applicability – organic & inorganic Can be very sensitive, accurate & precise Suitable for separation of nonvolatile species Has found numerous uses in industry, clinical settings, environmental areas, pharmaceuticals, etc.
3
4	"Modes of Separation" Modes of Separation Partition Chromatography – most used form of HPLC primarily for nonionic compounds of varying polarity with low MW (< 3000) Most common form is bonded phase chrom. using silica based packing materials functionalized by silylation (as for GC)
5	"Early work with partition chrom" Early work with partition chrom. was done with polar stationary phases (like bare silica) & non- polar solutes = normal phase chrom. Later bonded phases were introduced using C₁₈ groups à very non-polar with polar solvents = reversed-phase chromatography Today almost all partition chrom. done in reversed-phase mode with many different bonded phases (although C₁₈ very popular)
6	"-Si-CH₂-(CH₂)₁₆-CH₃" -Si-CH₂-(CH₂)₁₆-CH₃ 18 carbon chain Long chain acts as if it were an alkane coated on silica à analyte molecules partition into it, hence the name In chromatogram, most polar compounds elute first because they partition into C₁₈ least – like dissolves like – most non-polar compounds come out last
7
8	"Besides C₁₈ can have C₈" Besides C₁₈ can have C₈, C₄, C₃, C₂, C₁ plus functionalities like cyano (-C₂H₄CN), amino (-C₂H₄NH₂), diol (-C₃H₆O-CH₂-CHOHCH₂OH) Each has different polarity Can also do Ion Pair Chromatography or Paired-Ion Chromatography – type of RP-HPLC used to separate ionic species Still partition chrom. but use a reagent like a quaternary ammonium salt (C₄H₉)₄N⁺ to pair with analyte ions to separate by RP
9	"Adsorption Chromatography –" Adsorption Chromatography – bare silica or alumina to separate non-polar compounds because they adsorb to the stationary phase & are eluted by adjusting solvent strength of mobile phase – important non-linear appl. Adsorption chrom. = normal phase chrom.
10
11	"Ion Chromatography (Ion Exchange" Ion Chromatography (Ion Exchange) Historically was developed for the Manhattan Project (atomic bomb) Generally not automated because of the lack of good detectors until it was reinvented in 1970’s at Dow Chemical using conductivity detection & chemical suppression Stationary phases are resin beads of styrene-divinylbenzene functionalized with cationic & anionic groups developed for water purification in 1930’s
12	"weak base strong base" weak base strong base weak acid
13	"Can write reactions in general..." Can write reactions in general format xRSO₃^- H⁺ + M^x+ (RSO₃^-)_x M^x+ + xH⁺ solid solution solid solution Where R = polymer support (styrene divinylbenzene) Can write equilibrium expression for exchange [(RSO₃^-)_x M^x+]_s [H⁺]^x_aq K_ex = ----------------------------- tells affinity of [RSO₃^- H⁺]^x_s [M^x+]^x_aq resin for M⁺ compare to H⁺ here or any ion
14	"Ion Exchange Process" Ion Exchange Process Analyte ions (M^x+) are passed thru column & retained on an ion- exchange site. The mobile phase contains some H⁺ & this is increased sufficiently to cause exchange with M^x+.
15	"Back to equilibrium expression" Back to equilibrium expression [(RSO₃^-)_x M^x+]_s [H⁺]^x_aq K_ex = ----------------------------- [RSO₃^- H⁺]^x_s [M^x+]^x_aq Rearrange to [RSO₃^- H⁺]^x_s [(RSO₃^-)_x M^x+]_s ------------------ K_ex = --------------------- [H⁺]^x_aq [M^x+]^x_aq During elution [H⁺] is high & [RSO₃^- H⁺]_s is high Left hand side of equation essentially constant
16	"[(RSO₃^-)_x M^x+]" [(RSO₃^-)_x M^x+]_sC_s K = --------------------- = ----- [M^x+]^x_aqC_M K turns out to be a distribution ratio (partition) Order of affinity for sulfonated cation exchange Tl⁺>Ag⁺>Cs⁺>Rb⁺>K⁺>NH₄⁺>Na⁺>H⁺>Li⁺ Ba²⁺>Pb²⁺>Sr²⁺>Ca²⁺>Ni²⁺>Cd²⁺>Cu²⁺>Co²⁺>Zn²⁺>Hg²⁺
17	"Ion Chromatography Detection" Ion Chromatography Detection Basic detector is conductivity, but others are used such as UV-vis & atomic spectrometry (AA, AE) for metals Measure conductivity change in effluent when analyte passes through Problem – use high [H⁺] to elute small [M^x+] which makes it difficult to detect [M^x+] conductivity on high background of [H⁺] This problem hindered development of IC until the innovations made at Dow in 70’s
18
19	"Several ways now available to" Several ways now available to solve the conductivity problem from background ions 1) Suppressor column – Dow researchers used a second ion exchange column after the analytical column to neutralize the [H⁺] & remove its conductivity so M^x+ can be easily detected (e.g. if HCl is mobile phase use resin suppressor in OH^- form R⁺OH^-) H⁺Cl^- + R⁺OH^-_S H⁺OH^- + R⁺Cl^-_S
20	"Suppressor columns must be regenerated" Suppressor columns must be regenerated 2) Single Column IC – no suppressor column used, instead use low capacity analytical column to keep mobile phase concentration low & therefore the conductivity low – this is coupled with the use of a special conductivity detector that can null out high background of mobile phase without suppressing conductivity 3) Other Suppressor Options – membrane, electrochemical, hollow fiber, etc.
21
22
23	"Size Exclusion Chrom." Size Exclusion Chrom. (SEC) Packings are porous polymeric (resins) or silica based materials Two names used for the same process: 1) Gel filtration chrom. = aqueous solvent Gel permeation chromatography = non-aqueous mobile phase Column packing works like a molecular filter allowing small molecules access to every pore, retarding their progress – large molecules pass thru more quickly
24
25	"SEC" SEC
26
27	"Polymeric SEC packing" Polymeric SEC packing can be thought of as a ball of yarn with pores defined by the degree of crosslinking of the polymer chains
28	"Pellicular packings" Pellicular packings
29
30
31