Principle of Extraction (Overview) Extraction of Semivolatile Organics from Liquids

Transcription

1 Principle of Extraction (Overview) Extraction of Semivolatile Organics from Liquids 1

2 Extraction Chemical properties of the analytes and liquid medium Vapor pressure Solubility Molecular weight Hydrophobicity Acid dissociation Extractants Gas Liquid Supercritical fluid Solid Transport of chemicals In human body In the air-water-soil environment Between immiscible phases X A X B [ X] B K D = [ X] A Chemical Equilibrium 2

3 Volatilization Henry s s Law Constant: Volatility from the solution into air H' = K [ X] [ X] Vapor Pressure H = Solubility Vapor Pressure: Volatility from the pure substance into the atmosphere D = G L 3

4 Categorization of Vapor Pressure Vapor pressure Low: Medium High Solubility and Solubility 1x10-6 mmhg 1x10-6 1x10-2 mmhg > 1x10-2 mmhg Low: <10 ppm Medium ppm High > 1000 ppm 4

5 Henry s s Law Constant vs. Extraction techniques H analyte < H solvent Solute = nonvolatile in the solvent, preconcentration H analyte > H solvent Solute = semivolatile to volatile in the solvent P vap Low Medium High High H Low Medium S Low H High 5

6 Organic Solute in Water Nonvolatile: unimportant H < 3x10-7 atm.m 3 /mol Semivolatile: : slowly 3x10-7 < H < 10-5 atm.m 3 /mol Volatile: significant 10-5 < H < 10-3 atm.m 3 /mol Highly volatile: rapid H > 10-3 atm.m 3 /mol 6

7 Hydrophobicity n-octanol/water partition coefficient K = ow [ X] o [ X] w Highly hydrophilic: log K ow < 1 Highly hydrophobic: log K ow >

8 Acid-base Equilibria HA H + + A - K a = [ + ] [ - H A ] [ HA] Buffer ph = pk a + log [ - A ] [ HA] ph = pk a + 2 8

9 Sample Prep Aqueous Sample Liquid-Liquid Extraction (LLE) Solid-Phase Extraction (SPE) Solid Phase Microextraction (SPME) Stir Bar Sorptive Extraction (SBSE) Liquid-Phase Microextraction (LPME) 9

10 Liquid Liquid Extraction LLE 10

11 Solvent Extraction-LLE Popular technique For non - semi volatile organic compounds Partitioning the sample between two immiscible phases Aqueous phase: Sample matrix Organic phase: Organic solvent Like dissolves like A(aq) A(org) 11

12 LLE-Basic Theory Distribution Coefficient (K d ) K = D [ A] org [ A] aq K d is constant at a particular temperature 13

13 Separation by extraction Distribution equilibrium Partition of solute between two immiscible phases A aq A org K = (a (a A A ) ) org aq [A] [A] org aq [A] i Vaq i = [A] o VorgK + V aq 14

14 Example K of I 2 between organic solvent and H 2 O = 85. Find [I 2 ] remaining in the aqueous layer after extraction of 50.0 ml of 1.00 x 10-3 M I 2 with organic solvent of a) 50.0 ml, b) 2 x 25.0 ml, c) 5 x 10.0 ml a) [I b) [I a) [I ] ] ] = ( ) = ( ) = ( ) = = = M M M 15

15 LLE-Basic Theory The fraction of analyte extracted (E)( ) or Recovery (R) E E = R = R = = ( C V + C V ) o K D C o V V ( 1+ K V) D o o aq aq V = Phase ratio; V o /V aq 16

16 LLE-Basic Theory Distribution Ratio (D) If the analyte is partially dissociated in solution and exist as neutral species, free ions, ion-paired with counter ion D = Conc. of X in all chemical forms in the organic phase Conc. of X in all chemical forms in the aqueous phase Using D for K D 17

17 LLE ml Phase ratio; 0.1 < V < 10 One-step extraction; K d must be large, >10 K Ow (log K Ow ) Octanol-water partition coefficient 2-33 repeat extractions are required for quantitative recoveries E = 1- ( K ) + D V 1 1 n 18

18 How many extraction will be necessary? K D = 10 V = 10 #1 E = 99.0% K D = 100 V = 1 #1 E=99.0% K D = 1000 V = 0.1 #1 E=99.0% K D = 10 V = 1 #1 E = 90.9% #2 E = 99.2% K D = 10 V = 0.1 #1 E = 50.0% #2 E = 75.0% #3 E = 87.5% #4 E = 93.8% #5 E = 96.9% #6 E = 98.4% #7 E = 99.2% 19

19 How many extraction will be necessary? Typically, aqueous phase > organic phase V = (50:1000, 100:1000) K D = 1000 V = 0.1 #1 E = 99.0% K D = 1000 V = 0.05 #1 E = 98.0% #2 E = 99.9% K D = 100 V = 0.05 #1 E = 83.3% #2 E = 97.2% #3 E = 99.5% Multiple extractions are more efficient 20

20 Single vs. Multiple Extraction K D Single 1x150 ml V=6.67 Single 1x50 ml V=20 1x50 ml 2 nd extraction 1x50 ml 2x50 ml 1x50 ml 3 rd extraction 1x50 ml 3x50 ml %E %E 2 nd %E nd %E Add. %E Cum. %E 3 rd %E Add %E Cum. %E

21 LLE The net amount of analyte extracted depends on the K D The net amount of analyte extracted depends on the V org /V aq More analyte is extracted with multiple portions of extracting solvent than single portion of an equivalent volume of the extracting phase Recovery is independent of the concentration of the original aqueous sample 22

22 Selection of Extraction Solvents Immiscible with water (Low solubility) Have polarity and H-bonding H for good recovery of analytes (organic phase) Volatile for easy removal after extraction (pre-concentration if necessary) Compatible with method of analysis GC RP-HPLC 23

23 Solvent Modification Selectivity can be influenced by choices of additives affecting the equilibrium process Adjusting ph Ion pair Chelating agent Salting out Increase K D in organic phase 24

24 Example Extraction of amine (Aniline) from aqueous sample Buffer solution at least 1.5 ph units higher than its pk a, Amine will become un-ionized and will be extracted into the organic phase NH 3+ + H 2 O NH 2 + H 3 O + pk a =

25 Typical Solvents used in LLE Aqueous solvent Pure water Acidic solution Basic solution High salt (Salting out) Complexing agents (ion-pairing, chelating and chiral agents) Combination of two or more above Water immiscible organic solvent Diethyl ether Methylene chloride Chloroform Ethyl acetate Aliphatic ketones (C6+) Aliphatic alcohol (C6+) Toluene and xylenes Combination of two or more above 26

26 Emulsion Small droplets of organic phase floating in the immiscible aqueous phase Where the mass transfer occurs Vigorous shaking allows thorough interspersion between two immiscible phases; high efficiency Emulsion must be broken before collection Sample containing surfactants or fatty materials slow breaking or incomplete breaking of emulsion 27

27 Breaking Emulsions Add salt to aqueous phase Use a heating-cooling extraction vessel Filter emulsion through a glass wool plug Filter the emulsion through phase separation filter paper Centrifuge Add a small amount of a different organic solvent 28

28 Continuous LLE K D is very small Kinetics of the extraction is slow Sample is large, requiring too many extractions Fresh organic solvent is recycled continuously in the form of droplets passing through the sample aqueous phase 29

29 Continuous LLE Solvent heavier than water Solvent lighter than water G. LeBlanc, LC-GC, 19(11), (2001) 30

30 Continuous LLE - Solvent heavier than water 31

31 Continuous LLE - Solvent lighter than water 32

32 Advantage Continuous LLE Unattended operation Extract low K D Relatively low solvent usage Excellent extraction efficiency Disadvantage Time (18-24 hours) Very volatile compounds can be lost Thermally unstable compounds can be degraded 33

33 Disadvantage of LLE Time required; several successive extractions Seldom complete Use and disposal of large volumes of toxic organic solvents Formation of emulsion; hard to break emulsion Cumbersome glassware Labor-intensive Sample preconcentration is often required Not easily automated Cost 34

34 Extracting inorganic species Separating metals ions as chelates Organic chelating agents react with metals to form uncharged complexes that are highly soluble in organic solvents 2HQ 2HQ MQ 2 Organic phase 2H + + 2Q - + M 2+ MQ 2 Aqueous phase Q = 8-hydroxyquinoline 35

35 Extracting inorganic species Overall equilibrium 2HQ (org) + M 2+ (aq) MQ 2 (org) + 2H + (aq) [MQ [H + 2 org K = 2 2+ [HQ] org[m ] ] ] 2 aq aq [HQ] org is present in large excess with respect to [M 2+ ] aq [MQ 2 2 org K [HQ] org = K = 2+ [MQ [M ] 2 org = 2+ ] aq ] [M K + [H ] 2 aq [H ] aq + ] 2 aq 36

36 Extracting inorganic species [MQ [M ] 2 org = 2+ ] aq K + [H ] 2 aq Ratio of concentration of metal species in the two layers is inversely proportional to [H + ] 2 aq K varies from metal ion to metal ion, makes it possible to selectively extract one cation from another by buffering aqueous solution 37