Lecture # 20 – Analytical Separations Chapter 23 Analytical ...
Transcript of Lecture # 20 – Analytical Separations Chapter 23 Analytical ...
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Lecture # 20 – Analytical Separations
Chapter 23
Analytical Chemistry
Steps in Chemical Analysis
1. Formulating the Question 2. Selecting the Analytical Procedure 3. Sampling 4. Sample Preparation 5. Analysis 6. Interpretation and Reporting 7. Drawing Conclusions
Solvent Extraction
Hot Water Extraction
Solvent Extraction
Phase I (water)
Phase 2 (organic solvent)
Immiscible = don’t mix …much!
Liquid-Liquid Extraction
e.g. Separatory Funnel
S S
S S S
S S S
S S
S S
S S S S
S S
S
S = solute
Solvent Extraction Partition Coefficient
K = AS2 ≈ [S]2
AS1 [S]1
m = moles of S
q = fraction of S in “Phase 1”
1-q = fraction of S in “Phase 2”
K = (1-q)m/V2 qm/V1
Solvent Extraction Partition Coefficient
K = AS2 ≈ [S]2
AS1 [S]1
Fraction in Phase 1 = q = V1 (after one extraction) V1 + KV2
Fraction in Phase 1 = qn = V1 (after n extractions) V1 + KV2
n
2
Solvent Extraction pH and Solvent Extraction
Distribution = Total Concentration in Phase 2 Coefficient (D) Total Concentration in Phase 1
Example: Extraction of base (B)
D = [B]2 [B]1 + [BH+]1 K = [B]2/[B]1
Ka = ([H+][B]1)/[BH+]1 = K ⋅ Ka Ka + [H+]
αB = Ka/(Ka + [H+]) = K ⋅ αB
“ a neutral species is more soluble in an organic solvent and a charged species is more soluble in aqueous solution”
Solid-Phase Extraction
S K
Phase 2
Phase 1 S (in Solvent 1)
K1
K1 K1
K1
S
S
S
S
Solvent 2
S S S S
K2
K2
K2
K2
Solid-Phase Microextraction (SPME) Chromatography Solvent (Eluant)
Mixture (with analyte)
Column Packing (sorbent)
Eluate
Z
= Stationary Phase
Mobile Phase (solvent, gas)
Elu
tion
Chromatography
Packed Open Tubular
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Chromatography
Types of Chromatography (see Chapter 26)
1. Adsorption Chromatography 2. Partition Chromatography 3. Ion-Exchange Chromatography 4. Molecular Exclusion Chromatography 5. Affinity Chromatography
Chromatography
Adsorption Chromatography
Adsorbed
Mobile Phase (solvent or gas) Stationary Phase
e.g. SiO2
Chromatography
Partition Chromatography
Mobile Phase (gas) Liquid Stationary
Phase
Solid e.g. SiO2
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Chromatography
Ion-Exchange Chromatography
Mobile Phase (solvent/water) Stationary Phase
(= Resin with cations or anions)
+ + + + + +
+ + +
+ +
+
+
+ + + +
Chromatography
Molecular Exclusion = Gel Filtration or Gel Permeation
Chromatography
Affinity Chromatography
Mobile Phase (solvent/water) Stationary Phase
with Covalent Ligand
(e.g. Antibody)
Chromatography “Column Chromatography”
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Chromatography High-Performance Liquid Chromatography (HPLC)
Chromatography
Chromatography Chromatography Flow Rate
Length
Radius (r)
1 cm
V = πr2 ⋅ (1 cm)
e.g. r = 0.46 cm V = π(0.046 cm)2(1 cm) = 0.665 mL* *1 cm3 = 1 mL
If 20% of column volume is solvent, then
0.133 mL/cm
Chromatography
Flow Rate
Volume Flow Rate (uv): “how many milliliters of solvent per minute”
e.g. 1 mL/min
Linear Flow Rate (ux): “how many centimeters are traveled in 1 minute by the solvent”
e.g. 1 mL/min / (0.133 mL/cm) = 7.52 cm/min
tr = Retention Time
t’r = Adjusted Retention Time
= tr - tm tm
Time
Det
ecto
r Res
pons
e
The Chromatogram “a graph showing the detector response as a
function of elution time”
Unretained Mobile Phase (solvent or gas)
Vr = tr ⋅ uv
uv = Volume Flow Rate
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t’r2
Time
Det
ecto
r Res
pons
e
The Chromatogram
t’r1
Relative Retentionα = t’r2/t’r1 > 1
t’r = tr - tm
Time
Det
ecto
r Res
pons
e
The Chromatogram
tm
Capacity Factor: k’ = tr – tm = t’r tm tm
k’ = time in the stationary phase time in the mobile phase
= moles of solute in stationary phase moles of solute in mobile phase
k’ = CsVs
CmVm
k’ = Vs ⋅ K
Vm = t’r tm
Cs/Cm = partition coefficient
Retention Time and Partition Coefficient
α = t’r2 = k’2 = K2 t’r1 k’1 K1
Relative retention
Efficiency of Separation
Time
Det
ecto
r Res
pons
e
Peak Width
?
½ h w1/2
=2.35σ
w = 4σ
Resolution = Δtr = ΔVr = 0.589Δtr wav wav w1/2av
Efficiency of Separation 3σ
6σ
Resolution = 0.75
Resolution = 1.5
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Efficiency of Separation
dx
x c
x + dx d - dc
J (Flux)
Flux (J) = -D dc (mol/m2 ⋅ s) dx
D = diffusion coefficient (see Table 23-1)
“a band of solute broadens as it moves through a chromatography column”
Efficiency of Separation
x = 0
c = m e-x /(4Dt)
4πDt
2
c = concentration (mol/m3) t = time x = distance from center of band
σ = 2Dt
σ2 = 2Dt
Efficiency of Separation
Plate Height
σ2 = 2Dt x = ux ⋅ t
t = x/ux σ2 = 2D x ux
σ2 = 2D x ux
σ2 = H x H = plate height
Efficiency of Separation
Plate Height σ2 = H x H = σ2/x
Number of Plates (N) = Length of Column (L) H
N = L = Lx = L2 = 16L2 H σ2 σ2 w2
σ = w/4
N = 16 tr2 = tr2 w2 σ2
N = 5.55 tr2 w1/2
2
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Efficiency of Separation
Plate Height and Resolution
Resolution = N (γ – 1) 4
γ = separation factor = uA = tB
uB tA
Resolution ∝ N ∝ L
e.g. Doubling L increases resolution by 2
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Assigned Problems in Chapter 23:
Problems: 23-1, 23-8, 23-11, 23-19, 23-21, 23-27