Chem. 230

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Chem. 230 9/16 Lecture
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Announcements I

• First Homework Set (long problems) due today
  (solutions will be posted soon)
• Website now has Fall 2012 quizzes (as well as
  solutions) posted
• Specialized Topics sign up list available (some
  blank slots if you want your own topic if
  approved)
• Next Tuesday Exam 1 for first 40 minutes

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Announcements II

• Todays Lecture
• guest lecture (Dr. Justin Miller-Schulze on SPE)
• other advanced extraction techniques (primarily
  SPME)
• comparison of extraction with chromatography (to
  transition to chromatography theory)
• introduction to chromatography theory
• Last two topics (comparison of extraction with
  chromatography and chromatographic theory) not on
  Exam 1

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Advanced Extraction TechniquesSolid Phase Micro
Extraction (SPME)

• First described in Arthur, C. Pawlisyzn, J.
  Solid phase microextraction with thermal
  desorption using fused silica optical fibers,
  Analytical Chemistry (1990) 62, 2145-2148.
• Can be used for subsequent analysis by GC or
  HPLC, but most common with GC
• Typically, non-exhaustive type sampling (meaning
  only a portion of analyte in sample is trapped).
  Quantitation is based on keeping exposure to
  samples the same (easier with autosampler).
• While quantitation is often difficult,
  sensitivity is enhanced relative to SPE because
  whole trapped sample is injected.
• The other advantage of SPME is the reduction in
  required labor

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Advanced Extraction TechniquesSPME Procedure
Fiber Cross Section

• The needle pierces the septum to a sample (sample
  can be gas, liquid, or headspace)
• The sheath is removed allowing trapping of
  analytes on fiber
• Coating is similar to GC stationary phase but
  outside of solid core
• Stirring helps the transfer
• The sheath goes back and the needle is withdrawn
• The needle pierces the septum to a GC, the sheath
  is withdrawn and the analyte is desorbed by the
  heated GC injector

solid core
coating
Fiber
GC Inlet
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Advanced Extraction TechniquesSPME - Injection

• In HPLC
• Requires specialized injection valve
• Analytes desorbed from SPME fiber into solvent
  flowing around fiber (should be strong solvent)
• Use in HPLC is less developed
• Transfer off of fiber
• This may take time, particularly for less
  volatile compounds in GC and for compounds with
  polarity more like stationary phase than solvent
  in HPLC
• Good peak shape usually requires on column
  trapping
• In GC, this is done by splitless injections with
  the column initially at low temperatures

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Advanced Extraction TechniquesSolid Phase Micro
Extraction (SPME)

• Fiber Variables
• One can select fibers of different polarities and
  film thicknesses
• A polar fiber will selectively trap polar
  molecules
• Thinner films used for faster sorption/desorption
• A practical consideration is thermal stability

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Advanced Extraction TechniquesSolid Phase Micro
Extraction (SPME)

• Sample Types (GC analysis)
• Liquid Samples
• best for relatively clean samples at lower
  concentrations
• best if analyte has polarity like coating and
  different than solvent (e.g. non-polar analyte
  and coating in water)
• Headspace Sampling
• fiber is in space above liquid sample
• good for dirty samples (e.g. flower components)
• preferentially absorbs moderately volatile
  species
• Gas Samples
• In Fiber Derivatization (typically applied to
  polar organic compounds which often decompose on
  GC columns)

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Advanced Extraction TechniquesSPME vs. Liquid
Liquid Extraction for Water Phase in Synthetic
Diesel

• Synthetic diesel made from CO H2
• Products are CxHy H2O
• Some impurities (e.g. alcohols) end up in water
  phase

DCM extract
solvent peak
headspace SPME injection
benzene (1st) butanol (2nd)
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Advanced Extraction TechniquesSolid Phase Micro
Extraction (SPME)

• Areas of Applications (reviews on these areas)
• Environmental Analysis (VOCs in air, pesticides
  in water, soil/sediment analysis, toxic metals)
• Biological Samples
• Food Analysis
• Natural Products

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Advanced Extraction TechniquesSPME
Advantages/Disadvantages

• Advantages
• Listed as Solvent-less technique (at least
  great reduction in solvent injected into GC)
• Less interference from solvent peak
• Reduced injection of non-volatiles
• Less sample handling ( ability to automate)
• Can chose fibers for good selectivity
• Disadvantages
• More difficult for quantitative results
• Limited lifetime of fibers
• Memory effects (slow desorption from fibers)

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Advanced Extraction TechniquesOther Methods

• Emphasis toward microscale methods
• Liquid-Liquid Microextraction (drop scale liquid
  liquid extraction)
• Use of semi-permeable membranes (discussed in
  text)
• Stir-Bar Sorptive Extraction

1. Stir bar traps analytes
2. Stir bar transferred to GC inlet
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Advanced Extraction TechniquesSome Questions

1. A test for decomposition of a milk sample is made
   by measuring small aldehydes (e.g. butyraldehyde)
   by SPME through direct immersion in milk. A
   non-polar fiber is used and analysis is performed
   by GC with a non-polar stationary phase. Which
   of the following are advantages of using the SPME
   method
2. removal of interferents (other parts to milk)
3. 2 dimensions of separation (on SPME fiber and on
   GC column)
4. increase of concentrations by trapping on fiber
5. avoiding need for more labor intensive methods
   (e.g. liquid liquid extraction)
6. If a fiber sits in a solution long enough, the
   peak area will reach a constant (be independent
   of time). Why is this? Is this exhaustive
   extraction?
7. In SPME for HPLC, analytes are desorbed from the
   fiber into solvent that is injected into the HPLC
   column. Should the solvent be stronger or
   weaker than the sample solvent?
8. In comparing direct headspace injections with
   SPME headspace injections, later eluting peaks
   (by GC) are larger in SPME. Explain why.

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Chromatographic TheorySimple Separations vs.
Chromatography

• Simple separations generally involve one to
  several process steps that lead to two to several
  fractions.
• Simple separations are limited to coarse
  fractionation of samples.
• Chromatographic separations are generally capable
  of isolating more than 5 compounds.
• Once the number of simple separation steps goes
  over a few (maybe 5 maximum), it becomes a labor
  inefficient way of performing a separation.

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Chromatographic TheorySimple Separations vs.
Chromatography

• Example of separation of two compounds by LLE.
• Compound X has Kp 0.25 and Compound Y has Kp
  4. Extraction of X and Y using n washes with
  extractant phase (equal volumes and saving all
  extractant phase)
• To get efficient transfer of X means transferring
  a fair amount of Y also (poor selectivity)

Number Extractions (n) Fraction X transferred Fraction Y transferred
1 0.800 0.200
2 0.960 0.360
3 0.992 0.512
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Chromatographic TheorySimple Separations vs.
Chromatography

• Continuation of example
• Better selectivity at same efficiency can be made
  by adjusting extract volume and increasing number
  of extractions
• In past example, using Vraf/Vext 2.5/1 with 5
  extractions results in 99 efficient transfer of
  X, while only transferring 38 Y
• Table shows dependence of Y transferred on Kp
  values (assuming Kp(Y) 1/Kp(X)) and 99
  transfer of Y with 3 extractions (volumes
  adjusted to get 99 transfer of X)

Kp(X) Y also transferred
100 0.1
20 2.7
5 33
2 85
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Chromatographic TheorySimple Separations vs.
Chromatography

• Chromatography example
• Even column of poor efficiency can handle much
  more similar compounds
• Example KY/KX 1.25 ( a value)
• If we assume kX 4, and resolution 1.5
  (minimum for baseline), a plate number of 1000
  would be needed (not very high)

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Chromatographic TheorySimple Separations vs.
Chromatography

• Conclusions to example
• Unless order of magnitude differences in Kp
  values, simple separations have limited use (e.g.
  reduction of interfering substance).
• Simple separations are better for coarse
  separations
• Chromatographic separations can handle similar K
  values much better.

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Chromatographic Theory Chromatography vs. Other
Advanced Separation Techniques

• Chromatography is based on analyte partitioning
  between two phases
• Other methods use different mechanism for
  separation of analytes (e.g. electrolytic
  mobility in capillary zone electrophoresis)
• Some areas of overlap (e.g. Capillary
  electrochromatography and size exclusion
  chromatography)

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Chromatographic Theory Phase Definitions

• Mobile Phase (M subscript in later parameters)
• Fluid phase (gas, liquid or supercritical fluid)
  that moves through stationary phase
• Mobile phase defines the major classes of
  chromatography (GC, LC and SFC)
• Stationary Phase (S subscript)
• A non-moving phase (except in MEKC) to which
  compounds partition via absorption or adsorption
• Phase can be liquid (not very stable),
  liquid-like (most common), or solid (common for
  some applications)
• In past was second part of class name (for
  example GLC for gas-liquid chromatography)

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Chromatographic Theory More on Stationary Phases

• Stationary phases come in several arrangements
  in columns or on plates (used in thin layer
  chromatography)
• In columns, open tubular (coated walls), packed
  columns and monoliths are possible means of
  attaching stationary phase
• Packed columns contain packing material with the
  stationary phase either being the surface or
  being a coating on the surface
• Porous packing material is common
• Most common stationary phase is a liquid-like
  material chemically bonded to packing material or
  to wall (in open tubular chromatography).

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Chromatographic Theory More on Stationary Phases
Open Tubular (end on, cross section view)
Packed column (side view) (e.g. Silica in normal
phase HPLC)
Column Wall
Packing Material Stationary phase is outer
surface (although influenced by adsorbed solvents)
Mobile phase
Stationary phase (wall coating)
Bonded phase (liquid-like)
Expanded View
Stationary Phase Chemically bonded to packing
material
Packing Material
Note true representation should include
micropores in sphere
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Chromatographic Theory Definition Section

• Chromatograph instrument
• Chromatogram detection vs. time (vol.) plot

Chromatograph Components
Sample In
Chromatographic Column
Detector
Flow/Pressure Control
Mobile Phase Reservoir
Waste or fraction collection
Injector
Chromatogram
Signal to data recorder