Chem. 230 – 9/11 Lecture. Announcements I. HW Set 1 due Additional Resources (show students site) Application Paper (pass out). Announcements II. First Quiz First 30 minutes next Wednesday Will cover materials on Simple Extractions (in text and covered in lecture)
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First 30 minutes next Wednesday
Will cover materials on Simple Extractions (in text and covered in lecture)
Questions will be similar to those given as examples in lecture and in homework
You should be familiar with equations needed, but constants will be provided
Example Quiz + Solutions posted
Mixture of methylene blue (organic cation) and I2 in 90% water 10% methanol
Could in theory separate using either C18 or cation-exchange solid phase
C18 used in example (and methylene blue is a “sticky” molecule – so sticks to many surfaces, though should pass through C18 column)
As before, both efficient phase transfer and good selectivity are desired
To trap less polar compounds in polar solvents, hydrophobic stationary phases (also known as reversed-phase) are desired. (Example: pesticides in water)
To trap more polar compounds in less polar solvents, hydrophillic stationary phases (also known as normal phase) are desired. (Example: sugars in acetonitrile, steroids in hexane)
Trapping of polar compounds in polar solvents (or non-polar compounds in non-polar solvents) is difficult. “Breakthrough” often occurs.
Larger analyte – solvent polarity difference allows better trapping but is limited by analyte solubility.
To trap ionic compounds (usually in water), stationary phases with charged groups opposite in charge to analyte ions are used.
It may be possible to produce several fractions by increasing solvent strength or changing pH.
C18 (most commonly used); best for trapping compounds with alkyl groups
Phenyl: good for enhanced retention of aromatic compounds
“Stronger” solvent is less polar
Hydroxy (diol or SiOH)
“Stronger” solvent is more polar
Ion Exchange Stationary Phases
Sulfonate groups common for cation exchange
Ammonium groups –NR3+ common for anion exchange
Trapping occurs in low ionic strength solvents; release occurs in high ionic strength
Weak acids/bases need to be trapped in ion form but also can be released by pH adjustment
Breakthrough and Release
When SPE cartridges are used to trap and release compounds, losses can occur from incomplete trapping (breakthrough) or release of compounds.
Breakthrough can occur because the partitioning equilibrium is not strong enough or due to capacity of cartridge is exceeded (sample overload)
Breakthrough can be determined by measuring the concentration of solute passed through cartridge (either in whole sample or in intervals)
Release can be determined by secondary rinses of SPE cartridge
It is desired to trap benzoic acid in an aqueous phase on an SPE cartridge and release it to an aqueous phase. Is this possible?
Fish triglyerides are extracted in hexane. Describe a way to separate the triglyerides from more polar compounds (free fatty acids and steroids with OH groups).
Trapping of trace amounts of phenols in water is attempted. To concentrate phenols, large volumes of water are used followed by small volumes of acetonitrile. What is a concern?
Some of the phenols in water contain carboxylic acids. Suggest a way to trap both carboxylic acid-containing phenols and regular phenols while releasing them into two fractions for separate analysis. The pKa for carboxylic acids are about 4 and about 10 for phenols.
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 needle pierces the septum to a sample (sample can be gas, liquid, or headspace)
The sheath is removed allowing trapping of analytes on fiber
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
Sample Types (GC analysis)
Liquid Samples (best when analyte concentrations are low)
Headspace Sampling (avoids fiber fouling)
In Fiber Derivatization (typically applied to polar organic compounds which often decompose on GC columns)
Areas of Applications (reviews on these areas)
Environmental Analysis (VOCs in air, pesticides in water, soil/sediment analysis, toxic metals)
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
More difficult for quantitative results
Limited lifetime of fibers
Memory effects (slow desorption from fibers)
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
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:
removal of interferents (other parts to milk)
2 dimensions of separation (on SPME fiber and on GC column)
increase of concentrations by trapping on fiber
avoiding need for more labor intensive methods (e.g. liquid – liquid extraction)
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?
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?
In comparing direct headspace injections with SPME headspace injections, later eluting peaks (by GC)are larger in SPME. Explain why.
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.
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)
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)
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)
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.
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)
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)
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).
(end on, cross section view)
Packed column (side view) (e.g. Silica in normal phase HPLC)
Stationary phase is outer surface (although influenced by adsorbed solvents)
Stationary phase (wall coating)
Bonded phase (liquid-like)
Chemically bonded to packing material
Note: true representation should include micropores in sphere
Chromatograph = instrument
Chromatogram = detection vs. time (vol.) plot
Mobile Phase Reservoir
Waste or fraction collection
Signal to data recorder
V = t·F
V = volume passing through column part in time t at flow rate F
Also, VR = tR·F where R refers to retention time/volume (time it takes component to go through column or volume of solvent needed to elute compound)
VM = Vcolumn – Vpacking material – VS
VM = tM·F, where tM = time needed for unretained compounds to elute from column