23 Pre-Concentration Flashcards
How do yo carry out air sampling?
fill hollow tubes with sorbent
pump air through tubes
thermal desorption of analytes
What is pre-concentration and purification?
Increase in analyte concentration
reduce interference
What is the partition coefficent for?
Efficiency of extraction
If P is low then inefficient
large voume of solvent was used
extract must be reduced for instrumental analysis
What are the liquid phase micro-extraction techniques?
single drop micro-extraction SDME
Liquid liquid liquid micro extraction LLLME
Dispersive Liquid liquid micro extraction DLLME
Hollow fibre liquid phase micro extraction HF LPME
Cloud point extraction
Describe SDME and state it’s advantages and disadvantages
organic solvent loaded onto microsyringe
droplet at the tip is immersed into the aqueous solvent for extraction
droplet is drawn back into microsyringe for further analysis
Adv: minimum solvent used, no preconditioning req
Lim: easy dislodging
Small drop size confines extract efficiency
What is continuos flow SDME?
sample is constantly pumped around solvent drop
large smaple volume needed
higher enrichment
What di you call the study of sense of smell?
Olfactometry
How does headspace work and what are the factors that influence HS-SDME?
enrichment of volatile compounds in closed container
temperature
extraction time
droplet volume
Describe liquid liquid liquid micro extraction LLLME and what are the advantages and disadvantages?
analyte is extracted from aqueous layer into layer of organic solvent and back-extracted into a drop of water
Adv:
suitable for water solvable organic analytes
permit reversed phase separations
Lim: Extraction reproducibility (extract volume-> surface->efficiency of process) drop dislodgement (stir bar may stir vigorously)
How does milk demonstrate dispersion, emulsion, suspension, colloid and solution?
Emulsion is a form of dispersion Need polar and non-polar phase Solid in liquid need very small particles which form coilloid can contain Ca, Na
Describe DLLME and state its advantages and disadvantages
organic extraction solvent in 0.5-2 ml of water miscible disperser solvent (acetone / methanol is miscible with water and organic solvent)
rapid injection into aqueous sample = dispersion
separation of extraction solvent by centrifugation
Adv:
more robust than SDME
Short extraction time due to large s.a
Lim: identification of best combination of extraction solvent and disperser solvent
Unsuitable for complex matrices
Describe HF-LPME and state its advantages and disadvantages.
use of hollow microforous fibers such as polethylene, polypropylene, polyvinylidene fluoride
pores of fiber walls are impregnated with extraction solvent by immersion of the fiber into solvent
solvent phase is contained in fiber walls
Adv:
more stable than single drops
Fiber tips are disposable, no carry-over
Lim:
Air bubbles on surface reduce reproducibility and extraction efficiency
Pores can be easily blocked by hydrophobic substances
lumen filled with organic solvent in 2 phase
lumen filled with aqueous solvent in 3 phase
Describe cloud point extraction
Micelles are aggregates of surfactant molecules
Hydrophobic core in hydrophilic solvent
Hydrophilic core in hydrophobic solvent
CLoud point phenomena
Below cloud-point temperature, micelles form a colloidal solution
Above CPT, micells dehydrate and aggregate
separation of micelle rich and micelle poor phase
Describe SPE and state it’s advantages and disadvantages
Liquid sample containing analyte is loaded onto sorbet
