Unit 3 Flashcards
what is SPE?
what is the principal behind it?
solid phase extraction
process by which the analyte(s) suspended or dissolved in a liquid is selectively extracted with a sorbent material (liquid-solid extraction)
what does the selection of an appropriate sorbent in SPE depend on?
the mechanism(s) of interaction between the sorbent and analyte of interest
what are 2 main applications of SPE? describe them
- sample cleanup:
- isolation of the analyte(s) from the matrix and possible interfering molecules
- eg. weakly bound impurities washed out and desired analyte is eluted in a cleaner form - preconcentration of the analyte:
- when conc of the analyte is too low in the initial sample
- large amounts of sample is concentrated on the SPE phase
- extract is then eluted and concentrated to a small solvent volume
SPE is an alternative to what?
other extractions such as LLE which need large volumes of immiscible solvents
describe the analytical procedure for the determination of caffeine
- sample dilution
- SPE cartridge and conditioning
- sample load
- washing step
- caffeine elution
- filtration
- HPLC analysis
what are the 4 main steps of SPE?
what is the input and output of each?
- conditioning:
- input water
- output water - loading sample
- input sample (containing analyte
- output: interferences - washing:
- input water
- output: more interferences - eluting:
- input water
- output: analyte
how many wells are in a SPE plate?
96
what does the large diversity of interactions of intermolecular forces between analyte and sorbet indicate?
there can be many possible SPE mechanisms (ie. adsorption, exchange, partitioning)
for non polar analytes, what are good…
a) dissolving solvents
b) elution solvents
c) sorbents
a) methanol/water, acetonitrile/water
b) hexane and chloroform
c) octadecyl, octyl, ethyl, phenyl
for slightly polar analytes, what are good…
a) dissolving solvents
b) elution solvents
a) hexane and chloroform
b) methanol
anionic exchange resins are ____ (Acidic/basic)
basic
cationic exchange resins are _____ (acidic/basic)
acidic
what is HLB?
hydrophilic-lipophilic balance
the measure of the degree to which a surfactant is hydrophilic or lipophilic
describe the purpose of the conditioning step of SPE
example?
- activates and/or wets the pores of the stationary phase
- improves contact between sample and SPE phase
- example: C18 is a hydrophobic phase. The aqueous sample doesn’t come in contact with the C18 phase. MeOH is used to “wet” the phase
describe the purpose of the loading sample step in SPE
- liquid sample is passed through the SPE phase
- analyte is retained on stationary phase
- unretained sample matrix is discarded
what is the driving force of the loading sample step in SPE
what does it elute?
driving force: gravity, pressure, vacuum
this elutes the unretained sample matrix, which is discarded
when does low analyte recovery occur?
when the flow rate is too fast
or when the capacity of the SPE phase is too small for the sample size (overload)
describe the purpose of the washing step in SPE
- weakly bound interferences are washed off the cartridge
- uses a weak solvent
when can the cartridge go through drying?
after washing for some analytes
describe the purpose of the final elution step in SPE
- strong solvent is eluted to recover the analyte(s) of interest from the SPE phase
- fraction(s) containing the analytes are collected
eg. after extracting on C18 cartridges, the analytes are recovered using elution with methanol
what are benefits of SPE?
limitations?
benefits
- wide range of SPE phases available
- relatively fast; no reflux, shaking, or phase separation steps
- low solvent consumption
- pre-concentration of analyte
- can be automated
limitations
- needs optimization to prevent breakthrough/overload
- incomplete removal of interferences
what is SPME?
solid phase microextraction
what is used for extraction of analytes in SPME? how is it used?
- a polymer coated fused silica fiber that selectively adsorbs analytes of interest
- used directly in the liquid by immersion
- used from a solid or liquid by headspace extraction
in SPME, what happens after extraction?
analytes are desorbed from fiber by exposing fiber in the injection port of a GC or in the desorption chamber of a SPME interface
in SPME, the quantity of analyte extracted by the sorbent is proportional to what?
to its concentration in the sample medium in the case of equilibrium
in SPME, what happens to…
a) volatile analytes
b) non-volatile analytes
a) adsorbed onto the sorbent fiber (in air)
b) adsorbed onto sorbent fiber (in liquid)
