Membranes Flashcards
What is the definition of ‘membrane?’
structure with much greater surface area (lateral dimensions) than thickness, can transfer substances through with various driving forces
driving forces that can be used in membrane separations:
concentration gradient
applied pressure
electric potential
What are membranes used for in food analysis? (4)
sample prep (filter, fractionation, cleanup, sterilize…)
extraction
analyte identifiation/quantification
speciation studies
What is filtration?
segregation of phases (suspended solids in liquid or gas) by forcing it through a porous medium (filter)
separation based on size (also shape, charge)
a coffee filter has pore size of about _____
20 microns
The 2 flow types in filtration:
direct flow
tangential flow
What is direct flow filtration:
flow of feed is perpendicular to membrane; molecules that don’t fit through pores will accumulate on surface
What is tangential flow filtration?
feed is parallel to membrane, some will pass through (filtrate) while larger particles will keep flowing with rest of liquid (retentate)
Compare the performance of direct vs tangential flow filtration:
direct: faster at first, but then flow will slow and stop as the mass of retentate accumulates (clogs filter)
tangential: slower at first, but continues at same rate even with large volumes
Tangential flow is also called ______
Direct flow is also called _____
cross flow
conventional flow
Common designs of conventional filtration devices
buchner funnel + vacuum flask + filter paper
What AOAC approved food analysis method requires use of filtration?
measurement of soluble and insoluble dietary fibre; filter sample after enzymatic digestion
What are important characteristics of filter paper to consider? (6)
retention characteristics (pore diameter)
collection efficienty (% retained for specific size)
wet burst (pressure it can tolerate)
content of ash
content of trace elements
stability (vs pH, etc)
1 bar = ____ psi
14.5
2 common filter types used in the lab:
filter paper
syringe filter
What are 3 types of filter paper
cellulose paper (ashless; rinsed with acid and UP water)
glass fibre
silicon-treated cellulose, Polytetrafluorethylene
2 types of syringe filters:
hydrophilic: cellulose acetate, PA, PP, PES, Nylon, PVDF
hydrophobic membrane: PTFE
common issues with conventional filtration (5)
filter time (finer particles -> more time)
membrane clogging
pressure (vacuum) too strong -> damages filter
filter residue contaminates sample
adsorption of analyte
What are some filter materials with very low protein binding? (3)
cellulose acetate
PVF (polyvinylidene fluoride)
PES (polyethersulfone)
What is the protein binding ability of cellulose nitrate filter?
high protein binding
Put in decreasing pore size order:
microfiltration
nanofiltration
ultrafiltration
microfiltration (0.1 um)
ultrafiltration (0.001-0.1um)
nanofiltration (2nm)
microfiltation, nanofiltration, and ultrafiltration are usually ____ driven
pressure
applications of microfiltration: (4)
sterilization
removal of microparticulates
pretreatment for ultrafiltration
fractionation (separate different components one step at a time)
How does microfiltration achieve sterilization?
membranes with 0.2microns (or less) pore size will remove bacteria, mold, and yeast
T/F: ultrafiltration will achieve sterilization
True, even microfiltration can achieve sterilization (<0.2 microns)
Examples of ultrafiltration designs (2)
disposable UF
centrifugal UF
Applications of ultrafiltration: (3)
protein concentration
peptide fractionation
separate free vs bound molecules
How is ultrafiltration used for protein concentration?
protein solution added to tube with filter -> centrifuge (proteins retained due to large size) -> discard buffer that passed through, and resuspend retentate -> repeat as needed
How is peptide fractionation done?
use various ultrafiltration pore sizes: first filter out largest ones, then use smaller pore size to filter out smaller ones, and then an even smaller one, etc
How can ultrafiltration be used to separate food dyes bound to serum albumin?
human serum albumin is large protein. dye-protein complex is large and cannot pass through; unbound dye molecules can pass through filter (measure absorbance of filtrate)
What is the method used to separate molecules in solution based on size using a semi-permeable membrane? What is the driving force?
dialysis
concentration gradient
Describe the process of dialysis
sample placed in bag-shaped membrane, sealed and placed in water/buffer with gentle agitation
small molecules slowly diffuse out until reach equilibrium
examples of applications of dialysis:
carbohydrate purification (separate large vs small polysaccharides)
in-line dialysis for determine nitrate/nitrite in milk
T/F: dialysis is a fast method
False; relatively slow
What are hollow fibre membranes?
hollow fibre made of artificial polymers, porous structure
What is the flow type in hollow fibre membrane filtration?
cross-flow (tangential)
What is a supported liquid membrane?
formed by dipping (filling) a hollow fibre with organic solvent -> immobilized in the walls (capillary action)
then fill the fibre lumen with solvent (organic or aqueous) to form the acceptor phase
The 2 types of hollow fibre LPME:
2 phase
3 phase
describe 2 phase LPME:
hollow fibre is dipped with organic solvent to form supported liquid membrane
inside of fibre filled with organic solvent (acceptor phase)
dipped into sample, swish around, analyte extracted into acceptor phase
Describe 3 phase LPME:
hollow fibre is dipped with organic solvent to form supported liquid membrane
inside of fibre filled with aqueous solvent (acceptor phase)
dipped into sample, swish around, analyte extracted into acceptor phase
T/F: a 3 phase LPME uses an organic solvent as the acceptor phase
False; uses aqueous solution
What prevents the analyte from exiting the acceptor phase in 3-phase LPME?
once enter acceptor phase -> become ionized (cannot diffuse past organic SLM anymore)
How should you choose an acceptor phase for 3 phase LPME?
basic analyte -> use acidic solvent
acidic analyte -> use basic solvent
LPME methods are usually coupled to _____
GCMS or HPLC or LCMS
What is electromembrane extraction? What analytes is it used for
mini form of liquid-liquid extraction; similar set up as LPME
apply VOLTAGE -> electrical potential acts as driving force
used for charged compounds (can adjust voltage and charge)
For EME, the solution must be adjusted so the analyte remains in a ____ state
charged
For a basic (+) analyte, the anode is located in the ____, while the cathode is located in the ____
solution
acceptor phase
A cathode is (positive/negative) while an anode is (positive/negative)
negative
positive
