define membrane
- a structure through which transfer may occur under a variety of driving forces
- usually lateral dimensions are much greater than thickness
what are examples of driving forces?
concentration gradient
applied pressure
electrical potential
what are uses of membranes in food analysis?
- sample prep: filtration, fractionation, cleanup, sterilization
- extraction: liquid membrane extraction
- analyte identification and quantification: membrane for sensors
- specific studies
define filtration
the process of segregation of phases
eg. separation of suspended solids from a liquid or gas, usually by forcing a carrier gas or liquid through a porous medium
what is filtration primarily based on? what are other factors?
primarily based on size
other factors: shape and charge
what are two different types of filtration flows?
which one filtrates faster?
- direct flow filtration (conventional)
2. tangential filtration (Cross-flow)
describe direct flow filtration
what happens as volume filtered increases?
- flow is perpendicular to membrane
- molecules larger than pores accumulate at membrane surface
- as volume filtered increases, mass accumulated may clog the filter and block flow of liquid through membrane
describe tangential filtration
- no accumulation at membrane surface (b/c larger molecules flow in through)
- large volume of sample can be processed without clogging
describe the graphs of conventional and cross-flow filtration
conventional:
- beginning has high flow rate and low amount of particles at surface of membrane
- as particles accumulate, flow rate slows
cross-flow
- flow rate and particles at membrane surface are unchanged
what equipment are commonly used for conventional filtration?
- Buchner funnel
- filtration units
- filter paper
- syringe filters
what are characteristics of filter papers?
- retention characteristics: pore diameters in um
- collection efficiency: % retained for specific particle size
- wet burst: pressure in psi
- content of ash: for mineral analysis
- content of trace elements: for metal analysis
- stability: pH
what are 3 common types of material for filter papers?
- cellulose paper
- glass fiber
- silicon-treated cellulose, polytetrafluoroethylene (PTFE)
what are types of syringe filters?
- hydrophilic membranes: cellulose acetate, polyamid, etc…
- hydrophobic membranes: PTFE
what are common issues with conventional filtration?
- filtrating fine particles = needs more time
- membrane clogging
- pressure (vacuum) is too strong = damages filter
- contamination of sample with filter residues
- adsorption of the analyte
which ones have low or high protein binding?
a) cellulose acetate
b) polyvinylidene fluoride
c) poyethersulfone (PES)
d) cellulose nitrate
a, b, c have very low protein binding
d has high protein binding
define microfiltration
what size of molecules are rejected
- driven by pressure
- membrane-based
- small particles are dissolved
- macromolecules larger than 0.1 um are rejected
define ultrafiltration
what is the pore size range
process where a solution with a solute much bigger than the solvent molecule is removed from the solvent by using hydraulic pressure (which forces only the solvent to flow through a suitable membrane)
- pore size range is 0.001-0.1 um
define nanofiltration
what is the pore size range
- pressure drive
- membrane based
- separation process where particles and dissolved macromolecules smaller than 2nm are rejected
what are examples of applications for microfiltration
sterilization
removal of microparticulates
pretreatment for ultrafiltration
fractionation
how is culture media sterilized in food microbiology?
filtration through a membrane with 0.2um pore size
what are applications of ultrafiltration in food analysis?
- concentration of high molecular weights
- fractionation using sequential ultrafiltration
- removal of salts or low molecular wt interferences
- separate free from bound molecules
describe dialysis
- used to separate molecules in solution based on size using a semi-permeable membranen
- small molecules cross
the membrane via diffusion, but large molecules can’t - membrane is a bag in which the sample is placed
- membrane bag is placed in water/buffer solution
what does HF-LPME stand for?
hollow fiber liquid phase micro extraction
what two phases are in HF-LPME?
describe each
donor phase: where all molecules are
acceptor phase: only selected molecules
