Membranes

Question 1

What is the definition of ‘membrane?’

Answer 1

structure with much greater surface area (lateral dimensions) than thickness, can transfer substances through with various driving forces

Question 2

driving forces that can be used in membrane separations:

Answer 2

concentration gradient
applied pressure
electric potential

Question 3

What are membranes used for in food analysis? (4)

Answer 3

sample prep (filter, fractionation, cleanup, sterilize…)
extraction
analyte identifiation/quantification
speciation studies

Question 4

What is filtration?

Answer 4

segregation of phases (suspended solids in liquid or gas) by forcing it through a porous medium (filter)

separation based on size (also shape, charge)

Question 5

a coffee filter has pore size of about _____

Answer 5

20 microns

Question 6

The 2 flow types in filtration:

Answer 6

direct flow

tangential flow

Question 7

What is direct flow filtration:

Answer 7

flow of feed is perpendicular to membrane; molecules that don’t fit through pores will accumulate on surface

Question 8

What is tangential flow filtration?

Answer 8

feed is parallel to membrane, some will pass through (filtrate) while larger particles will keep flowing with rest of liquid (retentate)

Question 9

Compare the performance of direct vs tangential flow filtration:

Answer 9

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

Question 10

Tangential flow is also called ______

Direct flow is also called _____

Answer 10

cross flow

conventional flow

Question 11

Common designs of conventional filtration devices

Answer 11

buchner funnel + vacuum flask + filter paper

Question 12

What AOAC approved food analysis method requires use of filtration?

Answer 12

measurement of soluble and insoluble dietary fibre; filter sample after enzymatic digestion

Question 13

What are important characteristics of filter paper to consider? (6)

Answer 13

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)

Question 14

1 bar = ____ psi

Answer 14

14.5

Question 15

2 common filter types used in the lab:

Answer 15

filter paper

syringe filter

Question 16

What are 3 types of filter paper

Answer 16

cellulose paper (ashless; rinsed with acid and UP water)
glass fibre
silicon-treated cellulose, Polytetrafluorethylene

Question 17

2 types of syringe filters:

Answer 17

hydrophilic: cellulose acetate, PA, PP, PES, Nylon, PVDF

hydrophobic membrane: PTFE

Question 18

common issues with conventional filtration (5)

Answer 18

filter time (finer particles -> more time)
membrane clogging
pressure (vacuum) too strong -> damages filter
filter residue contaminates sample
adsorption of analyte

Question 19

What are some filter materials with very low protein binding? (3)

Answer 19

cellulose acetate
PVF (polyvinylidene fluoride)
PES (polyethersulfone)

Question 20

What is the protein binding ability of cellulose nitrate filter?

Answer 20

high protein binding

Question 21

Put in decreasing pore size order:
microfiltration
nanofiltration
ultrafiltration

Answer 21

microfiltration (0.1 um)
ultrafiltration (0.001-0.1um)
nanofiltration (2nm)

Question 22

microfiltation, nanofiltration, and ultrafiltration are usually ____ driven

Answer 22

pressure

Question 23

applications of microfiltration: (4)

Answer 23

sterilization
removal of microparticulates
pretreatment for ultrafiltration
fractionation (separate different components one step at a time)

Question 24

How does microfiltration achieve sterilization?

Answer 24

membranes with 0.2microns (or less) pore size will remove bacteria, mold, and yeast

Question 25

T/F: ultrafiltration will achieve sterilization

Answer 25

True, even microfiltration can achieve sterilization (<0.2 microns)

Question 26

Examples of ultrafiltration designs (2)

Answer 26

disposable UF | centrifugal UF

Question 27

Applications of ultrafiltration: (3)

Answer 27

protein concentration peptide fractionation separate free vs bound molecules

Question 28

How is ultrafiltration used for protein concentration?

Answer 28

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

Question 29

How is peptide fractionation done?

Answer 29

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

Question 30

How can ultrafiltration be used to separate food dyes bound to serum albumin?

Answer 30

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)

Question 31

What is the method used to separate molecules in solution based on size using a semi-permeable membrane? What is the driving force?

Answer 31

dialysis concentration gradient

Question 32

Describe the process of dialysis

Answer 32

sample placed in bag-shaped membrane, sealed and placed in water/buffer with gentle agitation small molecules slowly diffuse out until reach equilibrium

Question 33

examples of applications of dialysis:

Answer 33

carbohydrate purification (separate large vs small polysaccharides) in-line dialysis for determine nitrate/nitrite in milk

Question 34

T/F: dialysis is a fast method

Answer 34

False; relatively slow

Question 35

What are hollow fibre membranes?

Answer 35

hollow fibre made of artificial polymers, porous structure

Question 36

What is the flow type in hollow fibre membrane filtration?

Answer 36

cross-flow (tangential)

Question 37

What is a supported liquid membrane?

Answer 37

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

Question 38

The 2 types of hollow fibre LPME:

Answer 38

2 phase | 3 phase

Question 39

describe 2 phase LPME:

Answer 39

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

Question 40

Describe 3 phase LPME:

Answer 40

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

Question 41

T/F: a 3 phase LPME uses an organic solvent as the acceptor phase

Answer 41

False; uses aqueous solution

Question 42

What prevents the analyte from exiting the acceptor phase in 3-phase LPME?

Answer 42

once enter acceptor phase -> become ionized (cannot diffuse past organic SLM anymore)

Question 43

How should you choose an acceptor phase for 3 phase LPME?

Answer 43

basic analyte -> use acidic solvent acidic analyte -> use basic solvent

Question 44

LPME methods are usually coupled to _____

Answer 44

GCMS or HPLC or LCMS

Question 45

What is electromembrane extraction? What analytes is it used for

Answer 45

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)

Question 46

For EME, the solution must be adjusted so the analyte remains in a ____ state

Answer 46

charged

Question 47

For a basic (+) analyte, the anode is located in the ____, while the cathode is located in the ____

Answer 47

solution | acceptor phase

Question 48

A cathode is (positive/negative) while an anode is (positive/negative)

Answer 48

negative | positive