Measurement of ions Flashcards
what does sample clean up involve
- remove interfering species
2. remove species of interest
list the types of ion selective electrodes
- glass electrode
- solid state membrane electrodes
- ion exchange membrane electrodes
- gas sensing probes
what are the main characteristics of ion selective electrodes
- development of ISEs has concentrated on the search for membranes that exhibit both sensitivity and selectivity
- selectivity is most difficult to achieve - ISE electrodes can be very sensitive and very selective
- electrodes can be subject to fouling
describe the properties of the glass membrane electrode
- glass is largely composed of a silicate matrix that is permeable to na+, k+ and h+
- composition of glass determines permeability to each type of ion - most common type of ISE
- analyte ions interact with surface of silicate lattice
- na+ ions migrate through thin glass membrane
- potential difference caused by migrating ions
describe the errors in ph measurement that could occur in glass membrane electrode
- alkaline error- gives low readings at high na+ conc and k+ conc
- acid error/water activity error- at low ph, activity of h2o<1 because h2o is used to hydrate protons
- this results in the electrode reading being too high
describe the properties of solid state electrodes
- provides electrodes which give a nernstian response to a number of different cations and anions
- all solid state membrane electrodes consist of a robust body with a membrane fixed at one end
- fluoride electrode uses a single crystal of LaF3 as a membrane
- F- in LaF3 are good conductors via crystal defects
- unlike H+ and glass electrode, F- ions carry charge through the LaF3 membrane - selective to F- ions because lattice sites within the crystal are only available to similarly sized, shaped or charged ions
- no interference from Cl- or Br- - the only interference comes from hydroxy ions, but buffering generally stops this
describe the properties of ion exchange electrodes
- use porous membranes rather than membranes that are permeable to ions via adsorption
- 2 types: ion exchange and neutral carrier
- these have a polymeric matrix of PVC, silicone, rubber etc and contain a solvent and chelating agents which are selective to the species of interest
- cover a wider range of compounds than glass or solid state but performance is not as good in terms of selectivity and lifetime
- membrane is an inert physical support or a liquid ion exchange
- liquid ion exchange soaks into the membrane and is held bringing it into contact with the internal solution and external test solution
- most often used with ca2+, k+ and no3-
describe the properties of gas sensing probes
- based on electrodes but use 2 membranes rather than 1
- most selective out of all ISEs but have fairly long response times
- primarily used for estimation of dissolved CO2 or NH3 in solution
- also available for SO2, H2S and nitrogen oxides
describe the properties of biosensors
- can use the former ISEs
- the use of a biological molecule to act as a catalyst
- direct: urea detection uses immobilised urease enzyme
- indirect: trapped combination enzyme inhibition for the detection of toxic ions (arsenic, copper)
describe how ion chromatography works for ion exchange resins
- resin in equilibrium with anions in mobile phase
2. sample ion is displaced by mobile phase ion, then absorbs to next exchange group
what can ion chromatography be used for
can be used to quantify simple ions (ca2+)or more complex polyions (proteins, peptides)
what does the magnitude of charge, valency and size depend on in ion chromatography
- nature of the stationary phase
2. eluent ion concentration in the mobile phase
what are the two types of electrophoresis
Gel: SDS-PAGE- peptides and nucleotides
capillary- peptides
what can gel electrophoresis be used for
DNA fingerprinting, 1D and 2D electrophoresis of proteins in a sample
what can capillary electrophoresis be used for
proteomics, genomics, metabolomics, quantification
