Samenvatting analytische scheidingsmethoden deel 1 (Gas chromatography. Wordt gegeven in 3e bachelor 1e semester Ugent richting chemie. Kunnen klein beetje foutjes instaan, stuur gerust een berichtje want heb nog mijn eigen samenvatting liggen met extra notities op.
H1 Gas chromatography (GC)
1.1 Theoretical aspects
most efficient chromatographic technique because of faster diffusion reachable in a gas compared to other
mobile phases media
1.1.1 Introduction
based on different distribution of the components of a mixture over a gase phase and a stationary phase
gas-liquid & gas-bonded phase >> adsorption chromatography (adsorption slower process ⇒ broadening
of peaks )
long fused silica capilaries, inside coated with stationary phase (denk aan labo ding dat je vervangen
hebt)
most popular: WCOT columns (wall coated (rubber) open tubular columns)
PLOT = porous open tubulary column
1.1.2 Distribution proces
→ solutes moving through column: distributed between mobile & stat. phases
→ distribution process fast enough & [solute] low
enough ⇒ [solute] in both phases ≈equilibrium
→ dynamic equivalent of static seperation in a
funnel
Cs
K= CM
(s = conc stat phase) = distribution
constant
H1 Gas chromatography (GC) 1
, 💡 ΔG0 = −RTlnK
spontaan (=analyte favoring interaction with stationary phase):
ΔG0 < 0 → K > 1
retention factor k with column chromatography k = β ( phase ratio)= VM
VS
= dc
4df
MS
= massa / massa
MM → β is fixed column characteristic
K = kβ = k VVMS
→ dc = diameter column
→ RE verband K en k
→ df = film thickness
→ K is T afhankelijk: k ook (dalen met stijgende T in
→ k & β onafh. van lengte kolom en mobile phase
gas & liquid chromatografie)
velocity
1.1.3 Retention time, retention factor and seperation factor (selectivity)
difference in retention cause separation of the components
retention time (tR ) = time it takes after sample tR − tM t′R
k= = (geen eenheid)
tM tM
injection for the analyte peak to reach the detector
(rate of migration unretained species = average rate
dead time (tM ) ⇒
small peak on left for species not
of motion of mobile phase molecules)
retained (volatile solvent like DCM om je dingen in o
te lossen)
t′R = tR − tM (netto ret.tijd = hvlheid tijd het analyt
gespendeerd heeft in stationaire fase )
k best tussen 3 en 15
α ≧ 1 (1 indien geen scheiding)
→ hoe groter, hoe groter verschil in retentietijden
van de opl.
eenheid): degree of sep. of two adjavent solutes =
ratio net retention time of later eluting solute to that
of earlier eluting solute later/earlier = selectivity ⇒
2.1.4 Zone broadening and peak width
band broadening: when the solute emerges from the column it occupies a larger
volume dan when a solute is introduced into the column
H1 Gas chromatography (GC) 2
, → echt brede piek = shit scheiding, hoe scherper hoe beter ⇒ kan zorgen voor
overlap etc for closely eluting solutes
→ later eluting compounds have broader peak widths compared to te earlier eluting
ones
→ width of peaks not a constant
→ can occur outside or inside the column
for now: only consider inside (so assume inlet, connection and
detector systems make no signific. contributions)
→ Ct = concentration of substance at the outlet of column time t
→ Cmax = concentration of substance at peak maximum (t
= tR )
→ σ = standard deviation (of distribution in units of time)
peak width:
→ at half height: wh
= 2.355σ
→ at inflection point: w = 2σ
→ at base: wb = 4σ Gauss function to describe peak ⇒
represents concentration distribution along
axis of column according to:
(t−t r )2
Ct = Cmax e−
2σ 2
narrower peak = smaller
σ value
Gauss method fails in preparative chromatography where large amounts of sample
are used (here in analytical its ok); amount of analyte negligible t.o.v amount
stationary phase
1.1.5 Theoretical plate concept and column efficiency
The theoretical plate (N) is one way of expressing the ability of a molecule to achieve
relatively narrower or wider peak shapes
N= L
H , L = length of column (L hoger = betere scheiding)
→ more theoretical plates in column ⇒ narrower peaks (better separated from neighbouring peaks) ⇒ more
efficient column
→ no physical plates (gwn theoretisch concept)
H1 Gas chromatography (GC) 3
, → number of theoretical plates = column efficiency (N, geen dimensie)
N = ( tσR )2 = 16( wtRb )2 = 5.54( wtRh )2 =
L
H
wh = 2.355σ, wb = 4σ
H = height equivalent to theoretical plate (HETP, mm)→ totaal aantal theor. platen (N) hangt af van L
hoe kleiner H: hoe efficiënter (narrower peaks if all other tR naar seconden omzetten
factors equal)
1.1.6 Theoretical approach to the concept of resolution
As the purpose of chromatography is to separate different components from each
other, the interplay of N, k and a has to be considered for the separation of two peaks
tR2 − tR1
Rs (resolution, geen eenheid) als enige deftige beschrijving van de scheidingskwaliteit, Rs =
1/2(wb1 + wb2 ) (1
en 2 voor eerste en tweede piek)
→ measures the separation quality of two adjacent
peaks
→ R of 1.5 = baseline resolution
→ R =1 is genoeg voor gewoon identificatie
componenten
R is depending on all experimental parameters (column,T,..) & can be optimized in various ways
ΔtR → function of interaction of 2 components with stationary phase (selectivity)
wb → dependent on band broadening mech. (efficiency) which are functions of the
retention time (retention factor) of the components in the column
ASSUME: σ only due to chromatographic band broadening (INSIDE the column) + we do not take into
consideration extra band broadening mech: allow us to transform Rs (waarbij wb1 ong wb2)= Δt r
enkel afh van retentietijd en N2 van laatste piek (k2
en N2) (en alpha)!
→ Every interference with the chromatographic process which causes an increase in N, a or k, has a positive
influence on the resolution
→ high plate counts (column efficiency) beneficial for resolution
H1 Gas chromatography (GC) 4
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