GC Flashcards
What is the activity coefficient γ and saturation vapor pressure (p^0) called?
Effective volatility
where γ depends on the properties of analyte and choice of liquid phase (sta. phase)
p^0 depends on the properties of the analyte (and temperature)
How does temperature influence effective volatility?
An increase in temperature increases saturation vapor pressure (p^0) (as well as a decrease in molar mass).
How does activity coefficient γ vary?
The solute can be like (Roult’s law) or unlike (Henry’s law) the stationary phase.
in ideal solutions γ = 1 (Roult’s law) “like dissolves like”
in non-ideal solutions γ <1 or >1 (Henry’s law) when there is a large difference in polarity or strong analyte-stationary phase interaction.
p = X * γ * p^0
So strong analyte-stationary phase interaction will yield lower vapor pressure than if the analyte prefers to interact with itself.
How large liquid samples are usually injected
1 - 2 uL
Why can splitless injection in GC contribute to peak broadening?
The splitless injection has a considerably lower total flow rate in the injection chamber, which means the solutes are applied to the column over a longer period
What advantages does splitless injection have
- Allows transfer of main part of the sample into the column
- Sharp narrow bands when using a concentrating effect
- favourable for trace analysis (compared rto split injection)
- Good both for quantitative and qualitative analysis
- Good for diluted samples (trace analysis)
What limitations does splitless injection have
- Increased analysis time
- Large solvent peak
- Matrix effects
- Memory effects
What is the solvent effect (trapping)?
Both solvent and analytes are condensed at the column inlet. Creates narrower peaks.
What is the cold trapping effects?
Only analytes are condensed on the column inlet. Solvents stays in vapor form = bpA > bpSolvent. Creates narrower peaks.
which GC injection method should be for used for compounds which degrade close to bp?
On-column. Split/Splitless at high temperature should be avoided.
Explainthe principle of the Electron Capture Detector (ECD)
radio isotope source emitting ß-rays ionisation of the carrier gas plasma
containing e-, and the produced current is measured. A signal is a dip in current when an analytes passes the ionisation area.
Thermal conductivity detector mechanism (TCD)
measures the difference in heat
conductivity of pure carrier gas
and analyte containing carrier gas
Electron capture Detector good for which solutes (GC)
particularly sensitive to halogen-
containing molecules, conjugated carbonyls, nitriles, nitro compounds, and organometallic
compounds but relatively insensitive to hydrocarbons
Thermal conductivity detector good for which solutes (GC)
Universal: organic and inorganic compunds.
Explain the principle of Thermionic ionization detector TID/ nitrogen phosphor NPD mechanism (GC)
TID is a similar NPD.
NPD
The detector features a Rb2SO4-containing
glass bead at the burner tip. Ions such as NO2-, CN-, and PO2- produced by these elements
when they contact the bead carry the current that is measured
