CHEM 315 Study Guide - Quiz Guide: Gas Chromatography, Polyimide, Silanol
18 Jun 2018
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3 band broadening mechanism (Van Deemter eqn)
A: independent of flow-rate (molecules will choose different path)
B: Inversely proportional to flow rate (high B, low flow rate, and vice versa)
C: Proportional to flow rate (finite time required for solute to equilibrate between mobile
and stationary phase), slower linear flow, more complete eq’m, less boradening
Gas Chromatography
Mobile phase = gas
Stationary phase = non-volatile liquid or solid
Analyte = gas or volatile liquid
Gas analyte is transported through column by gaseous mobile phase
Mobile phase gas needs to be at high purity (carrier gas)
Stationary phase = non-volatile liquid bonded to column
Gas solid adsorption chromatography
Analyte is adsorbed directly on solid particles of stationary phase
Column needs to be hot enough to provide enough Vap pressure for gas to elute out
Open tubular column is often used in GC
Fused silica
e.g) Polyimide coating (up to 350 celsius) : you can control the thickness of wall
Higher resolution (A: this term is gone as analyte don’t have to go through in particles)
Shorter analysis time (flow rate)
Greater sensitivity (A term: amount of sample injected)
Lower sample capacity
* narrowed column diameter: higher resolution
* increase the column length, higher resolution, higher theoretical plate number (N), lowers H
* increase retention time, higher resolution
*diameter > 0.32mm cannot use MS as detector (needs vacuum)
Choice of liquid stationary phase (selectivity)
Like dissolves like (polarity)
Bleeding: GC columns can age, stationary phase can deteriorate, silanol group (Si-OH) is exposed
then *tailing* could occur.
Reduce bleeding
High temperature: Covalently attach stationary phase to silica surface, minimize
bleeding
oHigh temp -> Decompose -> Slow bleed -> elevated background ->
reduced signal to noise ratio -> detector contamination
New type of GC Stationary phase (ionic liquid, stable to high temperature)
oLow volatility at higher temperature, if you coat GC column with IL, you can offer novel
selectivity
Packed columns: fine solid particles coated with “non-volatile” stationary phase or solid
less resolution because of A term presence
Better sample capacity, gives broader peaks, longer tr
Can isolate mg amounts
find more resources at oneclass.com
find more resources at oneclass.com
Has solid that’s silanized to reduce hydrogen bonding
* uniform particle size will lower the A term, lower H term, higher resolution*
Temperature and Pressure programming
GC temperatures often increased DURING SEPARATION (cut down analyzing time), increase
analyte vap pressure.
Etched column (in a silicone chip)
GC columns comes with 2 temperature labels
1. Isothermal temperature limit: column kept at this temperature w/o damage
2. Programmed temperature limit: can be kept for few minutes before damage
Carrier gas
Plate height vs linear velocity -> minimum of curve = optimal flow rate
He is compatible with most detector
Sample Injection
Needle inserted into rubber septum -> sample evaporation minimized
Solvent is added to wash the sample, air is used to expel solvents
Split Injection
Used when analyte of interest is > 0.1% of sample, too much material
Quantitative analysis is inaccurate
Split ratio (Column:Vent)
Splitless Injection
Used when analyte of interest < 0.01% of sample
Detectors
Thermal Conductivity Detector (TCD)
Most commonly used with packed column with He (good heat conductivity)
Less sensitive than other detectors
Less conductive analyte will increase the electron resistance, voltage changes then this
becomes signal
Flame Ionization Detector (FID)
Eluent burned in mixture of H2 and air, which forms CH radicals
CH + O -> CHO+ + e-
Radical formation happens 1 in 105 carbon atms
Hence lower DL
Electron Capture Detector (ECD)
Compared to MS , extremely sensitive
Sensitive to halogen-containing molecules, conjugated carbonyls, nitriles, nitro
compounds and organometallic compounds
Insensitive to hydrocarbon, alcohol and ketones
Carrier gas must be N2 or 5% CH4 in Ar
Gas is ionized by high energy electrons, these electrons are attracted to anode to
produce small current
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
A: independent of flow-rate (molecules will choose different path) B: inversely proportional to flow rate (high b, low flow rate, and vice versa) C: proportional to flow rate (finite time required for solute to equilibrate between mobile and stationary phase), slower linear flow, more complete eq"m, less boradening. Gas analyte is transported through column by gaseous mobile phase. Mobile phase gas needs to be at high purity (carrier gas) Stationary phase = non-volatile liquid bonded to column. Analyte is adsorbed directly on solid particles of stationary phase. Column needs to be hot enough to provide enough vap pressure for gas to elute out. Open tubular column is often used in gc. Fused silica e. g) polyimide coating (up to 350 celsius) : you can control the thickness of wall. Higher resolution (a: this term is gone as analyte don"t have to go through in particles) Greater sensitivity (a term: amount of sample injected)
