Study Guide on GC Columns
1. Compare the Van Deemter plot for OTC vs. packed column? How does the efficiency
compares at optimum velocity in the plot for OTC vs. packed column? Is the efficiency
loss is more slow or fast in OTC? What is the practical consequence (advantage of this
trend) in OTC? How does the Van Deemter equation compares for packed versus OTC.

 How does the efficiency compares at optimum velocity in the plot for OTC vs. packed
column? Hmin is lower and Vopt is at higher velocity for OTC. Right had portion of OTC is
flatter than to packed column. At each flow rate, Hmin values are 3-4 times lower for
OTC.
 Is the efficiency loss more slow or fast in OTC?
Efficiency (N) is lost more slowly because the plate height (H) curve remains flat upon
increasing the flow rate in OTC.
 What is the practical consequence (advantage of this trend) in OTC?
Faster analysis time is possible when working at higher flow rate without sacrificing
efficiency.
Van Deemter equation:
H = A + B/u + Cu ------------> packed column
H = B/u + Cu
------------> OTC
2. List the 3 types of diatomaceous earth and study their properties in terms of surface
area, loading of stationary phase and the analyte type they are suitable for.
Chromsorb P: Pink diatomaceous earth derived crush fire brick; high surface area
(4.0m^2/g); load as high as 35 % w/w of liquid stationary phase; suitable for separation
of alkanes, but needs deactivation when polar compounds are injected
Chromsorb W: prepared from calcined diatomile; lower surface area (1.0 m^2/g),
loading is 12% w/w suitable for polar compounds
Chromsorb G: hardest and twice as dense as Chromsorb W; maximum S.P. loading is
5% w/w because of high density
3. Study the chemical reaction in which Diatomaceous earth is treated with silane reagent.
What is the advantage of silanization of diatoms rock particles?
To avoid peak tailing in GC, diatomaceous particles are generally pretreated
(thoroughly acid wash with HCl to remove the minerals) and silanized using
dimethylchlorosilane or HMDS to block the residual Si-OH groups with methylated
siloxane bonds, Si-O-Si-Me
4. Name the most polar and nonpolar stationary phase in GC? Provide chemical structure
for both. What are the bases of retention of chemical compounds in non-polar
stationary phase and polar stationary phase?
Nonpolar: Saturated hydrocarbons, Olefinic hydrocarbons, Aromatic hydrocarbons,
Halocarbons, Mercaptans, Sulfides, and CS2
Weak Intermediate Polarity: Ethers, Ketones, Aldehydes, Esters, Tertiary amines, Nitro
compounds (without α-H atoms), Nitriles (without α-atoms)
Strong intermediate polarity: Alcohols, Carboxylic acids, Phenols, Primary and secondary
amines, Oximes, Nitro compounds (with α-H atoms), and Nitriles (with α-H atoms)
Strongly Polar: Polyhydroxyalcohols, Amino alcohols, Hydroxy acids, Polyprotic acids, and
Polyphenols
5. Study the cross section area of WCOT, SCOT, FSOT and PLOT and understand the
differences.
 WCOT: Liquid S.P. is directly coated on the column wall (by passing a solution of liquid S.P.
(dissolved in an organic solvent), blowing the column dry with a stream of inert gas.
 PLOT: Solid particles are active S.P. A thin layer of solid adsorbent (porous solid support is
fixed to the inner wall of the capillary).
 SCOT: The liquid S.P. is coated on a solid support or solid particles (e.g. diatomaceous earth).
6. What is the major reason for bonding and crosslinking in GLC with open tubular
columns?
 To prevent peak tailing due to unreacted silanols and prevent “column bleed” at high
temperature. Column bleed is a phrase used to describe rise in baseline as the column is
degrading at high temperature.
7. Understand differences between different GC columns.
FSOT
WCOT
SCOT
Packed
Column
Length
(m)
Internal
Diameter
(mm)
Relative
Pressure
Relative
Speed
Efficiency
(plates/m)
Sample
Size
(ng)
Flexibility
Chemical
Inertness
10100
10100
10100
1-6
0.1-0.3
LOW
fast
10-75
YES
BEST
0.250.75
0.5
LOW
fast
>
fast
NO
>
2-4
HIGH
slow
500-1000
101000
101000
10-106
NO
LOW
20004000
10004000
600-1200
NO
POOREST
8. What is GSC? What type of molecules are typically analyzed by GSC and not by GLC and
why?
 GSC is Gas Solid Chromatography. In GSC, the column consists of a suitable stainless steel
tube packed with particles, which are uncoated. Gases are typically analyzed by GSC than by
GLC because gases are not sufficiently soluble in most liquid S.P. at temperature greater or
equal to 50 C to retain significantly by GLC. Also, gases have very little difference in polarity and
in GLC S.P. do not provide higher selectivity for gases. In GSC, the solid S.P. has a rigid geometry
which results in small molecules like gases to easily penetrate the pores of the molecular sieves.
9. What is PLOT column? What are the choices of packing material for PLOT columns?
Porous layer open tubular (PLOT) columns are defined as capillary columns where the
inner surface is coated with a layer of solid porous material.
10. What are the differences between pack and open tubular GC?
OTC has higher resolution but lower sample capacity than Packed Column (4
microliters versus 1 mL).
11. Why are gases more likely to be analyzed by GSC than by GLC?
Gases are not sufficiently soluble in most liquid S.P. at temperature greater or equal
to 50 C to retain significantly by GLC. Therefore, if we want to separate gases using GLC
we have to use subambient temperature. Gases have very little difference in polarity,
GLC S.P. does not provide higher provide higher selectivity for gases. Solid S.P.
(molecular sieves used in GSC) have rigid geometry so this results in small molecules like
gases to easily penetrate the pores of molecular sieves.
Packed Column
Open Tubular Capillary Column
Glass, Stainless-steel, 0.5-3m Length
Glass or Fused Silica 10-60m Length
Typically 4 mm i.d
Typically 0.2-0.3 mm i.d
Contain solid support
No solid support
Liquid phase on
solid support
Liquid or solid particles attached on the column
wall
Flow rate 20-70 mL/min
Flow rate 1-2 mL/min
Simple sample injection
Specialized injector required
Broad peaks
Narrow peaks
High sample Loading
Low sample loading
Less skill required
More skill required