Training - GC columns

Column installation

Injector installation tips

Make a nice square cut at column end
Ensure column nut and all other fittings are leak-tight
Always cut end of column after passing through ferrule
Column top end should be just above the bottom of the inlet liner

Correct column installation in injector

Effect of column placement in injector

Do not install the column too low or too high within the injector.

Effect of column placement in injector

Detector critical factors

Ensure minimal exposure between the column end and detector source
Look for crushed column remnants
Clean square cut at column end
Ensure column nut and all other fittings leak-tight
Cut column after ferrule is attached

Effect of column cutting

Column efficiency

What affects column efficiency?

Column efficiency is dependent on:

Flow rate/average linear velocity
Column diameter
Column length
Carrier gas molecular weight

Phase type

Effect of phase type on resolution

Basic resolution equation

Basic equation resolution

Selectivity

Type of stationary phase

Selectivity equation

Effect of phase type example using BPX1 and BPX50

Comparison of analyzing components using BPX1 and BPX50

Decane
4-Chlorophenol
Decylamine
Undecanol
Biphenyl
Pentadecane

Effect of phase type, comparing BPX1 and BPX50

Effect of phase type example using BP1 and BP20 (WAX)

Comparison of analyzing components using BP1 and BP20 (WAX)

p-Xylene
m-Xylene
Decane
Undecane

Effect of phase type, comparing BP1 and BP20 (WAX)

Column diameter

Effect of column diameter

Internal diameter (ID) can vary from 0.245 to 0.255 for a typical 0.25 mm ID column. When the inlet pressure and length are held constant, the flow rate can change by 16%.

The smaller the diameter, the greater the efficiency.

Decreasing column diameter results in:

Faster run times for a given resolution
Increased efficiency
Decreased capacity

Effect of column diameter

Film thickness

Effect of film thickness on retention

Retention (capacity) is dependent on:

Film thickness
Oven temperature

Retention equation

Increasing the film thickness will increase retention and improve resolution for volatiles.

Effect of film thickness on retention

Oven temperature

Effect of oven temperature on retention

Retention (capacity) is dependent on:

Film thickness
Oven temperature

Retention equation

Increasing the film thickness will increase retention and improve resolution for volatiles.

Effect of film thickness on retention

Carrier gas and flow rate

Effect of carrier gas

Hydrogen delivers the most time efficient separation.

Choice of carrier gas

	Advantages	Disadvantages
Hydrogen	Cheap Gives the most time efficient separation Still very efficient at high gas velocities i.e. 60 cm/sec	Can form an explosive mixture with air Some industries in some countries have regulated against the use of hydrogen Is a reductive gas
Helium	Very inert, will not react with analytes Gives a very time efficient separation Non flammable	Expensive A non-replenishable resource
Nitrogen	Cheap Very inert, will not react with analytes Non flammable	Very slow velocity to achieve good efficiency

Gas flow

Setting optimal flow rates

Hydrogen 35-45 cm/sec
Helium 30-40 cm/sec
Nitrogen 10-20 cm/sec

The lower the HETP, the better.

How to check carrier gas flow

Turn on the carrier gas and adjust the column pressure to the desired value. If pressure for the column has not been pre-determined, adjust flow rate temporarily to that recommended as listed in the tables below.

Cut the column end and check column flow by dipping the column end into a small vial containing a solvent (e.g. pentane). A stream of bubbles should be observed. If not, check for possible leaks in the GC inlet or for any sign of damage to the column.

Approximate column head pressure (gauge) for optimum flow rate, calculated for hydrogen at 100ºC and with atmospheric pressure detector.

Column ID (mm)	Column length (m)								Flow rate (mL/min)
	10	12	15	25	30	50	60	120
	Approximate column head pressure (psi (kPa))
0.1	39 (270)	44 (300)	50 (340)	-	-	-	-	-	0.7
0.15	17 (120)	19 (130)	-	32 (220)	36 (250)	50 (340)	-	-	1
0.22	-	8 (55)	-	14 (100)	16 (110)	24 (170)	27 (190)	-	1.54
0.25	-	-	7 (48)	-	12 (83)	-	21 (140)	33 (230)	1.75
0.32	-	3 (21)	4 (28)	6 (41)	7 (48)	10 (69)	12 (83)	-	2.24
0.53	-	0.72 (5)	0.9 (6)	1.5 (10)	1.8 (12)	3 (21)	3 (21)	-	3.71

Approximate column head pressure (gauge) for optimum flow rate, calculated for helium at 100ºC and with atmospheric pressure detector.

Column ID (mm)	Column length (m)								Flow rate (mL/min)
	10	12	15	25	30	50	60	120
	Approximate column head pressure (psi (kPa))
0.1	56 (390)	63 (435)	71 (490)	-	-	-	-	-	0.56
0.15	26 (180)	29 (200)	-	47 (325)	52 (360)	71 (490)	-	-	0.84
0.22	-	13 (90)	-	22 (150)	25 (170)	35 (240)	39 (270)	-	1.23
0.25	-	-	11 (76)	-	19 (130)	-	30 (210)	48 (330)	1.4
0.32	-	5 (34)	6 (41)	9 (62)	11 (76)	16 (70)	18 (124)	-	1.79
0.53	-	1.3 (9)	1.6 (11)	3 (21)	3 (21)	5 (34)	6 (41)	-	2.97

Column bleed and activity

Column bleed

What is normal column bleed?

Normal background signal generated by the column stationary phase

What is not normal column bleed?

High baseline at low temperatures
Discrete low level interfering peaks
Wandering or drifting baseline at any temperature

Why is low bleed better?

Improved signal-to-noise = Better sensitivity
Less spectral interference = More reliable library ("hits")
Cleaner detector = Lower maintenance

Column bleed comparison of BPX5 and competitors

Why BPX columns have low bleed

We substitute some of the oxygen with the aromatic ‘benzene’ into the polymer backbone, known as the silphenylene unit. The aromatic ring acts as an "energy sink". As the column is heated in the GC, the vibrational and rotational energies increase. The aromatic ring helps disperse this energy, thus preventing phase breakdown.

Feature: Silphenylene modified polysiloxane backbone
Benefit: Restricts degradation pathways

BPX column structure for lower column bleed

Phase operating at high temperatures (not necessarily maximum operating temperature)
Exposure to oxygen in the carrier gas at elevated temperatures
Reactive compounds injected onto the column

Formation of polysiloxane ring structures causing column bleed

Degradation: The polysiloxane chain breaks down to form very small, stable cyclic polysiloxane ring structures, which cause the rise in the baseline of your chromatogram (column bleed).
BPX technology limits this process: In BPX columns, the aromatic ring restricts the formation of these ring structures, therefore reducing column bleed, and allowing the phase to operate at much higher temperatures before column bleed becomes a problem.

Column activity

There are three categories of column activity: