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

 

◄Effect of column placement in injector

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

Selectivity

• Type of stationary phase

◄Effect of phase type example using BPX1 and BPX50

Comparison of analyzing components using BPX1 and BPX50

1. Decane
2. 4-Chlorophenol
3. Decylamine
4. Undecanol
5. Biphenyl
6. Pentadecane

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

Comparison of analyzing components using BP1 and BP20 (WAX)

1. p-Xylene
2. m-Xylene
3. Decane
4. Undecane

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

Column length

◄ Effect of column length on flow

The longer the column; the greater the efficiency.

Doubling the length increases resolution by 40%.

Film thickness

◄ Effect of film thickness on retention

Retention (capacity) is dependent on:

• Film thickness
• Oven temperature

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

Oven temperature

◄ Effect of oven temperature on retention

Retention (capacity) is dependent on:

• Film thickness
• Oven temperature

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

◄ 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

• 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

• 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:

• Reversible peak adsorption
• Non-reversible peak adsorption
• Breakdown

◄ BPX5 column features excellent chromatographic inertness

• Enables low detection levels to be achieved
• Ensures linear calibration plots
• Allows the analysis of all difficult environmental and toxicological compounds

Capillary columns and air

◄ Effect of air oxidation

Depending on the solute analysed, the effects of air oxidation of a column is generally observed first as either an:

• Increase in column bleed levels
• Increase in column activity

How air can get into your system

• Loss of carrier flow
• Leaking gas system
• Inexperienced operators
• Cylinder changeover
• Large air injections

Effect on a 5% Phenyl polysiloxane-silphenylene column using air as a carrier gas

Effect of subjecting columns to air as a carrier gas