Training - GC columns

Column installation

Injector intallation 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

    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

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

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


      Column length

      Effect of column length on flow

      The longer the column; the greater the efficiency.

      Doubling the length increases resolution by 40%.

      Effect of column length


      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

      Effect of 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

      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:

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

      column inertness comparison, analysis of EPA phenols - BPX5 and competitors

      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

      BPX5 column features excellent chromatographic inertness


      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

      Analysis of EPA 608 pesticides using ECD