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Best Practice for GC Sample Introduction

No matter what kind of GC inlet you are using we have some helpful hints that can be applied to achieve the best sample introduction every time.

1. Set the split ratio (Split) split flow

Some good rules of thumb when setting the split ratio are:

  • Typical split ratios are in the range 1:20 to 1:400.
  • When using thick stationary phase film columns (>0.5µm) or wide bore (0.533 mm i.d.) columns the sample capacity increases and lower split ratios are typical, 1:5 to 1:20.
  • With very narrow GC columns (<100 µm i.d.) split ratios can be as high as 1:1000+.

 

Figure 1:  Split/splitless inlet »

 

 

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2. Inject the correct volume

If the sample’s expanded volume is larger than the injection port (liner) volume, it flashes back up to the top of the injection port and out of the gas lines both coming into and leaving the inlet, where sample components will condense.  This unwanted situation, called backflash, can create a multitude of problems.

Backflash problems

  • Sample loss
  • Poor resolution
  • Peak shape problems (peak splitting, tailing, etc.)
  • Carry-over
  • Ghost peaks

How to prevent backflash

  • Choose a solvent with a high molecular weight
  • Increase the volume of your liner
  • Know the expansion volume
  • Lower the inlet temperature
  • Increase inlet pressure

CHROMacademy has a useful backflash calculator that can be used to optimise the injection volume for your GC method.

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Figure 2:  Backflash Calculator

Be aware of your column capacity, overloading a column will lead to poor chromatography.

Column ID (mm) Film Thickness (µm)
0.1 0.25 0.5 1.0
0.10 10 ng 30-40 ng 50-70 ng 100-200 ng
0.18 20-30 ng 60-80 ng 100-150 ng 250-350 ng
0.25 30-40 ng 125-175 ng 175-250 ng 400-500 ng
0.32 50-70 ng 200-250 ng 250-350 ng 600-800 ng
0.45 80-100 ng 300-400 ng 400-500 ng 800-1000 ng
0.53 100-120 ng 400-500 ng 500-700 ng 1000-1500 ng

Table 1:  Approximate GC sample capacity as a function of column diameter and film thickness (note that amounts are PER ANALYTE not per sample). 
For more accurate data please consult your column manufacturer. 


3. Purge the inlet (Splitless)

Achieve good inlet purge by:

  • Actuating the split (purge) valve to give a high split flow through the liner – flows of 100-200 mL/min are typical.
  • Optimise the splitless time. 

purge inlet

Figure 3:  Split less time TOO short (left) – loss of higher boiling analytes.  Splitless time TOO long (right) – broad solvent peak and rising baseline.

  • Typical splitless times lie in the region 20-90 seconds.

4. Focus the analyte (Splitless)

Focussing techniques usually involve setting the initial oven temperature at a suitably low value ensuring that condensation and re-concentration takes place in the column.

Focussing mechanisms:

Cold trapping – higher boiling analytes are condensed in a tight band in the temperature gradient between the inlet (~250 °C) and the column oven (~40 °C).

Solvent effect – low boiling (more highly volatile) components remain dissolved in the solvent which condenses on the inner wall of the GC column at low initial oven temperatures.  The solvent slowly evaporates to give a thin concentrated band of analyte

A good rule of thumb to aid analyte focussing is to match solvent and column polarity.


5. Choose the Correct Inlet Temperature

To choose an appropriate inlet temperature the following steps can be followed:

  • A good starting point for method development and for new analyte applications is
    250 °C.
  • Use a ‘scouting’ gradient can to estimate the elution temperature of the highest boiling component (Figure 4.)

Set the inlet temperature at least 50 °C above this temperature to ensure sufficient sample volatilisation.

 

Figure 4:  Scouting temperature gradient »

 

 

scouting temperature gradient

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