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The Correct Injection Volume for Split Injections

8 Dec 2023

We often see too much being injected into the hot injector during a classical split injection. 

  • This leads to unreproducible results (fluctuating peak areas), wear and contamination of the injection system. 
  • One possible consequence of this "backflash" are ghost peaks which - in the worst cases - appear for a long time in the subsequent chromatograms.
    • When partially unevaporated samples return to the split line, they condense in the filter (which should be replaced regularly during instrument maintenance!). Gradually - depending on the injector, flow and split conditions - the sample components are released and transported back into the injector and onto the column. These are visible as ghost peaks in the chromatogram.

However, with splitless injection larger volumes can be injected. If this problem occurs regardless, the wrong injection conditions have been chosen.

How do you determine the appropriate injection volume for split injections? 

The decisive factor is the volume of the vapor cloud that forms from the injected sample during rapid evaporation. The liner must be able to completely absorb the cloud! 

The maximum injection volume for split injection depends on: 

  1. Internal volume of the liner 
    • It should be noted that any constriction or filling reduces the volume available for the vapor cloud. Remember that the liner is already filled with carrier gas. 
  2. Solvent used 
    • Different solvents form very different vapor volumes.
    • As a rule of thumb: the more polar the solvent, the larger the vapor cloud during evaporation, headed by water.
  3. Injector temperature and carrier gas pressure 

Here are a few examples of vapor volumes (in μL) of different solvents when evaporating 1μL at 250°C at different carrier gas pressures:

  Expansion Volume in µL at various column head pressures
Solvent Density (g/mL) MW 5 psig 10 psig 15 psig
Heptane 0.68 100 219 174 145
Hexane 0.66 86 245 224 186
Toluene 0.87 92 303 242 201
Ethyl Acetate 0.90 88 328 261 217
Chloroform 1.49 119 400 319 266
Methylene Chloride 1.33 85 500 399 332
Methanol 0.79 32 792 629 525
Water 1.00 18 1776 1418 1179

Incidentally, the vapor cloud can be made smaller by pressure pulse injections.

The internal volume of the liner determines the correct injection volume. 

  • Use Restek’s "Solvent Expansion Calculator" for help. 
  • With our EZGC Method Translator and Flow Calculator we offer you another practical and free-of-charge tool for your method development and optimization.