Normal Alkanes or Paraffins are key to Kovats index and SimDist GC analysis. Wouldn’t it be nice to calculate and predict n-Alkanes instead of measuring them over and over again?

Now you can with StillPeaks Virtual GC software!

Virtual GC is a model of capillary GC that predicts CH4 to nC100 chromatograms on Methyl-Phenyl-Silicone capillary columns. The model is based on the Giddings peak move (time), Golay zone dispersion (width) and boiling point and polymer solubility (equilibrium).

With Virtual GC SimDist and Detailed Hydrocarbon Analysis by Kovats index without measuring n-Alkanes becomes possible.

PT diagram Methane to nC100 to predict n-Alkanes

PT diagram Methane to nC100 to predict n-Alkanes

Predict n-Alkanes for Detailed Hydrocarbon Analysis by Kovats Index (ASTM method D6730)

The Hydrocarbon group 1 and 2 samples need Kovats index data to identify peaks by methods known as Detailed Hydrocarbon Analysis (DHA). Kovats index is peak time interpolation between successive n-Alkanes. Without n-Alkanes, peaks cannot be identified by DHA.

Today GC is so repeatable that n-Alkanes from earlier measurement can be used to identify peaks in a sample without n-Alkanes. Run a sequence of samples including an n-Alkane standard and you are ready to calculate Kovats index. No n-Alkanes measured in your sequence? Use Virtual GC calculated times based on n-Alkane Boiling Point and Solubility in crosslinked polymers of Phenyl-Methyl-Silicone. Tweak times with column wear to get 1-2 index units precision. 

 

Predict n-Alkanes for SimDist Analysis (ASTM method D2887)

SimDist calibrates the time axis with n-Alkane boiling points. Normal operation involves a sequence with

  1. a blank,
  2. a n-Alkane standard and
  3. the samples.

 

Virtual GC saves time measuring the n-Alkane standard. And when it comes to analyzing crude: the high boiling point of nC100 requires a 425°C or more with column wear consequences to a level where repeatability may become compromised… In those cases the Virtual GC software can use the separated n-Alkanes in crude to assess column wear and calculate all n-Alkane times required for SimDist of Crude.

 

Run-to-Run stability with Ultra Thin Column Coatings

Thin films have a limit (see Poly(dimethylsiloxane) ALEX C. M. KUO). Ultra thin films are more amenable to thermal degradation. The problem of small loadabilty of thin films can (or cannot) be solved with a split injection with fast vent flow, a valve or syringe with femtoliter injection capability. StillPeaks Virtual GC calculates chromatograms on films down to 2 nm (0.002µm).

When coatings are so thin, run to run stability only allows the use of n-Alkanes present in the sample. One can see the heavy n-Alkane lose retention time run after run…

StillPeaks Virtual GC calculates the missing n-Alkanes, saving you additional measurements.

 

Predicting CH4 to nC100 chromatograms

StillPeaks Virtual GC predicts n-Alkanes from boiling point and polymer solubility (equilibrium), the Giddings peak move (time) and Golay zone dispersion (width). 

Virtual GC requires input of:

  • (pre-) Column dimensions L×dc×df , 
  • Phenyl% of the stationary phase(s), 
  • Inlet pressure / Outlet flow value,
  • Carrier gas type and
  • Temperature Program

So when these five GC method details are known, the finetuning can begin. 

 

Interested?

So are you interested in Simdis without measuring n-Alkanes? Or Kovats index without measuring n-Alkanes? Try predicting n-Alkanes with Virtual GC!
Read more here, contact us or download the software and ask for your free of charge trial license.