Retention index

A retention index (plural retention indices , also known as Kovats index ) enables a device-independent standardization of the retention times in gas chromatography . This makes it possible to use data from other laboratories for substance identification.

history

While working with the then new method of gas chromatography, Ervin Kováts developed the system of retention indices in 1958. They are considered to be groundbreaking findings for the use of gas chromatography as an analytical method. The concept was later refined. In 1979 ML Lee published his concept for what is now known as the Lee Index for aromatic hydrocarbons.

Kovats index

The Kovats index uses the relationship between carbon content and retention time of a substance in a gas chromatographic system to obtain a device-independent value for the identification of unknown substances.

For isothermal gas chromatography it is described by the following equation:

${\displaystyle I=100\times \left[n+(N-n){\frac {\log(t_{r({\text{unknown}})}')-\log(t_{r(n)}')}{\log(t_{r(N)}')-\log(t_{r(n)}')}}\right]}$

The following applies:

${\displaystyle I=}$ Kovat's retention time index,

${\displaystyle n=}$Number of carbon atoms of the first n-alkane ,

${\displaystyle N=}$ Number of carbon atoms of the second n-alkane,

${\displaystyle t_{r}=}$ Retention time

For temperature programmed gas chromatography the Kovats index is described by the following equation:

${\displaystyle I=100\times \left[n+(N-n){\frac {t_{r({\text{unknown}})}-t_{r(n)}}{t_{r(N)}-t_{r(n)}}}\right]}$

The following applies:

${\displaystyle I=}$ Kovat's retention time index,

${\displaystyle n=}$ Number of carbon atoms of the first n-alkane,

${\displaystyle N=}$ Number of carbon atoms of the second n-alkane,

${\displaystyle t_{r}=}$ Retention time

Lee index

Lee's retention time indices are determined in analogy to the Kovats indices for the reference substances benzene, naphthalene, phenanthrene, chrysene and picene.

Sources for retention indices

Retention indices make it possible to use data from other laboratories for substance identification. Sources for retention indices of chemical compounds are e.g. B. the NIST Chemistry WebBook and numerous publications such as Goodner (2008) or Ciaznska-Halarewicz et al. (2003)

Individual evidence

1. ↑ Ervin Kovats: Gas-chromatographic characterization of organic compounds. Part 1: Retention indices of aliphatic halides, alcohols, aldehydes and ketones. In: Helvetica Chimica Acta , 1958, 41 (7): 1915-32, doi : 10.1002 / hlca.19580410703 .
2. ↑ ML Lee, DL Vassilaros, CM White, M. Novotny: Retention indices for programmed-temperature capillary-c Column gas chromatography of polycyclic aromatic hydrocarbons. In: Analytical Chemistry , 1979, 51 (6), 768-773.
3. ↑ NIST Chemistry WebBook (available online)
4. ↑ KL Goodner: Practical retention index models of OV-101, DB-1, DB-5, and DB Wax for flavor and fragrance compounds. In: LWT-Food Sci Technol 2008, 41, 951-958.
5. ↑ K. Ciaznska-Halarewicz, T. Kowalska: A study of the dependence of the Kováts retention index on the temperature of analysis on stationary phases of different polarity. In: Acta Chromatographica , 2003, 13, 69-80.