Retention Index Problem

[crybaby]

Hi all, I have a problem with retention index and hope to get some comments from experts.

I am trying to do identification of peaks with GC-MS peak library search and retention index. I have run alkane ladder as the standard to calculate the retention index of peaks of interest. Let say for one of the peak of interest (at retention time 8.1314min), the library search gave a result of that peak being napthalene, the experimental retention index is 1210 whereas the library retention index for napthalene is 1179. I tried running napthalene by itself under the same experimental condition and its retention time is 8.1315min. So, it is obviously that the library search gives a good result but I don't understand why can the retention time different by so much.

I hope any expert can help me with this problem. Thanks.

[Don_Hilton]

One thing you have not mentioned is stationary phase. Retention indicies vary by stationary phase, so are you lookikng at tabultated RIs for the stationary phase you are using?

[Peter Apps]

And to add to what Don says; retention index also depends on temperature and programme rate, and on polar phases can vary a lot between different manufacturer's columns. Use literature RIs as a guide, not as gospel.

Peter

[wraymogg]

Hi,

exactly like Don and Peter said, the conditions must be identical.

For example if you look inside NIST MS Search, and query for Naphtalene, you will find:

2. Value: 1170 iu
Column Type: Capillary
Column Class: Standard non-polar
Active Phase: PONA
Column Length: 50 m
Column Diameter: 0.2 mm
Phase Thickness: 0.5 um
Data Type: Linear RI
Program Type: Ramp
Start T: 50 C
Heat Rate: 2 K/min

This means you will have to respect ALL of the conditions specified above.

Regards,
Vlad