Problem: Running Alkane standard C8-C20

[nazar_u]

Hi All

I use GC from Varian serial 3800. I run Alkane standard C8-C20 provided by Sigma-Aldrich (Components contains C8, C9, C10, C11, C12, C13, C14, C15, C16, C17, C18, C19, C20; 40 mg/l each, in hexane), but I was deeply disappointed about the result because my GC just showed 1 peak

Before I complain about the standard to Sigma-Aldrich, I need your suggestions and help.


The GC conditions are:
Temperatures;
Injector: 270 Celcius
Detector: 300 Celcius
Column: initial temperature was 50 and increase to 270 Celcius with ramp 5 Celcius/min and keep 270 Celcius for 23 minute or 44 minute (total time for running 65 minute)

Column pressure: 10 psi

Amount of sample: 0.2 microlitter

Split ratio: 80

Column specification:
Varian capillary column (CP-sil 5 CB), serial CP7690, 50 m x 0.32 mm, ID 5
micrometer

Any suggestions and helps would be appreciated. Thank you very much.

_________________
N_U, London

[AICMM]

nazar_u,

Shoot more or split less. 40 ppm X 0.2 uL is only 8 ng on column. At 80:1 you are only putting 0.1 ng of each on column which would be difficult for an FID.

I would suspect that your alkane mix is intended for splitless/split environmental analysis such as TPH, not petroleum analysis like D7096.

Best regards,

AICMM

[Peter Apps]

AICMM's advice is good (as always).

Do you really have a 5 um film in the column ?, if so the retention times will be very long, although you should certainly see the first few components of the sample. If you blow up the chromatogram until you can see the noise in the baseline you might be able to see some very small peaks for the alkanes.

Are you running with constant pressure or constant flow ?, if constant pressure the flow rate will be very low at the top of the temperature programme, which will add to the effect of the thick stationary phase.

Peter

[Consumer Products Guy]

I agree - I think you're only seeing the hexane solvent peak. You need more molecules on column.

[Pat Crowley]

I would say that a 5 micron film is way to thick. This is giving a phase ratio of just 16. If you are using a 50 metre column I would suggest a 0.25 or at most a 0.5 micron film thickness. You would probably get away with a 25 or 30 metre column as well, thus saving a considerable amount on retention time.

[nazar_u]

Thank you for all comments