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It depends on the range of hydrocarbons being analyzed. In general the column phase and the difference in boiling points will cause the hydrocarbons to separate and the hydrocarbons will not elute into one peak. You could try a short 0.53 mm ID deactivated piece of high temperature silica tubing at a constant high oven temperature to minimize separation. If the hydrocarbons are not separating then other molecules of interest also will not separate. There is not a column that will pass the hydrocarbons without separation while separating other molecules.
CO and CO2 will not separate on a DB-1 column. After going through the methanizer, there will be a single peak. The retention time will be dependent on the application parameters (carrier flow rate, column temperature, etc). Plot columns are the best column for separating CO and CO2, however they cannot be run at high temperatures so the range of hydrocarbons would need to be less than C5.
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My expected hydrocarbon in my sample will not exceed C3 . So as your recommendation plot column like AL2O3 will work for HC , CO and CO2 with methanizer .
Yes. C3 is low enough that you will not have a problem with a PLOT column.
Agilent has a CarboBOND PLOT column specifically for separating CO and CO2 at ppm levels using a Methanizer. An application note on that approach can be found at https://www.agilent.com/cs/library/applications/A01431.pdf . Agilent also has an application note based on a Varian Analyzer at https://www.agilent.com/cs/library/applications/SI-02091.pdf that could contain some helpful information.
Alox columns don't like CO2 as it can irreversibly change the columns selectivity (CO2 is build into the inorganic structure of the phase). A porous polymer like a PoraPLOT Q or any other Q-type can separate CO/CH4, CO2, C2s and C3s (A01610):
On a 10 m column CO and CH4 are no longer separated, same for higher temperatures. Be aware that a methanizer also burns the hydrocarbons and you will coking of it.
What is your intent here? Analyzing CO/CO2 in HC or just get one peak for all HCs? In the latter case use uncoated fused silica tubing between injector and detector and you will have one peak for everything.