Higher Alkanes

[Propanal]

Hello,

I wonder if anybody could help me. We use a 6890 GC and a 5975 MSD (both Agilent) together with a Innowaxcolumn. since last week I have large peaks of higher alkanes irrespective which inlet I use (split/splitless or PTV) starting at 180°C or if I do Headspace of air or just run the system. I suppose I can buy a new column. But I don´t want this to happen again. So, where can this be from?

Thank you,
unlucky propanal

[Consumer Products Guy]

First: let's establish whether you are using a quality oxygen trap on your carrier gas, and what final temperature you go to.

[Propanal]

hello,

thanks for the sudden reply. we are having a big universal trap, installed in april this year. the carrier gas is helium. the final temperature is 240 °C.


the technician suggested to condition the column at 245°C over 1,5 days. So I did. The result is very high baseline which jumps from 30.000 at 50°C start temperatur to 720.000 from 180°C on.

I did not uninstall the gas trap so far, maybe I should try, before the new column is bad, too.

I have had the problem with a DB 5ms, but with phtalates eluting at high temperatures instead of higher alkanes. I can not imagine that I ruined both columns by installing and uninstalling three to four times.

Propanal

[zokitano]

The maximum operating temperature for INNOWAX column is 260/270C
So if you conditioned this column at 245C for 1,5 days, I'm sorry but probably your column now is really out of use (you are gaining very high baseline -> bleeding from the column)

Best wishes

[Propanal]

Hello Zokitano,

thanks...
When I get a new Innowax, at which temperature should I condition this column and how long? What would you suggest?

Thank you again,
Propanal

[zokitano]

Hi,

Your first approach should be to look at the paperleaf inside your new column paperbox. There you should find maintenance instructions about caring the column, limit temperatures of use. But, eventually if the manufacturer does not provide you with such of information, try to search the WEB for your specific column to find additional information for it (for example the web site of the column's manufacturer)
I know that every new column before use should be conditioned over night at constant flow of mobile phase and constant temperature.
For example if you're planning to use the new column the next day, previous day set it at an ambient temperature - noncontrolled (if the lower temperature limit of the column is lower than 20C) or at temperature of 30 degrees celsius (again depends on the lowest working temperature prescribed from the manufacturer). Never use temperatures that are out of the temperature range of the column. Also, never use high temperatures to condition your column, although you're conditioning it over night (shorter than your previous conditioning of 1.5 days)

Hope that this could help you with your further columns and analyses

Best wishes

[Bruce Hamilton]

I'm not sure whether this thread has correctly identified your problem. I'd expect higher alkanes to come from contamination, rather than thermal degradation of PEG - but I've not used a MS to confirm.

If your InnoWax column is thin film - as I'd expect for MS, then it should easily have survived your 1.5 days at 245C - provided your carrier gas flows were maintained whenever the column was at above 40C temperature, and the carrier gas was suitably purified. Oxygen and temperature are the main killers of PEG columns.

A 1um film InnoWax column has a lower ( 240C ) MAOT, but if the carrier is pure it should cope, but just produce a higher bleed. The Innowax column is supposed to take higher MAOT ( 260/270 ).

Also, PEG columns have minimum temperatures, in this case 40C, so once you're sure you've completely flushed air out of the column, you should start at 40C, and slowly ramp up to your initial temperature over several hours. Hold there until baseline is stable, then slowly ramp up to final temperature and condition for a few hours until the baseline is stable.

Don't take PEG columns higher than you need to, the higher the temperature, the more likely the column will degrade with trace oxygen.

I'd be looking for hydrocarbons from other sources, such as contamination - eg on autosampler vials, injectors, septa, in your carrier gas, etc. One possible source is sample solvents, especially if you are preparing samples by concentration, and using low MW alkane or aggressive chlorinated solvents. I'd put in a very short low-bleed non-polar column and see what sort of baseline eventuates after overnight carrier flows at low oven and normal injector temperatures using blank injections.

If the same HCs appear in your first programmed run, start looking upstream. If they don't, your column may be dead or contaminated - and you need to discover why, unless you want to make Agilent even richer.

In my experience, Innowax columns die quickly if they see significant oxygen at any temperature above 160C, so carrier gas purity is a big deal, as is ensuring that the column is always full of inert gas.

Please keep having fun.

Bruce Hamilton

[gcguy]

You could check to see if the msd is contaminated by blanking off the transfer line and then heating and cooling the detector and monitoring the output signal.

GCguy

[Propanal]

Hello,

the alkanes are not from the transfer line.
I think about the gas trap. It is a new one, but maybe I installed it wrong and now something is desorbing from the trap and is carried over in the carrier gas. Is there anything I can do wrong by installing such a trap? It is leakfree, sure, but?
Tomorrow, I want ti install the new column. Should I uninstall the trap first?

Very unsure...

Propanal

[mbulentcakar]

Hi,
If you are suspecting the trap, then give it a try but if you want to check whether it is the source of the problem without uninstalling it you may follow the standard procedure for checking carrier gas purity.
You begin with two consecutive blank runs, with no waiting in between. Then you wait for 6 to 12 hours at a low temperature then one more blank run. If your higher alkane peaks get bigger, than your trap or carrier gas lines are contaminated, if not, than the problem lies some other place, on which I would bet.
Good luck,
Bulent