Aqueous samples on GC with RTX-1 or RTX-5 columns

Basic questions from students; resources for projects and reports.

3 posts Page 1 of 1
I want to play around with quantifying ammonia in water at very low levels. Of course, it has been drummed into me that GC columns don't like water, but I note that Restek's "volatile amine" compound has a chemistry that isn't terribly exotic.

I have some "junker" columns of unknown history sitting on my shelves, stuff like RTX-1 and RTX-5. I won't be sad if they die early. But are water injections really all that bad on a column? I know I'll have to use a reduced volume because the water will flash to steam and overload the inlet liner volume, but I can work around that.
Water injections will work on most crosslinked phases like what Restek produces, so damage from water is not the main concern, especially with a low injection volume or high split ratio.

What you will need to watch out for with those columns will be peak distortion due to the fact you are injecting highly polar solvent onto a highly non-polar stationary phase. If the oven temperature is below 100C, then the water can condense on the column. If the column is polar, then you get a nice smooth film of water on the initial portion of the column which can actually enhance the chromatography. If the column is non-polar, the water will tend to form droplets or an uneven film since it will tend to bead instead of making a film.(think water on a well waxed car finish) This can distort your peaks and lead to poor chromatographic performance.

To make a column work with a dissimilar solvent, you can add a guard column/retention gap to the head of the column that has a proper polarity deactivation layer on it to let the solvent film form on the cool column. Once the temperature ramp begins, it will again evaporate and pass through the column in a much better shaped peak. The same works for the analyte of interest, it can help to sharpen the peak with the proper guard column attached. For the ammonia you may want to add a base deactivated guard column to help it form a better peak on the non-polar column.
The past is there to guide us into the future, not to dwell in.
I see no problem with injecting water, except the danger of flashback - but you already knew that.

Good luck getting retention for NH3 on a non-polar column, unless you have LN2 or CO2 cooling on your oven.

I would also recommend a polar phase, for sure.
3 posts Page 1 of 1

Who is online

In total there are 10 users online :: 0 registered, 0 hidden and 10 guests (based on users active over the past 5 minutes)
Most users ever online was 599 on Tue Sep 18, 2018 9:27 am

Users browsing this forum: No registered users and 10 guests

Latest Blog Posts from Separation Science

Separation Science offers free learning from the experts covering methods, applications, webinars, eSeminars, videos, tutorials for users of liquid chromatography, gas chromatography, mass spectrometry, sample preparation and related analytical techniques.

Subscribe to our eNewsletter with daily, weekly or monthly updates: Food, Environmental, (Bio)Pharmaceutical, Bioclinical, Liquid Chromatography, Gas Chromatography and Mass Spectrometry.

Liquid Chromatography

Gas Chromatography

Mass Spectrometry