Chromatography Forum

Hi all,

Someone told me that splitless mode is better for complex mixtures like oil compared to split mode. He gave the logic that due to different boiling points, not all compounds get volatilized at once and the split valve remains open all the time so more volatile compounds might escape before the entering the column. And the fraction which enters in the column is not representative of what is actually in the sample Is this true?

What are the pros n cons of using splitless versus split mode for complex mixtures?

Further, I have straight crude oil samples and water column samples (which could have trace levels of hydrocarbons). I am thinking of using split mode for straight oil and splitless for water samples. Then we compares chromatograms for quantitative purpose. But again, someone told me that that could give some error in quantitative comparison between straight oil and water samples.

I will be grateful for your insight. Also. i would appreciate if you refer me to the literature.

Best regards,

Hey there,

I can give you an opinion based on personal experience. I always use split mode for samples with a large range in boiling point. I find splitless is to much of compromise. No matter how you tune the inlet parameters you will either lose the front end or the back end. I also find split to be a lot more reproducable for large boiling ranges.

Using split mode for crudes and splitless for dilute samples will show up the biases mentioned above. I run a 150:1 split for crudes and a 10:1 split for dilute samples and it works fine. Although of course the words dilute and trace are fairly ambiguous.

The water samples will probably give you all sorts of issues if you don't extract them into a solvent first.

Good Luck

Rich

Water on anything but a packed column can cause issues. I think the biggest issues is due to the expansion after volatilization - water expands immensely (1 ul will expand to 1010 ul of gas volume vs. like hexane where 1 ul expands to 140 ul).

Chroma11,

On-column (which really means temperature programmable on-column) is the best way to get the wide range of hydrocarbons that are present in oil samples.

For the water samples, the advice about extracting into a solvent is top notch. Then split (low split ratio) or splitless will both work well, depending on your objectives.

Best regards

On-column (which really means temperature programmable on-column)

Or better still Cold On column Carlo Erba (now Thermofisher) introduced the Cold On Column injector with secondary cooling that controls the evaporation zone using compressed air. This injector works great for a wide range of Hydrocarbons (I have a trace of up to C120 on waxes) It can even be used for direct Aqueus injection although you do suffer problems with inorganics precipitating at the evaporation point.

I would strongly advise against Splitless for Water injections, split if you have to, preferably with a special water injector liner. Extraction is the way to go.
Spilt and Splitless both suffer from discrimination on Hydrocarbon analysis, some designs suffer more than others. For wide range Hydrocarbons I have always recommended Cold On Column but I have seen good reults from PTV and temperature programmed on columns. Recovery of C40 + from Split/splitless is very poor .