Do we need extraction here ?

[Newchromatographer]

Dear friends , i would be very gratefull if you can help .There is an argure between me and my friend .He studies adsorption of modertly voltile chemical in water solution contain soil type to see just how much of the chemical is adsorped by soil ,,, ok .He filters the samples to remove solids and then extract the chemical by solid phase extraction and analyse by GC-FID .As u know We cann't just inject the filterate of water solution contain the chemical without doing extraction when use Gc because water is not detected by FID . The question is if we want to use HPLC -uV INSTEAD of Gc do we need also to do solid phase extration .In my opinion i think we don't need to do extraction with hplc because we have only water and chemical ( bearing in mind that we have already remove the soil by filteration !)

By the way i see a lot of papers do extraction for chemical in water using GC and i am surprise why they don't use hplc and save time from not doing extraction ( please not i am talking just about water contains chemical and the soil removed by filtration ) .

So please don't go far put your experience here

[danko]

As u know We cann't just inject the filterate of water solution contain the chemical without doing extraction when use Gc because water is not detected by FID.

But why would you want to detect the water, anyway? Isn’t it just the solvent in this context?

He studies adsorption of modertly voltile chemical in water solution contain soil type to see just how much of the chemical is adsorped by soil ,,, ok .He filters the samples to remove solids and then extract the chemical……….

As I understand the exercise; your colleague studies the adsorption of a certain chemical to soil, but the adsorbent (the soil) is removed (by filtration) before the extraction of the chemical takes place.
You see a problem in the conception?


Best Regards

[sassman]

If you use HPLC, then only filtering (or centrifuge) is required since a sample dissolved in water is compatible with HPLC. There is no need to do extraction in this case. Perhaps some people are using GC for analysis because their compound does not get good response on HPLC.

BTW, it is not the detection of water that is a problem for GC. The problem is that water has a very high degree of thermal expansion causing a small "explosion" inside the injection port as it vaporizes resulting in bad peak shape.

Danko: Most of the time, the concentration of chemical is determined in the aqueous phase and the remainder is assumed to be sorbed to the soil. A better procedure would be to extract the soil with solvent after removing the aqueous phase so that both the aqueous phase and sorbed analyte can be determined.

[Newchromatographer]

many thanks for you guys , i am gratefull for your answers especially sassman

thanks

[danko]

The problem is that water has a very high degree of thermal expansion causing a small "explosion" inside the injection port as it vaporizes resulting in bad peak shape.

I’d say it’s a question of injection volume, as well as injection port temperature. I'm not a GC expert but I’ve seen people injecting mixtures containing up to 80 % water.

Danko: Most of the time, the concentration of chemical is determined in the aqueous phase and the remainder is assumed to be sorbed to the soil. A better procedure would be to extract the soil with solvent after removing the aqueous phase so that both the aqueous phase and sorbed analyte can be determined.

If I were going to use something else than water for a sample solvent (e.g. ethanol, methanol, or whatever), I would certainly extract my analyte in that solvent from the beginning. Also, here one needs to consider the solubility of the compound of interest. Is it equally soluble in water and in the organic solvent? What if the compound stays adsorbed on the soil particles and never dissolves in the water?

I was just trying to emphasise the need for careful planning of the exercise in question.

Best Regards

[orcicdejan]

SPE (or some other extraction) might also be needed if you want to concentrate your sample (e.g. from 100 mL of extract to 1 mL of final solution to be injected).

[Don_Hilton]

The extraction with water has the same problem as any other solvent used for extration - it is all about partition coefficients. If the soils used all give the same partition coefficient and constant quantities of soil and water are used, this will work. The nature of the partition coefficient can be determined by spiking and extracting differing ratios of solvent (water) and soil.

On the matter of injecting water into a GC - Yes, it can be done. I've done it. If you are doing an analysis of distilled spirits, you are injecting a mixture that is mostly water - and you have no choice. But given the choise, I will design a sample preparation to avoid injection of water. I've even extracted a polar analyte out of hexane with propylene glycol to avoid injecting water. (Yes, I know that propylene glycol has some water in it.) I don't know if it was a better idea - but the analysis did what I needed it to and the report went into the file.

[orcicdejan]

But avoid injecting water onto polydimethylsiloxane-based columns... they don't appreciate it very much

[sassman]

If I were going to use something else than water for a sample solvent (e.g. ethanol, methanol, or whatever), I would certainly extract my analyte in that solvent from the beginning. Also, here one needs to consider the solubility of the compound of interest. Is it equally soluble in water and in the organic solvent? What if the compound stays adsorbed on the soil particles and never dissolves in the water?

As I understand the OP, the purpose of his experiment is to determine how much of the chemical is sorbed to the soil from an aqueous solution. Therefore, the first step must be equilibration with a solution of the compound in water. After analysis of the aqueous phase, it may be a good idea to extract the compound from soil into organic solvent (assuming that it is extractable with solvent) to be sure that what has disappeared from the water phase is actually sorbed and not degraded.

But avoid injecting water onto polydimethylsiloxane-based columns... they don't appreciate it very much

The water may cause problems with peak shape, but should not damage the column. What you need to worry about is what is dissolved in the water. Strong acids or bases are VERY bad for your column. High concentrations of salts are no volatile and will build up in the system.

[Jumpshooter]

The rationale and set of of experiments that were proposed and described by various posters were all well-observed. Ultimately, they bring to fore the "classical" issue of sample prep and analyte detection & quantification. Notwithstanding the unexpected result of poor chromatography caused by either chemical modification of the anaylte by extrac'n or (im)miscibility between extrac'n solvent and mobile phase used in the run. Additionally, key issues related to %Recovery of analyte [retained vs. isolated] can be illumined.