I am asking about the situation of how to find out the concentration of organic compound in a poor solvent for them - water.  I am looking forward to expertise input in giving me a reliable method. Thanks.

Dear Kan,

I think you can use the IR specroscopy. The intesity of characeristic lines in the specra is relaed with the concetration of the molecules. For example, you can prepare set of solutions with known concetrations and plot the calibration line (dependence of some characteristic line intesity on concentration). Using the calibration line you will determine the concetration in studied sample.

Sinserely,

Sergiy.

Hi Sergy,

Thanks for help. Would you further elaborate on how reliable IR could be?  The solubility of hexane in water is less than 1 ppm. I thought of using an LC-MS, but clearly, that's a bulky and expensive instrument. Other thoughts?

Regards,

Kan

Depending on the volatility of the compounds there are several approaches.   For volatile and semi-volatile compounds one can use headspace gas chromatogaphy (GC) or gas chromatogaphy-mass spectrometry (GC-MS).  For non-volatile compound (bp>200C) one can use a solvent extraction or solid phase extraction followed by GC or GC-MS.  There are numerous papers in the literature that describe the determination of organic compounds in water.  There are also US EPA methods. 

For some organics substrates in water, SBSE can be a good technic for sample preparation then analyse by GC/MS or HPLC/MS.

If it has low solubility, concentration is low, so, the best method is HPL.C-MS.

Ig it is soluble in solvents, may try GLC.

Luis, do you mean gas-liquid chromatography?

Of course.

Dear Kan,

You could also consider solid-phase microextraction (SPME) and then simple GC-FID (or GC-MS) depending how complex your samples are.  Headspace or direct immersion SPME could work. For headspace extractions, stirring and low heat tend to improve sample recoveries.  SPME approach could give your ppt-to-ppb range of detection limits with GC.  SPME is reusable and does not need solvents for extractions. 

Jacek

You would need to characterize the contaminants, then apply the proper technique for extraction and analysis.  If concentrations are expected to be low some type of enrichment procedure will be necessary in order to detect the compound of interest.  Many of the posts already cover this.  If you are looking at volatile organic compounds purge and trap GC/MS would be the gold standard, although good detection limits can be achieved with lower cost analytical instrumentation such as head-space GC with appropriate detector like FID, PID, ECD.  In this manner you could go low tech / low cost by just manually injecting headspace onto a field portable GC with appropriate detector.  Generally speaking though you really want to spend time and money up front characterizing what contaminant you have, then once you know what you have you can customize a low cost solution for quantitative analysis.  In your example of hexane though a low cost solution would be headspace GC/FID, you would need to start the GC off at a very low temp.  Many other alcohols and water soluble compounds can be measured in water with this set-up just by direct injection of the sample into the GC.

Here are 2 links to get you started, the 1st is how you should approach method development, the 2nd (NEMI) is called the National Environmental Methods Index.  This allows users to search for all published methods for a given analyte.

http://www2.epa.gov/sites/production/files/documents/schumacher.pdf

https://www.nemi.gov/home/

Thanks Peter.  By saying "..you would need to start the GC off at a very low temp", which part of the GC are you referring to and why?

The oven temp, if the oven temp to high to start the hexane peak would come out in the dead volume and would not be resolved or quantifiable.  A good trick with GC/FID is to inject ~ 2uL butane headspace - this is un-retained and will give you T=0, we optimize the GC to get u-opt using butane.  After optimizing your GC (flow rate / head pressure) using the butane, now you know what your T=0 , now inject standards, and optimize temps (inj port, oven, detector) to resolve your compounds of interest.

there are specific methods for your problem - Headspace analysis, SPDE, SPME and  other methods with preconcentration of a very small amounts of non-water soluble organic compouds. Your method depends on systems in your laboratory. All classical methods with temperatures, flows of carrier gas (....) are described in a lot of sheets on webs of firms like Restek, Agilent technologies, ...

A few years ago I worked with a system using stir bar sorptive extraction. It works like SPME but more sensitive as long as the logkow > 4; as it is in your case (low solubility compounds). It was marketed by  Gerstel, who sold also the GC MS setup (desorption of the bar by heating; cryo trapping using a nitrogen cooled injection system). Not cheap, but worked well (no affiliation).

Succes!

HPLC will be your best option, because you can use the sample directly and the detection would be good 

The organic solvents can be detrmined using Gas chromatography equipped with a head space & a capillary column preferably with mid polar stationery phase, though a temperature programming from low to high can be tried on other columns too. You can find good method and column reference in United states pharmocoepia under section of residual solvents/organic volatile impurities