Is there any standard method for it? I dissolve them in acetonitrile but when I mixed the solution with water, precipitation occurs. How can the precipitation be prevented?
Dissove them in ethanol, and ethanol and be mixed with water. Be careful with each of the apparent solubility of PAHs.
It depends of their solubility in water. Which PAH you want to spike in water, and which is the concentration needed for your study ?
Naphthalen is easy to spike in water at concentrations up to 25 ppm.
But for PAH with more than 2 or 3 cycles, difficulties start and you have to add verry low quantity of PAH.
It is very common error to try to spike too high concentration of hydrophobic analytes. You must look up the water solubility and never try to spike higher. It would be pointless and wrong to use too high concentrations. Acetonitrile is ok but as small amount as possible.
In my reserach I dissolve PAHs in methanol and then add to water. But as mentioned in previous comments, you cannot spike higher than the water solubility of each compound.
I have used a mixture of solvents on other semivolatile and relatively hydrophobic compounds, based on work done by the U.S. EPA Duluth Laboratory's investigation of vinclozolin in fish tissue using an HPLC protocol (Makynen, L. 1999, personal communication). I used a 0.05M phosphate buffer, dissolving 6.0g sodium phosphate monobasic in 1000ml Millipore deionized water. The buffer solution is brought to pH=3.3 by diluting with de-ionized water. Start with 40% phosphate buffer, 30% methanol, and 30% acetonitrile, by volume. Change the ratios, including removal of the buffer to eliminate the precipitation. I know this is trial and error, but it worked for vinclozolin and its degradates.
Thank you all for your precious suggestions. Some people reported work on higher concentrations, so I tried for that. With above replies, I realized that working beyond the water solubility is worthless.
You can also introduce the PAHs by phase partitioning from a polymer (typically silicone), which goes under the name "partitioning driven administering" or more recently "passive dosing". In this way you avoid the addition of co-solvent and you can control freely dissolved concentrations against a number of loss and binding processes. This provides some nice features in both analytical chemistry, biodegradation research and toxicological research. You can learn more about this by searching on the Web of Science for "passive dosing". You could also have a look at some of these papers:
Smith KEC, Dom N, Blust R and P Mayer. 2010. Controlling and maintaining exposure of hydrophobic organic compounds in aquatic toxicity tests by passive dosing. Aquatic Toxicology 98: 15-24.
Smith KEC, Oostingh G and P Mayer. 2010. Passive dosing for producing constant and defined exposures of hydrophobic organic compounds during in-vitro toxicity tests. Chemical Research in Toxicology 23: 55-65.
Gouliarmou V, Smith KEC, de Jonge LW and P Mayer. 2012. Measuring speciation of hydrophobic organic chemicals at controlled freely dissolved concentrations without phase separation. Analytical Chemistry 84, 1601−1608.
This is an old question, but to work at the solubility limit in water, I add the pure PAH to water directly (the total amount I would need for an experiment), and let it equilibrate over 24 hours stirring. Then I filter the water so that any undissolved PAH will be removed. I've found that even spiking at the solubility limit using a solvent, you will get some preceipitaiton becuase dissolution kinetics are slow.
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