Hi Ihsan,

this concentration you give is really low. You could try LC-MS analytics if available for you. You would need to desalt the sample first by using a desalting column. I once had a drug to analyze with a rather low concentration - I freeze-dried one mL solution and re-dissolved in 100 µL to get a signal in the UV-detector. Alternatively GC-MS possible.

Also, another aproach could be to derviatize the aldehyde group of your cinnamaldehyde with a strong fluorescent and measure UV-VIS. There are special kits available, but I'm not sure whether it would be sensitive enough for your sample.

Please find the attached link, there are some further information available.

http://www.abcam.com/aldehyde-quantification-assay-kit-colorimetric-ab112113.html

Thank you very much for your valuable contributions @Michael Päch, @Marcus Binner. 

In LC MS/MS our operators couldn't even create a calibration curve. One of my colleague also suggested the extraction in hexane and GC-MS and furthermore I will consider UV-VIS examination.

Thank you all, 

GC-MS is the method of choice, using either solvent extraction with a low-boiling non-polar solvent (e.g. hexane, dichloromethane) or solid phase microextraction. Micrograms per ml is well within the routine operation range of these methods.

A search for cinnamaldehyde GC-MS will provide several methods to choose from.