Détecteur à phosphore et azote

4 different dector were used. The FID, the  Olfactometric detector for phenolic and aldehydes compound. For the olfactometry, the eluant was diluted with a large volume of air (20ml/min) humidified with an aqueous copper sulfate solution to prevent fungi to grow. They also did some analysis with the MS detector. Finally, nitrogen- compounds were also analysed with NPD detector which we are going to detail a bit know. 

The NPD Nitrogen-Phosphorus Detector has a linear response selective to organic compounds containing nitrogen and/or phosphorus. The NPD also responds to normal hydrocarbons, but approximately 100,000 times less than nitrogen or phosphorus containing compounds. Due to its selectivity and sensitivity, the NPD is often used to quantify  compounds like drugs and pesticides. Nitrogen is the carrier gas of choice for the NPD detector, but helium is often used, especially when other detectors are installed on the same GC as the NPD.  If the injection and the calibration were well done, its possible to quantify solution up to a concentration of ppt ( 10-12 kg/L). Here we analyse thiazole and pyrazines in natural matrix like beer and chocolate and that kind of compound are always at trace levels in natural matrix   

But how does it works ?  

A concentration of hydrogen gas is used such that it is just below the minimum required for ignition. A rubidium or cesium bead, which is heated with a filament ignites the hydrogen (by acting catalytically), and forms a cold plasma. Excitation of the alkali metal results in ejection of electrons, which in turn are detected as a current flow between an anode and cathode in the chamber. As nitrogen or phosphorus analytes exit the column, they are adsorbed on the metallic bead before being exited by heating and eject electrons to, resulting in an increase  current detected by the collector electrodes .Due to his high sensitivity it’s important to calibrate the NPD detector with solution of know concentration. Since the alkali metal bead is consumed over time, it must be replaced regularly.  

Finally, with these method they could quantify pyrazine and thiazole at the range of 50 ppt with the optimization of liquid liquid extraction by playing on the pH and also replacing the steam distillation by a vacuum distillation. 

The main odorant compounds of these sample were aldehydes and they could be quantified around the ppm in each beer.

Phenols in beer could be quantified up to ppb concentration with the use of eugenol as intern standard and dodecane as extern standar before concentration.  

For the quantification of volatile compounds in the chocolate, decane was used as an external standard. They also used the standard addition method for compounds that were commercialy available. 

Thank you for listening !

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