Adsorption of Benzene using Activated Carbon - Increase in concentration!? I'm not quite sure where to post but here it goes. I'm performing an adsorption experiment of benzene from solution using commercial activated carbon. It is expected that the concentration of benzene would decrease with time since it gets adsorbed on the carbon, but in my case the concentration increased!!! I'll list the steps I used for my experiment first and all the details for preparation, experimentation and analysis: I'm performing an adsorption experiment of benzene from solution using commercial activated carbon. And I'm using a gas chromatography fitted with a flame ionization detector for the analysis and an auto-sampler. Also I'm using the internal standard method for making a calibration curve and determining my solution concentrations after adsorption. I've prepared a stock solution of benzene with ethanol as my solvent. I then made three standard solutions of different known concentrations by diluting the stock solution. I'm using glass volumetric flasks with glass stoppers. I used ethyl acetate as my internal standard which is also dissolved in ethanol. Similar to the benzene solutions, I made a stock solution and diluted it down to make a single standard solution of known concentration. The internal standard is not introduced into the adsorption experiment, it is only added for analysis. For the experiment part, I use 40mL glass vials with a PTFE screw caps and rubber seals. In the glass vial I added 50mg of activated carbon and filled it up with my standard solution and ensured minimum headspace to avoid any vapor losses. After specific times, I filter this solution and centrifuge it to discard any activated carbon particles. To create the calibration curve, I add 0.5mL of my internal standard and 0.5mL of my benzene standard solution into a 2mL glass vial with PTFE screw cap and rubber septum. This is then injected into the GC and I record the area of the ethyl acetate and benzene peaks. Taking the ratio of the area of benzene to the area of ethyl acetate as my y-axis and the concentration of my benzene standard solution on the x-axis I use a spreadsheet to calculate to the equation of the line. Using this equation I'm able to calculate the concentration of my experimental samples after adsorption. This is where things get weird. I did an experiment using 400ppm benzene solution and after 24 hours, I checked the concentration expecting the concentration to decrease or remain the same (logically it should decrease). BUT it increased! Can anyone help explain this? I've repeated the experiment several times and each time the concentration increases to higher than the original solution concentration.