I determined the desorption efficiency of BTEX based on the NIOSH 1501 standard at five different concentrations (0.5/2/4/50/100 mg/L). I injected 20 uL of DE stock solution onto 400 mg charcoal with a microliter syringe and allowed it to stand overnight. After that, I used dichloromethane to desorbed it and analyzed it by GC-FID. The result is showed in the picture below. I wanted to know why these lines are curved and how to apply them when analyzing samples. Thank you in advance.
Nguyen Binh, On what basis do you think that the lines illustarated above should be straight lines? The maxima on the desorption efficiency vs C line are also incomprehensible. Moreover, in order to make the problem clearer, it is worth specifying how desorption efficiency was defined. Usually the value of "efficiency" does not exceed 100% (1), but it depends on how this value was defined.
Apart from the above, you may wish to refine the methodology. 0.5-4 is a good range, but 50/100 is a very big jump to give you any meaning trends. What I mean is that the trends relay solely on one point (50 mg/L) and if you delete that, your conclusions would change completely.
Thanks to all of you Prashant Praveen Mirosław Grzesik
My procedure: Firstly, I put the front sorbent section of the charcoal tube (400 mg) into a vial. I used ORBO™ 32 Large Activated Coconut Charcoal (400/200). I prepared three tubes at each of five levels plus one blank. Then I injected 20 µL of 125/500/1000/12500/25000 mg/L directly onto the tubes with a microliter syringe and allowed them to stand overnight. After that, I added 2mL DCM to each vial. The vial was covered to avoid volatilization and stirred for a minute in vortex then it was subjected to an ultrasonic bath for 5 minutes. I repeated the extraction a second and third time by adding 2mL and 1mL DCM, respectively. Finally, the solution was filtered by a nylon filter of 0.45 µm and injected with 1.0 µL of the extract supernatant in the GC/FID. I calculated DE by dividing the area of a peak after extraction by the area of the standard solution. If I spike 20 µL of 125 mg/L, the standard solution will be 5 mg/L. The picture below shows the equation of how to calculate the sample concentration.
Hi Mirosław Grzesik
I guess that DE depends on concentrations, and if the lines are straight, I can apply it when I analyze samples in the air. Or if D is independent of concentration, I can use the pooled mean value over the concentration range. I don't know how to apply DE if the lines are curved (Do I need to find a math function to fit those DE values?) Or maybe each time I analyze a sample, I determine DE corresponding to the concentration of the sample?
I also don't know the reason why DE is higher than 100% at 50 mg/L (maybe due to contamination when I extract?)
Nguyen Binh, If, apart from desorption, there are other accompanying effects, e.g. contamination, then such results are distorted and cannot be interpreted. After the above explanations, your problem is more complicated and I thought at first. When I come up with something, I will write it.