I study the abilities of my microbial samples to degrade BTEX. I analyze the change in the concentration of each component by extracting 100 ul from the headspace of a sealed vial and analyzed by GC-FID.
Here are the equations of my standard lines plotted against 5 points of concentration and area of the peak:

Benzene: y = 0.0003x - 6.0626: R^2 = 0.9949
Toluene: y = 0.0004x - 25.957: R^2 = 0.9902
Ethylbenzene: y = 0.0007x - 63.233: R^2 = 1
p-Xylene: y = 0.0005x - 41.988: R^2 = 0.9995

My initial concentration for each component is 100 mg/L. At the end of the experiment, I obtained concentrations of -6 and -5 for benzene; -22 and -21 for toluene; -32 and -29 for ethylbenzene and -35 and -30 for p-xylene using the above equations. I haven’t read any paper that reports results with negative concentrations. Although I think that many researchers experience the same. May I know on how you deal with these data? I have read other papers that report not detected. Are negative results reported as not detected?

Similarly, using the equation below:

n-butylbenzene: y = 0.0004x + 41.375: R^2 = 0.9995

How do you report for a peak with a very small area or an undetected peak? Do you follow the standard curve of doing something else.

Many thanks.