Hello, My problem is as follows: The lab I am working on requires the construction of a calibration curve from the measured absorbance of samples of known phenol concentration to intrapolate the phenol concentration of two unknown samples. I have constructed the calibration curve and determined the equation of the line. The problem is that when I attempt to solve for the concentration of the unknown samples (in ppm), I get a negative value. I am not too sure how to correct this, I attempted changing the y-intercept to 0 but I do not think this is correct as it manipulates the "line of best fit". Is there anyone who knows what I am doing wrong?