How to calculate the detection limit?
Have a look at this: http://www.chem.utoronto.ca/coursenotes/analsci/stats/LimDetect.html

It is possible to calculate the limit of detection from the standard error of the regression: 

$$C_{\mathrm{LOD}} = \frac{3s_{y/x}}{b}$$ These values are calculated from the regression of all the data points, not the standard deviation of any subset of data.

However, in this case we have replicate blank measurements (four measurements at C=0), so we can also calculate the limit of detection as simply 3 times the standard deviation of the blank measurements:

$$y_{\mathrm{LOD}} = y_{\mathrm{blank}}+3s_{\mathrm{blank}}$$

(in instrument response units—convert to concentration from a known standard or from the regression equation)