I am having trouble figuring out how to calculate the detection limit. Here is the question:
You and your friend working in a different lab have been given an unknown water sample that contains potassium. You are both asked to measure the potassium concentration in the solution (10 replicate measurements). The methods that you and your friend are using are different. The results that were obtained are listed below. Determine:
(a) the concentration, standard deviation and relative standard deviation for the unknown as measured using the two methods (check for outliers!),
(b) calculate the detection limit (3sigma) for each method,
(c) compare the standard deviations and evaluate whether the two averages are significantly different (or not) at the 95% confidence level.
RESULTS:
Standard Method 1 Method 2
Concentration Intensity Intensity
(mg/L) (nA) (mV)
0.000 0.624 1.955
0.000 0.488 2.490
0.000 0.522 2.166
0.000 0.355 1.500
5.000 9.245 15.644
10.000 17.069 31.220
15.000 26.200 44.266
20.000 33.881 62.394
25.000 43.826 75.611
Replicate
1 26.544 46.977
2 25.449 49.120
3 21.053 50.998
4 24.353 46.615
5 23.899 49.326
6 24.010 46.666
7 25.554 45.291
8 23.549 42.995
9 26.008 43.678
10 24.404 49.012
So my problem is with part (b). I know the detection limit is calculated with the formula DL = 3sigma/slope
and that I can find the slope by calculating Sxy/Sxx
but I'm not sure what points I'm supposed to use? What points do I use for the standard deviation (sigma) and for Sxy/Sxx?
I've tried looking at examples in the textbook but nothing was really similar. I also tried searching online, but I couldn't find anything that helped me.