# Limit of Detection

## Exercise 1 – Limit of Detection (LOD)

There is lead in our drinking water and in the dust we breath. The CDC reports that half a million children have blood lead levels above the harmful threshold (50 µg L–1). Lead in our drinking water often comes from the pH-dependent dissolution of Pb2+ ions from water pipes made entirely of lead or from the lead solder connections. In fact, students in Portland are currently banned from using their schools’ water fountains due to high concentrations of lead. One way to measure lead in water (or blood) is by flame atomic absorption (AA) spectroscopy.

1. Why would it be important to know the LOD for your AA method?

1. Data from a batch of Pb standards measured by AA are shown below. What data are you missing in order to calculate the LOD?

Concentration (ppm)

Signal

NET (blank corrected) Signal

0

0.136

0.000

10.

0.721

0.585

1.0

0.195

0.059

0.10

0.142

0.006

0.010

0.137

0.001

1. Now, let’s calculate the LOD as a class…

## Exercise 2 – Complex matrices

Consider a water sample containing an analyte of interest within a matrix that was so complex (full of interferences) that you rightly suspect that matrix effects will bias your data and that a “true blank” (all of the matrix lacking only your analyte of interest) will be impossible to make.

1. What calibration method would you choose? Why?

1. How will you determine the limit of detection for your method? (Hint: you will need to come up with a way to estimate the standard deviation of the “true blank”)