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# Statistical Method (Concentration and Absorbance)

1. A gun shot residue containing barium and lead is analysed using atomic absorption. Residue from sample collected in Glasgow and Inverurie are also available

Glasgow gun shot residue (n-20) Ba (o.12+/-0,005%), Pb (5.2+/-2.0%)
Inverurir (n=5) Ba (0.22+/-0,001%), Pb (7.2+/- 1.0%)

(a) Describe the necessary parts of an atomic absorption spetrometer.

(b) Analysis of the unknown residue yields the following data.

Ba+2 [mg/L] Absorbance Pb [mg/L] Absorbance
0.0 0.000 0.0 0.00
0.1 0.101 2.0 0.10
0.2 0.216 4.0 0.21
0.3 0.308 6.0 0.29
0.4 0.392 8.0 0.40
0.5 0.474 10 0.49

The residue was measured 3 times and gave absorbances and standard deviations of 0.4+/-0.1 for Ba and 0.25+/- 0.05 for Pb

What were the concentrations of both elements in the residue if 10mg were dissolved in 100mL solution?

(c) Which statistical test is needed to identify the precision and which one identifies the differences in the mean?

#### Solution Preview

a.
Atomic absorption makes use of the light absorbance properties of an element. Each element absorbs wavelengths of light that correspond to the energy needed to make an electron move to a higher energy level. The amount of light absorbed is proportional to the number of atoms. This is a very sensitive technique and can detect up to parts per billion of a gram of sample.

The three main components of an atomic absorption spectrometer are a light source (to provide the light that the sample will absorb), a sample cell (a place that holds the sample, and a detector (a component that measures the light absorbed). For further details, see references 1-3.

(see figure in doc)

The light source commonly consists of a hollow cathode lamp with a tungsten anode and a cylindrical hollow cathode made of the element to be determine, i.e. if you would like to measure lead, you use a lamp with a lead cathode. When a high voltage is applied across the anode and cathode, the atoms in the cathode are excited and produce light.

To measure light absorbance, the sample is atomized (broken into free atoms). There are two methods for doing this:
- Electrothermal atomization: a drop of sample is placed into a graphite tube that is electronically heated
- Aspiration atomization: the solution is moved by aspiration into a flame

The detection system consists typically consists of a photomultiplier tube that translates the light into an electrical signal. Before the light goes to the photomultiplier, it is passed through a monochromator. This component selects a specific wavelength of light and allows ...

#### Solution Summary

The solution analyzes the atomic absorption occuring in a gun shot's residue. It uses statistical methods to determine the statistical significance.

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