What is the method used to detect lead in water/blood?

Flame Atomic absorption spectroscopy (FAAS)

This analytical technique is used to detect elemental species, especially metal, in the sample. It is able to determine the concentration of the particular metal.

First, the liquid solvent is evaporated, leaving the dry sample. Then, the solid sample is vaporized to a gas and mixed as an aerosol with combustible gases like acetylene and air or acetylene and nitrous oxide. Lastly, the compounds of the sample are broken into free atoms by using a flame of temperature from 2100 to 2800 degree C. Atoms of the element of interest (in this case, lead) will be reduced to its atomic state.

Radiation of 283.3nm is sent through using a hollow cathode lamp. A beam of light from the lamp is focused through the flame. The cathode of the lamp is made from the same element being determined. The beam passes through the flame into a monochromator and then a detector. The monochromator consists of mirrors which help to separate the beam from any background noise and light.

Free, unexcited atoms at ground-state of the element absorb light at a specific wavelength, and the reduction of light energy detected is the quantitative measure of the concentration of the element in the sample.

An example of a detector is the photomultiplier, which is an extremely sensitive detector for UV light and visible light. It can detect the amount of reduction of light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.

This diagram shows how the photomultiplier works:


Light enters the photomultiplier tube and strikes the photocathode layer. This causes electrons to be emitted.

The electrons will be directed towards the electron multiplier. The electron multiplier is made up of many electrodes, called dynodes. The dynodes all have different voltages, where each dynode has a higher positive voltage than the previous one. This is so that electrons can be multiplied.

The photoelectrons are electrostatically accelerated  towards the first dynode because it is positive. They hit the first dynode with greater energy than when they left the photocathode. More low energy electrons are released, which are in turn attracted to the greater positive field of the next dynode, and they are similarly accelerated by the positive field of the next dynode. This process keeps repeating and electrons will keep hitting dynodes and releasing more electrons until they are collected at the anode.

The accumulation of charge hitting the anode results in a sharp current peak, amount of light transmitted can be measured and thus the amount of light absorbed by the analyte can thus be determined.

http://www.galbraith.com/spectroscopy.htm
http://www.radio-electronics.com/info/data/thermionic-valves/photomultiplier/what-is-a-photomultiplier-basics.php
http://www.et-enterprises.com/photomultipliers/understanding-photomultipliers

Method for Quantitative Analysis
1) Select Method
-  Flame Atomic absorption spectroscopy of contaminated water.

2) Obtain a representative sample
-  Obtain contaminated water.

3) Prepare a lab sample
- Water is filtered through a 0.45 um membrane filter.

4) Define replicate samples
- Three 5ml water samples are used.

5) Dissolving samples
- Acidify the filtrate with 1:1 redistilled HNO3 to a pH of less than 2
- Discolouration in solution may indicate presence of metals (dilute if colour is too strong)

6) Eliminating Interferences
- Increase heat of flame to eliminate interferences.

7) Measuring amount of analyte
- Measure concentration of metal in solution using FAAS
- Calibration graph

8) Calculating results
- Using the calibration graph and lead standards, determine whether the water has been contaminated with lead.

9) Estimating the Reliability of Results
- Average of replicate samples

http://www.caslab.com/Test-Methods-Search/PDF/USGS-Method-I-2403.pdf
http://www.ffcr.or.jp/zaidan/FFCRHOME.nsf/7bd44c20b0dc562649256502001b65e9/146fd852cd5e269049256f32001a133e/$FILE/B20.pdf
http://www.ifrj.upm.edu.my/19%20%2803%29%202012/%2822%29%20IFRJ%2019%20%2803%29%202012%20Eka.pdf
http://sisbl.uga.edu/epaman4.html#partition

Why use FAAS?
Metals in their ground state absorb light at specific wavelengths. Metal ions in a solution are converted to atomic state by means of a flame. Light of the appropriate wavelength is supplied and the atoms will absorb them and be promoted to their excited state. This state does not last long and gets reverted soon, emitting back the photon that was absorbed. The amount of light absorbed can then be measured against a standard curve.

However, the disadvantage of using this technique is that when absorbance becomes higher than 0.5-1, the calibration line becomes a little non-linear. And some elements like B, Br and C cannot be detected at all using this technique.

http://faculty.sdmiramar.edu/fgarces/LabMatters/Instruments/AA/AAS_Theory/AASTheory.htm