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Instructor's Notes

Gunshot Residue Analysis by Electrothermal Atomic Absorption Spectrophotometry

References

Saferstein, Richard.  Criminalistics: An Introduction to Forensic Science, 7th edition, Prentice Hall: Englewood Cliffs, 2001; pp 435-438.

Encyclopedia of Forensic Sciences, Jay A. Siegel, ed., Academic Press, 2000; pp 953-962.

Krishnan, S.S. "Detection of Gunshot Residue:  Present Status" Richard Saferstein, ed. Forensic Science Handbook. (Prentice Hall: Englewood Cliffs, 1982) pp 572-591.

Midkiff, C.R.  "Analytical Methods for the Detection of Firearms Residues"  Mat H. Ho, ed. Analytical Methods in Forensic Chemistry. (Ellis Horwood: New York, 1990) pp 390-404.

Goal

To determine if someone recently fired a gun; to determine the lead content of handwashings.

Suggested Methods of Analysis

Electrothermal (graphite furnace) atomic absorption spectrophotometry

Solution Preparation

1% (v/v) Nitric Acid.  Dilute 2.5 mL concentrated nitric acid to 250 mL with water.

0.2% (v/v) Nitric Acid.  Dilute 20 mL 1% nitric acid to 100 mL with water.

Lead Stock Solution.  Dissolve 0.1598 g lead nitrate in 1% nitric acid.  Dilute to exactly 100 mL with 1% nitric acid.  The concentration of this solution is 1000 ppm.

Intermediate Lead Stock Solution.  Dilute 10 µL lead stock solution to 10 mL with 0.2% nitric    acid.  The concentration of this solution is 1000 ppb.

Lead Standards

Lead standards should range in concentration from 10 ppb to 250 ppb (the upper limit of the linear range for lead).  At these levels, contamination from reagents, glassware, pipet tips, storage containers etc. can be significant.  Clean with dilute nitric acid and rinse with pure water everything that may come in contact with these solutions.  Store the solutions in small polyethylene bottles.

To make a 5-mL standard solution, dissolve 0.05 g of ultrapure ammonium dihydrogen phosphate (matrix modifier) in a small amount of 0.2% nitric acid.  Quantitatively transfer the solution to a 5-mL volumetric flask, and then pipet an appropriate amount of intermediate lead stock solution into the flask.  Dilute to the mark with 0.2% nitric acid.  Don’t forget to prepare a blank containing matrix modifier and diluent only.

Hand-swabbing Samples

Place 1 mL of 0.8 M nitric acid (1/20 dilution of concentrated nitric acid) on a cotton ball (swab).  Wipe the thumb web, the palm, and the back of the hand with the swab.  The process should take about 30 seconds.  The subject should rinse the hand with water immediately after the sampling is completed.  Place the swab into a plastic centrifuge tube containing 25 mL of 1% nitric acid to extract any lead into the solution.  Wait 15 min, then centrifuge the tube and remove the supernatant.  Save the solution in a marked, plastic bottle.  Repeat the process for the other hand.

In order to judge whether or not a given person has recently fired a handgun, it is necessary to show that the amount of lead found is above that expected on hands that have not fired or handled a gun.  To find this background level several hand-swabbings of individuals who have not touched a firearm recently should be collected and tested.  Washing with soap and water effectively remove trace elements from the hands, so the persons being tested should not have washed up in the past hour or so.

To prepare the hand-swabbing samples for analysis mix 0.05 g ultrapure ammonium dihydrogen phosphate and 1.0 mL of hand-swabbing sample in a 5-mL volumetric flask and dilute to the mark with 0.2% nitric acid.  For samples containing more lead, add a smaller volume of handwashing solution (perhaps 0.25 mL) to keep the lead concentration within the linear range.  Adjust the sample volumes as needed.

Atomic Absorption Spectrophotometry

Follow the operating instructions for the spectrophotometer and the lead lamp.  Have the instrument perform background correction and report the peak area with an integration time of 2.0 s.  Set up the graphite furnace in the following manner:

  Step Number

 Furnace Temperature

 Time Ramp

 Time Hold

 Internal Gas Flow

 Read On

 Purpose

 1

 140

 5

 40

300 

 

 desolvation

 2

 850

 5

30

 300

 

 ash solids

 3

 1800

 0

3  

X

 atomization

 4

 2200

 5

 300

 

 clean furnace

 5

 20

 1

10

 300

 

 cool-down

Inject 20-µL aliquots of standard and sample solutions into the furnace. 

Lead has the tendency to contaminate the graphite; some lead remains even after the cleaning step, particularly when higher concentrations are involved.  Run the blank solution often to check for contamination.

Calculations

Prepare a calibration curve (peak area vs. concentration of lead in ppb), and calculate the concentration of lead and its uncertainty in the samples.  Account for sample dilution.  Calculate the amount of lead on each hand in µg.