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Nov 09 2020
Maximizing DNA Yield from Fired Cartridge Casings
Crimes involving guns have become increasingly prevalent in our society and investigative techniques for solving these crimes have been limited due to several factors. One factor is that the evidence left behind is commonly comprised of spent cartridge casings or stray bullets. These items are typically analyzed with firearms comparisons since DNA analysis has not proven to be successful in the past when using the typical collection technique of swabbing.
Some reasons for this include the fact that only a limited number cells are deposited on bullets when loading a gun, other factors such as the shedder status of the person handling the bullets and the environmental conditions.
Many laboratories have instituted policies where spent casings are not analyzed for DNA and investigators are left with firearms analysis only. The Broward Sheriff’s Office Crime Laboratory and Crime Scene Unit have worked together to optimize a DNA collection method that allows for approximately 41% higher DNA recovery when compared to swabbing techniques and yielded DNA profiles consisting of approximately 68% more alleles.
During her presentation at ISHI 31, Karin Crenshaw briefly described the optimized collection method itself, data results from several different controlled studies utilizing hundreds of bullets as well as observed results from forensic casework. As there was not enough time to answer all of the questions that came in during her presentation, we’ve compiled them here.
What kind of DNA extraction method did you use? Don’t solid phase extraction methods such as DNA IQ or Prepfiler wash away ions during the purification process?
We use the DNA Investigator Kit on the EZ1 system. Some ions are washed away, however, there is an overwhelming amount and it apparently interferes with the amplification process.
How many casings do you typically process together at one time?
We did a large study and found that about 5 was best; however, due to the large variability with handling times/shedder status etc., it is possible to obtain results with fewer casings.
How did you measure the numbers of cells you used and what was the source of cells?
We collected saliva samples, did a 1:1 dilution and took a small sample and counted them using a microscope. These were buccal cells.
Did you see correlation between DNA Quantity and number of alleles obtained?
The DNA quantity appeared to correlate pretty closely with the number of alleles obtained. The differences were mainly observed in the quality of the profiles (peak height ratios).
Are you using the whole filter or are you retaining any for future/defense testing?
We used the entire filter during our studies. In casework, some analysts take half and have gotten profiles and other use the entire filter (it will fit in a normal 1.5uL tube)
Do you have a firearms examiner examine casings before combining to ascertain that they all may have been fired in the same gun? If so, how do you prevent inadvertent contamination?
No, we do not. The Crime Scene Detective who collects the casings will determine the calibers of each and based on case scenario (to include location where they were found) will decide which casings can be combined. Casings of different calibers are never combined. If there is some inadvertent mixing, we have STRmix that can assist with analysis/statistics if needed.
What’s your minimum cut-off for DNA input?
We require 0.002 ng/uL for amplification (less than that and the sample is stopped, based on our validation of Quant Trio).
Will you do another study to further look at recovering DNA from the live rounds?
Our studies showed that live rounds gave a slightly lower recovery, and since fired casings are the typical evidence item at crime scenes, that is what we chose. Based on the results, I would expect live rounds to benefit in a similar way with the processing method we developed.
What was the success rate with the previous processing method for casework? Did you look at that? How many swab(s) do you use to collect per cartridge case?
We did not have any success with previous methods for years and had already implemented a policy where swabs of casings were not accepted at the lab. We do not swab, we are using this new method exclusively. Our comparisons to swabbing in our studies were using large numbers of cells and we used only one swab to collect the cellular material and in some cases used the one swab for multiple casings to make it equivalent to the Chelex methods.
Would your method possibly destroy fingerprint evidence? Is there a method you use to decide to see if a fingerprint examiner will look for potential prints or go straight to DNA analysis for fired casings?
We did not test this aspect, although my hypothesis is that it could destroy some fingerprint evidence. In our laboratory we had not had much success with fingerprints on casings and therefore it goes straight for DNA analysis/NIBIN.
How much DNA was amplified in the casework samples?
We amplified 15µL or up to 1 ng of DNA, depending on the quantification results. The details for each sample will be included in the publication.
Do you put a cover over the end of the cartridge casings to prevent the internal chemicals from contaminating the solution?
No, we did not, the pre-filter appears to be able to filter out any other potentially interfering chemicals as we have obtained good results.
Do you have concerns about the collection of casings? Do your LE and crime scene people wear masks to ensure the DNA you recover is not from them?
Crime scene does wear masks and gloves and they collect casings using the stick end of a swab to pick up the casings and place them directly into the glassine envelopes. In addition, we have a staff/visitor database that all samples are searched against in case of inadvertent contamination at the scene or in the lab.
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Using LIMS Technology (Laboratory information management system) to Obtain High Quality Nucleic Acid Samples