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Contamination prevention, chain of evidence and presumptive testing

Meet Forensic Scientist, Gabriella Mason-Buck, find out how to prevent contamination, check the chain of evidence and carry out presumptive testing.
Hello, Gabriella. Hi, David. I’ve got Natsumi here. She’s got some items of evidence from the Nelson Dock murder. I’m going to leave her in your hands now. Thank you. Thank you. Hi, nice to meet you, Gabriella. Nice to meet you, too, Natsumi. Hi. I’ve got this blood swab and metal bar here. Could we have a look at this evidence in this laboratory? Of course. Let me take those from you for now. I’m just going to put those down on the bench. Yes. But what we need to do first is we need to get you gowned up. So, if you’d like to follow me over here, and we’ll do that now.
OK, so what I need you to do is I need you to put on a face mask first. Ah, OK. So what this does is it protects the item that we’re about to analyse from any saliva while we’re talking. Usually in a case, we would not be talking, but obviously for the purpose of today, we’re talking. Yes, I understand. Then next, you have to put on a hair net. So what that’s going to do is it’s going to protect the item from any hair that you have. So if we do find any hair on the item, we can collect it and actually look at the DNA analysis for that if we need to at a later date. Like this? Perfect.
OK, so the next point is we’re going to do two sets of gloves. The first one is going to be a nice long pair of gloves. What that does is that protects all of your skin up and beyond the lab coat. You’re going to keep those gloves on for the remainder of the work, but then what you’re going to do is put another pair of gloves on top. Oh, wow. So these are the pair of gloves that you will change every time you do some different analysis. Uh-huh. OK? To avoid contamination? Avoids contamination, yes. So every time you touch a new item, you do something new, you use a new pair of gloves. OK.
Like this? Yeah, perfect. OK, and now you’re ready to go. So if you’d like to follow me over here, we’ll start the analysis. Yes please.
OK, Natsumi, as the DNA forensic scientist, it’s my responsibility to look at the items that were submitted for the Nelson Dock murder. Oh, OK. What we need to do today is we need to look at the packaging to check the chain of continuity, and we have to also look to see if we can identify any body fluids - in particular blood, because that’s the information that has been submitted to us by the police. And then what we’ll need to do is we’ll need to sample the items and send those off for DNA profiling. OK. First of all, what we’re going to look at is the swab. Yes.
So if I take this item here, the main things that we need to check on this item is that it’s secure and sealed correctly. I see. So having a look at the item, we have a seal number down here, and we also have some information on the packaging that I would like you to take a look at. Yep. The key pieces of information here are a description of the item, the time and date that it was seized and produced, where it was seized and produced from and who collected the item and their signature. I see. You’ll also see there’s an incident crime number on that item. So if you want to hand that one back over to me. Sure.
Thank you, Natsumi. What we’re going to do is take that to the bench, and we’re going to start processing the item. OK.
What we need to do is we need to make sure that we maintain the custody of this item. So I’m going to make a small incision with a clean scalpel into the packaging.
Then what I’m going to do is I’m going to carefully remove that swab, and we’re going to take a look at the inner information. If I hand that one to you, you can check the information on that swab OK.
I’m satisfied that the information on that swab matches that on the bag, so we’re going to start our processing. Right.
What we’re going to do today is we’re going to use a reagent called Kastle-Meyer reagent. So what that does is it uses the peroxidase capability of haemoglobin in blood, so the region that contains the iron in the blood. The red blood cells do not contain any DNA, but they are very good for presumptive testing due to the haemoglobin and the peroxidase-like activity. What I’m going to use are two reagents. The first is hydrogen peroxide, and the second is Kastle Meyer. What’s really good is to remember
that you use these in their alphabetic order: so ‘H’ for hydrogen peroxidase and then the Kastle-Meyer. Ah, I see. We also will need to conduct a positive and negative control. So using this positive control of blood, I’m going to spike some of this onto a filter paper to act as a positive control, and I’m also going to use water as a blank negative control.
It’s really important that everything you’re doing in the lab that you label everything before you start. Of course.
So I’m going to to take a pipette and I’m going to take 10 micrometres of the positive control of blood and place it down into the centre of the filter.
As I said, I’m then going to take 10 microliters of water to act as the negative control.
Then, we take our Kastle-Meyer reagents. First of all, add a couple of drops of hydrogen peroxide. What we want at this point is we want no colour change to occur. OK.
Now what we want to happen when the customary agent is added is for the positive control to go a nice, bright pink, and the negative control to remain colourless. I see.
What we also have to do is we have to time the reaction. So as soon as I add the drops, I will start the timer.
As you can see, within one second, the positive control turned pink. Ah, yes. And the negative control has remained colourless. Right. I’m satisfied that the reaction is working. Now, let’s examine the metal bar.

It is important to prevent contamination, check the chain of evidence and carry out presumptive testing. Watch Gabriella Mason-Buck explain more.

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