Chapter 17: Forensic Aspects of Fire and Explosion Investigation

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Welcome back to The Deep Dive, where we take complex evidence and distill it into pure essential knowledge.

Today, we're walking straight into what are probably the most challenging crime scenes in all of criminalistics.

Fire and explosion scenes.

Exactly.

These are incredibly difficult, largely because the destructive force used to commit the crime.

Well, it also consumes the very physical evidence you need to solve it.

It's the ultimate challenge, really.

In cases of arson or bombings, the perpetrator is basically relying on destruction and chaos to cover their tracks.

Right.

So the criminalist's job isn't to find, you know,

a perfectly intact piece of evidence.

It's often to find microscopic remnants of chemical materials left behind, traces that might be hours or even days old.

OK, let's just jump right in with a chilling example that completely changed how federal agencies approach this.

I'm talking about the 1995 Oklahoma City bombing.

An unimaginable act.

April 19th, 1995, a Ryder rental truck packed with explosives detonates outside the Alfred P.

Murrah Federal Building.

And the death toll was just staggering.

168 people, including 19 children who were in a daycare center inside.

The scale of that required this immediate, massive forensic response.

But the first big break in the case came from a surprisingly, well, a mundane piece of police work.

It did.

The genius of it was just solid investigation.

They traced the remnants of the truck axle back to the rental agency, which led them to an alias used by the perpetrator Robert Klang, who was, of course, Timothy McVeigh.

But the final piece of the puzzle, the thing that connected McVeigh to the scene so quickly was just sheer coincidence.

Incredible luck for the investigation, really.

Absolutely.

Just hours after the blast, McVeigh was pulled over by an Oklahoma Highway Patrol officer.

He was driving without a license plate.

A simple traffic violation.

That's it.

And that stop led to his arrest when the officer found he was carrying a loaded illegal firearm.

At first, this was completely separate from the bombing investigation.

But it was the detention that provided that crucial physical link.

So once he was detained and connected back to that truck, what was the single most critical piece of forensic evidence?

The thing that sealed his connection to the bomb materials.

There were two, really.

First, investigators found McVeigh's fingerprint on a receipt for two thousand pounds of ammonium nitrate.

The main ingredient.

A fertilizer grade chemical.

Right.

But second, and this was critical for the lab team, forensic analysts found PTN on the clothing he was wearing when he was arrested.

PTN.

That's pen for three three tall tetra nitrate.

A very powerful secondary explosive used in things like detonating cords.

Exactly.

So finding that residue meant he wasn't just near the raw fertilizer.

He was near the actual initiation system of a massive high explosive device.

That one detail just transforms the entire case from circumstantial evidence to a direct physical link.

It does.

And that intersection of rapid police work, catastrophic destruction, and microscopic chemistry is the perfect transition to our mission today.

We're going to take a deep dive into the chemical reactions, the investigative signs, the very precise rules for evidence collection, and the advanced lab analysis like gas chromatography and spectrometry that investigators use to solve these cases.

And this is where we really need to draw a clear line around the role of the criminalists, the forensic scientists.

Their job is very specific.

Detect and identify chemical materials and reconstruct any igniters they find.

But the ultimate legal determination of the cause, you know,

was it an accident or was it arson, that has to be made by an extensively trained fire investigator.

Because they're the ones on scene synthesizing everything.

Right.

They take the lab findings and combine them with all their on scene observations considering all the things that might not leave a chemical trace.

Things like faulty wiring, overheated motors, or even, you know, just a carelessly discarded cigarette.

It's a true collaboration.

All right.

Let's start at the very foundation, the science of combustion itself.

What makes fire?

Fire.

Well, at its core, it's a transformation process.

It's oxidation, where oxygen unites with another substance.

But the key is that it produces noticeable heat and light, what we call a flame.

So methane burning is rapid oxidation.

Iron rusting is slow oxidation.

What's the real difference there?

It all comes down to the dynamics of energy transfer and the speed of the reaction.

In any chemical reaction, you have to put energy in to break the existing chemical bonds.

Right.

And the fuel and the oxygen.

And then energy is liberated when new, more stable bonds are formed, like in carbon dioxide and water.

So when we talk about fire, we're really talking about what are called exothermic reactions.

Highly exothermic, yes.

This means that a lot more energy is released when those new bonds form, then was required to break the old ones.

That excess energy is what we see and feel as heat and light.

We call it the heat of combustion.

But not all exothermic reactions just burst into flames.

Rusting is exothermic, but it's slow.

That brings us to this idea of an energy barrier.

The activation energy.

Think of it as an invisible hill you have to push the reactants over to get them started.

For iron rusting, that hill is really small.

Normal room temperature is enough to get it going slowly.

But for fuels like methane or wood or gasoline, that hill is quite high.

You need that initial high temperature push a match, a spark to get the reaction over that threshold.

So if I have a pile of wood and air, it'll just sit there forever until I apply enough heat to get it over that activation energy.

Precisely.

And we define the ignition temperature as the minimum temperature where a fuel will spontaneously ignite and keep burning without you holding a flame to it.

So once it starts.

Once it starts, the heat being liberated by the combustion itself is enough to keep pushing new fuel molecules over that energy barrier.

It becomes a self -sustaining chain reaction until, you know, you either run out of fuel or you run out of oxygen.

And that gets us to the speed of the reaction, which is everything in a forensic context.

It is.

The faster the reaction, the more destruction.

And the rate is all about molecular collisions.

The hotter the molecules, the faster they move and the more often they collide.

There's a rule of thumb for this, isn't there?

There is.

A 10 degree Celsius rise in temperature, that's about 18 degrees Fahrenheit,

often doubles or even triples the reaction rate.

And that creates a vicious cycle.

Exactly.

The fire gets hotter, which makes the reaction go faster, which generates more heat, which makes the fire even hotter.

Now you mentioned something that's a non -negotiable requirement for a flaming fire.

The physical state of the fuel.

Absolutely essential.

For a sustained flaming combustion to happen, the fuel must be in gaseous state.

A gas.

Why is that?

Molecules just can't collide frequently or effectively enough to support a flame if the fuel is a bulk solid or a liquid.

The reaction is happening in the gas phase right above the fuel surface.

And then that means investigators have to understand how liquids and solids turn into that gas.

Let's start with liquids, like gasoline.

For liquids, we use the term flashpoint.

And this is a really important distinction.

The vapor to support combustion.

It's a readiness test in a way.

That's a great way to put it.

For gasoline, the flashpoint is incredibly low, around minus 50 degrees Fahrenheit.

Wow.

So on pretty much any normal day, gasoline is already producing enough vapor to burn.

Yes.

But just because it's producing the vapor doesn't mean it's on fire.

To actually start the fire, you need to reach the much higher ignition temperature.

For gasoline, that's 495 degrees Fahrenheit.

So the flashpoint tells you how dangerous the vapor is, but the ignition temperature tells you how hot the spark needs to be.

You've got it.

That's the vital distinction.

Okay, liquids are one thing, but what about solids like wood?

They don't just vaporize.

No, they undergo a chemical transformation called pyrolysis.

Pyrolysis.

Intense heat actually breaks down the solid fuel, the long cellulose molecules in wood, into smaller, volatile, combustible, gaseous products.

It's those gases from the decomposition of the solid that mix with oxygen and actually burn.

So understanding the depth of that pyrolysis, the charring, can tell an investigator about the fire's intensity and how long it burned in a certain spot.

It's a huge clue.

And even if you have the fuel in a gaseous state, you still need the right mix of fuel and air.

Correct.

The reaction can only happen within a very specific concentration range, which we call the flammable range.

So too little fuel, it won't ignite.

Right, that's too lean.

But if you have too much fuel vapor compared to the oxygen, it also won't burn.

That's too rich.

There's just not enough oxygen to complete the reaction.

For gasoline, that range is between 1 .3 and 6 .0 percent.

So if you had a room filled with 10 percent gasoline vapor, a fire would actually go out.

It would extinguish itself, a very precise window.

Let's quickly touch on a couple of variations that can complicate a scene.

First, glowing combustion.

Glowing combustion, or smoldering, is just oxidation happening right on the surface of a solid fuel, but without a visible flame.

Like hot coals?

Hot charcoal, the end of a cigarette.

It happens when the heat isn't high enough to cause pyrolysis and produce those flammable gases.

It can be a hidden danger, a fire that smolders for hours.

And what about the one that always comes up in suspicious fires?

Spontaneous combustion.

Ah, yes.

The conditions for true spontaneous combustion are actually very, very specific and limited.

It's a fire caused by a natural heat -producing process where that heat has no way to escape.

The classic example being hay in a barn.

Right.

Bacteria in poorly ventilated, damp hay can generate heat.

If it can't dissipate, it builds up and up until it reaches the ignition temperature of the hay.

Or a chemical example, like rags soaked in linseed oil.

Exactly.

Linseed oil oxidizes in the air.

If you crumple up those rags, the heat from that slow oxidation gets trapped and can build to the point of ignition.

But, and this is a critical point for investigators, this does not happen with common household oils.

No.

Not with your motor oil, not with your cooking oil.

Those are generally far too stable.

So an investigator has to be very skeptical when someone claims spontaneous combustion.

Okay, so once a fire is going, the whole building becomes this dynamic system.

And understanding how heat moves is the key to finding the origin.

Let's cover the three mechanisms, starting with conduction.

Conduction is simply heat moving through a solid object from atom to atom or electron to electron.

Metals are great conductors.

And things like wood or plastic are poor conductors or insulators.

Right.

And forensically, this can be a real trap.

A metal beam or even a pipe can conduct heat a huge distance from the fire's origin.

And start a new fire somewhere else.

Exactly.

It creates a secondary, unconnected ignition site that can easily trick an investigator into thinking the fire started in two places at once, suggesting arson.

Okay, next up is radiation.

Radiation is arguably the most important driver of fire spread.

It's heat transfer through electromagnetic waves, moving in a straight line from a hot surface to a cooler one.

So it can jump across empty space.

It can.

It's how a fire in one building can ignite the building next to it.

Any surface facing the fire absorbs this massive amount of radiant energy, and when it hits its ignition temperature, it just bursts into flame.

And the third mechanism is convection.

Convection is heat moving through a fluid, in this case the air and combustion gases.

Hot gases are less dense, so they rise very quickly.

And they collect at the ceiling.

They do, creating this incredibly hot, deadly layer of gas in the upper parts of a room.

And that superheated gas layer then acts like a giant broiler, radiating heat back down.

And that leads us to probably one of the most critical and misunderstood phenomena in fire investigation,

flash over.

A misunderstanding that has led to wrongful convictions.

Tragically, yes.

So define flash over for us.

What happens?

Flash over is a transitional phase.

It's the point where the room gets so hot, we're talking maybe 1 ,100 degrees Fahrenheit at the ceiling,

that the radiant heat from that gas layer causes the simultaneous ignition of all combustible fuels in the room.

Everything catches fire at once.

Everything.

The floor, the furniture, the curtains, they all hit their ignition temperature from the radiant heat and just ignite.

The Sackett Street fire in Brooklyn back in 1980 is the textbook case for how misinterpreting this can go horribly wrong.

It's a very sobering example.

In that fire, three men were convicted of arson and murder.

The primary physical evidence was what the fire marshal at the time called multiple unconnected burn sites.

And there was a puddle shape pattern on the floor.

Yes, a distinct puddle shape on the tiles.

Traditionally, an investigator sees that and their mind immediately goes to an accelerant, a flammable liquid being poured there.

But the lab tests at the time found nothing.

They found no accelerant residues.

Decades later, a forensic review by experts like John Lentini showed that those patterns, the deep charring, the so -called puddle shapes, were completely consistent with post -flashover conditions.

How so?

When a room goes to full flash over, the intense radiant heat can create very deep, uneven charring.

If a piece of carpet is burned away, the heat penetrates the subfloor deeper in that spot, creating what looks like a puddle.

It's an illusion.

So the key takeaway here is that flash over on its own can create what looks like definitive proof of arson, but it's just a natural effect of the fire's dynamics.

That's the vital principle.

Burn patterns are a hypothesis, they are not a conclusion.

Only the recovery and confirmed lab ID of an ignitable liquid residue can prove that an accelerant was used.

The men in the Sackett Street case spent decades in prison because that science wasn't understood.

That's right.

So with that major caution in mind, when an investigator does arrive, what are some of the things they look for that might genuinely point towards arson?

They look for the obvious things first.

You know, empty gasoline cans, remnants of an ignition device, or streamers' trails of paper or cloth used to spread the fire from one spot to another.

And of course, multiple separate, unconnected fires are a huge red flag.

Before they can even begin that search, they have to secure the scene.

And there's specific legal precedent here, right?

Michigan v.

Tyler.

A central Supreme Court ruling for the field.

The court decided that fire officials do not need a warrant to remain on the scene for a reasonable time after the fire is out.

Why is that?

The logic is about public safety and evidence preservation.

They need to find the cause to make sure it doesn't happen again, and those volatile chemical residues are evaporating by the second.

They have to be collected immediately.

But if they leave and want to come back days later for a deep dive search… Then they need a criminal warrant.

But that initial response period is protected.

And timeliness is just everything.

How long do they have?

Hours.

Maybe days, at best.

Gasoline evaporates incredibly quickly.

Cleanup has to be delayed.

And if accelerants soak into soil or plants,

bacteria start to eat them.

So what's the protocol for that?

You freeze the sample immediately.

It stops the bacterial degradation in its tracks.

Okay, so the search needs to focus immediately on finding the origin.

How do they do that?

The basic rule is that fire moves up.

So the origin is usually the lowest point that shows the most intense burning, the deepest char.

And there's a visual pattern to look for.

Often, yes.

You might see a V -shaped pattern on a wall where the point of the V is pointing down toward the origin of the fire.

And if a liquid accelerant was used, gravity gives another clue.

It does.

Liquids flow down.

So you might see charring that's more intense on the bottom of a table or a chair rather than on the top.

In a normal fire, heat rises so the tops of things burn more.

Once they've found that origin, it's all about collection.

What are the golden rules for packaging this evidence?

Three absolute non -negotiable rules.

First,

quantity.

You need to collect two to three quarts of ash, soot, and porous material from the origin.

You need enough bulk material to hold the residue.

Exactly.

Second, packaging.

The material must go immediately into an airtight container.

The industry standard is a new clean paint can with a friction lid.

And the big mistake to avoid.

Never, ever use plastic polyethylene bags.

Why not?

Two reasons.

One, the plastic lets the volatile vapors slowly escape.

And two, the plastic itself can react with the hydrocarbons contaminating your sample.

And the third rule.

Substrate control.

You have to collect a similar uncontaminated sample from another area of the scene.

To act as a baseline.

Yes.

It rules out background chemicals like carpet cleaner.

But more importantly, it lets the lab see what volatile compounds are produced when the carpet itself burns naturally.

You have to be able to tell the difference between the breakdown products of the material and a true added accelerant.

Okay, so the evidence arrives at the lab, properly packaged in those paint cans.

How does the hunt for those tiny volatile residues begin?

Well, before they go to the big instruments, they might use a screening tool like a portable vapor detector or a sniffer.

How does that work?

It's a handheld device that sucks in air from the can.

The air passes over a hot filament.

If there are combustible vapors, they burn on the filament, which makes it hotter, and you get a reading on a meter.

So it's a quick check, but it's not specific.

Not at all.

It just tells you something combustible is there.

And of course you have the trained canines who are still fantastic at pinpointing areas that need further analysis.

But for the definitive, conclusive identification,

the undisputed champion is the gas chromatograph, or GC.

Absolutely.

The GC is essential because things like gasoline aren't one single chemical.

They're a complex mix of hundreds of different hydrocarbons.

The GC's job is to separate that complex mixture.

How does it do the separation?

The sample is heated and vaporized, and an inert gas carries it through a long coiled tube called a column.

Different molecules travel through that column at different speeds based on their size and properties.

And they hit a detector at different times.

Right.

And that creates a chromatographic pattern.

That pattern is like a unique fingerprint for a specific petroleum product.

And you can classify those patterns.

We do.

Into light, medium, and heavy petroleum distillates based on their carbon count.

So we can tell gasoline from kerosene or from fuel oil.

The identification is made by comparing the pattern from the evidence to the pattern from a known standard.

And this is an important point.

You can't tell the brand of gasoline.

No, you can't.

The industry blending practices make that impossible.

So before the sample even gets into the GC,

you have to get the residue out of the debris.

What's the simplest way?

The simplest is the headspace technique.

You just heat the airtight can, which drives the volatile residues into the airspace, the headspace at the top.

Then you poke a syringe through the lid, pull out a small sample of that vapor, and inject it into the GC.

But the downside is you can only get a small amount of vapor.

It's not very sensitive, which is why the preferred method is the vapor concentration technique.

How does that improve things?

Well, instead of a syringe, you place a small strip coated with charcoal inside the can with the debris.

Then you heat the whole thing for maybe an hour.

And the charcoal acts like a sponge for the vapors.

A chemical sponge, yes.

It absorbs and concentrates all those volatile hydrocarbons.

Then you take the strip out and wash the concentrated residue off with a tiny amount of solvent.

And that's what you inject into the GC.

Right.

And the big advantage here is sensitivity.

This method is at least a hundred times more sensitive than the simple headspace technique.

It lets you find traces you'd otherwise miss entirely.

But what happens when the GC pattern is still a mess?

Yeah.

You know, you've got a little bit of accelerant mixed with all the breakdown products from the burnt carpet and plastic.

That's when you bring in the gold standard, gas chromatography mass spectrometry, or GCMS.

Okay.

What does the mass spec add to the process?

The GC still does its job, separating the mixture into individual components.

But as each component comes out of the GC, it flies directly into the mass spectrometer.

The MS then smashes those molecules into charged fragments or ions.

And it filters them somehow.

It acts as a perfect filter.

The analyst can program the MS to look only for the specific ions that are characteristic of gasoline while completely ignoring all the ions that come from the burning carpet.

So it's basically like putting on noise canceling headphones for chemistry.

You tune out all the background junk.

That's a perfect analogy.

It cleans up the data incredibly well, making it possible to identify trace amounts of accelerant in even the most complex and contaminated samples.

It's definitive.

All right.

Let's shift gears from the chemistry of fire to the chemistry of explosions.

We're now talking about a reaction that's all about one thing,

speed.

Extreme rapid rate, that's the defining characteristic.

An explosion is still a rapid exothermic oxidation reaction, but it produces massive quantities of gas and heat almost instantaneously.

And it happens so fast that the oxygen in the air can't even get involved.

Right.

It's too slow.

So most explosives have to carry their own oxygen supply, which is provided by oxidizing agents.

Like in black powder.

Exactly.

In black powder, the potassium nitrate is the oxidizing agent that provides the oxygen for the charcoal and sulfur fuels to burn.

Or even more efficiently, the fuel and the oxygen are locked together in the same unstable molecule like nitroglycerin.

Yes.

And when that molecule detonates, it rearranges itself into super stable gases like carbon dioxide and water vapor, releasing a colossal amount of energy and expanding in volume in just milliseconds.

Let's walk through the three devastating physical effects of that.

First, you have overwhelming pressure.

All that gas produced in a confined space, like a pipe bomb, creates internal pressures of hundreds of tons per square inch.

So the container fails.

It fails catastrophically.

That's the second effect, fragmentation or shrapnel.

The container fragments fly outward at incredibly high velocity.

And the third effect is the blast effect.

The blast effect is the most widespread source of damage.

The gases rush outward at speeds up to 7 ,000 miles per hour, compressing the surrounding air into a powerful supersonic shock wave.

And it's that shock wave that knocks down buildings.

That's what does it, the pressure wave.

And this difference in speed is how we classify the two main types of explosives,

low versus high.

Yes.

Low explosives decompose relatively slowly, up to about 1 ,000 meters per second.

The process is called deflagration.

It's basically a very, very fast fire.

A rapid subsonic burning.

And its effect is a propelling or throwing action, not a shattering one.

That's why it's used as propellant and bullets.

And critically, they have to be confined to be dangerous.

Absolutely.

If you just light black powder in the open, it just burns very quickly with a whoosh.

But if you seal it in a strong pipe, the pressure builds up and you get a devastating explosion.

Okay, so what are the main types of low explosives?

You have black powder, the classic mix, smokeless powder, which is the propellant in modern ammunition.

And then you see improvised things like chlorate mixtures, potassium chlorate mixed with sugar can be very effective.

And what about large scale gas air mixtures?

Those are a type of low explosive and they depend entirely on that flammable range we talked about.

If a room fills with natural gas and finds a spark, you get a huge explosion.

But if the mixture is rich with too much fuel, it doesn't all burn in the initial blast.

Exactly.

And when air rushes back into the vacuum created by the explosion, it mixes with that leftover hot gas and you often get a massive secondary fire, which can cause even more damage than the initial blast.

Okay, now let's contrast that with high explosives, a completely different animal.

High explosives detonate almost instantaneously at speeds up to 8500 meters per second.

This is detonation.

It's a supersonic shockwave.

And the effect is pure smashing and shattering.

Pure shattering.

And we subclassify them based on how sensitive they are.

Starting with primary explosives.

Primaries are ultra sensitive.

They'll go off from heat, shock, friction, almost anything.

They're too unstable to be the main charge.

So what are they used for?

They're used to initiate other explosives.

They're the key ingredient in blasting caps and primers, things like leadazide.

And they are used to set off the secondary explosives.

Right.

The secondaries are the main charge.

They're much more stable.

You can usually burn them in the open air without them exploding.

They need the intense shock from a primary explosive to get them to detonate.

And this category includes most of the big names.

Yes.

TNT, RDX, PTTN, and dynamite.

Let's talk about dynamite first.

Developed by Alfred Nobel, it was his solution to the problem of handling liquid nitroglycerin, which is incredibly unstable.

He found he could absorb it onto an inert material like wood pulp to make it much safer.

And now it's been largely replaced by ammonium nitrate explosives.

It has.

They're cheaper and safer.

This includes things like water gels and emulsions, and most importantly,

ANFO, ammonium nitrate soaked in fuel oil.

ANFO is huge in mining, but its ingredients are also way too easy to get.

That's the problem.

The ammonium nitrate is just agricultural fertilizer.

Which is what was used in the 1993 World Trade Center bombing plot.

Exactly.

They were using accessible, off -the -shelf components to try and make a massive bomb.

Shifting to homemade explosives favored by terrorists, let's talk about TATP.

TATP, triacetone triperoxide, is incredibly dangerous.

It's a potent, friction, and impact -sensitive homemade explosive made from common chemicals, acetone, hydrogen peroxide, and an acid.

It was used in some major attacks.

Yes, the 2005 London bombings, the 2016 Brussels bombings.

And TATP is the single reason we have our current airport security rules about liquids.

It's the direct link.

There was a thwarted plot in 2006 to detonate liquid explosives on airplanes.

The plot involved getting high -concentration hydrogen peroxide and mixing it with acetone to make TATP on the flights.

So the ban on liquids was a direct response to that specific threat.

It was.

And finally, a quick rundown of the main military explosives.

You need to know the acronyms.

RDX is the most popular and powerful military explosive, often found in its plastic form, C4.

TNT is another big one.

And PTN, which we mentioned with Oklahoma City, is the explosive core inside detonating cord.

And all of these secondary explosives, no matter how powerful, still need a detonator to set them off.

They do.

They need a blasting cap containing a primary explosive to give them that initial shock.

And those caps are initiated either by a fuse or, more often in criminal cases, by an electrical current from something like a clock or a cell phone.

So given that intense destructive power, searching an explosion scene has to be completely different from an arson scene.

What's the number one thing investigators look for?

The presence of a crater at the origin.

That instantaneous pressure excavates the surrounding material.

The crater is ground zero, and it's the most likely place to find any unconsumed explosive residues.

So how do you go about sampling the crater and everything around it?

It has to be slow and meticulous.

You start by collecting loose soil and debris from inside the crater.

Then you search for nearby porous materials, bits of wood, rubber, insulation, because they'll absorb and hold on to traces of the explosive.

And you have to collect things that have been thrown far away.

You do.

And you use wire mesh screens of different sizes to systematically sift through all the mechanism, a piece of wire, a fragment of a blasting cap, anything.

Packaging here is even more important than in arson cases.

What are the rules?

Airtight, sealed containers, always.

And for soil and loose debris, they have to be metal containers like paint cans.

And the rule against plastic is absolute here.

It is an absolute rule.

Plastic bags cannot be used.

Some explosive compounds can actually diffuse right through the plastic over time.

And as always, you have to package debris from different areas separately to prevent cross -contamination.

Okay, the evidence is in the lab.

What's the first step?

A careful microscopic examination.

The analyst is literally sifting through the debris under a microscope looking for any tiny, unconsumed particles of the explosive.

Low explosives are sometimes easier to spot because they have characteristic shapes and colors.

And for quick screening in the field or in the lab, they use an ion mobility spectrometer or IMS.

The IMS is a fantastic tool for trace detection.

It sucks in residues, vaporizes them, and then zaps them with electrons to turn them into charged ions.

And then it measures how fast those ions move.

Exactly.

The ions are sent through a tube called a drift region.

Based on their size and structure, they move at different speeds.

By measuring the time it takes them to get to the detector, the instrument can identify the substance.

It's fast, but it always needs to be confirmed.

So for those confirmatory tests, you have to extract the residue from the debris first.

You do.

And the solvent you use depends on the explosive.

For organic high explosives like RDX or TNT, you rinse the debris with acetone.

But for water -soluble explosives like those containing ammonium nitrate, you use water.

And once it's extracted, what's the sequence of analysis?

It usually starts with simple preliminary color spot tests.

These are quick chemical reactions that produce a characteristic color.

For example, TNT turns red when you add alcoholic potassium hydroxide.

And then you move on to more definitive techniques.

We use thin -layer chromatography, TLC, to separate components.

For more sensitive results, we use high -performance liquid chromatography, HPLC, which is great because it works at room temperature and doesn't risk destroying heat -sensitive explosives.

But for the absolute, legally unshakable confirmation… That comes from infrared spectrophotometry.

It shines infrared light through the sample, and the way the molecule absorbs that light creates a unique fingerprint spectrum.

It's definitive proof.

Wow.

We have covered a huge amount of ground today, from the microscopic principles of oxidation all the way to the massive destructive power of high explosives.

And I think the core knowledge really relies on a few key distinctions.

You have to remember that burn patterns alone, especially after a flash over like in the Sackett Street case, can be incredibly misleading.

Only a verified chemical signature from the lab can confirm arson.

Exactly.

And for explosions, it's all about speed.

Low explosives deflagrate and propel, and they need confinement.

High explosives detonate and shatter.

And none of the lab work matters if the collection isn't perfect.

That's right.

The golden rules, airtight metal containers, and proper substrate controls are what ensure the results from the GCMS and the IMS are valid in court.

And here's where it gets really interesting and maybe a final thought for you to take away.

We talked about how TATP, a homemade explosive, force a permanent change in global security with the liquid restrictions at airports.

This constant cat and mouse game, this adaptation by criminals creating new compounds and delivery systems, forces forensic science and security into a perpetual high stakes scientific race.

And that's what makes this field so critical.

That need for continuous technological evolution just to stay one step ahead of the next improvised threat.

Thank you for joining us for this deep dive into forensic fire and explosion investigation.

We hope you feel thoroughly well informed.

We'll see you next time.