Attending ex-girlfriend's wedding, arresting the groom's officer on the spot

Chapter 20 Solving the case is easy and easy
Generally speaking, there are two types of fingerprint collection in the past, physical methods and chemical methods.

Optical fingerprinting has also become more common in recent years as the use of lasers has become more widespread.

The criminal police in the second group of operations generally use physical methods to collect fingerprints. It's not that they don't want to use more methods, but they don't know much about other things.

Among the various methods of physical fingerprint collection, sticking with transparent glue is the most convenient and fast.

Of course, this fast fingerprint collection method has great disadvantages. The integrity of the fingerprint collection is not high, and it is very destructive to the fingerprint.

In addition, the application scenarios are also limited. Transparent glue can only collect fingerprints on hard non-absorbent materials such as glass, floor tiles, and stainless steel.

As for fingerprints on materials such as leather and paper, chemical methods are used.

Of course, in addition to transparent glue, the general powder method will also be used in the fingerprint collection method used by the second action group.

For example, common silver powder is widely used because of its strong adsorption.

Even a professional detective like Lu Chuan would use silver powder.

But at the scene of a fire, it is obvious that ordinary methods cannot be used to collect fingerprints.

Fingers are able to leave fingerprints on surfaces for two reasons.

One is that there are irregular lines on the surface of the fingers, and the existence of these lines can reduce the friction between the fingers and objects.

This is different from the understanding of ordinary people. It is common sense that rough surfaces can increase friction, but for fingers, the skin itself is rough and uneven.

The friction coefficient is already very large, and the uneven fingerprints reduce the contact area between the skin and the object, so the friction is reduced.

This is to protect the skin from friction.

The second is that there are many sweat glands and sebaceous glands on the fingerprint, and the mixture of the two substances is left on the surface of the object, and the fingerprint can be obtained by developing these substances.

What Lu Chuan is doing now is collecting fingerprints through physical and chemical methods.

Fingerprint collection at the fire scene has always been a difficult point in the field survey.

One is that after a fire, the fingerprint carrier itself will be damaged. In this case, the fingerprints cannot be collected even if the gods come.

Second, after burning, even if the fingerprint carrier itself is not completely damaged, the sweat stains that can develop fingerprints evaporate, and the proteins and amino acids in the oil secreted by the sebaceous glands are also damaged by high temperature.

The physical method is to collect fingerprints by using sticky adhesion through the fingerprints containing sweat and grease.

But at the scene of the fire, the sweat evaporates and the oil is destroyed. Naturally, fingerprints cannot be collected by this method.

Most of the chemical methods use chemical substances to react with proteins and amino acids in oils to develop images.

At the scene of excessive fire, it is obviously impossible to do it.

Therefore, in theory, among the fingerprint substances, the substances that cannot be damaged by high temperature become the key to collecting fingerprints.

And the premise of all this is to find fingerprints.

This is what Lu Chuan is doing now.

Following the fire point, Lu Chuan carefully swept it with an LV brand brush.

Every complete object surface is not spared.

Ren Qiang nodded frequently.

The most important thing for a criminal policeman is not how super logical thinking or reasoning ability is.

What is needed most is patience.

Just like what Lu Chuan is doing now, following the possible direction, not letting go of all the details, bit by bit, and doing it in a down-to-earth manner.

Only in this way can we ensure full coverage of the murderer's action path at that time.

This scan takes three hours.

The hard work paid off, and Lu Chuan discovered five groups of points where fingerprints could be extracted.

Afterwards, Lu Chuan took out a bottle of solution and began to extract fingerprints.

"This is... silver nitrate?"

Ren Qiang has worked as a criminal police officer for so many years, and he is also familiar with some of the properties currently under investigation.

Lu Chuan nodded: "Excessive fingerprints, only sodium chloride in sweat can be preserved at high temperature, and this substance can be extracted with silver nitrate solution."

Lu Chuan explained while operating: "But silver nitrate and sodium chloride have reacted, and black fingerprints appear."

At the scene of the fire, there are black and black fingerprints everywhere, almost like nothing.

"So the next step is to use copper powder to develop the fingerprints, so that we can extract the fingerprints we need."

"Why not use silver powder?"

Silver powder is cheaper and more absorbent.

"The extraction of this kind of fingerprint is not as strong as the adsorption force, the better. The fingerprint developed by silver nitrate is very fragile. Too strong adsorption force will cause the fingerprint to crack and cause unnecessary breakpoints."

Ren Qiang nodded as if realizing something, but the way he looked at Lu Chuan gradually changed.

In the current survey office in the morning, Ren Qiang heard from Lu Chuan that he had done research on fingerprint collection, but in fact he did not report particularly high expectations.

It's just that Lu Chuan has a precedent of meritorious service, and at least he is from a professional background, so he is better than himself and others.

With a try and see attitude.

As a result, Ren Qiang knew about Lu Chuan collecting fingerprints on the spot.

Lu Chuan's research on fingerprint collection is not ordinary, but extremely powerful.

He has worked with Liu Guodong all his life, so he has naturally seen each other take fingerprints.

Although he is two blocks away from him, but compared to Lu Chuan...

This Lu Chuan...is amazing.

After finishing everything, it was already 05:30 in the afternoon.

"Only four sets of fingerprints were collected..."

It's a pity that the collection of the fifth set of fingerprints failed. When Lu Chuan threw copper powder, his hand shook a few grams more.

But it was these few grams of copper powder that caused the fingerprints to break and completely unusable.

"Very good, these four sets are enough!"

Looking at the golden and clear fingerprints on the photo, Ren Qiang felt that there was no need for Liu Guodong and the others to compare them.

With such a clear fingerprint, even the new college students in his group can compare it with the naked eye.

Lu Chuan nodded and began to pack his survey box.

"Actually, it would be easier to extract the footprints at this kind of arson scene, but unfortunately the ground marks are too damaged."

Ren Qiang agreed with this. Since the murderer set fire to the warehouse, he would definitely leave footprints.

And in the process of igniting, it is likely to be covered by some collapsed objects, so as to be preserved.

However, due to the rescue at the scene and the trampling of people from the second operation group, the ground was already in a mess, and it was impossible to collect footprints.

At least, with Lu Chuan's current ability, they cannot be collected.

The system rewarded him with elementary footprint analysis skills, but he did not have the ability to collect footprints.

After finishing his work, Ren Qiang arranged for his car to send Lu Chuan back to the Criminal Investigation Detachment.

They have dinner at night, so naturally they can't be late.

As for the second group of operations: "All personnel are here to compare the fingerprints. Whether the suspect can be caught depends on these four groups of fingerprints!"

Returning to the detachment, Lu Chuan put the survey box back, went to the office to change into the police uniform, and put on casual clothes.

"How is the arson case going?"

(End of this chapter)