Chapter 20 Solving the crime is easy said and done
In the past, there were generally two categories of fingerprint collection: physical methods and chemical methods.
In recent years, as the application of lasers has become more widespread, optical fingerprint collection has become more common.
The criminal policemen of the second operation group usually use physical methods to collect fingerprints. It’s not that they don’t want to use more methods, but they are not very good at other methods.
Among the various methods of physically collecting fingerprints, using transparent glue is the most convenient and fastest.
Of course, this quick fingerprint collection method has major shortcomings. The integrity of the fingerprint collection is not high and it is highly destructive to the fingerprints.
In addition, the application scenarios are also limited. Transparent glue can only collect fingerprints on glass, floor tiles, stainless steel and other hard non-absorbent materials.
As for fingerprints on leather, paper and other materials, chemical methods must be used.
Of course, Operation Team 2 will use the fingerprint collection method. In addition to transparent glue, the general powder method will also be used.
For example, the common silver powder is widely used because of its strong adsorption power.
Even a professional detective like Lu Chuan would use silver powder.
But at the fire scene, it is obvious that ordinary methods cannot be used to collect fingerprints.
There are two reasons why fingers can leave fingerprints on the surface of objects.
First, there are irregular lines on the surface of the fingers. The existence of these lines can reduce the friction between the fingers and objects.
This is different from what most people understand. It is common sense that a rough surface can increase friction, but for fingers, the skin itself is very rough and uneven.
The coefficient of friction is inherently large, and uneven fingerprints reduce the contact area between the skin and the object, thus reducing friction.
This is to protect the skin from damage by friction.
The second is that there are numerous sweat glands and sebaceous glands on fingerprints. If the mixture of the two substances is left on the surface of the object, the fingerprint can be obtained by developing these substances.
What Lu Chuan is doing now is to collect fingerprints through a combination of physical and chemical methods.
Fingerprint collection at fire scenes has always been a difficult point in on-site investigation.
First, after a fire, the fingerprint carrier itself will be damaged. In this case, the gods will not be able to collect fingerprints.
Second, after being burned, even if the fingerprint carrier itself is not completely destroyed, the sweat that can develop the fingerprint evaporates, and the proteins and amino acids in the oil secreted by the sebaceous glands are also damaged by the high temperature.
The physical method is to collect fingerprints by using sticky adhesion to contain sweat and grease in the fingerprints.
But at a fire scene, sweat evaporates and grease is destroyed, so fingerprints cannot be collected using this method.
Most chemical methods use chemical substances to react with proteins and amino acids in oils to develop.
It is obviously not possible to do this at the scene of the fire.
So, theoretically speaking, the key to collecting fingerprints is the ones that cannot be damaged by high temperatures.
The premise of all this is to find fingerprints.
This is what Lu Chuan is doing now.
Following the area where the fire started, Lu Chuan used an LV brand brush to sweep over it carefully.
Every intact surface of the object is not spared.
Looking at this, Ren Qiang nodded frequently.
The most important thing about being a criminal police officer is not having super logical thinking or reasoning ability.
What is needed most is actually patience.
Just like what Lu Chuan is doing now, follow the possible direction, not neglecting all the details, and continue to do it step by step.
Only in this way can we ensure full coverage of the murderer's path of action at that time.
This sweep lasts for 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.
"Is this...silver nitrate?" Ren Qiang has been a criminal policeman for so many years and is familiar with some of the property currently under investigation.
Lu Chuan nodded: "Excessive fingerprints, only the sodium chloride in sweat stains can be preserved at high temperatures, and this substance can be extracted with silver nitrate solution."
Lu Chuan was operating. , while explaining: "But after the reaction of silver nitrate and sodium chloride, black fingerprints appeared."
At the fire scene, everything was pitch black and black fingerprints were almost as good as none.
“So next we need 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 has stronger adsorption power.
“For this kind of fingerprint extraction, the stronger the adsorption force, the better. Fingerprints developed with silver nitrate are very fragile. Too strong adsorption force will cause the fingerprint to break and cause unnecessary breakpoints.”
Ren Qiang nodded with understanding, but his eyes gradually changed when he looked at Lu Chuan.
In the morning at the site survey office, Ren Qiang heard Lu Chuan say that he was researching fingerprint collection, but he actually didn’t have particularly high expectations.
It's just that Lu Chuan has a history of meritorious service, and at least he is from a major, so he is better than himself and others.
With the attitude of giving it a try.
As a result, Ren Qiang knew that Lu Chuan had collected fingerprints on site.
Lu Chuan’s research on collecting fingerprints is not just ordinary, but extremely impressive.
He and Liu Guodong have worked together all their lives, and naturally they have seen each other collect fingerprints.
Although he is two blocks stronger than himself, compared with Lu Chuan...
This Lu Chuan... is amazing.
After finishing everything, it was already half past five 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 the copper powder, his hand shook and he added a few grams.
But just these few grams of copper powder caused the fingerprint to break, making it completely unusable.
“Very good, 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 to compare with Liu Guodong and the others.
With such clear fingerprints, even the new college students in his group can compare them with the naked eye.
Lu Chuan nodded and began to pack his survey box.
"Actually, it would be easier to extract footprints at this kind of arson scene, but unfortunately the ground marks are too badly damaged."
Ren Qiang agreed. Since the murderer set fire in the warehouse, he will definitely leave traces behind. footprints.
And during the fire process, it was likely to be covered by some collapse and thus preserved.
However, due to the rescue operations and the trampling of people from the second operation team, the ground was in a mess and there was no possibility of collecting footprints.
At least, Lu Chuan cannot collect it with his current ability.
The system rewarded him with basic footprint analysis skills, but he did not have the ability to collect footprints.
After finishing his work, Ren Qiang arranged for his car to take Lu Chuan back to the Criminal Investigation Detachment.
I have a dinner party in the evening, so I can’t be late.
As for the second operation team: "All personnel are here to compare fingerprints. Whether we can catch the suspect depends on these four sets of fingerprints!"
Returning to the detachment, Lu Chuan will find the box He put it back and went to the office to change out of his police uniform and put on his civilian clothes.
"How is the arson case handled over there?"
(End of this chapter)