What is the little piece of paper you wipe off when you go through security on an airplane?

When we go through security at the airport, we sometimes encounter a scene like this: the security personnel wipe your hand luggage, electronic devices, or even your clothes with a small white paper, and then put the test paper into the instrument next to it. What is the purpose of this seemingly ordinary step? What is the purpose of this small piece of paper? What is its principle?

The origin of the little white piece of paper

This small piece of white paper is actually a specially designed explosive trace detection paper, which is used to detect whether there are traces of explosives on the surface of luggage or items. Its use originated from the strict requirements for aviation safety, especially in the context of the increasing terrorist threats faced by the modern aviation industry, trace detection technology has become an important part of the airport security system.

In the history of aviation, many terrorist attacks have led to the continuous upgrading of aviation security measures. Especially in the early 21st century, as the methods of terrorist organizations became increasingly complex and covert, traditional security methods, such as metal detectors and X-ray machines, gradually became insufficient. Because these devices are mainly used to detect metal objects, and some new explosives (such as plastic explosives) may not contain metal components and are difficult to detect by traditional means.

"Roca's Law" mentioned in "Day and Night"

In order to meet this challenge, trace detection technology came into being. Its core concept is Rocard's law, which means "whatever touches, leaves a trace" . Simply put, when you touch an object, the secretions on your hands (sweat, dandruff, oil, etc.) will leave fingerprints on the surface of the object, and the particles of the object will more or less adhere to your hands. For example, after cutting an onion and rubbing your eyes with your hands, you may often burst into tears, because the substances in the onion have quietly run onto your fingers.

Working principle of explosive trace detection paper

The working principle of explosive trace detection paper is not complicated, but it is very effective. In simple terms, it uses the basic principle of chemical reaction to determine whether the item is safe by detecting the unique chemical composition of explosives.

We know that explosives usually contain certain specific chemical components, such as nitrates, nitro compounds, etc., which can trigger explosive reactions under certain conditions. Trace detection paper is coated with reagents that can react with these chemical components. When the paper is wiped over the surface of an object, if there are traces of explosives left on the object, these chemical components will adhere to the paper.

The paper is then placed in specialized testing equipment that can determine if there are traces of explosives by analyzing chemical reactions on the paper. For example, if a chemical reacts with the reagent on the test paper, the equipment will detect the reaction and sound an alarm. Based on the characteristics of the reaction, security personnel can determine whether this is a trace of a certain explosive.

Use an instrument to detect whether there are explosive particles on the paper

You may be worried about whether this detection method is really reliable? In fact, modern trace detection equipment is very sensitive and can detect extremely small traces of explosives. Even if there are only a few hundred millionths of a gram of explosive particles on the surface of luggage, the detection equipment can identify them. This high sensitivity is crucial for aviation security, because terrorists usually try to carry explosives in an extremely concealed way, and ordinary inspection methods may not be able to detect these threats.

The pros and cons of using trace detection paper in security inspection

The use of trace detection paper is not only for security reasons, but also because it is a fast and efficient means of inspection . Compared with completely unpacking luggage or conducting a full-body search, wiping items with a small piece of paper is obviously much faster. The inspection process usually only takes a few seconds and will not cause much trouble to passengers. At the same time, this inspection method is also non-invasive, and passengers do not need to worry about privacy issues. Security personnel only need to wipe the surface of the items, neither opening the luggage nor making physical contact with the passengers, thereby minimizing the invasion of personal privacy.

The inspectors use "one-touch communication" to quickly pass the release instructions to the release personnel, improving the efficiency of explosion-proof detection

Furthermore, time is of the essence at the airport. Every passenger wants to get through security and onto the plane as quickly as possible. The use of trace detection paper can significantly improve the overall efficiency of security checks. By quickly and accurately detecting traces of explosives, security personnel are able to process large numbers of passengers more quickly while ensuring that no dangerous items make it through security checks.

Although trace detection technology plays an important role in aviation safety, it is not perfect. Its limitations lie in the problems of false positives and false negatives . False positives refer to the detection equipment misreporting certain harmless substances as explosives, which may cause passengers to be further checked and cause unnecessary trouble. False negatives mean that the equipment fails to detect real traces of explosives. Although this situation is relatively rare, it still exists.

The small piece of paper that you wipe when you go through security check may seem simple, but it is actually an important application of modern technology in the field of aviation safety. Its existence not only protects our flights, but also reflects the wisdom and innovation of mankind in dealing with security threats. Next time when you encounter security personnel using this small piece of paper at the airport, you might as well be more understanding and patient, because it is silently providing security for your journey.

References

[1] Wang Yujia, Pan Xiaojia, Wei Shuoyun, et al. Research progress on the application of fluorescent quantum dots in trace detection of nitroaromatic explosives[J]. Journal of Analytical Science, 2023, 39(06): 716-724.

[2] Yuan Xi, Zheng Jian, Shi Liang, et al. Research on IMS explosive detector for on-site trace detection [J]. Nuclear Technology, 2006, (07): 544-547.

[3]https://mp.weixin.qq.com/s/gfyEupLorgMHkYW1e0TBoQ

[4]https://mp.weixin.qq.com/s/NpJ7OlEaeI2q98xjgdjBgA

[5]https://mp.weixin.qq.com/s/o43EyNQXnDCvmFgIz1n6pw

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Source丨Popular Science of Physical Science (ID: lxcp_cstam)

Editor: Yang Yaping

Proofread by Xu Lai and Lin Lin