On the use of X-rays to detect fingerprints.

There are almost no references in the specialized literature to the use of X-rays to identify fingerprints, with the exception of «Introduction to Forensic Science» by Peter de Forest [1].

Such an original method is recommended for visualizing traces on smooth non-porous and slightly porous surfaces, including human skin, after dusting them with lead powder. A similar method was experimentally tested in the Scientific and Technical Department of the Main Directorate of Internal Affairs of Leningrad and the Leningrad Region and the All-Russian Research Institute of the Ministry of Internal Affairs of the USSR in the mid-1980s. The possibility of detecting and recording latent fingerprints on paper, cardboard and skin using X-rays was studied. To “color” the traces, fingerprint powders with the addition of bismuth oxides and salts were used.

And just recently, Chris Worley, an analytical chemist at the Los Alamos National Laboratory in New Mexico, USA, reported at a meeting of the American National Chemical Society on a new method for detecting fingerprints using X-rays [2].

As is known, traditional methods of detecting fingerprints are associated with the use of powders, liquids or vapors of various chemical reagents to obtain a contrasting coloring of the papillary pattern of the fingerprint with its subsequent documentation. However, mechanical application of a solid coloring component can damage the fingerprint. In addition, in many cases, the use of these methods for developing fingerprints on surfaces such as textiles, fibrous paper, leather, some types of plastic, multi-colored surfaces and, finally, human skin is very problematic.

According to K. Worley and his colleagues, the new method of visualizing traces using X-rays is free from the above-mentioned drawbacks. Moreover, it is able to detect chemical impurities and/or markers contained in the trace substance, which can provide forensic scientists with additional information.

The new method uses the phenomenon of so-called micro-X-ray fluorescence (Micro X-ray Fluorescence – MXRF), which allows for the very rapid determination of the elementary composition of the chemical composition of a sample. In this case, it is a trace of a human hand. By irradiating it with a sharply focused X-ray beam, the expert does not subject the trace to any mechanical action, leaving it unchanged, absolutely suitable for further examination.

Of course, the new technology in no way replaces traditional methods, but rather complements them, allowing for the detection of traces containing certain impurities.

Using the phenomenon of micro-X-ray fluorescence, scientists were able to detect potassium, sodium and chlorine in the trace substance. And since the impurities of the said salts are located along the lines of the papillary pattern of the trace, there is a fundamental possibility of “drawing” the papillary pattern itself. According to K. Worley, he and his colleagues were able to identify handprints contaminated with various substances, including saliva, lotions and creams, without resorting to other methods.

Here it is necessary to make one important remark: according to the new technology fingerprints cannot be developed directly at the scene of the crime. Material evidence must be delivered to a laboratory equipped with the appropriate equipment. And this is complex and very expensive analytical equipment for X-ray fluorescence spectrometry.

The X-ray fluorescence spectrometry method is based on the dependence of the intensity of X-ray fluorescence on the concentration of a certain chemical element in the sample being studied. When the sample is irradiated with powerful X-ray radiation, characteristic fluorescence radiation of atoms occurs, which is proportional to their concentration in the sample.

Fig. 1. Electromagnetic radiation spectrum
 and the place of X-ray fluorescence in it

This characteristic radiation is decomposed into a spectrum, and then its intensity is measured using detectors and counting electronics. Mathematical processing of the spectrum allows for both quantitative and qualitative analysis.

According to the researchers, if further experiments confirm the effectiveness of the new method, then within 2-5 years we can expect the appearance of equipment suitable for use by practicing forensic scientists.

However, the new method also has its limitations, inherent in the method of X-ray fluorescence spectrometry itself.

Firstly, not all elements can be detected by this method. In general, the heavier the element, the easier it is to detect. Light elements: carbon, nitrogen and oxygen cannot be detected, while sodium, potassium and chlorine are detected quite reliably.

Secondly, some traces contain too little of the “control” substance to be detected.

In the near future, the authors propose to explore the possibility of combining fluorescent X-ray spectroscopy with other spectroscopic methods that allow detecting not only chemical elements, but entire and molecular structures, providing the forensic expert with valuable additional information.

For example, even now, the excessive presence of potassium in the trace substance may indicate the presence of potassium nitrate, a component of some explosive compositions, and a high level of sulfur and potassium is characteristic of black powder.

Even in cases where the pattern of the detected papillary pattern of the trace is not sufficient for confident identification of the individual, information on the chemical composition of the impurities may prove extremely valuable for the investigation as a whole.

K. Worley hopes that his research will eventually lead to the creation of a convenient portable device suitable for use directly at the crime scene.

The American researchers' report does not contain any information on how the papillary pattern is drawn using the signals produced by the X-ray fluorescence spectrometer. It remains only to make assumptions about the design of the device implementing this function.

It seems that the surface being examined is scanned in some way using a tightly focused source of soft X-ray radiation (5 — 10 keV), and the scanning results, combined with the output signals of the spectrometer, are displayed on a display in the appropriate coordinates.

In principle, a tightly focused beam of X-rays is quite feasible using the so-called Kumakhov optics – X-ray capillary optics, the principles of which were proposed in the 1980s at the Institute of X-ray Optics (at that time the laboratories of the I.V. Kurchatov Institute).

Kumakhov's optics is based on multiple total external reflection of X-rays. Multiple reflections allow the beam to be rotated by a significant total angle of the order of several degrees. This means that any X-ray entering an empty smooth tube, such as glass, at an angle smaller than the critical angle will be repeatedly reflected from the inner surfaces of the tube, creating a «virtual» X-ray source at its end.

This principle formed the basis for the development of the first Kumakhov capillary lens, created in the mid-80s of the 20th century.

Fig. 2. Scheme of operation of Kumakhov optics

Kumakhov's optics allow deflecting radiation at angles hundreds of times greater than the Fresnel angle. Waveguides based on glass capillaries with a diameter of about 1 µm have already been created. On this basis, X-ray and neutron lenses have been created for the first time in the world. X-ray lenses are a monolithic system of curved glass capillary waveguides of varying length and configuration.

Depending on the tasks, lenses can have different lengths and diameters from 1 to 3 cm.

One square centimeter can contain one hundred thousand channels of a special configuration (round, hexagonal, etc.), and 1…3 billion channels.

The lenses are designed to transport and control X-ray, gamma, and neutron radiation, such as focusing, monochromatization, and energy filtration. By directing X-ray radiation through a special capillary system, it is possible to focus an X-ray beam into a focal spot of different sizes.

Experiments conducted at accelerators in the USA and Europe have shown that Kumakhov lenses and half-lenses provide an energy flux density equal to that of a mid-generation accelerator. Kumakhov lenses allow one to create a focused beam with a diameter of several microns. Kumakhov capillary optics are the most efficient optics, which can be easily combined with traditional X-ray tubes with a finite-size anode.

Many modern X-ray fluorescence spectrometers use X-ray tubes with a Kumakhov lens to create an excitation beam.

Photo 1. MXRF spectrometer µ-EDX-1200/1300/1400 series
by the Japanese company SHIMADZU

For example, in the micro-X-ray fluorescence spectrometers µ-EDX-1200/1300/1400 series (photo 1) by the Japanese company SHIMADZU, local analysis is performed at a point with a diameter of only 50 microns!

Photo 2. MiniPal 4 desktop spectrometer
by PHILIPS Analytical B.V

The primary (exciting) radiation is focused using a Kumakhov lens – a polycapillary optical system.

The µ-EDX-1300 unit allows elemental analysis from sodium to uranium in the air atmosphere, and the µ-EDX-1200/1400 version – from aluminum to uranium.

K. Worley's hopes for creating portable equipment for detecting fingerprints using X-rays are well founded, if we take into account the MiniPal 4 desktop spectrometer (photo 2) from PHILIPS Analytical B.V. (power consumption 80 W, weight 28 kg, dimensions – 220 x 530 x 500 mm). The device is recommended for simultaneous analysis of samples in the range of elements to be determined from sodium to uranium.

High analytical results are achieved by using a new high-resolution silicon energy-dispersive detector that operates without liquid nitrogen cooling. Cooling is provided by a device that uses the Peltier effect.

The energy resolution of the detector is no worse than 145 eV on the 5.9 keV line.

In terms of resolution, the new device is not inferior to traditional X-ray fluorescence spectrometers that operate with semiconductor detectors cooled by liquid nitrogen.

The MiniPal 4 mini spectrometer is successfully used in industry, as well as in research and development, including as part of field laboratories.

Literature

1. Forensic Science. An Introduction to Criminalistics. P.R. de Forest, R.E. Gaensslen, H.C. Lee, McGraw Hill, Inc., 1998.
2.Physorg reference and information site.