Chemical-physical examinations
Chemical-physical examinations, popularly referred to simply as CHEMISTRY, have a continuous tradition that extends back several decades. Whether they were organized within a section, department, or service as an organizational unit, from their beginnings to the present day chemical-physical examinations have covered the widest area of work. This includes the examination of traces left after explosions, fires, traffic accidents, burglaries, property damage, and more recently, environmental pollution.
To ensure the highest possible level of objectivity, they apply numerous instrumental analysis methods in their work. The chosen method primarily depends on the type and quantity of the trace, and it is always necessary to select the method that minimizes the possibility of sample contamination. During forensic examinations non-destructive methods have priority whenever possible to preserve physical evidence.
Developed methods can analyse a wide range of materials, including: pigments, glass, construction materials, soils, minerals, metals, metal alloys and their corrosion products, organic and bioinorganic materials such as wood, textiles, paper, oil-based binders, sugars, adhesives, natural or synthetic coatings, adhesive tapes, cosmetics, polymers, flammable liquids, post-explosion residues, gases in diving cylinders, as well as a wide range of organic and inorganic chemicals.
Essential techniques used for the analysis of such samples include light microscopy (visible, ultraviolet, and polarized light), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy coupled with microscopy, gas chromatography - mass spectrometry (GC-MS), pyrolysis gas chromatography - mass spectrometry (Py-GC-MS), ion chromatography (IC), ultraviolet-visible spectroscopy (UV/Vis), ion trap mobility spectrometry (ITMS), gas analysers and instruments for measuring the density and kinematic viscosity of liquids. Additionally, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray fluorescence (µ-XRF) are also used.
The choice of method depends on the type and quantity of material as well as on the questions that must be answered, which arise from the event itself.
The goal of chemical-physical examinations is most often to:
Special challenges in this work include limited sample quantities, a wide range of analytes and matrices and the risk of contamination.
Paint Examination
Paint as a trace can occur in various criminal offenses, ranging from hit-and-run traffic accidents to burglaries and property damage. Paint traces can appear as single-layer or multilayer fragments, smears, liquid samples, or spray paint. Forensic identification and comparison of paint is a complex procedure that uses light microscopy to determine physical properties such as hue, morphology, surface characteristics and the layer arrangement in cross-section, while methods for determining chemical composition include vibrational spectroscopy and the Py-GC-MS system.
To obtain accurate and reliable results from instrumental techniques, the forensic expert preparing the paint samples must perform meticulous work. Before analysis, multilayer paint samples must be manually separated using a scalpel. The difficulty of the task lies in the fact that the average thickness of layers, for example in automotive paints, is only 20-30 µm.
Glass Examination
Broken windows, car glass, bottles, and other glass objects are important evidence in crimes such as burglaries, homicides, or hit-and-run traffic accidents. It is well known that any person located near a breaking glass object may collect tiny glass particles, which can remain on clothing, footwear, hair, or on tools used to commit the crime (burglary tools). The task of the forensic examiner is usually to compare two or more glass fragments to determine whether they originate from the same source.
Glass analysis includes determining visual characteristics such as colour, shape, thickness, or potential matching fracture lines, as well as measuring the refractive index and elemental composition of the glass. Measurements are performed using a glass refractive index measurement instrument (GRIM3), and results are expressed to five decimal places, which provides a high level of discrimination between samples. Elemental analysis is performed using µ-XRF.
Examination of Flammable Liquid Traces
The field of chemical-physical examinations includes the analysis of traces of flammable liquids and substances (gasoline, diesel fuel, fire-starter cubes, etc.) on objects collected from fire scenes, as well as on the hands and clothing of suspects.
For sample preparation, solid-phase microextraction (SPME) is primarily used, while analysis is carried out using GC-MS. The introduction of SPME as a sampling technique in 2004 represented a significant breakthrough in the examination of flammable liquid traces. First and perhaps most importantly, it significantly reduced the use of large quantities of carcinogenic hexane, previously used as an extraction solvent. Furthermore, sample preparation efficiency increased, which ultimately improved analytical results and increased the number of successfully resolved cases. Because SPME is a sensitive extraction technique suitable for sampling various substances from different matrices, it is also successfully used for the analysis of self-defence sprays (tear gas) and other volatile substances.
Explosives Examination
The development of methods for examining traces of explosives, explosive devices, and pyrotechnic materials was greatly influenced by the Croatian War of Independence. During and especially after the war, large quantities of explosives and explosive devices remained illegally in the possession of citizens, and they are still often used in criminal acts today.
Experts in this field gained extensive experience over many years through frequent on-site investigations of explosions. However, what was missing was a technique capable of detecting traces of explosives even after an explosion, since the thin-layer chromatography used at the time had a detection limit that was too high. For this purpose, an instrument for explosive detection based on ion trap mobility spectrometry (ITMS) was acquired. This instrument allows the detection of traces of approximately ten of the most common explosives, with detection limits reaching concentrations in the ppm and ppb range. Forensic examinations in this field also include determining the type and method of activation of explosive devices.
For faster and more accurate results, forensic experts have access to digital internal and international collections of chromatograms of flammable liquids, with about 1500 ignitable liquid chromatograms and nearly 1800 chromatograms of various matrices. Internal and ENFSI databases of IR and Raman spectra, with automated search capabilities, include more than 150 000 spectra of automotive paints, pigments, spray paints, tool paints, adhesive tapes, food products, and polymers. Internal databases of explosives, gunpowders, organic and inorganic compounds, and various other substances are continuously updated.
Forensic chemists are also responsible for training police officers who directly participate in fire and explosion scene investigations, as well as officers of different professional profiles.
Forensic experts are active members of the ENFSI Fire and explosion investigation working group (FEIWG) and Paint, glass and taggants working group (EPGTWG). To confirm their competence, forensic experts participate in international interlaboratory proficiency tests in the fields of flammable liquid trace analysis, paint, glass, explosives, and adhesive tapes. Successful test results demonstrate excellence maintained through continuous professional education, monitoring scientific literature, and participation in annual ENFSI meetings and international symposia.
Within the field of chemical-physical examinations, procedures, documentation preparation, and other tasks related to the quality management system are carried out in accordance with HRN EN ISO/IEC 17025:2017. So far, six qualitative methods for the detection and determination of explosives and flammable liquids and one quantitative method for cation determination using ion chromatography have been accredited.
Accredited methods:
To ensure the highest possible level of objectivity, they apply numerous instrumental analysis methods in their work. The chosen method primarily depends on the type and quantity of the trace, and it is always necessary to select the method that minimizes the possibility of sample contamination. During forensic examinations non-destructive methods have priority whenever possible to preserve physical evidence.
Developed methods can analyse a wide range of materials, including: pigments, glass, construction materials, soils, minerals, metals, metal alloys and their corrosion products, organic and bioinorganic materials such as wood, textiles, paper, oil-based binders, sugars, adhesives, natural or synthetic coatings, adhesive tapes, cosmetics, polymers, flammable liquids, post-explosion residues, gases in diving cylinders, as well as a wide range of organic and inorganic chemicals.
Essential techniques used for the analysis of such samples include light microscopy (visible, ultraviolet, and polarized light), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy coupled with microscopy, gas chromatography - mass spectrometry (GC-MS), pyrolysis gas chromatography - mass spectrometry (Py-GC-MS), ion chromatography (IC), ultraviolet-visible spectroscopy (UV/Vis), ion trap mobility spectrometry (ITMS), gas analysers and instruments for measuring the density and kinematic viscosity of liquids. Additionally, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDX) and X-ray fluorescence (µ-XRF) are also used.
The choice of method depends on the type and quantity of material as well as on the questions that must be answered, which arise from the event itself.
The goal of chemical-physical examinations is most often to:
- identify a specific substance, or
- compare two samples (regardless of their exact composition) in order to determine whether they originate from a common source.
Special challenges in this work include limited sample quantities, a wide range of analytes and matrices and the risk of contamination.
Paint Examination
Paint as a trace can occur in various criminal offenses, ranging from hit-and-run traffic accidents to burglaries and property damage. Paint traces can appear as single-layer or multilayer fragments, smears, liquid samples, or spray paint. Forensic identification and comparison of paint is a complex procedure that uses light microscopy to determine physical properties such as hue, morphology, surface characteristics and the layer arrangement in cross-section, while methods for determining chemical composition include vibrational spectroscopy and the Py-GC-MS system.
To obtain accurate and reliable results from instrumental techniques, the forensic expert preparing the paint samples must perform meticulous work. Before analysis, multilayer paint samples must be manually separated using a scalpel. The difficulty of the task lies in the fact that the average thickness of layers, for example in automotive paints, is only 20-30 µm.
Glass Examination
Broken windows, car glass, bottles, and other glass objects are important evidence in crimes such as burglaries, homicides, or hit-and-run traffic accidents. It is well known that any person located near a breaking glass object may collect tiny glass particles, which can remain on clothing, footwear, hair, or on tools used to commit the crime (burglary tools). The task of the forensic examiner is usually to compare two or more glass fragments to determine whether they originate from the same source.
Glass analysis includes determining visual characteristics such as colour, shape, thickness, or potential matching fracture lines, as well as measuring the refractive index and elemental composition of the glass. Measurements are performed using a glass refractive index measurement instrument (GRIM3), and results are expressed to five decimal places, which provides a high level of discrimination between samples. Elemental analysis is performed using µ-XRF.
Examination of Flammable Liquid Traces
The field of chemical-physical examinations includes the analysis of traces of flammable liquids and substances (gasoline, diesel fuel, fire-starter cubes, etc.) on objects collected from fire scenes, as well as on the hands and clothing of suspects.
For sample preparation, solid-phase microextraction (SPME) is primarily used, while analysis is carried out using GC-MS. The introduction of SPME as a sampling technique in 2004 represented a significant breakthrough in the examination of flammable liquid traces. First and perhaps most importantly, it significantly reduced the use of large quantities of carcinogenic hexane, previously used as an extraction solvent. Furthermore, sample preparation efficiency increased, which ultimately improved analytical results and increased the number of successfully resolved cases. Because SPME is a sensitive extraction technique suitable for sampling various substances from different matrices, it is also successfully used for the analysis of self-defence sprays (tear gas) and other volatile substances.
Explosives Examination
The development of methods for examining traces of explosives, explosive devices, and pyrotechnic materials was greatly influenced by the Croatian War of Independence. During and especially after the war, large quantities of explosives and explosive devices remained illegally in the possession of citizens, and they are still often used in criminal acts today.
Experts in this field gained extensive experience over many years through frequent on-site investigations of explosions. However, what was missing was a technique capable of detecting traces of explosives even after an explosion, since the thin-layer chromatography used at the time had a detection limit that was too high. For this purpose, an instrument for explosive detection based on ion trap mobility spectrometry (ITMS) was acquired. This instrument allows the detection of traces of approximately ten of the most common explosives, with detection limits reaching concentrations in the ppm and ppb range. Forensic examinations in this field also include determining the type and method of activation of explosive devices.
For faster and more accurate results, forensic experts have access to digital internal and international collections of chromatograms of flammable liquids, with about 1500 ignitable liquid chromatograms and nearly 1800 chromatograms of various matrices. Internal and ENFSI databases of IR and Raman spectra, with automated search capabilities, include more than 150 000 spectra of automotive paints, pigments, spray paints, tool paints, adhesive tapes, food products, and polymers. Internal databases of explosives, gunpowders, organic and inorganic compounds, and various other substances are continuously updated.
Forensic chemists are also responsible for training police officers who directly participate in fire and explosion scene investigations, as well as officers of different professional profiles.
Forensic experts are active members of the ENFSI Fire and explosion investigation working group (FEIWG) and Paint, glass and taggants working group (EPGTWG). To confirm their competence, forensic experts participate in international interlaboratory proficiency tests in the fields of flammable liquid trace analysis, paint, glass, explosives, and adhesive tapes. Successful test results demonstrate excellence maintained through continuous professional education, monitoring scientific literature, and participation in annual ENFSI meetings and international symposia.
Within the field of chemical-physical examinations, procedures, documentation preparation, and other tasks related to the quality management system are carried out in accordance with HRN EN ISO/IEC 17025:2017. So far, six qualitative methods for the detection and determination of explosives and flammable liquids and one quantitative method for cation determination using ion chromatography have been accredited.
Accredited methods:
- Detection of explosive substances and ammonium nitrate using ion trap mobility spectrometry (ITMS)
- Determination of explosive substances using infrared spectroscopy (FTIR with ATR technique)
- Qualitative determination of gasoline and diesel fuel components using SPME-GC/MS
- Quantitative determination of cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺, Ca²⁺) using ion chromatography
- Qualitative determination of flammable liquid components using GC/MS
- Qualitative determination of Rhodamine B using UV/Vis spectrophotometry