Validation of an analytical method for the refractive index measurement of glass fragments. Application to a hit-and-run incident

Abstract

A method to determine the refractive index (RI) of glass samples was validated in this research article. This validation was performed by studying analytical features such as the working range, precision, robustness, and bias. Locke silicone oil type B, glass standards type B and a sodium D source (589.3 nm) were chosen for a working RI range from 1.50225 to 1.52381, according to the typical RI values of glass samples of forensic interest. Robustness, calculated as Relative Standard Deviation (RSD) of slopes of ten calibration lines within a period of one month was 0.24%. Precision of the method (calculated for the two extreme glass standards B2 and B12 in the working range) was 0.21% and 0.14% (repeatability) and 0.12% and 0.09% (intermediate precision), respectively. A Certified Reference Material (CRM) of glass of known RI was employed to estimate the bias of the method, verifying a relative error of 0.00066%. The method was applied to eight validation samples (six glass fragments from different parts of an automobile, a glass container and an architectural tinted window), which were differentiated through their RI values. Finally, the procedure was applied to interpret the origin of glass evidence taken from a hit-and-run incident. A common origin of recovered and control glass fragments was supposed on the basis of the RI determination of these samples, together with the application of useful match criteria in forensic pairwise comparisons of glass fragments. Results were confirmed by means of a highly discriminatory technique such as Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS).

Forensic glass examinations—A review focused on elemental spectrochemical analysis

“…The standard test method (ASTM E1967‐19, 2019) utilizes automated oil immersion refractometry with phase contrast microscopy to measure the refractive index of micron‐size particles. Due to controlled manufacturing processes, the RI is relatively homogeneous across a single object, and measurements can be obtained with high precision and accuracy when proper sampling protocols are followed (Alamilla et al, 2013; ENFSI, 2016; Garvin & Koons, 2011; Hicks, 2013; Insana & Buzzini, 2017; Munger et al, 2014; Newton, 2011; Trimpe & Sammarco, 2017). Current sampling and statistical comparison recommendations for RI are based on experimental studies assessing the RI micro‐heterogeneity and other sources of measurement variations (ASTM E1967‐19, 2019; Garvin & Koons, 2011; Newton, 2011).…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…A study comparing the use of refractive index and LA-ICP-MS for the forensic analysis of glass particles may also be of interest to those working in the eld. 291 The classication and discrimination of automotive glass of relevance in New Zealand has been the subject of a recent study. 292 The elemental composition of 243 samples of automotive glass was assessed using direct sampling with LA-ICP-MS.…”