Precision benchmarking of the EDAX Orbis II for forensic glass comparison by micro-XRF

Introduction

Elemental-ratio comparison between glass fragments has long been a cornerstone of forensic glass examination [1]. Benchtop micro x-ray fluorescence (micro-XRF) enables the simultaneous detection of major, minor, and trace elemental constituents in the small glass fragments that are routinely encountered in forensic casework, while delivering high accuracy and precision. Because the method is completely non-destructive, it can be applied at any stage of the analytical workflow without altering sample shape or material properties.

Although accuracy is important for building and curating glass databases, precision is typically more critical for routine forensic comparison between unknown fragments and controls. An interlaboratory round-robin study conducted in 2009 demonstrated that micro-XRF can achieve high precision and reproducibility and performs well for glass comparison [2]. Building on that foundation, we evaluated the precision of the EDAX^® Orbis II using the same methodology to permit a direct comparison. Because eight of the nine instrument configurations in the round-robin study represented only three models—EDAX Eagle II, EDAX Eagle III, and EDAX PC [2]—all predecessors of the Orbis II, the study design provides an appropriate basis for assessing consistency across product generations.

Materials and methods

Seven replicate analyses were performed on full thickness fragments of the forensic glass standards FGS1 and FGS2 [3] using an Orbis II Super LZ micro-XRF analyzer, with instrument parameters listed in the Orbis II column of Table 1. The system is equipped with a C5 window silicon drift detector (SDD) that provides higher transmission for low-Z elements than the beryllium window SDDs used by the instruments in the round-robin study. Advances in detector technology permit higher throughput handling at constant time together with improved energy resolution, so an acquisition time of 600 live seconds was sufficient, which is at least 50% shorter than the conditions typically used in the round-robin.

Table 1. Instrument configurations used in the 2009 inter-laboratory round-robin study and in this work. Columns A – I summarize the round-robin configurations [2], and the Orbis II column lists the parameters used in the present evaluation.

To enable direct interlaboratory comparisons despite differences in instrument configuration and elemental response, results were normalized against SRM 1831 standard reference glass. For each element, the mean intensity measured on the forensic glass standard was divided by the mean intensity for the same element measured on SRM 1831 on the same day [2]. We evaluated the standard forensic elemental ratios Ca/Mg, Ca/Ti, Ca/Fe, Sr/Zr, Fe/Zr, and Ca/K. For each ratio we compared the Orbis II results to the round-robin averages and calculated z-scores, defined as the deviation of a reported value from the study mean divided by the study standard deviation [2]. All Orbis II z-scores met the acceptance criterion (|z| ≤ 3), as summarized in Table 2.

Table 2. Precision data for FGS1 and FGS2. Round-robin study average values were obtained from nine different instrument configurations, seven replicates per configuration for a total of 63 replicates [1,2]. Orbis II average values were calculated from seven replicates.

Results

In the 2009 round-robin, the majority of relative standard deviation (%RSD) values for these were <10% for both FGS1 and FGS2 ratios (measured on the Eagle series instruments; Table 3, columns A – C and E – G). Ratios involving Ti and Zr were higher, reaching as high as ~53% in FGS1, which is consistent with the low concentrations of Ti (69 ppm) and Zr (57 ppm) [3] and with the lower sensitivity of the older systems. In contrast, all Orbis II %RSDs were <10%, and most were in the range of ~2 – 4%, as shown in Table 3, Orbis II column. Even for Sr and Zr in FGS1, where the concentrations are at the trace level of 57 ppm and 49 ppm, respectively [3], the spectra display clear and well-resolved peaks (Figure 1), indicating improved sensitivity and counting statistics.

Table 3. %RSD values for elemental-ratio measurements in FGS1 and FGS2. Columns A-G report round-robin values [4] and the Orbis II column reports values from this work.

Conclusion

The Orbis II maintains excellent consistency with prior generation EDAX instruments, while delivering significantly improved precision for trace elements and doing so with at least 50% shorter acquisition time. Practically, this means higher-precision data can be obtained with greater efficiency, reinforcing micro-XRF as a high-throughput, non-destructive tool for forensic glass comparison and casework reporting.

References

1. ASTM International. (2025) Standard Test Method for Forensic Comparison of Glass using Micro X-ray Fluorescence (μ XRF) Spectrometry. West Conshohocken (PA): ASTM International. ASTM E2926- 17. https://doi.org/10.1520/E2926-17
2. Trejos T et al. Analytical and Bioanalytical Chemistry (2013) 405 5393–5409. https://doi.org/10.1007/s00216-013-6978-y
3. Latkoczy C et al. Journal of Forensic Sciences (2005) 50 1327–1341. https://doi.org/10.1520/JFS2005091
4. Valadez M et al. Trace Evidence Symposium 2011, 2011.