Comparison of high-resolution and dynamic reaction cell ICP-MS capabilities for forensic analysis of iron in glass

Forensic laboratories routinely conduct analysis of glass fragments to determine whether or not there is an association between a fragment(s) recovered from a crime scene or from a suspect to a particular source of origin. The physical and optical (refractive index) properties of the fragments are compared and, if a “match” between two or more fragments is found, further elemental analysis can be performed to enhance the strength of the association. A range of spectroscopic techniques has been used for elemental analysis of this kind of evidence, including inductively coupled plasma mass spectrometry (ICP-MS). Because of its excellent sensitivity, precision, and accuracy, several studies have found that ICP-MS methods (dissolution and laser-ablation) provide the best discrimination between glass fragments originating from different sources. Nevertheless, standard unit-resolution ICP-MS instruments suffer from polyatomic interferences including ⁴⁰Ar¹⁶O⁺, ⁴⁰Ar¹⁶O¹H⁺, and refractory oxide ⁴⁰Ca¹⁶O⁺ that compromise measurements of trace levels of Fe⁵⁶⁺ and Fe⁵⁷⁺, for example. This is a drawback in the analysis of glass fragments because iron has been previously identified as a good discriminating element. Currently, several techniques are available that enable reduction of such interferences. However, there are no data comparing detection limits of iron in glass using those techniques. The aim of this study was to compare, the analytical performance of high-resolution sector field inductively coupled plasma mass spectrometry (HR-SF-ICP-MS) and quadrupole ICP-MS equipped with a dynamic reaction cell (DRC-ICP-MS), for the detection of iron in glass, in terms of accuracy, precision, and method detection limits (MDLs). Analyses were conducted using conventional acid-digestion and laser-ablation methods. For laser-ablation analyses, carrier gases were compared to assess the effect on detection limits in the detection of iron isotopes. Iron polyatomic interferences were reduced or resolved by using a dynamic reaction cell and high-resolution ICP-MS. MDLs as low as 0.03 μg g⁻¹ and 0.14 μg g⁻¹ were achieved in laser-ablation and solution-based analyses, respectively. Use of helium as carrier gas improved detection limits for both iron isotopes in medium-resolution HR-SF-ICP-MS and in DRC-ICP-MS.