The quantitative determination of calcium in human fingernail keratin by laser-induced breakdown spectroscopy (LIBS) is compared with the results of inductively coupled plasma mass spectrometry (ICP–MS) in the present study. In a controlled setting, 75 fingernail samples were collected from type 1 (T1D) and type 2 (T2D) diabetes patients and healthy controls (C). The calcium concentrations were then determined independently by both methods. The LIBS calibration model was developed using reference samples determined by ICP-MS and based on the Ca II emission line at 396.847 nm, which was selected due to its spectral stability and reduced self-absorption under optimal plasma conditions. The model showed a good linearity (R² = 0.958), low relative prediction error (~6.9%) and the detection limits suitable for biological keratin matrices. Quantitative validation with diabetic samples demonstrated strong analytical agreement between LIBS and ICP-MS. Bland-Altman plots, Deming regression and Pearson correlation analysis (r > 0.998) revealed small systematic bias. A moderate decrease in calcium concentration was observed in the type 1 diabetes group by LIBS; however, this trend did not remain statistically significant after Tukey HSD correction. ICP-MS analysis of nail Ca concentrations between the diabetic and healthy control groups did not show statistically significant differences. Rather than signifying a genuine biological change, this discrepancy could be a consequence of matrix-related sensitivity. These findings indicate that calcium in nail keratin is stable regardless of metabolic state and is not an effective biomarker for diagnosing diabetes. As a stable internal reference element for biomedical research based on multi-element LIBS, calcium may serve as a promising internal reference element for future multi-element LIBS studies.

The quantitative determination of calcium in human fingernail keratin by laser-induced breakdown spectroscopy (LIBS) is compared with the results of inductively coupled plasma mass spectrometry (ICP–MS) in the present study. In a controlled setting, 75 fingernail samples were collected from type 1 (T1D) and type 2 (T2D) diabetes patients and healthy controls (C). The calcium concentrations were then determined independently by both methods. The LIBS calibration model was developed using reference samples determined by ICP-MS and based on the Ca II emission line at 396.847 nm, which was selected due to its spectral stability and reduced self-absorption under optimal plasma conditions. The model showed a good linearity (R² = 0.958), low relative prediction error (~6.9%) and the detection limits suitable for biological keratin matrices. Quantitative validation with diabetic samples demonstrated strong analytical agreement between LIBS and ICP-MS. Bland-Altman plots, Deming regression and Pearson correlation analysis (r > 0.998) revealed small systematic bias. A moderate decrease in calcium concentration was observed in the type 1 diabetes group by LIBS; however, this trend did not remain statistically significant after Tukey HSD correction. ICP-MS analysis of nail Ca concentrations between the diabetic and healthy control groups did not show statistically significant differences. Rather than signifying a genuine biological change, this discrepancy could be a consequence of matrix-related sensitivity. These findings indicate that calcium in nail keratin is stable regardless of metabolic state and is not an effective biomarker for diagnosing diabetes. As a stable internal reference element for biomedical research based on multi-element LIBS, calcium may serve as a promising internal reference element for future multi-element LIBS studies.