Sub-10 Nanomole Perchlorate δ ³⁷ Cl, δ ¹⁸ O, and Δ ¹⁷ O Measurements by ESI-Orbitrap-MS

Perchlorate (ClO₄^-) is found in both Earth and Martian surface environments, yet its formation mechanism remains enigmatic. ClO₄^- isotope composition can help to identify the formation mechanism/source, but the conventional gas-source isotope ratio mass spectrometry (IRMS) method requires micromole or larger sample sizes together with tedious mineral precipitation steps. Here, a capable method for measuring δ³⁷Cl, δ¹⁸O, and Δ¹⁷O values of trace ClO₄^- (<10 nmol) was established using the electrospray ionization Orbitrap MS (ESI-Orbitrap-MS) equipped with a dual-inlet system. After testing shot noise limit, acquisition error, and Allan variance, we have identified an optimal analysis duration of 11 min with 7 cycles of sample-reference measurement. A three-point calibration against USGS standards demonstrated that higher automatic gain control (AGC) targets (over 1 × 10⁶) can improve Δ¹⁷O precision, but inducing scale distortion caused by space-charge effects for the δ¹⁸O and Δ¹⁷O. Our method could reach the accuracy of 0.6‰ for Δ¹⁷O (AGC Target ≈ 2 × 10⁶) and 0.6‰ and 0.7‰ for δ³⁷Cl and δ¹⁸O (AGC Target ≈ 1 × 10⁶), respectively, in 8.40 nmol sample. Across a sample size range of 0.84 to 8.40 nmol of ClO₄^-, the uncertainties remained ≤1.4‰ under both optimal and nonoptimal electrospray conditions. Man-made ClO₄^- from commercial sources and natural ClO₄^- from the Atacama Desert exhibited IRMS differences within 1‰ after purification and concentration control by ion chromatography. The method enables perchlorate isotope application to atmospheric, environmental, and planetary samples previously inaccessible due to sample scarcity.