Multicollector Inductively Coupled Plasma-Mass Spectrometry with 10 Ω Faraday Cup Amplifiers for Ultrasensitive Mg Isotopic Analysis of Cerebrospinal Fluid Microsamples.

Magnesium isotopic analysis of cerebrospinal fluid (CSF) is a potentially interesting approach for studies on neurodegeneration. However, this type of analysis is challenging because of the invasiveness of the sampling and small sample volume. In this work, a novel analytical method was developed for ultrasensitive Mg isotopic analysis of CSF microsamples multicollector inductively coupled plasma-mass spectrometry (MC-ICP-MS) using high-gain 10 Ω Faraday cup amplifiers. The intermediate and internal errors on the δMg value were improved up to fourfold using 10 Ω resistors for the monitoring of both the Mg and Mg isotopes and up to twofold using a 10 Ω resistor for the most abundant Mg isotope and a 10 Ω resistor for the Mg isotope. Magnesium isotope ratios measured at a concentration level of 7-10 μg L were in good agreement with those obtained using the conventional method at a concentration level of 150 μg L. The expanded uncertainty for the quality control CSF material obtained at the ultratrace level was ±0.16‰. Ultrasensitive Mg isotopic analysis was carried out for CSF from hydrocephalus patients using only 5 μL of sample. δMg values thus obtained were not significantly different from those obtained using the conventional method using a sample volume of 400 μL instead ( ≤ 0.05). The Mg isotopic composition of the CSF from hydrocephalus patients ranged between -0.65 and 0.30‰, with a mean δMg value of -0.14 ± 0.27‰.