Multicollector Inductively Coupled Plasma–Mass Spectrometry with 10¹³ 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 via multicollector inductively coupled plasma–mass spectrometry (MC-ICP-MS) using high-gain 10¹³ Ω Faraday cup amplifiers. The intermediate and internal errors on the δ²⁶Mg value were improved up to fourfold using 10¹³ Ω resistors for the monitoring of both the ²⁴Mg and ²⁶Mg isotopes and up to twofold using a 10¹¹ Ω resistor for the most abundant ²⁴Mg isotope and a 10¹³ Ω resistor for the ²⁶Mg isotope. Magnesium isotope ratios measured at a concentration level of 7–10 μg L^–1 were in good agreement with those obtained using the conventional method at a concentration level of 150 μg L^–1. 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 (p ≤ 0.05). The Mg isotopic composition of the CSF from hydrocephalus patients ranged between −0.65 and 0.30‰, with a mean δ²⁶Mg value of −0.14 ± 0.27‰.