Multicollector Inductively Coupled Plasma–Mass Spectrometry with 10<sup>13</sup> Ω Faraday Cup Amplifiers for Ultrasensitive Mg Isotopic Analysis of Cerebrospinal Fluid Microsamples
Rosa Grigoryan, Marta Costas‐Rodríguez, Patrick Santens, Frank Vanhaecke
Abstract
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‰.