EA and GC-C-IRMS

This analytical section specialises in the isotopic analysis of organic pollutants, primarily pesticides, present in environmental samples (both liquid and solid). The study of isotopic fractionation (C & N) in these compounds provides insights into degradation pathways (e.g. microbial, photolysis, hydrolysis) and helps differentiate destructive processes (e.g. degradation) from non-destructive processes (e.g. sorption and dilution). This knowledge enhances our understanding of biological and geochemical mechanisms governing pesticide transport and their key attenuation pathways in the environment. The overarching research theme is to track and understand the dynamics of exogenous fluxes within terrestrial ecosystems.

The EA-IRMS system is an elemental analyser coupled with an Isotope Ratio Mass Spectrometer (IRMS). It measures %C and %N as well as δ¹³C and δ¹⁵N in solid and liquid samples. The sample is placed in a tin capsule and introduced into a combustion reactor, where it undergoes flash combustion at 1000°C, followed by reduction at 550°C in a secondary reactor. This process converts the sample into elemental gases: CO₂, N₂, and H₂O. Water is removed via a trap, while the remaining gases are separated using a molecular sieve chromatography column. Carbon and nitrogen percentages are determined using a thermal conductivity detector (TCD). After passing through the detector, the gases enter the IRMS via the Conflo IV interface, where they are ionised by an electron beam. The resulting ions are then separated based on their mass-to-charge ratio (m/z) within a magnetic field, and detected using Faraday collectors.

Applications

Unlike EA-IRMS, which measures the bulk isotopic composition of an entire sample, GC-C-IRMS enables the analysis of the isotopic composition of individual molecules—a technique known as Compound-Specific Isotope Analysis (CSIA). CSIA-based isotopic analyses are essential for various scientific fields:

• Environmental science: Tracing the origin and reactivity of pollutants by identifying contamination sources and degradation processes. CSIA studies on pesticides have demonstrated significant potential for evaluating degradation pathways and predicting transformation products.
• Climatology: Reconstructing past climate variations using natural archives, such as marine sediments.
• Biology: Studying trophic networks and dietary patterns of organisms. CSIA is widely applied in amino acid analysis.
• Petroleum geochemistry: Identifying oil sources and understanding hydrocarbon formation and alteration processes.

Overview

Once the sample is concentrated, it can be analysed using Gas Chromatography-Combustion – Isotope Ratio Mass Spectrometry (GC-C-IRMS). This technique provides δ¹³C and δ¹⁵N isotopic ratios for individual organic molecules in a sample. The method consists of three major steps:

1. Injection & Separation: The liquid sample is introduced via an automated sampler into a gas chromatographic system. The organic molecules within the sample are separated using a capillary column.
2. Combustion: Each separated molecule undergoes oxidation in a 1000°C reactor, converting them into CO₂, NOₓ, and H₂O gases. Water is removed via a trap at the reactor exit.
3. Mass Spectrometry: The remaining gases enter the IRMS via the Conflo IV interface. They are ionised by an electron beam, separated in a magnetic field, and detected using Faraday collectors based on their mass-to-charge ratio (m/z).

• Analysis of environmental samples, including water, soil, and sediments
• Identification of pesticides (e.g. chloroacetanilides, herbicides such as metolachlor, acetochlor, alachlor, pendimethalin, and oryzalin, as well as fungicides such as metalaxyl) and their degradation products
• Use of V-PDB (Vienna Pee Dee Belemnite) reference materials and certified IAEA-600 (caffeine) and USGS40/USGS41 (L-glutamic acid) standards