Db 5 analytical column

Samples from the tank experiments
were frozen at −20 °C immediately after sampling until
enough samples were collected for isotope analysis. For carbon and
nitrogen isotope measurements of BAM and metolachlor, the samples
from the diffusion cell experiments (40 mL) and tank experiments (1
L) were first filtered through a 0.2 μm Nalgene Rapid-Flow filter
(Thermo Fisher Scientific, Germany) and concentrated in ethyl acetate
after solid-phase extraction as detailed in the SI. All isotope measurements were conducted on a GC–IRMS
system in which a TRACE GC Ultra gas chromatograph (Thermo Fisher
Scientific, Italy) with a DB-5 analytical column (60 m, 0.25 mm i.d.,
1 μm film, Agilent Technologies, Germany) was coupled to a Finnigan
MAT 253 isotope-ratio mass spectrometer through a Finnigan GC Combustion
III interface (Thermo Fisher Scientific, Germany). For carbon-isotope
measurements of benzene, toluene, and ethylbenzene, a Velocity XPT
purge-and-trap sample concentrator with an AQUATek 70 liquid autosampler
(Teledyne Tekmar, Mason, OH) was connected to the gas chromatograph.
Detailed information about the method can be found in the SI.

Biodegradation
of BAM by Aminobacter sp. MSH1 has previously been
shown to induce strong carbon isotope fractionation with isotopic
enrichment factors ε_C = −7.8 ± 0.2‰
at high concentrations in batch experiments.^{40 (link)} For the carbon isotope measurements of BAM, samples concentrated
in ethyl acetate after solid-phase extraction (SPE) were measured
on a GC-IRMS system in which a TRACE GC Ultra gas chromatograph (Thermo
Fisher Scientific, Italy) was coupled to a Finnigan MAT 253 isotope-ratio
mass spectrometer (IRMS) through a Finnigan GC Combustion III interface
(Thermo Fisher Scientific, Germany). The separation was carried out
on a DB-5 analytical column (60 m, 0.25 mm i.d., 0.5 μm film,
Agilent Technologies, Germany). The typical uncertainty of carbon
isotope measurements is ±0.5‰. A detailed method description,
including sample preparation and SPE, is provided in the SI.