Operando isotope selective ammonia quantification in nitrogen reduction studies via gas chromatography-mass spectrometry

Rapid advances in electrocatalytic ammonia synthesis are impeded by laborious detection methods commonly used in the field and by constant risk of external contaminations, which generates misleading false positives. We developed a facile real-time GC-MS method for sensitive isotope NH3 quantification, requiring no external sample manipulations. This method ensures high detection reliability paramount to accelerate (electro-)catalyst screening.


S.1 -GC-MS layout and experimental method
A simplified representation of the presented GC-MS system is shown in Supplementary Fig. 1. The sample is first introduced in a switching multiport VICI Valco diaphragm valve, kept at a constant temperature of 80°C. A sample loop of 500 µL is filled with the analyte and then, at the moment of the injection, transported with a purified He carrier gas through two Select Low Ammonia (Agilent) capillary columns. Temperature and pressure profiles are provided in Supplementary Fig. 2. This ensures the best water/ammonia separation and an optimal peak shape. Once eluted out of the chromatographic column, the sample is partitioned between the pulse discharge detector (PDD) and the single quadrupole mass spectrometer (ISQ) via two restriction lines of one meter each and with internal diameter (ID) of 0.25 mm and 0.15 mm, respectively. Importantly, all the surfaces in contact with the analyte are inert or carefully passivated with inert coating (SilcoNert® 2000) to minimize any possible ammonia physisorption. The system was assembled by Interscience BV (Breda, The Netherlands).

S.2 -Estimated experimental time for cumulative detection methods
The required minimum time for an electrochemical nitrogen reduction reaction (NRR) experiment can be defined as the time required to reach the limit of detection of the analytical method used to quantify ammonia. NH 3 detection methods, as Berthelot method, 1 H NMR or UPLC-MS, rely on extensive NH 3 accumulation in a liquid medium (electrolyte or external acid trap).
As today, most reported ammonia electrochemical synthesis rate ( ) are on the order of 10 -10 mol NH3 cm -3 2 s -1 , 1 and electrochemical H-cells have a typical catalyst area to electrolyte volume ratio of 0.02-0.04 ( ) cm 2 mL -1 . Therefore, considering a practical limit of quantification (LOQ) of 10 µM, 2 the minimum time strictly necessary for a NRR experiment (t LOQ ) to reach such limit would be between 0.7 and 1.4 h, as calculated from equation 1. Additional time is then required for sampling, analyte preparation and analysis for multiple data points. Therefore, a single NRR experiment based on indirect cumulative detection methods can take several hours.

S.4 -Additional GC-MS measurements and calculations
To further clarify the peak identification and ammonia quantification, we hereby report a series of measurements performed under the following different conditions: 14 NH 3 alone, from certified calibration gas in dry and humidified conditions ( Supplementary Fig. 3); 15 NH 3 alone, from aqueous solution headspace ( Supplementary Fig. 4); 14 NH 3 / 15 NH 3 mixture from aqueous solution headspace (Fig. 3); ultrapure water headspace without ammonia ( Supplementary Fig. 5) and direct analysis of purified (Agilent OT3-4 filter) N 2 gas ( Supplementary Fig. 6).
When present in the analyte, ammonia ( 14 N or 15 N) is detected in the chromatograph as a peak with a retention time of 1.83 min, in the absence of interferences from water vapour, which is eluted at a later chromatographic elution time of about 2 min. Similarly, excellent water/ammonia separation is established at the MS detector for the m/z 16, 17 and 18. The outsized amount of water vapour (compared to typical ppm levels of ammonia produced in electrochemical nitrogen reduction experiments) carried during sampling of aqueous headspace is affected by several factors and therefore can largely vary over time, making unfeasible to reliably quantify NH 3 without a proper water/ammonia separation (e.g. in MS detectors without chromatographic separation).

Sample preparation
The concentration of 14 NH 3 and 15 NH 3 in solution was quantified using an 1 H NMR method with absolute quantification as previously described. 4 This method allows for quantification of 14 NH 3

H NMR data acquisition
1 H NMR spectra were acquired using a 400 MHz Fourier transform NMR spectrometer equipped with an autosampler and an autotunable, temperature regulated Agilent OneNMR room temperature probe. The temperature was set to 25° C and the receiver gain was optimized automatically. The excitation sculpting pulse sequence "waterES" was used to suppress the resonance of water during acquisition. The "waterES" pulse sequence has the following structure: waterES: d1-P90-G1-S180-P180-G1-G2-S180-P180-G2-aq where, G1,G2 are z-gradients of different strengths, P90, P180 are hard pulses and S180 is a selective 180° pulse. The acquisition parameter were: 0.75 s acquisition time, 0.05 s recycle delay and 1024 number of scans.

H NMR data processing
The data was processed with the software package MestReNova (version: 12.0.1-20560) using the automated tools available with this software. Unless otherwise noted an apodization of 4 Hz was applied followed by phasing and baseline correction. The peaks of NH 4 + (t/d ≈6.9 ppm, 4H) and MA (s, ≈6.21 ppm, 2H) were integrated using the line fitting tool (Lorentzian fit). The integrals of the 14 NH 4 + triplets (or 15 NH 4 + doublet) were added together to calculate the total 14 NH 4 + (or 15 NH 4 + ) integral (I NH4+ ). The concentration of NH 4 + was calculated with absolute quantification, from the ratio of the integral of NH 4 + and MA (I std ), according to equation 6.