Boron Isotope NMR Applications Quiz

Introduction to Quantitative Boron-11 NMR

• NMR (nuclear magnetic resonance) has historically had limitations in sensitivity compared to other analytical techniques.
• Recent advancements in NMR spectrometers have improved sensitivity.
• NMR is a valuable tool for analyzing structural and quantitative data from 1/2 spin isotopes.
• Quadrupolar nuclei (I > 1/2) have fewer research applications for quantitative analysis.
• Quantitative NMR (qNMR) is useful for determining the quantity of an unknown compound in a mixture using known standards or calibration curves.
• ¹H qNMR is widely used in organic molecules.
• qNMR uses direct proportionality between the integral of each signal and the number of protons.
• NMR is non-destructive and can simultaneously determine multiple analytes, even in complex mixtures.
• Boron-11 is a suitable element for this method.

Boron-11 NMR Method for Quantitative Analysis

• A new boron-11 NMR method (11B qNMR) was developed for determining boric acid in different commercial biocides.
• Limit of detection (LOD) 0.02% w/w
• Limit of quantification (LOQ) 0.04% w/w
• High linearity (R² > 0.99)
• High recovery (93.6%-106.2%)
• High precision (intra- and inter-day, 0.7 to 2.0%)
• Low uncertainty (3.7 to 4.4%)
• Minimal matrix effects
• Successful application using ICP-MS (inductively coupled plasma mass spectrometry) for comparison

Sample Preparation and Analysis

• Biocide pellets were ground, homogenized, and stored.
• Samples (150 mg) were extracted with D₂O.
• Vortex stirring, centrifugation, and supernatant transfer were used.
• Multiple extractions were necessary for higher concentrations.
• ICP-MS samples were prepared using 500 mg in 500 mL Milli-Q water with sonication for 30 min.
• NMR spectra were recorded on a Bruker Avance 300 spectrometer using a direct probe head.
• 11B chemical shifts are relative to BF₃·OEt₂ (0 ppm).
• Spectral parameters: TD, AQ, D1, FIDRES, NS, DE
• The impact of scans, acquisition time, and recycle delay on NMR signal was evaluated and optimized.

Method Validation

• Validation of the method was performed on a commercial biocide (0.01% boric acid).
• The linear range of the method was confirmed.
• The R² was 0.9996. Validation ensured reliability in routine analysis.
• Selectivity: No interfering signals were observed.
• Precision: Intra-day and inter-day precision were in line with guidelines.
• Trueness (recovery): Avg. recoveries at 0.5, 1.5 and 4.0% were 106, 94 and 101% respectively.
• LOD was 0.02% and LOQ was 0.04%, for a SNR of 3:1 and 10:1 respectively.
• Uncertainty was evaluated using a bottom-up approach with values of 4.4% (low conc.), 3.9% (1.5%), and 3.7% (4.0%).
• The matrix effect was validated.
• The method was validated using a calibration curve.
• Calibration curve for borax (sodium tetraborate) demonstrated successful application. (Linear)

Sample Analysis and Results

• 5 different commercial biocide samples were tested for boric acid content.
• Multiple extractions were required in certain samples.
• 11B qNMR spectra of the samples were analyzed.
• Quantitative results were compared with those by ICP-MS.
• Differences between methods were within acceptable limits (under 6%).

Conclusions

• A quantitative Boron-11 NMR method was developed for the determination of boric acid in commercial biocides.
• It is sensitive enough, fast, and reliable with minimal matrix effects.
• It can be applied to other complex matrices.