Single-Laboratory Validation Study of a Proton NMR Method for the Determination of L-Arginine, L-Citrulline, and Taurine Contents in Dietary Supplements
Authors: Lee Isaac , Vo Jennie , Gao Quanyin , Purohit Piyush , Zarraga Veronica , Babajanian Silva , Chang Peter , Swanson Gary 
Published: January 2020

Abstract

Background
A quantitative NMR (qNMR) method can provide rapid analysis compared to chromatographic methods. Sample preparation steps are relatively simpler and run time is shorter. Rapid analysis methods for release tests in quality control laboratories are very important for laboratory efficiency. Here, we describe a single-laboratory validation study for a rapid qNMR analysis of L-arginine, L-citrulline, and taurine in powdered and tablet dietary supplement products.

Objectives
This validation work is to provide documented evidence for the qNMR method validity as well as method performance.

Methods
The method used Bruker 400 MHz high-resolution proton NMR spectroscopy for simultaneous determination of L-arginine, L-citrulline, and taurine contents in dietary supplement product 1 (powder) and dietary supplement product 2 (tablet). The absolute NMR quantitation is based on a principle of universal proton response intensity correlation with the number of protons in each target analyte (amino acids) vs. that of a reference standard (maleic acid).

Results
The test method performance was validated with dietary supplement-1 (powder) and dietary supplement-2 (tablet). The linearity of the method was studied from about 360 mg/g to about 675 mg/g of L-arginine; from about 15 mg/g to about 30 mg/g of L-citrulline; and from about 20 mg/g to about 40 mg/g of taurine in dietary supplement-1, and from about 15 mg/g to about 30 mg/g of taurine in dietary supplement-2. The coefficients of determination (R2) are 1.0000 for L-arginine, 0.9967 for L-citrulline, and 0.9995 for taurine in dietary supplement-1 and 0.9903 for taurine in dietary supplement-2. The accuracies measured from the sample matrices are 102%, 101%, and 100% average recoveries for 80%, 100%, and 120% concentration levels of L-arginine, 105%, 105%, and 103% average recoveries for 80%, 100%, and 120% concentration level of L-citrulline, and 101%, 102%, and 100% average recoveries of taurine for 80%, 100%, 120% concentration levels in dietary supplement-1; and 95, 98%, and 93% average recoveries of taurine for 80%, 100%, 120% concentration levels in dietary supplement-2, respectively. The precisions (RSD) are 1% for L-arginine, 5% for L-citrulline, and 2% for taurine in dietary supplement -1, respectively; and 4% for taurine in dietary supplement-2. The ruggedness of the test method is within 2%, 4%, and 2% for L-arginine, L-citrulline, and taurine for dietary supplement -1, respectively, and within 4% for dietary supplement-2. The method is specific for the quantitation of each nutrient with no background interference from the matrix for the proton peaks of L-arginine, L-citrulline, taurine, and maleic acid (standard).

Conclusions
The test method is proven to be specific, precise, accurate, rugged, and suitable for intended quantitative analysis of L-arginine, L-citrulline, and taurine in powdered and tablet finished products.

Highlights
The simultaneous determination of all three nutrients of L-arginine, L-citrulline, and taurine using proton NMR provides rapid analysis for quality control release tests that is more efficient versus that of two HPLC methods. Previously, our laboratory was using one HPLC method to analyze L-arginine and L-citrulline while using a second HPLC method to analyze taurine. That approach required two HPLC instruments and two analysts for parallel analysis that takes 2 days using volatile and flammable solvents for extraction and chemical derivatization. This rapid NMR method can analyze the sample “as is” with results obtained in less than 4 h, and is efficient, safe, and environmentally friendly. The initial higher NMR instrument investment versus two HPLC instruments is rewarded with high returns for continued quality control tests.