Direct determination of free methionine in soy-based infant formula.
A method was developed for the direct determination of free methionine in soy-based infant formula, with analyte separation and quantitation by reversed-phase liquid chromatography (LC), and UV absorbance at 214 nm, respectively. Sample preparation required only dilution with mobile phase and syringe filtration. Using a 0.02M KH2PO4 mobile phase (pH adjusted to 2.9 with 85% o-phosphoric acid) and 0.7 mL/min flow rate, methionine eluted at approximately 8 min, and total run time was 14 min after column regeneration with acetonitrile-water. System linearity was demonstrated as peak area versus analyte concentration, ranging from 80 to 120% of the formula specification for free methionine (r > 0.999, and all residuals < 0.45%). Intermediate precision relative standard deviation values were < 1.5% for ready-to-feed and reconstituted powder samples, and recoveries ranged from 98.0 to 103.5% for inter-method comparison with an amino acid analyzer method. The limit of quantitation was 3 mg methionine/L in the "as fed" infant formula. Despite the relatively weak UV absorptivity of methionine, the 214 nm signal was sufficiently intense in the 30-65 mg/L (201-436 microM) range to afford quantitation by peak area proportionation versus a 2-point external standard calibration. This direct UV detection after reversed-phase LC separation provides a simple and accurate method for determining free methionine without derivatization.
Johns
Paul
P
Ross Products Division, Abbott Laboratories, 3300 Stelzer Rd, Columbus, OH 43219, USA. paul.johns@abbott.com
Phillips
Rosalyn
R
Dowlati
Lobat
L
D002853:Chromatography, Liquid; D006801:Humans; D007202:Indicators and Reagents; D007225:Infant Food / Q000032:analysis*; D007231:Infant, Newborn; D008715:Methionine / Q000032:analysis*; D015203:Reproducibility of Results; D013025:Glycine max / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D013454:Sulfoxides / Q000032:analysis
D007202:Indicators and Reagents; D013454:Sulfoxides; D008715:Methionine