Quantification of bovine plasma amino acids via liquid chromatography-electrospray ionization-mass spectrometry: Comparison of underivatized and precolumn derivatized methods.

The objectives of this experiment were to evaluate and compare underivatized (UND) and precolumn derivatized (DER) methods for quantification of bovine plasma AA by isotope dilution ratio via liquid chromatography-electrospray ionization (ESI)-single quadrupole mass spectrometry. Linearity of the mass-to-charge ratio signal and area signal sensitivity of (12)C were evaluated for each AA with 5-point standard curves (range: 1.1-500 µM). Plasma from lactating dairy cows was isolated by centrifugation and deproteinized using 1 N perchloric acid with a final concentration of 0.5 N. Deproteinized plasma was filtered and injected into a 50 × 2-mm column (Imtakt) or extracted, derivatized, and injected into a 250 × 3-mm column (EZ:faast, Phenomenex) and analyzed via liquid chromatography-ESI-single quadrupole mass spectrometry. Coefficients of variation and recovery rates were evaluated using 4 replicates of pooled plasma samples spiked with each AA at concentrations of 10, 20, and 50 µM. In addition, a subset of 24 plasma samples was used to directly compare methods using linear regression, correlation coefficient (r), concordance correlation coefficient (CCC), and Bland-Altman plot test. Both methods showed linearity within the dynamic range analyzed for all essential AA (coefficient of determination, R(2) ≥ 0.995) and most other AA, although the UND samples had poor linearity (R(2) ≤ 0.990) or peak resolution problems for Asp, Gly, Tyr, and Ser. Moreover, area signal sensitivity for (12)C AA was greater for DER samples than for UND samples [range: 2.2× (Pro) to 309.5× (Ala)]. Both methods had recovery rates ranging from 85.7 to 119.8.0%, and none differed from 100% except Gln [20 µM (85.7%) and 50 µM (87.6%)] and Val [50 µM (119.8%)] using the UND method. The UND method had a coefficient of variation ranging from 0.9% (Val) to 7.8% (His), whereas for the DER method the range was 2.2% (Glu) to 8.8% (Asp). The highest correlation coefficient (>0.90) and CCC (>0.90) were observed for Arg, Ile, Leu, Met, Thr, Trp, Val, and Gln, with the Bland-Altman plot test showing minimal mean bias for these AA. Lowest values were observed for His (r = 0.46; CCC = 0.45), Lys (r = 0.76; CCC = 0.75), Ala (r = 0.83; CCC = 0.73), and Glu (r = 0.65; CCC = 0.42). The UND method showed linearity, precision, and accurate recovery rates for most AA, with most essential AA having comparable values between the 2 methods. However, the DER method had greater (12)C AA area signal sensitivity, linearity, and recovery rates.