Determination of malondialdehyde in human plasma by fully automated solid phase analytical derivatization

Analytical derivatization (AD) increases the sensitivity of analysis by one to three orders of magnitude, stabilizes labile analytes and converts them into readily extractable products. Using a variant of this technique, we applied solid phase analytical derivatization (SPAD) to fully automate extraction, derivatization and liquid chromatography. The resulting device (AutoSPAD) determined malonyldialdehyde (MDA) from biological fluids. This biomarker of oxidative stress is highly water-soluble (500 g/L at pH 7), chemically labile and lacks any functionality that enables detection at high sensitivity. AutoSPAD utilizes column-switching technology to load DANSYL hydrazine onto the solid phase, pass the biological sample over the resulting reactor bed for derivatization on the surface to form a hydrophobic derivative suitable for increasing sensitivity of any other LC technique including LC–MS/MS. The hydrophobic solid phase retains the derivative during washing steps, following which AutoSPAD transfers the derivatized extract to the analytical column for separation and detection by fluorescence. In plasma, however, MDA exists both in free form and covalently bound to protein. Measuring MDA from plasma, therefore, required identification of appropriate protein precipitation and hydrolysis conditions. Under these conditions, the DANSYL derivative formed at only one aldehydic position but did not cyclize as reported for other reactions between hydrazine reagents and MDA. The calibration curve using ∼7 μL of plasma was linear (r2 = 0.999) in the physiological range (0.1–3 μg/mL) and the relative standard deviation of replicate determinations at 1 μg/mL was less than 5%.