Direct analysis of carbon isotope variability in albumins by liquid flow-injection isotope ratio mass spectrometry

We demonstrate the high precision C isotopic analysis of a series of purified albumins by liquid chromatography-combustion isotope ratio mass spectrometry (IRMS) by using direct aqueous liquid injection. Albumins from 18 species and albumens from chicken and turkey egg were obtained from a commercial source and shown to be of > 98% purity by capillary zone electrophoresis and high-performance liquid chromatography. One microliter of an aqueous protein solution with a total of < 40-pmol protein (2.5 mu g), which contained about 150-nmol C, was injected directly into a flowing stream of high-performance liquid chromatography grade water. The solution passed through a pneumatic nebulizer, was sprayed onto a moving wire, passed through a drying oven, and was combusted in a furnace. After the water of combustion was removed, the resulting CO2 gas was directed to a high precision IRMS instrument operated in continuous flow mode. The average precision across the 20 samples analyzed was SD(delta(13)C) = 0.45 parts per thousand, and the average accuracy was delta(13)C < 0.4 parts per thousand compared to aliquots analyzed by conventional preparation by using combustion tubes and dual inlet analysis. The observed isotope ratio range was about -22.5 parts per thousand < delta(13)C(PDB) < -16 parts per thousand as expected for modern materials from a natural source. These results demonstrate rapid, high precision, and accurate C isotopic analysis of untreated macromolecules in an aqueous stream by Liquid source IRMS.