Anal. Chem. 1996, 68, 3288-3289
Rubber Stoppers as Sources of Contaminants in Electrospray Analysis of Peptides and Proteins Xiaokui K. Zhang, Robert C. Dutky,† and Henry M. Fales*
Chemical Structure Section, Laboratory of Biophysical Chemistry, National Heart, Lung, and Blood Institute, Bethesda, Maryland 20892
Using MS/MS, we have identified ions at m/z 399, 421, 609.4, and 819.9 as those of tris(2-butoxyethyl) phosphate, originating in the gray rubber stoppers commonly used as closures in commercial sources of peptides and proteins. The compound is readily extracted by contact with organic solvents such as alcohol or acetic acid and even by water alone. Zinc salts of proteins are also observed, presumably from zinc oxide fillers present in the stoppers. We wish to call attention to complications that may occur during bioanalytical analyses when a certain style of closure, commonly used with commercial sources of peptides and proteins (Sigma Chemical Co., Peninsula Laboratories, etc.), is encountered. For example, while measuring the electrospray mass spectrum of a mixture of 5 µM equine apomyoglobin (Sigma Chemical Co., St. Louis, MO) and 20 µM L-methionyl-arginylphenylalanyl-alanine hydroacetate (MRFA, Finnigan Instrument Co., San Jose, CA) in 50% aqueous methanol (Mallinckrodt, Paris, KY) containing 0.5% acetic acid (Mallinckrodt), i.e., a standard tuning solution prepared as recommended by the manufacturer of our Finnigan TSQ-700 mass spectrometer, we noticed that the solution stored at 4 °C seemed to degrade over weeks of storage. The electrospray mass spectra (spray voltage, 4.5 kV; flow rate, 1 µL/min) of dilutions (10:1) show progressively intense, extraneous low-mass ions (
