Abstract: A protein or peptide is treated by a vapor containing an organic acid represented by the following general formula CF.sub.3 --(CF.sub.2)n--COOH (n is zero or more integer). The resulting reaction mixture is processed by a mass spectrometer to obtain a mass spectrum to measure a mass of respective chemical species contained in the reaction mixture. Alternatively, the reaction mixture is processed by an amino acid analyzer to determine an amino acid sequence of the protein or peptide from the carboxy-terminal. According to another method, the protein or peptide is treated by an anhydride of the organic acid represented by the following general formula CF.sub.3 --(CF.sub.2)n--COOH (n is zero or more integer). The resulting reaction mixture is processed by a mass spectrometer to obtain a mass spectrum to measure a mass of respective chemical species contained in the reaction mixture to determine an amino acid sequence of the protein or peptide from the carboxy-terminal.

Abstract: Novel immunoglobulin derivatives which are characterized in that in which the interchain disulfide bonds of immunoglobulin are predominantly cleaved, on the average 3 to 5 of the interchain disulfide bonds or the inter- and intra-chain disulfide bonds being cleaved, and so produced sulfur atoms (S-) are S-sulfonated (--S-SO.sub.3). The above immunoglobulin derivatives are prepared by reacting native immunoglobulin withA. a compound capable of forming tetrathionate ion, andB. a compound capable of forming in water, sulfite ion whereby cleaving on the average 3 to 5 inter-chain disulfide bonds, or inter- and intra-chain disulfide bonds, of the native immunoglobulin, and S-sulfonating (S-SO.sub.3) the so formed sulfur atoms. This invention also provides a water-soluble composition for injection containing said immunoglobulin derivative having the titre of anti-diphtheria of at least 1.0 I.U./ml at its concentration of 10.