Identification of Metal Cations, Metal Complexes, and Anions by Electrospray Mass Spectrometry in the Negative Ion Mode

• Published in: Analytical chemistry (Washington) 2000-03, Vol.72 (5), p.985-991
• Main Authors: Mollah, Sahana, Pris, Andrew D, Johnson, Steve K, Gwizdala, Albin B, Houk, R. S
• Format: Article
• Language: English
• Subjects: Analytical chemistry; Chemistry; Exact sciences and technology; Ions; Metals; Scientific imaging; Spectrometric and optical methods
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac9908647

Pneumatically assisted electrospray mass spectrometry (ES-MS) is used in the negative ion mode for aqueous metal (M) solutions in an excess of hydrochloric or nitric acid, where the major anion X = Cl- or NO3 -. A collision energy of ∼20 eV removes anion−solvent clusters for most elements and leaves negative complex ions of the general form (M n +X n +1)-. Complexation with anions prevents charge reduction reactions at least to n = 3, even in cases where the third ionization energy of M greatly exceeds the first ionization energy of the solvent. These negative ions thus preserve the charge state of the metal cation from the solution and allow identification of both cations and anions in a single set of electrospray conditions. Cations such as Fe3+ or Cu2+ that have a lower oxidation state in solution produce a distribution of negative ions, each with a single negative charge overall; e.g., an Fe3+ solution produces both FeIIIX4 - and FeIIX3 -. This distribution of FeIII and FeII species is attributed to electrochemical reduction of Fe3+ at the negatively charged ES needle. “Native” anions such as perrhenate or molybdate produce singly charged analogues such as ReO4 - or HMoO4 -. Metal−EDTA complexes are seen as MIIIY- or MIIHY-. The sensitivity for these “native” anions is suppressed by competition with the excess chloride or nitrate used to produce the metal-containing complex ions.