Characterization of CMPO and its radiolysis products by direct infusion ESI-MS G.S. Groenewold a,n , G. Elias a , B.J. Mincher a , S.P. Mezyk b , J.A. LaVerne c a Idaho National Laboratory, 2351 North Boulevard, Idaho Falls, ID 83415-2208, USA b California State University at Long Beach, Long Beach, CA 90840, USA c Radiation Laboratory, University of Notre Dame, Notre Dame IN 46556, USA article info Article history: Received 16 May 2012 Received in revised form 20 July 2012 Accepted 20 July 2012 Available online 26 July 2012 Keywords: Direct infusion electrospray Electrospray ionization mass spectrometry Metal separations ligand Radiolytic degradation Carbamoylmethylphosphine oxide CMPO Collision induced dissociation abstract Direct infusion electrospray ionization mass spectrometry (ESI-MS) approaches were developed for rapid identification of impurity compounds formed from octylphenyl-(N,N-(diisobutyl)carbamoyl- methyl) phosphine oxide (CMPO) during alpha and gamma irradiation experiments of this compound in dodecane. CMPO is an aggressive Lewis base, and produces extremely abundant metal complex ions in the ESI-MS analysis that make identification of low abundance compounds that are less nucleophilic challenging. Radiolysis products were identified using several approaches including restricting ion trapping so as to exclude the abundant natiated CMPO ions, extraction of acidic products using aqueous NaOH, and extraction of basic products using HNO 3 . These approaches generated protonated, natiated and deprotonated species derived from CMPO degradation products formed via radiolytic cleavages of several different bonds. Cleavages of the amide and methylene–phosphoryl bonds appear to be favored by both alpha and gamma irradiation, while alpha irradiation also appears to induce cleavage of the methylene–carbonyl bond. The degradation products observed are formed from recombination of the initially formed radicals with hydrogen, methyl, isopropyl and hydroxyl radicals that are derived either from CMPO, contacted aqueous nitric acid, or the dodecane solvent. & 2012 Elsevier B.V. All rights reserved. 1. Introduction Extractant ligands are hydrophobic organic compounds that function as aggressive Lewis bases, forming strong coordination complexes with metal cations. These properties make them useful for the selective separation of radionuclide metals from used nuclear fuel solutions using solvent extraction. One such example is octylphenyl-(N,N-diisobutylcarbamoyl)methyl phosphine oxide (CMPO, Fig. 1), which has been extensively studied as a mixture with tri-n-butyl phosphate for the separation of actinides and lanthanides from dissolved nuclear fuel in the TRUEX process (TRansUranic elements EXtraction) [1–8]. In solvent extraction processes it is critical that the partition- ing behavior of the solvent system remain constant, otherwise metal separation efficiency and selectivity can be compromised. A potential problem is that the radiation field associated with the radionuclides present in solutions of used nuclear fuel may cause homolytic cleavage of bonds in ligands forming free radicals. Subsequent radical recombination reactions can form radiolytic degradation products that may not have the same selectivity for the formation of coordination complexes [9]. For example, alpha radiation from 239 Pu and 241 Am caused significant radiation damage to the TRUEX solvent [10], which can result in poorer partitioning of the metal into the organic phase (a lower distribu- tion ratio). Other reports showed improved distribution ratios for TRUEX solvent that had been cleaned up using sodium carbonate, which suggested the formation of acidic compounds during irradiation [11,12]. These examples underscore the importance of understanding the degradation behavior of CMPO in radiation fields under highly acid conditions, in order to exert satisfactory process control. Accordingly the stability of CMPO is thus a key issue, and has been the subject of several research efforts [9]. Chiarizia and Horwitz observed formation of both octylphe- nylphosphinic acid and methyloctylphenylphosphine oxide in gamma radiolysis experiments in the presence of HNO 3 [13]. They proposed a degradation pathway that involved initial attack on the amide group by hydrolysis, resulting in the formation of octylphenylphosphinylacetic acid and diisobutylamine (Fig. 1d). The acetic acid derivative is unstable and de-carboxylates to methyloctylphenylphosphine oxide. This pathway was consistent with known behavior of amides and phosphine oxides; however they could not rule out homolytic cleavage of CMPO bonds followed by recombination reactions involving cleavage pathways ‘b’ and ‘c’. In a subsequent GC study of degraded CMPO solutions Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/talanta Talanta 0039-9140/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.talanta.2012.07.056 n Corresponding author. Tel.: þ1 208 526 2803; fax: þ1 208 526 8541. E-mail address: gary.groenewold@inl.gov (G.S. Groenewold). Talanta 99 (2012) 909–917