Clarifying the SF 5 CF 3 Record

Abstract

W. T. Sturges and colleagues report on the identification of SF5CF3 in the atmosphere and point out that this compound could be an important greenhouse gas (Reports, “A potent greenhouse gas identified in the atmosphere: SF5CF3,” 28 Jul., p. [611][1]). The identification is based on the mass spectrum of this compound, which the authors list as “68.995 (CF3+), 88.967 (SF3+), and 126.964 (SF5CF3+).” This latter assignment is wrong: the ion at the mass-to-charge ratio ( m/z ) of 126.964 is due to SF5+. This mistake may be just a typographical error, but it could be misleading by suggesting that a molecular ion for this molecule has been observed, which it has not. Incidentally, contrary to the authors' statement, a library mass spectrum for SF5CF3 is available in the National Institute of Standards and Technology (NIST) Mass Spectral Library (version 1.6). # Clarifying the SF5CF3 Record {#article-title-2} Sturges et al . indicate in their report an uncertainty as to the origin of SF5CF3. We know that one source of this compound is as a by-product of the manufacture of certain 3M fluorochemicals. The authors correctly suggest that concentrations of SF5CF3 detected in the atmosphere are so small as to be a minor contributor to overall radiative forcing. Despite this, they appropriately call attention to the prudence of limiting releases of SF5CF3. In fact, 3M had already implemented a comprehensive program to reduce all greenhouse gas emissions, including SF5CF3, from company operations. Since 1995, the company has reduced global greenhouse gas emissions by more than 40%, and additional reductions, on the order of 50% or more, are planned over the next 1 to 5 years. # Clarifying the SF5CF3 Record {#article-title-3} We thank Hites for noting the typographical error. Mass 126.964 is indeed attributable to the SF5+ ion, not SF5CF3+ as stated in our report ([1][2]). The molecular ion is not formed. It is true that SF5CF3 now appears in the latest version of the NIST Mass Spectral Library (version 1.6), but this version was not supported on our particular instrument at the time of the study. We can, however, confirm an excellent match between the spectrum we obtained from the pure compound and that given in the updated NIST library (see the [figure][3]). We remain confident in our identification of SF5CF3 in the atmosphere, as is also illustrated in the figure. ![Figure][4] Mass spectrum ( m/z 50 to 200) of SF5CF3. ( A ) Gas chromatography-mass spectrometry (GC-MS) measurements by this group (1) and ( B ) NIST Mass Spectral Library Version 1.6d entry 23164. The ions marked 1 to 3 correspond to CF3+, SF3+, and SF5+, respectively. The ion SF4+ ( m / z 107.966) is also observed at low abundance. The inset shows the GC-MS analysis of 200 standard cubic centimeters per minute of “clean” ambient air [Colorado Mountains, collected in 1994 ([1][2])] showing peaks of m / z corresponding to the same ions (1 to 3) in the traces from the bottom to top, respectively, at the retention time expected for SF5CF3. Regarding the letter from Santoro of 3M, he suggests a plausible origin of SF5CF3 in the atmosphere, presumably from the electrochemical fluorination production of widely used intermediates and products such as trifluoromethanesulfonic (“triflic”) acid and fluorosurfactants. This process has been in use since the late 1950s, which fits our observations of detectable SF5CF3 in air samples dating from the 1960s. What is not clear, however, is whether the amounts released by electrochemical fluorination can account for the year-on-year rise in emissions to a current rate of 270 metric tons per year. In the absence of actual emission data, the possibility of other sources cannot be ruled out. Efforts by private companies such as 3M to voluntarily reduce their greenhouse gas emissions are, however, to be commended. 1. [↵][5]1. W. T. Sturges 2. et al. , Science 289, 611 (2000). 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