Environ. Sci. Technol. 1995, 29, 286
Comment on "Pemxykobutyql Nitrate" SIR: Grosjean et al. ( I ) recently reported the synthesis and characterization of peroxyisobutyryl nitrate, PiBN (more properly termed peroxyisobutyric nitric anhydride), an analog of peroxyacetic nitric anhydride (PAN). They rely on gas chromatography with electron capture detection (GC/ECD) and a number of qualitative tests (thermal decomposition, alkaline hydrolysis) to determine the identity of PiBN. In their GC/ECD work, they purport to find that PiBN has the same retention time as PAN and conclude that PiBN is an interference in the analysis of PAN. Were this true, it calls into question measurements of PAN made with such packed column GClECD systems (the vast majority of all reported measurements). We do not believe it is consistent with chromatographic theory or with observations for PAN and PiBN to coelute in the chromatographic system used in their work. The gas chromatography of PAN-type compounds on packed and capillary columns has been studied by a number of groups (2) including the authors'. The findings of these previous studies are consistent with GC theory in that progressively longer retention times are observed for the homologous series PAN, PPN (peroxypropionic nitric anhydride),and PBN (peroxy-n-butyric nitric anhydride), while MPAN (peroxymethacrylic nitric anhydride) has a retention time intermediate to those of PPN and PBN as also reported by Grosjean et al. ( 3 ) . The structure of PiBN is that of PPN with an additional methyl group on the 2-carbon. It is completely outside the collective results of over 40 years of gas chromatography research that the addition of a methyl group to an alkyl substituent would result in a reducedretention time. Moreover, it goes against well-established theory of gas-liquid chromatography. The structure of PiBN is closely related to that of MPAN. While the site of unsaturation on MPAN will make it slightly more polar, and therefore better retained on Carbowax columns, this effect should be slight. Therefore, a PiBN retention time slightly less than that of MPAN would be predicted. Were the result of Grosjean et al., Le., coelution of PAN and PiBN, true it would be very significant. Such results are of tremendous interest since they are a prime means by which significant advances are made. However, the evidence for such results must be more solid and convincing than that presented by Grosjean et al. For example, GC/ MS would yield more convincing results as evidenced in the work of Bertman and Roberts on MPAN ( 4 ) . Given that the chromatographic peakidentified as PiBN has the same retention time as PAN and exhibits chemistry that is typical of PAN-type compounds, Le., thermally decomposes, and is destroyed in basic media, it seems reasonable to assert that the material in question is in fact PAN. This possibility would imply that PAN was a byproduct of the various synthetic routes used in the Grosjean et al. work and that, for some reason, PiBN did not elute from the GC column. Plausible mechanisms can be drawn to explain PAN formation in these systems. These conditions would explain the results of the various tests given in the paper. One piece of evidence pointing to the mistaken attribution of the PAN peak as that of PiBN is the result of the
286
1
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1, 1995
isobutanal/NO, irradiation experiment shown in Figure 2. The figure indicates that the reaction was run to completion with the PiBN yield being NO, limited, assuming an actual starting concentration of isobutanal of 2.0 ppm. Assuming the Monitor Labs NO, instrument with molybdenum catalyst was used in this experiment (notspecificallystated), there was 300 ppb of NO, (NO, products) in the chamber at the completion of the reaction. However, the authors indicate only 20 ppbv of PiBN was present at that time, corresponding to only 7% of the NO,. This is much lower than expected. Nearly all the measured NO, is found to be PAN in irradiations of acetaldehyde/NO, mixtures at long reaction times [cf. Shepson et al. (31.It would seem very possible that the product measured by the authors was in fact PAN, present at smallyield as a product of side reactions. One of us (T.E.K.) has recently conducted a CH30NO/ isobutanal/NO, irradiation, followed by injection of the product mixture into a capillary GClECD system. The column used was of moderate polarity, trifluoropropyl methyl silicone (RestekRTX-200),and gives an elution order for PAN-type compounds comparable to that of Carbowax. Samples after irradiation showed one large peak at a retention time intermedite to those of PPN and PBN, Le., essentially where we would expect PiBN to appear based on GC theory. Acetone was also measured with a yield of approximately 0.2. Furthermore, PAN was observed in yields between 1 and 2%, confirming the above assertion that PAN is a minor product in this system, probably from acetone degradation. In summary, the proof of coelution of PiBN and PAN offered by Grosjean et al. is nowhere near conclusive. On the contrary, their conclusion seems quite unlikely. We conclude that PiBN does not represent an interference in PAN measurements made by GCIECD.
+
Literature Cited (1) Grosjean, D.; Grosjean, E.; Williams, E. L., 11. Enuiron. Sci. Technol. 1994,28, 167-172. (2) Roberts, J. M. Atmos. Enuiron. 1990, 24A, 243-287, and references cited therein. (3)Grosjean D.; Williams, E. L., 11; Grosjean,E. Enuiron. Sci. Technol. 1993,27,830-840. (4)Bertman, S. B.;Roberts J. M. Geophys.Res. Lett. 1991,18,14611464. (5) Shepson, P. B.; Kleindienst, T. E.; Edney, E. 0.;Nero, C. M.; Cupitt, L. T.; Claxton, L. D. Enuiron. Sci. Technol. 1986,20, 1008- 1013.
James M. Roberts* and Steve B. Bertman Aeronomy Laboratory NOAA/ERL 325 Broadway Boulder, Colorado 80303
Paul B. Shepson Centre for Atmospheric Chemistry York University 4700 Keele Street North York, Ontario M3J 1P3 Canada
Tadeusz E. Kleindienst and David F. Smith ManTech Environmental Technology Inc. Research Triangle Park, North Carolina 27709 ES940103F
0013-936X/95/0929-0286$09.00/0
%i 1994 American Chemical Societv
