Atmospheric Environment 35 (2001) 4523–4524 Discussion Comment on ‘‘Formation of new particles in the gas-phase ozonolysis of monoterpenes’’ by Koch et al. (Atmospheric Environment 34 (2000) 4031–4042) E.G. Stephanou*, I. Kavouras, N. Mihalopoulos Environmental Chemical Processes Laboratory, Department of Chemistry, University of Crete, 71409 Heraclion, Greece Received 14 September 2000; received in revised form 26 February 2001; accepted 26 April 2001 We studied the interesting paper by Koch et al. (2000) entitled ‘‘Formation of new particles in the gas phase ozonolysis of monoterpenes’’. The authors attempted to identify a series of ozonolysis acidic products of monoterpenes by using gas chromatography mass spectrometry (GC/MS) in electron impact (EI) mode. In their paper, Koch et al. (2000) positively identified pinonic and pinic acids by using GC/MS analysis of their samples as well as that of authentic reference standards. In addition, they tentatively identified the rest of the obtained products among them norpinonic and norpinic acids, based on mass spectra, measured with GC/MS in the EI mode. Koch et al. (2000) compared their results to those previously reported in the literature, about the identification of norpinonic acid, by Kavouras et al. (1999a). They claimed (p. 4033) that the compound identified as norpinonic acid in ambient samples by Kavouras et al. (1999a) should be attributed to norpinic acid and not to norpinonic acid. We consider this statement as inaccurate for the following reasons: (a) The two mass spectra presented in the paper by Koch et al. (Fig. 3, p. 4037) were attributed to norpinonic and norpinic acids although, in our opinion, they do not contain sufficient information to support the identification of these two acids. One can observe their similarity and that their differ- ences concern the relative abundance of the same mass fragment peaks. These differences do not provide acceptable (McLafferty et al., 1996) evi- dence if authentic standards are not available to measure their mass spectra and chromatographic retention times. Koch et al. (2000) did not use in their work authentic standards for norpinonic and norpinic acids. (b) In both mass spectra (provided by Koch et al., 2000), the resolution is poor (actually expected from this kind of instrument) and, therefore, the detection of molecular ion was practically impos- sible. Due to the structural similarity of norpinonic and norpinic acids, the detection of molecular ion should be an essential parameter to identify accurately these two acids. (c) The results obtained (Koch et al., 2000) by GC/MS in the EI mode were not confirmed by another method such as chemical ionization (CI), or by using a high-resolution mass spectrometer instrument. (d) In a previous study (Kavouras et al., 1998) we reported the identification, in a forest atmosphere, of only pinonic acid, nopinone and pinonaldehyde by using the same analytical instrumentation (HP GC/quadrupole-MS, functioning in the EI mode) as in Koch et al. (2000). We compared the chromatographic and mass spectrometric charac- teristics of the above compounds obtained by GC/ MS in the EI mode with those obtained by using available authentic standards. We did not report pinic, norpinic and norpinonic acids, although we had the same kind of information as Koch et al. (2000). We considered that without the above- mentioned confirmation we should not provide structural information based solely on EI mass spectra without molecular ions. As we acquired a mass spectrometer of ‘‘Ion Trap’’ technology, with the option of CI, (Finnigan GCQ) we proceeded again to the analysis of the same samples. The new analysis of our samples provided mass spectra containing molecular ions, confirmed also by CI with methane. Both EI and CI mass spectra were *Corresponding author. Tel.: +30-81-393628; fax: +30-81- 393678. E-mail address: stephanou@chemistry.uch.gr (E.G. Stephanou). 1352-2310/01/$ - see front matter r 2001 Published by Elsevier Science Ltd. PII:S1352-2310(01)00256-4