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Nitric Oxide Chemical Ionization Mass Spectrometry of Long-chain Unsaturated Alcohols, Acetates, and Aldehydes Christian Malosse and Jacques Einhorn* INRA-CNRS, Laboratoire des Mgdiateurs Chimiques, Magny-les-Hameaux, 78470 St-Rgmy-les- Chevreuse, France
Nltrlc oxide chemical lonlzatlon mass spectrometry (CINO+-MS) has proved to be a highly efflclent tool for locatlng the CC double bond In longchaln monounsaturated alcohols, acetates, and aldehydes of type CH3(CH2),CH=CH(CH2),R (R = CH20H, CH20Ac, or CHO). The double bond asslgnment Is mostly provided by the presence of an acyllum Ion Cx+2H2x+s0+formed from the alkyl side of the molecule. When the double bond Is close to the tennlnal oxygen function ( y I2), a C,+3H2y+4NO+Ion may also be used as complementary dlagnostlc Ion. I n the case of dl- or trlethylenlc compounds, an acyllum Ion Is also formed whlch characterizes the external (unconlugated) double bond podtion. From the observed strong Influence of the sample pressure on the formation of the acyllum Ion and results of MS/MS experlments, two mechanlsms are proposed to explaln the origln of thls dlagnostlc Ion. Although some Instrument dependence may not be excluded, the method Is optimally appllcable to moderate sample loads (20-200 ng In our condltions).
INTRODUCTION Long-chain unsaturated alcohols, acetates, and aldehydes are the most common structures found thus far in insect pheromones (I). In several Lepidopteran species of economic importance, these compounds, generally included in multicomponent blends, can play a major role in sexual behavior. In order to assure efficacy and reliability of the synthetic lures used in agriculture or forest pest management, it is thus important to initially determine the pheromone structure and composition with the highest precision (2). In this context, the localization of the double bonds often represents the main difficulty due to the very small amounb of material available to 10 Kg per component). It is usually achieved by chemical derivatization ( 3 , 4 )or oxidative degradation (5) methods followed by gas chromatographymass spectrometry (GC-MS) analysis. To avoid this timeand material-exhaustive methodology, we have investigated the potentialities of gas-phase ion-molecule reactions for direct determination of the double bond position. Although electron impact mass spectrometry (EIMS) may be used to differentiate to some extent (6, 7) between positionally isomeric alkenes, the in situ chemical derivatization method is the most promising approach for unambiguous assignments. The recent concept of remote charge fragmentation associated with collisionally activated dissociation (CAD) spectra might also be considered (8-10), but its use seems limited to the presence of certain terminal functions (mainly acids and acid derivatives) or to certain positions of unsaturation (11). Various reagent gases such as vinyl ethers (12,13),amines (14, 15), isobutane (14), and the Fe+ cation obtained from Fe(C0)6 (16)have been used in the case of usually unfunctionalized monoolefins. CI-NO+-MS may also be used (14, 17,18). In the case of unsubstituted monoolefins, two complementary ions of general formula CnHhNO+ and a third one, C,-1H2,NO+, were found by Budzikiewicz and Busker (14)to determine the position of unsaturation.
The purpose of this study was to examine the possible application of CI-NO+-MSfor double bond location in longchain alkenes containing a terminal oxygen function, such as acetates (19),alcohols, or aldehydes
CH,(CHJ,CH=CH(CH2),R R = CHZOH, CHZOAC, CHO Investigations have also been performed to determine to what extent polyunsaturated compounds and double bond geometry can be analyzed using NO+ as reagent. EXPERIMENTAL SECTION The CI-NO+ mass spectra were recorded using Nermag R10-1OC GC-MS equipment (France) with the following conditions: T(source), 80 "C; filament current, 200 PA; electron energy, 95 eV; nitric oxide (99.9% from Air Liquide, France) at a 0.2 Torr pressure (or 1.6 X lo4 Torr in the source housing) giving rise to a (N0)2+/NO+ion ratio The 2 or E unsaturated alcohols, acetates, and aldehydes were synthesized by standard methods (20) in the Laboratoire des MBdiateurs Chimiques. Samples (50 ng) were introduced with a Ros injector (unlessotherwise specified) via a 25 m X 0.32 mm CPSIL 5CB (Chrompack,The Netherlands) GC capillary column at a temperature depending on the substrate. The MS-MS spectra were obtained with a triple quadrupole instrument MS/R-30-10 Nermag at Ecole Normale Supgrieure and at UniversitB P. et M. Curie, Paris. The source conditions were the same as those indicated above. Laboratory collision energy was 35 eV and argon was used as collision gas in the second quadrupole. Samples were introduced by GC or via direct inlet probe. Exact mass measuremenb were performed on a ZAB-2F reverse geometry mass spectrometer. RESULTS AND DISCUSSION Reactivity of Monounsaturated Acetates. The CINO+-MS spectra obtained from the complete E series of CI2-monounsaturated acetates (Figure 1) exhibit ions at m / z 256 corresponding to the (M + NO)+ adduct ion and m / z 225 (not detected for all isomers) (M - HI+. Ions at m / z 214 and 196 are also sometimes present which result from the (M + NO)+ adduct ion by loss of neutral CHzCO or AcOH, respectively. Two generally abundant ions are observed at m / z 165 and 166. The former corresponds to a ((M - H) - AcOH)' ion, whereas the second one may be formed from the undetected molecular ion M + (generated by a charge-exchange process) also by loss of an AcOH molecule. More interesting is the presence in the lower mass region of an abundant evenand/or odd-mass ion whose m / z value is clearly related to the position of the double bond. The existence of diagnostic ions of two types, i.e. even- or odd-mass ion, indicates a competition between two main processes depending on the position of the double bond relative to the oxygen function: y 3 2. In contrast to the diagnostic ions (even-mass ions containing NO) found in nonoxygenated alkenes (14), the diagnostic peak observed in the spectra of most of the positional isomers is due to an odd-mass ion. This ion, formed from the alkyl side of the molecule, is of CH3(CH2)$O+ structure (see Structure and Origin of the Diagnostic Ions section). Remarkably, no complementary ion which would contain (or originate from) the functionalized end of the chain is observed. However, homologous ions may be found, the
0003-2700/90/0382-0267$02.50/00 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 3, FEBRUARY 1, 1990
Table I. Main Ion Species from CI-NO+ Spectra of Monounsaturated Acetates ((M - H) -
(E)-2-undecenylacetate (Z)-4-dodecenylacetate (Zb5-dodecenylacetate (Z)-8-dodecenylacetate (Z)-g-dodecenylacetate (Z)-10-dodecenylacetate (Z)-5-tetradecenylacetate (E)-5-tetadecenylacetate (Z)-g-tetradecenylacetate (E)-6-tetradecenylacetate (Z)-8-tetradecenyl acetate (E)-8-tetradecenylacetate (Z)-g-tetradecenylacetate (E)-g-tetradecenylacetate (2) - 11-tetradecenyl acetate (E)-11-tetradecenylacetate (Z)-7-hexadecenylacetate (Z)-g-hexadecenylacetate (Z)-11-hexadecenylacetate a
(M + NO)+
M'+
242 (1) 256 (2) 256 (7) 256 (7) 256 (1) 256 (1)
212 (1)
211 (2)
226 n.d.a 226 n.d. 226 (
