Patrick R. Jones

The analysis of amines by gas chromatography–mass spectrometry (GC–MS) using electron ionization (EI) has always been a challenge for three primary reasons: These compounds are very polar and their chromatography is difficult; many of these compounds exist as hydrochloride salts that produce an equilibrium between the free base (R

Cl) in polar solvents; and to avoid problems with the presence of peaks due to air (

 35, which means that an important peak representing the diagnostic primary immonium ion (H

 30 is not observed. In the course of studying the fragmentation patterns of various compounds with an amine group, a fourth problem was discovered — the reaction of the analyte with common solvents. Methanol, ethanol, acetone, carbon disulfide, dichloromethane (methylene chloride), and chloroform as purchased from various suppliers were found to pose potential problems of undergoing reactions resulting in compounds other than the analyte being studied both in the solutions formed at room temperature and in the injection port of the GC system. This presentation is Part I of a multipart series with the focus on aliphatic primary amines in dichloromethane. This study also emphasizes the importance of finding a confirmation of a proposed structure after a logical solution has been reached. 

During the course of normal laboratory work involving electron ionization (EI) gas chromatography–mass spectrometry (GC–MS), it was found that the primary straight-chain aliphatic amine stearylamine appeared to decompose/rearrange into other compounds, or react with dichloromethane used as the diluent. A study was undertaken to see if other aliphatic amines behaved in the same way as stearylamine. The other straight-chain aliphatic amines studied were C

. These rearrangement/reaction phenomena were investigated to determine the identity of the reaction products and to determine what might be a possible mechanism of the reactions by which they were formed. In most of the cases in this study, data representing the target analyte were observed in the reconstructed total ion current (RTIC) chromatogram along with data representing the reaction product. 

The reactions of primary aliphatic amines with the dichloromethane occurred in solution before injection into the GC–MS system. These reactions were not an artifact of the injection method. The discovery and identification of these products also led to the realization that the mass spectra of several important compounds were missing from the NIST Mass Spectral Database. This situation has now been rectified. 

Table I lists all of the solvents and analytes and their purity, manufacturer, and purchased source. Although often not stated on the container or in the catalog, it was found that the dichloromethane contained a trace amount of cyclohexene as a stabilizer. More recent purchases did have this information printed on the bottle in some cases. All other chemicals were used as received without any further purification.  

The GC–MS analyses were carried out using a model 7890A gas chromatograph fitted with a split–splitless injector (Agilent Technologies, Santa Clara, California). The GC system was interfaced to a model 5975C Inert XL MSD EI mass spectrometer (Agilent Technologies). An Agilent split–splitless injector insert was used. A 30 m × 250 μm column with a 0.25-μm film thickness of XE-52 (SGE, Austin, Texas) was used for the chromatographic separations. The mobile phase was helium (99.995% purchased from California Welding Supply Co., Stockton, California) and passed through VICI Getter gas purification system (Valco, Inc., Austin, Texas).  

Table II:  Results of accurate mass assignments using MassWorks			

The mass spectrometer was tuned using the Agilent Standard Spectrum Tune file provided with GC, GC–MS ChemStation software (version E.02.00.493), and perfluorotributylamine (PFTBA, also known as FC-43). The following GC–MS settings were used:  

Oven program: On 40 °C for 3 min; 15 °C/min to 270 °C, hold for 2 min  

Solvent wash dispense speed: 2500 μL/min  

Sample wash dispense speed: 2500 μL/min  

Purge flow to split vent: 15 mL/min at 2 min 

Articles

The analysis of amines by gas chromatograph ;mass spectrometry (GC–MS) using electron ionization (EI) has always been a challenge

Interaction of Dichloromethane Solvent with n-Alkylamines Analyzed by Electron Ionization GC–MS

Determining an organic compound's structure from its mass spectrum is challenging. If, as in the case of soft ionization — chemical ionization (CI), atmospheric pressure chemical ionization (APCI), field ionization (FI), electrospray (ES), atmospheric pressure photoionization (APPI), or fast atom bombardment (FAB) — the mass spectrum only provides the mass of the intact molecule, the challenge can be even greater. At best, an elemental composition can be determined if the measured mass is sufficiently accurate; however, many of these elemental compositions represent large numbers of compounds. There are 127 compounds in the NIST08 Mass Spectral Database that have the elemental composition C

O. Without an accurate mass measurement, the challenge is even more overwhelming. The elemental composition C

O has a nominal mass of 156 Da. The NIST08 Mass Spectral Database has 684 compounds with this nominal mass. To get past this point of an elemental composition, fragmentation of the ion representing the intact analyte nolecule is eeded. Approaches to the interpretation of even the most information-rich spectra — considered by most to be spectra produced by electron ionization (EI) — usually begin with the explanation of fragment ions formed by the unimolecular decomposition of the ion representing the intact molecule (the molecular ion [M

Examination of the spectral difference between 8-methyl-4-nonanone and 4-decanone (Figure 26) shows that the relative intensity of the peak at 

 69 is much greater in the spectrum of 8-methyl-4-nonanone than it is in the spectrum of 4-decanone. This could lead to the conclusion that the same type of mechanism involved in the water loss from the acylium ion (through the formation of a secondary radical in the case of 4-decanone and a tertiary radical in the case of 8-methyl-4-nonanone) might be involved. Two possible mechanisms resulting in the expulsion of vinyl alcohol are shown in Figure 28. Both of these mechanisms are consistent with the increased abundance of the peak at 

 69 in the mass spectrum of 8-methyl-4-nonanone compared with the spectrum of 4-decanone based on the formation of a tertiary intermediate.  

Figure 26: Head-to-tail comparison of the EI mass spectra of 4-decanone (down) and 8-methyl-4-nonanone (up). 

This precursor-ion analysis showed that the ion with 

 87 was also a precursor of the ion with 

 87 (mentioned earlier, but whose origin has yet to be explained) could lose water to form the ion with 

 69. In the EI mass spectrum of 4-decanone, the intensity of the peak representing the ion with 

 87 is 16% of the intensity of the peak at 

 86, which is far too great to be an X+1 isotope peak of the 

 86 would represent an ion that has six atoms of carbon; however, it has been established that this 

 ion has 5 atoms of carbon, 1 atom of oxygen, and 10 atoms of hydrogen, and is formed by a γ-hydrogen shift-induced β cleavage involving the gamma carbon on the C

 side of the carbonyl. Most of the abundance of the ion with 

 87 is probably due to a double-hydrogen-shift rearrangement fragmentation, sometimes referred to as a 

Figure 27: Precursor-ion analysis for m/z 69. 

 87 represents a doublet produced by the X+1 ion of 

 86 and another ion that produces the ion with 

 69. A product-ion analysis was acquired using 

 87 as the precursor ion to provide information on the elemental composition and possible structure of the 

 87 ion. This analysis also proved that the ion with 

 69 (Figure 30), although it is obvious that the majority of the abundance of the ion with 

 69 is due to the fragmentation of the hexyl acylium ion with 

Figure 28: Two proposed mechanisms for the formation of the ion with m/z 69 from the hexyl acylium ion. 

The product-ion mass spectrum for the ion with 

 87 also shows that this ion will produce ions with 

 45, 71, and 72. A proposed mechanism for the formation of ions with 

 45 and 72 is shown in Figure 31. The ion with 

 71 could be formed by the loss of a hydrogen radical from the ion with 

Figure 29: Proposed mechanism for the formation of the ion with m/z 69 from the molecular ion of 4-decanone. 

To confirm the elemental composition of the ion with 

 69, this mass spectral peak was analyzed using the MassWorks software. The assigned accurate mass differed from the exact mass of the elemental composition of C

 by 0.42 millimass units. Also, an accurate mass assignment was made for the peak at nominal 

O (87.0810 Da) and was found to be 87.0790 Da; a variance of 3.5 millimass units from the 

O composition and –2.0 millimass units from the C

 value assigned to the doublet will be the weighted average of the masses of the two ions based on their abundances.  

Figure 30: Product-ion mass spectrum of the ion with m/z 87. 

EI mass spectral data for accurate mass determinations and relative intensities were acquired using an Agilent Technologies 7890A gas chromatograph (Wilmington, Delaware) coupled to an Agilent 5975C Inert XL MSD single-quadrupole mass spectrometer (Santa Clara, California). The product-ion and precursor-ion data were collected using an Agilent Technologies 7890A gas chromatograph coupled to an Agilent 5975B Inert XL MSD system that had been fitted with the Chromtech GmbH Evolution triple-quadrupole mass spectrometer upgrade (Chromsys LLC, Alexandria, Virginia).  

Figure 31: Proposed fragmentation pathways of the fragment ion with m/z 87. 

The GC conditions used in all data acquisitions on both instruments were the same (see Table II). Table III contains the acquisition parameters used with the single-quadrupole mass spectrometer; Table IV contains the acquisition parameters used with the triple-quadrupole mass spectrometer; and Table V contains the parameters used for the MS-MS experiments (precursor-ion and product-ion analyses).  

Table I: Structures and sources of the ions represented by the peaks in the mass spectrum of 4-decanone 

Accurate mass assignments of the ions produced in the EI mass spectrum of 4-decanone were made using the Cerno Bioscience (Danbury, Connecticut) MassWorks (Version 2, 0, 2, 0) software. The Agilent (single-quadrupole) GC–MS system was set to acquire mass spectral data in the raw scan mode with the threshold setting of 0. This resulted in 10 data points per 

 value. The software requires a special calibration of the mass spectral peak shapes acquired by the instrument used to acquire data for accurate mass assignment.  

Calibration spectra were acquired before and after the elution of the analyte to generate two calibration files in MassWorks (Figure 32). The procedure for acquiring calibration spectra during the mass spectral acquisition is described in Table III under the heading Timed Events. The calibration gas (PFTBA) valve was held open for 1 min to allow the pressure in the source to stabilize to provide constant mass spectral peak shapes. After the Agilent data file is opened with the MassWorks software, nine spectra are averaged at the end of the calibration acquisition period. These spectra and a table of elemental compositions for fragment ions of PFTBA are used to generate the special calibration file. The software automatically calculates the exact masses of the ions using their elemental compositions. The average spectrum selected is then calibrated using the expected mass spectral peak shape to correct the experimental data, which provides the accurate mass assignment. After the calibration file is created, it is saved and can be applied to the data file under the MS analysis mode of the MassWorks software. This procedure was replicated for the second calibration acquisition to provide two different calibration files.  

Table III: Single-quadrupole acquisition parameters 

The MassWorks CLIPS search function was used for mass spectral peaks that represent two different ions of different elemental composition but that have the same nominal mass (Figure 33). A doublet peak is selected and the CLIPS search function is opened using the assigned accurate mass. The mass tolerance, elements present, range of elements present, electron state, and double-bond equivalence range are selected to allow MassWorks to calculate possible elemental compositions using the selected accurate mass assignments.  

Table IV: Triple-quadrupole acquisition parameters 

The mixture search of the CLIPS search function is used for a doublet by providing elemental compositions of known ions represented by adjacent mass spectral peaks to the CLIPS search dialog box. For example, the peak at 

 57 is a doublet; therefore, the elemental composition for the ion represented by 

 58 is entered into the CLIPS search to provide the correct mass spectral peak shape for the software to calculate the two possible elemental compositions accurately. This is necessary because the peak at 

 58 is also a doublet, produced by the X+1 isotope peak from 

 57 and the peak represented by the ion with an elemental composition of C

. The accurate mass calculations for the ions in the mass spectrum of 4-decanone are found in Table I. Those values with an asterisk indicate that the EI mass spectra was of very low intensity, which means that there is a greater probability of an error in the assignment.  

As can be seen from Table I, each and every nonisotope nonbackground peak in the mass spectrum of 4-decanone has been rationalized. The analyte's structure has been confirmed based on these rationalizations and the examination of spectra of compounds thought to be similar to the analyte. The final step is to obtain an authentic sample of the analyte and perform an analysis on the same instrument that generated the spectrum being interpreted.  

Figure 32: Mass spectrum of 4-decanone acquired in the raw scan mode. Insert is an expansion of the region between m/z 112 and 115. 

The identification of the analyte as being 4-decanone would have been possible by using the database search utility, using the integer 

 value data acquired with a GC–MS instrument based on a transmission quadrupole because spectra of this compound are in the NIST08 Database. If a mass spectrum of the analyte is not in the NIST08 Database or some other available mass spectral database (as would have been the case if the unknown spectrum was that of structures 

), then the procedure described here will be very useful. The next step in this study will be to synthesize the four compounds represented by structures 

 (these compounds are not commercially available) and obtain their mass spectra to determine if assumptions made in this study are correct. Because there is only a single spectrum of 8-methyl-4-nonanone in the NIST08 Database and that spectrum may be saturated, it will be necessary to obtain additional data to conclusively prove the tertiary radical formation theory.  

Figure 33: Reconstructed total-ion-current chromatogram of data acquired for accurate mass determinations in the ions represented in the mass spectrum of 4-decanone. 

 are with the Pacific Mass Spectrometry Center, Chemistry Department, College of the Pacific, University of the Pacific, Stockton, California. 

(1) A. Lavanchy, R. Houriet, and T. Gäumann, 

(2) H. Budzikiewicz, C. Djerassi, and D.H. Williams, 

 (Holden-Day, San Francisco, 1967), p. 138. 

(3) W. Carpenter, A.M. Duffield, and C. Djerassi, 

(4) H.E. Audier, A. Milliet, and J.C. Tabet, 

(5) J.R. Dias, Y.M. Sheikh, and C. Djerassi 

The authors continue their discussion of the role of the analysis of fragment ions in the interpretation of an analyte's mass spectrum.

Anatomy of an Ion's Fragmentation After Electron Ionization, Part II

Consider the mass spectrum of hexacosane shown in Figure 1. It is easy to understand that the peaks separated sequentially by 14 

 units represent ions formed by the loss of successively larger alkyl radicals of the form C

 from different molecular ions. Different molecular ions fragment with the loss of different-sized radicals to produce correspondingly different-sized fragment ions. Clusters of peaks with lower intensities are observed near the peak representing each of these fragment ions. Careful examination of the spectrum reveals that each of the peaks representing fragment ions (which are primary carbenium ions) in the mass spectrum (at odd 

 unit higher, which is the carbon-13 isotope peak. Moreover, as the 

 value of the fragment ion decreases, peaks one, two, and in some cases three or four 

 units less begin to become obvious (see Figures 2 and 3 for an expanded view of the spectrum). 

 values of these primary carbenium ions become smaller, the intensities of adjacent peaks representing the ions at lower 

 values become greater relative to the intensity of the principal peak representing the primary carbenium ion. Also, the intensity of the peaks representing these primary carbenium ions at lower 

 values increases. It has been shown that the ion current for all the primary carbenium ions with an 

 is the total number of carbon atoms) is due only to fragmentation of molecular ions (1). The ion current for all the primary carbenium ions with an 

 is due to secondary fragmentation of the higher-mass fragment ions as well as fragmentation of molecular ions. Evidence for this is available from tandem mass spectrometry (MS-MS) studies such as the results shown in Figure 4. For example, some of the ion current at 

 57 results from the loss of a molecule of ethene (H

 85, a hexyl ion. Some of the ion current at 

 43 is a result of the loss of a molecule of propene (H

) from other primary carbenium ions with 

 85, these same hexyl ions. These other primary carbenium ions with 

 69 (not seen in the product-ion mass spectrum shown in Figure 4). Some of the ion current at 

 69 does result from the loss of a molecule of hydrogen from the primary carbenium ion with 

 71; however, the intensity of the peak at 

 69 cannot be rationalized by this fragmentation alone. Some of these proposed fragmentations of the ion with 

 85 are supported by the product-ion mass spectrum of 

 85 shown in Figure 4. Most of the ion current at one 

 unit less than that of the primary carbenium ion with 

 43 is the result of the loss of a hydrogen radical from the primary carbenium ion. 

The cluster of peaks immediately preceding a peak that represents a primary carbenium ion often are said to be signature peaks representing an alkyl ion as opposed to an acylium ion, which would have the same integer 

 both have a nominal mass of 57 Da). The presence of these peaks usually indicates that the ion is an aliphatic ion or that aliphatic ions are present in the mass spectrum. The absence of these peaks does not necessarily indicate that the 

 value peak represents an ion other than an aliphatic ion (such as an acylium ion RC≡O

The above etiological diatribe regarding the meaning of some of the peaks in the mass spectrum of a straight-chain aliphatic hydrocarbon should be helpful in understanding how to extract structural information from a mass spectrum of an unknown compound. 

In the evaluation of an electron ionization (EI) mass spectrum, the first step involves determining whether an M

 peak is determined, the nitrogen rule must be applied to determine if the unknown contains an odd number of nitrogen atoms. 

The next step should be to try to identify any odd-electron (OE

) fragment-ion peaks that may be in the mass spectrum. These peaks will be nonisotope peaks at even 

 values in the mass spectrum where the odd-electron fragment ion does not contain an odd number of nitrogen atoms. 

The presence of more than one odd-electron fragment-ion peak may indicate more than one set of γ hydrogens. Often an OE

 fragment ion will be formed by the expulsion of a molecule from the molecular ion, and this odd-electron ion will then lose a molecule to form another odd-electron ion. The presence of these odd-electron ion peaks also indicates that the analyte has nonhalogen heteroatoms present. If one of these odd-electron ion peaks has an 

 value of 58, there is a good probability that the analyte is an aliphatic ketone. A peak at 

 58 is considered to be diagnostic for aliphatic ketones. 

The next step is to determine which peaks in the mass spectrum represent logical losses from the molecular ion. A peak at [M – 15]

 indicates that there is a methyl group present on the analyte and that it is a special methyl such as that found in a methyl ketone. Peaks at [M – 29]

 values that represent ions that have masses of multiples of 14 

[14] + 1), can represent losses of larger carbonyl or aliphatic radicals (such as C

 ion formed by heterolytic cleavage of the molecular ion of an aliphatic ketone with a C

 chain on one side of the carbonyl or the heterolytic loss of CO from an acylium ion with 

 127 formed by homolytic cleavage with the loss of the moiety on the other side of another molecular ion of the same compound. This peak could also represent an ion formed by γ cleavage from molecular ions of the same aliphatic ketone where the carbonyl is in the number four position (Figure 5) (2). 

For a possible indication of which ion is represented, look for a peak that is 28 

 units higher in the mass spectrum. Be careful, the peak 28 

 units higher could represent an acylium ion that loses CO, or it could represent another aliphatic ion that loses H

. The mass spectra of ketones usually will have peaks representing acylium ions produced by homolytic cleavage of molecular ions, peaks representing aliphatic ions formed by the loss of CO from acylium ions formed by homolytic (alpha) cleavage in response to the charge and radical sites being associated with the carbonyl oxygen, and aliphatic ions that are formed by heterolytic cleavage of the molecular ion (Figure 6). At an integer 

 value resolution, the peaks representing acylium and aliphatic ions can often be isobaric. This complicates the problem. 

Consider the mass spectrum of 4-decanone (Figure 7). The M

 156. This is consistent with the nominal mass of the analyte being 156 Da. There appear to be two odd-electron ion peaks in the mass spectrum; one at 

 58. If this were an unknown spectrum, the peak at 

 58 would indicate that the analyte might be an aliphatic ketone. The fact that there are two odd-electron ion peaks indicates that there are two sets of γ hydrogens to participate in a γ-hydrogen shift-induced β cleavage. The molecule lost to form the first odd-electron ion has a mass of 70 Da. Assuming that this molecule is an olefin, it will contain five atoms of carbon. The olefin will have the general formula of C

 units. Dividing 70 by 14 yields the integer 5. The so-called 

 (shown in Figure 8) indicates that to have a loss of 70 Da, there must be six atoms of carbon on one side of the carbonyl. 

 86 then undergoes a γ-hydrogen shift-induced β-cleavage rearrangement to produce an ion of 

 58, a loss of 28 Da, meaning that there are three carbon atoms on the other side of the carbonyl carbon (Figure 9). This means that there are a total of 10 carbon atoms including the carbonyl carbon; three carbon atoms on one side of the carbonyl and six on the other side. The three carbon atoms must be an 

-propyl group; otherwise, there would be no γ hydrogen, which is required for a fragment ion with 

If this were a mass spectrum of an unknown compound, at this point it could be rationalized that the analyte was an aliphatic ketone with the carbonyl on the number 4 carbon atom. The analyte has a nominal mass of 156 Da. The three carbon atoms on the small side of the molecule constitute an 

-propyl group. The next step is to determine the arrangement of the six carbon atoms on the other side of the carbonyl. The number of possible arrangements is limited considering the requisite of the analyte having two sets of γ hydrogens. The possible configurations of the six-carbon-atom side chain of the analyte are shown in Figure 10. 

Figure 11 illustrates what products will be formed by homolytic and heterolytic cleavage of the molecular ion of this C

 ketone. It is now clear that R can be considered to have two atoms of carbon, and R' can be considered to have five atoms of carbon. This means that the R acylium ion will have an 

 value of 71, and the R' acylium ion will have an 

C-R ion, produced by heterolytic cleavage of the molecular ion with the loss of a H

=O radical or by the loss of a molecule of CO from the C

C-R' ion, produced by heterolytic cleavage of the molecular ion with the loss of a H

 value of 43. Examination of the mass spectrum in Figure 7 shows that these peaks are present; however, these peaks alone do not necessarily give an indication of the arrangement of the atoms in the six-carbon-atom side chain. 

It will therefore be necessary to look at some of the other peaks present in the mass spectrum. Reading the mass spectrum from right to left, there are peaks at 

 95. There is a group of peaks over the range 

 29 to 27, and the obvious group of peaks just following the peak with 

 39, 41, and 42). Also, the intensity of the peak at 

 87 is far too great to represent only the 

 86. Therefore, the significance of this ion must be explored. 

Aliphatic ketones with a γ carbon that is a non-ω carbon will undergo a γ cleavage (analogous to an α cleavage; not the same as a γ-hydrogen shift-induced β cleavage), resulting in a four-membered cyclic oxonium ion as shown in Figure 5 for a ketone with the carbonyl on the number 4 carbon atom (2). This mechanism dominates over β cleavage or even δ cleavage or ε cleavage. This was confirmed by the examination of the mass spectra of a number of aliphatic ketones with a branch containing four atoms of carbon and another branch with four or more carbon atoms. This γ cleavage could account for the peak at 

 99. If this is the case, then structures b and d in Figure 11 can no longer be considered to be candidates for this spectrum because of the substitution on the β carbon, which would mean that a loss due to γ cleavage would be less than 57 for molecular ions of these compounds. 

It has been reported, based on deuterium-labeling studies, that acylium ions will lose a molecule of water (3). Although this report did not propose a mechanism for this loss, one possibility is shown in Figure 12. This would explain the peak at 

 113. If this mechanism is true, then there should be a corresponding peak at 

 71 (Figure 13). Examination of the cluster of peaks just below 

It would be possible for a water loss to occur from the 3-methyl or 4-methyl pentyl acylium ions that could be formed from structures 

Still remaining are a number of peaks unrationalized and no unambiguous resolution to the configuration of the six-carbon-atom side chain of the aliphatic ketone. As a possible solution to these unresolved questions, product-ion mass spectra were obtained. Product-ion mass spectra were also used to confirm the formation of some of the ions that had already been rationalized. The details of how these spectra were obtained are found in the Experimental section of this presentation. 

The product-ion mass spectra obtained in this study were produced by a tandem-in-space triple quadrupole mass spectrometer. In such an instrument, product ions formed from the precursor ion isolated by Q1 (MS1) can themselves undergo subsequent collisionally activated dissociation (CAD) in the collision cell (q2). The conversion of precursor ion to product ion is not 100% during the acquisition of the product-ion spectra. The displayed product-ion mass spectrum may contain peaks representing precursor ions, primary product ions, secondary product ions, and tertiary product ions. This fact is an important consideration in the evaluation of these mass spectra. 

The first examination is that of the product-ion mass spectrum obtained for the ion with 

 113 (Figure 15). According to the above rationalizations, some of the acylium ions with 

 113 should undergo fragmentation to produce ions of 

 95 (loss of water) and others will produce ions of 

 57 and 43 in Figure 15 could represent ions formed by the loss of a molecule of ethene (H

 85 formed by the loss of CO from the acylium ion with 

 113. There is no other straightforward path for the formation of ions with 

 45, 67, and 69 may not be as easy. It is possible that the ion current at 

 69 is due to a loss of methane from the aliphatic ion with 

 67 could represent an ion formed by the loss of a hydrogen molecule from the ion with 

 69; however, this was not observed in the MS-MS experiment (Figure 16). A rationalization for the peaks at 

 41 and 42 may be a secondary fragmentation of the ion with 

 55 could also represent an ion formed by the loss of a molecule of hydrogen from the ion with 

Examination of the product-ion mass spectrum (Figure 16) of the precursor ion of 

 85 from the mass spectrum of 4-decanone also indicates that the ions with 

 43 are a result of the fragmentation of ions with 

 85 with the respective losses of ethene (28 Da) and propene (42 Da). Some of the ion current at 

 43 in the original mass spectrum is the result of heterolytic cleavage at the number 3 carbon atom of the molecular ion (Figure 11). The product-ion mass spectrum of 

 57. This is confirmed by examination of the product-ion mass spectrum of the ion with 

 57 (Figure 17) from the original EI mass spectrum (Figure 7). Figures 18 and 19 show possible mechanisms for the formation of ions with 

 41 (loss of methane) from the aliphatic ion with 

 57. There is no support for the formation of the ion with 

 69 from the product ion mass spectrum produced by the ion with 

 85 (Figure 16), and it will be necessary to look elsewhere for the sources of the ions with 

The product-ion mass spectrum of the ion with 

 57 (Figure 17) does not strongly support the formation of the ion with 

 55 as a product from the loss of a hydrogen molecule because of the relatively low intensity of the peak at 

 55; however, the indication that the ion with 

Examination of the product-ion mass spectrum for the precursor ion with 

 43 (Figure 20) confirms that the ion with 

The authors discuss the use of electron ionization to determine a compound's structure.