2540
Anal. Chem.
1982,5 4 , 2540-2547
Dimethyl Ether as a Reagent Gas for Organic Functional Group Determination by Chemical Ionization Mass Spectrometry T. Keough The Procter & Gamble Company, Miami Valley Laboratories, P.O. Box 39175, Cincinnati, Ohio 45247
Electron bombardment of dlmethyl ether at a source pressure of -0.2 torr ylelds C2H,0+ and C,H,O+ ions. These ions react wlth organlc molecules through a number of structurespeclflc ion-molecule reactlons such as hydrlde abstractlon, protonation, addltion, and cycloaddition. The relatlve reactivity of these two Ions depends upon the nature of the functional group(s) present In the organlc molecule. Consequently, dimethyl ether chemical lonlzation mass spectrometry Is a convenlent method for the dlfferentlatlon of, for example, lsomerlc alkenes and cycloalkanes, conjugated and nonconjugated dlenes, some polycyclic aromatlc hydrocarbons, aldehydes and ketones, aclds and esters, alcohols and ethers, and prlmary and secondary alcohols. Relatlve response factors exhlblt a strong dependence on the structure of the organlc molecule. Therefore, dlmethyl ether may be a useful reagent gas for some selectivo Ionization appllcailons. Other reagent gases whlch produce slmllar reactant Ions (formaldehyde yields CH,=OH+, dlethyl ether ylelds C4H,0+ and C,H,,O+), were found to be much less useful than dimethyl ether for organic-functional-group recognition.
One of the long-held goals of chemical ionization (CI) mass spectrometry is the development of functional-group-selective ion-molecule reactions. This goal has not been satisfactorily accomplished because most CI studies to date have relied on exothermic proton transfer as the method of chemical ionization. Exothermic proton transfer reactions proceed with near collision efficiency (1,2)but exhibit little functional-group selectivity. In recent years, a number of workers have developed novel ion-molecule reactions (other than proton transfer) that enable the differentiation of various isomers and the recognition of various organic functional groups. For example, vinyl methyl ether chemical ionization (3-5)provides a method for determining the location of double bonds in olefins and unsaturated fatty acids and fatty esters. Hydrogen/deuterium exchange in NH3 CI has been used to count active hydrogens in organic molecules (6-8). This provides a simple method for differentiating between primary, secondary, and tertiary amines. Ammonia (9)CI has been used to differentiate aldehydes from ketones since aldehydes form Schiff bases while ketones do not. Ammonia CI has also been used to differentiate some meta-disubstituted aromatics from the ortho and para isomers (IO). This is useful since more acidic reagent gases often allow the differentiation of ortho-disubstituted aromatics from the meta and para isomers (11). Nitric oxide (9,12)is one of the most promising CI reagent gases for organic-functional-group recognition since NO+ functions as an electrophile, hydride abstractor, or a oneelectron acceptor, depending on the nature of the organic molecule. Nitric oxide CI allows the differentiation of cycloalkanes from alkenes, aldehydes from ketones, alcohols from ethers, and primary, secondary, and tertiary alcohols. Unfortunately, nitric oxide is a powerful oxidizing agent toward the rhenium filaments used in chemical ionization sources. 0003-2700/82/0354-2540$01.25/0
To circumvent this problem, special ionization sources (Townsend discharge) are typically used for nitric oxide CI. We have found that dimethyl ether (DME) CI is a versatile technique for organic-functional-group recognition. The technique allows the differentiation of alkenes and cycloalkanes, conjugated and nonconjugated dienes, some polycyclic aromatic hydrocarbons, aldehydes and ketones, acids and esters, various alkyl esters, alcohols and ethers, and primary and secondary alcohols. Dimethyl ether CI appears to be as versatile as nitric oxide CI for organic-functional-group recognition. In addition, DME CI allows the differentiation of some isomers (conjugated and nonconjugated dienes, some polycyclic aromatic hydrocarbons, alkyl esters) that cannot be differentiated by nitric oxide CI. Dimethyl ether has the additional advantage of being innocuous toward rhenium filaments and, therefore, does not require the use of special ionization sources. Relative response factors in DME CI exhibit a strong dependence upon the nature of the organic sample to be ionized. Thus, DME CI may prove useful for some selective ionization applications such as the determination of ketones in the presence of aldehydes or determination of olefins and aromatics in the presence of alkanes. Finally, we have compared the reactivity of the simple oxonium ions, CH2=OH+, CH2=OCH3+, and CH3CH= OCH2CH3+. The C2H60+ion is the most useful for functional-group recognition, since C4H90+is not as reactive while the ion chemistry of CH2=OH+ is dominated by proton transfer reactions. EXPERIMENTAL SECTION Instrumentation. All chemical ionization mass spectra were obtained on a Hewlett-Packard 5985B GC/MS system with an ion source temperature of 250 "C. Samples were introduced by the gas chromatographic direct inlet (for volatile liquids or when we wanted to introduce measured quantities of material) or by the direct insertion probe (for liquids with low volatility and solids). Reagents. The dimethyl ether (99.9%)used in this study was obtained from Matheson Gas Products, Inc., Joliet, IL. Other reagent gases used were methane (99.9%, Matheson), anhydrous diethyl ether (reagent grade, Fisher Scientific Co., Fairhaven, NJ), and formaldehyde. Gaseous formaldehyde, generated by thermal decomposition of paraformaldehyde (95%, Matheson Coleman and Bell, Norwood, OH) at 120-150 O C within the GC oven, was bled into the ion source through the GC/MS direct transfer line. The other chemicals were obtained from various commercial sources and used without further purification. The chemical ionization mass spectra of these compounds did not reveal the presence of any significant levels of impurities. Procedure. Primary ionization of the dimethyl ether (DME) reagent gas, at an indicated source pressure of -0.2 torr, was accomplished with a beam of high energy (200-250 eV) electrons emitted from a heated rhenium filament. Ion source conditions (electron beam energy, repeller, and focus and drawout plate voltages) were optimized by maximizing the yield of the primary CI reactant ions, C2HSO+and CzH7O'. The distribution of reactant ions obtained by electron ionization of DME is strongly dependent upon source pressure, Figure 1. At an indicated pressure of -0.2 torr, the reagent ion spectrum is dominated by C2H60+and C2H70+. 0 1982 American Chemical Societv
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
2541
Table I. Summary of the Dimethyl Ether Chemical Ionization Mass Spectra of Alkanes I(M-H~/
compound n-hexane n-octane n-decane n-dodecane n-hexadecane cyclohexane cy clooctane 2,2,3,3-tetramethylpentane
ZI'
other ionsa
0.78
b b b b b
0.36 0.16 0.17 0.15 0.63 0.68 0.01
C,H,,' (0.31), C,Hl,+ (0.23), C7H1:
00
02
013
06
04
10
ndicated Pressure, torr
2,2-dimethylbutane
0.21
2,2,4-trimethylpentane
0.41
2,3-dimethylbutane 2-methvl~entane 2,2,4,6;6:pentamethylheptane
0.87 0.85 0.01
3,3,5-trimethylheptane
0.07
2,2,4,4,6,8,8hep tamethylnonane
0.00
~
Figure 1. Variation in the normalized intensity of reagent ions, gerierated by electron bombarldment of dimethyl ether, as a function of pressure.
0.54
0 4-
-2 - 0 3 t
0 2,
125
I50
175
200
225
Temperature
250
275
OC
Flgure 2. Variation of the normalized intensity of some ions in the dimethyl ether C I spectrurn of 6-undecanone as a function of ion source temperature.
With increased reagent gas pressure, these primary ions react with neutral DME (whiclh is present in large excess within the source) yielding solvated i'ons CzH50.C2He0" and C2H70C2H,0+ as well as (CH3)@+. The source pressure, which could be monitored directly with a thermocouple gauge, was typically set to -0.2 torr to minimize production of the solvated reactant ions. DME CI mass spectra exhibit a strong dependence on source temperature. This is illustrated by the spectra of 6-undecanone, Figure 2, obtained over a 125 "C temperature range. At 150 "e, the spectrum is dominated by the (M + C2H50)+ion while (M + H)+and (M + CzH7O)+ions are less abundant. With increasing temperature, the relative abundances of (M+ H)+ and (M + CH3)+increase at the expense of the (M + C2H50)+and (M C CzH7O)+adduct ions. The spectra can be considerably simplified by recording at high source temperature. To facilitatecomparison in this study, we recorded all spectra at the same temperature, 250 "C. Source conditions for the CI experiments using methane, formaldehyde, and diethyl ether as the reagent gases were established by using the procedure discussed above for DME. The major reactant ions generated from formaldehyde are CH20H+ and CH20H.CH20+,while the major ions generated from diethyl ether are CH3CH=OCzH5+,C4H110+,and C4Hl10C4Hlo0+.
RESULTS AND DISCUSSION Electron ionization of' DME a t a source pressure of -0.2 torr yields C2H50+and C2H70+as the major product ions,
(0.17),
C,H,+ (0.14) C,H'1,+r(0.54), C,H,+ (0.15) C,H,+ (0.32), C,H,,I (0.14) C,H,' (0.26), C,H,,+ (0.2119 C,Hi,+ (0.181, C"H.2 (0.131 C,Hllt'~0:45), C6H13t (Oa20), C,H,,+ (0.10) C,H,,+ (0.28), C4H,+(0.20), C,Hl,C (0.161, C,H,: (0.12) '
a Only ions with a relative abundance greater than 20% of,the.base peak are listed. The normalized intensity b I t / l " I z )of each fragment ion is given in parentheses. The major fragments are a distribution of C,Hz, ,I+ ions maximizing at C,H13+.
Figure 1. The C2H50+ion is unreactive toward DME at relatively low pressure because hydride abstraction from DME simply regenerates the reactant ion (1). At higher source + + (1) A \ + /o\ + / O \ + /ON pressures, C2H50+and C2H70+are solvated by DME. We also note the production of (CH3)30+which presumably results by methyl group transfer from C2H60+to DME (2). This r
7 +
process, which can be simply viewed as a nucleophilic displacement of a weak base (CH20)by a stronger base (DME), has been previously noted in low-pressure ICR studies of the reactivity of the C2H50+ ion (13). In the presence of organic samples, C2H50+functions mainly as an electrophile or hydride (or hydroxide, etc.) abstractor while C2H70+functions as an electrophile or a proton donor (proton affinity of DME = 189 kcal/mol ( 1 4 ) ) . Depending on the nature of the organic molecule, ionization may be effected by either (or both) of these reactant ions. Functional Group Recognition. Hydrocarbons. Table I contains the DME CI spectra of some normal, cyclic, and branched saturated hydrocarbons. The spectra of the n-alkanes exhibit significant (M - H)+ions (defining the molecular weight) and a distribution of intense CnHPn+*+ fragment ions. The spectra of the cycloalkanes are considerably simpler, exhibiting mainly the (M - H)+ ion, while the spectra of the branched alkanes are highly structure dependent. Some compounds exhibit intense (M - H)+ions while others do not. All of the spectra of the branched species exhibit intense ions
2542
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
corresponding to cleavage a t the branch points. Branched hydrocarbons are readily differentiated from n-alkanes by DME CI. In considering the origin of the product ions, it is instructive to consider the energetics associated with the various ionization processes. The enthalpy of formation of the CzH60+ ion was taken as 157 kcal/mol (13), while the enthalpies of formation of various product ions were estimated by using the group equivalent approach of Franklin (15) and values for l-CHZ+, 2-CH+, and 3-C+ of 221,208, and 197 kcal/mol, respectively (16). The enthalpies of formation of various neutral molecules were taken from a standard source (17) or estimated by using the thermochemical data of Benson (18). It should be stressed that the enthalpies of the various reactions listed in this paper are only estimates. The enthalpies of formation of the ions and neutrals correspond to values obtained at a standard temperature of 298 K while the present experiments were conducted at a considerably higher temperature. The purpose of these calculations is only to indicate whether a particular reaction is energetically likely (H < 0). The relative abundance8 of the (M - H)+ ions observed in the DME CI spectra of n-alkanes are comparable to those obtained by CH4 CI (19)but much lower than those obtained by NO CI (16). In the present study, the (M - H)+ ion apparently does not result from hydride abstraction by the CzH50+ion since that process (3) is calculated to be endo+ +
/o\ Iti'-";kcal
mole
'I
thermic. If the (M - H)+ ion is generated via hydide astraction by C2H60+,energetics require that rearrangement to a tertiary cation must accompnay the abstraction process (4). This type +
/o+ t
+
/o\
t
+
4,
AH = - 4 kcalmale
of rearrangement has been previously invoked to explain the formation of (M - OH)+ ions in the isobutane CI spectra of aliphatic alcohols (20). Since hydride abstraction by (4) is only slightly exothermic, we would not expect extensive fragmentation of the (M - H)+ ion. Formation of the intense CnH2n+l+fragment ions may, therefore, arise by a carboncarbon bond fission process ( 5 ) similar to that previously observed in the CH4 CI spectra of n-alkanes (19). +
/O+
(Mt
1-hexene
0.05
1-heptene 1-decene 1-dodecene 1-octadecene 1-eicose ne 2-decene 3-decene 4-decene cis,cis-2,4hexadiene trans,trans-2,4hexadiene 1,5-hexadiene 1,4-octadiene
0.07 0.01 0.00 0.00 0.00
compound
(Mt 13)"
(Mf 45)
0.55 0.28 0.03 0.01 0.01
0.03 0.02 0.04
0.06 0.03
0.08 0.08 0.10 0.12 0.05 0.05 0.13 0.13 0.14 0.33
0.24
0.04 0.03 0.04 0.03 0.03 0.13
0.04
0.27
0.22
0.14
0.02 0.05
0.12 0.22
0.63 0.15
0.01
H)"
0.04 0.06
0.04
0.00
0.03 0.04 0.05
0.02
a (M t 45)+= (M t C,H,O)+; (M t 13)+= (M t C,H,O - CH,OH)+. The other major fragment ions ~ + generally maxiwere a distribution of C , , H Z ~ -ions mizing at C,H,,+.
branched hydrocarbons which typically exhibit more extensive fragmentation than the spectra of the corresponding n-alkanes (19). We expect hydride abstraction from a tertiary carbon (8) to be slightly exothermic. As noted above, however, it
/+...A. k , t
+
/o\
t
(81
AH = - 2 kcallmole
is energetically unlikely that the fragment ions result from &elimination of an olefin (9). Instead, fragmentation
++A
-(+
- 3
(9)
AH = +23 kcal/mole
may result from carbon-carbon bond fission (10) as in the spectra of n-alkanes (19). For the branched hydrocarbons,
A A+ / k
+ -(+
+
Po\ (10)
AH
= -2 kcahole
151
=
-7 kcal mole
The DME CI spectra of the two cycloalkanes exhibit much less fragmentation than the spectra of the corresponding n-alkanes. The only major ion observed in each spectrum corresponds to (M - H)+. This ion again apparently does not result from simple hydride abstraction since that reaction (6) is endothermic. Hydride abstraction is presumably accomt
/o\
+
+
+ /o\ AH =
-
(6)
+ 11 kcal mole
panied by a ring contraction (7), yielding a tertiary carbonium
0 +:, +
1'121~
(MH)+
however, this process occurs predominantly at the branch 7 - wo\ points. AH
0+
Table 11. Summary of the Dimethyl Ether Chemical Ionization Mass Spectra of Alkenesal
D-.
+ A.
Table I1 contains the DME CI mass spectra of some alkenes and dienes. Alkenes, in contrast to alkanes, exhibit significant (M + C2H50)+and (M + CzH5O - CH,OH)+ adduct ions presumably formed by electrophilic attack at the double bond (11)and subsequent elimination of CHBOHto yield an al/-4l/-/
+
A
o\-
-
Ill)
t
AH
= - 3 1 kcal
mole
lylically stabilized cation (12). The (M + 45)+ ion generated H
-0,
H A
+
+
,'+.. - +CH,OH
AH
17)
ion. In the gas phase, tertiary alkyl carbonium ions are generally 10-15 kcal/mol more stable than secondary alkyl carbonium ions. Furthermore, acid-catalyzed ring contractions have been previously observed in solution (21). The spectra of the branched alkanes are highly structure dependent. 2,2,4-Trimethylpente, 2,3-dimethylbutane, and 2-methylpentane exhibit intense (M - H)+ ions. For these compounds, the (M - H)+ ion has a relative abundance that is greater than that observed for the n-alkane of the same carbon number. This is in contrast to the CHI CI spectra of
--+
fiz]
= +6 kcal mole
via (11) has sufficient excess internal energy to overcome the endothermicity of (12). The spectra also contain significant (M + H)+ ions, suggesting sample protonation by CzH,O+. The (M - H)+ ions were less abundant than (M + H)+ and the adduct ions. The major fragment ions correspond to CnHzn-l+ions and are presumably formed by carbon-carbon bond fission as in the spectra of the n-alkanes. Several results are apparent from the data in Table 11. First, there is a significant chain length effect on the relative abundance of the (M CzH5O - CH30H)+ ion. This ion is the only significant ion in the spectrum of 1-hexene while it
+
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
2543
Table 111. Summary o f t h e Dimethyl Ether Chemical Ionization Mass Spectra of Aromatic Hydrocarbons compouind benzene toluene 1,3,5-trimeth~rllbenzene hexamethylbenzene naphthalene anthracene biphenyl phenanthrene tetracene benz [a]anthra.cene chrysene tripheny lene
IiIZIi
'(M - H)+
M+
0.00 0.19 0.06 0.34 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
0.61 0.33 0.17 0.16 0.30 0.22 0.29 0.20 0.27 0.19 0.21 0.18
(M
a t designates that Diels-Alder additicin occurs readily in sc.Ation. occur readily in solution,
+ H)+
(M t 13)'
cycloaddition (M t 46)+ in solutiona
0.00 0.06 0.14 0.00 0.00 0.37 0.01 0.18 0.37 0.05 0.28 0.00 0.00 0.29 0.31 + 0.09 0.30 0.23 0.00 0.26 0.31 0.00 0.29 0.30 0.23 0.31 0.03 0.07 0.31 0.24 0.00 0.28 0.27 0.00 0.30 0.33 - designates that Die -Alder addit.Jn does not
-
20
+
I
0
100
1 200
unfavorable for the nonconjugated isomers. Table I11 contains the DME CI mass spectra of some aromatic hydrocarbons. All of the spectra exhibit intense molecular ions which are presumably formed by electon ionization. The abundance of these M+ ions suggests that aromatic hydrocarbons are not particularly reactive in DME CI. The spectrum of benzene exhibits a weak (M + H)+ adduct ion and (M CzHsO- CH30H)+with no indication of either (M - H)+ or (M + C2H50)+.These results suggest that hydride abstraction from an aromatic ring (14) is energetically unfa-
+
MZ Flgure 3. The dimethyl ether CI spectrum of 1-octene (upper) and cyctooctane (lower) at a source temperature of 200 O C .
is relatively minor in the spectrum of 1-eicosene. Second, the spectra of isomeric alkenes are quite similar, suggesting that DME CI will not prove generally useful for double-bond location. There is a steady decrease in the ratio of intensities of (M C2H50)+to (M[ C2H50 - CH30H)+as the double bond is moved from the one to four position in decene. Furthermore, there is ai steady increase in the ratio of (M H)+ to (M H)+ as the! double bond is moved from the one to four position in decene. However, the differences in these ratios are small, requiring careful measurements of the spectra of all of the isomers under identical conditions. Third, alkenes can be readily differentiated from cycloalkanes. This is illustrated in Figure 3 which compares the DME CI spectra of 1-octene (upper) with that of cyclooctane (lower). The cycloalkane exhibits an intense (M - H)+ ion while the alkene exhibits intense adduct ions and only a weak (M - H)+ ion. Finally, the DME CI spectra of conjugated dienes are significantly different from the spectra of the nonconjugated isomers. 1,5-Hexadiene and 1,4-octadiene yield adduct ions with relative abundances comparable to those observed in the spectra of the 1-alkenes of the same cairbon number. The nonconjugated double bonds appear to react independently of one another. On the other hand, the spectra of the conjugated dienes exhibit significantly increased abundances of the (M + C2H50)+ion compared to the spectra of either the nonconjugated dienes or the corresponding 1-alkene. The increased stability of the (M + C2H50)+ion may result because the conjugated diene undergoes a cycloaddition reaction (13)
+
+
+
that competes favorably with methanol elimination from the (M + C2H50)+adduct ion. Further support for this interpretation is provided by the DME CI spectra of some aromatic compounds discussed below. Cycloaddition is apparently
+
vorable and that (M C2H50)+is unstable and readily eliminates methanol (15).
-
This latter process, which yields a resonance stabilized C7H7+ion, is exothermic by 11kcal/mol. Methylenation of aromatic compounds has also been noted in low-pressure ICR experiments (22). The intense (M - H)+ ions observed in the spectra of toluene, 1,3,5-trimethylbenzene, and hexamethylbenzene suggest that, for these compounds, hydride abstraction occurs at the methyl group(s) and not the aromatic ring. The spectra of the polycyclic aromatic hydrocarbons are particularly interesting since some of the compounds (anthracene, tetracene, benz[a]anthracene) exhibit intense (M + C2H50)+adduct ions while others (naphthalene, phenanthene, chrysene, and triphenylene) do not. Those compounds that yield stable (M + C2H50)+adduct ions in DME CI are known to readily undergo Diels-Alder cycloaddition in solution (23). Those compounds that do not yield stable adduct ions in DME CI do not readily undergo Diels-Alder addition in solution. The solution-phase Diels-Alder reactivity of polycyclic aromatic hydrocarbons has been correlated with the loss in resonance energy (RE) accompanying the cycloaddition reaction (23). In general, appreciable reactivity is only observed when the loss in RE is less than -15 kcal/mol. Thus, anthracene (minimum loss in RE -12 kcal/mol) readily undergoes Diels-Alder addition while isomeric phenanthrene (minimum loss in RE -31 kcal/mol) does not. The observation of a stable (M + C2H5O)' adduct ion in the DME CI spectrum of anthracene, Figure 4 (upper), and not in the spectrum of phenanthrene, Figure 4 (lower),suggests that for anthracene, the adduct ion (a) is formed by a gas-phase cycloaddition process. Thus, DME CI readily allows the differentiation of anthracene and phenanthrene. These isomers
2544
ANALYTICAL CHEMISTRY, VOL. 54, NO. 14, DECEMBER 1982
are difficult to distinguish using other mass spectrometric techniques (24, 25). The DME CI spectra of the tetracyclic aromatic hydrocarbons also exhibit dramatic differences which can be correlated with known solution-phase reactivity. For example, the spectrum of tetracene (b) exhibits an intense (M
n
a
+
M/Z
Flgure 4. The dimethyl ether C I spectrum of anthracene (upper) and phenanthrene (lower). "
t
b
d
C *
ru
CzH5O)+adduct ion. There are two equally reactive rings in this molecule (the reactive ring(s) are indicated with pairs of asterisks). Solution-phase Diels-Alder addition at either ring results in a loss of RE of -13 kcal/mol(23). The spectrum of benz[a]anthracene (c) exhibits an adduct ion with a relative abundance that is significantly lower than that observed in the spectrum of tetracene. However, in solution, benz[a]anthracene has only one reactive ring (loss in RE -15 kcal/mol). Finally, the DME CI spectra of chrysene (d) and triphenylene do not exhibit intense (M + CzH60)+adduct ions. However, in solution, there are no reactive rings (loss in RE 125 kcal/mol) for either of these molecules. Carbonyl-Containing Compounds. The DME CI mass spectra of some aldehydes and ketones are listed in Table IV. In general, the spectra of aldehydes exhibit intense (M - H)+ ions which are virtually absent from the spectra of ketones. The DME CI spectra of ketones exhibit (M H)+ ions which are relatively less abundant in the spectra of aliphatic aldehydes. At this source temperature (250 "C) both classes of compounds exhibit (M + C2H60)+and (M + CzH70)+ adduct ions. It is clear that DME CI allows the differentiation of isomeric aliphatic aldehydes and ketones. In favorable cases, aromatic aldehydes and ketones can also be differentiated. The differences in reactivity between aliphatic aldehydes and ketones can be rationalized on the basis of the thermochemistry accompanying the various ionization processes (16-19). Hydride abstraction (16) from aliphatic aldehydes
+
/:\
-+
-o+
+
/'\
(161
AH = -1 3 kcal'mole
+
+
/%
+
0
Ay0
7
Py-.. + /O
99 YO efflclency) leaving CH, unreacted (
