- Email: [email protected]
Surface ionization mass spectrometry of organic compounds Part 4. Oxygen-containing organic compounds
International Journal of Mass Spectrometry and Zon Processes, 104 (1991) 129-136 Elsevier Science Publishers B.V., Amsterdam
129
SURFACE IONIZATION MASS SPECTROMETRY OF ORGANIC COMPOUNDS PART 4. OXYGEN-CONTAINING ORGANIC COMPOUNDS
TOSHIHIRO
FUJII* and KOUICHI
KAKIZAKI
National Institute for Environmental Studies, P.O. 16-2 Onogawa, Tsukuba, Zbaraki 305
(Japd YOSHIHIRO
MITSUTSUKA
Meisei University, Hino, Tokyo 191 (Japan) (Received 10 May 1990)
ABSTRACT Surface ionization organic mass spectrometry (SIOMS) was performed for 42 oxygencontaining organic compounds not previously investigated, ‘using a quadrupole mass spectrometer in which the thermionic ion source has a rhenium oxide emitter. The results are interpreted in terms of the modes of ion formation: molecular surface ionization, dissociative surface ionization and associative surface ionization. SIOMS is particularly well suited to molecular weight and structural determination of aliphatic aldehydes. However, in general, most of the oxygen-containing compounds have lower sensitivities than nitrogen-containing compounds, but still provide some applications for SIOMS. INTRODUCTION
The application of surface ionization (SI) in analytical chemistry and other chemical studies has increased significantly as a result of the comprehensive collection of mass spectral data for certain types of organic compounds. These collections are available for (1) nitrogen-containing compounds [l-3], (2) hydrocarbons and halogenated hydrocarbons [4] and (3) organometallic compounds [5]. Investigations into the general rules governing the ionization of compounds containing oxygen atoms are reported in the present study. Organic compounds with an oxygen heteroatom and proton-donor properties are considered to be of interest because the position of the heteroatom in the molecule presumably plays an important role, since this determines the electron density distribution (ionization energy). * Author to whom correspondence 0168-1176/91/$03.50
should be addressed.
0 1991 Elsevier Science Publishers B.V.
130
However, up to the present time, little research has been done using oxygen-containing compounds able to form positive ions by the SI method. The present experiments are a continuation of earlier work [2-41 and extend the measurements to include oxygen-containing compounds. It was found that aliphatic aldehydes are especially interesting. EXPERIMENTAL
Full experimental procedures have been reported previously [6]: these are based on surface ionization organic mass spectrometry (SIOMS) with a Finnigan Model 3300 gas chromatograph-mass spectrometer equipped with a home-made thermionic ion source. All the compounds used in this investigation were commercial products (Ieda Chemical Co., Tsukuba, Japan) and were used without further puritication. RESULTS AND DISCUSSION
The SI of 42 oxygen-containing organic compounds was investigated and results are presented in Table 1. In this table the final column cites the operational conditions for the solid samples. The appearance energy (AE) values [9], in those cases where they can be calculated from the available thermodynamic data [lo], are also listed in this column, in addition to the available ionization energy (IE) values of the dissociation product. The results were interpreted in terms of the three modes [9] of ion formation: molecular surface ionization (MSI), dissociative surface ionization (DSI), and associative surface ionization (ASI). For all the ion species observed, the dependence of the currents i on emitter temperature T basically follows the general rules [ 1l] established from the results of more than 110 organic compounds studied so far, namely that (1) the i vs. T curves are mountain-shaped with maxima for both the DSI and AS1 cases, while the i vs. T profile for the MS1 case is much flatter, and (2) the emitter temperature which provides the maximum intensity in the AS1 process is 360°C lower than that for the DSI and MS1 processes. Alcohols Six saturated aliphatic alcohols (methyl, ethyl, propyl, n-butyl, set-butyl and t-butyl alcohols), an aliphatic diol (ethylene glycol), an aliphatic trio1 (glycerin) and an unsaturated aliphatic alcohol (ally1 alcohol) were studied. These compounds gave no response except for t-butyl alcohol which forms the (M-OH)+ ions, whereas no other butyl alcohol isomer provides a signal. This
131
result is consistent with the well-known fact that the amount of energy needed to form the various classes of carbocations decreases in the order CH, > primary cation > secondary cation > tertiary cation. Abundant (M-OH)+ (C,HT) ions were produced from benzyl alcohol, but it is not certain whether these are benzyl-type or tropylium ions. Ethers
Neither saturated aliphatic ethers nor unsubstituted diary1 ethers exhibit peaks, but two kinds of aryl-alkyl ethers do. Both methoxybenzene and 4-methoxytoluene produce (M-H)+ ions, but their intensities differ substantially. In the former the peak is small, whereas it is very high in the latter, and the current density of the (M-H)+ ions increases when the ring is substituted by a methyl group. This enhancement may reflect an increase in the probability of hydrogen atom elimination from the molecule and the decrease in the IE value. This substituent effect has been observed previously [4]. Aldehydes and ketones
As can be seen from Table 1, the SI spectra of acetaldehyde, propionaldehyde and butyraldehyde are characterized by ions from the efficient DSI process; very intense (M-H)+ ions, which are directly characteristic of the molecular weight, and other relatively strong ion peaks, such as (M-CH3)+ and CHO+, which are diagnostic for structural determinations. It is concluded that (1) the SI spectra of these aliphatic aldehydes comprise principally the same characteristic set of lines, and (2) increasing the molecular weight of straight-chain aldehydes causes an increase in the ionization efficiency (sensitivity) of the (M-H)+ species. Four simple ketones (acetone, methylvinyl ketone, acetophenone, pbenzoquinone) were studied. The SIOMS of these compounds is virtually of no value with the exception of acetophenone which shows DSI ions at a very small response. A very efficient DSI process was found for benzoyl-containing compounds such as benzaldehyde and acetophenone. The molecules of these compounds desorb as C6H5CO+ ions with efficient ionization. At a higher emitter temperature, the C,H: peak was also obtained with low intensity; it is caused by decomposition of the metastable desorbed ions C6H5CO+ +
C,H,f + CO
This is in agreement with the results of Paleev [12].
132 TABLE 1 Surface ionization mass spectra and sensitivity for oxygen-containing Compound
Intensity
(m/s. IE%V))
W)
peakb
organic compounds
Emitter
Sensitivity’
SI/EI
temp.
(A Torr-‘)
004
Remarks’
(“C) A/coho/s r-Buy1
alcohol
57, (M-OH)+
830
5 x 10-Z
0.4
(74, 10.2) Benzyl alcohol
91, (M-OH)+
900
I.1 x lo-’
(108, 9.1) Methyl
alcohol
(m/z = 32, IE = 10.9eV)
No peak
(m/z = 60, IE = 10.2eV
No peak
(m/r
No peak
alcohol
xc-Butyl
alcohol
Ethylene
glycol (m/r
Glycerin
(m/r
Ally1 alcohol
IE[(M-H)]
= 6.82
No peak
(m/z = 46, IE = 10.5eV)
alcohol
n-Butyl
= 6.9eV
(79)
Ethyl alcohol Propyl
IE[(CH3)3]
(57) 1.4
= 74, IE = IO.OeV)
(m/z = 74, IE = 10.1 eV) = 62, IE =
No peak
10.1 eV)
No peak
= 92, IE = ?) (m/r
No peak
= 58, IE = 9.7eV)
No peak
Ethers Methoxybenzene
107, (M-H)+
760
1.7 x 10-j
8 x lO-3
(108, 8.2) CMetoxytoluene
121, (M-H)+
690
(108) 5.0
1.2
(122, 7.83) Diethyl
ether (m/r
Diphenyl
= 74, IE = 9.5eV)
No peak
ether (m/z = 170, IE = 8.8eV)
Aldehydes
No peak
and kerones
Acetaldehyde
43, (M-H)+
(44, 10.2)
29, CHO+
Propionaldehyde
57, (M-H)+
(58, 10.0)
43, CH,CO+
670
I x 10-2 1.5 x lo-’
Butyraldehyde
55, CH,=C(H)CO+ 43, CH,CO+ 57, (M-CH
830
)+
(29)
7 x 10-2
0.17
6.4 x IO-*
(43)
5.0 x 10-Z
I
3.1 x 10-2 1.1 x lo-2
29, CHO+ 900
105, (M-H)+
0.6
(106, 9.5) Acetone
43, CH,CO+
Methylvinylketone
43, CH,CO+
(70, ?) Acetophenone
105, C,H,CO
(120, 9.3)
119, (M-H)+
10-2
5 x 10-b
(72, 9, 8)
Benzaldehyde
I x (29) 0.3
900
29, CHO+
71, (M-H)
3 x 10-j 2.4 x lO-5
760 830
WW-WI = 7.22
TraLX
760
+
1.4 (77) 2.8 x IO-*
8 x lO-3 IWf
(55) 0.54
81
(105)
WM-CH311 = 7.22 T, = 25’C
p-Benzoquinone
(m/r
=
108, IE = 9.7eV)
No peak
133 TABLE 1 (continued) Compound
Intensity
(m/z, IEYeV))
(m/z)
peakb
Emitter temp.
SensitivityC (A Torr-‘)
SI/EI
830
100’
trace
760
0.1
0.33
900
100’
Remarkse
(n)*
(“C) Carboxylic acids and their derivatives Acetic acid 43, (M-OH)+ (60, 10.4) Acrylic acid 55, (M-OH)+ (72, 10.6) Levulinic acid 43, CH$O+ (116, ?) 99, (M-OH)+ 71, ? Benzoic acid 105, C,H,CO+ (122, 9.5) 77, C,H: p-Phthalic 105, C6H,CO+ acid (166, 9.9) Maleic acid (m/r = 116, IE = 10.7eV) Oxalic acid (m/r = 90, IE = 10.8eV) Phenols and their derivatives 107, (M-H)+ p-Methylphenol (108, 8.9) Aminophenol 109, M+ 108, (M-H)+ (109, ?) Nitrosophenol 107, (M-O)+ (123, ?) Phenol (m/z = 94, IE = 8.5eV) o-Chlorophenol (m/z = 128, IE = 9.3eV) o-Nitrophenol (m/z = 139, IE = 9.5eV) o-Catechol (m/r = 110, IE = 8.15eV) Heterocyclic compounds Tetrahydrofuran (72, 9.4) Furfural (96, 9.2) Tetrahydropyran (86, 9.3) Dibenzofuran (168, 7.9) Furan (m/a = 68, IE = Benzofuran (m/z = 118,
71, (M-H)+ 69, (M-3H)+ 95, (M-H)+ 81, (M-5H)+ 168, M+
620
830
(27) 0.076
T, = 25°C
T, = 55-z
83 58 IOd 7.4
(43)
(105)
WM-OWI = 7.22
100’
0.018
T, = 65’C T, = 130°C
0.21
(76) No peak No peak
760 900 830
100’ IOd 56 lOOf
0.0011
T, = 20°C
(107) 0.13
TX = lOO’=C
(109) 0.02
r, = 100°C
(123) No peak No peak No peak
900 830 760 830
9 x 10-Z 5.0 x 10-l lOOf
AE(MH+)
= 8.86
0.43 (42) 0.018
T, = 0°C
(96) 0.36
6 x 10m2
(41) 0.02
100’
T, = 110°C
(168) 8.9eV) IE = 8.3 eV)
No peak No peak
‘Ionization energy (I/?) values are from refs. 7 and 8. bPeaks have more than 5% base peak intensities; postulated structures are not supported by other evidence. ‘Current density at reference pressure of I Torr, determined from the heights of the lines in the mass spectrum. *n indicates the mass number of the peak in the EI spectrum for comparison. eT, is the temperature of the gas chromatograph oven in which the glass sampler containing the sample is placed. ‘The spectra of these compounds are expressed as relative intensities.
134
Carboxylic acids
In the ionization of aliphatic acids, all compounds except dicarboxylic acids (maleic acids and oxalic acid) exhibit (M - 17)+ ions with variable efficiency. Presumably these ions correspond to loss of OH, although the structure of the m/z 71 ions of laevulinic acid has not yet been determined. The aromatic carbonyl molecules studied form C6H5CO+ ions with high ionization efficiency, suggesting that C6H,CO+ ions are recorded upon the ionization of benzoyl-containing compounds. Again CsHc ions were also observed, presumably resulting from the decomposition of the C6H5CO+ ions. This process was observed with benzaldehyde. The radical ion CgH5CO+ (m/z 105) is expected to be produced by DSI from several compounds such as benzaldehyde, acetophenone, benzoic acid and phthalic acid. Results show that the intensity of this ion varies from one compound to another. Since the experiments were conducted under the same conditions, the difference in ionization efficiency of the radicals obtained from different compounds is attributable to the difference in the production rate of C,H,CO species (i.e. a different yield in the dissociative reaction on the emitter surface). The dissociation yield of phthalic acid was the smallest among these four compounds. Phenols
Phenol did not produce any detectable (M + H)+ ions, in contrast to the results of Zandberg and Rasulev [13]. This discrepancy is not due solely to instrumental differences and we are confident of our results. Our conclusion is also supported by thermodynamic considerations since calculations* show that the AE of (C,H, OHHf ) is 8.86 eV, too high for surface-ionization on the Re oxide emitter. The SIOMS of six ring-substituted phenols (RC6H,0H; R = CH,, NH,, NO, Cl, N02, OH) possessing different electronic properties were recorded. The SI spectrum of aminophenol is characterized by Mf and (M-H)+ ions, which are directly characteristic of the molecular weight. The currents of the M+ and (M-H)+ ions increase with increase in temperature up to T = 1170 K and their i(T) behaviours are similar over this temperature range. This indicates only a small difference between the IE values of the molecule and radical (M-H). p-Methyl phenol also produces (M-H)+ ions with low intensity. The efliciency of the formation of (M-H)+ ions depends significantly on the * The efficiency of AS1 ion formation depends on the appearance energy (AE), defined as ) = D(M-H) + IE(H) - PA(M) and calculated using literature values for the proton affinity (PA) of the molecule M and the bond dissociation energy D(M-H).
AE(MH+
135
electronegativity of R, with current densities of the (M-H)+ ions being in the order R = NH, > R = CH,. However, no ions could be obtained with R = N02, Cl, OH or NO at any temperature. These results are qualitatively correlated with the Hammett substituent constant. The same correlations were found for the effect of substituents on the composition of the SI mass spectra [3] of aniline derivatives. The SI spectrum of nitrosophenol is dominated by a (M - 16)+ peak which presumably corresponds to loss of oxygen. The spectrum of this compound shows the same characteristic ion species, (M-O)+, as those of nitrosamines
PI. Heterocyclic compounds
Tetrahydrofuran (THF) and tetrahydropyran (THP) form stable ions by loss of hydrogen atoms. In the mass spectrum of THF, the (M-H)+ and (M-3H)+ ions appear, while that of THP has (M-5H)+ as the highest peak. In this ion, the ring is, presumably, completely aromatized. Such behaviour is very similar to that of piperidine, the corresponding heterocyclic amine [3]. The mass spectral results of furan and furfural differ substantially. In the former, no peaks are observed, whereas the (M-H)+ peak is quite strong in the latter. This phenomenon may be compared with the formation of (M-H)+ ions in methyl-substituted benzene derivatives. Benzofuran forms no measurable ions, whereas dibenzofuran provides the MS1 ion. This result reflects the difference in the IE values of the two compounds. CONCLUSION
The principal generalizations governing the ionization of oxygen-containing compounds have been established on the basis of their SI mass spectra. In general, many oxygen-containing organic compounds show lower SI sensitivities than analogous nitrogen-containing compounds, but the SI mode gives greater output currents for three compounds (benzyl alcohol, 4methoxytoluene, benzaldehyde) than does the EI mode. This result demonstrates that SIOMS may be useful as an analytical method for certain kinds of oxygen-containing compounds. However, it should be noted that our comparisons were limited to the SI and EI sources we used and can have only qualitative significance. Aldehyde compounds generate intense DSI ions with a number of lines which are characteristic of the molecular weight and diagnostic for structural determinations. MS1 ions are produced by aminophenol and dibenzofuran for which the IE is not too large.
136
No associative ions were formed for the compounds was expected, as compounds with oxygen heteroatoms proton acceptor properties.
studied. This result have essentially no
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
E.Ya. Zandberg and U.Kh. Rasulev, Russ. Chem. Rev., 51 (1982) 819. T. Fujii and T. Kitai, Int. J. Mass Spectrom. Ion Processes, 71 (1986) 129. T. Fujii and H. Jimba, Int. J. Mass Spectrom. Ion Processes, 79 (1987) 221. T. Fujii, H. Ishii and H. Jimba, Int. J. Mass Spectrom. Ion Processes, 93 (1989) 73. T. Fujii and H. Ishii, Chem. Phys. Lett., 163 (1989) 69. T. Fujii, Int. J. Mass Spectrom. Ion Processes, 57 (1984) 63. H.M. Rosenstock, K. Draxl, B.W. Steiner and J.H. Herion, J. Phys. Chem. Ref. Data, 6 (1977) 783P. S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 18 (suppl. 1) (1988) 861P. T. Fujii, H. Suzuku and M. Obuchi, J. Phys. Chem., 89 (1985) 4687. R.C. Weast (Ed.), Handbook of Chemistry and Physics, 60th edn., CRC Press, Boca Raton, FL, 1979-1980, FL231-F237. T. Fujii and H. Arimoto, Am. Lab., (1987) 54. V.I. Paleev, Teor. Eksp. Khim., 14 (1978) 747. E.Ya. Zandberg and U.Kh. Rasulev, Dokl. Akad. Nauk. SSSR, 187 (1969) 777.
129
SURFACE IONIZATION MASS SPECTROMETRY OF ORGANIC COMPOUNDS PART 4. OXYGEN-CONTAINING ORGANIC COMPOUNDS
TOSHIHIRO
FUJII* and KOUICHI
KAKIZAKI
National Institute for Environmental Studies, P.O. 16-2 Onogawa, Tsukuba, Zbaraki 305
(Japd YOSHIHIRO
MITSUTSUKA
Meisei University, Hino, Tokyo 191 (Japan) (Received 10 May 1990)
ABSTRACT Surface ionization organic mass spectrometry (SIOMS) was performed for 42 oxygencontaining organic compounds not previously investigated, ‘using a quadrupole mass spectrometer in which the thermionic ion source has a rhenium oxide emitter. The results are interpreted in terms of the modes of ion formation: molecular surface ionization, dissociative surface ionization and associative surface ionization. SIOMS is particularly well suited to molecular weight and structural determination of aliphatic aldehydes. However, in general, most of the oxygen-containing compounds have lower sensitivities than nitrogen-containing compounds, but still provide some applications for SIOMS. INTRODUCTION
The application of surface ionization (SI) in analytical chemistry and other chemical studies has increased significantly as a result of the comprehensive collection of mass spectral data for certain types of organic compounds. These collections are available for (1) nitrogen-containing compounds [l-3], (2) hydrocarbons and halogenated hydrocarbons [4] and (3) organometallic compounds [5]. Investigations into the general rules governing the ionization of compounds containing oxygen atoms are reported in the present study. Organic compounds with an oxygen heteroatom and proton-donor properties are considered to be of interest because the position of the heteroatom in the molecule presumably plays an important role, since this determines the electron density distribution (ionization energy). * Author to whom correspondence 0168-1176/91/$03.50
should be addressed.
0 1991 Elsevier Science Publishers B.V.
130
However, up to the present time, little research has been done using oxygen-containing compounds able to form positive ions by the SI method. The present experiments are a continuation of earlier work [2-41 and extend the measurements to include oxygen-containing compounds. It was found that aliphatic aldehydes are especially interesting. EXPERIMENTAL
Full experimental procedures have been reported previously [6]: these are based on surface ionization organic mass spectrometry (SIOMS) with a Finnigan Model 3300 gas chromatograph-mass spectrometer equipped with a home-made thermionic ion source. All the compounds used in this investigation were commercial products (Ieda Chemical Co., Tsukuba, Japan) and were used without further puritication. RESULTS AND DISCUSSION
The SI of 42 oxygen-containing organic compounds was investigated and results are presented in Table 1. In this table the final column cites the operational conditions for the solid samples. The appearance energy (AE) values [9], in those cases where they can be calculated from the available thermodynamic data [lo], are also listed in this column, in addition to the available ionization energy (IE) values of the dissociation product. The results were interpreted in terms of the three modes [9] of ion formation: molecular surface ionization (MSI), dissociative surface ionization (DSI), and associative surface ionization (ASI). For all the ion species observed, the dependence of the currents i on emitter temperature T basically follows the general rules [ 1l] established from the results of more than 110 organic compounds studied so far, namely that (1) the i vs. T curves are mountain-shaped with maxima for both the DSI and AS1 cases, while the i vs. T profile for the MS1 case is much flatter, and (2) the emitter temperature which provides the maximum intensity in the AS1 process is 360°C lower than that for the DSI and MS1 processes. Alcohols Six saturated aliphatic alcohols (methyl, ethyl, propyl, n-butyl, set-butyl and t-butyl alcohols), an aliphatic diol (ethylene glycol), an aliphatic trio1 (glycerin) and an unsaturated aliphatic alcohol (ally1 alcohol) were studied. These compounds gave no response except for t-butyl alcohol which forms the (M-OH)+ ions, whereas no other butyl alcohol isomer provides a signal. This
131
result is consistent with the well-known fact that the amount of energy needed to form the various classes of carbocations decreases in the order CH, > primary cation > secondary cation > tertiary cation. Abundant (M-OH)+ (C,HT) ions were produced from benzyl alcohol, but it is not certain whether these are benzyl-type or tropylium ions. Ethers
Neither saturated aliphatic ethers nor unsubstituted diary1 ethers exhibit peaks, but two kinds of aryl-alkyl ethers do. Both methoxybenzene and 4-methoxytoluene produce (M-H)+ ions, but their intensities differ substantially. In the former the peak is small, whereas it is very high in the latter, and the current density of the (M-H)+ ions increases when the ring is substituted by a methyl group. This enhancement may reflect an increase in the probability of hydrogen atom elimination from the molecule and the decrease in the IE value. This substituent effect has been observed previously [4]. Aldehydes and ketones
As can be seen from Table 1, the SI spectra of acetaldehyde, propionaldehyde and butyraldehyde are characterized by ions from the efficient DSI process; very intense (M-H)+ ions, which are directly characteristic of the molecular weight, and other relatively strong ion peaks, such as (M-CH3)+ and CHO+, which are diagnostic for structural determinations. It is concluded that (1) the SI spectra of these aliphatic aldehydes comprise principally the same characteristic set of lines, and (2) increasing the molecular weight of straight-chain aldehydes causes an increase in the ionization efficiency (sensitivity) of the (M-H)+ species. Four simple ketones (acetone, methylvinyl ketone, acetophenone, pbenzoquinone) were studied. The SIOMS of these compounds is virtually of no value with the exception of acetophenone which shows DSI ions at a very small response. A very efficient DSI process was found for benzoyl-containing compounds such as benzaldehyde and acetophenone. The molecules of these compounds desorb as C6H5CO+ ions with efficient ionization. At a higher emitter temperature, the C,H: peak was also obtained with low intensity; it is caused by decomposition of the metastable desorbed ions C6H5CO+ +
C,H,f + CO
This is in agreement with the results of Paleev [12].
132 TABLE 1 Surface ionization mass spectra and sensitivity for oxygen-containing Compound
Intensity
(m/s. IE%V))
W)
peakb
organic compounds
Emitter
Sensitivity’
SI/EI
temp.
(A Torr-‘)
004
Remarks’
(“C) A/coho/s r-Buy1
alcohol
57, (M-OH)+
830
5 x 10-Z
0.4
(74, 10.2) Benzyl alcohol
91, (M-OH)+
900
I.1 x lo-’
(108, 9.1) Methyl
alcohol
(m/z = 32, IE = 10.9eV)
No peak
(m/z = 60, IE = 10.2eV
No peak
(m/r
No peak
alcohol
xc-Butyl
alcohol
Ethylene
glycol (m/r
Glycerin
(m/r
Ally1 alcohol
IE[(M-H)]
= 6.82
No peak
(m/z = 46, IE = 10.5eV)
alcohol
n-Butyl
= 6.9eV
(79)
Ethyl alcohol Propyl
IE[(CH3)3]
(57) 1.4
= 74, IE = IO.OeV)
(m/z = 74, IE = 10.1 eV) = 62, IE =
No peak
10.1 eV)
No peak
= 92, IE = ?) (m/r
No peak
= 58, IE = 9.7eV)
No peak
Ethers Methoxybenzene
107, (M-H)+
760
1.7 x 10-j
8 x lO-3
(108, 8.2) CMetoxytoluene
121, (M-H)+
690
(108) 5.0
1.2
(122, 7.83) Diethyl
ether (m/r
Diphenyl
= 74, IE = 9.5eV)
No peak
ether (m/z = 170, IE = 8.8eV)
Aldehydes
No peak
and kerones
Acetaldehyde
43, (M-H)+
(44, 10.2)
29, CHO+
Propionaldehyde
57, (M-H)+
(58, 10.0)
43, CH,CO+
670
I x 10-2 1.5 x lo-’
Butyraldehyde
55, CH,=C(H)CO+ 43, CH,CO+ 57, (M-CH
830
)+
(29)
7 x 10-2
0.17
6.4 x IO-*
(43)
5.0 x 10-Z
I
3.1 x 10-2 1.1 x lo-2
29, CHO+ 900
105, (M-H)+
0.6
(106, 9.5) Acetone
43, CH,CO+
Methylvinylketone
43, CH,CO+
(70, ?) Acetophenone
105, C,H,CO
(120, 9.3)
119, (M-H)+
10-2
5 x 10-b
(72, 9, 8)
Benzaldehyde
I x (29) 0.3
900
29, CHO+
71, (M-H)
3 x 10-j 2.4 x lO-5
760 830
WW-WI = 7.22
TraLX
760
+
1.4 (77) 2.8 x IO-*
8 x lO-3 IWf
(55) 0.54
81
(105)
WM-CH311 = 7.22 T, = 25’C
p-Benzoquinone
(m/r
=
108, IE = 9.7eV)
No peak
133 TABLE 1 (continued) Compound
Intensity
(m/z, IEYeV))
(m/z)
peakb
Emitter temp.
SensitivityC (A Torr-‘)
SI/EI
830
100’
trace
760
0.1
0.33
900
100’
Remarkse
(n)*
(“C) Carboxylic acids and their derivatives Acetic acid 43, (M-OH)+ (60, 10.4) Acrylic acid 55, (M-OH)+ (72, 10.6) Levulinic acid 43, CH$O+ (116, ?) 99, (M-OH)+ 71, ? Benzoic acid 105, C,H,CO+ (122, 9.5) 77, C,H: p-Phthalic 105, C6H,CO+ acid (166, 9.9) Maleic acid (m/r = 116, IE = 10.7eV) Oxalic acid (m/r = 90, IE = 10.8eV) Phenols and their derivatives 107, (M-H)+ p-Methylphenol (108, 8.9) Aminophenol 109, M+ 108, (M-H)+ (109, ?) Nitrosophenol 107, (M-O)+ (123, ?) Phenol (m/z = 94, IE = 8.5eV) o-Chlorophenol (m/z = 128, IE = 9.3eV) o-Nitrophenol (m/z = 139, IE = 9.5eV) o-Catechol (m/r = 110, IE = 8.15eV) Heterocyclic compounds Tetrahydrofuran (72, 9.4) Furfural (96, 9.2) Tetrahydropyran (86, 9.3) Dibenzofuran (168, 7.9) Furan (m/a = 68, IE = Benzofuran (m/z = 118,
71, (M-H)+ 69, (M-3H)+ 95, (M-H)+ 81, (M-5H)+ 168, M+
620
830
(27) 0.076
T, = 25°C
T, = 55-z
83 58 IOd 7.4
(43)
(105)
WM-OWI = 7.22
100’
0.018
T, = 65’C T, = 130°C
0.21
(76) No peak No peak
760 900 830
100’ IOd 56 lOOf
0.0011
T, = 20°C
(107) 0.13
TX = lOO’=C
(109) 0.02
r, = 100°C
(123) No peak No peak No peak
900 830 760 830
9 x 10-Z 5.0 x 10-l lOOf
AE(MH+)
= 8.86
0.43 (42) 0.018
T, = 0°C
(96) 0.36
6 x 10m2
(41) 0.02
100’
T, = 110°C
(168) 8.9eV) IE = 8.3 eV)
No peak No peak
‘Ionization energy (I/?) values are from refs. 7 and 8. bPeaks have more than 5% base peak intensities; postulated structures are not supported by other evidence. ‘Current density at reference pressure of I Torr, determined from the heights of the lines in the mass spectrum. *n indicates the mass number of the peak in the EI spectrum for comparison. eT, is the temperature of the gas chromatograph oven in which the glass sampler containing the sample is placed. ‘The spectra of these compounds are expressed as relative intensities.
134
Carboxylic acids
In the ionization of aliphatic acids, all compounds except dicarboxylic acids (maleic acids and oxalic acid) exhibit (M - 17)+ ions with variable efficiency. Presumably these ions correspond to loss of OH, although the structure of the m/z 71 ions of laevulinic acid has not yet been determined. The aromatic carbonyl molecules studied form C6H5CO+ ions with high ionization efficiency, suggesting that C6H,CO+ ions are recorded upon the ionization of benzoyl-containing compounds. Again CsHc ions were also observed, presumably resulting from the decomposition of the C6H5CO+ ions. This process was observed with benzaldehyde. The radical ion CgH5CO+ (m/z 105) is expected to be produced by DSI from several compounds such as benzaldehyde, acetophenone, benzoic acid and phthalic acid. Results show that the intensity of this ion varies from one compound to another. Since the experiments were conducted under the same conditions, the difference in ionization efficiency of the radicals obtained from different compounds is attributable to the difference in the production rate of C,H,CO species (i.e. a different yield in the dissociative reaction on the emitter surface). The dissociation yield of phthalic acid was the smallest among these four compounds. Phenols
Phenol did not produce any detectable (M + H)+ ions, in contrast to the results of Zandberg and Rasulev [13]. This discrepancy is not due solely to instrumental differences and we are confident of our results. Our conclusion is also supported by thermodynamic considerations since calculations* show that the AE of (C,H, OHHf ) is 8.86 eV, too high for surface-ionization on the Re oxide emitter. The SIOMS of six ring-substituted phenols (RC6H,0H; R = CH,, NH,, NO, Cl, N02, OH) possessing different electronic properties were recorded. The SI spectrum of aminophenol is characterized by Mf and (M-H)+ ions, which are directly characteristic of the molecular weight. The currents of the M+ and (M-H)+ ions increase with increase in temperature up to T = 1170 K and their i(T) behaviours are similar over this temperature range. This indicates only a small difference between the IE values of the molecule and radical (M-H). p-Methyl phenol also produces (M-H)+ ions with low intensity. The efliciency of the formation of (M-H)+ ions depends significantly on the * The efficiency of AS1 ion formation depends on the appearance energy (AE), defined as ) = D(M-H) + IE(H) - PA(M) and calculated using literature values for the proton affinity (PA) of the molecule M and the bond dissociation energy D(M-H).
AE(MH+
135
electronegativity of R, with current densities of the (M-H)+ ions being in the order R = NH, > R = CH,. However, no ions could be obtained with R = N02, Cl, OH or NO at any temperature. These results are qualitatively correlated with the Hammett substituent constant. The same correlations were found for the effect of substituents on the composition of the SI mass spectra [3] of aniline derivatives. The SI spectrum of nitrosophenol is dominated by a (M - 16)+ peak which presumably corresponds to loss of oxygen. The spectrum of this compound shows the same characteristic ion species, (M-O)+, as those of nitrosamines
PI. Heterocyclic compounds
Tetrahydrofuran (THF) and tetrahydropyran (THP) form stable ions by loss of hydrogen atoms. In the mass spectrum of THF, the (M-H)+ and (M-3H)+ ions appear, while that of THP has (M-5H)+ as the highest peak. In this ion, the ring is, presumably, completely aromatized. Such behaviour is very similar to that of piperidine, the corresponding heterocyclic amine [3]. The mass spectral results of furan and furfural differ substantially. In the former, no peaks are observed, whereas the (M-H)+ peak is quite strong in the latter. This phenomenon may be compared with the formation of (M-H)+ ions in methyl-substituted benzene derivatives. Benzofuran forms no measurable ions, whereas dibenzofuran provides the MS1 ion. This result reflects the difference in the IE values of the two compounds. CONCLUSION
The principal generalizations governing the ionization of oxygen-containing compounds have been established on the basis of their SI mass spectra. In general, many oxygen-containing organic compounds show lower SI sensitivities than analogous nitrogen-containing compounds, but the SI mode gives greater output currents for three compounds (benzyl alcohol, 4methoxytoluene, benzaldehyde) than does the EI mode. This result demonstrates that SIOMS may be useful as an analytical method for certain kinds of oxygen-containing compounds. However, it should be noted that our comparisons were limited to the SI and EI sources we used and can have only qualitative significance. Aldehyde compounds generate intense DSI ions with a number of lines which are characteristic of the molecular weight and diagnostic for structural determinations. MS1 ions are produced by aminophenol and dibenzofuran for which the IE is not too large.
136
No associative ions were formed for the compounds was expected, as compounds with oxygen heteroatoms proton acceptor properties.
studied. This result have essentially no
REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13
E.Ya. Zandberg and U.Kh. Rasulev, Russ. Chem. Rev., 51 (1982) 819. T. Fujii and T. Kitai, Int. J. Mass Spectrom. Ion Processes, 71 (1986) 129. T. Fujii and H. Jimba, Int. J. Mass Spectrom. Ion Processes, 79 (1987) 221. T. Fujii, H. Ishii and H. Jimba, Int. J. Mass Spectrom. Ion Processes, 93 (1989) 73. T. Fujii and H. Ishii, Chem. Phys. Lett., 163 (1989) 69. T. Fujii, Int. J. Mass Spectrom. Ion Processes, 57 (1984) 63. H.M. Rosenstock, K. Draxl, B.W. Steiner and J.H. Herion, J. Phys. Chem. Ref. Data, 6 (1977) 783P. S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.G. Mallard, J. Phys. Chem. Ref. Data, 18 (suppl. 1) (1988) 861P. T. Fujii, H. Suzuku and M. Obuchi, J. Phys. Chem., 89 (1985) 4687. R.C. Weast (Ed.), Handbook of Chemistry and Physics, 60th edn., CRC Press, Boca Raton, FL, 1979-1980, FL231-F237. T. Fujii and H. Arimoto, Am. Lab., (1987) 54. V.I. Paleev, Teor. Eksp. Khim., 14 (1978) 747. E.Ya. Zandberg and U.Kh. Rasulev, Dokl. Akad. Nauk. SSSR, 187 (1969) 777.