Investigation of Polytertiary Alkylamines Using Chemical Ionization Mass Spectrometry T. A. Whitney, L. P. Klemann, and F. H.Field' Corporate Research Laboratories, Esso Research and Engineering Company, Linden, N .J . 07036 The chemical ionization mass spectra of five polytertiary amines and of trimethylamine have been determined using nitrogen, methane, and isobutane as reactant gases. The amounts of fragmentation observed decrease in the order of reactants nitrogen > methane > isobutane. Even for the largest compounds investigated (triethylenetetramines), the (M 1)+ ions comprise about 50% of the amine ionization in the isobutane chemical ionization mass spectra. The spectra obtained by nitrogen chemical ionization are very similar to those obtained by electron impact. Reactions are postulated for the production of the major ions observed in the several spectra.
+
THISSTUDY was undertaken as part of the continuing investigation of chemical ionization mass spectrometry in progress in this laboratory. The types of compounds here investigated, polytertiary alkylamines, were chosen in part because they constitute a class of compounds which fragment very extensively under electron impact. We wished to determine whether the amount of fragmentation by chemical ionization would be smaller for these compounds as it is for other types of compounds, for example, paraffins (I), esters (2), alcohols U), and amino acids (4). Furthermore, we wished to investigate the gaseous ionic chemistry using different kinds of reagent gases. Finally, no satisfactory analytical method for this class of compounds exists. Conventional electron impact mass spectrometry provides little help because of extensive fragmentation, and we hoped that the results obtained in the present chemical ionization investigation might be useful in developing an analytical method. Previous chemical ionization studies on other compounds employing different reactant gases have demonstrated that the degree of fragmentation in chemical ionization varies with the type of reactant gas utilized (5). The use of methane produces increased fragmentation compared with isobutane although both compounds induce an even-electron, acidbase gaseous ionic chemistry. On the other hand, nitrogen as the reactant produces odd-electron, oxidation-reduction chemistry which is the chemical ionization mass spectrometry equivalent of electron impact mass spectrometry. Part of the purpose of the present study was to illustrate in detail the effect of three reactant gases on a single class of compounds.
EXPERIMENTAL
The polytertiary amines used in this study are listed, with their abbreviations, in Table I. Author to whom correspondence should be addressed a t the Rockefeller University, New York, N.Y. 10021 (1) F. H . Field, M. S. B. Munson, and D. A. Becker, Ad~urz.Chem. Ser., 58, 167-192 (1966). (2) M . S . B. Munson and F. H. Field, J . Amer. Chem. SOC.,88, 4337 (1966). (3) F. H . Field, ibid., 92, 2672 (1970). (4) G. W. A . Milne, T. Axenrod, and H. M. Fales, ibid., p 5170. ( 5 ) F. H. Field, Accouiits Chem. Res., 1, 42 (1968).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
Table I. Polyamine Nomenclature Compound Trimethylamine N,N'-Dimethylpiperazine
Abbreviation TMA DMP N,N,N',N'-Tetramethylethylenediamine TMED N,N,N',N",N"'-Pentamethyldiethylenetriamine P M D T Tris-(P-dimethylaminoethy1)amine iso-HMTT
N,N,N',N",N'",N"'-HexamethyltriethyleneTZ-HMTT
tetramine
Gaseous TMA was used as purchased from Matheson, DMP (Aldrich), TMED (Matheson), and PMDT (Ames Laboratories) were purified by distillation from calcium hydride. n-HMTT (Ames Laboratories) was obtained in >99.5% purity by vacuum fractional distillation on a 15plate Oldershaw column at a reflux ratio of 40:l. Tris(P-aminoethy1)amine (Dow Developmental Chemical SA1515 ) was methylated by the Eschweiler-Clarke (6) method, and pure iso-HMTT was obtained by preparative scale gas chromatography. The separation was effected using a 10-foot, S/s-inch diameter column of 15% Carbowax 20 MKOH on 60/80 Chromosorb W at 200 "C with a helium carrier gas flow of 80-90 cc/min. The spectra were obtained with the Esso chemical physics spectrometer, which has been described previously (7). All measurements were made with the source temperature at 150 f 5 "C. The methane and isobutane used were research grade materials obtained from Lif-0-Gen Co. of Lumberton, N. J. Nitrogen was J. T. Baker purified grade. The electron impact mass spectrum of TMED was obtained using a CEC Model 21-103 mass spectrometer. In the chemical ionization experiments, the pressures of the methane and nitrogen in the ionization chamber of the mass spectrometer were maintained at 1.00 + 0.05 Torr, while the pressure of the isobutane was held at 0.70 f 0.02 Torr. The introduction of the amine samples was accomplished in either of two ways. Trimethylamine gas was introduced at a pressure empirically adjusted to give a suitable ion intensity. The remaining compounds (all liquids) were introduced into the gas handling system through a gallium frit by means of a micropipet. A sample size of 1.75 microliters was generally employed. RESULTS AND DISCUSSION
The mass spectra of the six saturated tertiary amines investigated in this work are given in Tables 11-VII. For all compounds, spectra were obtained using nitrogen, methane, and isobutane as reactant gases. The values appearing in these tables are the relative intensities calculated as fractions of the ionization attributed to the amine additive. In addition, we have tabulated a quantity designated in Tables 111-VI1 as S, defined by
( 6 ) H. T. Clarke, H. B. Gillespie, and S. 2. Weisshaus, J. Amer. Chem. SOC., 55, 4571 (1933). (7) M . S. B. Munson and F. H. Field, ibid., 88, 2621 (1966).
Table II. Chemical Ionization Spectra of Trimethylamine RIn with reactant ms ~SO-C~HIO Nn CH4 mle ... 0.001 ... 119 0.002 ... ... 102 0,001 ... ... 101 0.016 ... 100 ,., *,. 0 024 98 ... 0.011 *.. 90 0.006 89 0:%3 0.081 ... 88 ... ... 0.001 87 ... ... 0.001 86 ... ... 0.001 75 *.. 0.003 74 *.. ... 0.001 72 0.001 ... 0:0i4 71 0.001 ... .,. 68 * . * 0.017 ... 61 0.453 0.035 60 0.925 0.127 0.072 59 ... 0.324 ... 0.226 58 57 .*. ... 0.054 ... 56 0.002 ... 0.007 ... ... 46 0.003 ... 45 0 024 0.004 ... 44 43 0.076 ... ... 0.167 42 ... ... 32 ... ... 0.089 0.195 ... 30 ZRIb 0.998 0.997 0:963 18163 ZI o 62494 1675 a RI = relative intensity. Summation of the tabulated RI values. Summation of ions attributed to amine additive. For trimethylamine the ZZ values vary because different amounts of trimethylamine gas were introduced into the mass spectrometer.
...
Table IV. Chemical Ionization Spectra of Tetramethylethylenediamine RI. with reactant gas Ion mle Nz CHI iso-C4Hlo (M + 5 7 ) + 173 155 .*. ... 0.019 (M 39)+ ... ... 0.015 118 ... 0.010 0.065 ( M I)+ 117 0.011 0.121 0.856 M+ 116 0.058 0.084 0.010 (M - I)+ 115 ... 0.302 0.026 73 ... 0.022 ,.. 72 0.052 0.408 0.009 71 0.031 0.010 ... 70 0.020 ... ... 59 0.028 ... ,.. 58 0.798 0.045 .., ZRIb 0.998 1 .ooo 1 .ooo 1.9 X 1OI6 1.1 X 10'6 1.8 X 1018 S= RI = relative intensity. Summation of the tabulated RI values, S = ZZ/(Z,)(u) where ZZ = summation of intensities attributed to TMED additive, I,,, = monitored ion current in amperes, and u = volume of added TMED in pl.
+ +
Table V. Chemical Ionization Spectra of Pentamethyldiethylenetriamine RI" with reactant gas
:
Table 111. Chemical Ionization Spectra of Dimethylpiperazine RIa with reactant gas mle Nn CHI iso-C4Hl0 172 ... *.. 0.014 157 ... 0.003 ... 155 *.. 0.006 153 143 116 115 114 113 112 101 99 98 72 71 70
ZRIb SC
... ...
0.071
0:os0 0.340 0.045
0.317 0.107 0.443
.,.
,..
0.040
...
0.040
... 0.013 ... *.. ... ...
o:Ois
0:%6 0.815 0.030 0.026
...
0.015
...
0.022 *.. 0.019 ... 0.124 ... 0.319 ... 0.999 0.960 0:981 3.7 x 10" 1.0 x 10'6 4.3 x 1018
RI = relative intensity. Summation of the tabulated RI values. S = ZZ/(Z,,,)(u) where ZZ = summation of intensities attributed to DMP additive, I,,, = monitored ion current in amperes, and u = volume of added DMP in ~ l . (I
where ZZ = summation of the intensities attributed t o the amine additive, Zm = monitored ion current in amperes, and u = volume of amine additive in microliters. The quantity is a measure of the intrinsic ionization sensitivity of the added amine since the absolute total ionization of the amine, ZZ,is proportional to the monitored ion current, I,,, (a small,
Ion
++ 57)+ 39)+ ( M + 1)+ M+
(M (M
(M - 1)+
+ 1 - 45)+ (M + 1 - 59)+ (M + 1 - 71)+ (M
(M - 101)+ (M
- 102)+
( M - 103)'
[(CHa)nN=CHzl+
m/e 230 212 175 174 173 172 131 130 129 128 115 103 102 99 73 72 71 70 59 58
ZRIb
... ... ... ... ... ... ... ... 0.023 0.079
o:oii 0.013 0.021 0.404 0.048 0.034 0.017 0.351 1.OOO
:
0 026 0.168 0.048 0.384
...
0.015 0.107 0.044 0.043 0.047 0.018
...
0:086 0.015
...
... ...
0.016 0.010 0.114 0.733 0.010 0.051 I
.
.
... ... ... 0.013 0.010
...
... ...
... ... ...
...
0: 980
Se
RI = relative intensity. Summation of the tabulated RI values. S = ZZ/(I,)(u) where ZZ = summation of intensities attributed to PMDT additive, I,,, = monitored ion current in amperes, and u = volume of added PMDT in pl. constant fraction of the unanalyzed ion current), and the volume of added amine, u. Since the primary aim of this research was to determine the relative spectra of the several compounds, the degree of care needed for quantitative mass spectrometric studies was not exercised, and thus the values of S obtained have only a semi-quantitative significance. No S values could be calculated for the trimethylamine spectra, since indeterminant amounts of the amine were introduced into the mass spectrometer. We observe from Tables 111-VI1 that the values of S generally fall in the range 10-l6to 10-l6,and the values do not show any consistent trend from one compound to another or from one reactant gas to another. The conclusion to be drawn from these results is that no very great differences in ionization sensitivity exist in the systems studied. ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1049
Table VI. Chemical Ionization Spectra of Isohexamethyltriethylenetetramine RIa with reactant gas Ion mle Nz CH4 iso-C,H,o 57)+ 287 ... ... 0.015
+ (M + I)+ M+
(M
(M - l)+ (M - 44)+
-I(CHa)zN=CHzl+ ., .
232 231 230 229 186 172 160 142 140 129 128 127 115 114 113 99 97 89 74 73 72 71 70 69 58 ZRIb Sc
... ... ... ... ...
0.080
...
... ... ...
o:Ok 0.023 0.020 0.020 0.014 ..*
0.037 0.241 0.050 0.251 0.074 0.038 0.018 0.011 0.016 0.012 0.012
0.097 0.630
0.011 0.027
...
...
... .
I
.
...
0.027 0.028 0.015
0.023 0.604 0.034 0.023
0.096 0.015
...
... 0.143
...
... ... ...
... 0.090
0:0k ...
... ... ... ... ...
... ... ... ... .
.
I
... ... ... ...
0.015 0.013
...
1.OOO
0.979 0.876 5.1 X 1015 2.6 X 10l6 1.6 X 10“
RI = relative intensity. Summation of tabulated RI values. S = ZZ/(Z,,,)(u) where 2 2 = summation of intensities attributed to iso-HMTT additive, 2, = monitored ion current in amperes and u = volume of added iso-HMTT in rl. Table VII. Chemical Ionization Spectra of n-Hexamethyltriethylenetetramine RIGwith reactant gas Ion iso-C4Hlo Nz CH, mle 287 ... 0.020 57)+
++ 29)+ (M + 1)’ M+ (M (M
(M - 1)+ (M - 44)’
259 232 231 230 229 186 174 172 160 129 128 127 115 114 113 103 101 99
... ... ... ...
...
... ... 0.050
0.014 0.035 0,208 0.041 0.285 0.031 0.012 0.033
...
0.loo 0.638 0.022 0.045
0:062
... ,..
0:029 0.050 0.025 0.113 0.014 0.014
...
01096 0.108
0,020 0.032
...
...
0.021
0.020
...
0.012 0.015 0.010
..* ... 0.042
...
... 0:023 ... 13 ... 0.021 72 0: 062 ... 0.430 0.016 ... 71 0.033 ... ... 70 0.057 0.140 .,. 58 ZRIb 1 .Ooo 0.999 0.999 5 . 8 x 1015 1.2 x 1016 3.4 x 1016 SC
RI
=
relative intensity.
* Summation of tabulated RI values.
S = ZZ/(Z,)(o) where ZZ = summation of intensities attributed to n-HMTT additive, 2, = monitored ion current in amperes, and u = volume of added n-HMTT in fil.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
As is readily apparent by inspection of the data given in Tables 11-VII, the degree of fragmentation obtained by the chemical ionization technique is markedly dependent upon the identity of the reactant gas used. Fragmentation is greatest for nitrogen as reactant, least for isobutane, and intermediate for methane. We consider first the nitrogen chemical ionization spectra, and we make the observation that at about 1 Torr pressure, nitrogen produces reactant ions at m/e 28 (Nz+), 42 (N3+), and 56 (N4+). In addition, with some additives hydrogen abstraction occurs to produce ions at m / e 29 (NzH’) and 43 (N3H+). Thus in general we run nitrogen chemical ionization spectra at mass numbers greater than 56. However, for trimethylamine a special effort was made to obtain ions of lower mje values. Ions in the trimethylamine spectrum with mje values equal to those of ions in the NZ spectrum were obtained by difference between the spectra in the presence and absence of the amine. Molecule ions, M+, are to be observed only for trimethyl amine, dimethyl piperazine, and tetramethylethylenediamine. The larger polyamines all fragment extensively, which is in keeping with the known sensitivity toward fragmentation of amines in electron impact ionization. In this regard it is of interest to compare spectra obtained by nitrogen chemical ionization and electron impact, and we list in Table VI11 nitrogen chemical ionization spectra obtained in this work along with available electron impact spectra. One sees that the two kinds of spectra are very similar, although there are some differences in details. The degree of similarity is close enough to indicate that nitrogen chemical ionization produces spectra which for practical purposes are essentially equivalent to those obtained under conventional electron impact conditions. That this is the case is not surprising insofar as odd-electron ions are initially formed in both ionization techniques, and the amount of energy transferred to the molecules being ionized is about the same in the respective techniques. Thus the recombination energy of Nz+is 15.3 eV (8), whereas the electron impact excitation energy distribution curve extends for several volts above the ionization potential (9), which would correspond to a total energy transfer of approximately 15 eV. The major ions in both the electron impact and nitrogen chemical ionization spectra of trimethylamine are those with m / e 59 (Mf), 58 [(M - 1)+], 42, and 30. The m/e 58 ion is formed by H-loss by a-cleavage, and Hvistendahl and Undheim (IO) have shown that in electron impact m / e 42 is formed both by CH1 and H loss from m / e 58 and by CH3 loss from m/e 57. M / e 30 is formed by CzH4 loss from m / e 58. Doubtless similar mechanisms occur in nitrogen chemical ionization. For dimethylpiperazine the M+ intensity at m / e 114 is large by both ionization techniques, and intense fragment ions are observed at m/e 71 and 70. Both ions probably arise by subsequent fragmentation of the products produced by the initial a-cleavage of the molecular iqn. The m / e 70 ion can be formed by the process (11, 12) shown in Scheme I, and we also include in this scheme a possible mechanism for the formation of the m/e 71 ion. ~~~
~
(8) E. Lindholm, Aduun. Chem. Ser. 58, 1 (1966). (9) See, for example, H. M. Rosenstock and M. Krauss, in “Mass Spectrometry of Organic Ions,” F. W. McLafferty, Ed., Academic Press, New York, N. Y.,1963,p 20. (10)G. Hvistendahl and K. Undheim, Org. Muss. Spectrom., 3, 821 (1970). (11) R. A. Saunders and A. E. Williams, “Advances in Mass Spectrometry,” Vol. 3, W. L. Mead, Ed., Elsevier Publishing CO., Amsterdam, 1964,p 681. (12) K. Biemann, “Mass Spectrometry,” Academic Press, New York, N. Y.,1962,p 185.
Table MI. Comparison of Electron Impacts and Nitrogen Chemical Ionization Spectra for TMA,DMP, and TMED DMPC(MW = 114) TMEDd(MW = 116) TMAb(MW = 59) RIe RI RI mle E1 CI d e E1 CI mle E1 CI 60 *.. 0.035 115 0.030 0.050 117 ... 0.011 59 0.164 0.127 114 0.431 0.340 116 0.051 0.058 58 0.392 0 226 113 0.021 0.045 99 0.013 ... 57 0.031 0.054 112 ... 0.040 72 0.023 0.052 56 0.020 ... 99 0.024 0.040 71 0.018 0.031 44 0.020 0.024 98 0.016 0.022 70 0.013 0.020 43 0.035 0.076 97 0,011 59 0.011 0.028 42 0.161 0.167 85 0.015 ... 58 0.852 0.798 41 0.024 ... 72 0.018 0.019 40 0.020 ... 71 0.224 0.124 32 0.008 0.089 70 0.162 0.319 30 0.117 0.195 ZRI’ 0.993 0.999 0.998 a E1 relative intensities calculated using published intensities for ions with m/e 2 30, 70,and 58 for TMA, DMP, and TMED, respectively. E1 data from Reference (8). e E1 data from Reference (9). d E1 data from CEC 21-103 mass spectrometer. I
.
e f
#
.
RI = relative intensity. Literature ion intensities displayed in the format used for the present study. ZRI = Summation of the tabulated CI relative intensities.
+
Table IX. (M 1)+ Produced from Polyamines with CHI and iso-C$Ilo (M 1)+ Intensity with Compound CHd iso-CdHlo TMA 0.453 0.925 DMP 0.317 0.815 TMED 0.121 0.856 PMDT 0.168 0.733 iso-HMTT 0.241 0.630 n-HMTT 0.208 0.638
SCHEME I1
+
m/e 56
SCHEME I
FH 3
CH3 CH,
FH3
/
FH3
CH3
m/e
70
CH3
m/e 72
m/e 71
For tetramethylethylenediamine, the dominant decomposition pathway involves ionization on nitrogen followed by a-cleavage of the C-C bond to form the mle 58 ion. For the higher acyclic amines the initial ionization can occur at any one of the nitrogen atoms present, which is followed by C-C a-cleavage to produce ions at m/e 115, 172, etc. in addition to ions with m/e 58. For the polyamines larger than TMED (PMDT, iso-HMTT, and n-HMTT) the most intense ion in the spectra is that with m/c 72, the dimethylaziridinium ion formed by a rearrangement process. Reactions illustrating these decomposition modes of the acyclic polyamines are given in Scheme 11. Note that the m/e 72 intensity in iso-HMTT is 0.604,which is higher by approximately 0.2 than the intensities of the ion in n-HMTT or in PMDT. The higher intensity in isoHMTT doubtless reflects the structure of this molecule
wherein the central nitrogen atom is substituted by three dimethylamino groups. When methane and isobutane are used as the reactant gases, additional information about the size and structures of the polyamine molecules can be obtained. In particular, molecular weight information is available from the relatively intense (M 1)+ ions formed by proton transfer reactions. The relative intensities of the (M 1)+ ions obtained using CHI and iso.C4Hlo are summarized in Table IX. This molecular weight information is of much use in the analysis of mixtures of polyamines, and it also is of value in the determination of structure. In studies with mixtures and with amines of uncertain structure we have found that with 1)+ isobutane as reagent the relative intensities of (M ions are of the order of 0.5 for polyamines containing up to six nitrogen atoms, which were the largest compounds in1)+ ions vestigated. We believe that the intensity of (M for polyamines using isobutane as reagent will be essentially independent of molecular size. Analogous behavior of the (M - 1)+ ions from paraffins using methane as reagent has previously been observed ( I ) . The spectra listed in Tables 11-VI1 exhibit ions in the
+
+
+
+
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1051
molecular weight region and above which are typically observed in methane and isobutane chemical ionization, The most intense of these for all compounds is the (M 1)+ ions formed with methane as reagent by hydride ion abstraction from any of the several CHa and CH2 groups in the molecules. The occurrence of this process is assisted by the possibility of referring the charge to the nitrogen atom to form an immonium ion. Thus, for example,
SCHEME 111
-
CH3\N-CH22H2-N
