monitoring of the 4308-A CD* emission for slightly deuterated species. Following the method of Johnson and Stross (32) the limit of detection for C6HI2in helium plasma at 1.0 torr was 7 X 10-9 gram sec-I. In the 0.25-2.0 torr range studied, increasing the pressure in the discharge increased the sensitivity, which is inversely proportional to the limit of detection. Also, no attempt was made in the present study to cool the phototube or otherwise reduce the noise level. Since the limit of detection is directly proportional to the noise area (32), a higher sensitivity would be expected if this were done. (A referee has suggested that the analytical technique described would be most suitable for use with capillary columns. Higher concentrations of sample in the effuent with lower He flow rates would allow for the entire output to be delivered into the low pressure discharge tube with a significantly improved sensitivity. This intriguing possibility was not examined in the present studies.)
o 100 %
C,H,
5 0 % %HI, v 25%
? I
A
C6H12
50%
75 %
100% C6DI2
E
: I
.-
Y)
E
5 0
n 10
*
2.5
CONCLUSION
It is evident from the present study that the fractional emission intensities of excited CH* and CD* fragments from labeled hydrocarbon species admitted to a low pressure microwave discharge in helium can be quantitatively related to the fractional deuterium content in the sample. From the analysis of the emission spectra of methanes and ethanes, it is apparent that isotope effects in fragmentation are observed which indicate that in the decomposition process, loss of H is favored over loss of D. The increase in ionic emissions and the decrease in atomic emission intensities of helium and argon upon addition of a trace hydrocarbon suggests a simple method to distinguish between ionic and atomic lines from rare gas discharges.
RECEIVED for review April 30, 1970. Accepted July 24, 1970. D. A. L. is a National Science Foundation Undergraduate (32) H. W. Johnson, Jr., and F.H. Stross, ANAL.CHEM., 31, 1206
(1959).
0
0 Thermal
2 4 Conductivity Detector Response
6 (mv)
Figure 10. Determination of extent of labeling in heavily deuterated C6HI2-C& mixtures: 4313-A emission response
Research Fellow, 1969-1970, and J. L. B. is an Alfred P. Sloan Fellow, 1968-1970. This research has been supported in part by funds made available by the United States Atomic Energy Commission (Contract AT(04-3) 767-81, the Alfred P. Sloan Foundation, the National Science Foundation (Grant No. GY-5881 and GY-7351), and American Cyanamid. Contribution No. 4044 from the Arthur Amos Noyes Laboratory of Chemical Physics.
Field Ionization Mass Spectra of Dialkyl Phthalates James C. Tou Chemical Physics Research Laboratory, The Dow Chemical Company, Midland, Mich. 48640 The field ionization mass spectrum of each of nine dialkyl phthalates shows an intense molecular ion peak, an intense metastable ion corresponding to the transition M f (M - R 2) .(R - 2) or M t 4 (M - OR) + .OR in cases of diallyl and diphenyl phthalates, and two characteristic peaks at m / e = 148, 149. An example is given of the qualitative analysis of a mixture to demonstrate the great usefulness of metastable ions in field ionization mass spectrometry. Beckey’s rules in the comparison of field ionization mass spectra with electron impact mass spectra are generally applicable to the phthalates studied. -f
+ + +
+
A HIGHELECTRIC FIELD has been considered as a much milder means for ionizing gaseous molecules ( I ) and radicals (2) than (1) For review see H. D. Beckey in “Mass Spectrometry,” R. I. Reed Ed., Academic Press, London and New York, 1965 pp
93-127.
(2) H. Butzert and H. D. Beckey, 2.Phys. Chem. (Frankfurt am Main), 62, 83 (1968).
electron impact. Field ionization (FI) mass spectrometry has the advantages of giving relatively stronger molecular ion peak as well as a simplified mass spectrum in the elucidation of organic molecular structure (3). The technique complements electron impact (EI) mass spectrometry. The E1 mass spectra of dialkyl phthalates have been studied extensively by many investigators (4-6). The intensities of the molecular ion peaks in the spectra show a rapid decrease with increasing size of the alkyl group. N o detectable molecular ion has been reported for a number of phthalates with unsaturated alkyl groups and alkyl groups whose carbon number (3) G. G. Wanless and G. A. Glock, Jr., ANAL. CHeM., 39, 2
(1967). (4) F.W. McLafferty and R. S . Gohlke, ibid., 31, 2076 (1959). (5) E. M. Emery, ibid., 32, 1495 (1960). (6) C. Djerassi and C. Fenselau, J. Amer. Chern. Soc., 87, 5756 (1965).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1381
Table I. R mol wt
o-allyl 246
o-rz-butyl 278
The FI Mass Spectra of Dialkyl Phthalates C6H4(COOR),
o-i-butyl 278
o-phenyl 318
o-c-hexyl 330
o-n-octyl 390
mje 28
29 30 41 42 43 44 55 56 57 58 66 72 74 81 82 83 84 85 93 94 98 105 112 113 114 123 148 149 150 166 187 188 189 190 205 206 207 221 222 223 225 226 230 231 233 246 247 248 249 250 25 1 261 264 265 275 276 277 278 279 280 28 1 291 292 305 318 319 320 330 331 332 334 335
1382
0.1
5e
4 2 0.1
57 14
0.4
4 2
p-i-butyl 278
9
0.4 6 0.3
3
0.4 2 0.3
41 3
0.5 0.6
0.3
100 23
0.6
0.3
3
17 1 5
o-it-nonyl 418
p-n-hexyl 334 6
1
0.8 0.6
5 2 100 34 2
1 0.8
38 6
20 4 L
0.5 0.8
6 8 0.8 100 10 0.8
6 2
100 15 2
5 1
19 7
3 5 0.5
0.7 1 2
7
0.8 2
0.3 3
2 6 7 0.8
0.9 0.6 0.1
3 0.9
34 5
3
2c 0.9 0.7
12 2
0.5
0.6 2
81 17 2 4
57 (Mt) 10 7 0.9
3
0.8 0.2
1
3 0.6. 0.1 3 0.5 100 (M:) 28 4
9 3 0.6
55 (Mt) 11 3
18 (Mt) 17 3
0.4 2
0.34
2 2
100 (Mt) 29 5 34 (Mt) 18 4
100 (Mt) 42
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
R mol wt 336 376 390 391 392 404 405 406 418 419 420 432 433 435
o-allyl 246
o-n-butyl 278
o-i-butyl 278
Table I. (continued) o-phenyl o-c-hexyl 318 330
o-iz-octyl 390
o-rt-nonyl p-i--butyl p-n-hexyl 418 278 334 8
20 100 (Mt) 52 11
;
)a
0.4 100 (Mt) 47 11
;
}a
0.4
b
0
c
5
5 0.5
2 0.2
Metastable Ion 0
2
10
0
+
7 Weakd & Broad 42) 0.9
7 0.6 Broad
4 0.6
14 2
(113+ + 71+ 44.6 (84+ --* 56+ 28) 0 . 7 37.3 a Due to impurity. b Mf + (M - R 2)+ .(R - 2) Mf --* (M - OR)+ .OR d The metastable ion peak is centered at m/e = 176.2, which might be generated from the transition of Mt + (M-OR 1)+ (OR - 1). e Split peaks.
+
+
+
+
+
is higher than five. All the phthalates studied give two important characteristic intense peaks. One is the peak at m/e 149, which is assumed to be a protonated phathalic anhydride even electron ion. The other one is the peak due to an ion corresponding t o the loss of one alkyl group with two hydrogen atoms rearranged back to the carboxylic group on which the leaving alkyl group was originally attached. The ions associated with the single bond cleavages as shown in Table I1 in the text and the consecutive fragmentation of the alkyl group were also noted. In their isotope labeling studies, Djerassi and Fenselau (6) found that the transferring reactions involving two hydrogen atoms and one hydrogen atom in the decomposition of dibutyl phthalate molecular ion which give rise to the intense peaks at m/e 223 and 149, respectively, were not specific but rather involved hydrogen from every carbon atom in the alkyl chain. A metastable ion involving double hydrogen rearrangement of the molecular ion was also observed in their logarithmic intensity scale mass spectrum. No metastable ion of the same transition was detected in other studies (4, 5, 7, 8), however. This paper presents the FI mass spectrometric studies of nine dialkyl phthalates, which have molecular ion peaks that are either weak or absent in their E1 mass spectra. An example is given for the analysis of a complicated mixture of phthalates, utilizing the strong metastable ion peaks observed in their FI mass spectra.
+
observation of an intense molecular ion peak, MT, in the FI mass spectra, while this ion peak is either absent or extremely weak in the E1 mass spectra. The FI mass spectra also show a 1) ion peak, presumably due to a considerably intense (M protonated molecular ion generated from an ion-molecular reaction on or near the surface of the field emitter, a Wollaston 1) ion is often observed wire in this experiment. The (M in the FI spectra of organic compounds containing a polar group (9-11). However this should not be misinterpreted as a molecular ion because, to the best of our knowledge, C-H cleavage is unlikely to occur in FI mass spectrometry. Furthermore this could be definitely identified by high resolution FI mass spectrometry (12,13) as an even electron ion. In contrast to E1 mass spectrometry, the intense ion peak corresponding to (M-R+2)+ observed in the E1 mass spectrum disappears or is extremely weak in the FI mass spectrum. On the contrary, the metastable ion associated with the transition, Mi. +(M - R+2)+ . (R - 2), is very strong in the F I mass spectrum as shown in Table I, but is weak or absent in the E1 mass spectrum (5,6). This metastable ion is very useful in the structure elucidation of dialkyl phthalate. An example will be given latter. The presence of this metastable ion might be due to the expected low frequency factor and possibly the low activation energy (14,15)for forming the highly strained activated complex (M+*) which leads to the reaction M t + M t * +(M - R+2)+ (R - 2). Hence the rates of decomposition increase slowly with respect to internal energy and
+
+
+
+
3
RESULTS AND DISCUSSIONS
The FI mass spectra of nine dialkyl phthalates are tabulated in Table I. The relative intensities of major monoisotopic ion peaks in the E1 ( 4 ) and FI mass spectra are calculated and compared in Table 11. One of the most striking differences is the (7) J. H. Beynon, “Mass Spectrometry and Its Applications to Organic Chemistry,” Elsevier Publishing Co., Amsterdam, 1960, p 377. (8) K. Bieman, “Mass Spectrometry,” McGraw-Hill Book Co., New York, N. Y . , 1962, pp 170-171.
(9) H. D. Beckey and G. Wagner, Z . Nuturforsclz., ZOa, 169 (1965). (10) J. N. Damico, R. P. Barron, and J . A. Sphon, J. Mass Spectrum. loiz Phys., 2, 161 (1969). (11) P. Brown, G. R. Pettit, and R. V. Robbins, Org. Muss Spectrum., 2, 521 (1969). (12) E. M. Chait, T. W. Shannon, W. 0. Perry, G. E. Van Lear, and F. W. McLafferty, J . Muss Spectrum. Ion Plzys., 2, 141 (1969). (13) P. Schulze, B. R. Simoneit, and A. L. Burlingame, ibid., p 183. (14) W. A. Chupka, J. Clzem.Phys., 30,191 (1959). (15) F. W. McLafferty, D. J. McAdoo, and J. S. Smith, J . Amer. Chem. Soc., 91,5400 (1969).
ANALYTICAL CHEMISTRY, VOL. 42,
NO. 12, OCTOBER 1970
1383
h 0
-
8
m 0
: '
i
o
0
Y
1384
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
0 2
0
80 70
t
; +
60
r-
50 40 30 20
10 0
.*2 0
60
100
140
180
220
260
m/ E
300
340
380
420
4 GO
500
_t
Figure 1. The E1 mass spectrum of a mixture of dialkyl phthalates thus the metastable ion transition ( k = 105 sec-loG sec) covers a wider energy range. However, as stated previously, this metastable ion is either weak or absent in the E1 mass spectrum. This is due to the fact that the molecular ion's internal energy in 70 eV E1 is distributed in a much higher energy range than in FI. Therefore, this mode of fragmentation has to compete with many other reactions with higher rates, which results in a relative decrease in the intensity of this metastable ion peak in the spectrum. As shown in Table I, the mentioned metastable ion was not observed in the cases of diallyl phthalate and diphenyl phthalate. This is not surprising when one considers the high stability of hydrogen atoms on allyl and phenyl groups, Instead, a metastable ion, intense in these two cases but weak in other cases, corresponding to the reaction M i -+ (M - OR)+ . O R was observed. Besides the ions generated from the alkyl group, the ions at mje 148, 149 were observed in all cases except that the ion m/e 148 was missing in the case of di-isobutyl terephthalate. The presence of these two ions in FI mass spectrum can be taken as an evidence of the compound being a phthalate. The structures of these two ions, mje 149,148, are assumed to be a protonated phthalic anhydride ion, a hydrogen rearrangement ion, and a phthalic anhydride ion, a nonrearrangement ion, respectively. Similar structures as noted in Table I1 are assumed for terephthalate cases. By analogy with the E1 fragmentation mechanism, it is reasonably assumed that the ions m/e 149, 148 are generated from multiple step rupture processes with and without hydrogen rearrangements. Beckey (16) defined quantities, R and Q, for comparison of single step decomposition involving rearrangement and nonrearrangement process in E1 and FI mass spectra, as
+
R
=
(Zi*i/lb~~>/(Ii~~/lb~~)
and Q
=
I'CFI n o r m a l ) / l i ( ~ metastable) ~
where 1's are the ion peak intensities, and the indices i and b refer to the ith fragment peak and the base peak. The quantities Rd and Ri,and Qd and Qr refer to nonrearrangement and (16) H. D. Beckey, H. Hey, K. Levsen and G. Tenschert, J . Muss Spectrum Ion PIzys., 2, 101 (1969).
rearrangement processes of R and Q, respectively. The rules found are Rd/Rt > 1, Qd > 1, Qi < 1. These quantities were calculated and listed in Table I1 for the reactions noted. It is obvious that the rules hold in general for the stated multiple step decomposition of phthalate molecular ions. Hence, the probability of having a rearrangement reaction in a E1 mass spectrum is higher than that in a FI mass spectrum, even though the rearrangement ion is observed to be relatively intense in FI mass spectrum. As noted in Table I and Table 11, some ion peaks are split and not resolved very well. This might be attributed to the ion decomposition in a cylindrical electrode which was maintained at about 100 V less than the ion accelerating voltage, 3 KV, with respect to ground. QUALITATIVE ANALYSIS OF A MIXTURE
The E1 and F I mass spectra of a sample supposedly to be octyl decyl phthalate, are shown in Figures 1 and 2. It is obvious from both spectra that the sample is not pure and contains phthalates with different lengths of alkyl groups. Evidence for the sample containing dialkyl phthalates is the presence of the peaks at mje 149, 148 and the hydrocarbon ion peaks at m/e 29, 43, 85 in the FI mass spectrum. Strong molecular ions and a 28 mass difference sequence are clearly shown in the FI mass spectrum. Six metastable ions corresponding to the transition, M t + (M - R+2)+ .(R - 2) were observed and grouped as follows:
+
- +++ --++
1. 390+ 279' 111; R = CSH1i 2 . 446' 307' 139; R = CloHzl 307+ 111; R = CsH17 3. 418" -418' 279' 139; R = CloHzl 4. 362+ 251' f 111; R = CsHli 362+ + 279' 83; R = CGHlO Hence four major components in the sample are dioctyl, didecyl, octyl decyl, and octyl hexyl phthalates. Because of the absence of the expected metastable ions, it can be ruled out that the molecular ions at m/e 418, 362 are due to dinonyl and diheptyl phthalates, respectively. The
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1385
4 18'
80
-
70
1
GO
C i-
.
50
-
3
4 0 -29'
I 30 20 10
0
20
60
100
140
180
220
-
260 d e
-
300
340
380
4 20
4 60
Figure 2. The FI mass spectrum of a mixture of dialkyl phthalates identification would be very difficult from the E1 mass spectrum alone. As noticed in Figure 1, certain weak molecular ions were observed in the E1 mass spectrum. However, the molecular ion of dioctyl phthalate, mje 390, was found to be absent in the spectrum previously reported (4). This might be due to the fact that the thermal energy transferred into the molecule prior to ionization (17) and the possibility of thermal decomposition is much less in the present experiment. CONCLUSIONS
EXPERIMENTAL The samples are commercial compounds. No further purification was performed. The FI and E1 mass spectra were obtained using the direct probe sample introduction and an Atlas CH4B mass spectrometer equipped with a EF4B ion source. The source was kept about 160 "C and electron energy at 70 eV. A voltage of 3 KV was applied on a Wallaston wire of radius of about 1.5 p and a voltage of -8 to -9 KV on its counter electrod:. The field on the wire surface is estimated to be 0.1 V/A. The wire (18) was usually conditioned with acetone at the source chamber pressure, 2 X 10-6 Torr and with the wire at +3 KV and its counter electrode at -7 KV for one to two nights to obtain operable sensitivity. It was found that the acetone sensitivity of the wire dropped after a sample had been run. The sensitivity can be regenerated to the original level by a vapor mixture of acetone and benzonitrile (19) at 2 X Torr. This mixture works better than acetone alone.
Besides their intense molecular ion peaks, the phthalates studied gave characteristic peaks at m/e 149, 148 and an intense metastable ion peak corresponding to the transition .(R-2) in their FI mass spectra. In M' ---t (M-R+2)+ the cases of dialkyl phthalate and diphenyl phthalate, an intense metastable ion corresponding to M t + (M-OR)' +ORwas exhibited instead, The length of the alkyl chain can be determined from the molecular ion and the metastable ions. Beckey's rules in comparison of E1 and FI mass spectra are generally applicable to the phthalates studied.
RECEIVED for review May 18, 1970. Accepted July 28, 1970.
(17) M. L. Vestal in "Fundamental Process in Radiation Chemistry," P. Ausloss, Ed., Wiley-Interscience, New York, N. Y . , 1968, p 59.
(18) J. C. Tou, L. B. Westover, and E. J. Sutton, J. Muss Spectrum. Ion Phys., 3, 277 (1969). (19) H. D. Beckey, E. Hilt, A. Maas, M. D. Migahed, and E. Ochterbeck, ibid., p 161.
+
+
1386
ACKNOWLEDGMENT The author would like to express his sincere appreciation to R. D. Beckrow for obtaining the mass spectra.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
