Anal. Chem. 1985, 57,684-692
684
Methane Enhanced Negative Ion Mass Spectra of Hexachlorocyclopentadiene Derivatives Elizabeth A. Stemmler and Ronald A. Hites* School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
The methane, electron capture, negative ion mass spectra of 24 bridged polycyclic Chlorinated pesticides are presented. The effects of ion source pressure and temperature upon the spectra are discussed. These spectra differ from spectra which have previously appeared in the literature, although they were obtained under similar conditions of pressure and temperature. The spectra presented here consist of molecular ions, [M H Ci]- ions and various fragment ions, while the previously reported spectra were dominated by low mass ions such as Ci- and by [M Ci]- adduct ions. I n order to better understand why similar operating conditions result in different modes of ionizatlon (electron capture vs. adduct ion formation), the spectrum of a-chlordane was examined with different concentrations of chloride ion and varylng amounts of sample molecules present In the ion source. Results of this study indicate that the [M Ci]-/M- ratio increases with increasing sample concentration and added chloride ion. The spectra reported here should be observed when working wtth the small size samples normally encountered when using capillary column GCMS.
+ -
+
+
The polychlorinated, bicyclic compounds shown in Figure 1are derivatives of hexachlorocyclopentadiene (1). Many of these compounds were introduced in the 1940s as insecticides. It was later discovered that some of these compounds (or their degradation products) were carcinogenic, and thus, with the exception of the endosulfans (18-20) and Pentac (23), their use has been banned or severely restricted (1). Although their use has been curtailed, these compounds are environmentally persistent and have been found in fish (2) and in sediment (3). The electron impact (4)and positive chemical ionization (5) spectra of these compounds have been reported. The derivatives of hexachlorocyclopentadiene,like many environmental contaminants, contain atoms or functionalities which stabilize a negative charge. Thus, the detection and identification of these compounds should be facilitated by the formation of negative ions. Negative ions may be generated (a) by electron capture (electron capture negative ion mass spectrometry, ECNIMS) using a nonreactive reagent gas such as methane or argon (6) or (b) by ion/molecule reactions (negative chemical ionization mass spectrometry, NCIMS) using a reactive reagent gas (7-9). Advantages of ECNIMS over electron impact and positive chemical ionization include selective ionization in the presence of complex matrices (10) and greater sensitivity (6). Dougherty et al. reported the methane negative ion (MNI) mass spectra of ten of these chlorinated bicyclic insecticides ( 1 1 ) . The mass spectra were dominated by [M + C1]- and other adduct ions in the high mass region. At low mass, ions such as C1-, HC12-, and H20Cl- were the most intense peaks in the spectra of six compounds. In our laboratory, using the same reagent gas and similar temperature and pressure conditions, we observed spectra which differed from the spectra reported by Dougherty et al. (11). Our results are in agreement 0003-2700/85/0357-0684$01.50/0
with the spectra of chlordane (12-14) and nonachlor (12)which have recently appeared in the literature. These spectra are dominated by molecular ions and fragment ions resulting from electron capturing processes. Adduct ions are not observed under our conditions. This paper will present the MNI mass spectra of the hexachlorocyclopentadiene derivatives given in Figure 1. Interpretively useful features of these spectra and the effects of pressure and temperature will be discussed. The questions of why different modes of ionization (electron capture vs. ion/molecule reactions) are observed under similar conditions of pressure and temperature and how the sample and chloride ion concentrations effect the [M + Cl]-/M- ratio will be addressed.
EXPERIMENTAL SECTION All studies were carried out on a Hewlett-Packard 5985B GC/MS system equipped for E1 and CI operation. Samples were introduced via a 30 m X 0.25 mm DB-5 fused silica column (J&W Scientific, Rancho Cordova, CA) with helium as the carrier gas. Methane reagent gas was introduced through a modified transfer line (15) at a flow rate sufficient to achieve the desired source pressure. During the experiments reported here the source pressure was monitored by measuring the voltage reading from a thermocouple gauge tube which was located on the side of the ion source. The opening where the direct insertion probe entered the source was plugged; this avoided variations in source conductance, and thus, the flow of methane necessary to achieve a given pressure. We have since installed a capacitance manometer in our system. Pressure measurements of 1 torr, measured by the thermocouple gauge, correspond to a pressure of 0.25 torr, measured with the capacitance manometer. Pressures reported in this paper are the thermocouple gauge measurements converted to the corresponding capacitance manometer pressures. The ion source temperature was varied between 250 "C and 100 "C and was monitored by a thermocouple located in the ion source block. Electron energy was typically 230 eV; electron emission current was kept at 300 PA. The mass axis was calibrated with the m/z 281 and 400 ions of perfluorokerosene-L (PCR Research Chemicals, Inc., Gainesville, FL), the m / z 123 ion of nitrobenzene (Aldrich),and the m/z 633 ion of perfluorotributylamine. After the system was tuned, a standard of 1ng/pL of chlordene (8) and a-chlordane (15) was run to check instrument performance. With the exception of Pentac (23) (Chemical Services, Inc., Westchester, PA), all compounds used in this study were obtained from the EPA Health Effects Research Laboratory, Office of Research and Development, Research Triangle Park, NC. The isotopically labeled compounds were also obtained from this laboratory. These compounds were synthesized from hexachlorocyclopentadiene which contained 92.9% 3'Cl (16). Octachlorofulvalene (24) was synthesized by partial dechlorination of Pentac (23) with triisopropyphophite (Aldrich) according to Mark (17). Dark blue crystals were obtained with the correct melting point. The methanedl was obtained from KOR Isotopes, Cambridge, MA. All standards were made up in hexane at a concentration of about 10 ng/fiL, and 1-WLinjections were used. Samples were injected in the splitless mode (0.7 min load). Methylene chloride-methane spectra were obtained by introducing methylene chloride (EM Science) such that it accounted for about 20% of the total ion source pressure as measured with the thermocouple gauge. At this concentration, the chloride ion 0 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
685
CI
CI
CI
CI
Ci-
"
6
7
8
9
10
"
"
"
"
H
-
-
-
N
N
N
N
"
~
W
W
W
W
"
8 z E 8 8 6 z 8 0 3 8 z 8 8 8 8 3 M/Z
Figure 2. Methane enhanced negative Ion mass spectra of heptachlor epoxide and [37Cl]heptachlor epoxide. Ion source temperature was 100 OC. I
CI
21
CI
22
23
24
Flgure 1. Derivatives of hexachlorocyclopentadlene: (1) hexachlorocyclopentadlene, (2) aklrln, (3) lsodrln, (4) dieldrin,(5) endrin, (6) endrin aklehyde, (7) endrin ketone, (8) chlordene, (9) a-chlordene, (10) y-chlordene, (11) l-hydroxychlordene, (12) heptachlor, (13) heptachlor epoxide, (14) oxychlordane, (15) a-chlordane, (16) y-chlordane, (17) fransnonachlor, (18) endosulfan I,(19) endosulfan 11, (20) endosulfan cyclic sulfate, (21) mirex, (22) kepone, (23) pentac, (24) octachlorofulvalene.
current was not depleted when compounds entered the ion source. The ion source temperature was 200 "C, and the ion source pressure was about 1torr. The CH2C12/CH4plasma showed ions at m / e 35, 71, and 119 (Cl-, HCl,, and CH2C13-)in a ratio of 1001.5:5.8. Direct insertion probe spectra were obtained by using submicrogram amounts of Pentac (23) and octachlorofulvalene (24) or several microgram quantities of a-chlordane (15). Ion source temperature was 200 "C and the probe held at 100 "C for 5 min and programmed up to 250 "C at 10 "C/min. RESULTS AND DISCUSSION The tabulated spectra of the 24 chlorinated bicyclic compounds, obtained at ion source temperatures of 250 "C and 100 "C, are shown in Tables I and 11, respectively. Ion intensities were normalized to the base peak in the spectra; only the mass and intensity of the lightest ion in each ion cluster are reported. In the following discussion, the effects of pressure and temperature upon the spectra will be reported. This will be followed by an examination of fragmentation patterns and their relationship to molecular structure. Features of these spectra which are interpretively useful and indicative of a particular group of compounds will be emphasized. Pressure. The effect of ion source pressure on sensitivity and on ion abundance was studied in the range of 0.1-0.8 torr at an ion source temperature of 100 "C using a-chlordane (15). Increasing the ion source pressure resulted in enhanced sensitivity, which has been attributed to an increase in collisional stabilization and/or a decrease in electron energy (18). Typically, maximum sensitivity was realized at a pressure of approximately 0.35 torr. The intensity of individual ions
relative to the total ion current varied by less than a factor of 5 over the range of 0.1-0.8 torr. While the ion source pressure has a large effect upon sensitivity, the appearance of the spectrum does not change very much with pressure. Temperature. The ion source temperature has a significant effect upon ion intensities, as has been noted by many investigators (19-22). The chlorinated bicyclic compounds show an enhancement in molecular ion formation at low ion source temperatures. The relative intensity of fragment ions increases with increasing ion source temperature (see Tables I and 11). These effects were observed when the ion source pressure was kept constant and when the reagent gas flow was held constant. Spectra obtained at different temperatures can differ dramatically, thus source temperature is an important parameter which must be carefully controlled. In our laboratory, it was concluded that the greatest structural information was usually obtained when spectra were run at an ion source temperature of 100 "C. Molecular Ions. At 100 "C, molecular ions are observed for all compounds except mirex (21),kepone (22), and pentac (23). The molecular ion is the most intense peak for endrin (5),endrin aldehyde (6),y-chlordene (lo),heptachlor epoxide (13), oxychlordane (14),a- and y-chlordane (15,16), transnonachlor (17), and endosulfan I and I1 (18,19).No [M - HIions are observed for any of the compounds. C&15 and C5C140Ions. To further understand the formation of the ion at m / z 235 and other fragment ions, the spectra of isotopically labeled heptachlor epoxide and trans-nonachlor were measured (see Figures 2 and 3). These compounds were synthesized from 92.9% [37Cl]hexachlorocyclopentadiene (16) yielding structures in which the hexachlorocyclopentadiene moiety contains chlorine-37 while remaining portion of the molecule contains chlorines of normal isotopic distribution. A fragment ion at m / z 235 (C5C15)is observed for all compounds containing a hexachlorocyclopentadiene moiety. Spectra of the isotopically labeled compounds show that the C5C15ion is shifted from m / z 235 to 245, and thus it contains five chlorine-37 atoms. The measured intensities are m/z 241 (7701,243 (39%), 245 (100%) (theoretical 6:38:100). These results indicate that this ion may result from a retro-Diels-
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
686
Scheme I11 M
+
H.-(M+H)
+
e --?+(M+H)-----+(MLH-C~)-~
C1
Scheme IV M
+
H - Cl.-(M+H-Cl)
+
1
+H. -c1,
(M+ZH-ZCI)
Scheme V
I
U
M'
CI
M/z
Figure 3. Methane enhanced negative ion mass spectra of fransnonachior and [37Ci]-frans-nonachlor. Ion source temperature was OC.
%i
Scheme I
te-cl
___+
c+f,
CI
CI
m/z
235
Scheme I1
lA
CI
(19)
m/z
216
e
m/z
235
Alder reaction preceded by or followed by chlorine loss (see Scheme I) or from another type of ring opening reaction (see below) producing an aromatic anion containing [4n 21 electrons. This m / z 235 ion is not observed for a- and ychlordene (8 and 9), which do not contain the hexachlorocyclopentadiene moiety. Pentac (23), for which loss of a chlorine would be more facile than cleavage of a carbon-carbon bond (see below) does not exhibit the m/z 235 ion either. The endosulfans (18-20) and endrin aldehyde and ketone (6,7) appear to yield this ion; however, it is confused by overlapping ion clusters, requiring high resolution data for accurate assignments. Another fragment ion of this type is observed in the spectrum of Kepone (22) and appears a t m / z 216 (CSCl4O). This ion may result from a retrocycloaddition reaction where the negative charge is retained on the carbonyl-containing five membered ring (see Scheme 11). This reaction would also give the m / z 235 ion observed in the spectra of Kepone (22) and mirex (21). [M + H - C1]- Ions. For many of the chlorinated bicyclic compounds, the first fragment ion appears 34 mass units lower than the molecular ion. Exceptions to this type of behavior include Pentac (23), which shows a loss of 35 from the molecular ion, endosulfan cyclic sulfate (20) and endosulfan I1
+
e ---+(M+zH-zc~)'
r (M-Cl).
(*
342
250
t
+
e --+(M+H--Cl)'
+
c1-
-"-
M+H- c 1)'
(19) (at 250 "C), both of which show losses of 36. A number of compounds, such as heptachlor (12), oxychlordane (14), a-chlordane (151, and trans-nonachlor (17) showed weak [M - 341 ions. Multiple losses of 34 are also observed. The loss of 34 mass units is accounted for by the loss of a chlorine radical and the addition of a hydrogen radical. A number of mechanisms exist which rationalize this loss. The most reasonable mechanisms involve reactions between sample molecules and hydrogen radicals, which are generated upon electron bombardment of the methane reageng gas. Reports of reactions between molecules and various alkyl and hydrogen radicals have appeared in the literature (23-26). These reactions are thought to occur rapidly prior to ionization and are characterized by: (a) a dependence upon emission current (with increasing emission currents, the intensity of ions resulting from molecules which have undergone radical reactions increases relative to molecular ions) and (b) reversion to simpler spectra lacking the radical additions when using a nitrogen plasma (25). Use of fully deuterated methane showed that the added species resulted from the methane plasma (23). In addition to our observations, examples of a loss of 34 mass units have been observed for the methyl derivative of a hydroxy polychlorinated dibenzofuran (27) and octachlorodibenzofuran (28). In our experiments, we have observed that [M - 341- ions increase relative to the molecular ion as the emission current increases. Experiments using argon or nitrogen as the reagent gas also show the [M - 341- ions; however, the positive chemical ionization spectrum of the reagent gas shows ions at m/z 18 and 41 for argon (HzO+and ArH+) and m / z 18,29, and 30 (HzO+,NzH+,and NzH2+)for nitrogen. Experiments using fully deuterated methane show ions of similar intensity resulting from losses of both 34 and 33 (M + H - C1 and M D - Cl). These results indicate that sources of hydrogen are present even in the absence of the hydrocarbon reagent gas. Three general mechanisms which account for this behavior are shown in Schemes 111-V. In Scheme 111, addition of a hydrogen radical could be followed by electron capture and chloride loss. Scheme IV shows an addition-elimination reaction which could occur prior to ionization by electron capture. A series of addition-elimination reactions occurring prior to electron capture could account for the multiple losses of 34. A third possibility is shown in Scheme V where dissociative capture yields a [M - Cl] radical which can interact with a hydrogen radical and then undergo electron capture. An ion-radical reaction is also possible (24). If ionization proceeded via Scheme 111, some evidence for (M + H)- or (M - C1)- ions should be observed. No evidence for (M + H)- ions has been observed and (M - C1)- ions appear at low abundance in only a limited number of cases. Scheme V is also possible; however, multiple losses of 34 would ne-
+
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985 687
I
I
I
M/Z
I
l l * l l l
I1
M/Z
Flgure 4. Methane enhanced negatlve ion mass spectrum of pentac.
Ion source temperature was 250 O C . Asterisk indicates ions of the CloCis-,H, series.
Flgure 5. Methane enhanced negative ion mass spectra of octachiorofulvalene. Ion source temperature was 200 O C . Asterisk indlcates ions of the CloC18-,H, series.
Scheme VI CI. ,CI
I C
h
CI
I
-c1
4 C
I
+e-8C1
m/z
CI
1 &
400
-x(Cl-H)
+e -(Z+x)Cl+xH
*
- \ / c10-x
m/z
x = l 366
x=z
338
x=3 x=4 x=5
298 264
M/Z
230
Flgure 6. Methane enhanced negative ion mass spectra of chlordene. Ion source temperature was 100 O C . Asterisk indicates ions of the
cessitate a series of dissociative capture reactions followed by radical attack. This scheme also suggests that the abundance of chloride ion and radical addition products should be comparable, which is often not the case. The addition-elimination type mechanism (Scheme IV) accounts for the M- ions, the [M H - C1]- ions, and the ion series differing by 34 mass units. CloCle_xH,Ion Series. A number of compounds show an ion series differing by 34 mass units. A good illustration of this behavior is the spectrum of Pentac (23) (see Figure 4) which shows a series of ions beginning at m/z 230 (CloC13H5) (addition of five hydrogens and loss of seven chlorides) and ending with m/z 400 (CloC18). The ions at m/z 264 (C10C14H4), 298 (ClOCl5H,),332 (CloClsHz),and 366 (CloC17H)fall id between. This series of ions results from the addition of up to five hydrogen atoms. In addition, other compounds with ten carbon skeletons yield the same series of ions. These are chlordene (a), 1-hydroxychlordene (1 l ) , heptachlor (12), aand y-chlordane (15,16), trans-nonachlor (17), mirex (21), and Pentac (23). A number of oxygenated ten-carbon compounds also show this ion series at m/z values 16 mass units higher: 1-hydroxychlordene ( l l ) , heptachlor epoxide (13), oxychlordane (14), and Kepone (22). The first ions appearidg in the spectrum of Pentac (23) result from the loss of one and two chlorine atoms to yield ions at m/z 435 and m/z 400. Loss of one chlorine would leave one of the five-membered rings with [4n + 21 electron-a favorable aromatic Huckel system (see m / z 435, Scheme VI). Loss of a second chlorine yields the most intense ion in the spectrum. This ion has the formula CloCls and is the first ion in a series of ions which have the general formula CloCls-xH,. The remaining ions result from the loss of one to five 34 mass units from m / z 400 (see m / z 366, 332, ..., 230, Scheme VI). The ion a t m/z 400 may be the radical anion of octachlorofulvalene (24). This compound is electron deficient and forms charge-transfer complexes with aromatic a donors (29). The methane enhanced negative ion mass spectra of octachlorofulvalene is shown in Figure 5. This compound yields the ion series C1oClhH, ( x = 0 to 4) supporting the assignment
+
C,oC18-xH, series. Scheme VI1 i c
CI
+ti
0
Clre
w
m/z
235
m/z
302
CI
t
-2C'-2H (MtH-CI)
of ion structures made in Scheme VI. The electron accepting ability of odachlorofulvalene is of interest with regard to losses of m / z 34. Previous radical additions were observed for compounds which form charge-transfer complexes (24). The ability to form charge-transfer complexes was used to explain the rapidity of the radical reactions and, thus, the ability of radical reactions to complete with ionization. The other ten carbon compounds which exhibit this series of ions can be classified as methanoindene (chlordene-like) or cage (mirex-like)structures. Because both structural types show the same ion series as Pentac (23) and octachlorofulvalene (24), these ions may be structurally similar, and the fragmentation of the methanoindene and cage compounds may be explained in the following manner: Methanoindene compounds, such as chlordene (8) (see Figure 6), may capture an electron and form the ring-opened intermediate shown in Scheme VII. The intermediate is generated when a carbon-carbon bond is broken and an allyl anion and an allyl radical are generated. The loss of one chlorine and two hydrogens yields the C10C14H4ion at m / z 264. The ion at 34 mass units lower (m/z 230) may result from a parallel reaction where the (M + H - C1) species captures
688
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Table I. Methane Enhanced Negative Ion Mass Spectra of Hexachlorocyclopentadiene Derivatives, Source Temperature 250 "C 5
10
C1 OC l 6 H 6
12C16H80
........ .. .
-. - -.. - -.-. . ....
--=. -.
270 M-CltH M-Cl M-C1-H
mie
INT
mle
mle
iiiiiiiii
INT
iiiiiiii:
378 344
235
...........
M-ZCltZH M-ZClIH M-2C1 M-2Cl-H M-ZC1-2H
201
._--.
---I---*. ---I----
M-3CltH M-3C1 M-3C1-H M-3C1-2H M-3C1-3H
._--.
---I----.
M-4Clt2H M-4CltH M-4C1 M-4C1-H M-4C1-2H M-4C1-3H
.__-.
...........
M-5C1+3H M-SCltZH M-5CltH u-qri
M-EI-H ...........
._--.
M-6C1 t 4 H M-6CltZH M-6C1
...........
M-7Clt5H M-7C1+3H M-7CltH
...........
.__..
M-8C1+6H M-8Clt4H
...........
___.
mle 235
- - - - - - -. 60
HCl2 c12 c1
...........
___.
Others
.....I.
235
. 2.1 7.1
_1 0_ 0_ _ _ - _ .
. _ - -_-!2 _ _ . 236 '270 '272 '306 k308
236 '268 '272 '304 '306
5.9 208 37 227 2.0 246 9 . 4 '272 *274
14 16 62 66
lo(
* Lowest mass and most intense ion in a complex cluster. Table TI. Methane Enhanced Negative Ion Mass Spectra of Hexachlorocyclopentadiene Derivatives, Source Temperature 100 "C 5
9 ClOCl6H6
:12C16H80
. -. .- -.-.- - - - -. mie INT .. -.-.-..-.... -. . .
..-. ..-.-. -.-.-.-. mle INT . .. .. ... ... .. . . 53 51
_____ ____
53
378
27
336
344 343 342
67 35 4.1
302
.___ 21 1.9
___.
336 302
.- - - .- - - - - - - . _ _ _ . .....
__ M-3CltH M-3C1 M-3C1-H M-3C1-2H M-3C 1 - 3 H ___________I
I
.--.-
iii/ii=i
4.31 5.3 281
280
___.
---I----
230 ~
246 245
___.
.........
...........
1
.....
I.
.
_ _ . _ _ _ . _ _ _ _ 8
M-7C115H M-7C113H M-7CltH
. . I. . .I.
mle 235
5.
HC12 c12 c1
19
...........
Others
... 1. 8.
* Lowest mass and most intense ion in a complex cluster
I !
.............
M-8Clt6H M-8Clt4H
1.1
I
....
..... I..
.
.. .... 4.e
lii
60
..... ....
i
INT iiiiij
.
264
_.
mie
63
I.
M-5Clt3H M-5C1+2H M-SCltH M-5C1 M-5C1-H M-6Clt4H M-6C112H M-6Cl
NT
4
264
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
.4-0 6_ -_3 3_ _- .4_4 _0 _ _-11- - - 4- -0 4- .
386
.___
372 371
____
338 337 336
314
.___ 63 3
314 313 312
298
212 211 210
48
IO
264
_ _ _ _ _I _ _ _
----I
I 7- - 1 - - 2.5
___
1.1
4.9
O.t
368
____. 335 334
2.0
6.1
---I-----
334
350 349 348
29 4.5 4.7
314
3.0
80
5.5 100 15 2.1 2.5
_____ ___
. - - _- _ ___
.___
435
8.
35
76
232
12
230
8.C
.__.
..._ _._.
196
196
1.2 __..
__..
_-- __.__
.___
..- .-.- -
13 4.1 4.7
7.0
___. 1'276 /*281
1 . 0 '337 20 * 3 4 1
___. '337 '340
1.4 5.1
14 20 5.7 17
- - - - .- - . 99 *232 '237 *239 *242 *268 *270
97 185 '268 '270
14
18
:10C18H40 - -.... - -.-. .
9C16H680:
iiii=iiii:
INT
2.2
1.0
-I-----
___.
13 ClOC17H50
404
l4
212 210
.--_
I- -
9.e
.___ 6.7
__-.
--*-I---I
370
. _ _ -_ _ _ .
280 279 278 277
11
0.6
3.i
. _ _ __ _ _ .
__--_--_ 246 245 244
406 405
. _ __ __ _ .
316
280 279
12 O.t
21 ClOC112
il=ii=ii:
INT
mle
IN1
iiiiiiii:
.___.
240 242
1.2 1.4
304 308
3.0
1.5
,350 '352
~
4 7:8
e
889
690
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
Scheme IX CI
1) M
+
e--+[M]'-M7
2) M
+
e +(M-Cl).
3) M
+
CI-+(M+Cl)-
4) M
+
MT---+MTd(M+C1)-7-
+
C1-
(M-C1).
Table 111. Adduct Ion Ratios and Molecular Anion Sensitivity of a-Chlordane Using Methane and Methane-Methylene Chloride MI2
Flgure 7. Methane enhanced negative ion mass spectra of mirex. Ion source temperature was 250 OC. Asterisk indicates Ions of the
methane 10 ng 500ng
C,,CI,-,H, series. [M + CI]-/M-
Scheme VI11
M-
methane-methylene chloride long 500 ng
0.0072 0.075 0.93 (f0.0003a) (f0.012) (*0.04) loo* 650 1
1.14 (f0.02) 48
a One standard deviation. *Integrated area of m / z 410, normalized to 100, for 10 ng with methane.
an electron and loses a chlorine and two hydrogens via a similar intermediate. The cage compounds, such as mirex (18) (see Figure 7)) may open by a retro [2 21 cycloaddition to form an intermediate (see Scheme VIII) which may undergo cleavage of a carboncarbon bond and a loss of three chlorines to yield an ion at m/z 435, as seen with Pentac. Loss of one more chlorine yields the CloClsion at m/z 400, which is followed by the remaining CloC1&x ions of this series, differing by 34 mass units. Many oxygenated methanoindene compounds and kepone also show ions which have the general formula CloCl~xHxO at m / z 280, 314, 348, 382, and 416. [M C1]- Ions. Significant differences were observed between the spectra reported here and those which appeared in the literature (11). The spectra differ in two respects: (a) many of the previously published spectra were dominated by low mass even electron negative ions such as Cl-, H,OCl-, Clz-, and HC1,- and (b) in the molecular ion region, the most prevalent ions previously reported resulted from adduction of the molecule with ionic species such as C1-, H,OCl-, OH-, HC12-, C10-, and C13-. Molecular ion intensities ranged from 0 to 39% of the intensity of the [M + C1]- ion. Dimers, which were temperature and pressure dependent, were also reported. Little fragmentation was observed, except displacement of chlorine by oxygen at (M - 19)-. In contrast, the spectra observed in our laboratory showed only low abundance ions corresponding to Cl-, Cl,, and HC1,. In the molecular ion region, molecular ions and the fragment ions described above dominate. The only adduct ions observed resulted from association with chloride ion and were observed for only a few compounds. In the concentration range normally used (up to about 10 ng injected), adduct ions were of low intensity ( 1% relative to M-) if observed at all. With our ion source modification (see Experimental Section),oxygen in the ion source was reduced, and (M - 19)- ions were not observed. Our spectra and the previously published spectra were obtained under similar conditions of pressure and temperature
+
+
N
and with the same reagent gas. There were, however, two major differences: (a) The previous spectra were obtained on an AEI MS-902 double focusing mass spectrometer equipped with an SRIC chemical ionization source, while our spectra were obtained on an H P 5985B quadrupole instrument. (b) We used capillary column gas chromatography to introduce, typically, up to 10 ng of sample while the direct introduction probe was used in the previous study to introduce microgram quantities of sample (11, 30). The disparity between these spectra is disturbing when considering gas-enhanced negative ion mass spectrometry as an analytical tool for identification purposes. In an effort to explain these spectral differences, we undertook a study to determine factors which enhanced the formation of adduct ions in our system. Scheme IX shows a series of reactions which may be occurring in the ion source. Reactions 1 and 2 show electron capture. Depending upon the molecule, the energy of the electron, and the collisional frequency with neutral molecules, molecular anion formation or dissociation to yield a chloride ion and an (M - C1) radical may occur. Adduct ions may be generated via interaction of sample molecules with this chloride ion (reaction 3) or by abstraction of chlorine from another sample molecule by the molecular anion (reaction 4). Factors which may influence the ratio of molecular ion to chloride adduct ion include the concentration of chloride ion, the concentration of electrons and their energy distribution, the concentration of sample molecules, and the rate of electron capture vs. the rate of chloride attachment. We chose to study variations in the ratio of adduct ions to molecular ion by varying the concentrations of chloride ion and sample molecules. a-Chlordane was selected for study because this compound contains sp3 hybridized sites which should facilitate chloride attachment, assuming the chloride ion interacts at a carbon center in a-chlordane, as has been reported (8). As mentioned above, adduct ions were not normally observed under our operating conditions. Initially, we felt that this was due to the lower amount of chloride ion generated under our operating conditions. It was suggested that the addition of methylene chloride would increase the abundance of adduct ion with a parallel increase in sensitivity (31). Thus, spectra were measured in the presence and absence of methylene chloride, under conditions which are reported to favor adduct ion formation (8),and comparisons were made regarding the [M Cl]-/M- ratio and sensitivity. The results of this study, carried out in the range of 10-500 ng injected, are tabulated in Table 111. In the absence of
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
891
$ 1 g
30000
u
M/Z
I
.
0
.
,
,
50
t'
,
I O
100
200 Amount Injootod (ng) 150
250
300
Flgure 8. Molecular ion and [M + CI]-/M- ratio vs. amount injected for a-chlordane.
added methylene chloride, the [M + Cl]-/M- ratio is less than 0.1 and increases as the amount injected increases (see below). In the presence of added methylene chloride, the [M + Cl]-/M- ratio is at least an order of magnitude larger compared with methane alone; however, this [M + Cl]-/M- ratio is not as high as that observed by Dougherty ([M + Cl]-/M- = 8) (11))suggesting that the high [M + Cl]-/M- ratios observed by Dougherty may not be attributed solely to high chloride ion concentration. Molecular ion sensitivities were at least an order of magnitude higher using methane compared with methane-methylene chloride. This study indicates that the [M Cl]-/M- ratio increases with an added source of chloride ion and that no analytical advantage is realized by working with methylene chloride instead of methane. Another study was undertaken where sample concentration was varied. Figure 8 shows a plot of the amount of a-chlordane injected vs. molecular ion abundance and [M + Cl]-/M- ratio using methane as the reagent gas. As the amount of compound injected increased, the molecular ion exhibited a nonlinear response. This behavior has been noted in prior work (6, 7, 32) and is attributed to saturation of the electron population. The onset of this nonlinear region and the [M + Cl]-/M- ratio varies with ion source conditions. The amount of adduct ion generated tracked this nonlinear behavior in molecular anion current. When low sample concentrations are used, adduct ions were not observed, either because of the low concentration of sample molecules or chloride anion or because the rate of electron capture is rapid relative to the chloride attachment process. However, with increasing sample concentrations, the [M + C1]- ion begins carrying a larger percentage of the ion current. This may occur because the percentage of sample available for ionization via the competing process of chloride attachment increases when electron capture processes are limited. Alternatively, higher sample concentrations may result in an increase in intermolecular chloride transfer resulting in ionization via reaction 4 in Scheme IX. Introduction of a larger quantity of a-chlordane (15) was accomplished by using the direct introduction probe. In this case, spectra were obtained which resembled Dougherty's spectra (see Figure 9) in terms of the [M + CI]-/M- ratio, although additions of oxygen, C10, HC12, or C13 were not observed. Our results indicate that both an increase in sample concentration and chloride ion concentration result in an increase in the [M + CI]-/M- ratio. These observations suggest that the difference between our spectra and those of Dougherty may be due to differences in sample concentration and/or differences in the amount of chloride ion present in the ion source. The previous spectra (11)were obtained by using the direct introduction probe technique which resulted in the
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Figure 9. Methane enhanced negative ion mass spectra of a-chlordane. Direct insertion probe. Ion source temperature was 200 "C.
introduction of larger @mountsof sample than are normally encountered using capillary column gas chromatography. With small sample sizes, we observe electron capture processes dominating over chloride attachment. The ions attributed to electron capture processes approach a limiting value with larger sample sizes, and the [M Cl]-/M- ratio increases. The amount of molecular ion relative to chloride ion and other fragment ions depends, in part, upon the electron population and energy distribution. If larger sample concentrations are used, or if ion source conditions favor chloride ion formation vs. molecular ion formation, then adduct ion formation may dominate the spectra. Under normal capillary column GC/MS conditions, they will not.
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ACKNOWLEDGMENT We thank Bruce D. McVeety and William J. Simonsick, Jr., for helpful discussions. Registry No. 1, 77-47-4; 2, 309-00-2; 3,465-73-6; 4,60-57-1; 5,72-20-8;6,7421-93-4; 7,53494-70-5; 8,3734-48-3; 9,56534-02-2; 10, 56641-38-4; 11, 2597-11-7; 12, 76-44-8; 13, 1024-57-3; 14, 27304-13-8; 15, 5103-71-9; 16, 5566-34-7; 17, 397654306; 18, 959-98-8; 19,33213-65-9; 20,1031-07-8; 21,2385-85-5; 22,143-50-0; 23, 2227-17-0; 24, 6298-65-3.
LITERATURE CITED (1) Balley, R. A.; Clark, H. M.; Ferrls, J. P.; Krause, S.; Strong, R. L. "Chemistry of the Environment"; Academic Press: New York, 1978; p 168. (2) Clark, J. R.; DeVault, D.; Bowden, R. J.; Weishaar, J. A. J. Great Lakes Res. 1984, 10, 38. (3) Frank, R.; Thomas, R. L.; Holdrinet, M.; Kemp, A. L. W.; Braun, H. E. J. Great Lakes Res. 1979, 5 , 18. (4) Damico, J. N.; Barron, R. P.; Ruth, J. M. Org. Mass Specfrom. 1968, 1, 331. (5) Biros, F. J.; Dougherty, R. C.; Dalton, J. Org. Mass Spectrom. 1972, 6 , 1181. (6) Hunt, D. F.; Crow, F. W. Anal. Chem. 1978, 5 0 , 1781. (7) Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russell, J. W. Anal. Chem. 1976, 48, 2098. (8) Tannenbaum, H. P.; Roberts, J. D.; Dougherty, R. C. Anal. Chem. 1975, 47, 49. (9) Smlt. A. L. C.; Field, F. H. J. Am. Chem. SOC. 1977, 99, 6471. (10) Dougherty, R. C. Blomed. Mass Specfrom. 1981, 8 , 283. (11) Dougherty, R. C.; Dalton, J.; Blros, F. J. Org. Mass Spectrom. 1972, 6 , 1171. (12) Jansson, B.; Wldeqvist, U. Inf. J. Envlron. Anal. Chem. 1983, 13, 309. (13) Dlxon, D. J. Appl. Mass Specfrom. Trace Anal. 1982, 139. (14) Ribick, M. A.; Dubay, G. R.; Petty, J. D.; Stalling, D. L.; Schmltt, C. J. Environ. Sci. Techno/. 1982. 16, 310. (15) Jensen, T. E.; Kamlnsky, R.; McVeety, B. D.; Woznlak, T. J.; Hites, R. A. Anal. Chem. 1982, 5 4 , 2388. (18) Chien, D. H. T.; Heys, J. R. J. Agrlc. Food Cbem. 1982, 30, 396. (17) Mark, V. Org. Synfh. 1986, 46, 93. (18) Gregor, I . K.; Gullhaus, M. I n f . J. Mass Spectrom. Ion Phys. 1984, 5 6 , 167. (19) Miwa, B. J.; Garland, W. A.; Blumenthal, P. Anal. Chem. 1981, 5 3 , 793. (20) Stan, H. J.; Keliner, 0. Blomed. Mass Specfrom. 1982, 9 , 483. (21) Greaves, J.; Bekesi, J. G.; Roboz, J. Biomed. Mass Specfrom. 1982, 9 , 406. (22) Crow, F. W.; BJorseth, A.; Knapp, K. T.; Bennett, R. Anal. Chem. 1981, 5 3 , 619. (23) McEwen, C. N.; Rudat, M. A. J. Am. Chem. Soc. 1979, 101, 6470. (24) McEwen, C. N.; Rudat, M. A. J. Am. Chem. Soc. 1981, 103, 4343.
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(25) Henis, N. B. H.; Busch, K. L.; Bursey, M. M. Inorg. Chim. Acta 1981, 53, L31. (26) Stocki, D.; Budzikiewicz, H. Org. Mass Spectrom. 1982, 17, 376. (27) Deinzer, M.; Griffin, D.; Miller, T.; Lamberton, J.; Freeman, P.; Jonas, V. Homed. Mass Spectrom. 1982, 9 , 85. (28) Rappe, C.; Buser, H. R.; Stalling, D. L.; Smith, L. M.;Dougherty, R. C. Nature (London) 1981, 292, 524. (29) West, R. Pure Appl. Chem. 1971, 28, 379. (30) Dougherty, R. C.; Bergner, A.; Levonowich,.P.; Roberts, J. D. Adv. Mass Spectrom. Biochem. Med. 1976, 1 , 181. (31) Dougherty, R. C., personal communication, Florida State University, 1983.
(32) Hass, J. R.; Friesen, M. D.; Busch, K. L.; Bursey, M. M. "American Society for Mass Spectrometry, 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, (1978) Abstracts, p. 390.
RECEIVED for review September 17,1984. Accepted November 13, 1984. The work was supported by the U.S. Department of Energy (Grant No. 80EV-10449) and the U.S. Environmental Protection Agency (Grant No. R808865).
Combination of Chemical Reduction and Tandem Mass Spectrometry for the Characterization of Sulfur-Containing Fuel Constituents Karl V. Wood*
EnginelFuels Laboratory, Chemistry Building, Purdue University, West Lafayette, Indiana 47907 R. Graham Cooks, James A. Laugal, and Robert A. Benkeser
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Tandem mass spectrometry has been combined with a caicium/mixed amines reduction system to characterize an SRC-I I middle distillate fraction for sulfur-containing polynuclear aromatlc hydrocarbons. Parent scans, which characterize a complex mixture for ail components which fragment to common structural moleties, were used to identify aikyibenzothlophenes and dibenzothiophenes as well as alkyibenrothiophene sulfones.
A chemical reduction scheme has been applied to a middle distillate of an SRC-I1 coal-derived liquid in order to facilitate characterization of sulfur-containing constituents by tandem mass spectrometry (MS/MS) (1-4). The identification of sulfur-containing polynuclear aromatic hydrocarbons (PNAs) in coal and liquids derived from coal (5-7) is of particular interest because of the environmental effect these constituents have after combustion (8). However, these compounds have proven difficult to identfy because of their low relative concentration and characteristic mass overlap with hydrocarbon components (9). The approach used here is an alternative to the rather complex separation schemes which have been used to address these problems by concentrating sulfur-containing PNAs prior to GC/MS (6, 7, 9). EXPERIMENTAL SECTION A Finnigan (Model4500) triple quadrupole mass spectrometer was used to characterize the reduction products (10, 11). This system consists of three coaxially arranged quadrupole rod assemblies. The first and third quadrupoles are conventional mass analyzers and the second quadrupole is used as a focusing collision cell. Samples were admitted into the mass spectrometer with the direct insertion probe. Negative ion chemical ionization (NICI) was employed with isobutane as the reagent gas (0.4 torr). Argon was used as the collision gas at a gauge pressure of 2.0 mtorr. The collision energy was set at 20 eV. The chemical reduction utilizes calcium metal (10-12 g) in a mixed amine solvent system (12) consisting of ethylenediamine and methylamine or n-butylamine. The calcium-mixed amine
reducing system seems superior to the lithium-ethylenediamine system in the reduction of polynuclear organic sulfur compounds. The lithium-ethylenediamine system has been reported to reduce dibenzothiophene to a plethora of products (13). The reduction of SRC-I1middle distillate which follows is typical of the general procedure. A 500-mL dry round bottom flask was fitted with a single Hershberg stirrer and an ethylene glycol cooled (-26 OC) Friedrichs condenser. The system was flushed with methylamine vapor and the flask then filled with 75 mL of condensed methylamine gas. Calcium metal shot (12.0 g), white sand (24 g), SRC-I1 middle distillate (3.3 g), and 75 mL of dry ethylenediamine were also added. The brown mixture was stirred for 24 h, after which time the methylamine was allowed to evaporate. Technical diethyl ether (100 mL) was added to the flask immersed in ice, and hydrolysis of the gray product was effected with a solution of NHlCl(27 g in 100 mL of water). The aqueous layer was separated, made acidic with concentrated HCl, and then extracted several times with diethyl ether. The ether extracts of the base-soluble layer were combined and dried over anhydrous sodium sulfate. The original ether layer (base insoluble fraction) was washed with aqueous HC1 followed by aqueous NaHC03and then also dried over anhydrous sodium sulfate. The solvent volume of both fractions was reduced by means of a vacuum jacketed distillation column to yield a brownish base-insoluble liquid fraction (1.6 g) and a brownish base-soluble liquid fraction (2.0 9) (14). The middle distillate SRC-I1sample was obtained from Charles E. Schmidt of the Pittsburgh Energy Technology Center. The sodium salts of benzenesulfinate and p-toluenesulfinate, as well as the other reagents, were obtained commercially and used without further purification.
RESULTS AND DISCUSSION Calcium has been shown to be an effective metal in reducing aromatic substrates (12). In the case of aromatic heterocycles containing a thiophene moiety, the thiophene ring is opened by calcium through reductive cleavage of the carbon-sulfur bond (14). For example, carbon-sulfur bond cleavage in benzothiophene yields o-ethylthiophenol. In basic media, the reduced benzothiophene exists as a thiophenolate anion which resists further reduction. The unsaturated substituent present
0003-2700/85/0357-0692$01.50/00 1985 American Chemical Society
