Anal. Chem. 1983, 55, 914-920
914
Correlation of Kinetic Energy Release with the Structure of Selected Chlorinated Compounds in Mass-Analyzed Ion Kinetic Energy Spectrometry Robert D. Voyksner' and J. Ronald Hass" Laboratory of Environmental Chemistry, National Instltute of Environmental Health Sciences, P.O. Box 12233, Research Triangle Park, North Carolina 27709
Maurice M. Bursey William Rand Kenan Jr. -Laboratories
of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27514
The klnetlc energy release for the loss of chlorlne from the molecular Ion can be used to classtfy compounds Into groups and usually can dlstlnguish between lsomerlc structures. Dlfferences In the kinetic energy release values were compared to dlfferences In relatlve retentlon times and differences In electron Impact relative peak helght ratios to demonstrate the techniques speciflcity and the complementary lnformatlon obtalned by kinetic energy release measurements. The klnetlc energy release values for C I D processes also prove useful In Isomer Identification In systems that we have examined. Klnetlc energy release measurements are rellable for quantltles down to 100 pg (from the caplilary gas chromatograph) for tetrachlorodibenzo-p-dloxln and some 70 other chlorlnated compounds.
Mass spectrometry has proven itself for the identification of organic compounds. Among other applications, environmental identifications are most commonly performed by some form of mass spectrometry because of the low sample requirements and the wealth of information generated by the technique. The information acquired in a typical gas chromatography/mass spectrometry (GC/ MS) analysis includes relative retention times (RRT) and mass and intensity information concerning the sample. More specific mass spectral techniques can provide elemental composition and the collision induced decomposition (CID) spectrum for a particular ion ( I , 2). Even with this wealth of information, difficulties can exist in identifying a particular isomer encountered in a sample. (This problem can easily be solved with additional information, such as nuclear magnetic resonance or infrared spectroscopy, if sample concentrations or quantities are not limited; but such abundance is seldom the case in environmental analysis.) In particular, the differences in the mass spectra of polychlorinated aromatic isomers are, a t most, minimal (3-6). Because fragment masses are identical, identification is based on slight differences in peak ratios or in the CID spectrum of the molecular ion. Probably the most useful data for isomer identification are relative retention time (RRT) values (7,8). Confirmation by unambiguous separation or mass spectral characterization must be made to validate the uniqueness of a particular RRT or peak ratio on which isomer identification is based. Seldom are all the isomers available to perform this validation. Kinetic energy release ( T ) measurements have been reported as a probe into ion structure (9-12). The kinetic energy IAlso at Department of Chemistry, University of North Carolina, Chapel Hill, NC 27514.
release in the decomposition of a metastable ion is derived from the excess energy e* of the activated complex and the reverse activation energy e,. Both e* and e, partition between the internal energy of the products and translational energy of their separation. Hence, T i s the sum of the contributions of the kinetic energy released from the reverse activation energy (Te)and that from the excess energy of the activated complex (T*). T is determined from the metastable peak width; the time interval which the mass spectrometer can detect these transitions is quite narrow (lo4 9). This means that for a set of sufficiently similar compounds, like isomers, e* is fixed by instrumental conditions; thus T i s sensitive to e,, which is an indicator of ion structure (13). Structural correlations with T have been reported for anisoles (I4),quinones (1.9, polychlorinated biphenyls (16), benzoylpyridines ( I 7), steroids (18),a variety of brominated compounds (19),and acetophenones and benzophenones (20). These studies have been performed on milligram quantities of sample, but with the development of GC/MS/MS and supporting software, the ability to perform T measurements on nanogram quantities from the column became practical (21).
This paper investigates of the use of T to differentiate between selected chlorinated compounds. The results demonstrate the selectivity of T as a probe for structure and the ability to obtain meaningful T values for low nanogram to high-picogram quantities of sample. Correlations of T with CID fragmentation processes, as well as the ratio of intensities of CID and unimolecular signals as a probe of ion structure, are presented.
EXPERIMENTAL SECTION A VG Micromass ZAB-2F mass spectrometer was used to perform all T and TCID measurements. The ion source was operated at 200 "C with 100 FA trap current, 70 eV electron energy, and 7 kV accelerating potential. The Finnigan-INCOS 2300 data system using previously described software was used for data acquisition and processing (22). The magnet was set to transmit the parent ion of interest while the computer scanned the electric sector, at a rate of 0.35-0.55 s/scan, over a small region centered over the fragment ion of interest. The unimolecular transitions were observed in the second field free region at a pressure of 7 X torr (as measured by the analyzer ionization gauge). The CID spectra were acquired at a gauge pressure of 1.5 X torr of helium. This resulted in an approximate pressure of 2 X torr in the cell (23) and an attenuation of the main beam by 50%. The chlorinated compounds were injected directly on 20M, DB-5 fused silica capillary column (J.W. Scientific). The oven was heated to a temperature, between 70 and 150 "C, then data acqusition commenced, and the GC temperature was programmed at 8 "C/min until all samples eluted. The complete peak profile was stored for each scan; then 5-25 scans were averaged, depending on the chromatographicpeak width, to obtain
0003-2700/83/0355-0914$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
70
-
60
-
.
50 40
.
-
0
.
I O O r a.
915
/,a,o-fic:i;r.
benzyl chloride
.
U 0 0
0
toluene
0 .
ilbenzyl chloride
50 0 0
Aliphatics
0 0
M o l e c u l a r wt.
Flgure 1. Group classification of caron-chlorine bond of selected compounds, as aromatic (O),aliphatic (0),or both (A)depending on kinetic energy release value associated with the unimolecular loss of chlorine from the molecular ion.
a T measurement. The T values were obtained by the following equation (24): lvcom 112 = (W1p - wmb)(Mz/M1)
Here WllZand Wmbare the widths at half height for the fragment and main beam, respectively, in eV, W,,,, 1/2 is the main beam corrected fragment peak width, Ml is the parent mass, Mzis the fragment mass, M3 is the difference of M1 and Mz, (eV) is the energy in electron volts acquired by the ion of mass Ml as it leaves the source, and E, is the elective sector voltage at which M I is observed. The half height value is more free of interferences than the 22.3% value (E), although the interferences are small in most cases. The chlorinated compounds used for the study were obtained from commercial sources and used without any purification.
RESULTS AND DISCUSSION Unimolecular Loss of Chlorine. Group Identity. The use of T for the unimolecular loss of chlorine can classify compounds into groups based on the type of carbon bound to the chlorine (Figure 1). There are two distinct groups, chlorine bound to aromatic carbon and chlorine bound to aliphatic carbon. The compounds with both types of bonds are intermediate in T value, but still clearly in the aliphatic group. The T values for aromatics range above 16 meV, the T values for aliphatic$ range from 4 to 14 meV, and the compounds with both types of chlorine have T values in the 14-18 meV range. Measuring T for an unknown would classify it as an aromatic or aliphatic. Some uncertainty would exist for some T values in the intermediate region. The peak shapes of the compounds which contain both aliphatic and aromatic C-Cl bonds were analyzed to determine if they were composite peaks, that is, the sum of peaks for the two kinds of C-C1 bonds. The peak shapes for benzyl chloride and 0-,m-, and p-chlorotoluene were used to determine their contribution to the peak for 0-,m-, and p chlorobenzyl chloride. This analysis was also performed to determine the c?xtent that peaks in benzyl chloride and 3,4dichlorotoluene modeled contributions to the peak profile for a,3,4-trichlorotoZuene. The results show a correlation between the composite peak with the peaks for the individual aromatic and aliphatic models (Figure 2). The results indicate that either chlorine can be lost, giving a T value corresponding to the type of C-Cl bond that is broken. The peak shape seems to indicate that loss of the aliphatic chlorine is favored by a factor of 2 to 3 over the aromatic chlorine loss. There is no evidence of isoimerization of these compounds; that would
roo[ 14-=,,k!z;o benzyl chloride
0 4 4 -
20ev
++ 20 ev
Figure 2. Deconvolution of the composite peak into components which correspond to aromatic and allphatic loss of chlorine from the molecular ion. (a) Models of benzyl chloride and o-chlorotoluene for the peak profile for a,odichlorotoluene. (b) Models of benzyl chloride and m-chlorotoluene for the peak profile for a,m-dichlorotoluene. (c) Models of benzyl chloride and p-chlorotoluene for the peak profile of a,p-chlorotoluene. (d) Models of benzyl chlorlde and 3,4dichYorotoluene for the peak profile to a,3,4-trlchlorotoluene.
require transformation to structures which would give T values similar to those expected. Isomer Idtmtity. The greater advantage of T measurements is in the determination of isomeric compounds. Conventional mass spectrometry can distinguish between similar compounds with different molecular weights, but easily can go astray in isomer determinations especially in aromatic substituted isomers. The complete list-of T values (Table I) for the unimolecular loss of chlorine or HC1 shows that T from a given isomer is usually different from T values for all the other isomers. This point is vital if T measurements are to be used with EX peak ratios or relative retention times (RIIT) for isomer identification. The first group of isomers (molecular weight 126) shows a T ratio of 1:2.6:3.7:4.7 for the a:ori,ho: para:meta isomers of chlorotoluenes (Figure 3a). The RRT values on the other hand were 1.63:1.00:1.03:1.06 (Figure 3b) and together with T values can distinguish the four isomers without a complete set of standards. There is no claim that the best chromatographic conditions were used and better separation could probably be achieved through changing the column or chromatographic conditions. Only chromatographic conditions which can separate the isomers in the mixture are required to get reliable T values. The ratios of the most intense peaks in the E1 spectra of these isomers are only slightly different. In most cases the differences observed are less than the standard deviations (which can be as high as 30% of the ratio given) of the measurements. This statement is even more valid if weak signals are used for isomer identification. The m / z 126/91 itensity ratios are 1.0:1.5:1.3:1.3, respectively, and the m / z 126/63 intensity ratios are 2.2:1.2:1.0:1.1, respectively (Figure 3c). A similar comparison is presented for other isomers (Table 11). For most cases the identification of isomers by T measurements is reliable. There are cases, such as dichbrotoluenes, where no method gives specific information; the T values, RRT, and the E1 peak ratios are all nearly equal. 13ut even in these cases T measurements can distinguish between groups of isomers. For example, benzyl chloride falls into one
916
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
Table I. Kinetic Energy Release for the Unimolecular Loss of Chlorine from Molecular Ions compound ethylene chloride methylene chloride 2-chloropropane 1-chloropropane 1-chloro-2-methylpropane 1-chlorobutane
2-chlorobutane 1,2-dichloroethane chlorocyclopentane 1-chloropentane chlorobenzene chloroform chlorocyclohexane 1-chlorohexane p-chlorotoluene o-chlorotoluene m-chlorotoluene benzyl chloride trichloroethene chlorocycloheptane 1-chloroheptane 1,5-dichloropentane (2-chloroethy1)benzene (1-chloroethy1)benzene p-chloroethylbenzene a -chloro-o-xylene a-chloro-m-xylene a-chloro-p-xylene 2-chloro-p-xylene 4-chloro-o-xylene m-dichlorobenzene o-dichlorobenzene p-dichlorobenzene 1-chlorooctane 4-chloro-a-methylstyrene
mol wt
T , meV
compound
mol wt
T , meV
62 84 78 78 92 92 92 98 104 106
6.8 11.5 2.6 20.6" 13.3" 38.2 M - HC1 6.5 M - HCl 25.7a 14.0 M - HCl 23.9 M - HCl 21.0 2.3 6.1 M - HC1 15.3 M - HC1 28.6 19.9 34.6 7.4 4.3 7.3 M - HC1 4.0 M - HC1 29.1 7.7 11.3 35.0 14.5b 14.6 13.8 22.7 27.8 23.7 18.2 29.3
2,5-dimethylbenzyl chloride 2,4-dichlorotoluene 2,6-dichlorotoluene 3,4-dichlorotoluene 2,5-dichlorotoluene 2,3-dichlorotoluene 3,5-dichlorotoluene o-chlorobenzyl chloride m-chlorobenzyl chloride p-chlorobenzyl chloride benzyl chloride 1-chloronaphthalene 2,5-dichloro-p-xylene a,&-dichloro-p-xylene
154 160 160 160 160 160 160 160 160
15.5 28.6 28.0 32.3 30.1 36.0 38.8 11.3b 13.8 12.4 4.0 41.7 32.3 12.6 16.9 28.6 30.0 29.7 31.0 37.0 36.3 7 .O 16.1 14.4 42.7 42.1 0.7 21.8 28.3 65.0 66.0 68.2 28.5 68.3 24.7
112
118 118 120 126 126 126 126 130 132 134 140 140 140 140 140 140 140 140 140 146 146 146 148 152
a M - HCl peak overlapped M - C1 signal. for the loss of HCl rather than C1.
44.8
1GO
( 1-chloromethy1)naphthalene
113,5-trichlorobenzene 1,2,4-trichlorobenzene 1,2,3-trichlorobenzene 2-chlorobipheny l
3-chlorobiphenyl 4-chlorobiphenyl a ,a ,a-trichlorotoluene oi ,3,4-trichlorotoluene a ,2,6-trichlorotoluene 2,3,6-trichlorotoluene 2,4,5-trichlorotoluene pentachloroethane 9-chlorofluorene 44 chloromethy1)biphenyl 1-chloroanthracene 2-chloroanthracene 9-chlorophenanthrene
160 162 174 174 176 180 180 180 188 188 188 194 194 194 194 194 200 200 202 21 2 212
1,2,3,4-tetrachlorobenzene
4,4'-dichlorobiphenyl 9-(chloromethy1)anthracene
212 214 222 226
Indistinguishable isomers based on 2' measurements alone. M - HCl T value
Benzyl chloride
p-chlorotoluene
LL
0
Energy(ev1 5020
[L
5040 5060 5 0 8 0
5020
5060
lliA 0
0:40 2:OO
3:20 4:40Scan time
m / z 6 0 100
Figure 3. (a) Peak profiles for the unimolecular loss of chlorine from the molecular ion for a-, 0 - , m-, and p-chlorotoluene. (b) The reconstructed gas chromatographic trace (by monitoring the loss of chlorine from the four chlorotoluenes). (c) The electron impact spectra of the chlorotoluenes.
group; 0-,m-, and p-chlorobenzyl chloride are in a second group; 2,4-, 2,5-, 2,6-, and 3,4-dichlorotoluene are in third; and 2,3 and 3,5-dichlorotoluene are in the fourth. While T measurements might not distinguish between the individual isomers in a group, they can a t least identify the proper group; then the other techniques (RRT and peak ratios) can further reduce the choice, since the information gathered from RRT or E1 spectra deals with other characteristics of the isomer. The use of RRT for a variety of columns can serve to yield
unique representation of the isomers or a t least divide the isomers into small groups. Table I1 shows that with the three pieces of information the identity of the dichlorotoluene isomers can be determined uniquely. There are few examples, such as 1-and 2-chloroanthracene and 9-chlorophenanthrene, where the only information gained from the T measurement is the identification of the carbon in the C-C1 bond as an aromatic carbon. The RRT distinguishes between 9-chlorophenanthrene and the two chloro-
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
917
Table 11. Comparison of T Values with Other Methods Used for Isomer Determination isomers
E1 peak ratio ( m / z used)a
re1 retention time
chlorotoluenes a/o/p/m
1.0/2.6/3.7/4.7
1.63/1.00/1.03/1.06
1.O/ 1.5/1.3/ 1.3 (126/91 )
chloroethylbenzenes and chloroxylenes 2-Etll-Etip-Et
1.0/1.5/1.8
1.5511.00/1.60
1. O i l .0/4.3 (1401105)
oc,p/a,o/a,m
1.9/1.9/2.9
1.6011.15/ 1.16
29/40
3.6/4.5
3.37/1.13
1.0/2.8/3.1/3.5 7.0/7.2/7.5
1.11/1.35/1.91/1.86 1.05/1.00/1.05
8.1/9.0/9.7
1.34/1.18/1.13
trichlorotoluenes a ,a ,ala,2,6/a ,3,4/2,4,5/2,3,6 dichlorobenzene o/p/m
1.O/ 2.1/ 2.3/ 6.0/6.1
1.00/1.80/2.1 O/ 1.59/ 1.49
1.0/1.3/1.6
1.0O/ 1.03/ 1.06
1.0/1.2/ 1.2 (146/ 111) l . O / l . l / 1 . 2 (146175 )
trichlorobenzene 135/123/124 chloroanthracene and 9-chloroghenanthrene 1-/2-19TCDD 1,2,3,4/2,3,7,8
1.OO/1.04/1.05
1.00/1.01/1.04
1.53/1.62/1.00 1.02/1.02/1.00
l.OO/l.OO/l.OO (212/176)
1.0/1.3/1.O/1.4
1/1
dichlorotoluenes a ,a/o/p/m 2,6/2,4/2,5 3,4/ 2,3/ 3,5
a
T ratio
From re€ 30.
2.2/1.2/1.0/1.1 (126163)
1.811.1/1.2 (140/51 ) 1.31-12.1 (140/105) 1.6/-/ 1.3 (140/51) 2.3/4.0 (140/105) 1.0/1.9 (140/51) 1.O/ 2.2/ 1.611.6 (160/ 125) 1.0/1.9/1.4/1.6 (160/89) 3.41 3.q 4 . 6 (160/ 125) 1.5/1.8/2.2 (160/89) 3.4/-/- (160/125) 1.81-/-(160189)
1.00/1.00/1.04 (212/106)
For loss of COCl.
anthracenes, but the latter two isomers could not be differentiated by any of the three techniques. The T values presented are an average of data from at least two runs with sample quantities ranging from 50 to 200 ng; the standard deviation is always less than 10%. In the picogram quantity range the standard deviation increased to 15% of the reported T value. High source and first field free region pressures due to the GC carrier gas can cause collision induced decompositions which could interfere with metastable T determinations. However, changes in source temperature or helium flow rate through the column into the source by 25% did not change ithe T value. The RRT usually had less than a 2% standard deviation, while the E1 peak ratios had a standard deviation as high as 30%. The low standard deviation of the RRT measurements make small ratios, such as 1.00/1.03, signijficant. If the sample has a 3-min retention time, then the components are separated by about 5 s, which is a reasonable quantity to base identification on. The ability to identify the carbon to which C1 is bonded as aliphatic or aromatic and to identify specific isomers has proven T measurements useful in analysis. With the examination of 70 chloro compounds, a number of generalities can be made. First, the values of T decrease in the following sequence: aliphatic < aliphatic and aromatic in the same molecule < aromatic. Second, isomeric chlorotoluenes and polychlorotoluenesshow the trend T ortho < T para < T meta. So for a given set of isomers some insight into the substitution pattern for the compound can be gained. This trend does not hold true for the xylenes and dichlorobenzenes. Third, as the number of similar C-C1 bonds increase, there is less variation in the values of T between isomers. The possibility that the signal is a composite of each type of C-Cl bond means that as the number of bonds increase there is an averaging out of the character of each bond. If there are four C-C1 bonds and the isomers that are to be distinguished differ in only one type of C-Cl bond, then the signal (if each loss has the same probability) would give a T value very similar to each other since T i s averaged in with the other losses which are identical. On the other hand for a monosubstituted isomer with different C-Cl bonds, the 'rvalue is completely due to differences, other chlorine losses are not averaged in with the measurement. Dection Limit. The ability to perform T measurements on low-nanogram to high-picogram quantities of samples off the GC demonstrates the usefulness of the technique in areas
Energy (ev)pd20 5660 51b0,
m/r90
91
92
Flgure 4. Peak profiles for the unlmolecular loss of chlorine from 'the molecular ion of o-chlorotoluene as a function of amount injected on the column.
involving trace organic analyses. Most samples tested showed 1 ng or lower peak profile detection limits. The peak profiles (Figure 4) show acceptable signal/noise levels down to 100 pg of p-chlorotoluene. The standard deviation for the T measurements for 10 ng is 2 meV, for 1 ng is 3.7 meV, and for 100 pg is 6.7 meV for p-chlorotoluene. Collision Induced Loss of Chlorine. Pressure Effects. The addition of collision gas to the collision cell results in a peak showing loss of chlorine by both unimolecular and CID processes. Tho T values show a monotonic increase with to 3.0 pressure (Figure 5) with a linear region from 0.8 X X torr. This pressure range corresponds to a main beam attenuation of 25-70%. It was desirable to work in this range since slightly different pressures or slight fluctuations of pressure over long periods of time could be corrected far more easily. The pressure in the collision cell should also affect the intensity of the CID process relative to that of the unimo-
918
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
Table 111. The Kinetic Energy _ _ Release for the CID Loss of Chlorine from the Molecular Ion at an Analyzer Pressure of 1.5 X Torr compound
TCID, meV
ethylene chloride methylene chloride 2-chloropropane 1-chloropropane 1-chloro-2-methylpropane 1-chlorobutane
2-chlorobutane 1,2-dichloroethane chlorocyclopentane 1-chloropentane chlorobenzene chloroform chlorocyclohexane 1-chlorohexane p-chlorotoluene m-chlorotoluene o-chlorotoluene benzyl chloride trichloroethane chlorocycloheptane (2-chloroethy1)benzene ( 1-chloroethy1)benzene p-chloroethylbenzene a -chloro-m -xylene a-chloro-p-xylene a -chloro-o-xylene 2-chloro-p-xylene 4-chloro-o-xylene m-dichlorobenzene p-dichlorobenzene o-dichlorobenzene 4-chloro-a-methylstyrene
2,5-dimethylbenzyl chloride
TCID, meV
%
change 34.8 15.9 18.4 M - HC1 38.1' M - HCl 15.6' 39.3 M - HCl 10.1 M - HC1 38.6a 12.6 M - HC1 28.8 M - HCl 17.7 2.1 7.5 M - HC1 21.3 M - HC1 38.3 48.5 35.0 23.1 18.7 5.9 M - HC1 32.0 23.6 38.5 42.6 38.5 32.1 50.0 51.6 37.0 39.8 29.4 58.7 25.0
'Partially resolved doublet between M - C1 and M - HCl. rather than C1.
411 38 607 85 17 3 55 50 -10 21 -1 5
-9 23 39 36
compound 2,4-dichlorotoluene 2,6-dichlorotoluene 3,4-dichlorotoluene 2,5-dichlorotoluene 2,3-dichlorotoluene 3,5-dichlorotoluene o-chlorobenzyl chloride m-chlorobenzyl chloride p-chlorobenzyl chloride benzyl chloride 1-chloronaphthalene a ,a-dichloro-p-xylene 2,5-dichloro-p-xylene ( 1-chloromethy1)naphthalene
192 178
1,2,4-trichlorobenzene 1,3,5-trichlorobenzene 1,2,3-trichlorobenzene p-chlorobiphenyl a,a ,a-trichlorotoluene a ,3,4-trichlorotoluene a ,2,6-trichlorotoluene 2,3,6-trichlorotoluene 2,4,5-trichlorotoluene 9-chlorofluorene pentachloroethane
121
4-(chloromethy1)biphenyl
120 86 56 36 61 31 61
1-chloroanthracene 2-chloroanthracene 9-chlorophenanthrene 1,2,3,44etrachlorobenzene 4,4' -dichlorobiphenyl 9-(chloromethy1)anthracene
40
76 21 2 567 20 315 109 10
26.4 24.6 33.3 37.0 45.3 64.3 30.5 40.8 32.0 18.9 66.3' 38.5 56.1 32.6 41.2 59.2 43.5 63.5'
%
change -7
18.1
-12 3 23 26 66 170 195 158 327 59 205 74 93 37 107 46 70 158
35.0 35.5 42.8 44.8 33.1 0.6 39.6
146 0.2 6 52 16 40
117
-b -b
-b
53.1
86
46.7
90
-b
Unresolved doublet. M - HCl TCID value for loss of HCl 50
z
7 c
-
40 30-
-C \
g 20c:
-
IO -
6 I
105x10-9
~XIO-~
5x10-'
5x10-'
5x10''
5x10-'
Analyzer pressure (torr) Analyzer pressure (torr)
Figure 5. Changes in kinetic energy release, for the C I D loss of chlorine from the molecular ion of o-chlorotoluene, as a function of pressure in the collision cell.
lecular. The plot of peak height vs. pressure (Figure 6) shows this effect. There is a consistent increase in signal up to a collision gas pressure of 3 x lo-' to 5 x torr. At pressures above this range the scatter due to the decreased mean free path of the ions increases faster than the signal due to collision induced fragmentation. The pressure chosen for analysis should permit good sensitivity and be in the range where T is linear. The TCID work was performed at 1.5 X torr, a t which the main beam was 50% attenuated and sensitivity was excellent. TCID Values. The TCID values for the same chlorinated compounds (Table 111) usually are greater than T values as predicted by theory. They display the same trends as the unimolecular losses did, but in some instances provide information which could identify the isomer when the unimo-
Flgure 6. The changes in peak height, for the loss of chlorine from the molecular ion of chlorotoluenes, as a function of collision gas pressure.
lecular measurements could not. For example, the unimolecular T values can only divide the ten isomers of the dichlorotoluenes into four groups, the TCID can further subdivide these groups. For group two, m-chlorobenzyl chloride can easily be distinguished from the ortho and para isomers. The group three isomers now can distinguish the 2,5-isomer from the 3,4-isomer, and both isomers from the 2,4- and 2,6-dichlorotoluene isomers. The group four isomers, 2,3- and 3,5-dichlorotoluene, can easily be distinguished, with T values of 45.3 and 64.3 meV, respectively. A number of the chloro compounds do not lose chlorine, or the loss is overwhelmed by an increased loss of HC1. Samples like 1- and 2-chloroanthracene and 9-chlorophenanthrene do not give a strong [M - C1]+ peak under CID conditions; the peak profile is a doublet due to the loss of both HC1 and C1, and no T values were determined.
ANALYTICAL CHEMISTRY, VOL. 55, NO. 6, MAY 1983
919
Table IV. List of TCID Ratios and Peak Height Ratios for the Loss of Chlorine from Selected Isomers at an Analyzer Pressure of 1.5 X Torr compound chlorotoluene a/o/m/p chloroethylbenzenes l-Et/Z-Et/p-Et chloroxylenes 2-p/a-p/4-o/a-o/a-m dichlorotoluenes a,a1o1~/m12,612,~/2,5/3,~/2,31~,5
dichlorobenzene o/p/m trichloro benzene I ,2,5/1,2,3/ 1,2,4 trichlorotoluene
W I D ratio
CIDIUNI peak ht ratio
1.012.012.712.2 1.0/1.4/ 1.6 2.1/1.6/2.2/1.4/1.8 1.O/ 1.611.7/2.1/1.3/ 1.41 2.011.812.413.4 111.311.4 1.4/1.1/1.o 1.0/2.0/ 1.912.512.4
11.21 1.411.711.0 1.1/1.0/4.4 1.ll2.9/ 1.113.21 1.2 4.2l6.5/8.8/lO.Oll.0/1.2/ 1.211.111.211.3 1.0/1,1/1.0 I .2/1.0/1.3 1.1/3.2/6.3/1.0/1.1
interference present
1.00/1.03/1.00a
1.0011.0411.0011.09
1.311.Ol 1.311.o
a,a,i~/a,2,6/a,3,4/2,4,5/2,3,6
chloroanthracene and 9-chlorophehanthrene 11219 TCDDl 1,2,3,4/2,3,7,8(M - C1, M - COC1) a
Peak height for CID includes loss of HCl.
The TCID measurements do not form a group plot as distinctive as that made from the unimolecular data (Figure 7a) but the difference between TCID and T shows a better correlation. The TCID values for aliphatic C-C1 bonds show the greatest increase in value over T values. Compounds with both aromatic and aliphatic C-C1 bonds show a moderate increase, and aromatic C-C1 bonds show little increase or no change from T measurements. The plot of ((TCID - r ) / T ) x 100 provides a better correlation with carbon type (Figure 7b). In general, less than 150% differences indicate an aliphatic C-C1 bond, values between 80 and 120% correspond to compounds with both aromatic and aliphatic C-C1 bonds. The values between 50 and -50% represent compounds with aromatic C-C1. Again, this classification group scheme is not as reliable as that from unimolecular T measurements and there are some exceptions to the rules. For example, T values for 4-chloromethylbiphenyl, 2,5-dimethylbenzyl chloride, and 9-chloromethylanthracene are too low for an aliphatic compound; and 1,3,5-trichlorobenzeneand 2-chloro-p-xylene had T values too high for correct classification as aromatic compounds. The TCID measurements for compounds which predominantly lost HC1 did not follow a characteristic trend. Many of the trends observed for T measurements also hold true for TCID imeasurements. However, there are more exceptions noted for TCID values. R a t i o o f (TCID T ) / Tf o r I d e n t i f i c a t i o n . The enhanced intensity of TCID over T for a constant sample size provides an added dimension for isomer specificity. In some cases the results (Table IV) provide extra information, but in general there is no new structurally specific information. The aromatic compounds with aliphatic C-Cl bonds have the largest ratio. This is of little consequence in the compounds analyzed, since ithe T values for the aliphatic compound differ greatly from thlose of the other aromatic isomers. There are slight differencles in the ratios for aromatic C-C1 bonds, but usually not enough to distinguish among individual isomers. These measurements follow the trend that the ratio of mixed aromatic and aliphatic C-C1 compounds aliphatic C-Cl compounds aromatic C-C1 compounds. This holds true for most compounds; however 1-and 2-chloroethylbenzene gave values lower than p-chloroethylbenzene. T e t r a c h l o r o d i b e n z o - p - d i o x i n Analysis. The identification of tetrachlorodibenzo-p-dioxin(TCDD) isomers present in a sample is of major importance in environmental analysis (26,27). With 22 TCDD isomers there are many possibilities for misidentification of the toxic 2,3,7,8-isomer (28). This can occur with the 1,2,3,4-isomersince the E1 spectrum is similar to that of the :2,3,7,8-isomer (29). A number of capillary columns can separate the 2,3,7,8-TCDDisomer from all others, enabling identification of that isomer with only a 2,3,7,8 reference. T values can be used for complementary infor-
*
I
.
.
... .
50 .-
2
c
.
C
0
2iO
240
360
250
300
Molecular wt.
A
+
2
100 0
.
-
1
A
0 0 . . W
0
.
.*...eo
0
A
0. .
Y
.
.
O
0..
. * :
I
I
100
150
200
Molecular w t
Flgure 7. (a) Group classiflcation carbon type based on CID kinetic energy releases for the loss of chlorine from the molecular ion of selected chlorinated compounds. (b) Group classification based on ((TCID - T ) / T ) X 100 for the loss of chlorine for the same chlorinated alcompounds. Aromatic C-CI compounds are represented by (0)3 iphatic C-CI compounds by (0),and mixtures of both bond types by
(A).
mation and for identification of the other isomers since all 22 TCDD isomers are difficult to separate on one column (7, 8). The T values for 1,2,3,4-TCDD are 86 meV for the loss of C1 and 67 meV for the loss of COCl from the molecular ion. The 2,3,7,8-isomer has values of 115 meV and 93 meV for these respective losses. The ratio of T(2,3,7,8-)/T(1,2,3,4)for the loss of C1 and COCl are 1.3 and 1.4, respectively. This difference is acceptable for proper identification down to quantities of about 1 ng with standard deviations less than 20% of the listed value (Figure 8). The TCID values for the losses of COCl and C1 are 144 and 179 meV, respectively, for the 1,2,3,4-isomerand 150 and 1196
920
ANALYTICAL CHEMISTRY, VOL. 55, NO. M-COCI
m / r 255
257
6,MAY 1983
styrene, 1712-70-5;2,5-dimethylbenzyl chloride, 824-45-3; 2,4dichlorotoluene, 95-73-8; 2,6-dichlorotoluene, 118-69-4;3,4-dichlorotoluene, 95-75-0; 2,5-dichlorotoluene, 19398-61-9;2,3-dichlorotoluene, 32768-54-0; 3,5-dichlorotoluene, 25186-47-4; ochlorobenzyl chloride, 611-19-8;m-chlorobenzyl chloride, 620-20-2; p-chlorobenzyl chloride, 104-83-6;benzyl chloride, 100-44-7;1chloronaphthalene, 90-13-1; 2,5-dichloro-p-xylene, 1124-05-6; a,a-dichloro-p-xylene,23063-36-7; (1-chloromethyl)naphthalene, 86-52-2;1,3,5-trichlorobenzene,108-70-3;1,2,4-trichlorobenzene, 120-82-1; 1,2,3-trichlorobenzene, 87-61-6; 2-chlorobiphenyl, 2051-60-7;3-chlorobiphenyl, 2051-61-8;4-chlorobiphenyl, 205162-9; a,a,a-trichlorotoluene, 98-07-7; a,3,4-trichlorotoluene, 102-47-6;a,2,6-trichlorotoluene,2014-83-7;2,3,6-trichlorotoluene, 2077-46-5;2,4,5-trichlorotoluene, 6639-30-1; pentachloroethane, 76-01-7;9-chlorofluorene, 6630-65-5;4-(chloromethyl)biphenyl, 1667-11-4; 1-chloroanthracene, 4985-70-0; 2-chloroanthracene, 17135-78-3;9-chlorophenanthrene, 947-72-8;1,2,3,4-tetrachlorobenzene, 634-66-2; 4,4'-dichlorobiphenyl, 2050-68-2; 9-(chloromethyl)anthracene, 24463-19-2; 1,2,3,4-tetrachlorodibenzo-p-dioxin, 30746-58-8;2,3,7,8-tetrachlorodibenzo-p-dioxin, 1746-01-6; chlorine, 7782-50-5.
M-CI
259
253
285
LITERATURE CITED
287
Figure 8. Peak profiles for the loss of CI and COCl from the molecular ion of TCDD: (a) 1,2,3,4-isomer, 100 pg injected on column; (b) 2,3,7,8-isomer, 100 pg injected on column; (c) 2,3,7,8-isomer, 5 ng injected on column.
meV for the 2,3,7,8-isomer. The CID/UNI peak height ratios for the loss of C1 for the 1,2,3,4- and 2,3,7,8-isomers are 0.5 and 0.65, respectively. The ratios are 4 and 5 for the loss of COC1. Neither TCID or CID/UNI peak height ratio provides enough information to distinguish these isomers. Either the complete unimolecular or CID spectrum of the molecular ion can be used to distinguish them; the spectrum of the 1,2,3,4-isomer contains a loss of CO from the molecular ion about 20 times more intense than that of the 2,3,7,8-isomer. Conceivably, an analysis based upon CO loss will be valuable for distinguishing among isomers of TCDD. T measurements are not an alternative to chromatographic identification. Good chromatography is necessary to implement this technique for mixtures. T measurements are not meant to be the first method used in mixture analysis, rather they are used to solve problems that cannot be resolved by common GC/MS techniques. T measurements are used to distinguish isomers which show similar spectral characteristics or those isomers for which no reference compounds are available. Also, T measurements can provide further confirmation for identification based on RRT or peak ratios. Registry No. Ethylene chloride, 75-01-4; methylene chloride, 75-09-2; 2-chloropropane, 75-29-6; 1-chloropropane, 540-54-5; l-chloro-2-methylpropane,513-36-0; 1-chlorobutane, 109-69-3; 2-chlorobutane, 78-86-4; 1,2-dichloroethane, 107-06-2; chlorocyclopentane,930-28-9; 1-chloropentane,543-59-9; chlorobenzene, 108-90-7; chloroform, 67-66-3; chlorocyclohexane, 542-18-7; 1chlorohexane,544-10-5;p-chlorotoluene, 106-43-4;o-chlorotoluene, 95-49-8; m-chlorotoluene, 108-41-8; benzyl chloride, 100-44-7; trichloroethene, 79-01-6; chlorocycloheptane, 2453-46-5; 1chloroheptane, 629-06-1; 1,5-dichloropentane, 628-76-2; (2chloroethyl)benzene, 622-24-2;(1-chloroethyl)benzene,672-65-1; p-chloroethylbenzene, 622-98-0; a-chloro-o-xylene, 552-45-4; achloro-m-xylene, 620-19-9; a-chloro-p-xylene, 104-82-5; 2chloro-p-xylene, 95-72-7; 4-chloro-o-xylene, 615-60-1; m-dichlorobenzene, 541-73-1;o-dichlorobenzene,95-50-1;p-dichlorobenzene, 106-46-7;1-chlorooctane, 111-85-3;4-chloro-a-methyl-
Klmble, 8. J. "High Performance Mass Spectrometry: Chemical Applications"; Amerlcan Chemlcal Society: Washington, DC; 1978; ACS Symposium Series, 70,pp 120-149. Kondrat, R. W.; Cooks, R. G. Anal. Chem. 1978, 5 0 , 51A-92A. Safe, S. Org. Mass. Spectrom. 1971, 5 , 1226. Brown, P. Org. Mass Spectrom. 1970, 3 , 639. Buser, H. R. Anal. Chem. 1978. 4 8 , 1553. Soltero-Rigau, E.; Kruger, T. L.; Cooks, R. G. Anal. Chem. 1977, 4 9 , 435-442. Buser, H. R.; Rappe, C. Anal. Chem. 1980, 5 2 , 2257-2262. Buser, H. R. Anal. Chem. 1977, 49, 918. Elder, J. F.; Cooks, R. G.; Beynon, J. H. Org. Mass Spectrom. 1976, 11, 423-428. Cooks, R. G.; Beynon, J. H.; Caprioli, R. M.; Lester, G. R. "Metastable Ions"; Elsevier: Amsterdam, 1973. Larka, 8. A.; Howe, I.; Beynon, J. H. Org. Mass Specfrom. 1981, 16, 465-466. Levsen, K. "Fundamental Aspects of Organic Mass spectrometry"; Verlag Chemie: New York, 1978; pp 237-242. Cooks, R. G.; Beynon, J. H.; Caprloll, R. M.; Lester, G. R. "Metastable Ions"; Elsevier: Amsterdam, 1973; p 118. Florenclo, H.; Vijfhuizen, P. C.; Heerma, W.; Dijkstra, G. Org. Mass Spectrom. 1979, 1 4 , 198-200. Proctor, C. J.; Kralj, B.; Larka, E. A.; Porter, C. J.; Maquestiau, A,; Beynon, J. H. Org. Mass Specfrom. 1981, 18, 312-322. Hass, J. R.; Bursey, M. M.; Levy, L. A,; Harvan, D. J. Org. Mass Spectrom. 1979, 14, 319-325. Schubert, R.; Grutzmacher, H. F. Org. Mass Spectrom. 1980, 15, 122-130. Larka, E. A.; Howe, I.; Beynon, J. H. Org. Mass Spectrom. 1981, 16, 465-466. Hass, J. R.; Tonduer, Y.; Voyksner, R. D. Anal. Chem., in press. Hoffman, M. K.; Friesen, M. D. Org. Mass Specfrom. 1976, 1 1 , 1101-1 104. Harvan, D. J.; Hass, J. R.; Blrch, D. J. S. Annu. Conf. Mass Spectrom. Allled Top., 26th 1978, 105-106. Weiss, M.; Karnofsky, J.; Hass, J. R.; Harvan, D. J. Annu. Conf. Mass Spectrom. Allied Top 27th 1979, 270-271. Morgan, R. P.; Beynon, J. H.; Bateman, R. H.; Green, B. N. Int. J . Mass Spectrom. Ion Phys. 1978, 2 8 , 171-191. Beynon, J. H.; Cooks, R. G. J . Phys. Educ. 1974, 7 , IO. Terwilliger, D. T.; Beynon, J. H.; Cooks, R. G. Proc. R . SOC.London, Ser. A 1974, 341, 135. Cairns, T.; Fishbein, L.; Mitchum, R . K. Biomed. Mass Spectrom. 1980, 7 , 484. McConnell, E. E.; Moore, J. A,; Haseman, J. K.; Harris, M. W. Toxicol. Appl. Pharmacol. 1978, 55, 335. Harless, R. L.; Oswald, M. K.; Wilklnson, M. K.; Dupuy, A. E.; McDanIel, D. D.; Tia, H. Anal. Chem. 1980, 5 2 , 1239. Buser, H. R. Anal. Chem. 1976, 4 8 , 1553. "Eight Peak Index of Mass Spectra", 2nd ed.; Mass Spectrom. Data Center: Awre, UK, 1974.
RECEIVED for review October 21,1982. Accepted January 24, 1983.
