Anal. Chem. 1008, 60,781-787
701
Interlaboratory Comparison of Methane Electron Capture Negative Ion Mass Spectra E. A. Stemmier,' Ronald A. Hites,*' B. Arbogast? W. L. B ~ d d eM. , ~ L. Deinzer,2R. C. D ~ u g h e r t y , ~ J. W. Ei~helberger,~ R. L. Foltz? C. Grimm; E. P. Grimsrud,6 C. S a k a ~ h i t aand , ~ L. J. Sears6
School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405, Department of Agricultural Chemistry, Oregon State University, Corvallis, Oregon 97331, US.Environmental Protection Agency, Office of Research and Development, Cincinnati, Ohio 45268, Department of Chemistry, Florida State University, Tallahassee, Florida 32306, Center for Human Toxicology, University of Utah, Salt Lake City, Utah 84112, and Department of Chemistry, Montana State University, Bozeman, Montana 5971 7
The electron capture negative ion (ECNI) mass spectra of 12 compounds were measured on quadrupole and magnetic sector Instruments. The compounds were selected as exampies of molecules that are often examined by ECNI mass spectrometry. Under similar conditions of sample concentrations and Ion source temperatures and pressures, the spectra showed good agreement in the molecular ion mass range. A Hewlett-Packard 59858 quadrupole instrument showed a lower abundance of low mass Ions (such as CI-) relative to the other Instruments. Other sources of spectral variability arose from lon/molecule reactions involving reagent gas impurities and from hydrogen incorporation reactions.
Electron capture negative ion (ECNI) mass spectrometry is a technique often noted for its sensitive and selective detection of electrophilic compounds (1, 2). Unfortunately, ECNI mass spectra are notoriously subject to variations in instrumental conditions and configuration (3,4). This problem can impair quantitative measurements and prevent the accumulation of reproducible data bases. Mass spectral differences can be caused by variations in ion formation, in ion transmission to and through the mass analyzer, and in ion detection as a function of mass. ECNI mass spectra are particularly difficult to reproduce because of the number of variables that affect the initial ionization process. In this paper, current knowledge regarding the variables which affect negative ion formation in a high-pressure ion source will be reviewed. This review will be followed by the results of a study comparing ECNI mass spectra of 12 compounds. These spectra were measured on both quadrupole and magnetic sector instruments using a predetermined set of operating conditions. From these data, spectral similarities and differences will be discussed. Difficulties associated with reproducing ECNI mass spectra and strategies which may be used to overcome these problems will also be addressed.
EXPERIMENTAL SECTION The instruments and operating conditions used in this study are shown in Table I. ECNI mass spectra were obtained by using methane as the reagent gas. A multicomponent standard, containing the compounds shown in Figure 1,was introduced to the mass spectrometers using 30-m, DB-5, fused silica capillary GC columns. The columns were directly coupled to the ion source. Helium was used as the carrier gas. Injections of 50-100 ng were made in the splitless mode. 2,2,4-Trimethylpentane was used as *Indiana University. *Oregon State University. U.S. Environmental Protection Agency. Flordia State University. University of Utah. Montana State University.
the solvent. On three instruments (HP 5985B, Finnigan 4500, and MS 80), spectra were also acquired using standard dilutions (see Table I). Spectra were acquired at a rate of 1 s/cycle over the mass range shown in Table I. Spectra were measured at the peak maxima and were background corrected.
THEORY ECNI mass spectral variations may be associated with variations in ionization, ion transmission, mass analysis, and ion detection. Parameters which are relevant to each of these areas are reviewed here with an emphasis on the initial ionization process. Formation of Negative Ions. Negative ions may be formed by resonance and dissociative resonance electron capture (5). These processes are shown by reactions 1 and 2. A stable negative molecular ion is formed if the vibra-
M
+
e-
-
MEM-]*
resonance electron capture (0-2 ev)
(1)
EM-XI
+
X-
dissociative electron Capture (0-15 eV)
(2)
tionally excited negative ion, initially formed by electron capture, is stabilized (most efficiently by a collision) before it undergoes autodetachment or dissociation. Dissociative electron capture (reaction 2) can occur by a direct rapid cleavage or by a slower process which may involve a molecular rearrangement (5). These reactions occur with electrons having energies between 0 and 15 eV. The products generated by electron capture reactions are determined by the electron's energy. Depending upon the negative ion lifetime, pressure can also affect the formation of molecular ions. Temperature can dramatically affect the production of fragment ions, particularly for reactions occurring near 0 eV (6). Operating the mass spectrometer's ion source at 0.1-1 Torr generates electrons in a medium which reduces their energy and collisionally stabilizes the negative ions formed by electron capture. The electrons are generated by bombardment of a buffer gas (such as CHI) with a beam of electrons. Ions, electrons, and radicals are formed, as shown by reactions 3 and 4. These species can undergo further collisions with the
neutral gas. The secondary electrons will undergo elastic and inelastic collisions and attain a distribution of energies that depends upon the ion source pressure, the nature of the gas, and the electric fields in the ion source (1). Negative ion formation is complicated by the high charge density of the ionized gas and the presence of reactive species
0003-2700/88/0360-0781$01.50/0Q 1988 American Chemical Society
782
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15. 1988
Table I. Instrumental Parameters Used in Interlaboratory Study HP 5985 IU
instrument locationb amt injected, ng ion source pressure, Torr pressure transducer emission current, p A electron energy, e\' accelerating voltage, kV mass range
FINN 4500 FSU
FINN 4500
uu
FINN 4000'
MS 80 EPA
OSU
50, 5, 0.5
100
100
100, 20, 10
20, 10, 5, 2.5
0.4
0.5 pirani gauge
0.4
-C
150
300
ion gauge 500
200 -
0.8 pirani gauge 300 70 -
70
70
60
30-600
30-600
30-600
cap mand 300
-
pirani gauge -
4
30-600
34-620
VG 7070 MSU 50 0.2 cap mand 2000 150 6 15-600
'FINN 4000 with 4500 source. bIU, Indiana University; FSU, Florida State University; UU, University of Utah; OSU, Oregon State University; EPA, EPA, Cincinnati, OH; MSU, Montana State University. Ion source pressure could not be measured directly; manifold Torr and ion source uressure is estimated at 0.1-0.4 Torr. dCauacitance manometer. uressure was 2 X cn
Saphthalene
Benzo(a)pyrene
CI
Diazepam
(2)
(1) Cl CI
-
(31
CI
The presence of reactive species such as oxygen, water, or radicals can also complicate negative ion formation. Ion/ molecule reactions give products such as [M - C1+ 01- ions from chlorinated aromatics and [M - HI- ions from compounds with proton affinities less than 0- or OH- (11) (see reactions 5 and 6). Charge exchange reactions are also
M-
CI CI
+ 02
M 6 2,2'.3,3'-Tetrachlorobiphenyl
'c I
2.2 4 . 4 , 5 , 5 ' - Hexachloro'biphenyl
(4)
(51
CI
i,2,3.?~~Tetrachlorod i b e n z o - p - d.ouin
Octachiorod : b e n r o - p - d:ox?n
(6)
(7)
0
p - Sitropheni-1-
Dibutyl p h t h a l a t e
(9)
(81
Decafluorotr pnen) lphosph ne
(11)
Pentach!oro. ?heno:
propionate CI
\/
CI
(10)
Enaosulfan
(12)
Compounds used for this interlaboratory study of methane ECNI mass spectra. Figure 1.
other than electrons. Because of the high charge density, the ionized gas is considered to be a plasma (7,8). In a plasma, the diffusion of charged species is ambipolar, meaning that electrons and positive ions diffuse at the same rate (assuming equal populations and energies) in order to maintain charge neutrality. Thus, the diffusion of electrons is slowed and that of positive ions is accelerated relative to their free diffusion rate. At 0.5-1.0 Torr, loss of ions by ambipolar diffusion to the walls provides a major, but not the sole, means of ion and electron loss (7,9). Because of the high charge density, rapid recombination reactions are also a dominant mode of ion and electron loss and will affect the concentration of positive and negative ions and electrons (7). Positive ion-electron recombinations and wall neutralizations may produce various radical species (10). The high charge density may also affect the extraction of ions from the ion source region, although no discussion of these effects has appeared in the literature in relation to ECNI mass spectrometry.
-+
(M - C1 + 0)+ C10
+ OH-
-+
(M - H)-+ HtO
(5) (6)
possible. Ions that show incorporation of alkyl species or hydrogen (10,12) have also been observed. These ions include [M + CH2]- and [M + H - XI-, where X is Br, C1, F, or CN. Ions such as [M + CH2]- have been attributed to reactions with radical species (produced by electron impact and ionelectron recombination) followed by electron capture. The formation of [M + H - XI- has recently been shown to occur via electron capture by a species generated by hydrogenation of the analyte on the ion source walls (9). When a sample molecule is introduced into the ion source, a number of competitive reactions may occur with electrons, positive ions, and other reactive species. Product ions will reflect the rate of these reactions and subsequent decompositions (7). Depending on the ion lifetime in the source and reaction rates, a series of consecutive reactions may also occur, particularly when rapid reactions, such as electron capture and positive-negative ion recombinations, are involved (9). Parameters that can be controlled in the ion source include its temperature and pressure, the sample concentration, the electron energy, the emission current, and the reagent gas. These parameters will influence the electron energy distribution, cross sections for negative ion formation, the charge density, the collisional frequency, and the type and concentration of reactive species present in the ion source. Numerous examples appear in the literature describing the effects of ion source temperature upon ECNI mass spectra (13-17). Higher ion source temperatures increase the abundance of fragment ions, such as C1-, while lower temperatures enhance molecular ion formation. Other effects &e observed. Reactions involving formation of [M + H - XI- ions are enhanced by lower ion source temperatures (18). Compounds that give both M- and [M - HI- ions show an enhancement of the [M - HI- ions at higher ion source temperatures (19). The ion source pressure is also an important parameter in negative ion formation. Increasing the ion source pressure will increase the number of ion-electron pairs that are produced until the electron beam is stopped by gas molecules. Increasing the ion source pressure will also increase the collisional frequency, affecting the rate of electron thermalization and collisional stabilization of vibrationally excited negative ions. Generally, increasing the ion source pressure increases the negative ion response until a pressure maximum is reached (20-22). The decrease at higher pressures probably results from ion transmission losses. Different reagent gases give different pressure maxima (20-22). The ion source pressure
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
also affects the abundance of fragment ions. Depending upon the reagent gas and the compound, molecular ion or fragment ion intensity may be enhanced as the ion source pressure is increased (21,22). The formation of [M H - XI- ions is enhanced by low ion source pressures (18). Sample concentration can also affect ECNI mass spectra. High sample concentrations can result in intermolecular reactions which may cause spectral variations (23). Certain reactions, such as those attributed to radical incorporation, are enhanced by low sample concentrations (IO). Other variations in ECNI mass spectra with changes in sample concentration have been noted (24).The electron energy and emission current determine the number of ion-electron pairs that are produced. The spectra of many compounds are unaffected by these parameters (18);however, ions showing radical incorporation are enhanced by an increase in emission current (10,12). The reagent gas, in conjunction with reagent gas pressure, can influence the relative abundance of ions in a mass spectrum (20-22). In addition, impurities present in the reagent gas can affect which ions are observed. For example, the presence of oxygen results in a large [M - HI- ion from diazepam (25). Extraction, Mass Analysis, and Detection of Negative Ions. Once negative ions are generated in the ion source, they are removed and undergo mass analysis. The relative abundance of ions generated in the source may be perturbed by the efficiency of ion extraction from the ion source region. Generally, the entire ion source is held at ground or a negative potential, and ions exit from the source with neutral reagent gas molecules; they then experience a net potential determined by the instrument configuration. Little is known about discrimination effects which may occur in this region of variable charge density and pressure. Lenses, used to maximize ion transmission to the analyzer, may also influence relative ion abundances. The mode of mass analysis may affect ion abundances because of differences in ion transmission and in ion transit time. Quadrupole systems can show significant variability because they are influenced by tuning parameters, fringing fields, and rod cleanliness (26). The mode of ion detection will also influence sensitivity to ions of different mass or energy. For example, systems using conversion dynode electron multipliers discriminate against low mass ions (27).In addition, when GC peaks are monitored, discrimination can occur by inadequate GC peak sampling (28).
I,-
783
I
+
RESULTS AND DISCUSSION To assess the variability of ECNI mass spectra measured on different instruments, a study was undertaken to compare the spectra of a number of common analytes under similar operating conditions. Participants in this study and their respective instruments are listed in Table I. Instrument types included two quadrupole systems (a Hewlett-Packard 5985B and three Finnigan 4500s) and two magnetic sector instruments (an MS-80 and a VG 70-70). In this study, a standard containing 12 compounds (see Figure 1)was introduced into the mass spectrometer by capillary column gas chromatography. Certain compounds were included because they undergo both electron capture and ion/molecule reactions depending upon the concentration of reactive species in the ion source. For example, diazepam (3) gives an [M - HI- ion when trace amounts of oxygen are present (25).The chlorinated aromatics show [M + 0 - C1]ions in the presence of oxygen ( I ) . Thus, ions from diazepam (3), the PCBs (4,5), and the dioxins (6,7) should reflect the amount of oxygen present in different instrumental systems. Other compounds, such as (decafluorotripheny1)phosphine (DFTPP) ( l l ) ,p-nitrophenyl propionate (9), and endosulfan
LM'
MSU, 1 2 O o C
I
(M-C~H~)'
uu, 120°c 365
167
275
I
FSU,
315346
' MI442
1
365
osu. 120°c
I
I
404
12OoC
I
1
442
167 365
404
I
IU. 120°c
W U U W r r r r r O O O O r W u l . J w 0 0 0 0 0
~ 0
N W 0
N N U . J 0 0
N W 0
W 0
W
W
W
w u l r l w
0
0
0
0
W 0
C + C e C W u l r l l n 0 0 0 0
M/Z
Flgure 2. Methane ECNI mass spectra of DFTPP (11) measured on six different instruments ( m l z 167 Is present at 1 % abundance in the I U spectrum).
I (12) undergo simple cleavage reactions, and their spectra should reflect the degree of fragmentation which is occurring. Endosulfan I (12) also gives [M - C1 + HI- ions and should indicate the prevalence of hydrogen incorporation reactions. Spectra were measured assuming that similar instrumental conditions, such as temperature and pressure, could be obtained on different instruments. Unfortunately, the manner in which temperature and pressure measurements are made varies from instrument to instrument. For example, on the H P 5985B and VG 70-70 the ion source pressure was monitored by a capacitance manometer, while on the Finnigan 4500's the ion source pressure is monitored by a Pirani gauge (calibrated for N,) located 10 cm from the ion source block. Thus, the pressure read on the Finnigan is nearly half the actual ion source pressure of methane. In addition, the ion source temperature indicated by a thermocouple in the ion source block of the Finnigan 4500 must be converted to obtain the actual (higher) ion source temperature. These pressure and temperature differences will contribute to the overall variability. The instruments also differed with regard to ion source and lens configurations. Both quadrupole systems used ion sources designed for chemical ionization. The H P source is located approximately 9 cm from the quadrupoles. The ion volume is held at the repeller potential (0 to -12.5 V) which determines the ion kinetic energy. Ions exit through a 0.1-cm orifice into a conical region. They then pass through a drawout lens, an einzel lens assembly (located 4.5 cm from the source), and an entrance lens before entering the quadrupoles. The Finnigan ion source is held at ground, and the ions exit through a flat plate. A series of three lenses is used to extract and focus the
784
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
Table 11. Methane ECNI Mass Spectra of Compounds 2-10 120 "C
HP 5985 IU M
+ 28 (280) + 15 (267) + 14 (266) M + 1 (253) M (252) M-1
M M
+ 15 (299) + 14 (298)
M (284) M-1 M - 35 M - 44 M - 58 M - 89
c1
c1
FINN 4500 OSU
VG 7070 MSU
0.3 2 0.7 25 100
0.7 0.9 2 24 100 0.8
150 "C MS 80
EPA
0.6 0.2 0.5 0.4 24 100
1 2
3 27 100
0.9 0.9 2 24 100
1
32 100 3
Diazepam (3) 0.9 0.2 100 3
(249) (240) (226) (195)
M (290) M - 1 (289) M - 34 (256) M - 35 (255) M - 70 (220)
FINN 4500 UIJ Benzo[a]pyrene (2)
+ 42 (294)
M M M
FINN 4500 FSU
83 3 5 6 15
M (358) M - 19 (339) M - 34 (324) M - 35 (323) M - 53 (305) M - 54 (304) M - 68 (290) M - 104 (254) C1
53 1 14
M (320) M - 19 (301) M - 35 (285) M - 70 (250) c1
79
M (456) M - 34 (422) M - 35 (421) M - 68 (388) M - 69 (387) M - 70 (386) M - 103 (353) M - 104 (352) M - 136 (320) M - 137 (319) M - 138 (318) M - 170 (286)
31 19 43 22 9 6
3
16 8 100 85 3 3 29
100 8 4
20
21
1
3 1 Tetrachlorobiphenyl (4) 3 3 1 1
100 36 34 3 2 1 0.7 0.7 100
3 100 Hexachlorobiphenyl (5) 53 2 2 2 1
100
100 8
100 3
1 2
3
3
9
3 0.6 1 0.4 4 100 52 0.8 16 2 0.5 0.1 8 0.5 64
100 30 10 2
100 51 0.2 8
50 8
1
3 1 100
12
Tetrachlorodibenzo-p-dioxin(6) 2
23 6 4 100
28 4 0.6 100
77 0.4 13 2 82
38 2 0.5 100
80 3 0.3 38
Octachlorodibenzo-p-dioxin(7)
c1
M + 15 (293) M + 14 (292) M + 1 (279) M (278) M - 56 (222) M - 57 (221) M - 72 (206) M - 102 (176) M - 112 (166) M - 113 (165) M - 115 (163) M - 116 (162) M - 129 (149) M - 130 (148)
3
4
19
4
29 6 4 3 0.7
4 4
0.7
1
2 4
0.6 1 0.3
1
3 1 2
100
100
0.1 0.2 2 0.9 0.6 0.2 0.6 0.2 0.9 0.5 0.4 0.8 100
27 12 8 8 8 16 10 8 100
6 46 3
51
Dibutyl Phthalate (8) 6 11
0.6 0.3
1
100
0.5
3
1
6 2
6 2
2
1
3
0.5 3
1 1
60
2 2
23
100
100
6 18 12 12 0.7 5 8 2 2 12 100
10 10 7 9 24 11
12 13 100
100
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL 15, 1988
785
Table I1 (Continued) 120 OC
HP 5985 IU
FINN 4500 FSU
FINN 4500
FINN 4500
uu
osu
VG 7070
150 "C MS 80
MSU
EPA
100 0.5
100
p-Nitrophenyl Propionate (9)
M (195) M - 16 M - 17 M - 29 M - 56 M - 57 M - 72 M - 73
(179) (178) (166) (139) (138) (123) (122)
100 0.5
100 3
100 3
100 2
1 n
L
3 4
62
25
2
1
24 31 3 3
8 56 0.7
95
1 0.5
73
Pentachlorophenol (10) M (264) M - 1 (263) M - 34 (230) M - 35 (229) M - 36 (228) M - 68 (196) M - 69 (195) M - 70 (194) M - 71 (193) M - 103 (161) M - 104 (160) M - 105 (159)
c1
9 88 76 31 9 4 2
0.2 6 21 5 11 6 2 3 1
7
14 13
18 25 5 15 13 3 3 3 2 3 100
10 2 0.6
1 24
100
ion beam. The entire Finnigan ion source assembly (source and lenses) is about 3 cm in length, an arrangement which is more compact than the H P configuration. The sector instruments used combination electron impact/chemical ionization ion sources in which the conductance out of the source was reduced by the ion exit slit. There are very large differences between the sector and quadrupole systems. Figures 2 and 3 show the spectra of DFTPP (11) and endosulfan I (121,respectively, measured on the six instruments used in this study. The tabulated spectra of the other compounds are given in Table 11. The spectrum of naphthalene (1) is not listed because this compound was not detected on any of the systems. Concentration studies did not show any striking spectral variations, although chromatographic peak patterns changed due to saturation of compounds such as DFTPP (It),hexachlorobiphenyl (5), and endosulfan I (12) a t higher concentrations. Chromatographic peaks were distorted (W-shaped) on the H P 5985B when 50 ng or more was injected. Normal peak shapes were observed under E1 conditions and when diluted samples were used. In general, the spectra show the same fragment ions. These include unusual ions such as the [M - C1+ HI- ion, seen in the spectrum of endosulfan I (12)and hexachlorobiphenyl(7). Although the appearance of specific ions shows good agreement between instruments, relative ion abundances vary. These variations occur in two areas: between the molecular ion and low mass fragment ions like C1- and between the molecular ion and other higher mass fragment ions. The most significant variation in relative abundances occurs between M- and some lower mass fragment ions. This variance is particularly apparent when the spectra measured on the HP5985 are compared with the other spectra. The abundances of C6F,- from DFTPP (1 1) and of C1- from endosulfan I (12)are significantly lower on the HP5985 (see Figures 2 and 3). Other differences include a lower abundance for the [M - COC,H,]- ion ( m / z 138) from p-nitrophenyl propionate (9) and the [M - OC4Hg- C4H9]-ion ( m / z 148) from dibutyl phthalate (8) for those spectra measured on the HP5985B. The enhancement of M- relative to C1- on the HP5985B
100
1oc
16 7 12
11 76
5 3 4
2
1
1 100
48
12OoC
MSU,
(M-Cl+H j M,\!,, 40&
-,'..I..I........
LI-
W
0
u
0
uu,
l
0
u
0
w
-
-
w
0
-,
120°c
r
u
0
r
l
0
r
u
0
-
8
20
~
p
~
K
g
g
~
M/Z
Flgure 3. Methane ECNI mass spectra of endosulfan I (12) measured on six different instruments.
was observed for all the chlorinated compounds that were measured. Figure 4 shows the relative abundance of Cl- (top)
~
;
g
786
ANALYTICAL CHEMISTRY, VOL. 60, NO. 8, APRIL
15, 1988
90 -
,
BO *
ci
"7'"
0' * 60
-
50
1-
4c
3c
20
J
-
'0
90
d
-
50 40 IU
30
FSu
uu
05u
MSU
EPA
Relative abundance of (M - H)- from diazepam (3) and pentachlorophenol (10) and of (M - CI + 0)-from 1,2,3,4-tetrachbrodibenzep dioxin (8) and 2,2',4,4',5,5'-hexachlorobiphenyl(5) as a function of instruments. Flgure 5.
20 10
0 Id
FSU
VU
CSU
EPA
MSd
ILi
FSU
OSU
MSU
1-1
12oOc
15OoC
180OC
Relative abundances of CI- and M-, as a function of instrument and ion source temperature Figure 4.
and M- (bottom) for the six chlorinated compounds, at different ion source temperatures, as a function of instrument. The relative abundances were determined from a summation of the peaks in each ion cluster. The HP5985B at Indiana University consistently shows a lower percentage of C1- and a bigger percentage of M- compared to the spectra measured on the other instruments. This has also been noted by other investigators (3). These differences in fragment ion abundance may be caused by differences in electron energy distribution, collisional stabilization, ion source temperature, or discrimination against certain ions during extraction, mass analysis, or detection. A previous study has suggested that the differences between H P and Finnigan instruments are due to differences in electron energy distributions (1);however, studies on an H P instrument have indicated that the relative abundance of chloride and other low mass ions (such as CJ?; from DFTPP) can be significantly enhanced by increasing the ion source temperature or increasing the potential placed on the ion focus (einzel type) lens (18). This latter mode of enhancement can dramatically affect the ECNI mass spectra, although the degree of enhancement is affected by the ion source temperature, sample concentration, and other lens potentiah. Low potentials favor M- abundance and yield spectra which are more easily reproduced over time. This observation suggests that the spectral differences observed here may result, in part, from differences in the transmission of ions such as C1-, in addition to variations in other ion source parameters.
Other differences in relative ion abundances are seen in the molecular ion region. For example, in the spectra of endosulfan I (12) and hexachlorobiphenyl (5), the abundance of the [M - C1 + HI- ion relative to the molecular ion varies considerably (see Figure 3 and Table 11). Since this ion is affected by changes in ion source pressure and emission current and since these are two parameters which are difficult to reproduce between instruments, this variance in relative ion abundances is not surprising. Some specific spectral differences occur because of reactions with species, such as oxygen, which may be present in the ion source. Figure 5 shows the relative abundance of [M - HIand [M - C1+ 01- ions in the ECNI spectra of compounds sensitive to these reactions. These ions are consistently higher in the FSU and EPA instruments, indicating that higher levels of oxygen are present. In addition to the ions described above, the FSU spectrum of DFTPP showed an [M + 321- ion which could have resulted from the addition of oxygen.
CONCLUSIONS This study demonstrates that the general features of ECNI mass spectra of several compounds may be reproduced on quadrupole and sector instruments; however, ion intensities are variable. In particular, differences are observed between M- and certain lower mass fragment ions formed by dissociative electron capture when the HP instrument is compared with magnetic sector and Finnigan instruments. In the molecular ion region, spectra show better agreement. Variations occur with ions, such as the [M + H - C1]- ion from endosulfan I, which are affected by temperature, pressure, and emission current. Other spectral differences result from the presence of reagent gas impurities, such as oxygen. Because ECNI mass spectra are dependent upon instrumental parameters, exact duplication of ECNI mass spectra on different instruments may be difficult to achieve. In order
Anal. Chem. 1988, 60, 787-792
to reproduce ECNI mass spectra on different instruments, an assessment must be made of the range of relative ion abundances that can be achieved, using lenses and ion source parameters, on different instruments. In addition, the presence of background impurities must be monitored and controlled. Registry No. 2, 50-32-8; 3,439-14-5; 4,38444-93-8; 5,3506527-1;6, 30746-58-8;7, 3268-87-9;8, 84-74-2;9, 1956-06-5;10, 87-86-5; 11, 5074-71-5; 12, 959-98-8; CHI, 74-82-8. LITERATURE CITED Dougherty, R. C. Anal. Chem. 1081, 53, 625-636A. Hunt, D. F.; Crow, F. W. Anal. Chem. 1978, 5 0 , 1781-1784. Oehme, M.; Stockl, D.; Knoppel, H. Anal. Chem. 1088, 58, 554-558. Stockl, D.; Budzlklewlcz, H. Org. Mass Spectrom. 1982, 17,
470-474. Chrlstophorou, L. G. EHP, Envlron. Health Perspect. 1080, 3 6 , 3-32. Chrlstophorou, L. 0.;McCorckle, D. L.; Christodoulldes, A. A. Electron Molecule Interactlons and Their Appllcations ; Chrlstophorou, L. G., Ed.; Academlc: Orlando, FL, 1984; pp 477-617. Slegel, M. W. Practlcal Spectroscopy Serles : Mass Spectrometty ; Merrit, C., McEwen, C. N. M., Eds.; Marcel Dekker: New York, 1979; Vol. 3, Part B, pp 297-306. Siegel, M. W. Int. J . Mass Spectrom. Ion Phys. 1983, 46,325-328. Sears, L. J.; Campbell, J. A.; Grlmsrud, E. P. Homed. Envlron. Mass Spectrom. 1087, 14,401-416. McEwen, C. N.; Rudat, M. A. J. Am. Chem. SOC. 1081, 103,
4343-4349. Jennlngs, K. R. Phllos. Trans. R . SOC. London, A 1070, 293,
125-133.
McEwen, C. N. Mass Spectrom. Rev. 1086, 5 , 521-547.
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RECEIVED for review August 10,1987. Accepted December 10,1987. We thank the U.S. Department of Energy (Grant No. 80EV-10449),the U.S. Environmental Protection Agency (Grant No. R808865) (R.A.H. and E.A.S.), and NIEHS (ES00040) (M.L.D. and B.A.) for support.
Electron Capture Negative Ion Mass Spectra of Halogenated Diphenylethane Derivatives E. 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 17 halogenated diphenyiethane, -ethene, and -ethanol derlvatlves (such as DDT, DDE, and dicofol) are reported. These three groups of compounds show spectral differences which reflect the nature of the base molecular structure. The diphenylethenes, where the ethene group provides conjugation between the aromatic rings, give intense molecular Ions and [M Clr Ions. Chlorine substitution on the aromatic ring Is necessary for molecular Ion stability. The dlphenyiethanes give complex spectra and weak molecular ions as a result of thelr ailphatlc character. The halogenated diphenylethanols also show significant fragmentation. Molecular ions and characterlstk bns that correspond to the dlhalobenzophenone anion are observed. The spectra of these compounds are sensitive to Isomeric substitution patterns; for example, para,pararvs ortho,para’ Isomeric pairs can be differentiated.
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Although the use of l,l-bis(4-chlorophenyl)-2,2,2-trichloroethane (DDT) (5,see Figure 1) has been banned in the United States since 1970,it continues to be a problem. Its degradation products are abundant in fish ( I ) , and atmos-
pherically transported DDT continues to be received in the U.S. and Canada from those countries that have not banned it (2). Mass spectrometry has been an important technique for that analysis and identification of DDT and related compounds. Their electron impact mass spectra have been reviewed by Hutzinger and Safe (3);in general, the molecular ions are weak, and no isomer specificity is observed (the para,para’ isomers give the same spectra as the ortho,para’ isomers). Furthermore, electron impact mass spectrometry offers no selectivity; that is, the sample matrix (for example, fish fat) ionizes just as well as the DDT-related analytes. For this reason, Dougherty et al. ( 4 , 5 )proposed the use of electron capture, negative ion (ECNI) mass spectrometry for the analysis of these compounds. With this technique, trace quantities of electrophilic compounds (such as DDT) can be detected in non-electron-capturing matrices. Dougherty et al. ( 5 )found that the ECNI spectra of DDT and related compounds were dominated by chlorine adduct ions ([M + Cll-) and that para,para’ and ortho,para’ isomers could not be distinguished from one another. Because these older data were obtained with sample sizes of 10-100 wg, we suspected that data obtained from more realistic sample sizes (0.1-10 ng) would be quite different (6). We were right. This
0003-2700/68/0360-0787$01.50/00 1988 American Chemical Society
