2346
Anal. chem. 1988, 6 0 , 2346-2353
(16) Day, R. J.; Forbes, A. L.; Hercules, D. M. Spectrosc. Lett. 1981, 74, 703-727. (17) Conzemius. R. J.; Zhao, S.; Houk, R. S.; Svec, H. J . Int. J . Mass Spectrom. Ion Phys. 1984, 67, 277-292. (18) Brunnix, E.;Rudstam, 0. Nucl. Instrum. Methods 1981, 73,131-140. (19) Hardin, E. D.; Fan, T. P.; Blakely, c. R.; Vestal, M. L. Ana/. &em. 1984, 56, 2-7.
(20) Busch, K. L.; Unger, S. E.; Vincze, A.; Cooks, R. 0.; Keough, T. J . Am. Chem. SOC. 1982, 704, 1507-1511.
for review May 18, 1987. Resubmitted May 21, 1988. Accepted August 1, 1988.
RECEIVED
Ion-Molecule Reactions in the Negative Ion Laser Mass Spectra of Aromatic Nitro Compounds Somayajula K. Viswanadham and David M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Emanuel M. Schreiber, Robert R. Weller,' and C. S. Giam Department of Industrial Environmental Health Sciences. Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania 15261
Negative Ion laser mass spectra of many aromatk nitro compounds show formatlon of an Ion correspondingto (M 0 H)- formed by an ion-molecule reaction. I t was establlshed that Interaction of NO,- wlth neutral molecules Is responslble for formation of (M 0 H)-, The presence of substituents like -CI and -SOs- in aromatic nltro compounds leads to formation of (M 0 C1)- and (M 0 SO,Na)-, respectively. The presence of labile hydrogens (-OH, -COOH, -NH-) causes Intense (M H)- peaks, suppressing (M 0 H)- ion formatlon. Use of defocused laser condltlons (30 pm above the sample) Is essential to obtaln reproducible production of (M 0 H)-. Formation of molecular anlons (We)was observed when the laser was focused directly onto thln (-0.1 pm) flims. The Intensity of the molecular antons was highly sensitive to both laser focus posltion and sample thickness.
+ -
+ + -
+
-
-
-
+
+ -
Polycyclic aromatic nitro compounds (nitro-PAH) have been found in air particulates ( I ) , diesel exhaust particles (2), carbon black (31, and in the atmosphere (4). Most analytical methods used to identify nitro-PAHs involve extraction followed by chromatographic analysis (2,5,6). Due to low levels of nitro-PAHs in environmental samples, in situ analysis is desirable. Laser mass spectrometry (LMS) has evolved as a powerful technique (7-11) for the analysis of nonvolatile and thermally labile organic compounds directly from solids without complicated sample preparation. The microprobe capability (12) of LMS (95% of the d3 compound. Samplesfor LAMMA analysis were prepared by dissolving the appropriate compound (-0.5 mg/mL) in a solvent (methanol or acetone); 5-10 pL of the solution was placed on a metal support, and the solvent was allowed to evaporate. Compounds 1,2,5,
0003-2700/8~/0360-2346$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
6, and 7 were volatile in the LAMMA-1000vacuum, so they were run on a liquid nitrogen-cooled sample stage (22). Other compounds were mounted on the conventionalLAMMA-1000sample holder. Negative ion laser desorption ion cyclotron resonance studies were performed by using a Nicolet FTMS-lo00 Fourier transform mass spectrometer (23) equipped with a 5-cm cubic cell and a 2.9-T superconducting magnet. Perfluorotributylamine served as the mass calibrant for all mass ranges. Laser desorption/ionization was performed with a Lambda-Physics EMG 101 eximer laser, using C02as the active medium and having a frequency of 10.6 pm (pulse width 1 ps). Samples were deposited on a probe tip and mounted on the end of a solids probe and introduced into the vacuum through an air lock. The spot size of the laser beam at the probe tip was 3.5 mm; the irradiance used was approximately lo6 W/cm2. High-resolution spectra were obtained with the FTMS-1000 by using standard heterodyne techniques, where the ion signal is mixed with an appropriate reference signal and filtered prior to digitization. Typically 64K data points were collected and zero filled prior to transformation. Kinetic studies on the ion-molecule reactions observed were also performed with a Fourier transform ion cyclotron resonance mass spectrometer. The FTMS-1000 was equipped with a 3.00-T superconductingmagnet and a 5-cm mesh, low noise analyzer cell operating at a trapping potential of -2 V. However, a 5-eV pulsed electron beam (5 ms) was used to produce negative ions for these studies. Samples were introduced in quartz capillaries mounted on a direct insertion inlet probe.
RESULTS AND DISCUSSION General Fragmentation Patterns. The ions observed in the negative ion laser mass spectra (LMS) of the aromatic nitro compounds studied (1-18) and their relative intensities are given in Table I. All spectra reported were taken with the h e r focused in front (-30 pm) of the sample surface. No odd-electron molecular ions (M-*) were observed for any compound under these conditions. Common fragment ions observed were CN- (mlz 261, CNO- (mlz 421, C3N- (m/z 501, C3NO- ( m / z 661, and C5N- ( m / z 74). These ions are commonly observed for compounds containing nitrogen and oxygen atoms and are not of diagnostic value. Characteristic ions formed in the negative ion LMS of the nitro aromatic compounds correspond to NO2- (mlz 46)) (M - NO)-, and (M + 15)-. The LMS of 9-nitroanthracene (3) is shown in Figure 1 as a typical example. The (M - NO)ions must be formed by rearrangement of the nitro group. A reasonable process is initial rearrangement to a nitrite (Ar0-N=O) followed by elimination of 'NO. Such rearrangements have been reported for nitro-substituted aromatics in electron impact mass spectrometry (24)(both positive and negative), photolysis (25),and thermal decomposition (26). Stabilization of the phenoxide ion by delocalization of its negative charge (27)into the aromatic ring is the driving force for the formation of (M - NO)-. The two fragment ions m/z 46 (NO2-) and (M - NO)- are typical of all simple nitro-substituted compounds studied. Observation of NO, and (M NO)- ions, along with the possible presence of CN-, C3N-, C3NO-, and CsN- in negative ion LMS, constitutes a reasonable method for identifying aromatic nitro compounds. Two compounds showed intense peaks other than those cited above. An intense (M - H)- peak was observed for 2,2'-dinitrobiphenyl (lo), along with a peak corresponding to (M - H20)-'. Also, N,N-dimethyl-3,4-dinitroaniline (15) showed (M - CH& as its base peak, along with a peak corresponding to (M + 0 - C2H5N)-. Three compounds showed (M - H)- peaks at ca. 10% of base: 2,9, and 16. The latter also showed (M + H)-. Ion-Molecule Reactions. The negative ion LMS of all aromatic nitro compounds studied showed formation of an ion corresponding to (M + 15)- (Table I), in addition to the fragment ions discussed above. In the case of 1,3,5-trinitro-
2347
1O(
mV
M.W: 223
(M-NO:
193
50
0 0
40
100
200
0
m/z Flgure 1. Negatlve ion LMS of 9-nitroanthracene with the laser focus -30 pm in front of the sample.
benzene (11) (MW 213) the (M + 15)-ion corresponds to a peak a t mlz 228. The position of this peak is shifted to mlz 230 (2 m u higher) in the case of 1,3,5-trinitrobenzene-2,4,6-d3 (12). This observation clearly indicates that formation of (M + 15)- involves removal of an aromatic hydrogen; i.e., the (M + 15)- ion corresponds to (M 16 - H)-.This implies that (M + 15)-ion formation in aromatic nitro compounds involves substitution of an oxygen atom for a ring hydrogen, probably via a nucleophilic substitution as suggested earlier by our group (19). High-resolution mass spectra could unequivocally establish the molecular formula of the ion at (M + 15)-. However, such high-resolution capability is not available on the LAMMA1000 used in the present study. Therefore, high-resolution studies were carried out on compounds 14 and 15 by using a Fourier transform mass spectrometer (FTMS-1000) equipped with a pulsed C02 laser (10.6 pm) for ionization (23). Formation of (M + 15)- ions was observed for both 2,3,5trinitronaphthalene (14) and 3,4-dinitro-N,N-dimethylaniline (15) in FTMS; the FTMS spectrum of compound 15 is shown in Figure 2 as an example. High-resolution spectra for the (M + 15)- ions of 14 and 15 gave peaks at m/z 278.0049 (calcd value for CI0H4N3O7is mlz 278.0086; error is 13.2 ppm) and a t m/z 226.0464 (calcd value for C8H8N305is mlz 226.0442; error is 9.8 ppm), respectively. These formulas are consistent with substitution of an oxygen atom for a ring hydrogen, supporting studies on the d3 compound reported above. I t is interesting to compare the FTMS data for 15 from Figure 2 and those for LMS from Table I. The (M - NO)peak at m / z 181 is intense in both cases, and the (M 0 H)- peak a t m / z 226 is clearly evident. However, a peak a t mlz 211 due to M-' is quite prominent in the FTMS, as is a peak a t m / z 256 corresponding to (M + NO2 - H)-; both are absent in LMS. It is not possible to determine from our experiments whether the difference between the two is due to the lasers used (IR vs UV), or their pulse width used, or a combination of them. However, it is clear that such comparisons represent interesting possibilities for future studies. The possibility of an impurity, corresponding to a higher nitro analogue, resulting in formation of a fragment ion having
+
+
2348
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
z
h
3
4 u
mm
dt-
E
.I
cN .l
Q,
0
3e
f
e 0 a
F;
2349
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988 100
H i
0
+
4
.,
i"
h
M-'
50
1 II 1
I,,! I,. ,,,,I
0 100
150
200
,
,,,
,
250
,
I , , , , , , , , I , , , , , , , , , I , , , , , , , ' , , I ,,.-
500
950
formation corresponding to the elimination of either substituent with the addition of oxygen was observed, in addition to (M + 0 - H)- (Table 11). For example, 4-chlor0-3~5-dinitrobenzamide (22) shows ions corresponding to (M 0 C1)- and (M + 0 - CONHJ- at m/z 226 and 217, respectively. In the case of sodium 2,4,6-trinitrosulfonate (23), the base peak in the negative ion spectrum corresponds to elimination of the -SO3- group with attachment of oxygen, (M + 0 S03Na)- at m / z 228. Other prominent ions in the LMS of 23 are due to the 2,4,6-trinitrosulfonate anion at mlz 292, NO;, and CN-. Mechanism of (M + 0 - H)-Formation. The formation of (M + 0 - HI- in the negative ion LMS of aromatic nitro compounds can result from reaction between ions derived from the nitro compound and the neutral molecules of the same compound. Several mechanistic possibilities were suggested in our preliminary communication (19).
0
450
400
m/z
Figure 2. Negative ion FTMS of N ,Ndimethyl-3,4dinitroaniline obtained with a pulsed CO, laser for Ionization. 50
ArH
I 35
I
CI
02Nc3N02 I
mV
MW:202
I 0
50
100
150
200
240
-/z
Figure 3. Negative ion LMS of 2,6dinitrochlorobenzene.
an m l z value equal to (M + 0 - H)-, must be considered. For example, a 1,2,3,5-tetranitrobenzene(MW 258) impurity in 1,3,5trinitroben~ene(11) might eliminate NO (a facile process in LMS), resulting in an ion at mJz 228; the m l z value would exactly match that of the (M + 0 - H)- ion from 1,3,5-trinitrobenzene. The purities of compounds 5,11, and 14 were checked by thin-layer chromatography,E1 mass spectrometry, proton NMR, and HPLC. None of these techniques showed any detectable impurities above 0.1'70. Repeated recrystallization of compounds 5, 11, and 14 did not result in any change in the relative intensities of the (M + 0 - H)- ions for these compounds, within experimental error. These observations and the consistent observation of (M + 0 - H)- in all compounds listed in Table I rule out the possibility that an impurity might be the origin of the (M 0 - H)- ions in the nitro-PAHs studied. Table I1 summarizes LMS data for five substituted nitro compounds. Two chloro-substituted nitro aromatic compounds, 19 and 20, showed formation of ions in LMS corresponding to (M - 19)-, in addition to the peak at (M + 15)-. This is seen by the peak a t m l z 183, shown in Figure 3 for compound 20. Absence of isotopic peaks characteristic of a chlorine atom indicates that the ion at m / z 183 corresponds to (M + 0 - Cl)-; formation of (M + 0 - C1)- ions seems to be a facile process for chlorine-substituted aromatic nitro compounds. When two different ring substituents are present in the nitrobenzene derivatives, as in 21 and 22, negative ion
+
ArH
ArH
+
0-•
+
0;'
+
NO;
-
-
-
CArNH3-*
0 '
I-* \02 3\NO2
-
ArO-
+
H*
(1)
ArO-
+
*OH
(2)
ArO-
+
HNO
(3)
These mechanisms involve the attack of 0-*,02-' or ,NOz- on the aromatic ring, resulting in the formation of an intermediate that loses 'H, 'OH, or HNO, respectively, to yield the phenoxide ion. The molecular anions' reacting with neutrals, leading to phenoxide ion formation,is also a possibility. These ion-molecule reactions could readily occur in the high-pressure region adjacent to where the laser interacts with the sample surface, as proposed by Hercules and co-workers (8). This region is best thought of as a rapidly expanding gas, going from near-liquid to vacuum in several cLm; it should readily give rise to chemical reactions. Nucleophilic substitution and adduct formation reactions involving the three "reagent" ions of eq 1-3 are known for a variety of substrates in negative ion chemical ionization (28-38) and atmospheric pressure ionization (39) mass spectrometry. No peaks corresponding to 0- or 0,- were observed in the negative ion LMS of any nitro compound studied at the laser power densities used for the experiments reported here; NO, ions were observed in nearly every case. Ions due to (M + NO2- H)- were seen for compounds 14 and 15 in their negative ion FTMS spectra obtained with the COz laser, as illustrated in Figure 2. Lack of 0-' or 0,' ions and the observation of NOz- and its adduct ions (M + NO2 - H)-• indicate that reaction 3 is the most likely mechanism, even though other reactions cannot be ruled out on the basis of the absence of the above ions. To obtain further evidence for the involvement of NO2- ions in the formation (M + 0 - H)-, a gas-phase kinetic study was done on 2,6-dinitrochlorobenzene (20) using the FTMS-1000 under E1 conditions. This study involved initial generation of negative ions (M-', (M - NO)-, NO-, and C1- for 20) from the target compound by a pulsed E1 source; the ions were trapped in the ion cyclotron resonance (ICR) cell while a continuousbackground pressure (-5.0 X lo* Torr) of neutral molecules was maintained. E1 ionization was used because this method is known not to produce any ions from ionmolecule reactions in the pressure range used for these experiments. The ions generated in the cell were detected after varying (60 ps to 1s) delay times. Collisions inside the cell between the primary ions and neutrals should result in the formation of ion-molecule reaction products as a function of reaction time (delay before detection). The intensities (as a percentage of total ion current) of all ions formed were
2350
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
Scheme I
2,6 -Dl NITROCHLORO BENZENE NO;
1 0060
100
IONS PRESENT I N THE CELL
' 200
300
. 500
400
600
700
900
800
1000
m Sec D E M Y BEFORE DETECTION
Flgwe 4. Intensity of negative ions from 2,6dinitrochlorobenzene as
a function of delay time before detection on FTMS NIEI.
(Intensities
as percentage of total ion current).
I
2,6--DlNITROCHLORO B E N Z E N E NO- Ions o b s e n t in t h e c e l l .
00
$ '"1
-Y-
&,-------
__zc..
s+ No; O T
-'
. I .
lu.o-cli c
'
L--. 0060
100
:
200
_t
e---
c__-
300
400
500
600
700
800
900
1000
m 5eu
3 e ay
I -e
b e f o r e detect
0"
F W e 5. Intensity of negative ions from 2,6dinitrochlorobenzene after ejecting NO, ions from the cell as a function of delay time before detection on FTMS NIEI (intensities as percentage of total ion current).
monitored as a function of reaction time in the ICR cell. Figure 4 shows a plot of these intensities for 20 as a function of reaction time before detection. It is clear that the intensity of the (M 0 - C1)- ion increases with increasing delay time, reaching a maximum at a delay time of ca. 0.5 s; there is a corresponding decrease in the intensity of NO< ions in the same period. Reaction times longer than 0.5 s did not show any appreciable change in the intensities of these two ions. This observation clearly indicates direct involvement of NO, ion in the formation of (M + 0 - C1)- as proposed in mechanism 3. Figure 4 also shows a decrease in the intensity of C1- ion with increasing reaction time in the range 60 ~s to 100 ms. A corresponding increase in the intensity of (M - NO)- is noticed in the same time period. Beyond a reaction time of 100 ms, there is no appreciable change in the intensities of these two ions as a function of reaction time. These observations suggest the following parallel reaction for 20: M C1- = [M'Cl]-- (M - NO)- NOCl (4) which seems to achieve equilibrium after a reaction time of 100 ms. The relationship between C1- and (M - NO)- ions is independent of the reaction involving NOz- ions (vide infra, Figure 5). No significant change was observed in the intensity of M-' ion as a function of delay time in this time period, indicating that the molecular ion is neither a reactant nor product ion of these reactions. Double resonance experiments (40) also were performed on 2,6-dinitrochlorobenzeneby using FTMS uner negative ion
+
+
+
electron ionization (NIEI) conditions. Primary negative ions M-', (M - NO)-, NOz-, and C1- were obtained from 2,6-dinitrochlorobenzene by a pulsed E1 source (60 ms) while a frequency pulse was applied at the ICR resonance frequency of NOz-. This has the effect of ejecting NOz- ions from the cell as they are formed, minimizing their opportunity to react with 2,6-dinitrochlorobenzene neutrals. All other ions were trapped in the cell, and the kinetic study of Figure 4 was repeated. Figure 5 shows a plot of ion intensities (as a percentage of total ion current) as a function of delay before detection. It is clear from Figure 5 that ions corresponding to (M + 0 - C1)- at m / z 183 are absent when NO2- ions are ejected from the cell; however, the reaction proposed in eq 4 proceeds. The observations presented here qualitatively support reaction 3; namely, the formation of (M + 0 - C1)- by the reaction of NOz- ions with neutral 2,6-dichlorobenzenemolecules. Similarly these experiments support nucleophilic substitution by NO2- ions as the general mechanism for the formation of (M 0 - H)- in the LMS of nitro-substituted compounds. One can generalize the mechanism as follows: NO2- ArX ArO- NOX (5)
+
+
-+
+
where X = H, C1, SO3-, -COOMe, CONH2. The elimination of C1 and SO3- at intensities comparable to the elimination of 'H (Figure 3) raises question about splitting out molecular HNO in a single step, as suggested in reaction 3, as does the observation of (M + NOz - H)- in Figure 2. The reaction between NO2- ions and neutral aromatic compounds may proceed through formation of a Meisenheimer type intermediate, as has been proposed for gas-phase nucleophilic reactions of alkoxide ions with fluorobenzenes (41) and fluoride ion reactions with pentafluorophenylethers (42). The Meisenheimer complex could lose HNO in a single step or lose 'H and 'NO successively in a stepwise process, as shown in Scheme I. Elimination of H' by the attack of nucleophiles 0-*,'OH, and O2-* with a variety of organic compounds is known (43) in the gas phase, even though such reactions involving NOz- ions were not reported. A signal due to the Meisenheimer intermediate ion (M + NO& is not seen in the laser mass spectra of the aromatic nitro compounds studied. However, an ion corresponding to the elimination of 'H from (M + NO& ion leading to (M + NOz- H)-' at m/z 256 is noticed in the laser desorption FTMS spectrum of compound 15 (Figure 2). In order to obtain support for the occurrence of a stepwise mechanism in the formation of (M + 0 - H)-, as proposed in Scheme I, the (M + NO2- H)-• ion of compound 15 was subjected to collision induced dissociation. The (M + NOz - H)' ion was isolated in the FT-ICR cell by ejecting all other ions from the cell with standard double resonance techniques and was excited to a value below 700 eV before introduction of argon collision gas by a pulsed-valve inlet system. ions formed in the cell due to collisions were then detected. The collision induced dissociation spectrum of (M + NOz - H)-' thus obtained showed formation of an (M + 0 - H)- ion at
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
2351
+
m / z 226 directly from the (M NOz - H)-• ion, supporting the stepwise mechanism proposed for its formation. This, however, does not rule out parallel elimination of HNO in a single step. The phenoxide ion formed after elimination of HNO from the Meisenheimer type complex is highly stable (44) due to resonance stabilization of the negative charge on the oxygen atom. The presence of nitro groups either ortho or para to the oxygen further stabilizes the phenoxide ion. This greater stability of the nitro-substituted phenoxide is probably the driving force for the elimination of aromatic hydrogens for the formation of (M + 0 - H)- in nitro aromatic compounds. Substituent Effects on the Formation of (M 0 - H); To determine the effect of substituents on the formation of (M + 0 - H)- in aromatic nitro compounds, the negative ion LMS of compounds 24-35 were studied. All compounds have a group containing a labile hydrogen atom, such as -OH, -COOH, or -NHz. The labile group is substituted directly on the aromatic ring for all compounds except 28, where it is isolated by the methylene group. The ions observed for these compounds and their relative intensities are given in Table 111. Elimination of a labile proton in the nitro-substituted phenols 24-27 and 29-31 leads to formation of the phenoxide ion (M - H)-. An electron-withdrawing nitro group on the aromatic ring enhances phenoxide ion formation by increasing the proton acidity. Thus, loss of an acidic proton becomes a major ion formation pathway in the nitro-substituted phenols, anilines, and benzoic acids. Formation of (M + 0 - H)- was not detected for any compound in groups 24-27 and 29-31. However, when the -OH group is separated from the aromatic ring by a methylene bridge as in 28, (M + 0 H)- ion formation is observed. Formation of (M - H)- by elimination of a hydroxyl hydrogen in 28 cannot resonancestabilize the resulting anion. It is clear that there is competition between ion-molecule and proton-transfer reactions in aromatic nitro compounds; it is also clear that the latter type is preferred in compounds having acidic protons (phenols, carboxylic acids). As the acidity becomes weaker, ion-molecule reactions appear (benzylic protons) and finally dominate for protons of very low acidity (aromatic ring hydrogens). The competition appears to be between a reaction whose rate is independent of proton acidity (ion-molecule reaction), and one that depends highly on proton acidity (proton ionization). This is consistent with a pseudounimolecular reaction for (M - H)- formation and a classical binary collision for ion-molecule reactions. An interesting fragmentation process is observed in the negative ion LMS of compound 28. This involves elimination of Hzfrom (M 0 - H)-, M-', (M - NO)- and (M - NOz)-. The mechanism for loss of Hz from the (M + 0 - H)- ion, resulting in a stable ion "a", is shown in Scheme 11. The nitrobenzoic acids 32-35 show ions corresponding to (M - H)-. Elimination of COz from (M - H)- results in the base peak for most of these compounds. No ion corresponding to (M + 0 - H)- is observed for any nitrobenzoic acid. Electron capture negative ion spectra of the isomeric nitrobenzoic acids show differences in fragmentation of the molecular anions (45, 46). o-Nitrobenzoic acid eliminates NO to yield an intense (M - NO)- ion that decomposes (45) by loss of COz. simultaneous two-stage cleavage processes (M - NO)- and (M - HN02)- occur for the meta and para isomers, respectively (46). Such differences were not observed in the
+
+
00
m/z Figure 6. Negative ion LMS of g-nitroanthracene with the laser focused on the sample surface.
negative ion LMS of the isomeric nitrobenzoic acids, making them qualitatively indistinguishable with laser ionization. Nucleophilic substitution reactions resulting in formation of the (M + 0 - HI-ion in negative ion LMS also are observed for nitro-substituted heterocyclic compounds. Formation of (M 0 - H)- was observed with high intensity for 6- and 8-nitroquinolines and 5-nitro-l,lO-phenanthroline, as shown in Table IV. The presence of labile hydrogens in 4-nitroimidazole (38) and 5-nitroimidazole (40) leads to formation of (M - H)- ions; (M + 0 - H)-is absent in these cases. Other common fragment ions in the heterocyclic compounds include (M - NO)-, NOz-, and CN-. The above observations indicate the generality of the fragmentationprocess in aromatic nitro compounds upon laser irradiation. Formation of ions corresponding to (M - NO)-, NOz-, and CN- can be used as an aid in identifying the aromatic nitro compounds upon laser irradiation. Formation of (M - H)- is common if functional groups containing labile hydrogens are present on the aromatic ring, while (M + 0 X)- ions are commonly seen in the absence of such labile groups. Effect of Irradiance and Spot Size on the Sample. All spectra discussed above were taken with the laser focused approximately 30 pm above the sample surface (defocused mode); formation of M-' is not seen under these conditions. However, the aromatic nitro compounds studied (1-18) show formation of molecular anions to varying degrees when the laser is focused directly on the sample surface. Figure 6 shows the negative ion LMS of 9-nitroanthracene (3, MW 223) taken when the laser was focused on the sample. Formation of the molecular anion (M-') is clearly evident; its intensity is very sensitive to sample thickness and to the position of the laser focus with respect to the sample surface. Molecular anions can be seen for these compounds only when the laser is tightly focused on the sample surface to a spot size of 4-6 pm and only when the sample layer is less than 0.1 pm thick. Slight defocusing of the laser or a thick sample layer significantly decreases formation of the molecular anion. Molecular anion formation was not observed for the same nitro compounds when the LAMMA-500 instrument was used (19). The primary difference between the LAMMA-500 and
+
2352
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
8
woo
hl
m
8
f
c-
r3
mm
mw
cy m
m
n
h
c - c -
m
m
d
C
m
m
0
2
oowm mt-m
m
z m
L O P -
N c u
N 00
Q,
cu
0 -
nn
N
n
n n* n
In n
ANALYTICAL CHEMISTRY, VOL. 60, NO. 21, NOVEMBER 1, 1988
LAMMA-1000 is the way in which the laser impinges the sample. In the LAMMA-500 the laser is focused on the back of the sample, while the ions are extracted from the front, normal to the surface. Samples for the LAMMA-500 must be thin enough for the laser to penetrate. In contrast, in the LAMMA-1000 the ions are extracted normal to the surface from the same side that the laser impacts. This difference in the geometry of the instruments could be aiding the formation of M-’ with the LAMMA-1000 only, as discussed in detail in an earlier communication (47). However, the observation that molecular anions are seen only under focused conditions with the LAMMA-1000 could be explained in the following simplified explanation. When the laser impinges a thin organic sample supported on a metal foil, the beam penetrates the sample to the metal surface, ejecting electrons from the metal. The energies of these electrons are moderated by collisions within the “plume” produced by laser irradiation under these focused conditions. Capture of these low-energy electrons (- 1 eV) by neutral nitro-substituted aromatic compounds is possible, leading to formation of molecular anions. When the sample thickness is greater than the ablation depth of the laser (-0.1 pm), no electrons are produced from the metal surface, and thus no molecular anions are formed. Negative ion laser mass spectra of nitroaromatics studied by using quartz substrate did not show the formation of molecular anions under any conditions, which is in agreement with the suggested explanation. It is recognized that this represents an oversimplificationof what is a complicated process and that it clearly needs further investigation. Electrons produced by photoionization of the nitro compounds to form positive ions are not considered in the model proposed. It has been our experience that such a process is not very important for molecular anion formation in the LMS of organic compounds (47).Another mechanism has been suggested (47)to explain the formation of PAH molecular anions involving charge transfer between C; (carbon cluster ions formed in LMS) and the neutral PAH, with a third body taking up excess energy. Another possibility is initial formation of vibrationally excited molecular anions by charge transfer with subsequent stabilization by collisions in the plume. Similar mechanisms cannot be ruled out for the formation of molecular anions of nitro-PAHs. The sample preparation method most commonly used in LMS involves evaporating a few drops of sample solution on a metal support. With this sample preparation method, most nitro compounds studied tend to form thick crystalline particles on evaporation of the solvent, as seen by microscopic examination. Due to formation of an uneven surface on sample evaporation, the spectrum obtained will be a convolution of spectra at various laser focus positions with respect to the sample surface. Thus, the reproducibility of molecular anion formation is very poor, even when the laser is focused on the sample and (M + 0 - H)- was commonly observed in these aromatic nitro compounds.
ACKNOWLEDGMENT We thank Robert B. Cody and James A. Kinsinger of Nicolet Analytical Instruments for providing the FTMS spectra of some of the nitro aromatic compounds obtained with COz laser ionization. Thanks also are due to Peter Sturrock of Georgia Tech for the sample of some nitrophenols and to A. G. Sharkey for useful discussions. Registry No. 1, 98-95-3;2,86-57-7;3,602-60-8;4,5522-43-0; 5, 528-29-0; 6, 99-65-0; 7, 100-25-4; 8, 602-38-0; 9, 605-71-0; 10, 2436-96-6; 11,99-35-4; 12,14702-07-9;13,2243-95-0;14,87185-24-8; 15,35998-95-9;16,610-39-9;17,602-01-7;18,81-20-9;19,2578-45-2; 20, 606-21-3; 21, 2552-45-6; 22, 20731-63-9; 23, 5400-70-4; 24, 88-75-5; 25,100-02-7;26,88-74-4;27,100-01-6;28,79544-31-3; 29,
2353
577-71-9;30,573-56-8; 31,66-56-8; 32,528-45-0; 33,552-16-9; 34, 121-92-6;35,62-23-7;36,613-50-3; 37,607-35-2; 38,3034-38-6; 39, 4199-88-6; 40, 6146-52-7; NO,, 14797-65-0.
LITERATURE CITED (1) Jager, J. J. Chromatography 1978, 152, 575-578. (2) Schuetzle, D.; Lee, F. S.-C.; Prater, T. L.; Tejada, S. B. Int. J . Environ. Anal. Chem. 1981, 9 , 93-144. (3) Fitch, W. L.; Evenhart, E. T.; Smlth, D. H. Anal. Chem. 1978, 50, 2122-21 26. (4) Oehme, M.; Mano, S.;Stray, H. HRC CC, J . High Resolut. Chromatatogr. Common. 1982, 5 , 417-423. (5) Ramdhal, T.; Kueseth. K.; Becher, G. HRC CC, J. High Resdut. Chromatogr. Chromatatogr. Commun. 1982, 5 , 19-26. (6) Ramdhal, T.; Urdar, K. Anal. Chem. 1982, 5 4 , 2256-2260. (7) Denoyer, E.; van Grieken, R.; Adams, F.; Natusch, D. F. S. Anal. Chem. 1982, 5 4 , 26A-41A. (8) Hercules, D. M.; Day, R. J.; Balasanmugam, K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982. 5 4 , 280A-305A. (9) Heinen, H. J. Int. J . Mass Spectrom. Ion Phys. 1981, 38, 309-322. (10) Hercules, D. M. Pure Appl. Chem. 1983, 55, 1869-1885. (11) Conzemius, R. J.; Capellan, J. M. Int. J . Mass Spectrom. Ion M y s . 1980, 34, 197-271. (12) Spurny, K. P.; Schormann. J.; Kaufmann, R. Fresenius’ 2. Anal. Chem. 1981, 308, 274-279. (13) Wechsung, R.; Hlllenkamp. F.; Kaufmann. R.; Nltsche, R.; Unsold, E.; Vogt, H. Microsc. Acta, Suppl. 1978, No. 2, 281-296. (14) Bowie, J. H.; Nissuey, B. Org. Mass Spectrom. 1972, 6 , 429-442. (15) Bowie. J. H. Mass Spectrom. Rev. 1984. 3 , 161-207. (16) Budzikiewicz, H. Angew. Chem., Int. Ed. Engl. 1981, 2 0 , 624-637. (17) Bowie, J. H.; Stapleton, B. J. Austr. J . Chem. 1975, 2 8 , 1011-1015. (18) Stapleton, B. J.; Bowle, J. H. Org. Mass Spectrom. 1976, 1 1 , 429-435. (19) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1983, 55, 2424-2426. (20) Heinen, H. J.; M e r , H.; Vogt, H.; Wechsung, R. Int. J . Mass Spectrom. Ion Processes 1983, 47, 19-22. (21) Bunal, E.; Symons, E. A. Can. J. Chem. 1968, 44, 771-774. (22) Novak, F. P.: Hercules, D. M. Presented at the Pittsburgh Conference and Exposition, New Orleans Feb 25-Mar 1, 1985; Abstract 366. (23) Wilkins, C. L.; Weil, D. A.; Yang, C. L. C.; Ijames, C. F. Anal. Chem. 1985, 57, 520-524. (24) Bowle, J. H.; Blumenthal, T.; Walsh, 1. Org. Mass Spectrom. 1971, 5 , 777-783. (25) Iokl, Y. J. Chem. SOC.,Perkin Trans. 2 , 1977, 1240-1242. (26) Hoch, S. W.; Schollkopf, V. Justus Liebigs Ann. Chem. 1978, 1823- 1834. (27) Sykes, P. A G M e Book to Mechanisms in Organic Chemistry, 4th ed.; Longman: London, 1975; p 60. (28) Jennlngs, K. R. I n Mass Spectrometry; Specialist Periodic Reports: Johnstone, R. A. W., Ed.; The Chemical Soclety: London, 1977; Vol. 4, pp 211-215. (29) Levonowich, P. F.; Tannenbaum, H. P.; Dougherty, R. C. J . Chem. Soc., Chem. Commun. 1975, 597-598. (30) Hunt, D. F.; Harvey, T. M.; Russel, J. W. J. Chem. Soc., Chem. Commun. 1975, 151-152. (31) Yinon, J.; Harvan, D. J.; Hass, J. R. Org. Mass Spectrom. 1982, 17. 321-326. (32) Bowie, J. H.; Stapleton, B. J. Aust. J. Chem. 1977, 30, 795-800. (33) Bouma, W. J.; Jennings, K. R. Org. Mass Spectrom. 1981, 16, 33 1-335. (34) Bruins, A. P.; FerrerCorrela, A. J.; Harrlson, A. G.; Jennlgs, K. R.; and Mitchum, R. K. Adv. Mass Spectrom. 1978, 7A, 355-358. (35) Hunt, D. F.; Mcewen, C. N.; Harvey, T. M. Anal. Chem. 1975, 47, 1730-1734. (36) Dougherty, R. C.; Dalton, J.; Biros, F. J. Org. Mass Spectrom. 1972, 6 , 1171-1181. (37) Hunt, D. F.; Stafford, G. C., Jr.; Crow, F. W.; Russel, J. W. Anal. Chem. 1976, 48, 2090-2105. (38) Grimsrud, E. P.; Chowdhury, S.; Kebarle, P. Int. J. Mass Spectrom. Ion Processes 1986, 6 8 , 57-70. (39) Dzidlc, I.; Carroll, D. I.; Stillwell, R. N.; Hornlng, E. C. Anal. Chem. 1975, 47, 1308-1312. (40) Camisarow, M. 6.; Grassi, V.; Parisod, G. Chem. Phys. Lett. 1978. 5 7 , 413-416. (41) Briscese, S. M. J.; Riveros, J. M. J . Am. Chem. SOC. 1975, 9 7 , 230-231. (42) Ingemann, S.;Nlbberlng, N. M. M.; Sullivan, S. A,; Dupuy, C. H. J. Am. Chem. SOC. 1982, 104. 6520-6527. (43) Harrison, A. G. Chemical Ionization Mass Spectrometry; CRC: Boca Raton, FL, 1983; p 79. (44) March, J. Advances in Organic Chemistry, McGraw Hill: New York, 1977; p 241. (45) Bowie, J. H. Org. Mass Spectrom. 1971, 5 , 945-951. (46) Bowle, J. H.; Hart, S. G. Int. J. Mass Spectrom. Ion Processes 1974, 13, 319-325. (47) Balasanmugam, K.; Vlswanadham, S. K.; Hercules, D. M. Anal. Chem. 1986, 5 8 , 1102-1108.
RECEIVED for review September 9, 1987. Resubmitted July 6,1988. Accepted July 25,1988. This work was supported in part by Grant CHE-8411835 from the National Science Foundation.
