Characterization of polycyclic aromatic hydrocarbons by laser mass

Tracy Donovan McCarley, Robin L. McCarley, and Patrick A. Limbach. Analytical Chemistry 1998 ... Luc Van Vaeck , Herbert Struyf , Wim Van Roy , Fred A...
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1102

Anal. Chem. 1986, 58, 1102-1108

Characterization of Polycyclic Aromatic Hydrocarbons by Laser Mass Spectrometry Kesagapillai Balasanmugam, Somayajula K. Viswanadham, a n d David M. Hercules*

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260

The positive- and negative-ion laser mass spectra (LMS) of a series of polycyclic aromatic hydrocarbons (PAHs) are described. PAHs were analyzed using both the LAMMA-500 and LAMMA-1000 laser mass spectrometers. There were no significant differences In positive ion spectra obtained using the two instruments. The positive-ion LMS of ail PAHs are dominated by the odd-electron molecular ion M+-; in most cases a peak corresponding to (M i- H)' Is also observed. Addltlon of NH,CI to PAHs increases the Intensity of (M H)' relative to M'.. Ions correspondingto (M H)', (M H,)'., and (M H,)' are observed for most compounds. The poshive ion LMS of alkyl- and aryl-substituted PAHs show intense ions corresponding to loss of alkyl or aryl groups from M'.. No molecular anion was observed In the negative-ion laser mass spectrum of any PAH using the LAMMA-500. A series of PAHs were analyzed by mixing with N,N,N',N'-tetramethyl-p -phenylenediamine (TMPDA). The spectra showed no molecular anions; however, PAHs having CH,, alkyl, or CH groups show an ion corresponding to (M H)-. I n contrast, molecular anions M-e were observed in the negative-ion LMS of ail PAHs with the LAMMA-1000. Possible reasons for the differences between the two instruments are discussed.

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Polynuclear aromatic hydrocarbons (PAHs) have been studied extensively by mass spectrometry (1). Positive-ion electron impact (EI) (2), chemical ionization (CI) (3), field desorption (FD) (4),laser (LMS) (5), and field ionization (FI) (6) mass spectrometry have gained wide acceptance for the analysis of PAHs. Negative ions of fused benzenoid systems and their derivatives are known to be stable in solution. Attempts to synthesize supercharged (di-, tri-, tetraanion) PAHs have been successful. Studies of temporary anion states of aromatic compounds by electron transmission spectroscopy (7)indicate that gas-phase negative ions of benzene and related compounds are stable. Negative ion chemical ionization spectra of PAHs have been reported (8-10), where formations of M-., (M - H)-, and (MH)- anions were observed. Molecular anion formation by organic compounds in LMS is rarely seen. It is, therefore, of interest to study PAHs that produce stable anions in solutions and in the gas phase to compare with negative-ion laser mass spectrometry. Here we report characterization of a series of PAHs by both positive- and negative-ion LMS. The positive-ion LMS of all PAHs are dominated by the odd-electron molecular ion M+-. In most cases, a fairly intense peak corresponding to (M + H)+ is observed. Addition of NH&l to PAHs increases the intensity of (M + H)+ with respect to M+.. No molecular anion was observed in the negative-ion laser mass spectrum of any compound measured using the LAMMA-500. However, PAHs having CH2,alkyl, or CH groups show ions corresponding to (M - H)- when mixed with N;N,N',N'-tetramethyl-p-phenylenediamine (TMPDA). The negative-ion LMS of all PAHs obtained by the LAMMA-1000 show a molecular anion corresponding to M-.. 0003-2700/86/0358-1102$01.50/0

EXPERIMENTAL SECTION Laser mass spectra were obtained with both the LAMMA-500 and LAMMA-1000 laser microprobe mass spectrometers; both have been described elsewhere (11, 12). In the case of the LAMMA-500, a frequency quadrupled Q-switched Nd-YAG laser (265 nm, 15-ns pulse width) is focused onto the sample using one of three microscope objectives: lox, 32X, 1OOx. In most cases the 32X objective was used. Ions were accelerated (3 keV) into the drift tube of a time-of-flightmass spectrometer. The output from a 17-stage electron multiplier coupled with a transient recorder functioned as a storage buffer for selected portions of the mass spectrum. The timing sequence was triggered by the laser pulse, and in all cases, mass spectra were displayed on an oscilloscope. Power densities used were typically 106-10s W/cm2; the spot\size on the sample was approximately 2 gm. Ions produced by laser irradiation are extracted 180° to the incident beam in the LAMMA-500 (transmission mode). To obtain spectra, samples were dissolved (1mg/l mL) (or dispersed) in toluene, and 10 WLof this solution was deposited onto Formvar-filmed copper grids. In the case of the LAMMA-1000, the ion acceleration voltage was 4 keV, and the laser was focused to a spot about 5 W r n in diameter. In the LAMMA-1000, ions are extracted 45O to the incident beam and 90° to the sample surface (reflection mode). Other variables are the same as for the LAMMA-500. To obtain laser mass spectra using the LA'MMA-lOOO,lO-~L toluene solutions (1 mg/l mL) of samples were deposited on a zinc foil. The spectra reported in the figures were obtained by a single laser shot. Relative intensities reported in the tables are the averages of six to eight individual specta. Variations in the relative intensities to the extent of *15-20% are commonly noticed from shot to shot. PAHs were purchased from Chem Service, West Chester, PA, and were used without further purification. The compounds studied were (molecular weight in parentheses) anthracene (178), phenanthrene (178), anthracene-d,, (188), pyrene (2059, fluoranthene (2029, 2,3-benzofluorene (216), chrysene (228), triphenylene (228), benz [alanthracene (228),benzo[a]pyrene (252), triptycene (254), 3-methylcholanthrene (268), 1,12-benzopetylene (276), 1,2:3,4-dibenzanthracene(278), 1,2:5,6-dibenzanthracene (278), coronene (300),decacyclene (450), and rubrene (532). RESULTS AND DISCUSSION Positive-Ion Spectra. The positive-ion laser mass spectra (LMS) of anthracene obtained at three different laser powers are shown in Figure 1; peaks observed in the molecular-ion region for anthracene and other PAHs are tabulated in Table I. The major positive-ion LMS peak of anthracene near threshold energy corresponds to M+- (Figure la); other ions observed near threshold energy are C3H+( m / z 37), C6H+( m / z 61), and C7H+( m / z 85). As the laser power is increased, the intensities of carbon cluster ions such as CnH2+,C,H+, and C,+ increase relative to the intensity of M+. (Figure lb,c). When n is odd, the intensity of C,H+ is always higher than those for C,+ or C,H,+; when n is even, the relative intensities of carbon clusters (C,+, C,H+, C,H2+) vary. The absolute intensity of (M + H)+increases with laser power (Figure 1); however, the relative intensity of (M + H)+, with respect to M+-,remains essentially constant above threshold. Generally, all PAHs show an intense peak corresponding to M+.. In most cases peaks corresponding to (M - HI+, (M - Hz)+.,and (M - H3)+are observed; (M - Hz)+.is stronger 0 1986 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986

"""1

1103

Positive ion LMS

dlO-anthracene + NH4CI

dlO-anthracene

M +'

1

rl+

IL 250

b601

-

180

190

LEO

200

190

200

m/z

Plgure 2. Positive-ion LMS of (a) anthracene-d,, and (b) anthracene-dlQ and ",GI (-5 X 10' W/cm2) using the LAMMA-1000. Anthracene and NH,CI mixture was prepared by depositing NH,CI from water-methanol solution on top of anthracene.

+

85

I

0

20

L

+

+

17

-

compound may occur (Le., Ar H+ ArH+), where the protons are produced originally from the PAH. A third possibility is that transfer of a proton occurs between two PAH molecules during laser irradiation, producing a pair of ions, (M H)+ and (M - H)-. These three possibilities are discussed below. No peaks corresponding to H+ are observed in the positive-ion LMS of any PAH at laser powers where (M + H)+ is clearly seen, indicating that addition of "free" protons (Ar H+ ArH+) is unlikely. The absence of ions corresponding to (M - H)- in the negative-ion spectra of almost all PAHs studied indicates that (M H)+ production by proton transfer from a second molecule of the same PAH (pair production) is also unlikely. Ions corresponding to C,H+ and C,H2+ are observed in the positive-ion LMS of PAHs even at threshold energy, peaks at m / z 37,61, and 85 in Figure la. Thus, (M H)+ ions are probably produced by the protonation of PAHs by ions such as C,H+ and/or C,H2+. Figure 2 shows the positive-ion spectra of anthracene-dlo and a mixture of anthracene-dlo and NH4Cl in the molecular ion region. A strong peak corresponding to M+-is observed for anthracene-&, (Figure 2a); the intensities of ions corresponding to (M H)+ and (M D)+ are very small [3% (corrected for 13C)and 10% of M+., respectively]. However, the intensity of (M + H)+ ions increases tremendously relative to M+. (65% of M+ after correction for 13C contribution) in the presence of NH4Cl (Figure 2b). Similar results were obtained for 2,3-benzofluorene and 3-methylcholanthrene when mixed with either NH4Cl or H3B03. The above results demonstrate that (M + H)+ ion production in LMS is high in the presence of a proton source. Ions corresponding to H+ (m/z 1)were not observed in the positive-ion LMS of PAHs mixed with NH4Cl or H3B03. Ions corresponding to NH4+were observed; the intensities of ions corresponding to C,H+ and C,H2+ were small. These results indicate that species such as NH4+and H,BO, are probably the proton donors, supporting the argument that protonation of PAHs by proton donors is a major mechanism in LMS. A 1:l mixture of anthracene-dlo and pyrene was analyzed by use of positive-ion LMS. Ions corresponding to (M + D)+ were observed for pyrene in the mixture, in addition to M+. and (M + HI+; similarly, ions corresponding to (M H)+ were detected for anthracene-dlo, in addition to M+. and (M + D)+. Ions corresponding to (M D)+ for pyrene and (M + H)+ for

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+

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2% 020

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Figure 1, Positive-ion LMS of anthracene as a function of laser power (LAMMA-500): (a) -5.4 X lo7 W/cm2; (b) -5.3 X lo8 W/cm2; (c) -1.8 X IOe W/cm2.

than the others. Since ions corresponding to (M - H)+, (M - H2)+-,and (M - H3)+are seen for almost all PAHs, the loss of hydrogen from M+- is probably a general process leading to benzyne-type structures. An ion corresponding to (M H)+ is seen in the positive-ion LMS of most PAHs without an added proton donor (Table I). For example, the relative intensity of (M + 1)+ (with respect to M+-)in 3-methylcholanthrene is 70% (Table I); the 13C isotope contribution from M+. is only 23%, meaning that much of the (M + ')1 peak must be due to (M + H)+. Several possibilities exist for the protonation mechanism in aromatic hydrocarbons. Protonation of neutral PAH molecules may occur by reaction with carbon clusters, such as C,H+ and C,H2+ (n Z 11,which are produced from the PAH. Another possibility is that addition of a free proton to the aromatic

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ANALYTICAL CHEMISTRY, VOL.

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Table I. Positive-Ion Spectra of Some Polycyclic Aromatic Hydrocarbons" relative intensities of the ions compd (mol wt) phenanthrene (178) anthracene (178) 2,3-benzofluorene (216) benz[a]anthracene (228) chrysene (228) 3-methylchlolanthrene (268)

(M + 1)'

(M+ H)+b

59 (179) 36 (179) 33 (217) 21 (229) 20 (229) 70 (269)

44 21 14 1 0 47

Mt*

(M - H)+ (M - HJt-

100 (178) 15 (177) 100 (178) - 10 (177) 100 (216) 11 (215) 100 (228) 21 (227) 100 (228) 34 (227) 100 (268) 99 (267)

other ions (mass, re1 intens)

(M - HJ+

16 (176) 8 (176) 27 (214) 74 (226) 92 (226) 22 (266)

00 (175) 00 (175) 2 (213) 16 (225) 12 (225) 29 (265)

(M - CHd)+. (252, loo), (M - CH,)+./(M + H CH4)+(253, 87)

1,12-benzoperylene (276) 1,2:3,4-dibenzanthracene(278) 1,2:5,6-dibenzanthracene(278) coronene (300) decacyclene (450)

37 (277) 36 (279) 24 (279) 38 (301) 00 (451)

13 12 00 12 00

100 (276) 100 (278) 100 (278) 100 (300) 100 (450)

19 (275) 37 (277) 12 (277) 21 (299) 00 (449)

23 (274) 79 (276) 52 (276) 28 (298) 64 (448)

00 (273) 19 (275) 00 (275) 8 (297) 00 (447)

(M - 2H,)+. (454, 32),

rubrene (532)

00 (533)

00

58 (532)

00 (531)

00 (530)

00 (529)

(M - Ph)+*(455, IOO),

-

(3 - 3Hz)+ (252, 12) (M - 2Ph)+ (378, 67) (M 3Ph) (301, 12)

"Spectra were obtained just above threshold power; intensities of carbon clusters are small under the experimental conditions chosen and they are not tabulated; ion masses are given in parentheses. *Corrected for 'Y! contribution. POSITIVE

ION

LMS

NEGATIVE

400

ION LMS

501 rubrsna

3-methylcholonlhrene

b (M-cH.,J+'

M+.

252

268

24

mV 200

25

300

4 00

5 00

250

260

270

Figure 3. Positive-ion LMS of (a) rubrene (-5 X IO7 W/cm2) (b) 3-methylcholanthrene (-5 X to7 W/cm2)(LAMMA-500). anthracene-dlo were hardly observed for these compounds when run separately. Carbon cluster ions such as Cn+,C,H+, and C,D+ were observed for the mixture; the intensity of C,H+ ions was always higher than that of C,D+. Similarly, for both anthracene-dlo and pyrene, the inbnsity of (M + H)+ ions was always higher than that of (M + D)+. Thus, the relative intensities of the (M + H)+ and (M + D)+ ions tracked the relative intensities of C,H+ and C,D+ ions for the anthracene-dlo-pyrene mixture. No peaks were observed in the spectrum corresponding to H+ or D+ ions for the mixture. These results strongly support the suggestion that (M + H)+ and (M D)+ ions are produced by protonation of the aromatic hydrocarbons by C,H+ and C,D+ ions, respectively. Although the experiments do not rigorously eliminate other processes, collectively the data strongly support the idea that protonation of aromatic hydrocarbons is caused by reaction with carbon cluster ions. The positive-ion spectra of typical alkyl- (3-methylcholanthrene) and aryl (rubrene)-substituted PAHs are shown in Figure 3. Intense peaks are observed for ions corresponding to loss of alkyl or aryl groups from M+- for these compounds. In the case of rubrene (Figure 3a), loss of one phenyl group from M+. leads to the base peak at m / z 455. Loss of another phenyl group gives the peak at m/z 378; loss a third phenyl group gives only a very weak peak at mlz 301. The base peak in the positive-ion LMS of 3-methylcholanthrene (Figure 3b) corresponds to M+. ( m / z 268); the strong peak at m / z 252

+

48

IO

2

m/z

11

Figure 4. Negative-ion LMS of anthracene obtained using the LAMMA-500 (-2.8 X lo8 W/cm2). corresponds to loss of methane from M+. (Figure 3b). As observed for other PAHs, ions corresponding to (M + H)+ ( m l z 269) , (M - H)+ ( m / z 267), (M - Hz)+,( m / z 266), and (M - 3H)+ ( n / z 265) are also observed. Loss of aliphatic hydrogens probably accounts for the high relative intensity of (M - H)+ in 3-methylcholanthrene. Loss of CH, and CH3 from (M + H )' and M+., respectively, is also seen at m / z 253 (Figure 3b). Positive-ion chemical ionization spectra of PAHs using methane as the reagent gas show formation of abundant (M H)+ions and less significant M+. in the molecular ion region (13). Mixed charge exchange chemical ionization ( 1 4 , 1 5 ) of PAHs (MCECI) using a methane-argon mixture as the reagent gas show (M + H)+ and M+. ions in greater abundance. Thus, the results from LMS are qualitatively comparable to those of MCECI. The molecular ions in MCECI are produced by a charge exchange process between Ar+. and the PAH, whereas in LMS, M+. ions are probably formed by photoionization (16, 17). However, (M + H)+ ions in these two ionization methods are produced by ion/molecule reactions with proton donors such as CH5+in MCECI and C,H+ in LMS. Negative Ion Sgectra. Experiments with the LAMMA500. Figure 4 shows the negative-ion LMS of anthracene obtained at moderate laser power density; it is typical of spectra obtained for all PAHs using the LAMMA-500. At threshold laser power, only carbon clusters such as C4H-, C5-, C6H-, and C8H- are observed. A t high laser powers, carbon cluster ions Cn-, C,H- of higher order (n = 2-12) are observed;

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986

a

1

Table 11. Negative-Ion Laser Mass Spectra of Some Polyaromatic Hydrocarbonsa,* mol wt ( M - H ) -

compound

anthracene pyrene

1 01 0

I

'

5

10

15

20

25

LASER POWER DENSlTYlxlO 'W/Cm

b I

2,3-benzofluorene triphenylene benzo[a]pyrene tryptycene 1,l-binaphthyl 3-methylcholanthrene 1,12-benzoperylene

atlve-ion LMS of anthracene as a function of laser power (LAMMA500): (a) C,H-, n = odd; (b) C,-/C,H-, n = even.

however, no molecular anion M-. was observed for any aromatic hydrocarbon studied at any laser power. Figure 5 shows the intensity variation of some carbon clusters as a function of laser power density. These results are typical of all PAHs studied. When n is odd, only C,- ions are observed; no ions corresponding to C,H- are detected. However, when n is even, C,H- ions are also observed; sometimes the intensity exceeds that of C,- ions. The intensity ratio of C,-/C,H- (n = even) increases with increasing laser power density (Figure 5a). At lower laser power density the C;/C,Hratio is less than unity, indicating preferential formation of C,H- to C; near threshold energy for even values of n. The negative-ion LMS of other PAHs show similar results, i.e., only carbon clusters over a wide range of laser power densities. The observation of intense C,H- ions only for n = even can probably be explained by the stability of the acetylene-like structures, which are possible for these ions; the structure of C,- is not clear. Nfl,"fl'-Tetramethyl-p-phenylenediamine (TMPDA) is known to form donor-acceptor (electron-transfer) complexes with aromatic compounds (17). In an attempt to produce molecular anions from aromatic hydrocarbons, PAHs were mixed with TMPDA, and LMS were obtained using the LAMMA-500. No molecular anions were observed for any PAH studied. However, the mass spectra of all PAHs containing either CH, or aliphatic CH groups showed anions corresponding to loss of a proton, (M - H)-. Carbon clusters such as C,- and C,H- were observed, in addition to (M - H)ions. Similar results were obtained for tryptycene, 2,3benzofluorene, and 3-methylcholanthrene. It is clear that

178 202 216 228 252

254

254 268 276 1,2:3,4-dibenzanthracene 278 300 coronene

22 100 62c 0.0 0.0 60 0.0 0.0 0.0 0.0 0.0

M-.

(M + H)-

9 11 38 100 24e

40e 45 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

100 100 100 75c 25c 100

"All relative abundances are expressed as percentage of the base peak. bPeaks below m / z 5 (M - H)- are carbon clusters. 'Base peak corresponds to carbon clusters in the lower mass ranges. TMPDA acts as a base in the LMS experiment rather than producing molecular anions by electron transfer. The basic behavior of TMPDA is confirmed by the presence of peaks corresponding to (M H)+ for TMPDA in the positive-ion LMS of the mixture of TMPDA and tryptycene. The positive-ion spectrum of TMPDA alone shows only M+. ions in the molecular ion region. Thus, attempts to produce molecular anions through complex formation and subsequent electron transfer were not successful. However, the results show that TMPDA is a potent base under LMS conditions causing ionization of aliphatic protons; the observation of (M - H)ions for the above three compounds may be accounted for by the presence of weakly acidic hydrogens (18) in these molecules. TMPDA is known to form proton transfer complexes with aromatic compounds having acidic hydrogens in solution (17) but is also known to form electron donor-acceptor complexes as well. The above results indicate preference for proton transfer over electron transfer. The reason for the more rapid kinetics for the proton transfer reaction may be related to the fact that TMPDA is a very strong base in its lowest electronically excited singlet state. Experiments with the LAMMA-1000. Although no significant differences were observed between the positive-ion spectra obtained by use of the LAMMA-500 and the LAMMA-1000, the negative-ion LMS of all aromatic compounds obtained using the LAMMA-1000 showed ions corresponding to M-., in contrast to the LAMMA-500. The results obtained for the negative-ion LMS of PAHs using the LAMMA-1000 are tabulated in Table 11. Figure 6 shows the LAMMA-1000 negative-ion LMS of 1,12-benzoperylene, 1,2:3,4-dibenzanthracene, and benzo[a]pyrene as typical examples. All compounds show ions corresponding to M-a; in few cases, M-is the base peak at threshold laser power densities (Table 11). The above results clearly indicate that molecular anion formation takes place to a significant extent in the LAMMA-1000 but not in the LAMMA-500. Possible causes for the difference between the two instruments are discussed below. Three possible mechanisms can be envisioned for the formation of molecular anions on laser irradiation: (1) electron transfer between neutral PAH molecules and carbon cluster ions (M + C,-. M-.* + C,, where M--* represents an excited molecular anion); (2) attachment of an electron having energy greater than the electron affinity of the neutral PAH, followed by stabilization of the excited mdecular anion; (3) resonance electron capture of thermolized electrons. Charge transfer between PAHs and carbon cluster ions is a likely mechanism for molecular anion production, since carbon cluster ions are observed even at threshold laser power densities in both instruments. Detection of molecular anions

+

LASERPOWER DENSITY(~IO'W/~ I Flgure 5. Intensity variation of carbon clusters observed in the neg-

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986

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a

N E G A T I V E ION LMS I21

uw a 7 8 112 -BLNZOPLRYLEM

b 121

128 145

I I

1,~~,4~l8ENZANTHRACENE

short-lived < T~ < lom6s) (these can be stabilized in high-pressure swarm experiments); (3) long-lived ( 7 , < s) (these can be conveniently studied with conventional time-of-flight mass spectrometers). Thus, if molecular anions are extremely short lived, they cannot be detected; long-lived molecular anions can be detected easily using the time-of-flight analyzer. 7,7,8,8-Tetracyanoquinodimethane(TCNQ) forms a very stable anion because of the presence of four cyano groups in the molecule; it is probably one of the most stable organic anions known. The negative ion (M-.) of TCNQ was detected as base peak in LMS using either the LAMMA-500 or the LAMMA-1000. This result strongly suggests that highly stable organic ions can be detected by either instrument and that lack of M-e in the LMS of PAHs obtained with the LAMMA-500 must be due to ion instability in that instrument. If the anions produced in LMS are moderately short lived, the stability of a molecular anion will be related to the process by which it is formed. Stabilized molecular anions can be produced by the processes shown below: (a) the capture of an electron by charge transfer between M(PAH) and C,(carbon cluster ions) with a third body taking up the excess energy M C,- A -+ M-- C, A*

+

+

+

A = third body (b) the capture of an electron by charge transfer between M and C,- with vibrational excitation of the molecular anion and its subsequent stabilization by collision with another species M + C,-- [M-*]* C, A M-. C, A*

I

M278

+

0

r

+

I

C

produced by charge transfer will be related to the anion lifetime ( T J ; the molecular anion must be stabilized (i.e., M-.* must be deactivated) to be detected. Since molecular anions are observed only using the LAMMA-1000, production of M-. in that instrument requires some stabilizing process that is not operative in the LAMMA-500. The lifetime (7,) of an anion is an important factor in detecting that ion using a time-of-flight analyzer. The lifetime of a negative ion depends on the energy of the electron attaching to the molecule and the processes available to dissipate excess energy. Depending on the magnitude of T,, three classes of negative ions have been distinguisted (19): (1)extremely short-lived < r, < s) (these are observed in electron scattering and/or dissociative studies); (2) moderately

+

+ +

In these processes a third body takes up the excess energy of the electron transfer process; such processes require relatively high pressure (20). If the molecular anions are produced by the above processes in the LAMMA-1000, this would require a mechanism that would allow for more collisions between species in the LAMMA-1000 than in the LAMMA-500. In the LAMMA-500 the laser strikes the sample normal to the sample plane, on the side away from the ion lens, and penetrates the sample; the ions are extracted 180" to the incident beam. Ions produced during the initial part of the laser interaction ( 10 ns) move away from the ion lens of the mass spectrometer, since they are ejected before the laser penetrates the sample. Movement of products from the target in the direction away from the ion lens is facilitated by the high-pressure gradient between the liquidlike interaction region and the spectrometer vacuum. A simple illustration of this process is shown in Figure 7a. Only ions produced in the final stage (Le,, after penetration-stage 3) of the laser interaction will move toward the ion lens. Thus, there is dual expansion of the products in LAMMA-500 toward and away from the analyzer. The expanding plume from the laser produces a pressure defined as recoil pressure (21); part of the plume will be under this pressure. However, in the case of the LAMMA-500, the recoil pressure developed prior to laser penetration will be directed away from the ion lens and, in essence, would not last long enough so that species in the back part of the plume could be detected by the time of flight (TOF) analyzer. Effectively, the number of collisions between the species in the laser plume seen by the LAMMA-500 will be low (weak interaction), and the molecular anions produced in the LAMMA-500 will not be collisionally stabilized. Therefore, such ions will dissociate and will not be detected by the TOF analyzer. In the case of the LAMMA-1000 the laser strikes the sample 45O to the sample plane put on the same side as the ion lens, as shown in Figure 7b. Thus, the plume produced by the laser expands toward the ion lens during the entire interaction N

Flgure 6. Negative-ion LMS of (a) 1,12-benzoperylene, (b) 1,2:3,4dibenzanthracene, and (c) benzo[a]pyrene (-7 X 10'to 5 X lo7 W/cm2)(LAMMA-1000).

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ANALYTICAL CHEMISTRY, VOL. 58, NO. 6, MAY 1986

a

a 4001

I

lncidrnl /beern low

1107

NEGATIVE ION LMS

I

MM178

me

ANTHRACENE

I

200 73

Lamma

m'

170

500

97

b

I.

0.

I

3

\--

b

mv

bachtng

NEGATIVE ION LMS I21

Ill

I31

Lamma 1000

25

Figure 7. Illustration of the movement of laser-produced plume (a) in the LAMMA-500 and (b) in the LAMMA-1000.

I

@ 0

0

Mw:202 WRENE

process (i.e,, there is no dual expansion). Part of the laser plume near the sample surface will be under the high recoil pressure developed by plume expansion. This high pressure region will ensure multiple collisions between species in the plume, i.e., long interaction time. Thus,moderately short-lived anions can be collisionally stabilized in the LAMMA-lo00 and molecular anions (M-.) observed for the PAHs. Two other possible methods of anion formation are (1)the attachment of high-energy electrons to neutral PAHs, followed by stabilization of the activated molecular anion and (2) resonance capture. The two likely sources of electrons in LMS are the metal support for the sample and the electrons formed by photoionization of PAHs, i.e.

M

hr

M+. + e-

The electrons formed by photoionization will have energies greater than 1 eV because photoionization of a PAH in LMS using 265-nm radiation will require at least two photons (-4.7 eV each), since all PAHs studied have ionization potentials ca. 7 eV. The electron affinity of PAHs is relatively low, generally less than 1 eV (22, 23). Thus, if the electrons involved in molecular anion formation have energies higher than the electron affinities of PAHs, autodetachment of electrons (24) from anions is likely. Similarly, photodetachment is a factor that may limit molecular anion formation. However, autodetachment and photodetachment should probably affect molecular anion formation in both instruments to the same extent, since the same laser is used at approximately the same power density; it should produce similar environments with respect to electron density and the number of photons available for photodetachment. However, stabilization of excited molecular anions by collision is possible in the LAMMA-1000, as discussed earlier. Thus, this process might also be contributing to the molecular anion formation in the LAMMA-1000 but not in the LAMMA-BOO. Alternately, the collision processes available in LAMMA-1000 could reduce the energy of the electrons; resonance capture of the thermal electrons by the neutral PAH might also lead to the formation of molecular anions in LAMMA-1000.

201

I

0

Figure 8. Negative-ion LMS of (a)anthracene and (b) pyrene (-5 X I O 7 W/cm2),obtained using the LMMA-1000.

Needless to say, the above ideas represent proposals for explaining the differences in spectra observed between the two instruments; we feel that they are the most reasonable of the simple explanations that could be offered. We recognize that they probably represent an oversimplification of what is a complicated process and that it clearly needs further investigation. Investigations are currently under way in our laboratory in an attempt to clarify additional details of the mechanism. Other Negative Ions. Figure 8 shows the negative-ion LMS of pyrene (mol wt 202) and anthracene (mol wt 178) obtained near threshold energy. Both compounds show ions corresponding to (M + H)- and (M - H)-, which are more intense than M-.. The observation of ions corresponding to (M D)- and (M - D)- in the negative-ion LMS of anthracene-d,, indicates that the protons involved come from the compound itself. The peak at m/z 166 for anthracene (Figure 8a) can be rationalized as the fluorene anion. Ions such as (M + H)- and (M - H)- are observed in the negative ion chemical ionization mass spectra of PAHs; (M + H)- and (M - H)- are proposed to be produced by an ion/molecule reaction between H- and M and OH- and M, respectively (9). The absence of H- and OH- ions in the negative ion LMS of PAHs excludes an ion/molecule reaction between M and H-/OH- in LMS. The most likely process by which (M + H)- and (M - H)are produced in LMS is reaction between M and C,H-/C,as follows:

+

M

M

+ C,H-

e (M

+ H)- + C,

+ C,H-/C,- * (M - H)- + C,H,/C,H

(1) (2)

1108

Anal. Chem. 1986, 58, 1108-1112

This is supported by the observation of ions corresponding to C,H- and C,- a t threshold energy; the intensity of C,Hions is always higher than that of C-, near threshold. These reactions also can account for the observation of ions corresponding to (M - H)- for 2,3-benzofluorene and tryptycene which have acidic hydrogens (17), as discussed earlier.

ACKNOWLEDGMENT We wish to thank K. Jordan and F. Novak for helpful discussions. Registry No. Anthracene, 120-12-7; pyrene, 129-00-0; 2,3benzofluorene, 243-17-4; triphenylene, 217-59-4; benzo[a]pyrene, 50-32-8; tryptycene, 477-75-8; 1,l-binaphthyl, 604-53-5; 3methylcholanthrene, 56-49-5; 1,12-benzoperylene, 191-24-2; 1,2:3,4-dibenzanthracene, 215-58-7; coronene, 191-07-1.

LITERATURE CITED (1) Bartle, K. D.; Lee, M. L.; Wise, S. A. Chem. SOC. Rev. 1981, 10, 113-158. (2) Stenhagen, E.; Abrahamsson, S.; McLafferty, F. W. "Registry of Mass Spectral Data"; Wiley: New York, 1974; Vol. 1-4. (3) Hunt, D. F.; Stafford, G. C.; Crow, F. W.; Russel, J. W. Anal. Chem. 1976, 48,2098-2105. (4) Barofsky, D. F.; Barofsky, E.; Held-Aigner, R. Adv. Mass Spectrom. 1978, 7 ,109-116. (5) Balasanmugam, K.; Viswanadham, S. K.; Hercules, D. M. Anal. Chem. 1983, 55, 2424-2426.

(6) Scheppele, S. E.; Grizzle, P. L.; Greenwood, G. J.; Marriot, T. D.; Perreira, N. B. Anal. Chem. 1976. 48, 2105-2113. (7) Burrow, P. D.; Ashe, A. J.; Bellville, D. J.; Jordan, K. D. J. Am. Chem. SOC. 1982, 104, 425-429. (8) Buchanan, M. V.; Glerich, G. Org. Mass Spectrom. 1984, 19, 486-489. (9) Gehme, M. Anal. Chem. 1983, 55, 2290-2295. (10) Lida, Y.; Daishima, S. Chem. Lett. 1983, 273-276. (11) Kaufmann, R.; Hiilenkamp, F.; Weschung, R. G. Med. Prog. Techno/. 1979, 6 , 109-121. (12) Hercule, D. M. Pure Appl. Chem. 1983, 55, 1869-1885. (13) Lee, M. L.; Novotny, M. V.; Bartle, K. D. "Analytical Chemistry of Polycyclic Aromatic Compounds"; Academic Press: New York, 1981; pp 269-278 - - - - . -. (14) Lee, M. L.; Hites, R. A. J . Am. Chem. SOC. 1977, 99,2008-2009. (15) Arsenault, G. P. J. Am. Chem. SOC. 1972, 94,8241-8243. (16) Brocklehurst, 6.; Gibbons, W. A.; Lang, F. T.; Porter, G.; Savadatti, M. I . Trans. Faraday SOC. 1966, 62, 1793-1801. (17) Alchala, A.; Tamir, M.; Ottolenghi, M. J. Phys. Chem. 1972, 76, 2229-2235. (18) Popl, M.; Dolansky, C.; Mostecky, J. J . Chromatogr. 1974, 91, 649-658. (19) Chrlstophorou, L. G.; Grant, R. W. Adv. Chem. Phys. 1977, 36, 413-520. (20) Chrlstophorou, L. G.; Goans, R. E. J. Phys. Chem. 1974, 60, 4224-4250. (21) Knox, B. E. Oyn. Mess Specfrom. 1971, 2, 61-96. (22) Pelllzzari, E. D. J. Chromatogr. 1974, 98,323-361. (23) Sowada, U.; Hoiroyd, R. A. J . Phys. Chem. 1981, 85, 541-547. (24) Budzikieicz, H. Angew. Chem., I n t . Ed. Engl. 1981, 20, 624-637.

RECEIVED for review June 21, 1985. Resubmitted December 9, 1985. Accepted December 9, 1985.

Ion Yields of Impurites in Gallium Arsenide for Secondary Ion Mass Spectrometry Yoshikazu Homma* and Tohru Tanaka

NTT Electrical Communications Laboratories, Musashino-shi, Tokyo 180, J a p a n

Relative secondary Ion yields In GaAs are measured for posltive Ions of 11 elements and negative Ions of 10 elements Ar', and C s' bombardment. I t was found that the under 02+, Ion ylelds of negative molecular ions consisting of hnpurlty and matrix elements under Cs+ bombardment are relatively hlgh compared to the monatomlc Ions for 11, 111, and I V group elements in the periodic table. The use of these molecular ions for elements having low electron afflnity in comhatlon with monatomlc Ions for elements having hlgh electron afflnity makes it possible to obtain high sensltivity detection of a varlety of elements. For posltlve secondary ions, Impurity ion yields can be Interpreted by the local thermodynamic equllibrlum (LTE) model. The relative Ion ylelds for 44 elements were calculated by using the LTE model.

Although secondary ion mass spectrometry (SIMS) is a very high sensitivity technique for both surface analysis and bulk analysis, serious problems arise when quantitative measurements are carried out with this technique. Since secondary ion yields strongly depend on the chemical state of the ion bombarded surface, the ion yield of a given element varies from material to material. Therefore, highly accurate quantitative analysis cannot be performed without using calibration standards for each material. With the advancement of the GaAs crystal growth and device fabrication technologies, the quantitation of SIMS analysis has been required for impurities in GaAs crystals. To

perform quantitative analysis of GaAs impurities, ion-implanted GaAs samples ( 1 , 2 )or heavily impurity-doped GaAs samples (3) whose impurity concentrations are determined by chemical analysis have been used as calibration standards. In 1984 a round robin study was held in Japan with the participation of SIMS analysts, spark source mass spectrometry (SSMS) analysts, and chemical analysts using heavily doped GaAs crystals (4).Reliable standard GaAs samples for SIMS were obtained as a result of this study. From a fundamental viewpoint, accurate ion yield data are desirable for the interpretation of the ionization mechanism in semiconductors. Although the ion yields of pure elements (5) and impurity elements in steels (6) and glass and silicate (7) have been intensively studied, few systematic measurements of secondary ion yields in semiconductors have been reported. This is due to a lack of reliable standards for semiconductors. Leta and Morrison's work (1)reporting the use of the ion-implanted standards has been the most intensive report yet published with respect to semiconductors. However, their work lacks transition-metal data and information regarding negative ion yields under cesium ion bombardment, both of which are important in GaAs impurity analysis. Therefore, the round robin GaAs standard samples will offer useful data concerning secondary ionization. In this study, we will present the secondary ion yield for various elements in GaAs by using the standardy samples obtained through the round robin study. We also test the validity of the thermodynamic model based on the SahaEggert equation proposed by Andersen and Hinthorne (8,9)

0003-2700/86/0358-1108$01.50/00 1986 Amerlcan Chemical Soclety