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AMI. Che” 1991, 63,2064-2068
a substantial improvement of the chemical or functional group selectivity. (c) Additional Molecular Information. The added time dimension provides new and complementary functional group or atomic information that broadens the scope of selectivity of existing flame-based detectors as well as allowing for new pulsed-flame detection schemes. For example, PFPD can be used as a sensitive halogen-selective detector (6) or nitrogen-selectivedetector, as was recently found in our laboratory. Similarly, pulsed FIDs can potentially be used as a halogenselective detector due to their expected delayed action as flame retardants. (d) Lower Hydrogen Consumption. Since there is no need to hold or keep a continuous flame, the hydrogen flow can be arbitrarily reduced. Our pulsed FID typically works with 2 mL/min hydrogen flow. This flow can be further reduced by using H, as the carrier gas or at a lower repetition rate or by reducing the flame chamber volume. This hydrogen fuel saving is of special importance in field portable detectors. (e) Detection of Unseparated Mixtures and Solutions. The added dimension of time dependence allows the detection of unseparated mixtures, as demonstrated in Figures 2 and 3. The ratio between the (wavelength dependent) early hydrocarbon peak and delayed sulfur peak in time permits its quantification since the hydrocarbon peak can serve as an internal standard. The road to flame photometric detection of HPLC is now open. We believe that pulsed flame is a new technology that can be applied to all the flame-based detectors such as FID, FPD, TID, FIRED, and AA. At this early stage, the limited information gathered from the operation of our PFPD is insufficient to assess all the possibilities. However, it seems that one of its most promising directions might be the detection of SFC (7)and HPLC (8), where the added selectivity is a highly desirable feature. The spectroscopic detection of flame-generated species can also greatly benefit from the time separation of flame interferences. In addition to the usual GC detection, the reduced H2 consumption is of large importance in mobile detectors. We note that pulsed flames offer unique possibilities in combustion and flame ignition research. Laser ignition-detection schemes (9) with unequaled time resolution of up to 10 ps are estimated to be possible with
radical vibrational energy distribution information. Finally, we also note that the pulse operation mode can simplify the size and cost of flame-based detectors by reducing the size of the hydrogen bottle or generator and by replacing the complex and delicate two photomultipliers and their highvoltage power supply (in a differential FPD (IO))with a single silicon photodiode.
ACKNOWLEDGMENT The contribution of Eran Yavin to the early experiments of PFPD is greatly appreciated. This work was inspired by several stimulating discussions with Brad E. Farch and laser-pulsed flame ignition experiments performed at the US. Army BRL at Aberdeen Proving Ground and also with J. B. Morris and R. J. Locke. Registry No. S,1104-34-9;P,1123-14-0. LITERATURE CITED (1) Dressler, M. Selective Gas Chmarupaphlc Derectws: Elsevier: Amsterdam, 1986. (2) Amirav, A. Pulsed Flame Detector Method and Apparatus. Israel Patent Application No. 95617, Sept 1990. US., European, and Japan Patent application submission Aug 1991. (3) (a) Farwell, S. 0.; Gage, D. R.; Kagel, R. A. J . Chromatogr. Scl. 1981, 79, 358. (b) Farwell, S. 0.; Barinaga, C. J. J . Chrmtogr. Sei. 1988, 24, 483. (4) Patterson, P. L.; Howe, R. L.; Abu-Shumays, A. Anal. Chem. 1978, 50, 339. (5) Patterson, P. L. Anal. Chem. 1978, 50, 345. (6) Bowman, M. C.; Beroza, M. J . Cbrmtogr. Sei. lB69, 7, 484. (7) Richter, B. E.; Bornhop. D. J.; Swanson, J. T.; Wangsgaard, J. Q.; Andersen, M. R. J . Chrometcgr. Sci. 1989, 27, 303. (8) McGuffin, V. L.; Novotny, M. Anal. Chem. 1981, 53, 946. (9) (a) Morris, J. B.; Forch, B. E.; Mirlolek, A. W. Appl. Spectrosc. 1990, 44, 1040. (b) Forch, 8. E.; Mlrlolek, A. W. Combust. Fkme 1991, 85, 254. (10) Am, W. A.; Miller, 6.; Sun, X. Y. Anal. Chem. 1990, 62, 2453.
Eitan Atar Sergey Cheskis Aviv Amirav* School of Chemistry Sackler Faculty of Exact Sciences Tel Aviv University Ramat Aviv 69978 Tel Aviv, Israel RECEIVED for review January 14,1991. Accepted May 17,1991.
Preforming Ions in Solution via Charge-Transfer Complexation for Analysis by Electrospray Ionization Mass Spectrometry Sir: Electrospray (ES) ionization is rapidly developing as a method to produce gas-phase ions from analyte species in solution for subsequent analysis by mass spectrometry. The combination of ES ionization with mass spectrometry (MS), first demonstrated by Fenn and co-workers ( 1 , 2 ) ,has proven useful in the analysis of involatile, polar, and thermally labile compounds, especially high molecular weight biopolymers. Electrospray also serves to interface the mass spectrometer with a variety of liquid phase separation methods, including HPLC and CZE (see refs 3-1 for recent ESMS reviews). Electrospray ionization can be viewed as an ionization process involving two steps. First, highly charged droplets of a solution containing the analyte are dispersed a t atmospheric pressure. This usually is accomplished by application of a high potential difference (typically 3-5 kV) between a 0003-2700/91/0363-2064$02.50/0
capillary needle, through which the analyte solution is flowing at a low rate (typically 1-10 pL/min), and the atmospheric sampling aperture of the mass spectrometer, which are typically separated by 0.5-2.0 cm. This dispersal is followed by droplet evaporation and finally ion evaporation or desorption to yield gas-phase ions that can be sampled and analyzed by the mass spectrometer. While the detailed mechanism for ion evaporation or ion desorption is currently at issue (3-9,it has become clear that best ESMS results, in terms of both sensitivity and detection limits, are achieved for compounds that are already ions in solution. This is in direct analogy to FAB and SIMS where best performance is observed for preformed ions (8). Species that are ionic in solution and have been analyzed by ESMS include, for example, metal salts (I, 9-11) and organic salts (e.g., alkylphosphonium salts (12),and 0 1991 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
alkylammonium halides and alkyl sulfates (13)). Compounds with functionalities that can be ionized via solution-phase acid/base chemistry, such as carboxylic acids and tertiary amines, are also amenable to ESMS. The latter category of compounds includes peptides and proteins, which contain basic amino acid residues, and oligonucleotides,which contain acidic phosphate groups and are usually detected'as the (M + nH)"+ and (M - nNa)" species, respectively (see refs 3-7). Some polar molecules are also ionized efficiently by ES via attachment of ions other than a proton. For example, Na+ or CH3COO-ions, which are either added to or already present in the analyte solution, are sometimes observed to attach to the analyte molecules (14). It is also possible, in the negative-ion mode, to form anions from species in solution that have high electron affinities by operating at high needle voltages such that a corona discharge is formed at the needle tip (I, 15). However, gas-phase ion-molecule reactions rather than solution chemistry accounts for formation of these species. Since the applicability of ES ionization is limited to compounds that are ionic in solution or that can be ionized in solution by acid/base chemistry or by adduct formation, several important classes of compounds, such as the polycyclic aromatic hydrocarbons (PAH's), cannot, at present, be analyzed with this technique. Additionally, the ions formed in solution via acid/base chemistry or adduct formation are limited in the positive-ion mode to cationized species (e.g., (M + H)+and (M + Na)+) and in the negative-ion mode to the deprotonated molecule, Le., (M - H)-, or an adduct ion (e.g., (M + CH3COO-)-). Radical cations, M + , and radical anions, M-, which directly provide the molecular weight and might be of use in other mass spectrometric analyses, such as MS/MS (16),are typically not formed by ES ionization. Also, decomposition reactions involving the analyte, such as dehydrogenation (13,can take place in attempting to ionize certain types of compounds by adding an acid or base to solution. In this paper we demonstrate the use of charge-transfer complexation (18)to form radical cations and anions in solution from neutral compounds for subsequent analysis by ESMS. This method of preforming ions in solution has been demonstrated by both De Pauw (19)and DiDonato and Busch (20) to enhance molecular ion formation in FABMS. Charge-transfer complexes are formed by electron transfer between an electron donor (D) and electron acceptor (A), as shown in eq 1. In nonpolar solvents, charge-transfer com-
+A
-
-
(D*+)solv*te+ (A*-)solv*te (1) plexes can exist as the neutral complex (D'+A'-). In polar solvents, such as methanol, however, the complex can be dissociated, due to the solvation energy of the ions formed, into the respective cationic and anionic species (18). Since a number of compound classes show charge-transfer behavior (18),this methodology has the potential to expand the utility of ES ionization. Electron donors and acceptors are usually compounds of low ionization energy (IE) and compounds of high electron affinity (EA), relative to one another, respectively. Typical electron donors include PAH's and other aromatic species that contain electron-donating groups such as -OH, -OCH3, -N(CH3)2, and -CH3. Common electron acceptors usually contain several electron-withdrawing groups, such as -NOz, -CN, or halides. For example, the quinones p-chloranil (tetrachloro-1,4-benzoquinone) and 2,3-dichloro5,6-dicyano-l,4-benzoquinone (DDQ) are good acceptors. The data presented here, obtained for both positive and negative ions, demonstrate the utility of charge-transfer complexation for ESMS analysis of compounds that display both acid/base and charge-transfer behavior and also for the analysis of compounds that can be ionized via charge-transfer complex-
D
@*+A*-)
2065
ation but are not otherwise readily amenable to analysis by
ESMS. EXPERIMENTAL SECTION All experiments were carried out on a modified version of a Finnigan-MAT ion trap mass spectrometer (ITMS) adapted to sample from ambient air. Detailed descriptions of this ES/ITMS instrument and its operation have been presented elsewhere (17, 21,22). Continuous infusion employing a syringe pump (Harvard Apparatus, Inc., Cambridge, MA) was usually used to deliver solutions containing the analytes to the electrospray needle, held at h3 kV, depending on the mode of analysis, at a rate of 1 pL/min. The amount of material consumed to acquire the spectra shown, as quoted in the figure headings, corresponds to the amount of analyte that flowed from the ES needle during the data acquisition period. For the flow injection experiment described, the syringe pump was used to deliver solvent to the needle at a constant rate (2.5 rL/min) through a Rheodyne (Cotati, CA) Model 7520 injector with a 0.5-pL internal sample chamber and then through a short length (ca. 15 cm) of 100 pm i.d. silica capillary to which the ES needle was connected (17). All analytes were obtained from commercial suppliers and used without further purification. Stock solutions of the analytes were prepared by dissolution of small amounts (ca. 1-3 mg) of each in 25-100 mL of HPLC grade methanol or CH2Cl,. The various analyte/solvent combinations investigated were prepared from these stock solutions to give final analyte concentrations of ca. 25-150 pmol/KL. Solvent composition was typically adjusted to 50/50 methanol/ CHIClz (v/v) so as to ensure dissolution of the analytes and solvation of ions formed via charge transfer and to provide a stable spray. RESULTS AND DISCUSSION Charge-transfer complexation can be used to enhance the utility of ESMS both for compounds that can be charged in solution by acid/base chemistry or adduct formation as well for compounds that cannot be charged in this fashion. For example, aromatic amines are basic and can be protonated in solution, but many such compounds can also be ionized in solution via charge-transfer complexation (18). This is demonstrated by using the compound N,N,N',N'-tetramethyl1,4-phenylenediamine(TMPD, MW = 164). Figure l a shows the positive-ion ES mass spectrum obtained when TMPD is sprayed from a solution composed of CH2C12/methanol/acetic acid (50/50/0.5 v/v). The major ion in the spectrum is that corresponding to (TMPD + H)+,at m/z 165. Two other ions of lesser abundance, the radical cation, TMPD'+ ( m / z 164) and a fragment ion ( m / z 150), are noted in this spectrum. Addition of acid to the CHzClz/methanolsolvent system was used to enhance the abundance of the protonated molecule. However, a similar spectrum, with somewhat reduced abundance of (TMPD + H)+,is obtained even without the addition of acetic acid owing to traces of acid in the solvents used. Spraying this compound from a neutral or basic solvent system would further suppress the signal due to the protonated molecule. Experiments have shown that the fragment ion observed in this spectrum is formed upon injection of the protonated species into the ion trap (i.e., injection-induced fragmentation (see e.g., ref 17)) and is not a product of solution chemistry. Through adjustment of the voltages on the ES interface lenses and the amplitude of the rf voltage applied to the ring electrode of the ion trap during ion injection, this fragmentation is minimized. The origin of the radical cation, in the absence of a known charge-transferreagent, is uncertain but is not without precedent. We have noted, for example, the formation of radical cations from metalloporphyrins sprayed under similar conditions (17). Both the porphyrins and TMPD are characterized by relatively low ionization energies (IEs) (ca. 6.5 eV for both (23))compared with other organic molecules. Compounds with low IE's could possibly donate an electron to a solvent molecule or a contaminant in the solution that has a relatively high electron affinity or is
2068
ANALYTICAL CHEMISTRY, VOL. 63,NO. 18, SEPTEMBER 15, 1991
I'"
120
140
180
160 mlz
200
mh M+.
I+.
228
164
(b)
(b)
A 180
200
mh Figure 1. Positiveion ES mass spectra obtalned by continuous infusion of (a)a 104 pmoilpL solution of TMPD in CH,Cl,/methanol/acetic acid (50/50/0.5v/v/v) and (b) a CH,Ci,/methanol (50/50v/v) solution containing a mixture of TMPD (104 pmol/pL) and DOQ (1 13 pmoi/pL). Approximately 1.4 and 0.71 pmol of TMPD were consumed to acquire
the spectrum in (a) and (b), respectively. easily reduced. Another possible mode for formation of the radical cation might be an electrochemical oxidation in the ES needle. The positive-ion ES mass spectrum in Figure l b demonstrates that TMPD can also be ionized via charge-transfer complexation. This spectrum was obtained by spraying TMPD from a CH2C12/methanol(50/50 v/v) solution to which was added a molar amount of 2,3-dichloro-5,6-dicyano-1,4benzoquinone (DDQ) approximately equal to that of TMPD. The radical cation TMPD'+ (m/z 1641, the major ion observed in this spectrum, is formed by electron transfer from TMPD (the donor, D), to DDQ (the acceptor, A) in solution, as described by eq l. Neither the protonated molecule nor fragment ions are observed in this spectrum. (The origin of the ion of lesser abundance a t m / z 195 has not been determined, but it may be a product of the reaction of the radical cation with the solvent.) Enhancement of the signal from TMPD in the spectrum in Figure 1b compared to the signal in the spectrum in Figure l a is due in part to formation of only one ion from TMPD rather than three. Note that just 710 fmol of TMPD was consumed to acquire the spectrum in Figure Ib. On the basis of the signal-to-background of this spectrum and experimenb that used much shorter sampling times, the detection limit or "figure of merit'' (21) for analysis of TMPD employing the charge-transfer reaction was determined to be in the low-femtomole range. This level of detection compares well with the levels we have determined for a number of other compound classes on this ES/ITMS system (17, 21, 22). Moreover, this level of detection is better than we obtained when forming and detecting the protonated TMPD species. One point of emphasis is the ability to form either the protonated molecule or radical cation from a particular analyte for analysis by ESMS. Such flexibility in formation of the molecular species can be of use, for example, in MS/MS
0
2
mh FIgW 2. Posithre-btl ES specta obtained by COnthuoUS infusion of (a) a 46 pmd/& sokrtion of 2,3-benranthraceneR CH,Cidrnethanci (50/50v/v) and (b) a CH2Ci,/methanol(5O/50v/v) solution containing a mixture of 2,3-benranthracene(46 pmol/pL) and Dw (56pmol/pL). Approximately 0.82 pmoi of 2,3-benzanthracene was consumed to acquire the spectrum in both (a) and (b).
analyses. The MS/MS spectrum of M + from a particular compound can often provide structural information complementary to that from the MS/MS spectrum of (M + H)+ (16). Furthermore, formation of the molecular ion, rather than the protonated molecule or other adduct, allows for direct, unambiguous, molecular weight determination. In contrast to TMPD, and other compound classes that can be ionized in solution via acid/base chemistry, polycyclic aromatic hydrocarbons (PAHs) are not readily charged in solution. Typical compounds showing such behavior are neutral and relatively nonpolar and therefore currently not amenable to analysis by ESMS. If, however, such a compound shows charge-transfer behavior, as do the PAH's, the present method can be used to preform ions from the neutral species in solution. As an example, the ES mass spectrum in Figure 2a is that obtained from 2,3-benzanthracene dissolved in CH2C12/methanol(50/50 v/v). No ions are observed for the analyte when spraying from this solvent or when acetic acid or trifluoroacetic acid (up to 1% by volume) are added to the solvent in an attempt to protonate the molecule. Figure 2b shows the ES m w spectrum obtained for 2,3-benzanthracene after an q u a i molar amount of DDQ was added to the sample used to acquire the spectrum in Figure 2a. In this case an abundant ion at m / z 228 is observed, which corresponds to the radical cation of this PAH formed via charge-transfer complexation with DDQ. A similar spectrum, showing only the radical cation, was obtained from benzo[ghi]perylenewhen DDQ was used as the electron acceptor. However, no ions attributable to the analyte could be observed in attempting to analyze anthracene or the isomeric phenanthrene in this manner. The success or failure to ionize these PAH's via charge-transfer complexation appears to correlate with their respective IE's. That is, radical cations are observed from the compounds of lowest IE (23), viz. 2,3-benzanthracene (6.79-7.04eV) and benzo[ghi]perylene (7.15 eV), but no ions
ANALYTICAL CHEMISTRY, VOL. 63,NO. 18, SEPTEMBER 15, 1991
mlz
22
1, '30
mlz
240
250
Figure 3. Negativaion ES mass spectra obtained by continuous infusion of (a) a 113 pmoVpL sdutkn of Dw in C~Cl&neu"ethaol (50/50 v/v) and (b) a cH,c~/melhanol(50/50v/v) sdutkn containing a mixtue of TMPO (104 pmol/pL) and DOC2 (1 13 pmol/pL). Approxknately 0.77 pmol of DDO was consumed to acquire the spectrum in both (a) and (b). The solution used to acquire the spectrum in (b) was the same as that used to acquire the positive-ion spectrum in Figure lb.
are absented from the compounds of higher IE, viz.anthracene (ca 7.35-7.47 eV) and phenanthrene (7.85-8.25 eV) when mixed with DDQ. This would indicate the degree of electron transfer is too small and/or solvation of the potential ionic species is too weak for discrete ions to be formed by chargetransfer complexation for these latter two compounds with DDQ. These species and other PAHs with a relatively high IE are not, however, necessarily precluded from analysis by this method. A number of different solvent systems should be tested as well as electron acceptors of higher electron affinity. Since spectroscopic data (18) indicate that these compounds form charge-transfer complexes with a number of species, there is a good probability that a donor/acceptor pair and solvent system can be found that will facilitate formation of the discrete ionic species in solution. However, because formation of ions via charge-transfer requires the proper electron donor/acceptor pair, this method of ionization is compound specific. By changing the solution chemistry, that is, by changing the electron acceptor (or donor) added to the ES solvent, one can obtain preferential compound ionization. By the nature of charge-transfer complexation, both a positive and negative ion are formed by the reaction. Therefore, for example, formation of TMPD'+ in solution via charge transfer, as witnessed by the spectrum in Figure lb, must be accompanied by formation of the radical anion of the electron acceptor, in this case DDQ. The negative-ion ES mass spectrum in Figure 3a was obtained by spraying DDQ from a CH2C12/methanol (50/50 v/v) solution. The major ions observed is this spectrum correspond to the radical anion, DDQ' (m/z 226) and the requisite isotope peaks. The isotope peak pattern confirms that this species contains two chlorine atoms. The abundance of the signal due to DDQ' is, however, highly variable under such conditions and appears to depend
2087
on sample preparation and adjustment of the voltage on the ES needle. The origin of the other cluster of ions in this spectrum ( m / z 207) is uncertain, but the isotope pattern indicates this species contains chlorine. One possible explanation for this ion cluster is the species (DDQ - C1 + 0)-, which is known to be formed from compounds of this type in atmospheric pressure ionization sources via gas-phase ion/molecule reactions with oxygen (24). Gas-phase formation of (DDQ - C1+ 0)in this experiment would indicate that a corona discharge was operative. The presence of a discharge can be confirmed by monitoring the low-mass ions 02-,CN-, and C1- typically formed in such a discharge, but such experiments were not carried out in this case. The molecular anions of the electron-acceptor compounds 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetracyanoethylene (TCNE) were also observed when sprayed from this solvent. As with formation of TMPD'+ in the positive-ion mode in the absence of a discrete charge-transfer reagent, DDQ', TCNQ', and TCNE'- could each be formed by charge transfer with a solvent species or contaminant in the solution or possibly via an electrochemical process. The low needle voltage employed in this study (i.e. -3 keV) and the observation of decreasing molecular anion signal with increasing ES needle voltage would indicate that a corona discharge, and therefore, gas-phase ionization, might not be involved. On the other hand, if the ion cluster at m/z 207 is due to (DDQ - C1+ 0)-, some gas-phase ionization must be taking place. When an electron donor is added to a solution of DDQ, a somewhat different mass spectrum is obtained. Instead of DDQ'-, the molecular anionic species observed corresponds in mass to (DDQ 1)-( m / z 2271, as demonstrated by the negative-ion ES mass spectrum in Figure 3b. This spectrum was obtained with the same sample used to obtain the positive-ion ES spectrum in Figure Ib. Observation of the ion at m / z 227 suggests that the original molecular anion formed from DDQ by charge transfer, undergoes reactions in solution. One plausible scenario for formation of this species is reaction of the molecular anion to form the dianion which then undergoes protonation in solution, as shown in eqs 2 and 3.
+
+ -
2DDQ'DDQ2-
H+
DDQ2-
+ DDQ
(DDQ2-
+ H+)-
(2)
(3) Spectroscopic data for a similar charge-transfer reagent pair (TMPDlp-chloranil)has been interpreted as indicating formation of the dianion of the electron acceptor by disproportionation (18). The dianion formed would be expected to be quite reactive and thus easily protonated, yielding the singly charged anion. Consistent with this explanation is the fact that when TCNE, which is not a quinone (and therefore not expected to undergo the same solution reactions as DDQ), is used as the charge-transfer reagent, the molecular anion is observed. Since formation of ions in solution via charge-transfer complexation is essentially instantaneous on the time scale of these experiments, this method has the potential to be used on-line following a separation method as a selective and sensitive method to preform ions in solution for ESMS. This potential of charge-transfer complexation as a postcolumn "derivatization" reaction for ES is demonstrated by the data in Figure 4, obtained from the injection of a sample of TMPD (29 pmol) into a solvent stream containing DDQ (63 pmol/rL). Figure 4a shows the total ion current (TIC) profile and Figure 4b the extracted ion current profile for TMPD'+ ( m / z 164) obtained in this experiment. This experiment effectively simulates the elution of a separated compound (the injected species) into a reaction region containing the derivatizing reagent (the charge-transfer reagent) just prior to the ES needle. Since the reaction to form the ions is rapid, the analyte
2088
ANALYTICAL CHEMISTRY, VOL. 63, NO. 18, SEPTEMBER 15, 1991
molecular species for further analyses such as MS/MS. In addition, the rapid nature of the chargetransfer reaction offers a convenient, sensitive and compound specific method with which to ionize species on-line following a separation method for analysis by ESMS. Registry No. DDQ, 84-58-2.
,y"'] (b) 2
m h 164
1 00 2:12
50 1:lO
\
150 3:14
Scan Time (min)
/I ';''
'','.'I
120
'
'
140
: ''i.'
, '
..'!
i
140
'3
'
';' i
1Qo
260
'
i
220
mh Figure 4. (a) Total Ion current profile and (b) extracted ion current profile for m / z 164 obtained In the positive-ion mode for injection of 29 pmol of TMPD (dissolved in CH,Cl,/methanol (50/50 vlv)) into a CH,Cl,lmethanol (50150 v/v) stream, flowing at 2 pL/min, containing DDQ (63 pmoIlpL). (c) The ES mass spectrum obtained near the peak maximum (scan 83) in the extracted-ion current profile in (b).
and reagent can be mixed just prior to spraying. Also, the method should be quite selective since the signal from a particular analyte (that does not charge in solution via another mechanism) will only be observed when it can be ionized by charge-transfer complexation. As Figure 4b shows, the majority of the ion current in the TIC peak (Figure 4a) is due to eluting TMPD'+, charged in solution via interaction with the DDQ in the solvent stream. When DDQ is absent from the solvent, the molecular species is mainly (TMPD + H)'+. The ES mass spectrum in Figure 4c, obtained in the scan near the maximum of the extracted-ion current profile peak in Figure 4b, shows that the only ionic species observed from TMPD is the radical cation and, based on the signal-tobackground, much lower levels of TMPD could be detected. The data presented above demonstrates that ionization of compounds in solution via charge-transfer complexation expands the utility of ESMS. This method of preforming ions presents a sensitive and selective way in which to ionize species in solution, both for compounds normally ionized via acid/base chemistry as well as for those compounds that are otherwise not responsive in ESMS. Formation of the molecular ion rather than a pseudomolecular species allows for direct molecular weight determination as well as providing a different
LITERATURE CITED Yamashita. M.; Fenn, J. B. J. Phys. Cbem. 1984, 88, 4451. Yamashita, M.;Fenn, J. B. J. Phys. them. 1984, 88, 4471. Fenn, J. 8.; Mann, M.; Meng, C. K.; Wong, S. K.; Whilehouse, C. M. Science 1989, 246, 64. Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S . K. Mass. Spectrom. Rev. 1990, 9 , 37. Smith, R. D.; Loo, J. A,; Edmonds, C. G.; Barinaga, C. J.; Udseth, H. R. Anal. Chem. 1990, 62, 802. Huang. E. C.; Wachs, T.: Conboy, J. J.; Henion, J. D. Anal. Chem. 1990, 62, 713A. Mann, M. Org. Mass Spectrom. 1990, 25, 575. Busch, K. L.; Unger, S. E.; Vlncze, A,; Cooks, R. G.; Keough, T. J. Am. Chem. Soc. 1982, 104, 1507. Ikonomou, M. G.; Blades, A. T.; Kebarle, P. And. Chem. 1990, 62, 957. Jayaweera, P.; Blades, A. T.; Ikonomou, M. G.; Kebarie, P. J . Am. Chem. Soc. 1900, 112, 2452. Blades, A. T.; Jayaweera, P.: Ikonomou, M. G.; Kebarle, P. J. Chem. Phys. 1990, 9 2 , 5900. Udseth, H. R.; Loo, J. A.; Smith, R. D. Anal. Chem. 1989, 61, 228. Conboy, J. J.; Henion, J. D.; Martin, M. W.; Zweigenbaum, J. A. Anal. Chem. 1990, 62, 800. Covey, T. R.; Sushan, B. I.; Thomson, B. A,; Henion, J. D. proceedngs of the 37th ASMS Conferenceon Mass Spectromeby and AHM Topics, Miami Beach. FL; American Society for Mass Spectrometry: East Lansing, MI, 1989; pp 558-559. Hiraoka, K.; Kudaka, I. Rapid Commun. Mass Spectrom. 1990, 4 , 519. Busch, K. L.; Glish, G. L.; McLuckey, S.A. Mass SpectrometrylMass SpeCtron?eby;VCH Publishers: New York, 1988. Van Berkel, G. J.; McLuckey, S. A,; Glish, G. L. Anal. Chem. 1991, 63, 1098. Foster, R. Organic Charge Transfer Complexes; Academic Press: New York, 1989. De Pauw, E. Anal. Chem. 1983, 55, 2198-2199. DiDonato, G. C.; Busch, K. L. Anal. Chim. Acta 1985, 171, 233-239. Van Berkel, G. J.; Glish, G. L.; McLuckey, S. A. Anal. Chem. 1990, 62, 1284. McLuckey, S. A.; Van Berkei, G. J.; Glish, G. L.; Henion, J. D.; Huang, E. C. Anal. Chem. 1991, 63, 375. Levin, R. D.; Lias, S. G. Zonizatlon Potential and Appearance Potential Measurements, 7971- 1981. U.S. Government Prlntlng Office: Washington, DC, 1982. Dzidic, I.; Carroi. D. I.; Stillwell, R. N.; Horning, E. C. Anal. Chem. 1975, 47, 1308. * To whom correspondence should be addressed.
Gary J. Van Berkel* Scott A. McLuckey Gary L. Glish Analytical Chemistry Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831-6365 RECEIVED for review April 1, 1991. Accepted June 14, 1991. This research was sponsored by the United States, Department of Energy Office of Basic Energy Sciences, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
