1133
Anal. Chem. 1907, 59, 1133-1138 in a variety of situations to evaluate its effectiveness. For this purpose, diverse samples (bovine liver, kidney, and brains, human urine, wines, and drinking water and wastewater) were analyzed. In the analysis of biological samples (liver, kidney, brains, and urine), as well as wine samples, a preliminary step is the destruction of organic matter. From the diverse methods tested, the most adequate is the procedure described by Bajo (27) in which a mixture of concentrated nitric acid and H2O2 is used. Wines analyzed show alcoholic grades between 16 and 18% and an important sugar content (sweet wines). Wastewater collected in the Guadalhorce River (MBlaga, Spain) shows an appreciable amount of nitrite and organic matter, due to residues of diverse food industries. Results obtained in these analyses have been compared with determinations obtained by use of atomic absorption (AAS), with full agreement. The results obtained are depicted in Table 11.
ACKNOWLEDGMENT The authors thank the Comison Asesora de Investigaci6n Cientifica y TBcnica for supporting this study. Registry No. Ga, 7440-55-3; Zn, 7440-66-6; SATCH,4136111-9; HzO, 7732-18-5. LITERATURE CITED (1) Hart, M. M.; Adamson, R. H. R o c . Natl. Acad. Sci. U . S . A . 1971, 6 8 , 1623. (2) Hayes, R. C. J. Excerpta Med., Sect. 23 1970, 18,740.
Zweidinger, R. A.; Barnett, L. Anal. Chem. 1973, 45, 1563. Newman, R. A.; Brody, A. R.; Krakoff, I. H. Cancer (Philadelphia) 1979, 4 4 , 1728. Kelsen, D. P.; Alcock, N.; Yeh, S.; Brown, J.; Young, C. Cancer (phila-
dephia) 1980, 4 6 , 2009. Trace Elements in Human Health and Disease; Prasd, A. S., Oberlease, D., Eds; Academic: London, 1976; Vol. 1. Falchuck, N. H. NewEngl. J. Med. 1977, 296, 1129. UreAa, M. E.; Garcia, A.; Cano, J. M. Anal. Chem. 1985, 5 7 , 2309. Cano, J. M.; Urefia, M. E.; Garcia, A. Anal. Chem. 1986, 58, 1449. Lloyd, J. B. F. Nature (London), Phys. Sci. 1971, 231. 64-65. Lloyd, J. B. F. J. Forensic Sci. SOC. 1971, 1 7 , 83-94. Lloyd, J. B. F J. Forensb Sci. SOC. 1971, 11, 153-170. Lloyd, J. B. F. J. Forensic Sci. SOC.1971, 11, 235-253. Lloyd, J. B. F. J. Forensic Sci. SOC. 1972, 12, 83. John, P.; Soutar, I.Anal. Chem. 1976, 4 8 , 520. Vo-Dinh, T.; Gammage, R. B.; Martinez, P. R . Anal. Chem. 1961, 53, 252-258. Vo-Dinh, T.; Gammage, R. B.; Hawthome, A. R. Environ. Sci. Techno/. 1976, 72, 1297-1302. Vo-Dinh, T.; Martinez, P. R. Anal. Chim. Acta 1981, 125, 13-19. Lloyd, J. B. F. Analyst (London) 1980, 105, 97-109. Andre, J. C.; Bandot, Ph.; Niclause, M. Clin. Chim. Acta 1977, 7 6 , 55-66. Green, G. L.; O'Haver, T. C. Anal. Chem. 1974, 4 6 , 2191. Garcia Borrbn, J. C.; Escribano, J.; JimBnez, M.; Iborra, J. L. Anal. Biochem. 1982, 125, 277-285. Eastwood, D.; Fortier, S.H.; Hendrick, M. S. I n t . Lab. 1978, (July/August), 51. Robert, H. Christenson; McGiothlin, C. D. Anal. Chem. 1982, 5 4 . 20 15-20 17. MontaAa, M. T.; Gbmez Ariza, J. L.; Garda de Torres, A. A n . Quim., B 1984. 80, 129. O'Haver, T. C.; Green, G. L. Anal. Chem. 1976, 48. 312-318. Bajo, S.;Souter, U.; Aeschliman, B. Anal. Chim. Acta 1983, 749, 321.
RECEIVED for review July 8,1986. Accepted December 9,1986.
Laser Photodissociation of Gaseous Ions Formed by Laser Desorption Clifford H. Watson, Gokhan Baykut, and John R. Eyler* Department of Chemistry, University of Florida, Gainesville, Florida 32611
Both pulsed and gated contlnuous-wavecarbon dioxide lasers have been employed to desorb Ions and then to photodlssoclate them In a Fourler transform Ion cyclotron resonance mass spectrometer. Pulsed COP laser lrradlatlon was most successful In laser desorption experiments, while a gated continuous-wave laser was used for a majority of the successful Infrared multiphoton dissociation studles. Fragmentatlon of Ions with m / r values In the range 400-1500 was Induced by Infrared multiphoton dissociation. Such photodlssoclatlon was successfully coupled with laser desorptlon for a number of dlfferent classes of compounds. Either two sequential pulses from a pulsed carbon dloxlde laser (one for desorption and one for dlssoclatlon) or one desorption pulse followed by gated contlnuous-wave lrradlatlon to bring about dlssoclatlon was utilized.
The technique of laser desorption (LD) has been widely used in mass spectrometry (1, 2) to desorb and ionize high molecular weight or other nonvolatile samples, most often using time of flight (3-5) or Fourier transform ion cyclotron resonance (FTICR) (6-8) mass spectrometers for mass analysis. A sample is inserted into the vacuum system of a
mass spectrometer where it is desorbed and ionized by highly focused laser irradiation, most often with a power density of at least IO8 W/cm2. The primary advantage of laser desorption is that abundant molecular or pseudomolecular ions are produced for many different classes of compounds. Positive pseudomolecular ions are most often formed by attachment of a cation, typically a proton, potassium ion, or sodium ion, to the parent molecule. Negative pseudomolecular ions can also be formed by laser desorption, usually by loss of a proton. Often little fragmentation occurs and the strong molecular ion signal (M+, (M + H)', (M + Na)+, (M K)', M-, or (M - H)-) provides molecular weight information. Although an abundant molecular ion peak is important in identifying the molecular weight of a compound, fragmentation is often desirable to characterize the molecular and/or ionic structure. Several techniques have been applied to achieve fragmentation; most notable are collision-induced dissociation (9,lO) and photodissociation (11, 12). Thus, it would be advantageous to combine one of these techniques with laser desorption to obtain both molecular weight and structural information. One possible drawback of collisioninduced dissociation is difficulty in dissociating larger ions (13-15) because of the inability to impart sufficient internal
0003-2700/~7/0359-1133$01.50/0 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59,
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laser Figure 2. Configuration used for single laser experiments wlth a pulsed GO, laser. During the first laser pulse the beam was focused through the lens and formed ions by laser desorption. A rotatable mirror directed the second, unfocused laser beam into the cell to fragment trapped ions by multiphoton dissociation.
energy to them in collisions with much lighter target gases. Also, energy sufficient to dissociate a smaller ion could be “lost”, that is, randomized along the many vibrational modes of a significantly larger ion resulting in a lifetime before dissociation which is longer than the ion’s transit time in a conventional mass spectrometer. However, irradiative pumping of moderately large molecular compounds by infrared lasers has been observed to bring about photofragmentation. In this paper the successful combination of the laser desorption and photodissociation (PD) techniques is reported.
EXPERIMENTAL SECTION Experiments were carried out in a Nicolet FT/MS-1000 mass spectrometer. The important features of this technique have been described elsewhere (16-21). The mass spectrometer was operated in the “broad-band” mode which corresponds to a mass range 17-3000 daltons. Ions were detected by the standard frequency “chirp” excitation method. Because of the high ionization efficiency of laser desorption, only one ion formation/detection event was performed for each spectrum obtained. No signal averaging was necessary. Typically, for each spectrum 16 384 data points were acquired and this data set was zero-filled once prior to performing the Fourier transform. The number of data points was increased when higher resolution was required. The electron beam voltage and current were optimized for each electron impact spectrum. No electron beam was required in t h e laser desorption experiment. A typical event sequence shown in Figure 1illustrates the LD/PD experiment. The standard laser desorption interface supplied by the manufacturer was used. As shown by the solid line in Figure 2, light from a Lumonics TE 860 grating-tuned pulsed COz (infrared)
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
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Flgure 4. (Top) Ion/molecule product formed by reaction of cisdlchloro-transdlhydroxobis(2-propanamine)platinum(IV)wlth its 50-eV electron Impact fragments. (Bottom) Fragmentation pattern produced upon irradiatbn wlth CW COPlaser: same experimental condltions and delay times as In the top spectrum.
photofragments originating from some unknown source. Samples were obtained from commercial sources where available or submitted from various research laboratories. Sample purity was confirmed by wide mass range spectra and the samples were used without further purification.
RESULTS AND DISCUSSION Laser Desorption with the (Gated) Continuous-Wave Laser. Initial attempts to perform LD with the CW laser were of limited success. Although K+ and (M K)+ from a sucrose sample were observed, low signal-to-noise (SIN) ratios and lack of reproducibility hindered these experiments. A 25-ms gated pulse from the CW COz laser used for LD had a focused power density of ca. 2 X IO3 W/cm2. This is substantially below the currently accepted lower limit (23)for reliable LD results. It was observed, however, that reasonably intense and reproducible ion signals could be obtained for preformed ions by using the gated CW laser. For example, both the organic cation and perchlorate anion of rhodamine-6G perchlorate were observed in positive and negative LD mass spectra, respectively. Laser Desorption with the Pulsed Laser. The pulsed laser had sufficient power to produce abundant ions and substantial molecular ion intensity has been observed in approximately 90% of samples analyzed in this laboratory to date. Excellent S / N ratios and fair reproducibility were usually obtained. Photodissociation of Large Ions. Isomeric differentiation studies by photodissociation (22)and comparison of photodissociation with collision-induced dissociation in FTICR (24) have shown the usefulness of the former in obtaining structural
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Figure 5. (Top) Protonated [N ,N’-bis(4,6dlmethoxysalicylidene)-4(trlfiuoromethyl)-o-phenylenedlimlnato] cobalt(1I ) (CoSALOPH) formed by reaction of the neutral molecule with fragment ions produced by electron impact at 50 eV. (Bottom) Fragmentation obtained upon irradlation with the CW COPlaser; same experimental conditions and delay times as in the top spectrum.
information about relatively small gaseous ions. The work reported here resulted from successful attempts to extend PD methods to much larger ions. These larger ions, with their many degrees of freedom and higher density of states, are expected to appear “black” in the infrared region (12);that is, their infrared absorption spectrum should be nearly continuous, leading to a high prabability that they will absorb infrared laser photons. The question of whether internal excitation produced by infrared absorption would be relaxed by radiative or collisional processes faster than it might accumulate in certain modes leading to bond rupture was of great interest. Initially, three different larger molecular weight samples gently heated on a solids probe and ionized by electron impact were studied. The three ions discussed below underwent photodissociation after being subjected to gated output of 0.5-2-9duration from the CW laser under conditions similar to those reported (22) for smaller gaseous ions. An ion of nominal mlz 442 was produced from cis-dichloro- trans-dihydroxobis(2-propanamine)platinum(IV) (CHIP) by ion/molecule reactions (IMR) of the major electron impact (EI) fragment ion with the neutral molecule. As shown in Figure 4 this ion dissociated to produce two daughter photofragment ions, mlt 366 and m / z 311,by loss of various ligands.
m / z 441 + nhv
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(1)
Shown in Figure 5 is the photodissociation of protonated [NJV’-bis(4,6-dimethoxysalicylidene)-4-(trifluoromethyl)-o-
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phenylenediiminato]cobalt(II) (CoSALOPH) which is also produced by ion/molecule reactions of electron impact fragments with the neutral molecule. This ion dissociated by loss of a methyl group.
m / z 562 + nhu
-
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The ion produced by electron attachment to tris(perfluoro-n-nony1)triazine dissociated as shown in Figure 6.
m / z 1488 + nhu
-
m / z 1069
(3)
A similar pathway was indicated by the collision-induced dissociation spectrum of this ion. Other CID pathways were also observed and the dissociation behavior was similar to that presented in ref 24. The above examples show that infrared multiphoton dissociation of large molecular ions can occur readily. This dissociation technique was next coupled with production of high molecular weight ions via laser desorption. Use of a Single Laser for Desorption/Dissociation. Photodissociation of small gaseous ions by a pulsed laser has been reported (25),so an attempt was made both to form ions and to photodissociate them with sequential pulses from the same pulsed laser using the irradiation scheme shown in Figure 2. Use of the same laser for ion formation and dissociation is most desirable, offering obvious advantages in reducing both experimental complexity and cost. As reported in the Experimental Section, a large K+ signal always appeared during
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Figure 7. (Top) tert-Butylpyridinium cation formed by laser desorption from its perchlorate salt. (Bottom) Fragmentation achieved with a single laser pulse from a pulsed CO, laser; all other conditions are identical with those in the top spectrum.
the pulsed laser desorption experiment for both the focused and unfocused pulse. This K+ signal was reduced somewhat by adjusting various delay times and ejecting this ion during the laser beam duration and for 50 ms following the end of the laser pulse. Time-resolved LD mass spectrometry ( 4 ) has shown that significant amounts of K+ are formed for up to 30 ms following the laser desorption pulse. Under these ejection conditions, photodissociation of the tert-butylpyridinium (TBP) cation (from the perchlorate salt) produced by laser desorption was observed. As shown in Figure 7 the TBP cation, m / z 136, dissociates under laser irradiation with loss of C4Hsto produce an ion of m / z 80, which is most likely protonated pyridine.
m / z 136 + nhu
-
m / z 80
(4)
Use of Two Lasers for Desorption/Dissociation. [N,N'-Bis (4,6-dimethoxysalicy1idene)-4-(trifluoro-
methyl)-o-phenylenediiminato]cobalt(II)(CoSALOPH): This compound was selected as a test case since its PD pathways were known (see eq 2 and Figure 5 ) and significant amounts of (M + H)+were observed following laser desorption. Photodissociation behavior observed following laser desorption was identical with that seen when electron impact was used to produce the ions (eq 2 and Figure 5). Sucrose: Observation of the molecular ion produced by LD (23) and LD followed by E1 (26) has been reported for sucrose. The negative ion mass spectrum following laser
ANALYTICAL CHEMISTRY, VOL. 59, NO. 8, APRIL 15, 1987
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m / z 612 + nhv
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(5)
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