Competitive Ionization ofTetraphenylporphyrin in a Laser-GeneratedMetal Ion Plasma

Junggi Ha, Jeremiah D. Hogan, and David A. Laude, Jr.Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas, USA

The ionization of tetraphenylporphyrin (TPP) in a laser-desorbed metal ion plasma isexamined by Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Com­petitive reaction pathways observed to generate abundant molecular ion species includeelectron detachment, cation attachment, charge exchange, metallation, and transmetallationin the positive ion mode and electron capture, metallation, and transmetallation in thenegative ion mode. In general, cation attachment reactions dominate positive ion spectrabelow the laser irradiance threshold for plasma ignition, although the metallation productfrom [TPPj+' reaction with the metal atom, M, is observed. Negative ion products are notobserved in the FT-ICR spectrum when a plasma is not formed. Under plasma ignitionconditions, positive ion spectra include [TPPj+' formed by charge exchange with M+, whichis also present in the spectrum. Negative ion spectra are dominated by [TPPj-', which isformed by attachment to thermal electrons generated in the plasma. Metallation reactionsinvolving IPP and the metal substrate are examined. Positive ion metallation products areobserved both in the absence of a plasma through reaction of [TPPj+' with M and by asecond pathway under plasma ignition conditions through reaction of TPP with M+. Innegative ion mode, metallation is only observed under plasma ignition conditions throughreaction of [TPP]-' with M. Observation of metallated products is found to be consistentwith formation of stable metal oxidation states in the metallated porphyrin. (J Am Soc MassSpectrom 1993, 4, 159-167)

The fundamental importance of metalloporphyrinchemistry to the fields of biology and geologyhas prompted the development of analytical

techniques to characterize structure, physical proper­ties, and chemical reactivity. Thus, with the demon­stration that direct insertion probe electron ionizationmass spectrometers would generate spectra featuringmolecular ions of both free and metallated porphyrins,a rich history involving mass spectrometry of por­phyrins has developed [1-6]. Numerous soft ioniza­tion techniques have been successful in generatingabundant positive and negative molecular ion speciesfor a wide array of porphyrins. More recently, frag­mentation of the pyrollic macrocycle for sequencingpurposes has been accomplished with H 2 and NH 3chemical ionization [7-11].

Porphyrin mass spectrometry is intriguing becauseof the facility with which molecular species are gener­ated by competing ionization pathways. For example,the large, delocalized aromatic ring structure accom­modates large amounts of internal energy to allow the

Address reprint requests to David A. Laude, [r., Department ofChemistry and Biochemistry, The University of Texas at Austin,Austin, TX 78712.

© 1993 American Society for Mass Spedrometry1044-0305/93/$6.00

molecular ion to dominate 70-eV electron ionizationspectra. Mass spectra from field desorption [12],plasmadesorption [13], fast atom bombardment [14-19], andlaser desorption ionization (LDI) [20-23] sources alsogenerate proton and cation attachment products. Inaddition, the ionization potential (H') of porphyrins issufficiently low (e.g., the IF for porphine is 6.6 eV [24])that electron detachment or gas-phase charge exchangewith surrounding organic and metal ions will occurunder appropriate conditions. The relatively high elec­tron affinity of porphyrins, 1.5-1.8 eV (25], is sufficientfor electron capture products to dominate negative ionspectra [26-29].

In the work presented here, this ensemble of com­petitive gas-phase porphyrin reactions is examined byLDIjFourier transform ion cyclotron resonance (FT­ICR) mass spectrometry. Laser desorption ionizationwas selected because all of the reagents required forthe reactions noted previously can be formed abovethe sample surface under plasma ignition conditions.The formation of a plasma is a general consequence oflaser ablation of metal substrates above a thresholdpower density that typically exceeds approximately108 WIcm2 [30-32]. Laser plasma ignition is oftenexploited to generate metal ions to be used as reactive

Received April 30, 1992Revised July 31, 1992

Accepted August 5, 1992

160 HA ET AL. JAm Soc Mass Spectrom 1993, 4, 159-167

species in gas-phase ion-molecule reaction studies[33-35]. The FT-ICR technique has been demonstratedby several groups to be an excellent detector of LDIproducts, specifically LDI of porphyrins [20-23, 34].For example, Brown and Wilkins [21] showed thatLDI/FT-ICR of free porphyrins and metalloporphyrinsat high laser fluence yielded primarily the porphyrincation and anion with modest side-chain fragmenta­tion. At lower laser irradiance in the presence of analkalai metal salt, cation attachment processes werefavored. More recently, Nguyen et a1. [22] usedLDI/FT-ICR to examine dimethyl 8-acetyl-33,7,12,17­tetramethylporphyrin-2,18-dipropianoate. They foundthat the porphyrin cation and anion dominated at lowlaser irradiance, with extensive fragmentation at higherlaser irradiance. No metallation products were ob­served at laser irradiances to 3.8 X 107 Wjcm2

• Irikuraand Beauchamp [34] demonstrated that selective met­alia ted porphyrins form from laser-desorbed metalcations that subsequently reacted with gas-phase por­phine in the trapped ion cell; estimated metal­porphyrin bond energies for these cations were greaterthan 127 kcaljmol; however, generation of metallatedanions directly from the metal was not observed, withthe explanation given that metal anions, M-, were notformed in good yield by LDI.

Even with the LDI/FT-ICR experiments performedto date, an understanding of the factors that contributeto competitive cation attachment, charge exchange,electron capture or detachment, metallation, and trans­metallation reactions is incomplete. From an analyticalperspective, appropriate control of the experiment, in­cluding sample preparation and laser conditions, iscritical to provide the necessary reagents to generatethe porphyrin product of interest for structure determi­nation or subsequent reaction. Of more general interestis whether reactions occurring in the high-energylaser-generated plasma are in any way consistent withthe results observed for porphyrin chemistry that oc­curs in solution. As will be discussed, it is found thatproduct stabilities for metallation reactions that occurin a laser-desorbed plasma correlate well with metallo­porphyrin products that exhibit stable metal oxidationstates in solution. This finding is important because itsuggests that plasma ignition LDI might be used forgenerating important bioporphyrin species for subse­quent gas-phase reactions in the trapped ion cell withdetection by FT-ICR. In particular, this technique maybe a convenient method for synthesizing any desiredmetallated porophyrin directly from the porphyrinprecursor without any other pretreatment or synthesis.

Experimental

Instrumentation

Experiments were conducted with an FT-ICR instru­ment, featuring a 3.0-T superconducting magnetic, a5-cm cubic trapped ion cell, a vacuum chamber diffu-

sion pumped to below lO-8-torr pressure, and aprobe-mounted fiber-optic-based laser desorption in­terface [36]. Data acquisition and processing were con­trolled with an Extrel FTMS-2000 data system andsoftware. A pulsed Nd:YAG model DCR-ll from Spec­tra Physics (Mountain View, CA) was used to desorband generate the plasma.

Sample Preparation

Samples of tetraphenylporphyrin (TPP), obtained fromAldrich Chemical Co. (Milwaukee, WI) and used with­out additional purification, were dissolved in chloro­form and spray deposited onto spinning cylindricalprobe tips fashioned from stainless steel. In early ex­periments, stainless steel probe tips were sandblastedbetween sample depositions to eliminate cross contam­ination. This had the effect, however, of providing asource of Al20 3 on the probe surface that under cer­tain conditions participated in the LDI process. Forsome experiments it was desirable to provide a sourceof potassium ions to enhance cation attachment pro­cesses, and KBr was dissolved in methanol and aerosolsprayed onto the probe tip prior to deposition of TPP.

A fiber-optic-mounted sample probe used for allLDI/FT-ICR experiments is described elsewhere [36].Briefly, TPP-sprayed probe tips were mounted on a0.25-in. o.d. probe that was positioned in a 0.75-in. o.d.probe assembly that also housed the fiber-optic. This0.25-in. probe could be rotated and translated to pro­vide several hundred laser firings on fresh sample. Theprobe was inserted into the analyzer chamber alongthe z-axis centerline of the trapped ion cell to within 2em of an 80% transmissive mesh trap plate. Pressurein the analyzer chamber was allowed to return below2 x 10- 8 torr prior to executing the LDI/FT-ICRexperiment.

Data Acquisition and Processing

The FT-ICR pulse sequence commenced with a singlelaser firing of a 9-ns Q-switched, 1064-nm pulse thatdelivered up to 23 m] of laser energy (measured with aGentec model 200 [oulcmeter) through the fiber-opticto the sample. With the fiber positioned within 1 mmof the probe tip, unfocused light exiting the fibercreates a spot measuring 0.9 mm2 on the probe surfacethat corresponds to a laser irradiance from below 107

to 3 X 108 W/cm2.

As a more direct measure of the extent of plasmaformation associated with each firing of the laser, aLeCroy (Chestnut Ridge, NY) model 9400 digital stor­age oscilloscope recorded the negative excursion of theeffective trap potential as the laser-desorbed plasmapassed through the trapped ion cell. This trappingpotential depression, which accompanies several of theLDI/FT-ICR spectra to be shown, provides a qualita­tive measure of the charged-particle density. Supportfor this assumption comes from earlier work that indi-

J Am Soc Mass Spectrom 1993, 4, 159-167 IONIZATION IN LASER-GENERATED ION PLASMA 161

~ 600 700mi.

Figure 1. LDIjFT-ICR spectra of TPP deposited with KBr on asandblasted stainless steel probe tip. A laser power density ofapproximately 3 X 108 W/cm2 was used; K+ and Br" wereejected from the cell following desorption for positive and nega­tive ion spectra, respectively. (a) Positive ion mode with A ~

[TPP - 77]+; B ~ [(TPP - 2H + AI) - 77]+; C ~ [(TPP - 2H +Fe) - 77]+; D = TPP+'; E ~ [TPP - 2H + AJ]+; F = [TPP +K]+; G ~ [TPP - 2H + Fe]+. (b) Negative ion mode with A =[TPP - 77]-; B ~ [(TPP - 2H + Fe) - 77]-; C ~ TPP-; D =

(TPP - 2H + Fe]-.

700

D

F G

c

D

600mi.

E

B

cB

A

a

b

500

Results and Discussion

In this report the following nomenclature is used:

TPP, free-base TPP[TPP]+', TPP radical cation[TPP]-', TPP radical anion[TPP + M]+, cationized TPP[TPP + M]-, anionized TPP[TPP - 2H + M]+, metallated TPP anion[TPP - 2H + M]-, metallated TPP cation.

cated that the potential depression created at biasedtrap plates by selected electron or metal ion popula­tions increased with increasing laser irradiance [32].

Following the laser event, a several hundred mil­lisecond-delay period allowed system base pressurestoreturn to the low 1O-8-torr region. For many experi­ments} as will be specified later, double-resonanceevents'at the cyclotron resonance frequencies of strongmatrix ions and suspected precursor ions would beapplied to eject ions from the cell. Either the Extrelfrequency synthesizer was used to facilitate sequentialejection of unwanted ions during the delay or a sec­ondary synthesizer output, consisting of superimposedwaveforms at multiple frequencies, was created with aLeCroy model 9100 arbitrary function generator andapplied to the excite plates [37].

Broadband cyclotron excitation prior to detectionwas accomplished with a linear frequency sweep at1000 Hz//ks over a 2.66-MHz range. Nominal massresolution to about 1500 Da was achieved in broad­band mode with 128K data points acquired for an800-kHz bandwidth. Transients were processed withsine bell apodization, augmentation with an equivalentnumber of data points, and magnitude-mode Fouriertransformation.

Negative ion FT-ICR spectra were acquired bymaintaining trapping potentials at - 0.75 V continu­ously, except during the quench event. Positive ionspectra were acquired with a trap potential continu­ously applied at 0.75 V. Spectra are the result of asingle laser firing on fresh sample or, for depth-profil­ing studies, successive laser firings in the same probeposition.

As will be discussed, metal valency determineswhether the metallated TPP ion is a radical.

General Features of Tetraphenylporphyrin Spectra

Presented in Figure 1 are representative positive andnegative ion FT-ICR spectra generated by LOI at 3 X108 W/cm2 on a sample consisting of TPP depositedwith KBr on a sandblasted stainless steel probe. Awealth of molecular ion species generated by competi­tive gas-phase reactions in the plasma are shown. Asexpected, the positive ion spectrum exhibits [TPP]+' asthe base peak at m/z 614. In contrast with the obser­vations of Nguyen et al. [22], metallation reaction

products yielding ions at m/z 639, 664, 667, and 668corres~onding to [TPP - 2H + M]+, where M =27Al,52 Cr, Mn, and 56 Fe, respectively, were observed andin fact dominate the spectra for many metal substrates.These metallation reactions occur with concomitantloss of hydrogen. The 52 Cr, 55Mn, and 56Fe were gener­ated from the stainless steel substrate, and 27Ai waspresent from prior sandblasting of the probe tip. Acation attachment reaction also yielded an ion at m/z653 that corresponded to [TPP + K]+. The fragmenta­tion pattern in the mr z 537-591 region correspondedto loss of a phenyl group from [TPP]+' and from themetallation products, but fragmentation associated

162 HA ET AL. JAm Soc Mass Spectrom 1993, 4, 159-167

Distinction between the two processes should be possi­ble but in practice is difficult to accomplish. For exam­ple, although a wide range of metal IPs surround theIP of TPP, the laser plasma ignition process generatesions with kinetic energies extending to the hundreds ofelectron volts [32, 36, 48-50]. As a consequence, kineti­cally driven endothermic processes may interfere [51,52]. Charge exchange might be distinguished by per­forming double-resonance experiments, as was donewith cation attachment reactions; ejection of suspected

The macrocyclic center is the likely site for potassiumtransfer and attachment because the pyrrole function­ality is more prone than the benzene ring to attach­ment reaction with metal cations [48]. Potassium at­tachment to octaethylporphyrin is additional evidencethat the porphyrin ring is the reaction site. The obser­vation that cleavage of a phenyl ring does not accom­pany potassium attachment suggests that low-energyprocesses involved do not seriously perturb the molec­ular orbital system of neutral porphyrin.

In contrast with cation attachment, the chemistryassociated with formation of [TPP]+' in the LDI plasmais not as well documented. Given the low IP of TPP,[TPP]+' is likely to form either due to charge exchangewith metal ions in the plasma (reaction 2) or electrondetachment as a consequence of thermal effects at thesurface or in the plasma (reaction 3):

with the cation attachment product [TPP + K]+ wasnot observed.

The negative ion spectrum of TPP was similar to thepositive ion spectrum and included [TPP]-' as the basepeak and fragment ions corresponding to loss of phenylgroups from all molecular species. In contrast withIrikura and Beauchamp [34], negative ion metallationreaction products were observed in excellent yield.Notable distinctions from the positive ion spectrumincluded the absence of cation attachment productsand a negligible metallation product ion from the[TPP]-' reaction with AI.

Of the ions produced in Figure 1, the process lead­ing to formation of [TPP + K]+ at mjz 653 is moststudied and best understood because it is also thevehicle by which most infrared LDIjtime-of-flight andLDIjFT-ICR mass spectra of organic molecules areproduced [38-44]. Specifically, a weak ionic associa­tion results from the gas-phase ion-molecule reaction[20-23, 35] between an intact neutral TPP and a pre­cursor ion from the salt. The [TPP + K]" formation inFigure 1 was shown by double-resonance ejection ex­periments to involve K2Br" as the primary reactiveion, as shown in reaction 1 [45-47]:

(4)

(5)

TPP + A- --> [TPP] _.+ A

TPP + e - --> [TPP]-

This process, which is the subject of considerable inter­est in porphyrin solution chemistry, involves expulsionof H 2 and replacement by a metal [1,4). Metallation ofthe porphyrin ring can also be observed in the gasphase, although a systematic study has not been con­ducted. Recent studies by lrikura and Beauchamp [34)of reactions involving laser-desorbed metals and back­ground neutral porphine under controlled conditionsdefine at least one mechanism (reaction 6) as involvingthe metal cation reaction with the neutral gas-phaseporphyrin. An inability to directly observe metallatedporphyrin anion products was attributed to a lack ofM-; however, as we will show, M- is not essential toform the metallated porphyrin anion because, as shownin reaction 7, an alternate pathway exists under plasmaignition conditions in which desorbed M reacts withTPP-' formed by reaction 5. Also presented in reaction

TPP + M+--> [TPP - 2H + M)+

or [TPP - 2H + M)+'+ H 2 (6)

[TPP]-' + M --> [TPP - 2H + M)-

or [TPP - 2H + M]- + H 2 (7)

[TPP]+' + M --> [TPP - 2H + M) +

or [TPP - 2H + M)+ + H 2 (8)

Ejection studies confirm that neither Br" (EA = 3 ev),which is formed in large quantities, nor adduct ions,such as KBri participate in gas-phase reactions withthe free porphyrin in the cell. Instead, the likely mech­anism for ion formation is electron capture. As will bedemonstrated, if plasma ignition of the stainless steelsurface occurs, a large population of low-energy elec­trons is generated that will readily attach to desorbedTPP. Confirmation that plasma ignition is a prerequi­site for [TPP]-' formation will be demonstrated later.

One additional mechanism for porphyrin ion forma­tion shown in Figure 1 is metallation of the porphyrinring (reactions 6-8):

M+ precursors with consequent disappearance of[TPP)+' from the spectrum would indicate that chargeexchange was occurring in the cell. These experimentswere unsuccessful, although the failure could be dueto exchange reactions occurring in the plasma adjacentto the sample surface rather than in the cell.

The presence of a strong [TPP)_. in the negative ionspectrum (Figure Ib) is explained by the high electronaffinity (EA) of TPP (EA = 1.7 eV). Again, the mecha­nism for molecular ion formation could be chargeexchange with an anion (reaction 4), most likely fromKBr-derived ions because M- was rarely observed, orsimply due to electron capture (reaction 5):

(2)

(3)

TPP + M+ -> [TPP] +.+ M

TPP -> [TPP] +.+ e-

J AM Soc Mass Spectrom 1993, 4, 159-167 IONIZA nON IN LASER-GENERATED ION PLASMA 163

8 is the positive ion analog in which M participates inthe metallation process.

a

[(TPP-2H+VO)+Kj+

[TPP - 2H + VO]+ is observed at mjz 679. As laserpower increases, a strong plasma is formed, as indi­cated by a larger negative depression in the potentialprofile in Figure 2b. This negative deflection is indica­tive of either an excess of electrons in the plasmareaching the trap plate or a slightly better detector

100 300 m/z 500 700Figure 2. LDI/Ff-ICR spectra of [TPP - 2H + VOl with sam­ple preparation and conditions as in Figure 1. (a) First laser shotat 7 X 107 W/cm2 on fresh samrle in the positive ion mode; (b)first laser shot at 3 X 108 W/cm on fresh sample in the positiveion mode; (c) second laser shot at 3 X 108 W/cm2 on freshsample in the negative ion mode. The inset accompanying eachspectrum is the potential drop at the front trap plate as laser-de­sorbed ions enter the cell.

[TPP-2H+VOr

[TPP-2H+VO]+

.. -r-­~ -1-1-

(TPP-2H+VOj+

[(I'PP-2H+VO)-Oj+ I

Ii I I IIH "'I

... ~~.

b

e

Plasma Effect on Laser DesorptionIonization Pathways

We find that the most important factor in the LDI/FT­ICR experiment in determining the reaction pathwaysthat will be observed is whether a laser-induced plasmais formed at the desorption site. The two experimentalparameters that contribute to formation of this plasmaare laser irradiance, which must be above the plasmaignition threshold for a particular metal, and samplepreparation, which ensures that the metal substratereceives a sufficient thermal spike for ignition to occur.In our work, sufficient sample covered the metal sur­face that the first laser firing on fresh sample did notgenerate a plasma; however, subsequent laser firingson exposed metal produced a plasma and simultane­ously desorbed porphyrin in the selvage region of thedesorption site. Thus, throughout this report, spectraare distinguished as resulting from a laser firing onfresh sample under conditions that generate no plasmaand subsequent laser firings on exposed metal sub­strate that do generate a plasma.

At this point, an explanation can be offered for thedifferences between the LDIjFT-ICR spectra observedby Nguyen et al. [22] and our own and, specifically,the absence of metallation products at laser irradiancesto 3.8 X 107 W j cm2

• One possibility is that the por­phyrin studied by Nguyen et aL [22] is more suscepti­ble to cleavage than TPP at the higher irradiancesrequired for plasma ignition. Another explanation isthat conditions for plasma were never achieved inNguyen et al. [22] because laser irradiances were rela­tively low, and sample preparation was described asthick (0.5 mm). As mentioned previously, it is for laserfirings on exposed metal surface that we observedplasma ignition and the resulting positive and nega­tive ion porphyrin metallation products.

As evidence of the distinction in reaction pathwaysbased on laser irradiance and sample preparation, theLDI/FT-ICR spectra in Figures 2 and 3 are presented.First, the spectra in Figure 2 are acquired at increasinglaser irradiance for a [TPP - 2H + VO](VO =

vanadyl) and KBr sample on a sandblasted stainlesssteel probe tip. As laser irradiance was increased from1 X 106 to 3 X 108 Wjcm2, FT-ICRspectra, along withlaser plasma current (as measured by a potential dropon the front trap plate), were simultaneously meas­ured. Up to a threshold of approximately 107 W j cm2,

no ion current was detected on the trap plate; how­ever, small amounts of K+ and AI+ were observed inthe positive ion mode. Above 107 W/cm2

, a smallcurrent was detected on the front trap plate, and posi­tive ion FT-ICR spectra were obtained with good sig­nal-to-noise ratio, as shown in Figure 2a. Cation attach­ment products, such as [(TIP - 2H + YO) + K]+ and[(TPP - 2H + VO) + AJ]+,are favored, although some

164 HA ET AL. J Am Soc Mass Speclrom 1993,4,159-167

Figure 3. Companion positive and negative ion LDljFT-ICRspectra of [TPP - 2H + VOl deposited with KBr on a sand­blasted stainless steel probe tip. Sfectra in (a)-(dJ are fromsuccessive firings at 3 X 108 Wjern in the same sample probeposition, Each inset is the potential drop at the front trap plate asIaser-desorbed ions enter the cell.

response for electrons; otherwise the quasi-neutralplasma would not exhibit a net change in chargeincident on the plates, and a flat response profilewould result. Associated with the much larger plasmawas a marked decrease in the FT-ICR cation attach­ment products relative to formation of intense [TPP ­2H + VO]+ in Figure 2b and [TPP - 2H + VOrshown in Figure 2c. These data indicate that in factthere is a strong correlation between the onset ofplasma ignition and the commencement of reactionconditions under which charge exchange, electron de­tachment, or electron capture are promoted.

Additional evidence of the role that plasma ignitionplays in the formation of molecular ion species wasobtained from depth-profiling studies of TPP de­posited on a metal substrate. The LDI/FT-ICR spectraobtained from successive firings in the same probeposition with the laser operating at high laser irradi­ance are shown in Figure 3. A series of four successiveshots are presented with spectra acquired in the posi­tive and negative ion modes. Potential depression pro­files accompany each spectrum and are a measure ofthe extent of plasma ignition, In evaluating the data itis important to recognize that although the FT-ICRtrapped ion cell selectively captures ions of a singlepolarity for detection, both positive and negative ionsare formed as a result of each laser desorption event.Thus, the total ion population that exists as the resultof each laser firing is best represented, as shown inFigure 3, as a combination of the two spectra.

Several distinctions between positive and negativeion mode FT-ICR spectra are noted from the depth­profiling experiment. In positive ion mode, [TPP - 2H-1- Va]" is generated from an initial firing that pro-

positivea

c

J.d

.'"

[TPP-2H.j.VOI+

! ,

400 600

negativea

- d

.'""

t I

duces little plasma, but negative ions are not observed.The FT-ICR spectra for all subsequent laser firings inwhich a significant laser plasma is generated at thesample surface exhibit very different features. In thesespectra, an intense [TPP - 2H -1- VO]- signal domi­nates negative ion spectra for numerous laser firings.The companion positive ion spectra for these laserfirings exhibit significant metal cation M + and [TPP ­2H + VO]+ products in a ratio that is strongly depen­dent on laser irradiance. Little cation attachment isobserved in these spectra. These results caution againstassuming that a probe should be rotated to a freshsample position without first evaluating both positiveand negative ion spectra.

An explanation for the different product ions ob­served in the spectra shown in Figures 2 and 3 can begiven simply in terms of the desorbed charged andneutral particle populations that are generated in thepresence and absence of plasma ignition conditions. Ifthere is no laser plasma, either because of reducedlaser irradiance or because the sample buffers themetal surface from the photon flux, then the primaryreactive ions and neutral particles generated are asdepicted in Figure 4a and include M, neutral por­phyrin and porphyrin cation from the sample and C+and A ~ from adventitious salts, In contrast, whenplasma ignition conditions are achieved on subsequentlaser firings, the particle pool shown in Figure 4b isaugmented by e " and M+ populations. Product ionsobserved from reactions within this second ion pooldiffer in that the salts have been cleaned from thesurface by the first laser firing, and, more important,an enormous thermal electron and singly charged metalion population is generated as a result of plasmaignition.

A quick reexamination of the spectra in Figures 2and 3 indicates consistency with the reactants depictedin Figure 4. For example, the positive ion spectra inFigures 2a and band 3a are derived from the particlepool in Figure 4a. Cation attachment products (reac­tion 1) from salts are observed. Metal cations are notobserved in the absence of a plasma, which impliesthat the [TPP - 2H + VOl t peak in Figure 2b mustresult from electron detachment (reaction 2) ratherthan charge exchange.

Metallation Reaction with Tetraphenylporphyrin

The competitive reactions suggested by Figure 4 alsoapply to metallation reactions involving TPP and themetal species arising from the substrate by laser des­orption processes. Shown in Figure Sa are LDl spectraof TPP on stainless steel, which in the absence of aplasma, generate the reactant pool in Figure 4a. Thesole reaction products result from cation attachment(reaction 1) to form [TPP -1- K]+ and [TPP + Na]" andelectron detachment (reaction 3) to form [TPP]+'. Prod­ucts observed in the depth-profiling spectra in Figure5b-d arise from the reactant pool depicted in Figure

J Am Soc Mass Spectrom 1993,4, 159-167 IONIZATION IN LASER.(;ENERATED ION PLASMA 165

a

b

fiberOpti~

MP+ P

+ "AI A- M C+c-rM p+ P-tpA- P

M r!'<_A- * f:4. C+C t. It p+lo.,1.-P+p PPM

A- ~A c.+: P A.. C+ P+M P P+ A- 'p.M + 't.:+ P

A- M P+ cfl P+ P P ~M P+ P P ~- ~-tP+ P+ p.

A- c- P+ p+r,(f R- M C+

\

M P M P P+A·A- M P+ C

M P+ P Psample A- c+

M

fiberOPti~

,.... n

Figure 4. Depiction of predicted particle pools generated from laser desorption of TPP on metal substrates. (a) Particle desorptionproducts in the absence of laser plasma ignition include porphyrin (P), porphyrin cation (P+), neutral metal (M), and salt cations (C+)and anions (A -). (b) Particle desorption products following laser plasma ignition include porphyrin (P), porphyrin cation (P+), neutralmetal (M), metal cation (M+), and thermal electrons (e-).

166 HA ET AL. J Am Soc Mass Spectrom 1993, 4, 159-167

apositive

aneeative product, either [TPP - 2H + M(II)]~ or [TPP - 2H +

M(I)]-, respectively, can be detected. Thus, for exam­ple, because Al forms a stable III state, it is onlyobserved in positive ion spectra. In contrast, Fe and Cr,which form stable 1, II, and III states, will generateboth positive and negative ion metallation spectra.

_.~

'I .,

c

.1

Figure 5. Companion positive and negative ion LDI/FT-ICRspectra of TPP on a sandblasted stainless steel probe tip. Spectrain (al-Id) are from successive firings at 3 X 108 WIcm' in thesame sample probe position. Spectra in (a) result from LDI onfresh sample, with product ions formed in the absence of plasmaignition conditions. Spectra in (b)-(d) exhibit product ions formedunder plasma ignition conditions.

4b. The presence of thermal electrons from plasmaignition alters the product ion distribution by openingnegative ion reaction pathways. The [TPP]-' formedfrom electron capture (reaction 5) is the precursor ionfor negative ion reactions, including porphyrin metal­lation (reaction 8). Also present as a result of plasmaignition is a metal cation M+ population that partici­pates in charge exchange, metallation, and transmetal­lation reactions that compete with electron capture forthe desorbed TPP. The extent of competition dependson the availability of M+ because under plasma igni­tion conditions, the ion can achieve kinetic energies ofhundreds of electron volts that effectively remove itfrom the reactant pool detected by FT-ICR; however,provided that sufficient low-energy cations (less thanthe trapping potential) are generated, the competitionwith electrons will depend on the inherent reactivity ofthe gas-phase metal species with the porphyrin.

As the differences between positive and negativeion spectra in Figure 1 suggest, the observation ofgas-phase porphyrin metallation reactions is stronglydependent on the metal. For example, although Cr andFe from stainless steel are observed to undergo metal­lation reactions to produce positive and negative ionspecies, Al metallation is only observed in the positiveion spectrum. One explanation is that, as in solution,observation of metallation products is dependent onthe stability of a particular metal oxidation state. Ingeneral, if the II or III oxidation state of a metal isstable in the porphyrin, then a positive ion metallationproduct, either [TPP - 2H + M(III)j+ or [TPP - 2H +M(II)]+~ respectively, can be detected by FT-ICR. If theII or 1 state is possible, then a negative ion metallation

Conclusions

The ionization of TPP and [TPP - 2H + VOl in alaser-desorbed metal ion plasma was detected by FT­ICR with abundant molecular ion species generated bycompetitive processes, including electron detachment,cation attachment, charge exchange, metallation, andtransmetallation in the positive ion mode and electroncapture, metallation, and transmetallation in the nega­tive ion mode. The generation of e" and M+ from themetal substrate by laser plasma ignition is a control­ling factor in the spectra that are generated. In general,cation attachment reactions dominate positive ionspectra below the laser irradiance threshold for plasmaignition, although the metallation product from [TPP]+.reaction with the metal atom M can be observed.Under plasma ignition conditions, positive ion spectrainclude [TPP]+' formed by charge exchange with M+and a strong metallation product. Negative ion prod­ucts are absent from the FT~ICR spectrum when aplasma is not formed; however, under plasma condi­tions, the intense negative ion spectra that are ob­served feature (TPP]-', which is formed by attachmentto thermal electrons. Metallation products are alsoobserved, evidently from reaction of [TPP]-' with neu­tral M. Support for the role that plasma ignition playswas obtained from depth-profiling studies of TPP de­posited on a metal substrate in which the initial firingon fresh sample did not ignite the metal substrate, butsubsequent laser firings on exposed metal did yield adense plasma of e " and M+ that participated in reac~

tions with TPP.Metallation reactions between TPP and the laser-de­

sorbed metal substrate were examined. Positive ionrnetallation products are observed both in the absenceof a plasma through reaction of [TPP]+' with M, andby a second pathway under plasma ignition conditionsthrough reaction of TPP with M+. In negative ionmode, metallation is only observed under plasma igni­tion conditions through reaction of [TPP]-' with M.The TPP reactivity with the metals is consistent withformation of stable metal oxidation states in the metal­lated porphyrin. Specifically, the metalloporphyrinmust exist as either [TPP - 2H + M(II)J+' or [TPP ­2H + M(III)]+ to be observed in positive ion spectraand as [TPP - 2H + M(IW or [TPP - 2H + M(II)]-'to be observed in negative ion spectra. This explainsthe appearance of stainless steel metallation productsin both positive and negative ion spectra but the ap­pearance of aluminum metallated porphyrin only inthe positive ion mode.

J Am Soc Mass Spectrom 1993, 4, 159-167 IONIZATION IN LASER-GENERATED ION PLASMA 167

From an analytical perspective, the results obtainedsuggest that appropriate control of the experiment,especially sample preparation and laser conditions, iscritical to provide the necessary reagents to generatethe porphyrin product of interest for structure determi­nation or subsequent reactions. Of more general inter­est is the observation that reactions occurring in thehigh-energy laser-generated plasma are consistent withthe results observed for porphyrin chemistry that oc­cur in solution. This finding is important because itsuggests that plasma ignition LOI might be used forgenerating important bioporphyrin species for subse­quent gas-phase reaction in the trapped ion cell withdetection by FT-ICR.

AcknowledgmentsThis work is supported by the Welch Foundation (F-1138), theTexas Advanced Technology and Research Program, and theNational Science Foundation (CHE9013384) and (CHE9057097).

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