1500 Volume 51, Number 10, 1997 APPLIED SPECTROSCOPY0003-7028 / 97 / 5110-1500$2.00 / 0q 1997 Society for Applied Spectroscopy

Potential for an Atmospheric-Pressure Low-PowerMicrowave-Induced Plasma Ionization Source for MassSpectrometry

NOHORA P. VELA,* JOSEPH A. CARUSO,² and R. DUANE SATZGERDepartment of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, Ohio 45221-0172 (N.P.V., J.A.C.); andU.S. Food and Drug Administration, National Forensic Chemistry Center, 1141 Central Parkway,Cincinnati, Ohio 45202 (R.D.S.)

The potential for using the microwave-induced plasma (MIP) as anionization source is further explored . This source operates at at-mospher ic pressure, minimizing pumping problems and, throughpower and gas-¯ ow adjustment, offers the possibility of selectingfrom elemental spectra to fragmentation spectra resembling thosefrom electron impact sources. The effect of microwave power, car-rier gas-¯ ow rate, and injector-probe con® guration in the produc-tion of fragment ions is demonstrated with the use of per¯ uorotri-butylamine and tetramethyltin. Initial potential for liquid-sampleintroduction to the MIP is accomplished by using a direct-injectionnebulizer (DIN) at higher reproducibility levels than in earlier stud-ies.

Index Headings: Microwave-induced plasma; Mass spectrometry;Atmospheric-pressure ionization; Fragmentation spectra.

INTRODUCTION

Plasma sources such as inductively coupled plasmas(ICPs) and microwave-induced plasmas (MIPs) are wellrecognized atomization and ionization sources for ele-mental analysis.1 They have been successfully coupled togas chromatography (GC), liquid chromatography (LC),and supercritical ¯ uid chromatography (SFC) in order toprovide elemental speciation information.2 The possibil-ity of modifying these ionization sources to use them toobtain molecular and structural information as well hasbeen the topic of several publications.3± 6 Poussel et al.3

described the use of an MIP produced at low pressure( , 0.1 mbar) through a surface-wave propagation processas an ionization source to provide molecular information.Olson and co-authors4 have demonstrated the use of alow-pressure helium MIP for the fragmentation of organ-ic compounds. Results indicated that by varying the MIPpower it was possible to change the fragmentation patternfor several low-molecular-weight hydrocarbons. Thesestudies demonstrated some capability of the MIP as asoft-ionization source and reported results comparablewith those obtained from electron impact (EI) sources.

Shen and Satzger described the use of a low-poweratmospheric-pressure MIP source for mass spectrometry(MS) with direct introduction of gaseous and liquid or-ganic samples.5 By using a low-power (30 W) He plasma,they evaluated the effect of different carrier gases onthe fragmentation pattern for per¯ uorotributylamine

Received 12 August 1996; accepted 3 March 1997.* Current address: ManTech Environmental Research Service Corpo-

ration, R. S. Kerr Environmental Research Laboratory, 919 Kerr Re-search Drive, Ada, OK 74821-1198.

² Author to whom correspondence should be sent.

(PFTBA). This ionization source was also shown to havepotential for obtaining structural information for volatile,low-molecular-weight compounds. Further, aqueous sam-ple introduction was accomplished by passing the solu-tion directly through an aluminum oxide tube, which waspart of the interface. However, without nebulization, theMIP power was not applied ef® ciently to the analyte,7,8

leading to elevated relative standard deviations (RSDs)of 40% related to plasma instability and/or the introduc-tion of nonuniform sample droplets.

To further study and improve on the MIP as an ion-ization source, we report a direct-injection nebulizer(DIN) as a sample introduction device to the plasma.While unnecessary for gaseous sample introduction sinceno aerosol needs to be formed, the DIN does provide anability to vary the analyte fragmentation. Some of theadvantages of this nebulizer for liquid-sample introduc-tion include production of ® ne droplets (1± 20 m m in di-ameter),9 low dead volume, and high analyte transportef® ciency.10± 12 These characteristics are bene® cial forcoupling chromatography with plasma detection to min-imize band broadening and to improve sensitivity.13

Additionally, if viable for sample introduction to thelow-power MIP, then microbore LC or capillary electro-phoresis (CE) will be potentially useful with low totalliquid ¯ ow rates of 30± 100 m L/min (including CE buffer¯ ow). The effect of the microwave power, carrier samplegas-¯ ow rate, and injector-probe con® guration in the pro-duction of fragment ions with the use of PFTBA andtetrabutyltin is described here. Preliminary liquid-sampleintroduction to the MIP using the DIN is also discussed.

EXPERIMENTAL

Instrumentation. A VG PlasmaQuad PQ1 (VG Ele-mental, Winsford, Chesire, England) mass spectrometerwith an m/z range from 4 to 246 amu was used (an un-fortunate limitation of commercially available elementalmass spectrometers). The HPLC system includes an IscoModel 2350 pump and Model 2360 gradient programmer.The MIP system consists of a 120-W microwave gener-ator (Kiva Instruments, Inc., Rockville, MD) and a mod-i® ed Beenaker microwave TM010 cavity (92.5 mm i.d. and2 cm depth) that incorporates an injector probe.6 A dia-gram of the interface is presented in Fig. 1, and a detaileddescription is given by Shen and Satzger.5 Plasma tuningwas always set to minimum re¯ ected power for all for-ward powers studied. Three different injector probes wereused in this experiment, and they are described as fol-

APPLIED SPECTROSCOPY 1501

FIG. 1. Atmospheric-pressure ionization (API) microwave-inducedplasma source.

FIG. 2. Effect of MIP power on the fragmentation of PFTBA. Gaseoussample introduction through different injector probes. (A) Probe A:stainless steel tube, 1/16 in. o.d.; (B) probe B: stainless steel tube, 1/16in. o.d. with a ceramic tube inside (1/32 in. o.d.); and (C probe C:stainless steel tube, 1/8 in. o.d. with a ceramic tube inside (1/16 in.o.d.).

lows: Probe A: stainless steel tube, 1/16 in. o.d.; ProbeB: stainless steel tube, 1/16 in. o.d. with a ceramic tubeinside (1/32 in. o.d.); and Probe C: stainless steel tube,1/8 in. o.d. with a ceramic tube inside (1/16 in. o.d.).

Reagents and Standards. Per¯ uorotributylamine orPFTBA with a molecular weight of 671 amu, which iscommonly used for calibration with EI MS, was utilizedfor this study. PFTBA was purchased from Sigma Chem-ical Company (St. Louis, MO). Tetramethyltin and tet-rabutylammonium hydroxide were purchased from John-son Matthey (Alfa Products, Danvers, MA) and SigmaChemical Company, respectively, and used without fur-ther puri® cation. A 0.025 M tetrabutylammonium hy-droxide solution was prepared in distilled deionized water(18 Megohm, Barnstead).

Operating Parameters. A helium MIP with a sheathgas ¯ ow rate of 5 L/min was used. The sample carriergas was nitrogen, and the ¯ ow rate was varied between50 and 135 mL/min. The effect of the MIP power on thefragmentation of PFTBA was evaluated by increasing thepower from 20 to 32 W.

Sample Introduction. PFTBA and tetramethyltin areintroduced to the system at ambient temperature as gas-eous samples by passing nitrogen through the head spaceof an Erlenmeyer ¯ ask containing the liquid analyte. AnLC pump and the DIN were used for the introduction oftetrabutylammonium hydroxide with the use of a liquid¯ ow rate of 20± 30 m L/min.

Data Acquisition. All data acquisition was performedin the survey scan mode using the software supplied withthe instrument. Responses at m/z 69, 100, 114, 119, 131,150, 181, and 219 amu (major fragments in the EI massspectrum for PFTBA) were selected to evaluate the extentof fragmentation. A minimum of six replicates for eachset of conditions was run. Signal intensity was normal-ized for the most intense peak in each spectrum.

RESULTS AND DISCUSSION

Effect of MIP Power in the Fragmentation ofPFTBA. Since the instrument used for the experimentsis a modi® ed ICP-MS system (and also different from thesystem used earlier5), the mass range is limited to 246amu. Figure 2 shows the effect of the MIP power on theproduction of fragment ions from PFTBA using the three

different injector-probe con® gurations. Figure 2A indi-cates that with the use of injector probe A and low-MIPincident power (20 or 22 W), the base peak in this masswindow is at m/z 131. With an increase in the power to24 or 26 W, there is more decomposition and the basepeak shifts to m/z 69. Figure 2A also shows that, withthe use of injector probe A, there is minimum formationof fragment ions at higher m/z, even at a low MIP powerof 20 W. Thus, there were no experiments with probe Aat MIP powers higher than 26 W.

The use of a ceramic tube inside the stainless steel tube(injector probes B and C) appears to reduce the interac-tion between the plasma and the sample with a corre-sponding reduction in the extent of fragmentation at agiven power level. Figures 2B and 2C display the effectof MIP power for the injector probe designs B and C,respectively. Figure 2B shows that, for MIP powers of22, 24, and 26 W, the base peak is at m/z 131. For MIPpowers higher than 28 W, the base peak is at m/z 69.Results with probe C suggest that, at MIP powers be-tween 22 and 26 W, and considering the mass range lim-itations of this instrument, there is a shift to higher massfragments (m/z 219). Ions at or above m/z 250, whichcould not be monitored by using the system, would beexpected.

A comparison of Figs. 2A, 2B, and 2C shows a markeddifference in the relative abundance of the different frag-ment ions with injector probe design. The base peak isat m/z 69, 131, and 219 for injector probes A, B, and C,

1502 Volume 51, Number 10, 1997

FIG. 3. Effect of nitrogen ¯ ow on net signal intensity for the frag-mentation of PFTBA. MIP incident power: 24 W; helium gas: 5 L/min.Injector probe design used: C.

FIG. 4. API spectrum for tetramethyltin, (CH3)4Sn, MW 5 180. MIP incident power: 24 W; nitrogen carrier gas: 40 mL/min; helium gas: 5 L/min.

respectively, with MIP powers of 24 or 26 W. At thelower MIP power of 22 W, the base peak is at m/z 131for both injector probes A and B and at m/z 5 219 forinjector probe C. The fragmentation pattern can be variedby changing the incident MIP power and by using dif-ferent injector probes. A combination of injector probe Cand an MIP power of 24 W was selected for further ex-periments. RSDs for signal intensities varied from 4.1 to14.9%. Clearly, with the power dependence shown, forthe source to be useful it will be necessary to well-reg-ulate and couple the MIP power.

Effect of Nitrogen Flow. Shen and Satzger5 evaluatedthe effect of different carrier gases such as helium, argon,and nitrogen in the low-power microwave API ofPFTBA. Their results indicated that nitrogen carrier gasproduced higher m/z fragments; therefore, nitrogen wasalso used as a carrier gas for this work. Figure 3 shows

the results for PFTBA. For gas ¯ ow rates of 60± 75mL/min, the base peak in this mass window is at m/z219. An increase in nitrogen ¯ ow rate to 90 or 135mL/min shifts the base peak to m/z 131 and 69 amu,respectively. This result indicates that a higher carriergas-¯ ow rate may contribute to smaller fragment ion for-mation. A gas-¯ ow rate of 75 mL/min appears to be thebest for structural information, since it gives similar sig-nal intensities for most of the fragments obtained fromPFTBA.

API Spectrum for Tetramethyltin. Figure 4 presentsthe API spectrum for gaseous tetramethyltin (MW 5 180)at 24 W power and a nitrogen carrier gas ¯ ow of 60mL/min. This spectrum shows ions containing the Sn iso-tope pattern. The isotopic abundances at the elemental tinmasses taking m/z 120 as 33% are 119 (8.6% expected,11.7% calculated), 118 (24% expected, 26% calculated),117 (7.6% expected, 10.5% calculated), 116 (14.2% ex-pected, 18% calculated), 122 (4.7% expected, 4.8% cal-culated), and 124 (6% expected, 6.4% calculated). Whilethe calculated vs. the expected percentages are not ascoincident as they might be, there are a variety of pos-sible complications from gas-phase interactions that willrequire further study.

Fragments corresponding to successive methyl grouplosses may be responsible for ions at m/z 165, 150, 135,and 120 and are in agreement with the data reported inthe literature for tetramethyl compounds of the group IVelements.14 Also, no such structure is seen with organo-tins using the standard ICP MS con® guration where onlythe elemental masses are observed. The base peak is(CH3)3Sn1 with a relative abundance of 50.1%.14 The oth-er major fragments reported are (CH3)Sn1 , (CH3)2Sn1 ,SnH1 , Sn1 , and SnCH2

1 with relative abundances of18.1, 11.3, 10.1, 3.8, and 3.7%, respectively. The signalobserved at m/z 183 might correspond to the hydratedtrimethyltin ion (CH3)3Sn´H2O1 with the corresponding

APPLIED SPECTROSCOPY 1503

tin isotopes at m/z 181 and 179. Another possible hy-drated ion, (CH3)3SnCH2´H2O1 , was also detected atm/z 197 with the analogous tin isotopes at m/z 195 and193. The possibility of hydrated ions has been consideredin this case since (1) no gas curtain or countercurrent ¯ owwas used, (2) there was no heat applied to the interface,and (3) the presence of ion-solvent clusters (X1 (H2O)n)has been reported with atmospheric-pressure ioniza-tion.15± 17 The isotope signals at m/z 122 and 124 and thosecorresponding in the base peak region at m/z 165 aresomewhat high, and further study is necessary.

Liquid Sample Introduction. The modi® ed atmo-spheric-pressure low-power MIP as a possible ionizationsource for liquid-sample introduction was demonstratedby using a solution of tetrabutylammonium hydroxide,TBAH (MW 5 259), that was introduced into the DINwith the use of an LC pump. The rationale for this ex-periment was to improve on the reproducibility of earlierwork5 where the RSDs were, on the average, 40%. Thebase peak in this study was at m/z 142, and some othernotable peaks in the mass spectrum of TBAH were atm/z 184, 186, 142, 128, and 100, possibly representing(C4H9 )2 ± N1 5 CH± C3H7 , (C4H9 )3N1 H, (C4H9 )2 ± N1 5CH2

1 , C4H9 ± N1 H5 CH± C3H7, and C4H9 ± N1 H5 CH± CH3,respectively. Shen and Satzger5 reported, for the samecompounds and similar operating conditions, a base peakat m/z 186 (they did not use any carrier gas or nebulizer).Also, they were able to detect a quasimolecular peak (M±OH) 1 at m/z 242. Because of instrument-scanning limi-tations, we were not able to see the peak at m/z 242 orthe molecular ion that would occur at m/z 259. The dif-ferences are possibly due to the introduction of nitrogenthat was used in these experiments to operate the DIN.The results discussed above indicate that an increase inthe nitrogen ¯ ow causes a higher degree of fragmenta-tion. A comparison with the National Institute of Stan-dards and Technology (NIST) data base reference spec-trum for tetrabutylammonium bromide was favorable. Itincluded peaks at m/z 186, 185, 184, 142, 128, and 100as the results in this study show, but owing to the dif-ferent source and differing conditions, the relative inten-sities differ. Also, the NIST spectrum has the base peakat m/z 142, as in this study. However, thus far the advan-tage of using the DIN is the improvement in the % RSD(8.2%) as compared with the much higher earlier results5

of 40% RSD.5 While these results are positive, much ad-ditional study will be required before the DIN-MIP com-bination can be fully assessed for its potential as a liquid-sample introduction source for MS.

The ability to vary the fragmentation from lower tohigher mass fragments as suggested with PFTBA indi-cates there may be potential to quantify at elementalmasses (as with ICP-MS) while providing structural in-formation through tuning the source. If this ultimatelyproves viable, it will provide a new departure for ele-

mental speciation studies, since current elemental speci-ation studies must utilize standard retention times forqualitative identi® cation.

CONCLUSION

The potential of MIP as a soft-ionization source forMS has been further demonstrated. Control in the frag-mentation process is obtained by varying MIP power andcarrier gas-¯ ow rate and by changing the injector-probecon® guration of a DIN nebulizer. Increases in the powergenerally yield a greater degree of fragmentation of theanalyte. However, with the use of an insulator (the ce-ramic tube), the extent of fragmentation is decreased, andlarger fragments are observed. The stability of the plasmawith aqueous solutions and the possibility of acquiringstructural information are suggested; however, additionalstudy will be required to fully assess the liquid-sampleintroduction potential.

ACKNOWLEDGMENTS

The authors are grateful to the National Institute of EnvironmentalHealth Sciences for providing research support through Grant No.ES-03221 and No. ES-04908. We are also grateful to the NIH-BRSshared Instrument Program for providing the VG Plasma Quad throughGrant S10RR02714. A special thanks to Wei-Lung Shen and Lisa Olsonfor assistance with the MIP MS system. The authors also wish to thankDr. Dennis Fine for his collaboration in interpreting the MS spectrumof organic compounds.

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