Negative Ion Chemical Ionization Mass Spectrometry of Volatile Metal Chelates S. R. Prescott, J.
E. Campana, and T. H. Risby”
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802
The negative Ion chemlcal ionlzatlon mass spectra for 34 volatile metal chelates are reported. Also a sensitivity study uslng a gas chromatograph chemlcal lonlration mass spectrometer computer system comparing detection limits for several Cr( P-diketonates), in both negatlve and posltlve ion detection modes is described. This study Illustrates the ability of negative ion chemical ionlratlon mass spectrometry to provide two to three orders of magnitude Increase in specific Ion currents for compounds contalnlng electronegative functlonai groups.
The purpose of this paper is to report on negative ion chemical ionization mass spectra for several transition metal P-diketonates together with a preliminary study on the use of various reagent gases. Also a relative sensitivity study will be discussed using time resolved scans to compare relative sensitivities in the positive and negative ion detection modes for several fluorinated and nonfluorinated chromium P-diketonates. Finally a quantitative determination of the detection limits for these same chelates using a gas chromatograph chemical ionization mass spectrometer system will be described.
Chemical ionization mass spectrometry has been shown to be a potentially sensitive method of analysis for trace metals (1-4). Detection limits in the picogram range have been reported for several of the first, second, and third row transition metals (4). This earlier work used the more conventional positive ion detection mode in which ionizatiod of the sample occurs by the transfer of a proton from a hydrocarbon reagent ion such as CH5+or C4H9+to the sample molecule. In this paper the, results of a companion study utilizing negative ion chemical ionization mass spectrometry are presented. Recently it has been suggested that negative ion chemical ionization mass spectrometry is more sensitive than positive ion chemical ionization mass spectrometry for those compounds which contain heteroatoms that promote anion formation (5-10). In electron impact mass spectrometry, negative ions are formed by three generalized mechanisms which are dependent on the electron energies and pressure: (1) resonance capture; (2)dissociative resonance capture; and (3) ion-pair production (11). Historically, negative ion electron impact mass spectrometry has been limited by ion currents which are two or three orders of magnitude lower than the corresponding positive ion currenb (12). The higher pressures available in a chemical ionization source produce larger numbers of low energy electrons, which are secondary and thermalized primary electrons. It has been found that a t the normal chemical ionization source pressures (0.7-1.5 Torr) the secondary electron current is several times more intense than the primary electron current, which indicates that the primary electrons are undergoing multiple collisions with the reagent gas (13). Under these conditions it is apparent that molecules which have large cross sections for electron capture (and/or large electron affinities) should form anions efficiently. Often these same molecules will have greater cross sections for electron capture than for proton transfer. This advantage has been exploited successfully in the electron capture gas chromatographic detector. Although the formation of negative ions by chemical ionization mass spectrometry has been studied for several years, it has only recently been shown quantitatively that, for selected molecules, electron capture currenb can exceed their corresponding positive ion currents by two or three orders of magnitude (8).
Chelate Preparation. The 2,4-pentanedionates (acac), the l,l,l-trifluoro-2,4-pentanedionates (tfa), and the 2,2,6,6-tetramethyl-3,5-heptanedionates (thd) of VO(II),Mn(II),Ni(II),Cu(II), Zn(II), Pd(II), Pt(II), Cr(III),Mn(III), Fe(III), Co(III), Rh(III), and Ru(1II) were prepared by methods reported previously (14-16). The chelates were purified by either recrystallization or reduced pressure sublimation. Chromium tris(l,1,1,5,5,5hexafluoro-2,4-pentanedionate)[Cr(hfa)3]and chromium tris(1,1,1,2,2,2,3,3-heptofluoro-7,7-dimethyl-4,6-octanedionate) [ C r ( f ~ d )were ~ ] synthesized by modification of conventional methods and purified by sublimation at reduced pressures (17,
EXPERIMENTAL
18).
Chemicals. 2,4-Pentanedione, H(acac), and 2,2,6,6-tetramethyl-3,5-heptanedione,H(thd) were obtained from Polysciences, Inc., Warrington, Pa.; 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl4,6-octanedione, H(fod), 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, H(hfa), and l,l,l-trifluoro-2,4-pentanedione, H(tfa), from Pierce Chemical Co., Rockford, Ill. Reagent Gases. Methane (99.0%) and isobutane (99.0%)were purchased from Air Products and Chemicals, Inc., Allentown, Pa.; nitrogen (99.99%)was obtained from Philip Wolf and Sons, Inc., Lewistown, Pa.; and argon-10% methane was purchased from Airco Industrial Gases, Rivertown, N.J. Apparatus. The gas chromatograph-mass spectrometercomputer system consists of the following components: gas chromatograph with automatic injection system (HewletbPackard High Efficiency Gas Chromatograph 402B, Hewlett-Packard Automatic Injector 7171A); quartz capillary interface (General Electric Company); high temperature fine control bellows valve (Nupro B.M. Series); chemical ionization mass spectrometer (ScientificResearch Instruments Corporation Biospect System); and computer system (Modcomp II/III, “Lab Box” system and graphics terminal Techtronix 4006-1). Gas Chromatograph and GC/MS Interface. The column is connected so that the column eluent passes directly into the GC/MS interface without passing through a detector. In all the studies which used the gas chromatograph for sample introduction, the carrier gas was also the reagent gas. The column effluent was stream split so that most of it went to atmosphere and 10 mL/min passed into the CI source. This flow rate results in a source pressure of 1 Torr which produced the optimum signal. The GC/MS interface originally consisted of a length (50 cm) of silanized glass lined stainless steel tubing (Supelco Inc., 0.16-mm i.d.) but after a period of time the integrity of the glass lining was found t o deteriorate. The inlet was then replaced by a quartz capillary tube (6-mm o.d., 0.25-mm id.). The interface was connected to the CI source through a fine control stainless steel needle valve which was adjusted so that the required flow rate of column effluent entered the CI source. The interface and needle ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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Table I. Instrumental Conditions Cr(hf a),
Cr(fod), Column temperature Interface temperature Source temperature Methane flow rate Retention time Splitting ratio Sample size Solvent Column
170 "C 190°C 170°C 40 mL/min 1 min 7: 1 0.2 PL
Hexane 3% ov-101 on Gas Chrom Q 1m, 4 mm (i.d.)
valve were wrapped with a heating tape and temperatures were measured at four different points with thermocouples. These temperatures were maintained at approximately 20 "C higher than the column temperature. Mass Spectrometer Modifications. The chemical ionization masa spectrometer was modified so that the detection system could monitor either positive or negative ions in a manner similar to that which has been reported previously (19). In addition, a high voltage relay was used which enables the conversion from positive to negative ion detection systems to be made easily. Another modification was made which enables the quadrupole and detection systems to be computer controlled. Computer Facilities. The GC/MS system is interfaced to The Pennsylvania State University Department of Chemistry's Modcomp II/III computer via the department's "Lab Box" system. The authors have developed a GC/MS software package, CAD, for data acquisition, mass spectrometer control, data manipulation, and display through a graphics terminal in their laboratory (20). Procedure. Methane, isobutane, nitrogen, or argon-10% methane were used as reagent gases (21). Aliquots of known solutions of the metal chelates in toluene or hexane were evaporated onto a direct insertion probe, and the probe was inserted directly into the ionization source. The following are the source temperatures used to sublime the chelates into the mass spectrometer: 140-180 OC (acac), 100 "C (fod), 80 "C (hfa), 130-140 "C (tfa),and 140-160 OC (thd). All other conditions were identical with those previously reported (1,2). The mass-to-charge ratios of the various peaks in the mass spectra were determined by the mass marker, which was calibrated with methyl stearate and Lu(thd)B. A study comparing the relative sensitivities for several Cr(P-diket~nates)~ in the positive vs. the negative ion detection modes was carried out in the following manner: 1.0 FL solutions of Cr(aca~)~, C r ( f ~ d )Cr(hfa)3, ~, Cr(tfa)3,and Cr(thd)3in toluene or hexane (10 ppm) were injected into sections (1cm) of glass capillaries. The solvent was evaporated using an infrared lamp and the capillaries inserted into the CI source via the solids probe. The quadrupole controller was adjusted so that there was unit resolution at the region of the parent molecular ion and a 10-amu region encompassing the parent ion region was repetitively scanned (1s/scan). The resulting time resolution spectra were integrated and relative sensitivities calculated. Quantitative determinations of detection limits for Cr(fod)B, Cr(hfa)3,and Cr(tfa), were made in a similar manner except that the sample was introduced from the gas chromatograph. The gas chromatographic conditions are listed in Table I. Because of their lower volatility,the detection limits for Cr(acac)s and Cr(thd), were obtained using a heated solids probe. Aliquots of the chelates in toluene or hexane were evaporated onto 1-cm capillary sections, placed on the probe, and inserted directly into the CI source. The probe was then heated while the quadrupole repeatedly scanned the parent ion region. This method proved to be reproducible and thus the decomposition problems associated with these chelates when injected into gas chromatographic columns were avoided. Stock solutions were prepared by dissolving a known amount of chelate in toluene or hexane to yield a 10-ppm solution (with respect to Cr). Subsequent dilutions yielded sample solutions ranging in concentration from 10' to 1 PPb. 1502
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
Cr(tfa),
40°C
140 "C 165 "C 140 "C 40 mL/min 6 min
6 5 "C
70°C 40 mL/min 3 min
7:l
6:l 0.2 PL
0.2 PL
Toluene 15% OV-101 on Chromsorb WHP 1m, 4 mm (i.d,)
Toluene 15% OV-101 on Chromsorb WHP 1 m, 4 mm (i.d.) 51 1
I
L
0
I
a.m.u.
Flgwe 1. The methane negative ion chemical ionization mass spectrum
of Cr(tfa)s
RESULTS AND DISCUSSION Reagent Gases 1. Methane. The methane used in our laboratory contains trace amounts of water (-1%) causing intense ions to be observed at 19,37, and 55 m u , respectively, in the positive ion detection mode. These ions correspond to the protonated water monomer, dimer, and trimer. This observation is not surprising since methane is a stronger Bronsted acid than water and will upon collision transfer a proton. CH,' + H,O + CH, t H,O+ C,H,+ + H,O- C,H, t H,O' In the negative ion mode a rather intense ion was observed at 32 amu which was determined to be the superoxide anion,
02'.This ion was due to the water present in the methane and was eliminated by passing the reagent gas through a drying tube containing activated molecular sieves. 2. Nitrogen, isobutane, argon/methane (10%). There was no evidence of reagent ions associated with any of these gases in the negative ion mode. Parent anions were observed for the chelates and there appeared to be no difference except that with nitrogen the maximum signal for the parent ion occurred at a slightly lower source pressure which suggests it is a more efficient moderator of the electron energy. In all cases the major ion observed, disregarding the anion resulting from the ligand itself in a few cases, was the molecular parent ion formed by resonance capture of low energy electrons. An example of the negative ion chemical ionization mass spectrum for Cr(tfa), using methane as the reagent gas is shown in Figure 1. It can be seen from this spectrum that, apart from the parent ion there is only one other ion of major importance (>5% parent ion intensity). This ion occurs at 153 amu and is attributed to the ligand H(tfa). The extreme simplicity of the Cr(tfa), spectrum is not surprising since resonance capture involves electrons with energies near 0 eV. The chelates based on H(acac), H(fod), H(hfa), H(tfa), and H(thd) likewise exhibited little or no fragmentation. Prior to drying the methane, several of the chelate spectra showed another ion in the molecular parent ion region resulting from
Table 11. Relative Sensitivities from Time Resolved Scans
Metal chelate Cr( hfa), t fa 1 3
Cr( fod), Cr( thd )3
Cr(acac),
acac
Mol
Relative sensitivity Positive Negative mode mode
wt, g
(M
673 511 937 601 349
+
1)’ 1 1 1 10 10
(MI5000 100 10 1 1
tfa
an ion-molecule reaction between the superoxide ion ( O i )and the chelate to form an associated ion [M + O,]-. This ionmolecule reaction was observed with all the metal l,l,l-trifluoro-2,4-pentanedionates, but only with a few of the divalent 2,4-pentanedionates or 2,2,6,6-tetramethyl-3,5-heptanedionates hfa (VO, Zn, and Ni). Charge delocalization caused by the presence of many electronegative fluorine atoms contained by the metal l,l,l-trifluoro-2,4-pentanedionates most certainly contributed to the stability of the ion-molecule product. In order to verify this assumption, another fluorinated P-diketonate Cr(hfa), was studied. The Cr(hfa), spectrum also contained an [M + OJ ion which disappeared when the thd methane was dried. The fact that the [M + 02]ion was observed with several of the divalent 2,4-pentanedionates and 2,2,6,6-tetramethyl-3,5-heptanedionates results from the divalent metal chelates containing two unoccupied coordination sites. Several first row transition metal chelates of the ligands H(hfa) and H(tfa) have been analyzed by 70-eV negative ion FH3 C H mass spectrometry (22,23). In most cases the spectra were CH,-C-C~--?C-CF,CF,CF~ it fod simple with large ion currents located in the molecular parent 3 o, ion region. However, in other cases the most dominant peaks Figure 2. Structure of various Pdiketonates observed were fragments resulting from fluorine migrations. Another interesting result was that during the analysis of Co(hfa), the ratio of negative to positive ions in the ion source in the sensitivity of these metal chelates in the negative ion was (1:3). This observation indicates that the fluorinated mode. The expected decrease in the electron capture cross P-diketonates have large electron capture cross sections, and section of the Cr(thd)3and Cr(acac)B, due to the presence of if large low energy electron currents can be generated (i.e. the two electron donating tert-butyl groups on the H(thd) ligands and two-electrons donating methyl groups on the H(acac) CI source), high sensitivities can be achieved. ligands, is demonstrated by the increased sensitivities of A sensitivity study was undertaken using solutions of Cr(tfa)3 (10 ppm) in toluene to determine the sensitivity Cr(thd)3 and Cr(acac)a in the positive ion mode. Cr(fod), which contains a ligand having an electron releasing tert-butyl difference between the positive and negative ion modes. The method of injecting solutions directly into the CI source was group on one end and an electron capturing heptafluorinated propyl group on the other end is sensitive to both proton used which has been previously discussed ( 3 ) . While the method of direct injection into the CI source is useful, it suffers transfer and electron capture. from problems of perturbation of the CI source which are more A quantitative sensitivity study was made using C r ( a ~ a c ) ~ , acute in the negative ion mode. Unfortunately, together with Cr(fod),, Cr(hfa)3,Cr(tfa),, and Cr(thdI3 in toluene or hexane the negative ions, electrons are also transmitted by the (1ppb-1 ppm by weight in terms of Cr). As discussed earlier, quadrupole mass filter and impinge on the ion multiplier there were some problems associated with the direct injection causing a noise level of 100 mV to be observed when the ion port related to the amount and type of solvent injected into multiplier and electrometer are maximized. This “system” the mass spectrometer. Because of the difficulties associated noise is directly proportional to the electron density in the with this sample introduction system, it was decided to couple CI source and, when injections are made through the direct a gas chromatograph to the mass spectrometer in order to injection port, the “system” noise increases dramatically separate the solvent and the chromium chelate. The resulting because of ionization of the solvent. It was for this reason area of the mass chromatogram was measured and a calithat it became apparent that an alternative method of sample bration curve plotted. The relationship between the area of introduction would be needed to determine the sensitivity the mass chromatograms and the concentration of the Crdifference between the positive and negative ion modes. (P-diketor~ate)~ was linear for three to four orders of magnitude Another routine method for determination of sensitivity in both the positive and negative ion modes. The minimum was undertaken by the use of time resolved spectra (24-28). detectable limits, after taking into account the split ratio of A series of Cr(P-diketonates) was analyzed by this method with the sample prior to entrance into the mass spectrometer for the results summarized in Table 11. (Figure 2 illustrates the the chelates introduced through the gas chromatograph, are structures of the ligands involved in this study.) The increased summarized in Table 111. As expected, the minimum deelectron capture cross section of the Cr(tfa), and Cr(hfa), due tectable limits in the negative ion mode for the fluorinated to the presence of nine and 18 electronegative fluorine atoms chromium chelates are greater than those for Cr(acac)3 and Cr(thd)3. This sensitivity increase is attributed to the large on each metal chelate, respectively, is reflected in the increase ,@
ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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Table 111. Minimum Detectable Amounts Minimum detectable amount, g Cr
Positive
Negative
Metal chelate
mode (M + 1)’
mode
Cr( hfa), Cr(tfa), Cr( fod), Wthd), Cr(acac),
1.3 x 10-9 1.2 x lo-”
2.4 X 10-1,
1.9 x lo-’* 8.6 X lo-’’ 8.6 X
(MI-
2.9 x 1043 1.9 x 10-1, 8.6 X lo-” 8.6 x lo-’’
cross section for electron capture provided by the fluorine atoms on the H(fod), H(hfa), and H(tfa) ligands which promote anion formation. Increased sensitivity towards cation formation due to the electron releasing inductive effect of the methyl and tert-butyl groups associated with the chelates Cr(acac)3 and Cr(thd)3is demonstrated by the higher sensitivities in the positive ion mode.
CONCLUSIONS
-
inductive effect. As a result the probability for electron capture is as follows: hfa > tfa > fod > thd acac.
ACKNOWLEDGMENT We thank G. W. Gokel, P. C. Jurs, R. A. Olofson, and M. Shamma for their helpful discussions.
LITERATURE CITED T. H. Risby, P. C. Jurs, F. W. Lampe, and A. L. Yergey, Anal. Chem., 48, 161 (1974). T. H. Rlsby, P. C. Jurs, F. W. Lampe, and A. L. Yergey, Anal. Chem., 48, 726 (1974). S. R. Prescott, J. E. Campana, P. C. Jurs, T. H. Risby, and A. L. Yergey, Anal. Chem., 48, 829 (1976). J. J. Dulka and T. H. Risby, Anal. Chem., 48, 640A (1976). R. C. Dougherty, J. Dalton, and F. J. Biros, Org. Mass Spectrom., 8, 1171 (1972). H. P. Tannenbaum, J. D. Roberts, and R. C. Dougherty, Anal. Chem., 47, 49 (1975). R. C. Dougherty, J. D. Roberts, and F. J. Biros, Anal. Chem., 47, 54 (1975). D. F. Hunt, G. C. Stafford, Jr., F. H. Crow, and J. W. Russel, Anal. Chem., 48, 2098 (1976). D. F. Hunt, T. M. Harvey, and J. W. Russell, J. Chem. Soc., Chem Commun., 151 (1975). D. F. Hunt, C. N. McEwen, and T. M. Harvey, Anal. Chem., 47, 1730 (1975). J. 0. Dillard, Chem. Rev., 73, 589 (1973). C. E. Meiton, “Principals of Mass Spectrometryand Negative Ions”, Marcel Dekker Inc., New York, N.Y., 1970, Chapter 7. C. E. Klotz, “Fundamental Processes in Radiation Chemistry”, P. Ausloos, Ed., John Wiley and Sons, New York, N.Y., 1966, Chapter 1. R. W. Moshier and I?.E. Severs, “Gas Chrwnatogaphy of Metal Chelates”, Pergammon Press, Elmsford, N.Y., 1965, references cited therein. G. Guiochan and C. Pommler, “Gas Chromatography In Inorganics and Organometallics”, Ann Arbor Science Publishers, Ann Arbor, Mlch., 1973, references cited therein. S.R. Prescott and T. H. Risby, in preparation. R. E. Sievers, R. W. Moshler, and M. L. Morris, Inorg. Chem., 1, 966 (1962). R. E. Slevers, J. W. Connoliy, and W. D. Ross, J. Gas Chromatogr.,5 , 241 (1967). A. L. C. SmW, M. A. J. Rossetto, and F. H. Field, Anal. Chem., 48, 2042 (1976). J. E. Campana, “CAEA Publication No. 456-76”, pp 1-5. T. H. Risby, “CAES Publication No. 486-76”, p 5. I. W. Fraser, J. L. Garnett, and Ian K. Greggor, J. Chem. Soc., Chem. Commun., 365 (1974). I.W. Fraser, J. L. Garnett, and I.K. Greggor, Inorg. Nucl. Chem. Left., 10, 925 (1974). A. E. Jenklns and J. R. Majer, Talanta, 14, 777 (1967). A. E. Jenkins, J. R. Majer, and M. J. A. Reade, Tabnta, 15, 1213 (1966). J. R. Majer, M. J. A. Reade, and W. I. Stephen, Tabnta, 15. 373 (1966). R. Belcher, J. R. Majer, R. Perry, and W. I. Stephen, Anal. Chim. Acta, 43, 451 (1968). E. R. Kowalski, T. L. Isenhour, and R. E. Slevers, Anal. Chem., 41, 998 (1969).
On the basis of this study, we feel that it is possible to postulate the mechanisms which account for the relative sensitivities (Table 11). In the positive ion mode, the chelate is protonated either at the methylene carbon or at the oxygen atoms. The reason why the 2,4-pentanedionate or 2,2,6,6tetramethyl-3,5-heptanedionateproduces a larger ion current that the 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate, l,l,l-trifluoro-2,4-pentanedionate, or 1,1,1,5,5,5hexafluoro-2,4-pentanedionateis that the electron releasing inductive effect of the methyl or tert-butyl groups stabilize the cation whereas the trifluoromethyl or heptafluoropropyl groups have electron withdrawing inductive effects which destabilize the cation and decrease the probability of protonation. The presence of the tert-butyl group does not have the effect of increasing the stability of the cation as compared to the methyl group because of the reduced effect over an extra carbon atom and increased steric hindrance. Therefore the probability for proton transfer is as follows: acac > thd > fod > tfa > hfa. In the negative ion mode, the chelate captures an electron which is stabilized either in the A electron system on the P-dienolate ring or on the oxygens. Once again the side groups affect the stability of the anion and the trifluoro methyl or heptafluoropropyl groups stabilize the ions through electron RECEIVED for review April 2, 1977. Accepted June 29, 1977. withdrawing inductive effects while the methyl or tert-butyl Work supported by the U.S. Environmental Protection groups destablize the anion through an electron releasing Agency, Grant No. R803651-20.
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