Anal. Chem. 1986, 58,679-684 (23) Horning, E. C.; Stillwell, R. N.; Nowlin, J. G.; Carrol, D. 1. Anal. Chem. 1881, 53, 2007-2013. (24) Smith, D. E.; Smith, J. S.; Jerolamon, D.; Weston, A. F.; Richton, D.; Brozowski, E. J. Presented at the 26th Annual Conference on Mass Spectrometry and Allied Topics, St. Louis, MO, May 28-June 2, 1978. (25) Munson. 8. I n “Interactions Between Ions and Molecules”; NATO Advanced Studies Institute, Series B.; Ausloos, P., Ed.; Plenum: New York, 1975; pp 505-525. (26) Ceili, R.; Weddle, G.;Ridge, D. p. J . Chem. PhYS. 1980, 73, 801-812. (27) Denbigh, K. G. Trans. Faraday SOC.1940, 36, 936-947.
679
(28) Sternberg, J. C.; Gallaway, W. S.; Jones, D. T. L. I n “Gas Chromatography”; Brenner, N., Callen, J. E., Weiss, M. D., Eds.; Academic Press: New York, 1962; pp 231-267. (29) Utterback, D. F.; Millington, D. S.; Gold, A. Anal. Chem. 1984, 56, 470-473.
RECEIVED for review July 26, 1985. Accepted November 20, 1985. This work was supported in part by a grantfrom the National Science Foundation, CHE-8312954.
Electron, Chemical, and Thermal Ionization Mass Spectra of Alkali Halides F. Aladar Bencsath* and F. H. Field
The Rockefeller University, New York, New York 10021
We report the electron Ionization (E1 ), various chemical Ionlzations (CI), and thermal lonlzatlon spectra of alkali hallde salts obtained wlth an unmodlfled commercial mass spectrometer. The salts studied were CsI, CsBr, CsCI, CsF, RbI, K I , NaI, LII, LiF, and NaF. The posltlve E1 and C-I spectra consist of alkali ions and alkali Ion clusters. The negative Ion CI spectra conslst of hallde Ions and halide Ion clusters. Molecular Ions and quasi-molecular ions are observed wlth only small Intensities. The C I sensitlvlty Is about an order of magnitude higher than the E1 sensltlvlty. Useful E1 spectra are obtained from 0.5 pg of sample, and a tenth as much can be detected. The thermal lonlzatlon spectra consist solely of the alkali cations. Thermal ionization sensitivity was 5-6 orders of magnitude higher than that of EI.
Because of their low volatility, the metal salts of mineral acids are generally considered to be intractable for routine analytical work with commercial mass spectrometers oriented for the analysis of organic compounds. Measurements of salts are usually made with mass spectrometers incorporating specialized sources such as high-temperature Knudsen cell (1-3), radio frequency spark (4),field desorption (5), and thermal ionization (6). One finds, however, some mention in the literature of the adequacy of more or less conventional commercial mass spectrometers to cope with salts. Several years ago we pointed out (7) that a slightly modified commercial quadrupole mass spectrometer could be used to determine isotopic ratios in lithium. Hunt and co-workers (8) obtained excellent CI spectra with intense molecular ions from potassium benzoate, and White (9) obtained E1 spectra of alkali metal salts of several organic acids, the spectra containing the metal cation and clusters involving the metal cation. Rosenstock and co-workers (IO)using a noncommercial mass spectrometer found in 1955 that copper halide salts placed on an electrically heated platinum filament evaporated sufficiently in the mass spectrometer source to give rise to characteristic spectra. In 1977 Soltmann and co-workers (11) used an activated field emitter wire to volatilize inorganic alkali metal salts in a slightly modified field desorption source, and the resultant E1 spectra did not differ from those obtained by Knudsen cell mass spectrometry (12). Although their source and probe setup was not the kind used in general purpose mass spectrometers, they pointed out that their 0003-2700/86/0358-0679$01.50/0
method, based on the rapid sample volatilization from a heated thin wire, might be used with commercial instruments. Volatilizing samples from a rapidly heated wire is the basis of the desorption chemical ionization (DCI) method (13), which as proved to be useful for the analysis of involatile and/or thermolabile organic compounds. Because of the development of this method in the last half decade, the DCI probe has become available as a standard or optional sample introduction device in commercial mass spectrometers designed for organic samples. We recently had occasion to attempt to obtain with our DCI probe and commercial mass spectrometer the mass spectrum of a biochemical sample that apparently had been badly contaminated with potassium iodide. We observed abundant K+ and I- ions in the positive and negative spectra, respectively. This interesting finding prompted us to undertake the systematic study reported here. The first series of measurements were made with our mass spectrometer focused to collect ions produced by electron ionization and chemical ionization from gaseous salt molecules evaporated from the DCI probe. We think it of interest and little recognized that spectra of alkali halide salts can be obtained from a completely unmodified contemporary commercial mass spectrometer. Furthermore, the spectra themselves are of interest in that little information seems to be available on the E1 spectra of the alkali halide species emitted from a hot wire, and no information at all is available about the positive and negative CI spectra of such species. In the course of making these measurements, we by chance discovered that our mass spectrometer could be refocused to collect ions formed on the DCI probe wire by thermal ionization. A study was then made using this mode of operation of the instrument.
EXPERIMENTAL SECTION Apparatus. Mass spectra were obtained with a VG 70-250 double-focusing magnetic instrument. Spectra collection and data manipulation were accomplished by the data system of the instrument. The VG DCI probe with programmable heater controller was used for sample introduction. Sample Loading and Running. Aqueous stock solutions (2 pg/pL) of CsF, CsC1, CsBr, CsI, RbI, KI, NaI, and LiI were prepared for the E1 and CI experiments. For thermal ionization experiments, another set of stock solutions (100 ng/pL) was prepared of those salts listed above and additionally of NaF and LiF. The salts were obtained from Matheson, Coleman and Bell, Norwood, OH; Mallinckrodt, Inc., St. Louis, MO; and Alfa Products, Beverly, MA, and they were all analytical reagent grade 0 1986 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
or equivalent. For the E1 and CI experiments the stock solutions were diluted so that 4 p L of solution contained the desired amount of salt to be inserted into the mass spectrometer. The dimensions of the DCI probe tip assembly are such that three complete windings of the 0.005 in. (0.127 mm) diameter Pt wire fills the space between the support wires. A 4-pL sample of solution applied to the wire filled the space defined by the Pt coil with a fluid bead of uniform shape. For the E1 runs 8 pg of salt comprised the sample; for CI runs 4 pg was used. For the much more sensitive thermal ionization experiments a new Pt probe coil was fabricated with a coil diameter of 0.8 mm. Dilutions of the salt solution prepared for thermal ionization were made so that the desired amount of salt to be inserted into the mass spectrometer was contained in 1pL. The amount of salt inserted was usually 10 ng. Before a run the coil was heated by the maximum current supplied by the heater controller for 15-20 min until the alkali ion intensities were reduced to a relatively low, constant base line level. The solvent in the bead of solution in the probe coil was evaporated in a current of warm air, which produced a uniform distribution of solid salt on the coil. In making a run for all modes of ionization, the current through the coil was increased at a linear rate from 0 to 1.3 A in 16 s and then held at this value until the end of the run. The end of the run was taken as the time when the ion intensity had fallen to 10% of its maximum value. Memory effects in the E1 and CI measurements were eliminated by limiting the sample size to 8 pg and 4 pg, respectively, and by carefully cleaning the probe wire after each run by flaming it with a torch. Temperature Calibration of the DCI Probe. The temperature of the DCI probe as a function of the heating current was of particular interest. We visually estimated the temperature of the Pt coil carrying a given current by comparing its color with that of a thermocouple junction heated by the flame of a torch. The Pt coil was inside the vacuum envelope of the mass spectrometer (viewed through a glass window). Measurements were made with two different pressures in the mass spectrometer envelope: IO4 torr to simulate E1 and thermal ionization conditions and 0.5 torr of CH4or N2 to simulate CI conditions. The coil temperature as a function of current is given in Figure 1. The error bars represent the extreme temperature values from replicate color matches at a given coil current. The eye is quite sehsitive to small color changes, and we estimate the accuracy of our temperature measurements to be f 4 0 "C. The power supply for the DCI probe in the VG 70-250 mass spectrometer provides a maximum current of 1.3 A, and this limit is represented by the dotted vertical line in Figure 1. One sees that this maximum current corresponds to a temperature of 900 "C under E1 and thermal ionization conditions and 700 "C under CI conditions. Mass Spectrometer Parameters. The instrumental conditions in both the E1 and CI modes were as follows: source temperature, 250 "C; emission current, 0.5 mA; ionizing energy, 200 V. The gain of the electron multiplier was 2 X lo5 a t the 1400 V used. The ion current amplifier was set to lo4 A/full scale (10 V), i.e., lo7 V/A. Methane was used as reactant gas for positive chemical ionization and electron capture negative ionization. Its flow was adjusted until the pressure in the source envelope was between 8X and 9 X mbar. The pressure in the source was measured with a Texas Instruments quartz spiral gauge and stainless steel tube passing through the sample probe insertion assembly. For this measurement the source end of the stainless steel tube was equipped with a ceramic tip matching the sample probe receptacle on the ion source. For methane, a source envelope pressure of (8-9) X 10" mbar corresponded to a source pressure of 0.5 f 0.05 torr. For OH-/NCI a mixture of CH4and NzO (4:l) was used. This mixture has been used in our laboratory for general OH-/NCI work. The instrumental conditions for the thermal ionization experiments were rather different from those used in the E1 and CI experiments in that the ion beam focusing conditions were different. To collect thermal ionization ions, changes had to be made in the position of the ion exit slit, in the potentials on the beam centering electrodes, and in the potentials on the beam focusing electrodes. The need for different focusing parameters
Table I. Electron Ionization Positive Spectra (8 fig Samples) re1 abundance (70of base)b X' MX'. M2Xt M3Xz+
sample
TIC"
M'
LiI NaI KI RbI CsI CsBr CsCl CsF
5900 11000 11200 9200 11900 6700 5900 5100
44 100 100 100 100 100 100 100
100 44 31 27 24 5 3 0
3 12 8 7 8 0 0 1
42 15 8 5 6 0 3 1
1 0 0 0 0 0 0 0
" Integrated total ion current. Minimum amount tabulated
=
1%.
Table 11. Methane Positive CI Spectra (4 pg Samples) sample
TIC"
M'
re1 abundance ( % of baseIb X' MX'. MXH' MzX' M3Xz+
LiI NaI KI RbI CSI CsBr csc1 CsF
11200 40 600 61 200 69 700 115000 60 700 81 700 92 100
29 100 100 100 100 100 100 100
7 5 2 1 1 0 0 0
2 4 2 2 1 0 0 0
13 5 1 1 1 0 0 1
100 13 5 2 1 0 2 2
4 0 0 0 0 0 0 0
"Integrated total ion current. bMinimum amount tabulated = 1%.
Table 111. Electron Capture Negative CI Spectra (4 pg Samples) sample
TIC"
LiI NaI KI RbI CSI CsBr CsCl CsF
5 500 17600 21 200 27 300 31200 3800 2400 0
re1 abundance ( % of base)b XMXP MZXS 100 100 100 100 100 100 100
14
5 6 3 3 0 0
1 0 0 0 0 0 0
"Integrated total ion current. *Minimum amount tabulated = 1%.
must result from the fact that the ions collected in the E1 and CI experiments are coming from one spatial location (the gas phase in the ionization chamber) and the ions in the thermal ionization experiments are coming from another spatial location (the surface of the Pt wire of the DCI probe). The discrimination between the two kinds of ions was quite sharp: when the tuning was set to maximize thermal ions, less than 1% of gaseous ions present could be collected, and conversely. In addition to these changes in focusing conditions, for the thermal ion experiments the electron emission current was set to zero and the gain of the ion current amplifier was decreased by a factor of 10 (to lo6 V/A).
RESULTS AND DISCUSSION As is depicted in Figure 1,the temperature of the DCI probe under E1 and thermal ionization conditions reaches about 900 O C when the maximum heater current from the VG power supply is applied to it. Since earlier investigators (2,3)used Knudsen cells at temperatures between 500 and 1000 "C in their high temperature work, and since the best-emitter temperatures for sodium halides in Soltmann and co-workers' E1 study (11)fell into the only slightly higher range of 600-1150 "C, our DCI probe temperature range is comparable with those used previously to volatilize alkali metal halides.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
lnnl
50
-
881
m / z 133 Cs+
1200
T "C
50 IO 09
IO
II 12 13 14 PROBE CURRENT (AMP)
15
o
16
Figure 1. Temperature of the DCI probe tip wire vs. heating current. Curve A was obtained at a pressure of 2 X lo-' torr, Le., under E1 conditions. Curve B was obtained at a pressure of 0.5 torr with methane (e)or nitrogen (V),i.e., under CI condltlons. The dotted vertical line at 1.3 A represents the maximum current provided by the DCI probe heater supply in the mass spectrometer.
Table IV. OH- Negative CI Spectra (4 wg Samples) re1 abundanceb sample
LiI NaI
KI RbI CSI CsBr CSCl CsF
TIC"
X-
MXOH-
MXC
MzXC
11100 25400 25500 19900 15400 4900 3400 0
100
0
100 100 100 100 100 100
2 3 1 3 5 7
26 15 10 6 9 I3
1 0 0 0 0 0
7
0
Integrated total ion current.
Minimum amount tabulated =
1%.
The results of our E1 and CI studies are given in Tables I-IV. Each quantity tabulated is the average of two to five replicate runs. The relative abundances given are calculated from the summed intensities of the isotopic ions involved in each ion species. The quantity tabulated as JTIC (integrated total ion current) is obtained by integrating the mass chromatograms (such as those in Figure 2) of the sample ions in a spectrum across the scans comprising the evaporation of the sample and then adding the integrals. The reproducibility in the JTIC in successive replicate runs is such that average deviations from average are about 30%, which becomes somewhat poorer when the replicates are made on different days. The deviations from average of the relative intensities are a few percent or less for the major ions and about 30% for the minor ions. The electron ionization spectra are given in Table I. The total ion current is largely constant for the iodides with different cations, except that the JTIC for LiI is low by about a factor of 2. The M+ ion (alkali metal ion) is most intense for all compounds except LiI, for which I+ is most intense. The variations in the relative intensities of M+ and X+ from one compound to the next are compatible with one's expectations based on considerations of ionization potential trends in M and X. The intensities of MX+- are small, which manifests small stabilities for these molecule ions. All of the compounds except CsBr exhibit M2X+ions, and LiI exhibits a low intensity Li312+ion. The relative intensity of the Li21+ ion is 42%, and the intensities of the analogous ions are smaller for salts with larger cations. Presumably the reaction producing the M2X+ions is M2X2(g) + e-
-
-
[M2X2+-] M2X+ + X.
(1)
and analogously for the formation of Li31,+. The trend of increasing tendency to form MX2+ions with smaller cations
0.5
20 40 60 80 io0 120 140 160scans I
2
1.5
2.5
3
3.5
Figure 2. Mass chromatograms of Cs', CsI'., and Cs$: CsI; E1 mode.
minutes 8 pg of
may be at least partly the result of increasing polarization and bonding of the X atoms, which, of course, reaches its maximum with Li+. Interestingly, during the evaporation of the salts, no meaningful change occurs in the relative intensities of the several ions for a given compound in spite of the continuously rising temperature. An example of this behavior for three ions from CsI is given in Figure 2. The identical mass chromatograms for Cs+ and Cs21+are evidence that the CsJ+ is not formed by a gas-phase ion/molecule reaction between Cs+ and CsI, and they also suggest that CsI and CszIz are evaporating simultaneously. As a confirmation of this conclusion, we plotted the ratio IcsI+/Ics+against scan number for three replicate evaporations of CsI samples. No meaningful trend in the plot could be observed. A linear regression made on the data gave A(s1ope) = -0.00092, B(intercept) = 0.033, and r(corre1ation coefficient) = -0.21. These figures also demonstrate no significant variation of the intensity ratio as a function of scan number (wire temperature), which result requires a simultaneous evaporation of the two species. The intensities of M2X+ions in our spectra are low in comparison with those obtained with Knudsen cell high-temperature mass spectrometry (14),which is surely the result of the lower, nonequilibrium pressure of the salts in our experiments. The intensities of the E1 salt spectra are fairly high; the integrated total ion currents are about 5 times less than those obtained from identical amounts of organic materials. A 0.5-pg sample of CsI was enough to obtain a characteristic spectrum, and 0.1 of this m o u n t was detectable. The mass spectrometer parameters used in these experiments produced medium instrumental sensitivity. The methane positive CI spectra are given in Table 11. The most notable difference between these spectra and the E1 spectra is that the intensities (JTIC) are significantly higher in the CI spectra. Thus, making an adjustment for the differing sample sizes used in the two types of spectra, one sees that the CI intensity gain in the iodide series is a factor of about 20 for Cs and Rb, 10 for K and Na, and 3 for Li. As in the E1 spectra, M+ is the most intense ion in all the compounds except LiI, for which LizI+ is most intense. The relatively high proclivity of LiI to produce cluster molecules noted in the E1 spectra is also manifested in the CI spectra. Presumably the reaction producing the M+ ions is
MX
+ CHS+ MXH+
-
-
MXH+
+ CHI
M+ + HX
(24 (2b)
and analogously for the formation of M2X+and M3X2+.C2H5+ may also serve as a reactant ion in reaction 2a. The low intensity of the MXH' ion shows that the ion as formed by proton transfer from CH5+is not very stable with respect to decomposition (reaction 2b). The decrease in intensity of MXH+ in progressing from LiI to KI may be attributed to the decrease in the energies of the gaseous cation (for Li+, Na+, and K+ AH,= 162, 144, and 122 kcal/mol, respectively (15)).
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
The X+ ions are produced with much lower abundances in CI than in EI. Energetic reasons may again be invoked; the thermochemistry of reaction 2b producing M+ is surely more favorable than that of the reaction
-
+
MXH' Xt MH (2C) producing X+. The small amounts of MX+. shown in Table I1 are probably produced by the reaction of some odd-electron ion produced in the methane reactant gas, perhaps C2H,+.. The electron capture negative CI spectra are given in Table 111. The absolute intensities lie intermediate to those obtained with positive E1 and CH4 positive CI. The trend of the total ioh current is interesting in that it rises for the iodides as the cation increases in size from Li to Cs, but then falls off sharply for the Cs salts as the anion decreases in size from I to F. This decrease in intensity with decreasing anion size may tentatively be rationalized on the basis of the concept advanced by one of us (16) that the dissociative electron capture in a molecule with no low-lying vacant orbitals depends upon a competition between the electron affinity of the electronegative atom and the dissociation energy of the bond which ruptures as a result of electron capture. Energies do not seem to be available for all of the gaseous molecules, atoms, and ions involved in the compounds listed in Table 11, but from the partial information that is available (1517) one calculates that the bond dissociation energy decreases rather sharply as the anion increases in size from F to I with a given cation. Thus in the series LiF, LiC1, and LiI, D(LiX) = 137,114, and 86 kcal/mol, respectively, and for CsF and CsC1, D(CsX) = 122 and 104 kcal/mol, respectively. The variation of the electron affinities of the halogens lies within a rather narrow range (EA(F, C1, Br, I) = 79,84,76,71 kcal/mol, respectively), and we suggest that the failure to observe F- ions from CsF is the consequence of the high bond dissociation energy in this compound (122 kcal/mol). Weaker bond energies when F is replaced by C1, Br, and I permit tlie corresponding ions to be observed with increased intensities. These considerations make it dubious that any alkali fluoride will produce F- ions by electron capture. The X- ion dominates the spectra for all of the compounds for which ions are formed, and no MX-. ions are observed. Thus the electron capture is completely dissociative. Cluster ions are observed in LiI with greatest intensity. The OH- negative chemical ionization spectra are given in Table IV. The absolute intensities are roughly comparable with those obtained by electron capture. The X- ion is dominant, and LiI continues to exhibit the largest cluster ion intensities. However, for unknown reasons the cluster ion intensities for the other compounds are higher than those produced by the other modes of ionization. Small intensities of the MXOH- association ions are also observed. The intensities in the series CsI, CsBr, CsC1, and CsF decrease in a manner very similar to that found in the electron capture spectra including no formation of F- from CsF. We suggest that the X- ions are produced by a nucleophilic displacement reaction OH-
+ MX
-+
MOH
+ X-
(3)
and here also it is possible that the magnitude of the bond dissociation energy in MX may determine whether reaction 3 occurs and with what rate. Our failure to observe F- from CsF raises the possibility that F- will not be produced from other alkali fluorides. A typical thermal ionization run (using CsC1) is shown in Figure 3. The upper box displays the mass chromatogram of the Cs+ ions. The decay of the Cs+ ion signal to 10% of its peak value required about 4 min, which was significantly longer than the time required to evaporate micrograms of sample in the E1 and CI modes. The lower box displays the
m/z
133
I133
20 40 60 80 100 120 140 160 180 200 I 2 3 4 5
scans minutes
m/z Figure 3. CsCl thermal ionization run, 10 ng of CsCI: (upper Box) mass chromatogram of m / z 133 (I,,, in arbitrary units); (lower box) mass spectrum at the maximum of the mass chromatogram curve.
Table V. Comparison of Thermal Ionization and Electron Ionization Sensitivities compound LiI NaI
KI RbI CSI CsBr csc1 CsF
IM+0 1.5 x 2.6 X 4.9 x 2.1 x 2.8 X 2.7 x 3.7 x 3.4 x
105
lo6 106 106 lo6 106 106 106
TIS/EISb 2.0 x 2.0 x 3.5 x 1.8 X 1.9 x 3.2 X 5.3 x 5.3 x
105 106
106 lo6
106 lo6 106 106
a Integrated intensity of the metal cation obtained from 10 ng of sample. Thermal ionization sensitivity divided by electron ionization sensitivity. Computed from the integrated ion intensities in the second column of this table and the JTIC values from Table I. The different sample sizes and detection sensitivities have been taken into account.
mass spectrum a t the peak of the sample evaporation. Although the detection sensitivity was 10 times lower than that used for E1 runs and 800 times less sample was inserted in the thermal ionization run, the thermal ionization intensity of the Cs+ ions is still about 4 times higher than the E1 sensitivity. Thus the response for Cs+ ions in the two modes differs by a factor of 32000. Because of the longer sample life in the thermal ionization mode, the sensitivity ratio is even larger when the integrated intensity values are used for its calculation. The results of such calculations are given in Table V. The thermal ionization mass chromatograms of the several cations (such as that for Cs+ in Figure 3) were integrated from the scan at which the ions first appeared to scan 220, at which time the ion intensities had reduced to about 10% of their maximum values. The integrated intensities obtained for the compounds studied are given in Table V. The values tabulated are the averages of three to five replicates. Table V also contains a comparison of the integrated intensities by thermal ionization with the TIC values by EI, and one sees that the thermal ionization sensitivity is 105-106times greater. Perhaps because of the high sensitivity of thermal ionization, impurities sometimes manifest themselves rather strongly. For example, in the CsCl spectrum given in Figure 3, three peaks in addition to that of Cs+ may be observed. These correspond to Na+ (m/z 23) and the isotopes of K+ (m/z 39 and 41). After running several water samples of different purity and origin, we concluded that in this case the source of the sodium was the deionized water used for the preparation of the CsCl solution while the source of the potassium ions
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
683
2400 Pt wire and the relative inefficiency of the electron ionization process. Presumably in thermal ionization the transport of
MX from the solid to the gas phase entails the formation of an M+ gaseous ion with high probability. By contrast, in
2000
/I153 ARB.
1f
-
1200
UNITS
800
400
I/ U
0
I
40
80
120
160
200
240
ng CsI
Flgure 4. Instrument response vs. C s I amount In thermal ionization mode,
I ,33
in arbitrary units.
was the platinum wire itself. Most of the background sodium could be eliminated by sonicating the probe tip in methanol and then flaming it, but the potassium ions persisted after this cleaning procedure. The high K+ background sets a sensitivity limit for analytical use; for the insertion of 10 ng, KI produces spectra a t the evaporation maximum with K+ ion intensities about 10 times higher than background, and the ratio between the integrated ion intensities in sample and background is about 20. Thus the minimum detectable amount of KI is about 1 ng. We determined the ion intensity vs. sample relationship for the thermal ionization method in order to establish the dynamic range for practical applications. For that purpose, different amounts of CsI between 0 and lo00 ng were run, and the integrated ion current at m/z 133 was plotted against the amount of sample (Figure 4). The plot is of the form of a saturation curve, and internal standards will be necessary for accurate quantitative work. Samples larger than about 200 ng do not effect any significant increase in the integrated ion current; thus the upper limit of the dynamic range is approximately 200 ng. The maximum Cs+ ion intensity occurs with 40 ng of CsI; larger amounts increase only the width of the mass chromatogram but not its height. We suggest the following as an explanation of these phenomena. Thermal ionization is a surface phenomenon. Until the surface of the Pt wire is fully covered with sample, the addition of more sample will increase the ion current. If the amount of sample is increased to give more than monolayer coverage, the excess sample will only contribute to an increase of the duration of the ion current, not its intensity. We attribute the change in behavior at 40 ng of CsI to the transition from monolayer to multiple layer coverage. As a check on this hypothesis we have calculated the amount of CsI required to provide monolayer coverage of our Pt wire, and the value we obtained assuming ionic radii of Cs+ and I- of (1.69 and 2.16) X cm, respectively (18),is 7 ng. Given the very elementary quality of our calculation, the agreement of this value with the value of 40 ng deduced from the ionization behavior is satisfactory. The limiting integrated intensity setting in a t about 200 ng can be explained by the hypothesis that CsI layers formed by sample amounts in excess of 200 ng are bound weakly enough that they evaporate before they can make a contribution to the formation of Cs+ by thermal ionization. The decreasing response with added sample between 40 and 200 ng of sample may be explained by postulating a decrease in the binding of successive CsI layers, which results in increasing evaporation and smaller contributions to the formation of Cs+ by thermal ionization. The very much greater sensitivity of thermal ionization compared with E1 ionization may be attributed to the combination of better collection of ions coming directly from the
electron ionization the entities initially produced in the gas phase are MX or M2Xzmolecules, and the conversion to M+ or M2X+ ions requires that the molecules be struck by one of the bombarding electrons. Under the conditions obtaining in our experiments this will happen to only a small fraction of the gaseous molecules. We pointed out earlier that the sensitivity by chemical ionization is greater than that by electron ionization, and we suggest that this results from a higher ionization efficiency in the CI mode. The alkali halides used in the main body of this work were chosen to form a logical set: all cations were combined with one anion and all anions were formed with one cation. However, this set does not include the least volatile of the alkali halides, which are LiF (bp 1679 "C (19))and NaF (bp 1695 "C (19)). We found that appropriate ions were produced by both of these compounds in both the electron ionization and the thermal ionization modes, and thus the conditions in our experiments are sufficient to produce spectra from all the alkali halides. As an exercise to investigate in a preliminary way the analytical utility of our equipment and technique, we attempted to construct an ion intensity response curve for Na+ ions from NaI using the thermal ionization mode. Solutions were made of mixtures of 20 ng/pL of RbI with amounts of NaI varying between 0 and 80 ng/pL. The RbI served as an internal standard. Thermal ionization spectra of the several solutions were run, and the integrated intensities at m J z 23 and m / z 85 were calculated. The response curve in the form of J123/J185 vs. ng of NaI is nicely linear (correlation coefficient = 0.997). The good linearity indicates that analytical utility of the method may be anticipated.
CONCLUSION Our results indicate that the analysis of alkali halide salts with our commercial mass spectrometer designed for organic materials will be feasible. We think that similar results will be obtained with contemporary instruments from other manufacturers. Of the five modes of ionization we have examined, the thermal ionization mode will because of its high sensitivity be most useful for the identification and assay of cations. Either of the two negative chemical ionization modes and possibly the positive electron ionization mode will be useful for the identification and assay of anions. ACKNOWLEDGMENT We thank Yiu Ting Ng for the mass spectrometric measurements and Gladys McMilleon for typing this manuscript. Registry No. CsI, 1789-17-5;CsBr, 7787-69-1;CsCl, 7647-17-8; CsF, 13400-13-0;RbI, 7790-29-6;KI, 7681-11-0;NaI, 7681-82-5; LiI. 10377-51-2;LiF, 7789-24-4;NaF, 7681-49-4. LITERATURE CITED Friedman, L. J. Chem. Phys. 1955, 23, 477. Milne, T. A.; Klein, H. M. J. Chem. Phys. 1960, 33,1628. Berkowitz, J.; Tasman, H, A.; Chupka, W. A. J. Chem. Phys. 1962, 36,2170. Buck, R. 0.; Hass, J. R. Anal. Chem. 1970, 45, 2208. Schulten, H. R.; Rollgen, F. W. Org. Mass Spectrom. 1972, 6, 885. Moore, L. J.; Machlan, L. A. Anal. Chem. 1972, 44, 2291. Lloyd, J. R.; Field, F. H. Siomed. Mass Spectrom. 1981, 8, 19. Hunt, D. F.; Shabanowltz, J.; Botz, F. K. Anal. Chem. 1977, 49, 1160. White, E. V. Org. Mass Spectrom. 1978, 13, 495. Rosenstock, H. M.; Sites, J. R.; Walton, J. R.; Baldock, R. J. Chem. Phys. 1955, 23,2442. Soltmann, B.; Sweeley, C. C.; Holland, J. F. Anal. Chem. 1977, 49, 1184. Porter, R. F.; Chupka, W. A.; Inghram, M. G. J. Chem. Phys. 1955, 23,1347. Cotter, R. J. Anal. Chem. 1980, 52, 1589A. Mllne, T. A. J. Chem. Phys. 1960, 32, 1275.
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(15) Rosenstock, H. M.; Draxl, K.; Seiner, B. W.; Herron, J. T. J. Phys. Chem. Ref. Data, Suppl. 1977, 6(1), 1774-1782. (16) Field, F. H. “Chemical Ionization: Negative AsDects”: DreSented at the 28th Annual Conference on Mass Spectrometry and Allied ToDics, New York, May 1980. (17) Benson, S. W. “Thermochemical Kinetics”; Why: New York, 1976. (18) Paullng. C. “The Nature of the Chemical Bond”, 2nd ed., Cornell University Press: Ithaca, NY, 1944; p 346.
(19) Weast, R. C., Ed. “CRC Handbook of Chemistry and Physics”, 59th ed.; CRC Press: West Palm Beach, FL, 1979.
RECEIVED for review July 29,1985. Accepted October 28,1985. This work was supported in Part by a grant from the Division of Research Resources of the NIH.
Application of Pattern Recognition to Metal Ion Chemical Ionization Mass Spectra R. A. Forbes, E. C. Tews, and B. S. Freiser* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
M. B. Wise Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
S. P. Perone* Lawrence Livermore National Laboratory, Livermore, California 94550
Pattern recognitlon methods were used to evaluate the information content of mass spectrometry data obtalned using transltlon-metal ions as an ionization source. Data sets conslsting of the chemlcal lonlzatlon mass spectra for Fe+ and Y+ with 72 organics (representlng the SIXclasses alkane, alkene, ketone, aldehyde, ether, and alcohol) and 24 alkanes (representing the three subclasses ilnear, branched, and cyclic) were subJectedto pattern recognition analysis using a k-nearest neighbor approach with feature weightings. The reactivltles of Fe+ and Y+ toward the classes of compounds studied were characterized by uslng classiflcatlon accuracles as a measure of selectlvlty, and Important chemlcal Informatlon was extracted from the raw data by empirical feature seiectlon methods. A total recognitlon accuracy of 81% was obtalned for the recognition of the SIXclasses and 96% accuracy was obtained for the recognitlon of the three subclasses of alkanes.
Electron impact (EI) ionization mass spectrometry has become a standard means for the classification of unknown compounds according to functionality or structure (1-5). The differentiation of isomeric molecules, however, remains a difficult problem and subtle differences in molecular structure often cannot be distinguished by electron impact. The need arises, therefore, for a more selective form of ionization. Chemical ionization (CI) has the potential for such an increased selectivity since it is possible to adjust the reactivity of the CI reagent for selectivity in a way that is not possible for the E1 mass spectrometry experiment (6-8). In our laboratory the reactivities of laser-generated transition metal ions toward various types of compounds have been studied for several years (9-13). A major goal of this work has been to evaluate the utility of metal ions as selective reagents for mass spectral identification of the functionality and structure of unknown compounds. In view of the potentially large data matrix generated from the reactions of different metal ions with various organic compounds, the application of pattern recognition techniques provides a
particularly useful means for achieving these goals. Pattern recognition has been applied to a wide variety of chemical problems and numerous reviews on the subject have been published (14-21). Some of its more recent applications include recognition of organic compounds by using Fourier transform infrared spectrometry, interpretation of gas chromatography data, nuclear magnetic resonance spectral interpretation, and analysis of electrochemical systems (22-32). Many applications of pattern recognition to mass spectral data have recently appeared in the literature. Electron impact ionization is by far the most widely used ionization means and has been employed in studies ranging from the analysis of complex mixtures using gas chromatography/mass spectrometry to the recognition of steroids and the use of mass spectrometric data to predict the biological activity of antibiotics (33-36). Pattern recognition has also been applied to the experimental optimization of field desorption and fast atombombardment mass spectrometry and for the location of homoconjugated triene and tetraene units in aliphatic compounds using NO chemical ionization (37, 38). Because pattern recognition is a well-established tool for interpretation of mass spectral data, it was the goal for the work described here to use this tool to enhance our understanding of the information content of a new and important advance in chemical ionization mass spectrometry. This was a particularly efficient approach because of the potentially enormous data matrices. It also provided a unique opportunity to apply pattern recognition to an emerging data base where the scientist is near the bottom of the “learning curve”. This work illustrates how such an application can enhance the rate of climbing that “learning curve”. One of the goals of pattern recognition is to minimize the number of features required to effect class separation within a data set while maximizing the recognition accuracy through the elimination of features detrimental to class separation. Thus, an empirical feature selection algorithm is often used to map a classification problem down from the space of all features to a space of smaller dimensionality, which consists of only important, relevant features. This procedure not only enhances the ratio of patterns to features in order to have a
0 1986 American Chemical Society 0003-2700/86/0358-0684$01.50/0
