1015
Anal. Chern. 1984, 56,1015-1020
Sampling of Ions from Volatile Solutions by Electrohydrodynamic Mass Spectrometry Sandra L. Murawski and Kelsey D. Cook* University of Illinois, School of Chemical Sciences and Materials Research Laboratory, 53 Roger Adams Laboratory, Box 49, 1209 West California Street, Urbana, Illinois 61801
Modlflcatlons to an electrohydrodynamiclonlratlon source are described whlch allow collection of spectra from aqueous solutlons. The spectra are characterlzed by peaks slgnlflcantly narrower than those obtalned wlth glycerol solutlon, suggestlng that aqueous sampling results In a narrower klnetlc energy dlstribptlon. Deposition of Internal energy may calso be reduced relatlve to glycerol. Detected sodlum Ions are appreclably solvated, though the exact relatlonshlp between Ion lntensltles and solvent structure Is not yet clear. Similarly, detalls of spectra obtalned from mixed glycerol-water solvent can tentatlvely be ascrlbed to preferential solvatlon, though alternative explanations cannot yet be ruled out. Resutts wlth organlc solutes conflrm that the lonlzatlon mechanlsm Is very soft.
Numerous approaches to desorption ionization have been developed in recent years with the primary aim of relieving volatility constraints on the mass spectrometric experiment (1). Such relief is essential to the extension of mass spectrometry to the study of thermally labile, nonvolatile samples. Several of these new approaches employ a liquid matrix, thereby exploiting the low-energy processes of solution phase ionization (via salt dissociation or ion attachment) to minimize energy deposition and resulting fragmentation (1-4). Among the liquid desorption techniques are fast atom bombardment (FABMS) (4, 5), "liquid" secondary ion (liquid SIMS) ( 6 ) , thermospray (7), liquid ion evaporation (8), and electrohydrodynamic ionization (EHMS) (3, 9). In some of these techniques bombardment of the sample with high-energy atoms (FABMS) or ions (SIMS) aids in the desorption of ions, while in others (liquid ion evaporation, thermospray, and EHMS) field-assisted desorption is utilized. In liquid ion evaporation, this field-assisted desorption is accompanied by atmospheric pressure ion-molecule interactions, while in the thermospray method the energetics of field evaporation are convoluted with those of heating. By contrast, in EHMS field evaporation is the principal desorption mechanism p d may in fact be the only process operative. Here, the field is applied externally between an emitter at the tip of a solution-filled syringe and a surrounding extractor electrode. As a result, preformed ions are extracted directly from solution. The mechanism of E H ionization has been discussed by Stimpson and Evans (3). Little or no fragmentation except for desolvation processes has been observed for ions sampled by the E H process. Because solution is introduced directly into the E H ion source, high pressure in the source, especially locally near the emitter tip, can result in ion scattering and/or sparking between the emitter and extractor plate (10). Furthermore, rapid evaporation of volatile solvents can promote freezing of solution a t the emitter tip, interrupting emission. Thus, until quite recently (111, glycerol has been the solvent of choice for EHMS because of ita good solvent properties and low volatility (vapor pressure = 3 X lo-* torr at 20 "C, resulting in back-. .. .
ground source pressures of to lo4 torr (12)). However, analysis is thereby limited to those samples that are soluble and nonreactive in glycerol. In addition, glycerol solution chemistry is not well-characterized, making interpretation of the relation between spectra and solution chemistry difficult. Obviously, it would be desirable to analyze samples in more conventional solvents such as water or methanol. In 1980, Zolotoi et al. (11)obtained EH mass spectra from aqueous solutions of NaI and saccharose through the use of a stainless steel capillary (internal diameter 0.3 mm) into which a steel wire (0.22 mm) had been inserted. This reduced the exposed capillary opening from a circle (0.28 mm2) to an annulus (0.13 mm2), thereby reducing solution flow and vacuum load. However, they report that the total ion current was discontinuous, with pulses of emission lasting 1s with a repetition frequency of 0.1 to 1 Hz. Pulse amplitude and frequency were found to increase with increasing NaI concentration. This paper reports an alternative approach to EHMS of volatile solutions, involving instrumental modifications that allow more nearly continuous sampling of ions. This is possible mainly through the use of emitters with very small capillary openings (-0.002 mm2).
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EXPERIMENTAL SECTION Mass spectra were obtained with a double-focusing mass spectrometer (AEI MS902) equipped with an EH ion source described elsewhere (10,12). Several modifications were made on the original source to facilitate sampling of ions from volatile solutions. Most importantly, the emitter described previously (0.2 mm i.d. (12)) was replaced by one with a much smaller aperture. Capillary bonding tools made of carbides of tungsten or titanium (Gaiser (Ventura, CA) and Small Precision Tools (San Rafael, CA)) with inside diameters of 25 pm were selected for use (Figure 1). These tools are similar to those used as liquid metal ion emitters for SIMS primary ion sources or as fast atom sources for FABMS in what has been called a capillaritron ion source (13-15). In a further modification, the diameter of the extractor electrode aperture was increased from 5.08 mm to 9.19 mm to reduce or eliminate sparking between the emitter and extractor. With the larger aperture, it was necessary to use extractor potentials between -1 and -4 kV, somewhat higher than those previously used (12, 16). As in prior experiments, the emitter potential was set at about +8.2 kV, and the collector was fixed at ground potential. Full energy ions (Le., those that had not undergone any metastable evaporative loss of solvating molecules prior to the electrostatic analyzer (ESA)) were detected by empirically matching the ESA and emitter potentials (17). Typical scan rates of 100-200 s/decade were employed, except where noted. A Varian VK-12A Model 917-1000 cryopump with a water vapor pumping speed of 3800 L/s was utilized to maintain source torr during analysis of aqueous solutions. pressure below 1 X For mixed glycerol-water solutions, the oil diffusion pump used previously (12) was sufficient to evacuate the source housing to the desired pressure. Further cryogenic evacuation was thus unnecessary and in fact undesirable because the relatively nonvolatile glycerol would contaminate the cryoarrays. In some (thus far unpredictable) cases, capillary action was insufficient to maintain aqueous solution at the emitter tip. In 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50. NO. 0. MAY 1984
XI0
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Fbum 1. Capllaty bond@ mol emW (coin included for &e refer-
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thcee esses,tbe positive pressure feed eystem described previously (18)WBB used In this system,a rotary-blinear drive continuously advances the syringe plunger. Sample consumption on the order of 4 pL/min was required to maintain emission for aqueous solutions. Except where noted, aqueous samples were degassed by the freezepumpthaw method. Those few samples run without degaasing tended to bubble in the syringe during pump down of the source. Excessive bubbling resulted in loss of sample by spraying into the source prior to analysis or erratic sample flow to the tip of the capillary. On the other hand. it was noted that in some canes formation of a single bubble stabilized flow, and therefore emission, without the need for mechanical solution feed. Mixed glycerol-water solutions were prepared hy two procedures which gave roughly equivalent spectra. In the first, a glycerol-analyte solution wan degassed under vacuum and low heat and then diluted with an appropriate amount of water that had been previously degassed by freezepump-thaw, Alternatively, a mixed solvent solution of analyte wan prepared and degassed by freezepumpthawing the entire mixture. Solution conductance measurements were made with a bipolar pulse conductance instrumentoperated in the voltage p h mode (19,20).
Nal (Mallinckrodt) and glycerol (Aldricb) were reagent grade chemicals and were uned'as received. Malachite Green (oxalate salt, Sigma) was also used without further purification. The diqustemaryammonium salt [(CHdJCA"CZCHdd12 was kindly provided by R G.Cooks of Purdue University. Aqueous solutions were prepared with water from a Millipore Continental water purification system. RESULTS AND DISCUSSION Studies with Aqueous Sodium Iodide. Attampta to analyze a solution of NaI in water a t a concentration similar to that used previously in glycerol (0.68 M or 5.0 mol % (IO)) were unsuccessfuldue to severe sparking. Subsquently, the concentration of analyte was adjusted so that the conductance of the solution WBB similar to that of a typical glycerol solution used for EHMS (-2 X lo4 W' cm-'). This required an aqueous NaI concentration of 2.0 mM (3.8X mol %). Ion emission was obtained from this solution but differed markedly from that obtained with glycerol. Emission was characterized by random instability (although no apparent sparking was observed) fluctuating between 5 X and 2 X A. This is much lower than typical ion emission currents (1W to lo* A) for glycerol. For data from typical experimenta, the average relative standard deviation of mean analyte ion intensities ( ~ ( o , / f , 3 ' 4 / i V where , 43''' is the standard deviation of the mean (x,) of three replicate measurements of the intensity of the ion i and N is the number of ions in the spectrum) was 4C-70% for water, 88 compared to about 30% for spectra obtained from comparable glycerol Solutions. Probably due to the lower concentration of the
e 2. portion of an OscillOgapMc reading of a positiM )on EH mss specrnm ot 2.0 mM aqueous NaI (degassed). -8 are labelad according to the fmwlng bn series (where W denotes water and n denotes the number of sohrent molecules in a detected cluster): a. = [W. Na]': bn = [W. HI': c. = [W. NaJ]'. Unlabeled peaks can be assigned to metastable bns arising from desolvation: [W. Na]' [W.-, Na]+ and [W. HI+ [W. .. HI+. m
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analyte used in water, the electron multiplier voltage had to be increased from 2.5 kV to 3.0 kV in order to obtain signals of intensity comparable to those obtained with glycerol This may also have been due in part to the ditribution of aqueous analyte among a larger number of solvated peaks (seebelow). Figure 2 shows an oscillographic recording of a spectrum of a degassed 2.0 mM aqueous solution of NaI. Figure 3 compares excerpts from similar spectra with corresponding data obtained before degassing. Two major series of peaks are observed in all these spectra, corresponding to solvated protons and sodium ions. The relative intensity of the proton peaks is diminiihed by degaasing. It is believed that this may be due to the removal of dissolved COzby the f r e e z e p u m p thaw method, which increases the pH of the solution. It has not been possible to test this hypothesis by detecting the bicarbonate anion because of sparking encountered while attempting to obtain negative ion spectra. Two additional features are evident in the spectrum of Figure 2. First, fullenergy ion peaks observed in the aqueous spectrum are narrower than corresponding peaks in glycerol spectra, resulting in increased resolution. (Thisis particularly evident for peaks a t higher maas, such as those obtained from poly(ethylene glycol) oligomers (Figure 4).) Because the ESA slit width and band-pm are the enme as for glycerol, the initial kinetic energy spread for aqueous ions must be narrower. this could result from translational cooling if evaporating solvent in the high-pressure region near the emitter tip forms a supersonic jet (21). Alternatively, sampling of ions from water may be an intrinsically more nearly monoenergetic process due to the different types of solvent-solvent interactions that
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
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Figure 5. Portions of oscillographic recordings showing a comparison of peak widths for metastable Ions of comparable nominal mass. These metastables arlse from the following processes occurring in the second field-free region (G denotes glycerol and W denotes water): (a) [G3 Na]' [G2 Na]+; (b) [W, Na]' [W, Na]'.
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100
300
200
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m/z
Flgure 3. Poskive ion EH mass spectra of (a) an undegassed and (b) a degassed aqueous solution 2.0 mM in NaI. Peaks are labeled as in Figure 2. The intensities represent an average of three spectra.
1 .1 1 1 1 1 1 1 1 1
937
937
Figure 4. Portions of oscillographic recordings showing a comparison of peak widths for an ion sampled from (a) glycerol solution and (b) aqueous solution. The ion shown is attributable to the potassium adduct of the 20-mer of poly(ethy1ene glycol).
must be overcome during desorption or due to the different tip geometry. The data do not indicate which (if either) of these possibilities is correct. A similar peak narrowing is observed for metastables, which result from evaporation of one or more solvent molecules in the second field-free region of the spectrometer (between the ESA and magnetic sector). Such mestastables are generally detected as broadened peaks due to the angular and kinetic energy distributions of the daughter ions resulting from internal energy release upon breakup. Figure 5 compares the width of peaks arising from metastables from aqueous and glycerol solutions for two ions of approximately the same nominal m/z. This peak narrowing is likewise apparent when peaks corresponding to metastables that have experienced roughly the same proportional mass loss (e.g., [W, + Na]+ [W4 + Na]+ and [Gs+ Na]+ [G4+ Na]+) are compared. The angular distribution of metastable ions from water solutions is not likely to differ greatly from that from glycerol. Thus, these results indicate that metastables from water have a narrower kinetic energy spread. This spread arises from release of internal energy upon fragmentation (desolvation, here). Part of the internal energy is needed to overcome the endothermicity of desolvation; only internal energy in excess of that amount is available for redistribution as fragment kinetic energy. If removal of a solvent molecule from a large cluster involves disruption primarily of solvent-solvent in-
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Degree of Solvotion ( n )
Flgure 6. Dependence of relative Intensity on degree of solvation for EH spectra of Na ions sampled from 2.0 mM aqueous NaI (degassed). The results represent an average of three scans.
(b)
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teractions (asin the secondary solvation sphere), then the heat of solvent vaporization may provide a qualitative measure of the relative endothermicity of desolvation. On comparison of heats of vaporization (AHvap is 18.19 kcal/mol for glycerol and 9.717 kcal/mol for water (22)),it seems reasonable, then, to assume that less energy is required for water desolvation than the corresponding glycerol reaction. Therefore, if waterand glycerol-solvated ions were formed with roughly equal internal energies during desorption, one would predict that water-solvated ions would possess a greater excess internal energy, which would result in a wider energy spread following redistribution. The fact that the spread is actually narrower would therefore seem to indicate that less internal energy is initially deposited in water-solvated ions during the desorption process. Again, this may be intrinsic to the sampling process or may result from cooling associated with supersonic expansion (if it occurs) in the high-pressure region of the emitter tip. Further study is needed to better characterize the desorption process from water. The second feature apparent in Figure 2 is the solvation of the analyte ions. If relative intensity (average from three spectra) vs. degree of solvation is plotted for the Na ions, the results in Figure 6 are obtained. The fluctuations in individual ion intensities measured at normal scan rate are evident from the relatively large error bars (representing the standard deviation of the mean of triplicate measurements of each ion intensity) in Figure 6. In fact, the positions of the dip (at degree of solvation n = 6) and the maxima (at n = 5 and 7) varied over a range of about 2-3 units among the individual spectra comprising the figure. However, the general shape of the curve is suggestive of the solvation sphere characteristic of ionic solutions. X-ray studies indicate that the primary solvation number of the aqueous sodium ion in solution is either 4 or 6 (23-27). By contrast, studies of gas-phase Na+-bound water clusters (28-29) indicate a continuous de-
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ANALYTICAL CHEMISTRY, VOL. 5 6 , NO. 6 , MAY 1984
crease in -AGO and -A"'with increasing n for the process
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Na+.(H20)n-1 H20 s Na+.(H20), suggesting no favored cluster size beyond n = 1. Thus, the data of Figure 6 would seem to be more reflective of liquid solvent structure than gas-phase ion stability. However, in considering this interpretation it must be remembered that the ions detected in these spectra are generally those that have not undergone any evaporative loss of solvating molecules prior to the ESA. The presence of metastable peaks in spectra confirms the occurrence of desolvation in the second field-free region. If desolvation also occurs more rapidly (e.g., in the first field-free region prior to the ESA), resulting "fast metastable" ions will have energy lower than the nominal acceleration potential and will not be detected unless the voltage of the ESA ("energy fiiter") is lowered proportionately. Thus, if heavily solvated ions are more likely to undergo desolvation, they may be less likely to be detected as full energy ions. In any event, fast desolvation may complicate the relationship between spectra and solution structure. As a test for the presence of fast mestastables in previous EH experiments with glycerol, it has been possible to obtain spectra at ESA voltages corresponding to ion energies substantially lower than the acceleration potential (17). However, efforts to obtain fast metastable EH spectra from aqueous solutions have so far been generally unsuccessful. This may be due to an absence of such ions (which seems unlikely in view of the detection of conventional metastables from the second field-free region). Alternatively, the narrower energy spread of ions sampled from water (described above) may necessitate relatively critical setting of the ESA potential for each fragment ion. In this case, linked scanning capabilities may be necessary to assess contributions from fast metastables. Two other factors should be considered in interpreting these spectra. First, the degree of solvation of ions may affect mass spectral sampling efficiency and sensitivity (16). Finally, the possibility of cluster growth by ion-molecule reactions in the region of relatively high pressure at the emitter tip must be considered. Zolotoi et al. (11)analyzed solutions of several different concentrations of NaI in water by continuous discharge EHMS and attributed a maximum in the Naf.(HzO), distribution (at n = 3 or 4) to a competition between such reactions and evaporative loss of water from ions with higher n. Clearly, further solvation studies, including studies of the kinetics of the metastable desolvation process, are necessary to determine the extent to which spectra reflect solution structure. Studies with Mixed Aqueous-Glycerol Solvent. If EH spectra at least qualitatively reflect solution chemistry, then studies in mixed solvents may provide direct information on competitive solvation. Figure 7a shows the EH mass spectrum of a 2.5 mol 70solution of NaI in 50/50 mol % glycerol/water. The major peaks observed are due to Na+-glycerol adducts. Surprisingly, no peaks were observed corresponding to solvation by either water or mixed glycerol/water clusters. Even increasing the water content to 80 mol % did not result in detection of Na ion-solvent clusters containing water (Figure 7b). However, more highly solvated Na+-glycerol adducts were detected in greater abundance in this solution, suggesting that the reduction in solution viscosity by addition of water may facilitate extraction of larger clusters from solution. Also evident in Figure 7b is a series of ions that can be assigned to solvated cobalt. These evidently arise from degradation of the bonding tool (see below). Although the absence of water in solvated sodium clusters may reflect a preference for glycerol solvation due to the thermodynamics of competitive solvation, again the sampling artifacts discussed above cannot be ruled out. As for pure water, fast metastables were not detected in these mixed
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Figure 7. Positive ion EH mass spectra of (a) 2.5 mol % NaI in 50150 mol % glycerol/water and (b) 1.0 mol YO NaI in 20/80 mol % glycerol/water. Peaks are labeled according to the following ion series (where G denotes glycerol and n denotes the number of solvent Na]'; b, = [G, CoI2+ molecules in a detected cluster): a, = [G, (impurltks, See text). Unlabeled peaks can be assigned to ions of the HI+, [G, Na - XH,O]+, [G, Na CH20]+ (impurity form: [G, Na]' [G,-, Na]+ (metastable ion). Peaks in glycerol), and [G, labeled with a question mark are unidentified. The results represent an average of two spectra.
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solvent experiments. In this case, the additional possibility that aqueous ions may be sampled from mixed solvents with energies slightly different from glycerol adducts must be considered. Further studies are under way to determine if the optimum operating conditions for sampling from other solvents are different from those for glycerol. Studies with Organic Solutes. A principal feature of EHMS has been its ability to sample labile analytes of relatively high molecular weight without fragmentation. The data for poly(ethy1ene glycol) in Figure 4 suggest that this capability is retained in the modified source. As a more rigorous test, spectra were obtained from an aqueous solution of the polar dye Malachite Green (I) with NaI as supporting electrolyte
(Figure 8). A molecular ion was observed at m / z 329, but detection of this ion was dependent upon careful adjustment of the emitter potential so that the ion energy and ESA voltage were well matched. As before, the results here indicate that ions sampled from aqueous solution have a narrow kinetic energy spread. Several slow scans (160-280 s/decade) were made across the molecular ion region in order to measure the intensity of the 13C isotope peak. The experimental intensity agreed with the theoretical value to within 20% (which is reasonable in view of the low overall intensities involved), indicating that good precision can be obtained with time
ANALYTICAL CHEMISTRY, VOL. 56, NO. 6, MAY 1984
( E )( m / z 6 0 )
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Flgure 8. Portions of a slngle positive ion EH mass spectrum of (a) a degassed and (b) an undegassed aqueous solution 0.5 mM in Malachite Green (with 10.2 mM NaI as supporting electrolyte). Peaks are labeled according to the following ion series (where C denotes the molecular cation (I), W denotes water, and n denotes the number of solvent molecules in a detected cluster): a, = [W, + C + HI2+; b = [C]’. Unlabeled peaks can be assigned to hydrated ions of the form [W, -I- Na]’, [W, + HI’, and [W, -I- Na]’ [W,-, 4- Na]’. -+
and/or signal averaging. In addition to C+, a series of lowintensity peaks beginning at m / z 165 and separated by 9 mass units was observed in the spectra. These could be assigned to the protonated, doubly charged species (11) solvated by
I
( m / z 165)
water molecules. When the solution was not degassed, these peaks were more intense than the molecular ion (Figure 8b), again reflecting the role of degassing in adjusting the solution PH. In general, doubly charged ions such as those observed for Malachite Green are relatively rare in mass spectrometry (30). Often doubly charged species decompose to separate the charge centers, resulting in detection of singly charged ions. However, in EHMS the energy deposition is low enough that doubly (and other multiply) charged ions have been observed (31-34). In fact, in the analysis of a series of diquaternary ammonium ions in glycerol (31), decomposition was observed only for the diquat (1111, in which the distance between charges
(m)
(m/z 97)
is small. The EHMS of a glycerol solution of this diquat was dominated by the fragment ions IV (m/z 60) and V ( m / z 136) and their solvated adducts. Simple charge separation could not account for the hydrogen atom at the cleavage site on each
(PI ( m / z
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136)
fragment. The possibility of gas-phase unimolecular fragmentation with rearrangement was ruled out by analyzing a deuterated analogue of this diquat. The possibility of sample contamination was also ruled out (by comparing proton magnetic resonance spectra of aqueous solutions of I11 and V). Furthermore, from the lH NMR data it was clear that products IV and V were not formed by chemical reaction in aqueous solution, at least in the absence of an applied E H field, Unfortunately, because of the difficulty of obtaining IH NMR spectra from viscous solutions, comparable experiments in glycerol were not conclusive. Thus, two possible sources of ions IV and V remained: reaction with the glycerol solvent or gas-phase ion-molecule reactions. Either of these mechanisms could be affected by the E H field. As a test for the possibility of field-induced reaction during desorption from water, the diquat was dissolved in water and its EH spectrum obtained. Both C2’ at m / z 97 and CI’ at m / z 321 and their solvated products were observed. The fragment ions IV an V were present only at very low intensity (
