Anal. Chem. 1901, 63, 2357-2365 (10) Katakuse, 1.; Ichlhara, T.; Fujlta, Y., Matsuo, T.; Sakurai, T.; Matsuda. H. Int. J . Mss Spectrom. Ion phys. 1986, 69, 109-114. (11) Demlrev, P.: Olthoff, J. K.; Fenselau. C.; Cotter, I?. J. Anal. Chem. 1987, 59, 1951-1954. (12) Cotton, F. A.; Wllkenson. G. In Advanced Inorgenic Chemistry; Wlley: New York, 1988; p 211. (13) Ebei. M. F.; Lkbl, W. J . €leclron Spectrosc. Relet. phenom. 1979, 16, 463-470. (14) Retch, 1.: Yarrhemski, V. G.; Nefedov, V. 1. J . €kctron Spectrosc. Rebt. phenom. 1988, 46, 255-267. (15) Hannawak files: average value for the varlous known alumlnum oxides. Joint Committee for Powder DiffractionlStandards.Powder Dif-
2357
fraction and RetrlevallDlsplay System, Versbn 2.10. PDF-2 Data Base, International Centre for Dtffractbn Data, Swarthmore. PA, 1989.
RECEIVED for review April 10, 1991. Accepted July 29,1991. This work was presented at a meeting of the Deutsche Physikalische Geseuschaft, Munich, Gemany, March 12-16, 19g0, and in part$ by Grant p6854 from the Austrian “Fonds zur Forderung der Wissenschaftlichen Forschung”.
Initial Characterization of a Plasma Gun Source for Atomic Spectroscopy Joel M.Goldberg* and David S . Robinson’
Department of Chemistry, University of Vermont, Burlington, Vermont 05405-0125
A plasma gun Is descrlbed and evaluated as an emission source wltabk for direct soM mmpllng methods of spectrochemkd analysk. Atomlzatlon occurs In the radially contlned currmt-carrylng portkn of the plasma, and emlsdon is measured In the expelled, cooling plasma plume; this allows both spatial and temporal segregation of these competing proceoes. Temporally and spatlally resolved spectroscoplc studles of the emlssion from the plasma gun source are presented. The lnltlal plasma front Is expelled from the discharge tube at about 3000 m/s and then slows to about 700 m/s at a hdght of 8-10 mm. Plasma fonnatbn and hnpkskn processes result in the expulslon of a sharp, rapidly propagating pulse of plasma, whlch appears to combine or coillde wlth plasma that Is b l n g contlnuously expelled due to the oscillatory heatlng and coding of the plasma In the dlscharge tube. Spatially resolved emlolon measurements show slgnlflcant Increases In both continuum and ilne emlsslon at heights greater than 10 mm. It is suggested that this behavior le due to both colllslonal and optical reexcitation processes. It k shown that masking of emhrlon from the lower r.gkn, of the pknw (0-10 mm) and gatlng omlsdon from the plum after the flrd curnnt half-cycb (after about 75 ps) can enhance V( I I ) llne-to-background ratlos by almost 10-fold. Applicatlon of the source to the direct qualltative analysls of a USGS reference rock sample (MAQ-1, Marlne Mud) Is presented.
The determination of elemental concentrations directly in refractory, nonconducting solids remains as one of the unsolved challenges in atomic emission spectrochemical analysis. Direct solid sampling (DSS) approaches based on segregation of the atomization and excitation steps have produced some of the most promising results (1-5); however, there are very few sources that are suitable atomizers for the analysis of very refractory materials (6, 7). We have been investigating the properties of an imploding thin-film plasma (ITFP) source that shows promise for development as an atomization source
* To whom correspondence should be addressed. ‘Current address: Sentex Sensing Technology, Inc., 553 Broad Ave., Ridgefield, NJ 07657.
capable of analyzing solid refractory materials. The ITFP is created by discharging a high-voltage capacitor bank through a cylindrically symmetric thin conductive film. The thin film is coated on the inner wall of a nonconductive substrate, and the plasma that is formed implodes and is then confined radially. Solid powders deposited on the surface of the thin film in the discharge tube come into intimate contact with this hot plasma and are rapidly converted to an atomic/ionic vapor. We have studied this plasma source in considerable detail (8-12)and found that: (1) its extremely high power densities enable the complete atomization of compounds as refractory as vanadium carbide, (2) axial observation of emission from the plasma is not suitable for analytical measurements due to severe self-reversalof even analyte ion linea, and (3) confinement of the plasma for in situ reexcitation is hampered by significant axial expansion of the plasma. Coupling this atom cell with an external reexcitation source, then, would require transport of the vapor produced by the ITFP. In this report, we describe an ITFP source that has been modified so that plasma may expand axially in only one direction, allowing measurement of emission from the expelled plasma plume as well as direction of the sampled vapor into an appropriate reexcitation source (such as an ICP). Due to the high velocity with which this device expels plasma, it is referred to as a plasma gun. We report here on the initial spectroscopiccharacterization of the plasma gun as an atomic emission source for direct solid sampling.
EXPERIMENTAL SECTION The dischargecircuit and chamber have been described in detail previously (8). Discharge Cassette. The plasma gun discharge cassette shown in Figure 1 is a modification of the original ITFP discharge cassette described previously (8). The cassette consists of a lower brass electrode support block (marked a), a brass side support block (marked d), an upper brass electrode support block (marked b), and two polycarbonate insulating blocks (marked c). Electrical contact with the capacitive discharge circuit is made via three hemispheric dimples milled into the bottoms of the lower and side brass support blocks (a and d, respectively),which mate with three raised electrodes inside the discharge chamber (not shown). Electrical contact with the discharge tube is made by using graphite electrodes that press fit into the lower and upper brass support blocks (a and b, respectively). Polycarbonate shields (marked e) provide electrical insulation as well as ensure good
0003-27001@1/0363-2357$02.50/0 0 1991 American Chemical Sockty
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ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
Table I. Discharge Conditions Discharge Tube Properties substrate polycarbonate thin film silver 3.8 (length) x 1.3 (0.d.) X 0.95 (id.) tube dimensions, cm film mass, mg 3.2 0.28 film thickness, pm resistance, R 1-2
c m
Discharge Conditions charging voltage, kV 4-7 capacitance, pF 50 residual inductance, pH 6 energy, J 400-1225 discharge frequency, kHz 8.7 support gas argon pressure atmospheric Figure 1. Plasma gun cassette: (a) lower brass electrode support block, (b) upper brass electrode support block, (c) polycarbonate insulating blocks, (d) brass sMe support block, (e) polycarbonate elec-
trode shields.
electrical contact of the electrodes with the brass support blockes. The lower graphite electrode is essentially a solid graphite plug that press fits into the discharge tube, providing electrical connection to the discharge circuit while preventing axial expansion of the plasma out of the bottom of the discharge tube. The upper graphite electrode also press fits into the discharge tube but is hollow, thus allowing plasma to escape out of the top of the discharge tube. Since the discharge is quite violent, special care was taken to ensure that the structural integrity of the cassette was maintained. The upper brass electrode block (marked b) was secured to the rest of the cassette with four steel screws with polycarbonate insulating caps. Also, the lower brass electrode block (marked a) was secured to the polycarbonate insulating blocks (marked c) with two sets of anchoring steel screws: one set (shown) entering the front of the block and another set (not shown) entering the bottom of the block and anchoring into the polycarbonate insulator block. Optical and Electrical Monitoring. Measurements of the discharge current were made with a Rcgomki coil (Pearson, Model 1025); plasma voltage waveforms were measured with a 1OOO:l high-voltage probe (Tektronix,Model P1099). Net plasma voltage waveforms were obtained by subtracting voltage waveforms measured at each chamber electrode. Emission from the plasma plume was observed normal to the discharge tube axis. All temporally integrated spectra were obtained by using a Jarrell-Ash, 1-m, Czerny-Turner mount spectrograph (Model 75-150) having a first-order reciprocal linear dispersion of about 0.8 nm/mm. Spectra were recorded on Kodak SA-1 photographic plates which were then developed according to the manufacturer's instructions and scanned/digitized by using a modified laser scanning microdensitometer (LKB, Model 2202) interfaced to a microcomputer (13). An entrance slit width of 100 pm was used in all experiments. All temporally resolved emission profiles were obtained by wing an Instruments SA, Inc. 0.64-m, Czerny-Turner mount monochromator having a first-order reciprocal linear dispersion of about 0.64 nm/mm. Emission intensity was monitored with a 1P28A photomultiplier tube (Hammamatsu) biased with a regulated high-voltage power supply (Fluke, Model 410B). Photocurrent was measured as the voltage drop across a 1-kQload resistor. Entrance and exit slit widths of 100 pm were used in all experiments. Spatially integrated spectra and emission profiles were obtained by placing the discharge chamber on the optical rail at a distance from the entrance slit sufficientto ensure that emission from the entire plasma volume was observed. Spatially resolved spectra and emission profiles were obtained by using over-and-under mirror optical systems designed either to produce a high-fidelity image of the plasma on the spectrograph focal plane ( 1 4 1 5 ) or to provide a high degree of spatial zone discriminationusing both an external slit on the optical rail (placed at the tangential focus) and the monochromator entrance slit (at the sagittal focus) (16).
Both of these optical systems have been described in detail (8, 9).
All temporally resolved emission, plasma voltage, and discharge current waveforms were recorded by using a digital storage oscilloscope (Nicolet, Model 4094-2) at a sampling rate of 2 megasamples/s. These waveforms were then downloaded to an XT-clone microcomputer and processed by using SpectraCalc or LabCalc software (GalacticIndustries, Salem, NH). All emission profiles are averages of four replicate determinations. All analyte emission profiles shown were background corrected by subtracting an emission profile (also an average of four replicate determinations) obtained from discharges without analyte. Experimental Procedures. Details of the discharge conditions used in this study and thin-film discharge tube properties are presented in Table I. Silver thin-film coated polycarbonate discharge tubes were prepared by using procedures described previously (8).Solid powder sampleswere deposited on the inner wall of the dischargetube by micropipet from a stirred 2-propanol slurry; the alcohol was then evaporated from the discharge tubes in a drying oven. Due to the potentially lethal nature of the electrical discharges used to generate these plasmas,special care was taken in operating the capacitive dmharge source to ensure the safety of the operator. Safety interlocks on the capacitor bank and discharge chamber prevented charging of the capacitors if the possibility of accidental access to components at high-voltage existed. Also, prior to removing the plasma gun cassette from the chamber after a discharge, any charge remaining on the capacitor bank was dissipated through a high-power resistive damping safety network built into the discharge circuit (this is described in detail in ref 8). Support gas and any noncondensed sample vapor from the plasma gun discharge were exhausted from the chamber into a fume hood (located directly above the chamber) similar to those commonly used with flame and ICP sources. Materials and Reagents. Graphite electrodes were machined from spectroscopic grade graphite rods (Ultra-Carbon, Bay City, MI). All reagents and powder samples were reagent grade. Vanadium carbide powder sampleswere commercially sized (ATM Corp.); only the smallest particle size fraction (less than 5 pm) was used in this study.
RESULTS AND DISCUSSION General Characteristics. The general operating properties of the plasma gun are quite similar to those found with the ITFP system (8-12). Measurements of the discharge current and plasma voltage waveforms were performed for a number of discharge conditions, revealing power dissipation behavior very much the same as observed previously with I T P s (IO). Thus,the solid sampling capabilities of the ITFP should be unaffected by the different discharge orientation and expansion properties of the plasma gun system. The very high peak power densities characteristic of both systems (ca. 10 MW/cm3) should be sufficient for efficient atomization of even the most refractory materials. Figure 2 is an illustration of the plasma gun system. With the plasma gun, sample is initially confined in very close
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wavelength(s), identifier species nm
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OCTOBER 15, 1991 2358
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W
-10
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0
50
100
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Time-resolved current (a), emisslon (b), and line-tackground ratk (c)proflles obtained by using 7kV, 50-pF discharges. The e” at the V(I1) 3 1 0 . 2 h m Hne was monitored for dWwges both wRh and wRhout a VC sample containing 1.5 pg of vanadium, Flgurr 4.
proximity to the imploding plasma. For the discharge conditions used here, the sample is in intimate contact with the plasma throughout the implosion process (the first 20 ps of the discharge). Only after the plasma has imploded will plasma begin to be expelled out of the upper electrode. Since the current path through the plasma is limited to the region of the discharge tube, observation of emission from the expelled plasma plume in a direction normal to the tube axis should help reduce the continuum background measured as well as minimize line self-reversal. Since the current-carrying plasma fireball is not observed when emission measurements are made in the expelled plasma plume and since the plasma plume is essentially unconfined (and can expand freely), this region is best suited for analyte emission measurements. The plasma gun itself, then, affords some segregation (both spatial and temporal) of atomization and measurement processes. Temporally Integrated Spectra. Temporally integrated spectra obtained by using 6-kV, 50-pF discharges are shown in Figure 3. Overlaid are spectra obtained both with and without a sample (1.5 pg of vanadium as VC). The blank spectrum (dark line trace) shows only atomic and ionic emission lines due to the vaporized silver thin film. Note that there are no emission lines from argon support gas species. The silver neutral resonance lines (marked A and B) are self-reversed due to the approximately 3 mg of silver that is
vaporized by the discharge. Numerous vanadium ion and atom lines are present in the sample spectnun (light line trace) with good line-to-background ratios. If the blank spectrum is subtracted from the sample spectru, the spectrum shown in the inset is obtained. This spectrum only shows emission lines from the analyte (vanadium). Many of the major lines are marked in the inset and are identified in Table 11. Notice that all of the identified lines are ion lines with the exception of the poorly resolved multiplet marked 1. Detailed classification of all of the lines in this spectral region reveals the strongly ionic character of the vanadium emission from the plasma, with the few observableatomic lines showing up only with a very low intensity. These spectra differ markedly from ITFP spectra obtained under similar conditions ( 8 , I Z ) . The plasma gun spectra have a much lower continuum background and many analyte emission lines are observable. There is little evidence of self-reversal of any analyte lines, as was evident in ITFP spectra. The plasma gun spectra shown here resemble the spectra obtained from indirect measurements of the plasma expelled from the ITFP discharge tube (9). Thus, the plasma gun system should enable analytical emission measurements
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ANALYTICAL CHEMISTRY, VOL. 83,
NO. 20, OCTOBER 15, 1991
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Figure 5. Spatially resolved emission spectral map (lower 17 mm of plasma plume) for a 7-kV. 50-pF discharge with 1.5 pg of vanadium as VC. For clarity of presentation, the spectra are ordered along the z axis according to decreasing displacement from the base of the plasma plume. to be made directly on the expelled plasma plume without requiring reexcitation. Temporally Resolved Emission. Temporally resolved emission profiles of the 310.23-nm vanadium ion line for a 7-kV, 50-pF discharge obtained with a VC sample containing about 1.5 pg of vanadium are shown in Figure 4 along with the resultant line-to-background ratio waveform. Emission from the plasma plume is not observed until about 21 ps after initiation of the discharge; this corresponds well with the implosion times observed previously with ITFPs using these discharge conditions (11). This delay in plasma expulsion ensures that sample (placed inside the tube) has ample time to interact with the plasma and to be converted to an atomic/ionic vapor before being expelled with the metal-vapor plasma plume. Note also that although the plume carries no current from the capacitive discharge source, emission from the plume oscillates at a frequency similar to that of the discharge current. This oscillation is initially out of phase with the current but “catches up” by the third current halfcycle. The oscillating emission is likely due to the changing temperatures and pressures of the current-carrying plasma inside the discharge tube, expelling “pulses” of plasma with each current half-cycle. Even though emission from the current carrying plasma is not observed,the continuum background emission maximum swamps the V(I1) emission early in the discharge. This results in subunity line-to-background ratios during the first 75 ps. In contrast with the time-resolved emission behavior of and@ emission from the ITFP source, there is no evidence of selfreversal (i.e., negative net line), even during this early portion of the discharge. After the first 75 ps, the continuum background emission decays very rapidly due to the cooling and expansion of the plasma plume. Emission from the V(I1) species, however, decays much more slowly and line-tobackground ratios greater than unity are observed throughout the remainder of the discharge, reaching a maximum of greater than 5 at about 125 ps. Time-gating out the large continuum emission pulse should result in improved detectability. Spatially Resolved Spectra. We expected that the emission from the plasma gun plume would be heterogeneous both with respect to space and time due to the rapid expansion of the plasma as it is expelled from the discharge tube. As discussed above, continuum background was found to be most intense early in the discharge. Investigation of the spatial structure of the emission from the plume may enable additional spatial discrimination of the analyte emission from the
Waveleiglh A
Flgwe 6. Spatially resolved emission spectral map ( 1 0 - 3 0 ” region of phsma plume) for a 6 k V , 50-pF discharge with 1.5 pg of vanadium as VC. Note that spectra are ordered along the I axis according to increasing displacement from the base of plasma plume. continuum background. A spatial map of the emission observed from a 100-pm-wide region in the center of the plume is shown in Figure 5 for a 7-kV, 50-pF discharge with about 1.5 pg of vanadium as VC. These spectra were obtained by using the high-fidelity spatial imaging system and then recorded at 1-mm intervals using the laser-scanning microdensitometer. Presented in the figure are spectra from 1mm-high zones at 17 vertically displaced locations in the plume; the spectra are arranged along the z axis according to decreasing vertical displacement for clarity of presentation. In the regions nearest the base of the plasma plume, continuum background is quite large and the spectra are reminiscent of those obtained with the ITFP source (8, 12) (i.e., small, broad lines superimposed on a large background). The continuum intensity drops off rapidly, however, as displacement from the plasma base increases, reaching a minimum at about 9 mm. At displacements greater than 10 111111, however, small increases in a continuum emission are evident with increasing displacement. More significantly, vanadium and silver line emission also begin to increase in intensity at displacements greater than 10 mm, improving analyte line-to-background ratios. Vanadium ion line-to-backgroundratios in the 1 6 ” zone are comparable to those in the 6-7-mm region. This increase in both background and line emission in the upper regions of the plume was not predicted. We expected that the expelled plasma plume would expand rapidly and cool, producing a decreasing continuum background at increasing displacementsfrom the upper electrode and, w i b l y , better analyte line-to-background ratios at the higher displacements due to the cooler environment. Based on the decreasing line widths observed at the higher displacements, it is fair to assume that the plasma environment in these regions is cooler and less dense. However, the increased line and background intensities found at the highest displacements cannot be explained as easily By lowering the discharge chamber 1cm, we were able to probe the plasma plume up to a height of 30 mm; the spatial map obtained in this region using a 6-kV, 50-lrF discharge is shown in Figure 6. Here, the spectra are oriented with increasing displacement between 10 and 30 mm above the base of the plasma plume. In this upper region, emission intensitiea continue to increase until about the 25-mm zone; above this zone, emission intensity decreases slightly until the uppermost observation zone is reached. In these spectra, we see very significant increases in line-to-background ratio as emission is measured higher in the plasma. It is emission from these upper zones that is responsible for the intense analyte line emission observed in the spatially integrated spectrum shown
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
2381
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Time,ps ne*u 7. Tk?e-resohred cdnuum emlsskn proflles from U V , 50-pF discharges measured at stated displacements above the base of the plasma plume. in Figure 3. This is especially evident for the weak vanadium lines between about 295.0 and 308.0 nm; these lines are only observed with any reasonable line-to-background ratio a t heights greater than about 20 mm. Analytically, then, there should be significant advantage to spatial masking out the lower 10 mm of emission from the plasma plume; this would remove much of the most intense continuum background with only minimal loss of analytically useful line radiation. Spatially and Temporally Resolved Emission: Continuum Background. The spatially resolved spectra presented above indicate that spatial masking of emission from the lower regions of the plume should enhance analyte lineto-background ratios due to the increased line emission found in the upper zones. Temporally resolved emission from the plasma plume suggests that temporal masking of the earlytime emission should also improve line-to-background ratios. In order to deconvolute the benefits of spatial and temporal masking techniques on analyte detectability as well as to better understand the expulsion properties of the plasma gun (and, possibly, the mechanism of emission enhancement in the upper spatial zones), studies of spatially and temporally resolved emission from the plume were performed by using the photoelectric zone-discriminating optical system. For these studies, temporally resolved emission profiles were recorded at a number of vertical locations along a 100-pm-wide region in the center of the plasma plume (similar to the spatially resolved study discussed above). The external slit of the zone-discriminating optical system selected a 0.5"-high spatial region for observation. Spatially and temporally resolved continuum background profiles (at 310.23 nm) obtained with this system using 6-kV, 50-pF discharges are shown in Figure 7. Only emission from the first 75 ps of the discharge is shown and all profiles have been scaled on the basis of their relative intensities in order to clearly show the initial propagation characteristics of the plasma. Since appearance of continuum emission at each spatial zone signals the arrival of the plasma a t that zone,
Flgwe 8. Plot of plasma arrival times versus location above the base of the plasma plume, indicating propagation velocity of the lea* edge of the expelled plasma. The data were obtained from the waveforms presented In Figure 7.
vertical propagation of the plasma is evidenced by the progressive delay in continuum emission at increasing displacements from the base of the plasma plume. If the plasma arrival time in each zone (determined as the time at which 10% of the peak continuum emission is attained) is plotted with respect to height above the base of the plasma, the propagation characteristics of the leading edge of the plasma may be quantified; such a plot for these waveforms is shown in Figure 8. These arrival time data indicate that the leading edge of the plasma initially has a very high propagation velocity (as evidenced by the arrival time data from the 0-8-mm zones), estimated at 3000 m/s. At heights greater than about 8 mm, however, the leading edge propagation velocity decreases by more than a factor of 4 to about 700 m/s. The initial velocity (in the 0-8-mm zones) can really only be estimated from the data as there is significant uncertainty in the arrival times in these lower zones; there is, however, a clear decrease in the rate of plasma propagation at heights greater than 10 mm. From the waveforms shown in Figure 7, there appear to be two sources of continuum emission: a broad emission pulse that predominates low in the plume and follows the current waveform (about 45 degrees out of phase), and an initially short emission pulse that first appears in the 4-mm zone and broadens considerably as it propagates upward. The broad pulse is likely due to pressure pulses of plasma material expelled from the periodic heating and cooling of the plasma inside the discharge tube (these periodic emission pulses are also observed during the second and third current half-cycles in the lowest zone). The intensity of this pulse decreases very rapidly with increasing displacement and is just barely detectable in the 8-mm zone. The sharp emission pulse is first clearly observed in the 4-mm zone, where it is almost equal in intensity to the broad continuum pulse. The emission pulse not only propagates very rapidly, but also is localized to a distinct spatial region, propagating upward independent of the discharge current. These propagation characteristics are more clearly seen in the temporal slices of the continuum emission presented in Figure 9a,b. In these plots, emission intensities at each spatial zone are plotted at selected times. In order to show more clearly the plasma propagation properties using the same scale, the emission signal was normalized so that the maximum intensity in each spatial zone was set to a value of 1. The plot in Figure 9a shows emission spatial profiles at three times early in the discharge: 23, 25, and 27 ps. Here, the initial bulk propagation of the plasma may be seen as the emission maximum shifts from 4 to about 7 mm over the 4-ps time
2362
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991 60 i
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above the base of the piasma plume at early times low in the plume (a)and at later times over the entire Observation window (b). The data were obtained from the waveforms presented in Figure 7 after intensfties in each spatial zone were normalized 80 that they ranged from 0 to 1. frame shown in the plot; this corresponds to a bulk propagation velocity of about 750 m / s and is considerably slower than the propagation of the leading edge of the plasma. The plot in Figure 9b shows emission spatial profiles over the entire observation window for four times later in the discharge: 30, 40,50, and 60 ps. Again, upward propagation of the plasma is clearly seen by the progressive movement of the emission maxima at successive times to higher displacements; propagation continues at approximately the same velocity as was determined for the early-time plot. The plot for the last time period,60 wa,shows a maximum intensity at the highest spatial zone (25 mm) and indicates that a significant amount of the emitting plasma has propagated out of the observation window. Note that the bulk plasma propagates as an expanding shell, with the emission pulse spanning only about 3.5 mm a t 23 ps but expanding to cover more than 10 mm at 60 ps. The broader emission pulse is also observable in the plots shown in Figure 9. Recall that this emission pulse is observed only at lower displacements (0-8 mm) and its intensity follows the discharge current. In Figure 9b, this behavior is shown by the emission features in the 0-6.” region where the intensity of the emission band changes with time but remains within the same spatial region. There is no indication of upward propagation of this emission feature, most likely due to the continuousnature of the expulsion of plasma that c a w s this emission. Based on these observations, it is reasonable to attribute the broad, nonpropagating emission pulse to the oscillatory heating and cooling of the plasma in the discharge tube. Since
the emission is directly associated with the heating cycles of the current-carrying plasma, it is not surprising that the intensity of this emission is greatest nearest the base of the plasma plume and then decays rapidly within the first 5-10 mm. No clear propagation is observed with this pulse as emission is from plasma that is continuously expelled from the discharge tube. The sharper, propagating emission pulse is most likely due to the initial formation and implosion of the plasma and probably is representative of plasma shot out of the discharge tube due to the extreme pressure pulse associated with plasma formation processes. This pulse of plasma material travels upward a t an initially very high velocity, and ultimately slows and spreads out at displacements greater than about 10 mm. If the continuum emission at each spatial zone is integrated over the entire discharge, the spatial map of integrated continuum background shown in Figure 10 results. Here, a very intense continuum emission is observed in the lowest spatial zones, which drops off very rapidly in intensity until reaching a minimum at 8 mm. At displacements greater than 8 mm, however, the background intensity increases until a maximum is reached at the 20.” zone. This behavior is the same as that observed in the spatially resolved photographic spectra presented earlier. Based on the temporally and spatially resolved studies presented thus far, it appears that a significant change occurs in the plasma plume at a height of about 8-10 mm, as this is the spatial zone where a drastic reduction in plasma propagation is observed as well as where the plasma emission begins to increase in intensity at increasing displacements. From the waveforms shown in Figure 7, it looks as though the plasma material responsible for the two emission pulses collides or merges approximatelyin this spatial zone. Collision of these two plasma pulses at the 10-mmspatial zone would result in an increased continuum emission due to the increased plasma density and the resultant increases in collision frequency. Increases in collisional activity would also slow the upward propagation of the sharper pulse of plasma, thus reducing the propagation velocity as well as broadening the pulse spatially and temporally. Thus, masking of continuum background is best accomplished via spatially masking out emission from regions between 0 and 8 mm above the base of the plasma; although the continuum background does increase in intensity in the higher (10-26 mm) zones, the increase is quite small relative to the intensities observed in the lowest zones. Furthermore, spatial masking should be more effective than temporal masking as it removes the continuum emission pulses that are observed in the lowest spatial zones during
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991
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Time, ~s
Figure 11. Tiwesolved contlnuum (marked b, light line traces) and V(I1) net line (marked a, dark line traces) emission profiles from &kV, 50-pF discharges measured at stated dlspfacements above the base
of the plasma plume. V(I1) net line profiles were obtained by using 1.2 pg of vanadium as VC and have been background-corrected.
the second and third current half-cycles. Spatially and Temporally Resolved Emission: Analyte. The usefulness of spatial and/or temporal masking for the reduction of continuum background emission depends strongly on the spatial and temporal distribution of emission from the analyte. Figure 11 shows spatially and temporally resolved emission waveforms (at 310.23 nm) obtained by using &kV, 50-pF discharges; waveforms marked b (light line traces) were obtained without any analyte present, and those marked a (dark line traces) were obtained with about 1.2 pg of vanadium as VC deposited in the discharge tubes and then background-corrected. Emission at each of the six spatial zones has been scaled for clarity. As shown earlier in Figures 7 and 10, the continuum background is most intense in the zone nearest the base of the plasma plume and then decreases significantly in intensity until about the IO-" zone, where it begins to increase in intensity again. The V(I1) emission tends to follow a similar trend with very intense emission in the lowest zone as well as in the 15-, 20-, and 25-mm zones and very little emission in the 5- and 10." zones. Although the V(I1) emission is very intense in the 0-mm zone, it is still lower in the intensity than the background. V(I1) emission is very weak in the 5-mm zone and only rises above the background during the second current half-cycle. In the 10mm zone, there is clear indication (as evidenced by a negative emission signal) that self-reversal occurs coincident with the background emission pulse. Spatial masking of the emission from these regions, then, should enable significant reduction in the continuum background signal with little decrease in the V(I1) emission signal. In the higher spatial zones (15,20, and 25 mm above the
were obtained from net line and continuum background proRles shown in Figure 11.
plasma base), analyte emission is at least as intense as the V(I1) emission observed nearest the base of the plume. In the 15-mm zone, this analyte emission pulse is nearly temporally coincident with the background emission pulse, almost decaying completely within the first 75 bs of the discharge. In the two uppermat zones, however, very intense V(II) emission is observed at times as late as 150 1.1s. The temporal characteristics of the analyte emission in these uppermost zones is especially interesting, as it seems to oscillate with the discharge current (although time-delayed by about 25 ps). This is curious behavior in light of the fact that this emission is displaced from the current-carryingplasma in the discharge tube by over 25 mm as well as in considerationof the complete absence of significant V(i1) emission at those times in the two zones directly below them (10- and 15-mm zones). While this behavior may be attributed to the same collisional mechanism presented earlier to explain the increased background emission viewed in these zones, the degree of enhancement of analyte emission suggests that optical excitation may be the predominant reexcitation mechanism for these species. This is not an unreasonable assumption, especially considering the very intense emission from the discharge tube during the second and third current half-cycles that is directed upward into the expanding plasma plume. The absence of any significant analyte emission in the 10- and 15-mm zones after the initial emission pulse indicates that there could be high concentrations of ground-state analyte species in those zones that could be optically excited and then fluoresce as they reach the uppermost zones. If collisional reexcitation were solely responsible for the enhanced emission, such a moderate increase in continuum relative to analyte emission would not be expected. The overall effects of time and spatial discrimination on V(I1) 310.23 nm line-to-background ratios (LIB)are shown in Figure 12. In this figure, spatial maps of LIB are presented for three different time gates: 0-75, 75-150, and 0-150 ps. When integrated over the entire discharge, the LIB is less than unity for all zones under 15 mm and then increases to about 3.5 in the uppermost zone. Simple spatial masking then should enable at least a 2-fold enhancement in LIB. Recognizing that low LIB'S should be observed early in the discharge as well (see 0-75-ps time-gate plot), LIB enhancement should also be observed for integration periods that exclude emission during the first current half-cycle. The spatial map of LIB for the 75-150-ps integration period shows the most significant improvement in LIB,with LIB values typically greater than 5 in spatial zones a t 10 mm and above. Thus, both spatial and temporal masking of emission can result in
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9
ANALYTICAL CHEMISTRY, VOL. 63, NO. 20, OCTOBER 15, 1991 Table 111. Qualitative Analysis of USGS MAG-1 species
Si(1) AUI) MdI) MdII) Ti(I1) MnU)
V(I1)
0' 2860
3045
3230
3415
Wavelength. p\
Flgwe 13. Temporally integrated spectra of l-mg samples of USGS MAG1 rock standard obtained from 6 k V , 50-pF discharges. Spectrum A is a microdensltometer trace of emission from a region near the base of the plasma plum, while spectnm B Is spatially integrated. Insets I and I1 show expansions of regions with Mg(I1) and Mn(1) emission and V(I1) emission, respectively. The V(I1) lines marked in inset I1 are identified in Table 11.
a significant (almost a 10-fold) improvement in LIB if emission from regions above 10 mm is integrated over a time gate that excludes the first 75 ps of the discharge. Qualitative Analysis: Direct Solid Sampling. In order to investigate the utility of the plasma gun source as an atom cell suitable for direct solid sampling methods, spectra were acquired from a "real world" solid sample. Figure 13 shows emission spectra obtained from 1-mg samples of MAG-1, a USGS rock standard of marine mud, using 6-kV, 50-pF discharges. This sample is almost two-thirds SiOzand Al2O3and was chosen as an example of a sample that normally would require extensive sample processing prior to analysis. The 1-mg samples used to generate these spectra were deposited from alcoholic slurries into the plasma discharge tube; the slurries were prepared directly from the sample powder as supplied by the USGS. Spectrum A is a densitometer trace taken from a region near the base of the plasma plume from a spatially resolved spectrum recorded with the high-fidelity imaging spectrographicsystem. Spectrum B is a densitometer trace from a spectrum acquired by using no spatially resolving optics. As expected, spectrum A consists of small sample emission lines superimposed on a very intense continuum background. A rapid survey of the emission lines in both spectra quickly allowed identification of a number of major and minor constituents in the MAG-1 sample; lines from these species are identified in the figure and listed in Table 111. Note that Al, Si, and Mg atomic resonance lines are easily located in spectrum A as they are strongly self-reversed; even in spectrum B there is significant self-reversal of the resonance lines of these species. These elements are present at very high levels (70)in the sample, and thus, the self-reversal of these lines is not surprising. There is significant ionic character to the analyte emission as well. The ionic resonance lines for Mg (see inset I) are strongly self-reversed in spectrum A again, this is not surprising when the high level of Mg in the sample is considered. The intense continuum background level as well as the significant line broadening observed in spectrum A makes identification of species present at lower concentrations difficult. Titanium, for example, is easily detected in the spatially integrated spectrum but is obscured by the high background between the two severely broadened silver resonance lines in
wavelength(s), nm 288.16 308.22 309.27 285.21 279.55 280.27 334.90 336.12 337.28 279.83 280.106 see Table I1
concn (17) 23.3% 8.7% 1.8% 0.52%
770 ppm 132 ppm
spectrum A. Careful examination of this region in spectrum A shows the Ti(I1) lines as small reversals of the background. Manganese, present at 770 ppm, is also observed in spectrum A as reversed resonance lines in the background between the two broadened Mg(I1) lines (see inset I). These manganese lines are very difficult to pick out in spectrum B, as they are swamped by the intense emission from the Mg(I1) lines. Ionic emission from vanadium, present at 132 ppm, is not detectable in spectrum A due to the high background from the broadened nearby Al(1) resonance lines, but is observed in spectrum B (see inset 11). Clearly, the spatially integrated spectrum contains many more intense sample emission lines and a substantially decreased background than does the spectrum from the region nearest the base of the plasma. In fact, the spectral characteristics that diminish the utility of spectrum B (Le., high background, line reversal, line broadening) can be attributed to emission features from the lower regions of the plasma plume. Thus,even though elements present in the ppm range are detectable in spectrum B, detectability should be able to be improved significantly via spatial masking of the emission from the lower regions of the plasma plume. Still, although drastically reduced, self-reversal of major constituent resonance lines is observable even in spectra from displacements as great as 14 mm above the base of the plasma plume. Spatial masking alone, then, will not completely eliminate the deleterious spectral features that are characteristic of emission from the lowest regions of the plasma plume. As discussed previously, we would expect the greatest improvements by using both spatial and temporal discrimination.
CONCLUSIONS The results of the studies presented here show that the plasma gun has the potential for direct application as a source for direct solid sampling methods. However, even though the emission characteristics are improved considerably relative to those of the ITFP,as configured, there are still considerable limitations to the direct use of the emission from the plasma gun plume. The excitation characteristics limit detectability to the ppm level with 1-mg samples; this can be improved via spatial and temporal masking of the emission, but it is unlikely to push detectabilities into the ppb range. Spectra from the source tend to be highly ionic and line-rich, which results in spectral overlap problems with complex, real samples (e.g., the 3.8% Fe in the MAG-1 sample presented many situations where spectral overlap obscured detection of analyte present at the ppm level). Still, the ability of the plasma gun to directly atomize even the most refractory solids suggests that further study and development of the source is warranted. Improvement of spectral characteristics via altering the discharge source parameters as well as thin-film material and thickness is currently under investigation in our laboratory. Also, studies of segregated atomization and excitation systems using the plasma gun as an atom cell with ICP and MIP
Anal. Chem. 1001, 63,2365-2370
excitation are currently underway in order to evaluate the utility of the directional expulsion properties of the plasma gun.
ACKNOWLEDGMENT We thank Walter Weir, Michael Hamblin, and Bradford Williams for their help with the design and construction of the apparatus. LITERATURE CITED (1) Allen, 0. M.; Coleman, D. M. Appl. Speclrapc. 1987, 41, 381-387. (2) Beckwith, P. M.; Mulllns. R. L.: Cobman, D. M. Anal. Chem. 1987, 59, 163-167. (3) AKOWS~W~. P. Anal. m.1987, 59, 1437-1444. (4) Thompson, M.; Chenery, S.; Brett, L. J . Anal. At. Specirom. 1989, 4 , 11-16. ( 5 ) Darke, S. A.; Long, S. E.; Plckford, C. J.; Tyson, J. F. J . Anal. A t . SpeCtrOm. 1089, 4,715-719. (6) Clerk, E. M.; Sacks, R. D. Spectrochlm. Acta, Part 8 1080, 358,
471-488. (7) White, J. S.; Scheeline, A. Anal. Chem. 1987, 59, 305-309.
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(8) CarIWy, K. P.; Goldberg, J. M. A M . C b m . 1888. 58, 3108-3115. (9) Carney, K. P.; Goldberg, J. M. Anal. Chem. 1986, 58, 3115-3121. (10) Goldberg, J. M.; Camry, K. P. Spsctrochlm. Acta, Part8 1990, 458,
1 167- 1 175. (11) Goldberg, J. M.; Carney, K. P. spactroddm. Acta, P a r t 8 1990, 458, 1177-1186. (12) w g , J. M.; Carney, K. P. Spsctrocham. Acta, Part B 1901. 468, 393-406. (13) Robinson, D. S.;Mam, K. J.; Dorman, F. L.; Goldberg, J. M. Appl. spectrosc. 1090, 44, 1584-1587. (14) Goldstein, S. A.; Welters, J. P. S p s c M m . Acta, Part B 1978, 318, 201-220. (15) Goldstein, S. A.; Walters, J. P. Spectrochlm. Acta, Part B 1976, 318, 295-316. (16) Salmon, S. G.;Holcombe, J. A. Anal. Chem. 1078, 50, 1714-1716. (17) Menhelm, F. T.; Hetheway. J. C.; Flanagan, F. T.; Fletcher. J. D. In OeSCrtptkns anUAna@k of Ebht New USGS Rock Standards: Flenagen. F. T. Ed.; U.S. Government Prlntlng Offlce: Washington, DC, 1976;pp 25-28.
RECEIVED for review April 24,1991. Accepted July 29,1991. We gratefully acknowledge financial support for this study from the National Science Foundation (Grant R11-8610679).
Temperature-Jump Relaxation Kinetics at Liquid/Solid Interfaces: Fluorescence Thermometry of Porous Silica Heated by a Joule Discharge S. W. Waite and J. M. Harris* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
E. H. Ellison and D. B. Marshall* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300
Temperature-jump reiaxatlon technlques are adapted to meawrlng the kinetics of reversible reactions at Ilquld-rolld Interfaces. I n the present work, we determine whether fad temperature changes can be created wtthin a packed bed of sillca gel by using Joule discharge heatlng. Fast heating of the surface of porourdyccr dopond8 on the soiutkn within the pores carrying current through the particle, since heat flow from the surroundlng solution Into the interior of the diica partido would generally bo r k w . To characterize the rate of heating of the silica surface, fluorescence thermometry was omployed to measure the temperature rise at a ilquid/soild interface on a mlcrosecond time scale. Q,lO-Dlchioroanthracone (9,lO-DCA) was found to be a suttabk probe molecule for fast fluorescence moa(wr(HMnl8 of temperature changes at alkylated silica surfaces. For 10-pm silica particles, we observe an exponentlal relaxatlon of the temperature-dependent fluorescence Intenrlty wtth a decay time that corresponds to the rate of energy deposition In the sample. These results indlcate that ionic current flows through the siHca, uniformly heatlng the surface area of smaller partlciesit0 silka. Larger particle silica shows a slower thermal roiaxatkn Indicating nonconductlvo domains that are heated on a slower tlme scale by thermal conduction.
INTRODUCTION A number of phenomena important to analytical chemistry
occur a t interfaces between liquids and dielectric solids including adsorption, desorption, monolayer self-assembly, and reactions of immobilized reagents. A wide range of processes
rely on selective interactions at these interfaces, such as surface passivation, stabilizing composite materials, selective adsorption for chemical separations, and surface modification for control of heterogeneous reactions. While a great body of information exists for these phenomena, most of our understanding of dielectric solid-liquid interfaces has been inferred from measurements of surface concentrations, interfacial equilibria, isotherms, or other time-independent behavior of these systems. The subtlety of chemical interactions that play a role in these phenomena often cannot be sorted out with equilibrium measurements, where only the average, steady-state chemistry of the system is observed. Kinetic measurements coupled with surface-selective spectroscopies offer the opportunity for understanding mechanisms responsible for the steady-state behavior and determining the dispersion of rates in cases where inhomogeneities lead to a distribution of transition-state energies. Among the measurement challenges that must be addressed to observe chemical reaction kinetics at dielectric solid-liquid interfaces is the need to rapidly change surface activities, in order to observe the rates of surface reactions. This is particularly challenging for nonconductive solids where electrochemical perturbation to surface activities is not feasible, and where the rate of change of chemical composition at the interface is limited by molecular diffusion through a stagnant solvent layer adjacent to the surface. Several strategies for measuring reaction rates at dielectric liquid-solid interfaces have been developed. For sufficiently slow kinetics, competitive reactions between a surface ligand and a mixture of reagents provide useful information, for example, about the relative rates of chlorosilane condensation reactions to silica (I). Similarly, one can use flow methods to change the com-
0003-2700/01/0363-2385S02.50/0 Q lQ9l American Chemical Sockty
