Anal. Chem. 1994,66, 2022-2030
Inductively Coupled Plasma Optical Emission Spectrometry Using Nebulizers with Widely Different Sample Consumption Rates John W. Oleslk,' Jeffery A. Klnrer, and Bryan Harkleroad Laboratory for Plasma Spectrochemistry, Laser Spectroscopy and Mass Spectrometry, Department of Geological Sciences, The Ohio State University, Scott Mil, 1090 Carmack Road, Columbus, Ohio 43210 A pneumatic, high-efficiency nebulizer (HEN), consumingonly 50 rL of sample per minute, produced detectionlimits for most lines that were within a factor of 3 of those exhibited by commonly used pneumatic nebulizers that use 1.0 mL of sample per minute. Detection limits were within a factor of 8 for all linesobserved, despite the 20 times smaller sample colrsumption rate for the HEN compared to commonly used nebulizers. The aerosol properties were measured and related to the observed vertical emission profdes, Ar emission intensities, a d ion to atom emission intensity ratios. The aero801 properties and plasma excitationconditions were used to understand theorigins of the nebulizer-dependent sensitivities, background, and background fluctuations that control detection limits. More than 90%of the primary aerosol volume produced by the HEN had drop diameters less than 10 pm. The analyte transport efficiency was 20%for the HEN compared to 1.5-2% for the commonly used nebulizers. More emission intensity was produced per microgram of analyte entering the plasma when the HEN was used in place of the other nebulizers. There are many situations where low concentration detection limits are desired but sample consumption rates are limited. The total volume of sample available for analysis may be small. Optimum chromatographic or electrophoretic separations often require capillary columns and low liquid flow rates. Handling of hazardous materials and chemical waste is minimized by using small volumes. The goals of this study were to (1) evaluatea new pneumatic nebulizer for ICP-OES which requires only small sample volumes (50 pL/min) to obtain low concentration-based detection limits and (2)determine why the detection limits are similar to those obtained using conventional pneumatic nebulizers requiring 20 times higher sample volumes. Comparisons were made among commonly used pneumatic nebulizers (Perkin Elmer cross-flow and Meinhard TR-30A3) that typically use sample uptake rates of 0.5-3 mL/min and a Meinhard high-efficiency nebulizer (HEN) which was operated at 0.05 mL/min. The HEN is a concentric, pneumatic nebulizer but with a smaller center liquid carrying tube and smaller gas orifice ring than the TR-30-A3. As a result, the HEN operates at a higher back pressure (170-200 psi for an argon nebulizer gas flow rate of 1.O L/min) than the cross-flow or TR-30 nebulizers. ICP-OES detection limits were measured and the factors controlling detection limits, sensitivity, background, and relative standard deviation of the background, were investi2022
AnalyNcalChe~by,Vol. 66, No. 13, July 1, 1994
gated. Emission lines with a wide variety of excited-state energies from elements with a wide variety of ionization energies were monitored. Primary and tertiary aerosol dropsize distributions, analyte transport rates, aerosol transport rates, vertical emission profiles, Ar emission, and ion to atom emission intensity ratios were measured in order to understand the key factors controlling the nebulizer-dependent detection limits.
EXPERIMENTAL SECTION Nebulizer/Spray Chamber Systems. The three nebulizers were used with a Perkin Elmer ryton, Scott-type double-pass spray chamber. In each case the nebulizer gas flow rate was 1.0 L/min. The mass flow controller in the Perkin Elmer Optima 3000 ICP Optical Emission Spectrometer was used to control the nebulizer gas flow rate when the cross-flow and TR-30-A3 nebulizers were used. The back pressures were 26 and 3 1 psi for the cross-flow and TR-30 nebulizers, respectively. A separate Ar supply, controlled by a Brooks Model 5850C/5876 mass flow controller, was used for the HEN because a high back pressure (approximately 170-200 psi) was necessary to maintain a gas flow rate of 1 .O L/min. Liquid samples were delivered to the nebulizers using a Gilson Minipuls I11 peristaltic pump. The uptake rates were 1.0, 1 .O, and 0.050mL/min for the cross-flow, TR-30, and HEN nebulizers, respectively. Peristaltic pump tubing with inner diameters of 0.76 and 0.25 mm were used to generate flow rates of 1 .O and 0.05 mL/min, respectively. Drop-Size Distribution Measurements. A Malvern Mastersizer with a 300 mm focal length lens was used to measure percent volume based drop-size distributions using laser Fraunhofer diffraction. Five repetitions (of 1000-30700 scans each) of each measurement were made. Typical short-term precision of the individual points in the percent volume based drop-size distribution data was generally better than 2% RSD (for percent volume values greater than 2%). Short-term precision in Sauter mean diameters was typically better than 2%. Sauter mean diameters of the primary aerosol generated by the cross-flow nebulizer, measured by three different researchers on three different days, had a relative standard deviation of less than 6%. Primary aerosol measurements were made 0.75 in. from the tip of the nebulizer. Tertiary aerosol measurements were made 0.5 in. above the top of the spray chamber (with no torch in place). The Malvern instrument provides percent volume based drop-size distributions. Absolutevolume based 0003-2700/94/03662022$04,50/0
@ 1994 American Chemical Soclety
primary aerosol dropsize distributions were obtained by multiplying the percent volume based dropsize distributions by the liquid sample uptake rate. Absolute volume based tertiary aerosol dropsize distributions were obtained by multiplying the percent volume based dropsize distributions by the measured aerosol transport rate (described below). Transport Rate and Efficiency Measurements. Transport rate measurements were made using the method described by Bates,’ which was based on the description by Smith and Brownera2A 1000-ppmSr solution was aspirated for a known amount of time. Tertiary aerosol was collected on a 0.3-pm filter placed above the spray chamber but not connected to it. The filter was washed, and the solution analyzed via ICP optical emission spectrometry. Olson et al.3 previously described a closed system based on similar concepts. Sample Solutions. Three multielement solutions were used. One solution contained 0.1 ppm Sr, 0.1 ppm Mg, 1 ppm Cu, 1 ppm Cr, and 1 ppm Zn in 2% (v/v) nitric acid (made in the laboratory from solids). A second solution contained 0.1 ppm Ba, 0.1 ppm Ca, 1 ppm Mn, 1 ppm Na, 1 ppm La, 1 ppm Li, and 5 ppm K in 2% (v/v) nitric acid (made by diluting PE Pure Zodiac Vis Wavecal Mix by a factor of 20). The third solution contained 5 ppm P, 5 ppm K, 5 ppm S, 1 ppm As, 1 ppm La, 1 ppm Li, 1 ppm Mn, 1 ppm Mo,1 ppm Ni, 1 ppm Sc, and 1 ppm Na in 2% (v/v) HCI (made by diluting a Spex Instrument Check Standard by a factor of 20). OpticalEmission Measurements. A Perkin-Elmer Optima 3000495 was used for emission measurements. The power was 1.1 kW, and the outer and intermediate gas flow rates were 15 and 1.O L/min, respectively. The total exposure time for each line was 50 s (the Optima software automatically chose the on-detector exposure time and number of exposures to make on the basis of a preshot intensity measurement). Peak intensity measurements were made. Because the number of detector pixels across each spectral peak was small, a linear interpolation can lead to errors in the estimate of peak height.6 The Optima software fit a parabolic function to three points nearest the peak in order to estimate the actual peak intensity. The height above the load coil (ALC) was optimized for best detection limits for K I, Cu I, Zn I, Sr 11, and Mg I1 for the cross-flow nebulizer using the directed search between 5and 20-mm ALC under control of the Optima software. Observation heights for K I (typicalof low-energy atom lines), Cu I (typical of high-energy atom lines), and Sr I1 (typical of ion lines) were optimized when the HEN and TR-30 nebulizer were used. Emission from all lines was acquired from 12 mm above the load coil when the cross flow nebulizer was used and 7 mm above the load coil when the HEN was used. Emission from all lines except Li, Na, and K was collected at 15 mm above the load coil when the TR-30-A3 nebulizer was used. The alkali element lines were viewed at 12 mm above the load coil. (1) Bates, L. C. Sample A e r w l Characterization and Study of Ib Effect on
Inductively Coupled Plasma Atomic Emission Spectrometry. Ph.D. Thesis, University of North Carolina-Cbapel Hill, 1991. (2) Smith, D.D.;Browner, R. F. Anal. Chem. 1982, 55, 374. (3) Olson, K. W.; Haas, W. J., Jr.; Farurel, V. A. Anal. Chem. 19n, 49,632-637. (4) Bamard, T. W.; Crockat, M. I.; Ivaldi, J. C.; Lundberg. P. L. AMI. Chem. 1993,65, 1225-1230. (5) Bamard. T.W.; Crockett, M. I.; Ivaldi, J. C.; Lundberg, P. L.; Yatar, D. A.; k i n e , P. A.; Sauer. Anal. Chem. 1993, 65, 1231-1239. (6) Lepla. K. C.; Horlick, G. Appl. Specfrarc. 1990, 44, 1259-1269.
Table 1. 8p.ctral Um chrrrctuhtka element wavelength ionization (nm) energy (eV) (I or 11)
K (1) Li (I) N a (1) c u (1) Mg (1) Zn (1) As (1) Ba (11) Sr (11) La (11) Ca (11) sc (11) M8 (11) Mn(I1) Cr (11) Mo (11) Ni (11)
766.49 670.78 588.99 324.15 285.21 213.86 193.70 455.40 421.55 408.67 393.37 361.38 280.27 257.61 205.55 202.03 221.65
5.21 5.70 5.58 6.11 6.54 7.65 7.44 6.77 7.10 1.64
excitation energy (eV) 1.62 1.85 2.10 3.82 4.30 5.80 6.40 2.72 2.94 3.03 3.15 3.45 4.42 4.8 1 6.03 6.13 6.63
The Optima transfer optics were designed to introduce some a~tigmatism.~As a result, light is collected from an observation zone about 5 mm high in the plasma. This provides compromise conditions for analysis as well as a reduction of the effect of signal fluctuationsdue to incompletely desolvated droplets and vaporizing particle^.^^^,^ Vertical Emission Profiles. Vertical emission profiles were obtained sequentially by adjusting the first transfer mirror, under control of the Optima software. At a setting of 0 mm above the load coil, the light is partially blocked by the coil. Spectral Lines and Characteristics. Spectral lines used in this study are listed in Table 1 together with the excitation energy and the energy needed to produce the ions (for ion lines). Calculation of Detection Limits. Sample solutions and blanks were used to determine the sensitivity. Ten measurements of the appropriate blank were used to determine the standard deviation of the background in concentration units. Thedetection limits werecalculated to be 3 times the standard deviation of the background in concentration units.
RESULTS AND DISCUSSION DetectionLidts. Detection limits obtained when the three different nebulizers were used are shown in Table 2. The data in Table 2 are arranged so that the required ionization and excitation energy generally increase as one moves down the table. The most striking result is that the detection limits for the three different nebulizers were very similar despite the fact that the liquid sample uptake rate is 20 times smaller for the HEN. The detection limits using the three nebulizers are always within a factor of about 8, and for a majority of the lines investigated (1 1 of 17) the detection limits using the different nebulizers are all within a factor of 4 of each other, as shown in Table 3. Looking more closely, some patterns emerge. Detection limits for atom and ion lines with low excitation energies are consistently best for the TR-30 nebulizer and poorest for the HEN nebulizer. The detection limits for the HEN and cross(7) Cicerone, M. T.;Famworth, P. B. Specfrochim.Acta, 1989,44B, 897-907. (8) Olesik, J. W.; Smith, L. J.; Williamsen, E. J. Anal. Chem. 1989.61, 20022008.
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Table 2. Detectlon Llmitr ( W L ) Obtained Udng M DlIf.nnt NebuHr1~rs4~ element wavelength (I or 11) (nm) cross-flow TR-30-A3 HEN
K (1) Li (I) Na (1) c u (1) Zn (1) As (1) Ba (11) Sr (11) La (11) Ca (11) s c (11) Mg (11) Mn(I1) Cr (11) Mo(I1) Ni (11)
766.49 670.78 588.99 324.75 213.86 193.70 455.40 421.55 408.67 393.37 361.38 280.27 257.61 205.55 202.03 22 1.65
39 1.2 120 0.8 0.6 110 0.16 0.048 0.79 0.28 I .5 0.078 0.3 18 13 5.8
36 0.7 91
120 4.0 390 1.2 1.3 64 0.25 0.03 2.4 0.32 0.24 0.1 0.37 8.3 6.0 2.1
3
1.6 71 0.08 0.015 0.39 0.04 0.23 0.066
0.11 18 5.6 4
I Cross flow
I /
z >
10
*-
.
al
.. . I --
0
20
HEN
0 Numbers in bold indicate lowest detection limits. Data displayed in ppb units. In order to avoid use of scientific notation, the number of significant figures is not appropriate in all cases.
C
0 1
Table 3. Relative Detect1011W r a element wavelength lowest DL (I or 11) (nm) nebulizer cross-flow
K (1) Li (I) Na (I) Cu (I) Zn (1) As (1) Ba (11) Sr (11) La (11) Ca (11) Sc(I1) Mg (11) Mn (11) Cr (11) M O(11) Ni (11)
766.49 670.78 588.99 324.75 213.86 193.70 455.40 421.55 408.67 393.37 361.38 280.27 257.61 205.55 202.03 221.65
TR-30 TR-30 TR-30 cross flow cross flow HEN TR-30 TR-30 TR-30 TR-30 TR-30 TR-30 TR-30 HEN TR-30 HEN
1.1 1.7 1.4 1 .o 1 .o 1.7 2.0 3.2 2.0 7.1 6.4 1.2 2.7 2.1 2.2 2.8
HEN
1.0 1.0 1.0 3.8 2.7 1.1 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.2 1.0 1.9
3.3 5.7 4.3 1.5 2.2 1.0 3.1 2.0 6.2 8.2 1.0 1.5 3.4 1.0 1.1 1.0
flow nebulizers are best (compared to the TR-30) for highenergy atoms and high-energy ion lines. In order to understand why the detection limits for the HEN nebulizer using a 0.05 mL/min uptake rate are so similar to those for the two nebulizers using a 1.0 mL/min uptake rate, the aerosol characteristics of each nebulizer and their effect on sensitivity and background emission must be considered. Aerosol Characteristics. Key characteristics of aerosols generated for introduction into ICPs include the primary dropsize distribution produced by the nebulizer, the tertiary dropsize distribution at the entrance to the plasma, the analyte transport rate, and the aerosol transport rate. Aerosol measurements should always be viewed with a certain amount of caution as different measurement techniques often produce different absolute results. However, the trends within a set of measurements using the same technique are insightful. The primary aerosol drop-size distributions for the three different nebulizers on a percent volume basis are shown in Figure 1. The Sauter mean diameters (D3.2) for the primary aerosol were 9.7, 6.2, and 2.6 pm for the cross-flow, TR-30, Analyticsl Chemistty, Vol.
100
Drop Diameter (pm)
TR-30
I, Detection limits relative to smallest nebulizer-dependent detection limit obtained.
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Flgure 1. Percentvolume basedprhnary aerosoldrop-sizedistributions for three different nebulizers.
and HEN nebulizers, respectively. The D3,2 values for the TR-30 nebulizer are within the range of values previously observed for nebulizers of the same type.'s9J0 Canals et a1.I0 also reported a larger 03.2 value for the cross-flow nebulizer compared to theconcentric nebulizer, although theexact values are not comparable to those reported here because we used a higher gas flow rate. Unlike the aerosols produced by the cross-flow and TR-30 nebulizers, most of the aerosol produced by the HEN nebulizer was contained in droplets less than 10 pm in diameter. Tarr et al." noted that it is very unusual to produce a primary aerosol of such small-sized droplets using either ultrasonic or pneumatic nebulizers. No aerosol characteristics have been previously reported for this nebulizer for identical gas and liquid flow rates. However, we measured a D3,2 of 6.6 pm when the liquid and gas flow rates were 0.3 and 0.1 L/min, respectively. With the use of a different Malvern instrument approximately 3 years earlier, the measured D3.2 was 7 pm using the same nebulizer with gas and liquid flow rates of 0.3 and 0.1 L/min, re~pectively.~Furthermore, we observed a continuous decrease in D3.2 as the gas flow rate through the HEN was increased from 0.3 to 0.5 to 0.75 to 1.O L/min. This is somewhat in contrast to the observations of Shum et who observed little change in D3.2 as a function of gas flow rate when they used a direct injection nebulizer at high gas/ liquid flow rates. The percentages of sample aerosol volume contained in drops with diameters less than 10 pm were 21, 46, and 93% for the cross-flow, TR-30, and HEN nebulizers, respectively. The primary aerosol drop-size distributions for the three nebulizers, on an absolute volume basis (displayed on a (9) Hobb, S.E.; Oleaik, J. W. Unpublished results. (IO) Canals, A.; Wagner, J.; Browner, R. F.;Hemandis. Specrruchim.Acra 1988. 43B, 1321-1335. (11) Tarr, M. A.; Zhu, G.; Browner, R. F. A d . Chem. 1993,6S,1689-1695.
(12) Shum, S.C. K.; Johnson, S.K.; Pang, H.-M.; Houk, R. S. Appl. Spcefrosc. 1993, 47, 575-583.
10
1
100
Drop Diameter (pm)
Figure 2. Absolute volume basedprknaryaerosddropskedistributlons for three different nebulizers.
1
10
100
Drop Diameter (pm)
Figure 3. Absolutevolume basedtertiary aerosoldrop-sitedistributions for the cross-flow (CF) and TR-30 nebulizers.
logarithmic scale), are shown in Figure 2. The volumes of primary aerosol contained in drops less than 10 pm in diameter were about 0.2,0.46, and 0.047 mL/min, for the cross-flow, TR-30, and HEN nebulizers, respectively. The percentages of aerosol volume contained in drops with diameters less than 5 pm were approximately 10,20, and 71%, corresponding to 0.096, 0.20, and 0.035 mL/min for the cross-flow, TR-30, and HEN, respectively. Furthermore, it is likely that a significant volume of primary aerosol produced by the HEN was contained in drops with diameters less than 1 pm, which are not accurately measured by the Malvern instrument. The analyte transport rate is the amount of analyte that enters the plasma per unit time. The transport rates for analytes present in the sample at a concentration of 1 pg/mL were 0.015, 0.020, and 0.010 pg/min for the cross-flow, TR-30, and HEN nebulizers,respectively. So, despite a factor of 20 less sample being used per minute by the HEN compared to the cross-flow or TR-30 nebulizer, the analyte transport rate for the HEN is only a factor of 2 lower than that for the TR-30 and a factor of 1.5 lower than that for the cross-flow nebulizer. Losses of even small drops in the spray chamber were less severe when the HEN was used. The TR-30 nebulizer produced more than 5 times as much primary aerosol volume in drops with diameters less than 5 pm than the HEN, but the TR-30 analyte transport rate was only a factor of 2 larger. The cross-flow nebulizer produced almost 3 times as much primary aerosol volume in drops with diameters less than 5 pm than the HEN, but the cross-flow analyte transport rate was only 50% higher. This is probably due to less drop coalescence when the HEN was used because of the smaller number of primary drops produced. Figure 3 shows the absolute volume based tertiary aerosol drop-sizedistributionsfor the cross-flow and TR-30 nebulizers. We were unable to make reliable measurements of the tertiary aerosol produced by the HEN and the Scott-type spray chamber. Either the number of detectable droplets was too
low or their size was too small to be efficiently detected by the Malvern Mastersizer. The primary aerosol measurements suggest that there was probably aerosol with drops below the minimum detectable size (the percent volume for the lowest size interval was more than 70% as large as the maximum percent volume per interval). Furthermore, significant evaporation of the primary aerosol droplets likely occurred in the spray chamber. As a result, it is not entirely surprising that the tertiary aerosol produced by the HEN was not detectable by the Malvern instrument despite the fact that the analyte transport rate is only a factor of 2 smaller than when the TR-30 nebulizer was used. The Sauter mean diameters were 2.6 and 3.4 pm for the tertiary aerosols produced by the cross-flow and TR-30 nebulizers, respectively. The tertiary aerosol produced by the cross-flow nebulizer had a smaller D3.2 than the aerosol produced by the TR-30 nebulizer, although the D3,2 of the primary aerosol was larger when the cross-flow nebulizer was used in place of the TR-30 nebulizer. Secondary and tertiary processes will depend on a number of factors including the drop velocities and the angular distribution of the primary aerosol. The total volume of tertiary aerosol contained in drops less than 4 pm in diameter was slightly higher for the cross-flow nebulizer than for the TR-30 nebulizer (9.2 versus 8.3 pL/ min). Conversely, the volume of tertiary aerosol contained in drops with diameters between 4 and 20 pm was significantly smaller for the cross-flow nebulizer than the TR-30 nebulizer (8.2 versus 11.4 pL/min). On the basis of these data, the number of incompletely desolvated droplets in the ICP was probably higher when the TR-30 nebulizer was used. The Scott-type spray chamber is particularly effective at removing most droplets greater than 20 pm in diameter (as is seen by comparing the primary aerosol dropsize distribution to the tertiary). Most of the primary aerosol produced by the cross-flow and TR-30 nebulizers was contained in droplets larger than 20 pm while almost all of the primary aerosol droplets produced by the HEN nebulizer were less than 20 pm. Therefore, the transport efficiency of the aerosol produced by the HEN nebulizer would be expected to be much higher than when the cross-flow or TR-30 nebulizers are used. The analyte transport efBciency is the fraction of analyte pumped to the nebulizer that reaches the plasma. The analyte transport efficiencies of the cross-flow, TR-30, and HEN nebulizers were lS%,2.0% and 20%, respectively. The high analyte transport efficiency is due mainly to the small averagesize of the primary aerosol. However, the droplet velocity and number density will certainly affect the transport efficiency, as noted above. The comparison of the cross-flow and TR-30 nebulizers at an uptake rate of 1.O mL/min to the HEN at 0.05 mL/min may be biased in favor of the nebulizer used with a low liquid flow rate. In general, the Sauter mean diameter of the primary aerosol produced by pneumatic nebulizers used for ICP spectrometry will decrease and the analyte transport efficiency will increase as the gas/liquid flow rate ratio is increased.'~91'0,*3-'6 So, the mean diameter of primary aerosol (13) Sharp, B. L. 1.AMI. At. Specrrom. 1988,3,613-652. (14) Sharp, B. L. J. Anal. At. Spctrom. 1988, 3, 939-963. (15) Canals, A.; Hemandis. V.; Browner, R. F. Spcctrmhlm. Acro 1990,59140I.
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produced by a TR-30 nebulizer with a low liquid flow rate will be smaller than if a high liquid flow rate is used. Bates’ observeda decrease in the Sauter mean diameter of the primary aerosol from 10.9 to 10.0 pm when the liquid uptake rate was decreased from 1.0 to 0.4 mL/min while the nebulizer gas flow rate was 1.0 L/min. Furthermore, the drop number density will decrease when the liquid uptake rate is reduced, which will decrease the chances of drop collision and coagulation. Bates’ observed an increase in transport efficiency from 1.&%to3.2% when the uptakerate was decreased from 1.O to 0.4 mL/min using a Meinhard TR-30 nebulizer and a Scott-type spray chamber. Farino et al.17 reported similar trends. The transport efficiency of droplets of all sizes through the Scott-type spray chamber is generally low.’ Therefore, it may be possible to further increase the efficiency of the HEN nebulizer by using a more open spray chamber. The amount of liquid aerosol entering the plasma and its size distribution affect excitation and ionization in the plasma.1.18-22 Solvent enters the plasma as both aerosol and vapor. Saturated Ar contains approximately 20-30 mg of water vapor per liter of Ar at temperatures of 25-35 0C.18*23 When the liquid sample uptake rate is 1.0 mL/min (approximately 1 g/min), the percentage of liquid sample converted to water vapor in the spray chamber is small (less than a few percent, 30 mg or less per gram of water pumped to the nebulizer). Therefore, the total amount of water aerosol entering the plasma per minute can be estimated from the measured analyte transport rate. The analyte transport rates were 15 and 20 pg/min for a 1000 pg/mL analyte solution, when the cross-flow and TR-30 nebulizers, respectively, were used. Therefore, the aerosol transport rates were approximately 15 and 20 mg/min (or pL/min) for the crossflow and TR-30 nebulizers, respectively. The amount of water vapor entering the plasma was similar or larger than the amount of water aerosol entering the plasma. However, the effects of aerosol on plasma conditions and analyte emission appear to be more dramatic than those due to water vapor.18 The difference in the volume of aerosol entering the plasma in large droplets for the TR-30 versus the cross-flow nebulizer is moredramatic than the difference in aerosol transport rates. While the aerosol transport rate was only 25% larger for the TR-30 nebulizer, the volume of aerosol contained in droplets with diameters greater than 10 pm was almost 300% larger when the TR-30 nebulizer was used (3.9 versus 1.4 pL/min, for the TR-30 and cross-flow nebulizers, respectively). This indicates that the secondary and tertiary processes are different for the two nebulizers. The aerosol transport rate for the HEN is more difficult to estimate. The total amount of aerosol generated was 0.05 mL/min (approximately 50 mg/min). If insignificant amounts of aerosol evaporation occurred in the spray chamber, then (16) Canals, A.; Hemandis, V.; Browner, R. F. J. Anal. At. Spectrom. 1990, 5, 61-66. (17) Farino, J.; Miller, J. R.; Smith, D. D.; Browner, R. F. Anal. Chem. 1987,59, 2303-2309. (18) Long, S.E.; Browner, R. F. Spectrochim. Acra 1988,438, 1461-1471. (19) Olesik, J. W.; Den, S.-J. Spectrochim. Acta 1990, ISB, 731-752. (20) Bates, L. C.; Olcsik, J. W. J. Anal. At. Spectrom. 1990, 5, 239-247. (21) Olesik, J. W.; Fister, J. C., 111. Spectrochim. Acta 1991, 46E, 851-868. (22) Fister, J. C., III; Olesik J. W. Spectrochim. Acta 1991, 468, 869-883. (23) Browner, R. F.; Smith, D.D. AMI. Chem. 1983,55, 374-376.
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~ n a ~ y t i #emistry, ca~ VOI. 00, NO. IS,J
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1994
the aerosol transport rate would be 10 mg/min (based on an analyte transport rate of 10 pglmin for a 1000 pg/mL Sr solution at an uptake rate of 0.050 mL/min). However, significant aerosol evaporation is likely. If the aerosol was the main source of water vapor and the Ar was saturated with water vapor (approximately 20-30 mg/min), 40-6076 of the liquid primary aerosol would be converted into water vapor. In this case, the aerosol transport rate could be as low as 4 or 2 mg/min. Theevaporation rates would be drop dependent, further complicating estimation of the aerosol transport rate. It is possible that the evaporation could lead to cooling of each drop and the surrounding gas so that the evaporation rate is reduced.” Evaporation of water on the walls of the spray chamber could also contribute to the water vapor load. Therefore, while the exact aerosol transport rate was unknown when the HEN was used, the rate was between 2 and 10 mg/min. Effect of Sample Aerosol on Plasma Excitation and Ionization. The amount of solvent aerosol entering the plasma, and the number of large droplets that remain incompletely desolvated in the ICP, will likely affect excitation and ionization. Complete vaporization and atomization of the sample aerosol requires less than 1% of the power applied to the p l a ~ m a . Therefore, ~ ~ . ~ ~ one must consider local effects to explain the large influence changes in solvent loading have on excitation and i o n i ~ a t i o n . ~Local ~ . ~ effects ~ on excitation and ionization in time and space are produced by incompletely desolvated droplets.22J5 In turn, sensitivities,vertical emission profiles, and background are dependent on the properties of the aerosol introduced into the plasma. The effect of solvent ionization and excitation conditions in the plasma can be assessed via ion to atom emission intensity Fister and Olesik22 observed that Mg 11/ Mg I emission intensity ratios fluctuate widely as incompletely desolvated droplets pass through the observation zone. They concluded that time-integrated ion to atom emission intensity ratios are often dictated by the fraction of time an incompletely desolvated droplet is in the observation volume. The Mg I1 280.27 nm (or 279.55 nm) to Mg 1285.13 emission intensity ratio has often been used because the excitation energies for the atom and ion lines are very similar. As seen in Figure 4, the Mg ion to atom emission intensity ratio was highest at all heights when the HEN nebulizer was used, next highest when the cross-flow nebulizer was used, and lowest when the TR-30 nebulizer was used. The wavelength dependence of the spectrometer response was not calibrated. Therefore, the raw Mg I1 to Mg I emission intensity ratios can only be used in a relative sense, not for comparison to values obtained from models assuming local thermodynamic equilibrium, to assess “robustness” of the plasma.26 The grating efficiency of the echelle spectrometer falls off rapidly as the diffraction angle varies from the blaze angle (Figure 3 of ref 4). Assuming the ICP continuum emission intensity should be the same at 280.27 and 285.13 (24) Boumans, P. W. J. M.; de Boer, F. J. Spectrochim. Acta 1976,31,355-375. (25) Hobb,S. E.; Olesib J. W. Spectrochim.Acta 1993,48B, 817-833. (26) Mermct, J. M. Spectrochim. Acta 1989, 448, 1109-1 116. (27) Alder, J. F.; Bombclka, R. M.; Kirkbright, G. F. Spectrochim. Acta 1980, 358, 163-175. (28) Mcnnet, J. M. Anal. Chim. Acta 1991, 250, 85-94. (29) Pousscl, E.; Mermet. J. M.; Samuel, 0.Spectrochim. Acta B 1993,488, 743755.
18000 1
=
n 16000
"
4/\
1
a I
TR-30
' 0
5
0
10
15
20
25
Height above load coll (mm)
Flgwe 4. Mg I1 280.27 nm to Mg I 285.13 nm emissionintensity raw versus height above the bad cdl when the three different nebukers were used. Corrected values take into account differences in spectrometer efficiency (seetext).
0
5
10
15
20
25
Height above load coil (mm)
Figwe 5. Ar I 420 nm emission intenshyversus heigMabove the bad coil when the three different nebulkenr were used.
nm gives a relative spectrometer response factor of 1.85.30 With this response factor, the measured Mg II/Mg I emission intensity ratio values were corrected, as shown in the right Y-axis of Figure 4. The peak Mg II/Mg I emission ratios were 5.3,4.8, and 7.8 for measurements when the cross-flow, TR-30, and HEN nebulizers were used, respectively. The value predicted by LTE models for a pure Ar plasma is between 10 and 11. The values observed here are similar to those reported by Mermet.2B Furthermore, the value obtained by Ivaldi when emission was viewed at 12 mm ALC using an Optima 3000 and cross-flow nebulizer was 5.2,9O similar to the value of 4.9 that we observed. The rapid rise in the ion to atom emission intensity ratio occurred lowest in the plasma when the HEN nebulizer was used. These obervations are consistent with the number of incompletelydesolvated droplets in the ICP being largest when the TR-30 nebulizer was used and smallest when the HEN nebulizer was used. Most Ar exists as atoms in the ICP. Therefore, changes in Ar emission intensity are almost entirely due to variations in the fraction of Ar atoms that are excited. When the gas temperature increases, the gas density will decrease, so the number of Ar atoms in the observation zone will decrease. As a result, increases in Ar emission intensity when the plasma "temperaturen increases underestimate the change in excitation. Furthermore, much of the Ar emission originates away from the center of the plasma, so changes in solvent aerosol loading will affect the line-of-sight emission intensity far less than the intensity in the radial center of the ICP. The Ar emission intensity at all the heights in the plasma (Figure 5 ) was highest when the HEN nebulizer was used, lower when thecross-flow nebulizer was used, and lowest when the TR-30 nebulizer was used. These data are consistent with more extensivecooling of the plasma due to higher solvent (30) Ivaldi, J. C. Personal communication.
0
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Height above load coil (mm) Figure 6. Vertical emisslon intensity profiles for (a) U I 070 nm, (b) Cu I 324 nm, and (c) Zn I 213 nm using three different nebulkers.
loading when the TR-30 nebulizer was used. The peak Ar emission intensity undoubtedly peaked below the top of the load coil. The lower intensity observed at 0 mm ALC is due to partial blocking of the light. Vertical Emission Profiles. Vertical emission profiles provide insight into why the heights in the plasma for optimum detection limits were different for the various nebulizers. Fister and Olesik found that the height of peak emission intensity was related to the number of remaining incompletely desolvated droplets.22Olesikand DenI3found that vertical emission profiles probably peaked below the load coil when dry sample was introduced from a spark source rather than as a liquid aerosol. Long and Browner" observed shifts in the location of peak emission intensity as large as 8 mm when dry versus wet samples were introduced into the plasma." Olesik and Hobbs3I showed that the height of peak emission intensity could be varied from below the load coil to 30 mm above the load coil by controlling the sample drop size, which in turn affected where droplet desolvation was completed in the ICP. Therefore, the vertical emission profiles are also useful to further assess the effect of sampleaerosolon plasma excitation and ionization. The Li I (670 nm) vertical emission profiles (Figure 6a) were typical of low-energy atom lines (including Na and K). It is likely that the Li 670 nm emission intensity peaked below (31) Oleaik,J. W.; Hobbs, S. E. The Monodisperse Dried Microparticulate Injector (MDMI): Initial Characterization of a N e w Tool for Studying Fundamental Processes in Inductively Coupled Plasmas. AMI. Chrm.. submitted for publication.
AmMiml a m b b y . Voi. 88, No. 13, Ju& 1, 1994
2027
2000
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I
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Flguro 7. Mg I1 280.27 nm emission Intensity versus height above the load coil for the three different nebulizers.
the load coil in all cases (recall that light is partially blocked when the spectrometer views 0 mm above the load coil). The low-energy atom line emission intensities are highest for the TR-30 nebulizer and lowest for the HEN nebulizer. The height for optimumdetection limits (7 mm ALC for the HEN, 12 mm ALC for the cross-flow and HEN nebulizers) was well above the height of peak emission intensity because the background falls rapidly as the height above the load coil is increased. The Cu I (324 nm) vertical emission profiles (Figure 6b) peaked about 6 mm higher in the plasma when the cross-flow and TR-30 nebulizers were used than when the HEN was used. This is consistent with aerosol transport rates in the order of TR-30 > cross-flow > HEN and the number of incompletely desolvated droplets in the plasma likely being lowest when the HEN was used. Thevertical emission profile may have peaked slightly higher in the plasma when the TR30 nebulizer was used, in comparison to when the cross-flow nebulizer was used. The heights for optimum detection limits (7 mm ALC for the HEN, 12 mm for the cross-flow, and 15 mm for the TR-30 nebulizer) were near the location of peak Cu I emission intensity for each nebulizer. Despite having the lowest analyte transport rate, the peak Cu I emission intensity was highest for the HEN at heights below 15 mm ALC. The Zn I (213 nm) vertical emission profiles (Figure 6c) were typical of high-energy atom lines (including As). The emission profile is less sharply peaked than the profiles from low- or medium-energy atom lines. This is likely due to the greater importance of excitation for the high-energy lines. Therefore, the height of peak emission intensity probably occurred closer to the locations of maximum excitation, rather than the height of the highest atom number density. The Zn I emission intensity was highest at all heights in the plasma when the HEN was used despite the lower analyte transport rate for the HEN. The Zn I emission intensities were very similar for the cross-flow and TR-30 nebulizers even though the analyte transport rate was approximately 25% higher for the TR-30. This is again consistent with highest plasma “temperatures” when the HEN was used and lowest “temperatures” when the TR-30 was used. The Mg I1 (280.27 nm) vertical emission profiles (Figure 7) were typical of ion line behavior. The location of peak emission intensity was lowest in the plasma when the HEN was used, higher when the cross-flow nebulizer was used, and highest in the plasma when the TR-30 nebulizer was used. This is once again consistent with the number of incompletely desolvated droplets in the plasma, at any fixed height, being 2028
Analytical Chemistiy, VOI. 66, No. 13, Ju& 1, 1994
Sa Sr Mg Mn Cr Mo Ni Flgws 8. Relative responslvities (signaVanaIyte transport rate), normalized to the values for the cross-flow nebulizer, when three dlfferentnebulizerswereused.(Farboththeatomandk-~nlnesshown, excltationenergy and ionization energbs generally Increase from lefi to rlght.) Key: TR-30 : cross-flow HEN
.;
a.
largest when the TR-30 nebulizer was used and smallest when the HEN was used. The height to observe ion lines for optimum detection limits was lower (7 mm ALC) when the HEN was used than when the cross-flow (12 mm ALC) and TR-30 (15 mm ALC) nebulizers were used. In each case, this height was near the height of maximum ion emission intensity. At 3 mm ALC, the Mg I1 emission intensity was 19 times higher for the HEN than the TR-30 nebulizer. At 12-18 mm ALC, the Mg I1 emission intensity was 2-3 times higher for the HEN than the TR-30. Responsivity. The amount of signal produced per microgram of analyte that enters the plasma per second (responsivity) is a relative measure of the efficiency of conversion of sample into analyte emission.20 The responsivity will be dependent on plasma excitation and ionization conditions which are in turn affected by solvent loading. Figure 8 shows the relative responsivities for the analyte lines. In all cases, including the low-energy atom lines from K, Li, and Na, the largest responsivity is observed for the HEN. This is consistent with a greater extent of excitation when the HEN is used, compared to the cross-flow or Meinhard nebulizer. The greater extent of excitation overcomes the expected lower analyte density (due to a lower analyte transport rate and lower gas density). Responsivities for emission from atoms with low ionization energies and low excitation energies (K, Li, Na) when the HEN was used were most similar to those when the cross-flow or TR-30 nebulizer was used. This is likely because the fraction of K, Li, and Na that exists as atoms (rather than ions) is much lower when the HEN is used in place of the cross-flow or TR-30 nebulizer. Sensitivity. Sensitivities (emission intensity per ppm in the sample) depend on the number of atoms (ions) in the observation volume and the fraction that is excited and emits light. The number of atoms (ions) in the observation volume in turn depends on the analyte transport rate, height above the load coil, plasma gas temperature, diffusion rates, and relative numbers of atoms versus ions. A picture consistent with the observed line-dependent relative sensitivities can be developed from the observed emission intensities, analyte
*
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Fl@m0. Analytical characteristics of atom emission and relative detectkn IkniEB. (a)retIathgensltMtles(signal/mcentratkmh sample), (b) relative background emission intensities, (c) relative standard deviation of the background, and (d) relative detectkn limb. For a, b, and d, thesensltlvlties, backpund emlssbn I n t e w s , and detectkn lknlts were normalizedto thoseobservedwhen the cross-llow nebulizer was used. (For both the atom and ion llnes shown, excltation energy and ionlzatlon energies generally Increase from left to rlght.) Key: m-3064 ; cross-flow . ;EN 0. G 5
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d o Ba Sr Mp Mn Cr
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Flguro 10. Anatytical characteristics of lon emisslon and relative detectknIknlts: (a)relatlvesensltMtles(signal/concentratkmInsample), @) relative background emission Intensities, (c) relative standard devletkm of the background, and (d) relative detection limb. For a, b, a d d , the sen&Mth, backgroundemissknlntensltles,and detection limitswere normallzedto those observedwhen the cross-flow nebulizer was used. (For both the atom and ion lines shown, excitation energy and hizatbn energies generally increase from left to right.) Key: m-30N: cross-flow . ;HEN
a.
transport rates, liquid aerosol transport rates, ion to atom emission intensities, and argon emission intensities. For the following discussion, comparisonsare made for measurements made at the nebulizer-dependent heights in the plasma where best detection limits were observed. In general, as seen in Figures 9a and loa, the relative sensitivities are highest for the HEN compared to those for the cross-flow and TR-30 nebulizers, except for low-energy atom lines (Li, K, Na). Emission from low-energy excited state atoms is known to be most intense in "cool" regions of the plasma, particularly near incompletelydesolvated droplets. While the fraction of atoms that are excited will be small in these regions, the atom number densities are large.25 The larger atom number density dominates over the smaller extent of excitation so that the emission intensity is high.
The relativesensitivities observed when theTR-30 nebulizer was used was generally higher than when the cross-flow nebulizer was used. The relative sensitivitiesof lines with low excitation energies and low ionization potentials (Li, K, Na) when the TR-30 nebulizer was used were better than those observed when the HEN was used. The higher analyte transport rate for the TR-30 nebulizer overcomes the similar or higher responsivities when the cross-flownebulizer or HEN was used. The emission sensitivities observed when the HEN was used were much higher than expected if the cross-flow or TR-30 nebulizer was used with a sample uptake rate of 0.05 mL/min. The relatively high sensitivity observed when the HEN was used is likely due to the higher fraction of atoms and ions that were excited together with the high analyte transport efficiency (but lower analyte transport rate). The fraction of atoms and ions that were excited was higher when the HEN was used because the aerosol transport rate, and probably the number of incompletelydesolvateddroplets,was smaller. The high analyte transport efficiency is likely due to a combination of a primary aerosol with a surprisingly small average drop size and less extensive coagulation of droplets. The tertiary aerosol average drop size may have also been smaller when the HEN was used. Koropchak and Aryamanya-MugishaS2 found that the amount of signal produced per unit mass of analyte transported into the plasma was drop-size dependent. The relative sensitivity (and responsivity) was a factor of 2 to almost 7 higher when the HEN was used, relative to the other nebulizers, for ion lines or medium- to high-energy atom lines. This is consistent with a case where the fraction of atoms (ions) excited is more important (and higher when the HEN was used) then the number of atoms (ions) in the observation zone (which was likely lower when the HEN was used). Background. The relative background emission intensities, shown in Figures 9b and lob, were consistently about a factor of 4-8 higher when the HEN was used, compared to the crossflow or TR-30 nebulizer. This is likely due to two factors. First, the emission was observed lower in the plasma when the HEN was used (7 rather than 12 or 15 mm above the load coil). The off-axis continuum emission intensity increases as one looks lower in the plasma. Second, background emission intensity is higher when the solvent aerosol loading (and the number of incompletely desolvated droplets in the plasma) is small than when it is large. The relative background emission intensity for the TR-30 nebulizer was lower by 2040% (except for Cu) than when the cross-flow nebulizer was used. This is likely due to the higher liquid aerosol transport rate (20 versus 15 mg/min) and larger number of incompletelydesolvateddroplets, which lead to bulk and localized cooling, in the plasma when the TR-30 nebulizer is used. Relative Standard Deviation of the Background. The detection limit depends on sensitivityand absolute fluctuations in the background emission intensity (which in turn are the product of average background intensity and the relative standard deviation of the background). In general, the relative (32) Koropchak, J. A.; Aryamanya-Mugisha, H. J. AMI. AI. Specrrom. 1989,4, 291-295.
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standard deviation of the background was lowest when the HEN was used, higher for the cross-flow nebulizer, and highest for the TR-30 nebulizer (Figures 9c and 1Oc). The small %RSD of the background for the HEN is likely due to two factors. The emission was viewed at 7 mm (rather than 12 or 15 mm) above the load coil when the HEN was used. Much of the background at 7 mm above the load coil probably originates from points off-axis in the plasma where the sample aerosol has less effect than points on-axis. There are probably few incompletely desolvated droplets in the plasma when the HEN is used (for reasons discussed above). Therefore, the fluctuations in background intensity in the center of the plasma may also be smaller when the HEN is used. The %RSD of the background is largest for the TR-30 nebulizer. On-axis emission contributes significantly to the total background emissin observed at 12 and 15 mm above the load. There are probably significant number of incompletely desolvated droplets or vaporizing particles at 15 mm above the load coil which lead to large fluctuations in the "temperature'' in the center of the plasma. Therefore, the background produced in the center of the plasma probably also has large fluctuations. Relative Detection Limits. Detection limits were generally best for the TR-30 nebulizer, except for lines with high excitation energies (Cu, Zn, As, Ni). This was likely the result of higher analyte and aerosol transport rates when the TR-30 is used, which lead to a large concentration of analyte in the observation zone together with low average background emission intensities. The high aerosol transport rates, incompletely desolvated droplets, and vaporizing particles also lead to higher %RSDsfor the background and a lower fraction of atoms and ions that are excited. When high-energy lines were used, detection limits were best for the HEN or crossflow nebulizer. For these nebulizers, the fraction of atoms and ions excited was higher than when the TR-30 nebulizer was used. However, the small number of incompletely desolvated droplets and smaller aerosol transport rate led to higher average background emission intensities than when the TR-30 nebulizer was used. Nebulizer-Dependent Optimization. While the primary aerosols generated by the cross-flow nebulizer, the TR-30 nebulizer, and the HEN are quite different, the same spray chamber was used. Because the primary aerosol produced by the HEN consists of much smaller droplets, on average, it may be possibleto further increase analyte transport efficiency
by using a different spray chamber,without deleterious effects. Furthermore, the optimum ICP power or gas flow rates may be different for the HEN. However, a number of factors including precision and chemical matrix effects, which were not considered in this study, would need to be assessed to find optimum analytical conditions (depending on the data quality objectives).
CONCLUSIONS Detection limits using the Meinhard HEN with a sample uptake rate of 0.050 mL/min were similar to those obtained when the Perkin Elmer cross-flow or Meinhard TR-30-A3 nebulizer was used with an uptake rate of 1.O mL/min. The detection limits are similar because the sensitivity was higher when the HEN was used while the %RSD of the background was lower (although the average background emission intensity was higher). The excellent detection limits provided by the HEN are due to high analyte transport efficiency (20% compared to 1.5% and 2.0% for the cross-flow and TR-30 nebulizers, respectively) and a high responsivity (analyte emission intensity/amount of analyte entering the plasma). The higher transport efficiency when the HEN was used resulted in an amount of sample entering the plasma that was only 50% lower than when the cross-flow nebulizer was used despite the factor of 20 times smaller sample uptake rate for the HEN. The higher responsivity obtained when the HEN was used was likely due to smaller bulk and localized cooling of the plasma as a result of the smaller aerosol transport rate and probably smaller average tertiary aerosol drop size compared to the case for the cross-flow or TR-30 nebulizer. The HEN provides a convenient way to obtain excellent concentration detection limits and reduce sampleconsumption by a factor of 20 compared to that for commonly used pneumatic nebulizers. ACKNOWLEDGMENT Support for this research was provided by the National ScienceFoundation (Grant CHE 9217 170),the Perkin-Elmer Corp., and The Ohio State University. The Optima 3000 was provided by the Perkin-Elmer Corp. J. E. Meinhard Associates provided the HEN nebulizer. Recehred for revlew December 16, 1993. Accepted Aprii 11, 1994.' ~
Abstract published in Advance ACS Absrracrs, May 15, 1994.
