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(4) Cobourn. W. G.; Djuki6-Husar, J.; Husar, R . B. J . Geophys. Res 1980, 8 5 , 4487-4494. (5) Kiyoura. R.; Urano, K. Ind. Eng. Chem. Process Des. Dev. 1970, 9 , 489-494. (6) Cobourn, W. G.; Husar, R. B.; Husar, J . D. Afmos.Environ. 1978, 12, 89-98, (7) Tanner, R. L.; D’Ottavio, T.; Garber, R.; Newman, L. Atmos. Environ. 1980, 121-127. (8) Cobourn, W. G.; DjukiB-Husar, J.; Husar, R. B.; Kohli, S. Atmos. Environ. 1981, 15, 2565-2571. (9) Lindqvist, F. Afmos. Environ. 1985, 19, 1671-1680.
James J. H u n t z i c k e r Department of and Sciences oregon~~~d~~~~ center 19600 N.W. Von Neumann Dr. Beaverton, Oregon 97006 RECEIVED for review September 26,1985. Accepted December 16, 1985.
AIDS FOR ANALYTICAL CHEMISTS Rapid Throughput Nebulizer-Spray Chamber System for Inductively Coupled Plasma Atomic Emission Spectrometry D. R. Luffer a n d E. D. Salin* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 The inductively coupled plasma (ICP) provides a relatively stable, high-intensity signal that can be integrated for very short time periods until a point when quantum noise is introduced (1, 2 ) . There appears t o be little advantage in measuring the signal over periods longer than 10 s because of the introduction of drift or flicker noise terms ( 2 , 3 ) . With respect to the sample introduction system, it would be advantageous t o minimize the nebulizer-related low-frequency noise, which has been established as a principal source of uncertainty in ICP measurements (4). Compensation for these noise sources is theoretically feasible if standard and blank solutions can be run often; however, the cost is an accompanying increase in analysis time. Over the past few years, several modifications of the pneumatic nebulizer and spray chamber sample introduction system for the ICP have been proposed (5-8) in the hope of increasing sample throughput. It has been noted (7-9) that mixing of nebulizer aerosol and dead volume trapped aerosol results in an exponential decay of signal with time when transferring from analyte t o blank solution and a logarithmic rise of signal when transferring from blank t o analyte. These phenomena are referred to as the memory effect. Wohlers (10) reports a clearing time of 45 s using a Meinhard pneumatic nebulizer (1I ) , while Dobb and Jenke (9) report clearing times for some elements in excess of 200 s for the Scott spray chamber (12). In both cases, the clearing time is defined as the time interval necessary for the signal t o decay t o 0.1% of its original intensity. Ramsay e t al. (8)suggested that the memory effect is a combination of these two exponential decays, each with a different time constant. Several attempts have been made to reduce, or compensate for, the memory effect; these include modified nebulizer configurations (7,8), optimization of the length of the aerosol carrier tube (13), modifications of the spray chamber ( 5 , 6), and mathematical correction (9). We have designed a system that consists of an upright spray chamber and a Meinhard nebulizer. The chamber is characterized by radically smaller volumes and is devoid of the baffles (12) found in the interior of the Scott chamber. The system was designed with the expectation that a greatly reduced clearing time would provide the following three advantages if the system could provide signal t o noise ratio performance comparable to a conventional system: (I) higher sample throughput; (2) precise spectral calibration for rapid scanning systems using standards; (3) reduction of long-term 0003-2700/86/0358-0654$0 1.50/0
drift errors and low-frequency noise by recalibration. In this paper we will compare the performance of the new sample introduction system to a conventional system using a single nebulizer throughout.
EXPERIMENTAL SECTION Apparatus: The experimental apparatus consists of a conventional source (Plasma Therm, Inc., Kresson, NJ, Model HFP-2500D with an AMN 2500E automatic matching network) operated with 1.25-kW forward power. The plasma gas flow is 14 L/min with a 0.8 L/min auxiliary flow. A Jarrell-Ash 1.0-m spectrometer with a 0.8-nm spectral band-pass was equipped with a 1P28 PMT operated at 600 V for all measurements. Data were acquired by an AIM-65 single-board microcomputer networked to a Z-80 based system for storage and processing (14). A Scott type double pass spray chamber from Plasma Therm was used as a spray chamber in the “conventional” system. The same Meinhard concentric glass pneumatic nebulizer (Model TR-50-B4) was used for all experiments and was operated at 25 psi with a gas flow rate of 0.35 L/min. The spray chamber (Figure 1) was constructed from two concentric glass tubes. The inner tube was blown into the shape of a funnel and the mouth covered by a coarse glass frit (Acme Glass Inc., Model EA-7176-27),which allowed smooth drainage of waste liquid and provided the back pressure necessary for aspiration of aerosol into the plasma. A plastic screw cap is used to secure the funnel, thus allowing volume adjustments. The volumes chosen were 8, 16, 24,32, and 40 mL. The outer tube is sealed with a cap made of Teflon into which the nebulizer is inserted at an angle with respect to the surface of the cap. The angles chosen were 30°,36’, 45’, 57’, and 70’. The two parameters were varied in a search for the optimum conditions. Figures of Merit. The figures of merit examined were operationally defined as follows: ( I ) Short-term SNR is the average signal divided by the noise in the signal. When the blank is observed, the signal is the average blank value. When the analyte is observed, the signal is considered to be the total (unbackground corrected) signal. At this concentration level there is no significant difference between this value and the background-corrected SNR. Each datum is a 0.3-s integration at 0.3-s intervals, and the averages were calculated from acquisitions over 1 min (Figure 2). Noise was obtained by averaging 50 signal values on the base line (blank) or plateau (analyte) and calculating the standard deviation of the data set. (2)Rise time is the time interval between the first signal found three standard deviations above the average blank value of the base line and the first signal found three standard deviations below the average analyte value of the plateau. 0 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL.
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MARCH
1986
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Table I. Scott Chamber and Experimental System Performance rise, s fak s SNRO SNRn AVG Std Dev AVG Std Dev blank analyte
system Scott exptl, 8 mL exptl, 16 mL exptl, 24 mL exptl, 32 mL exptl, 40 mL
5.0 2.8 5.8 2.8 3.2 3.5
36.9 14.5 15.6 21.7 22.9 23.2
1.5 0.9 1.6 1.1 0.7 0.9
75 200 170 80 80 90
10.3 2.2 2.0 1.5 1.9 4.2
50 80 00 40 30 40
nAll SNRs are short-term SNRs. 15.5 c m
-
300
i
3
0
B
5
23
io
38
45
53
6:
68
?5
TIME (min)
Figure 3. Long-term signal to noise ratio behavior of three spray
chamber configurations: (a) squares, 8-mL spray chamber volume and nebulizer at 30' angle of incidence; (b) dots, Scott spray chamber: (c) triangles, spray chamber as in (a) with heated aerosol outlet tube. Flgure 1. Spray chamber design: (a) aerosol outlet tube; (b) nebulizer: (c) Teflon cap with O-ring; (d) coarse glass frit (e) O-ring; (f) plastic screw cap. 1 min
I
\
, i .
I
.
I
.,
I
TI ME
Flgure 2. Rise and fall of signal with change of concentration (blank and 100 ppm Cu). See text for details.
(3)Fall time is the time interval between the first signal found three standard deviations below the average analyte value of the plateau and the first signal found three standard deviations above the average blank value of the new baseline. ( 4 ) Long-term SNR is the signal to noise ratio calculated as in (1);however the signal has been integrated for 10 s with a 1-s interval between each integration. Ten such integrations comprised one data set. The interval between data sets was 5 min. The entire experiment extended over a period of 75 min. Blank solution was aspirated for 30 min before the start of experiments to allow the instrument to achieve a stable condition. Procedure. The aspiration tube of the nebulizer, supported by a glass capillary, was moved between two beakers by a Technicon AutoAnalyzer sample changeover system with a 50% duty cycle. One beaker contained a 100 ppm copper solution and the other a 0.5 N "0% blank. Figure 2 is a graphical representation of the data acquired in this manner. Data sets of 1500 points were acquired for each volume and angle combination. The data was evaluated with respect to the figures of merit defined above. The same procedure was followed for the Scott spray chamber system. The Cu 324.754-nm line was used for all measurements.
RESULTS AND DISCUSSION T h e results obtained for the clearing of the Scott spray chamber, summarized in Table I, are not consistent with the
values found by Dobb and Jenke (9);however they agree very well with those quoted by Browner and Boorn (15). It became apparent that the parameters to optimize were the fall time and the relative precision of the analyte signal. We initially optimized the fall time, ignoring SNR, and found that smaller nebulizer angles yield the best results. Since the angle is measured with respect to the cap made of Teflon, the smaller the angle, the higher the aerosol stream hits the wall of the chamber, and the sharper (closer to 90') the angle of impact on the vertical axis. We believe that a low-angle (close to horizontal) flow of aerosol produces a cyclone effect that fractionates droplets in a manner similar to t h a t of a baffle or impact bead. Not surprisingly, the smaller chamber volumes were cleared more quickly than the larger ones. T h e best performance was recorded for the combination of the smallest volume and the smallest angle. The smallest angle results are summarized in Table I. The short-term SNR values of the conventional system listed in Table I can be compared directly with the results of the experimental system since data were acquired in the same manner for both. It is clear that there is an improvement in the S N R of the small chamber system by a factor of 2.5 for the smallest volume-angle combination. The preceding results concerned the clearing time as well as the short-term precision of the new system. After the conditions for a minimum fall time were established, it was necessary to examine signal to noise behavior over a longer period of time. The results of this study can be seen graphically in Figure 3. The time interval corresponds to the time between long-term SNR sets. T h e experimental spray chamber performance (represented by squares) indicates a steady decrease of SNR with time. On comparison of these results with those of the Scott chamber (represented by dots), it is clear that the long-term precision of the new system is inferior to that of the conventional system. The average SNR value of 140 for the Scott system indicates t h a t a relatively
656
Anal. Chern. 1986, 58,656-658
high-performance nebulizer was being used. The major source of noise in the small chamber was determined to be condensation of droplets in the aerosol outlet tube of the chamber. This does not occur in the Scott system because of separation of primary from tertiary aerosols (16). In an attempt to eliminate this condensation, a solution of surfactant was run through the chamber; however the improvement was shortlived and condensation was detected again almost immediately. An alternative solution that met with considerable success was the heating of the outlet tube with a strip of nichrome ribbon. The temperature in the tube was kept a t 50 O C and prevented the formation of large drops. The long-term study u(as carried out as before and the results are represented in Figure 3 by triangles. The improvement is striking. The long-term SNR values average 230 for the heated experimental system vs. an average SNR of 140 for the Scott system over the same length of time. This accompanies an improvement in the clearing time by a factor of 2.5, as well as an improvement in short-term precision. In addition, we observed t h a t when the tube is heated, the blank signal consistently stabilizes in 10-12 min as opposed to 30 min or more without heating. I t is clear t h a t a nebulization system characterized by a small volume and a cyclone-type of aerosol flow shows great promise in the areas of increased sample throughput and long-term stability.
ACKNOWLEDGMENT The authors wish to thank Scott McGeorge for his contributions during the developmental stages of this work and
Robert Sing for his assistance with the data processing algorithm.
LITERATURE CITED McGeorge, S.W.; Salin. E. D. Spectrochim, Acta, Part 6 1985,406, 447-459. McGeorge, S. W.; Salin, E. D. Appl. Spectrosc., in press. Belchamber, R. M.; Horlick, G. Specfrochim, Acta, Part 6 1982,376, 71-74. Duursma, R. P. J.; Smit, H. C.; Maessen, F. J. M. J. Anal. Chim. Acta lg8l. 733. 393-408. Schutyser,'P.; Janssens, E. Specfrochim. Acta, Part B 1979, 346, 443-449. Belchamber, R. M.; Horlick, G. Spectrochim. Acta, Part6 1982,376, 1075- 1078. Layman, L. R.; Lichte, F. E. Anal. Chem. 1982. 54, 638-642. Ramsay, M. H.; Thomuson, M.: Coles, B. J. Anal. Chem. 1983, 55. I 626- i629. Dobb, D. E.: Jenke, D. R. Appl. Spectrosc. 1983,37,380-384. Wohlers, C. C. ICP I n f . Newsl. 1977, 3 , 37-50. Meinhard, J. E. ICp I n f . Newsl. 1978,2, 163-165. Scott, R. H.; Fassel. V. A.; Knisley, R. W.; Nixon, D. E. Anal. Chem. 1974,46, 75-80. Whaley, B. S.;Snable, K. R.; Browner, R. F. Anal. Chem. 1982,5 4 , 162-165. Sing, R. L. A,; Salin, E. D. Talanfa 1984,37,565-571. Browner, R. F.; Boorn, A. W. Anal. Chem. 1984, 56, 875A-888A. Browner, R. F.; Boorn, A. W. Anal. Chem. 1984,56, 786A-798A.
RECEIVED for review July 18, 1985. Accepted September 26, 1985. This research was funded by grants from the Natural Sciences and Engineering Research Council of Canada (Grant A1126) and the Government of Quebec (Fonds F.C.A.C. EQ1642). Additional assistance was provided by the Ontario Ministry of the Environment (Project No. 176).
Improvement of Trace Aluminum Determination by Electrothermal Atomic Absorption Spectrophotometry Using Phosphoric Acid C. L. Craney,* Kurt Swartout, F. W. Smith, 111, and C. D. West Department of Chemistry, Occidental College, Los Angeles, California 90041 The aluminum concentration of precipitation is a necessary measurement for the determination of noncrustal contributions to the precipitations' trace element content (1,2). The measurement of aluminum concentration is part of an ongoing study of the trace elements in remote U.S. west coast precipitation. The determination of aluminum in natural waters by graphite furnace atomic absorbance spectrophotometry (GFAAS) has been discussed in a number of papers and various procedures for overcoming interferences have been suggested (3-5). The literature on aluminum determination suggests the use of the highest possible char temperature (6) to remove matrix components and the rapid generation of the atomic vapor in an isothermal furnace (5). The literature also notes a variability in the results (7) depending upon the specific graphite tube, its age, and type. The fewest number of manipulations and chemical additions are desirable (8). As most of the samples can be expected to have low aluminum concentrations, high sensitivity is also desirable. This report focuses on the use of phosphoric acid as a matrix modifier for the determination of trace aluminum concentration in natural waters. Phosphate salts or phosphoric acid have been employed in other systems: lead in chloride matrix (9,l o ) , fish tissue ( I I ) , blood (12),and a variety of elements ( 4 ) (including aluminum) in natural waters. The literature indicates t h a t the addition of phosphate allows higher char
temperatures (12)and delays the onset of atomization in the high-temperature step (12).
EXPERIMENTAL SECTION Apparatus. The instrument used was an Instrumentation Laboratories (IL) Model 551 atomic absorption spectrophotometer and a Model 655 controlled-temperature furnace. The spectrophotometer used an IL aluminum hollow cathode lamp at a wavelength of 309.3 nm with a slit width of 0.15 nm. Initially the D2arc was used for the background correction. Its use, however, was found unnecessary for the samples of interest. Background correction adversely affected the precision and was discontinued. Furnace Operation. Pyrolytically coated graphite tubes from IL No. 29669 (lot IV 36/391), and P / N 12426800 (lot VI1 63 780) were used for the experiments. The furnace was operated in the following cycle: dry, 90 O C (20 s ramp), 130 "C (25 s ramp); char, 1500 "C (20 s ramp), 1520 "C (25 s hold, pressurize to 20 psi with argon); atomize, 2550 O C (0 s ramp), 2550 O C (5 s hold, no argon flow); cool down about 30 s with argon purge. The drying conditions were set as per the manufacturer's manual with an indicated argon flow of 30 scfh. The char temperature chosen was the highest char temperature possible without loss of analyte. The highest controlled temperature (i.e., using a thermocouple) for our instrument (2550 "C) and fastest heating rate possible were used for atomization. Procedures. In low level aluminum analysis, contamination
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0003-2700/86/0358-0656$01.50/00 1986 American Chemical Society
