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Anal. Chem. 1981, 53, 2030-2033
Continuous Flow Cold Vapor Atomic Absorption Determination of Mercury C. E. Oda and J. D. Ingle, Jr.” Department of Chemistty, Oregon State University, Cowallis, Oregon 9733 1
Three dlfferent deslgns for continuous flow reduction vessels are described for the determlnatlon of ultratrace concentratlons of mercury. Sample and reductanl solutions are continuously fed to the reductlon vessels where the mercury Is reduced and volatlllred. The volatllized mercury Is swept into an absorption cell where the atomlc absorption at 253.7 nm is measured. The best design is based on strlpplng of the mercury from a thin stream of solutlon with a countercurrent flow of gas over the solutlon stream. The detectlon limit for mercury Is 0.03 ppb.
The cold vapor atomic absorption (CVAA) method is the accepted method for determination of ultratrace concentrations of Hg(I1) in solution (1-4). The method is based on the chemical reduction of mercuric ions to elemental mercury with typically SnClz or NaBH4. The elemental mercury is then swept out of solution with a carrier gas into a long path absorption tube where the atomic absorption at 253.7 nm is measured. Normally a discrete sample method is used in which all the mercury from a given volume of solution is swept once through the absorption tube to produce a peak response or is recirculated through the absorption cell to produce a continuous signal. Many improvements to the method ( 5 , 6 ) have reduced the detection limit to as low as 0.002 ppb in a 1-mL sample. This research is concerned with the development and characterization of a continuous sample introduction reduction vessel for C V M . With discrete sample reduction vessels, the sample and reducing solution must be added to the reduction vessel with syringes or pipets and then the reaction mixture must be removed after analysis. In contrast, continuous sample introduction eliminates the time-consuming solution introduction and removal tasks and thus simplifiesthe analysis procedure and reduces the dependence on operator skill. This method of sample introduction makes it easier to automate analysis for uninterrupted and/or remote monitoring of trace mercury levels and could be used as an HPLC detector specific for mercury. EXPERIMENTAL SECTION Mercury solutions and the 1% SnClzreductant solution were prepared and preserved as previously described (5,s).A block diagram of the instrument is shown in Figure 1. The double-beam spectrophotometer and 60-cm path length, 2 mm i.d. heated absorption cell are similar to the instrumentation previously described (5, 6) with a few modifications described below. The monochromator was replaced by an interference filter (Pomfret = 2537 h 20 A, bandResearch Optics, Inc. Stamford, CT, A, width = 120 f 20 A, peak transmittance = 18.5%). The preheating tube was similar to that previously described (6) but smaller (4 mm i.d., 5 cm long tube filled with glass wool). A constant current circuit shown in Figure 2 was constructed to stabilize the current to the mercury pen lamp. The base of the transistor is biased at a constant voltage by the zener diode which in turn clamps the transistor emitter to a constant voltage. Thus the current through R2 and R3 and the lamp is maintained at a constant value determined by the combined resistance of R2 and R3 and independent of the lamp resistance. The output of this circuit is connected to the Hg pen lamp through the lamp 0003-2700/81/0353-2030$01.25/0
switching circuit previously described (6). For all measurements, R2 was adjusted to yield a 7 mA lamp current. The time constant of the linear amplifier was 1s for all measurements. The absorption cell and preheating tube was maintained at 135 “C to prevent condensation of water vapor in the cell and to vaporize water mist carried to the absorption cell from the reduction vessel. The block diagram for the continuous flow reduction vessel and associated pumps and plumbing is shown in Figure 3. The peristaltic pumps were a calibrated three-channel Pharmacia Peristaltic Pump P-3 (Piscataway, NJ) for the inlet flow and a Cole-Panner Masterflex Model 7020C peristaltic pump (Chicago, IL) for the cell outlet flow. A 1.17 mm (0.046 in;) i.d. plastic “T’ was used to bring the reducing and mercury solutions together. PVC Tygon tubing (0.25 in. i.d.) was used for connection of all paths of nitrogen flow, while in. i.d. microbore tubing was used for interconnection of all paths of solution flow with each pump requiring its own special internal tubing. The optimal design of a continuous sample flow reduction vessel should provide the following desired conditions: (1)minimal solution and vapor dead volume to reduce response times, (2) efficient mixing of Hg(I1) species and reducing solutions for maximum chemical interaction between the two species, (3) the vessel must be easily cleaned or self-flushing, preferably with nonwetting surfaces to reduce contamination and response time, and (4) maximization of the area of air-liquid interface to promote Hg volatilization into the inert gas stream. Three different basic reduction vessels were constructed and tested and were based on (1) mist formation and subsequent diffusion of Hgo out of the droplets, (2) vigorous bubbling of an inert gas through the solution mixture, and (3) passing a stream of gas over a thin f i i of solution. The reduction vessel based on mist formation was constructed with a Beckman Model 4030 medium bore Oz/Hz burner (Fullerton, CA). A plastic “ T and 0.040 in. tubing brought the sample and reductant solutions together into the nebulizer (no pumps were used). The nozzle of the burner was directed into a 10 cm long, 3/s in. i.d. Tygon tube section that terminates at the absorption cell. The expended solution was drained by the carrier gas pressure through a small stainless steel tube inserted through the Tygon tube section. A minimum Nz gas flow rate of about 2 L/min connected to the oxidant inlet of the burner was necessary to maintain aspiration at about 0.5-1.0 mL/min for each solution. The reduction based on bubbling aeration is a modification of the discrete sampling vessel previously described (a modified 1.0 cm diameter course frit sealing tube (5)) with the addition of solution entrance and drainage ports (see Figure 4). The sample and reducing solutions are brought together and directed to the inlet port with the arrangement shown in Figure 3. Thus the sample continuously flows into and out of ports situated on opposite sides of the vessel and vigorous bubbling of the solution liberates the Hg vapor which is carried to the absorption cell. The drainage pump was set to the minimum flow rate (about 15 mL/min) that would maintain a constant bubbling level in the vessel. The design for the thin stream reduction vessel with countercurrent gas flow is shown in Figure 5. Here the carrier gas is passed over a thin stream or layer of solution containing the dissolved elemental mercury flowing in the opposite direction. The analyte and reductant solution are brought together with a “T” before the solution inlet port as shown in Figure 3. RESULTS AND DISCUSSION Ideally the absorbance signal output displayed on a recorder would be a square wave if the blank and analyte solution were 0 1981 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
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To Exhaust Vent
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Figure 1. Block diagram of double beam CVAA system.
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Figure 2. Schematic of constant current lamp supply R1, 22K; R2, 5K pot; R3, 1.8K; D, 36 V zener; C, 0.1 KF, 600 V; Q, RCA SK3111.
pumped alternately through the continuous flow reduction vessel for equal lengths of time. The maximum height of the square wave represents the difference in absorbance between the blank and analyte solutions and will be denoted the plateau absorbance (A,,). The solution and carrier gas displacement lag times or the nonzero response time of the reduction vessel results in rounded corners and nonvertical sides in the absorbance profile. The response time is defined as the time elapsed after the mercury solution first enters the cell to the point at which the absorbance reaches 99% of the maximum ox plateau absorbance.
The concentrationof mercury present in the absorption cell is given by cg = kF,c,/Fg (1) where cg = concentration of Hg(0) atoms in the carrier gas, mol/L, F, = flow rate of solution into the reduction vessel, mL/min, c, = concentration of Hg2+ions in solution, mol/L, Fg = flow rate of carrier gas through the absorption cell, mL/min, and k = efficiency factor or the fraction of mercuric ions in solution that are reduced and transferred to the carrier gas that enters the absorption cell, dimensionless. The efficiency factor will be less than one due to incomplete reduction or volatilization of the mercury during the residence time of the sample solution in the reduction vessel or to mercury vapor carried out through the solution drainage tube. Application of Beer's law (eq 2) converts the Hg(0) concentration in the gas to absorbance units (A.U.) which may be measured instrumentally A = ebc, (2) where A = absorbance, A.U., e = molar absorptivity of mercury atoms at 254 nm, 4.1 X lo6mol-' L cm-* (7), and b = cell path length, 60 cm or
A = (4.1 X lo6 mol-' L cm-')(60 cm)kF,c,/Fg ( 3 )
Peristaltic Pump I -
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Flgure 3. Block diagram of peristaltic pump setup for continuous sample flow reduction vessel.
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
A = (2.5 X lo* mol-l L)kF,c,/F,
(4)
Equation 4 can be used to predict the maximum plateau absorbance (A,) if k is set equal to 1. The efficiency of the reduction vessel under a given set of experimental conditions is calculated from the ratio of A, to A,. The effect of sample, reductant, and carrier gas flow rates on the calibration sensitivity (plateau absorbance/ppb), detection limit (concentration where A , equals twice the standard deviation in the blank absorbance base line), response time (R),and efficiency were evaluated for each of the three basic flow cell designs and variations of these designs. The thin stream reduction vessel was found to be the best design and is discussed first below. Figure 6 illustrates the tradeoff between calibration sensitivity and response time with carrier gas flow rate. The efficiency factor was relatively constant (lO-ll%) above 60 mL/min which indicates the velocity of nitrogen passing over the air stream has little effect on k. Likewise the product, Fg X A,,, is relatively constant above 60 mL/min. This is predicted by eq 4 and indicates the decrease in absorbance with increasing flow rate is a simple dilution of mercury vapor in the carrier gas. The decrease of response time with increasing carrier gas flow rate is expected since the displacement time is reduced for the carrier gas previously in the reduction vessel, absorption cell, and connecting tubing before the introduction of mercury solution. A flow rate of 80 mL/min was chosen for further studies as an acceptable compromise between sensitivity and response time. The effect of analyte flow rate on the plateau absorbance is shown in Figure 7. The absorbance increases with flow rate as predicted from eq 4. However, the increase is not linear and the plateau absorbance levels off at higher flow rates apparently because the efficiency k decreases with the reduced residence time at higher flow rates. A flow rate of 4.8 mL/min was chosen for further studies since higher flow rates waste solution with little increase in plateau absorbance. The reductant flow was 40% of the sample flow rate in the above studies. The plateau absorbance was independent of the angle from vertical of the thin stream reduction vessel from 15to 50’ and decreases slightly above this range and 15’ was chosen for further studies. Increasing the diameter of the reduction vessel from 5 to 10 mm produced no significant difference in plateau absorbance but increased the response time about 50%. An increase in the length of the reduction vessel from 3.5 to 5.5 in. increased the plateau absorbance only 13%. Studies with water dyed with food coloring showed that slow solution movement from the pump resulted in laminar flow and very inefficient mixing as the combined reductant and sample solutions meet at the “T” and move to the reduction vessel, Unlike the bubbling aeration vessel where agitation and volatilization occur simultaneously in the reduction vessel, mixing must be achieved before the solutions enter the thin stream vessel. A 6 cm section of glass tubing with indentations in the wall and a 0.2 mL volume glass mixing chamber with a 2 x 5 mm Teflon coated stirbar were inserted in the stream to induce turbulence. These devices changed A, and R by 20% or less and were not used for further studies. A typical chart recorder tracing of the absorbance plateau is depicted in Figure 8 along with the final conditions used for analysis. The response time is 0.9 min (0.35 min to 90% of A,). The rise to the plateau was found not to be exponential, With the conditions shown in Figure 8, the calibration plot is linear from a detection limit of 0.03 ppb up to above 5 ppb Hg with a calibration slope of 0.0065 A.U./ppb. The peak-to-peak base line noise in the blank was 4-5 X A.U. and the relative standard deviation for measuring concentrations from 0.3 to 5 ppb was 2-4%.
The bubbling aeration vessel produced results similar to those obtained with the thin stream vessel. The response time and calibration sensitivity varied with carrier gas and analyte solution flow rate in a similar manner to that shown in Figures 6 and 7, respectively. At a compromise carrier gas flow rate of 120 mL/min and a sample flow rate of 4.8 mL/min, the response time was 1.3 min, the calibration sensitivity was 0.014 A.U./ppb, the efficiency factor was 28%,and the detection limit was 0.02 ppb. Compared to the thin stream vessel, the bubbling aeration vessel was slightly better in efficiency (factor of 3) and in calibration sensitivity (factor of 2), but the detection limit was only 33% better due to greater base line noise. However it was determined to be the second best design because of longer response times and due to spurious peaks and noise that appeared on the absorbance signals. The response time is longer because of the greater volume of solution maintained in the cell which must be displaced by the entering analyte solution (i.e., the cell acts like an exponential dilution flask). This greater volume and hence longer residence time in conjunction with the good mixing due to vigorous bubbling does account for the higher efficiency of the cell. The greater noise and spurious peaks are due to generation of mist by the bubbling in cell. Foaming and bursting of bubbles carried to the top of the reduction vessel caused occasional solution carryover into the absorption cell with resultant spurious peaks. These spurious peaks were much less frequently observed with the thin stream reduction vessel. Reducing the distance between the inlet and exit ports reduced the solution dead volume in the cell and decreased the response time but also increased the base line noise due to a smaller physical buffering effect on the bubbling. Reduction of the diameter of the reduction cell from 1.0 to 0.6 cm to reduce the solution dead volume reduced the response time but decreased the calibration sensitivity and increased the frequency of solution carryover into the absorption cell. The design based on mist formation was abandoned for several reasons. Memory effects and drift were significant and resulted from continuous formation of large droplets on the walls of the Tygon tubing immediately surrounding the nebulizer head. The sensitivity was over an order of magnitude worse than for the thin stream reduction vessel due in part to the dilution of the volatilized Hg in the more than 2 L/min of carrier gas required to induce efficient nebulization. The base line noise and drift were relatively large (typically 0.001-0.002 A.U. peak-to-peak) and probably due in part to mist entering the observation cell. The low calibration sensitivity and high base line noise would have resulted in a high detection limit. The uptake of the solution (0.2545 mL/min of each solution) was based on a vacuum drawn by the nebulizer and did not necessarily ensure consistent and equal flow through sample and reductant tubes leading to the “T”.Slight differences in solution viscosity or particulate obstructions would cause considerable variability in results. CONCLUSIONS Compared to the discrete sampling reduction vessel approach, the continuous flow sampling reduction approach provides some advantages. The operation is simpler and less dependent on operator skill in that the sample tube need only be moved between sample and blank solutions as in a conventional flame AA spectrometer. This means the continuous sample scheme is easier to automate and more suitable for remote monitoring. The primary disadvantages of the continuous sample reduction vessel approach compared to the discrete sampling approach are reduced calibration sensitivities,worse detection limits, and longer analysis times. The calibration sensitivity of 0.007 A.U./ppb and detection limit of 0.03 ppb are sig-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981 Nitrogen Outlet ( t o absorption cell 1
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Flgure 7. Effect of solution flow rate on absorbance. N2 flow rate of 80 mL/min with 1 ppb Hg(I1).
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Flgure 8. Typical chart recorder response tracing of 1 ppb Hg(I1) for the thln stream vessel: N, flow rate, 80 mL/mln; sample solution flow, 4.8 mL/min; diameter of reduction vessel, 0.5 cm; length of reductiori vessel, 3.5 in.; angle of reduction vessel, 15' from vertical; reductant flow rate, 1.9 mL/min.
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Flgure 5. Thin stream reduction vessel. 1.25
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nificantly worse than the respective values of 0.03 A.U./ppb and 0.003 ppb obtained with the discrete sampling cell with the same spectrophotometer system. However, the detection limit is more than adequate to determine if mercury levels
in water exceed the acceptable drinking water limit of 2 ppb. The reduced calibration sensitivities are due in part to i n complete volatilization (- 10%)of mercury from the reaction solution. Also the base line absorbance noise is a factor of 2-4 greater with continuous sample introduction even though a larger electronic time constant was employed. This increased noise is apparently due to pressure fluctuations and mist generated by the continuous flow reduction vessels. The response times of 0.9 min results in a typical analysis time about 2 min since both a blank and sample solution must be run. The sample analysis time with the discrete sampling reduction vessel is about 1min because a blank run does not have to be made unless there is mercury in the reagent blank. Further studies will be directed at designing a better reduction cell which provides higher volatilization efficiency to improve the detection limit and a smaller response time. Shorter response times can be achieved with a corresponding loss in sensitivity by using higher gas flow rates (e.g., R = 0.4 min for 150 mL/min N2flow rate). The reduction vessel design should be also applicable to continuous generation of hydrides of As and Se with Nal3H4 and subsequent detection by atomic absorption or plasma emission. ACKNOWLEDGMENT We thank Riley Chan for construction of the constant current circuit. LITERATURE CITED (1) "Standard Methods tor the Examination of Water and Wastewater", 14th ed.; American Public Health Association: Washlngton, D.C., 1976; pp 156-159. (2) Ure, A. M. Anal. Chlm. Acta 1975, 76, 1-26. (3) Chllov, S. Talsnta 1875, 22, 205-232. (4) Hatch, R.; Ott, W. L. Anal. Chem. 1888, 40, 2085-2087. (5) Hawiey, J. E.;Ingle, J. D., Jr. Anal. Chem. 1974, 46, 719-723. (6) Christmann. D. R.; Ingle, J. D.. Jr. Anal. Chim. Acta 1978, 86. 53-62. (7) Stainton, M. P. Anal Chem. 1971, 43, 625-627.
RECEIVED for review June 20,1981. Accepted July 29,1981. Presented in part at the 62nd Canadian Chemical Conference, Vancouver, BC, 1979.
