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Anal. Chem. 1981, 53, 1450-1453
Table IV. Nitrogen in Kjeldahl Digested Sediments (gg/g) present method
indophenol blue method
present method
indophenol blue method
1.96 6.21 1.67 3.31 2.47
2.20 6.50 1.70 3.40 2.40
2.78 0.27 2.30 0.25 0.21
2.70 0.30 2.30 0.30 0.20
Table V. Ammonia in Environmental Samples amt of NH, as amt of NH, in particulatea in air, rainwater, Pg/m3 MmL 0.44 5.80 0.70 6.55 0.50 5.63 0.35 0.65 5.79 0.49 0.54 a
0.59 0.50 0.38 0.57 0.15 0.82 0.39 1.77 0.14 0.10 0.10
Collected on Hi-vol filters.
(twice the standard deviation at or near zero concentration) are close to 0.10 pg/mL). The sensitivity as calculated from the slope of the standard curve is 0.26 pg NH,/mL. It may be further increased by using a longer absorption cell. Comparison of Proposed Method w i t h Manual Methods. The sensitivity of the automated method is significantly greater than that of Muroski and Syty's manual method and
27% greater than Cresser's improved method. The base line stability is superior. The ammonia absorption signals can be recorded with equal ease and sensitivity at the 193.7- and 197.3-nm arsenic lines. The proposed method is faster and more sensitive than Cresser's improved method which requires 30 min of thermostating and is subject to the effect of variables such as configuration, depth and dead volume of the bubbler, slit width, heat of neutralization, etc. Although Muroski and Syty's manual method (9) is fast, it is 15-fold less sensitive, As opposed to the transient signals generated by the manual methods, the proposed method produces steady-state signals and provides complete recording of sequential events in a continuous manner. This feature is very useful in assessing the performance of the system. The appearance of signal peaks and base line assists in diagnosing problems, if any should arise. The most desirable feature of the automated method is its manipulation-free unattended operation. The system is very rugged, and up to 140 determinations can be performed in 1 man day. The sampler module can be eliminated if fully automatic operation is not desired. In this mode, the sample can be presented manually to the sample probe. LITERATURE CITED (1) Crowther, J.; Evans, J. Analyst(London) 1980, 705,841. (2) Vogel, A. I. "A Textbook of Quantitative Inorganlc Analysis", 3rd ed.; London W-1,1961;p 783. (3) Orion Research "Analytical Methods Guide", 9th ed.; 1978;p 33. (4) Bouyoucos, S. A. Anal. Chem. 1977, 49, 409. (5) Obrink, K. J. Biochem. J . 1955, 59, 134. (6) Cresser, M. S. Anal. Chlm. Acta 1976, 85, 253. (7) Cresser, M. S. Lab. fract. 1977, 19. ( 8 ) Cresser, M. S. Analyst (London) 1977, 702,99. (9) Muroski, C. C.; Syty,A. Anal. Chem. 1980, 52, 143. (10) Takahashi, M.; Tanabe, K.; Saito, A.; Matsumoto, K.; Haraguchi, H.; Fuwa, K. Can. J. Spectrosc. 1980, 25, 25. (11) Vijan, P. N.; Wood, G. R. At. Absorpt. News/. 1974, 73,33.
RECEIVED for review January 30, 1981. Accepted April 20, 1981.
Determination of Mercury at the Ultratrace Level by Atmospheric Pressure Helium Microwave-Induced Plasma Emission Spectrometry Kiyoshl Tanabe, Koichi Chiba, Hlrokl Haraguchi, * and Kellchiro Fuwa DepaHment of Chemistty, Faculty of Science, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Atmospheric pressure heilum microwave-induced plasma emission spectrometry has been applied to the determlnation of mercury, where the cold vapor generation technique was employed for generation of mercury from the solutlon. The detectlon limit was 4 pg/mL or 8 pg, and the dynamic range of the calibration curve was 2 X lo5. The relative standard devlation of 10 replicate measurements using 100 pg/mL standard solution was about 2%. The Interferences of some catlons with the mercury determlnatlon were negligible except for Pt2+ and Pd2+. The present method was applied to the determination of mercury In bovine liver (SRM 1577 from NBS), and anaiytlcai data were consistent with the certifled value.
Current environmental concern with the danger of mercury pollution has accelerated progress of analytical methods for
mercury. Especially in the field of atomic absorption spectrometry, the cold vapor generation technique has been developed to determine mercury a t the sub-part-per-billion (ng/mL) level (1-4), and such a method has been extensively applied to mercury analysis in various samples. Furthermore, such a technique has also been used for atomic fluorescence spectrometric determination of mercury, and has also provided great detection capability of mercury a t the sub-part-perbillion level (5,6).In spite of extensive studies, the cold vapor generation technique utilizing the amalgamation trap for preconcentration has been required for the determination of mercury in some natural samples (e.g., seawater), which contain mercury only at a few parts-per-trillion (pg/mL) level (7-9). However, this kind of preconcentration methods has many chances of loss or/and contamination of mercury. Therefore, improved methods are required for the direct determination of mercury at such low-level concentrations without resorting to tedious preconcentration procedures.
0003-2700/81/0353-1450$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
Recently, the present authors reported vacuum-ultraviolet
(VUV) atomic absorption spectrometry of mercury with cold vapor generation technique (IO),where the resonance line a t 185.0 nm used for the absorption measurements. The VUV atomic absorption spectrometry was more sensitive by about 5 times than the conventional methods utilizing the atomic line at 253.7 nm. This method, however, was still not sensitive enough to determine mercury directly at the parts-per-trillion level. On the other hand, in the field of atomic emission spectrometry, various types of plasmas, such as inductively coupled plasma (ICP), microwaveinduced plasmas (MIP), and direct current plasma (dc plasma), have been used for the determination of mercury (11-21). The use of the cold vapor generation technique hari also improved the analytical sensitivity of mercury by plasma emission spectrometry. Lichte and Skogerboe reported the detection limit of 60 pg (6 pg/mL) by argon MIP emission spectrometry with the cold vapor generation technique (15),and Braman obtained the detection limit of 0.4 ng (4 pg/mL) by dc helium plasma emission spectrometry with the membrane probe cold vapor generation technique (17). I n 1976, a new type of cavity (TMolo type) for MIP was developed (22)and the helium plasma at atmospheric pressure sustained by the cavity has been investigated as an excitation source for both metallic and nonmetallic elements (21,23-29). According to these investigations, the excitation efficiency of this new helium MIP is generally about an order of magnitude higher for most elements than those of the plasmas previously used for atomic emission spectrometry. Quimby et al. used the atmospheric pressure helium MIP as an element-selective detector for gas chromatography and obtained the detection limit of 1 pg/s for mercury (21). However, the determination of ultratrace mercury in solution by atmospheric pressure helium MIP emission spectrometry with the cold vapor generation technique has not been investigated yet. Hence, the present authors applied the atmospheric pressure of helium MIP to the determination of mercury with the cold vapor generation technique to realize the direct determination of mercury at the parts-per-trillion level without any preconcentration method.
EXPERIMENTAL SECTION Chemicals. A stock solution of mercury (10 pg/mL) was prepared by dissolving mercury(I1) chloride in 3% (w/v) hydrochloric acid. All the standard solutions were obtained by properly diluting the mercury stuck solution with 1% HC1 solution, while standard solutions with concentrations lower than 10 ng/mL were diluted with subboiling water to avoid contamination from hydrochloric acid. All the mercury solutions prepared were analyzed within 6 h. A 3% SnClz in 1% HC1 solution was used as the reducing agent. Instrumentation. For the present experiment, the measurement system used previlously for the determination of nitrogen was employed without any modification (28). The microwave generator (Model MR-3S from Ito Chotanpa Co., Ltd., Japan), which provided 20-200 W of microwave power at 2.45 GHz, was generally run at 75 W forward power in the present experiment. Reflected power could be tuned to a minimum (13 W) with the tuning screws on the cavity, and no additional tuning device was used. The carrier helium gas for the gas generation was commonly used as the plasma gas. The flow rate of helium gas was in the range of 0.25-1.0 L/min, when a silica tube of 6 mm 0.d. and 3 mm i.d. was used for the plasma discharge tube. The cavity (laboratory constructed) was mounted on a pedestal which could move both vertically and horizontally for the adjustment of the observation position. The 1.1image of the plasma (axially viewed) was focused on the entrance slit of the monochromator (Model JE-50E from Nippon Jarrell-Ash Co., Ltd.). The slit width and height of the entrance slit were 10 pm and 1.5 mm, respectively, and those of the exit slit were 10 pm and 10 mm, respectively. In the present system, the spectral band pass of the monochro-
100
P
E
-
ij!I!
50
1451
1
+l.5
-15
0 Position (mm)
Flgure 1. Spatial distributions of emission intensities of He I, Hg I, and Hg 11: (0)He I 388.9 nm, (U) Hg I 253.7 nm, (A) Hg I1 194.2 nm.
mator was 0.02 nm. The mercury atomic line at 253.7 nm was used for the analysis. The time constant of the readout system was 0.3 s. Procedures. At first, the carrier gas was flowed through the bypass. Then the washed reaction vessel was connected to the gas generation system, and the carrier gas was directed to the reaction vessel by switching the two synchronous electromagnetic three-way valves. First, the plasma was extinguished by the excws air in the vessel, and so the gas flow was switched back from the reaction vessel to the bypass. After a few seconds, self-initiation of the plasma occurred, and then the carrier gas was directed again to the reaction vessel. This procedure had to be repeated a few times until the air was almost completely removed and the plasma could be stably sustained with the carrier gas flowing through the vessel. Next, 0.5 mL of the reducing solution (the tin chloride solution) was injected with a syringe into the reaction vessel through the side wall rubber septum and was bubbled with helium gas from about 15 s to remove mercury and air dissolved in the reducing solution. The completion of the degassing was confimed by monitoring the emission signal at 253.7 nm. After the degassing, 2 mL of sample solution was injected with a syringe into the vessel through the side wall septum. The generated mercury vapor was led into the plasma with carrier helium gas (which was also the plasma gas at the same time) and the emission signal was observed. The signal was measured as the peak height on the recorder chart. The silicone rubber septum could be used for about 50 measurements.
RESULTS AND DISCUSSION Spacial Distributions of Emission Intensities in the Plasma. The spacial distributions of the emission intensities of the mercury atomic line (Hg I; 253.7 nm), mercury ionic line (Hg 11; 194.2 nm), and helium atomic line (He I; 388.9 nm) in the plasma were investigated in order to find the optimum position for the measurement. During the measurement of the emission intensity distributions, mercury vapor was constantly introduced into the plasma by the vaporization of metallic mercury at room temperature with flowing the carrier/plasma helium gas (0.5 L/min). The results are illustrated in Figure 1. The abscissa of Figure 1 indicates the distance from the center of the discharge tube; 0 mm corresponds to the center, k1.5 mm corresponds to the wall of the discharge tube. It should be noted that the emission intensities in Figure 1are not real ones, Le., they are normalized by taking the maximum intensity for each line as 100. As can be seen from the figure, the center of the plasma, at which the helium atomic line was most intense, was slightly apart from the center of the discharge tube (30). The emission intensity profile of the mercury atomic line was broad as twice as that of the helium atomic line and provided two peaks. It showed the large dip near the center of the plasma. The large dip of the emission profile, which means the less population of mercury atoms, may be caused by extensive ionization of atoms near the center of the plasma. Therefore, the position for the measurement of mercury atomic line was preferable near 0.5 mm from the center of the
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
loo/
A 1
Table I. Detection Limits and Standard Deviations at Different Helium Gas Flow Rates He gas flow detection limit, pg/mL std dev? % rate, L/min 0.25 4.2 ( 8 . 4 pg) 3.1 0.5 4.1 (8.2 pg) 1.9 1.0
6.4 ( 1 2 . 8 pg)
6.1
Standard deviation obtained at 0.1 ng of Hg/mL. Detection limit represented by the absolute amount of mercury. a
-15
0 4.5 Position (mm) Flgure 2. Dependence of analytical sensitivity on the measurement position investigated at three different helium gas flow rates: (A)0.25 L/min, ( 0 )0.5 L/min, (m) 1 L/min.
discharge tube a t which the mercury atomic line was most intense. On the other hand, the emission intensity profile of the mercury ionic line which was also broad similar to that of the atomic line has only one peak near the center of the plasma. However, the actual intensity of the ionic line was about 2 orders of magnitude weaker than that of the atomic line. Thus, the mercury ionic line could not be used for the analysis. Optimization of Experimental Conditions. According to the previous works employing mercury cold vapor generation techniques (2-4), the influences of the carrier gas flow rate, the solution volume in the reaction vessel, and the volume of the reaction vessel on the analytical sensitivity must be generally taken into account. Furthermore, i t has been reported that the sensitivity of the helium MIP emission spectrometry is significantly dependent on the plasma gas flow rate (23,24,28). Therefore, the experimental conditions for the cold vapor generation and the plasma parameters were examined to achieve better analytical sensitivity for mercury analysis. Volume of the Reaction Vessel. In general, the use of the smaller volume reaction vessel provided better sensitivity in the gas generation experiment. This may be due to less dilution of generated mercury vapor in the dead space of the reaction vessel. Therefore, a 20-mL reaction vessel was used in this experiment. Solution Volume. Using the smaller solution volume is generally advantageous to obtain better Sensitivity. It was reported in the previous paper (28) that the use of solutions more than 3 mL made the sensitivity worse in the analysis of nitrogen with the similar experimental system. Consequently, the volumes of the reducing solution and the sample solution were made up to be 0.5 and 2 mL, respectively. Flow Rate of Helium Gas. The dependence of analytical sensitivity on the measurement position in the plasma was investigated a t three different flow rates of helium gas, Le., 0.25,0.5, and 1L/min, where the 0.5 ng/mL mercury standard solution was used. The results are shown in Figure 2. The results obtained at the flow rates of 0.25 and 0.5 L/min are almost identical with each other. At the flow rate of 1L/min, however, analytical sensitivity was significantly decreased at any point in the plasma. Particularly, the sensitivity was poor near the center of the discharge tube, at which the maximum sensitivity was obtained at the lower flow rates. This may be interpreted by the cooling effect of helium gas on the plasma or by the short residence time of mercury in the plasma because of too rapid gas flow near the center of the discharge tube. The optimum measurement position was near 0.3 mm from the center of the discharge tube in both cases of the flow rates
of 0.25 and 0.5 L/min. However, at the former flow rate, the analytical signal profiles were broad and the standard deviation of the measurements was not good. Therefore, the flow rate of carrier/plasma helium gas was chosen to be 0.5 L/min. Detection Limit and Dynamic Range. The detection limits (defined as the signal level of mercury corresponding to twice the standard deviation of the blank signal) and dynamic ranges were investigated a t three different flow rates of helium gas. The results are summarized in Table I. As can be seen from Table I, the best detection limit and the dynamic range of 8 pg (or 4 pg/mL) and 2 X lo5,respectively, were obtained at the gas flow rate of 0.5 L/min. The detection limit of 4 pg/mL may be the best one obtained in the measurement system without any preconcentration. The dynamic range of 2 X lo6 is about 3 orders of magnitude wider than that obtained with atomic absorption spectrometry (28). The relative standard deviation of 10 repeated measurements using the 0.1 ng/mL mercury standard solution was about 2%. Blank Problems. When the analysis is preformed a t extremely low concentration, the blank from the solutions, reagents, and environments significantly influence the analytical results. Furthermore, the blank restricts the detection limit of the method. In the helium MIP emission spectrometry of mercury, there are two possibilities for blank contributions, Le., contaminated mercury and dissolved nitrogen in the solutions. The latter provides the emission of the NO band near 253.7 nm. Actually, 2 mL of distilled water gave the apparent signal equivalent to about 0.03 ng of mercury. In order to elucidate the origin of this blank signal, the spectral profiles of the blank signal and the signal of 0.5 ng/mL mercury standard solution were measured. The results are shown in Figure 3 (top). The spectral profile of the blank signal had no peak at 253.7 nm, but it showed some background emission which decreased toward the shorter wavelength. The characteristics of the profile indicates that the blank signal is mainly due to the band emission. As can be seen in Figure 3 (bottom), the interfering band emission is ascribed to that of NO double-headed bands in the “y system”, which have the band heads at 255.0, 255.9, 258.8, and 259.6 nm. This problem can be overcome by injecting the solutions in the reverse order as follows (see Experimental Section): First, sample solution should be taken in the reaction vessel and degassed for about 10 s. Then, the reducing solution previously degassed should be injected into the vessel. As an alternative method to reduce the influences of the band emission due to the dissolved nitrogen, simultaneous background correction may be taken into account. If the problem of the NO band emission could be avoided, the detection limit of the present method may be improved to less than 1 pg (
