Anal. Chem. 1980, 5 2 , 1391-1392
busted by the quartz 900 "C method. Table I1 shows that the 6 values for the ARO-1 and ARO-2 samples obtained by the quartz and glass methods are identical, whereas the dynamic combustion technique shows more erratic results. The 6 values for the heteroatomic compounds are in good agreement with the glass method, especially when considering the fact that the analyses on the dynamic method were done by a different laboratory where a slight deviation in the calibration of the standards might account for the difference in the 6 values. The 6 values for the saturated hydrocarbons are in excellent agreement with the glass method; however, the standard deviation ( l o ) for the glass 550 "C method is 8 to 10 times better than for the dynamic method. T h e 6 values obtained by the dynamic method for the aromatic hydrocarbons are significantly different from values obtained by the glass or quartz method. This difference has also been observed for asphaltenes (not reported here). The dynamic combustion seemed in all cases to give more negative 6 values (up to 2%0 difference has been observed). We do not have a good explanation for the difference, however, because the standard deviation ( l o ) for the glass 550 "C method is 8 to 10 times better than for the dynamic method, and because there is an excellent agreement between the glass and the quartz methods, we believe that the glass 550 "C method is a very reliable technique.
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A 9/1 methane/C02 mixture was separated cryogenically on a vacuum line and the methane was combusted by the glass 550 "C method for 5 h. For comparison, the same mixture was separated on a gas chromatograph (GC) and the separated gases were combusted and trapped as C02immediately as they exited the GC. Table I1 shows that although the GC method gives more reproducible numbers, the two average 6 values are reasonably close. Therefore, since a combined GC-combustion train is expensive and not commonly available, the glass 550 "C method seems to be a very good alternative.
ACKNOWLEDGMENT I thank Craig Schiefelbein for his help in preparing and analyzing the samples, and Cities Service Company, Energy Resources Group, for supporting this investigation.
LITERATURE CITED ( 1 ) Frazer, J. W. Microchim. Acta 1962, 993-999. ( 2 ) Stuermer, D. H.; Peters, K. E.; Kaplan, I. R. Geochim. Cosmmhim. Acta 1978, 42, 989-997. (3) Craig, H. Geochim. Cosmochim. Acta 1953, 3 , 53-92. DesMarais, D. J.; Hayes, J. M. Anal Chem. 1976, 48, 1651-1652. (4) (5) Ebel, S. Fresenius' 2.Anal. Chem. 1973, 264, 16-28.
RECEIVED for review February 19, 1980. Accepted April 18, 1980.
Cold-Vapor Determination of Mercury Kimberly E. Lawrence, Morris White, Richard A. Potts, and Richard D. Bertrand* Department of Natural Sciences, University of Michigan-Dearborn,
Since the early work of Hatch and Ott ( I ) , the determination of trace amounts of mercury by cold-vapor atomic absorption has become widely used. One method is based on the reduction of mercury(I1) to the elemental state by tin(I1) chloride, vaporization of the mercury, and subsequent measurement of the vapor in an atomic absorption spectrophotometer. Detection limits for this method are usually about 1.0 ppb mercury(I1); however, Hawley and Ingle (2) report a detection limit of 0.001 ppb in a 1-mL sample. Several modifications of the cold-vapor method have been reported, each with advantages and disadvantages. T h e complexity of the reduction cell is a common disadvantage for some of these methods. We report a simple, efficient, inexpensive, precise, and sensitive method for the cold-vapor determination of trace quantities of mercury. All instrumental modifications are made with materials that can be readily found in most laboratories. Approximately 50 samples can be analyzed in 1 h. T h e calibration curve is linear over the range 0-70 ppb mercury(I1). The precision of the method is about 2% (RSD) for 1.0 mL containing 10 ng mercury(I1). The method may be particularly suitable for clinical applications.
EXPERIMENTAL Apparatus. All atomic absorption measurements were made at 253.7 nm using a Varian AA-175AB spectrophotometer with a Varian mercury hollow cathode lamp. The reduction vessel was a 16 X 100 mm disposable culture tube fitted with a Vacutainer septum. Disposable 21-gauge by 1.5-inchand 16-gaugeby 1.5-inch Becton-Dickinson syringe needles were used as the entrance and exit ports of the reduction vessel, respectively. The carrier gas was compressed air and the flow rate was measured with a flow meter attached to the exit of the absorption cell. Tygon tubing (30 cm X ' / 4 inch i.d.) was used to connect the exit port of the 0003-2700/80/0352139 1$01 .OO/O
490 1 Evergreen Road, Dearborn, Michigan 48 128
reduction cell to the entrance port of the absorption cell. The cylindrical absorption cell (170 X 16 mm) with closed ends was from the Varian Model 64 Generation Kit. The mercury containing standards and tin(I1) chloride reducing solution were mixed in the stoppered reduction cell using a Vortex-Genie mixer at approximately 50 Hz.The absorption signals were recorded either by using the "peak signal" mode or a Varian A-25 recorder. Reagents. The recommendations of Feldman (.3) were followed regarding the preparation and preservation of dilute mercury solutions. Reagent grade chemicals and distilled deionized water were used in the preparation of all solutions. Solutions were prepared fresh daily except for the tin(I1) chloride which was prepared every other day. A stock mercury(I1) chloride solution (100 pg/mL) in 5% (v/v) hydrochloric acid was used to prepare the 100 ng Hg(II)/mL working solution which was 5% (v/v) nitric acid. The mercury standards (0-100 ng/mL) were prepared by appropriate dilution with a 5% (v/v) nitric acid/O.Ol% (w/v) potassium dichromate solution. The reducing solution ( 4 ) was 10% (w/v) tin(I1) chloride, 5% (w/v) sodium chloride, and 10% (v/v) sulfuric acid. Procedure. A calibration curve was obtained using a series of standards 0-100 ng/mL in mercury(I1). A 1.0-mL aliquot of each standard was pipetted into a disposable culture tube and immediately stoppered with a Vacutainer septum. At least five replicate analyses were carried out on each mercury standard. Through the septum of the stoppered reduction vessel was injected 1.0 mL of the tin(I1) chloride reducing solution using a syringe. The contents of the vessel were vortexed at 50 Hz for 10 s, followed by a 20-s equilibration period during which the entrance and exit syringe needles were inserted through the septum. The needle supplying air (21 gauge X 1.5inch) was inserted to its full extent above the level of the solution; however, the exit needle (16 gauge X 1.5 inch) was inserted so as to just penetrate the septum. This allows the mercury vapor to be more quickly swept from above the solution. At this point, air flow was started, the mercury vapor being swept from the vessel to the absorption cell, and the 0 1980 American
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maximum absorbance was recorded or displayed on the digital readout. Absorbance maxima were observed within 3 s after the start of air flow, and they returned to the base-line value within 7 s. The mercury reduction vessel was removed from the system and an empty, stoppered culture tube was substituted. The system was flushed with air for 15 s to eliminate any residual mercury vapor. The next sample could then be analyzed. Fogging of the absorption cell does not occur; hence, no drying tubes were required. Syringe needles and tubing were changed periodically. Test trials indicated up to 50 prepared samples could be analyzed per hour. RESULTS AND DISCUSSION General. The effect of various instrumental and physical parameters on the sensitivity, detectability, and reproducibility of the method were investigated. Under optimum operating conditions, the method yields an absolute detection limit of 1.0 ng mercury(I1) and a sensitivity of 0.4 ng for a 1.0-mL sample. The slope of the calibration curve is 0.0079 (jzO.0001) ppb-' and is linear up to 70 ppb. The relative standard deviation (RSD) of the measurements for 10 ppb mercury(I1) is approximately 2%. Although the parameters governing this method have been optimized to provide the lowest detection limits, it should be pointed out that further improvements in the detection limit could be made by using long, narrow cells with an intense mercury pen lamp (2) or by further minimizing the dead volume in the apparatus. Vortex Time. When tin(I1) chloride solution is injected into the reduction vessel, mercury(I1) ions are readily reduced to mercury(0) which diffuses from the aqueous phase and is swept by the carrier gas to the absorption cell. Since the mercury vapor is only swept from above the solution, it is necessary to maximize the production of the reduced vapor. It follows that the mixing of the mercury standards upon addition of the tin(I1) chloride solution is an important variable in the method, as rapid equilibration of gas and aqueous phases is favorable. The absorbances of five 1-mL samples containing 10 ng mercury(I1) were measured after vortex periods ranging from 0 to 90 s and were found to plateau after a 10-s vortex period a t a 50-Hz vortex frequency. Flow Rate. Hawley and Ingle (2) have stressed the importance of sweeping the mercury vapor from the reduction vessel to the absorption cell so as to achieve the most concentrated mercury plug. Assuming an efficient diffusion of mercury atoms from solution, the rate of carrier gas flow will determine how quickly the plug of mercury is swept through the absorption cell. The effect of air flow velocity was examined by measurement of the absorbance produced by five 1.0-mL samples containing 10 ng mercury(I1) at each flow rate.
The air flow rates were varied from 0.5 to 2.5 L/min. The optimum flow velocity was found to be 1.0 L/min and was used in all determinations. S a m p l e a n d R e d u c t a n t Volumes. Since a maximum quantity of reduced mercury vapor above the aqueous phase is desirable, the sample volume was expected to be an important variable. Samples containing 10 ng mercury(I1) in L O - , 2.0-, and 3.0-mL solutions were prepared and five replicate analyses performed on each using 1.0 mL of reducing solution. The results show that peak absorbances and reproducibility decrease with increased sample volume. A 1.0-mL sample volume was chosen for convenience, although sample volumes of less than 1.0 mL may improve the sensitivity of the method. The volume of reductant added to 1.0 mL of sample was varied from 0.1 to 1.0 mL. The volume of reductant added appears to have no effect on peak absorbance. This may be attributed to the efficiency of mixing as equilibration of the vapor and aqueous phase is easily obtainable in the dead volume available above the solution. Needle Size. The size of the needle through which air entered the reduction vessel was not varied. The size and position of the exit needle were varied so as to provide maximum absorbances and best precision. The depth to which the needle is inserted is important since the vapor should be swept rapidly from the vessel as a plug. I t is found that the best results were obtained when the needle was inserted only to the point where it just penetrates the septum. When the needle is inserted to its full extent, air turbulence prevents rapid removal of the mercury vapor. The inside diameter of the exit needle was determined to also be important. Onemilliter samples containing 10 ng of mercury(I1) were prepared and analyzed using exit needles of varying inside diameters. The carrier gas flow rate was adjusted to 1.0 mL/min for each needle size. It was found that peak absorbances increased more than 10% in going from 20-gauge needles to 16-gauge needles. Larger inside diameter needles were unavailable for study, but it is presumed increased sensitivity would be obtained with them. LITERATURE CITED (1) Hatch, W. R . ; Ott,W. L. Anal. Chem. 1968, 40, 2085-2087. (2) Hawley, J. E.; Ingle, J. D.. Jr. Anal. Chern. 1975, 47, 719-723. (3) Feldman, C. Anal. Chern. 1974, 46, 99-102. (4) Tong, S.L. Anal. Chern. 1978, 50, 412-414.
RECEIVED for review March 7,1980. Accepted April 28,1980. Support for this work by the University of MichiganDearborn Campus Grants Committee is gratefully acknowledged.
