Membrane probe-spectral emission type detection system for mercury

Robert S. Braman , David L. Johnson , Craig C. Foreback , James M. ... G. P. Cobb , A. W. Moore , K. T. Rummel , B. M. Adair , S. T. McMurry , M. J. H...
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Membrane Probe-Spectral Emission Type Detection System for Mercury in Water Robert S. Braman Department of Chemistry, Cinioersity of South Florida, Tampa, Flu. 33620

A new method for mercury detection and analysis is presented. Mercury compounds are reduced to metallic mercury, diffused into a helium carrier gas stream through a rubber diaphragm immersed in sample solutions and then passed through a dc discharge. The 2537 mercury emission line intensity i s observed. The lower limit of detection is 4 parts per trillion for a concentration sensing probe and is 4 x 1 0 - 1 0 gram in a batchwise analysis modification of the technique. Since only volatilized mercury diffuses through the diaphragm, both free and total mercury may be determined on the same sample. Dimethyl mercury is detected. Operating characteristics, calibration, and use of the method for the analysis of some environmental samples are presented. The membrane technique holds promise as a new analytical procedure for volatile materials in solution. OWINGTO THE TOXICITY of mercury and its compounds, in,. terest in their detection and determination has been of long standing. Much of the older analytical work is aimed at trace analysis. Considerable current interest lies in the analysis of environmental samples for traces of mercury. Low concentrations of mercury or small amounts are determined largely by spectrophotometric methods using the dithizone mercury complex (1-4). Spectrophotometric techniques are often complicated by separation or masking steps to eliminate interferences. When these techniques are applied to very small sample sizes, blank errors or background corrections can become troublesome, particularly if considerable preconcentration is required, Lower limits of detection are approximately 0.5 pg of mercury. A catalytic method developed by Pavlovic and Asperger (5) is subject to similar separation and interference problems and has similar sensitivity. Neutron activation analysis has been used (6, 7) for detection of microgram sized samples but the procedures are inconvenient, especially when interferences must be removed. Flame emission and conventional atomic absorption analysis do not have very low limits of detections, approximately 2 . 5 ppm and 10 ppm, respectively (8). The most sensitive methods for low concentrations to date have been based upon absorption by mercury of UV radiation from a mercury vapor lamp. Early work was reported by Woodson (9). A number of what are essentially modifica-

(1) Yikimura and V. L. Miller, A/ta/. Cliirn. Ac/u, 27, 325 (1962). (2) W. H. Gutenmann and D. J. Lisk, J. Agr. Food Clwni., 8, 306 ( 1 960). (3) “Official Methods of Analysis,” 8th Ed., Association of Official Agricultural Chemists, Washington, D. C., 1955. (4) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” 3rd ed., Interscience, New York, N. Y.,1965. ( 5 ) D. Pavlovic and S. Asperger, ANAL.C w . , 31, 939 (1959). (6) B. Sjostrand, ibid., 36, 814 (1964). (7) C . K . Kim and J. Silverman, ibid., 37, 1616 (1965). (8) H. H. Willard, L. L. Merritt, Jr., and J. A. Dean, “Instrumental

Methods of Analysis,” 4th Ed., Van Nostrand, Princeton, N. J., 1965. (9) T. T. Woodson, Rer. Sci. I/istr.urn.,10, 308 (1939).

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tions of the same method have been reported (10-13) and are now called flameless atomic absorption methods. Concentration limits of detection for these range from 0.02 to 1.0 pg per liter depending upon sample sizes taken. The lower limit of detection on an analyte weight basis is 5 to 10 nanograms (14). Metals other than mercury are not an interference in thi: method but volatilized materials absorbing in the 2537-A region are, if present. The dc discharge spectral type detector (15) is useful for the analysis of small amounts of heteroatom-containing organic compounds; lower limits of detection range from to lo-“ gram for many compounds. Because of this and the high degree of selectivity available in emission type detection systems, the method was chosen for application to mercury detection and analysis. In order to use an emission type detector, mercury metal vapor or a volatile mercury compound must be separated from its sample matrix materials. It must then be passed through ,an electrical discharge in helium carrier gas and the 2537-A Hg emission line observed in an appropriate optical system. The analysis of water or air samples by total sample injection onto a gas chromatography column of appropriate type appears feasible. Nevertheless, the analysis of large water samples is clearly a problem because of the need for eventual removal of 1 ml or more of water from the column. The analysis of 10-p1size samples would be more suitable, but the to lo-’* gram of mercury in the samples could be lost in the separation process. Two techniques for separating mercury from aqueous samples were investigated prior to study of the membrane. Vacuum evaporation of water samples, and reduction of the residue with sodium borohydride in a small tube was first tried with some success. Mercury vapors were readily detected but reproducibility was poor, largely because water from the reducing agent solution interfered in discharge operation. A direct volatilization method similar to that used in the flameless atomic absorption method was next tried. Helium carrier gas was passed through a sample solution treated with sodium borohydride and then through a drying tube and into the detector cell. Larger sample sizes could be accomodated and limits of detection were low but sample handling, necessity for replacement of the drying tube, and time required for analysis rendered this approach less convenient than the membrane-emission detector probe technique subsequently developed. Selection of the diffusion of mercury through a membrane from air or water solution into the carrier gas stream was based upon prior reported work. Volatile materials are well (IO) C. W. Zuehlke and A. E. Ballard, ANAL. CHEM., 22, 953 (1950). ( 1 1 ) 0. Lindstrom, ihid., 31, 461 (1959). (12) M. J. Fishman, ibid., 42, 1462-3 (1970). (13) W. R. Hatch and W. L. Ott, ibid., 40, 2085 (1968). (14) L. P. Morgenthaler, “The Determination of Trace Quantities

of Mercury by Atomic Absorption Spectrometry,” McKeePedersen Instruments, Applications Notes, Vol. 5, Nov. 1970. (15) R. S. Braman and A. Dynako, ANAL. CHEM., 40, 95 (1968).

ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971

DISCHARGE POWER

TO STIRRER MOTOR

900 v @

FROM

He TANK RECORDER 200 rnl BEAKER

MEMBRANE IOVER SURFACE A N D DOWN SIDE1

Figure 1. Apparatus arrangement for membrane probeemission type detector known to diffuse through membranes. Some uses of membranes in analysis have been reported. Oxygen diffuses through thin layers of Teflon (Du Pont) in the polarographic membrane electrodes (16). Membranes are also used to separate carrier gases from eluant in interfacing mass spectrometers with gas chromatography systems (17,18). Carbon dioxide membrane electrodes are used (19). An entire latex rubber balloon was used in initial experiments with promising results. Probe designs providing more control over membrane surface area and more convenient to use were then constructed and used throughout the study. The membrane probe technique reported here for the determination of mercury has eventually proved to be more selective with lower sample size limits of detection than any other method for this element. EXPERIMENTAL

Apparatus. The apparatus arrangement used is shown in Figure 1. It consisted of the helium carrier gas source, a membrane type probe (or batchwise cell) and emission type detector cell, and a conventional optical and electronic system. The probe type cell design is shown in Figure 2a. Latex rubber balloons, 1.7 cm in diameter (not inflated) and 18.5 cm long and 4 mils thick were cut off to 2 inches long (16) D. E. Carritt and J. W. Kanwisher, ANAL. CHEM., 31, 5-9 (1959). (17) S. R. Lipsky, D. G . Horvath, and W. J. McMurray, ibid., 38, 1585 (1966). (18) J. T. Watson and K. Biemann, ibid., 36, 1135 (1964). (19) J. W. Severinghaus and A. F. Bradley, J . Appl. Physiol., 13, 515 (1968).

Tz-4 40

-

Figure 3. Batchwise cell (half size) and pulled over the glass probe end. The balloons fit tightly enough over the 4-cm diameter probe end so as to serve as their own leak-tight seal. A short end of the balloon tip extended below the probe. Although the membrane area influences the total rate of mercury difl'usion, difficulties arising from the elasticity of the rubber were not observed. The carrier gas inlet pressure was kept just above ambient. Helium carrier gas flowed over the inner surface of the rubber diaphragm and carried diffused mercury and other diffused vapors into the quartz D C discharge cell shown in Figure 2b. This cell was constructed entirely of quartz for optical transmission and to minimize absorption of mercury on metal surfaces. Polyethylene tubing, 1/4-in.o.d., was a convenient connector. It stretched tightly over the 6-mm glass tubing. A batch analysis type cell was also constructed and is shown in Figure 3. It was easily substituted for the concentration probe cell by using the polyethylene connectors. The same carrier gas, monochromator, and electronic equipment were used. A Heath Company scanning monochromator, Model EU 701-30 photomultiplier module (1P28 tube), a model EU 703-31 Heath amplifier, and a 0-10 mV strip chart recorder were used. An unregulated discharge power supply, a voltage doubler followed by a pi filter, was constructed. TO FLOWMETER

He

-

6 mm TO DISCHARGE CELL

1

-

Figure 2. Probe cell and discharge design

DISCHARGE

PROBE FROM

--

__r

OUARTZ CELL

LATEX BALLOON

TO

.

--

MONOCHROMCTER

:

P,

[

-

ITIGHTLY ATTACHED1

ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971

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Pi

3.8 Ppe

CI b

B

A

C Figure 4. Selected response recordings

Tap water, X scan, 0.036 ppb A scale) Standard, X scan, 0.096 ppb (lo-' A scale) Standard, X scans, 3180 ppb A scale) D. Standard, X scan, approximately 0.2 ppb

A. B. C.

I T - -I -

I

I

I

I

I

I

I

I

I

I

I

6min

5

4

3

2

1

0

Figure 5. Mercury detection system response rate Continuous scan at 2537 Hg, 40-fi slit)

A Hg line, 23

'C, 10-7 A scale, 0.48 ppb

Reagents. Sodium borohydride from Alfa Inorganics, selected as the reducing agent, exhibited a zero blank value for mercury. No other reagents were necessary for most work except where pH adjustments were made. Stannous chloride was tried, but the sample on hand was found to be highly contaminated with mercury. J. T. Baker Co. ultrapure helium is of sufficient purity to permit use as a carrier gas without cryogenic purification or other treatment. Instrument Settings. Photomultiplier voltage, discharge voltage, slit width, amplifier gain, and range of the strip chart recorder all control the total signal gain. The present apparatus was operated with the following ranges of conditions: Slit width 25-100 microns, PM voltage 750-800 V., dc discharge 900 V, 31.5 watts/lineal inch of discharge, to A scale on the amplifier, 0-10 mV recorder range. Full scale on the recorder equaled 20% of the amplifier out put. Procedure. SOLUTION PROBECELL. Helium carrier gas is turned on and adjusted to a flow rate of 80 to 100 ml/ 1464

A scale)

minute. The outlet of the detector cell is restricted by a pinch clamp so as to raise the internal pressure of the system to slightly above ambient. This helps to decrease diffusion of air into the carrier gas through small leaks, but does not affect detector response. After 3-5 minutes, the system is sufficiently flushed out with helium to permit the dc discharge to remain on. A determination of residual mercury can be made on the system by placing distilled water (pr tap water) in the sample beaker !nd scanning the 2537-A region. A peak will occur at 2537 A superimposed on the N24th positive band system (see Figure 4). Residual mercury, if present from previous analyses, rapidly decreases to a low value or zero. Analysis of aqueous samples is accomplished by placing 200-ml samples in a 400-ml beaker, 3 t o 10 drops of 2 x NaBH4 in distilled water is added, and the emission response is recorded. Sqlutions are stirred during the analysis. Response at 2537 A rises to a maximum in 2 to 3 minutes after addition of the reducing agent as is shown in Figpre 5. Data may be taken by recording response at 2537nA as a function of time or by scanning through the 2537-A region several times over a 2- to &minute period. The scanning method appears best for the 0- to 0.2-ppb range so that a base-line technique of emission peak area may be used to calculate responses. This also affords observation of any background radiation which may be present. Dissolved nitrogen is usually present. At higher concentrations changes in background will be sufficiently small as to be negligible. Examples of the types of data obtainedoareshown in Figure 4. A decrease in the response of 2537 A is observed after the initial rise to a maximum. This is due to the removal of mercury from the sample during analysis. The maximum response is usually constant over several minutes before the decay is observed. Response values taken in this time period are used in calibration based upon concentration and analysis. After each analysis the cell is washed with distilled water and is allowed to stand with a blank water solution for 3-5 minutes, or until residual mercury is cleared from the system. The apparatus is then ready for the next sample. If the interior of the diffusion cell and detector cell become overloaded with mercury or if cleaning is desired, bromine water wash may be used followed by water. Solvents may be used but the last wash should be with water. Organic layers absorb nearly completely 10- to 100-nanogram amounts of mercury passed through the system. Calibration is obtained by addition of appropriate volumes of dilute (10 ppm)

ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971

Figure 6. Effect of temperature on batchwise cell response Top curve: 75 "C, 95.8 ng Hg2+, 10-%4 scale Middle curve: 41 "C, 95.8 ng Hg2+,lO-?-A scale Bottom curve: 23 "C, 95.8 ng Hg*+,lO-?-A scale

HgClz solutions to 200 ml of distilled water and then analysis by this procedure. Sample volumes down to 50 ml have been used with the probe. BATCHWISE ANALYSISCELL.Samples may be analyzed by diffusing all the mercury in a small sample volume through a membrane. From 2 to 5 ml of sample solution is placed on top of the membrane in the batchwise analysis cell shown in Figure 3. The cell is heated to 60-80 "C by means of an electrical tape wound around the cell. A stirrer is mounted above the cell. One or two drops of 2 x NaBHa solutiop are added to the sample. The emission response at 2537 A is recorded as a function of time. Data of the type shown in Figure 6 are obtained. The area under the curve is proportional to the total amount of mercury in the sample. Calibration is obtained by simply adding known amounts of mercury from dilute solutions to a blank water sample. RESULTS AND DISCUSSION Response Curves, Precision, and Limits of Detection. Several response curves were obtained in the study of the technique. Samples of known mercury content were prepared by injecting microliter volumes of an approximately 10 ppm (as Hg) standard mercuric chloride solution into water of nil or very low mercury content just prior to analysis. Local tap water was found to be very low, 10 ppt or less, in mercury and served as a convenient blank source. One microliter of the 10 ppm Hg solution contains 10 nanograms (ng) of mercury and produces a solution at 50 ppt Hg when added to 200 ml water. Table I gives the experimental conditions, least squares line equations and pertinent statistical data from the response curve study. Precision of the slopes ranged from 2 to 5 % relative and is a fair measure of the precision of the method for the concentration ranges and sample sizes used. Series 1 was carried out using sample volumes measured by filling the sample cell to the 200-ml mark, a less precise method than the sample weighing procedure used in series 2. The improved precision of the technique is reflected in the 0.01 ppb estimate of the standard deviation of the individual values. Series 3 affords an evaluation of the lower limit of detection

of the method on a concentration basis. The standard deviation in the individual points is near 4 parts per trillion. The standard deviation in the individual points for the batchwise analysis method is 3.8 X 10-10 gram. The approximate flow rate of mercury through the detector was calculated from data on the decrease in the signal as a function of time. This was found to be 1.7 X g/sec at Ct = 0.7 ppb. At a carrier gas flow rate of 4 3 ml/minute and with a detector active volume of 0.32 ml, 5.3 X lo-" gram of mercury is present in the detector at time t. Since the concentration limit of detection for 200-ml sample volumes is on the order of 2 X lo-" g/ml, this should yield a flow rate of 5 X 10-l0 g/sec with 1.5 X 10-l2 gram in the detector active volume at the limit of detection. This is in fair agreement with the sample size limit of detection of 3.8 X 10-lo gram over a 30-second response time, calculated from the response curves series 4 in Table I. Membrane Function. Membrane operation is analogous to the plating out of a metal at an electrode. The rate of diffusion of mercury across the membrane is the effect controlling the detector response in a stirred solution. If dN/dr is rcte of mercury loss to the carrier gas inside the probe, then from Fick's law: d Ndt

dC DAdX

Where A is the membrane area, dCjdX is the concentration gradient across the membrane, and D is the diffusion coefficient for mercury metal in the membrane. If dCjdX is linear across the diffusion layer, then

where d is the thickness of the layer and C" the mercury concentration at the water-carrier gas interface. The diffusion layer includes the membrane but possibly also solution outside the membrane. If C" may be considered to be very small. then

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Table I. Response Curves and Statistical Data for Mercury in Water Analyses Response curves Conditions 0.1 to 1.4 ppb Hg, probe type cell 24-11 slit, 800 V, PM, 23'/2 "c 10-7-A scale range, ng/200 ml = 15.94 cm (peak right) -20.2 chart speed 1 in./minute, s (slope) = 0.30 80-100 ml/min, He s (ng/200 ml) = 0.08 12 = 12

Series 1

Series 2

0.1 to 1.O ppb, probe type cell ppb = 0.04594 X cm (peak height 0 + 0.01 s (slope) = 0.00176 s (intercept) = 0.02 I2 = 9 0.001 to 0.100 ppb, probe type cell cm2(peak area) = 18.90 x ppb 0.21 s (slope) = 0.99 s (intercept) = 0.052 s (area) = 0.074 cm2 s (PPt) = 4 I1 = 7 0-10 ng, batchwise cell cm2(peak area) = 0.574 x ng 0.16 s (slope) = 0.027 s (intercept) = 0.15 s (area) = 0.22 cm2 s(ng) = 0.38 I1 = 7

Series 3

10-7-A scale range, chart speed '/z in./minute, 122 mlimin, He 4@1 slit, 740 V, PM,

+

Series 4

+

Table 11. Effect of He Flow Rate on Response (96 4 2 0 0 ml sample) Flow rate, ml/min Peak height, cm, 10-6 scale 114 6.4 loo 6.4 72 6.5 64 6.4 46 6.4 35

6.3

(3) and the rate of mercury transfer to the carrier gas stream is a linear function of the concentration of mercury metal in the sample solution. This was confirmed by experiment. Values of D for mercury were calculated from experimental cm2/sec for an assumed 4-mil data and found to be 1.2 X value of d and 20 cm2 for A . The technique operates in a manner similar to controlled potential coulometry in the batchwise analysis technique. Mercury leaves the system through diffusion and thus: DAt Ct = Coe - -V

(4)

gives the concentration at any time t where C o is the mercury concentration at time t = 0, A is membrane area, Vis sample volume, and D is the apparent diffusion coefficient. Figure 6 which illustrates a typical response for the batchwise type cell indicates conformance of experimental results to Equation 4. Only one type of membrane material other than latex rubber was tested. Saran wrap, 1 mil thick, was used in a few experiments in the batchwise analysis cell. Saran diffused mercury much more rapidly than the latex rubber, probably because of its decreased thickness. It will be suitable for use in batchwise analyses but is more fragile. 1466

40-11 slit, 740 V, PM,

IO-'-A scale range, chart speed 1/2 in./minute, 122 ml/min, He Scan rate 0.02 A/sec

75 "C

35-11 slit, 740 V, PM, IO-7-A scale range,

chart speed 1/2 in./minute, 122 ml Heimin, Scan rate 0. 05 A/sec

As predicted from Equation 3, response is a function of membrane area. Comparison of calibration plots for two different balloons, approximate areas 18 and 23 cm2 indicated a linear relation of mercury diffusion rate to membrane area. Detection System Characteristics. Equation 3 predicts that carrier gas flow rate or pressure will have no influence on the rate of diffusion. The rate of mercury transfer across the diffusion membrane layer, then, controls the rate of flow of mercury through the discharge. If the detector responds to rate of flow, then carrier gas flow rate should have no effect on response. This was found to be the case experimentally as shown in Table 11. Nevertheless, it was expected that the detector would be concentration sensitive since the excitation process does not destroy mercury. Further investigation is clearly needed to evaluate detector operation. Stirring rate influences response but only if very slow rates are used. If stirring is halted altogether, the rate of mass transport of mercury through the solution to the diffusion layer becomes important. An increase in slit width increases response. Slit widths larger than 200 microns admit too much background radiation. Temperature Effects. Temperature influences the diffusion rate of mercury. Consequently, the concentration sensing probe must be operated under temperature controlled conditions. All calibration runs were made at 23.5-24.0 "C, room temperature. To determine the temperature effect on probe response, several duplicate samples were analyzed in the range 24-50 "C. The temperature effect from 24 to 40 "Cwas to increase the probe signal by 2.04% per "C increase in temperature, the expected temperature effect on diffusion coefficients. The decay rate of the peak signal was increased by increasing the temperature. At 50 "C, water diffusion through the membrane starts to quench the discharge. The batchwise analysis cell operation is also influenced by temperature. Temperature increases decrease the analysis

ANALYTICAL CHEMISTRY, VOL. 43, NO. 11, SEPTEMBER 1971

time because of the increased diffusion rate. Figure 6 shows the sharpening effect of elevated temperature on the Hg response us. time curve. Batchwise analyses were carried out at 70-80 "C to take advantage of the improved response time. The response of the system in terms of area per ng Hg is not influenced by temperature. Quenching of the discharge was not observed at the elevated temperatures, presumably because of the smaller membrane area than was present in the probe cell temperature study. Limitations and Interferences. The mercury detection and analysis technique is subject to the chemical limitation that mercury must be mercury metal or a volatile compound. Sodium borohydride is capable of reducing mercurous and mercuric compounds in mildly acidic to alkaline media. Acidic solutions rapidly hydrolize sodium borohydride and even though some reduction occurs, there may be reoxidation of mercury prior to detection. This may be avoided by buffering the solution with borax prior to using sodium borohydride. Alternatively, a different reducing agent, providing it is sufficiently low in mercury content, can be used. The following materials did not interfere with the analysis technique: sea water, dissolved organic solvents, EDTA, oxalic acid, NO3-, H2P04-, CN-, Fea+, Zn2+, and borax. Oxidizing agents which react with sodium borohydride will interfere unless additional sodium borohydride is added to reduce these materials. Mercury not in solution or not readily convertible to mercury will not be detected. Dimethyl mercury diffuses rapidly enough for easy detection, but a paint fungicide, di(phenylmercury)dodecenylsuccinate, diffused too slowly through the membrane for easy analysis. Prior oxidation of this type of mercury compound will apparently be needed before analysis. Applications. Samples from natural water sources in and around the Tampa Bay area were analyzed using the concentration probe cell. Analyses required approximately five minutes per sample. Mercury concentrations ranged from 0.01 to 0.16 ppb. Samples ranged in quality from fresh river water to highly polluted salt water from port areas. N o interferences were detected based upon recovery of added known amounts of mercury prior to analysis. Several commercial chemical products were analyzed after suitable sample preparation. Commercial bleach was found to contain 125 ppb Hg. Sufficient reducing agent was added to reduce the hypochlorite content of the sample and to provide an excess for mercury reduction. Highly acidic waster water samples were analyzed after addition of sufficient borax solution to the sample to adjust the pH to 7-8. The borax solution contained insignificant amounts of mercury. It is particularly important to check on the mercury content of reagents used in sample preparation. There may also be wide variations in mercury content of reagents from different sources. For example, one source of SnC12 had 2.5 ppb Hg, another had 0.3 ppm Hg. The analysis of some common

laboratory reagents were as follows: EDTA 2.0 ppm Hg; concd H2SO4, 4.4 ppb; concd H N 0 3 , 5.1 ppb; (NH4)&08, 55 ppb; K M n 0 4 ,80 ppb; NaOH, 34 ppb; K2Cr2Q7,less than 1 ppb, and NHaOH, less than 1 ppb. Several urine samples and a pooled blood serum sample were directly analyzed by the technique. No interferences were observed. Mercury in the urine samples ranged from 3 to 10 ppb. The method was also suitable for the determination of dimethyl mercury in low concentration in aqueous solutions. Dimethyl mercury is detected without reduction by sodium borohydride with the same sensitivity as mercury. SUMMARY

Combination of a membrane probe with a dc discharge emission type detector has expanded its capabilities. Extrdction, preconcentration, and gas chromatographic separations are avoided. Separation by means of a membrane permits transfer of analytes directly from samples into the helium carrier gas stream of the detector. Diffusion of water and dissolved gases is insufficient to quench the discharge. Thus, the dc discharge detector can be used for analysis of aqueous samples directly without separation for any diffusible material with a specificity limited only by the emission wavelength specificity available in the dc discharge detector. The technique has been applied here to the determination of mercury in water. The method is specific, comparatively free of interferences, rapid, and convenient to perform. Limits of detection are equal to 10 to 20 times lower than those of the flameless atomic absorption methods (depending upon the published limits of detection used for comparison.) Both concentration measurements and batchwise analyses can be made. The technique has been applied to the detection of mercury in fresh water, sea water, laboratory chemicals, commercial chemical products, food samples, and urine. Limitations are based upon the necessity for converting all mercury compounds into mercury metal by reduction (except those mercury compounds which diffuse through the membrane). Complexing agents do not prevent the reduction of mercuric or mercurous salts to mercury metal. The diffusion technique described here obviously serves as a model for many other water or air analysis applications. Dissolved oxygen, nitrogen, sulfur dioxide, carbon dioxide, nitrogen oxides, and many volatile organic compounds are detectable. Certain inorganic compounds are also detectable. The potential uses are multifarious. The technique is being studied further.

RECEIVED for review April 8, 1971. Accepted June 8, 1971. Presented at the Meeting-in-Miniature, Florida Section, American Chemical Society, May 1971. Equipment used in this research was provided by National Science Foundation Grant G P 8598.

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