The mean long term drift of the Figure 3 operational amplifier integrator starting from a null was 0.4 mV/15 minutes while the mean long term drift starting with 10.000 volts o n the integrating capacitor was 0.001 volt/l5 minutes. CONCLUSIONS
Operational amplifiers with Field Effect input transistors are now available for performing reliable integrations of chemical laboratory signals without requiring chopper stabilization. Although the integrator described here, which uses a Burr-Brown Model 3420K operational amplifier, possesses drift characteristics which are acceptable for most
applications even less drift may be obtained if desired by use, at a slightly greater cost, of a Burr-Brown 3420L operational amplifier. The total material cost of the laboratory integrator including power supply and cabinet is approximately one hundred and fifty dollars. Because of the small number of components and simplicity of the circuit, construction of the integrator is a relatively easy task. Care should, however, be taken during construction to provide proper shielding, particularly of the summing point circuitry from noise pickup. RECEIVED for review September 24, 1971. Accepted December 14, 1971.
Inexpensive Mercury-Specific Gas Chromatographic Detector James E. Longbottom Enuironmental Protection Agency, National Encironmental Research Center, Analytical Quality Control Laborator),, Cincinnati, Ohio 45268
MERCURY in some form or another has been found present in most phases of our environment. After Jenson and Jernelov ( I ) established that methylation of the inorganic form of the compound t o the toxic methyl mercury can occur in natural waters, the heretofore unrestricted dumping of elemental mercury into our streams was sharply curtailed. Massive surveys are currently under way to assess the extent of mercury pollution and t o further define the complex interrelationships between inorganic and organic forms of mercury. The gas chromatograph has been used t o identify specific organomercurials. Westoo ( 2 , 3 ) and later Newsome ( 4 ) reported the separation and detection of methyl mercury salts as the chloride using an electron capture detector (ECD). Nishi and Horimoto ( 5 , 6 )and Sumino (7,8) used the E C D for detecting a wide variety of alkyl and aryl mercury salts, while Tatton and Wagstaffe ( 9 ) used the E C D after first forming the dithizonate derivatives of organomercurials salts. The direct determination of dimethyl mercury and similar non-salts was virtually ignored, however, until Bache and Lisk ( I O ) reported the chromatograph of dimethyl mercury and the detection of many organomercurials with the mercury-specific emission spectrometer. This detector gave interference-free chromatograms of dimethyl mercury and the organomercury salts without risk of the detector poisoning reported with use of the electron capture (9). Our intention a t the National Environmental Research Center, Cincinnati, was to achieve the sensitivity and selectivity of the emission spectrometer for considerably less cost by adapting mercury meter t o a gas chromatograph. The meter used as the detector was a Coleman Model 50 Mercury Analyzer System, designed specifically for use with ( 1 ) S. Jensen and A . Jernelov, Nature: 223,753 (1969). ( 2 ) G. Westoo. Acto. C ~ J WS cZn. d . , 20, 2137 (1966). ( 3 ) [bid., 22.2277 (1968). (4) W . H. News0me.J. Agr. FootiC/rcni..19,567:(1971). ( 5 ) S. Nishi and Y . Horimoto. J o p . A I I N I 17, . . 75 (1968). (6) [bill.. p 1247. (7; K.Sumino. KobeJ. ,ck~~tl.Sci..14,115(1968). (8) [bid., p 1 3 1. (9) J. O'G. Tarton and P. J. Wagstaffe, J . Cl?ron?atogr.,44, 284 (1 969). ( I O ) C. A . Bache and D. J. Lisk. ASAL.CHEM..43,950(1971).
FROM
FID
7
e+=-
>
e
-
MAGNESlbM PERCHLORATE
iANH 1
u i ' Figure 1. Condensor assembly used for drying flame effluent before it enters the detector E X H A U S T ( P U M P OFF1 AIR I N T A K E ( P U M P O N )
-
L
TO C O N D E N S E R (PUMP ON1
Figure 2. Tee arrangement for exhaust dilution or vent the Hatch and Ott (11) wet chemical method for determining total mercury. The instrument consists of a pump that draws vapor through a 15-cm cell where UV absorbance is continuously monitored at 254 nm. The system is very sensitive for elemental mercury but requires the prior reduction of all mercury to the elemental state. This was accomplished after (1 1) W . R. Hatch and W . L. Ott, ANAL.CHEM.; 40,2085 (1968). ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, MAY 1972
1111
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I 10.000~ 100010010-
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0'
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, 1
2 3 4 5 L O G A R I T H M OF MEASUREMENT UNITS
Figure 3. Amount injected L'S. detector response for diethylmercury
TIME
(MINUTES)
Figure 4. Chromatogram of 1.0 nanogram of four dialkyl compounds: (1) (2) (3) (4)
Dimethyl mercury Diethyl mercury Dipropyl mercury Dibutyl mercury
gas chromatography with a combustion furnace and also with a flame. The copper oxide and nitrogen oxidation furnace method described by Wenninger (12) was adapted to a GC using a Dohrmann Model SlOO Combustion Unit. Quartz glass was also tried as a catalyst with oxygen-nitrogen combustion gas. Both catalytic systems successfully converted the organic mercury compounds to free mercury at about 800 "C, but the C u O oxidized a 5-111 injection of benzene more successfully than did quartz. The benzene had been proposed as typical of potential interferences for the UV method. The solvent itself gives a large response but the interference is eliminated if it is successfully oxidized to carbon dioxide. Combustion with the flame was accomplished using a flame ionization detector (FID) contained in a Perkin-Elmer Model 900 G a s Chromatograph. The combustion gas mixture of air and hydrogen was left a t a ratio that had been used for previous FID work, although this resulted in the incomplete combustion of the benzene solvent. The detector output was fixed a t 250 mV and response attentuation was accomplished with a variable range recorder. (12) J. A. Wenninger. J . Ass. OfJic. A g r . Clicrn.,48,826 (1965). 1112
Direct attentuation of the normal instrument background resulted in a high noise and drift level which was traced t o the presence of water vapor in the sample line. The combustion furnace produced a small amount of water vapor that was easily removed by inserting a 4-in. section of tygon tubing packed with anhydrous magnesium perchlorate between the furnace and detector. When the FID was used for combustion, the large amounts of water produced by the flame required the addition of a cold water condenser. The assembly shown in Figure 1 was inserted between the flame and the mercury detector. The magnesium perchlorate in either mode required replacement daily. Since the detector worked best with the high flow rate controlled by its one-speed internal pump, the chromatographic effluent was diluted with room air with a tee arrangement. The tee configuration shown in Figure 2 was used with the flame and served a second purpose. In the partial combustion of benzene, soot formed and collected in the transfer line. The soot absorbed mercury vapor and led to erratic results, but could be vented from the system by shutting off the pump during the solvent elution. The total flow rate through the cell with the condenser in place was about 600 ml/min. When the detector was modified in the manner described above, it was capable of detecting as little as 0.02 ng of dimethyl mercury. Replicate injections of 5-ng amounts of diethyl mercury were reproducible, with a relative deviation of 1.72%. The response given by individual organomercurials stoichiometrically related t o the percentage of mercury in the compound, and sensitivity was determined by the sharpness of the peaks. The linear range of the instrument extends through three orders of magnitude. For diethylmercury the detector response is linear from 0.05 ng through 100 ng as shown in Figure 3. The chromatography of several diakylmercury compounds is shown in Figure 4. The 6-ft column described by Dressman (13) consisting of 5% DC-200 and 3 z QF-1 copacked on G a s Chrom Q was used for the separation. Column temperature was held at 60 "C for two minutes and then programmed to 180 O C a t 20 "C/min. These mercurials were combusted with a flame, and the detector output was monitored with a 2-mV range recorder. Since as little as 0.02 ng of dimethyl mercury can be detected from a gas chromatograph with the MAS-50, concentrations of 1-10 ng/liter can be detected in environmental samples. Extracts of natural water samples have shown them t o be completely free of interferences. The detector system has also been applied to the detection of methyl mercury in sediment samples. Although not sensitive enough to serve in place of the electron capture detector in the analysis for the halides of methyl mercury, it has been used t o confirm the presence of methyl mercury a t levels of 0.010 pg/g or higher, and may have a direct application to fish analyses. The extracts of the sediment samples, which had been prepared for electron capture detection, yielded only the single methyl mercury peak when analyzed with the MAS50 detector.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 6, M A Y 1972
RECEIVED for review December 13,1971. Accepted February 25, 1972. Mention of trade names or commercial products does not constitute endorsement or recommendation for use from the Environmental Protection Agency. (13) R. C . Dressman, J . C/rro/nu~oyr. Sci., in press.
