A simple mercury vapor detector for geochemical prospecting

GEOLOGICAL SURVEY CIRCULAR United States Department of the Interior

ROGERS C. B. MORTON, Secretary

Geological Survey

William T. Pecora, Director Abstract---------------------------------- Introduction -- ------------- -------------- -- General description------------------------ Instrument design (physical) ----------------

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2. Functional diagram of mercury vapor detector-----------------------------------------
3. Photograph of amalgamative mercury-extraction stage (thermoamalgamator) -------------
4. Circuit diagram of mercury vapor detector--------------------------------------------
5. Circuit diagram of power supply -----------------------------------------------------
6. Temperature-saturation curve for mercury vapor in air--------------------------------

C 1 a r o stat series 51 high-voltage potentiometer, 5 megohms, CM 33507 Clarostat series 62JA 10-turn potentiometer, 100,000 ohms Resistor, 500,000 ohms, !4 watt Resistor, 250,000 ohms, !4 watt Ohmite potentiometer type 2541, 250,000ohms Same as R Resistor, 50,000 ohms, !4 watt Same as R7 Resistor, 8,000 ohms, 2 watts Ohmite potentiometer type 1041, 100,000ohms Amphenol potentiometer type 994P trimmers;

PARTS LIST

Ill Instrument design (electronic)--------------- Instrument operation----------------------- Calibration-------------------------------- References--------------------------------

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Amphenol potentiometer type 994P trimmers, 1,000 ohms Amphenol potentiometer type 994P trimmers, 2,000 ohms Amphenoi potentiometer type 994P trimmers, 5,000 ohms Ohmite potentiometer model E, type 0122, 3,500 ohms, 10 watts Resistor, 1,500 ohms, 10 watts Resistor, 20,000 ohms, 10 watts 6201 twin-triode vacuum tube 10-microampere taut band meter RCA 935 photocell

By W. W. Vaughn

Although the geochemistry of mercury and its geologic environmental behavior as a free element are not fully understood, mercury is recognized to be an indicator element for mineral deposits and, in certain geologic surroundings, for some heavy metals in direct association with it (Saukov, 1946; Hawkes and Williston, 1962; Williston, 1964; Williston andMorris, 1965; Erickson and others, 1964, 1966; Friedrich and Hawkes, 1966). Of the procedures available for mercury determinations, the large-volume low-temperature atomic-absorption technique seems to be the most practical for geochemical studies. The sensitivity and reliability of this technique have been demonstrated (Vaughn and McCarthy, 1964). The minimal version of the apparatus described here will be helpful to most people who are interested in determining the mercury content of materials occurring naturally.

A single-neam atomic-absorption mercury vapor detector with a needle-type meter indicator has been developed for rapid and accurate mercury analysis. The primary objective was to build a very simple and portable instrument for vehicular operation that would meet the requirements for extensive geochemical investigations in the field. The system (fig. 1) consists of a two-stage quartz-enclosed thermoamalgamator (~ operated by a radio-frequency induction heater (£) an absorption chamber (£) with a constant-temperature ultraviolet light source, and a b a 1anced bridge differential amplifier (~. Normally the instrument, which has a sensitivity of 1 part per billion in a 1-minute analytical period, requires a 0.25-gram rock sample crushed to 80-mesh size. The sample; upon being heated to approximately 500°C, releases mercury as a vapor. The mercury vapor is carried up the quartz tube by an airstream and selectively trapped by amalgamation with silver while contaminants pass through. The mercury is then thermally released from the amalgam into the absorption chamber and measured quantitatively. The instrument response is calibrated in parts per billion using a known volume of saturated mercury vapor. A T -valve allows the airstream to be directed through the chamber or diverted to bypass the chamber.

A functional diagram of the detector is shown in figure 2. Mercury vapor and other volatile materials (~) from the heated rock sample (£) are drawn into an upright quartz tube (£) where the separation and amalgamation take place. Dust and, to some extent, chemical contaminants are arrested by the lead-acetateimpregnated glass-wool filter plug (d). The mercury vapor from the airstream is extracted by the silver mercury trap (~. and con tam in at in g substances (smoke, particulate matter) are shunted through the bypass (!). The trap is then heated to 500°C, in approximately 7 seconds, and the mercury vapor is released through the absorption chamber. The unexcited atoms in the mercury vapor attenuate the light beam from the ultraviolet lamp (g), which reduces the current in the photocell (!!). This simultaneous action produces a meter reading at the amplifier proportional to the mercury concentration in the chamber. The mercury vapor and airstream are moved through the "gas train" in the instrument at a flow rate of 2,000 cubic centimeters per minute. Maximum absorption and peak meter reading are usually reached in about 5 seconds.

The amalgamative mercury-extraction stage, here called a thermoamalgamator, is built on a standard laboratory-type double-pole double-throw knife-blade switch on a bakelite base (fig. 3). A 3/4-inch hole to receive the quartz tube is drilled through the body of the switch parallel to the long axis and is located so that the detent positioning system is retained. Soft copper tubing, having 1/8-inchOD (outside diameter) and 0.030-inch wall thickness, is wound to form the radio-frequency induction heating coils, which have a (fl, .S/8-inch inside diameter with seven full turns spaced 0.015 inch apart. The coils are connected to the terminal posts at the ends of the switch with a No. 2 Exacto wire clamp. The coils serve to con centra t e the radio-frequency electromagnetic radiation in a pattern experimentally determined to be optimum for this heating application. The common switch terminals are connected to the output terminals of a 0.5-megacycle 750-watt induction heater with !.l-inch-square bus bars sawed from a 2 -.inch aluminum L-beam. The ends of the copper tubing extend one-half an inch past the clamp so that flexible plastic tubing may be attached for the cooling system. Water is circulated through the coils at a rate of 1 quart per hour.

Figure 1.-Mercury vapor detector, including noble-metal amalgamator (a}, radio-frequency induction heater (b), absorption chamber (c), and differential amplifier (d). -

Figure 2.- Functional diagram of mercury vapor detector, including mercury vapor and other volatile rnaterials (a), heated rock sample (b), upright quartz table (c), glass wool filter plug impregnated with lead acetate @l, silver mercury trap (~), bypass (fl, ultravioletlamp (g_), and photocell (~). rf, radio frequency .

The quartz tube (228 mm long by 15-mm OD by 1-mm wall thickness) containing the mercury trap and filter plug may be inserted after the axes of the copper-tube coils and the hole through the switch have been alined to allow 0.002-inch radial clearance between the tube and copper coils for thermal expansion. The quartz tube is constricted to hold a

perforated ceramic disk, size 00, which supports the silver. The silver is cut from 6-inch-square sheets, 0.001 inch thick, into strips one-eighth inch wide. Loosely folded, 1 gram of the strips is pressed down the quartz tube to form a right cylinder 1 inch long.

The amplifier and power supply are constructed on an aluminum chassis, 3x13x17 inches, which is attached to a standard 19-inch faceplate 10 inches high. The instrument controls that are used routinely and the indicating micro am met e r are mounted on the faceplate. The a b sorption chamber is constructed from two pieces of !4-inchaluminl\m L-beam, 2x3x18 inches, joined by a' rabbet along one edge of each piece. The two sections slide together and are held firmly by end plates recessed to fit the chamber cross section. Thus two walls of the chamber can be easily removed as a unit and the internal surfaces cleaned without disconnecting the circuitry.

A vaneaxial exhaust fan (1,300 rpm, 68 cfm) is attached to the ultraviolet-lamp end of the chamber through an adaptor plate. The fan is operated from a ¥ariac and is positioned so that the "interference from ultraviolet-absorbing ozone, which is created continuously by the lamp, will be removed from the light path. The mass and large surface area of the chamber serve as an effective heat sink to keep the ultraviolet -lamp at an even temperature and a constant intensity. The octal base of the photocell· at the opposite end of the chamber extends about 1 inch. through a semipermanent 0-ring seal for easy insertion in its receptica:l on· the amplifier chassis. The chamber assembly is attached to the amplifier by a mounting plate and four !4-inch wing bolts.

Tube fittings are placed at both ends of the chamber and connected to the T -valve with plastic tubing. The same tubing is used to connect the output of the amalgamative stage to the T -valve.

The electronic circuitry for the mercury vapor detector consists of a single-photocell balanced bridge and a high-gain twin-triode differential amplifier with both halves of the triode conducting at all times (Westinghouse Electric Corporation, 1948; Valley and Wallman, 1948; fig. 4). The photocell, which is physically in the absorption chamber, is electronically

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Figure 4.-Circuit diagram of mercury vapor detector. mu a, microamperes.

In the quiescent state, with no mercury present, the current in the photocell resulting from excitation by the ultraviolet lamp is balanced by adjusting R 1 and R2; this balance nullifies the circuit. The bias developed from the voltage drop across grid resistors R3, R4, and R5 controls the current in V and as there is no difference between the plate potentials, there is no meter indication. In this condition! the gr:ld voltage relative to the common cathode for both V1 and V2 should be slightly negative, -0.2 to i -0.4 volt de.

When mercury vapor enters the absorption cham-, her, strong absorption of the ultraviolet light occurs, 1 &nd the current in the photocell and the right half of . the bridge circuit is sharply reduced. The bias on V2 then becomes more negative while the plate potential increases in a positive direction and causes the meter to deflect to the right. This deflection is proportional to the mercury concentration in the absorption chamber when the instrument is calibrated.

A packaged 300-v de 50-milliampere solid-state power supply, as well as the supply diagrammed in figure 5, has been used satisfactorily with the differential amplifier.

The ultraviolet-lamp power supply is a ballasttype transformer available from the lamp manufacturer.

A. Induction heater (radio frequency) and amalgamative stage:

1. Ad just the water flow through radio-frequency
2. Turn the on-off switch (green light) on inducand V ; ·

coils to trickle (a few drops per second).

tion heater to on and allow 45 seconds for the instrument to reach ambient temperature. The high-voltage power switch (red light), which provides power (heat) for the sample holder and the silver mercury trap, may now be turned on and operated as desired. (For convenient operation, a foot switch can be substituted for the high-voltage power switch in the instrument.)

3. Set the knife-blade switch, which is on the

B. Amplifier unit:

1. Set the range switch selector to position 5.
2. Turn the on-off switch to on.
3. Adjust the variac blower control to 45.
4. Turn the on-off switch on the ultraviolet-lamp
5. As the ultraviolet lamp warms up, the emitted

C. The instrument response is divided irr.o five ranges, in order of decreasing sensitivity, with alternate range factors of 5 and 2. Note.-Range 2 seems to be the one most used for sample analysis; however, if the senritivity of this range is too great, a second or third equal measure of the sample may be run on a less sensitive range.

The mercury vapor detector is calibrated with mercury-saturated air, which is obtained b:r placing a few grams of mercury in a 60-milliliter serum bottle having a sleeve-type rubber-membrane stopper. The air is withdrawn from the bottle with a hypodermic syringe by puncturing the membrane to insert the needle. The membrane seals . itself hermetically as the needle is removed. Figure 6 is the temperature-sE'turation curve for mercury vapor in air. front of the radio-frequency induction unit, in the neutral position. The switch alternately applies power to the steel sample holder and the silver mercury trap. This action, coordinated with on-off high-voltage power switch, sequentially releases mercury from the rock in the sampleholdertothemercury trap, and from themercurytrap into the absorption chamber where the measurement takes place.

power supply to on. (The warmup time for the ultraviolet lamp is approximately 15 minutes.)

light becomes more intense, and the meter needle drifts to the left. The range switch may be turned to progressively lower numhers (greater sensitivity) and the fine zero control adjusted to maintain E'n approximate zero setting during this warrrup period.

B+ After the instrument has been adjusted to a stable zero reading, inject a sample of the air from the serum bottle into the lower end of the quartz tube with the hypodermic syringe. Turn on the high-voltage switch on the induction furnace and heat the silver mercury trap for approximately 10 seconds or until a peak meter reading has been observed. In a similar manner, inject

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different volumes of mercury-saturated air for all ranges of sensitivity until sufficient information has been obtained to prepare calibration curves of nanograms of mercury versus meter readings, in microamperes for each range.. The meter readingi~ may be used to numerically quantitize the unknown mercury value derived from a crushed rock sample. Erickson, R:'L., Marranzino, A. P., Oda, Uteana, and and Janes, W. W., 1964, Geochemical exploration near the Getchell mine, Humboldt County, Nevada: U.S. Geol. Survey Bull. 1198-A, p. A1-A26. Erickson, R. L., VanSickle, G. H., Nakagawa, H. M., McCarthy, J. H., Jr., and Leong, K. W., 1966, Gold geochemical anomaly in the Cortez district Nevada: U.S. Geol. Survey Circ. 534, 9 p. Friedrich, G. H., and Hawkes, H. E., 1966, Mercury as an ore guide in the P achuca- Real Del Monte district Hidalgo, Mexico: Econ. Geology, v. 61, no. 4, p. 744~ 753. Hawkes, H. E., and Williston, S. H., 1962, Mercury vapor as a guide to lead-zinc-silver deposits: Mining Cong. Jour., v. 48, no. 12, p. 30-32. Sau~ov, A. A., 1946, Geokhimiya rtuti [The geochem-IStry of mercury]: Acad. Sci. U.S.S.R., Inst. Geol. Sci., Trans. 78, Mineralog. -Geochem. Ser 17 129 p. '

Valley, G. E., and Wallman, Henry, 1948, Vacuumtube amplifiers: New York McGraw-HillBookCo Inc 743 p. , ., ., Vaughn, W. W., and McCarthy, J. H., Jr., 1964, An instrumental technique for the determination of submicrogram concentrations of mer~ury in soils, rocks, and gas, in Geological Survey J·esearch 1964: U.S. Geol. Survey Prof. Paper 501-D,p. 0123-0127. Westinghouse Electric Corporation, 1S'·~8. Industrial electronics reference book, prepare1 by electronic engineers of the corporation: NewYC'":"k, John Wiley & Sons, Inc., 680 p. Williston, S. H., 1964, Themercuryhakmethodof exploration: Eng. Mining Jour., v. 16f, no. 5, p. 98- 101. Williston, S. H., and Morris, M. H., 19(5, Method and apparatus for measurement of mercury vapors: U.S. Patent 3173016.

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