GEOLOGICAL SURVEY CIRCULAR 540 

A Simple Mercury 

Vapor Detector for 

Geochemical Prospecting 

A Simple Mercury 

Vapor Detector for 

Geochemical Prospecting 

By W. W. Vaughn 

----------------------------~ GEOLOGICAL SURVEY CIRCULAR 540 

Washington 1967 
United States Department of the Interior 

ROGERS C. B. MORTON, Secretary 

Geological Survey 

William T. Pecora, Director 

First printing 1967 

Second printing 1968 

Third printing 1971 

Free on application to the U.S. Geological Survey, Washington, D.C. 20242 
CONTENTS 

Page 

Abstract----------------------------------

Page 

1 

1 

1 

1 

Instrument design (electronic)--------------- 5 

Introduction -------------------------------

General description------------------------

Instrument design (physical) ----------------

Instrument operation----------------------- 6 

Calibration-------------------------------- 6 

References-------------------------------- 8 

ILLUSTRA TI~ONS 

Page 

Figure 1. Photograph of mercury vapor detector-------··----------------------------------------- 2 

R1 

R2 

R3 

R4 

Rs 

R6 

R7 

R8 

R9 

R1o 

Rll 

2. Functional diagram of mercury vapor detector----------------------------------------- 3 

3. Photograph of amalgamative mercury-extraction stage (thermoamalgamator) ------------- 4 

4. Circuit diagram of mercury vapor detector-------------------------------------------- 5 

5. Circuit diagram of power supply ----------------------------------------------------- 6 

6. Temperature-saturation curve for mercury vapor in air-------------------------------- 7 

PARTS LIST 

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 R5 

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; 

200 ohms 

Ill 

R12 

R13 

R_14 

R1s 

R16 

Rl7 

V1, Vz 

M1 

p1 

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 

A Simple Mercury Vapor Detector for Geochemical Prospecting 

By W. W. Vaughn 

ABSTRACT 

The detector utilizes a large-volume atomic-absorption technique for quantitative determinations of mercury vapor thermally 

released from crushed rock. A quartz-enclosed noble-metal 

amalgamative stage, which is temper at u r e controlled and is 

actuated by a radio-frequency induction heater, selectively traps 

the mercury and eliminates low-level contamination. As little as 

1 part per billion of mercury can be detected in a 1-gram sample 

in a 1-minute analytical period. 

INTRODUCTION 

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. 

GENERAL DESCRIPTION 

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 1 anced 

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 

1 

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. 

INSTRUMENT DESIGN (PHYSICAL) 

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 
N 

Figure 1.-Mercury vapor detector, including noble-metal amalgamator (a}, radio-frequency induction heater (b), absorption 

chamber (c), and differential amplifier (d). -
f 

h -Exhaust 

Amplifier 

0 0 0 0 0 

Power supply 

High-temperature glass rod 

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 . 

.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. 

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 

• 

3 

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 
Figure 3.-Amalgamative mercury-extraction !jta:ge (thermoamalgamator). 

4 
¥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. 

INSTRUMENT DESIGN (ELECTRONIC) 

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 

connected as one arm of the bridge circuit. Two 

series variable resistors (R ( and·R2 )1in th~ opposite 

arm are the coarse and fine zero controls for the instrument. The lower arms .of the bridge circuit (R3, 

R4,· and Rs) are also the bia·sing resistors for the 

amplifier. · 

A 6201- VS;CUUm tube, a ruggedized ver~ion of a 

12AT7 with low _microphonics, is connected as a tlifferential amplifier. The overall circuit volta~e is adjusted by R 15. a variable bleeder resistor in the 

voltage divider system of the power supply. The current in the two triodes of .the differential arn..,lifier is 

normalized by adjusting the plate voltage on each tube 

with a common variable resistor (R6)~ The adjustment 

of Rs in the lower right-hand ar.mofthe bridge allows 

for the change in sensitivity which is exp!ec.ted when 

the photocell ·is replaced or has lost sensitivity owing 

to natural aging. All the variable circuit parameters 

are interrelated and must be alternately ~c justed to 

attain optimum--instrument pedormance .. A 10-microampere taut band meter, shunted by the appropriate 

range resistors, between the plates of the triodes indicates 'the amplified change in current irfthe bridge 

circuit caused by absorption of the ultraviolet light in 

the chamber. 

r---------------------------------~--~~--~-++..~vdc 

8-

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

5 
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 1 and V 2; · 

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 V 1 and V 2 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 

V 2 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. 

INSTRUMENT OPERATION 

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

1. Ad just the water flow through radio-frequency 

coils to trickle (a few drops per second). 

2. Turn the on-off switch (green light) on induction 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 

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. 

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 

power supply to on. (The warmup time for 

the ultraviolet lamp is approximately 15 

minutes.) 

5. As the ultraviolet lamp warms up, the emitted 

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. 

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. 

CALIBRATION 

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. 

20 h-40 ma 

~~1111 450 v 

40ma 

choke 2,500 ohms 3,100 ohms 

2 mf 2 mf 

600 v 600 v 

B+ 

300-v de 

OA2 regulator tube 

OA2"regulator tube 

8-

~Figure 5.-Circuit diagram of power supply. h, henrys; rna, milliamperes; mf, microfarads. 

6 
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 

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. 

50~---------------~--------------~---------------~~---------------~---------------~---------------~--------------~ 

a: 

1&.1 

~ 

1&.1 

::E 

i= 

z 

~20 

0 

iii 

::J 

0 

a: 

1&.1 

Q. 

en 

::E 

4( 

a: 

CJ 

0 

~ 10 

z 

~ 9 

z 

0 

i 7 

~ 

z 

1&.1 

0 

z 

0 

0 

> a: 

::J 

0 

a: 

1&.1 

::E 

3 

0~--------~5~---------1~0~--------~1~5~--------~20~--------~25~--~~--~~~--------~35 

TEMPERATURE, IN DEGREES CENTIGRADE 

Figure 6.-Temperature-saturation curve for mercury vapor in air. 

7 
REFERENCES 

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 geochemIStry of mercury]: Acad. Sci. U.S.S.R., Inst. Geol. 

Sci., Trans. 78, Mineralog. -Geochem. Ser 17 

129 p. • , 

8 

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. 

U.S. GOVERNMENT PRINTING OFFICE: 1971 0- 425-214