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A sensitive photo-acoustic mercury detector
Analytica Elsevier
Chimico Scientific
A SENSITIVE
136 (1982)
Acto,
Publishing
321-327
Company,
Amsterdam
PHOTO-ACOUSTIC
-. Printed
MERCURY
in Thr
Netherlands
DETECTOR
J. E. PATTERSON DSIR
Chemistry
(Keceived
Iliuision,
7th October
Petone
(New
Zealand)
1981)
Apparatus for the photo-acoustic determination of mercury vapour is described. A detection limit of better than 0.2 ng of mercury and a linear working range up to 150 ng can be rcaliscd in the presence of substantial levels of ambient noise. Nitrogen carrier gas effectively quenches the mercury fluorescence in a tubular cell attached along-
side a modulated fluctuations, in
germicidal
mercury
discharge
a partially closed cell, which
lamp. The released heat results in pressure are detected by using a microphone and
simple electronics. A narrow-band amplifier and a lock-in detector allow signals to be recorded. Very low cost and simplicity are important features. A gold foil collection is suitable for labor;rLov and field system IS used Lo increase selecLiviLy. The appararus applications.
The photo-acoustic effect [l] was applied to the optical study of gases by Tyndall in 1881 [2] but no account has been published incorporating the later observation by Wood [3] that secondary resonance radiation (fluorescence) of mercury vapour in an irradiated vessel, weakens as the pressure of admitted air rises. The energy removed from the exciting light by the mercury vapour is spent in increasing the velocity of air molecules and mercury atoms. In a closed vessel containing air, therefore, the pressure rises when traces of mercury are irradiated by a mercury light source. \Vith a modulated mercury light source, the resultant pressure fluctuations can be monitored with the aid of a microphone. Quenching of atomic fluorescence is most efficient when the electronic energy of excited atoms can be carried away by molecules [4] in the form of vibrational energy and ultimately as heat. Nitrogen is a practical gas to employ for this purpose. By taking advantage of the quenching process, the presence of mercury vapour in a nitrogen carrier can readily be dctccted at the nanogram level by using a microphone and a simple electronic circuit to monitor the pressure fluctuations in a suitable cell. This provides the basis for an inexpensive and simple mercury analyzer with potential applications both in the analytical laboratory and as a field exploration instrument. Two important factors contribute to the success of this method of mercury determination. Firstly, low-pressure mercury sources emit most of their light at 253.7 nm; secondly, because the photo-acoustic effect is a
0003-2670/82/0000-OOOO/$O2.75
Q 1982
Elsevier
Scientific
Publishing
Company
332
response to the total p;& of the spectrum
energy absorbed it will be stronger in the ultraviolet than in the visible and infrared regions. The spectral
energy distribution of a germicidal mercury lamp and the energy of a photon as a function of wavelength are shown in Fig. 1. These factors compound to provide a selective and sensitive means of detecting mercury vapour at trace levels. Roe and Cary [ 51 used photo-acoustic spectroscopy to detect mercury down to about 0.05 ng, but no description of their apparatus has been published. The simple equipment dcscribcd in this paper should allow this technique ‘ti be applied generally. Detection limits in acoustically noisy environments are better than 0.2 ng of mercury, with a linear working range of up to 150 ng. The equipment is readily combined with simple gold collection of mercury to improve selectivity. CONSTRUCTION
OF TIIE
SYSI’EM
Most germicidal mercury discharge lamps emit approximately 95% of their light at 253.7 nm. These lamps fit standard fluorescent light fittings and provide a good, if rather large, source of mercury resonance radiation. .4 simple tubular photo-acoustic cell can be devised which receives a significant proportion of the available radiation. The only additional optical component required is some aluminium foil for use as a multiple pass reflector and to prevent eye damage from emitted U.V. light. The cell is a 450-mm length of silica tubing (6.7 mm id., 0.7 mm wall thickness). It is bonded (Fig. 2) longitudinally to the germicidal lamp (Thorn I-iytck 15W 92-2013) with strips of adhesive tape which encompass only the plastic and parts of the lamp in order to avoid U.V. irradiation. Two plastic
100
Fig. 1. Photon energy plotted against wavelength. Also shown, in bar form, energy distribution of the linrs emitted by a germicidal mercury lamp.
is the relative
323
Fig. 8. _Arrnngcmcnt of componcrnts was mounted in a standard fluorescrnt
spacers
prevent glass-to-glass
aluminium 1S \V unit
of thr photo.ncoustic lamp holder.
mercury
detector.
‘I’tlc~ lamp
contact.
foil 0.02 mm thick, running on half-wave
The assembly is then enclosed in bright held on with bands of tape. The lamp is a 50 Hz rcctificd ax. with ;I stanckard 20 \V
ballast. The diode in series with the lamp avoids the need to derive a lOO-Iiz reference signal for the lock-in amplifier and gives a hotter defined off-period for the lamp. A push button replaces the usual starter for reliable operation on reduced power. The lamp should be of a type that does not also produce ozone. Care should be taken to ensure that the aluminium foil does not touch any live contacts. By noting the pin orientation of the lamp relative to the silica tube, the asscrnbly can be constructed so that it can rotate into a standard fitting thereby improving elcctriwl safety and convenience. The silica tube is tcrminakd at one end by a 3lillipore Swinnes 25mm polypropylene membrane filter assembiy and at the other end by a pyres T-tube. One arm of the T-tube is terminated by a second filter assembly while the other arm is linked to a brass microphone housing. The microphone housing is fabricated from a 0.5-in. Ml? brass union [6 ] which provides escellent isolation from estemal noise. The microphone is an inespensive 0.5-in. (nominal) clcctrct unit (ECM 1015) with a built-in preamplifier requiring a 1.5 V supply. This is provided from the power supply by using a light emitting diode as a voltage regulator. The electronic circuitry required (Fig. 3) is simple and uses commonly available devices. Hecause of the evolutionary nature of this design, further simplifications and improvements can hc made, such as a quad operational
Fig. 3. Circuitry for the photo-acoustic mercury detector. The power supply (not shown) WM a well filtered voltage doubler based on a 6.3-V filament transformer. A bypassed voltage divider and a T41C amplifier wired as a voltage follower impedance ground reference point, set at half the supply voltage.
provided
a low-
amplifier IC to replace the 741C amplifiers and FET switches in a better lock-in detector. The signal from the built-in microphone preamplifier is a.c.-coupled, using an internal 10 PF capacitor, to a 50 Hz twin T narrow band amplifier with its gain at the pass frequency set by the 470 kohm feedback resistor. It is fortunate that a 50 Hz filter can be realised using preferred component values as shown. For 60 Hz operation, the capacitors in the twin T filter should be altered to 0.56 PF. The output of the narrow band amplifier is a.c.-coupled to an amplifier with variable gain. This amplifier is then a.c.coupled to a simple lock-in detector [ 71 using a pair of small signal complcmentary silicon transistors apprcximately matched for gain. A low-pass filter provides a suitable output for a O-100 mV range chart recorder with a differential input. Switching the 470 kohm resistor to 47 kohm can reduce gain for high level determinations_ The 50-Hz reference signal for the lock-in amplifier is derived from the power supply transformer_ A phase shifter (Fig. 3) is necessary to match the timing of the sample and reference signals. The single W180° stage used here is sufficient because the recorder terminations can be reversed if the signal polarity is unsuitable. The reference amplitude should be about 3 V RMS. Correct phase can be set by using a continuous flow of nitrogen saturated with mercury vapour from a source of 0°C and adjusting the phase control for maximum signal amplitude. Readjustment should be unnecessary.
The membrane filters provide an acoustically enclosed cell assembly while permitting the passage of nitrogen and mercury vapour. Filters removed inside tubing from unused cigarettes, cut to 10 mm in length, and inserted with an internal diameter of 4.3 mm are equally effective. For .selectivity, gold collection of mercury is recommended, although release of mercu13 vnpour
with
sodium
tctrahydroborate
has provided
a direct
detection
limit
of better than 0.2 ng of mercury. The simple collector shown, based on a packed gold foil plug (length 25 mm) inside a silica tube (3.5 mm i.d.), gives a satisfactory performance with gas flame or clcctrical heating to drive off the collected mercury. A nichromc winding dissipating 40 \\: for 10 s was sufficient for sharp peaks. A nitrogen flow of 0.3 1 min-’ is about optimum, becausct higher flo\vs increase background noise while lower flows cause peak broadening. The apparatus is assembled on a 600 X 240 X 12 mm particle board base. No special acoustic isolation was used other than a foam rubber mounting for the microphone housing. Operation in a normal noisy laboratory environment is entirely practical ; the present equipment tolerates noise from fans, a vacuum pump, closing doors and collcagues.
In preliminary
trials,
vapour
from
the head
space
above
mercury
kept
at.
0°C was withdrawn through a rubber septum using a calibrated syringe. _-I small leak in the septum restored lost pressure resulting from the withdrawal of vapour. Results obtained with volumes of 1 ml are shown in Fig. 4:\ where each peak represents about 2 ng of mercury. Some losses at this level are inevitable, for example, by warming of the syringe by hand. The advantage of this procedure is speed which is zn asset when a new instrument is being optimized. \
.I-
-._-
uJLL -._rmc
-
---
Tume
Fig. 4.(A) Sequence of l-ml aliquots of mercury vapour in near equilibrium with mercury at 0°C; the pcnks rcprcscnt roughly 2 ng of mercury each, and the hascline spikcts occurred when the recorder was turned on or off. (B) Response for Z-16 ng of mercury evolved from the gold foil collector. Vapour samples were taken from mercury-saturated air in equilibrium with mercury at 0°C.
326
To obtain a calibration plot, volumes up to 8 ml of air saturated with mercury at 0°C were injected into the nitrogen carrier gas flowing at 0.3 1 min-i , collected on the gold plug, and released by electrical heating. Typical results are shown in Fig. 4B. Raising the mercury to 20°C showed that the calibration was linear up to at least 150 ng. To obtain a more accurate calibration over a wide range, it is necessary to use the slower chemical release of mercury by tin(I1) chloride reduction with prior gold collection. The results obtained in this way show an excellent linear response over the same range. Typically, mercury (ng) = 1.599 reading + 0.540, u = 0.999. DISCUSSION The apparatus described has high sensitivity even in the presence of substantial levels of ambient noise. A detection limit of better than 0.2 ng of mercury can be realised without special acoustic precautions_ Presumably, with such precautions, a much lower limit can be reached, although chemical difficulties will increase for any method at such levels. To reach extremely low concentrations at the expense of larger solution volumes, the gold foil preconcentration step is employed with the added advantage of increasing the selectivity for mercury_ The linear calibration range can be extended at the expense of poorer detection limits by increasing the tube diameter so the sample is diluted into a larger cell volume. The lost sensitivity could be restored by surrounding this tube by a number of light sources. The response time is also slowed, however, as the volume increases. The upper concentration limit is approached when extinction of the received light is nearly complete and no light can reach the axis of the sample tube. Another limit is the maximum vapour pressure of mercury allowed at the cell temperature. The apparatus described here is a simple, cheap but very sensitive unit for the determination of mercury. It is expected that such equipment could be used in geochemical exploration as an item of field equipment. Considerable scope exists for further development of this equipment for a wide range of applications_ The author thanks Byron Weissberg for his continued encouragement and glassblowers Keith Holden and Grant Franklin for glass and silica ware created during development of this equipment. REFERENCES 1 A. C. Bell, Philos. Mag., 11 (1881) 510. 2 J. Tyndall, Proc. R. Sot. London. 31 (1881) 307. 3 R. W. Wood, Physical Optics, 3rd revised edn., Dover pp.591.592.
Publications,
New
York,
1967,
4 G. F. Kirkbright and M. Sargent, Atomic Absorption and Fluorescence Academic Press. London, 1974, p. 91. 5 D. K. Hoe and R. A. Gary, 175th A.C.S. National Meeting March Abstract ANAL. 47. 6 H. E. Eaton and J. D. Stuart, Anal. Chcm., 50 (I 978) 587. 7 P. Williams. J. Sci. Instrum., 42 (1965) 474.
Spectroscopy, 13-17,
1978.
Chimico Scientific
A SENSITIVE
136 (1982)
Acto,
Publishing
321-327
Company,
Amsterdam
PHOTO-ACOUSTIC
-. Printed
MERCURY
in Thr
Netherlands
DETECTOR
J. E. PATTERSON DSIR
Chemistry
(Keceived
Iliuision,
7th October
Petone
(New
Zealand)
1981)
Apparatus for the photo-acoustic determination of mercury vapour is described. A detection limit of better than 0.2 ng of mercury and a linear working range up to 150 ng can be rcaliscd in the presence of substantial levels of ambient noise. Nitrogen carrier gas effectively quenches the mercury fluorescence in a tubular cell attached along-
side a modulated fluctuations, in
germicidal
mercury
discharge
a partially closed cell, which
lamp. The released heat results in pressure are detected by using a microphone and
simple electronics. A narrow-band amplifier and a lock-in detector allow signals to be recorded. Very low cost and simplicity are important features. A gold foil collection is suitable for labor;rLov and field system IS used Lo increase selecLiviLy. The appararus applications.
The photo-acoustic effect [l] was applied to the optical study of gases by Tyndall in 1881 [2] but no account has been published incorporating the later observation by Wood [3] that secondary resonance radiation (fluorescence) of mercury vapour in an irradiated vessel, weakens as the pressure of admitted air rises. The energy removed from the exciting light by the mercury vapour is spent in increasing the velocity of air molecules and mercury atoms. In a closed vessel containing air, therefore, the pressure rises when traces of mercury are irradiated by a mercury light source. \Vith a modulated mercury light source, the resultant pressure fluctuations can be monitored with the aid of a microphone. Quenching of atomic fluorescence is most efficient when the electronic energy of excited atoms can be carried away by molecules [4] in the form of vibrational energy and ultimately as heat. Nitrogen is a practical gas to employ for this purpose. By taking advantage of the quenching process, the presence of mercury vapour in a nitrogen carrier can readily be dctccted at the nanogram level by using a microphone and a simple electronic circuit to monitor the pressure fluctuations in a suitable cell. This provides the basis for an inexpensive and simple mercury analyzer with potential applications both in the analytical laboratory and as a field exploration instrument. Two important factors contribute to the success of this method of mercury determination. Firstly, low-pressure mercury sources emit most of their light at 253.7 nm; secondly, because the photo-acoustic effect is a
0003-2670/82/0000-OOOO/$O2.75
Q 1982
Elsevier
Scientific
Publishing
Company
332
response to the total p;& of the spectrum
energy absorbed it will be stronger in the ultraviolet than in the visible and infrared regions. The spectral
energy distribution of a germicidal mercury lamp and the energy of a photon as a function of wavelength are shown in Fig. 1. These factors compound to provide a selective and sensitive means of detecting mercury vapour at trace levels. Roe and Cary [ 51 used photo-acoustic spectroscopy to detect mercury down to about 0.05 ng, but no description of their apparatus has been published. The simple equipment dcscribcd in this paper should allow this technique ‘ti be applied generally. Detection limits in acoustically noisy environments are better than 0.2 ng of mercury, with a linear working range of up to 150 ng. The equipment is readily combined with simple gold collection of mercury to improve selectivity. CONSTRUCTION
OF TIIE
SYSI’EM
Most germicidal mercury discharge lamps emit approximately 95% of their light at 253.7 nm. These lamps fit standard fluorescent light fittings and provide a good, if rather large, source of mercury resonance radiation. .4 simple tubular photo-acoustic cell can be devised which receives a significant proportion of the available radiation. The only additional optical component required is some aluminium foil for use as a multiple pass reflector and to prevent eye damage from emitted U.V. light. The cell is a 450-mm length of silica tubing (6.7 mm id., 0.7 mm wall thickness). It is bonded (Fig. 2) longitudinally to the germicidal lamp (Thorn I-iytck 15W 92-2013) with strips of adhesive tape which encompass only the plastic and parts of the lamp in order to avoid U.V. irradiation. Two plastic
100
Fig. 1. Photon energy plotted against wavelength. Also shown, in bar form, energy distribution of the linrs emitted by a germicidal mercury lamp.
is the relative
323
Fig. 8. _Arrnngcmcnt of componcrnts was mounted in a standard fluorescrnt
spacers
prevent glass-to-glass
aluminium 1S \V unit
of thr photo.ncoustic lamp holder.
mercury
detector.
‘I’tlc~ lamp
contact.
foil 0.02 mm thick, running on half-wave
The assembly is then enclosed in bright held on with bands of tape. The lamp is a 50 Hz rcctificd ax. with ;I stanckard 20 \V
ballast. The diode in series with the lamp avoids the need to derive a lOO-Iiz reference signal for the lock-in amplifier and gives a hotter defined off-period for the lamp. A push button replaces the usual starter for reliable operation on reduced power. The lamp should be of a type that does not also produce ozone. Care should be taken to ensure that the aluminium foil does not touch any live contacts. By noting the pin orientation of the lamp relative to the silica tube, the asscrnbly can be constructed so that it can rotate into a standard fitting thereby improving elcctriwl safety and convenience. The silica tube is tcrminakd at one end by a 3lillipore Swinnes 25mm polypropylene membrane filter assembiy and at the other end by a pyres T-tube. One arm of the T-tube is terminated by a second filter assembly while the other arm is linked to a brass microphone housing. The microphone housing is fabricated from a 0.5-in. Ml? brass union [6 ] which provides escellent isolation from estemal noise. The microphone is an inespensive 0.5-in. (nominal) clcctrct unit (ECM 1015) with a built-in preamplifier requiring a 1.5 V supply. This is provided from the power supply by using a light emitting diode as a voltage regulator. The electronic circuitry required (Fig. 3) is simple and uses commonly available devices. Hecause of the evolutionary nature of this design, further simplifications and improvements can hc made, such as a quad operational
Fig. 3. Circuitry for the photo-acoustic mercury detector. The power supply (not shown) WM a well filtered voltage doubler based on a 6.3-V filament transformer. A bypassed voltage divider and a T41C amplifier wired as a voltage follower impedance ground reference point, set at half the supply voltage.
provided
a low-
amplifier IC to replace the 741C amplifiers and FET switches in a better lock-in detector. The signal from the built-in microphone preamplifier is a.c.-coupled, using an internal 10 PF capacitor, to a 50 Hz twin T narrow band amplifier with its gain at the pass frequency set by the 470 kohm feedback resistor. It is fortunate that a 50 Hz filter can be realised using preferred component values as shown. For 60 Hz operation, the capacitors in the twin T filter should be altered to 0.56 PF. The output of the narrow band amplifier is a.c.-coupled to an amplifier with variable gain. This amplifier is then a.c.coupled to a simple lock-in detector [ 71 using a pair of small signal complcmentary silicon transistors apprcximately matched for gain. A low-pass filter provides a suitable output for a O-100 mV range chart recorder with a differential input. Switching the 470 kohm resistor to 47 kohm can reduce gain for high level determinations_ The 50-Hz reference signal for the lock-in amplifier is derived from the power supply transformer_ A phase shifter (Fig. 3) is necessary to match the timing of the sample and reference signals. The single W180° stage used here is sufficient because the recorder terminations can be reversed if the signal polarity is unsuitable. The reference amplitude should be about 3 V RMS. Correct phase can be set by using a continuous flow of nitrogen saturated with mercury vapour from a source of 0°C and adjusting the phase control for maximum signal amplitude. Readjustment should be unnecessary.
The membrane filters provide an acoustically enclosed cell assembly while permitting the passage of nitrogen and mercury vapour. Filters removed inside tubing from unused cigarettes, cut to 10 mm in length, and inserted with an internal diameter of 4.3 mm are equally effective. For .selectivity, gold collection of mercury is recommended, although release of mercu13 vnpour
with
sodium
tctrahydroborate
has provided
a direct
detection
limit
of better than 0.2 ng of mercury. The simple collector shown, based on a packed gold foil plug (length 25 mm) inside a silica tube (3.5 mm i.d.), gives a satisfactory performance with gas flame or clcctrical heating to drive off the collected mercury. A nichromc winding dissipating 40 \\: for 10 s was sufficient for sharp peaks. A nitrogen flow of 0.3 1 min-’ is about optimum, becausct higher flo\vs increase background noise while lower flows cause peak broadening. The apparatus is assembled on a 600 X 240 X 12 mm particle board base. No special acoustic isolation was used other than a foam rubber mounting for the microphone housing. Operation in a normal noisy laboratory environment is entirely practical ; the present equipment tolerates noise from fans, a vacuum pump, closing doors and collcagues.
In preliminary
trials,
vapour
from
the head
space
above
mercury
kept
at.
0°C was withdrawn through a rubber septum using a calibrated syringe. _-I small leak in the septum restored lost pressure resulting from the withdrawal of vapour. Results obtained with volumes of 1 ml are shown in Fig. 4:\ where each peak represents about 2 ng of mercury. Some losses at this level are inevitable, for example, by warming of the syringe by hand. The advantage of this procedure is speed which is zn asset when a new instrument is being optimized. \
.I-
-._-
uJLL -._rmc
-
---
Tume
Fig. 4.(A) Sequence of l-ml aliquots of mercury vapour in near equilibrium with mercury at 0°C; the pcnks rcprcscnt roughly 2 ng of mercury each, and the hascline spikcts occurred when the recorder was turned on or off. (B) Response for Z-16 ng of mercury evolved from the gold foil collector. Vapour samples were taken from mercury-saturated air in equilibrium with mercury at 0°C.
326
To obtain a calibration plot, volumes up to 8 ml of air saturated with mercury at 0°C were injected into the nitrogen carrier gas flowing at 0.3 1 min-i , collected on the gold plug, and released by electrical heating. Typical results are shown in Fig. 4B. Raising the mercury to 20°C showed that the calibration was linear up to at least 150 ng. To obtain a more accurate calibration over a wide range, it is necessary to use the slower chemical release of mercury by tin(I1) chloride reduction with prior gold collection. The results obtained in this way show an excellent linear response over the same range. Typically, mercury (ng) = 1.599 reading + 0.540, u = 0.999. DISCUSSION The apparatus described has high sensitivity even in the presence of substantial levels of ambient noise. A detection limit of better than 0.2 ng of mercury can be realised without special acoustic precautions_ Presumably, with such precautions, a much lower limit can be reached, although chemical difficulties will increase for any method at such levels. To reach extremely low concentrations at the expense of larger solution volumes, the gold foil preconcentration step is employed with the added advantage of increasing the selectivity for mercury_ The linear calibration range can be extended at the expense of poorer detection limits by increasing the tube diameter so the sample is diluted into a larger cell volume. The lost sensitivity could be restored by surrounding this tube by a number of light sources. The response time is also slowed, however, as the volume increases. The upper concentration limit is approached when extinction of the received light is nearly complete and no light can reach the axis of the sample tube. Another limit is the maximum vapour pressure of mercury allowed at the cell temperature. The apparatus described here is a simple, cheap but very sensitive unit for the determination of mercury. It is expected that such equipment could be used in geochemical exploration as an item of field equipment. Considerable scope exists for further development of this equipment for a wide range of applications_ The author thanks Byron Weissberg for his continued encouragement and glassblowers Keith Holden and Grant Franklin for glass and silica ware created during development of this equipment. REFERENCES 1 A. C. Bell, Philos. Mag., 11 (1881) 510. 2 J. Tyndall, Proc. R. Sot. London. 31 (1881) 307. 3 R. W. Wood, Physical Optics, 3rd revised edn., Dover pp.591.592.
Publications,
New
York,
1967,
4 G. F. Kirkbright and M. Sargent, Atomic Absorption and Fluorescence Academic Press. London, 1974, p. 91. 5 D. K. Hoe and R. A. Gary, 175th A.C.S. National Meeting March Abstract ANAL. 47. 6 H. E. Eaton and J. D. Stuart, Anal. Chcm., 50 (I 978) 587. 7 P. Williams. J. Sci. Instrum., 42 (1965) 474.
Spectroscopy, 13-17,
1978.