Sensitive Simple Mercury Photometer Using Mercury Resonance Lamp as a Monochromatic Source Clement Ling Kodak (Australasia) Pty., Ltd., Research Laboratories, P . 0 . Box 90, Coburg, Victoria, AustraIia
A sensitive mercury photometer using a mercury resonance lamp as a source of sharp monochromatic resonance radiation is described. Parameters pertaining to sensitivity are discussed. Null method of increasing the sensitivity is also described. The potential application of the photometer in mercury isotopic analysis is suggested. Method for correcting nonatomic absorption is also proposed by means of which mercury may be analyzed in the presence of interfering substances. The instrument is compact enough to be portable. The detection limit (for 1% absorption) of the single beam system is 0.3 nanogram.
A MERCURY PHOTOMETER basic9lly consists of a source emitting mercury resonance line 2537 A, a n absorption cell into which mercury vapor is introduced, and a photoelectric detector to which an output indicator is connected. All mercury photometers exploit the fact !hat mercury vapor absorbs resonance radiation a t 2537 A. The conventional mercury photometer employs a low pressure mercury vapor lamp as a source of 2537 A emission line which is isolated from the undesirable attendant emission lines by means of a filter and/or narrow band pass photoelectric detector. The main disadvantages are: 1 . The low pressure Hg vapor lamp needs the application of heat to produce a satisfactory Hg vapor pressure for operation; its emission is then maintained by a n arc discharge; both of these are subject to temperature variation. 2. The emitted resonance line is broadened and selfreversed and optimum sensitivity cannot be achieved under this condition. 3. 6 h e method of isolating the resonance wavelength 2537 A is not completely satisfactory, because it does not remove completely the unwanted nonresonance energy, and in addition it reduces the intensity of the resonance wavelength. This causes a decrease in sensitivity and introduces nonlinearity in the calibration curve and therefore results in less accuracy. It will be shown later that, by considering the parameters of Beer’s law, improvement in sensitivity can be achieved by optimizing the geometry of the absorption cell. Also, it will be seen that the above disadvantages can be eliminated and ultimate spectral sensitivity can be achieved by the use of a mercury resonance lamp as a source of sharp resonance radiation. The properties of the mercury resonance lamp have been extensively investigated by Wood ( I ) , who in 1912 first made $ quantitative study of absorption of mercury vapor a t 2537 A a t room temperature; many investigators (2-4) subsequently made studies on the absorption coefficient of mercury vapor a t 2537 A but so far no commercial instrument has made use I ) K. W. Wood, Phil. Mug.. 3,690 ( 1 912). ( 2 ) A. L. Hughes and A. R. Thomas, Phys. ReL:, 30, 466 (1927). ( 3 ) A. R. Thomas, P h y s . R w . 3 5 , 2253 (1930). (4j .% W. %rmanskq.Ph.ys. I. Rec.,X, 219(1930).
of the mercury resonance lamp as a sharp monochromatic source. Consideration of the sharp line monochromatic emission of the mercury resonance lamp suggests its application also in the construction of a simple instrument for analyzing mercury isotopes. PRINCIPLE
Normal atoms, when excited by a suitable exciting source, will undergo transitions mainly involving the ground states, and will spontaneously re-emit energy corresponding to this transition in ail directions and the energy re-emitted is the resonance wavelength. A mercury resonance lamp is a highly evacuated fused silica vessel containing mercury vapor a t room temperature. The resonance lamp when irradiated by a suitable mercury arc lamp--e.g., low pressyre-type meLcury lamp-will emit reson:nce radiations 1850 A and 2537 A in all directions (the 1850 A will be absorbed by oxygen under normal conditions). The line width emitted by this type of resonance lamp a t room tempe!ature is determined by Doppler’s effect only (about 0.002 A), and is a constant, irrespective of the line width of the exciting source. (Change in the line width of the exciting source will cause a change in the emitted intensity of the resonance lamp only.) In atomic absorption spectroscopy, measurement is made of the absorption coefficient at the centre of the absorption line (5)and the spectral requirements provide that the emission line width must be smaller than the absorption line width and that it must be constant. These spectral requirements are met with by the resonance lamp which offers, in addition, monochromatic radiation in all directions. The resonance lamp thus disposes of the necessity of monochromatic devices such as a monochromator, filter or narrow band pass photodetector. The fact that the lamp emits in all directions may be exploited to great advantage: More cells may be added around the resonance lamp thus making possible simultaneous multiple analysis (6). A much simplified double beam system may be instituted-e.g., beam splitting and precise optics are unnecessary. DESCRIPTION OF THE PHOTOMETER
The photometer constructed (see Figures 1, 2, and 3) consists of a n exciting source, a mercury resonance lamp, an absorption cell, and a photodetector enclosed in lightproof housing to exclude ambient radiation. The exciting source (a G.E. Ozone lamp GE OZ 4511) whose radiation is collimated by a 3-cm focal length “suprasil” lens, irradiates the mercury rFsonance lamp which re-emits resonance radiation 2537 A in all directions. The re-emitted resonance radiation from the resonance lamp is collected in two channels opposite t o each other: one, the sample path, the other, a reference path.
(
798
ANALYTICAL CHEMISTRY
( 5 ) A. Walsh, Spectrol,him. Ar/a. 7, io8 (1955). 16) C. Ling, invention Report K 27951 i iY66i Patent aoplieci
Tor.
Exciting Source
8
Cell Detector Resonance Lamp
Figure 1. Schematic representation of photometer
The reference path is used only when the high sensitivity of the “null system’’ is required. In the reference path there is a light guide with windows made from Wratten 18B glass filter, and an attenuator for balancing the energy against the sample beam. The attenuator is provided with fine and coarse adjustment. The sample path contains a n absorption cell (l-cm diameter x 35 cm length) into which mercury vapor is introduced. The cell has two windows, one of fused silica and situated in the resonance lamp end, while the other, made of Wratten 18B glass filter, is placed at the detector end. The cell has a side arm at the center which accepts a sample tube (Quickfit MF24/0) and two other side a r m near the ends of the cell, each connecting with a stopcock which should be closed during analysis. One stopcock is connected to a vacuum pump for removal of mercury. The windows are held by nuts screwed onto detachable collets which grip the cell. The collets can be removed when the cell needs cleaning. The detectors used in both beams are RCA 1P28 phot? multipliers, and are selected so that their sensitivity a t 2537 A is roughly matched. The more sensitive photomultiplier should always look a t the reference path because of the provision made for attenuation.
M g n Considerations. The design of the photometer is aimed at optimum sensitivity, minimum noise, and compliance of Beer’s law. The relevant parameters are itemized and discussed as follows: CELLDIMENSIONS. There are two conditions under which mercury may be analyzed: Mercury vapor in terms of weight regenerated by heat after it has been extracted by a collectore.g., cadmium sulfideimpregnated asbestos (7).gold leaf, palladium chloride (8), etc. ; mercury concentration-i.e., weight of mercury vapor in a given volume of matrix. By considering Beer’s law, it may be seen that the a b sorbance value for a given weight of mercury vapor is independent of the path length of the cell, but is inversely proportional to the cross-sectional area of the cell. Thus,. the sensitivity of the instrument may be increased simply by reducing the diameter of the cell. In practice, it is limited by signalinoise consideration and the fact that (7) A. E. Ballad and C. D. W. Thornton, IND.EM. C H ~ .ANAL. , Eo.,13,893 (1941). (8) C. H. James and J. S. Webb, Instifurion of Mining & MefoC lurgy, 633 (1964).
. . ,.
I
t Figure 2. hull sv-tern, ~irnpI-1miwurv phntorneter. Agscmhled v i e r
VOL 39, Nr.. 7, J !tat i r 5 7
799
Figure 3. Null system, simple mercwy photometer. Exploded view the cell n lust have a minimum volume before mercury will diffuse fratm the sample tube into the cell. For thi!itype of analysis, the convenient dimensions of the cell are 1 (:m in diameter and about 35 cm in length. In estirnating mercury in air without prior treatment, for example, the sensitivity depends only on the length ofthecell and is liniited by the signal/noise consideration only. STRAY 1LADIATION. The presence of stray radiation causes an undesi rable curvature and reduction in slope of the analytical calibration curves. It is important therefore to reduce the;amount of stray radiation to a minimum. Quantit,ariue Examination of Siray Radiation. There are four nlain sources of stray radiation: 1. Am1iient radiation 2. Refl,:cted radiation from the exciting source 3. Flucirescence of the fused silica excited by UV radiation 4. Fluairescence of mercury vapor in the resonance lamp These rnay be individually identified as follows: The a nibient radiation is measured by the output of the photomul tiplier with the source off. The reflected radiation from the exciting source is measured by introducing a blank resonance lamp (a lamp which contains no mercury vapor) instead of the resonance lamp, and measuring the output with the souriz on. The visible component of the reflected radiation from the exciting source may be obtained by inserting a U V absorbing filter-e.g., ordinary crown glassbetween 1the exciting source and the photomultiplier (this assumes I:here is no ambient radiation; however, correction for ambient radiation may be applied where necessary). Finally,, the output due to the fluorescence of fused silica and mercury vapor in the resonance lamp may be obtained from the difference of the output measured with the UV filter placed between the exciting source and the resonance lamp, ani1 the output with the UV filter placed between the resonance: lamp and the photomultiplier. (Fluorescence of mercuiy vapor is caused by the transitions of the higher energy leigels. These take place only if the transition from the grout id state is first initiated. By removing the energy at 2537 PL the transition from the ground state is prohibited and therefore no fluorescence will take place.) Three Itypes of stray radiation from the resonance lamp with the c:xciting source were identified with a spectrograph. The re:sonance lamp excited by a G.E. germicidal lamp was expo,sed . .in the spectrograph for one hour. The s r c t r o gram sevealed two nonresonance mercury lines 43% A and 4047 A and an over-exposed resonance line 2537 A together with faint mercurx lines in \he ultraviolet region. The mercury lines 4358 A and 4047 A may be due to the reflected ~
~~
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ANALYTICAL CHEMISTRY
radiation from the exciting source (Type 2) and/or may be due to fluorescence of the mercury vapor in the resonance lamp. A spectrogram taken of the resonance lamp under the same exposure condition but with the UV filter placed between the exitin$ source an$ the resonance lamp, revealed that the 4358 A and 4047 A lines were slightly reduced in intensity, whereas the resonance lines 2537 A and the mercury lines in the ultraviolet region were completely absent. It was concluded therefore that the stray radiation was largely reflected. Quantitatiue Eoaluarion of the Various Types of Stray Radiation. It should be noted that the magnitude of the types of stray radiation is largely dependent on the geometric desim of the housing and on the soectral reswnse characteristicof the detector. As TVLX1 radiation is associated with the diffuse nature of the ex$& source, it was decided to evaluate it with and without collimation. This was performed by inserting a short (3 cm) focal length “suprasil” lens between the source and the resonance lamp. The Types 3 and 4 radiation are not so easily controlled as they are associated with the resonance line (2537 A). Reduction of this radiation can be achieved by using an a b sorption cell with windows made of Wratten 188 filters. Wratten 18B filter cuts off mainly the visible and transmits 2537 A. As can be seen from the data of Table I: 1. The use of an absorption cell increased the resonance lamp output by a factor of 10 and collimation with the lens further increased the output by another factor of two. 2 & 3. These tests indicated the magnitude of stray radiation due to fluorescence of the fused silica and fluorescence of the mercury vapor in the resonance lamp. Further, the collimator had completely removed the stray radiation due to the reflected visible component of the exciting source. 4 & 5. The system in which the absorption cell had 18B windows and in which the resonance lamp was excited by collimated radiation was most satisfactory for minimizing the stray radiation. The stray radiation measured so far belonged only to the visible components. Measurement of Stray UV Radiation. A quantity of mercury vapor sufficient to absorb all the resonance radiation was under these.~ conditions the total introduced into the cell; . . stray radiation was found to be 1.5% T. By comparison with the visible stray radiation as shown in Table I (5 d), the UV component was considerably higher. This was because the 18B filter reduced 2537 A by 60% and U V radia~
1. 2. 3.
4.
5.
Table I. Output of Resonance Radiation from Resonance Lamp and Stray (Nonresonance) Radiation Exciting Source: G.E. Ozone Lamp, 18.5 V, 300 mA E.H.T. to Photomultiplier = 500 V (nominal) output 2 Stray radiation With Without With Without Types of Radiation collimation collimation collimation collimation Resonance and stray Total output from resonance lamp 2 . 0 2 volts 1 . 1 volt a. Without absorptioi cell 19.5 volts 1 0 . 2 volts b. With absorption cc:ll ... 0.5 Not Type 1 only 50 mV UV absorbing filter bztween exciting detectable source and resonance lamp (with windowless ct:ll) 0.8 150 mV 2.2 225 mV Types 1 , 2, & 3 UV absorbing filter between resonance lamp and windowless cell Resonance and stray Without UV absorbing filter 1 7 . 2 volts 9 volts a. Cell with one fused silica window 15.4volts 7 . 9 volts b. Cell with two fused silica windows 1 2 . 2 volts 6 . 1 volts c. Cell with one 18B filter window 7 . 5 volts 3 . 8 volts d. Cell with two 18B filter windows Types 1 , 2, and 3 With UV filter in pos tion 3 2.2 0.7 125 mV 200 mV a. Cell with one fused silica window 1.9 0.8 125 mV 150 mV b. Cell with two fused silica windows 0.4 0.8 50 mV 50 mV c. Cell with one 18B filter window 0.7 0.3 20 mV 25 mV d. Cell with two 18B filter windows
tion between 3000 to 3600 A by about 3 0 x . By substituting one 18B window with fused silica, the total stray radiation was reduced by half to about 0.7%. The 18B window was placed a t the photomultiplier end t o provide additional protection against ambient radiation. EXCITING SOURCE. Spectrally, the requirement of an exciting source is that it must emit sufficient energy in the region corresponding to the ;ibsorption line width in the resonance lamp so that it will rem-emit the resonance energy with sufficient intensity. Somc broadening of the line and partial self-reversal in the exciting source will not affect the spectral purity of the resonance line of the resonance lamp, as the line width in a resonance lamp is a constant. Moreover, it is essential that the exciting source must be stable. The physical shape of the source and also its power requirements should also be considered. Although no exhau:,tive attempt has been made in search for such a lamp commercially available, we have found a G.E. Ozone lamp meets the requirements admirably. The Ozone lamp was found to strike a t 18.5 V and run at a current of about 300 mA. It should be noted, however, that there is a variation in the individual lamps. The lamp may be connected directly t o a dry cell or accumulator. (Further information may be obtained from G.E. Lamps Bulletin.) MERCURY RESONANCE LAMP. A mercury resonance lamp is a highly evacuated fused silica vessel containing mercury vapor a t room temperature. The mercury atoms are capable of absorbing resonance radiation from a mercury exciting source, and spontaneously re-emit the resonance radiation in all directions. Fluorescence may occur in the resonance lamp if the excited atoms are further excited by nonresonance radiation from the exciting source (if the intensity of the source is strong enough). This undesirable property may be minimized by reducing the current to the exciting source. The presence of a foreign gas-e.g., H2,co2-(9)will cause quenching of the resonance radiation. Quenching is a process involving collisions of the second kind in which the excited atom collides with a normal atom or a molecule, and gives up a quantum of energy t o the unexcited atom. This (9) A. C. G . Mitchell and M , W. Zemansky, “Resonance Radiation and Excited Atoms,” Cambridge Press, 1934, p. 187.
converts it to translational energy, hence no resonance radiation is emitted. For this reason, the resonance lamp must be carefully constructed so that the vessel is thoroughly clean and highly evacuated. In a n early model, Araldite was used t o cement the parts of the lamp and, after a few weeks, the output from the lamp was reduced considerably. It was possible that there may have been a leakage in the lamp; however, the possibility that foreign gases were released from the Araldite a s a result of the prolonged intense UV irradiation cannot be dismissed. The resonance lamp was constructed subsequently by fusion of the fused silica. No deterioration of the re-emitted energy was evident. The body of the lamp is a fused silica cylinder. It will be seen that a “null system” may be readily applied. The entrance window is of plane fused silica. To reduce internal reflection of the exciting beam t o a minimum, the cylinder is made longer than the diameter of the absorption cell. Ideally the cylinder should have a tapering end, as stray radiation due to internal reflection of the exciting beam is completely eliminated by this geometry. The most intense region of the re-emitted radiation in the lamp is at the layer immediately incident to the exciting beam. The absorption cell window should therefore preferably be directed to this region. The exciting beam is collimated by the lens and aperture so that the beam will graze the cylinder of the lamp. The amount of mercury vapor needed in the lamp is only the mercury vapor pressure a t room temperature. However, to avoid clean-up of the atoms a few milligrams of clean mercury is introduced by distillation. Because the lamp contains a low pressure of mercury vapor (-0.001 mm) a t room temperature, the emitted line width is determined entirely by Doppler’s effect. The absorption line width under analytical conditions, in addition to Doppler broadening, is broadened by collisions with the air molecules which are present in the absorption cell, together with the mercury vapor. Therefore, optimum sensitivity is reached and Beer’s law will be obeyed (5). PHOTOELECTRIC DETECTOR AND INDICATOR.An RCA 1P28 photomultiplier was used as a detector. The output from the resonance lamp and the sensitivity of 1P28 enabled a microammeter to be used as the current measuring indicator, The output may also be connected t o a recorder directly. The 1P28 photomultiplier must be operated a t a minimum of 500 V. Care must be taken to ensure that. the output VOL. 39, NO.
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801
from the photomultiplier does not exceed 20 V. Above this value the photomultiplier will be saturated. If the voltage to the 1P28 is already at a minimum of 500 V, its output may be conveniently reduced by decreasing the intensity of the exciting source by reducing its operating current or by the use of an optical attenuator. A filter is not recommended for the reason discussed previously under Stray Radiation. Null System. The photometer is conveniently adaptable to null method of measurement as illustrated in the prototype
%T 1 2 3 4 5 6 1 2 3 4 5 6 7 8 9 10
Blank
0.0003 (0.3 ns)
model. The conventional null system is operated by balancing the output of the sample and reference channels so that the noise from each channel is minimized. If the two photomultipliers are connected to a common power supply, then the noise due to this factor can be nullified. Using such a system the limiting factor in obtaining high gain is only the difference in the variation of the photomultiplier characteristics. The principle is incorporated in the mercury photometer by
Table 11. Calibration Curve” Average Absorbance, A absorbance, 2
99.5 100.0 99.5 100.0 99.5 100.0
0.002 0 0.002 0 0.002 0
98.0 98.5 98.0 99.5 98.0 98.0 99.0 98.0 98.0 99.0
0.009 0.007 0.009 0.002 0.009 0.009 0.004 0.009 0.009 0.004
73.5 71 .O 74.0 70.5 71 .O 72.0 71 .O 70.0
0.134 0.149 0.131 0.152 0.149 0.143 0.149 0.154
46.5 45.0 47.5 48.0 47.5 48 5 48.5 46.5 46.5 47.5
0.332 0.347 0.323 0.319 0.323 0.314 0.314 0.332 0.332 0.323
29.0 40.5 30.5 31 .O 31 .O 34.0 32.0 32.0 30.5 31.5
0.538 0.392 0.516 0.509 0.509 0.468 0.495 0 495 0.516 0.502
20.0 ... 23.0 ... 25.0 22.5
0.699
29 0 22 0 23 $.
0 538 0.658 0.629
I 7 U
x 100 2
0.001
0.001
100%
0.007
0.0026
37.1
0.145
0.0079
5.5%
0.326
0.0096
2.9%
0.494
0.381
7.7%
0.638
0,0466
7.3%
0.01 1 2 3 4 5 6 7 8 0.02 1 2 3 4 5 6 7 8 9 10 0.03 1 2 3 4 5 6 7 8 9 10
0.04 1
2 3 4 5 6
0.638 0.602
0,648
7
e
9 10 a
A straight line was obtained by ploiting average absorbance values against tne weight of mercury vaporized. .- . I _ _1 ----I-
nl3nnl ABSORPTION CELL
:s I
I
\
@-
DETECTOR
! S I
I
’
@(E Figure 4. Schematic representation of mercury photometer with provision for correcting nonatomic absorption A. Argon-filled mercury resonance lamp V. High vacuum mercury resonance lamp
E. Exciting source 0. Optical attenuator C. Cell containing mercury and nitrogen S . Shutter
comparing the transmitted radiation from the absorption cell with radiation from the resonance lamp passing through a light guide and suitable optical attenuation devices. Preliminary examinaiion revealed that with the null method at least one order of magnitude increase in sensitivity may be obtained over the single beam system. CALIBRATION OF THE INSTRUMENT A calibration curve was obtained with mercury ranging from 10-50 nanograms (0.01-0.05 pg). The mercury in the form of sulfide was d e h e r e d to the sample tube from a micrometer syringe. A porticln (0.1 ml) was taken from 0.1 ppm Hg to give 10 nanograms, and 50 nanograms Hg was obtained with 0.1 ml of 0.5 ppm Hg etc. T o prepare sulfided 0.1-ppm Hg solution, 1 ml of 10 ppm Hg (from mercuric chloride salt) was added with 1 ml of 20 ppm of hydrated sodium sulfide and brought up to 100 ml in a volumetric flask. The sample tubes containing mercury solution were evaporated to dryness in a desiccator under reduced pressure and heated with infrared lamp at -100’ C. The results are shown in Table 11. PORTABILITY The instrument may be made portable as all power requirements may be battery operated, and the whole assembly is a compact unit. :. ISOTOPIC ANALYSIS The use of sources emitting spectra of only one isotope for the analysis of isotopes has been suggested by Walsh (5), and the determination of isotopic composition of mercury vapor has been carriecl out by Osborn et al. (IO) who used enriched mercury isotclpe in the arc lamp to obtain the isotopic component of the hyperfine structure in the resonance line. This method suffered the disadvantage that great care had to be taken to emure no broadening took place as the isotopic components are very close to each other. The use of resonance lamp containing mercury isotope will prove to be a simple means of isotopic analysis. Moreover, it should be possible to analyze all mercury isotopes by applying Leeman effect as observed by Mrozowski ( I I ) . ( I O ) K. R. Osborn and F1. E. Guniung, J Opt. Soc. Am., 45, 552 \ 1955). ( 1 1 ) €3. Mr07owski, Bull 4cad. Pol. {I930 and 1931).
CORRECTION FOR NONATOMIC ABSORPTION Although absorption of mercury atomic vapor is specific at the resonance wavelength 2537 A,there are some substances, such as a wide range of common organic compounds, which interfere by absorbing at this wavelength. In addition, fumes and suspended matter in the vapor phase cause apparent absorption of the radiation by physical scattering. It is necessary to make correction for such interference or “nonatomic absorption.” Two methods of correction by absorption measurement have been used. One method, which requires a monochromator, uses a continuous source tuned at 2537 A (12). The other method uses a double beam system. One beam measures the atomic and nonatomic absorption and the other measures only nonatomic absorption after the mercury vapor in the sample has been removed with palladium chloride (8),the differential output being due to mercury vapor alone. A simple method (patent applied for) to correct for nonatomic absorption is based on the use of a mercury resonance lamp emitting a broadened line as a source for correcting nonatomic absorption. The resonance line from a resonance lamp may be broadened at will by introducing various nonquenching foreign gases at various pressures (13) (Lorentz broadening). If the broadened resonance line is passed through mercury vapor in a nitrogen atmosphere at room temperature, the center portion of the broadened line, which corresponds to the analytical absorption line width, will be completely absorbed. The transmitted broadened line under this condition will now be insensitive to mercury vapor, but is sensitive to nonatomic absorbing species. Because this transmitted broadened line is immediately adjacent to the center line, the sensitivity to nonatomic absorption will be the same as that measured with resonance line of Doppler width. In a preliminary investigation, a broadened resonance line was emitted by a resonance lamp filled with spectroscopically pure argon at one atmosphere pressure. The broadened resonance line was passed through a cell with a LO-cm path (12) A. E. Ballard. 3 ‘ A . Stewar: A 0
