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Investigation of a high-intensity spectral lamp for atomic absorption spectrometry—II
Spectrochimica Acto, Vol. 36B.No. 12.pp. 1173-1176. 15’81. Printedin GreatBritain.
0584-8547/81/121173-O4SO2.00/0 @ 1981PergamttPressLtd.
Investigation of a high-intensity spectral lamp for atomic absorption spectrometry-II. Radio frequency electrodeless discharge lamps for alkaline earth metals and lithium V. V. ALEXANDROV,A. I. BEZLEPKINand A. S. KHOMYAK Scientific Council on Spectroscopy, Academy of Sciences of the U.S.S.R., Pr. Sapunova, 13-15, 103012, Moscow, U.S.S.R. (Received 4 February 1981) Abstract-The design of RF electrodeless-discharge lamps for alkaline earth metals and Li with Ma-glass envelopes in discussed. A study of the emission characteristics of the lamps has been carried out. Operating conditions providing high intensity without significant broadening of resonance lines have been established. The emission instability of the lamps for the above mentioned elements was found to be l-1.6% for a 30 min period. The lamps yield intensities a factor of 10-20 higher than the high-intensity combineddischarge lamps for the same elements. The minimum operating lifetime of the lamp is about 150 h.
VARIOUStypes of spectral lamps, in particular hollow cathode lamps, are widely used for atomic absorption measurements. Investigations aimed at designing new models of spectral lamps, characterized by resonance lines of high intensity and stability as well as small line width, cannot be restricted to one type of light source only, the hollow-cathode lamp, for instance. Spectral light sources for easily sputtered and fusible elements may well be represented by a radio frequency electrodeless discharge lamp (EDTL), which can be easily and reproducibly made. Lamps for a number of elements are recognized for their extremely high resonance line intensity and were first applied to atomic absorption analysis in the U.S.S.R. [l-3]. Today, commercial BCB - 2 series electrodeless lamps for 29 elements are available[4]. In this respect it should be noted, however, that previous investigations have not covered a wide group of elements, more specifically the alkaline earth metals (barium, strontium, calcium and magnesium) and the alkali metal lithium. Leading manufacturers in other countries do not fabricate RF lamps for the above noted elements either. The reason is that there has not been found a suitable envelope material, namely glass that would be capable of withstanding temperatures over about 400-65O”C created by atomic vapour of the above mentioned elements. In our laboratories an investigation has been carried out to measure the performance of RF electrodeless discharge lamps for the mentioned elements with an envelope made of commercial molybdenum glass. The results of the measurements are presented in this work. It should be noted, that the influence exerted by high-temperature metal vapour on the glass envelope is either reduced or fully eliminated. EXPERIMENTAL
For excitation of an RF electrodeless discharge, the PPBL - 3-standard generator with the lamp in its inductor was employed. The excitation frequency of the generator changed from 100 to 110 MHz. An experimental system with a 3MP-3 monochromator was used to precisely determine the emission intensity and spectral composition. [l] N. P. IVANOV, L. I. MINERVINA and S. V. BARANOV, Chimitcheskie reactive i preparati, Trudi ZREA, vip. 27. Moscow (1965). [2] B. V. Lvov, Atomic Absroption Spectrochemical Analysis. Nauka, Moscow (1%6). [3] N. P. IVANOV, .J. Anal. Chem. (USSR) 21, 1129 (1966). [41 S. V. BARANOV, N. P. IVANOV and L. G. POFRALYDI, Poslednije dostizhenija v oblasti AA analysa, LDNTP. Leningrad (1%9).
I174
V. V. ALEXANDROV, A. I.BEZLEPKIN and A. S. KHOMYAK
For scanning, we varied the pressure in a pressure-chamber in which a FabryPerot interferometer was placed. The final design of an RF electrodeless discharge lamp is presented in Fig. 1. The lamp envelope (I) is cylindrical in shape. It is made of conventional C52-1 glass and tilled with a noble gas under low pressure. A tubular portion of the lamp is sealed at one end and carries a flat window (2), the latter being of a suitable material for efficiently transmitting wavelengths between 0.21 and 2 pm. A metal sleeve (3) with its inner surface covered with a layer of a pure element (4) is fixed inside the envelope by means of two quartz or ceramic tubes (5), which are located between the constricted portion of the envelope and the bottom (6) with the exhaust tube (7), intended for gas evacuation by means of a vacuum system. Positioning the lamp into a generator cavity leads to excitation of an RF discharge in the 811gas. Heating the metal sleeve in the RF field causes evaporation of atoms of the element, which are further excited in the RF discharge.
Fii. 1. Diagram of RF electrodeless lamp. 1. Envelope; 2. Window; 3. Sleeve; 4. Alkalineearth element; 5. Tubes; 6. Bottom; 7. Exhaust tube; 8. Base.
RESULTS AND Drscussro~
It is typical of EDTLs that the analytical line intensity is in a certain correspondence to the type and pressure of fill gas. The interdependence of these parameters is less appreciable in comparison with hollow cathode lamps and highintensity combined-discharge lamps. The type and pressure of the filler were determined so as to achieve minimum starting voltage and maximum emission intensity. Argon was selected for this purpose, since the use of other gases led to undesirable results as far as line to background ratio was concerned. Optimum results were obtained under 400 Pa argon pressure. In the process of modelling, the emission characteristics of the lamps of interest have been studied. In Fig. 2 emission intensity is plotted against the output of the generator. The curves show that thermal evaporation is the principal mechanism for supplying a cloud of ground state atoms, because the order of the emission characteristics of the above mentioned elements in Fig. 2 corresponds to the order of the temperatures at which a saturation vapour pressure of 133 x 10e3 Pa is reached, viz. Mg: 382°C Sr: 456°C Li: 46O”C, Ca: 517°C and Ba: 5’37°C. Because of the steep slope of the curves, the output power of the generator must be rigorously stabilized; for changing the oper$ing current by only 1.5% causes a change of 11% in the line intensity of Mg 28.52 A. Therefore a stabilizing rectifier had to be used to make the current output of the generator stable enough. Then the instability of the Li, Ba, Sr, Ca and Mg resonance line intensities was kept limited to l-1.6% over a 30 min period. The following operating conditions were considered optimum for the lamps in the
Investigation
of a high-intensity
I50
100 Generator
Fig. 2. Analytical line intensities
1175
spectral lamp-II
current,
of lamps for different current.
I 200
I
i !: i0
mA
elements
as function
of generator
PPBL - 3 generator, if the requirements of high intensity, long operating lifetime and elimination of significant sputtering of the elements were taken into account: Mg50 mA; S-130 mA; Li--150 mA; Ca-160 mA; Ba-190 mA. For these conditions the signal-to-noise ratio ranges from 70 to 350, the value being different for each element. The reduced background noise is due to comparatively high purity of the spectrum at the analytical lines for argon filled lamps (see Fig. 3).
Colcum
Barlum
Strontium
L
Llthlum
Magnesium
Fig. 3. Emission spectra of RF lamps for alkaline earth metals and lithium.
1176
V. V.
ALEXANDROV, A. I. BEZLEPKIN and
A. S.
KHOMYAK
Table 1. Analytical line width as a function of a generator current Element resonance line (A) Barium 5536 Strontium 4607 Magnesium 2852 Calcium 4227
Generator current (mA) 50
0.02
60 70
100
120
Line width FWHM 0.004 0.013 0.04t 0.05t 0.06t
130
150
160
180
(A)* 0.013
0.005 0.013
0.02
0.026
0.07
*Full width at half maximum. Self-reversed line.
To prove the suitability of the experimentally established operating conditions, measurements were carried out to determine the relation between the analytical line width for the relevant element and the generator output current. The data listed in Table 1 show that the line width ranges from 0.004-0.05 8, and that there is no self-reversal observed at the recommendated operating conditions, except for the Mg 2852 8, line at generator currents of 60-70 mA. The lamps described above yield intensities a factor of 10-20 higher than high-intensity combined-discharge lamps for the same elements, the factor differing from element to element. The operating lifetime of the lamps for the elements noted above is about 150 h. CONCLUSION In this investigation practical models of radio frequency electrodeless lamps for alkaline earth metals and lithium have been developed. The lamps realize some significant advantages. They are characterized by high analytical line intensities along with a small line width. They possess features which result in good stability and long life, and thus are suitable for applications in atomic absorption and atomic fluorescence analysis. Acknowledgements-We are grateful to V. V. constructive criticism regarding this work.
NEDLER and V. B. BELYANIN
for helpful discussions
and
0584-8547/81/121173-O4SO2.00/0 @ 1981PergamttPressLtd.
Investigation of a high-intensity spectral lamp for atomic absorption spectrometry-II. Radio frequency electrodeless discharge lamps for alkaline earth metals and lithium V. V. ALEXANDROV,A. I. BEZLEPKINand A. S. KHOMYAK Scientific Council on Spectroscopy, Academy of Sciences of the U.S.S.R., Pr. Sapunova, 13-15, 103012, Moscow, U.S.S.R. (Received 4 February 1981) Abstract-The design of RF electrodeless-discharge lamps for alkaline earth metals and Li with Ma-glass envelopes in discussed. A study of the emission characteristics of the lamps has been carried out. Operating conditions providing high intensity without significant broadening of resonance lines have been established. The emission instability of the lamps for the above mentioned elements was found to be l-1.6% for a 30 min period. The lamps yield intensities a factor of 10-20 higher than the high-intensity combineddischarge lamps for the same elements. The minimum operating lifetime of the lamp is about 150 h.
VARIOUStypes of spectral lamps, in particular hollow cathode lamps, are widely used for atomic absorption measurements. Investigations aimed at designing new models of spectral lamps, characterized by resonance lines of high intensity and stability as well as small line width, cannot be restricted to one type of light source only, the hollow-cathode lamp, for instance. Spectral light sources for easily sputtered and fusible elements may well be represented by a radio frequency electrodeless discharge lamp (EDTL), which can be easily and reproducibly made. Lamps for a number of elements are recognized for their extremely high resonance line intensity and were first applied to atomic absorption analysis in the U.S.S.R. [l-3]. Today, commercial BCB - 2 series electrodeless lamps for 29 elements are available[4]. In this respect it should be noted, however, that previous investigations have not covered a wide group of elements, more specifically the alkaline earth metals (barium, strontium, calcium and magnesium) and the alkali metal lithium. Leading manufacturers in other countries do not fabricate RF lamps for the above noted elements either. The reason is that there has not been found a suitable envelope material, namely glass that would be capable of withstanding temperatures over about 400-65O”C created by atomic vapour of the above mentioned elements. In our laboratories an investigation has been carried out to measure the performance of RF electrodeless discharge lamps for the mentioned elements with an envelope made of commercial molybdenum glass. The results of the measurements are presented in this work. It should be noted, that the influence exerted by high-temperature metal vapour on the glass envelope is either reduced or fully eliminated. EXPERIMENTAL
For excitation of an RF electrodeless discharge, the PPBL - 3-standard generator with the lamp in its inductor was employed. The excitation frequency of the generator changed from 100 to 110 MHz. An experimental system with a 3MP-3 monochromator was used to precisely determine the emission intensity and spectral composition. [l] N. P. IVANOV, L. I. MINERVINA and S. V. BARANOV, Chimitcheskie reactive i preparati, Trudi ZREA, vip. 27. Moscow (1965). [2] B. V. Lvov, Atomic Absroption Spectrochemical Analysis. Nauka, Moscow (1%6). [3] N. P. IVANOV, .J. Anal. Chem. (USSR) 21, 1129 (1966). [41 S. V. BARANOV, N. P. IVANOV and L. G. POFRALYDI, Poslednije dostizhenija v oblasti AA analysa, LDNTP. Leningrad (1%9).
I174
V. V. ALEXANDROV, A. I.BEZLEPKIN and A. S. KHOMYAK
For scanning, we varied the pressure in a pressure-chamber in which a FabryPerot interferometer was placed. The final design of an RF electrodeless discharge lamp is presented in Fig. 1. The lamp envelope (I) is cylindrical in shape. It is made of conventional C52-1 glass and tilled with a noble gas under low pressure. A tubular portion of the lamp is sealed at one end and carries a flat window (2), the latter being of a suitable material for efficiently transmitting wavelengths between 0.21 and 2 pm. A metal sleeve (3) with its inner surface covered with a layer of a pure element (4) is fixed inside the envelope by means of two quartz or ceramic tubes (5), which are located between the constricted portion of the envelope and the bottom (6) with the exhaust tube (7), intended for gas evacuation by means of a vacuum system. Positioning the lamp into a generator cavity leads to excitation of an RF discharge in the 811gas. Heating the metal sleeve in the RF field causes evaporation of atoms of the element, which are further excited in the RF discharge.
Fii. 1. Diagram of RF electrodeless lamp. 1. Envelope; 2. Window; 3. Sleeve; 4. Alkalineearth element; 5. Tubes; 6. Bottom; 7. Exhaust tube; 8. Base.
RESULTS AND Drscussro~
It is typical of EDTLs that the analytical line intensity is in a certain correspondence to the type and pressure of fill gas. The interdependence of these parameters is less appreciable in comparison with hollow cathode lamps and highintensity combined-discharge lamps. The type and pressure of the filler were determined so as to achieve minimum starting voltage and maximum emission intensity. Argon was selected for this purpose, since the use of other gases led to undesirable results as far as line to background ratio was concerned. Optimum results were obtained under 400 Pa argon pressure. In the process of modelling, the emission characteristics of the lamps of interest have been studied. In Fig. 2 emission intensity is plotted against the output of the generator. The curves show that thermal evaporation is the principal mechanism for supplying a cloud of ground state atoms, because the order of the emission characteristics of the above mentioned elements in Fig. 2 corresponds to the order of the temperatures at which a saturation vapour pressure of 133 x 10e3 Pa is reached, viz. Mg: 382°C Sr: 456°C Li: 46O”C, Ca: 517°C and Ba: 5’37°C. Because of the steep slope of the curves, the output power of the generator must be rigorously stabilized; for changing the oper$ing current by only 1.5% causes a change of 11% in the line intensity of Mg 28.52 A. Therefore a stabilizing rectifier had to be used to make the current output of the generator stable enough. Then the instability of the Li, Ba, Sr, Ca and Mg resonance line intensities was kept limited to l-1.6% over a 30 min period. The following operating conditions were considered optimum for the lamps in the
Investigation
of a high-intensity
I50
100 Generator
Fig. 2. Analytical line intensities
1175
spectral lamp-II
current,
of lamps for different current.
I 200
I
i !: i0
mA
elements
as function
of generator
PPBL - 3 generator, if the requirements of high intensity, long operating lifetime and elimination of significant sputtering of the elements were taken into account: Mg50 mA; S-130 mA; Li--150 mA; Ca-160 mA; Ba-190 mA. For these conditions the signal-to-noise ratio ranges from 70 to 350, the value being different for each element. The reduced background noise is due to comparatively high purity of the spectrum at the analytical lines for argon filled lamps (see Fig. 3).
Colcum
Barlum
Strontium
L
Llthlum
Magnesium
Fig. 3. Emission spectra of RF lamps for alkaline earth metals and lithium.
1176
V. V.
ALEXANDROV, A. I. BEZLEPKIN and
A. S.
KHOMYAK
Table 1. Analytical line width as a function of a generator current Element resonance line (A) Barium 5536 Strontium 4607 Magnesium 2852 Calcium 4227
Generator current (mA) 50
0.02
60 70
100
120
Line width FWHM 0.004 0.013 0.04t 0.05t 0.06t
130
150
160
180
(A)* 0.013
0.005 0.013
0.02
0.026
0.07
*Full width at half maximum. Self-reversed line.
To prove the suitability of the experimentally established operating conditions, measurements were carried out to determine the relation between the analytical line width for the relevant element and the generator output current. The data listed in Table 1 show that the line width ranges from 0.004-0.05 8, and that there is no self-reversal observed at the recommendated operating conditions, except for the Mg 2852 8, line at generator currents of 60-70 mA. The lamps described above yield intensities a factor of 10-20 higher than high-intensity combined-discharge lamps for the same elements, the factor differing from element to element. The operating lifetime of the lamps for the elements noted above is about 150 h. CONCLUSION In this investigation practical models of radio frequency electrodeless lamps for alkaline earth metals and lithium have been developed. The lamps realize some significant advantages. They are characterized by high analytical line intensities along with a small line width. They possess features which result in good stability and long life, and thus are suitable for applications in atomic absorption and atomic fluorescence analysis. Acknowledgements-We are grateful to V. V. constructive criticism regarding this work.
NEDLER and V. B. BELYANIN
for helpful discussions
and