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An improved power supply for the magnet in a.c. modulated Zeeman atomic absorption spectrometry
Specwochimica AcWVol. 37B.Na.6.pp.527-531.1962 Printed inGreatBritain.
~n2nwJs27-osslu.~ @1582PergunonPrcl~Lcd.
TECHNICAL
NOTE
Au improved power supply for the magnet in a.c. modulated Zeeman atomic absorption spectrometry (Receiued 3 Lkcember 1981) 1. INTROLXJCTI~N
THE ADVANTAGES
for Zeeman atomic absorption (ZAA) spectrometry of an a.c. modulated magnetic field over systems utilizing a d.c. electromagnet of a permanent magnet have been discussed previously [ l-61. In a strong a.~. field sensitivity is equal to normal AA. High background absorption is corrected and analytical curves of maximum linearity are obtained. When the field is applied to the atomizer, it is possible to
correct for structured background. a.c. driven electromagnets for ZAA described in the literature normally use sine wave modulation. UCHIDAand HATT~RJ[7] report a magnetic field of only 0.43 kG. OTMJBAet al. [8] constructed a 10 kG a.c. magnetic field around a 10 mm burner. The same authors described an a.c. magnetic field of 6 kG and 3 kG applied to a flame and an electrothermal atomizer, respectively 191. STEPHENS[51 applied a 1.5 kG magnetic field to a conventional hollow cathode lamp. BRODIEand LIDDELL[~]report the application of a 7.1 kG a.c. field to a 60 mm burner and the same authors described a 10 kG magnetic field applied to a small graphite furnace[4]. However, an important limitation in the application of sine wave modulation is the short time available to perform the intensity measurement at zero field. The a.c. magnets described so far all use the 50 Hz frequency of the mains voltage that drives the power supply. In a sine wave magnetic field the modulation frequency of the light intensity is then twice this frequency, i.e. 100 Hz. The alternate measurements of background absorbance and background plus analyte are then performed with a temporal separation of 5 ms. The zero field measurement must be performed in about 0.1 ms. Longer measurement times result in lower analytical sensitivity and in curvature of the analytical curve. In principle, there are two possibilities to extend the period of zero magnetic field. One is to use half wave rectification of the a.c. current. This approach has been chosen by FERNANDEZ et al.[6]. As a result, the intensity modulation frequency becomes equal to the a.c. current modulation frequency of the magnet. The authors stipulate a field strength of 8 kG at 60 Hz, thereby separating the alternate absorbance measurements by 8.3 ms. The other alternative has been chosen by LIDDELLand BRODIE[~]who reported a
modification of the power supply to pause at zero field for about 1.5 ms each half cycle. The intensity modulation frequency remained at 100 Hz and the peak field dropped to 7.3 kG. However, technical data of the power supply were not provided. This note describes a simple power supply for an a.c. magnet. The 100 Hz intensity modulation is retained, whereas the time available to perform the zero measurement is lengthened in comparison to the sine wave magnetic field.
[II M.T. C. DE LOOS-V•
LLEBREOT and L. DE GALAN, Spectrochim. Actu C. DE LOOS-V• LLEBREOT and L. DE GALAN, Spectrochim. Actu f31K. G. BRODIEand P. R. LIDDELL.Anal. Chem. 52. 1059 (1980). f41P. R. LIDDELL and K. G. BRODIE. And. Chem. 52, 1256(1980). [Sl R. STEPHENS, Tolontn 26.57 (1979).
121M. T.
field
22~.495(1978). JSB,495(1980).
161P. J. FERNANDEZ.W. BOHLER,M. M. BEATYand W. B. BARNFIT, Atom. Spectrosc. 2.74 (1981). t71Y. UCHIDAand S. HA~ORI, OYO Bum 44,852 (1975). VI v. OTRUBA, J. JAMBOR.J. HoRAK and L. SOMMER. ScriptclFuc. Sci. Nat. Uniu Emo, 6, I (1976). 191 V. OTRUBA.J. JAMBOR. J. KOMAREK,J. HORAKand L. SOMMER. Anal. Chim. Acta 101.367(1978). 527
528
Technical Note 2. EXPERIMENTAI.
The magnet used in this study has been described previously (21. A simplified block diagram of the power supply is presented in Fig. I. The magnet is connected to the mains voltage. The current through the coils of the magnet is controled by two thyristors connected anti parallel with respect to each other. By triggering the thyristors at the right moment the current through the magnet can be kept at zero during a selectable period. Figure 2 shows the mains voltage. The first thyristor is triggered T milliseconds after zero crossing of the mains voltage. The thyristor becomes conducting and the current flows through the magnet until zero current is reached. With the same delay after the next zero crossing the other thyristor is triggered and the current Rows through the coils of the magnet in the opposite direction. The current is kept at zero during a selectable period by adjustment of the delay (7’) of the trigger pulses. The current wave form is close to sine wave. Calculation gives the following result. i = Rs +:,Li
(- R sin ~7’ + OL cos UT) exp
+Rsinot-oLcosot
where U is the amplitude of the sine wave voltage (U = I!2 UC,,), R is the resistance of the coils. o is the angular frequency and L is the inductance. The first positive part of eqn (1) describes the positive part of the current wave. The negative part shows the same form. In the present system U,s. = 220 V. R = 2 R, L = 36 mH and the frequency is 50 Hz. When the trigger pulse appears with a delay T = 4.75 ms after zero crossing of the mains voltage, current is zero during 0.5 ms and the maximum current is 25 A. With a further delay of the pulses the zero current period is extended, but the maximum current is reduced. The maguetic field strength follows the current immediately[Z] so that the zero field period is lengthened at the cost of lower maximum fieldstrength. Fiie 3 shows oscilloscope traces of the shape of some
Fii. 1. Block diagram of the power supply of the magnet.
I
’
trigger
T 5
10
15
I 20
pulses t.n-s
Fig. 2. Modified sine wave modulation of the magnet. Mains voltage and current through the coils of the magnet are presented in the upper trace. The lower trace shows the position of the pulses that trigger the thyristors. The zero current period depends on the delay time T of the pulses.
Technical Note
529
6hC
6kC
1srns
lOkG
05l?lS
Fig. 3. Oscilloscope traces of the shape of the 50 Hz magnetic field at a field strength of 6, 8 and IO kG.
magnetic fields. A magnetic field of 10 kG is reached with a zero field period 0.5 ms. For a field of 8 and
6 kG the zero field time is 1.8 and 3.1 ms respectively. The time available to perform the maximum field measurement depends on the shape of the atomic line, the Zeeman splitting pattern and the strength of the magnetic field. For most transitions the maximum field measurement can be performed during about 1 ms in a 50 Hz sine wave field of 10 kG. For this reason it is not useful to extend the zero field measurement period far beyond 1 ms. Maximum sensitivity and optimum signal to noise ratio will be obtained in a magnetic field of 8 to 10 kG. The electrical diagram of the power supply is presented in Fig. 4. Cooling of the thyristors is not necessary because the magnetic field is applied to the graphite furnace only during the atomizing time.
3. CONCLUSION The power supply presented is not expensive, simple and easy to construct. It works instantaneously and produces no direct current in the mains supply lines. The 10 kG magnetic field is strong enough to be used in ZAA. The modification of the 50 Hz sine wave magnetic field makes it possible to extend the zero field intensity measurements to 0.5 ms whereas the 100 Hz modulation in the light intensity is maintained. Further lengthening of the zero field measurement time can easily be achieved at the cost of reduced field strength. Longer zero field time at high field strength requires supply voltages over 220 V or a redesign of the magnet coils. The alternate measurements of background absorbance and background plus analyte are performed with a temporal separation of 5 ms. With an a.c. system faster background correction can only be obtained in a magnetic field of higher frequency. However, there are two disadvantages to such a system. The time available to perform the measurements at maximum field and zero field is reduced and the power supply for the magnet becomes very complicated. When background correction appreciably faster than twice the mains frequency is desirable, d.c. driven magnets are more promising because fast rotation of a polarizer is rather simple. Analytical results obtained with the modified sine wave magnetic field will be published elsewhere [ 101.
4. SUMMARY
This note describes a simple power supply for a magnet to be used in a.c. ZAA spectrometry. The modified sine wave shape of the 50 Hz 10 kG magnetic field allows
[IO] M. T. C.
DE
LOOS-VOLIXRRECT and L. DE GAI.AN, Spectrochim. Acto
B, in press.
.5
I
.5 I
I
FROM ATOMIZER POWER SUPPLY .5
.5 I
,,,
Fig. 4. Electrical diagram of the power supply of the magnet. Diodes: IN 4448, thyristors: AEG type T22N 700 EOB.
.5
,I,.
I
“,
,,I
I
~“,
,..
Technical Note
531
ms period at zero field to perform the zero field intensity measurement. At lower maximum field strength, the zero field period can be extended.
a 0.5
Laboratorium voor Magnet&he Techniek, Afdefing Elektrotechniek, Technische Hogeschool Delft, 2600 GA Delft, The Netherlands
J.
W. M.
VAN UFFELEN,
T. C. DE LOOS-VOLLEBREGT and L. Luboratorium voor Analytische Scheikunde, Technische Hogeschool Delft, 2600 GA Delft, The Netherlands M.
*Authorto whom correspondence should be addressed.
DE GALAN,*
~n2nwJs27-osslu.~ @1582PergunonPrcl~Lcd.
TECHNICAL
NOTE
Au improved power supply for the magnet in a.c. modulated Zeeman atomic absorption spectrometry (Receiued 3 Lkcember 1981) 1. INTROLXJCTI~N
THE ADVANTAGES
for Zeeman atomic absorption (ZAA) spectrometry of an a.c. modulated magnetic field over systems utilizing a d.c. electromagnet of a permanent magnet have been discussed previously [ l-61. In a strong a.~. field sensitivity is equal to normal AA. High background absorption is corrected and analytical curves of maximum linearity are obtained. When the field is applied to the atomizer, it is possible to
correct for structured background. a.c. driven electromagnets for ZAA described in the literature normally use sine wave modulation. UCHIDAand HATT~RJ[7] report a magnetic field of only 0.43 kG. OTMJBAet al. [8] constructed a 10 kG a.c. magnetic field around a 10 mm burner. The same authors described an a.c. magnetic field of 6 kG and 3 kG applied to a flame and an electrothermal atomizer, respectively 191. STEPHENS[51 applied a 1.5 kG magnetic field to a conventional hollow cathode lamp. BRODIEand LIDDELL[~]report the application of a 7.1 kG a.c. field to a 60 mm burner and the same authors described a 10 kG magnetic field applied to a small graphite furnace[4]. However, an important limitation in the application of sine wave modulation is the short time available to perform the intensity measurement at zero field. The a.c. magnets described so far all use the 50 Hz frequency of the mains voltage that drives the power supply. In a sine wave magnetic field the modulation frequency of the light intensity is then twice this frequency, i.e. 100 Hz. The alternate measurements of background absorbance and background plus analyte are then performed with a temporal separation of 5 ms. The zero field measurement must be performed in about 0.1 ms. Longer measurement times result in lower analytical sensitivity and in curvature of the analytical curve. In principle, there are two possibilities to extend the period of zero magnetic field. One is to use half wave rectification of the a.c. current. This approach has been chosen by FERNANDEZ et al.[6]. As a result, the intensity modulation frequency becomes equal to the a.c. current modulation frequency of the magnet. The authors stipulate a field strength of 8 kG at 60 Hz, thereby separating the alternate absorbance measurements by 8.3 ms. The other alternative has been chosen by LIDDELLand BRODIE[~]who reported a
modification of the power supply to pause at zero field for about 1.5 ms each half cycle. The intensity modulation frequency remained at 100 Hz and the peak field dropped to 7.3 kG. However, technical data of the power supply were not provided. This note describes a simple power supply for an a.c. magnet. The 100 Hz intensity modulation is retained, whereas the time available to perform the zero measurement is lengthened in comparison to the sine wave magnetic field.
[II M.T. C. DE LOOS-V•
LLEBREOT and L. DE GALAN, Spectrochim. Actu C. DE LOOS-V• LLEBREOT and L. DE GALAN, Spectrochim. Actu f31K. G. BRODIEand P. R. LIDDELL.Anal. Chem. 52. 1059 (1980). f41P. R. LIDDELL and K. G. BRODIE. And. Chem. 52, 1256(1980). [Sl R. STEPHENS, Tolontn 26.57 (1979).
121M. T.
field
22~.495(1978). JSB,495(1980).
161P. J. FERNANDEZ.W. BOHLER,M. M. BEATYand W. B. BARNFIT, Atom. Spectrosc. 2.74 (1981). t71Y. UCHIDAand S. HA~ORI, OYO Bum 44,852 (1975). VI v. OTRUBA, J. JAMBOR.J. HoRAK and L. SOMMER. ScriptclFuc. Sci. Nat. Uniu Emo, 6, I (1976). 191 V. OTRUBA.J. JAMBOR. J. KOMAREK,J. HORAKand L. SOMMER. Anal. Chim. Acta 101.367(1978). 527
528
Technical Note 2. EXPERIMENTAI.
The magnet used in this study has been described previously (21. A simplified block diagram of the power supply is presented in Fig. I. The magnet is connected to the mains voltage. The current through the coils of the magnet is controled by two thyristors connected anti parallel with respect to each other. By triggering the thyristors at the right moment the current through the magnet can be kept at zero during a selectable period. Figure 2 shows the mains voltage. The first thyristor is triggered T milliseconds after zero crossing of the mains voltage. The thyristor becomes conducting and the current flows through the magnet until zero current is reached. With the same delay after the next zero crossing the other thyristor is triggered and the current Rows through the coils of the magnet in the opposite direction. The current is kept at zero during a selectable period by adjustment of the delay (7’) of the trigger pulses. The current wave form is close to sine wave. Calculation gives the following result. i = Rs +:,Li
(- R sin ~7’ + OL cos UT) exp
+Rsinot-oLcosot
where U is the amplitude of the sine wave voltage (U = I!2 UC,,), R is the resistance of the coils. o is the angular frequency and L is the inductance. The first positive part of eqn (1) describes the positive part of the current wave. The negative part shows the same form. In the present system U,s. = 220 V. R = 2 R, L = 36 mH and the frequency is 50 Hz. When the trigger pulse appears with a delay T = 4.75 ms after zero crossing of the mains voltage, current is zero during 0.5 ms and the maximum current is 25 A. With a further delay of the pulses the zero current period is extended, but the maximum current is reduced. The maguetic field strength follows the current immediately[Z] so that the zero field period is lengthened at the cost of lower maximum fieldstrength. Fiie 3 shows oscilloscope traces of the shape of some
Fii. 1. Block diagram of the power supply of the magnet.
I
’
trigger
T 5
10
15
I 20
pulses t.n-s
Fig. 2. Modified sine wave modulation of the magnet. Mains voltage and current through the coils of the magnet are presented in the upper trace. The lower trace shows the position of the pulses that trigger the thyristors. The zero current period depends on the delay time T of the pulses.
Technical Note
529
6hC
6kC
1srns
lOkG
05l?lS
Fig. 3. Oscilloscope traces of the shape of the 50 Hz magnetic field at a field strength of 6, 8 and IO kG.
magnetic fields. A magnetic field of 10 kG is reached with a zero field period 0.5 ms. For a field of 8 and
6 kG the zero field time is 1.8 and 3.1 ms respectively. The time available to perform the maximum field measurement depends on the shape of the atomic line, the Zeeman splitting pattern and the strength of the magnetic field. For most transitions the maximum field measurement can be performed during about 1 ms in a 50 Hz sine wave field of 10 kG. For this reason it is not useful to extend the zero field measurement period far beyond 1 ms. Maximum sensitivity and optimum signal to noise ratio will be obtained in a magnetic field of 8 to 10 kG. The electrical diagram of the power supply is presented in Fig. 4. Cooling of the thyristors is not necessary because the magnetic field is applied to the graphite furnace only during the atomizing time.
3. CONCLUSION The power supply presented is not expensive, simple and easy to construct. It works instantaneously and produces no direct current in the mains supply lines. The 10 kG magnetic field is strong enough to be used in ZAA. The modification of the 50 Hz sine wave magnetic field makes it possible to extend the zero field intensity measurements to 0.5 ms whereas the 100 Hz modulation in the light intensity is maintained. Further lengthening of the zero field measurement time can easily be achieved at the cost of reduced field strength. Longer zero field time at high field strength requires supply voltages over 220 V or a redesign of the magnet coils. The alternate measurements of background absorbance and background plus analyte are performed with a temporal separation of 5 ms. With an a.c. system faster background correction can only be obtained in a magnetic field of higher frequency. However, there are two disadvantages to such a system. The time available to perform the measurements at maximum field and zero field is reduced and the power supply for the magnet becomes very complicated. When background correction appreciably faster than twice the mains frequency is desirable, d.c. driven magnets are more promising because fast rotation of a polarizer is rather simple. Analytical results obtained with the modified sine wave magnetic field will be published elsewhere [ 101.
4. SUMMARY
This note describes a simple power supply for a magnet to be used in a.c. ZAA spectrometry. The modified sine wave shape of the 50 Hz 10 kG magnetic field allows
[IO] M. T. C.
DE
LOOS-VOLIXRRECT and L. DE GAI.AN, Spectrochim. Acto
B, in press.
.5
I
.5 I
I
FROM ATOMIZER POWER SUPPLY .5
.5 I
,,,
Fig. 4. Electrical diagram of the power supply of the magnet. Diodes: IN 4448, thyristors: AEG type T22N 700 EOB.
.5
,I,.
I
“,
,,I
I
~“,
,..
Technical Note
531
ms period at zero field to perform the zero field intensity measurement. At lower maximum field strength, the zero field period can be extended.
a 0.5
Laboratorium voor Magnet&he Techniek, Afdefing Elektrotechniek, Technische Hogeschool Delft, 2600 GA Delft, The Netherlands
J.
W. M.
VAN UFFELEN,
T. C. DE LOOS-VOLLEBREGT and L. Luboratorium voor Analytische Scheikunde, Technische Hogeschool Delft, 2600 GA Delft, The Netherlands M.
*Authorto whom correspondence should be addressed.
DE GALAN,*