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Simple background monitoring device for atomic absorption spectrometry
Analytica C&mica Acta, 109 (1979) 177-179 0 Elsevier Scientific Publishing Company, Amsterdam-
Printedin The Netherlands
Short Communication SIMPLE BACKGROUND MONITORING ABSORPTION SPECTROMETRY
ROBERT
F. M. HERBER*
DEVICE
FOR ATOMIC
and JAN L. M. DE BOER**
Coronel Labomtory for Occupational and Environmental ifygiene. 1st Const. Huygensstraat 20, Amsterdam (The Netherlands) (Received
Faculty
of hfedicine,
2nd January 19’79)
Summary. A cheap device which recorder is described_
can be used to follow
transient signals with a chart
Delrterium or hydrogen background correction is commonly used during the atomic absorption spectrometry of complex materials by electrothermal atomization. In most commercial apparatus, however, only one type of output can be obtained, the corrected signal or the background signal or the total signal. Maessen and Posma [l] showed that fast transient signals, such as those coming from volatile elements, cannot be followed by commercially available apparatus. They therefore used a look-in-ampli~er and waveform recorder instead of the commercial amplifier. Further, de1 Castilho and Herber [Zf showed this combination to be a valuable tool for following the signals from corrected, background or analytc absorption. It was possible to obtain simultaneously digital or analog signals with medium time response (ca. 50 ms) and the correctid fast signal. The lock-in-amplifier and waveform recorder combination however, is rather expensive. This report describes an alternative, much cheaper, background monitoring device which can be used in conjunction with a chart recorder. In Fig. 1, connections to the Varian AA6 CSB (BC6 correction) system for background monitoring are illustrated. The background signal can be taken from TP 52 and the corrected signal from TP 62. This allows the existing Varian system to present a total, background or corrected signal in digital (time constant = 50 ms) or analog (time constant Z 300 ms) form, and permits the system described in this report (time constant 40 ms) to provide background or corrected information. These possibilities only apply to the Varian IM60-BC6 system. For other background-corrected apparatus, the appropriate connections should be made. For apparatus without background correction, the monitor can be used to follow the analyte signal in a fast mode; connection should then be made directly after the photomultipliex-preamplifier output. Connection should always be made before any damping amplifier. **Present Amsterdam,
address: Laboratory The Netherlands.
for
Analytical
Chemistry,
Nieuwe
Achtergracht
166,
178 Corrected signal to momtor
BC6
IMGD
f
t to background monitor
Fig. 1. Connection BC-6 system.
points for background
or corrected
signal monitor on existing Varian
TP 52 L-2 0
In impedance
tronstormcr
+
I_lSV
Fig. 2. Electronic circuit for background
(alter,
tlme constant =LOms
or corrected signal monitor.
Figure 2 shows the electronic circuit of the monitor. The impedance transformer lowers the high impedance of the photomultiplicr and has an amplification of unity. The zero offset compensates any d-c. level. The low-pass
filter has a time constant of 40 ms; the amplification is also unity. The time constant can be changed by varying the 0.47+ F capacitor and/or the 82-kS2 resistor. The monitoring system follows the background (or corrected) signal simultaneously with the digital (or analog) IM6D read-out, and so the signal can be reproduced on a fast chart recorder. This makes it possible to follow the separation between peaks, the ashing procedure and any manually controlled ashing/atomization steps on the chart paper. The system can be used to follow fast transient signals from volatile elements such as lead and cadmium, and enables them to be monitored in the corrected mode, and to follow the ashing/atomization steps in the background mode, while the commercial instrument is in the corrected mode. Figure 3(a) shows a recorder trace obtained with the monitor in background mode for cadmium in whole blood [ 2 1. The blood was diluted 5-fold with water and 5 ~1 of the diluted blood was injected into the furnace (Varian CRA 63). The atomization cycle used was 6.8 V for 4.2 s. Peak A is probably the organic matrix, while peak B is probably caused by inorganic salts. The cadmium peak appears between peaks A and B [21_ Changing the temperature setting czxuses a much greater change in the height of peak A than in peak B.
179
(bl
T
time (5) d
1
time
(sf 4
Fig. 3. Chart recorder tracings of (a) background monitor signal from cadmium in blood of A and B, see text) and (b) corrected monitor signal from cadmium in blood. The arrows indicate the start of the atomization cycle.
(for meaning
TABLE
1
Effect of ashing voltage on determination Voltage
0.58 0.62 0.67 0.70 0.75
Peak A A.U. 2 s.d.= 180 f 17 53 f 20 6 .t 4 1
aEach result is the mean
of
of cadmium in blood
Peak B A.U. 2 s.d.a
Corrected Cd absorbance
C.V.
24 2 27 2 28 z 7*1 lo?
0.072 0.079 0.089 0.091 0.070
6 10 9 7 10
1 3 4 1
6 peak height measurements
(%I
in arbitrary units (A-U.).
The monitoring device was used to find conditions for a good separation of analyte and matrix peaks and to find conditions for good precision with reasonable peak separation. The results obtained by using various ashing voltages are shown in Table 1. It shows that 0.70 V gives the highest corrected cadmium absorbance, with a 7% coefficient of variation. However, at higher voltage, there is a sharp decrease in the analyte signal, possibly because of loss of analyte [3] ; thus the 0.70 V is a critical value, and it might be more reliable to use 0.67 V, with a 9% coefficient of variation and a peak A/peak B ratio of 0.2. In practice, this ratio is usually 0.5-2; too low an ashing voltage leads to a high peak A/peak B ratio which can lead to incorrect background correction [41. Figure 3(b) shows a recorder trace of a cadmium signal from a blood sample (concentration 0.5 pg Cd 1-i. 5fold diluted) monitored in the corrected mode, under the recommended conditions.
The authors are greatly indebted to H. J. Pieters for his technical assistance. REFERENCES 1 2 3 4
F. P. C. P.
J. M. J. Maessen de1 Castilho and W. Fuller, Anal. Bailey and T. A.
and F. D. Posma. Anal. Chem., 46 (1974) 1439. R. F. M. Herber, Anal. Chim. Acta, 94 (1977) 269. Chim. Acta, 62 (1972) 442. Kilroe-Smith, Anal. Chim. Acta, 77 (1975) 29.
Printedin The Netherlands
Short Communication SIMPLE BACKGROUND MONITORING ABSORPTION SPECTROMETRY
ROBERT
F. M. HERBER*
DEVICE
FOR ATOMIC
and JAN L. M. DE BOER**
Coronel Labomtory for Occupational and Environmental ifygiene. 1st Const. Huygensstraat 20, Amsterdam (The Netherlands) (Received
Faculty
of hfedicine,
2nd January 19’79)
Summary. A cheap device which recorder is described_
can be used to follow
transient signals with a chart
Delrterium or hydrogen background correction is commonly used during the atomic absorption spectrometry of complex materials by electrothermal atomization. In most commercial apparatus, however, only one type of output can be obtained, the corrected signal or the background signal or the total signal. Maessen and Posma [l] showed that fast transient signals, such as those coming from volatile elements, cannot be followed by commercially available apparatus. They therefore used a look-in-ampli~er and waveform recorder instead of the commercial amplifier. Further, de1 Castilho and Herber [Zf showed this combination to be a valuable tool for following the signals from corrected, background or analytc absorption. It was possible to obtain simultaneously digital or analog signals with medium time response (ca. 50 ms) and the correctid fast signal. The lock-in-amplifier and waveform recorder combination however, is rather expensive. This report describes an alternative, much cheaper, background monitoring device which can be used in conjunction with a chart recorder. In Fig. 1, connections to the Varian AA6 CSB (BC6 correction) system for background monitoring are illustrated. The background signal can be taken from TP 52 and the corrected signal from TP 62. This allows the existing Varian system to present a total, background or corrected signal in digital (time constant = 50 ms) or analog (time constant Z 300 ms) form, and permits the system described in this report (time constant 40 ms) to provide background or corrected information. These possibilities only apply to the Varian IM60-BC6 system. For other background-corrected apparatus, the appropriate connections should be made. For apparatus without background correction, the monitor can be used to follow the analyte signal in a fast mode; connection should then be made directly after the photomultipliex-preamplifier output. Connection should always be made before any damping amplifier. **Present Amsterdam,
address: Laboratory The Netherlands.
for
Analytical
Chemistry,
Nieuwe
Achtergracht
166,
178 Corrected signal to momtor
BC6
IMGD
f
t to background monitor
Fig. 1. Connection BC-6 system.
points for background
or corrected
signal monitor on existing Varian
TP 52 L-2 0
In impedance
tronstormcr
+
I_lSV
Fig. 2. Electronic circuit for background
(alter,
tlme constant =LOms
or corrected signal monitor.
Figure 2 shows the electronic circuit of the monitor. The impedance transformer lowers the high impedance of the photomultiplicr and has an amplification of unity. The zero offset compensates any d-c. level. The low-pass
filter has a time constant of 40 ms; the amplification is also unity. The time constant can be changed by varying the 0.47+ F capacitor and/or the 82-kS2 resistor. The monitoring system follows the background (or corrected) signal simultaneously with the digital (or analog) IM6D read-out, and so the signal can be reproduced on a fast chart recorder. This makes it possible to follow the separation between peaks, the ashing procedure and any manually controlled ashing/atomization steps on the chart paper. The system can be used to follow fast transient signals from volatile elements such as lead and cadmium, and enables them to be monitored in the corrected mode, and to follow the ashing/atomization steps in the background mode, while the commercial instrument is in the corrected mode. Figure 3(a) shows a recorder trace obtained with the monitor in background mode for cadmium in whole blood [ 2 1. The blood was diluted 5-fold with water and 5 ~1 of the diluted blood was injected into the furnace (Varian CRA 63). The atomization cycle used was 6.8 V for 4.2 s. Peak A is probably the organic matrix, while peak B is probably caused by inorganic salts. The cadmium peak appears between peaks A and B [21_ Changing the temperature setting czxuses a much greater change in the height of peak A than in peak B.
179
(bl
T
time (5) d
1
time
(sf 4
Fig. 3. Chart recorder tracings of (a) background monitor signal from cadmium in blood of A and B, see text) and (b) corrected monitor signal from cadmium in blood. The arrows indicate the start of the atomization cycle.
(for meaning
TABLE
1
Effect of ashing voltage on determination Voltage
0.58 0.62 0.67 0.70 0.75
Peak A A.U. 2 s.d.= 180 f 17 53 f 20 6 .t 4 1
aEach result is the mean
of
of cadmium in blood
Peak B A.U. 2 s.d.a
Corrected Cd absorbance
C.V.
24 2 27 2 28 z 7*1 lo?
0.072 0.079 0.089 0.091 0.070
6 10 9 7 10
1 3 4 1
6 peak height measurements
(%I
in arbitrary units (A-U.).
The monitoring device was used to find conditions for a good separation of analyte and matrix peaks and to find conditions for good precision with reasonable peak separation. The results obtained by using various ashing voltages are shown in Table 1. It shows that 0.70 V gives the highest corrected cadmium absorbance, with a 7% coefficient of variation. However, at higher voltage, there is a sharp decrease in the analyte signal, possibly because of loss of analyte [3] ; thus the 0.70 V is a critical value, and it might be more reliable to use 0.67 V, with a 9% coefficient of variation and a peak A/peak B ratio of 0.2. In practice, this ratio is usually 0.5-2; too low an ashing voltage leads to a high peak A/peak B ratio which can lead to incorrect background correction [41. Figure 3(b) shows a recorder trace of a cadmium signal from a blood sample (concentration 0.5 pg Cd 1-i. 5fold diluted) monitored in the corrected mode, under the recommended conditions.
The authors are greatly indebted to H. J. Pieters for his technical assistance. REFERENCES 1 2 3 4
F. P. C. P.
J. M. J. Maessen de1 Castilho and W. Fuller, Anal. Bailey and T. A.
and F. D. Posma. Anal. Chem., 46 (1974) 1439. R. F. M. Herber, Anal. Chim. Acta, 94 (1977) 269. Chim. Acta, 62 (1972) 442. Kilroe-Smith, Anal. Chim. Acta, 77 (1975) 29.