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Evaluation and application of internal standardization in atomic absorption spectrometry with electrothermal atomization
Analytica Chimica Ada. 107 (1979) 129-138 o Elsevier Scientific Publishing Company, Amsterdam
-
Printed in The Netherlands
EVALUATION AND APPLICATION OF INTERNAL STANDARDIZATION IN ATOMIC ABSORPTION SPECTROMETRY WITH ELECTROTHERMAL ATOMIZATION
TAKE0
TAKADA*
and KUNIO
NAKANO
Department of Chemistry, College of Science, Rikkyo Ikebukuro, Toshima-ku. Tokyo I71 (Japan) (Received
19th September
(St. Paul’s) University,
Nishi-
1978)
SUMMARY A new dual-channel system developed for use in atomic absorption spectrometry is used to assess the internal standard technique for electrothermal atomization. Cobalt was found to be a suitable internal standard for iron determinations, and is used for determination of 7-330 ng Fe ml-’ in water samples. With use of the in+xrnal standard technique, fluctuations caused by atomizer variables are reduced and interferences from many cations are also decreased.
Compared with conventional flame methods, electrothermal atomization makes the determination of trace metals more rapid and much more sensitive_ However, the absorbance from electrothermal atomizers is variable, because of factors such as atomizer temperature, heating rate, sample amount, etc. This is considered to be the cause of the decreased accuracy and precision of the analytical results obtained when such atomizers are used. In order to minimize the variations in signals arising from atomizer conditions, a definite amount of an element can be added to each sample as an internal standard. As the absorbances of the analyte and standard elements are considered to be simiiariy influenced by the various factors, the ratio of the absorbances may be free from the influence of variable conditions. The use of an internal standard to increase the precision of emission spectroscopic methods was introduced by Gerlach and Schweitzer [l] _ The application of an internal standard to &omic absorption spectrometry has been difficult because the instruments are monochromatic, and few examples are known [Z--4] _ Rules for sekcting internal standard elements have not yet been completely discussed, nor has any practical anolication --1~~---------
to electrothermal
at.0m-i~ ahsnrntinn r-----
heen
nuhlished. L-----------
In the present paper the utilization of an internal standard method is described, whereby most of the errors associated with the atomizer and atomization process are compensated. Samples of water were selected for’ investigation because total iron in water can be measured rapidly by elec-
130 trothermal atomic absorption spectrometry. The optimum conditions for the analysis, including the choice of the proper internal standard, are described. EXPERIMENTAL
Apparatus A dual-channel atomic absorption spectrometer was designed in this laboratory and assembled by Nippon Jarrell-Ash Co. The burner assembly was replaced by a carbon-tube atomizer (V&an-Techtron model 63). The instrument has two separate channels, one of which is used for the analyte element and the other for the element added; the ratio of the absorbances at the two wavelengths is measured_ A block diagram of photometric system is shown in Fig. 1. Two hollowcathode lamps are placed at right angles to each other, one for the analyte element, one for the standard element. The hollow-cathode lamps are alternately pulsed by a square-wave current. The light from the hollowcathode lamps is combined by a half-mirror and after, passing through the small carbon tube, it is split again into two beams by a second half-
mirror.
Each beam falls in the slit of a monochromator
and each is detected
with a photo-detector_ The sample and internal standard signals are amplified synchronously with each lamp current by each lock-in amplifier_ The intencitipc “.I.LU
gr~ u*_
pnnvprtd ~ULII~LUCU
intn ahenrhanmx hxr AAJYVLI”.TVLVcAAI~~U VJ
trvn “..V
nneratinnal VybLULUaV”U’
cxmnlifierc u”‘y~‘~~L’u_
The ll.L
sum, difference and ratio of the two absorbance signals are computed by analog computer. Each absorbance signal and the ratio of the signals are recorded on a three-pen recorder (Model B 361, Rika Denki Co.). Absorbances were obtained by measurement of peak heights. The atomizer consists of a pair of carbon rods which hold a carbon tube,
i monochrcrnato A
7
half mirror
I
I
8
11 phaseI!
/chmJW-B
j
I
I
I
i I
Fig. 1. Block diagram of dual wavelength system. H.C.L. = hollow-cathode AMP = amplifier_
recorder
lamp;
I
H!
131 a n d a p o w e r s u p p l y u n i t . T h e c a r b o n r o d h o l d e r is c o o l e d b y w a t e r . N i t r o g e n gas (4.0 1 r a i n - ' ) was e m p l o y e d f o r s h e a t h i n g . T h e p o w e r u n i t p r o v i d e s t h e n e c e s s a r y c u r r e n t f o r t h e c a r b o n t u b e in a p r e s e l e c t e d s e q u e n c e o f d r y i n g , a s h i n g a n d a t o m i z i n g . T h e a t o m i z i n g c y c l e c a n be o p e r a t e d in t w o m o d e s , a " s t e p " m o d e w h e n a c o n s t a n t s e l e c t e d v o l t a g e is a p p l i e d f o r a p r e s e t p e r i o d o f t i m e , o r a " r a m p " m o d e w h e n t h e v o l t a g e is l i n e a r l y i n c r e a s e d to a chosen voltage at a preselected rate. T h e t e m p e r a t u r e o f t h e c a r b o n t u b e was m e a s u r e d w i t h a C h i n o IR-PH R a d i a m a t i c p y r o m e t e r , w h i c h has a t e m p e r a t u r e r a n g e o f 1 0 0 0 - - 3 5 0 0 ° C .
Reagents and procedure Standard solutions. In all cases, s t o c k s o l u t i o n s o f c o p p e r , c o b a l t , a n d n i c k e l w e r e p r e p a r e d f r o m t h e i r a n a l y t i c a l r e a g e n t - g r a d e c h l o r i d e s in d o u b l y distilled, d e i o n i z e d w a t e r . T h e final s t o c k s o l u t i o n s ( 1 0 0 p g m l - ' o f e a c h e l e m e n t ) w e r e a d j u s t e d t o 0.1 M in h y d r o c h l o r i c a c i d a n d various o t h e r c o n c e n t r a t i o n s w e r e m a d e b y d i l u t i o n w i t h 0.1 M h y d r o c h l o r i c acid. A s t a n d a r d s o l u t i o n o f i r o n ( 1 0 0 p g F e ml-~) was p r e p a r e d b y dissolving a n a l y t i c a l - r e a g e n t g r a d e a m m o n i u m i r o n ( I I ) s u l p h a t e in d o u b l y distilled w a t e r a c i d i f i e d t o 0.1 M in h y d r o c h l o r i c acid. T h e w o r k i n g s t a n d a r d s o l u t i o n s w e r e p r e p a r e d daily. Water sample preparation. I r o n - f r e e 6 M h y d r o c h l o r i c a c i d (10 ml) was a d d e d t o 1 0 0 m l o f s a m p l e s o l u t i o n , w h i c h was t h e n e v a p o r a t e d t o 20 ml. A f t e r c o o l i n g , t h e s o l u t i o n was t r a n s f e r r e d t o a 50-ml v o l u m e t r i c flask a n d a k n o w n a m o u n t of an internal standard added.
Operation T h e c a r b o n t u b e was a d j u s t e d f o r m a x i ; , : : ' m signal, t a k i n g c a r e t h a t t h e o n l y r a d i a t i o n f r o m b o t h l a m p s r e a c h i n g t h e d e t e c t o r was t h a t passing t h r o u g h t h e c a r b o n t u b e . B e f o r e use, t h e t u b e was p u r i f i e d b y h e a t i n g t o a v e r y high t e m p e r a t u r e . T h e s a m p l e was i n j e c t e d f r o m a 5-pl p i p e t t e . T h e w a v e l e n g t h s a n d l a m p c u r r e n t s u s e d w e r e : Fe, 2 4 8 . 3 n m , 16 m A ; Co, 2 4 0 . 7 n m , 15 m A ; Ni, 2 3 2 . 0 n m , 15 m A ; Cu, 3 2 4 . 7 n m , 8 m A . T h e d r y i n g , ashing, a t o m i z a t i o n c y c l e was d o n e a u t o m a t i c a l l y . RESULTS AND DISCUSSION T h e i n t e r n a l s t a n d a r d f o r a p a r t i c u l a r e l e m e n t m u s t h a v e t h e s a m e relative p r o p e r t i e s w i t h r e s p e c t t o t h e m a t r i x a n d a t o m i z e r . T h e s e can be d e t e r m i n e d e x p e r i m e n t a l l y b y m a k i n g a b s o r b a n c e m e a s u r e m e n t s o n t h e e l e m e n t s as a function of the atomization conditions.
Drying and ashing conditions B e f o r e use, t h e c a r b o n t u b e was p u r i f i e d a t a high t e m p e r a t u r e . T h e s a m p l e was t h e n a p p l i e d , d r i e d , a s h e d a n d f i n a l l y a t o m i z e d . T h e o p t i m u m d r y i n g a n d ashing c o n d i t i o n s w e r e e s t a b l i s h e d as follows.
132 F o r e a c h m e a s u r e m e n t , 5 ul o f an 0 . 0 5 u g m l - i i r o n s o l u t i o n w a s i n j e c t e d . N o s i g n i f i c a n t c h a n g e in t h e i n t e n s i t y w a s o b e r v e d w h e n t h e d r y i n g t i m e w a s varied from 10 to 20 s at a drying voltage of 2.0 V with an atomizing temperature of 2630°C. Therefore, the voltage for the drying cycle was chosen as 2 . 0 V f o r 1 4 s s o as t o r e m o v e s o l v e n t b y g e n t l e b o i l i n g , t h u s p r e v e n t i n g l o s s e s b y s p u t t e r i n g . A s h i n g w a s a c c o m p l i s h e d in 5 - - 1 0 s a t a v o l t a g e o f 2 . 0 V. C o n s e q u e n t l y , all o f t h e f o l l o w i n g e x p e r i m e n t s w e r e c a r r i e d o u t w i t h a s h i n g a t 2 . 0 V f o r 1 0 s_
F u r n a c e t e m p e r a t u r e a n d a t o m i z a t i o n behavior o f e l e m e n t s T o o b t a i n t h e r a t e o f a t o m i z a t i o n a n d t h u s t h e n u m b e r o f f r e e a t o m s in a n a t o m i z a t i o n cell, t h e b e h a v i o r o f v a r i o u s m e t a l s w a s s t u d i e d w i t h r e s p e c t t o t h e i r v o l a t i l i t y . F i g u r e 2 s h o w s t h e c h a n g e in t e m p e r a t u r e o f c a r b o n t u b e with step atomization voltage. The measurements pertain to the temperature o f a s p o t o n t h e o u t s i d e o f t h e c a r b o n t u b e . A l t h o u g h t h e tern p e r a t u r e d i f f e r e n c e b e t w e e n t h e o u t s i d e a n d i n s i d e o f t h e t u b e w a s n o t e s t i m a t e d , it c a n b e c o n s i d e r e d t h a t t h e r e is l i t t l e d i f f e r e n c e o f t e m p e r a t u r e b e c a u s e t h e tube has a small cross-section and high electric resistance at this position,
3000i
/
A 2000 ".?_. 0,. E
/ 100( /
0
/
/
/
/
/
1
5.0
/
v
0
\x\
a Q.
5 A t o m i z i n g voltoge ( V )
10
0 1420
l
r
2030
2630
Atormzotion
temp.
(°C)
Fig. 2. Variation o f carbon t u b e t e m p e r a t u r e w i t h a t o m i z a t i o n voltage during preset t i m e o f 3 s. Drying, 2 . 0 V, 1 4 s; ashing, 2 . 0 V, 1 0 s. Fig. 3. Variation o f t h e appearance t i m e o f t h e a b s o r p t i o n signal w i t h c a r b o n t u b e t e m perature. D r y i n g and ashing as f o r Fig. 2; 5-ul s a m p l e ; c o n c e n t r a t i o n s (~g m l -~) F e 0 . 0 2 , C o 0 . 0 5 , Ni 0 . 2 0 , Cu 0 . 0 5 .
3240
133
and the temperature of the whole tube rises rapidly and homogeneously
as power is supplied. The precision of the temperature values was +. 5%. Figure 3 shows that the appearance time (i.e. the time between starting the atomization phase and the appearance of the absorption signal) of the absorption signal of various elements depends on the atomization temperature. The elkments were atomized by employing identical heating procedures (step mode, 4-10 V; 10 s). It is clear from the results that increasing atomization temperatures are accompanied by marked decreases in the appearance time, though considerably less so in the higher temperature range. Iron and cobalt have almost identical thermal behavior, unlike the other elements investigated. Thus it should be possible to use cobalt as an internal standard
for iron.
Choice of in ternal standard Effect of atomization temperature. The effect of atomization temperature on the absorbance of cobalt, nickel and copper alone, and when used as internal standards for iron absorbance measurements, is illustrated in Fig. 4. When the atomization temperature was raised from about 1800 to 29OO”C, a gradual increase in the peak absorbance of iron was shown in the absence of an internal standard. However, as demonstrated in Fig. 4, the absorbance of iron and cobalt varied similarly when the temperature was changed and the ratio of iron to cobalt absorbances remained almost constant from 2100 to 2900°C. In contrast, Fig. 4 shows that copper would be a poor internal standard for iron.
.__._&___‘__
1810
.
.
.._._.
c.c!...a .. . .. . ---..-
2350 Temp
2090 PC)
Fig. 4. Effect of the atomization temperature on absorbance and absorbance ratio. Drying and ashing as for Fig. 2; 3-s atomization. Concentrations (pg ml-‘): Fe 0.02, Co 0.05, Ni 0.20, Cu 0.05; ~-PI sample.
134 Effect of atomization time. In the step-mode operation, a constant selected voltage is applied for a preset time; when this time is changed from 1 to 10 s the furnace temperature increases rapidly. Therefore, the effect of atomization time on absorption intensity was studied. Figure 5 shows the change in absorbance of each element and absorbance ratio of iron to each element with a preset atomization time, holding all other conditions constant, and with an atomization voltage of 9 V. Atomization temperature increases with increasing atomization time, as would be expected, increasing by 900°C during the 9 s. As can be seen from Fig. 5, metal atom production is also affected by the atomization time used; atomization is achieved in 3-4 s, but the temperature of the furnace is increased until all the element to be measured is atomized. The recommended instrumental conditions are summarized in Table 1. Effect of amount of sample. Aliquots of 5,10,15 and 20 ,~l of 0.02 pg Fe ml-’ standard solutions were used to examine the effect of amount of sample introduced into the carbon tube on the absorbance_ The atomization temperature was 2630°C and the atomization time 3 s. Figure 6 shows t.hat the absorbance of iron increases proportionally with the amount of sample introduced, but that the absorbance ratios remain relatively constant, again demonstrating the value of the internal standard method. The ratio of iron to cobalt absorbanced showed little change for 5-20 ~1 of sample.
i
i
1
10
5 Atonwotm
tame (5
I
Fig. 5. Effect of the atomization time on absorbance and absorbance ratio. Concentrations (pg ml-‘): Fe 0.02, Co 0.05, Ni 0.20, Cu 0.05; ~-PI sample.
135 TABLE Operating
1 conditions
for measurement
of iron
Fe wavelength
248.3
nm, lamp
current
18 mA
Co wavelength
240.7
nm, lamp
current
16
Drying Ashing Atomizing Nitrogen sheathing
(A
channel)
mA (B channel)
2.0 V for 14 s 2.0 V for 10 s 9.0 V for 3 s 4 1 min-’
gas
20
co
0 Amount
Fig.
6. Effect
Fig.
4.
Fig.
7. Effect
Concentrations
of sample
solutw7
volume
of the sample
of hydrochloric (pg
ml-‘):
on absorbance
acid concentration
Fe 0.04.
Co 0.04;
and absorbance
on absorbance
020
010 HCI
(~1)
ratio.
(t.4)
Conditions
and absorbance
as
ratio.
5-~1 sample.
Interference studies Several possible interferences were examined. Chemical and physical interferences are assumed to occur, such’as the influence of anions, cations, pH and refractory matrices, and the formation of stable oxides or other molecules. Acid interferences. The absorbances of aqueous 0.04 pg Fe ml- ’ and 0.04 pg Co ml-’ solutions containing various concentrations of hydrochloric acid were measured under the standard conditions in Table 1. As seen in Fig. 7, the absorbance of iron is slightly increased by addition of hydrochloric acid, but the absorbance change of cobalt is small. Thus the absorbance ratio increases at relatively low acidities, so that the internal
136
standard is without effect. It is thought that this results from the iron being hydrolysed at low acid concentrations_ Consequently, hydrochloric acid should be used to keep the pH below 1, to suppress interference by hydrolysis. Cation interferences_ Interference by cations is common in electrothermal atomic absorption spectrometry. In this study, the effect of nine cations on the iron: cobalt absorbance ratio was measured_ The solutions used contained Fe (0.05 pg ml-‘) and Co (0.05 pg ml-‘) in 0.1 M hydrochloric acid- Other cations were added mostly as chlorides; silicon was added as sodium silicate. Figure 8 shows that copper, cadmium and chromium cause little enhancement of absorbance ratio except at high (500 ppm) concentrations. When the sample contains sodium, potassium or lead at 500 pg ml-‘, noticeable reduction of the absorbance ratio occurs. The presence of more than 50 fig ml- 1 magnesium greatly increases the absorbance ratio because magnesium causes considerable depression of cobalt absorbance_ On the contrary, it was found that the presence of about 5 pg ml-’ calcium decreased the absorbance ratio_ This may result from a change in the vaporization rate of either iron or cobalt from the carbon tube. It was thought that one of the principal advantages of the internal standard method would be a correction of such matrix effects. Indeed, in flame atomic absorption spectrometry, an internal standard largely overcomes these interference problems [ 51.
0
005
050
Metal
Fig_ 8. Effect Concentrations
mn cont.
50
500
500
(pg ml-‘)
of cations on the iron: cobalt absorbance (pg ml-‘): Fe 0.05, Co 0.05.
Fig. 9. Calibration tions as in Table
Iron conc.(pg
ratio.
Conditions
ml-9
as in Table
1.
graph, for the direct and internal standard methods for iron. Condil_
137 TABLE
2
Comparison of measurements of iron in waters by the direct and internal standard methods. Five analyses were done in each case. TPTZ methoda (ng ml-‘)
Sample
Boiler water
Glacier water
1 2 15 32
1 2 3
98 35 110 23 7 28 331
Range (ng ml-’ )
Mean
R.s.d_ (S)
Range (ng ml-* )
Mean
R.s.d_ (a)
101-113 32-36 104-115 24-28
107 34 107 26 8 27 321
4.3 4.7 4.6
100-105 33-35 110-114 26-27 8-9 27-28 319-326
103 34 112 27 8 27 322
2.5 2.6 1.6 1.7 1.7 2.0 1.0 1.9
8-9 25-30 305-331
Mean aSpectrophotometric
Internal standard method
Direct method
determination
s”-: 814 4.7 5.6
(see text).
Obviously, however, in electrothermal work, internal standard and analyte elements must have the same physical and chemical properties with respect to the matrices, as well as similar general atomization characteristics. For all elements except calcium, the absorbance ratio of iron to cobalt is independent of the type or amount of matrix until the amount of matrix is 100 times that of iron. Hence cobalt is considered to be the most suitable internal standard for the determination of iron in water. Precision and accuracy For the determination of iron in water, the step mode atomization procedure was employed and the time and voltage settings in Table 1 were used. The calibration curves for the direct and internal standard methods are given in Fig. 9 for comparison. The calibration lines are based on replicate (5) measurements of five standards of different concentrations. With cobalt as an internal standard, the iron values are more reproducible than those obtained by the single-channel method. Table 2 summarizes the precision and accuracy data obtained with the internal standard and direct methods, under the recommended conditions. A series of iron determinations on different water samples utilizing a cobalt internal standard gave relative standard deviations of l.O-2.4%. The direct method under the same conditions had relative standard deviations of 4.3-8.4%. The median deviation for the direct method was 5.6%, and for the internal standard method was 1.9%. Again the internal standard method improves the precision of the measurements. The accuracy of the method was ascertained by comparison with a spectrophotometric procedure based on the iron(2, 4, 6-tris(2’-pyridyl)-s-triazine complex [6, 71 in lOO-mm cells at 595 nm.
138 Conclusions For internal standardization in electrothermal atomic absorption spectrometry, the choice of the internal standard element is extremely important. The first requirement is that the analyte and standard element are quite similarly influenced by changes in experimental variables. Therefore, the physical and chemical properties of both elements must be very similar. By using cobalt as an internal standard, the determination of iron is made more accurate and precise by reducing the influence of atomizer temperature variations, minimizing the effect of changing the amount of sample injected and, sometimes, decreasing the interferences from cations. Another advantage is that the internal standard will also correct for sample dilution errors because both the internal standard element and the sample element will be subject to the same dilution effect. REFERENCES 1 IV. Gerlach and E. Schweitzer. Foundations and Methods of Chemical Analysis by the Emission Spectrum, Vol. 2, Voss Adam Hilger, London, 1929. p_ 2. 2 L. R. P. Butler and A. Strasheim, Spectrochim. Acta, 7 (1965) 1207. 3 F. J. Feldman, Anal. Chem., 42 (1970) 719. 4 K. Nakano. T. Takada and T. Satho, Nippon Kagaku Zasshi, 91 (1970) 293. 5 T. Takada, unpublished data. 6 F. H. Case and E. Koft, J. Am. Chem. Sot., 81(1959) 905. 7 P. F. Collins, H. Diehl and G. F. Smith, Anal. Chem., 31 (1959) 1862.
-
Printed in The Netherlands
EVALUATION AND APPLICATION OF INTERNAL STANDARDIZATION IN ATOMIC ABSORPTION SPECTROMETRY WITH ELECTROTHERMAL ATOMIZATION
TAKE0
TAKADA*
and KUNIO
NAKANO
Department of Chemistry, College of Science, Rikkyo Ikebukuro, Toshima-ku. Tokyo I71 (Japan) (Received
19th September
(St. Paul’s) University,
Nishi-
1978)
SUMMARY A new dual-channel system developed for use in atomic absorption spectrometry is used to assess the internal standard technique for electrothermal atomization. Cobalt was found to be a suitable internal standard for iron determinations, and is used for determination of 7-330 ng Fe ml-’ in water samples. With use of the in+xrnal standard technique, fluctuations caused by atomizer variables are reduced and interferences from many cations are also decreased.
Compared with conventional flame methods, electrothermal atomization makes the determination of trace metals more rapid and much more sensitive_ However, the absorbance from electrothermal atomizers is variable, because of factors such as atomizer temperature, heating rate, sample amount, etc. This is considered to be the cause of the decreased accuracy and precision of the analytical results obtained when such atomizers are used. In order to minimize the variations in signals arising from atomizer conditions, a definite amount of an element can be added to each sample as an internal standard. As the absorbances of the analyte and standard elements are considered to be simiiariy influenced by the various factors, the ratio of the absorbances may be free from the influence of variable conditions. The use of an internal standard to increase the precision of emission spectroscopic methods was introduced by Gerlach and Schweitzer [l] _ The application of an internal standard to &omic absorption spectrometry has been difficult because the instruments are monochromatic, and few examples are known [Z--4] _ Rules for sekcting internal standard elements have not yet been completely discussed, nor has any practical anolication --1~~---------
to electrothermal
at.0m-i~ ahsnrntinn r-----
heen
nuhlished. L-----------
In the present paper the utilization of an internal standard method is described, whereby most of the errors associated with the atomizer and atomization process are compensated. Samples of water were selected for’ investigation because total iron in water can be measured rapidly by elec-
130 trothermal atomic absorption spectrometry. The optimum conditions for the analysis, including the choice of the proper internal standard, are described. EXPERIMENTAL
Apparatus A dual-channel atomic absorption spectrometer was designed in this laboratory and assembled by Nippon Jarrell-Ash Co. The burner assembly was replaced by a carbon-tube atomizer (V&an-Techtron model 63). The instrument has two separate channels, one of which is used for the analyte element and the other for the element added; the ratio of the absorbances at the two wavelengths is measured_ A block diagram of photometric system is shown in Fig. 1. Two hollowcathode lamps are placed at right angles to each other, one for the analyte element, one for the standard element. The hollow-cathode lamps are alternately pulsed by a square-wave current. The light from the hollowcathode lamps is combined by a half-mirror and after, passing through the small carbon tube, it is split again into two beams by a second half-
mirror.
Each beam falls in the slit of a monochromator
and each is detected
with a photo-detector_ The sample and internal standard signals are amplified synchronously with each lamp current by each lock-in amplifier_ The intencitipc “.I.LU
gr~ u*_
pnnvprtd ~ULII~LUCU
intn ahenrhanmx hxr AAJYVLI”.TVLVcAAI~~U VJ
trvn “..V
nneratinnal VybLULUaV”U’
cxmnlifierc u”‘y~‘~~L’u_
The ll.L
sum, difference and ratio of the two absorbance signals are computed by analog computer. Each absorbance signal and the ratio of the signals are recorded on a three-pen recorder (Model B 361, Rika Denki Co.). Absorbances were obtained by measurement of peak heights. The atomizer consists of a pair of carbon rods which hold a carbon tube,
i monochrcrnato A
7
half mirror
I
I
8
11 phaseI!
/chmJW-B
j
I
I
I
i I
Fig. 1. Block diagram of dual wavelength system. H.C.L. = hollow-cathode AMP = amplifier_
recorder
lamp;
I
H!
131 a n d a p o w e r s u p p l y u n i t . T h e c a r b o n r o d h o l d e r is c o o l e d b y w a t e r . N i t r o g e n gas (4.0 1 r a i n - ' ) was e m p l o y e d f o r s h e a t h i n g . T h e p o w e r u n i t p r o v i d e s t h e n e c e s s a r y c u r r e n t f o r t h e c a r b o n t u b e in a p r e s e l e c t e d s e q u e n c e o f d r y i n g , a s h i n g a n d a t o m i z i n g . T h e a t o m i z i n g c y c l e c a n be o p e r a t e d in t w o m o d e s , a " s t e p " m o d e w h e n a c o n s t a n t s e l e c t e d v o l t a g e is a p p l i e d f o r a p r e s e t p e r i o d o f t i m e , o r a " r a m p " m o d e w h e n t h e v o l t a g e is l i n e a r l y i n c r e a s e d to a chosen voltage at a preselected rate. T h e t e m p e r a t u r e o f t h e c a r b o n t u b e was m e a s u r e d w i t h a C h i n o IR-PH R a d i a m a t i c p y r o m e t e r , w h i c h has a t e m p e r a t u r e r a n g e o f 1 0 0 0 - - 3 5 0 0 ° C .
Reagents and procedure Standard solutions. In all cases, s t o c k s o l u t i o n s o f c o p p e r , c o b a l t , a n d n i c k e l w e r e p r e p a r e d f r o m t h e i r a n a l y t i c a l r e a g e n t - g r a d e c h l o r i d e s in d o u b l y distilled, d e i o n i z e d w a t e r . T h e final s t o c k s o l u t i o n s ( 1 0 0 p g m l - ' o f e a c h e l e m e n t ) w e r e a d j u s t e d t o 0.1 M in h y d r o c h l o r i c a c i d a n d various o t h e r c o n c e n t r a t i o n s w e r e m a d e b y d i l u t i o n w i t h 0.1 M h y d r o c h l o r i c acid. A s t a n d a r d s o l u t i o n o f i r o n ( 1 0 0 p g F e ml-~) was p r e p a r e d b y dissolving a n a l y t i c a l - r e a g e n t g r a d e a m m o n i u m i r o n ( I I ) s u l p h a t e in d o u b l y distilled w a t e r a c i d i f i e d t o 0.1 M in h y d r o c h l o r i c acid. T h e w o r k i n g s t a n d a r d s o l u t i o n s w e r e p r e p a r e d daily. Water sample preparation. I r o n - f r e e 6 M h y d r o c h l o r i c a c i d (10 ml) was a d d e d t o 1 0 0 m l o f s a m p l e s o l u t i o n , w h i c h was t h e n e v a p o r a t e d t o 20 ml. A f t e r c o o l i n g , t h e s o l u t i o n was t r a n s f e r r e d t o a 50-ml v o l u m e t r i c flask a n d a k n o w n a m o u n t of an internal standard added.
Operation T h e c a r b o n t u b e was a d j u s t e d f o r m a x i ; , : : ' m signal, t a k i n g c a r e t h a t t h e o n l y r a d i a t i o n f r o m b o t h l a m p s r e a c h i n g t h e d e t e c t o r was t h a t passing t h r o u g h t h e c a r b o n t u b e . B e f o r e use, t h e t u b e was p u r i f i e d b y h e a t i n g t o a v e r y high t e m p e r a t u r e . T h e s a m p l e was i n j e c t e d f r o m a 5-pl p i p e t t e . T h e w a v e l e n g t h s a n d l a m p c u r r e n t s u s e d w e r e : Fe, 2 4 8 . 3 n m , 16 m A ; Co, 2 4 0 . 7 n m , 15 m A ; Ni, 2 3 2 . 0 n m , 15 m A ; Cu, 3 2 4 . 7 n m , 8 m A . T h e d r y i n g , ashing, a t o m i z a t i o n c y c l e was d o n e a u t o m a t i c a l l y . RESULTS AND DISCUSSION T h e i n t e r n a l s t a n d a r d f o r a p a r t i c u l a r e l e m e n t m u s t h a v e t h e s a m e relative p r o p e r t i e s w i t h r e s p e c t t o t h e m a t r i x a n d a t o m i z e r . T h e s e can be d e t e r m i n e d e x p e r i m e n t a l l y b y m a k i n g a b s o r b a n c e m e a s u r e m e n t s o n t h e e l e m e n t s as a function of the atomization conditions.
Drying and ashing conditions B e f o r e use, t h e c a r b o n t u b e was p u r i f i e d a t a high t e m p e r a t u r e . T h e s a m p l e was t h e n a p p l i e d , d r i e d , a s h e d a n d f i n a l l y a t o m i z e d . T h e o p t i m u m d r y i n g a n d ashing c o n d i t i o n s w e r e e s t a b l i s h e d as follows.
132 F o r e a c h m e a s u r e m e n t , 5 ul o f an 0 . 0 5 u g m l - i i r o n s o l u t i o n w a s i n j e c t e d . N o s i g n i f i c a n t c h a n g e in t h e i n t e n s i t y w a s o b e r v e d w h e n t h e d r y i n g t i m e w a s varied from 10 to 20 s at a drying voltage of 2.0 V with an atomizing temperature of 2630°C. Therefore, the voltage for the drying cycle was chosen as 2 . 0 V f o r 1 4 s s o as t o r e m o v e s o l v e n t b y g e n t l e b o i l i n g , t h u s p r e v e n t i n g l o s s e s b y s p u t t e r i n g . A s h i n g w a s a c c o m p l i s h e d in 5 - - 1 0 s a t a v o l t a g e o f 2 . 0 V. C o n s e q u e n t l y , all o f t h e f o l l o w i n g e x p e r i m e n t s w e r e c a r r i e d o u t w i t h a s h i n g a t 2 . 0 V f o r 1 0 s_
F u r n a c e t e m p e r a t u r e a n d a t o m i z a t i o n behavior o f e l e m e n t s T o o b t a i n t h e r a t e o f a t o m i z a t i o n a n d t h u s t h e n u m b e r o f f r e e a t o m s in a n a t o m i z a t i o n cell, t h e b e h a v i o r o f v a r i o u s m e t a l s w a s s t u d i e d w i t h r e s p e c t t o t h e i r v o l a t i l i t y . F i g u r e 2 s h o w s t h e c h a n g e in t e m p e r a t u r e o f c a r b o n t u b e with step atomization voltage. The measurements pertain to the temperature o f a s p o t o n t h e o u t s i d e o f t h e c a r b o n t u b e . A l t h o u g h t h e tern p e r a t u r e d i f f e r e n c e b e t w e e n t h e o u t s i d e a n d i n s i d e o f t h e t u b e w a s n o t e s t i m a t e d , it c a n b e c o n s i d e r e d t h a t t h e r e is l i t t l e d i f f e r e n c e o f t e m p e r a t u r e b e c a u s e t h e tube has a small cross-section and high electric resistance at this position,
3000i
/
A 2000 ".?_. 0,. E
/ 100( /
0
/
/
/
/
/
1
5.0
/
v
0
\x\
a Q.
5 A t o m i z i n g voltoge ( V )
10
0 1420
l
r
2030
2630
Atormzotion
temp.
(°C)
Fig. 2. Variation o f carbon t u b e t e m p e r a t u r e w i t h a t o m i z a t i o n voltage during preset t i m e o f 3 s. Drying, 2 . 0 V, 1 4 s; ashing, 2 . 0 V, 1 0 s. Fig. 3. Variation o f t h e appearance t i m e o f t h e a b s o r p t i o n signal w i t h c a r b o n t u b e t e m perature. D r y i n g and ashing as f o r Fig. 2; 5-ul s a m p l e ; c o n c e n t r a t i o n s (~g m l -~) F e 0 . 0 2 , C o 0 . 0 5 , Ni 0 . 2 0 , Cu 0 . 0 5 .
3240
133
and the temperature of the whole tube rises rapidly and homogeneously
as power is supplied. The precision of the temperature values was +. 5%. Figure 3 shows that the appearance time (i.e. the time between starting the atomization phase and the appearance of the absorption signal) of the absorption signal of various elements depends on the atomization temperature. The elkments were atomized by employing identical heating procedures (step mode, 4-10 V; 10 s). It is clear from the results that increasing atomization temperatures are accompanied by marked decreases in the appearance time, though considerably less so in the higher temperature range. Iron and cobalt have almost identical thermal behavior, unlike the other elements investigated. Thus it should be possible to use cobalt as an internal standard
for iron.
Choice of in ternal standard Effect of atomization temperature. The effect of atomization temperature on the absorbance of cobalt, nickel and copper alone, and when used as internal standards for iron absorbance measurements, is illustrated in Fig. 4. When the atomization temperature was raised from about 1800 to 29OO”C, a gradual increase in the peak absorbance of iron was shown in the absence of an internal standard. However, as demonstrated in Fig. 4, the absorbance of iron and cobalt varied similarly when the temperature was changed and the ratio of iron to cobalt absorbances remained almost constant from 2100 to 2900°C. In contrast, Fig. 4 shows that copper would be a poor internal standard for iron.
.__._&___‘__
1810
.
.
.._._.
c.c!...a .. . .. . ---..-
2350 Temp
2090 PC)
Fig. 4. Effect of the atomization temperature on absorbance and absorbance ratio. Drying and ashing as for Fig. 2; 3-s atomization. Concentrations (pg ml-‘): Fe 0.02, Co 0.05, Ni 0.20, Cu 0.05; ~-PI sample.
134 Effect of atomization time. In the step-mode operation, a constant selected voltage is applied for a preset time; when this time is changed from 1 to 10 s the furnace temperature increases rapidly. Therefore, the effect of atomization time on absorption intensity was studied. Figure 5 shows the change in absorbance of each element and absorbance ratio of iron to each element with a preset atomization time, holding all other conditions constant, and with an atomization voltage of 9 V. Atomization temperature increases with increasing atomization time, as would be expected, increasing by 900°C during the 9 s. As can be seen from Fig. 5, metal atom production is also affected by the atomization time used; atomization is achieved in 3-4 s, but the temperature of the furnace is increased until all the element to be measured is atomized. The recommended instrumental conditions are summarized in Table 1. Effect of amount of sample. Aliquots of 5,10,15 and 20 ,~l of 0.02 pg Fe ml-’ standard solutions were used to examine the effect of amount of sample introduced into the carbon tube on the absorbance_ The atomization temperature was 2630°C and the atomization time 3 s. Figure 6 shows t.hat the absorbance of iron increases proportionally with the amount of sample introduced, but that the absorbance ratios remain relatively constant, again demonstrating the value of the internal standard method. The ratio of iron to cobalt absorbanced showed little change for 5-20 ~1 of sample.
i
i
1
10
5 Atonwotm
tame (5
I
Fig. 5. Effect of the atomization time on absorbance and absorbance ratio. Concentrations (pg ml-‘): Fe 0.02, Co 0.05, Ni 0.20, Cu 0.05; ~-PI sample.
135 TABLE Operating
1 conditions
for measurement
of iron
Fe wavelength
248.3
nm, lamp
current
18 mA
Co wavelength
240.7
nm, lamp
current
16
Drying Ashing Atomizing Nitrogen sheathing
(A
channel)
mA (B channel)
2.0 V for 14 s 2.0 V for 10 s 9.0 V for 3 s 4 1 min-’
gas
20
co
0 Amount
Fig.
6. Effect
Fig.
4.
Fig.
7. Effect
Concentrations
of sample
solutw7
volume
of the sample
of hydrochloric (pg
ml-‘):
on absorbance
acid concentration
Fe 0.04.
Co 0.04;
and absorbance
on absorbance
020
010 HCI
(~1)
ratio.
(t.4)
Conditions
and absorbance
as
ratio.
5-~1 sample.
Interference studies Several possible interferences were examined. Chemical and physical interferences are assumed to occur, such’as the influence of anions, cations, pH and refractory matrices, and the formation of stable oxides or other molecules. Acid interferences. The absorbances of aqueous 0.04 pg Fe ml- ’ and 0.04 pg Co ml-’ solutions containing various concentrations of hydrochloric acid were measured under the standard conditions in Table 1. As seen in Fig. 7, the absorbance of iron is slightly increased by addition of hydrochloric acid, but the absorbance change of cobalt is small. Thus the absorbance ratio increases at relatively low acidities, so that the internal
136
standard is without effect. It is thought that this results from the iron being hydrolysed at low acid concentrations_ Consequently, hydrochloric acid should be used to keep the pH below 1, to suppress interference by hydrolysis. Cation interferences_ Interference by cations is common in electrothermal atomic absorption spectrometry. In this study, the effect of nine cations on the iron: cobalt absorbance ratio was measured_ The solutions used contained Fe (0.05 pg ml-‘) and Co (0.05 pg ml-‘) in 0.1 M hydrochloric acid- Other cations were added mostly as chlorides; silicon was added as sodium silicate. Figure 8 shows that copper, cadmium and chromium cause little enhancement of absorbance ratio except at high (500 ppm) concentrations. When the sample contains sodium, potassium or lead at 500 pg ml-‘, noticeable reduction of the absorbance ratio occurs. The presence of more than 50 fig ml- 1 magnesium greatly increases the absorbance ratio because magnesium causes considerable depression of cobalt absorbance_ On the contrary, it was found that the presence of about 5 pg ml-’ calcium decreased the absorbance ratio_ This may result from a change in the vaporization rate of either iron or cobalt from the carbon tube. It was thought that one of the principal advantages of the internal standard method would be a correction of such matrix effects. Indeed, in flame atomic absorption spectrometry, an internal standard largely overcomes these interference problems [ 51.
0
005
050
Metal
Fig_ 8. Effect Concentrations
mn cont.
50
500
500
(pg ml-‘)
of cations on the iron: cobalt absorbance (pg ml-‘): Fe 0.05, Co 0.05.
Fig. 9. Calibration tions as in Table
Iron conc.(pg
ratio.
Conditions
ml-9
as in Table
1.
graph, for the direct and internal standard methods for iron. Condil_
137 TABLE
2
Comparison of measurements of iron in waters by the direct and internal standard methods. Five analyses were done in each case. TPTZ methoda (ng ml-‘)
Sample
Boiler water
Glacier water
1 2 15 32
1 2 3
98 35 110 23 7 28 331
Range (ng ml-’ )
Mean
R.s.d_ (S)
Range (ng ml-* )
Mean
R.s.d_ (a)
101-113 32-36 104-115 24-28
107 34 107 26 8 27 321
4.3 4.7 4.6
100-105 33-35 110-114 26-27 8-9 27-28 319-326
103 34 112 27 8 27 322
2.5 2.6 1.6 1.7 1.7 2.0 1.0 1.9
8-9 25-30 305-331
Mean aSpectrophotometric
Internal standard method
Direct method
determination
s”-: 814 4.7 5.6
(see text).
Obviously, however, in electrothermal work, internal standard and analyte elements must have the same physical and chemical properties with respect to the matrices, as well as similar general atomization characteristics. For all elements except calcium, the absorbance ratio of iron to cobalt is independent of the type or amount of matrix until the amount of matrix is 100 times that of iron. Hence cobalt is considered to be the most suitable internal standard for the determination of iron in water. Precision and accuracy For the determination of iron in water, the step mode atomization procedure was employed and the time and voltage settings in Table 1 were used. The calibration curves for the direct and internal standard methods are given in Fig. 9 for comparison. The calibration lines are based on replicate (5) measurements of five standards of different concentrations. With cobalt as an internal standard, the iron values are more reproducible than those obtained by the single-channel method. Table 2 summarizes the precision and accuracy data obtained with the internal standard and direct methods, under the recommended conditions. A series of iron determinations on different water samples utilizing a cobalt internal standard gave relative standard deviations of l.O-2.4%. The direct method under the same conditions had relative standard deviations of 4.3-8.4%. The median deviation for the direct method was 5.6%, and for the internal standard method was 1.9%. Again the internal standard method improves the precision of the measurements. The accuracy of the method was ascertained by comparison with a spectrophotometric procedure based on the iron(2, 4, 6-tris(2’-pyridyl)-s-triazine complex [6, 71 in lOO-mm cells at 595 nm.
138 Conclusions For internal standardization in electrothermal atomic absorption spectrometry, the choice of the internal standard element is extremely important. The first requirement is that the analyte and standard element are quite similarly influenced by changes in experimental variables. Therefore, the physical and chemical properties of both elements must be very similar. By using cobalt as an internal standard, the determination of iron is made more accurate and precise by reducing the influence of atomizer temperature variations, minimizing the effect of changing the amount of sample injected and, sometimes, decreasing the interferences from cations. Another advantage is that the internal standard will also correct for sample dilution errors because both the internal standard element and the sample element will be subject to the same dilution effect. REFERENCES 1 IV. Gerlach and E. Schweitzer. Foundations and Methods of Chemical Analysis by the Emission Spectrum, Vol. 2, Voss Adam Hilger, London, 1929. p_ 2. 2 L. R. P. Butler and A. Strasheim, Spectrochim. Acta, 7 (1965) 1207. 3 F. J. Feldman, Anal. Chem., 42 (1970) 719. 4 K. Nakano. T. Takada and T. Satho, Nippon Kagaku Zasshi, 91 (1970) 293. 5 T. Takada, unpublished data. 6 F. H. Case and E. Koft, J. Am. Chem. Sot., 81(1959) 905. 7 P. F. Collins, H. Diehl and G. F. Smith, Anal. Chem., 31 (1959) 1862.