Houba, V.J.G.

Lee, J.J., van der

Novozamsky, I.

Temminghoff, E.

van der Lee, J.J.

NARCIS 

Background correction in the determination of Cd and Pb by flame AAS in plant and soil samples with high Fe levels.

Name not available

Wageningen Yield 

Background Corrections in the Determination of Cd and Pb by 
Flame AAS in Plant and Soil Samples with High Fe Levels 
J. J. VAN DER LEE,* E. TEMMINGHOFF, V. J. G. HOUBA, and I. NOVOZAMSKY 
Department ofSoil Science and Plant Nutrition, Agricultural University, De Dr6ijen 3, 6703 BC Wageningen, The Netherlands 
Plant leaf digests and soil extracts were analyzed for Cd and Pb by 
flame AAS in three ways: with deuterium background correction, with 
the Smith-Hieftje background correction system, and without any back- 
ground correction. Background correction proved to be necessary for 
both elements and for both types of material. The two background cor- 
rection systems gave equal results, provided that Fe concentrations were 
low. High Fe concentrations caused spectral interference (i.e., structured 
background), which was overcompensated by the deuterium system. The 
Smith-Hieftje system, however, produced correct results. 
Index Headings: Background correction, Smith-Hieftje system, deute- 
r ium system, Cd and Pb determination, flame AAS. 
INTRODUCTION 
Since the recognition of Cd and Pb as physiologically 
and environmentally toxic metals, much attention has 
been paid to their determination in various matrices, 
such as blood, tissue, plant leaves, soil, etc. Both ele- 
ments can be determined by a variety of methods, of 
which voltammetry and atomic absorption spectrometry 
(AAS) are most frequently applied. In the latter tech- 
nique, a flame atomizer is preferred with respect o a 
graphite furnace--if concentrations are high enough-- 
since flame AAS is more rapid and less prone to inter- 
ferences. 
Among many other elements, Cd and Pb are deter- 
mined in the course of an annual Dutch interlaboratory 
analysis program for soils and plant material. Within 
this framework, it was noticed that the results for Cd in 
a Dutch river clay soil showed remarkably large varia- 
tions. The same was true for Pb, although to less extent. 
All laboratories used flame AAS for the final determi- 
nation, but with different apparatus and different pre- 
treatment procedures. The only relevant common factor 
appeared to be the extremely high HCl-extractable Fe 
content of this soil (25 g per kg). It was assumed that 
such an amount of Fe could cause line and/or back- 
ground interference. Therefore, the participants took 
measurements with and without background correction 
on standard Cd solutions to which a large amount of Fe 
had been added. Without background correction, the 
results were higher than the nominal Cd concentrations. 
Received 1 August  1985; revision received 25 August  1986. 
* Author to whom correspondence should be sent. 
Application of deuterium arc background correction, on 
the contrary, resulted in lower figures, which indicates 
overcompensation. This gave cause for testing a double- 
pulsed hollow-cathode-lamp system (the so-called Smith- 
Hieftje background correction system 1) in flame AAS 
measurements of Cd and Pb in the above-mentioned 
synthetic solutions, in plant material, and in soil sam- 
ples. 
EXPERIMENTAL 
Apparatus. Measurements were taken on an S-11 
atomic absorption spectrophotometer (Instrumentation 
Laboratory, Andover, MA), equipped with both a deu- 
terium background corrector (D2) and a Smith-Hieftje 
background correction system (SH). A stoichiometric air/ 
acetylene flame was used. For Cd the wavelength 228.8 
nm was chosen; and for Pb, 217.0 nm. The slit width 
was adjusted so as to give a bandpass of I nm. 
For reference purposes, some measurements were tak- 
en on a PE 460 atomic absorption spectrophotometer 
(Perkin-Elmer, Norwalk, CT), equipped with an HGA 
76 graphite furnace, an AS-1 autosampler, and a deu- 
terium background corrector. 
Reagents. Standard solutions for Cd and Pb were pre- 
pared from Titrisol ampoules (Merck, Darmstadt, GFR; 
Nos. 9961 and 9969, respectively) and diluted appropri- 
ately. We prepared standard Fe solutions by dissolving 
FeSO4.6H20 in 0.12-M HC1 at appropriate dilution. All 
reagents were analytical grade, and "demi-dest" (i.e., 
distilled after demineralization) water was used 
throughout. 
Pretreatment Procedures. Plant Material. Leaf sam- 
ples of different plant species were air-dried, ground, 
and dry-ashed in duplicate according to a procedure 
adopted from the Comit6 Inter-Instituts d'Etudes des 
Techniques Analytiques du Diagnostic Foliaire (C.I.I.)? 
In this method, 2 g of sample is heated at 450°C for 2 h, 
then taken up in HC1 and filtered. The filter is heated 
for 0.5 h at 550°C, and the remainder is treated with HF, 
evaporated to dryness so as to volatilize the silicates 
present, and taken up in HC1. The combined solutions 
are made up to 100 mL with water. Because of the low 
Cd levels, 5 g was weighed out in the present procedure. 
TABLE I. Interference of iron on the cadmium signal in flame AAS (0.1 mg L ~ Cd in 0.12-M HCI; Fe added as FeSO4.6H20; IL S-11; 228.8 
nm; air/acetylene; mean of 5 readings; no background correction). 
Fe added (mg L ~) 0 
Absorbance x 103 (stoichiometric) 102 
Absorbance x 10 .~ (reducing) 104 
Absorbance x 10 .~ (oxidizing) 100 
10 20 50 100 200 500 1000 2000 2500 
101 102 103 101 100 106 109 119 122 
122 
130 
388 Volume 41, Number 3, 1987 0003-7028/S7/4103-0388$2.00/0 APPLIED SPECTROSCOPY 
© 1987 Society for Applied Spectroscopy 
TABLE II. Influence of iron on cadmium measurements by flame AAS 
with different background correction systems (Fe added as FeCI3.6H20; 
IL S-11; 228.8 nm; stoichiometric air/acetylene; mean of 5 readings). 
Solution 
Apparent Cd concentration (t~g L -1) 
(mean _+ standard eviation) 
Code Cd Fe Without With With 
no. (/~g L-') (mg L-') BG D= SH 
A 40 2500 56 _+ 1 22 _+ 1 41 ± 1 
B 0 2500 11 -+ 1 -17 -+ 1 -1  _+ 1 
C 70 2500 80 _+ 2 56 ± 1 71 _+ 2 
D 200 0 198 ± 9 201 ± 2 202 ± 3 
Soils. The soils were treated as follows: 
1. Digestion: different soil samples were digested ac- 
cording to a standard procedure in use in the authors' 
laboratory. In this method, 3 g of soil is repeatedly 
heated in Teflon ® beakers with concentrated HNO~ 
and H202 at 170°C and taken to dryness. Thereafter, 
the soil cake is heated several times with an HNO~- 
HF mixture until the major part of the silicates has 
disappeared. The remainder is suspended in HC1, di- 
luted with water to 100 mL, and filtered. 
2. Extraction: 5 g of soil is shaken with 50 mL of 0.43-M 
HNO3 and filtered. 
RESULTS AND DISCUSSION 
The Cd absorbance signal, measured in a stoichio- 
metric air/acetylene flame without background correc- 
tion, was not affected by Fe concentrations up to 200 
mg L -1. From 500 to 2500 mg L -1, the apparent Cd signal 
increased gradually (Table I). The upper value was cho- 
sen on the basis of the Fe content of the Dutch river 
clay soil mentioned in the introduction. It also appears 
that variations in the air/acetylene ratio do not have 
much influence on the signal. 
For the highest Fe concentration, measurements were 
taken with some Cd-containing solutions for different 
background correction conditions (Table II). The results 
without background correction show the same tendency, 
as indicated in Table I. The D~ background correction 
TABLE III. Influence of iron on Cd and Pb measurements in plant 
material with and without background correction (Cd: 228.8 nm, sam- 
ple: tobacco leaves; Pb: 217.0 nm, sample: apple leaves). 
Added Fe 
concen- Cd (mg 
tration 
(mg L -~) Without D2 
kg-') Pb (rag kg -1) 
SH Without D2 SH 
0 3.9 3.3 3.3 23.1 21.2 20.9 
55 3.8 3.3 3.4 23.4 20.8 21.0 
110 3.8 3.3 3.4 23.1 20.5 20.8 
165 3.7 3.2 3.4 22.5 20.3 20.6 
220 3.8 3.2 3.4 22.6 20.4 21.0 
320 3.7 3.2 3.4 22.8 19.7 21.0 
420 3.7 3.2 3.4 23.5 20.1 20.9 
620 3.7 3.0 3.4 24.0 19.5 20.7 
820 3.8 3.1 3.4 24.4 19.4 20.5 
1000 3.7 3.1 3.4 24.4 19.5 20.4 
gave values that were too low; this result can be inter- 
preted as overcompensation f the interfering effect. In 
the literature, the problem of overcompensation a d 
negative values because of the occurrence of atomic lines 
within the spectral bandpass has already been recog- 
nized2 5 The degree of overcompensation s roughly equal 
to the value found for solution B, which supports the 
assumption that the interference is due to Fe. The Smith- 
Hieftje system turned out to be able to compensate for 
this interference to the right degree. This may very well 
indicate a structured background, of which the average 
absorbance as measured by the D2 lamp is higher than 
the actual background just under the Cd absorption peak. 
Measurements at 228.8 nm with an Fe hollow cathode 
lamp on a 2500 mg/L Fe solution gave an appreciable 
absorbance, whether D2 correction was applied or not, 
and zero absorbance when SH correction was applied. 
With the use of the same Fe lamp at the same wave- 
length and no background correction, a 1000 mg/L Cd 
solution gave essentially no absorption signal. Thus, in 
the 228.3-229.3-nm region there must be one or more 
resonance lines of Fe. Scanning of the emission spec- 
trum from the used Fe hollow cathode lamp revealed 
one line at 228.72 nm. This is in accordance with the 
table of Zaidel, quoted by Michaud and Mermet, ~ who 
TABLE IV. Determination of Cd and Pb in plant material (leaves) with and without background correction (mean of duplicate 5 g weighings). 
Cd content (mg kg 1) Pb content (mg kg -~) 
Mean of Mean of 
int. coll. int. coll. 
Fe study, or study, or 
content Without GFAAS certified Without GFAAS certified 
Plant material type (mg kg -~) BG D2 SH 1.1. extr. value BG D2 SH 1.1. extr. value 
Tobacco 740 3.7 3.2 3.1 3.3 3.0 8.2 4.5 4.3 3.9 4 
Apple (Golden Delicious) 260 0.40 0.25 0.25 0.25 0.20 13.8 12.8 13.9 6 16 
Apple (Cox) 1055 1.2 1.0 0.9 0.61 0.50 23.1 20.4 20.7 24.3 22 
Vine 256 0.5 0.3 0.2 0.28 0.20 19.0 16.0 15.7 16.0 18 
Lettuce 510 1.5 1.1 1.1 0.87 0.95 8.8 6.4 6.2 5.4 6 
Grass 200 0.30 0.24 0.22 0.25 0.20 3.0 3.6 4.3 2.9 4.5 
CRM 060 1189 4.0 3.2 3.2 n.d." 2.20 b 60 56 58 n.d. ~ 63.8 b 
Lagarosiphon mj +0.10 +3.2 
CRM 061 4650 1.7 0.7 1.0 n.d. 1.07 b 62 57 59 n.d. 64.4 
Platihypnidium riparioides +0.08 +3.5 
CRM 062 140 0.2 0.2 0.2 n.d. 0.10 b 27.2 28.3 27.9 n.d. 25.0 b 
Olea europaea _+0.02 _+1.5 
n.d. = Not determined. 
b Certified value _+ 95% conf. int. 
APPLIED SPECTROSCOPY 389 
TABLE V. Influence of background correction on Cd measurements in iron-rich clay soils (air/acetylene flame; IL S-11; Cd 228.8 nm; Pb 217.0 
nm; Fe 248.3 nm; mean of duplicate weighings). 
Soil Fe Cd (mg kg-') Pb (mg kg-') 
Code Type Pretreatment (g kg -~) Without BG D2 SH Without BG D2 SH 
OMC-2 River clay Digestion 26.0 1.87 0.52 0.74 38.5 27.7 31.2 
OMC-202 River clay Digestion 41.9 1.97 0.36 0.64 46.0 34.4 39.3 
OMC-203 River clay Digestion 23.4 1.32 0.14 0.34 29.0 20.4 23.8 
OMC-204 Sea clay Digestion 30.0 1.40 0.47 0.68 66.4 58.7 62.4 
OMC-205 Sea clay Digestion 28.0 1.27 <0.02 0.28 30.6 21.6 24.4 
OMC-2 River clay Extraction 1.73 0.50 0.50 0.50 18.3 18.0 18.2 
OMC-202 River clay Extraction 2.83 0.63 0.52 0.58 23.8 23.0 23.5 
OMC-203 River clay Extraction 1.71 0.49 0.36 0.32 15.8 15.0 15.4 
OMC-204 Sea clay Extraction 6.64 0.95 0.63 0.62 56.3 54.0 54.5 
OMC-205 Sea clay Extraction 2.67 0.35 0.32 0.29 18.8 19.5 18.5 
give several ground-state transitions for Fe around the 
wavelength of 228.8 nm. Similar results were found for 
the Pb line at 217.0 nm. 
Addition of Fe to plant leaf digests gave similar re- 
sults: (too) high values without any background correc- 
tion, low values with D2 correction, and intermediate 
values with SH correction, for Cd as well as for Pb (Ta- 
ble III). Note, however, that both background correction 
systems give equal results as long as the Fe concentra- 
tion is not higher than about 100 mg L -1. 
In addition to these fortified digests, Cd and Pb were 
determined in several plant materials with different in- 
trinsic Fe levels. For this purpose, the plant materials 
of Table IV were purchased either from C.I.I. or from 
the European Community Bureau of Reference (CRM 
060, 061, and 062) because of their known levels of Cd 
and Fe. Although the Fe concentrations in the digests 
were far too low to have any influence on Cd or Pb, as 
in the foregoing experiments, the results with back- 
ground correction were, for both systems, significantly 
lower than without background correction (Table IV). It 
should be noticed that this effect is more pronounced for 
Cd than for Pb, and that both types of background cor- 
rection give the same results. The only exception is CRM 
061, which behaves just like the pattern that appeared 
from Table III, and for which material the certified Cd 
value shows a remarkably good agreement with the value 
obtained by SH correction. It is certainly more than pure 
coincidence that this is also the only sample with an 
extremely high Fe content. For the other samples, the 
Fe content indeed seems to be too low (concentrations 
in the digests, 7-60 mg/L) to play a role; presumably the 
plant material matrix causes a smooth continuous back- 
ground absorption profile, to which both correction sys- 
tems respond to the same degree. 
For reasons of comparison, Table IV shows the aver- 
age Cd and Pb values of some leaf samples from an 
international collaborative study on plant material anal- 
ysis. Statistical analysis revealed that the difference with 
"no BG" is significant, the difference with D2 is not sig- 
nificant, and the difference with SH is not significant at 
the 5% level. 
Table IV also shows results for Cd and Pb determi- 
nations by graphite furnace AAS after matrix elimina- 
tion by a liquid-liquid extraction procedure with dieth- 
ylammonium-N,N-diethyldithiocarbamate in xylene. 
These values are closer to the D2 and SH results than 
to the uncorrected ones. All differences were found to 
be insignificant at the 5% level, including the differ- 
ences with the results of the international collaborative 
study. 
Finally, some Dutch clay soils with high iron content 
were analyzed for Cd and Pb after digestion with 
HNO3 + H202 + HF and after extraction with 0.43-M 
HN03 (Table V). After digestion, the Cd as well as the 
Pb content consistently showed the same trend as de- 
scribed for the fortified synthetic solutions. After ex- 
traction, however, the results showed a tendency similar 
to that for plant material. This is evidently due to the 
lower apparent Fe content, since 0.43-M HNO3 does not 
extract so much iron from the soil. 
Thus, background correction is always necessary in 
the flame AAS determination of Cd and Pb in plant and 
soil material. A double-pulsed hollow-cathode-lamp 
background correction system gives more accurate re- 
sults than does a deuterium arc background corrector in 
the case of a structured background ue to spectral in- 
terference of high concentrations of iron. At normal Fe 
concentrations, the D2 system is to be preferred because 
of the inherent loss of sensitivity of the SH system. 
ACKNOWLEDGMENTS 
The authors are grateful to Mr. B. J. Hoekman (Laboratory for Soil 
and Crop Testing, Oosterbeek) for putting the soil samples at their 
disposal, and to Mr. Ch. Koot for the graphite furnace determinations. 
1. S. B. Smith and G. M. Hieftje, Appl. Spectrosc. 37, 419 (1983). 
2. Members of the C.I.I., l'Agronomie Tropicale 24, 827 (1969). 
3. C. Hendrikx-Jongerius and L. de Galan, Anal. Chim. Acta 87, 259 
(1976). 
4. R. Goguel, Geostandards Newsletter 7, 341 (1983). 
5. G. Wibetoe and F. J. Langmyhr, Anal. Chim. Acta 16S, 87 (1984). 
6. E. Michaud and J. M. Mermet, Spectrochim. Acta 37B, 145 (1982). 
390 Volume 41, Number 3, 1987 


Background correction in the determination of Cd and Pb by flame AAS in plant and soil samples with high Fe levels.

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