Direct Determination of Copper in Urine Samples by Electrothermal

Direct Determination of Copper in Urine
Samples by Electrothermal Atomic
Absorption Spectrometry Using a
Platform and Palladium Matrix Modifier
Application Note
Atomic Absorption
Authors
Introduction
Michel Hoenig
Electrothermal atomization — atomic absorption spectrometry (ET–AAS) has several advantages over flame atomization including extremely small sample volume,
improvement of up to 2–3 orders of magnitude in sensitivity, and the elimination of
lengthy pretreatment for the destruction of organic matter.
Paul Van Hoeyweghen
Institute of Recherches Chimiques
Ministere de l’Agriculture
Museumlaan 5 — 1980 Tervuren
Belgium
In recent years, several studies have pointed out the difficulties associated
with analysis of complex salt matrices such as urine or sea water. These types
of samples contain rather large amounts of salts which can have a distinct
effect on the accuracy of an analysis.
First, the relatively high background levels generated during the atomization step
can interfere with the determination of volatile elements because of incomplete
matrix removal at the low pyrolysis temperature that must be used [1].
Second, the premature loss of the analyte as a volatile halide may be a significant
interference in some situations [2].
Finally, matrix interferences produced by large amounts of salts volatilized
simultaneously with the analyte add to the problems [3,4].
However, work with modern instruments which use the L’vov platform conditions has
shown that, within limits, determination by furnace AAS can be relatively independent
of the matrix.
One of the latest and most often used approaches is to delay sample vaporization by
means of the platform technique, combined with the addition of a suitable chemical
modifier to the sample.
Copper is one of the elements most frequently and easily
determined by atomic absorption spectrometry. However, its
determination by ET-AAS is not free of interferences, since
copper can only be thermally pretreated to about 1000 °C
without losses, so that is is not always possible to remove
the matrix completely.
Table 2.
Sampler Parameters
Sampler Automix
Cu Standard 20 µg/L-1
Blank H2O
Modifier Pd 0.2% in HNO3 2%
The determination of copper in chloride was studied by Ediger
[5]. Addition of ammonium nitrate as a chemical modifier
facilitates the removal of the chloride matrix during pyrolysis,
and the atomization of the copper is consequently less susceptible to interference. Nitric acid may be used to the same
end; the background signal is then considerably reduced [6].
In this study we developed a rapid method for interference-free
determination of copper in urine samples.
Sampler volumes (µL)
Solution
Blank
Modifier
Blank
Standard 1
Standard 2
Standard 3
Sample
–
3
6
9
5
2
2
2
2
2
Hot inject
Temperature
Injection rate
Yes
120
1
2
Experimental
A Hewlett-Packard 82905-A printer was used for plotting
absorbance-time profiles.
All analytical work was performed on an Agilent SpectrAA-10
Spectrometer equipped with Agilent GTA-96 graphite furnace
and programmable sample dispenser. Pyrolytically coated
tubes and solid pyrolytic graphite platforms were used
throughout.
All measurements were performed in the peak height mode.
Details of electrothermal program and sampler parameters
are given in Table 1 and Table 2.
The copper aqueous standards were prepared in 1% nitric
acid (Suprapur Merck) from commercial standard solution
(Titrisol Merck). Palladium matrix modifier solution (0.2% Pd)
was prepared by dissolving the palladium oxide (Specpure
Johnson and Matthey) in nitric acid.
Table 1
A copper hollow cathode lamp, operated at 2 mA current, spectral
width of 0.5 nm (reduced slit height, 0.5 R) and copper resonance
line at 324.8 nm were used for all experiments.
Operating Parameters for Determination of Copper
Furnace Parameters (with platform)
Step
Temperature Time (s)
Gas flow
no.
(°C)
(s)
(L,min-1)
Gas
type
Read
command
1
2
3
4
5
6
7
Argon
Argon
Argon
Argon
Argon
Argon
Argon
No
No
No
No
Yes
Yes
No
500
1100
1100
1100
2700
2700
120
20
5
5
2
0.8
3
12.9
3
3
3
0
0
0
3
Results and Discussions
Sampler
The undiluted urine samples were placed directly in the
autosampler microvials. Premixed chemical modifier (palladium
nitrate in 2% HNO3) was placed in the modifier vessel.
Solutions are taken into the capillary and dispensed at 120 °C on
the platform. The programmable “hot injection” facility of the
GTA 96 was used to avoid overflowing the platform [7,8]. In this
case, the ashing ramp starts directly after the dispensing.
2
Importance of Reduced Slit Height
Choice of Modifiers
Use of the recommended 0.5 nm spectral width with normal
slit height in conjunction with the platform provided a blank
signal due to the emission of the tube wall and the platform,
as illustrated in Figure 1. To overcome this problem, the
reduced slit height must be used, but the hollow cathode
lamp current must be consequently lowered to 2 mA to
attain good energy balance between the deuterium arc and
the hollow cathode sources. With this arrangement, the
baseline, enhanced during the entire atomization step with
use of normal slit height, remains at instrument zero level. It
must be noted that furnace alignment is then critical and
that it is necessary to check the absence of any spurious
signal with blank firing before the analysis.
Nitric acid was used to remove a large amount of the chloride
matrix present in the urine samples [2]. The absorbance-time
profiles obtained with only nitric acid showed that the atomization rate of copper in urine samples is much faster than in
aqueous standards (Figure 2).
Figure 2.
Absorbance-time profiles of Cu without modifier in aqueous
standard (1) and urine (2).
3 - background absorbance of urine
4 - background absorbance of aqueous standard
The slope of the standard addition curve in urine is steeper
than that of the aqueous standards working curve. In these
conditions the direct calibration procedure leads to erroneous
results for urinary copper determinations (Figure 3).
Figure 1.
Influence of slit height on copper response.
1 - 120 pg Cu, 0.5 nm normal slit height
2 - 120 pg Cu, 0.5 nm reduced slit height
3 - blank firing, 0.5 nm normal slit height
4 - blank firing, 0.5 nm reduced slit height
Figure 3.
3
Working curves for determination of copper.
Platform atomization, without modifier.
1 - aqueous standards
2 - standard additions in urine sample
Calculated Cu content with direct calibration is 83 pg for 5 µL
sample (or 16.6 µg/L-1). Calculated Cu content with standard
addition method is 64 pg for 5 µL sample (or 12.8 µgIL-1).
Consequently we checked all the known chemical modifiers to
attain a uniform atomization rate for copper in both media. Only
palladium appeared to be an adequate modifier to this end. The
optimal volume of the modifier added was 4 µL for a 5 µL
sample using a premixed solution of 0.2% Pd in 2% nitric acid.
Performances
Under the described conditions, the characteristic mass for copper
is about 5 pg for 0.0044 absorbance units. For 5 µL undiluted urine
sample we consider that real copper concentrations equal to or
greater than 4 µg/L-1 can be easily determined with sufficient
accuracy.
Compared to the situation illustrated in Figure 2, absorbancetime profiles of copper in simple aqueous standards and urine
samples are similar in the presence of the palladium modifier
(Figure 4). The calibration curve for copper in the simple nitric
acid medium achieved with the same modifiers is parallel to
the standard additions curves obtained for the urine samples
(Figure 5). Direct comparison of urine samples with acidified
aqueous standards then becomes possible. This was checked
with urine samples spiked with copper.
Three injections of the same sample provided relative standard deviations from 0.2 to 3%, depending on the magnitude
of measured concentrations.
Conclusions
The results obtained in this study suggest that modern
ET–AAS is a very convenient and sensitive technique for urinary copper analysis. Interference-free determination was
achieved in this case using platform and chemical modifiers.
The very long lifetime of platforms guarantees numerous
copper determinations: the graphite tubes tested plus platforms
provided a lifetime in excess of 500 firings.
The proposed method is simple, rapid and accurate, and thus
suitable for routine analysis of copper in urine.
Figure 4.
Absorbance-time profiles of Cu with palladium modifier in
aqueous standard (1) and urine (2).
3 - background absorbance of urine
4 - background absorbance of aqueous standard.
Figure 5.
Working curves for determination of copper.
Platform atomization with palladium modifier
1 - aqueous standards
2 - standard additions in urine sample
Calculated Cu content is 64 pg for 5 µL sample (or 12.8 µglL-1) for
both methods.
4
References
1.
M. Hoenig, R. Wallast, Spectrochim. Acta 37B, 399 (1982).
2.
W. Slavin, G. R. Carnrick, D. C. Manning, Anal. Chem.
56, 163 (1984).
3.
M. Hoenig, K. Meeus-Verdinne, P. O. Scokart Analusis
12, 279 (1984).
4.
M. Hoenig, Y. Van Elsen, R. Van Cauter, Anal. Chem. 58,
777 (1986).
5.
R. D. Ediger, G. E. Peterson, J. D. Kerber, Atom. Absorpt.
Newsl. 13, 61 (1974).
6.
E. Pruszkowska, G. R. Carnrick, W. Slavin, Anal. Chem.
55, 182 (1983).
7.
M. Hoenig, Varian Instruments At Work, AA-49, (1985).
8.
M. Hoenig, Varian Instruments At Work, AA-61, (1986).
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November 1, 2010
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