Direct Determination of Manganese in Sea Water by

Direct Determination of Manganese in
Sea Water by Electrothermal Atomic
Absorption Spectrometry with
Deuterium Background Correction Using
a Platform and Platinum Matrix Modifier
Application Note
Atomic Absorption
Authors
Introduction
Michel Hoenig
The determination of manganese in sea water samples using electrothermal atomic
absorption spectrometry (ET-AAS) has been investigated by many workers [1-9]. Sea
water has been found to be difficult to analyze because of the matrix. If the matrix is
vaporized simultaneously with the analyte, the result is a large background signal
which is often beyond the correcting capabilities of current deuterium devices. The
presence of large amounts of chlorides has also been shown to provide interferences,
making direct analysis difficult [10].
Paul Van Hoeyweghen
Institute of Recherches Chimiques
Ministere de l’Agriculture
Museumlaan 5 — 1980 Tervuren
Belgium
To reduce the problems associated with the determination of manganese in sea water,
most authors have used matrix modification [2,4,7] or have extracted the analyte from the
sea water matrix [3,5]. Few workers have been successful with the direct manganese
determination after volatilization of the matrix during the pyrolysis step [1,4,6]. Slavin and
Manning [11] have shown that by using the platform, the interference of a salt matrix on
cadmium, lead and thallium was greatly reduced. The same authors [12] have also
shown that direct manganese determination in sea water is possible by using platform
and Zeeman background correction. The reported sensitivity of this method is about
2.2 pg Mn for 0.0044 absorbance unit.
We describe in this work a rapid method for the direct determination of manganese
in sea water at sub µg L-1 levels using deuterium background correction, platform
and platinum as matrix modifier.
Experimental
First, it was important to determine the correct electrothermal
program using the platform technique. The ash study illustrated
in Figure 1 showed that manganese can be thermally pretreated
up to 1000 °C without losses for both aqueous standard and sea
water sample (curves a and b). At this temperature the background absorbance generated by the salt matrix during the atomization step (curve c) is still relatively high (about 0.2 Abs). The
addition of platinum as matrix modifier to the sample permits a
pyrolysis temperature up to 1400 °C without loss of manganese
(curves d and e).
All the absorbance measurements reported here were made
using an Agilent SpectrAA-10 Spectrometer with an Agilent
GTA-96 graphite furnace and programmable sample dispenser.
The furnace was fitted with pyrolytically coated tubes and
solid pyrolytic graphite platforms.
A Hewlett-Packard 82905-A printer was used for plotting the
absorbance-time profiles.
All measurements were performed in the peak-height mode.
A manganese hollow cathode lamp operated at 4 mA current,
spectral width of 0.2 nm and manganese resonance line at
279.5 nm were used throughout.
The manganese aqueous standards were prepared in
5% nitric acid, (Suprapure Merck) from commercial standard
solution (Titrisol Merck). Platinum matrix modifier solution
(0.1% Pt) was prepared by dissolving the platinum metal in
the aqua regia, followed by adequate dilution.
Natural sea water samples were collected in the NE Atlantic
and the North Sea. They had a salinity of approximate 3.5%.
After sampling, the sea water was filtered through a nominal
0.45 µm membrane filter and acidified (5% HNO3).
Results and Discussion
Figure 1.
A review of papers which report direct determination of manganese in sea water reveals a contradictory situation and a
very difficult analysis. Earliest papers reported a reduced sensitivity for manganese in a sea water matrix and attributed it
to covolatilization of analyte with the salt matrix. More recent
reports suggest that this reduced sensitivity is a vapor-phase
binding of a portion of the manganese by chlorine [13,14]. The
experiments of Ediger et al [15] established that the decrease
in signal for manganese in the presence of large amounts of
NaCl is a chemical interference. The authors showed that it is
necessary to char away as much as possible of the sea water
matrix to get maximum sensitivity for manganese and to be
relatively free of interference. Pritchard and Reeves [16]
reported that sodium chloride is vaporized from a heated
graphite surface in a few seconds at temperatures a little
higher than 700 °C. According to Nakahara and Chakrabarti
[17], the large amounts of NaCl present in sea water are
volatilized below 950 °C: this temperature seems to be in
agreement with our own experiences. On the other hand,
Hoenig et al. [18] showed that other sea water matrix elements remain in the tube up to 1700 °C. At higher pyrolysis
temperatures the background signal disappears entirely.
Ash temperature
The effect on the atomization signal is plotted as a function of
different char temperatures for an aqueous standard and a sea
water sample. Manganese concentration was 2 µg L-1 in both
samples (5 µL dispensing).
(a) — Aqueous standard (5% HNO3)
(b) — Sea water (5% HNO3)
(c) — Background absorbance
(d) — Aqueous standard (5% HNO3) + Pt (2 µL 0.1%)
(e) — Sea water (5% HNO3) + Pt (2 µL 0.1%)
Finally, the ashing temperature of 1300 °C was set for both
media (aqueous standards and sea water samples). With this
ashing temperature the background absorbance has decreased
to an acceptable value of about 0.04 absorbance. It must be also
noted that in the presence of the platinum modifier the manganese peak-height absorbance signal is enhanced by about
30%. The best sensitivity was obtained with an atomization
temperature of 2600 °C. Details of electrothermal program and
sampler parameters are given in Table 1. Figure 2 shows the
absorbance-time profiles of manganese for both media with
electrothermal program described.
2
Table 1.
Operating Parameters for Determination of Manganese
A recent review of the literature [19] reported that the base
line level of total manganese in unpolluted deep sea samples
was expected to be about 0.05 µg L-1. Numerous trace metal
measurements in coastal waters reported levels ranging from
0.5 to 50 µg L-1.
Furnace parameters (with platform)
Step
no.
Temperature
(°C)
1
2
3
4
5
6
7
500
1300
1300
1300
2600
2600
120
Time
(sec)
40
8
5
2
0.7
2.5
12.5
Gas flow
(L/min-1)
Gas type
Read
command
3
3
3
0
0
0
3
Argon
Argon
Argon
Argon
Argon
Argon
Argon
No
No
No
No
Yes
Yes
No
The sensitivity of the manganese determination was
1.2 pg Mn for 0.0044 absorbance. For 5 µL natural sea water
samples we then estimate that real manganese concentration
greater than 0.5 µg L-1 can be easily determined with good
accuracy and precision (between 3 and 8% depending on the
magnitude of the measured absorbance signal).
Sample parameters
(Sampler automixing, Mn standard 2 µg L-1)
Blank
Standard 1
Standard 2
Standard 3
Sample
Solution
Blank
–
4
8
12
5
5
For samples that range lower than 0.5 µg L-1 the problem is
more complicated. The dispensing of larger sample volumes
on the platform (or multiple injection facility) is simple. Up to
20 µL of sea water sample, the background generated during
the atomization step is not excessive and it is easily corrected
by the deuterium device.
Modifier
(Pt 0.1% in HN03 2%)
2
2
2
2
2
However, in this case a larger amount of salt matrix remains
on the platform after the ashing step and produces a nonspectral interference resulting in a reduced sensitivity for
manganese.
The direct calibration method using simple standard solutions then becomes inadequate because the slope of the
single element standard curve is steeper than the working
curve slope established in sea water. This slope decrease
is variable and proportional to the sample volume of sea
water. In such cases only the standard addition method is
valid. However, for samples larger than 20 µL the curve
slope is too low to obtain accurate results, and then the
20 µL method must be considered as a limit. In this case,
manganese concentrations of about 0.2 µg L-1 can be
determined with a sufficient accuracy.
Figure 2.
Absorbance-time profiles for manganese in presence of platinum
matrix modifier. Manganese concentration was 1.6 µg L-1 in both
samples (5 µL dispensing).
Conclusions
No major analytical problems were encountered in the manganese sea water analysis using the ET-AAS with platform and
platinum matrix modifier. The 5 µL method using a direct calibration procedure is very rapid, simple and accurate and thus
suitable for routine analysis. For very low manganese concentrations (below 0.5 µg L-1) it is necessary to analyze larger sea
water volumes. In this case a matrix effect occurs and the standard addition method must be used. The long lifetime of solid
pyrolytic platforms permits numerous determinations without
decrease of absorbance signal. The platforms tested provided a
lifetime in excess of 500 firing.
In the previous paragraph we showed that the background
absorbance of 5 µL sea water samples did not exceed
0.04 absorbance. Consequently, it is evident that major analytical
problems will not be due to spectral interference.
The calibration curve of manganese in a simple nitric acid
medium is parallel with the standard addition curves obtained
for 5 µL sea water samples. In such cases the direct calibration
procedure against simple reference solutions is valid because
of the absence of non-spectral interference.
3
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