Application
Note: 40833
The Analysis of Nitrogen, other Nutrient
and Toxic Elements in Fertilizers using the
Thermo Scientific iCAP 6500 Duo ICP
Matthew Cassap, Applications Specialist, Thermo Fisher Scientific, Cambridge, UK.
Instrumentation
Key Words
• ICP
• iCAP 6000 Series
• Fertilizer Analysis
• Nitrogen Analysis
• Nutrient Analysis
Introduction
Fertilizers are used to provide major plant nutrients
(N, P, K), secondary plant nutrients (Ca, S, Mg) and
micronutrients such as B, Mn, Fe, Cu, Zn, Mo and Se.
Accurate determination of the composition of fertilizers is
essential so that the correct dose can be applied to the soil. An
insufficient application of a fertilizer may result in poor crop
yield. In contrast, an excessive application may result in
environmental damage such as eutrophication - caused by
dissolved phosphates and nitrates entering water courses or
land contamination from non-nutrient elements within the
fertilizers.
ICP enables the cost effective analysis of fertilizers due to
the powerful multi-element capability of this technique. In the
past, complex torch designs and sparging systems have been
required in order to determine nitrogen by ICP. However, this
additional hardware can be expensive and can also prolong the
time required for sample analysis. These factors are no longer
an issue when using the Thermo Scientific iCAP 6000 Series
ICP, which can enable the simultaneous analysis of all the plant
nutrition elements (including nitrogen) as well as potential
harmful elements (As, Cd, Cr) in fertilizers.
Principle
For this analysis, 1 gram of sample was digested in 20 ml of
1:1 hydrochloric acid and deionized water. The samples were
then made up to 100 ml with deionized water and analyzed
directly on the Thermo Scientific iCAP 6500 Duo ICP.
The Thermo Scientific iCAP 6500 Duo ICP was selected for
this analysis due to the instruments ability to read signals from
both the axial and radial plasma view, which enables both
major constituent and trace elements to be effectively
determined in the same sample. The Thermo Scientific iCAP
6000 Series comprises of a range of high performance ICP
emission spectrometers which enable high sample throughput,
application flexibility and low cost of ownership. The range
employs a high resolution Echelle Spectrometer with an
advanced Charge Injection Device (CID) detector. The
advanced CID detector enables higher sensitivity and lower
noise than any of its predecessors. The CID detector is also a
non-blooming device and in this application it has the
advantage of being able to measure high concentrations of
major nutrient elements and low levels of micronutrient
elements at the same time without saturation.
The full wavelength coverage of the unique CID detector
allows the optimum wavelength to be selected whilst the high
sensitivity of the axial torch provides the lowest possible
detection limits for this application.
Method
Reagents
Hydrochloric Acid 37 % A.R grade
Inter-laboratory performance samples
MAGRUDER-Fertilizer check sample numbers 2003/08,
2004/10, 2005/04
Sample preparation
The samples were well mixed to ensure homogeneity and then
weighed (1 g) into a beaker. Hydrochloric acid (20 ml of 50
%) was added to the beaker and the initial reaction was
allowed to subside before being boiled for 10 minutes. The
sample was filtered into a 100 ml volumetric flask and made
up to volume with deionized water.
Standard preparation
A stock standard solution was prepared from a well
characterized inter-laboratory performance sample using the
same method that was used to prepare the samples. This was
then diluted to the appropriate concentrations for the range of
expected results. Additional nitrogen standards were prepared
from ammonium dihydrogen orthophosphate using the same
method used for the sample preparation, but replacing the 1
gram of sample with an appropriate amount of ammonium
dihydrogen phosphate.
N.B. All deionized water used was (as far as possible) free
from nitrogen. This can be achieved by either boiling or
bubbling an inert gas though the water. In this analysis the
water was heated for 15 minutes, covered and allowed to cool
before use.
Pump winding
Method development
The standard sample introduction kit was used (glass
cyclonic spray chamber, standard glass concentric nebulizer
and 2 mm centre tube) as the samples contained low
amounts of total dissolved solids. Wavelengths were selected
for each of the elements of interest (Table 1). Using the auto
optimization function of the Thermo Scientific iTEVA
software, the plasma parameters were optimized to produce
a set of optimum parameters (Table 2) for the elements of
interest, then the standards and samples were run. The sub
array plots of each of the wavelengths were examined and
the optimum wavelength and background correction points
were selected. Figure 1 shows the subarray plot of the
nitrogen peak at 174.272 nm for each of the samples
analyzed. It is from here that the integration area and the
background correction points can be examined, and
repositioned if necessary. A detection limit study was carried
out by running a blank sample with ten replicates. The
standard deviation of the replicates was multiplied by three
to give the instrument detection limit (IDL). The IDL was
then multiplied by the dilution factor to provide the method
detection limit (MDL) of the element in the solid.
Element
Wavelength
Plasma View
As
B
Ca
Cd
Co
Cr
Cu
Fe
K2O (K)
Mg
Mn
Mo
N
Na
Ni
P2O5 (P)
Pb
S
Se
Zn
189.042 nm
249.678 nm
422.673 nm
214.438 nm
238.892 nm
267.716 nm
324.751 nm
259.940 nm
766.490 nm
285.213 nm
259.373 nm
202.030 nm
174.272 nm
588.995 nm
221.647 nm
177.495 nm
220.353 nm
180.731 nm
196.090 nm
202.548nm
Axial
Radial
Radial
Axial
Radial
Radial
Radial
Radial
Radial
Radial
Radial
Axial
Axial
Radial
Axial
Radial
Axial
Radial
Axial
Radial
Pump Rate
Nebulizer
Spray Chamber
Center Tube
RF Power
Nebulizer Gas Flow
Coolant Gas Flow
Auxiliary Gas Flow
Purge Gas Flow
Analysis Mode
Torch Orientation
Integration Times
Low Wavelengths
High Wavelengths
Orange/White Tygon Sample
White/White Tygon Drain
50 rpm
Standard Glass Concentric
Standard Glass Cyclonic
2 mm
1350W
0.65 l/min
16 l/min
0.5 l/min
5 l/min
Speed
Duo
Axial
Radial
15 seconds
7.5 seconds
5 seconds
2.5 seconds
Table 2. The parameters determined using the auto optimization feature of
iTEVA.
Results
The instrument was calibrated and the samples run in one
continuous run, the data obtained is shown in Table 4.
The calibration plot for nitrogen is shown in Figure 2.
Furthermore, it is possible to check the performance of the
calibration standards on-screen by reviewing the
percentage difference between the stated and the actual
concentrations and the associated correlation coefficient
for the curve fit.
Table 1. The wavelength and plasma view used.
Figure 2. The calibration plot of nitrogen
Figure 1. The subarray plot of nitrogen 174.272 nm
Element
Wavelength
MDL
(ppm)
As
B
Ca
Cd
Co
Cr
Cu
Fe
K2O (K)
Mg
Mn
Mo
N
Na
Ni
P2O5 (P)
Pb
S
Se
Zn
189.042 nm
249.678 nm
422.673 nm
214.438 nm
238.892 nm
267.716 nm
324.751 nm
259.940 nm
766.490 nm
285.213 nm
259.373 nm
202.030 nm
174.272 nm
588.995 nm
221.647 nm
177.495 nm
220.353 nm
180.731 nm
196.090 nm
202.548nm
0.4
1
2
0.01
0.4
0.3
0.6
0.6
20
0.3
0.09
0.02
250
3
0.04
20
0.2
1
0.2
0.09
Conclusion
In conclusion, all of the results obtained using the iCAP 6500
Duo are in alignment with the mean results (within 2 standard
deviations of the mean) of certified values. Any exceptions to
these results have occurred if the certified value was calculated
from results submitted from less than 5 laboratories (leading to
inaccurate statistics) or if a different method was used to
determine the results. In these cases, the results obtained on an
iCAP 6000 Series instrument are approaching 2 standard
deviations of the mean. The elements with data calculated from
less than 5 laboratories are As, B and Se in sample ID 2003/08,
Se in sample ID 2004/10 and Se in sample ID 2005/04.
The analysis of nitrogen was carried out in the axial view,
which enabled excellent instrument detection limits (2.5 ppm)
and a typical RSD of 0.2 % at N 174.272 nm with a standard
axial torch. There was no requirement to use expensive modified
or custom torches that exclude atmospheric nitrogen from the
plasma. Furthermore, no ionization buffer was used during the
analysis, so the torch life time will be maximized.
The Thermo Scientific iTEVA software enables correction
factors to be entered in to a method so that an element result
can be corrected and reported as a compound i.e. P as P2O5 or
K as K2O.
The iCAP 6500 Duo ICP enables simultaneous analysis for
all of the elements reported. In addition, the instrument also
offers the best analysis solution for the sample matrix analyzed.
The radial view allows for the analysis of major elements and
the axial view allows for the analysis of trace elements.
Table 3. The result of the MDL study.
Element
As
B
Ca
Cd
Co
Cr
Cu
Fe
K2O (K)
Mg
Mn
Mo
N
Na
Ni
P2O5 (P)
Pb
S
Se
Zn
Unit
ppm
%
%
ppm
ppm
ppm
%
%
%
%
%
ppm
%
%
ppm
%
ppm
%
ppm
%
2003/08
2004/10
2005/04
Cert
Found
Cert
Found
Cert
Found
0.8
3.4
1.12
2.17
6.91
54.19
0.004
0.86
18.30
0.95
0.16
4.75
15.83
0.10
26.28
4.73
8.63
12.72
1.79
0.024
<MDL
0.002
1.12
2.20
6.21
59.29
0.004
0.89
19.08
0.92
0.16
4.04
15.85
0.11
24.75
4.72
7.95
12.58
0.31
0.028
44.89
0.13
6.49
1.15
56.86
185.12
0.120
0.47
18.65
2.21
1.53
14.33
3.46
0.56
89.67
9.71
21.64
8.03
2.47
0.452
44.15
0.14
6.39
1.00
52.28
178.16
0.110
0.43
18.24
2.06
1.33
12.07
3.08
0.60
75.75
9.04
21.91
8.15
2.09
0.452
1.26
0.004
0.14
0.74
7.61
8.58
0.002
1.10
4.10
0.14
0.55
0.74
29.36
0.04
6.81
2.97
3.07
9.37
1.19
0.010
1.32
0.004
0.14
0.87
4.94
11.13
0.002
1.01
4.33
0.12
0.52
1.37
30.65
0.04
8.76
3.08
2.42
9.54
1.01
0.010
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Table 4. Results of the sample analysis.
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