ICP-OES Analysis fo FeCr Alloys Prepared by Sodium Peroxide Fusion

a p p l i c at i o n N o t e
ICP-Optical Emission Spectroscopy
Author
Chady Stephan
PerkinElmer, Inc.
Woodbridge, Ontario, Canada
ICP-OES Analysis
of FeCr Alloys
Prepared by Sodium
Peroxide Fusion
Introduction
Ferroalloys, alloys of iron with sufficient
amounts of one or more other elements
present (V, Mo, Cr, etc.), are important as
a source of various metallic elements in
the production of all types of steels. They
usually have low melting points and can
be incorporated readily into molten steel
where they provide a certain chemical
composition with specified properties.1
The ferrochrome alloy is produced by electric arc melting of the mineral chromite
(FeCr2O4). It is largely used for the production of stainless steels. Increasing the
amount of chromium content in the steel allows for increased corrosion and
oxidation resistance. Other alloying elements, such as nickel (Ni) or molybdenum
(Mo), aid in making the steel more passive and increasing its stainless properties.2
The presence of unwanted impurities in the metal and slag may alter reaction
temperatures and cause undesirable reactions of their own.1 The chemical analysis
of the chromite ore and its final products are mandatory to assess the quality
of the chromite ore and to optimize the grade of stainless steel production.
Analysis of waste products is also needed to optimize recovery of chromium and
other additives.2
Chromite does not dissolve well in aqua regia or nitric acid,
the typical acids used for digestion of samples for atomic
spectroscopy.1 The traditional method for the dissolution
of chromite and ferrochrome is time consuming and includes
the use of hydrofluoric (HF) and perchloric (HClO4) acids.
Sodium peroxide fusions offer an alternative to working with
these high risk acids. Fusions are also less time consuming,
providing a safer and more efficient method to obtain full
dissolution of ferrochrome samples.3
In this work, we describe the performance of the Optima™
8300 inductively coupled plasma-optical emission spectrometer
(ICP-OES) in analyzing ferrochrome samples prepared by
sodium peroxide fusion. The revolutionary radio frequency
(RF) generator in the Optima 8300 spectrometer features flat
induction plates that replace the traditional helical load coil.
The Flat Plate™ plasma induction plate technology delivers a
low-flow operating system that does not require coil cooling
and is capable of operating at a plasma argon flow as low
as 8 L/min.
Several key parameters were evaluated for this particular
application: accuracy and precision by measuring the recovery
and the relative standard deviation of three standard reference materials (SRMs), stability by plotting the recovery of
continuing calibration verification (CCV) samples versus time
for more than 12 hours, and method detection limits (MDLs)
calculated as three times the standard deviation of the
measured concentrations of 10 fused matrix blank replicates.
Experimental Conditions
Instrumentation
All samples were analyzed for elemental content using the
PerkinElmer® Optima 8300 ICP-OES (Figure 1) with a singleslot one-piece torch (Part No. N0780130). A SeaSpray™
nebulizer (Part No. N0775345) and baffled cyclonic spray
chamber (Part No. N0776053) were used for their known
ability to handle samples with high levels of dissolved solids.
Instrumental parameters used for the analysis are listed in
Table 1. The wavelengths and viewing mode are listed in
Table 2 (Page 3).
By combining a segmented-array charge coupled device
(SCD) detector and an Echelle optical system, the Optima
8300 ICP-OES can measure all the wavelengths simultaneously.
Its wavelength flexibility allows the end users to easily add
new elements or wavelengths as their program changes.
The Flat Plate technology generates a unique transversely
symmetrical plasma that exhibits an extremely flat bottom
which caps off the sample and prevents it from escaping
around the outside (Figure 2). The symmetry and stability
of the Flat Plate plasma, in conjunction with the new torch
geometry, allows for a high level of stability and precision.
2
Figure 1. Optima 8300 spectrometer – the most economical ICP-OES,
capable of running at a plasma flow rate as low as 8 L/min.
Figure 2. Low-flow operation with Flat Plate plasma induction plate
technology.
Another benefit of using the Optima 8300 ICP-OES for fusion
samples includes a 40 MHz free-running solid state RF generator.
It is capable of handling the high power required to generate
a robust plasma which is essential for precise analysis of
complex matrices such as fusion samples.
Table 1. Instrumental operating conditions.
Parameter Value
Instrument Optima 8300 ICP-OES
Nebulizer SeaSpray
Spray Chamber Baffled cyclonic
Torch Single slot quartz torch
Injector
2.0 mm alumina
Power (W) 1500
Plasma Gas (L/min) 10
Aux Gas (L/min) 0.3
Neb Gas (L/min) 0.65
Sample Uptake Rate (mL/min) 1.5
Sample Tubing Samples and standards
(Black/Black)
Internal standard (Green/Orange)
Internal Standard 5 ppm yttrium + 10 ppm
cesium (as an ionization buffer)
Table 2. Wavelength and viewing mode for the elemental
analysis of dissolved ferrochrome samples.
Analyte Wavelength
Viewing Mode
Al394.401 Axial
Ca317.933 Radial
Co228.616 Axial
Cr206.158 Axial
Cu327.393 Axial
Fe239.562 Axial
K 766.490Radial
Mg279.077 Radial
Mn257.610 Axial
Mo202.031 Axial
Ni231.604 Axial
P178.221Axial
S 181.975Axial
Si251.611 Axial
Ti334.940 Axial
V290.880Axial
Zn206.200 Axial
Table 3. FeCr alloy (SRM 64c) recovery – all values are in
mg/kg; dashed lines indicate no certified value was provided
by the manufacture.
Analyte
Certified Measured Recovery (%)
Al ---
658
---
Ca ---
295
---
Co 510
520
102
Cr 680000
655000
96.3
Cu 50
< MDL
---
Fe 249800
260000
104
K
---
4.20
---
Mg ---
23120
---
Mn 1600
1650
103
Mo ---
165
---
Ni 4300
4050
94.2
P
200
191
95.6
S
670
731
109
Si 12200
11450
93.9
Ti 200
205
102
V
1500
1587
106
Zn ---
< MDL
---
Sample Preparation
All samples were prepared by sodium peroxide fusion. In a
zirconium crucible, precisely 0.2 g of finely ground sample
was fused with 3 g of sodium peroxide (Na2O2) and 0.5 g of
sodium carbonate (Na2CO3). The fused mixture was poured
into a beaker containing 250 mL of a 20% acid mixture of
1:1 hydrochloric (HCl) and nitric (HNO3) acids.
Table 4. Cr ore (SARM 8) recovery – all values are in mg/kg.
Analyte Certified Al
Measured Recovery (%)
5590053250 95.3
Ca 18581875 101
Co ---265 --Cr
335050345000
103
PerkinElmer NIST Traceable Quality Control Standards for
ICP were used as the stock standards for preparing working
standards. A four-point calibration curve was used for each
measured element.
Cu ---103 ---
Results
Mn 17401820 105
®
Accuracy and Precision
The method accuracy was monitored by measuring the
recovery of two different SRMs: a high-carbon ferrochromium
(FeCr) alloy (SRM 64c) (Table 3) and chromium ore South
African Reference Material 8 (SARM 8) (Table 4). The method
precision was monitored by measuring the variation of a
FeCr slag SRM (SARM 77) analyzed 55 times over a period
of 12 hours (Table 5 – Page 4). Both SRMs showed excellent
recoveries for all analyzed elements (± 10%).
Fe
141300145600
103
K ---182 --Mg 8859094110 106
Mo ---105 --Ni ---1460 --P
S
Si
---17.3 --341359 105
2010019510 97.1
Ti 14401390 96.5
V 780821 105
Zn
---
< DL
---
3
Table 5. Analysis of FeCr slag (SARM 77) over 12 hours
(n=55) – all values are in mg/kg.
Average Analyte Certified Measured SD
RSD
(%) Recovery
(%)
Al 145540 149800 1283 0.86
103
Ca 26015 24440 319 1.3
93.9
Co --- 79.4 1.9 2.4
---
Cr 85530 93370 442 0.47
109
Cu --- 125 8.6 6.9
---
Fe 53100 52210 2049 3.9
98.3
K
--- 1060 14 1.3
---
Mg 138640 143800 1200 0.83
104
Mn --- 1474 24 1.6
---
Mo --- 73.1 6.0 8.2
---
Ni --- 303 16 5.1
---
P
--- < MDL --- ---
---
S
--- 2736 95 3.5
---
Si 125410 128000 2008 1.6
102
Ti --- 3201 22 0.69
---
V
--- 536 6.5 1.2
---
Zn --- < MDL --- ---
---
Method Detection Limits
The method detection limits (MDLs) were calculated based
on ten replicate measurements of the matrix blank. The MDL
was calculated by multiplying the standard deviation of the
ten replicate measurements by three (Table 6). The low-flow
plasma was able to achieve MDLs less than or within a
reasonable range of the traditional plasma flow rate. This
would suggest that the lower Ar flow and hence lower cost
of ownership can be achieved without sacrificing performance.
Table 6. Method detection limits for low argon plasma gas
flow. All values are in the calibration units of mg/L.
Analyte MDL
Al 0.0055
Ca (Radial) 0.045
Co 0.0015
Cr 0.0034
Cu 0.0019
Fe 0.018
K (Radial)
0.17
Mg (Radial)
0.037
Mn 0.0010
Mo 0.0019
Ni 0.0022
Stability
P
0.017
The instrument stability was assessed by plotting the recovery
of SARM 77 as a CCV versus time for more than 12 hours
(Figure 3). There was little change in concentration (< 4%)
for all elements measured. The excellent instrument stability
allows laboratories to confidently run samples without the
need for time consuming recalibration or optimization.
S
0.0036
Si 0.029
Ti 0.0008
V
0.0015
Zn 0.0012
Conclusions
Figure 3. Instrument stability showing the % recovery of CCV (FeCr slag)
over time. R = radial viewing.
4
The Optima 8300 spectrometer demonstrated its ability to
run heavy matrix samples using 44% less argon than typical
ICP-OES systems while maintaining superior performance.
By incorporating Flat Plate™ technology, a robust matrixtolerant plasma was generated and fusion samples were run
successfully with only 10 L/min argon plasma gas flow. The
Optima 8300 system maintained remarkable accuracy and
precision throughout the 12-hour testing period, demonstrating excellent calibration stability. The method detection
limits, when analyzing at a low plasma argon flow, were
comparable to higher traditional argon flow ICP-OES systems.
References
1.The New Encylopædia Britannica: Micropedia Ready
Reference. 15th ed., Vol. 3-4, Chicago: Enclyopædia
Britannica, 1998. s.v. “Ferrochromium”.
2.Davis, J.R., ed. Metals Handbook Desk Edition. 2nd
ed., Materials Park, OH: ASM International, 1998. s.v.
“Ferroalloy”.
3.Pitre, Janice, Melanie Bedard, and Aaron Hineman.
”Dissolution Made Easy Using Peroxide Fusions for
ICP-OES Analyses for Chromite Ores, Ferrochromes
and Chromium Slags.” http://www.claisse.com/claisse_
highlights_details.php?no=63.
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