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 | Description of ICP Optical Emission Spectrometry (ICP-OES) |
1. Introduction
It has been 25 years since ICP optical emission
spectrometer (ICP-OES) began to be widely used,
and is now one of the most versatile methods of
inorganic analysis. Its features are often compared
to atomic absorption spectrometer. Compared to
atomic absorption spectrometer, in which the excitation
temperature of air-acetylene flame measures 2000
to 3000 K, the excitation temperature of argon
ICP is 5000 to 7000 K, which efficiently excites
many elements. Also, using inert gas (argon) makes
oxides and nitrides harder to be generated.
2. ICP Optical Emission Spectrometry Principle
ICP, abbreviation for Inductively Coupled Plasma,
is one method of optical emission spectrometry.
When plasma energy is given to an analysis sample
from outside, the component elements (atoms) are
excited. When the excited atoms return to low energy
position, emission rays (spectrum rays) are released
and the emission rays that correspond to the photon
wavelength are measured. The element type is determined
based on the position of the photon rays, and the
content of each element is determined based on
the rays' intensity.
To generate plasma, first, argon gas is supplied to torch coil, and high
frequency electric current is applied to the work coil at the tip of
the torch tube. Using the electromagnetic field created in the torch
tube by the high frequency current, argon gas is ionized and plasma is
generated. This plasma has high electron density and temperature (10000K)
and this energy is used in the excitation-emission of the sample. Solution
samples are introduced into the plasma in an atomized state through the
narrow tube in the center of the torch tube.
3. Analytical Chemical Features of ICP-OES
The following features of ICP-OES distinguish
it from atomic absorption spectrometer used for
similar purposes.
1. Simultaneous, sequential analysis of multiple
elements possible
2. Wide linear region of analytical curve
3. Few chemical interference or ionization interference, making analysis
of high-matrix samples possible
4. High sensitivity (low limit of detection for majority of elements
is 10ppb or lower)
5. High number of measurable elements - elements that are difficult to
analyze in atomic absorption spectrometry such as Zr, Ta, rare earth,
P and B can be easily analyzed
6. Stable
The majority of the above features are derived
from the structure and characteristics of the light
source plasma.
4. Equipment
Equipment for ICP optical emission spectrometry
consists of a light source unit, a spectrometer,
a detector and a data processing unit. There are
several types of equipment based on differences
in the spectrometer and the detector. The most
common type is shown in Figure 1.
1) Sequential type
A spectrometer with a Czerny-Turner monochrometor, and a detector with
a photomultiplier is most common for this type. With this equipment,
programmed wavelength of the spectrometer is consecutively varied to
measure multiple elements. This causes rather long measuring time,
however, with its high resolution spectrometer, it is favorable for
measurement of high-matrix samples.
Figure 1: Sequential Type ICP-OES
2) Simultaneous Type
This type typically uses an echelle cross disperser in spectrometer and
semi-conductor detector such as CCD for the detector. Echelle cross
disperser disperses light of measurable wavelength range two-dimensionally
by combining prism and echelle diffraction grating. Combination of
echelle cross disperser and a CCD detector enables multi-element measurement
at any wavelength. The most notable feature of this equipment is the
high-speed measurement, providing information on all 72 measurable
elements in measurements of 1 to 2 minutes normally.
Figure 2: Simultaneous ICP-OES
5. Applications
3) Steel Analysis
One major application field of ICP optical emission spectrometry is material
analysis. The below example is an analysis of a steel sample.
* Equipment: Sequential Type ICP-OES, SPS3000
* Sample: JSS Standard Steel Sample 150, 0.5g dissolved in 100ml mixed
acid of chloric acid and nitric acid.
* Standard solution: 0.5g pure iron (99.99%), dissolved into 100ml solution
is used as blank, and the standard curve is created by standard solutions
of 8 measured elements (matrix matching).
* Analysis conditions: With a high-matrix sample such as this, spectral
overlap with the matrix element (iron in this case) occurs, so the spectral
profile is inspected and the most suitable analysis wavelength is chosen
for measurement.
* Analysis result: The results from the 3 measured samples and the certified
value of the standard sample is shown on the chart, showing extremely
good matches.
| Sample |
500-2 |
501-2 |
502-2 |
| Element |
Certified
value |
Measured
value |
Certified
value |
Measured
value |
Certified
value |
Measured
value |
| Si |
0.29 |
0.29 |
0.27 |
0.22 |
0.26 |
0.22 |
| Mn |
0.49 |
0.50 |
0.74 |
0.74 |
0.70 |
0.70 |
| P |
0.025 |
0.025 |
0.024 |
0.024 |
0.019 |
0.019 |
| Ni |
0.10 |
0.10 |
0.062 |
0.059 |
0.050 |
0.046 |
| Cr |
1.10 |
1.11 |
1.03 |
1.04 |
1.00 |
1.00 |
| Mo |
0.19 |
0.19 |
0.17 |
0.17 |
0.18 |
0.17 |
| Cu |
0.12 |
0.12 |
0.10 |
0.11 |
0.068 |
0.068 |
| V |
0.006 |
0.007 |
0.007 |
0.007 |
0.004 |
0.004 |
unit:%
Chart1: Analysis result of high
tensile structural steel series
2) Bastnasite Analysis
Bastnasite is a mineral of lanthanide rare earth type. There are many
issues with analyzing rare earth with atomic absorption spectrometer,
such as difficulty to obtain a light source lamp and difficulty with
atomization due to oxides easily generated from rare earths, but these
problems are eliminated for the most part with ICP-OES, which is widely
used in this field. One major difficulty with analyzing rare earths is
that they tend to have a high number of emission rays and has a high
possibility of interference by nearby rays on the analysis wavelength.
* Analysis conditions: 0.5g Bastnasite is dissolved
into 100ml with hydro fluoric acid, nitric acid
and hydrogen peroxide solution.
* Analysis result: Analysis of non-rare earth and rare earth elements
is shown on the chart below. Quantitative determination is conducted
from 30% concentration cerium oxide to 0.11% europium oxide.
unit:%
Chart 2: Analysis result of Bastnasite
3) Hair analysis
Human hair has attracted attention because it is thought to contain a
person's health history on some level and is thought to act as an excretory
organ for heavy metal in the body. However, there are problems because
there are few usable samples and knowledge about multiple elements
is required. With simultaneous analysis equipment, we can collect useful
information with a small amount of sample.
* Equipment: Simultaneous ICP-OES, Vista-PRO
* Sample: After washing, 0.5g of hair is measured, and then resolved
in a Teflon pressurized container with nitric acid into 10ml solution.
Partial spectral profile and analysis results shown below.
unit: ug/g
Analysisi Result
6. Conclusion
ICP optical emission spectrometry is now highly
rated as a multipurpose analysis technique and
there are over 2,000 units of ICP-OES in use in
Japan. It is well regarded as an environmental
measurement technique, along with atomic absorption
spectrometry and ICP mass spectrometry, and its
use is expected to expand even further in the future.
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