3.1: Introduction to ICP-OES

ICP-OES (Inductively Coupled Plasma–Optical Emission Spectroscopy) is a powerful tool for detecting trace metals in water, food, soil, and biological samples. By the end of this module, you should be able to: - Explain the principle of atomic emission. - Identify major components of an ICP-OES instrument. - Recognize common matrix effects and strategies to mitigate them. - Justify the choice of calibration strategy for a given sample.

For an on-line introduction to much of the material in this section, see Atomic Emission Spectroscopy (AES) by Tomas Spudich and Alexander Scheeline, a resource that is part of the Analytical Sciences Digital Library.

---

Principles of Atomic Emission

When atoms are excited (by heat or electrical energy), electrons jump to higher energy levels. As they relax back down, they emit photons at element-specific wavelengths.

• Each element has a unique “fingerprint” of emission lines.
• The intensity of the emission is proportional to the number of atoms in the excited state (and thus the concentration in the sample).
• ICP-OES detects these emissions across many wavelengths simultaneously, enabling multielement analysis.

---

ICP-OES Instrument Overview

    

Nebulizer and Spray Chamber

• Converts liquid samples into an aerosol (fine mist).
• Carries aerosol droplets into the plasma.
• Transport efficiency (how much sample makes it into the plasma) depends on solution properties like viscosity and dissolved solids.

Plasma Torch   

• Argon gas is ionized inside a quartz torch by a radio-frequency (RF) coil, creating a plasma at ~6000–10,000 K.
• The plasma serves as both atomizer (breaking samples into free atoms) and excitation source (promoting electrons to higher energy states).

Optical System

• A monochromator or polychromator separates emitted light by wavelength.
• Detectors measure intensity at selected wavelengths corresponding to target analytes.

---

Sample Introduction & Matrix Effects

Understanding and correcting for Sample Introduction and Matrix Effects is critical for collecting reliable data from ICP-OES.

• Sample Introduction Effects: The delivery of sample to the plasma at a constant rate and the maintenance of a constant plasma temperature are critical to getting reliable results in ICP-OES.  
• Matrix Effects: In real samples, the matrix (everything in the sample besides the analyte of interest) can alter signal intensity in a number of ways. These are called matrix effects.

Effect (category)	Mechanism (what’s happening)	Typical example	Primary fixes
Physical (transport/nebulization)	Viscosity/density/surface tension differences alter aerosol delivery; high TDS or organics load the plasma	Hard water (Ca²⁺/Mg²⁺); organics from filters	Dilute; matrix/acid match; optimize sample-intro hardware/flows; internal standard for physical drift only.
Chemical (plasma / ionization / loading)	Concomitant elements change excitation/ionization equilibria, or matrix consumes energy	High Na⁺/K⁺ suppressing analyte; high Fe loading	Dilute; matrix match; ionization buffer (e.g., Cs); standard additions when matching is impractical. (Internal standard is generally not reliable here unless it behaves identically to the analyte.)
Spectral overlap / background	Another line or structured background overlaps analyte wavelength; background slope/curvature	Fe line near analyte line; carbon-rich backgrounds	Choose alternate wavelength; background correction; inter-element correction (IEC) if unavoidable; verify with spike recovery.
Acid strength mismatch (a special physical case)	Different %HNO₃ changes viscosity/transport vs. standards	Standards at 2% HNO₃; samples at 0.2%	Acid match standards and samples; then confirm with spike recovery.
Other sources of information:  Thermo Fisher Scientific,  Inorganic Ventures,  Agilent

Key takeaway: Calibration only works if the standards and samples behave the same in the instrument. Sample Introduction problems and Matrix effects disrupt this assumption. 

---

ICP-OES Instruments: More Details

Plasma Source

A plasma is a hot, partially ionized gas that contains an abundant concentration of cations and electrons. The plasma used in atomic emission is formed by ionizing a flowing stream of argon gas, producing argon ions and electrons. A plasma’s high temperature results from resistive heating as the electrons and argon ions move through the gas. Because a plasma operates at a much higher temperature than a flame, it provides for a better atomization efficiency and a higher population of excited states.

A schematic diagram of the inductively coupled plasma source (ICP) is shown in Figure 3.1.2 . The ICP torch consists of three concentric quartz tubes, surrounded at the top by a radio-frequency induction coil. The sample is mixed with a stream of Ar using a nebulizer, and is carried to the plasma through the torch’s central capillary tube. Plasma formation is initiated by a spark from a Tesla coil. An alternating radio-frequency current in the induction coil creates a fluctuating magnetic field that induces the argon ions and the electrons to move in a circular path. The resulting collisions with the abundant unionized gas give rise to resistive heating, providing temperatures as high as 10000 K at the base of the plasma, and between 6000 and 8000 K at a height of 15–20 mm above the coil, where emission usually is measured. At these high temperatures the outer quartz tube must be thermally isolated from the plasma. This is accomplished by the tangential flow of argon shown in the schematic diagram.

Figure 3.1.2 . Schematic diagram of an inductively coupled plasma torch.

Types of Multielemental Analysis

Atomic emission spectroscopy is ideally suited for a multielemental analysis because all analytes in a sample are excited simultaneously. If the instrument includes a scanning monochromator, we can program it to move rapidly to an analyte’s desired wavelength, pause to record its emission intensity, and then move to the next analyte’s wavelength. This sequential analysis allows for a sampling rate of 3–4 analytes per minute. 

Another approach to a multielemental analysis is to use a multichannel instrument that allows us to monitor simultaneously many analytes (simultaneous analysis). A simple design for a multichannel spectrometer, shown in Figure 3.1.3 , couples a monochromator with multiple detectors that are positioned in a semicircular array around the monochromator at positions that correspond to the wavelengths for the analytes.

Figure 3.1.3 . Schematic diagram of a multichannel atomic emission spectrometer for the simultaneous analysis of several elements. Instruments may contain as many as 48–60 detectors.

Detection Limits for ICP-OES

ICP-OES can be used to detect elements across the periodic table; however, it is most sensitive to elements with low ionization energies such as metallic elements and heavier elements. The figure below shows the published detection limits for ICP-OES under ideal conditions using the most sensitive instruments available.

Figure \(\PageIndex{1}\): Atomic Spectroscopy limits of detection (LOD) in units of \(\frac{ng}{mL}=ppb\). Data from Parsons et. al., App. Spectosc. 1981, 37, 411-418. (CC-BY-SA, Kathryn Haas)

---

Optimization and Analysis of ICP-OES Data

Atomic emission is used widely for the analysis of trace metals in a variety of sample matrices. The development of a quantitative atomic emission method requires several considerations, including choosing a source for atomization and excitation, selecting a wavelength and slit width, preparing the sample for analysis, minimizing spectral and chemical interferences, and selecting a method of standardization.

Settings and Method Choices

• Choice of Atomization and Excitation Source (Plasma vs Flame): Except for the alkali metals, detection limits when using an ICP are significantly better than those obtained with flame. Plasmas also are subject to fewer spectral and chemical interferences than flame. For these reasons a plasma emission source is usually the better choice.

• Selecting the Wavelength and Slit Width: The choice of detection wavelength is dictated by the need for sensitivity and the need to avoid interferences from the emission lines of other constituents in the sample. Because an analyte’s atomic emission spectrum has an abundance of emission lines—particularly when using a high temperature plasma source—it is inevitable that there will be some overlap between emission lines. For example, an analysis for Ni using the atomic emission line at 349.30 nm is complicated by the atomic emission line for Fe at 349.06 nm. A narrower slit width provides better resolution, but at the cost of less radiation reaching the detector. The easiest approach to selecting a wavelength is to record the sample’s emission spectrum and look for an emission line that provides an intense signal and is resolved from other emission lines.

• Sample Preparation: Flame and plasma sources are best suited for samples in solution and in liquid form. Although a solid sample can be analyzed by directly inserting it into the flame or plasma, they usually are first brought into solution by digestion or extraction.

Data Analysis Choices

• Background Correct to Minimize Spectral Interferences: The most important spectral interference is broad, background emission from the flame or plasma and emission bands from molecular species. This background emission is particularly severe for flames because the temperature is insufficient to break down refractory compounds, such as oxides and hydroxides. Background corrections for flame emission are made by scanning over the emission line and drawing a baseline (Figure 3.1.4 ). Because a plasma’s temperature is much higher, a background interference due to molecular emission is less of a problem. Although emission from the plasma’s core is strong, it is insignificant at a height of 10–30 mm above the core where measurements normally are made.

Figure 3.1.4 . Method for correcting an analyte’s emission for the flame’s background emission.

Minimizing Chemical Interferences

Flame emission is subject to the same types of chemical interferences as atomic absorption; they are minimized using the same methods: by adjusting the flame’s composition and by adding protecting agents, releasing agents, or ionization suppressors. An additional chemical interference results from self-absorption. Because the flame’s temperature is greatest at its center, the concentration of analyte atoms in an excited state is greater at the flame’s center than at its outer edges. If an excited state atom in the flame’s center emits a photon, then a ground state atom in the cooler, outer regions of the flame may absorb the photon, which decreases the emission intensity. For higher concentrations of analyte self-absorption may invert the center of the emission band (Figure 3.1.5 ).

Figure 3.1.5 . Atomic emission lines for (a) a low concentration of analyte, and (b) a high concentration of analyte showing the effect of self-absorption.

Chemical interferences when using a plasma source generally are not significant because the plasma’s higher temperature limits the formation of nonvolatile species. For example, \(\text{PO}_4^{3-}\) is a significant interferent when analyzing samples for Ca²⁺ by flame emission, but has a negligible effect when using a plasma source. In addition, the high concentration of electrons from the ionization of argon minimizes ionization interferences.

Standardizing the Method

From Equation \ref{10.1} we know that emission intensity is proportional to the population of the analyte’s excited state, \(N^*\). If the flame or plasma is in thermal equilibrium, then the excited state population is proportional to the analyte’s total population, N, through the Boltzmann distribution (Equation \ref{10.2}).

A calibration curve for flame emission usually is linear over two to three orders of magnitude, with ionization limiting linearity when the analyte’s concentrations is small and self-absorption limiting linearity at higher concentrations of analyte. When using a plasma, which suffers from fewer chemical interferences, the calibration curve often is linear over four to five orders of magnitude and is not affected significantly by changes in the matrix of the standards.

Emission intensity is affected significantly by many parameters, including the temperature of the excitation source and the efficiency of atomization. An increase in temperature of 10 K, for example, produces a 4% increase in the fraction of Na atoms in the 3p excited state, an uncertainty in the signal that may limit the use of external standards. The method of internal standards is used when the variations in source parameters are difficult to control. To compensate for changes in the temperature of the excitation source, the internal standard is selected so that its emission line is close to the analyte’s emission line. In addition, the internal standard should be subject to the same chemical interferences to compensate for changes in atomization efficiency. To accurately correct for these errors the analyte and internal standard emission lines are monitored simultaneously.

Representative Method: Determination of Sodium in a Salt Substitute

The best way to appreciate the theoretical and the practical details discussed in this section is to carefully examine a typical analytical method. Although each method is unique, the following description of the determination of sodium in salt substitutes provides an instructive example of a typical procedure. The description here is based on Goodney, D. E. J. Chem. Educ. 1982, 59, 875–876.

Description of Method

Salt substitutes, which are used in place of table salt for individuals on low-sodium diets, replaces NaCl with KCl. Depending on the brand, fumaric acid, calcium hydrogen phosphate, or potassium tartrate also are present. Although intended to be sodium-free, salt substitutes contain small amounts of NaCl as an impurity. Typically, the concentration of sodium in a salt substitute is about 100 μg/g The exact concentration of sodium is determined by flame atomic emission. Because it is difficult to match the matrix of the standards to that of the sample, the analysis is accomplished by the method of standard additions.

Procedure

A sample is prepared by placing an approximately 10-g portion of the salt substitute in 10 mL of 3 M HCl and 100 mL of distilled water. After the sample has dissolved, it is transferred to a 250-mL volumetric flask and diluted to volume with distilled water. A series of standard additions is prepared by placing 25-mL portions of the diluted sample into separate 50-mL volumetric flasks, spiking each with a known amount of an approximately 10 mg/L standard solution of Na⁺, and diluting to volume. After zeroing the instrument with an appropriate blank, the instrument is optimized at a wavelength of 589.0 nm while aspirating a standard solution of Na⁺. The emission intensity is measured for each of the standard addition samples and the concentration of sodium in the salt substitute is reported in μg/g.

Questions

1. Potassium ionizes more easily than sodium. What problem might this present if you use external standards prepared from a stock solution of 10 mg Na/L instead of using a set of standard additions?

Because potassium is present at a much higher concentration than is sodium, its ionization suppresses the ionization of sodium. Normally suppressing ionization is a good thing because it increases emission intensity. In this case, however, the difference between the standard's matrix and the sample’s matrix means that the sodium in a standard experiences more ionization than an equivalent amount of sodium in a sample. The result is a determinate error.

2. One way to avoid a determinate error when using external standards is to match the matrix of the standards to that of the sample. We could, for example, prepare external standards using reagent grade KCl to match the matrix to that of the sample. Why is this not a good idea for this analysis?

Sodium is a common contaminant in many chemicals. Reagent grade KCl, for example, may contain 40–50 μg Na/g. This is a significant source of sodium, given that the salt substitute contains approximately 100 μg Na/g.

3. Suppose you decide to use an external standardization. Given the previous questions, is the result of your analysis likely to underestimate or to overestimate the amount of sodium in the salt substitute?

The solid black line in Figure 3.1.6 shows the ideal calibration curve, assuming we match the standard’s matrix to the sample’s matrix, and that we do so without adding any additional sodium. If we prepare the external standards without adding KCl, the emission for each standard decreases due to increased ionization. This is shown by the lower of the two dashed red lines. Preparing the standards by adding reagent grade KCl increases the concentration of sodium due to its contamination. Because we underestimate the actual concentration of sodium in the standards, the resulting calibration curve is shown by the other dashed red line. In both cases, the sample’s emission results in our overestimating the concentration of sodium in the sample.

Figure 3.1.6 . External standards calibration curves for the flame atomic emission analysis of Na in a salt substitute. The solid black line shows the ideal calibration curve assuming we match the matrix of the samples and the standards using pure KCl. The lower of the two dashed red lines shows the effect of failing to add KCl to the external standards, which decreases emission. The other dashed red line shows the effect of using KCl that is contaminated with NaCl, which causes us to underestimate the concentration of Na in the standards. In both cases, the result is a positive determinate error in the analysis of samples.

4. One problem with analyzing salt samples is their tendency to clog the aspirator and burner assembly. What effect does this have on the analysis?

Clogging the aspirator and burner assembly decreases the rate of aspiration, which decreases the analyte’s concentration in the flame. The result is a decrease in the emission intensity and a negative determinate error.

Example 3.1.1

To evaluate the method described in Representative Method 10.7.1, a series of standard additions is prepared using a 10.0077-g sample of a salt substitute. The results of a flame atomic emission analysis of the standards is shown here [Goodney, D. E. J. Chem. Educ. 1982, 59, 875–876].

added Na (µg/mL)	Ie (arb. units)
0.000	1.79
0.420	2.63
1.051	3.54
2.102	4.94
3.153	6.18

What is the concentration of sodium, in μg/g, in the salt substitute.

Solution

Linear regression of emission intensity versus the concentration of added Na gives the standard additions calibration curve shown below, which has the following calibration equation.

\[I_{e}=1.97+1.37 \times \frac{\mu \mathrm{g} \ \mathrm{Na}}{\mathrm{mL}} \nonumber\]

The concentration of sodium in the sample is the absolute value of the calibration curve’s x-intercept. Substituting zero for the emission intensity and solving for sodium’s concentration gives a result of 1.44 μgNa/mL. The concentration of sodium in the salt substitute is

\[\frac{\frac{1.44 \ \mu \mathrm{g} \ \mathrm{Na}}{\mathrm{mL}} \times \frac{50.00 \ \mathrm{mL}}{25.00 \ \mathrm{mL}} \times 250.0 \ \mathrm{mL}}{10.0077 \ \mathrm{g} \text { sample }}=71.9 \ \mu \mathrm{g} \ \mathrm{Na} / \mathrm{g}\nonumber\]

---

References & Resources

• Harvey, D. Analytical Chemistry 2.1. LibreTexts. Chapter 10: Spectroscopic Methods.
• Atomic Emission Spectroscopy (AES) module by Tomas Spudich and Alexander Scheeline (Analytical Sciences Digital Library).
• CHEM401L Module 5: Tainted Tap! What’s in Your Water? (course manual).