Inductively Coupled Plasma Optical Emission Spectroscopy
ICP-OES is used to identify the chemical constituents of a sample. The sample is often digested using strong acids into a homogenous solution. It is then diluted and fed into a plasma, and the emission spectra is recorded. Concentration is quantified by comparing the measured intensity of the spectra in characteristic regions to the signal intensity generated for standards of known concentration.
Practice
Sample Preparation
In this example, 2.5 mg of LiNi0.5Mn1.5O4 was disgested in 250 μl of nitric acid and 500 μl of hydrochloric acid at 80°C. The sample was subsequently diluted to 250 ml in a volumetric flask to create a 10 mg/L solution of LiNi0.5Mn1.5O4 in water. PPM concentrations of each individual analyte were calculated using the table below.
| Sample | Concentration (mg/L) | Molar Mass (g/mol) | Molarity (mMol/L) |
|---|---|---|---|
| LiNi0.5Mn1.5O4 | 10 | 182.69 | 0.0547 |
| Analyte | Stoichiometry | Molarity (mMol/L) | Molar Mass (g/mol) | Concentration (mg/L) |
|---|---|---|---|---|
| Li | 1 | 0.05474 | 6.941 | .3799 |
| Ni | 0.5 | 0.02737 | 58.69 | 1.606 |
| Mn | 1.5 | 0.08211 | 54.93 | 4.510 |
Standard Selection
Standards should be selected such that they span both higher and lower than the expected sample concentration. At least three, but preferably 5 standards should be made up to analyze a sample. Additionally, a blank standards should be made using the same processing conditions as the sample to eliminate the impact any trace contaminations in the acid. In this case, three standards were used, in addition to a blank made by the dilution 250 μl of nitric and 500 μl of hydrochloric acid to 250 ml of total volume.
| Standards | Blank (mg/L) | Low (mg/L) | Medium (mg/L) | High (mg/L) |
|---|---|---|---|---|
| Li | 0 | 0.2 | 0.4 | 1 |
| Ni | 0 | 0.5 | 1.5 | 3 |
| Mn | 0 | 2 | 5 | 10 |
Standards often contain between ~0.1-30 ppm of the desired elements. At too high of a concentration, the response becomes non-linear, and this non-linear regime can't be used. At too low concentrations, signal to noise ratio and sample stability are reduced. Minimum concentrations can be identified in https://www.inorganicventures.com/periodic-table. Best practices for making up standards will be added later.
Running ICP
A water cooler is turned on, and argon tanks are opened up, and the ICP's software should be started. To start, the intake valve should be connected to deionized water, and the outflow valve to waste. At this point, the plasma torch should be ignited, and deionized water should be run through the machine for 30 minutes. At this point the machine is ready.
Next, the programmed sequence should be started. The ICP will request solutions, a common sequence being the blank, low standard, medium standard, high standard, sample 1, sample 2, etc. Standards should be rerun after every 5 samples. To turn off the machine, present a blank standard for 5 minutes, followed by deionized water for 5 minutes. Cut off the argon torch, dissasemble the inlet/outlet tubing, turn off the water cooler, cut off the argon, and dispose of the waste.
Analysis
It is critically important that there be a linear relationship between the nominal concentrations of the standards and the measured intensity. This ensures the machine is functioning appropriately and standards were made up without error. An R2 for this relationship of 0.9999 for the standards is required to have strong confidence in the interpolated sample concentration.
