ICP-OES Analysis
Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) is a widely used analytical technique for precise elemental analysis across various scientific and industrial applications. Utilizing high-temperature plasma and advanced optical detection, ICP-OES measures the total elemental composition of samples, typically in aqueous or acidic solutions. It is capable of analyzing a broad range of materials, including water, soil extracts, digested plant and animal tissues, food and feed samples, and chemical solutions, as long as they can be converted into a liquid form. Known for its high sensitivity and accuracy, ICP-OES can detect trace elements at concentrations as low as parts per billion (ppb), making it particularly valuable in environmental monitoring, pharmaceuticals, agriculture, and manufacturing. Its ability to perform simultaneous multi-element analysis improves efficiency and reliability, even in complex sample matrices.
Sample Preparation
When preparing samples for ICP-OES analysis, it is essential to first determine the sample type. Solid samples require dissolution or digestion to convert them into a liquid form suitable for analysis, while liquid samples may need filtration and possibly acidification to ensure compatibility with the instrument and calibration standards.
For solid samples, acid digestion is typically used to break the material down into a liquid. Common acids for this purpose include nitric acid (HNO₃), which is effective for both organic and inorganic materials, and hydrochloric acid (HCl), which works well for dissolving oxides and carbonates. Please note, the laboratory is not equipped to handle hydrofluoric acid (HF). The digestion process can be facilitated using a microwave digestion system or hot plate digestion to ensure complete dissolution.
Our laboratory provides a range of digestion and extraction methods for ICP analysis, tailored to different types of solid samples. For detailed information on available analyses and methodologies, please visit our Plant and Soil webpages. If you require a specific method, we may be able to accommodate your request - contact our laboratory to discuss your needs.
Liquid samples often require acidification to adjust their pH and matrix composition to align with ICP calibration standards. The most commonly used calibration standards in the laboratory are made with 2% HNO₃ acid. Acidifying the sample to 2% HNO₃, helps preserve the sample and stabilize the analytes. The matrix of your sample should closely match that of the calibration standards to ensure accurate results. If your sample has an unusual matrix, it is advisable to contact the laboratory before submitting.
Removing particulates is a critical step, as suspended solids can clog the nebulizer during analysis. Any remaining particulates must be removed by filtration, typically using a filter with a pore size of 0.45 µm or finer. Centrifugation can also help achieve a clear, particle-free sample. Ensure the prepared sample volume meets minimum laboratory requirements - at least 10 mL. Smaller volumes can be accepted but will require dilutions, which can affect detection accuracy at low concentrations.
The Limit of Detection (LOD) represents the smallest amount of a substance that can be reliably detected by the instrument. It is calculated as a multiple of the standard deviation of blank intensities, relative to the sensitivity defined by the slope of the calibration function. In the instrument software, the LOD is determined using a specific formula that incorporates these factors to ensure accurate detection. Since the LODs are recalculated with each calibration for each element during every analysis, they may vary slightly from day to day. The laboratory can provide an approximate range of LODs to give you a general understanding, but it’s important to note that these values will fluctuate slightly due to daily calibration differences.
Finally, proper labeling and storage are key to maintaining sample integrity. Each sample should be clearly labeled with a unique identifier and stored in clean, acid-washed centrifuge tubes or containers. Refrigeration is typically recommended, and if submission delays are expected, freezing the samples is an option to preserve the samples long term.
Communication with the laboratory is essential to discuss specific elements and analytical goals. If you are uncertain about any preparation step, please contact the laboratory.
Methods
The ICP-OES is a sophisticated analytical instrument used to determine the elemental composition of a sample. It works by using high-temperature plasma as a source to excite atoms and ions, causing them to emit light at specific wavelengths characteristic of each element. The intensity of this light is then measured to determine the concentration of elements within the sample.
The process begins with sample introduction, where a liquid sample is converted into a fine aerosol using a nebulizer. This aerosol is carried into the plasma by an inert gas, typically argon. The plasma, generated using a high-frequency radiofrequency (RF) coil, creates a high-temperature environment (6,000–10,000 K) that atomizes the sample and excites the atoms and ions.
As the excited atoms and ions return to their ground states, they emit light at wavelengths unique to each element. This emitted light is directed into a spectrometer, where it is separated into its component wavelengths. Detectors measure the intensity of light at each wavelength, which is directly proportional to the concentration of the corresponding element in the sample.
Interpreting Results
Elemental determinations by ICP are typically reported in parts per million (ppm) based on the sample’s weight (for solid samples) or volume (for liquid samples). If dilution factors were applied during sample preparation, these are factored into the final reported values. ICP values reflect the elemental concentration on an atomic weight basis, not as molecular species. For instance, phosphorus is reported as elemental phosphorus rather than phosphate. Converting elemental concentrations to other forms requires accounting for the atomic weight of the element relative to the molecular weight of the compound of interest. Additionally, for elements present in high concentrations, remember that 10,000 ppm on a weight/weight basis equals 1% by weight. For example, a calcium concentration of 20,000 ppm corresponds to 2% calcium by weight in the sample material.
FAQ
"What sample sizes do you need?"
We typically request a minimum sample volume of 10 mL, with 20 mL being ideal for optimal analysis. However, if a smaller volume is all that is available, we can still accommodate the testing. For specific minimum sample requirements based on the test, please refer to the back of the water job request sheet.
"How quickly will I receive results?"
You can request a turnaround time when you submit samples. A small set will likely take 1-2 weeks, but a larger sample set may take several weeks to complete.
"Can you test for specific elements?"
Yes, we offer single elements, custom sets, or our standard 15 or 27 element suite. We do offer a few elements outside of our standard 27 elements, please consult our laboratory for more information.
“How do I submit samples?”
Please bring or ship your samples to our main office: Room 135 of the Crops Research Building. We are open 8:00 am - 4:30 pm, Monday to Friday. Please fill out either a Water Job Request sheet or Special Job Request sheet and submit that form with your samples. We have printed forms in our office as well.
“What kind of ICP-OES instruments do you use?”
Thermo Scientific iCAP 7600 Duo ICP-OES.