August 29, 2014

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ICP Analysis

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Sample Preparation

Samples submitted for ICP analysis must be FULLY PREPARED and should match the acidity and background matrix of our calibration standards. Typical methods of sample preparation include extraction, dissolution, or acidification and should result in a solution free of particulates. Typical acid matrices include 10% HCl (1 volume of concentrated HCl in 10 volumes of solution), 25% HNO3, and 1% HNO3.

We prefer samples be at least 10ml in volume, but can generally get by with 5ml samples. Sample volumes smaller than 5ml will necessitate dilution and the corresponding change in reporting limits. Contact the Laboratory to discuss your analytical goals and the appropriate matrices to accommodate those goals. If you have a matrix unlike our conventional standards, there is an additional charge for preparing matching standards.

All quality control samples (outside reference checks, replicates, spikes, and blanks) are the responsibility of the submitter. A mismatch in the acidity can lead to a difference between the viscosities of the samples and standards. The viscosity (and other factors) affect the nebulization properties of a solution, which has a significant impact on the accuracy of the analysis. In addition to the acid matrix, the calibration standards should approximate the kind and amounts of elements in the samples to be analyzed as closely as possible. For example, a large difference in the amount of calcium between the calibration standards and samples can bias the determinations of some of the elements by as much as 20%. For this reason, we have many suites of calibration standards that are formulated to match typical types of samples that we work with.

You should be aware that while ICP values are estimates of ‘total’ elemental values in solution, the sample preparation technique determines the fraction of the elements in the sample that are brought into solution.

Analyses

‘ICP’  or ‘ICAP-OES’ are acronyms for Inductively Coupled Argon Plasma Optical Emission Spectrometer. Both of our ICPs provide ‘total’ elemental analysis of materials in aqueous solution, typically an acidic solution.  Many different types of materials are suitable for analysis by ICP – water, soil extracts, plant or animal tissue digests, food and feed digests, chemical solutions, and many other materials that can be brought into aqueous solution. 

Methods

An ICP works by injecting a nebulized mist from a liquid into the center of an argon plasma. A plasma is created from a flow of gas within a high energy field (in the case of an ICP, by a strong alternating current of RF energy flowing in a coil just outside of the gas flow) which ionizes the gas and causes intense heating. Temperatures inside an ICP plasma reach 10000 K – as hot as the surface of the sun. When the mist of the sample enters the plasma, the intense heat causes the dissociation of most chemical compounds, and the energy that the component atoms absorb causes them to undergo excitation and ionization energy transitions.  These transitions produce spectral emissions characteristic of the elements being excited. The spectra produced by the plasma is broken down into individual spectral lines by the ICP’s spectrometer, and the ICP’s computer translates the spectral lines into concentrations for a specified suite of elements.  In contrast to flame emission spectroscopy, the plasma is not an oxidizing environment (like a flame) and there are fewer chemical reactions to confound the analysis. Also, ICP has a large linear range – around 4-6 orders of magnitude for most elements. This means that fewer dilutions are required to accommodate samples with a wide range of concentrations. 

Interpreting Results

Elemental determinations by ICP are typically reported in ppm (parts per million) on a sample weight (for once-solid samples) or sample volume (for liquid samples) basis. If we know the dilution factors for the samples, we can apply those factors and report the values in ppm on a sample weight basis. For example, results for plant samples that we have prepared are reported as ppm on a weight element / dry sample weight basis.  Note that ICP values are expressed on an atomic weight basis, not as any other molecular species. For example, phosphorus is reported as elemental phosphorus, not as phosphate. To convert elemental values to other forms you need to factor in the atomic weight of the element as a fraction of the molecular weight of the species in question. For elements in high concentrations, consider that 10000ppm on a weight/weight basis is the same as 1 percent by weight. That is, a calcium value of 20000ppm is the same as 2% calcium (by weight) in the material.