The MRL has an Inductively Coupled Plasma (ICP) Atomic Emission Spectrometer that is available for use by qualified users on a recharge basis. Our Instrument is a TJA High Resolution IRIS model that can simultaneously view emissions from as many elements and lines as desired.

The ICP uses a very hot argon plasma to excite atoms into high energy states. As these atoms relax they emit light at characteristic wavelenghts, or lines. The ICP can measure about 70 elements in aqueous solutions, including most metals. It cannot measure H, He, C, N, O, F, Ne, Cl, and some other elements. Total dissolved solids in sample solutions need to be limited to less than 1%. The optimum working range for element concentrations is from 0.5 to 100 PPM by weight. Determinations down to 100 PPB or less are possible for many elements in clean solutions. Measurements of element concentrations over 100 PPM are routine, but at this level the signal to element concentration response may not be linear. All sample solutions must be absolutely free of particulates, sediments, precipitates, suspended gels, or lipids.

The ICP can be used for qualitative analysis to determine what elements are present in a sample. But, most often the ICP is uses for the quantitative determination of how much of each element is present in a sample. If a number of variables are held constant, then the amount of light emitted by a given line is directly proportional to the amount of the emitting element in the sample. By comparing the intensity of light from an unknown sample to that emitted by samples of known concentration, it is possible to determine the concentration of the unknown.

The Recharge Rate for the ICP is $20.00 per hour ($29.20 off campus hourly rate). It takes about one hour to warm up and standardize the instrument, about 5 minutes per sample, and about 20 minutes to shut down the instrument. ICP users need to bring their own calibration standards.

More Information

Inductively Coupled Plasma-Atomic Emission Spectroscopy (294Kb):
A good introductory Paper by Thomas Manning & William Grow. This is available for download in Adobe Acrobat format. To view it you will need Adobe Acrobat Reader, which can be downloaded here.

Linearity, Accuracy, Precision, & Detection Limits of the TJA IRIS HR ICP

ICP Spectrometer manufacturers all proudly claim that their instruments are linear to four or five decades. But they don't say for which decades or for which lines. In practice, the signal from most emission lines is linear with the element concentration up to somewhere between 50 to 250 PPM. Different lines for the same element may have different linear ranges, primarily depending of the relative intensity of the lines. Above some concentration, the signal begins to fall off. This is usually due to limitations in the detector and to atomic self absorption.

TJA defines linearity for their ICP as being when the measured concentration falls within 5% of the true value. Using this definition, they say that most lines are linear to about 100 PPM. Clearly the definition of what is acceptable as linear will effect the size of the linear range and the accuracy of any results.

The linear range of an emission line can be affected by the sample matrix and should be examined for each line in each broad category of samples. If at all possible, samples and standards should be prepared to have concentrations that stay within the linear range of the emission lines used.

With good line selection, standards that match the sample well, and good technique it should be possible for the TJA IRIS ICP Spectrometer to give results with an accuracy of from two to five percent. It will be seen in practice that some emission line consistently produce more accurate readings than other lines. Some of the factors that contribute to the making of a good line are the Relative Line Intensity, element mapping, interferences from other nearby lines, the sample matrix, and the background elements. In other words, the choice of the most accurate line(s) will vary with the nature of the sample being measured. The way to determine the best line(s) for a sample is to run several different known materials and to see which lines for each element yield the best results.

TJA specifies that our ICP will drift at no more that 2% an hour. This drift may be due to changes in room temperature, changes in ambient pressure, and limitations in the sample introduction system (SIS).

The precision for readings on the ICP Spectrometer are shown as percent Relative Standard Deviation (%RSD) and are calculated for each line in each run. Some lines tend to have lower %RSDs than other lines for the same element, for many of the same reasons that some lines have better accuracy that other lines. In general, %RSDs of between 0.4 and 0.7 are normal. %RSDs larger than 2% are probably indicative of a problem with the SIS; possibly the pump tube, the nebulizer, or the spray chamber. Any run with a %RSD larger than 2%, for most of the lines, should be repeated. This is especially true for standardizations.

The SIS has a large influence on the lower detection limit for any given element. The SIS could be modified to put more sample into the plasma, which would lower the detection limits, but so doing would reduce the stability of the mass flow into the plasma and lower the precision of the ICP Spectrometer. Our SIS has been setup for maximum precision at the expense of slightly higher detection limits. In general, most elements will be clearly measurable at 0.5 PPM and higher. Many elements will be detectable down to 0.1 PPM or less and may be measured in this range if the sample is clean enough. If very low detection limits are needed, ICP-MS or AA may be better analytical techniques.

Sample Requirements

All of the standards, blanks, and unknowns used on the ICP Spectrometer should be clear, homogeneous, and aqueous. Many problems can be avoided if the samples are sufficiently diluted to be below 250 PPM for each element of interest and if the total dissolved solids do not exceed 1%. Likewise, samples and standards must be free of all particles, precipitates, and sediments. If there are any particles or other solid material in the sample or standard, it must be filtered before introduction into the ICP Spectrometer.

Most tests require from 25 to 50 mls of solution. With care and advance preparation, tests can be done with 10 ml or less. Since solutions are usually dilute, most users prefer to bring an excess of sample.

The sample introduction system on our instrument is all plastic and designed to allow the use of sample solutions with strong acids, including HF. Sample introduction hardware is available for the use of organic liquid matrices, but is not installed on this ICP Spectrometer. Samples containing organic solvents cannot be run on this device.

Biological samples often contain lipids and other materials that will clog the sample delivery system. They may not be used. Check with MRL staff for more information.

Questions

If you have additional questions after you have checked out the information at the above links, please feel free to e-mail questions to us at jdoyle@mrl.ucsb.edu

ICP Links

High-Purity Standards: ICP standards, with good selection and fair prices. http://www.hps.net/

Mike Cheatham's Inductively Coupled Plasma Optical Emission Spectroscopy Page http://www.geochemistry.syr.edu/cheatham/icpoesins.html