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Article written by Charles Torok, Quality Control Chemist, Analytical Products Group, Inc.
Complete article from the Edition 22 APG eNewsletter
ICP-OES is a powerful tool for quickly identifying and quantifying metals in a sample. An operator of an ICP-OES has to be aware of some common issues that can occur in running the instrument. The torch, the tubing, and the sample preparation are some of the common areas in which problems can arise.
Torch
A dirty torch can affect readings and sensitivity as metal deposits on the torch will act as if they are part of the sample. This is most evident when running of blanks where readings will be present when there should not be any. In truly severe cases, if build-up occurs in the wrong area it can cause instrument shutdown if the optical sensor is blocked. So it is important to regularly clean the torch as prescribed by the manufacturer.
Tubing
Regular rotation and changing of peristaltic pump tubing is required. Stretched or squeezed tubing can result in variable quantities of sample being introduced to the instrument during a run. This will decrease the accuracy and affect the precision of measurements.
The end of the Teflon or HDPE tubing that is joined with pump tubing must be clean cut and unpinched. If the end of the tubing that is inserted into the pump tubing is pinched, the flow of sample into the instrument can be impeded and if a sharp edge is present it can even slice the pump tubing during insertion. This can result in varied flow during a run and even cause leaks in the tubing resulting in air bubbles or loss of sample. Any of these results will affect the readings of the instrument.
Particles can clog the tubing or the nebulizer. To reduce the chance of this, filter samples before running if there is a chance of any solids being present.
Sample Preparation
If the instrument is in proper running order and a valid testing method is in place, the most important thing for the operator to do is to take the proper care in preparation of calibration, quality control, and test samples. Being sure to carefully dilute to the proper concentrations is critical to acquiring reliable and accurate data.
The calibration curve that is built during a run is the translation of instrument response to concentration. If the calibration curve is inaccurately built then all readings that follow will be unreliable. The QC sample, which is of a known value, is important in verifying that the calibration curve is correct. The value the instrument yields on this QC sample should be the same as its known value when taking into account the labs acceptable level of uncertainty. If the calibration curve is well built and the test samples are properly prepared then meaningful values on the unknowns can be achieved.
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