The Analyst’s Guide to Preventing ICP-OES Calibration Mistakes
The Analyst’s Guide to Preventing ICP-OES Calibration Mistakes
How to reduce rework when preparing calibration standards
Having to restart an analysis, including preparing new standards and generating a new calibration, could cost you considerable time and money, especially if your calibration involves multiple standards.
Often, lower than expected sensitivity and a poor fit on your calibration graph are the first signs of trouble.
Common sources of error
Listed below are some common sources of error associated with the manual preparation of calibration standards. Ensure that your lab’s standard operating procedures include steps to minimize or eliminate these potential sources of calibration error.
Pipettes that are out of calibration
The relevant international standard for pipette calibration is ISO 8655-2:2002.
Here’s a pictorial guide.
Contaminated glassware from inadequate cleaning/soaking
For guidance on glassware cleaning, refer to Annex A of the international standard ISO 4787:2010 Laboratory glassware—Volumetric instruments—Methods for Testing of Capacity and for Use.
Here’s a simple video.
Or refer to this application note from Corning.
Cross-contamination between stock solutions of different elements
This can often be due to poor analytical practice (e.g., double-dipping a previously used pipette).
Remember to change the pipette tip before picking up and dispensing the next component of the solution you’re preparing.
Always have dedicated pipettes for each element.
Using the wrong stock solution when preparing a multi-element standard
This can be a problem if you are using single-element stock solutions.
Try to work from a multi-element stock standard to minimize the number of components you have to use, reducing the chance of error.
Ensure all solutions are clearly labeled and try to line them up in a logical order.
Missing or doubling up on a required element in a multi-element standard
Try to work from a multi-element stock standard to minimize the number of components you have to use, reducing the chance of error.
Ensure all solutions are clearly labeled and try to line them up in a logical order.
Stock solutions that are past their expiration date
Check the expiration dates before preparing your standards.
Ensure there is a process and a person responsible for regularly checking stock solutions and replenishing them when they are close to expiration.
Degradation of stock solutions or standards due to incorrect storage
Store standards at controlled room temperature per USP 35 General Notices and Requirements (10.30.60).
Do not freeze, heat, or expose to direct sunlight. Minimize exposure to moisture or high humidity.
Poor reagent quality
Check the level of impurities in the selected reagents from the Certificate of Analysis before purchase/use.
Try alternate brands or a higher purity reagent if the levels are unacceptable for your application.
Use Agilent’s certified reference materials, which provide the assurance of NIST traceability and are manufactured using high-purity starting materials to minimize impurities.
Instruments that are out of wavelength calibration
Use the instrument performance tests that are built into most ICP-OES instruments. A wavelength calibration test is usually one of the standard tests.
Incorrect acid used to stabilize a standard solution
Always check the matrix that the stock standard is prepared in and match that matrix when stabilizing the prepared standard. For example, H2SO4 should not be used for Pb standards.