Linearity and Detection Limits

Defining ICP Performance Characteristics

The following steps are intended as a practical guide for the determination of an ICP’s performance characteristics:

1. Read the operating manual and familiarize yourself with the software, key instrumental parameters and preferred settings before the instrument is installed.

Most instruments are supplied with optimization and wavelength or mass calibration standards that will be used during set-up by the service technician and are intended for use on a regular basis by the operator. Discuss the optimization process with the manufacturer as well as the preferred settings for the key instrumental parameters.

The remaining steps assume that the operator fully understands and is able to perform the optimization process that has been defined by the manufacturer as well as the spectral limitations of the instrument.

2. Select the lines to be studied for each element (‘lines’ is used in this document to mean either wavelength or mass).

Line selection is based upon spectral interference issues, detection limit requirements and working range requirements. Select as many lines as possible within practicality for each element. The greater the number of lines, the greater the flexibility.

3. Prepare single element standards over the anticipated working range for each element. The range of standards depends upon the analytical requirements. The following ranges are suggestions only:
   •  Radial view ICP-OES:  0.0, 1, 10, 100, and 1000 µg/mL
   •  Axial view ICP-OES:  0.0, 0.1, 1, 10, and 100 µg/mL
   •  Quadrupole (R~ 300) mass filtered ICP-MS:  0, 1, 10, 100, and 1000 ng/mL
   
   This step is important because these data can be used to determine instrument detection limits (IDL), linear working ranges, and spectral characteristics such as background equivalent concentrations (BEC) and spectral interferences. With most modern (if not all) instruments, the spectra obtained for each element at each concentration can be saved for review later. In addition, the software will calculate the IDL and BEC plus the linear regression of each line will establish the linear working range. All of this is typically done for the operator by the software that comes with the instrument. If at all possible, attempt to:
• Use single element standards that have the trace metals impurities reported on the certificate of analysis. Most chemical standards manufacturers provide this information with their single element standards. These data are important in identifying direct spectral overlap interferences and in not identifying an impurity as an interference of this type.
• Store all spectra on computer and collect the spectra for all lines of interest on each and every solution. This means that if you are interested in possibly using up to 6 lines for roughly 72 elements, then each solution spectrum totaling 72 x 6 = ~ 432 lines per solution and ~ 432 x 5 = 2160 spectra for each element need to be stored for future reference. Most ICP-MS applications would require far fewer data to be collected due to the reduced number of lines available and/or feasible.
• Wash blank acid solution through the instrument for several minutes ‘between elements’ and always analyze a blank at the beginning of each element concentration series. Look for the presence of the prior element analyzed to confirm that it has been completely washed out of the introduction system.
4. Having the data available on a desktop computer is convenient and allows the analyst to construct potential spectra by calling up the element and the anticipated concentration for each element in the analytical sample. Having several lines available makes the job of line selection easy as well as the estimation of the line’s sensitivity and linearity. Constructing these composite spectra from pure single element solutions eliminates confusion as to the identity of the line. The following example is intended to illustrate the process:

Was this helpful?  Tell a Friend | Let us know