ICP-MS - The Analysis: Calibration
Because the response of the mass spectrometer in counts per second is directly proportional to the concentration of a given element in a sample, it is relatively easy to calibrate the system using a series of external standards of differing concentrations. Any sample entered into the mass spectrometer under exactly the same conditions will return a count rate, which can be converted directly to concentration for each element from a calibration curve.
The analytical challenge is to ensure that the conditions are identical for each sample. There are many variables that can play a significant role in the analysis of a sample and allowances must be made for all of them. Possible factors affecting the sampling conditions of the ICP-MS include, but are not necessarily limited to:
- Variations in plasma ionization efficiency
- Possible clogging or erosion of cone apertures, resulting in different interface transport efficiency
- Different matrix or matrix concentration in the samples, resulting in possible matrix suppression
- Laboratory temperature or humidity fluctuations, resulting in more or less recombination of elements into molecular species
- Molecular species may be formed by elements in the sample. These species may interfere with another element in an unexpected way. An example of this would be the formation of a chromium dimer (Cr2) in a high chromium matrix. One of the possible Cr2 molecules has a mass of 103, which can interfere with rhodium-103 (the only isotope of that element).
These variations may appear to make the accurate analysis of samples difficult or even impossible. This is not the case. By the application of a few simple methods, they can all be corrected for. At MURR, the following steps are followed in order to minimize difficulties:
- We prepare an external calibration series containing every element that is to be analyzed. The series is designed to cover a range of concentrations that completely brackets the concentration of analyte in the sample. If the sample is found to fall significantly outside of this range, then it will be diluted and run again so that it falls within the range.
- Internal standards are added to all samples and standards. The internal standards consist of one or more elements that are known to be absent in the sample. They are added to each sample at a known concentration so that any variation in the intensity of their signal can be corrected to the known concentration. By applying the same correction to the other elements in the solution, the correct concentration can be calculated easily.
- Spiked samples: For potentially difficult matrices, one duplicate of one sample can be prepared with a known spike of every element of interest added to it. This can be used to measure the recovery efficiency of each element so that obvious discrepancies can be discovered and investigated further.
A typical calibration graph showing the response
from standards of zero to 10 ppb on the VG Axiom