ICP-MS is powerful technique capable of measuring very low levels in a wide variety of sample types, limited only by cleanliness and the presence of interences. This article will examine the types of interferences that are encountered and various ways of dealing with them using a quadrupole ICP-MS instrument: mathematical correction equations, matrix removal, and cell-based ICP-MS. The strengths and limitations of each method will be discussed.

Nov 01, 2010

Spectroscopy

Since its inception as an analytical technique, inductively coupled plasma–mass spectrometry (ICP-MS) has gained popularity due to its ability to rapidly measure trace levels of most elements on the periodic table. As the technique has matured, new advances have allowed ever-lower levels to be measured, limited primarily by the cleanliness of the laboratory environment and reagents. However, there has been one obstacle that has continuously plagued the technique: interferences. Although ICP-MS has relatively few interferences compared to ICP-optical emission spectrometry (OES), they are problematic when measuring low levels. Much time and effort has been devoted to removing the effects of interferences, including the development of new classes of instruments: high-resolution ICP-MS and cell-based ICP-MS. This article will discuss various manners of removing the effects of interferences for analyses performed on quadrupole ICP-MS instruments, both with and without cells.
m/z 58 will have contributions from both Fe and Ni.
Polyatomic interferences result from the combination of two or more isotopes from different elements, which usually occur in the plasma. The elements that form the polyatomic interferences usually result from the sample matrix, sample diluent, and argon itself. An example of a polyatomic interference is ArCl
Let's look at Cd as an example. The most abundant isotope of Cd is Let's look at this in mathematical terms:
where Rearranging terms:
To figure out the contribution of Sn to the signal at
where or
Substituting this expresession into equation 2 yields the following:
This equation can be entered into the instrument software so that the correction is automatically performed online. This strategy is also applicable to polyatomic interferences, where alternate isotopes of the interference can be measured. For example, Cl contains two isotopes: Correction equations work well and can correct for a wide range of interferences and concentrations. However, they do have their limitations. First, if a correction equation is used but there is no interference, the equations tend to over-correct, leading to low or negative concentrations. Also, if the interference concentrations are very high, the equations might not compensate for the elevated levels adequately. Finally, equations can become complicated if the alternate isotope also has an interference. For example, consider the ClAr
Despite their complexity, correction equations work well when measuring moderate analyte concentrations, generally more than 1 ppb. Correction equations have even been published as part of regulated methods, such as U.S. EPA Methods 200.8 and 6020 for the analysis of waters, soils, and solid wastes. Therefore correction equations are a valid, well-established, and well-understood way to correct for interferences. |