Reducing the Effects of Interferences in Quadrupole ICP-MS - ICP-MS is powerful technique capable of measuring very low levels in a wide variety of sample types, limited only by cleanliness and the pr
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Reducing the Effects of Interferences in Quadrupole ICP-MS
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.


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.

Types of Interferences

There are two main types of interferences: isobaric and polyatomic. Isobaric interferences refer to different elements whose isotopes share a common mass. For example, both Fe and Ni have isotopes at mass 58. Therefore, any signal measured at 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+ on As+ , both of which occur at m/z 75. The ArCl+ forms from a combination of Ar in the plasma and Cl from the sample matrix or diluent; if no Cl is present, then ArCl+ will not form, and As will be interference-free.

The Easy Solution . . . : Because ICP-MS has the ability to measure multiple isotopes, the easy solution is to measure another isotope that does not suffer from interferences. For the Fe/Ni example, this would be easy: 60 Ni does not suffer from any isobaric interferences. Because all elements (with the exception of In) have at least one isotope that does not have an isobaric interference, this would appear to be a simple solution.

. . . Isn't Always So Easy: Unfortunately, the solution isn't always this simple: 60 Ni suffers from the polyatomic interference CaO+ , which originates from calcium-containing samples. Also consider the ArCl+ interference on As: As has only one isotope, so choosing As isotope without interference is not an option.

But There Is Another Solution: We can take advantage of the fact that most elements have multiple isotopes to correct mathematically for both isobaric and polyatomic interferences. By considering the natural abundance of different isotopes and measuring the intensity of a noninterfered isotope, we can calculate the extent of the interference and subtract this contribution to yield the concentration.

Let's look at Cd as an example. The most abundant isotope of Cd is m/z 114 (28.73%). But Sn also has a minor isotope at m/z 114 (0.65%). This means that any intensity at m/z 114 is a contribution from both Cd and Sn. However, Sn has other isotopes which are interference-free, such as m/z 118 (24.23%). Therefore, we can measure Sn at m/z 118 and calculate its contribution to the signal at m/z 114.

Let's look at this in mathematical terms:

I(m/z 114) = I(114 Cd) + I(114 Sn) [1]

where I = intensity

Rearranging terms:

I(114 Cd) = I(m/z 114) − I(114 Sn) [2]

To figure out the contribution of Sn to the signal at m/z 114, we take into account the natural abundance of both 114 Sn and 118 Sn and the signal intensity of 118 Sn:

I(114 Sn) = [A(114 Sn)/A(118 Sn)] I(118 Sn) [3]

where A = abundance

or

I(114 Sn) = [0.65/24.23] I(118 Sn) [4]

I(114 Sn) = 0.0268 I(118 Sn) [5]

Substituting this expresession into equation 2 yields the following:

I(114 Cd) = I(m/z 114) − 0.0268 I(118 Sn) [6]

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: m/z 35 (75.77%) and m/z 37 (24.23%). Therefore, if correcting for the 35 Cl40 Ar+ interference on As at m/z 75, the signal for the 37 Cl40 Ar+ ion at m/z 77 can be used, as described earlier.

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+ interference on As described above. The 37 Cl40 Ar+ ion at m/z 77 would be used for the correction. However, because Se also has an isotope at m/z 77 (7.63%), an additional term must be used to correct for the isobaric interference on the polyatomic interference. This can be accomplished by using the 82 Se isotope (8.73%), which yields a final equation of:

I(75 As) = I(m/z 75) − 3.127 [I(77 Se) − [0.874 I(82 Se)]] [7]

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.


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