4.2.5  Mass Interferences and Reaction Cells

ICP-MS instruments separate and detect analytes based on the atoms mass to charge ratio. Since the plasma in an ICP system is adjusted to maximize signally charged species, sample identity is directly related to atomic mass. While atomic emission spectrometry, ICP-AES, can be relatively free from spectral interferences (with monochromator systems that produce nm resolutions to three-decimal places), certain elements have problematic interferences in ICP-MS analysis due to the limited unit resolution (one amu) of most mass filters (especially the most common quadrupole mass filters). All ICP systems are subject to the nebulizer interferences given in the previous chapter. Spectral interferences are divided into three categories: isobaric, polyatomic, and doubly-charged species. Isobaric interferences occur in mass analyzers that only have unit resolution. For example, ⁴⁰Ar⁺ will interfere with ⁴⁰Ca⁺ and ¹¹⁴Sn⁺ will interfere with ¹¹⁴Cd⁺. High-resolution instruments will resolve more significant figures of the cation’s mass and will easily distinguish between these elements. Polyatomic interferences result when molecular species form in the plasma that have the same mass as the analyte of interest. Their formation can be dependant on the presence of trace amounts of O₂ and N₂ in the Ar or sample, certain salts in the sample, and the energy of the plasma. For example, ⁴⁰Ca¹⁶O⁺ can overlay with ⁵⁶Fe⁺, ⁴⁰Ar²³Na⁺ with ⁶³Cu⁺, and ⁸⁰Ar²⁺ and ⁸⁰Ca²⁺ with ⁸⁰Se⁺. The final type of interference occurs with doubly charged species. Since mass analyzers separate atoms based on their mass to charge ratio, ¹³⁶Ba²⁺ interferes with the quantification of ⁶⁸Zn⁺ since their mass to charge ratios are identical. The presence of any of these types of interferences will result in overestimation of the analyte concentration. Fortunately there are several ways of overcoming these interferences.

The easiest, but most expensive, way to overcome all three spectral interferences is to use a high-resolution mass analyzer, but, at a minimum, this can double to quadruple the cost of an analysis. Most inexpensive alternatives include the use of interference equations to estimate the concentration of the interfering element or polyatomic species, the use of a cool plasma technique to minimize the formation of polyatomic interferences, and the use of collision and/or reaction cells prior to the entry to the mass filter. These three techniques will be discussed in detail below.

Interference Equations: Most elements are present on the Earth in their known solar abundance (the isotopic composition of each element that was created during the formation of our solar system). Important exceptions are elements in the uranium and thorium decay series, most notably lead. For these elements, the isotopic ratios are dependent upon the source of the sample. For example, lead isotope ratios found in the environment can be attributed to at least three possible sources: geologic lead, leaded gasoline, and mined lead shot from bullets.

Interference equations are mathematical relationships based on the known abundances of each element that are used to calculate the total concentration of all of the isotopes of a particular ion. Isobaric correction is relatively easy when two or more isotopes of each element (the analyte and the interfering isotope) are present in the solar abundance. There are two ways to correct for this type of interference: (1) the analyte of interest can be monitored at a different mass unit (different isotope), or (2) the interfering element can be quantified as a different isotope (mass unit) and the result can be subtracted from the analyte concentration. Polyatomic interferences can be corrected for in the same manner but to a less effective degree. This type of correction is illustrated in the following example taken from the ICP-MS primer from Agilent Technologies Company, a manufacturer of ICP-MS systems.

Example 4.1 Arsenic is an important and common pollutant in groundwater and an industrial and agricultural pollutant. The analyte of interest is ⁷⁵As but ⁴⁰Ar³⁵Cl has an identical mass on a low-resolution mass filter system, and since most water samples contain chloride, this interfering ion will be present in varying concentrations. These can be corrected for by doing the following instrumental and mathematical analysis. Note that all analysis suggested below require external standard calibration or for the instrument to be operated in semi-quantitative mode (a way of estimating analyte concentrations based on the calibration of a different element or isotope).

1. Acquire data at masses 75, 77, 82, and 83.
2. Assume the signal at mass 83 is form ⁸³Kr and use this to estimate the signal from ⁸²Kr (based on solar abundances).
3. Subtract the estimated contribution from ⁸²Kr from the signal at 82. The residual value should be the counts per second for ⁸²Se.
4. Use the estimated ⁸²Se data to predict the size of the signal from ⁷⁷Se on mass 77 (again, based on solar abundances).
5. Subtract the estimated ⁷⁷Se contribution from the counts per second signal at mass 77. The residual value should be from ⁴⁰Ar³⁷Cl.
6. Use the calculated ⁴⁰Ar³⁷Cl data to estimate the contribution on mass 75 from ⁴⁰Ar³⁵Cl.
7. Subtract the estimated contribution from ⁴⁰Ar³⁵Cl on mass 75. The residual is ⁷⁵As.

This process may seem complicated but is necessary to obtain accurate concentration data for As in the absence of a high-resolution mass filter. It should also be noted that this type of analysis has limitations. (1) If another interference appears at any of the alternative mass units used, the process will not work. (2) If the intensity of interference is large, then a large error in the analyte concentration will result.

Cool Plasma Technique: The ionization of Ar-based polyatomic species in the normal “hot” plasma can be overcome by operating the radiofrequency at a lower wattage (from 600 to 900 W) and therefore the lowering the temperature of the plasma. This technique, a function of all modern ICP-MS systems, allows for the removal of polyatomic interferences in the analysis of K, Ca, and Fe. One downside is the tendency to form more matrix induced oxide cations.

Collision/Reactor Cells: The limitations of the two techniques described above, and the price of high-resolution mass spectrometry, led to the development of collision and reaction cells in the late 1990s and early 21th century to remove these interferences. Numerous Ph.D. dissertations, as well as research and development programs in industry, are active in this area and there are books specifically devoted to this topic. Two basic types of approaches have been used, (1) a collision cell that uses He to select for an optimum kinetic energy by slowing interfering ions relative to the analyte and only allowing the passage of the higher energy analyte and (2) reaction cells that promote reactions between a reagent gas and the interferences in order to remove them from detection.

The actual collision/reaction cell is a quadru-, hexa- or octa-pole that is considerable smaller than the subsequent quadrupole (mass filter) and is enclosed in a chamber that can contain higher pressures then the surrounding vacuum chamber. No mass separation occurs in the multi-pole since only a DC current is applied to the poles. Instead, the main purpose of the multi-pole is to keep the beam focused/contained to provide a space for the necessary collisions or chemical reactions to occur. While the number of poles in the reaction cell varies with different instruments, the larger number of poles allows for a more effective cell since the cross-sectional area of the ion beam is larger for an octa-pole over a hexa- or quadu-pole. The majority of collision/reaction cells can be operated in either mode by altering the gas utilized by the system. The price of the instruments increases slightly with the addition of these cells, however removing interferences with a collision cell is still less expensive than the alternative; a high-resolution mass filter.

Collision Cells: In a collision cell a non-reactive gas, usually He, is used to remove polyatomic ions that have the same mass to charge ratio as the analyte of interest. These multi-pole collisions cells are relatively small as compared to the mass filtering quadrupole and confine the ion beam from the plasma. Helium gas is added to the cell while the analyte of interest (an atomic species) and the interferent (a polyatomic species) move through the chamber. Polyatomic species are larger then atomic species and therefore collide with the He gas more often. The net result of these collisions is a greater reduction in the kinetic energy (measured in eVs) of the polyatomic species in relationship to the atomic species. As the polyatomic and analyte ions exit the collision cell, they are screened by a discriminator voltage. A discriminator voltage is the counterpart to an accelerating lens and contains a slit with a positive voltage; this process is commonly referred to as kinetic energy discrimination. When a positive voltage is applied to this gate, only cations possessing sufficient kinetic energy will pass through the slit. Smaller cations retaining more of their energy, after being subjected to the collisions with He, will pass through the slit while larger polyatomic cations that have been slowed by the He collisions will be repelled by the voltage. The polyatomic species that do not pass into the mass analyzer collide with the walls of the chamber, are neutralized and removed by the vacuum system. Common interferences that are removed in this manner are sample matrix-based interferences such as ³⁵Cl¹⁶O⁺ from interfering with ⁵¹V⁺, ⁴⁰Ar¹²C⁺ from interfering with ⁵²Cr⁺, ²³Na⁴⁰Ar⁺ from interfering with ⁶³Cu⁺, ⁴⁰Ar³⁵Cl⁺ from interfering with ⁷⁵As⁺, and plasma-based interferences such as ⁴⁰Ar¹⁶O⁺ and ⁴⁰Ar³⁸Ar⁺+. Interfering polyatomic species can be reduced down to ppt levels through kinetic energy discrimination. An animation of a collision cell is shown in Animation 4.1.

Animation 4.1. A Collision Cell.

Reaction Cells: The physical structure and design of a collision cell, depending on the manufacturer, is similar or identical to that of a reaction cell. However, instead of utilizing an inert gas such as helium, more reactive gases are introduced into the cell. H₂ is the most common reactive gas but CH₄, O₂, and NH₃ are also used. Table 4.1 shows a variety of reaction gases and their intended use.

Table 4.1 Reagent Gases used in Collision and Reaction Cell ICP-MS Systems. (Source: Koppenaal, et al., 2004, J. Anal. At. Spectrom., 19, 561-570)

Collision Gas	Purpose
He, Ar, Ne, Xe	Used as a collision gas to decrease the kinetic energy of the polyatomic interference
H2, NH3, Xe, CH4, N2	Used in charge exchange reactions
O2, N2O, NO, CO2	Used to oxidize the interference or analyte
H2, CO	Used to reduce the interference
CH4, C2H6, C2H4, CH3F, SF6, CH3OH	Used in adduction reactions to remove interferences

The purpose of the reactive gas is to break up or create chemical species, through a set of chemical reactions, and change their polyatomic masses to one that does not coincide with the mass of the analyte of interest. These cells have significantly extended the elemental range of ICP-MS to include some very important elements; the most important being ³⁹K⁺, ⁴⁰Ca⁺, and ⁵⁶Fe⁺ which had previously been difficult to measure due to the interferences of ³⁸Ar¹H⁺, ⁴⁰Ar⁺, and ⁴⁰Ar¹⁶O⁺ respectively. The removal of interferences can be divided into three general categories: charge exchange, atom transfer, and adduct formation (i.e. condensation reactions).

A generic reaction for a charge transfer reaction would be

A⁺ + B⁺ + R ⇒ A⁺ + B + R⁺

where A⁺ is the analyte, B⁺ is the isobaric interferent, and R is the reagent gas. An example of a charge exchange reaction is removal of the cationic Ar dimer in the analysis of selenium.

⁸⁰Se⁺ + ⁸⁰Ar²⁺ + H² ⇒ ⁸⁰Se⁺ + ⁴⁰Ar⁴⁰Ar + H₂⁺

The neutral Ar dimer is now removed by the vacuum and ⁸⁰Se⁺ is easily transported through the mass filter. Another specific case would be the interference of ⁴⁰Ar⁺ with the measurement of ⁴⁰Ca⁺. The reaction is

⁴⁰Ca⁺ + ⁴⁰Ar⁺ + H₂ ⇒ ⁴⁰Ca⁺ + ⁴⁰Ar + H₂⁺

In this reaction, the interfering cationic species is neutralized and removed by the vacuum and does not enter the MS. It should be noted that in charge exchange reactions, the ionization potential of the reagent gas must lie between the ionization potentials of the interfering ion and the analytes. Such a requirement is not necessary for atom transfer and adduction formation/condensation reactions. Two such reactions follow.

Atom Addition Reaction
A⁺ + B⁺ + R ⇒ AR⁺ + B
Fe⁺ + ArO⁺ + N₂O ⇒ FeO⁺ + ArO⁺ + N₂

In this case the interference of ArO⁺ with the measurement of Fe is removed by oxidizing the Fe to its oxide that has a different mass from the argon oxide. Another example is given below for the removal of ⁹⁰Zr interference in the detection of ⁹⁰Sr.

⁹⁰Sr⁺ + ⁹⁰Zr⁺ + 1/2O₂ ⇒ ⁹⁰Sr⁺ + ⁹⁰ZrO⁺

These chemical reactions in the cell create cations that can potentially interfere with other analytes, hence it is not uncommon for these problematic analytes to be measured singularly (no multi-elemental analysis). As a result, the reaction cell mode is frequently utilized for singular applications or for argon interferences (ex. ⁴⁰Ar⁺ and ⁴⁰Ar¹⁶O⁺) with hydrogen since the products of the reaction do not interfere with other analytes of interest. If possible, operating the collision/reaction cell in the collision mode is preferable since the interferences are removed from the system. After the spectral interferences have been removed by either process, the ion beam is separated by mass to charge ratio with the mass filter. An animation of a typical reaction occurring in a reaction cell with H₂ is shown in Animation 4.2.

Animation 4.2. Reaction Cell.

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Section 4.2.6

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