Questioning the Relationship Between Analyte Ion Mass and ICP-MS Matrix Effects

Matrix effects-changes in analyte sensitivity induced by a high concentration of matrix elements-can reduce accuracy in inductively coupled plasma–mass spectrometry (ICP-MS). It has long been accepted that matrix effects are more severe for light analyte ions in the presence of heavy matrix ions. However, new studies by Shi Jiao and John Olesik in the Trace Element Research Laboratory (TERL) at The Ohio State University (OSU) in Columbus, Ohio, carried out using current ICP-MS instruments, show that matrix effects are not strongly dependent on analyte ion mass. These study results have implications for understanding the fundamental causes of matrix effects in ICP-MS, and for the choice of internal standards. Jiao and Olesik spoke to Spectroscopy about this work.

What is the current understanding of the relationship between analyte ion mass and matrix effects?Shi Jiao: Space charge repulsion of ions in the positive ion beam downstream of the skimmer has been considered the most likely cause of mass-dependent matrix effects for many years. If space-charge effects are responsible, then signals from light analyte ions would be expected to be more severely affected than signals from heavy analyte ions. In addition, a heavy matrix element (that is efficiently ionized in the ICP) should induce more severe loss of analyte ion signal than an equimolar concentration of a light matrix element. This is entirely consistent with the results that Tan and Horlick reported in 1987 (1).

What made you question the accuracy of that understanding?John Olesik: In addition to research on ICPs and instrument development, the TERL provides elemental analysis for a wide variety of clients and collaborators in many disciplines at OSU and outside of OSU, including other universities, organizations, and companies. We had been using multiple internal standard elements so that an internal standard could be used to correct for matrix effects on signals from analyte ions with similar masses. However, we kept seeing similar results (within 10-20%) regardless of the mass of the internal standard element that we used. There also were data in the literature that showed a lack of influence of analyte mass but with much more limited sets of data than from the Tan and Horlick study (1).

You looked at matrix effects using different instruments that have different optics. What did you find?Jiao: Two ICP-MS instruments with very different ion optics (PerkinElmer ELAN 6000 and Thermo Element 2) both showed little correlation between the severity of matrix-induced changes in the analyte ion signals and analyte ion mass, in contrast to the data in the 1987 study (1). However, matrix effects induced by equimolar (5 mM) concentrations of the matrix element were more severe as the mass of the matrix element increased on both instruments. The ELAN 6000 ICP-MS uses a grounded stop with a positively charged (+4 to +10 V), cylindrical lens. The lens voltage is varied as different mass ions are measured. The Element 2 ICP-MS uses a cylindrical, -2000 V extraction lens followed by a set of focusing lenses to shape the ion beam.

What do you believe to be the role of the optics in the differences you saw?Olesik: I believe that matrix-induced changes in the efficiency of ion transmission from the ICP to the mass spectrometer are responsible for the matrix mass dependent matrix effects in ICP-MS. That would lead to the reasonable conclusion that the ion optics should be important. Also, if a fixed lens voltage is used on the ELAN 6000 ICP-MS that should focus heavy ions well but not light ions, then matrix effects do depend on the analyte ion mass. However, it is surprising that the matrix-induced changes in analyte signals would be so similar for two instruments with such different ion optics. Another question is why matrix effects are much more severe for heavy analyte ions using the newer ICP-MS instruments. Perhaps it is related to the orders of magnitude higher sensitivity of the newer ICP-MS instruments.

What did you see when you looked at different matrix concentrations? 

Jiao: Matrix effects became less severe as the matrix element concentration decreased, as expected, but the severity of matrix effects was still similar for analytes with different masses.

How were the recovery levels of analytes of different masses affected by the use of a single internal standard?

Jiao: By using a single internal standard, more than 80% of analytes with masses from 7 to 238 that we investigated had recoveries within ±20% error, no matter whether the internal standard was low, mid, or heavy mass.

What do your studies indicate about the choice of an internal standard?

Olesik: A single internal standard can often be used for analytes with a range of masses to correct (typically within 10 to 20%) changes in analyte ion signals that are caused by the presence of high concentrations of concomitant elements. If better accuracy is required, then it may be necessary to experimentally determine which internal standard elements are best, closely matrix-match samples and standards, use standard additions, or use isotope dilution.

In some cases, it may not be appropriate to use a single internal standard for all analytes. For example, high concentrations of high ionization energy elements, such as C, can enhance ionization of analyte elements with high ionization energies (such as As, Se, P, S, and Hg). Then an appropriate internal standard just for elements with high ionization energies may be necessary, particularly for samples with variable concentrations of C, for example.

Were you able to identify an optimum lens voltage to reduce matrix effects?

Jiao: It is possible to reduce the severity of matrix effects on both instruments by adjusting the lens voltages. Presumably, if the beam is wider (less well focused), the number of ions per unit volume will decrease and then space charge repulsion should be less severe. However, using voltages that would dramatically reduce the severity of matrix effects will often result in a 10× or more loss of sensitivity.

You also looked at the effect of nebulizer gas flow rate. What did you find?

Jiao: Matrix effects became less severe, as expected, at lower gas flow rates. When the nebulizer gas flow rate is decreased, the location along the axis of the ICP where the temperature dramatically increases moves farther away from the sampling orifice. As a result, there is more extensive diffusion of ions from the sample, in effect diluting the sample in the plasma, before the ions reach the sampler orifice. The relative contribution of matrix ions (rather than plasma ions such as Ar+) to the ion beam current is reduced. Diluting the sample in the plasma also reduces analyte sensitivity. However, when measuring analyte concentrations that can still be easily measured with 5 to 10× lower sensitivity, it may be useful to operate at lower nebulizer gas flow rates to reduce the severity of matrix effects. At reduced nebulizer gas flow rates there was still no correlation between severity of matrix effects and analyte ion mass.

What are your next steps in this work?

Olesik: The results of this work show that much is still not known about the processes that control ICP-MS signals and those have direct implications on practical use of ICP-MS. Our focus in these studies has been to characterize matrix effects as a function of analyte ion mass, matrix ion mass, matrix concentration, lens voltages, and nebulizer gas flow rate.

The results we’ve talked about today are based on a small portion of the data that Jiao has acquired. There are also some questions about elements that are not extensively ionized in the plasma that we have not yet addressed. We are also well into similarly comprehensive experiments using the PerkinElmer NexION ICP-MS (which has a three-cone interface and turns the ions at a right angle using a dc quadrupole lens). We plan to submit manuscripts with the results from each instrument to the Journal of Analytical Atomic Spectrometry.

Ultimately, we need to know how the ion trajectories through the interface and then from the skimmer through the ion optics to the mass spectrometer are affected by high concentrations of concomitant elements in the sample. Experimental and ion trajectory modeling approaches to gain that knowledge are both very challenging but could provide very important new insight. Paul Farnsworth’s group at Brigham Young University (BYU) is doing some beautiful (but still very challenging) experiments using laser-induced fluorescence and optical-based velocity measurements in the interface and downstream of the interface. Ross Spencer, also at BYU, is modeling the processes in the interface between the sampler and skimmer. Their results to date suggest that the assumption that neutral gas flow controls the flow of ions through the interface is not entirely correct. In that case, matrix effects could occur in the interface. We are also continuing to try to model ion trajectories through the ion optics to gain insight into potential explanations for the lack of analyte ion mass dependence for matrix effects.

Reference

(1) S.H. Tan and G. Horlick, J. Anal. At. Spectrom.2, 745–763 (1987).