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Simpler removal of spectral interferences in ICP-MS with the Agilent 7700 Series ORS3
By Ed McCurdy
Agilent ICP-MS Product Marketing
Traditional strategies for minimizing spectral interferences
ICP-MS is a widely used technique for metals analysis, particularly in laboratories where rapid multi-element measurement is required, such as environmental and food analysis contract laboratories. However, the sample types analyzed in such laboratories can give rise to spectral interferences in ICP-MS, due to matrix-based molecular (or polyatomic) ions that overlap many analyte masses.
Techniques to reduce or avoid such interferences have included eliminating certain acids (such as HCl or H2SO4) from the sample preparation, measuring a minor isotope that does not suffer from interferences, or correcting the data through the use of mathematical corrections. All of these techniques have limitations, and may compromise some aspect of the analysis. For example, eliminating HCl from the sample preparation compromises the chemical stability of many elements including Hg, Ag and Mo.
Collision/reaction cells in ICP-MS
Since their introduction in 1999, collision/reaction cells (CRCs) have become widely used in ICP-MS, offering another means of reducing spectral interferences. Most CRCs operate using reactive cell gases, targeted at a small number of known or predicted polyatomic ions. Such gases (usually H2, NH3, O2 or CH4), remove interferences by reacting with the polyatomic ion but not the analyte (or vice versa). While reaction gases can remove some interferences, they must be targeted at polyatomic ions that are known to be present, and each reaction gas only reacts with certain polyatomic ions.
Comparing helium collision mode and reaction mode for removing polyatomic interferences
The Agilent 7700 Series ICP-MS features a 3rd generation Octopole Reaction System (ORS3) cell, which is designed to provide effective removal of spectral interferences in collision mode, using an inert cell gas – helium. Helium (He) mode has the key benefit that it is non-specific, meaning that it filters out all polyatomic ions, not just reactive ones. The process of interference removal in He mode is the rejection of the polyatomic ions using a process known as Kinetic Energy Discrimination (KED). KED makes use of the fact that polyatomic (or molecular) ions are larger than analyte ions of the same mass, and so collide more frequently with the He cell gas. With each collision, the ions lose some energy so, by the cell exit, the polyatomic ions have less residual energy than the analyte ions and can be stopped from entering the quadrupole by applying a fixed bias voltage “step.”
Because He mode is non-specific, it is suitable for multi-element analysis in unknown or variable samples of the types found in the labs mentioned above; in these samples, a reactive gas will fail to remove unreactive interferences, so residual overlaps will cause errors in the reported results. The second key benefit of He cell gas is that it is inert, so it does not create any new interferences by reacting with other ions in the cell. Again, this is a major limitation of reactive cell gases, with the result that they cannot be used successfully when the sample matrix is complex or variable.
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Figure 1. Apparent Cr concentration due to matrix-based interferences from a series of matrices, measured in He, H2 and no gas mode on the 7700x. (Click here to see this image larger.)
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Benefit of He mode for removal of interferences on 52Cr in variable samples
The relative performance of He mode, reaction (H2) mode and no gas mode (all using the Agilent 7700x ICP-MS) was evaluated by measuring the background signals (expressed as background equivalent concentration, or BEC) for several potentially interfered elements in a series of single-component blank matrices. The matrices measured were 5% HNO3, 5% HCl, 1% H2SO4, 1% methanol, 200 ppm Na, 200 ppm Ca, 500 ppm P, and a mixed matrix containing all of the single components. The BEC in each gas mode was plotted against the matrix solution, to show the residual interferences present and which matrix gave rise to them.
Figure 1 shows the apparent concentration of chromium (measured at its preferred isotope at mass 52) in each of the matrices, measured using each cell gas mode; all of the samples measured were matrix blanks, so the Cr concentration should have been zero in each case. Cr has been difficult to determine accurately by ICP-MS, due to the presence of polyatomic interferences on the major isotope at mass 52 (from 40Ar12C), and the secondary isotope at mass 53 (from 37Cl16O). These matrix-derived interferences would bias results, and the error would vary according to the amount of carbon and chloride in each sample (Ar is always at a high level, as argon is used as the plasma support gas in ICP-MS). From the plot in Figure 1, it is apparent that He mode gave low and consistent blanks in all the matrices, demonstrating effective removal of the interferences (from ClO and ArC) that gave an increased blank level in no gas mode. By contrast, while H2 mode was effective at removing the ArC interference, it actually increased the apparent Cr concentration in the 5% HCl matrix. This is due to the creation of a new cell-formed polyatomic ion – 35Cl16O1H – produced when H2 is present in the cell.
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Figure 2. Helium mode shown at the bottom is by far the best at removing matrix interferences that would otherwise produce numerous false positives in multi-element analysis. Inset spectrum shows high sensitivity and good isotopic template fit for 10 ppb spike in He mode. (Click here to see this image larger.)
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Effective removal of all polyatomic interferences to improve multi-element analysis
The comparison shown in Figure 1 indicates the ability of He mode to remove multiple polyatomic interferences on 52Cr, but most labs use ICP-MS as a multi-element technique. The spectra shown in Figures 2a, 2b and 2c compare the performance of the different cell gas modes for multi-element analysis. In these spectra, all of which are of the same sample, displayed on the same mass and intensity scales, a slightly simpler matrix (composed of 5% HNO3, 5% HCl, 1% H2SO4, 1% isopropyl alcohol) was used, in order to minimize the trace element contamination from the matrix components. Figure 2a shows the spectrum from mass 44 to 82 (covering the elements Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As and Se) for this mixed matrix, measured in no gas mode. There is a peak visible at almost every mass and, since these were blank matrices, this indicates the presence of matrix-based interferences on every useful isotope of the elements listed; the color of the spectrum indicates which matrix gave rise to each interference. As shown in Figure 2b, some of these interferences were reduced in H2 mode, but many remained and some new polyatomic ions were formed. By contrast, the spectrum in Figure 2c shows that He mode effectively removed all the matrix-based polyatomics. High sensitivity was maintained in He mode, as shown by the inset spectrum of a 10 ppb spike measured under the same conditions.
Most universal method to remove interferences
The data comparisons in Figures 1 and 2 illustrate two of the unique capabilities of He cell gas; it removes polyatomic interferences on multiple analytes and in multiple matrices, and it is inert so it does not create any new interferences. This means that 7700x ORS3 users can report reliable multi-element data across a range of sample types, without the need for specific instrument conditions for each analyte or sample type.
If you need consistent and effective removal of interferences, especially for complex or unknown samples, examine the new Agilent 7700 Series ICP-MS. For more information, including a look inside the cabinet with our 360° interactive animation, please visit our product page or contact your Product Specialist.
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