Further Reading
7.3 Inductively Coupled Plasma–Mass Spectrometry
7.3.2 Interferences
the length of a curved surface inside a horn-shaped detector. Electron multipliers can provide a signal amplification of 105–108.
Among the other less commonly used detectors are Faraday cup collector, photo- graphic plates and scintillation-type transducers.
A more serious type of interference is caused by the formation of polyatomic and adduct species formed in plasma, such as 40Ar 16O⫹and 40Ar 40Ar⫹. This type of interference is caused by the species in plasma gas, solvent, matrix, atmosphere and reagents used in sample dissolution. In this respect, choice and concentration of
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Table 7.1 Selected isobaric interferences in ICP-MS.
Percent abundances shown in parenthesis have been taken from Ref. 10, rounded to two digits at most after the decimal point
Analyte isotope Isobaric interferant(s)
40Ca (96.94) 40Ar (99.60),40K (0.01)
48Ti (73.8) 48Ca (0.19)
50Ti (5.4) 50V (0.25),50Cr (4.35)
54Fe (5.8) 54Cr (2.37)
58Ni (68.08) 58Fe (0.28)
64Zn (48.6) 64Ni (0.93)
70Ge (21.23) 70Zn (0.6)
74Ge (35.94) 74Se (0.89)
76Se (9.36) 76Ge (7.44)
87Rb (27.84) 87Sr (7.00)
92Mo (14.84) 92Zr (17.15)
94Zr (17.38) 94Mo (9.25)
96Mo (16.68) 96Ru (5.52),96Zr (2.80)
113Cd (12.22) 113In (4.3)
115In (95.7) 115Sn (0.34)
122Sn (4.63) 122Te (2.60)
123Sb (42.64) 123Te (0.91)
138Ba (71.70) 138La (0.09),138Ce (0.25)
180Hf (35.10) 180Ta (0.012),180W (0.13)
Figure 7.12 Relative concentrations of MO⫹, M⫹and M2⫹ along the height of Ar plasma (Adapted with permission from Thermo Electron Corporation, UK)
acid(s) used in sample dissolution becomes an important issue. Some possible sources for elements that can form polyatomic or adduct species are shown in Table 7.2. Selected interferences caused by polyatomic and adduct ion formation are given in Table 7.3.
Interferences caused by polyatomic, metal oxide and hydroxide ion formation has now been well documented in literature.11 Some selected examples are given in Table 7.4.
7.3.2.2 Non-spectral Interferences
Non-spectral interferences take place when analyte concentration measured is altered for any reason; the result is positive or negative error. Most of these kinds of inter- ferences take place during sample transport and introduction into plasma or by vari- ations in plasma conditions. In order to minimize these type of interferences, it is necessary to take precautions to keep the plasma robustness and sample transport and introduction under control. It is desired in ICP-MS measurements that total dissolved
Table 7.2 Some sources of elements to form polyatomic and adduct species causing interference in ICP-MS Element Possible source(s)
Ar Plasma gas
B LiBO2, Li2B4O7(fusion)
Cl HCl, HClO4
F HF
H Water, solvent, acids and organic sample matrix N Air, HNO3, NH3
O Air, water, solvents, HNO3, HClO4, H3PO4and H2SO4
P H3PO4
S H2SO4
Table 7.3 Selected interferences caused by polyatomic and adduct ion formation in ICP-MS. Percent abun- dances shown in parenthesis have been taken from Ref. 10, rounded to two digits at most after the decimal point
Analyte Interferant(s)
42Ca (0.65) 42Ar 2H⫹
44Ca (2.09) 12C 16O16O⫹
51V (99.75) 35Cl 16O⫹,34Cl 16O1H⫹,14N 37Cl⫹
52Cr (83.79) 40Ar 12C⫹,36Ar 16O⫹,36S 16O⫹,35Cl 16O1H⫹
55Mn (100) 40Ar 14N1H⫹
56Fe (91.72) 40Ar 16O⫹
63Cu (69.17) 31P 16O2⫹
64Zn (48.6) 31P 16O16O1H⫹,32S 16O16O⫹,32S 32S⫹
75As (100) 40Ar 35Cl⫹
solids in the sample solution be lower than 0.2%. Higher amounts will cause deposits in sampling and skimmer cones, lowering sensitivity. In addition, longer washout times are needed to prepare sample introduction system for next sampling. High amounts of solids also affect plasma conditions; this will also alter the analyte signal since plasma equilibria for ionic species will be affected. It is often required that the sample solution is diluted. For the reasons above, diluted samples are easier to han- dle. This approach will result in reduction of sensitivity; in addition, water blanks should be low and under control. However, since ICP-MS is a very sensitive tech- nique, in most cases successful results can be obtained by this approach in despite of reduced concentrations due to dilution. On the other hand, sample introduction sys- tems should also be under a good control. Lack of precision and accuracy in sample transportation is often a major source of error. Peristaltic pumps should be working well and other sample introduction systems should be in good condition.
7.3.2.3 Approaches for Elimination of Interferences
Several approaches are used to eliminate interferences in ICP-MS.
● Using another isotope of analyte
● Correction by computation
● Elimination of solvent
● Reaction/collision cells.
Naturally, one solution is to use another isotope of analyte that is not affected by the spectral interference; this is usually done at the cost of reduced sensitivity because of lower abundances.
When the spectral interference is caused only by the species that are not in sample matrix, blanks may be used for correction as long as their signals are not very high.
Problems caused by isobaric and doubly charged species can also be corrected by computation. If the only cause of interference is an isobaric isotope, intensity of
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Table 7.4 Selected interferences caused by ions of metal oxides and hydroxides. Percent abundances shown in parenthesis have been taken from Ref.10, rounded to two digits at most after the decimal point
Analyte Interferant(s)
43Ca (0.14) 27Al 16O⫹
56Fe (91.72) 40Ca 16O⫹
59Co (100) 43Ca 16O⫹,42Ca 16O1H⫹,40Ar 18O1H⫹
63Cu (69.17) 47Ti 16O⫹
66Zn (27.9) 50Ti 16O⫹
75As (100) 59Co 16O⫹,43Ca 16O16O⫹
109Ag (48.16) 92Zr 16O1H⫹
134Ba (2.42) 102Ru 16O16O⫹
169Tm (100) 153Eu 16O⫹
another isotope of interfering element is measured; since signals would be propor- tional to respective abundances of isotopes, the signal caused by interfering isotope is computed and subtracted from the total signal at analyte mass. Similar computa- tions can also be carried out for interference caused by doubly charged ions.
Presence of solvent in aerosol introduced to plasma has several adverse effects.
First of all, solvent is a potential source of elements that will form interfering species as shown in Table 7.2. Second, solvent cools the plasma, affecting equilibrium of atoms and ions. It is therefore desirable to remove solvent prior to transport of sam- ple into plasma; in other words, it is nearly ideal to introduce sample in the form of a dry aerosolto plasma. Techniques such as LA produces dry aerosols and are there- fore advantageous for solid samples.
In order to eliminate solvent prior to introduction to Ar plasma, several approaches may be used. Ultrasonic nebulizers usually have online solvent removal systems. After nebulization, the aerosol is passed through a heated zone; solvent evaporates here. In the next stage, the transport tube is cooled and solvent is sepa- rated by condensation; dry aerosol is thus formed to be introduced to plasma. In some systems, wet aerosol is passed through a tube surrounded by a non-polar mem- brane such as PTFE; organic solvents pass through this membrane and pumped out by a flushing argon stream. This technique is called membrane desolvation; it is schematically shown in Figure 7.13. Using membrane desolvation, losses may take place if analyte is in the form of organometallic compound with low polarity.
Use of a reaction cell orcollision cell is another effective way to eliminate spec- tral interferences. The principle of a reaction/collision cell is to selectively affect and prevent the formation of interfering species. This is done in a section prior to mass analyzer. Gases such as NH3, H2and He are used. As the interfering compounds are converted to new species that will not interfere, the formation of analyte ions are not affected. In case of NH3, this molecule reacts with Ar⫹(products are NH3⫹and Ar);
formation of Ca⫹is not affected since the reaction involved is very slow. Therefore, interference of 40Ar⫹(99.60) on 40Ca⫹(96.94) is reduced or eliminated. Effect of collision cell on an aqueous solution containing a mixture of helium and hydrogen is illustrated in Figure 7.14. Interfering species such as 40Ar 16O⫹for 56Fe (91.72)
Figure 7.13 Schematic representation of principle of membrane desolvation to produce dry aerosol. A⫹OS, aerosol ⫹organic solvent; FAr, flushing argon; HOT, heated outer tube; PTFE-M, polytetrafluoroethylene membrane; DA, dry aerosol; and AP, argon plasma
and 40Ar 12C⫹for 52Cr (83.79) are converted to hydrogen or helium compounds of Ar; spectral interferences are eliminated. Reaction/collision cells are now an integral part of ICP-MS instruments.
Non-spectral interferences require precautions to keep the plasma robustness and sample transport under control. Regarding plasma conditions, diluted samples are easier to handle.
Handling interference problems in ICP-MS measurements requires that a thor- ough information regarding sample composition is available. Actually, this statement is meaningful for any kind of trace element analysis. However, in case of ICP-MS analysis, knowledge, intuition and proper remedies are very important, as this tech- nique is presently the most sensitive one in its field. Dealing with low analyte levels also require cleaner laboratory environment, water and reagents of high purity.