Quadrupole-based ICP

An overview of commercial ICP mass spectrometers from different companies (quadrupole based ICP-MS with and without collision/reaction cell, double-focusing sector field instrumentation with single and multiple ion collectors, time-of-flight (ToF), ICP-ion trap-MS and non-commercial ICP-Fourier transform ion cyclotron resonance (FTICR) mass spectrometers is given in Figure 5.2. By using ion traps and FTICR mass spectrometers in ICP-MS isobaric interferences of atomic ions... 

Figure 5.2 Overview of ICP-MS instrumentation quadrupole-based ICP-MS with and without colli-sion/reaction cell, sector field ICP-MS with single ion collector (ICP-SFMS) and multiple ion collector ICP-MS (MC-ICP-MS) and time-of-flight ICP-MS.

Figure 5.3 Instrumental outline of a quadrupole-based ICP-MS without collision/reaction cell (Agilent 7500). (Reproduced by permission of Agilent.)...

The mass range of most quadrupole based ICP-MS instruments is limited (1-300 u). By ICP-MS nearly all elements in the mass range of 6u (for lithium) to 238 u (for uranium) can be measured. Light elements at low concentration levels, such as H, O, N, C and noble gases, cannot be determined because an atmospheric pressure (argon) plasma ion source always contains traces of these light elements in the air and also in aqueous solution acidified with HN03. 

As discussed before, quadrupole based ICP-MS allows multi-element determination at the trace and ultratrace level and/or isotope ratios in aqueous solutions in a few minutes as a routine method with detection limits of elements in the sub pgml-1 range and a precision for determined trace element concentration in the low % range (RSD - relative standard deviation). The precision for isotope ratio measurements varies between 0.1% and 0.5% RSD. This isotope ratio precision is sufficient for a multitude of applications, e.g., for evidence of contamination of sample with depleted or enriched uranium in urine (this technique is used in the author s laboratory in a routine mode14) or the isotope dilution technique for the quantitative determination of trace element and species concentration after doping the sample with enriched isotope spikes. 

As an alternative to commercial quadrupole based ICP-MS measurements at low mass resolution ( / 300), in 1996 Yiang and Douglas15 proposed a quadrupole ICP-MS system which allows a maximum mass resolution of 9000. At a mass resolution of 5000 the sensitivity was comparable to that of a commercial double-focusing sector field ICP-MS operated at the same mass resolution. Due to the very high continuum instrumental background of about lOOOcps no commercial high resolution quadrupole instrumentation with an inductively coupled plasma source exists and the development of high resolution quadrupole based ICP-MS has ceased. 

The figures of merit of quadrupole-based ICP-MS, such as the precision of isotope ratio measurements and the detection limits, can be improved significantly, especially for elements which are difficult to determine due to the appearance of isobaric interferences (e.g., by the trace, ultratrace and/or isotope ratio measurements of Ca, Fe, S, As, I or Se).16-22 The occurrence of interference problem can be minimized by the insertion of a collision/reaction cell in ICP-MS as the result of defined collision induced reactions using selected collision/reaction gases or gas mixtures (such as H2, He, NH3, 02, CH4 and others). For each analytical problem, which is different, e.g., for U or... 

The figures of merit of several commercial quadrupole based ICP mass spectrometers with and without a collision/reaction cell, produced by different companies, are summarized in Table 5.1. 

Table 5.1 Figures of merit for quadrupole based ICP mass spectrometers. 

Figure 5.23 LA-ICP-MS by combining a quadrupole based ICP-MS with a CETAC LSX 2 13 laser ablation system.

Figure 9.5 Impurities in platinum and palladium nanoclusters measured by quadrupole-based ICP-MS and double-focusing sector field ICP-MS.

Multi-element determination of REE concentrations by quadrupole based ICP-MS... 

Multi-element analysis on 57 tree bark samples collected at different locations (near power plants, closed to a motorway, in urban or in uncontaminated rural areas) in the UK in order to study environmental contamination was performed by quadrupole based ICP-MS (Elan 6000, Perkin Elmer Sciex) after microwave induced digestions of samples and dilution. The measured concentration ranges of 52 elements varied from the sub-ngg-1 range (e.g., Hf or Pt) to the low % range (e.g. for Ca) and are summarized in Table 9.24.4 Oriental Tobacco Leaves SRM (CTA-OTL-1) was employed to validate the analytical ICP-QMS method. 

Figure 1.14 Transient signals for Cu and Cli produced by an electrothermal vaporization device and detected with (a) a quadrupole-based ICP-MS instrument and (b) a ICP-TOFMS instrument, when five isotopes were measured simultaneously.

Quadrupole mass spectrometers were used in both the early ICP-MS instrument development research and the first commercial instruments. To date, quadrupole-based ICP-MS instruments continue to be predominant. During the last several years, ICP ion sources have been coupled with mass spectrometers of several different designs, including double-sector, single-sector, time-of-flight, ion trap, and Fourier transform ion cyclotron resonance. 

The ideal internal standard is the same element as the analyte because it has similar mass, ionization energy, and chemical properties. Therefore, isotope dilution based calibration provides high accuracy as long as isotope equilibration is attained and the measured isotopes are free of spectral overlaps [192,193]. Standards do not need to be matrix-matched. Quadrupole-based ICP-MS instruments can typically provide isotope ratio precision of 0.1% to 0.5%. Much better isotope ratio precision can be obtained by using simultaneous MS detection, such as a multicollector-based instrument or perhaps time-of-flight MS. In comparison to thermal ionization mass spectrometry, ICP-MS provides much higher sample throughput and simpler, faster sample preparation. 

---