Solution Based Calibration in LA-ICP-MS

Calibration and quantification procedures are easier in LA-ICP-MS compared to other solid-state mass spectrometric techniques because the laser ablation and the ICP ion source operate at normal pressure and the laser ablation of solid samples and ionization of analytes are separated in space and time. Therefore the advantage of solution calibration in ICP-MS can be applied in this solid-state analytical technique. The introduction of solution based calibration, which is only possible in LA-ICP-MS, was an innovative step in the development of this sensitive mass spectrometric technique. A number of different calibration approaches using aqueous standard solutions in the dual gas flow technique have been discussed by various authors.74 75 In the dual gas flow injection technique , the nebulized standard solution and the laser ablated sample material are mixed in the -piece and the two gas flows from the nebulizer (e.g. ultrasonic nebulizer) and laser ablation chamber are added. Using solution based calibration with the addition of a standard solution, Leach et alP determined minor elements in steel reference materials with a relative accuracy of a few %. In comparison to the so-called dual gas flow technique proposed in the literature, where the argon flow rates through the nebulizer and ablation cell add up to 11 min-1 (e.g. 0.451 min-1 and 

Three different calibration strategies for solution based calibration in LA-ICP-MS have been developed in our laboratory. These are similar to the solution calibration in solution analysis by ICP-MS external calibration if a high purity matrix-matched blank target is available,29,71 the standard addition technique (e.g., for high purity platinum)76 or the isotope dilution technique.43 

External Calibration Technique for Solution Based Calibration in LA-ICP-MS 

Bings77 quantified 11 trace metals in lubricating oil with the aid of solution based calibration in LA-ICP-MS using a ToF mass spectrometer. It has been shown that good analytical performance in terms of precision (6 % on average) and low detection limits in the ng g-1 range can be obtained with this easy and fast quantification procedure in LA-ICP-MS.77 

On line additions of aqueous standard solutions for the calibration of LA-ICP-MS including a comparison of wet and dry plasma conditions are discussed by O Connor et alJ For solution calibration of standard solutions the authors used a 100 p,l PFA nebulizer together with a cyclonic spray chamber or a MCN-6000 sample introduction system with desolvator, to study the wet and dry plasma, respectively. A polypropylene Y piece was applied to mix the laser ablated material and the nebulized standard solutions. The authors found that the on line addition of water is the preferred mode of operation for quantification by LA-ICP-MS, i.e., wet plasma is more stable (improved standard deviation of sensitivity ratios). 

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A comparison of sensitivities for several elements in LA-ICP-MS and ICP-MS - relevant for solution-based calibration in LA-ICP-MS described below - are illustrated in Figure 6.14. 

Another calibration technique - standard addition - minimizes matrix effects because analytes with well defined increasing concentrations are added to a set of sample solutions to be analyzed. The measured calibration curve in the standard addition mode plots the measured ion intensities of analytes versus the concentration added to the sample solution. The concentration of analytes in the undoped sample is then determined by extrapolation of the calibration curve with the x-axis. Matrix matching is subsequently performed and the matrix effects (signal depression or interference problems) are considered. An example of the standard addition technique is described in Section 6.2.6 using solution based calibration in LA-ICP-MS. 

Further details of different strategies in solution calibration are described in the literature.1 29 76 79 Precise and accurate measurements of isotope ratios, which is one of the major advantages of mass spectrometric techniques, are a requirement for the application of isotope dilution techniques in trace analysis, which is also the main goal of the application of isotope dilution in solution based calibration in LA-ICP-MS. 

Online isotope dilution technique LA-ICP-MS is proposed as the method of choice for quantitative element mapping and imaging even if no reference materials are available (42). For online solution-based calibration with LA-ICP-MS, a microflow total consumption nebulizer DS-5 (CETAC Technologies, Omaha, NE, USA) was inserted into the laser ablation chamber (21). It is more convenient to use solution-based calibration because such calibration can be performed easily, quickly and in any concentration range for many elements. The application of the online isotope dilution technique in LA-ICP-MS using a microflow nebulizer inserted into the laser ablation chamber to biological standard reference materials (e.g., apple leaves) yielded accurate analyte concentrations (42). 

Figure 6.22 Calibration curve of Ir in high purity platinum measured by LA-ICP-MS (solution based calibration in the standard addition mode).

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