Elemental speciation and miniaturised sample introduction studies for inductively coupled plasma mass spectrometry

Pearson, Gareth F.

Abstract

The work presented in this thesis describes the development of sample introduction for inductively coupled plasma mass spectrometry (ICPMS). The ultimate aim is to develop methods for conducting rapid speciation analysis. This is approached in two ways; firstly using liquid chromatography (LC) employing a column with a monolithic silica stationary phase, and secondly by the development of a highly efficient sample introduction system designed for interfacing an electrophoresis microfluidic chip with ICPMS.

Chapter 1 gives a comprehensive introduction to the project. The background and fundamentals of ICPMS are presented, with particular attention drawn to the sample introduction systems used. A brief history of elemental speciation analysis is discussed along with elements that have found particular interest. Arsenic speciation analysis is considered in detail since it was a focus of the work presented on LC - ICPMS. Separation schemes used in conjunction with ICPMS for speciation are then reviewed, including the principles and application of chromatographic and electrophoretic techniques.

Chapter 2 details the instrumentation and standard operating procedures used throughout this work. Operating parameters for each instrument are given along with details of calibration, optimisation and maintenance. The use of reagents, gases and certified reference materials are also described in this section. Common procedures for trace elemental analysis, including liquid handling and equipment cleaning are considered.

Chapter 3 describes the rapid separation of arsenic species by application of a commercial monolithic silica column (Chromolith(tm)). The optimisation of the chromatographic conditions suitable for use with this column and ICPMS detection is described. Separation is achieved by ion-pair chromatography using a mobile phase consisting of 2.5 mM tetrabutylammonium bromide, 10 mM phosphate buffer (pH 5.6) and 1.0 % (v/v) methanol. The analysis time required for separation with conventional columns is greatly reduced with the separation being completed within 3 minutes. This affords a dramatic reduction in detector idle time and an increase in sample throughput. Detection limits of 0.107, 0.084, 0.120, 0.121 and 0.101 micro-g As /L for As(III), arsenobetaine (AsB), dimethylarsinate (DMA), monomethylarsonate (MMA) and As(V), respectively have been achieved. The precision of the method, based upon analysis of 15 micro-g As /L, is better than 5.9 % for all species. The technique is applied to the speciation analysis of urine and food samples in an ingestion experiment carried out in collaboration with De Montfort University to investigate the effect of rice consumption on arsenic exposure in humans. For the first time this study reveals that ingestion of American long grain rice, widely consumed in the UK and purchased from a local supermarket, significantly increases the excretion of DMA in human urine. This is consistent with a recent study which reported that DMA was the predominant arsenic species in rice from the USA, which also happened to contain the highest mean arsenic level in the grain compared to rice grown throughout Europe, Bangladesh and India.

Chapter 4 introduces the development of a sample introduction system specifically designed for interfacing laboratory on a chip devices with ICPMS. Low flow sample introduction is discussed in which it is possible to allow the entire sample aerosol to enter the plasma. The design and optimisation of an evaporation chamber is presented in order to prevent the losses associated with traditional spray chambers. A low flow micro-concentric nebuliser (MicroMist) is used to generate the sample aerosol. No sample recondensation is ever observed in the evaporation chamber at flow rates up to 20 micro-l/min. A comparison with a commercial low volume cyclonic spray chamber (Cinnabar) is performed. The optimised evaporation chamber results in a 400 % increase in signal intensity and instrumental detection limits are improved by an average 34 %.

Chapter 5 describes the development of a nebuliser designed specifically for use with microfluidic devices and its incorporation into a sample introduction system for ICPMS. This system uses an extremely low flow micro-cross-flow nebuliser (MCFN) sited directly at the liquid exit of the chip. The evaporation chamber described in Chapter 4 has been incorporated into this highly efficient microchip interface. The optimised system has been shown to achieve a sensitivity of 13 500 cps for 10 micro-g/L indium at an extremely low flow rate of 5 micro-l/min. The stability of the sample introduction over 10 min is 2.6 % RSD (n = 453) and sample volumes of greater than 40 nl can be analysed. The advantages and potential applications, in particular for elemental speciation, are discussed. The system is then applied to sample introduction for microchip electrophoresis.

Chapter 6 reviews the use of hydrodynamic sample injection for microfluidic devices. The performance of a manual switching valve is evaluated, achieving a reproducibility of 5.10 % RSD (n = 6). Suggestions for improvement of the sample injection specifically for small ionic species are discussed. The use of microvalves formed in situ within the microchip channels is investigated. Photo-polymerisation with a UV laser source is performed to produce rigid (highly cross-linked), non-stick (highly fluorinated) monoliths which do not adhere to the glass channel walls. Incorporation of these microvalves into a microchip in order to create a discrete sample loop is discussed.

Chapter 7 summaries the general conclusions of the work presented and gives suggestions for future work

Citation

Pearson, G. F. (2007). Elemental speciation and miniaturised sample introduction studies for inductively coupled plasma mass spectrometry. (Thesis). University of Hull. Retrieved from https://hull-repository.worktribe.com/output/4213169

• Thesis Type: Thesis
• Deposit Date: Oct 2, 2012
• Publicly Available Date: Feb 22, 2023
• Keywords: Chemistry
• Public URL: https://hull-repository.worktribe.com/output/4213169
• Additional Information: Department of Chemistry, The University of Hull
• Award Date: Jul 1, 2007