A novel calibration strategy for laser ablation ICP-MS
Sample introduction by laser ablation has many desirable features: reduction of the time involved in sample pre-analysis processing, avoiding the use of hazardous reagents and reducing the risk of contamination by reagent impurities. It is also possible to produce spatial analytical profiles across small sections of samples. Laser spots of < 10 μm diameter are possible With the latest commercial instrumentation. Additionally, for plasma spectrometry, the presence of molecular species derived from the plasma gases and the solvent vapour results in interferences, particularly for elements with an atomic mass of less than 80. Sampling with a laser removes the need for a solvent. The type of laser used for sampling is an important consideration. Ultraviolet lasers give better coupling between the laser and sample with ablation being mainly photochemical in nature. With infrared lasers, coupling with some samples is inefficient and is generally thermal m nature leading to poor crater definition. Calibration is one of the main difficulties associated with quantitative analysis by laser ablation. The majority of papers associated with the use of lasers for solid sampling give reference to the difficulty of reproducible calibration and in particular the lack of matrix matched standards The most commonly used calibration method to date involves the use of the National Institute of Standards and Technology (NIST) standard reference materials, particularly the 600 series glass standards. The disadvantages associated with these standards are: the analyst has no control over the elemental make up of the standard, they are relatively expensive and most importantly the matrix is fixed and cannot be matched to the sample. This thesis describes a calibration technique based on the ablation of aqueous standards. Water is transparent to the commonly used UV laser wavelengths, 193,248 and 266 nm resulting in poor energy coupling between the laser and the aqueous standard. The addition of a photo-stable chromophore results in modification of the standards absorption coefficient and enables them to mimic the behaviour of solid samples. the benefit of such standards is that they are easy to produce in any analytical laboratory. The elemental and matrix composition can be controlled by the analyst. The standards also offer the advantage of a constantly renewable surface. Initial work involved design and set-up of an optical system and laser to couple the laser with an ICP-MS. Poly( sodium 4-styrene-sulphonate) was identified as a suitable chromophore. The main criteria for the additive being that it absorbed at the excimer laser wavelengths and had an acceptable lifetime to allow adequate analytical data to be generated Investigation into the characteristics of the chromophore including effect of concentration, laser energy and laser frequency were investigated. Calibration and validation of the aqueous calibration technique was demonstrated by comparison with NIST standard reference materials. The absorption coefficient of the aqueous standard was matched with that of the NIST reference material. Both samples were then analysed by ICP-MS. The count rates observed were initially found to be similar for both samples, however the signal for the aqueous standard remained stable but the signal for the NIST glass decreased. This was thought to be due to the laser channelling into the solid sample causing loss of focus. The aqueous standard in effect provides a constantly renewable surface and no loss of focus. An internal standard was used to correct for the differing sensitivities obtained. The final part of the work involved application of the calibration method to two biological matrices: Bone samples from patients with osteoporosis and porcine liver samples. Elemental profiles across the samples are presented which are in general agreement with the expected and certified concentrations.