+44 (0)1509 263171
Please use this identifier to cite or link to this item:
|Title: ||A novel calibration strategy for laser ablation ICP-MS|
|Authors: ||Masters, Barrie J.|
|Issue Date: ||1997|
|Publisher: ||© Barrie James Masters|
|Abstract: ||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.|
|Description: ||A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.|
|Appears in Collections:||PhD Theses (Chemistry)|
Files associated with this item:
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.