Classification of Sample Types

All of the calibration techniques described above may be applied to liquid samples with no particular problems. Sometimes it is necessary to dilute very viscous samples or to reduce long equilibration times by using a shaker. Liquid samples also show a wide range of linear relationships between concentration in the sample and peak area - the headspace linearity. Solid samples can also be analysed by HS-GC, but only if they behave as a partition system, similar to liquid samples, owing to inherent calibration problems. However, most solid samples behave as a nonlinear adsorbent. Additional problems are caused by slow diffusion in a solid matrix; size, porosity and specific surface of solid samples are therefore very important parameters. Bulky solid samples are not amenable to HS-GC at all unless they are pulverized, for example by freeze grinding, with loss of the volatiles avoided by chilling the sample with liquid nitrogen or dry ice. There are many problems to be taken into account and most techniques for solid samples try to establish a partition system, which can then be treated like a liquid sample.

Polymers and plastic materials often behave as partition systems if heated above the glass transition temperature. A classical example is the determination of vinyl chloride monomer in a polyvinyl chloride (PVC) resin above the glass transition temperature of 85°C. Such solid samples can be handled as a quasiliquid sample with all types of calibration techniques. Even the technique of standard addition may be applied, because the analyte may be added to the gas phase and not necessarily into the sample using the existing equilibrium system to achieve homogeneous partitioning from both directions. However, considering the variety of plastic materials - pure resins, preforms, copolymers, complex mixtures with all type of additives and finished products - any new solid sample has to be checked carefully for this property. An example for a systematic approach to develop a suitable quantitative method for a solid sample is given below.

The most common procedure for headspace analysis of solids is the solution approach, where the sample is dissolved in an appropriate high boiling solvent, which is eluted late in the chromatogram and may be removed by column backfiushing. The disadvantage is the reduced sensitivity due to the dissolution.

Insoluble samples can often be handled as a suspension in water or an organic solvent, using the displacement effect of the solvent. This suspension approach works well where the analytes are superficially adsorbed. It is obvious that the samples should be a powder rather than a bulky material, to provide the necessary large surface. Calibration in this case is straightforward by using an external standard in the same solvent. The insoluble solid sample remains as a slurry in the headspace vial and causes no matrix effect. A smaller amount of solvent is sufficient here to dissolve only the displaced volatiles, compared to the solution approach where the whole sample must be dissolved. But the resulting smaller dilution effect may be even further minimized by reducing the volume of the high boiling solvent such that only the surface of the sample is wetted by the solvent, which then works as a surface modifier. This surface modification technique provides a homogen eous surface with constant adsorptivity. In this way a much better sensitivity can be achieved owing to the smaller amount of such a liquid displacer. The absence of any residual adsorptivity, however, has to be confirmed, e.g. by the MHE technique. In this case the solid sample with its homogeneous surface behaves like a partition system, where the partition coefficient remains constant over a wide range.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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