Carrier flow optimisation

There are also advantages with glass fibre-reinforced materials. For example, the typical processing temperature with 30% glass fibre-reinforced PEI (360-380 °C) blended with 21% PPO can be reduced to 310-340 °C. This flow-optimised technology can find applications in, for example, automotive headlamp reflectors, lamp bases, throttles, chip carriers and aerials. 

Carrier gas flow should be optimised for a particular column and a particular carrier gas. This is most important for open tubular columns. Fig. 5 shows the relationship between efficiency expressed as the height equivalent of a theoretical plate versus carrier gas velocity (Van Deemter plot) for a 28 m by 0.25 mm internal diameter wall-coated open tubular column of Carbowax 20M. 

To design and optimise flow-based procedures, however, the analytical chemist needs a practical index that indicates the extent of the dispersion process and thus the concentrations of sample, carrier and reagent(s) along the sample zone. This index may also provide an approximation of the improvement in analytical sensitivity that could, in principle, be attained simply by reducing sample dispersion. 

There are many reports in the literature about interfaces and devices for preparation of blood samples on chip. Eor example, Jandik et al. used a laminar fluid diffusion interface to replace centrifugation and consequently reduced the preparation time from between 30-60 minutes to only five minutes . The H-filter has also been used to clean up biological samples. A sample, e.g. blood, is put into a reservoir at one end of one post of the H, and a diluent such as water or saline is placed in the reservoir at the other end. The two parallel laminar streams will flow along the crossbar of the H as a resnlt of hydrostatic pressnre. Smaller, more mobile analyte molecnles will cross the interface between streams qnickly, whilst heavier particles remain in the carrier stream. Conseqnently, by controlling the fluid velocity and the length of the channel, the process can be optimised. There are also devices for separating plasma from blood on a chip . ... 

Once a column has been chosen, the two variables which can be modified to optimise the separation of components are temperature and carrier gas flow-rate. The latter is often preset although the newer microcomputer controlled instruments allow the flow rate and temperature to be changed between runs for automatic optimisation. Accurate control of column temperature is important in order to obtain reproducible chromatograms. 

The separated components emerging from a GC column are present in pico-gram amounts in the carrier gas stream. If the column eluant is to be coupled to a mass spectrometer then the volume flow-rate of carrier gas should be minimised for a given separation to achieve high sensitivities in the ion source and reduce pumping requirements. GC parameters are selected to obtain symmetrical sharp peaks which are eluted in the minimum carrier gas volume and with the best peak height to width ratio that can be achieved. Clearly, the components need to be resolved, as co-eluting peaks would not produce pure mass spectra which could then be compared to a library of reference spectra. A number of factors need to be considered to optimise the GC system ... 

Optimise the column and injector temperature to give good resolution and peak shape for the components of the standard mixture. Then inject the mixture (1-5 pi) at various carrier gas flow-rates. Determine the value of N... 

To find optimal conditions for a given analysis, the principal parameters can be varied in a rational way. In the case of an ICP, to which the further discussion will be hmited, this concerns the flow rate of aerosol carrier gas, the RF power input, the observation height above the induction coil, and the sample uptake rate. Numerous simple optimisation schemes have been described in the literature, especially for ICP analysis. In addition, sophisticated and complex diagnostics have been suggested in the past to characterise the performance of ICP-AES systems. 

Analysis of cement after optimisation of the operational variables (forward power, sample uptake rate, flow rates of intermediate plasma gas, outer plasma gas, aerosol carrier and spray chamber, nebuliser and type of torch). Cement ICP-OES Modified simplex... 

The experimental conditions for a time-based device used for the determination of tin by hydride generation flow injection atomic absorption techniques were optimised by simplex (sample and reagent concentrations and volumes, carrier gas flow rate and atomiser conditions). 

The efficiency of a given column is dependent on a number of factors, including the nature and flow-rate of the carrier gas, column dimensions, liquid-phase thickness and column temperature. By optimising these, it may be possible to increase the resolution attainable quite considerably. On the other hand, this improved resolution may be bought at the expense of increased analysis time. In practice, it may be desirable to compromise and select conditions for an analysis which give adequate resolution in a reasonable time. 

The response is subject to some experimental variables, chief among which are the flow-rates of hydrogen, air and the carrier gas. The instrument manufacturer s instructions should be followed closely to optimise these, although there is usually some leeway, as it is not easy to give general guidelines for instruments of different makes. 

In direct thermal desorption (DTD) a few mg of a solid (typically <10 mg) are loaded into the cold injector, the carrier gas is temporarily halted, the injector is rapidly heated to the desired temperature (usually 50 to 200°C for polymer analysis), the carrier gas is resumed and the thermally extracted components are swept onto the column (Fig. 2.47b). The flow-path is simplified with few possibilities for sample loss and few parameters to optimise. The... 

This temperature is maintained until all of the components are apparently eluted. Helium is used as a carrier gas at a flow rate of 40-50 ml/min. Hydrogen and air flows are optimised and maintained at those values. 

---