Measuring and testing – Gas content of a liquid or a solid – By pressure of the gas
Patent
1996-05-15
1999-01-12
Houtteman, Scott W.
Measuring and testing
Gas content of a liquid or a solid
By pressure of the gas
73 2335, 73 2342, C07C 1135
Patent
active
058593608
DESCRIPTION:
BRIEF SUMMARY
This is the national filing of international application Ser. No. PCT/EP95/03627, filed Sep. 15, 1995.
1. Technical Field
The present invention relates to a method and a device for controlling pressure and flow rate of carrier gas in apparatus for gas chromatographic analysis with capillary column.
2. Background Art
As it is well known analysis time and separation efficiency and response of detecting system in an apparatus for gas chromatographic analysis are influenced not only by variations of temperature in the column, but also by variations of carrier gas flow rate passing through the column itself, variations that are in turn dependent on temperature. For this reason it has been endeavoured, both in past and in recent times, to control and program besides the temperature in column, the carrier gas flow rate as well.
Actually, one of the most usual problems arising in capillary gas chromatography is caused by the variation of carrier gas flow rate while programming temperature in the column, this programming being required to separate the samples containing components with a high range of volatility and largely utilized in practice. While programming temperature, the viscosity of the carrier gas increases and, if such gas is fed by constant pressure, its flow through the column decreases. This influences the response of gas chromatographic detectors sensitive to the flow rate, meaning that the response results variable as a function of the temperature program selected. Besides, a decrease in the flow rate of carrier gas increases analysis time and reduces the detectability of eluted components in the conclusive period of same analysis. Maintaining a constant flow rate of carrier gas at the column exit eliminates this inconvenience.
Feeding carrier gas with constant flow rate, instead of feeding at constant pressure, does not present a solution to the problem when the column operates in association with injectors of the split/splitless type, certainly well known, which present different exit lines open to the atmosphere. Maintaining a constant carrier flow at the entrance of the injector does not guarantee a constant flow through the column because the fluid resistance in the column increases along with oven temperature, whereas the other exits present unvaried fluid resistance.
For this and further reasons there is an ever increasing tendency to control carrier gas flow through the column not only and not always in order to keep it volumetrically constant, but also even to run a prefixed program of volumetric flow rate variation.
Harris & Habgood (Programmed Temperature Gas Chromatography, Wiley, N.Y.(1966) 23-31) recalled notably that the flow of a gas in a capillary column is ruled by Poiseuille's equation: ##EQU1## in which the volumetric flow rate in Fo.sup.T outlet is correlated to inlet pressure p.sub.i and outlet pressure p.sub.o, to inner diameter d and column length L, as well as to viscosity .eta. of the fluid, or rather of the carrier. As viscosity .eta. depends upon temperature, also flow rate Fo.sup.T varies according to temperature variation and the above mentioned publication presents semiempiric mathematic correlations to be applied so as to take into account the variations of viscosity with temperature, in order to achieve the desider control of flow rate, keeping in mind the temperature variations to which the column is subjected. In this way there is obtained the equation regulating the variations of inlet pressure as a function of the temperature to be applied to achieve determinate flow values at the exit of the column, equation that shall be called afterwards, in its various formulations "equation of carrier gas flow rate variation as a function of inlet pressure, of column temperature and system structural parameters". The volumetric flow rate at exit can be expressed as a function of the effective values of column temperature and of pressure outlet.
It is,however, more convenient to express it in pressure and temperature standard conditions (equations 2.07 and 2.08 in above ment
REFERENCES:
patent: 5339673 (1994-08-01), Nakagawa et al.
Patent Abstract of Japan, vol. 15, No. 460 (P-1278), 21 Nov. 1991.
Colombo Pieralbino
Magni Paolo
Munari Fausto
Trestianu Sorin
Zilioli Giacinto
Fisons Instruments S.p.A.
Houtteman Scott W.
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