Measuring and testing – Gas analysis – Gas chromatography
Reexamination Certificate
2001-04-23
2002-12-10
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Gas chromatography
C073S023350, C073S864330, C422S070000, C210S660000
Reexamination Certificate
active
06490910
ABSTRACT:
The invention relates to apparatus and a method for investigating the properties of a solid material by inverse chromatography, and in particular by inverse gas chromatography.
In conventional gas or liquid chromatography, the composition of an unknown multicomponent gas or liquid is determined by injecting the unknown material into a carrier fluid (the “mobile phase”, typically helium or nitrogen) which passes through an absorptive column (the “stationary phase”). The retention time due to mobile-stationary phase interactions (solid-vapour, vapour-liquid or solid-liquid adsorption) of the various eluted components allows for the composition of the unknown material to be deduced by reference to calibration data. In contrast, in inverse gas or liquid chromatography the properties of an unknown solid material in a column are investigated by passing a known probe through the column. For inverse gas chromatography, the carrier again is typically an inert gas such as helium or nitrogen, and the probe may be a material which is a gas under ambient conditions, or a solvent vapour.
The length of time that the probe is retained in the column is determined by the physicochemical interactions between the probe and the solid material packed in the column. This interaction is governed by a number of mechanisms, the most important of which are surface adsorption, and where the probe is partially soluble in the solid material, bulk solubilisation. Thus, the study of the retention behaviour of a probe can provide important information on the surface and bulk properties of the material in the column.
The use of inverse gas chromatography in investigating various properties of solids, such as adsorption isotherms, adsorption thermodynamics, surface energetics, acid-base interactions, glass transition temperatures, surface area, diffusion coefficients and porosity is described in the book “Physicochemical Measurement by Gas Chromatography” by J. R. Conder and C. L. Young., John Wiley & Son, 1979 and the article “Daryl Williams, Inverse Gas Chromatography, Characterisation of Composite Materials: Materials Characterisation Series—Surfaces, Interfaces, Thin Films, Chapter 5, 80-103 Butterworth-Heinemann, 1994. This article also discusses in detail the most appropriate experimental method, for example “elution at infinite dilution”, “elution of a characteristic point” or “frontal analysis”, for determining each of these properties. In addition to influencing the method chosen, the nature of the property under examination will also affect the choice of probe.
In most instruments used for inverse gas chromatography, the probe is a single compound which is introduced into a single component carrier gas flow that is then eluted through the column. For example, J. Chrom. A. 715, 1995, 279-285 discloses an injection system for an inverse gas chromatograph composed of a closed loop vapourisation chamber with a gas sampling valve, which provides multiple injections of the same sample into a single component gas flow at precisely controlled intervals.
A further variation is disclosed in U.S. Pat. No. 4,806,315 (Daigle). This reference is concerned with the ability to measure samples containing water more accurately by adding water vapour to the carrier gas flow, to form in effect, a two component carrier gas flow (e.g., water and helium). However, in the method disclosed, water passes into the flow via a water permeable membrane, and therefore the ratio of the two components (i.e., the humidity of the carrier) is fixed by the physical properties of the membrane. The only way to vary the humidity of the carrier is to dismantle the equipment and replace the membrane with one of a different type and size.
U.S. Pat. No. 4,869,094 (Gilber) relates to a method of determining sorption isotherms of food by inverse gas chromatography. In the method disclosed, a temporary front of water is established in a carrier gas which can be regarded as a two component gas flow composed of water and carrier gas. Although this reference discloses the possibility of investigating the behaviour of a solid material through inverse gas chromatography, under conditions of humidity, it does not disclose any method by which the humidity level can be varied. The two component carrier flow is formed by passing helium through a water trap, and thus the humidity of the flow is fixed at 100%. The only way therefore of changing the concentration of water vapour in the gas flow would be to change the temperature of the water bath surrounding the pre-column and the experimental column. This would, of course, necessitate a change in the temperature of the sample in the column, thereby influencing the behaviour of the material in the column since adsorption phenomena are highly temperature dependant.
According to the present invention there is provided apparatus for investigating the properties of a solid material comprising:
means for generating a flow of a carrier fluid, comprising at least two fluid components;
means for generating a second fluid flow comprising the said carrier fluid components and probe material;
a column, for holding the solid material;
means for selectively passing the flow of carrier fluid and the second fluid flow through the column;
means for controlling the temperature of the column;
a detector for detecting the passage of the probe material through the column, and
means for controlling the relative proportions of the said fluid components in the flow of carrier fluid and the second fluid flow.
Preferably, the carrier fluid is a gas. The probe material may be a gas under normal conditions (e.g. methane) or may be a vapour of a substance normally liquid under normal conditions (e.g. octane).
In a particular preferred embodiment of the invention, one of the said carrier fluid components is water i.e. the means for controlling the relative proportions of the carrier fluid components comprises means for controlling the relative humidity of the carrier and second fluid flows. The ability to vary the humidity of the carrier enables the solid material to be investigated under a wide range of conditions and enables the simulation of conditions which may arise in use. This is particularly important where for example, the solid material is a compound intended for pharmaceutical use.
In another embodiment of the invention the means for generating a flow of carrier fluid comprises:
means for generating a first sub-flow relatively rich in a first said component (for example a stream of an inert gas such as helium);
means for generating a second sub-flow relatively rich in a second said component (for example a saturated stream of the same gas);
means for combining the first and second sub-flows,
wherein the means for controlling the relative proportions of the said fluid components comprises at least one control valve for varying the mixing ratio of the said first and second sub-flows (for example so as to control the humidity of the carrier).
In a further preferred embodiment, a third sub-flow, relatively rich in the probe material is also provided (for example, of humidified helium containing the desired probe material) and mixed with the first and second sub-flows in a desired ratio.
It is therefore possible for variations in the proportions of the carrier components to be made without the need to vary other experimental parameters such as column temperature. Furthermore, since the proportion of components present is not predetermined by the intrinsic properties of the apparatus, as in U.S. Pat. No. 4,806,315, but rather is varied simply by adjusting a control valve, the relative proportions of the fluid components may be varied both before and during an investigation.
Preferably, the second sub-flow comprises both the first and second components (for example, inert gas and water vapour) and the means for generating the second sub-flow comprises a container for the second component in liquid form. The container has an inlet connected to a fluid line, and a vapour space over the liquid, so that a flow of the said first componen
Butler David A.
Levoguer Carl
Williams Daryl R.
Pillsbury & Winthrop LLP
Surface Measurement Systems Limited
Wiggins David J.
Williams Hezron
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