Optics: measuring and testing – By polarized light examination – With birefringent element
Patent
1994-08-19
1996-01-09
Gonzalez, Frank
Optics: measuring and testing
By polarized light examination
With birefringent element
356128, 356130, 356365, G01N 2141, G01N 2143
Patent
active
054833442
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of high performance liquid chromatography which determines the composition of a liquid medium by means of its refractive index.
The invention also relates to a process for measuring the difference between the refractive indexes of two media traversed by a light beam.
More precisely, the invention relates to a phase modulation and an interference fringe differential refractometer.
The use of the present invention allows improvements in high performance liquid chromatographs, known as HPLC, to be achieved.
2. Description of the Prior Art
Most detectors in use in the in this field are photometers and refractometers but also for very specific applications, electrochemical, conductometric or other detectors may be used.
Photometers have the advantage of exhibiting a high sensitivity and a great stability. They may be used in the case in which the eluates to be detected absorb the light within the range of wavelengths between about 190 and 700 .eta.m. But these photometers have major drawbacks: they are not universal devices and, in a single analysis, there may be eluates which absorb the light at different wavelengths or which even practically do not absorb it, which has the disadvantage, notably in preparative chromatography, of letting impurities pass unnoticed.
Furthermore, in the case of preparative chromatography, the photometer is rapidly saturated at around an optical density of the order of 2.
By dividing for example by 10 the length of the optical path in the vessel, this drawback is decreased but still more sensitivity is lost.
The main advantage of refractometers is that they are practically universal. Most of the devices which are currently manufactured use a light beam deviated through a double prismatic circulating vessel. In these detectors, a light source casts a beam onto a double photodetector after passing successively through a diaphragm, possibly concentrating lenses, a rotating glass strip with parallel faces for adjusting the optical zero of the device, i.e. for balancing the light intensity lighting up the two photodetectors, a double prismatic vessel, one for the reference liquid and the other for the phase to be analyzed.
When the refractive index of the phase to be analyzed varies, the prismatic section of the two successive vessels is such that the beam deviates from one photodetector to the other, according to the sign of difference of the indexes between the two vessels. However, in some cases, notably in preparative chromatography, where high concentrations may be found in one of the two vessels, or if an elution gradient is created, the index variations may be such that the beam may deviate to the point of saturation of the device, which means that the deviated beam eventually only lights up one of the two cells. The chromatogram is thus clipped and several peaks with a common base may no longer be distinguished.
As it is the case for photometers, these drawbacks may be decreased by reducing the deviations, but here again at the expense of a sensitivity loss. Saturation is then avoided, but the smaller peaks, i.e. impurities, are no longer distinguished in preparative chromatography.
Whatever the effort the problem created by an elution gradient is always present.
A refractometric system with a monochromatic source has also been proposed, whose beam is divided to pass through two vessels in parallel, one containing a reference liquid and the other, the phase to be analyzed, then the two beams are reassembled to light up a photodetector. Interferences occur because of the variation of the optical path as a function of the index variation. One may consider that the sinusoid followed by the intensity is linear in the vicinity of the zero difference index, which gives an acceptable sensitivity but does not solve the problems of saturation and of use of a gradient, and, as in the previous cases, saturation is avoided to the detriment of sensitivity.
French patent FR-2,596,526 illustrates
REFERENCES:
patent: 3472598 (1969-10-01), Hossmann
patent: 3680963 (1972-08-01), Edwards et al.
patent: 4289403 (1981-09-01), Allington
patent: 4571082 (1986-02-01), Downs
patent: 4690562 (1987-09-01), Davies et al.
patent: 4787746 (1988-11-01), Couillard
patent: 5073024 (1991-12-01), Valette et al.
patent: 5168325 (1992-12-01), Yoder-Short
Beauducel Claude
Couillard Francois
Frot Didier
Gonzalez Pierre
Couillard Francois
Eisenberg Jason D.
Gonzalez Frank
Institut Francais du Pe'trole
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