Optics: measuring and testing – By particle light scattering – With photocell detection
Patent
1997-05-13
1998-06-23
Font, Frank G.
Optics: measuring and testing
By particle light scattering
With photocell detection
356345, G01B 902
Patent
active
057710961
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for obtaining information on the electromagnetic spectrum of a sample.
The method according to the invention offers a fast scanning of signals in order to obtain information on the electromagnetic spectrum of a sample without loosing valuable information.
The prior art documents WO-A-92/14997, EP-A-0019692, U.S. Pat. No. 5,355,086 and U.S. Pat. No. 5,253,183 describe various approaches in reducing trasformation artifacts due to different error sources in the measuring spectrum and illustrates alternative methods to that of the present invention.
The present invention is especially suited for use in instruments for use in quantitative determination of components in liquids or solids on the basis of e.g. the absorption spectrum thereof. In typical scanning instruments, the resolution on the absorption axis of the absorption spectrum of the sample will depend on the number of scans taken. This means that obtaining a large resolution on the absorption scale--which is a pre-requisite for performing quantitative analysis--will require a relatively longer measuring time compared to that of qualitative analysis.
For e.g. standard FTIR instruments, obtaining a satisfactory resolution on the absorption axis of the transformed spectrum may easily require performing 2-3 times the number of the scans required for qualitative analysis and, thus, a measuring time 2-3 times larger. However, using the method of the invention, the scanning procedure may be speeded up by a factor of 2 or 3 while retaining the resolution on the frequency axis of the absorption spectrum of the sample. Thus, using the method of the invention, a FTIR instrument may be used for quantitative analysis (that is, with a satisfactory resolution on the absorption axis of the transformed spectrum) using substantially the same measuring time and without loss of resolution on the frequency axis of the transformed spectrum.
Thus, the method of the present invention provides a method which enables typical instruments primarily used for qualitative analysis to also be used for quantitative analysis without loss of resolution and without any substantial increase in the measuring time.
The scanning of e.g. interference signals in FTIR instruments has normally been performed by scanning a number of times over a certain length of the shape of the interference signal. The scan length depends on the resolution required or desired on the frequency scale of the absorption spectra obtained from the instrument. The interference signal typically comprises a well defined maximum amplitude region, and the scan is usually performed symmetrically around this region.
In addition, a smoother absorption spectrum may be obtained using a FTIR instrument by the so-called zero filling method in which the parts of the flanges of the signal outside the scanned part of the signal is assumed to be zero. This artificial extension of the scanned signal smooths the spectra obtained. However, as no additional information from the interference signal is used in this method, no additional information is present in the Fourier transformed spectrum of the zero filled signal.
A faster method of obtaining a large resolution on the frequency scale of an absorption spectrum obtained using a FTIR instrument is performing an asymmetric scanning of the interference signal (phase correction). The interference signal is scanned from a position close to the maximum amplitude region, across this region and further out the opposite flange. From the information in the maximum amplitude region and the scanned flange portion, the shape of the other flange portion (to the width of the scanned flange portion) is calculated. Thus, from an asymmetric scan, the information of a wider, symmetric scan may be generated without actually performing the wide symmetric scan.
The most common way of using FTIR instruments is for qualitative measurements of samples. For qualitative measurements, a high resolution on the frequency scale of the Fourier transformed spectrum is cruci
REFERENCES:
patent: 5251008 (1993-10-01), Masutani
patent: 5253183 (1993-10-01), Inoue et al.
patent: 5355086 (1994-10-01), Ratzel
Font Frank G.
Foss Electric A/S
Kim Robert
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