Multiple pathlength spectrophotometer

Details Multiple pathlength spectrophotometer Multiple pathlength spectrophotometer

2001-05-02

2003-11-04

Nguyen, Thong (Department: 2872)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrophotometer

C356S312000, C356S340000

Reexamination Certificate

active

06643016

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to ultraviolet-visible spectrophotometers and absorbance detectors, and, more particularly, to a novel method and accompanying apparatus for extending the linear dynamic range of such detectors by separation of incident light before a series of variable path length cells.
DESCRIPTION OF THE PRIOR ART
A widely used method for monitoring various characteristics of a sample of interest relies upon obtaining accurate measurements of light absorption by the sample. Such measurements are commonly performed as a function of wavelength. For example, the concentration of a solute in a solution can be determined quantitatively by comparing a measured intensity of light transmitted through a sample to a reference light intensity, or alternatively the qualitative identity of the solute may be inferred by considering the various specific wavelengths of light that are absorbed by the sample. In many laboratory and industrial uses of spectrometry the relative intensities or absorbance values at selected wavelengths are needed and the analysis of every point in a complete spectrum is not necessary. Often the point of an analysis is to derive a concentration of one or more components that absorb light. In other cases, points from the absorbance spectrum may be used to define a quality indicator using regression or chemometric techniques. These quantitative analysis applications of spectrometry may require the use of absorbance or intensity values obtained at widely different wavelengths with absorption or emission values that vary substantially. A spectrometer is normally configured to provide the highest accuracy at one wavelength, causing measurements at other wavelengths to be less accurate.
Many spectrometers are available that use a single path length with a single light source and many detectors including photodiode arrays such as Kuderer, U.S. Pat. No. 4,958,928, Kuderer, U.S. Pat. No. 5,116,123, and Bilhorn, U.S. Pat. No. 5,173,748. These systems typically use a monochrometer between the sample and the detector to separate out a single wavelength for measuring absorbance. By scanning the monochrometer, absorbance can be measured at different wavelengths, but not simultaneously. Optical Coating Laboratories, however, has disclosed a miniature spectrometer with optical filters instead of a monochrometer as disclosed in Anthon, U.S. Pat. No. 6,057,925. In Wang, U.S. Pat. No. 5,408,326, a two-wavelength absorbance detector was disclosed that uses two independent light sources and a single sample pathlength to measure two values simultaneously. Additionally, prior art multiple wavelength systems such as the Ocean Optics PC2000 unit or the CVI SM200S unit extract substantial information as a function of wavelength simultaneously, but resolution is limited because intensity information is gathered by 12-bit or 16-bit count CCD elements. CCD based systems inherently have limited dynamic range due to the limits of charge accumulation in the device well and the analogue-to-digital conversion resolution. CCD elements with a 12-bit well depth or a 16-bit well (deep well) depth are available, which provide 4096 and 65536 increments of intensity, respectively. In addition to CCD arrays, diode array instruments can be used, but both generally require significant processor overhead to complete the measurement. Photodiodes or photomultipliers provide much higher, sensitivity, dynamic range and linearity compared to CCD elements. This is discussed in Perkini Elmer Technical Document, Choosing the Detector for your Unique Light Sensing Application, by Larry Godfrey and is available on the World Wide Web at http://opto.perkinelmer.com/library/papers/tp4.htm.
Certainly in industrial measurement and control it is often necessary to reduce a group of measurements at different wavelengths to one or two easy to understand and control process parameters. In fact, other characteristics of the sample of interest may be investigated by performing more complex analyses of the absorbance data. Several patents have addressed methods for extracting performance indicators from absorbance data. For example, Richardson et al., U.S. Pat. No. 5,242,602, teach the use of chemometrics or linear regression techniques with multiple ultraviolet-visible absorbance measurements to derive water treatment performance indicators. International Patent Application WO 96/12183 discloses a method of determining quality parameters and the organic content in pulp and paper mill effluents by applying chemometric methods. In the method disclosed in WO 96/12183 the chemometric algorithms are applied directly to the spectroscopic data. The spectra are subjected to data treatment using values from several discrete wavelengths from each particular spectrum. U.S. Pat. No. 6,023,065, issued to Garver, discloses a method for monitoring and controlling a characteristic of process waters that uses at least two measurements of ultraviolet light absorption to construct a ratio for computing an empirical value of the characteristic of the effluent or process. Garver taught that the use of at least one absorbance ratio to derive a performance indicator improved the information extraction from the absorbance spectrum by providing a means to model non-linear processes and decouple covariant absorbance data. Feedback control is used for adjusting feed input components in accordance with the computed empirical value of the characteristic such that a target measurement of the characteristic is obtained while the excess amount of the input component is kept to a minimum.
According to the method disclosed in U.S. Pat. No. 6,023,065, accurate real-time absorbance data for up to eight different predetermined wavelengths of ultraviolet light are required to obtain empirical values for a plurality of effluent characteristics including: pulp final target brightness; yellowness; residual peroxide; brightness efficiency, yellowness efficiency; and delignification efficiency. For this reason, it will be appreciated that single-wavelength units do not provide sufficient information to determine multiple performance indicators that are dependent on more than one input. Furthermore, the generation of ultraviolet-visible absorbance ratios can multiply signal noise when the absorbance value is in the denominator of the ratio. A very low absorbance at a long wavelength, for example, may be used in a denominator to represent color and an intense absorbance at a short wavelength may be used to represent a bleaching agent such as hydrogen peroxide, for example. In this case the ratio of high absorbance to low absorbance is substantially less accurate than the high absorbance value or the low absorbance value. For example, if the actual ratio is A
230
/A
350
, and the error is expressed as err
230
and err
350
the measured ratio=[A
230
±err
230
]/[A
350
±err
350
]. In a simplified e spectrometer error is 0.01 absorbance units at all wavelengths and absorbance values and absorbance measurements were 1.000 at 230 nm and 0.08 at 350 then the error at 230 nm is 1%. the error at 350 nm is 12.5% and the error in the ratio is ~15%. This simplified example highlights the need for highly accurate absorbance values at different wavelengths when functions with division of absorbance values are used. In practice, different types of accuracy, resolution, and linearity increase error at both high and low absorbance values. An absorbance detection system is typically optimized for measurements between 0.3 and 0.9 absorbance units.
In general, measurements of a quantitative nature entail a prior calibration of the instrument response using at least two different calibration standards of the sample of interest to prepare an absorption curve. Preferably, the prior calibration of the instrument response is such that the light absorption by the fluid sample tends toward an amount of absorption approximately central to an approximately linearly varying region of the absorption curve for the sam

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