Ultrasensitive spectrophotometer

Details Ultrasensitive spectrophotometer Ultrasensitive spectrophotometer

2002-04-29

2004-05-25

Evans, F. L. (Department: 2877)

Optics: measuring and testing

By dispersed light spectroscopy

Utilizing a spectrophotometer

C356S244000

Reexamination Certificate

active

06741348

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention is spectrophotometry. Devices and methods of the invention are applicable to all uses of spectrophotometry, i.e., the measurement of light absorption or scattering in liquids, gases and solids, in addition to absorption, reflection, and scattering of light at interfaces. A wide range of spectroscopic and analytical instruments and devices may benefit from the invention. Exemplary applications of the invention include Ultra Violet-Visible (UV-Vis), Infrared (IR), Atomic Absorption (AA), circular dichroism (CD) spectrophotometers, and High Performance Liquid Chromatography (HPLC).
BACKGROUND OF THE INVENTION
A fundamental property of a sample, be it gas, liquid or solid, is its tendency or lack of tendency to absorb or scatter light at certain wavelengths. Characterization of the tendency of a sample to absorb, scatter or transmit is the basis for spectrophotometry. Example applications include chemical and biological sample analysis. Other example applications include manufactured product testing and the testing of air or water quality.
The point of any application of quantitative spectrophotometry is the ability to numerically characterize a sample in order to discover sample properties or to differentiate it from another sample. Irrespective of the application, the critical aspects of quantitative spectrophotometry are sensitivity, precision, and accuracy. The sensitivity of a spectrophotometric measurement directly relates to the ability to detect small differences between samples having similar absorption properties. The greater the sensitivity, the smaller the difference that can be detected. The precision of a spectrophotometric measurement may be considered as a function of the ability to repeat the same measurement for an identical sample at different times. The accuracy of a spectrophotometric measurement may be considered as a function of the ability to correctly determine the numerical measure of the sample composition. The latter is critical, for example, when attempting to quantify an unknown element in a sample. Over a given range of concentration, the quantification is characterized by certain levels of precision and accuracy. However, below the lower limit of the concentration range, both precision and accuracy are adversely affected. This lower limit is the detection limit of the particular spectrophotometric instrument. As sensitivity increases, the detection limit decreases. Improvements in sensitivity, while retaining high levels of precision and accuracy are desirable.
One known application of spectrophotometry is spectrophotometric chemical analysis. Consideration of this technology is useful to illustrate the problems encountered when practical devices are used to measure light absorption. Spectrophotometric chemical analysis is a standard method for the determination of concentrations of light absorbing substances in liquids and gases. If solutions are studied, the substances are referred to as solutes. In practice, the quantity measured is the Absorbance (A), which is defined by the Beer-Lambert law as A=−log T, where T is the Transmittance. The Absorbance, which is given in Absorbance Units (AU), is proportional to C, the concentration of the absorbing substance by the relationship A=&egr;LC, where L is the length of the light path through the sample and &egr; is a proportionality constant called the Absorptivity, which is specific to the absorbing substance. In order for the equations to be valid, terms A and T must relate only to absorption of light by the solute. Correction must be made for any interference, i.e., absorption other than that attributable to the solute. In practical devices, that type of interference can arise from various sources such as absorption/scattering attributable to the solvent or light reflected by portions of the device being used to measure absorption.
Spectrophotometers generally include a controlled optical system, a sample, detection system, and means for data analysis. The optical system produces a controlled beam or beams to pass through the sample or samples and then be collected by detectors. Detector outputs, which are proportional to the light powers, are then used for data analysis. A typical spectrophotometer has a dual beam optical system and is equipped with two cells, designated Sample and Reference. The power of light emerging from the cells results in detector currents, i
S
and i
R
, which are converted to voltages, V
S
and V
R
, respectively. For the precise measurement of A, interference corrections are performed by making two separate determinations. First, the ratio Q
0
=V
S0
/V
R0
is determined with pure solvent in both S and R cells. Second, the ratio Q=V
S
/V
R
is determined with solution in the S cell and pure solvent in the R cell. Thus, one calculates T=Q/Q
0
and A=−log T. Care must be taken when discussing the Absorbance because some systems give a response that is not identical to A as defined herein. Such a response may be useful as a qualitative indicator for monitoring purposes and it is often referred to as an “Absorbance”. Absorbance values referred to in this application concern the absorbance values as defined by the Beer-Lambert Law, a quantitative measurement.
Others have recognized some sensitivity limits in spectrophotometry and some attempts have been made to reduce noise. Different spectrophotometric devices will have different limits. The sensitivity limits vary depending on the spectral region in question. Consider a UV-visible scanning instrument, of the type that is widely used for chemical analyses. This instrument uses a Tungsten lamp source to cover the visible range. The detectors are either photodiodes or photomultipliers. The generally accepted standard noise specification (Absorbance standard deviation) for high quality commercial units is &sgr;
A
=5×10
−5
AU (at 500 nm wavelength, 1 sec time constant). There is some misconception that this noise originates in the detectors as shot noise. However, with the use of a light meter equipped with a Silicon photodiode detector, it is easy to monitor the power output of a Tungsten lamp with a regulated power supply in a laboratory setting. Analysis of such results obtained by us shows that the Relative Noise Standard Deviation, &sgr;
V
/V, is about 5×10
−5
, which (from the Beer Lambert Law) equals a noise level Standard Deviation of about 2×10
−5
AU, similar to the commercial noise level specification. Also, this noise is independent of the light power received by the detector in contrast to the basic characteristics of shot noise. Of course, other light source types will have different noise characteristics.
Furthermore, this noise level is about 100-fold greater than the calculated shot noise with detector current of 1-2 &mgr;A, as in the present embodiments. Thus, source noise is a more important factor than detector shot noise in determining spectrophotometer sensitivity. That source noise limits performance was recognized by Haller and Hobbs. See, K. L. Haller and C. D. Hobbs, SPIE Vol. 1435, pp. 298-309 (1991).
Where source noise is determined to be dominant, steps can be taken to reduce the noise. Use is made of the fact that source noise is coherent in the two beams of a dual beam spectrophotometer, in which case, it is known that at least some of the noise can be canceled. Various noise cancellation circuits have been proposed. The detector circuit of Hobbs (U.S. Pat. No. 5,134,276) has been cited in the patent literature and elsewhere. Noise cancellation occurs because the source and reference currents are balanced at a node in the circuit. To accomplish this, the reference current is divided by use of a differential transistor pair that acts as a current splitter. The differential voltage controls fractions of current through the two legs of the current splitter across the transistor bases. Current balance can be achieved manually by applying an external differenti

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