Circular dichroism measurement system

Claim 2

In a circular dichroism measurement system that includes a zone to receive a dichroic sample, a light responsive detector, and means for transmitting a beam of light to the detector via the sample, said means including a polarization modulator located in the path of said beam and operable to cause the resultant output of the detector to vary cyclically in correspondence to transmission of right- and left-circularly polarized light, the improvement comprising: signal processing means having electrical connection to the detector for processing said resultant output to obtain processed output values substantially proportional to log IL and log IR, respectively, in alternating sequence; where IR the detector output during an interval of detection of transmission of predominantly right circularly polarized light; IL the detector output during an interval of detection of transmission of predominantly left circularly polarized light; the sequential alternation of output values constituting an AC output component superposed upon a DC component; and AC circuitry responsive to said AC output component to isolate the magnitude of the difference between said processed output values; said signal processing means and AC circuitry including a synchronous filter comprising a coupling network including a serieS resistance and parallel capacitors, the network also including switching operated in synchronism with the polarization modulator to effect alternating series-circuit connection of said resistance with said respective capacitors, in such manner that the AC circuitry output is characterized by a substantially rectangular wave train wherein alternate half cycles have magnitudes corresponding to the absorbance of the sample for light of left- and right-circular polarization respectively.

Claim 3

The improvement of claim 1 wherein said processing means comprises a logarithmic amplifier.

Claim 4

The improvement of claim 1 wherein said circuitry includes means to display said magnitude with a positive or negative value therefor depending upon which one of said values log IL and log IR is larger than the other.

Claim 5

The improvement of claim 1 wherein said circuitry includes a differential amplifier having first and second input terminals, a resistance bridge including a potentiometer having a movable element, a reference signal source, a drive responsive to the output of said amplifier to move said potentiometer element, said bridge being electrically connected with said reference signal source, with said amplifier input terminals, and to receive a version of said processed output in such manner that said movable element is driven to a position wherein the bridge is balanced and said position is representative of the magnitude of sample circular dichroism.

Claim 6

The improvement of claim 4 wherein said reference signal and said version of said processed output have substantially the same wave shape.

Claim 7

Apparatus to receive a sample, for measurement of circular dichroism of such sample, such sample having a characteristic transmission value for predominantly right-circularly polarized light, and a characteristic transmission value for predominantly left-circularly polarized light, said apparatus comprising: means for generating a first electrical signal which is substantially proportional to each of such transmission values in alternation; signal-processing means, connected to receive said first signal, for producing a modified electrical signal substantially proportional within a predetermined additive constant to the logarithm of said first signal, and for extracting and processing the AC component of said modified signal to form an output signal substantially proportional to said AC component only; and display means responsive to said output signal, for indicating a measured value of such circular dichroism.

Claim 8

Apparatus to receive a sample, for measurement of circular dichroism of such sample, such sample having a characteristic transmission value for predominantly right-circularly polarized light, and a characteristic transmission value for predominantly left-circularly polarized light, said apparatus comprising: means for generating a first electrical signal which is substantially proportional to each of such transmission values in alternation; signal-processing means, connected to receive said first signal, for producing a modified electrical signal substantially proportional within a predetermined additive constant to the logarithm of said first signal, and for extracting and processing the AC component of said modified signal to form an output signal substantially proportional to said AC component only; and display means responsive to said output signal, for indicating a measured value of such circular dichroism.

Claim 9

The apparatus of claim 6, wherein said generating means comprises: means for producing a beam of light whose intensity is substantially proportional to each of such transmission values in alternation; and a light-sensitive detector receiving said beam of light and, in response thereto, generating said first electrical signal.

Claim 10

The apparatus of claim 7, wherein said beam-producing means comprises: means defining a zone adapted to receive such a sample; a source of light disposed to direct light along an optical path which traverses such zone; and in combination, a polarizing element and a cyclically variable optical retarder, disposed along said optical path, adapted to coact with a simple which may be placed in said zone and with light from said source to produce said beam of light.

Claim 11

The apparatus of claim 8, also comprising wavelength-selecting means, disposed along said optical path, for limiting the width of the band of wavelengths of said beam of light reaching said detector.

Claim 12

The apparatus of claim 9, also comprising wavelength controlling means responsive to a human operator; wherein: said wavelength-selecting means is responsive to said wavelength-controlling means; and said display means is also responsive to said wavelength-controlling means, to also indicate a parameter of said limited-width band of wavelengths; whereby said measured value of circUlar dichroism may be associated with said parameter.

Claim 13

The apparatus of claim 10, wherein: said parameter is a nominal wavelength characterizing said limited-width band of wavelength characterizing said limited-width band of wavelengths; and said controlling means is programmable by said operator to cause said selecting means automatically to select different bands of wavelengths sequentially; whereby said display means automatically records, in association, sequential measured values of circular dichroism at said different bands of wavelengths, and the corresponding sequential values of said nominal wavelength.

Claim 14

The apparatus of claim 8, wherein: said polarizing element receives light from said source, and produces linearly polarized light therefrom; said variable retarder receives said linearly polarized light and produces therefrom light which is alternately predominantly right- and predominantly left-circularly polarized; said zone is disposed along said optical path between said retarder and said detector, whereby a sample which may be positioned in said zone may receive said light from said retarder, and transmit at least some of said predominantly right- and predominantly left-circularly polarized light along said optical path toward said detector, in proportion respectively to such characteristic transmission values.

Claim 15

The apparatus of claim 8, wherein: said zone receives substantially unpolarized light from said source, said substantially unpolarized light being regarded as consisting of left- and right-circularly polarized constituents of substantially equal intensity; a sample which may be positioned in said zone may transmit at least some of said right- and left-circularly polarized constituents along said optical path toward said variable retarder, in proportion respectively to such characteristic transmission values; said polarizing element has a transmission axis; and said polarizing element receives and selectively transmits, along said optical path toward said detector, said plane-polarized components predominantly corresponding alternately to said right- and left-circularly polarized constituents, with intensities as established after transmission through said zone.

Claim 16

The apparatus of claim 7, wherein said beam-producing means comprises: a source of light disposed to direct light along an optical path; and means disposed on such optical path to receive said light and to produce therefrom elliptically polarized light which is alternately predominantly left- and predominantly right-circularly polarized, and to direct said elliptically polarized light along said optical path to such zone; means defining a zone adapted to receive such a sample, such that a sample which may be positioned in said zone may receive said elliptically polarized light, and transmit at least some of said elliptically polarized light, in accordance with such characteristic transmission values, through said zone and along said optical path toward said detector; elliptically polarized light transmitted through said zone forming said beam of light.

Claim 17

The apparatus of claim 7, wherein said beam-producing means comprises: means defining a zone adapted to receive such a sample; a source of light disposed to direct light toward said zone; said zone-defining means being adapted to permit a sample which may be positioned in said zone to receive said light and transmit through said zone right- and left-circularly polarized constituents of said light in proportion to such characteristic transmission values; and means disposed to receive light transmitted through said zone and to produce therefrom modified light whose intensity is alternately substantially proportional to the intensities of said right- and said left-circularly polarized constituents transmitted through said zone; said modified light forming said beam of light.

Claim 18

The apparatus of claim 6, wherein said processing means comprises: an electrical amplifier having a first and a second terminal; means defining a feedback path between said terminals; an electrical element, connected in said feedback path, having a substantially logarithmic electrical response characteristic; and means for applying said first electrical signal to said first terminal; whereby said modified signal appears at said second terminal.

Claim 19

The apparatus of claim 6, wherein said signal-processing means comprises: means, connected to receive said modified signal, for selectively attenuating a DC component of said modified signal; and means, connected to said selectively attenuating means, for directing relatively unattenuated components of said modified signal to said display means.

Claim 20

The apparatus of claim 16, wherein said electrical element comprises a solid-state device.

Claim 21

The apparatus of claim 16, wherein said electrical element is a diode.

Claim 22

The apparatus of claim 6, wherein said processing means comprises: an electrical amplifier connected to receive said first electrical signal and in response thereto to form a second signal, said amplifier characterized in operation by a gain parameter relating said first and second signals; means for establishing said gain parameter; means, responsive to said second signal, for establishing a control signal to control said gain-parameter-establishing means, so as to maintain said second signal at a substantially constant nonzero value, even if such characteristic transmission values be unequal and said first signal be thereby cyclically varying; said gain-parameter-establishing means characterized by an approximately logarithmic variation of said control signal with said gain parameter, over an operating range; and said modified signal being produced in proportion to said control signal.

Claim 23

The apparatus of claim 20, wherein said generating means comprises: means for producing a beam of light whose intensity is substantially proportional to each of such transmission values in alternation; and a light-sensitive detector receiving said beam of light and, in response thereto, generating said first electrical signal; and wherein: said detector comprises the photocathode of a photomultiplier, and first electrical signal is an electron stream ejected from said photocathode; said amplifier comprises dynodes and an anode of said photomultiplier; said gain-establishing means comprises a voltage supply connected to apply voltages between one of said dynodes and said photocathode, and between certain of said dynodes; said control signal being a substantially linear function of said voltages; said second signal being an electrical current from said anode; said control-signal-establishing means comprising amplifying means adapted to receive said current and to regulate said voltage supply.

Claim 24

The apparatus of claim 17, wherein said selectively attenuating means comprises a capacitor.

Claim 25

The apparatus of claim 17, wherein said selectively attenuating means comprises an inductor.

Claim 26

The apparatus of claim 23, wherein said inductor forms a part of a transformer.

Claim 27

A process for determination of circular dichroism in a sample, comprising the steps of: forming a first signal alternately proportional to the transmission by such sample of light of two distinct polarization states; treating said first signal to form therefrom a second signal substantially proportional to the logarithm of said first signal; and isolating and using a time-varying part of said second signal as a measure of such dichroism.

Claim 28

A process for determination of circular dichroism in a sample, comprising the steps of: forming a first signal alternately proportional to the transmission by such sample of light of two distinct polarization states; treating said first signal to form therefrom a second signal substantially proportional to the logarithm of said first signal; and isolating and using a time-varying part of said second signal as a measure of such dichroism.

Claim 29

Apparatus for measuring circular dichroism in a sample, comprising: a source of substantially randomly polarized light of a relatively wide band of wavelengths; a monochromator disposed to receive said light and select therefrom selected light of a relatively narrow band of wavelengths, said Band having a nominal wavelength; a polarizer, disposed to receive said selected light, and adapted to reject components of said selected light having linear polarizations perpendicular to an axis which is fixed in said polarizer and which is substantially perpendicular to the direction of propagation into said polarizer of said selected light; an element having cyclically varying linear birefringence, disposed to receive from said polarizer unrejected components of said selected light, and adapted to form therefrom elliptically polarized light whose ellipticity varies cyclically in correspondence with said cyclically varying birefringence, elliptically polarized light being understood to be composed of unequal amounts of circularly polarized light of opposite sense; means for supporting a sample in position to receive said elliptically polarized light, and to transmit as an attenuated beam at least a part of said elliptically polarized light, whereby any circular-dichroic characteristic of such sample is manifested as unequal transmission of said circularly polarized components of opposite sense, and in turn whereby the intensity of said attenuated beam varies cyclically in accordance with said varying ellipticity and such unequal transmission; photoelectric detector means disposed to receive said attenuated beam and to produce in response thereto an electrical signal whose magnitude is substantially proportional to said intensity of said attenuated beam, whereby said magnitude varies cyclically in correspondence with said cyclically varying ellipticity and such unequal transmission; an electrical amplifier connected to receive said signal at an input terminal and to produce in response thereto an amplified signal at an output terminal; a solid-state diode connected in a feedback circuit between said input and output terminals, and said diode having a substantially logarithmic voltage-vs.-current characteristic over an operating range, whereby said amplified signal is substantially proportional to the logarithm of said magnitude, and in turn whereby said amplified signal varies cyclically, in correspondence with said cyclically varying ellipticity and with such unequal absorbance of such sample for said circularly polarized components of opposite sense, each such unequal absorbance value being related to corresponding said transmission value as: absorbance -log10 (transmission); AC sensitive display means, not responsive to DC components of said amplified signal, but responsive solely to the magnitude of cyclically varying components of said amplified signal which are present in correspondence with said cyclically varying ellipticity, for indicating a measured value of said dichroism, the magnitude of said varying components being substantially linearly related to the difference between such unequal absorbance values for said circularly polarized components of opposite sense, said difference being definable as such circular dichroism, circular dichroism AL-AR'' where AL and ARare respectively absorbance values of such sample for left- and right-circularly polarized light; scanning means for continuously adjusting said monochromator to select sequentially light of different wavelength bands having successive adjacent nominal wavelengths, whereby said display means indicates a continuous sequence of said measured values corresponding respectively to said successive nominal wavelengths; and means operating synchronously with said scanning means, for controlling said display means to indicate also said successive nominal wavelengths, simultaneously with said corresponding measured values; whereby each of said measured values may be associated with the corresponding nominal wavelength.

Claim 30

Apparatus for measuring circular dichroism in a sample, comprising: a source of substantially randomly polarized light of a relatively wide band of wavelengths; a monochromator disposed to receive said light and select therefrom selected light of a relatively narrow band of wavelengths, said Band having a nominal wavelength; a polarizer, disposed to receive said selected light, and adapted to reject components of said selected light having linear polarizations perpendicular to an axis which is fixed in said polarizer and which is substantially perpendicular to the direction of propagation into said polarizer of said selected light; an element having cyclically varying linear birefringence, disposed to receive from said polarizer unrejected components of said selected light, and adapted to form therefrom elliptically polarized light whose ellipticity varies cyclically in correspondence with said cyclically varying birefringence, elliptically polarized light being understood to be composed of unequal amounts of circularly polarized light of opposite sense; means for supporting a sample in position to receive said elliptically polarized light, and to transmit as an attenuated beam at least a part of said elliptically polarized light, whereby any circular-dichroic characteristic of such sample is manifested as unequal transmission of said circularly polarized components of opposite sense, and in turn whereby the intensity of said attenuated beam varies cyclically in accordance with said varying ellipticity and such unequal transmission; photoelectric detector means disposed to receive said attenuated beam and to produce in response thereto an electrical signal whose magnitude is substantially proportional to said intensity of said attenuated beam, whereby said magnitude varies cyclically in correspondence with said cyclically varying ellipticity and such unequal transmission; an electrical amplifier connected to receive said signal at an input terminal and to produce in response thereto an amplified signal at an output terminal; a solid-state diode connected in a feedback circuit between said input and output terminals, and said diode having a substantially logarithmic voltage-vs.-current characteristic over an operating range, whereby said amplified signal is substantially proportional to the logarithm of said magnitude, and in turn whereby said amplified signal varies cyclically, in correspondence with said cyclically varying ellipticity and with such unequal absorbance of such sample for said circularly polarized components of opposite sense, each such unequal absorbance value being related to corresponding said transmission value as: absorbance -log10 (transmission); AC sensitive display means, not responsive to DC components of said amplified signal, but responsive solely to the magnitude of cyclically varying components of said amplified signal which are present in correspondence with said cyclically varying ellipticity, for indicating a measured value of said dichroism, the magnitude of said varying components being substantially linearly related to the difference between such unequal absorbance values for said circularly polarized components of opposite sense, said difference being definable as such circular dichroism, circular dichroism AL-AR'' where AL and ARare respectively absorbance values of such sample for left- and right-circularly polarized light; scanning means for continuously adjusting said monochromator to select sequentially light of different wavelength bands having successive adjacent nominal wavelengths, whereby said display means indicates a continuous sequence of said measured values corresponding respectively to said successive nominal wavelengths; and means operating synchronously with said scanning means, for controlling said display means to indicate also said successive nominal wavelengths, simultaneously with said corresponding measured values; whereby each of said measured values may be associated with the corresponding nominal wavelength.