Correction method of detection signal value in spectrophotometer and spectrophotometer having correction function of detection signal value

Abstract

The purpose is to reduce the influence on the measurement due to high order diffracted light without arranging a filter for removing high order diffracted light between a diffraction grating and a PDA. The correction method includes a correction coefficient determination step of determining a correction coefficient related to a ratio of a portion of a detection signal value to the detection signal value, the portion of the detection signal value being derived from a second order diffracted light of light in the first wavelength range contained in the detection signal value of a long wavelength side photodiode for detecting light in the second wavelength range in the photodiode array, and a correction unit configured to obtain a corrected detection signal value derived from light in the second wavelength range from a different detection signal value of the long wavelength side photodiode by using the correction coefficient determined by the correction coefficient determination step.

Claims

1. A correction method of a detection signal value of a photodiode array of a spectrophotometer in which when a certain range of a measurement wavelength range on a short wavelength side is defined as a first wavelength range and a range of the measurement wavelength range on a longer wavelength side than the first wavelength range is defined as a second wavelength range, light of the measurement wavelength range in which a longest wavelength in the second wavelength range is substantially twice a longest wavelength of the first wavelength range is spectrally dispersed by a diffraction grating and guided to the photodiode array, the correction method comprising: a correction coefficient determination step of determining a correction coefficient related to a ratio of a portion of a detection signal value to the detection signal value, the portion of the detection signal value derived from a second order diffracted light of light in the first wavelength range contained in the detection signal value of a long wavelength side photodiode for detecting light in the second wavelength range in the photodiode array; and a correction step of obtaining a corrected detection signal value derived from light in the second wavelength range among detection signal values of the long wavelength side photodiode using the correction coefficient determined in the correction coefficient determination step.

2. The correction method as recited in claim 1, wherein the correction coefficient determination step includes: a first measurement step of measuring the detection signal value of the long wavelength side photodiode derived from the light in the second wavelength range by making light in the measurement wavelength range spectrally dispersed by the diffraction grating incident on the photodiode array and removing the light in the first wavelength range from light incident on the long wavelength side photodiode by using a second order diffracted light cutoff filter which does not allow transmission of the light in the first wavelength range but allow transmission of the light in the second wavelength range; a second measurement step of measuring the detection signal value of the long wavelength side photodiode by making the light in the measurement wavelength range spectrally dispersed by the diffraction grating incident on the photodiode array without using the second order diffracted light cutoff filter; and a step of obtaining the correction coefficient from a ratio of a measurement value of the first measurement step to a measurement value of the second measurement step.

3. The correction method as recited in claim 2, wherein in the first measurement step, the detection signal value of the long wavelength side photodiode derived from the light in the second wavelength range is obtained in consideration of transmittance of the second order diffracted light cutoff filter.

4. The correction method as recited in claim 3, wherein the correction coefficient determination step further includes a transmittance measurement step of spectrally dispersing light in a wavelength range not including the first wavelength range but including the second wavelength range using the diffraction grating, making the spectrally dispersed light incident on the photodiode array, measuring a detection signal value of the long wavelength side photodiode when the second order diffracted light cutoff filter is not used and a detection signal value of the long wavelength side photodiode when the second order diffracted light cutoff filter is used, and obtaining the transmittance from a ratio of the measurement values.

5. A spectrophotometer comprising: a light source in which when a certain range of a measurement wavelength range on a short wavelength side is defined as a first wavelength range and a range of the measurement wavelength range on a longer wavelength side than the first wavelength range is defined as a second wavelength range, the light source being configured to emit light in the measurement wavelength range in which a longest wavelength in the second wavelength range is substantially twice a longest wavelength of the first wavelength range; a flow cell arranged on an optical path of light from the light source and configured to flow a sample; a diffraction grating configured to spectrally disperse the light that passed through the flow cell for each wavelength component; a photodiode array provided with a plurality of photodiodes for detecting a light quantity of incident light and configured to detect light spectrally dispersed by the diffraction grating for each wavelength component, the photodiode array including a long wavelength side photodiode for detecting light of the second wavelength range; and a computer, the computer being configured to: retain a correction coefficient related to a ratio of a portion of a first detection signal value of the long wavelength side photodiode to the first detection signal value, the portion of the first detection signal value being derived from a second order diffracted light of light in the first wavelength range that contributes to the first detection signal, and obtain a corrected detection signal value representing a detection signal value contributed by light in the second wavelength range of a second detection signal value of the long wavelength side photodiode by using the retained correction coefficient.

6. The spectrophotometer as recited in claim 5, wherein a filter for removing high order diffracted light is not arranged between the diffraction grating and the photodiode array.

7. The spectrophotometer as recited in claim 5, wherein a first light source configured to emit light in the measurement wavelength range and a second light source configured to emit light in the second wavelength range are included as the light source.

8. The spectrophotometer as recited in claim 6, wherein a first light source configured to emit light in the measurement wavelength range and a second light source configured to emit light in the second wavelength range are included as the light source.

9. The spectrophotometer as recited in claim 7, wherein the first light source is a deuterium lamp and the second light source is a halogen lamp.

10. The spectrophotometer as recited in claim 8, wherein the first light source is a deuterium lamp and the second light source is a halogen lamp.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic configuration diagram showing an embodiment of a spectrophotometer.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

(2) An embodiment of a spectrophotometer will be described with reference to FIG. 1. Note that the spectrophotometer described here is merely an example of the configuration of a spectrophotometer to which the present invention can be applied, and the type and arrangement of the light source and the optical system can be changed as necessary.

(3) In this embodiment, a certain range of the short wavelength side in the measurement wavelength range is defined as the first wavelength range, and a certain range of the longer wavelength side than the first wavelength range is defined as the second wavelength range. The longest wavelength in the second wavelength range is approximately twice the longest wavelength in the first wavelength range. The spectrophotometer of this embodiment is provided with a first light source 2 configured to emit light in the measurement wavelength range (first wavelength range and second wavelength range) and a second light source 4 configured to emit light in the second wavelength range. In this embodiment, a deuterium lamp that emits light of 200 nm to 800 nm is used as the first light source 2, and a halogen lamp that emits light of 400 nm to 800 nm is used as the second light source 4. That is, the measurement wavelength range in this embodiment is 200 nm to 800 nm.

(4) The light emitted from the first light source 2 and the light emitted from the second light source 4 are combined by a half mirror and irradiated to the flow cell 10 via the condensing lens 8 as measurement light. The light that passed through the flow cell 10 is introduced into a detection unit having a mirror 14, a diffraction grating 16, and a PDA 18 via an inlet slit 12. The measurement light introduced into the detection unit via the inlet slit 12 is reflected by the mirror 14 and guided to the diffraction grating 16 and spectrally dispersed for each wavelength component. The light of each wavelength component spectrally dispersed by the diffraction grating 16 enters a predetermined photodiode of the PDA 18 arranged to detect light of each wavelength component, and is detected.

(5) The detection signal of each photodiode of the PDA 18 is introduced to the processing unit 20. The processing unit 20 is provided with an operation unit 22, a correction coefficient retaining unit 24, and a correction unit 26. The processing unit 20 is realized by, for example, a dedicated computer or a general-purpose personal computer. The operation unit 22 and the correction unit 26 are functions obtained by executing a program stored in a storage device of such a computer by an arithmetic element. The correction coefficient retaining unit 24 is a function realized by a region of a part of the storage device of such a computer.

(6) The operation unit 22 is configured to calculate the absorbance spectrum, etc., of a sample solution flowing through the flow cell 10 by calculation based on the detection signal fetched from the PDA 18. Here, as the detection signal value of the PDA 18, in addition to the detection signal value derived from the first order diffracted light of each wavelength component spectrally dispersed by the diffraction grating 16, a detection signal value derived from high order diffracted light generated in the diffraction grating 16 is included. Therefore, from the detection signal value fetched from the PDA 18, correction is performed to remove the detection signal value derived from the high order diffracted light. The operation unit 22 is configured to obtain an absorbance spectrum, etc., based on the corrected detection signal value.

(7) The correction unit 26 is configured to perform correction of the detection signal value fetched from the PDA 18 using the following equation.
I′.sub.λ=I.sub.λ−K×I.sub.λ  (1)

(8) In the above equation (1), “I.sub.λ” is a detection signal value of a photodiode that detects light of a wavelength λ, “I′.sub.λ” is a detection signal value after correction of the photodiode that receives the light of a wavelength λ, and “K” is a correction coefficient. The correction coefficient K is a coefficient indicating the ratio of the detection signal value derived from the second order diffracted light to the detection signal value of each photodiode, and is an actual measurement value obtained in advance by measurement. The correction coefficient K is retained in the correction coefficient retaining unit 24.

(9) Since the calculation of the absorbance spectrum, etc., based on the detection signal value corrected by the correction unit 26 is performed, the spectrophotometer of this embodiment does not have a filter for removing high order diffracted light between the diffraction grating 16 and the PDA 18. Therefore, a frame member, a window plate, etc., for holding such a filter becomes unnecessary, and the light reflected by these members does not enter the PDA 18 as stray light, so that the stray light is reduced and the detection sensitivity is improved.

(10) Next, a method of determining the correction coefficient K using the spectrophotometer will be described below.

(11) As described above, most of the high order diffracted light incident on the PDA 18 is second order diffracted light, and the contributions of the other high order diffracted light is slight as compared with the second order diffracted light. Therefore, by removing the detection signal value derived from the second order diffracted light from the detection signal value of the PDA, a value close to the detection signal value derived from the first order diffracted light incident on the PDA can be obtained.

(12) Here, it is known that the second order diffracted light of the wavelength λ/2 is incident on the first order diffracted light of the wavelength λ incident on the PDA 18 in an overlapped manner. Therefore, the detection signal value I.sub.λ of the photodiode detecting the light of wavelength λ can be expressed by the following equation by using the detection signal value I′.sub.λ derived from the first order diffracted light of the wavelength λ and the detection signal value I.sub.iiλ/2 derived from the second order diffracted light of the light of the wavelength λ/2.
I.sub.λ≈I′.sub.λ+I.sub.ii/2  (2)
Here, if
(I.sub.iiλ/2)/I.sub.λ=K  (3)
the equation (2) is the same as the equation (1). That is, K is a coefficient representing the ratio of the detection signal value I.sub.iiλ/2 derived from the second order diffracted light of the wavelength λ/2 light to the detection signal value I.sub.λ of the photodiode for detecting the light at the wavelength λ.

(13) In order to obtain K, the first light source 2 and the second light source 4 are individually lighted, and the detection signal value of the PDA 18 is measured. The measurement will be described below using each light source.

(14) <First Light Source: Deuterium Lamp>

(15) In a state in which only the first light source 2 which is a deuterium lamp is lit, the detection signal value of the PDA 18 is measured. The deuterium lamp emits light having a wavelength of 200 nm to 800 nm. The second order diffracted light is detected in such a manner as to be overlapped with the first order diffracted light having a wavelength twice the wavelength of the second order diffracted light. Therefore, when a deuterium lamp is used, although the second order diffracted light does not overlap the light of 200 nm to 400 nm (first wavelength range), the second order diffracted light of 200 nm to 400 nm (first wavelength range) overlaps with the light of 400 nm to 800 nm (second wavelength range).

(16) Therefore, in the range of 200 nm≤λ<400 nm, the detection signal value derived from the second order diffracted light is 0. Therefore, the above formula (3) is expressed as follows:
I.sub.λ≈I′.sub.λ+0
Therefore, K becomes zero (K=0).

(17) On the other hand, as for the range of 400 nm≤λ≤800 nm, the correction coefficient K is determined by comparing the detection signal values between when using the second order diffracted light cutoff filter that does not transmit light of wavelength 200 nm to 400 nm (first wavelength range) but transmits light of 400 nm to 800 nm (second wavelength range) and when not using the same.

(18) First, the detection signal value I.sub.1λ is measured without using the second order diffracted light cutoff filter. Since the measured detection signal value I.sub.1λ includes the detection signal value I.sub.1iiλ/2 derived from second order diffracted light, the measured detection signal value I.sub.1λ can be expressed as follows.
I.sub.1.sub.λ≈I′.sub.1.sub.λ+I.sub.1iiλ/2=I′.sub.1.sub.λ+K×I.sub.1.sub.λ  (4)

(19) Next, the detection signal value I.sub.1λ(filter) is measured by using the second order diffracted light cutoff filter. Since the measured detection signal value I.sub.1λ(filter) does not include the detection signal value derived from the second order diffracted light, it can be said that this detection signal includes only the detection signal value Iζ.sub.1λ derived from first order diffracted light. However, considering the transmittance T in the second order diffracted light cutoff filter at 400 nm to 800 nm (second wavelength range), the detection signal value I.sub.1λ(filter) can be expressed as follows.
I.sub.1.sub.λ(filter)=T×I′.sub.1.sub.λ  (5)

(20) Therefore, from the above equations (4) and (5), the correction coefficient K can be expressed as follows:
K=1−I.sub.1.sub.λ(filter)/(T×I.sub.1.sub.λ)  (6)

(21) Here, in the case of not considering the transmittance T of the second order diffracted light cutoff filter, K can be obtained by setting T in the above equation (6) to 1. Also, on the assumption that the transmittance T of the second order diffracted light cutoff filter in this second wavelength range is uniform, its representative value and average value (for example 0.97) may be used. In the case of not considering the transmittance T (T=1) or using the representative value of T or the like, it is not necessary to perform measurement using the second light source 4 (halogen lamp) described below.

(22) <Second Light Source: Halogen Lamp>

(23) Next, in order to accurately obtain the transmittance T in the above equation (5), measurements of the detection signal values are performed in the case in which the second light source 4 is lit and the above second order diffracted light cutoff filter is used and in the case in which the second light source 4 is lit and the above second order diffracted light cutoff filter is not used. The second light source 4 which is a halogen lamp emits light of 400 nm to 800 nm (second wavelength range). When only the second light source 4 is turned on, since there is no light of wavelength 200 nm to 400 nm (first wavelength range), the second order diffracted light does not enter the photodiode detecting light of this second wavelength range.

(24) First, the detection signal value I.sub.2λ is measured without using the second order diffracted light cutoff filter. Since the measured detection signal value I.sub.2λ does not include the detection signal value derived from the second order diffracted light, the measured detection signal value I.sub.2λ can be expressed as follows.
I.sub.2.sub.λ=I′.sub.2.sub.λ  (7)

(25) Next, the detection signal value I.sub.2λ(filter) is measured by using the second order diffracted light cutoff filter. The measured detection signal value I.sub.2λ(filter) can be expressed as follows.
I.sub.2.sub.k(filter)=T×I′.sub.2.sub.k  (8)

(26) From the above equations (7) and (8), T can be obtained as follows:
T=.sub.2 .sub.k(filter)/I.sub.2.sub.λ  (9)
By applying T obtained by the above equation (9) to the above equation (6), it is possible to obtain an accurate correction coefficient K in consideration of the transmittance T of the second order diffracted light cutoff filter.

DESCRIPTION OF REFERENCE SYMBOLS

(27) 2 first light source 4 second light source 6 half mirror 8 condensing lens 10 flow cell 12 inlet slit 14 mirror 16 diffraction grating 18 photodiode array (PDA) 20 processing unit 22 operation unit 24 correction coefficient retaining unit 26 correction unit