GB2070765A

GB2070765A - Spectrophotometry

Info

Publication number: GB2070765A
Application number: GB8103257A
Authority: GB
Original assignee: Bodenseewerk Perkin Elmer and Co GmbH
Current assignee: PE Manufacturing GmbH
Priority date: 1980-02-28
Filing date: 1981-02-03
Publication date: 1981-09-09
Also published as: DE3007453A1; JPS56153244A

Abstract

A spectrophotometer for double wavelength spectrophotometry to determine the concentration of a component of a sample from the height of an absorption band of the absorption spectrum in the presence of an interfering component which also absorbs in the range of the same absorption band comprises an optical system 18 for producing a measuring light beam from a source 16 and directing it onto the entrance slit of a monochromator 22 having a wavelength drive 24 for scanning a wavelength range associated therewith and means 30 for controlling the wavelength drive to define two different wavelengths for the output light beam, the first ???m located in the absorption band to be measured of the sought component of the sample and the second ???r outside this absorption band. A detector 28 is exposed to the output light beam from the monochromator after passage through the sample 26 and produces signals corresponding to the two wavelengths which are applied under the control of means 34 responsive to the wavelength drive to means 32 for storing the signals. Linear combining means 44 establishes the difference between the two stored signals to indicate the concentration of the sought component of the sample. <IMAGE>

Description

SPECIFICATION Spectrophotometry This invention relates generally to spectrophotometers and, in particular, spectrophotometers for used in double wavelength spectrophotometry. Spectrophotometers are generally employed for determining the presence of a sought component in a sample from the height of an absorption band characteristic of the component.

Normally in conventional spectrophotometrythe total absorption of the sample is measured at a single wavelength by comparison with a reference sample. For this purpose a measuring light beam is directed through the sample under test and a reference light beam is directed through a reference sample. Both beams are then projected onto a common detector, the two light beams being rendered effective alternately by a light beam chopper apparatus. In such a conventional instrument, spectrophotometric errors can occur from differences in the effective path lengths of the two light beams. For example, the effective path lengths can differ because of different measuring cells or from different arrangements of the measuring cells.Also the determination of the sought component can be quite difficult if the sample solution further contains a background factor which can be caused, for example, by turbidity or by the presence of mutually interacting components.

In double wavelength spectrometry, light is alternately directed through a sample under test at two different wavelengths. One wavelength corresponds to the absorption band of the sought substance, whereas the other wavelength is outside that absorption band and thus provides a reference signal. The second wavelength is preferably selected such that the absorption value of the pure interfering component is exactly the same as at the first wavelength. Hence the sought component can be determined from the absorption spectrum of the unknown sample from the difference of the absorption at the first and second wavelengths. Alternatively, if there is no second wavelength at which the interfering component absorbs the same as the first wavelength, then a factor K is determined for an appropriately selected second wavelength.

Preferably the factor K is chosen to be representative of the ratio of the absorption value of the interfering component at the first wavelength to its absorption value at the second wavelength. Thus during the actual test of an unknown sample, the absorption measured at the first wavelength is corrected by the absorption measured at the second wavelength multiplied by the factor K.

The sought component is generally determined with a conventional spectrophotometer by analysing the recorded spectra of the pure sought component, the pure interfering component and the unknown sample.

The spectra of the two pure components are recorded. The first wavelength corresponding to a maximum in the absorption band, is selected for measuring the wavelength of the spectrum of the sought component.

The second wavelength is selected from the spectrum of the interfering component at a wavelength which is outside the absorption band but at which the interfering component absorbs to the same degree as at the first wavelength. Finally, the spectrum of the unknown sample is recorded and the difference of the ordinate values thereof is measured at the first and the second wavelengths, which difference is representative of the presence of the sought component in the unknown sample. This procedure is complicated and wearisome.

In an alternative method the light emanating from a light source emitting a continuous spectrum is divided into two light beams, each being directed onto a respective grating monochromatorto produce two light beams at different wavelengths. The two light beams are then passed through a rotating light chopper having the sequence "first light beam, zero signal, second light beam, zero signal". The sequential light beams are directed from the chopper through a measuring cell containing the unknown sample and onto a photo-multiplier. While such a method facilitates sample evaluation, it is nevertheless quite expensive and requires the use of two grating monochromators. Furthermore, it is a major drawback that a user needs a specially adapted spectrophotometer and cannot use existing instruments.

It is an object of this invention to provide automatically operated double wavelength spectrophotometry using a standard spectrophotometer. This is achieved in accordance with the invention by the use of a spectrophotometer which includes an optical system for producing a measuring light beam, and directing it onto the entrance slit of a monochromator having a wavelength drive for scanning a wavelength range associated therewith, a detector exposed to the output light beam from the monochromator after passage through a sample and producing a corresponding signal, means for defining two different wavelengths for the output light beam, means, responsive to the wavelength drive, for applying the signals from the detector at the two wavelength values to means for storing the signals and means for forming a linear combination of the two stored signals.It will be understood that the spectrophotometer itself is unmodified but that additional components are also included.

The invention will now be described in more detail with reference to the accompanying drawings, in which: Figure 1 is a graphic representation of absorption spectra illustrating the principle of double wavelength spectrophotometry; Figure 2 is a block diagram of a spectrophotometer, in accordance with the present invention; and Figure 3 is a schematic view of a unit for use in the spectrophotometer shown in Figure 2.

An ultra-violet absorption spectrum of an unknown sample is generally indicated as 10 in Figure 1.

Ordinarily the concentration of the component is determinable from a light beam including the absorption spectrum which is characteristic of a sought component. Often, however, the sample also comprises a second component, which also absorbs in the same wavelength range as the sought component, such a second component being referred to as an "interfering component". In Figure 1, the absorption spectrum of the sought component is shown as 12 and the absorption spectrum of the interfering component is shown as 14. Using a known concentration of a pure solution of the sought component a first wavelength k m is determined, at which wavelength the absorption band 12 has its maximum, assuming that this wavelength is now already known.Then the spectrum 14 of the pure interfering component is recorded and a wavelength , r, outside the absorption band, is determined, at which wavelength the interfering component absorbs as strongly as at the first wavelength k m. The height of the absorption band effected by the sought component can be determined from the difference of the ordinate values of the spectrum 10 at the wavelenghts X m and X r.

If, however, there is no wavelength outside the absorption band of the sought component at which the interfering component absorbs as strongly as at the wavelength k mis appropriately selected at a point located outside the absorption band of the sought component and where the interfering component is noticeably absorbed. A factor ordinate value at k m ordinate vaiue at k r can then be formed from the spectrum of the interfering component. The spectrum of the unknown sample is evaluated so that the ordinate value at X m is corrected for the value of the ordinate at X r multiplied by the factor K.That is, the linear combination A(km) - KA(Ar) is formed wherein A (R m) represents the graph of the absorption spectrum.

A spectrophotometer, embodying the principles of the present invention is shown as a block diagram in Figure 2 and comprises a light source 16 and an optical system 18 for producing a measuring light beam 20 emanating from the light source 16. The measuring light beam 20 is directed onto the entrance slit of a monochromator 22. The monochromator 22 has a wavelength drive 24 for scanning a wavelength range in the conventional way. The output light beam from the monochromator 22 is passed through a sample 26. A detector 28 is exposed to the measuring light beam 20 which has passed through monochromator 22 and sample 26.

The elements described above form part of the conventional construction of a spectrophotometer and being well known in the art, require no further detailed description.

The spectrophotometer further includes means 30 such as a conventional indexing apparatus for defining two wavelengths. Alternatively the means 30 can comprise a pair of micro-switches adapted to be tripped by a camming mechanism co-operating with the wavelength drive 24. Memory means 32 are provided for storing signals generated by the detector 28. In addition, means 34 for applying the signals supplied by the detector 28 at the two defined wavelength values to the memory means 32 are also included. The memory means 32 can be any conventional semi-conductor device designed for retaining a number of representation values. In Figure 2 the applying means 34 are represented by two switches 36 and 38 which are closed by the wavelength drive 24 at each of the defined wavelengths, as indicated by the two dotted lines 40 and 42.

Furthermore, the spectrophotometer includes means 44 for forming a linear combination of the two stored signals. The means 44 can, for example, be any known semi-conductur device capable of performing arithmetic functions.

In the circumstances illustrated in Figure 1, where the interfering component absorbs at X r as strongly as at X m, the linearly combining means 44 can simply be a difference forming means. When working with analogue signals, for example, the difference forming means can be a differential amplifier. Under other circumstances the linear combining means can be designed as shown in Figure 3. As shown, the means 44 includes a difference forming means 46, represented as a differential amplifier connected to receive one stored signal A (k m) directly at one input. The other stored signal A(k r) is applied to a multiplying means 48 in conjunction with a signal representative of the factor K. The output of the multiplying means 48 is then applied to the other input of the amplifier 46. A signal representative of the linear combination A(X m) - KAZAK r) thus appears at the output 50 of the amplifier.

The factor K can be determined automatically by a quotient forming means 52 to which the signals stored in the memory means 32 are applied, for forming a ratio signal. The ratio signal, represents the ratio of the signals of the detector 28 at the first and second wavelengths, i.e. X m and k r. Means 54 is provided for storing this ratio signal i.e. the factor K and means 56 is provided for applying that stored ratio signal to the multiplying means 48.

Use of spectrophotometer as just described results in a method for determining the concentration of a component of a sample by double wavelength spectrophotometry from the height of an absorption band of the absorption spectrum in the presence of an interfering component which also absorbs in the range of this absorption band which method includes the following steps: Firstly, a first wavelength X m is defined by, for example, the means 30, at which wavelength the absorption band 12 to be measured of the sought component of the sample is located. Next a second wavelength X r is selected which is outside the absorption band 12 and the ratio K of the absorption values of the pure interfering component of the first wavelenth X m and the second wavelength X r is determined.

Preferably as in Figure 1, the second wavelength X r is selected so that the ratio K is equal to one, i.e. the absorption values are equal.

The monochromator 22 is then set to the first wavelength X m, by means of the wavelength drive 24 and the signal obtained from the detector 28 at this wavelength is applied to the memory means 32 for storing this signal. The monochromator 32 is next set to the set wavelength X r by means of the wavelength drive 24 and the signal obtained at the second wavelength is also applied in the memory means 32 for storing this signal. Finally one stored signal is subtracted from the other to give the required result.

The order in which the first and second wavelengths are provided and the respective signals are generated can, of course, be reversed.

The ratio K of the absorption values at the first and second wavelengths is determined by the following method. A solution with pure interfering component is placed, as sample 26, into the spectrophotometer.

The monochromator 22 is set to the first wavelength X m, by means of the wavelength drive 24. The signal of the detector 28 obtained at the first wavelength X m is stored in the memory 32. Then the monochromator 22 is set to the second wavelength by means of the wavelength drive 24. The signal of the detector 28 obtained at the second wavelength A r is detected. The ratio signal, which represents the ratio of the signals obtained at the two wavelengths, X m and X r, is produced by the quotient forming means 52. This ratio signal is stored as the factor K for forming the linear combination A(h m) - KA(h r) in the linear combining means 44 with the measurement of a sample to be tested.

The switch 58 in Figure 2 which is opened after the measurement with the pure interfering component is to ensure that the stored value K is the same as before with the subsequent measurement of unknown samples.

Claims

1. A spectrophotometer for double wavelenth spectrophotometry comprising an optical system for producing a measuring light beam, and directing it onto the entrance slit of a monochromator, having a wavelength drive for scanning a wavelength range associated therewith, a detector exposed to the output light beam from the monochromator after passage through a sample and producing a corresponding signal, means for defining two different wavelengths for the output light beam, means responsive to the wavelength drive for applying the signals from the detector at the two wavelength values to means for storing the signals and means for forming a linear combination of the two stored signals.

2. A spectrophotometer as claimed in claim 1 wherein the means for forming a linear combination is a difference forming means.

3. A spectrophotometer as claimed in claim 1 or claim 2 wherein the means for forming the linear combination comprises means for multiplying one stored signal by a factor K and means for forming the difference of the other stored signal and the one stored signal multiplied by the factor K.

4. A spectrophotometer as claimed in claim 3 further comprising means to which the stored signals are applied, for forming a ratio signal, representative of the ratio of the signals of the detector at the two wavelengths, means for storing the ratio signal and means for applying the stored ratio signal as the factor K to the multiplying means.

5. A method for determining the concentration of a component of a sample by double wavelength spectrophotometry from the height of an absorption band of the absorption spectrum in the presence of an interfering component which also absorbs in the range of the same absorption band by means of a spectrophotometer as claimed in claim 1, the method comprising the following steps; defining two wavelengths, the first located in the absorption band to be measured of the sought component of the sample and the second outside this absorption band, determining a ratio K of the absorption values for the pure interfering component at the first wavelength and the second wavelength, setting the monochromator to the first wavelength by means of the wavelength drive, applying the signal obtained from the detector at the first wavelength to the means for storing the signal, setting the monochromator to the second wavelength by means of the wavelength drive, applying the signal obtained at the second wavelength to the means for storing this signal and linearly combining by means of the linear combination forming means the signal obtained at the first wavelength and the signal obtained at the second wavelength multiplied by the said ratio K, the signals being combined with opposite signs.

6. A method as claimed in claim 5 wherein the second wavelength is selected so that at this second wavelength the absorption value of the pure interfering component is equal to the absorption value of the interfering component at the first wavelength whereby the said ratio of the absorption values equals unity.

7. A method as claimed in claim 5 wherein the determination of the ratio K of the absorption values for the pure interfering component at the first and second wavelengths comprises the following steps; placing a solution with the pure interfering component as the sample into the spectrophotometer, setting the monochromator to the first wavelength by means of the wavelength drive, storing the detector signal at the first wavelength, setting the monochromator to the second wavelength by means of the wavelength drive, detecting the detector signal obtained at the second wavelength, producing a ratio signal representative of the ratio K of the signals obtained at the two wavelengths and storing the ratio signal for forming the linear combination during the measurement of a sample to be tested.