WO1999028714A2

WO1999028714A2 - Spectrometers

Info

Publication number: WO1999028714A2
Application number: PCT/GB1998/003527
Authority: WO
Inventors: Simon Justin George; Roger Nevin Frank Thorneley
Original assignee: Plant Bioscience Limited
Priority date: 1997-11-27
Filing date: 1998-11-26
Publication date: 1999-06-10
Also published as: WO1999028714A3; AU1251099A; GB9725174D0

Abstract

A Fourier-transform optical spectrometer is provided with means (50) for restricting the bandwidth of light incident on a detector (36) for the spectrometer output to a smaller bandwidth than that transmitted from a light source (30) providing the spectrometer illumination. It is then possible to increase the source brightness to enhance the detector signal without saturating the detector. As a result the signal to noise ratio can be increased.

Description

SPECTROMETERS

The present invention relates to spectrometers, in particular Fourier-transform (FT) spectrometers. Spectrometers according to the invention are particularly, although not necessarily exclusively, useful in the measurement of infra-red spectra.

Fourier-transform infra-red (FT-IR) spectroscopy has become a standard technique for measuring infra-red spectra. In common with all optical spectrometers, the basic principle behind an FT-IR spectrometer is that light from an appropriate source is transmitted through a sample material to an appropriate detector. In contrast to conventional optical spectrometers, which use a monochromator to vary the wavelength of light incident on the sample and build up the spectrum over a few minutes, FT-optical spectrometers use, in place of the monochromator, an interferometer with a moving mirror. A simplified schematic layout of such an FT instrument is shown in Fig. 1. An interferometer 2 receives light from a source 4, via an appropriate reflector 6 so that the light entering the interferometer is parallel. The brightness of the light reaching the interferometer is controlled by an aperture 8 in front of the source. On exit from the interferometer 4 the light passes through an optical cell 10, in which the sample to be investigated is mounted, and then to a detector 12, via a further reflector 14. The interferometer 4 has a beamsplitter 16 to divide the light into two orthogonal beams, each directed against a plane mirror, one of the mirrors 18 being fixed and the other mirror 20 being movable. The mirrors reflect the beams back towards one another and the moving mirror is swept through its range of movement to generate an interferogram, typically within a fraction of a second, which can be converted to a spectrum using standard techniques. Unlike conventional spectrometers using monochromators, in FT spectrometers the total light from the source can be incident on the sample, and hence the detector throughout a measurement .

The general principles, as well as many examples of FT optical spectrometers, are well known in the art, and no detailed explanation is required here. In the infra-red region, FT optical spectroscopy is seen to have a number of advantages over conventional optical measurements. These include the speed of data collection, accuracy of wavelength calibration and inherent high sensitivity. Another advantage is that when there is a desire to observe small features in the spectrum, which may well be smaller than the noise level for a single FT-IR scan, due to the speed of data collection, it is practical and straightforward to enhance spectrum signal to noise by measuring and averaging hundreds or even thousands of interferograms.

However, this approach to addressing the noise problem is not satisfactory for all potential applications of this spectroscopic technique. For instance, there is an interest in the use of FT spectroscopy to take time- resolved kinetic measurements, especially in the infra-red spectrum, where changes in spectral features are monitored as a function of time, typically over a period of seconds. Often the spectral features of interest are of low intensity and many of the features may be smaller than the noise level for a single scan using conventional FT-IR spectrometers, so there is a desire to enhance signal to noise performance. However, the approach mentioned above for achieving this enhancement is incompatible with time- resolved measurements, because it assumes a steady state for the period during which the multiple readings are being taken.

The present invention is concerned primarily with the problem of noise in FT optical spectrometers, and has the general aim of enhancing the spectrum signal to noise without having to resort to the generation of a large number of interferograms .

We have found that by using a spectrometer in which the light incident on the detector is substantially restricted to one or more limited bandwidth regions of the spectrum, an improvement in signal to noise performance can be realised over known spectrometers.

Accordingly, in one aspect the present invention provides an optical spectrometer comprising a light source and an interferometer for transmitting light through a sample, a detector for receiving transmitted light from the sample, and means for restricting the bandwidth of light incident on the detector.

The present proposal has developed from an understanding that the primary limiting factor for noise levels in many spectrometers, particularly FT-IR instruments, is the detector, in particular the noise background characteristic thereof. Thus, if the signal can be increased relative to the background noise generated by the detector, enhancement of spectrum signal to noise can be attained. Many types of infra-red detector, for example solid-state mercury cadmium teluride (MCT) detectors typically employed for high sensitivity measurements, exhibit a constant noise background characteristic, independent of incident light level. Other detectors exhibit "photon statistics" noise, which increases in proportion to the square root of incident light intensity. In both cases the signal level can be increased relative to the noise level by increasing the light incident on the detector. However, it has been found that if the brightness of the source is simply increased, the detector becomes saturated before any significant improvement in signal to noise performance is seen.

By limiting the bandwidth of light incident on the detector as proposed now, we have found that the brightness of the source can be increased, for example by opening the aperture of the known type of spectrometer discussed above, while still ensuring that the total light incident on the detector is maintained below the acceptable maximum, avoiding saturation. This increase in useful incident light gives a corresponding improvement in signal to noise performance.

In practice, to enable the source brightness to be increased by a sufficient degree to achieve a noticeable enhancement of signal to noise, the light incident on the detector should be substantially restricted to one or more regions of the spectrum not exceeding in total a bandwidth of 1000cm^"1. Thus, in the present invention, the means for restricting the light incident on the detector is arranged to transmit light in one or more wavenumber ranges, not exceeding in total a bandwidth of 1000cm^"1, while preventing or at least significantly attenuating the transmission of light outside of these ranges. With this proposal, the need for a large number of readings to achieve satisfactory results is alleviated, and it becomes practical to use spectrometry to study weak samples that need to be measured quickly. The proposed spectrometer is therefore useful for, among other applications, time-resolved kinetic measurements, the measurement of unstable samples, and may also be used to accurately measure spectra at significantly greater dilution than previously possible.

Preferably, the total bandwidth of the region (s) of the spectrum incident on the detector does not exceed 800cm^"1, more preferably 500cm^"1 and even more preferably the region (s) are restricted to a total bandwidth of less than or equal to 200-300cm^"1. The bandwidth used may be narrower than this, but practical minimum limits may be imposed by the type of measurement being taken. For example if looking at infra-red bands, which are typically 10-40cm^"1 in width, bandwidths less than 100-200cm^"1 may not be desirable.

In principle, the means for restricting the incident light on the detector can be disposed anywhere in the light path from the source to the sample or the sample to the detector. Preferably, however, it is placed close to the detector, close to the source or adjacent the sample .

It is preferred that the means for restricting the incident light on the detector comprises one or more filters, eg. infra-red filters, but other optical devices, for example, a monochromator using a prism or optical grating to give the desired bandwidth, may be used as alternatives or in addition to the filter (s) . The filter (s) or other optical device (s) used can be selected to be appropriate for the desired measurements, the width of the spectral band or bands incident on the detector and their position in the spectrum being selected to include the wavelength region or regions of interest .

For many measurements it will be sufficient for the light incident on the detector to be restricted to a single continuous range of wavenumbers, using for example a band-pass filter or separate high- and low-cutoff filters to achieve this restriction. However, there may be some instances where it is preferable for the light incident on the detector to comprise two or more spaced apart regions of the spectrum, for example where the features of interest are widely spread in the spectrum. In this case, the means for restricting the bandwidth of light incident on the detector may, for example, comprise two or more filters

(e.g. a band-pass filter to define upper and lower cut-offs and a band-stop filter to define intermediate cut-offs) .

In practice, it is desirable for a single spectrometer to be capable of being used in a wide range of measurements. Preferably, therefore, means are provided for adjusting the wavenumber region (s) of the spectrum incident on the detector. There are many appropriate ways of doing this. By way of example, the spectrometer may be provided with a plurality of interchangeable filters (or other optical devices) , preferably mounted for example on a rotary or otherwise moveable carriage which can be moved to bring a selected one of the filters into the light path of the spectrometer.

According to another aspect of the invention, there is provided a method of inspecting a sample using an optical spectrometer comprising a light source and an interferometer for transmitting light from the light source through the sample to a detector, wherein the bandwidth of the light transmitted from the light source is restricted in its path to the detector.

The invention is exemplified in detail below, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation of a known FT-IR spectrometer;

Fig. 2 is a schematic representation of a FT-IR spectrometer according to an embodiment of the present invention; Fig. 3 is a plot of maximum detector response against wavenumber, showing results for a conventional FT- IR spectrometer (solid line) and a spectrometer as shown in Fig. 2 (broken line), in both cases in the presence of a 50μm water sample in a CaF₂ window sample cell; and Fig. 4 is a plot of absorbance against wavenumber, showing results for a conventional FT-IR spectrometer (lower solid line) and a spectrometer as shown in Fig. 2 (upper solid line) ;

Fig. 5 is a schematic illustration of the detector with a replaceable filter; and

Fig. 6 is a schematic illustration of the filter holder for the replaceable filter.

Referring to Fig. 2, there is schematically shown an FT-IR spectrometer, eg. a Bruker IFS-66 spectrometer, adapted to embody the present invention, having an infrared source 30, an interferometer 32, an IR optical cell 34, and an MCT detector 36. There is an adjustable aperture 38 in front of the source 30 and two reflectors 38,40, between the source 30 and interferometer 32, and between the optical cell 34 and detector 36 respectively. The reflector 38 in front of the source 30 reflects the light from the source 30 towards the interferometer in a parallel beam, and the reflector 40 in front of the detector reflects the light from the optical cell 34 so that it converges to the detector 36. Although shown, for simplicity, as a parallel beam, the light from the interferometer will usually be focussed onto the sample in the optical cell 34.

The interferometer 32 comprises a beam splitter 42, a fixed mirror 44 and a moving mirror 46. Light entering the interferometer is divided into two orthogonal beams by the beam splitter 42, one beam directed towards each of the mirrors 44,46. The light is reflected back from the mirrors 44,46, which are plane, to combine and provide the desired interference pattern. In use, the moving mirror is traversed through a range of movement to produce an interferogram, from which the spectrum can be deduced.

The features describes so far are essentially those of a conventional FT-IR spectrometer, and may be entirely standard.

The spectrometer illustrated in Fig. 2, however, in contrast to known spectrometers, comprises an infra-red optical filter 50 disposed in the path of the light transmitted from source 30 to detector 36, in this example directly in front of the detector 36. It can be positioned elsewhere in the optical path, however, as exemplified by the alternative locations 36a, 36b, close to the light source and close to the sample respectively. The filter can be of standard construction.

The effect of this filter 50 is to prevent the complete spectrum of light from the source reaching the detector. In this example, there is a single narrow-band filter which restricts the light incident on the detector to a relatively narrow wavenumber range of, for example, a width of about 200cm^"1. In practice, the width of the band, and its position in the spectrum are selected in accordance with the area of interest in the spectrum. For example for IR spectroscopy the band will be in the IR part of the spectrum, typically in the mid infra-red region (eg. 3000- 1000cm^"1' . Preferably the filter is completely non- transmitting outside of the selected ranges.

In use, the selected filter is mounted in front of the detector 36, and the spectrometer aperture 38 in front of the source 30 is opened to increase the light intensity. Some further increase in intensity may also be achievable by, e.g. increasing the source voltage. Standard calibration techniques can be used to ensure that the detector 36 is not saturating, the source intensity preferably being increased to just below the limiting maximum of the detector. For an MCT detector this limiting intensity is usually in the region of lmWcm^"2.

In experiments, using a Bruker IFS-66 FT-IR spectrometer, significant improvements in signal to noise have been observed using 200cm^"1 narrow band IR filters in the 2500-1500cm^"1 region of the IR spectrum.

Fig. 3 shows the maximum detector response for a 50μm water sample in a CaF₂ window cell. The solid line shows the response when no filter is used and the broken line the response in the presence of a 2083-1856cm^"1 70% transmitting filter. This illustrates how a much greater light throughput is achievable over the restricted wavenumber region when the filter is used. Fig. 4 shows an example of the effect of the filter on signal to noise, in the wavenumber region of interest, for a 20ms scan. The lower of the two lines shows this region in the absence of a filter, with a very noticeable noise characteristic. The upper line in Fig. 4 shows the same region, but with the filter in place. As can be seen, the peak to peak background noise for the 20ms scan is reduced from about l.lxlO^"3A to about 1.4xlO^~4A, i.e. by an order of ten.

In the experiments conducted, even with the spectrometer aperture fully open, the detector did not become saturated. In this example, therefore, the maximum possible intensity of the source can be seen to limit the improvement in signal to noise performance. Increasing the intensity of the source, eg. by increasing the supply voltage or using a brighter source may therefore yield further improvements .

Figs. 5 and 6 illustrate how the wavenumber range investigated can be varied by changing the optical filter. Parts already described are indicated by the same reference numbers. Support structure 54 for the detector 36 also has a mounting 56 into which a filter holder 58 can be slid. The filter 50 is located in an aperture 60 in the holder and secured there by a clamping screw 62, so that it can be easily replaced by grasping the finger grip 64 to slide the holder out of the mounting. When the holder is slid into place the filter is automatically aligned optimally in the light path. It will be appreciated that the above description of a preferred embodiment is given by way of example, and various modifications and variations are possible. For instance, while the filter is described as being directly in front of the detector in the above example, it can in principle be anywhere in the path of light between the source and the detector.

Claims

1. An optical spectrometer comprising a light source and an interferometer for transmitting light from the light source through a sample, a detector for receiving transmitted light from the sample, and means for restricting the bandwidth of light incident on the detector to a smaller bandwidth than that transmitted from the light source.

2. An optical spectrometer according to claim 1 wherein the means for restricting the bandwidth is arranged to confine the transmission of light to the detector to one or more wavenumber ranges not exceeding in total a bandwidth of 1000cm^"1, while at least attenuating significantly the transmission of light outside the chosen range or ranges.

3. An optical spectrometer according to claim 2 wherein the total of said restricted bandwidth range or ranges does not exceed 800cm^"1, and preferably does not exceed 500cm^"1.

4. An optical spectrometer according to claim 3 wherein the total of said restricted bandwidth range or ranges is less than or equal to 300cm^"1.

5. An optical spectrometer according to any one of the preceding claims wherein the restricted bandwidth lies in the mid infra-red region of 3000-lOOOcm^"1.

6. An optical spectrometer according to any one of the preceding claims wherein the means for restricting the bandwidth is located close to the detector, or close to the light source, or adjacent the sample.

7. An optical spectrometer according to any one of the preceding claims wherein the means for restricting light incident on the detector comprises one or more optical filters and/or monochromators .

8. An optical spectrometer according to any one of the preceding claims having a plurality of interchangeable optical devices for selecting alternative wavenumber ranges for transmission to the detector.

9. An optical spectrometer according to claim 8 having a mounting for receiving alternative said optical devices .

10. A method of inspecting a sample using an optical spectrometer comprising a light source and an interferometer for transmitting light from the light source through the sample to a detector, wherein the bandwidth of the light transmitted from the light source is restricted in its path to the detector.

11. A method according to claim 10 wherein the bandwidth of the light transmitted to the detector is confined to one or more wavenumber ranges not exceeding in total a bandwidth of 1000cm^"1, while at least attenuating significantly the transmission of light outside the chosen range or ranges.

12. A method according to claim 10 wherein the total of said restricted bandwidth range or ranges does not exceed 800cm^"1, and preferably does not exceed 500cm^"1.

13. A method according to claim 12 wherein the total of said restricted bandwidth range or ranges is less than or equal to 300cm^"1.

1 . A method according to any one of claims 10 to 13 in which the restricted bandwidth of light transmitted is restricted to one or more ranges within the mid infra-red region of 3000-lOOOcm^"1.