DE112014007080B4 - Grating spectrometer with switchable light path - Google Patents

Abstract

Spectrometer for optical emission spectrometry with a radiation emitting source (11), an entrance slit (2), a grating (1) and detectors (3, 4, 5, 6), the radiation from the entrance slit (2) under a The first angle of incidence (α1) falls on the grating (1) against a grating normal (N), characterized in that a first mirror (7) is provided at a point at which the zero-order radiation reflected on the grating (1) falls on the first mirror (7), - a second mirror (8) is provided at a point at which the radiation reflected in the zero order on the grating (1) from the first mirror (7) onto the second mirror (8) - wherein the second mirror (8) is aligned such that the radiation reflected at the second mirror (8) falls on the grating (1) at a second angle of incidence (α2), and that at least one diaphragm (9) is provided that is in the optical path between the grating (1), the first mirror (7), the second mirror (8) and the grating (1) can be switched on to optionally interrupt this path, - the grating (1) is a concave grating with a radius of curvature R, and the grating (1), the entrance slit (2) and the detectors (3, 4, 5, 6) are set up in a Rowland arrangement, and that the sum of the distances between the grating (1) and the first mirror (7), between the first mirror (7) and the second mirror (8 ) as well as the second mirror (8) and the grating (1) assumes the value R▪ [cos (α1) + cos (a2)].

Description

The present invention relates to an optical spectrometer according to the preamble of claim 1.

Optical emission spectrometry (OES) uses grating spectrometers to determine the element content in a sample based on an analysis of the radiation emission of excited atoms. Large spectral ranges have to be measured simultaneously - starting from deep UV to near IR. Due to the enormous line densities and complexities of the atomic emission spectra, in addition to a large bandwidth, a high spectral resolution is required in order to prevent the overlapping of neighboring extraneous lines. This is especially true for the OES analysis of metals.

$$\mathrm{\Delta \beta }\text{/}\mathrm{\Delta \lambda }=\text{N/}\left[\text{d}\cdot \text{cos}\mathrm{\beta }\right],\text{ f}\ddot{\text{u}}\text{r kleine }\mathrm{\Delta \lambda }$$

$${\mathrm{\lambda }}_{\text{G}}:=\text{d/N}\cdot \left[\text{sin}\mathrm{\alpha }+1\right]$$

mit λ= Wellenlänge, N= Beugungsordnung, d= Abstand der Gitterfurchen, α= Einfallswinkel, β= Beugungswinkel und λ_G= Grenzwellenlänge.A diffraction grating causes a dispersion of the spectrum according to the equations: $$\mathrm{\lambda }\cdot \text{N}=\text{d}\cdot \left[\text{sin}\mathrm{\alpha }+\text{sin}\mathrm{\beta }\right]$$

$$\mathrm{\Delta \beta }\text{/}\mathrm{\Delta \lambda }=\text{N /}\left[\text{d}\cdot \text{cos}\mathrm{\beta }\right],\text{f}\ddot{\text{u}}\text{r small}\mathrm{\Delta \lambda }$$

$${\mathrm{\lambda }}_{\text{G}}:=\text{d / N}\cdot \left[\text{sin}\mathrm{\alpha }+1\right]$$

with λ = wavelength, N = diffraction order, d = spacing of the grating furrows, α = angle of incidence, β = diffraction angle and λ _G = cutoff wavelength.

The angular dispersion Δβ / Δλ (equation 2) indicates the difference in the diffraction angle Δβ for two wavelengths that differ by the small amount Δλ. The spectral resolution of the spectrometer is essentially determined by the angular dispersion of the diffraction grating.

The limit wavelength λ _G characterizes the wavelength for which the diffraction angle 90 ° is reached (equation 3). Wavelengths greater than λ _G are no longer diffracted at this grating. The cut-off wavelength must therefore be above the longest wavelength of the spectrum to be displayed. Equation 3 says that for the diffraction of long wavelengths, the spacing of the grating grooves d must be large and the diffraction order N must be selected as low. For a high angular dispersion, however, exactly the opposite procedure is necessary. According to equation 2, a high angular dispersion is achieved through a small groove spacing or a high diffraction order.

The largest wavelength to be measured determines the groove spacing of the grating and thus also determines the angular dispersion. The requirements for large spectral coverage and high angular dispersion can therefore not be met at the same time.

According to the state of the art, two approaches are taken to resolve the conflict. The first way is to use different diffraction orders of a diffraction grating at the same time. Parts of the spectrum with higher demands on the angular dispersion are measured in a higher diffraction order. Here the problem arises that, for example, at the diffraction angle of the wavelength λ in the second diffraction order, the wavelength 2λ simultaneously appears in the first diffraction order. Measures must therefore be taken in order to detect the overlapping diffraction orders separately. Echelle spectrometers follow this approach. Here, many overlapping diffraction orders are mapped spatially separated on a two-dimensional array detector.

From the US 2005/0 179 895 A1 and the DE 10 2006 017 705 A1 such spectrometers for optical emission spectrometry are known.

It has been found that echelle systems are less suitable for the OES of metals because of crosstalk effects which reduce the analytical performance.

The pamphlet GB 765 441 A shows another spectrometer in Rowland setup.

The second approach is to combine several spectrometer units simultaneously in one device, with the respective diffraction gratings having different groove spacings. In this way, selected parts of the spectrum can be displayed with a higher angular dispersion than the main spectrum. However, the use of several spectrometer units measuring simultaneously in one device has disadvantages. All units must be optically coupled to the radiation source in the same way. In addition, the construction effort and the use of materials are significantly increased, resulting in high costs.

the US 2001/0 046 047 A1 shows an instrument for spectroscopy that combines several spectrometer units.

It is therefore the object of the present invention to provide an inexpensive and compact arrangement of an optical spectrometer which splits up a broad, extended wavelength range, the angular dispersion being as high as possible. This object is achieved by an optical spectrometer with the features of claim 1.

In a spectrometer for optical emission spectrometry, the beam path of the radiation emitted by a source in the UV to IR wavelength range runs through an entrance slit and a grating to detectors, with the radiation from the entrance slit falling onto the grating at a first angle of incidence against a grating normal during operation. Because a first mirror is additionally provided at a point at which the radiation reflected in the zero order on the grating falls on the first mirror, a second mirror is also provided at a point at which the radiation reflected in the zero order on the grating falls from the first mirror onto the second mirror, the second mirror being aligned such that the radiation reflected at the second mirror falls onto the grating at a second angle of incidence, further at least one diaphragm is provided which is in the optical path between the grating , the first mirror, the second mirror and the grating can be switched on to optionally interrupt this path, and the grating is a concave grating with a radius of curvature R, and the grating, the entrance slit and the detectors are set up in a Rowland arrangement, as well as the sum of the distances between the grating and the mirror 7th , between the mirror 7th and the mirror 8th as well as the mirror 8th and the grid the value R▪ [cos ( α1 ) + cos ( α2 )], either a spectrum based on the first angle of incidence or two superimposed spectra based on the first angle of incidence and on the second angle of incidence can be generated at the location of the detectors. According to the grating equation, the resolution of these two partial spectra depends on the number of lines in the grating. If the entire spectrum is to be displayed continuously on the detectors, the number of lines and thus the resolution are limited. Since the spectrum according to this invention can be divided into two partial spectra, a grating with a higher number of lines can be used, which offers a higher angular dispersion.

If a filter can also be switched on in the optical path between the entrance slit and the grating, as soon as the diaphragm is switched off, the wavelengths of the spectrum can be absorbed in this filter that occurs at the first angle of incidence at the location of the detectors. When the diaphragm is switched off and the filter is switched on, only the spectrum of the second angle of incidence falls on the detectors.

In this way, the total spectrum to be measured can be divided into two sections, which are measured one after the other by the detectors. The angular dispersion and thus the spectral resolution in each of the two sections can be significantly greater compared to a conventional arrangement in which the entire spectrum has to be mapped onto the detectors.

Preferably, α2> α1 is chosen, since in this case the filter element can be a simple long-pass filter. From Eq. 1 shows that with α2> α1 this is the case α1 corresponding spectrum shorter-wave than that to α2 appropriate spectrum. Hence this can be too α1 belonging spectrum can easily be suppressed by a long-pass filter with a suitably selected filter edge. Depending on a control, either the diaphragm or the filter are switched on and the spectrum is switched over automatically. The control can preferably act on a common actuator.

Preferably the source is a spark excitation source or an arc excitation source or an inductively coupled plasma (ICP).

Finally, one or both mirrors can be designed with focusing properties, for example as cylindrical or spherical mirrors, in order to improve the imaging properties at the angle of incidence a2.

• 1: den Strahlengang eines erfindungsgemäßen Spektrometers in einer ersten Betriebsart; sowie
• 2: den Strahlengang des Spektrometers aus 1 in einer zweiten Betriebsart.

An exemplary embodiment of the invention is described below with reference to the drawing. Show it:

• 1 : the beam path of a spectrometer according to the invention in a first operating mode; as
• 2 : the beam path of the spectrometer off 1 in a second mode of operation.

the 1 shows schematically the beam path and the arrangement of the optical components of a spectrometer according to the invention in a plan view of the diffraction plane. The spectrometer has a Rowland arrangement in which a concave, reflective grating 1 with a given radius of curvature R determines the Rowland circle with the radius ½ R and the center M. The grating normal N draws the optical symmetry axis of the grating 1 the end. The angles α = angle of incidence and β = angle of diffraction mentioned at the beginning are measured against the normal N of the grating.

There is an entrance slit on the Rowlandkreis 2 as well as several detectors 3 , 4th , 5 and 6th arranged. The detectors 3 - 6 are designed in this embodiment as CCD line sensors in a linear arrangement.

The spectrometer also has a first mirror 7th and a second mirror 8th on. In the light path between the first mirror 7th and the second mirror 8th is a switchable shutter 9 arranged that completely block the light path at this point depending on the switch position (as in 1 is shown) or can release.

Next is in the light path between the entrance slit 2 and the grid 1 a switchable filter 10 provided, which is in a first switch position as in 1 shown is located completely outside the light path and is in a second switching position within the light path. The filter 10 is a long-pass filter that allows all wavelengths larger than a certain wavelength to pass and absorbs all wavelengths that are smaller than the certain wavelength.

The light path of the radiation to be analyzed from a source 11 from the entrance slit 2 runs in 1 so under the angle α1 to the grid normal N to the grid 1 . Here is a part in the zeroth order under the angle - α1 to the first mirror 7th reflected from there in the direction of the second mirror 8th , but the part of the zeroth order is at the aperture 9 completely absorbed.

Another part of the radiation is on the grating 1 bent in the first order and at the angle dependent on the wavelength β1 spectrally broken down and then hits the sensors 3 - 6 , which are arranged on the Rowland circle and on which images of the entrance slit arise in the various wavelengths in a known manner.

The wavelength range that is made up in the arrangement 1 on the detectors 3 - 6 falls, the entire wavelength range to be analyzed is not from UV to red, but only the short-wave part, for example from 150 nm to 350 nm.

the 2 shows the arrangement 1 in another switch position. Here is the switchable iris 9 moved out of the light path so that the path between the first mirror 7th and the second mirror 8th free is. In addition, this is a filter 10 driven into the light path so that only the longwave part is from the source 11 outgoing radiation the light path from the entrance slit 2 to the grid 1 can happen. the short-wave part of the spectrum is absorbed.

The radiation emerges in the position 2 so from the source 11 through the entrance slit 2 and is in the filter 10 filtered. The long wave part then falls under the angle α1 on the grid 1 . Since the diffraction angle β depends on the wavelength, the long-wave part of the radiation is diffracted in the first order, but lies outside the range that the detectors can use 3 - 6 capture. The long wave part is also at the angle -α1 in zero order with no dispersion on the grating 1 reflected and on the first mirror 7th steered. From there the long wave part then becomes the second mirror 8th which reflects the radiation back onto the grating 1 throws back, but now at a different angle of incidence α2 , which in this embodiment is greater than α1 .

The one under the angle α2 on the grid 1 falling radiation is diffracted in the first order and then falls on the detectors at the wavelength-dependent diffraction angle β2 3 - 6 . The long-wave part of the radiation from the source is now present there 11 goes out. The spectrum, for example from 300 nm to 800 nm, can now be recorded. The filter 10 prevents the short-wave part of the spectrum from reaching the detectors 3 - 6 falls and superimposes the desired signal.

If one considers the positions of the diaphragm when measuring a sample during an emission of radiation 9 and the filter 10 switches, the short-wave part can be measured once, as in 1 , and then the long wave part, as in 2 . The aperture 9 and the filter 10 are expedient with a common actuator 12th switched simultaneously.

In this way, an extended spectrum can be measured with a grating and a compact detector arrangement, and this with a spectral resolution that would otherwise require a detector area that is twice as large or a second dispersive arrangement. The spectrometer created in this way can therefore be lighter, more compact and cheaper with high resolution.

Claims (7)

Spectrometer for optical emission spectrometry with a radiation emitting source (11), an entrance slit (2), a grating (1) and detectors (3, 4, 5, 6), the radiation from the entrance slit (2) under a The first angle of incidence (α1) against a grating normal (N) falls on the grating (1), characterized in that - a first mirror (7) is provided at a point where the zero-order radiation reflected on the grating (1) falls on the first mirror (7), - a second mirror (8) is provided at a point at which the radiation reflected in the zero order on the grating (1) from the first mirror (7) onto the second mirror (8) - wherein the second mirror (8) is aligned such that the radiation reflected at the second mirror (8) falls on the grating (1) at a second angle of incidence (α2), and that at least one diaphragm (9) is provided that are in the optical path between the grating (1), the first spi egel (7), the second mirror (8) and the grating (1) can be switched on for the optional interruption of this path, - the grating (1) is a concave grating with a radius of curvature R, and the grating (1), the Entrance slit (2) and the detectors (3, 4, 5, 6) are set up in Rowland arrangement, and that the sum of the distances between the grating (1) and the first mirror (7), between the first mirror (7) and the second mirror (8) as well as the second mirror (8) and the grating (1) assumes the value R▪ [cos (α1) + cos (a2)]. Spectrometer after Claim 1 , characterized in that a filter (10) can optionally be switched into the optical path between the entrance slit (2) and the grating (1). Spectrometer after Claim 2 , characterized in that the filter (10) is a long pass filter. Spectrometer according to one of the preceding Claims 2 or 3 , characterized in that either the diaphragm (9) or the filter (10) is switched on as a function of a controller. Spectrometer according to one of the preceding claims, characterized in that the source (11) is a spark excitation source or an arc excitation source or an inductively coupled plasma (ICP). Spectrometer according to one of the preceding claims, characterized in that the following applies: α2> α1. Spectrometer according to one of the preceding claims, characterized in that at least one of the mirrors (7, 8) has a curved surface.

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