Chopper arrangement for atomic absorption spectrophotometer

Abstract

An atomic absorption spectrophotometer incorporating discrimination against "background" absorption, i.e., absorption not caused by the resonant line absorption by the element being measured. The device includes a resonant line emitting source (e.g., a hollow cathode lamp) and a continuous spectrum light source (e.g., a deuterium lamp), a monochromator and a detector system. The improvement comprises a specific chopper arrangement which sequentially causes: (a) the resonant line light from the hollow cathode lamp to go to the sample path, (b) this light to go to the reference path, (c) the continuous spectrum light from the deuterium lamp to go to the reference path, and finally (d) the continuous spectrum light to go to the sample path. By comparing the light intensities during intervals (c) and (d) the effect of the background absorption can be determined and compensated for so as to determine the relationship between (a) and (b) free of the effect of such background absorption. The specific improvement includes a sector mirror and disk-shaped mask which are conjointly rotated in the angularly intersecting paths of the radiation from the cathode lamp and the deuterium lamp.

What is claimed is: 
     
       1. In a double-beam atomic absorption spectrophotometer having a sample beam path passing through an atomized sample substance and a reference beam path, including a line-emitting first light source which emits a resonance line of an element of interest desired to be measured and a second light source emitting a continuous spectrum, a monochromator for selecting a limited spectral range containing the said resonance line from the entire continuous spectrum, a detector impinged upon by the sample and reference beams of light, and a signal analyzer circuit connected to generate an output signal from the detector corrected with respect to the background absorption, a chopper arrangement by which, in a predetermined cyclical sequence of four successive intervals, light from the line emitting first light source is directed to the sample and reference paths and light from the continuous spectrum second light source is similarly directed to the sample and reference paths, said chopper arrangement comprising:   a sector mirror and an opaque mask rotating coaxially and conjointly therewith;   said sector mirror having two angularly spaced reflecting sectors, arranged respectively on its opposite faces and, angularly interposed between said sectors, a pair of light-transmitting cutout portions;   said mask comprising two partially cutout portions, including an arcuate aperture and a cutout portion of different radii, extending across a respective one of the transmitting cutout portions of the sector mirror, and a completely open sector in aligned position with the reflecting sector on the side facing the mask;   and means for causing the beams of light from the first and the second light source to impinge obliquely upon said second mirror substantially reflectingly symmetrically to the sector mirror in the range of the reflecting sectors, so that one of said partially cutout portions of the mask passes one of the beams of light impinging thereon and the other passes the other beam of light to the sample path and the reference path during intervals when transmitting cutout portions of said sector mirror allow the beams of light .[.of.]. .Iadd.to .Iaddend.pass through said sector mirror.   
     
     
       2. An atomic absorption spectrophotometer as claimed in the claim 1, in which: said reflecting sectors (32,34) of said sector mirror are diametrically opposite each other with respect to the axis of rotation (28) and said light-transmitting cutout portions (36,38) are interposed therebetween.   
     
     
       3. An atomic absorption spectrophotometer as claimed in claim 2, in which: the reflecting sectors of the sector mirror are symmetrically disposed with respect to a first diametral line normal to said axis of rotation and the light-transmitting cutout portions are symmetrically disposed with respect to a second diametral line normal to the first diametral line and to the axis of rotation.   
     
     
       4. An atomic absorption spectrophotometer as claimed in the claim 2, in which: both of the beam axes of the two beams of light (10,20) impinging upon the sector mirror from the first and second light source, respectively, are in a plane containing the axis of rotation (28) of the chopper arrangement (18) and are inclined towards this axis of rotation (28) with respect to the surface normal.   
     
     
       5. An atomic absorption spectrophotometer as claimed in claim 4, in which: said arcuate shaped aperture (44) of said mask (30) extends angularly across the one cutout portion (36) of the sector mirror (26) in the radial range of the point of intersection of the beam of light (20) on the side of the mask through the plane of the mask (30);   a cutout portion (50) at the periphery of said mask is in the radial range of the point of intersection of the beam of light (20) reflected on the reflecting sector (34) on the side of the mask though the plane of the mask (30) which angularly extends across the other cutout portion (38) of the sector mirror (26), and   said completely open portion (51) extends radially across both points of intersection and is angularly aligned with the mirror (34) on the sector on the side of the mask.   
     
     
       6. An atomic absorption spectrophotometer as claimed in claim 5, in which: at the edges of the reflecting sectors (32,34) there are provided absorbing sectors (40,42) for generating a dark signal each.   
     
     
       7. An atomic absorption spectrophotometer as claimed in claim 2, in which: said sector mirror (26) comprises a disk-shaped carrier of light-transmitting material on which the reflecting sectors (32,34) are applied as bilaterally reflecting layers, and that these reflecting layers are covered on one respective side each by an absorbing layer.   
     
     
       8. An atomic absorption spectrophotometer as claimed in the claim 7, in which said mask comprises a thin circular disk. .Iadd.9. In a double-beam spectrophotometer optical testing system having a sample beam path passing through a sample substance and a reference beam path, including a line-emitting first light source which emits a resonance line of an element of interest desired to be measured and a second light source emitting a continuous spectrum, a monochromator for selecting a limited spectral range containing the resonance line from the entire continuous spectrum, a detector impinged upon by the sample and reference beams of light and a signal analyzer circuit connected to generate an output signal from the detector corrected with respect to the background absorption,   a chopper arrangement by which, in a predetermined cyclical sequence of four successive intervals, light from the line emitting first light source is directed to the sample and reference paths and light from the continuous spectrum light source is similarly directed to the sample and reference paths,   said chopper arrangement comprising: two disc-like elements having a common axis and being mounted in offset relationship along their common axis and being rotatable coaxially and conjointly, each of said disc-like elements being mounted so as to be rotatable through each of said beam paths;   one of said elements having a light transmissive portion and a reflecting portion positioned to periodically, alternately pass and reflect said beams of light;   the other of said elements having a light transmissive portion and a light blocking portion positioned to periodically, alternately pass and block     
     
     
        one of said beams of light. .Iaddend. .Iadd.10.  A spectrophotometer according to claim 9 wherein said spectrophotometer is a double-beam atomic absorption spectrophotometer and wherein said sample beam path passes through an atomized sample substance. .Iaddend. .Iadd.11. An atomic absorption spectrophotometer according to claim 10 wherein said light transmissive portions are cutout portions for allowing the beams of light to pass therethrough.