GB1455327A - Atomic absorption spectrophotometers - Google Patents

Abstract

1455327 Absorption spectrometers BODENSEEWERK PERKIN-ELMER & CO GmbH 24 Jan 1974 [25 Jan 1973] 03340/74 Heading G1A In an atomic absorption spectrometer, light 10, Fig. 1, from a first source is passed along a measuring path 54 through the sample (e.g. a flame) or along a reference path 60 not including the sample, in dependence on the position of a chopper 18, so that measurements of absorption can be compensated for source brightness variation. The first source has a line spectrum and is chosen such that the sample under test absorbs a line of the radiation To compensate for background absorption by the sample light 20 from a second source, which emits a continuous spectrum, is also passed along paths 54 and 60 by the chopper. The radiation 20 is derived from the second source via a monochromator (not shown) which passes a narrow band of the continuous spectrum, this band including the frequency of the line from the first source which is absorbed by the sample. The chopper is shown in Fig. 1 and 2, and has two portions which are rotated together on axle 28. The first portion has two quadrant mirrors 32, 34 which are disposed on opposite faces of support 26. The rear faces of the mirrors are non-reflective, and each mirror also has a non-reflective portion on its front face, shown at 40, 42 in Fig. 2, to provide dark signals. The second chopper portion is a mask having apertures 44 and 50 in an otherwise non-transmissive disc 30. As the chopper rotates (anti-clockwise in Fig. 2) it controls the following sequence: (a) light 10 goes to path 54, (b) light 10 goes to path 60, (c) light 20 goes to path 60, (d) light 20 goes to path 54. Fig. 3 shows the circuitry. A hollow cathode source HC emits light 10 via flame 66 on path 54, and also along path 60, while a continuous spectrum deuterium lamp gives light 20 via the flame on path 54, and also on path 60. The signals from the detector (not shown) pass to switches 70, 76 controlled in phase with the chopper. During periods (c) and (d) above, the detector output of path 54 is amplified at 68, and is fed via a controlled switch 74 to 72 where it is compared with the reference path to set the gain of amplifier 68; this compensates for background absorption. During periods (a) and (b) the detector signals pass to log amplifier 78 in turn where the reference path is compared with the measuring path to form a ratio so that source brightners variations are removed and absorption due to the sample absorption line alone is measured.