Patent GB-1245513-A (GB)
Title: Improvements in or relating to spectroscopy
IPC: G01N21/31
Filed: 1969-04-29
Abstract: 1,245,513. Spectroscopic analysis. COMMONWEALTH SCIENTIFIC & INDUSTRIAL RESEARCH ORGANISATION. April 29, 1969 [May 1, 1968], No. 21901/69. Heading G1A. A method of atomic absorption spectroscopic analysis of a sample substance 36 for a particular element comprises introducing the sample substance into a flame 14 and directing radiation from a lamp 10 through the flame to a photodetector 18, the current flow in the lamp being periodically varied between a relatively low level and a relatively high level so that radiation generated during a period of relatively low discharge current passes through residual atomic vapour left in the lamp by the preceding period of relatively high discharge current, thereby causing at least partial absorption of the resonance lines emitted during the period of relatively low discharge current. The concentration of the element is determined by comparing the intensities of resonance radiation received at the photo-detector firstly in the presence, then in the absence, of the sample in the flame 14. As shown, the lamp 10 comprises a pair of electrodes at least one of which consists of the element whose concentration in the sample 36 is to be measured, and the lamp is powered from a supply 23 comprising a D.C. source 24 and a pulsed source 25 controlled by a trigger circuit 26. The resulting power supply to the lamp is represented by Fig. 2b. Each current pulse generates a pulse or cloud of vapour within the lamp, and when the current pulse is terminated the vapour cloud strongly absorb any resonance radiation emitted by the D. C. discharge. However, as the vapour cloud disperses, the emitted intensity of resonance radiation increases to an amount corresponding to the unabsorbed level of the D. C. discharge, so that the resulting intensity variation of emitted resonance radiation is represented by Fig. 2c. In the absence of any sample from the flame 14, Fig. 2c represents the output from the photo-multiplier 18, and this output is fed to a gate 30 which is controlled by the trigger 26 so that the photo-multiplier output signal is blocked during its recurring peaks. The output of gate 30 is therefore represented by Fig. 2d, and this output is fed to a synchronous detector 32 again controlled by the trigger 26. The detector 32 accepts signals only while gate 30 is open, thus helping to eliminate background noise, and its output is represented by Fig. 2e. Either the peak or the average value of this voltage signal is measured by the output circuit 34, and comparison of values of this measured voltage with and without a sample in the flame gives a measure of the amount of the test element in the sample. In further embodiments Figs. 3 and 5 (not shown), the gating of the photo-multiplier is achieved using pulse circuits containing monostable and bi-stable multi-vibrators driven by a variable frequency pulse generator. The sample may be in gaseous or liquid form, contained in a cell, and there may be a broad band absorption or interference filter 16 located between the sample and the photomultiplier which photo-multiplier preferably has its maximum sensitivity at or near the U. V. region of the spectrum.