Patent GB-1431207-A (GB)
Title: Methods of analyzing line spectra
IPC: G01N23/205
Filed: 1973-06-19
Abstract: 1431207 Analysing line spectra INTERNATIONAL BUSINESS MACHINES CORP 19 June 1973 [26 June 1972] 28925/73 Heading G4A A method of analysing the line spectra of powdered crystalline specimens represented in groups in a data store as intensity representations of the density distribution of the specimens, each group including observed data relating to a standard specimen and known data relating to the same standard specimen, comprises (1) retrieving from the store one of the groups and testing the data for run variations to locate data relating to spectral lines, (2) generating a correction equation based on a comparison of the data of the standard specimen as measured with its actual known representation, (3) applying this correction equation to the data for other spectral lines of the group to obtain an indication to the location, intensity and peak region of those spectral lines. As described the stored data is derived using an X-ray generator (11, Fig. 1, not shown) and a Guinier-De Wolff camera (13) accepting four samples (14) mounted, e.g. by dry pressing, in a holder, one of the samples being a standard specimen such as silicon powder. The diffracted X-rays are incident on X-ray films (16), the resulting exposed film containing four independent line spectra being scanned by a step scan digital microdensitometer, the outputs from which are fed to nine track magnetic tape (19) for storage as eight bit bytes with an odd parity bit. A zero angle marker is provided for each sample and the microdensitometer may include a binary coded decimal counter to indicate the departure in film position from the zero marker, film density being converted to binary form using a logarithmic amplifier and an analogue/ digital converter. Processing of the recorded data.-The data is converted from binary form to a usable form and stored on an auxiliary disc work file. The file relating to the known spectra is first analysed. The zero position is ascertained and all subsequent intensity positions are scaled with respect to this zero. All regions containing lines are extracted (see below) and the data for each line is smoothed in both the forward and backward directions by setting its intensity to the average of the immediately preceding and immediately following data values. The centroid position within the area is determined by scanning the product of position and intensity readings and dividing it by the sum of the intensity readings. By comparison with the normal positions of the standard sample measured lines of the standard closest to the known lines are derived. A check is then made as to whether a more intense line exists in a window surrounding this line and if so it is used to derive constants A0 and A1 in a correction equation of the form y=A0+A1x where y is an actual value for the standard sample and x is the observed value. The straight line for the best fit using the least squares technique is derived using five of the lines of the standard known sample. The files for the other three specimens are then processed and the correction equation is applied to the measured spectral lines. The resulting output data is then compared with a master file to determine the compounds which give rise to the signals. Sensing of a region containing a line.-The centre of the zero marker is first found by stepping a position pointer through the first hundred positions of the data relating to density intensity until an intensity reading of ten or less is found. This is assumed to be the blank area between the zero marker and the region containing lines and background. The position of the position pointer between the beginning of the data and this low intensity position having maximum intensity is assumed to be the centre of the zero marker. From this position the position pointer is incremented so that the data points are sequentially examined with an ascending run counter being incremented as the position pointer is stepped along each time a new intensity value exceeds the preceding one. If the intensity value then starts to decrease, if the count in the ascending run counter exceeds a predetermined level, a peak representing a spectral line is presumed to have been encountered. Consequently a line counter is incremented and the starting position for the peak is the position pointer value before the run starting location. The value for this location and the intensity value of the peak are both stored. A descending run counter is then incremented for each intensity value less than the previous one. When the intensity value becomes lower than that of the start of the peak the associated pointer position is stored as the end of the peak provided the descending run counter has reached a predetermined count. The system then processes the next peak to determine the next spectral line position. If the intensity values start to increase before the level of the start of the peak is reached this value corresponds to the starting position for a secondary peak and if a predetermined number of increasing intensity levels are encountered, peak ending position to the first peak is stored as the position before the start of the run corresponding to the secondary peak and processing for the secondary peak is restarted at this position. When the position pointer has a value greater than the last data position sensing of the line is stopped.