Peak area planimeter method

Planimetry. The planimeter is a mechanical device which enables the peak area to be measured by tracing the perimeter of the peak. The method is slow but can give accurate results with experience in manipulation of the planimeter. Accuracy and precision, however, decrease as peak area diminishes. 

Another requisite is to measure the areas of peaks accurately. In our office, areas of exothermic peaks on DSC charts have been determined for many years by weighing their cutouts. At one time,an attempt was made to use a planimeter, but this method was not used long. In recent years personal computers have come into wide use. The authors have also connected the DSC apparatus directly to a computer which collects and analyzes the data and prints out the results. This system marks a great advance in that peak areas can be measured accurately. 

Basically, there are three approaches to peak area measurement (a) the cut and weigh method, which is accurate to within 5% (b) the planimeter method, which is accurate to within 1% (c) the integrator method, which is more accurate than method (b). It should be noted that when computer-equipped apparatus is used, enthalpy calculations are carried out automatically if the apparatus is correctly calibrated using calibration reference materials. 

Recorder—A recording potentiometer with a full-scale response time of 2 s or less must be used. If a manual method of area measurement, such as a planimeter, is employed, the chart speed must be at least 152 cm/h (60 in./h) to minimize errors in peak area measurements. This requirement is waived if a ball-and-disc integrator or an electronic integrator is employed. 

Two methods used to find the area under the photometer trace are peak-height-times-half-width approximations and actual measurements with a polar planimeter. Both methods are time consuming and offer little increase in total accuracy over the peak center method. Another method involves computer fitting an assumed scattering function, usually a Gaussian or Lorentzian (though more exotic functions have also been used) to the scan data. The integrated area under the mathematical curve is then calculated. 

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