TA Tip on the Sample Robot: Change in Moisture Content before Analysis - METTLER TOLEDO

TA Tip on the Sample Robot: Change in Moisture Content before Analysis

Sample changers are often used in thermal analysis when large series of samples have to be measured. The advantage is that the samples can be prepared all together at the same time and the analyses performed overnight or even over the weekend. Especially in lengthy thermogravimetric experiments, the samples ready for measurement on the turntable are however exposed to atmospheric conditions for longer periods. During this time the samples may lose volatile components such as moisture, or conversely, if hygroscopic, absorb moisture. Both processes have a direct influence on the accuracy of the analytical results. There are two ways to prevent such effects from occurring with samples stored on the turntable of the sample robot:

  1. The samples are hermetically sealed in aluminum crucibles and the lid is not pierced until just before the measurement. To do this, the sample robot must be equipped with the crucible lid piercing kit (see Fig. 1). This method is particularly suitable for the determination of moisture, for example in pharmaceutical substances.
  2. The samples are stored in ceramic crucibles with special aluminum lids. The sample robot removes the lid during the measurement. The following experiments performed with polyamide 6 illustrate the effect of sample storage in an open crucible and demonstrate the importance of the crucible lid piercing kit.

 

 

Determination of the Moisture Content of Polyamide 6

Depending on the environmental conditions, polyamide 6 can absorb up to 10% moisture, which of course results in large changes in its physical properties. With material used for clothing, this storage effect may even be desirable. The moisture content of the sample was analyzed thermogravimetrically by performing multiple determinations with the sample robot. Small granules of PA 6 weighing 8 to 10 mg that had been stored at 100% relative humidity were placed in 40-µl aluminum crucibles. The crucibles of 8 samples (Samples A) were hermetically sealed with the special lids (for automatically piercing prior to measurement); 8 other crucibles (Samples B) were sealed with standard lids that had 0.35-mm holes. The A and B samples were analyzed alternately in order to demonstrate the effect of storage time on the sample robot turntable. Each sample was heated from 30 to 230 °C at 5 K/min and the weight loss curve recorded. Figure 2 summarizes and compares the measured curves all together in one diagram

The B samples lose moisture and dry during storage because the lid is effectively open. Only 4.6% moisture was for example measured in the B sample stored for 20 hours. In contrast the hermetically sealed A samples show a constant moisture content of 9.23% (standard deviation 0.11%). The slight differences in the TGA curves of the A samples are due to minor differences in the sample material.

Figure 3 summarizes the results; the measured moisture content of each sample is plotted against storage time on the sample turntable. The drying rate can be estimated from an exponential regression curve over the B samples. For example, about 6% of the moisture originally present in the sample has already been lost after storage for one hour in an open crucible.

Conclusions

By definition, thermogravimetric measurements have to be performed with open crucibles. Samples that undergo weight changes under ambient conditions (e.g. due to changes in moisture content, vaporization of volatile components) must therefore be measured immediately. When series of samples are measured with the sample robot, the special lid piercing kit is very advantageous. It ensures that the samples remain hermetically sealed during storage on the turntable until the measurement is performed. Otherwise weight changes are measured that do not properly represent the original sample.

TA Tip on the Sample Robot: Change in Moisture Content before Analysis |  Thermal Analysis Application No. UC 206 | Application published in METTLER TOLEDO Thermal Analysis UserCom 20