Home made efficient tandem coupling of TG/DTA and MS equipment

In principle, the coupling of these two units is straightforward, a portion of the exhaust gas from the TG/DTA unit is diverted for sampling in the MS quadrupole unit for subsequent m/z analysis.

Depending on the TG/DTA model, it may be possible to find a specific commercial accessory to make this connection between both units. However, this accessory is not always available for a specific TG/DTA model. Usually, this accessory is designed for TG/DTA equipment including a horizontal type furnace or hot chamber with no moving parts attached to this part of the equipment. In this case, the differential thermal balance TG-DTA2000SE (Sirius) from Netzsch is a moving vertical electric furnace that uses a retractable head that allows access to the sample location for measurement analysis. The exhaust is located on the top of this moving head that covers the sample stage.

The differential thermal balance TG-DTA2000SE (Sirius) by Netzsch

There is no coupling accessory for this equipment in order to take in-line samples for the MS quadrupole. Then, a T-shaped connector can be placed in the exhaust output and feed the MS device directly. However, by doing that, there is a remarkable delay between the maximum observed peak of the DTG trace (derivative of the TG trace or mass flow) and the total/partial pressure detected by the MS equipment. The thermal degradation of the standard calcium oxalate material shows especially a large delay of 84 seconds for the peak related to the water portion release.

TG and calculated DTG traces from the TG/DTA equipment and partial pressure measured in the MS equipment. During the test, a 100 ml/min inert gas was used as carrier gas flow.

Despite sampling being done as close as possible to the exhaust of the TG/DTA equipment, this delay could be alleviated if sampling could be done internally close to the sample stage by using a metallic capillar. In this way, the gas volumetric flow for sampling would be instantaneous equal inside the TG/DTA and the fused glass silica capillar from the MS equipment. The scheme below depicts the conventional attachment and the capillar based sampling procedure.

(Left) conventional sampling from T-shaped connector and (right) sampling using inner capillar close to the sample stage. Red and green arrows denotes fast and slow flow, respectively

After that, the problem becomes how to hold this concentric capillar inside the main exhaust line considering that the oven head of TG-DTA2000SE is a mobile part. It could be problematic because the capillar input is placed just 1/2 cm above the sample boats and, therefore, a tight holding between the accessory and system is necessary. The tight holding was solved by designing a helmet for the oven head where external piping, valve and concentric capillar are attached and moving solidary to the oven-head.

Helmet holding exhaust and capilar lines in the head of the movable oven

The improvement minimizing the delay upon calcium oxalate gas releaseases measured in both equipment is remarkably good compared to the conventional sampling strategy.

TG and calculated DTG traces from the TG/DTA equipment and partial pressure measured in the MS equipment using inner capillar for the connection of both equipment.

A blueprint for the helmet design can be downloaded here.

This TG/DTA helmet accesory was used recently for detecting released gas compounds from hybrid halide perovskite.^{1, 2} Please cite one of these articles if you find useful the blueprint of the helmet for coupling your devices.