What is the difference between decomposition and separation

The information can only be obtained with adequate accuracy if the sample is chemically stable during the melting process. Many organic materials, however, start to decompose as soon as melting begins. As a result, the DSC curve exhibits two overlapping thermal events that cannot easily be separated.

Furthermore, decomposition leads to contamination of the sample and in turn to a change in the measured melting point [1]. One possibility to separate melting and decomposition is to measure the sample at a high heating rate. This approach takes advantage of the effect that the temperature of the melting process is not or is only slightly dependent on the heating rate whereas a chemical reaction is shifted to higher temperatures at higher heating rates.

If the heating rate is sufficiently high, it should therefore be possible to shift a decomposition reaction to a temperature high enough to separate the processes of melting and decomposition.

Measurements using conventional DSC, however, show that the technical limit of the DSC is frequently reached before the two processes can in fact be separated.

In both curves, it is apparent that melting and decomposition overlap; the thermal events cannot be correctly evaluated in either of the curves. Much higher heating rates are therefore necessary to separate the two effects.

In this article, Flash DSC 1 measurements are shown in which the melting and decomposition effects of an organic substance are separated and individually analyzed. The sample mass was approximately 4 mg.

Silicone oil was applied to the UFS 1 chip sensor of the Flash DSC 1 on the sample side and on the reference side in order to ensure good contact between the sample and the sensor [3]. Samples with a mass of about 3 ng were positioned on the sensor using a hair. The curves indicate that this heating rate is still too low to separate the two effects. The two curves however show that the melting peak is shifted to higher temperature.

The reason for this is that the decomposition reaction is shifted to higher temperature. The degree of decomposition in the melting range is therefore lower so the sample is less contaminated and melts at a higher temperature. At this heating rate, the melting process is completely separated from the decomposition process. The Flash DSC 1 allows physical effects to be separated if the maximum possible heating rate of 2.

Both effects can then be correctly and quantitatively evaluated. Automated Reactors and In Situ Analysis. The weaker the molecular bond, the more prone the molecule is to degradation. Decomposition is more pronounced at higher temperatures. The longer samples are exposed to high temperatures, the more pronounced the degradation.

That is why one sees more decomposition with hot splitless injection than hot split, because the residence time is much longer. Figure 1 : Endrin can decompose to endrin aldehyde and endrin ketone, both of which elute later. Activity in the sample path can come from many sources.

Metal surfaces can be extremely active because they provide many mechanisms that catalyse encourage, initiate, accelerate decomposition. Glass liners are used in inlets to shield sample vapours from the surrounding metal inlet weldment, yet some of the sample vapours still make it to the metal prior to entering the column. Even the glass surfaces themselves are not inert. Glasses commonly used for inlet liners contain oxides of boron, sodium, aluminium, magnesium, etc.

Even the surface of highly purified quartz pure silicon dioxide is acidic and that can accelerate. Activity is also directly related to the area of the exposed active surface. That is why glass wool can be such a problem for labile compounds, especially if it has many exposed broken ends. Sometimes it is impossible to have a completely inert system; some decomposition is inevitable. Two such banned pesticides that are prone to degradation and rearrangement reactions are endrin and DDT.

Endrin degrades to endrin aldehyde and endrin ketone which, because of molecular rearrangement, are more highly retained elute at higher temperatures than the parent compound, endrin Figure 1.

If degradation exceeds this for degradation check standards, then data cannot be reported and system maintenance must be performed and the samples re-run. Maintenance in this case usually involves replacing the liner and trimming the head of the column, because both can become more active as more samples are run, especially if they are of soil extracts. Figure 3 : Dicumylperoxide thermally decomposes into two much smaller molecules.

This is because the degradation happens in a matter of seconds as the vapours pass through the inlet. Dicumylperoxide can degrade in a hot GC inlet into, among other things, acetophenone and dimethylbenzenemethanol. Essentially, the parent molecule fragments in half Figure 3 , yielding products that elute much earlier than the parent.

B Degradation inside the column starts to happen when some minimum temperature is reached and continues until the parent compound is gone. The broad peak many times wider than typical peak width with lack of parent peak indicates total decomposition within the column. In Figure 4 A , the early-eluting peaks represent the degradation products. The shape of the peaks is sharp.