Why Choose TGA-GCMS Over Other Options for Evolved Gas Analysis?

Interview conducted by Stuart MilneNov 29 2016

Noah Menard, Laboratory Manager for Veritas Testing and Consulting, talks to AZoM about the new TGA-IST16-GC/MS system from Mettler Toledo and how it compares to other options for evolved gas analysis.

What are some of the key features you look for in a TGA?

The most important part of a TGA is the balance. This needs to be highly accurate and sensitive to identify alterations in mass as the entire point of a TGA is to identify weight changes. By accurately determining the change in weight, a user can then have a good idea of the composition of the sample and how each part is affected by the change in temperature or time. To be sure of what those components are, we need to use an EGA technique and work backwards from the offgas. Many evolved gas applications look at components that are coming off at a certain temperature.

In addition to an accurate and high level of sensitivity, the ability to hold a large sample amount is also a significant factor. As with all of the evolved gas analysis techniques, the more sample you are able to load into the TGA results in stronger signals for the corresponding hyphenated technique, especially when looking at events that are a fraction of a percent. Gas flow through the furnace and temperature control are also important.

What is a Hyphenated Technique

A hyphenated technique involves the coupling of one or more instruments together in order to obtain a better picture of the material properties. It gets its name from the hyphen used in designating it – for example TG-IR for TGA coupled to FTIR. In some pharmaceutical areas, this can also be called an orthogonal technique. The majority of these techniques generally involve some form of intermediate interface to regulate material transfer; these can be simple heated lines up to highly precise systems with regulated gas flow, temperature and sampling devices. Evolved gas analysis, which looks at the outgassing components of material, is a prime example and involves methods such as TGA-GC/MS, TGA-MS, and TGA-IR. Other techniques such as Photo-DSC, RAMAN-DSC, UV-DMA, or even GC/MS are also considered hyphenated techniques.

How does TGA-GCMS compare with TGA-IR and TGA-MS?  Why would you choose one technique over the other?

There are different advantages to each of the evolved gas techniques. The three most common involve connecting to a FTIR, a MS, and/or a GCMS. TGA-FTIR allows for the analysis of functional groups and isomers of molecules, but is somewhat limited for a few reasons. The wavelengths measured by a standard IR may not be enough for an adequate identification. Another issue is the possible masking of components when CO₂ and H₂O comes off of the sample within the TGA. Out of the EGA techniques, IR is the least sensitive and requires more sample due to the lower detection limits of IR. While IR vapor libraries exist, they are not as extensive as those of a GC/MS. This is probably the most common technique due to simplicity and cost.

TGA-MS allows you to look at mass ions and fragments. This can be very useful when you have an idea of what you are looking for or for simple outgassing issues, but can become difficult when there are multiple overlapping events or a large amount of sample released at one time. Libraries exist but are somewhat limited. It is certainly more sensitive than the FTIR method, but you have to be concerned about the amu limit of the instrument and flooding the sensor. This is a great technique for gases like CO₂, water, and very light materials.

Analysis of commercial almond flavored electronic cigarette solution as an example of the ability of TGA-GC/MS to identify components in complex matrices. Separating the propylene glycol/glycerin solution of flavorings and nicotine makes a good model system for difficult samples.

GC/MS alone is one of the most sensitive and commonly used techniques due to its ability to separate out components from complex materials. This is what makes it a workhorse in analytical laboratories. When paired with a TGA, you are able to then analyze each weight loss event while being able to prevent sample overload by different injection methods, as well as taking advantage of the high instrument sensitivity. Each segment can be analyzed individually for a further understanding of the material composition. Since GC/MS is used so universally, there are extensive libraries that can identify each component with little user interaction. When you are looking at a fraction of a percent weight loss in a TGA, a GC/MS coupling excels as it can easily detect and separate the released gas.

How does the new TGA/DSC3+ - IST16 - GC/MS compare to other options for evolved gas analysis (such as TGA-FTIR and TGA-MS)?

TGA-IR and TGA-MS both do not sample, but run the evolved gas mixture from the TGA directly. Only when you start using chromatographic techniques does sampling become an option, but older approaches only used 1 or 2 samples per run. Because the strength of TGA-GC/MS is the ability to separate components, more samples help resolve the mixtures. The Mettler-Toledo system, which couples a TGA/DSC3 to an IST 16 to a GCMS allows collecting of 16 independent samples from the TGA run. On a normal run where we are looking for components under 600 C, we can sample every 40 degrees. Of course, we can also concentrate our sampling in a region of interest to get even more information.

While TGA-IR and TGA-MS do allow for real time scanning, it is a continual stream of gas flow which does not allow targeted sampling. As there may be large amounts of molecular fragments being transferred, masking of the signal due to the earlier evolved gas flooding the system may occur. With the IST system taking designated sampling locations, this problem can be averted. This also helps with liquids and other similar materials that greatly expand when evaporation, as the sample loop holds a small amount; when run through a GC-MS split/splitless mode the risk of flooding of the detectors becomes even easier to avoid.

What is unique about the SRA storage interface that sits between the METTLER TOLEDO TGA/DSC3+ and the Agilent GC/MS?

The first unique capability is the ability to store up to 16 gas samples in the internal loops. Many of these tests have more than one location of interest, so it fits well with most laboratory requirements. Some models that facilitate evolved gas analysis testing run a continual stream of gas from the TGA, which can cause the user to need to repeat the run more than once to obtain all the data required for a full analysis. At this point, it may become harder to identify which component is coming off at a location when there are multiple weight loss events occurring. As smaller molecules will move through the GC/MS faster, a cluster of higher mass molecules may form giving an inexact idea of what came off at each location.

In addition, these can tie up the TGA until the entire process is complete and need continuous user interaction, which ties up personnel and decreases laboratory throughput. Another advantage is after the TGA run the IST system can be disconnected from the TGA. The IST system can coordinate the sample injections and let the gas analysis continue automatically without any further interaction until all sample loops have been analyzed, while the TGA can then be used for other projects. Depending on the sample requirements, the transfer line temperatures and storage chamber temperatures can be controlled through the software. In addition, the user has the ability to make the collection loop trigger via the STARe software, or take samples through the interface by inputting at what time to take the sample.

What is the advantage of storing evolved gas in up to 16 loops?

The IST-16 system allows us to take several measurements during the course of a TGA run. For some materials there may be several weight loss events during the course of heating. With sixteen loops we can take samples from all the different locations of interest in a single run, allowing greater confidence in the results as well as determine how the molecule fragments during decomposition. For an example, taking several samples during a single weight loss event can show what fragments come off first and the decomposition process based on the progression through the weight loss.

Is there any risk of sample transformation during storage step?

The majority of the changes occur in the TGA during the decomposition or outgassing of the material. If it is a solvent, the system will easily be able to store the sample without changes. As you begin to break down the sample, the fragments often come off in discrete segments that are already stable. Even if these fragments could decay over time, it is very unlikely that the time in the loop is long enough to induce significant changes.

Is it possible to analyze high boiling compounds with IST16 (bp > 200°C)?

This is not a problem in the system, as the heated chamber allows the sample to remain in gas form while awaiting injection into the GC. The GC parameters can be optimized for that analysis while the IST loops hold the samples.

Is it possible to perform more than 16 injections in the GC column from one TGA experiment?

Well, it depends.  One approach is to use the multi-injection mode, which can be used to collect and inject samples in a short amount of time during the TGA run. This sends pulses of gas into the GC at a user set rate and period of time. There is a slight loss in precision of result, but it is offset by the ability to take several more samples than normal.

Can you tell us about a success story where the instrument has been used to solve a particularly difficult problem?

While there are several, two stand out. One application was for an issue with piping in a chemical plant. Certain pipes were turning brittle and cracking after a certain time within the plant. This was localized to only one plant, and the problem only appeared intermittently. While DSC testing indicated some difference between fresh material, known good material, and corrupted material replication of the problem proved elusive. We were able to rule out thermally induced degradation, and evidence pointed towards chemical degradation. As it was a chemical plant, we began to wonder if there were leaks or release of solvent vapours as direct sample soaking in solvents appeared to degrade the material well past the actual degraded sample. Since these problems took several months to years to appear, there was no real way to directly measure what occurred. In this case, we were able to heat the tubing to degradation and analyze the gas composition. From this we were able to determine that a solvent by-product produced in the plant was being released into the atmosphere in small amounts intermittently, and collecting in the tubing which in turn caused the cracking and degradation observed.

IST16 principle

Another example was in pharmaceutical manufacturing. In many cases, there are several solvents used in the creation of medicines, and improper removal can cause residual solvents to remain in the final product. This is obviously a sub-optimal situation for the manufacturer as the FDA takes a dim view of this. We found that when heated in the TGA, we could identify not one, but four solvents associated with the weight loss. From that, we could tell the client where in the manufacturing process the issue was occurring. Other techniques may miss these because of the low levels of each solvent or because of overlapping fragmentation patterns in TG-MS.

What was the value of knowing the solution to the customer who requested the data?

Most importantly it indicated possible health and safety issues for workers within the plant. In addition it was an indication that processes had to be re-evaluated as it was causing plant infrastructure problems. While the identification of the problem had some short term benefits, the customer was able to implement changes that would prevent future problems, which have the possibility of becoming progressively worse as time passed. As these were a high throughput plants, even partial or temporary shutdowns could cause problems.

How easy is the system to operate for someone who is not an expert in chromatograph and thermal analysis?

All of the systems are user friendly, and tie together very well due to the level of automation. From the TGA software we can designate time based locations for sampling, which then sends a signal to the IST system to take a sample at the chosen time. From that, the IST system ties automatically into a user defined GC sequence based on the number of active sample loops. The IST system sends a signal to the GC to initiate the method; when the method is done and reset the IST-16 will then trigger the next sequence until all of the loops have been discharged and analysed. Out of the entire process the hardest step may be determining your GC method for proper compound separation. However, in many cases, a general temperature ramp in the GC is sufficient. With the continued development in modern GC-MS software, this is also easy to apply to the desired amount of loops.

About Noah Menard

Noah has over a decade of experience in hyphenated techniques and thermal analysis.

He is the laboratory manager for Veritas Testing and Consulting, where he oversees lab operations.

In addition to TGA-GC/MS, Veritas Testing and Consulting has a range of thermal and rheological equipment (DSC, TGA, TMA, and DMA) along with other hyphenated techniques such as TGA-IR, Photo-DSC, and Photo-DMA.