Hyphenation refers to the process of connecting two or more instruments together for the purpose of increasing the amount of information obtainable from a sample. 

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Hyphenation refers to the process of connecting two or more instruments together for the purpose of increasing the amount of information obtainable from a sample.  Hyphenating thermogravimetric analyzers (TGA) with mass spectrometers (MS) and Fourier Transform Infrared (FTIR) spectrometers has a long history. Zitomer1 was the first to report the hyphenation of a MS to a thermobalance in 1968, while Kiss2 reported the same for an FTIR and thermobalance in 1969.  (For a history of evolved gas techniques, the reader is referred to Thermal Methods of Analysis, W.W. Wendlandt, 2nd edition, 1974.) The ability to combine spectroscopic data for the purpose of chemical analysis, with the mass loss data from the TGA has borne fruit over numerous material science fields.  For example, the pharmaceutical industry routinely monitors for the presence of solvent in their products.  Evolved gas analysis can provide information on degradation pathways, reaction products, and/or chemical composition.

In addition to MS and FTIR, the desire to hyphenate a TGA to GC-MS instruments is also of great interest to users.  But whereas MS and FTIR are continuous techniques (i.e.  they continuously collect spectra during the execution of the TGA experiment), GC-MS is typically not continuous.  On the other hand, GC-MS will give the most complete chemical information about the evolved gas.

Further information on all three techniques can be found in the TA Instruments webinar linked below.  You can also click on the individual links for each technique to learn more about TA Instruments various solutions for hyphenation.

1 Zitomer, F., Anal. Chem., 40, 1091 (1968)
2 Kiss, A.B., Acta Chim. Acad. Sci. Hung., 61, 207 (1969)

Fourier Transform Infrared Spectroscopy, or FTIR, refers to a spectroscopic technique where a sample is irradiated with infrared radiation of a specific wavelength range.  Depending upon the chemical nature of the sample being irradiated, some wavelengths will be absorbed, and some will not, thereby producing an absorption spectrum.  These spectra are typically searchable in a library for the purpose of identification.  The webinar linked to below gives an overview of the technique.

Most benchtop instruments for TGA hyphenation are mid-IR instruments, meaning they operate over a wavenumber range of 40 to 4000 cm-1.  In order for a molecule to absorb IR radiation, it must have an electric dipole moment.  Therefore symmetric molecules, such as nitrogen or oxygen, will not be IR active.

Mass spectrometry (MS) is an analytical technique whereby a gas is ionized and then undergoes fragmentation which can then be separated according to their mass-to-charge ratio and proportionally detected depending upon their abundance. For example, in a MS equipped with an electron ionization (EI source) incoming gaseous molecules interact with electrons in the 40 to 70 eV energy range generated by a heated filament which gives off electrons. The now ionized gaseous species are passed through a mass analyzer, whereby all but the chosen mass-to-charge species are allowed to pass through to the detector. The scanning mass analyzer (a quadrupole in the case of the Discovery II MS) continuously scans over the unit’s mass-to-charge range, typically 1-300 atomic mass units (amu) for benchtop instruments used in tandem with TGA, selectively allowing ions of different masses through to the detector over very short intervals of time. As the species impact the detector, they are counted based upon their abundance, thereby producing multiple spectra as the TGA scans in temperature.

The technique is very sensitive – capable of detection limits in the ppb range, depending upon the gas being analyzed. The webinar linked below gives an overview of the technique.

Gas chromatography-mass spectrometry (GC-MS) refers to an analytical technique whereby the chemical components of a gas are separated by passing through a chemically activated column.  The interior of the column is coated with a stationary phase and the gas components that pass through it show various affinities for this stationary phase.  The more affinity the gas has for the stationary phase, the longer it will stay within the column. After passing through the column and being chemically separated, the gases are ionized and sent through a mass spectrometer for detection.

When coupled with TGA, this technique will give the most chemical information of the techniques discussed.  However, because of the time required for the gas to pass through the column, it is a non-continuous method as opposed to MS and FTIR which are continuous methods.

Multiple hyphenation typically refers to TGA/FTIR/MS and TGA/FTIR/GC-MS setups.  Hyphenating more than one analytical technique to a TGA allows for the maximum data from a single sample and greater flexibility in measurement capability. One can vary the number and type of analytical techniques utilized depending upon the experimental needs.

Concerning TGA/FTIR/MS multiple hyphenation setup, since MS units typically do not require much gaseous sample (units draw about 1 sccm of gas through the capillary), the rest of the effluents exiting the TGA can be easily sent to an FTIR.  Whereas when gas chromatography is introduced as in TGA/FTIR/GC-MS, dedicated hardware is required to ensure efficient flow of gas through the FTIR gas cell and into the GC-MS.  Such hardware is also required for standalone TGA/GC-MS interfacing.

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    Thermogravimetric Analyzers/Fourier Transform Infrared Spec. (TGA/FTIR)Thermogravimetric Analyzers/Mass Spectroscopy (TGA/MS)Thermogravimetric Analyzers/Gas Chromatography-Mass Spec. (TGA/GC-MS)Multiple Hyphenation (TGA/GC-MS/FTIR)

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