depends on the modulation frequency.
When helium is used in the sample chamber, the modulation frequency range of the MTEC model 200 extends from below 1 Hz to nearly 10 kHz. The use of air results in a high frequency cut-off at approximately 3 kHz. Measurements at higher frequencies in air are difficult due to a substantial loss in sensitivity after the detector goes through its first Helmholtz resonance.13 This type of resonance is due to gas oscillations in the tube between the sample and microphone chambers. Acoustic resonances in the sample chamber itself do not occur because the chamber dimensions are too small when a sample is in place.
Helium purging of the sample and microphone volumes enhances sensitivity by a factor of 2 to 3, allows higher frequency operation, and removes moisture and CO₂. Moisture and CO₂ cause spectral interference as well as photoacoustic signal generation interference. The latter interference is caused by a phase difference between photoacoustic signals generated by absorption in gases or vapors versus solids. The phase shift of gas or vapor signals leads to increased noise in spectra.
Many samples such as coal evolve water vapor after being sealed in the sample chamber. In such cases, a cup of desiccant is placed in the sample holder beneath the sample cup. Magnesium perchlorate is an excellent desiccant for this purpose and typically can be used for a day of operation without renewal.
It is important to note that moisture and CO₂ bands in spectra can be due to the presence of vapor and gas in both the FTIR optical path and the photoacoustic detector. The source location can be identified by recognizing that contamination in the FTIR causes negatively pointing (transmission-like) moisture and CO₂ bands in FTIR-PAS spectra, whereas bands are positively pointing (absorbance-like) if contamination is in the detector itself. These observations should be used as a guide in purge and desiccant operations.
A final instrumental consideration is provision for normalizing spectra to account for spectral variations in the FTIR source and optics, and for any sensitivity changes that may occur from day to day due to changes in source intensity or optical alignment. Normalization is performed by computing the ratio of the sample spectrum to a carbon black spectrum. The latter spectrum is best obtained with a MTEC reference standard consisting of an absorber element with a stable carbon black coating that is permanently mounted and protected in a dedicated sample holder. Loose carbon black is not a good standard for general use because its signal intensity varies as the powder settles and it is easily spilled or blown out of the sample cup. In some instances, a glassy carbon or graphite standard is desirable (See Section V.E.).
Many FTIR data systems erroneously label the normalized PAS spectrum as a "transmittance" spectrum rather than an absorbance spectrum. The FTIR data system should be commanded to change the label to absorbance prior to processing the data because many computer processing routines will either not operate or will produce incorrect results if a spectral file carries the wrong ordinate axis designation.