Using the Agilent Cary 670 FTIR Spectrometer to observe rotational and isotopic bands in CO through high resolution FTIR Spectroscopy

Importance of Topic

Fourier transform infrared (FTIR) spectroscopy at high spectral resolution enables detailed study of rotational–vibrational structure in gas-phase molecules. Precise measurement of isotopic variants of carbon monoxide (CO) is essential in atmospheric monitoring, isotope chemistry and fundamental molecular research. Advances in optical design are required to maintain signal-to-noise performance when using small apertures for high resolution.

Objectives and Study Overview

This study demonstrates the capabilities of the Agilent Cary 670 FTIR spectrometer to resolve rotational and isotopic bands of CO. The goal was to observe weak isotopomer peaks (C13O16, C12O18) alongside the main C12O16 band in a short acquisition time and to quantify wavelength accuracy, precision and spectral resolution.

Used Instrumentation

The following instrument and features were employed:

• Agilent Cary 670 FTIR spectrometer with large 57 mm collection optics
• Retro-reflected, AC-powered infrared source for stable emission and high throughput
• Frictionless air-bearing interferometer with dynamic mirror alignment for velocity control
• MCT narrow-band detector at 25 kHz speed with a 17.4 kHz filter
• Gas cell: KBr windows, 10 cm path length, 4 Torr CO sample

Methodology

Prior to measurement, the instrument was purged with nitrogen. Spectra were recorded using:

• Resolution: 0.09 cm⁻¹, aperture setting 0.10 cm⁻¹
• Sample and background scans: 4 each (≈40 s total acquisition)
• Apodization: boxcar, zero-filling factor: 16, UDR: 2

Data analysis focused on the 2250–2000 cm⁻¹ region to identify P- and R-branch transitions of the C≡O stretch around 2143.3 cm⁻¹.

Key Results and Discussion

Strong rotational structure of C12O16 and weaker isotopomer bands were resolved:

• C12O16 fundamental at 2143.3 cm⁻¹ with well-defined P and R branches
• C13O16 observed at 2095.7 cm⁻¹ and C12O18 at 2092.1 cm⁻¹, matching reduced-mass predictions

Wavelength performance metrics, determined from three replicate scans of R-5, R-6 and R-7 transitions, were:

• Accuracy: <0.001 cm⁻¹ deviation from HITRAN reference
• Precision: standard deviation <0.001 cm⁻¹
• Spectral resolution: full width at half maximum <0.06 cm⁻¹

These results illustrate outstanding signal-to-noise and rapid data collection at high resolution.

Benefits and Practical Applications

The Cary 670 FTIR’s high throughput and stability allow:

• Rapid analysis of trace isotopomers in gas-phase samples
• Accurate wavelength measurement for QA/QC and reference standards
• Environmental monitoring of atmospheric CO isotopic composition

Future Trends and Opportunities

Emerging directions include:

• Integration with real-time data processing and machine-learning algorithms
• Extension to other molecular isotopologues and trace gases
• Field-deployable high-resolution FTIR systems for remote sensing

Conclusion

The Agilent Cary 670 FTIR spectrometer achieves high optical throughput and precise control to resolve rotational-vibrational and isotopic bands of CO with <0.06 cm⁻¹ resolution and <0.001 cm⁻¹ wavelength accuracy and precision in a 40 s acquisition. Its performance supports advanced gas-phase spectroscopy applications.

References

1. Rao KN and Mathews CW, editors. Molecular Spectroscopy: Modern Research. Academic Press, New York, 1972.
2. Rothman LS et al. The HITRAN 2004 molecular spectroscopic database. J Quant Spectrosc Radiat Transfer. 2004;96(2):241–250.