Confirmation of Reaction Mechanism Using 13CO2 (GCMS)

A variety of analytical methods are utilized to measure reaction products in research into artificial photosynthesis, in which high-energy substances are manufactured using photo catalysts and solar energy. These methods include GC, HPLC, and absorption photometry. Experiments utilizing isotope-labeled samples are an effective way of confirming and clarifying reaction mechanisms. Mass spectrometers demonstrate their worth in such measurements. This Data Sheet presents an example of the use of GC-MS to measure carbon monoxide, methanol, formaldehyde, formic acid, and other substances generated in carbon dioxide photo catalytic reduction under a methanol environment.

(1) Confirmation of Photo Catalytic Reduction Reaction Products Using ¹³CO₂*

Fig. 1 shows examples of the use of GC-MS to measure carbon monoxide (molecular weight: 28), generated in a carbon dioxide reduction using a photo catalyst. Fig. 1A is the extracted ion chromatogram for CO in experiments using unlabeled CO₂ (methanol solvent). Fig. 1B is the extracted ion chromatogram for CO in experiments using ¹³CO₂ (methanol solvent).

In Fig. 1B, a peak can be confirmed at m/z 29 (¹³CO). However, a peak is likewise confirmed at m/z 28 (CO). From this, it is evident that in this reaction, the carbon monoxide generated from the carbon dioxide is part of the total carbon monoxide. The reaction mechanism for the photo catalytic reaction can be inferred by measuring the formaldehyde, a reaction product of ¹³CH₃OH, and the formic acid, a reaction product of ¹³CO₂ in a similar manner.

Fig. 1: Extracted Ion Chromatograms, Measuring Gas Phase Samples Generated by a Photo Catalytic Reaction

Note: This data was provided by Professor Osamu Ishitani of the Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology. Sekizawa,K; Maeda,K; Domen,K; Koike,K; Ishitani,O. J.Am.Chem.Soc, 2013, 135, 4596. Artificial Z-Scheme Constructed with a Supramolecular Metal Complex and Semiconductor for the Photocatalytic Reduction of CO2

Table 1: Analytical Conditions for Carbon Monoxide (CO)

(2) Analysis of Methanol, Formaldehyde, and Formic Acid in Solution

Fig. 2 shows a sample measurement in which a mixed aqueous solution of methanol, formaldehyde, and formic acid was introduced directly to the GC-MS. A polar column was used for the separation, enabling these three components to be measured in the same analytical run. With these conditions, the ¹³C labeled compound and the non-labeled compound have the same retention time. The mass spectra for each of the ¹³C labeled components is shown in Fig. 3. With the ¹³C labeled samples, since the mass spectra obtained is only shifted by an m/z of 1, the existence of isotopes can be confirmed from the presence of the molecular ion.

Fig. 2: Total Ion Current Chromatogram for a Standard Solution (Unlabeled CH₃OH, HCHO, HCOOH)

Fig. 3: Mass Spectra

 

Table 2: Analytical Conditions for Methanol, Formaldehyde, and Formic Acid

GC-MS	GCMS-QP2010 Ultra
Column	Rtx-WAX ®(Length: 60 m, 0.32 mm I.D., df=0.5 µm, RESTEK)

 

[GC]		[MS]
Injection unit temperature	250 °C	Interface temperature	250 °C
Column oven temperature	70 °C → (10 °C/min) → 240 °C (5 min)	Ion source temperature	200 °C
Injection mode	Split (50:1)	Measurement mode	Scan
Control mode	Linear velocity (40 cm/sec)	Measurement range	m/z 10 to 100
Carrier gas	Helium	Event time	0.3 sec
Injection volume	0.5 µL	Ionization method	EI
		Ionization voltage	70 V
		Emission current	150 µA