Formaldehyde Analysis in Non-Aqueous Methanol Solutions by Infrared Spectroscopy and Electrospray Ionization

Abstract

We present the analysis of formaldehyde (HCHO) in anhydrous methanol (CH₃OH) as a case study to quantify HCHO in non-aqueous samples. At higher concentrations (C > 0.07 M), we detect a product of HCHO, methoxy methanol (MM, CH₃OCH₂OH), by Fourier transform infrared spectroscopy, FTIR. Formaldehyde reacts with CH₃OH, CD₃OH, and CD₃OD as shown by FTIR with a characteristic spectral feature around 1,195 cm^-1 for CH₃OH used for the qualitative detection of MM, a formaldehyde derivative in neat methanol. Ab initio calculations support this assignment. The extinction coefficient for 1,195 cm^-1 is in the order of 1.4 × 10² M^-1cm^-1, which makes the detection limit by FTIR in the order of 0.07 M. For lower concentrations, we performed the quantitative analysis of non-aqueous samples by derivatization with dinitrophenylhydrazine (DNPH). The derivatization uses an aqueous H₂SO₄ solution to yield the formaldehyde derivatized hydrazone. Ba(OH)₂ removes sulfate ions from the derivatized samples and a final extraction with isobutyl acetate to yield a 1:1 methanol: isobutyl acetate solvent for injection for electrospray ionization (ESI). The ESI analysis gave a linear calibration curve for concentrations from 10 to 200 µM with a time-of-flight analyzer (TOF). The detection and quantification limits are 7.8 and 26 μM, respectively, for a linear correlation with R ² > 0.99. We propose that the formaldehyde in CH₃OH is in equilibrium with the MM species, without evidence of HCHO in solution. In the presence of water, the peaks for MM become less resolved, as expected from the well-known equilibria of HCHO that favors the formation of methylene glycol and polymeric species. Our results show that HCHO, in methanol does not exist in the aldehyde form as the main chemical species. Still, HCHO is in equilibrium between the production of MM and the formation of hydrated species in the presence of water. We demonstrate the ESI-MS analysis of HCHO from a non-aqueous TiO₂ suspension in methanol. Detection of HCHO after illumination of the colloid indicates that methanol photooxidation yields formaldehyde in equilibrium with the solvent.

Keywords: ESI-TOF; hemiformal; methoxy monoglycol; methoxylated methylene glycol; methoxymetanol.

FIGURE 1

Flow diagram for formaldehyde derivatization for ESI-TOF MS analysis.

FIGURE 2

FTIR spectrum of methoxymethanol prepared by dissolving formaldehyde in anhydrous methanol CH₃OH. Pathlength 153 μm determined by interferometry.

FIGURE 3

Effect of deuterium on the FTIR spectrum of MM. Methoxymethanol prepared by dissolving formaldehyde in (−− −, black) anhydrous CH₃OH, in (⋅⋅⋅⋅, red) CD₃OD, and in (---, blue) in CD₃OH. The spectra were normalized to the strongest peak and corrected by the background of CH3OH, CD₃OH, and CD₃OD.

FIGURE 4

ESI spectra and calibration curve for formaldehyde in methanol after derivatization to formaldehyde 2, 4-Dinitrophenyl hydrazone. (A) Spectrum for a derivatized 150 µM formaldehyde sample in MeOH containing DNPH, m/z = 197 and FDH, m/z = 209.01. (B) Calibration curve obtained for formaldehyde in methanol after derivatization plotting the intensity of the parent ion of FDH, m/z = 209. (C) Spectrum for 2, 4-DNPH (standard). (D) Spectrum for FDH (standard).

FIGURE 5

Formaldehyde derivatization reaction used in this work. We modified the method to enable analysis from non-aqueous methanol because the aqueous acid solution complicates the separation of the formaldehyde 2, 4-dinitrophenyl hydrazone (FDH).

FIGURE 6

(A) Mass spectra for the formaldehyde 2, 4-Dinitrophenyl hydrazone region obtained for different solutions. (B) Histogram showing the abundance of formaldehyde 2, 4-Dinitrophenyl hydrazone (m/z = 209) obtained for different solutions.

FIGURE 7

Proposed equilibria for the analysis of formaldehyde. In anhydrous methanol solutions, we demonstrate the formation of MM by FTIR reaction I) and can be used to prove the presence of HCHO qualitatively. In aqueous solutions, MM formation competes with the formation of methylene glycol and subsequent species II) and the derivatization reaction III). In acid aqueous media reaction III) ultimately yields the derivatized FDH used for ESI analysis.