On-Line Derivatization for Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry:  Detection of Aliphatic Aldehydes and Amines via Reactive Coupling of Aromatic Photo Ionization Labels

Abstract

Resonance-enhanced multiphoton ionization time-of-flight mass spectrometry (REMPI-TOFMS) is a powerful technique for the on-line analysis of aromatic compounds with unique features regarding selectivity and sensitivity. Aliphatic compounds, however, are difficult to address by REMPI due to their unfavorable photo ionization properties. This paper describes the proof of concept for an on-line derivatization approach for converting nonaromatic target analytes into specific, photoionizable aromatic derivatives that are readily detectable by REMPI-TOFMS. A multichannel silicone trap or poly(dimethylsiloxane) (PDMS) open tubular capillary was used as a reaction medium for the derivatization of volatile alkyl aldehydes and alkylamines with aromatic "photoionization labels"and to concentrate the resulting aromatic derivatives. The aldehydes formaldehyde, acetaldehyde, acrolein, and crotonal, which when underivatized are poorly detectable by REMPI, were converted into their easily photoionizable phenylhydrazone derivatives by the on-line reaction with phenylhydrazine as reagent. Similarly, the methyl-, ethyl-, propyl-, and butylamines were converted into their REMPI-ionizable benzaldehyde alkylimine derivatives by the on-line reaction with benzaldehyde as reagent. The derivatives were thermally desorbed from the PDMS matrix and transferred into the REMPI-TOFMS for detection. The REMPI-TOFMS detection limits obtained for acetaldehyde; acrolein; crotonal; and methyl-, ethyl-, propyl-, and butylamine using this photo ionization labeling method were in the sub-parts-per-million range and, thus, readily below the permissible exposure limits set by OSHA.

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“…Given the repeated measures design, we used the 80 th percentile from a one-sided chi-squared distribution (i.e. with γ = 0.2) as the estimate of measurement precision (Limentani et al 2005). Matching this, we used the 80 th percentile of distribution (i.e.…”

Advances in structure elucidation of small molecules using mass spectrometry

“…In the case of GC-MS, the aim is to increase volatility of the compound and protect reactive groups to avoid thermal breakdown or reactions with the column material. For LC-MS studies, derivatizations are performed to improve ionization characteristics for poorly ionizable compounds [444–447]. The obtained products must be hydrolysis stable as for example in the tert -butyldimethylchlorosilane products from a N -methyl- N -[ tert -butyldimethyl-silyl]trifluoroacetimide derivatization [448].…”

Derivatization in Mass Spectrometry—8. Soft Ionization Mass Spectrometry of Small Molecules

“…To detect aliphatic aldehydes and amines by this method, their transformation into easily photoionizable phenylhydrazones and benzaldehyde alkylimines was suggested by reaction with phenylhydrazine and benzaldehyde, respectively. 183 This "photoionization labeling" may be performed in liquid solution or on-line in a poly(dimethylsiloxane) (PDMS) matrix. In the latter case, the analytes from the sample gas as well as the derivatization reagent are co-adsorbed in a PDMS trap, which is heated to accomplish the reaction and further thermally desorbed into the mass spectrometer.…”