A stepwise surface ionization method for ion mobility spectrometry.

Rationale: The detection of organic nitrogen compounds in exhaled breath is expected to provide an early warning of diseases such as kidney disease. Detecting these trace disease markers in exhaled breath with complex composition and high moisture content is a challenge. Surface ionization (SI) shows a highly selective ionization of organic nitrogen compounds, and it is a good candidate for breath analysis combined with ion mobility spectrometry (IMS).

Methods: A stepwise SI method of low-temperature adsorption/high-temperature ionization was proposed, and trimethylamine (TMA) was detected when combined with an ion mobility spectrometer. TMA at different concentrations and humidity levels and spiked in human breath was detected to evaluate the method's properties.

Results: TMA with concentrations from 2 to 200 ppb was detected. The peak intensity of the TMA characteristic ions was linearly related to the "e" exponent of the concentration with a curve fit of 0.996. A standard deviation of less than 0.306% was obtained with 10 replicate analyses of 10 ppb TMA. The signal intensity difference between dry and wet (relative humidity > 93%) TMA samples is only 2.7%, and the recovery rate of the sample was 106.819%.

Conclusions: SI-IMS based on the stepwise SI method has the advantages of low ionization temperature, high detection sensitivity, strong resistance to humidity interference, and good repeatability. It is a promising method for detecting organic nitrogen compounds in exhaled breath.