Ultrasonic nebulization for the elemental analysis of microgram-level samples with offline aerosol mass spectrometry
Abstract. The elemental composition of organic material in
environmental samples – including atmospheric organic aerosol, dissolved
organic matter, and other complex mixtures – provides insights into their
sources and environmental processing. However, standard analytical
techniques for measuring elemental ratios typically require large sample
sizes (milligrams of material or more). Here we characterize a method for
measuring elemental ratios in environmental samples, requiring only
micrograms of material, using a small-volume nebulizer (SVN). The technique
uses ultrasonic nebulization of samples to generate aerosol particles
(100–300 nm diameter), which are then analyzed using an aerosol mass
spectrometer (AMS). We demonstrate that the technique generates aerosol from
complex organic mixtures with minimal changes to the elemental composition
of the organic material and that quantification is possible using internal
standards (e.g., NH 4 15 NO 3 ). Sample volumes of 2–4  µL 
with total solution concentrations of at least 0.2 g L −1 form sufficient
particle mass for elemental ratio measurement by the AMS, despite only a
small fraction ( ∼  0.1 %) of the sample forming fine
particles after nebulization (with the remainder ending up as larger
droplets). The method was applied to aerosol filter extracts from the field
and laboratory, as well as to the polysaccharide fraction of dissolved
organic matter (DOM) from the North Pacific Ocean. In the case of aerosol
particles, the mass spectra and elemental ratios from the SVN–AMS agree with
those from online AMS sampling. Similarly, for DOM, the elemental ratios
determined from the SVN–AMS agree with those determined using combustion
analysis. The SVN–AMS provides a platform for the rapid quantitative
analysis of the elemental composition of complex organic mixtures and
non-refractory inorganic salts from microgram samples with applications that
include analysis of aerosol extracts and terrestrial, aquatic, and
atmospheric dissolved organic matter.