Ultrasonic nebulization for the elemental analysis of microgram-level samples with offline aerosol mass spectrometry

<p>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&thinsp;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., <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msubsup><mi mathvariant="normal">NH</mi><mn mathvariant="normal">4</mn><mn mathvariant="normal">15</mn></msubsup><msub><mi mathvariant="normal">NO</mi><mn mathvariant="normal">3</mn></msub></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="48pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="233606c5f2e3a1b80f616614fce08fc7"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="amt-12-1659-2019-ie00001.svg" width="48pt" height="17pt" src="amt-12-1659-2019-ie00001.png"/></svg:svg></span></span>). Sample volumes of 2–4&thinsp;<span class="inline-formula">µL</span> with total solution concentrations of at least 0.2&thinsp;g&thinsp;L<span class="inline-formula">⁻¹</span> form sufficient particle mass for elemental ratio measurement by the AMS, despite only a small fraction (<span class="inline-formula">∼</span>&thinsp;0.1&thinsp;%) 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.</p>