Characterization of the planar differential mobility analyzer (DMA P5): resolving power, transmission efficiency and its application to atmospheric relevant cluster measurements

The planar differential mobility analyzer (DMA), functioning as a particle sizer, exhibits superior transmission and selection accuracy at ambient pressure relative to its cylindrical counterparts. It also presents integration potential with atmospheric pressure interface mass spectrometry (API-MS) for enhanced cluster detection with an additional ion mobility dimension. In this study, the performance of a commercially available planar DMA (DMA P5) was evaluated. The device is capable of sizing particles below 3.9 nm, with larger sizes measurable through a sheath gas flow restrictor. The resolving power was appraised under various recirculation arrangements, including suction and counterflow modes along with different sheath flow rates, using electrosprayed tetra-alkyl ammonium salts. The peak resolving powers for tetrahexylammonium (THAinline-formula⁺) achieved in suction and counterflow modes were 61.6 and 84.6, respectively. The DMA P5 offers a sizing resolution that is 5 to 16 times greater than that of cylindrical DMAs. Resolving power displayed a near-linear relationship with the square root of the applied voltage (inline-formula $M2inlinescrollmathml\sqrt{V_{\mathrm{normal\; DMA}}})$ 39pt15ptsvg-formulamathimg80007ad383e6b71675eacd78e3538eb7 amt-16-5995-2023-ie00001.svg39pt15ptamt-16-5995-2023-ie00001.png in counterflow mode. Conversely, the resolving power for THAinline-formula⁺ ceased its linear enhancement with inline-formula $M4inlinescrollmathml\sqrt{V_{\mathrm{normal\; DMA}}}$ 36pt15ptsvg-formulamathimg0cf914a8eedbb77c756e9c3b22f14657 amt-16-5995-2023-ie00002.svg36pt15ptamt-16-5995-2023-ie00002.png beyond a inline-formulaV_DMA of 3554.3 V, entering a plateau which is ascribed to the perturbations in sample flow impacting the laminar nature of sheath flow. The DMA P5 transmission efficiency reaches 54.3 %, markedly surpassing that of conventional DMAs by nearly 1 order of magnitude. Moreover, the mobility spectrum of various electrosprayed tetra-alkyl ammonium salts and the mass-to-charge versus mobility 2D spectrum of sulfuric acid clusters were characterized using the DMA P5 MS system.