DMA evaluation with cluster standards
1 / FWHM Diff = CV 1/2 ; C = 1.55 Volt − 1/2 (2)
The value of C analogously obtained for the prior Perez DMA model was 1.35 V − 1/2 , indicating a superior performance of the LT version, perhaps as a result of the better laminarization or the larger convergence angle. One can see at all pump settings that the **data** for the lower voltages (smaller clusters) approach best the diffusive limit. Yet, past a certain voltage, the resolution shows a drastic reduction. This reduction cannot be due to turbulence, since each series is obtained at fixed flow rate with only the size of the cluster varying. We believe it is primarily an artificial consequence of the fact that the larger clusters are not completely resolved from each other (top spectrum in Fig. 4 above 0.4 kV), so that either the tails from neighbor cluster peaks or ion impurities between main members of the cluster series create a background that is not accounted for by the Gaussian fitting. The resolutions plotted in Fig. 5 for each Q cannot accordingly be trusted past the voltage at which they fall markedly below the diffusion line, which starts at the voltage at which subsequent peaks are no longer baseline separated from each other. This corruption naturally sets in at smaller voltages for smaller pump speeds, so the upper envelope of the various **data** sets provides a more reliable measure of the real resolution, uncorrupted by peak merging. For instance, the spectrum of Fig. 4 corresponding to a pump speed of 9000 rpm shows isolated peaks for all the clusters included (monomer to octamer), so all the open circles of Fig. 5 ought to give uncorrupted resolutions. The resolutions measured at 9000 rpm for three of the **data** exceeds 37, well above the best value of about 30 observed with the prior Perez DMA. If the zero drift correction had not been implemented, the maximal resolving power would have been 34, still substantially higher than in our earlier study with the shorter laminarization trumpet. The fact that the measured resolution at increasing voltages falls clearly below the diffusive limit indicates that there is some level of non-ideality with this instrument. In other words, while the ideal peak width is 0 for non-diffusing particles in the limit q/Q→0, the actual value is non zero: