Approaching a Universal Pneumatic Nebulizer – the next step

1. Tracey Cyclonic Spray Chamber With Helix SeaSpray Concentric Glass Nebulizer With EzyFit And EzyLok Approaching a Universal Pneumatic Nebulizer – the next step Jerry Dulude and Ron Stux (USA), Vesna Dolic and Peter Liddell (Australia), Glass Expansion (www.geicp.com) What we want to find out What we have already learned Figure 3: At 60 psi, lower uptake rate reduced the background more than at 30 psi Figure 5: Overall, 60 psi and 0.4 mL/min yielded best detection limits Glass Expansion currently designs nebulizers to accommodate spectrometer manufacturers’ recommended gas pressures, gas flows, and sample uptake rates. However, it is desirable to design a single nebulizer that when run under high pressure conditions, generates a finer mist and more of it. In earlier studies (Winter Conference 2006), we compared 60 psi and 30 psi nebulizers for a variety of performance parameters on two different ICP-OES systems. In this study we continue work on the high pressure nebulizer to identify the important characteristics for overall performance improvement. In previous work we studied inter-nebulizer variation to demonstrate the degree with which we can discern differences between designs. We also showed that a 60 psi nebulizer designed to be operated at 2 mL/min produces comparable signal (60 to 140%) when operated at 0.4 mL/min (Figure 2). In general, RSDs were better at 60 psi but detection limit results were somewhat mixed. What we found in this study • In this study, we show that a nebulizer designed to operate at standard (30 psi) pressure consistently produced lower intensity at lower uptake. While this might be expected, it does stand in stark contrast to the results obtained at 60 psi (Figure 2). We also noted these interesting characteristics: • transport efficiency did not appear to depend on nebulizer pressure at a constant (0.7 L/min) argon flow (Table 1). • the background improved much more at lower uptake when using 60 psi than when using 30 psi (Figure 3). • the lowest absolute background was achieved at 60 psi and 0.4 mL/min (Figure 4). • overall, the highest absolute intensity was actually achieved at 60 psi and 0.4 mL/min uptake rate (Figure 2). • 8 of 16 tested elements gave best detection limits at 60 psi and 0.4 mL/min. Of the remaining 8 elements, 7 were less than a factor of 2 worse than the best result (Figure 5). Figure 4: Overall, 60 psi and 0.4 mL/min yielded the lowest blank intensity What we are using DISCUSSION and CONCLUSIONS We used a Tracey™ Cyclonic spray chamber and SeaSpray™ concentric glass nebulizers (Glass Expansion, Inc., Pocasset, MA, shown in Figure 1), a PerkinElmer Optima 2100 with axial view, and standard conditions except for nebulizer pressure and sample uptake, the parameters under test. We were somewhat surprised to find that at constant argon flow, the transport efficiency as measured by weighing the amount of liquid aspirated and the amount collected in the drain did not depend on the argon pressure. This suggests that a nebulizer designed to operate at higher pressure delivers a finer aerosol to the plasma than a nebulizer that is designed to operate at lower pressure. The higher pressure appears to result in a smaller mean droplet size regardless of the uptake rate. As a result, the amount of sample that can be excited in the plasma at 60 psi and 0.4 mL/min is comparable to the amount at 30 psi and 2 mL/min. However, solvent removal efficiency is improved due to the smaller mean droplet size. This also explains why results at 60 psi and 2 mL/min are overall worse than at all other conditions tested. The higher total solvent loading experienced by the plasma as a result of the reduced droplet size leads to greatly elevated backgrounds and higher noise but essentially with no improvement in signal intensity. We believe that this study shows that there are significant benefits to operating at higher nebulizer pressure and lower sample flow. The next step in this series of tests will be to investigate how these performance characteristics affect ICP-MS performance. Figure 1: Sample Intro System Figure 2: Overall, 60 psi and 0.4 mL/min yielded the highest element intensity Table 1: The transport efficiency appears related only to the sample uptake rate at constant Ar flow