Aerosol transport efficiency

Most ultrasonic nebulizers use a somewhat larger sample uptake rate (2-3 mL/min) than pneumatic nebulizers. Typically the spray chamber and/or a tube following the spray chamber is heated to evaporate water partially from the aerosol. Because the aerosol transport efficiency is higher when an ultrasonic nebulizer is used, particularly with a heated spray chamber, a system to remove solvent (typically a condenser and/or membrane separator) is essential to prevent deleterious cooling of the ICP by excess water. 

Spray Chambers and Desolvation Systems. A nebulizer must produce droplets less than 10 /im in diameter in order to achieve a high aerosol transport efficiency (the percentage of the mass of nebulized solution that reaches the plasma), and rapid desolvation, volatilization, and atomization of the aerosol droplets. Pneumatic nebulizers, especially, produce highly poly dispersive aerosols with droplets up to 100 jwm in diameter and these large droplets must be removed by a spray chamber. 

Delivery of sample at a rate of 1 mL/min yields an analyte transport efficiency of about 20%. By reducing the delivery rate of sample to the transducer, high transport efficiencies can be obtained. Using a sample delivery rate of 5-20 iJiL/min, an aerosol transport efficiency of close to 100% can be obtained. Between samples, the surface of the transducer is flushed with large quantities of water to remove all traces of the previous sample. 

Sensitivity and detection limits of ICP-MS are governed by the absolute amount of analytes introduced to the plasma per time unit. Hence, sample transport efficiency of the ICP-MS introduction system will critically affect detection limits in CE-ICP-MS. A general drawback of CE is that concentration-based detection limits are limited by the small sample injection volumes and the electrophoretic peak width. Interfaces employing nebulisers in combination with spray chambers yield analyte transport efficiencies of < 100%, depending on the nebuliser and solution flow rate. Consequently, the sensitivity of CE-ICP-MS can be improved by using introduction systems with 100% aerosol transport efficiency, such as the direct injection nebuliser and the direct injection high-efficiency nebuliser. ... 

The introduction of hydrides into plasma-based instmmentation has also been achieved. The sensitivity increases markedly when compared with conventional nebulization because of the improved transport efficiency of the analyte to the atom cell (close to 100%). Often, a membrane gas-liquid separator is usee ensure that aerosol droplets of liquid do not reach the plasma. 

The concerns addressed in our feasibility studies of He-jet activity transport were 1) Will the He-jet technique work at the beam intensities that exist at LAMPF 2) What transport efficiencies can be expected for both fission and spallation products 3) What is the time dependence of the activity transported and 4) What aerosols and/or aerosol conditions are optimum ... 

Desolvation systems can provide three potential advantages for ICP-MS higher analyte transport efficiencies, reduced molecular oxide ion signals, and reduced solvent loading of the plasma. Two different approaches have been used for desolvation in ICP-MS. The heated spray chamber/condenser combination has been discussed it is the most commonly used system. The extent of evaporation of the solvent from the aerosol and cooling to reduce vapor loading varies from system to system. The second approach is the use of a membrane separator to remove solvent vapor before it enters the ICP. 

Heated Spray Chambers. The use of a heated spray chamber to evaporate the aerosol partially leads to reduction in drop size and therefore higher analyte transport efficiencies. Often the drying of the aerosol droplets is incomplete. 

Direct Sample Insertion. In direct sample insertion (DSI) [82], the sample is placed on a rod, metal loop, or cup on a rod. After desolvation (by inductive heating of the rod or use of a heat gun), the sample is inserted into the plasma. The advantages of the DSI system include nearly 100% sample transport efficiency into the ICP and use of a single power source. The most exciting capability of DSI is preconcentration using aerosol deposition that can provide two orders of magnitude of improvement in ICP-MS detection limits [83]. Detection limits as low as 0.06 parts per trillion were obtained. 

Many experimental parameters and components affect sensitivity, including the analyte transport efficiency of the sample introduction system and the mean size and size distribution of the aerosol entering the ICP. The plasma torch design, rf generator, load coil, interface between the atmospheric pressure ICP and mass spectrometer, ion optics, mass spectrometer itself, and detector also affect sensitivity. 

Over the years many analytical spectroscopists have attempted to improve upon this situation, but the only reliable way to improve transport efficiency with pneumatic nebulizers is apparently to restrict the aspiration rate.17,18 Reduced aspiration rate means that the nebulizer energy is distributed to less aerosol per unit time, resulting in a finer droplet size distribution finer droplets (e.g. < 2 pm in diameter) are more likely to be transported through the spray chamber. Alternatively, the determinant may be introduced to the flame in gaseous form, or in a small cup. Such approaches are discussed in Chapter 6. However often the approach taken is to use electrothermal atomization rather than a flame,6,19 but this is outwith the scope of the present small volume. 

Some organic solvents, for example isobutyl methyl ketone (4-methylpen-tan-2-one) and ethyl acetate, produce particularly fine aerosol and very high transport efficiency as a consequence.8 Such solvents are therefore particularly useful for solvent extraction where very low detection limits are required. 

The large transport efficiencies through the capillary for the aerosol gas-jet technique can be explained in terms of the laminar flow profile of the gas inside the capillary [32], According to Bernoulli s law, the gas pressure at the center of the capillary, where the velocity is highest, is lower than near the capillary walls. Thus, when the sub pm-sized aerosol particles drift away from the center of the capillary, they are subject to a restoring force toward the center of the capillary. Transport efficiencies of over 50% have routinely been achieved for transport capillary lengths over 20 meters. 

For many years pneumatic nebulizers of the V-groove, Meinhard and cross-flow type have been the most widely used sample insertion devices for aerosol generation. The interaction geometry between the gas and liquid sample streams allows pneumatic nebulizers to be classified into two major groups, namely (a) pneumatic concentric nebulizers, which involve concentric interaction and (b) cross-flow nebulizers, which involve perpendicular interaction between the liquid and gas streams. Pneumatic nebulizers are well established and widely used on account of their simplicity, robustness, ease of use and low cost however, they provide low transport efficiency and tend to be clogged by high salt-content solutions [4]. 

Ultrasonic nebulization is known to provide a higher analyte transport efficiency than pneumatic nebulization (normally 8-15 times higher) this results in improved sensitivity and lower detection limits, which is especially important for the analysis of species at trace or ultratrace levels [31-35]. Ultrasound-assisted generation of smaller drops and the use of a desolvation system to remove most of the solvent load allow the production of fine, dry analyte-enriched aerosol for insertion into a detection system some authors, however, ascribe most of the sensitivity increase of USNs to the desoivation system aione [36]. 

The pneumatic nebulizer has for many years been the most universal sample insertion device for plasma-based spectrometry. The inherent lack of transport efficiency, coupled with the continuing need for increased sensitivity, has promoted research into the use of ultrasonic nebulizers to boost detection capabilities. Such research has focused on various aspects including fundamental aerosol properties [86-88], instrument development [89], nebulizer comparisons [90,91], desolvation effects [92,93], direct nebulization applications [94,95] and speciation [96]. 

In this device the liquid sample is sprayed into a heated spray chamber, where the nebulizer gas transfers the aerosol through the membrane desolvator. An argon flow removes the solvent vapour from the exterior of the membrane. If compared to conventional pneumatic nebulizers, this system enhances analyte transport efficiency and limits solvent loading to the plasma. Oxide and hydride polyatomic ion interferences are significantly reduced, improving the detection limits by an order of magnitude. 

Transportation Efficiency of Rapidly Decaying Nuclides by Aerosol... 

Fig. 3.9 Comparison of the transportation efficiencies (including the survival yields), measured at the exit of a straight open cylindrical tube, for relatively short-lived tracers, which are carried either as gaseous molecules or as aerosols. The values are plotted versus the hold-up time of the gas divided by the mean lifetime of the particular nuclide.

Transport efficiency is defined as the amount of the original sample solution that is converted to an aerosol and then transported into the plasma source. 

Inductively coupled plasma atomic emission spectrometry (ICP-AES) involves a plasma, usually argon, at temperatures between 6000 and 8000 K as excitation source. The analyte enters the plasma as an aerosol. The droplets are dried, desol-vated, and the matrix is decomposed in the plasma. In the high-temperature region of the plasma, molecular, atomic, and ionic species in various energy states are formed. The emission lines can then be exploited for analytical purposes. Typical detection limits achievable for arsenic with this technique are 30 J,g As/L (23). Due to the rather high detection limit, ICP-AES is not frequently used for the determination of arsenic in biological samples. The use of special nebulizers, such as ultrasonic nebulization, increases the sample transport efficiency from 1-2% (conventional pneumatic nebulizer) to 10-20% and, therefore, improves the detection limits for most elements 10-fold. In addition to the fact that the ultrasonic nebulizer is rather expensive, it was reported to be matrix sensitive (24). Inductively coupled plasma atomic emission spectrometry is known to suffer from interferences due to the rather complex emission spectrum consisting of atomic as... 

Introduction of gaseous samples into the plasma offers several advantages over liquid aerosol introduction. The transport efficiency for introduction of gases approaches 100%, whereas in pneumatic nebulization more than 95% of the sample solution is discarded. When more analyte is transported into the plasma, improved signal-to-noise ratios and detection limits may be obtained. 

---