Spray chambers desolvating

To assist in the deposition of these larger droplets, nebulizer inlet systems frequently incorporate a spray chamber sited immediately after the nebulizer and before the desolvation chamber. Any liquid deposited in the spray chamber is wasted analyte solution, which can be run off to waste or recycled. A nebulizer inlet may consist of (a) only a nebulizer, (b) a nebulizer and a spray chamber, or (c) a nebulizer, a spray chamber, and a desolvation chamber. Whichever arrangement is used, the object is to transfer analyte to the plasma flame in as fine a particulate consistency as possible, with as high an efficiency as possible. 

In ICP-MS mostly liquids are analyzed. The aqueous solution is nebulized using an effective pneumatic nebulizer (e.g., Meinhard or MicroMist nebulizer with cyclonic spray chamber, ultrasonic nebulizer (USN), microconcentric nebulizer with desolvator (e.g., Aridus or APEX), the latter for the introduction of small volumes of solution)3 as described in Section 5.1.6. 

A frequently used micronebulizer with heated spray chamber and membrane desolvator is the Aridus from CETAC Technologies, Ohama, NE. The experimental setup of the Aridus II microconcentric nebulizer is shown in Figure 5.16. 

On line additions of aqueous standard solutions for the calibration of LA-ICP-MS including a comparison of wet and dry plasma conditions are discussed by O Connor et al.ls For solution calibration of standard solutions the authors used a 100 (xl PFA nebulizer together with a cyclonic spray chamber or a MCN-6000 sample introduction system with desolvator, to study the wet and dry plasma, respectively. A polypropylene Y piece was applied to mix the laser ablated material and the nebulized standard solutions. The authors found that the on line addition of water is the preferred mode of operation for quantification by LA-ICP-MS, i.e., wet plasma is more stable (improved standard deviation of sensitivity ratios). 

The combination of the ultrasonic nebulizer, heated spray chamber and condenser/desolvator leads to improvements in detection limits by a factor of about 10 compared to that of a pneumatic nebulizer without a desolvation system. This is the main reason ultrasonic nebulizers are used despite their higher cost (approximately U.S. 15,000 in 1998). 

There are several drawbacks to ultrasonic nebulizer/desolvation systems. Precision is typically somewhat poorer (1% to 3% relative standard deviation) than for pneumatic nebulizers (0.5% to 1.0% relative standard deviation) and washout times are often longer (60 to 90 sec compared to 20 to 30 sec for a pneumatic nebulizer/spray chamber without desolvation). Furthermore, chemical matrix effects are dependent on the amount of concomitant species that enter the ICP per second. Therefore, use of any sample introduction device that increases the amount of sample entering the plasma per second also naturally leads to more severe matrix effects when the sample contains high concentrations of concomitant species. 

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. 

It must be noted that increasing the amount of solvent in the ICP produces a higher load on the plasma and an increase in reflected powers, causing plasma instability. Desolvation of the sample aerosol may overcome this hurdle and is achieved by the use of a cooled spray chamber. In addition, Peltier coolers and membrane dryers [23] have been used for desolvating liquid aerosols and eliminate approximately 90% of the aerosol (Fig. 10.3). Spray chambers may also im-... 

The vast majority of ICP-based analyzes are performed on liquid samples that are introduced to the plasma in the form of an aerosol. In this case, sample introduction system consists of four parts (i) a nebulizer, which generates an aerosol (ii) a spray chamber, which filters the aerosol and transports it to the plasma (hi) a desolvation system to reduce the mass of solvent reaching the plasma (iv) an injector tube to introduce the aerosol into the plasma base. 

Although there has been limited use with CE interfaces, the direct injection nebulizer (DIN) was first described by Shum et al. - and later used by Liu et al. for CE (Fig. 2E). In this design, the nebulizer introduces the sample very near the plasma inside the ICP torch and eliminates the spray chamber assembly. Close to 100% analyte transport efficiency can theoretically be obtained with the DIN, but the nebulizer is restricted to very low liquid flow rate and thus is well matched to CE interfacing. This design does induce local plasma cooling due the lack of desolvation and detection limits are only slightly improved over other nebulizer designs. 

To assist in the deposition of these larger droplets, nebulizer inlet systems frequently incorporate a spray chamber sited immediately after the nebulizer and before the desolvation chamber. Any liquid deposited in the spray chamber is wasted analyte solution, which can be run off to waste or recycled. 

The Thermospray jet is introduced into a spray chamber which is heated sufficiently to complete the vaporization process. Helium is added through a gas inlet in sufficient quantity to maintain the desired pressure and flow rate. The fraction of the solvent vaporized in the thermospray vaporizer and the temperature of the desolvation chamber is adjusted so that essentially all of the solvent is vaporized within the desolvation region. The Thermospray system allows very precise control of the vaporization so that all of the solvent can be vaporized while most of even slightly less volatile materials will be retained in the unvaporized particles. 

The apparatus in its simplest form is shown schematically in Figure 2. The Thermospray vaporizer is installed in the heated desolvation chamber through a gas-tight fitting. The helium is introduced through a second fitting and flows around the Thermospray vaporizer and entrains the droplets produced in the Thermospray jet. In general the carrier gas flow required is at least equal to the vapor flow produced by complete vaporization of the liquid input. The heated zone within the spray chamber should be sufficiently... 

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. 

Cross contamination encountered with desolvation systems has been greatly reduced by using a concentric sheath to prevent deposits on tube walls. It is important to note that nebulisers and spray chambers operate interactively and must be optimised as a unit rather than individually. There are, however, certain parameters that need to be considered in relation to the spray chamber ... 

The behaviour of solvents for the analysis of metal ions is important because the determination of the correct concentration is paramount to whether the ICP-OES can handle a solvent or not. The journey from liquid to nebulisation, evaporation, desolvation, atomisation, and excitation is governed by the physical nature of the sample/solvent mixture. The formation of the droplet size is critical and must be similar for standards and sample. The solution emerging from the inlet tubing is shredded and contracted by the action of surface tension into small droplets which are further dispersed into even smaller droplets by the action of the nebuliser and spray chamber which is specially designed to assist this process. The drop size encountered by this process must be suitably small in order to achieve rapid evaporation of solvent from each droplet and the size depends on the solvent used. Recombination of droplets is possible and is avoided by rapid transfer of the sample droplets/mist to the plasma torch. The degree of reformation depends on the travel time of the solution in the nebuliser and spray chamber. For accurate analysis the behaviour must be the same for standards and samples. 

In an APCI interface the column effluent enters a heated nebulizer where the pneumatically assisted desolvation process is almost completed. While still in the spray chamber, ionization of analytes is initiated by corona discharge. The ionization mechanisms in APCI are almost identical to those in conventional medium pressure chemical ionization (7). Positive ion formation can be achieved by proton transfer, adduct formation or charge exchange reactions, while in the negative mode ions are formed due to proton abstraction, anion attachment and electron capture reactions. The APCI interface is compatible with flow rates exceeding 1 ml/min and will... 

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