Desolvation systems

The use of a desolvating system improves the sensitivity and can extend the matrix effect with respect to conventional sample introduction apparatus.85 The current trend is towards total consumption systems. Fundamental studies concerning the analysis of microsamples with common micronebulizer based systems have been reviewed by Todoli and Mermet.85... 

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. 

Mixed Gas Plasmas. Water loading can be reduced by a desolvation system (condenser or membrane separator) only if the vast majority of the water can be removed. One way to eliminate the introduction of water into the plasma during measurement of the analyte signals is with electrothermal vaporization, laser ablation, or other direct solid sampling techniques. Mixed gas plasmas,... 

C. Fragniere, M. Haldimann, A. Eastgate, U. Krahenbuhl, A direct ultratrace determination of platinum in environmental food and biological samples by ICP-SFMS using a desolvation system,. /. Anal. Atom. Spectrom., 20 (2005), 626 D630. 

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. 

Finally, several desolvation systems have been proposed to reduce the solvent loading to the plasma, thus improving its performance and avoiding interferences. Most desolvation systems comprise two steps (i) heating, in which the solvent is totally or partially evaporated from the aerosol droplets and (ii) removal of solvent vapor from the aerosol stream. More detailed information can be found elsewhere. ... 

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]. 

Microwave-induoed plasma atomic emission speotrometry is a useful analytioal technique for the determination of alkali and alkaline earth metals (e.g. that of sodium in water-soluble organio pharmaceuticals [42] with detection limits of 0.91-3.0 ng/ml). Also, it allows samples with organio contents up to 5% to be inserted without a desolvation system. Beryllium in spiked water samples was determined in this way with detection limits at the level of parts per trillion (ppt) and a preoision of 1.8% as relative standard deviation (RSD) [43]. 

Nebulizer. For the present studies, the sample aerosol was generated with a 3-MHz ultrasonic nebulizer, as illustrated in Figure 2, and the sample aerosol was desolvated prior to its injection into the plasma. A complete description of the nebulizer and desolvation system has been given elsewhere (2). The main virtue of ultrasonic nebulizers is that their efficiency of nebulization (mass of sample injected into the plasma/ mass of sample fed to the nebulizer) is 10 to 12 times greater (2) than that of common pneumatic nebulizers which are suitable for use with... 

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 ... 

Figure 7.32 Schematic of a commercial USN, the CETAC U-5000AT + and desolvation system. [Courtesy of CETAC Technologies, Omaha, NE (www.cetac.com).]...

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. 

Apart from this, memory effects may exist if long tubes and large vessel surfaces are used in the nebulization and desolvation system. This type of interference is not the fault of the plasma source, and can be easily avoided and controlled. 

Ultrasonic nebulizers are free of these difficulties as aerosol production is independent of gas flow, and droplet size is related to ultrasonic frequency. Droplets with diameters of 1 pm can be obtained with frequencies above 1 MHz. The efficiency is high enough to require the addition of a desolvation system to eliminate water. The desolvation system consists of an oven associated with a cooling system to trap the water vapor. Detection limits are usually improved by a factor of 10 in comparison to a pneumatic nebulizer. Although ultrasonic nebulizers are attractive in this respect, their use is as yet not widespread, due to their prohibitive cost. 

Urine QCs (see below), standards, samples, and ultrapure water blanks are acidified with ultrapure nitric add. The volume of urine depends on the concentration range and instrument setup. For a magnetic sector instrument with an Aridus desolvation system and nitrogen addition, 2-4 mL of urine is the typical volume required for samples with uranium concentrations as low as 0.003 pg/L. For samples with uranium concentrations <10 ng/L, 4mL of urine is preferred. Four milliliters of addified urine is the maximum volume that will also fit in the Eichrom 0.21 g TRU resin SPE columns without the additional funnel. Three hundred seventy-five microliters of concentrated nitric acid per milliliter of urine is added to each unknown urine, QC sample, or water blank before introduction onto the column. 

If an Aridus desolvating system is used, then sweep gas and nitrogen addition gas should be optimized for the Aridus system. The optimal values depend entirely on the unit, the conditioning of the membrane, and so on. If either an Aridus or Apex is... 

Intermediate accuracy is accomplished by permitting the elution of uranium from the chelation resin cartridges to be slightly concentrated rather than diluted, and analyzed with a Q-ICP-MS and a partially desolvat-ing introduction system, rather than a desolvating system on an MS-ICP-MS. However, such an instrumental setup permits greater access of laboratories with more limited funds to acquire useful uranium isotope ratio results and is fit for the purpose as intended. 

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