Concentric nebulizer

Concentric nebulizers Solution sample introduction has been associated with pneumatic nebulizers (concentric and cross-flow) almost universally for routine analysis, due to their simplicity and low cost. However, they provide low analyte transport efficiency (< 5%) to the plasma, and may be prone to clogging. 

Precision For absorbances greater than 0.1-0.2, the relative standard deviation for atomic absorption is 0.3-1% for flame atomization, and 1-5% for electrothermal atomization. The principal limitation is the variation in the concentration of free-analyte atoms resulting from a nonuniform rate of aspiration, nebulization, and atomization in flame atomizers, and the consistency with which the sample is heated during electrothermal atomization. 

Nebulizers can be divided into several main types. The pneumatic forms work on the principle of breaking up a stream of liquid into droplets by mechanical means the liquid stream is forced through a fine nozzle and breaks up into droplets. There may be a concentric stream of gas to aid the formation of small droplets. The liquid stream can be directed from a fine nozzle at a solid target so that, on impact, the narrow diameter stream of liquid is broken into many tiny droplets. There are variants on this approach, described in the chapter devoted to nebulizers (Chapter 19). 

The drop in pressure when a stream of gas or liquid flows over a surface can be estimated from the given approximate formula if viscosity effects are ignored. The example calculation reveals that, with the sorts of gas flows common in a concentric-tube nebulizer, the liquid (the sample solution) at the end of the innermost tube is subjected to a partial vacuum of about 0.3 atm. This vacuum causes the liquid to lift out of the capillary, where it meets the flowing gas stream and is broken into an aerosol. For cross-flow nebulizers, the vacuum created depends critically on the alignment of the gas and liquid flows but, as a maximum, it can be estimated from the given formula. 

Under these conditions, the flow of sample liquid is predicted to be shown and this is similar to the flows observed with concentric nebulizers. 

Using Poiseuille s formula, the calculation shows that for concentric-tube nebulizers, with dimension.s similar to those in use for ICP/MS, the reduced pressure arising from the relative linear velocity of gas and liquid causes the sample solution to be pulled from the end of the inner capillary tube. It can be estimated that the rate at which a sample passes through the inner capillary will be about 0.7 ml/min. For cross-flow nebulizers, the flows are similar once the gas and liquid stream intersection has been optimized. 

Figure 19.7 shows a typical construction of a concentric-tube nebulizer. The sample (analyte) solution is placed in the innermost of two concentric capillary tubes and a flow of argon is forced down the annular space between the two tubes. As it emerges, the fast-flowing gas stream causes a partial vacuum at the end of the inner tube (Figure 19.4), and the sample solution lifts out (Figure 19.5). Where the emerging solution meets the fast-flowing gas, it is broken into an aerosol (Figure 19.7), which is swept along with the gas and eventually reaches the plasma flame. Uptake of sample solution is commonly a few milliliters per minute.

The dimensions of concentric-tube nebulizers have been reduced to give microconcentric nebulizers (MCN), which can also be made from acid-resistant material. Sample uptake with these microbore sprayers is only about 50 xl/min, yet they provide such good sample-transfer efficiencies that they have a performance comparable with other pneumatic nebulizers, which consume about 1 ml/min of sample. Careful alignment of the ends of the concentric capillary tubes (the nozzle)... 

In a concentric-tube nebulizer, the sample solution is drawn through the inner capillary by the vacuum created when the argon gas stream flows over the end (nozzle) at high linear velocity. As the solution is drawn out, the edges of the liquid forming a film over the end of the inner capillary are blown away as a spray of droplets and solvent vapor. This aerosol may pass through spray and desolvation chambers before reaching the plasma flame. 

Three common types of nozzle are shown diagrammatically. Types A and K are similar, with sharp cutoffs on the ends of the outer and inner capillaries to maximize shear forces on the liquid issuing from the end of the inner tube. In types K and C, the inner capillary does not extend to the end of the outer tube, and there is a greater production of aerosol per unit time. These concentric-tube nebulizers operate at argon gas flows of about 1 1/min. 

The flows of gas and liquid need not be concentric for aerosol formation and, indeed, the two flows could meet at any angle. In the cross-flow nebulizers, the flows of gas and sample solution are approximately at right angles to each other. In the simplest arrangement (Figure 19.11), a vertical capillary tube carries the sample solution. A stream of gas from a second capillary is blown across this vertical tube and creates a partial vacuum, so some sample solution lifts above the top of the capillary. There, the fast-flowing gas stream breaks down the thin film of sample... 

This arrangement provides a thin film of liquid sample solution flowing down to a narrow orifice (0.007-cm diameter) through which argon flows at high linear velocity (volume flow is about 0.5-1 1/min). A fine aerosol is produced. This particular nebulizer is efficient for solutions having a high concentration of analyte constituents. 

The high potential and small radius of curvature at the end of the capillary tube create a strong electric field that causes the emerging liquid to leave the end of the capillary as a mist of fine droplets mixed with vapor. This process is nebulization and occurs at atmospheric pressure. Nebulization can be assisted by use of a gas flow concentric with and past the end of the capillary tube. 

HR-ICP-MS EEEMENT-2 (Pinnigan MAT, Germany) equipped with a standard introduction system (quartz water-cooled spray chamber, concentric nebulizer, torch with 1.5 mm i.d. injector and nickel cones) was used for measurements. The following operating conditions were used RP power 1150 W, coolant gas flow rate 16 1 min k auxiliary gas flow rate 0.85 1 min nebulizer gas flow rate 1.2 1 min k Sample uptake rate was 0.8-1 ml min k Measurements were performed with low and middle resolutions. Rh was used as an internal standard. Por calibration working standard solutions were prepared by diluting the multielemental stock solutions CPMS (SPEX, USA) with water to concentration range from 5 ng to 5 p.g I k... 

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

These solutions are not always practicable and HPLC flow rates of up to 2 mlmin may be accommodated directly by the use of electrospray in conjunction with pneumatically assisted nebulization (the combination is also known as lonspray ) and/or a heated source inlet. The former is accomplished experimentally by using a probe that provides a flow of gas concentrically to the mobile phase stream, as shown in Figure 4.8, which aids the formation of droplets from the bulk liquid, and will allow a flow rate of around 200 p. min to be used. 

Factors may be classified as quantitative when they take particular values, e.g. concentration or temperature, or qualitative when their presence or absence is of interest. As mentioned previously, for an LC-MS experiment the factors could include the composition of the mobile phase employed, its pH and flow rate [3], the nature and concentration of any mobile-phase additive, e.g. buffer or ion-pair reagent, the make-up of the solution in which the sample is injected [4], the ionization technique, spray voltage for electrospray, nebulizer temperature for APCI, nebulizing gas pressure, mass spectrometer source temperature, cone voltage in the mass spectrometer source, and the nature and pressure of gas in the collision cell if MS-MS is employed. For quantification, the assessment of results is likely to be on the basis of the selectivity and sensitivity of the analysis, i.e. the chromatographic separation and the maximum production of molecular species or product ions if MS-MS is employed. 

Atmospheric-pressure chemical ionization (APCI) An ionization memod in which a liquid stream is passed through a heated capillary and a concentric flow of a nebulizing gas. Ions are formed by ion-molecule reactions between me analyte and species derived from me HPLC mobile phase. 

As a starting material, TTBP (Titinium tetraisoproxide, Aldrich Chem. Co. ltd., 98%) was dissolved in distilled water by adding nitric acid. Total titanium ion concentration was fixed at 0.5M. The precursor solution was converted into droplets by ultrasonic nebulizer of 1.7MHz. These droplets were transported to the reaction region by carrier gas. 

Monitor serum potassium in patients receiving high-dose or continuous nebulization of a short-acting p2-agonist. Serum potassium concentrations should be obtained upon admission, and if hypokalemic, every 4 hours (after each 30 to 40 mEq or mmol of replacement) until the patient s potassium is stable. Potassium should be monitored every 3 to 6 months after discharge. 

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