Sample uptake

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)... 

The transfer efficiencies for ultrasonic nebulizers (USN) are about 20% at a sample uptake of about 1 ml/min. Almost 100% transfer efficiency can be attained at lower sample uptakes of about 5-20 pl/min. With ultrasonic nebulizers, carrier gas flows to the plasma flame can be lower than for pneumatic nebulizers because they transfer sample at a much higher rate. Furthermore, reduction in the carrier-gas flow means that the sample remains in the mass measurement system for a longer period of time which provides much better detection limits. 

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... 

A further procedure involves the inclusion of an HPLC column separation analysis. However, the rate of sample uptake must be controlled by the analysis time on an HPLC column. 

The nebulizer capillary position may be adjustable on a screw thread to permit optimization of sample uptake and drop size. Alternatively or additionally, an impact bead may be placed in the path of the initial aerosol to provide a secondary fragmentation and so improve the efficiency of nebulization. Such a device is illustrated in Fig. 2.9. 

Adjusting the nebulizer. Measure the sample uptake rate using a 10 ml measuring cylinder and stop-watch. Report the result in ml min-i Compare this with your previous reading. Note that the sample uptake rate and flame conditions are interdependent and that adjusting one will require re-optimization of the other. Therefore, do not adjust the sample uptake rate once the fuel/air ratio has been optimized. 

The sample is typically pumped at a rate of 0.4 to 1.0 mL/min to a nebulizer that produces an aerosol with a range of drop sizes from submicrometer to 40 x in diameter [4,5]. Recently, nebulizers with small dead volumes that can be used with sample uptake rates as low as 10 xL/min have been introduced. The aerosol is modified as it passes through a spray chamber. Most aerosol drops that are too large to be vaporized effectively in the plasma (>20 xm diameter) are eliminated in the spray chamber. The spray chamber also limits the total amount of solvent liquid aerosol and vapor that enters the plasma. The aerosol exiting the spray chamber enters the hot, atmospheric pressure plasma gas (typically argon). 

Figure 7 Effect of nebulizer gas flow rate and sample uptake rate on primary and tertiary aerosol drop size distributions. A Meinhard TR-30 nebulizer was used with a double-pass spray chamber, (a) Primary aerosol produced by nebulizer as a function of nebulizer gas flow rate for a 1-mL/min sample uptake rate, (b) Tertiary aerosol exiting spray chamber as a function of nebulizer gas flow rate, (c) Primary aerosol as a function of sample uptake rate at a nebulizer gas flow rate of 0.8 L/min. (d) Tertiary aerosol exiting spray chamber as a function of sample uptake rate. (From Ref. 18.)...

The primary aerosol droplets also become slightly smaller as the sample uptake rate is decreased. However, the Sauter mean diameter is not as sensitive to changes in sample uptake rate as it is to the nebulizer gas flow rate. For example, when the uptake rate was decreased from 1.0 to 0.6 mL/min, the Z>3 2 value decreased by only 4% (10.9 to 10.5 at a nebulizer gas flow rate of 0.8 L/min) [5]. 

As the sample uptake rate is increased, the amount of analyte transported into the ICP increases [19], but not proportionately (Fig. 3.8). The efficiency of analyte transport improves as the sample uptake rate is decreased. The analyte transport efficiencies were 60%, 14%, and 3% at sample uptake rates of 10, 100, and 1000 pL/min, respectively. As a result, detection limits obtained using a sample uptake rate of 50 to 85 pL/min are similar to those for a 1-mL/min uptake rate [12]. 

Figure 8 Analyte transport rate (expressed as equivalent volume of sample solution) as a function of sample uptake rate. A Cetac microcentric nebulizer (MCN) was used in a double-pass spray chamber. (From Ref. 422.)...

If the sample uptake rate is decreased, the number of droplets per cubic centimeter in the spray chamber decreases, droplet-droplet collisions resulting in coagulation are less likely, and the analyte transport efficiency increases, as shown... 

Table 1 Analyte Transport Efficiency as a Function of Sample Uptake Rate for Three Different Nebulizers Used with a Scott Double-Pass Spray Chamber0...

As can be seen from the results in Table 3.1, the analyte transport efficiency is similar for both conventional and micro- or high-efficiency nebulizers when compared under identical flow rates. The increase in analyte transport efficiency with decrease in the sample uptake rate (sometimes called starving the nebulizer because uptake rates less than the natural aspiration rate are used) was reported long ago [21,22]. So the main advantage of the newer micronebulizers is that their internal volume is small, a feature that becomes more important as the uptake rate is reduced. A capillary can also be inserted into a conventional concentric, pneumatic nebulizer to decrease its internal dead volume [23,24]. 

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. 

Attainable detection limits depend on the amount of analyte that enters the ICP per second, the efficiency of aerosol conversion into analyte ions in the ICP, and the transmission efficiency of ions from the plasma to the MS detector. The detection limits also depend on the variation of the background and the integration time. Typical pneumatic nebulizer/spray chamber systems operated at sample uptake rates from 0.1 to 2.0 mL/min introduce an amount of analyte equivalent to that in 10 to 30 JiL/min of sample solution into the ICP. At a sample uptake rate of 1 mL/min, only 1% to 2% of the analyte enters the plasma most of the sample is lost in the spray chamber and exits through the drain. Concentration based detection limits can be improved by approximately a factor of 10 by using a high-... 

One source of poor precision in this method is the differing viscosities found after dilution of the oil samples with solvent. These differences give small changes in sample uptake rate with respect to both other samples and to standards. The method of standard additions may be used to overcome this problem but with a considerable increase in analysis time. 

The module is attached to the main instmment and is monitored by the same. With the DTE pipette (Fig. 17b) the sample and reference solution are taken up at the same time. The user inserts the relevant electrolyte slide into the sample uptake unit. The sample and reference solution can be applied simultaneously to the slide. The slide remains in the incubator for about 1.5 minutes where it is incubated at 25°C. An electrometer measures the potential difference between the two half-cells. The measured voltage value is transmitted to the main DT instrument which calculates the relevant concentration from this value. The result is printed out. 

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