Adaptation of a new pneumatic nebulizer for sample introduction in ICP spectrometry

Abstract

A new pneumatic nebulizer, the pneumatic extension nozzle (PEN), originally developed in technical laboratories, mainly for the production of rock and glass wool, is applied for sample introduction in ICP-OES. The droplet formation process under various operating conditions and geometries was investigated using a transparent enlarged model of the PEN. For the application in ICP spectrometry the geometry of the PEN was optimized and miniaturized with the aid of similarity theory. In comparison with a standard concentric nebulizer the miniaturized PEN generates an aerosol with a twice higher mass fraction of droplets with diameters D < 10 μm. An empirical model for the prediction of the mean droplet diameter is presented, using the nozzle diameter as a linear size scale. This model enables a better fit to experimental data compared with existing models. Applying the miniaturised PEN to a simultaneous ICP-OES instrument lowers the detection limits up to a factor of 3.5 depending on the element. The new pneumatic extension nozzle can be easily adapted to existing ICP-OES instruments.

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Introduction

The typical method of sample introduction in ICP spectrometry is the generation of an aerosol from a solution by means of a pneumatic concentric nebulizer. This aerosol is modified and conditioned using a spray chamber. Finally, the desired fine droplets are transported into the plasma.^1–8

Besides the two main types of pneumatic nebulizer (concentric nebulizer, cross-flow nebulizer), special nebulizers are applied for particular applications. If high analyte flows are required in order to achieve lower detection limits (e.g., potable water analysis) mainly ultrasonic nebulizers⁹ are applied. In cases where only small sample amounts are available, micro-flow nebulizers^10,11 are used. Various Babington nebulizers,^12–14 for instance the V-groove nebulizer, were developed for the introduction of highly concentrated salt solutions. From this nebulizer type only the so called Conespray nebulizer¹ is regular supplied by a company as standard equipment.

The typical radiofrequency power used in ICP spectrometry is 1.3 kW. This power allows for a mass flow input of up to 120 μl min⁻¹ into the plasma without the extinction of the plasma. Only droplets with a diameter less than 10 μm are suitable in ICP spectrometry,¹ due to the retarded evaporation of larger droplets.

All standard concentric nebulizers (CPN, see Table 1) used in ICP spectrometry are operated at sample uptake rates between 0.8 ml min⁻¹ and 1.5 ml min⁻¹. Typically, the mass fraction of the desired droplets in the generated aerosol amounts to about 40%.^4,15 Thus, these nebulizers produce more small droplets than can be used for sample introduction. The aerosol leaving the spray chamber consists mainly of the desired fine droplets D < 10 μm. In view of the separation of bigger droplets all chamber types used in ICP spectrometry show reasonable precipitation behaviour for bigger droplets. These droplets are deposited at the walls of the chambers. However, 95% by mass of the small droplets are also lost while passing the chamber. The sample flow finally entering the ICP, in most cases, is only about 20 μl min⁻¹ or even less. In order to explore the full potential of the ICP, this far too low flow rate has to be increased and the complete sample introduction system must be improved. The goal is clearly defined by the necessity of introducing the sample at a flow rate of about 100 μl min⁻¹, consisting exclusively of droplets smaller than 10 μm.

Table 1 Geometric dimensions of the nebulizers examined

	PEN	CPN
Internal capillary diameter	d C = 200 μm	d C = 250 μm (internal) dC,e = 360 μm (external)
Gas orifice diameter	d O = 200 μm	d O = 400 μm
Distance capillary–orifice	a = 50 μm	a = 0 μm
Annular slot width (gas)	—	s = 20 μm
Cross sectional area of gas exit	A S = 3.1 × 10^−2 mm^2	A S = 2.4 × 10^−2 mm^2

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The aerosol losses in the chamber may be avoided by the application of the various types of direct injection nebulizers.^10,11 Direct injection nebulizers with small orifice and annular gas slot dimensions may be directly connected to the ICP torch without intermediate separation in a spray chamber. These devices must be equipped with proper pump systems, e.g., HPLC pumps for constant low flow rates due to their large pressure drop in the sample duct. The unfavourable droplet size distribution gives rise to comparatively high noise of the signal. Therefore, direct injection nebulizers are an excellent tool for handling small sample amounts and low flow rates, but lowering the detection limits is difficult to achieve.

Two approaches could possibly be used to increase the mass flow of small droplets at the exit of the spray chamber. One is the improvement of the separation characteristic of the spray chambers, i.e., the sharpness of separation. In the past, the improvement in spray chambers was mainly carried out empirically. In previous papers it could be shown that detailed knowledge of the flow behaviour of the aerosol stream and of the droplet deposition onto the walls of the spray chamber could be obtained by means of numerical flow simulations.¹⁶ Combining computational fluid dynamics (CFD) with mathematical strategies of the evolution theory a spray chamber could be improved considerably by applying computer experiments instead of classical testing.¹⁷ The other way to improve the analyte flow rate to the plasma is the generation of an aerosol consisting of a greater amount of useful droplets.

In the last decades a large number of different nebulizers were introduced mainly in order to increase the aerosol yield at the outlet of the chamber. However, no real breakthrough was achieved. All of them were more or less strongly modified variations of nebulizers already used earlier for spectroscopic applications. More than a century ago (1879) Gouy described a sample introduction system containing a pneumatic concentric ring-slit nebulizer. Due to the robustness and simplicity of the concentric pneumatic nebulizer, compared with other more sophisticated nebulizers, like the ultrasonic or thermospray nebulizer, it is still the standard nebulizer in ICP spectrometry. Besides producing small droplets a newly designed nebulizer should be as easy to handle as the existing concentric nebulizer.

The fundamentals of the concentric nebulizer were first described in engineering sciences in 1939 by Nukijama and Tanasawa¹⁸ with an empirical equation for the prediction of the mean droplet diameters. The relationships can be found in many monographs on atomic spectrometry.^5,19,20 In the present paper the validity of this equation for the prediction of the mean droplet diameter for pneumatic nebulizers will be questioned by the experimental data and the improved relationships for drop size prediction proposed.

Theoretical background

In pneumatic nebulizers the energy for droplet generation is provided by the gas flow.^21,22 The liquid is exposed to a gas stream at comparatively high relative velocity v_g. At the capillary outlet of a concentric nebulizer, when operated at typical conditions for plasma spectroscopy, the liquid emerges with negligible velocity as an annular film and disintegrates to primary droplets of comparatively coarse size. A reduction of the mean drop diameter occurs if the high relative velocity v between gas and droplets is increased. Among other parameters, the mean droplet size, expressed, for example, by the mass mean D₅₀ or the Sauter mean diameter D₃₂, is also determined by the liquid outlet diameter d_C of the capillary. The Sauter mean diameter D₃₂ is the diameter of a droplet with the same surface to volume ratio as the entire droplet distribution.

According to the literature, the mechanism of droplet formation at a pneumatic nebulizer, as shown in Fig. 1, is determined by the following parameters: one linear dimension of the nebulizer, in this case the nozzle diameter d_C of the liquid outlet, the gas velocity v_g, the gas density ρ_g after expansion, the liquid density ρ₁, the liquid viscosity η_l as well as the liquid mass flow rate m₁. It can be assumed that the gas viscosity η_g does not influence the mechanism of droplet generation, since the gas Reynolds number Re_g = v_gd ρ_g/σ typically reaches values >1000. Thus, a dimensionless relationship for the Sauter mean diameter of the droplet size distribution

can be derived. This relationship contains the gas Weber number We_g = v²ρ_gd_C/σ, the liquid to gas mass flow ratio μ = ṁ₁ /ṁ_g and the Ohnesorge number On = η_l/(d_Cρ_l σ)^1/2, which dominates the influence of the liquid viscosity η_l. Also, the gas flow is closely related to the pressure and orifice geometry. In order to express the effect of the differential gas pressure, Δp_g ∼ v²ρ_g, it seems to be more useful to replace the gas Weber number by the more practical gas Laplace number Δp* = Δp d_C/σ ∼ We_g. For the comparison of different nebulizer types the density ratio ρ_g/ρ_l and also the adiabatic exponent κ usually remain unchanged during the investigation and are not considered any further. If the nebulizer is operated at constant non-dimensional numbers Δp_g*, μ and On, the related Sauter mean diameter D₃₂/d_C is determined by the geometrical design of the nebulizer only.

Fig. 1 Sketch of a classical concentric pneumatic nebulizer (CPN).

Considering relationship (1), the nondimensional Sauter mean diameter D₃₂/d_C can be predicted for pneumatic nebulization by the semi-empirical equation valid for given ratios of ρ_g/ρ_l and given adiabatic exponents κ:

For each nebulizer design, the parameters C₁, C₂, m and j are different and have to be determined by drop size measurements. These parameters are already known for a concentric nebulizer design and can be taken from the literature.²³

The pneumatic extension nozzle (PEN)

Fig. 2 shows the design of a PEN (see also Table 1) including the geometrical parameters: inner diameter of the capillary d_c, diameter of the orifice d₀ and the distance between the capillary and the orifice a. A similar principle was already applied in 1966 by Walz²⁴ for the production of rock and glass wool. In order to generate approximately monosized droplets Gañán-Calvo and Barrero²⁵ examined pneumatically extended laminar micro-jets disintegrated by the Rayleigh mechanism at comparatively low gas differential pressures. They applied a nozzle similar to the PEN but within the laminar flow regime.

Fig. 2 Illustration of the functional principle of a pneumatic extension nozzle PEN.

The liquid discharges from a capillary positioned inside the nebulizer chamber pressurised by the gas. At comparatively small gas differential pressures, a single liquid jet is formed. Along with the gas the liquid jet passes the orifice located at a distance a opposite the capillary outlet. The emerging stream within the nebulizer chamber is accelerated and extended by the pressure gradient at the orifice and shear forces due to the faster gas flow pulling the jet. Outside the nozzle the jet disintegrates into droplets by the Rayleigh mechanism.²¹ This, so called laminar jet disintegration regime is characterised by a narrow droplet size distribution. Fig. 3(a) shows the laminar jet disintegration at a low gas differential pressure Δp_g. At higher Δp_g the jet is forced into asymmetric oscillations, shown in Fig. 3(b). A further rise of Δp_g results in a turbulent, i.e., more chaotic, nebulization process, as demonstrated in Fig. 3(c). This leads to smaller droplets but to a wider drop size distribution (DSD). This regime is of relevance in this paper, as a miniaturised PEN was operated in this regime at Δp_g* ≥ 500 for the purpose of sample introduction in ICP OES.

Fig. 3 Different disintegration regimes with increasing gas pressure Δp_g*: (a) laminar jet disintegrating by the Rayleigh mechanism, Δp_g* = 270; (b) oscillating jet, Δp_g* = 420; (c) turbulent jet, Δp_g* = 700.

Experimental

Optimization of the nebulizer geometry

Current literature gives insufficient knowledge about the influence of the geometrical dimensions of the PEN on the resulting DSD at Δp_g* ≥ 200. The distance a between the capillary outlet and the orifice appears to be an important parameter, affecting the aerosol generation and the DSD, respectively. Therefore, fundamental study on the geometrical effects was accomplished at an enlarged nebulizer with d_C = d_O = 2 mm. Fig. 4 shows the orifice geometries examined with different inlet radii at the gas orifice. The distance a, as well as the radius r of the orifice contour, were varied during this investigation. A laser diffraction spectrometer (LDS; Malvern Instruments Type 2600) was used in order to measure the DSD of the aerosol and the Sauter mean diameter D₃₂, respectively. The LDS is an integrating method detecting a droplet collective across the entire space where the laser penetrates the aerosol cone. Data on d₃₂ were extracted from whole drop size distribution (DSD) readings (Malvern). For the same adjustments, such as to flow rates and air pressures, at least 7 DSD spectra were taken. The largest and smallest values of d₃₂ were eliminated and from the remaining five an arithmetic mean value was taken and presented in the following figures. The maximum deviations of d₃₂ were less than ±5% of the mean.

Fig. 4 Orifice geometries of the PEN tested: (left) rounded orifice with r = d_O; (right) sharp-edged orifice with r ≈ 0. Distance a was also varied in trials.

Fig. 5 illustrates the dependence of the related Sauter mean diameter D₃₂/d_C on the related distance a/d_O. The gas Laplace number was varied in the range of 200 ≤ Δp_g* ≤ 800 at a constant liquid to air mass flow ratio μ = 1 and Ohnesorge number On = 0.003 by the adjustment of the liquid feed flow. Therefore, a sharp edged orifice with a radius of r ≈ 0 was used. A related distance a/d_O < 0.1 seemed not to be useful. In this range a strong increase of the mean droplet size, obviously due to higher gas energy dissipation, can be observed. The smallest relative Sauter mean diameters D₃₂/d_C are achieved within the range of 0.2 ≤ a/d_O ≤ 0.3 at all gas Laplace numbers investigated. The influence of a/d_O on D₃₂/d_C becomes more pronounced with decreasing Δp_g*. At Δp_g* < 400 the reduction of the related distance from a/d_O = 0.4 to a/d_O = 0.3 results in a distinct decline in D₃₂/d_C. This can be explained by the change in the disintegration regime which occurs at this point.²⁴ The transition from point 1 to point 2, as shown in Fig. 5, by lowering the distance a leads to a change of the disintegration regime from aerodynamic wave formation to turbulent jets, even at a constant and relatively low gas Laplace number of Δp_g* = 200. The droplet size reduction can be explained by the formation of multiple ligaments observed at the turbulent jet disintegration regime.

Fig. 5 Sauter mean diameter D₃₂/d_c depending on the related distance a/d_O at different gas pressure Δp_g (measurements on Malvern LDS).

This behaviour could be observed at a transparent enlarged model of the PEN with a capillary diameter of d_C = 10 mm and a capillary to orifice diameter ratio of d_C/d_O = 1. This PEN model was operated in a range of characteristic numbers typical for nebulizers applied for sample introduction. The related distance a/d_O = 1 was kept constant during the observation. Fig. 6 shows the liquid discharged from the capillary before passing the orifice at different gas pressures, Δp_g*, at constant liquid flow rate. These pictures were taken with a high speed camera (HighSpeedStar 1-10k CCD from LaVision, Göttingen, Germany) at a repetition rate of 2000 frames s⁻¹ and a resolution of 512 × 512 pixel. The illumination was realised with a self-made LED panel²⁶ consisting of 952 single LEDs (light emitting diodes) arranged in an array. An electronic circuit enabled light pulses with a minimum duration of 10 μs when triggered from outside. At gas Laplace numbers of Δp_g* = 270 and Δp_g* = 420 the jet discharges from the capillary as a single stream as in Fig. 6 (a) and (b). Below the orifice, the jet disintegrates due to the aerodynamic oscillations. With increasing gas Laplace numbers the jet becomes more turbulent. At the given Ohnesorge number of On = 0.0013 the turnover from the aerodynamic oscillation regime to turbulent nebulization is reached at a gas Laplace number of approximately Δp_g* = 500. The generation of droplets in this regime is characterised by the formation of a multitude of comparatively thin and irregular streams, following an irregular distortion of the primary jet, as shown in Fig. 6 (c) and (d). Due to the multiple relatively fine ligaments, the droplets become smaller and the break up length becomes very short.

Fig. 6 Photos of the liquid stream discharged from the capillary with increasing gas pressure, gas Laplace number respectively, at an enlarged and transparent model of PEN, On = 0.0013 and a/d_O = 1.0; d_O = d_C = 10 mm; (a) Δp_g* = 270, (b) Δp_g* = 420, (c) Δp_g* = 560, (d) Δp_g* = 700.

A further essential geometrical parameter of the PEN is the orifice inlet radius r. In this study we compared two different orifice contours (sharp and rounded edges) as shown in Fig. 4. The Sauter mean diameter D₃₂ was again measured at different related distances a/d_O, Δp_g* and mass flow ratios μ using the LDS method. One characteristic result of this investigation is presented in Fig. 7. At a rounded orifice with r/d_O = 1, the related distance a/d_O has no substantial influence on D₃₂/d_C. At a related distance of a/d_O = 0.25 the sharp-edged orifice generates a spray with a smaller related Sauter mean diameter D₃₂/d_C compared with the rounded orifice.

Fig. 7 Sauter mean diameter D₃₂versus. the related distance a/d_O at Δp_g* = 800, d_c = 200 μm, μ = 1 and On = 0.003 for a sharp-edged (r/d_O ≈ 0) and a rounded-edged (r/d_O = 1) orifice, measured by LDS method (Malvern).

Miniaturization of the pneumatic extension nozzle

Changes to the geometrical design revealed that finest aerosols are generated at sharp-edged orifices with r/d_O ≈ 0 and a related distance in a range of 0.2 < a/d_O < 0.3. With this information, a prototype of the PEN was designed and manufactured, see Fig. 8.

Fig. 8 Picture of the PEN prototype including a close up of the platinum capillary.

Almost the entire nebulizer was produced of PEEK (polyether-ether-ketone) with the exception of the exchangeable capillary. In order to obtain a high precision manufactured part as required at a bore hole diameter of d_C = 200 μm, the capillary was made of platinum (see also close up photo in Fig. 8). The PEN consists of seven components, as illustrated in Fig. 9. The sample is supplied to the nebulizer by a tube with an inner diameter of 1 mm and a length of 55 mm. The lower end of the tube is screwed into an adapter sealed by a PTFE ring. The liquid flows downstream from the tube, passes the adapter and the capillary. Finally, the liquid discharges from the capillary located inside the nebulizer chamber closed by the nebulizer cap. In order to design a nebulizer with a low pressure drop at the liquid duct, almost the total capillary bore length had an internal diameter of 1 mm. On a length of 0.5 mm in front of the outlet, the capillary diameter is restricted to the outlet diameter of d_C = 200 μm. The gas enters the nebulizer housing by a gas inlet port with an inner diameter of 3 mm. It passes the adapter through six holes of 1.5 mm in diameter and flows into the nebulizer chamber. The gas, along with the liquid, passes the sharp-edged orifice with a diameter of d_O = 200 μm located at the bottom of the cap. The optimum distance a is adjustable by moving the nebulizer cap on the adapter’s external thread. An orifice with d_O = 200 μm has an optimum capillary distance of a = d_O/4 = 50 μm, but even larger distances up to a ≤ 100 μm can be adjusted. The design of the PEN enables an easy adaptation to typical ICP spray chambers.

Fig. 9 Sectional drawing of the PEN prototype.

Comparison of the PEN and the CPN

In order to classify the efficiency of the PEN in terms of generating fine aerosols a comparative examination of drop sizes was conducted. Therefore, the PEN prototype was compared with a frequently used concentric pneumatic nebulizer CPN (Precision Glasblowing Type A-1). The geometric dimensions of these nebulizers are given in Table 1. Deionised water and pressurized air were used to run the nebulizers. The liquid was fed to the nebulizers using a HPLC-pump (double piston pump with low pulsation, Knauer Type K-501). Both nebulizers were provided with air from the local compressed air system. The volumetric gas flow rate was controlled by a mass flow meter (Bronkhorst V_max = 1 l_n min⁻¹). The droplet size distributions of the aerosols were measured by means of a LDS again. The measuring position was located 20 mm below the nebulizer and remained constant during the test. It can be expected, that the disintegration process is completely terminated at this distance.

An ICP spectrometer (IRIS) from Thermo Finigan San, CA95134-1991, running at 1.15 kW RF power, was used for the comparative measurements of the detection limits attainable with the original concentric nebulizer and the PEN.

Results and discussion

As the CPN had a larger internal diameter, d_c = 250 μm, of the liquid outlet compared with the PEN nozzle with a diameter of d_c = 200 μm, it was necessary to compensate for this difference by considering the droplet size in non-dimensional parameters. The investigations were performed by operating both nebulizers at identical characteristic numbers. The Ohnesorge number was kept constant during the whole test at On = 0.008. The gas Laplace number and the liquid to air mass flow ratio were varied in a range of 500 ≤ Δp_g* ≤ 700 and 0.5 ≤ μ ≤ 2.5, respectively. Fig. 10 shows the related Sauter mean diameter D₃₂/d_cversus the liquid to air mass flow ratio μ at a constant gas Laplace number of Δp_g* = 600 and an Ohnesorge number of On = 0.008. At all adjustments of μ the smallest related Sauter mean diameters D₃₂/d_c were achieved with the PEN.

Fig. 10 Related Sauter mean diameter D₃₂/d_cversus the liquid to air mass flow rate ratio for PEN and CPN at Δp_g* = 600 and On = 0.008.

For the purpose of sample introduction, however, the fraction of useful droplets generated by the nebulizer is important. Therefore, cumulative mass distributions at Δp_g* = 600, μ = 0.5 and On = 0.008 were measured and are presented versus the related droplet diameter D/d in Fig. 11. The PEN generates a higher fraction of appropriate droplets compared with the CPN over the whole range examined at equal characteristic numbers. In addition, the aerosol produced by the PEN has a more narrow droplet size spectrum, which is characterized by a steeper gradient of the distribution curve. Assuming a uniform liquid duct discharge diameter of d_C = 200 μm for both nebulizers at a critical droplet diameter of D ≤ 10 μm, the related drop size range is D/d_c < 0.05. As shown in Fig. 11, a CPN with a reduced diameter d_c = 200 μm produces a mass fraction of usable droplets of about 0.36. In contrast to this, the PEN generates a mass fraction of approximately 0.76 at the same characteristic numbers. The aerosol contains a more than twice as high mass fraction of desired droplets.

Fig. 11 Cumulative drop size mass distribution Q₃ as a function of the related droplet diameter D/d_c for the PEN and CPN at Δp_g* = 600, μ = 0.5 and On = 0.008.

The design of the PEN has several advantages in contrast to the most frequently used nebulizers like the CPN. It is, for example, possible to operate it with different capillary and orifice diameters using the same nebulizer body, by changing the screw cap and capillary. Thus, the nebulizer equipment can be adapted to the sample properties. The easy to clean construction of the nebulizer is another advantage. Since the PEN was made of PEEK and platinum the nebulizer is very robust in handling and it is well suited for routine employment.

In the case of spectroscopy the calculation of the mean drop size is predominantly carried out by the empirical equation of Nukiyama and Tanasawa¹⁸ (N-T equation) proposed in 1939. The N-T equation is:

According to eqn. (3), the dimension of the nebulizer has no effect on the Sauter mean diameter D₃₂. However, in the field of engineering science the influence of the nebulizer outlet diameter on the droplet size has been considered for a long time.^22,23 Thus, the N-T equation, which is deduced from experiments of larger nebulizers, cannot reliably predict the droplet size generated by the small nebulizers used in ICP spectrometry. A better prediction of mean drop sizes can be expected from equations taking into account the linear dimension of the nebulizer as well as considering all significant parameters represented by characteristic numbers. Eqn. (2) is based on significant dimensionless characteristic numbers including the nebulizer outlet diameter d_c as a linear size parameter and considers the momentum transfer within the jet. The parameters C₁, C₂, m and j are dependent on the nebulizer geometry. With the aid of the previously conducted investigations, the parameters were fitted to data from experiments in this study. For the CPN these values are: C₁ = 0.35, C₂ = 0.25, m = −0.75 and j = 1. The values for the PEN are: C₁ = 0.4, C₂ =0.4, m = −0.6 and j = 1. The difference between our data and the parameters described in the literature²³ is negligible, only the parameter m differs slightly.

Droplet size predictions for different gas Laplace numbers 500 ≤ Δp_g* ≤ 700 and varied mass flow ratios 0.5 ≤ μ ≤ 1.5 were performed using the N-T eqn. (3) and eqn. (2) as well. Fig. 12 shows a comparison of the calculations and the experiment at Δp_g* = 600. It demonstrates that the experimental data of the Sauter mean diameters D₃₂ cannot be estimated by the N-T equation. Especially at higher μ, which is a frequent case in spectroscopy, the N-T equation shows significant disagreement with the measured values of D₃₂. This was already mentioned by Browner³ and Sharp.¹In contrast, eqn. (2), based on a dimensionless number, achieves good conformity between the predicted and the measured droplet sizes for both nebulizers. Fig. 13 shows that eqn. (2) enables the estimation of D₃₂ for the used CPN with satisfying agreement with the measured values over the entire range of 500 ≤ Δp_g* ≤700 and 0.5 ≤ μ ≤ 1.5. This graph also demonstrates the fact that the N-T equation does not sufficiently predict the droplet size for pneumatic nebulizers used in spectroscopy. The same is true for the PEN.

Fig. 12 Comparison of droplet size prediction models with data from measurements; the CPN and the PEN were operated at Δp_g* = Δpd_C/σ = 600, droplet size measurement by LDS.

Fig. 13 Comparison of measured and calculated Sauter mean diameter D₃₂ for the CPN at different gas Laplace numbers 500 ≤ Δp_g* ≤ 700 and 0.5 ≤ μ ≤ 1.5 with two different prediction methods.

Owing to the large amount of appropriate droplets in the aerosol generated by the PEN, a higher mass flow rate of the analyte to the ICP can be expected. Consequently, the sensitivity of the spectroscopic method was expected to increase. The influence of the PEN on the performance of the spectroscopic method was investigated by a comparison of both nebulizers using the same instrument and an earlier optimized Scott type double pass spray chamber. In these trials, the detection limits (N = 25, 3s values) of ten elements were determined from the blank solution. In order to find the optimal operating conditions for the PEN connected to the ICP, the nebulizer gas flow rate was varied between 0.4 l min⁻¹ and 0.6 l min⁻¹ and the sample uptake rate between 0.2 l min⁻¹ and 2 ml min⁻¹. Thus, the liquid to air mass flow ratio was varied in a range of 0.2 ≤ μ ≤ 2.8. The lowest detection limits with an assembled PEN were found at a sample uptake rate of 1.3 ml min⁻¹ and a gas flow rate of 0.5 l min⁻¹ at a RF power of 1300 W. The integration time was set to 30 s, an ICP standard solution (Merck IV) was applied and the element lines used for the investigation of the detection limits are shown in Fig. 14.

Fig. 14 Improvement factors of the detection limits achieved at the given element lines with the PEN in comparison with the standard concentric pneumatic nebulizer CPN.

These were the same conditions found earlier for the combination of the spray chamber and the standard concentric nebulizer. It can be stated that considerable lowering of the detection limits in ICP spectrometry is possible by replacing a standard concentric pneumatic nebulizer by a newly designed pneumatic extension nozzle. It was found that in comparative measurements at same flow conditions the detection limits can be enhanced by a factor up to 3.5 (Fe). The increased sensitivity is responsible for this improvement while the noise of the blank or a signal at low concentration is comparable for both nebulizers. The Δp_g* for the PEN under the conditions used for the comparative investigation was about 570 and compared to the previous experiments the conditions can be regarded as “turbulent”. Fig. 14 shows the element lines investigated and the improvement factor of the detection limits, i.e., the ratio of the detection limit with the CPN to that with the PEN, for the elements when the standard nebulizer is replaced by the PEN. This replacement is comparatively easy, as the existing pump and gas supply devices are also well suited to the new system which, in addition, turned out to be very robust in operation.

Conclusion and outlook

The application of the pneumatic extension nozzle (PEN) for sample introduction in ICP spectrometry results in a lowering of the detection limits down to one third compared with the classical concentric pneumatic nebulizers (CPN) for various elements investigated. This improvement is achieved on the basis of a higher aerosol mass fraction of desired droplets in the size range d < 10 μm generated by the PEN. The PEN enables easy adaptation to various ICP instruments since the nebulizer shows very robust operation behaviour at the typical Argon flow rates and sample uptake rates used in ICP spectrometry. It may be expected that the risk of blocking should be lower for the PEN because it has a comparatively large gas orifice diameter compared with the small annular slot width of the CPN. Additionally, due to the modular design and the rugged material used, the PEN can be handled and cleaned easily.

The Sauter mean diameter D₃₂ of the aerosol produced by the PEN depends on the distance a between the capillary and the orifice. Smallest D₃₂ were achieved using a sharp edged gas orifice compared with a rounded orifice and a distance a of approximately a quarter of the orifice diameter d_O.

In contradiction to the well known Nukiyama Tanasawa equation, the equation presented based on non-dimensional numbers shows satisfying agreement of measured and predicted mean droplet diameters for the CPN as well as for the PEN.

The PEN may also be interesting for the application as a direct injection nebulizer. For future examinations the nebulizer will be modified for sample introduction in flame atomic absorption spectrometry (FAAS).

Acknowledgements

The work was funded by the Deutsche Forschungsgemeinschaft (German Research Council, grant No. WA 979/7-1), and by Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie and the Ministerium für Schule und Weiterbildung, Forschung und Technologie des Landes Nordrhein-Westfalen”. The authors want to express their gratitude for this financial support.

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