Aerosol Generation and Sample Transport
Barry L. Sharp and Ciaran O’ Connor
4.1
Introduction
Sample introduction is the principal means by which the analyst can tailor the perform-ance of inductively coupled plasma–atomic emission spectrometry or mass spectrometry (ICP-AES or MS) to particular analytical tasks. Whereas formerly sample introduction was seen as a limiting part of the instrumentation, now it is this aspect that brings enormous versatility to the applications of these techniques. Interfaces for solid, liquid and gaseous samples have been described and in turn these have been coupled to a variety of separation techniques including gas chromatography (GC), liquid chromatography (LC), capillary electrophoresis (CE), gel plates and field flow fractionation. Developments in sample introduction for the ICP source have been largely responsible for bringing about conver-gence between the historically separate fields of elemental and organic analysis.
General accounts of sample introduction have been provided by several authors includ-ing, for liquids: Browner [1], Montaser [2], Sharp [3,4] and Mora [5]; and for solids Durrant [6], Russo [7] and Günther [8].
4.2
Sample introduction characteristics of the ICP source
4.2.1
Particle size distribution
The convention in discussing the sample aerosol is to describe the aerosol produced by the aerosol generator as the primary aerosol, and that entering the plasma as the tertiary aerosol [1,9]. It is important in any discussion of sample introduction techniques to remember that the ICP only responds to the latter, having no knowledge of any prior process.
surviving in the Ar plasma [13]. A narrow particle size distribution is also advantageous since it facilitates optimization in terms of the height of viewing (AES), or depth of sam-pling (MS), provides a uniformity to the pattern of matrix interferences, and maximizes the signal-to-noise ratio [14]. Hobbs and Olesik studied the fate of individual particles passing through the ICP and provided a useful model of desolvating/vaporizing particles and their effect on the local plasma conditions and the observed analytical signal [15].
Recent work in the field of laser ablation (LA) has suggested that for the plasma to process particles in a truly composition independent fashion requires particles⬍90 nm in size (based on the analysis of glass) [16]. This is well below the median diameter of the particle size distributions produced by common sample introduction systems and is therefore an impractical working standard. It is a further reminder however that the analytical signal is heavily dependent on the nature of the sample introduction process and reflects nuances that are not always obvious from cursory observation. Careful calibration, with appropriate attention to solvents and matrix composition, is required to control these subtle and some-times unsuspected variances.
Although it is convenient to discuss particle size distributions in terms of particle diam-eter, the analytical signal is dependent on the mass distribution which introduces a ‘d3’ term into the equation. It is worth noting that a 10m particle, which is only marginally transported and processed by the sample introduction system and plasma, contains 1000 times the analyte of the more desirable 1m particle. Thus, any tendency to polydispersity above a few microns is highly detrimental. Another common measure of particle size is the volume-to-surface area mean diameter or Sauter mean diameter given the symbol D3,2[1]. See Section 4.3.1 for some typical particle size distributions.
4.2.2
Plasma loading
The ICP is a remarkably robust source, not least because power is not directly coupled into the central channel where the sample is introduced. Nevertheless, for a given plasma power there is a maximum plasma loading that can be tolerated, above this level the plasma robustness is reduced leading to a greater propensity for matrix interferences and ulti-mately instability and extinction of the discharge [17]. A typical nebulizer and spray cham-ber combination will deliver about 20l min⫺1(20 mg min⫺1) of liquid aerosol to the plasma accompanied by an equivalent mass flux of vapour. Fig. 4.1 shows the achievable vapour loading at saturation for 1 l min⫺1of argon (a typical sample introduction flow rate) at various temperatures. Saturation of the liquid aerosol with vapour is guaranteed by the large surface area of the aerosol and the increased vapour pressure inside the droplets due to the surface tension. The maximum rate of sample input for an aqueous aerosol has not been investigated as such, but indirect evidence from using more efficient types of sample introduction (e.g. the ultrasonic nebulizer, USN) suggest that the maximum lies within a factor of ~5 of this typical value. Practically the limit is more likely to be set by matrix problems rather than simple liquid flux.
generator has a robust impedance matching capability. In contrast, a relatively small flux of volatile solvent can cause extinction of the plasma. The reasons for this are twofold. Firstly, 20l min⫺1of water, for example, translates to 0.6 l min⫺1of vapour at 7000 K, so a signifi-cant increase in the central channel flow arises when liquid is vaporized. Secondly, when this vapour is derived from a volatile solvent, it is produced low down in the central chan-nel and can diffuse out into the energy coupling region of the discharge. A high enthalpy solvent or one that produces stable fragments can depress the electron energies to the point where the plasma equilibrium is destroyed.
The evaporation of a droplet with an initial diameter r0is described by the equation [14,18]:
(4.1)
where:
r is the radius at time t
D is the vapour diffusion coefficient is the surface tension
ps is the saturated vapour pressure is the density
M is the molecular weight R is the gas constant T is the temperature
Thus a variety of factors determine how much liquid is partitioned to the vapour phase in a given time. Vapour flux is usually controlled by cooling or aerosol ‘drying’, but for the rea-sons discussed above, care is needed when combining hot surfaces with aerosols as the
r r D p M RT t 3 03 s 2 6 ⫽ ⫺ s r ⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ 0 100 200 400 700 0 20 40 60 80 100 Temperature (°C) Mass of water (mg) 800 600 500 300
flash evaporation of large aerosol droplets produces pressure pulses that result in noise in the analytical signal.
4.3
Liquid aerosol generation
The processes of aerosol generation and transport have been widely studied and some excellent reviews produced that explore the underlying theory [1–4,19–22].
The discussion here will be restricted to identifying only the key factors so that the basis of successful designs may be understood. Whilst understanding the theory is certainly a necessary starting point, being able to produce the design in suitable materials, within tight tolerances and to an acceptable cost are equally important.
Many processes have been invoked to produce the primary aerosol [1,2,4], but commer-cially pneumatic nebulization and ultrasonic nebulization are the only ones that have made significant impact and so the discussion here is restricted to these types of device.
4.3.1
Pneumatic nebulization
A typical pneumatic nebulizer for ICP spectrometry operates at 0.6–1.2 l min⫺1gas flow with a liquid flow rate of 0.5–1.5 ml min⫺1. These are here defined as standard conditions, lower flows, notably of the liquid, are referred to as micro-flow conditions.
The processes responsible for producing aerosol droplets have been extensively studied [23,24] and are qualitatively described by three mechanisms:
● Low velocity disintegration of large free, or attached, droplets of liquid (Rayleigh–Taylor instability).
● Disintegration of liquid filaments stretched out from the liquid body by the gas flow.
● Film formation followed by break up of wave-like structures formed on the surface of the film.
The gas provides the energy; surface tension provides the ‘returning’ force for the oscillations and viscosity the damping force. Thus generally, high-velocity gas combined with low surface tension and viscosity liquid produces the finest aerosols. Film-formation accounts for the smallest droplets, whilst Rayleigh–Taylor instability may lead to larger droplets.
where:
dlis the liquid outlet diameter (cm) lis the liquid surface tension (dyne cm⫺1)
land gare the liquid and gas densities, respectively (g cm⫺3) V is the difference in velocity between the liquid and gas (cm s⫺1) Qland Qgare the liquid and gas flow rates, respectively (cm3s⫺1) lis the liquid viscosity (poise)
This equation shows that to produce a fine primary aerosol requires:
● A high gas-to-liquid ratio (commensurate with the central channel flow limitation of the ICP).
● A small liquid channel diameter (this being large enough to avoid adventitious blockage).
● A small gas flow channel area to maximize V.
● Low viscosity.
● Low surface tension.
● High gas density (so Ar is more efficient than e.g. He).
The finer aerosols produced by adhering to these axioms improve transport efficiency and enable equivalent detection limits to be achieved at much lower uptake rates.
Typical primary aerosol drop size distributions for some common nebulizers are shown in Fig. 4.2 [2,26,27]. Note the finer aerosol produced by the high-efficiency nebulizer with the normal device.
4.3.1.1
Pneumatic nebulizer designs
A wide variety of pneumatic nebulizer designs have been described (oscillating capillary, sin-gle bore [28,29], HEN [27,30], direct injection high-efficiency nebulizer (DIHEN) [31–36], etc.), but here the focus is on devices that are commercially available and are therefore in widespread use. Pneumatic nebulizers are described by their basic geometry and thus Fig. 4.3 a–e shows the configurations for the concentric (similarly for micro-concentric), DIHEN, Babington (Cone–Spray version), cross-flow and Parallel Path designs. Choosing between
Drop diameter (µm) Drop diameter (µm) Drop diameter (µm) % Volume Cross flow (a) (b) (c) TR-30 1 10 100 1 10 100 1 10 100 HEN 0 10 20
