ICP - Ultrasonic Nebulizer

The ultrasonic nebulizer is an atomization device that is popular for use when enhanced sensitivity is required for analysis applications.

The apparatus consists of a piezoelectric crystal transducer, which is driven by an ultrasonic generator operating at a frequency of 200 kHz to 10 MHz.

Sample is delivered to the front surface of the transducer through tubing from a peristaltic pump at a flow rate of up to 1 mL/min.

An aerosol is formed by the standing longitudinal waves at the surface of the transducer crystal. When the amplitude of the wave is large enough to disrupt the film of solution deposited on the surface of the transducer, an aerosol is generated, which is removed from the nebuHzer by the argon flow.

Delivery of sample at a rate of 1 mL/min yields an analyte transport efficiency of about 20%.

By reducing the delivery rate of sample to the transducer, high transport efficiencies can be obtained. Using a sample delivery rate of 5-20 uL/min, an aerosol transport efficiency of close to 100% can be obtained. Between samples, the surface of the transducer is flushed with large quantities of water to remove all traces of the previous sample.
The aerosol production rate is high and is independent of aerosol carrier gas flow rate, more sample is transported to the plasma at lower injector gas flow rates than that obtained with conventional pneumatic nebulizers.

This results in enhanced sensitivity obtained using this nebulizer, with a corresponding improvement in detection limits for essentially all elements measured.

ICP - Direct Injection Nebulizer

The direct injection nebulizer (DIN) is a total consumption device.

The entire aerosol generated by the nebulization process is injected directly into the plasma.

No spray chamber is used to classify the size of the droplets formed by nebulization, so this technique is 100% efficient. Precision and washout times were significantly improved using this approach. Use of this device also leads to reduced memory effects.
Because the DIN operates at 100% efficiency, a significant solvent load is introduced into the plasma. This can result in increased interferences from molecular compounds formed from solvent constituents.

Detection limits achieved by this method are equivalent to or better than those obtained by other nebulization techniques.
An advantage of the DIN is its ability to function with micro-size samples. Sample volumes as small as a few microliters can be analyzed, making this approach an ideal solution to many special analytical applications.
However, a major drawback to this device is its intolerance to suspended matter in the sample solution. Transport of sample, which is forced through a microcapillary as part of the nebulization process, can be seriously affected by partial clogging from particles or fibers.

This alteration of the sample transport will have a direct impact on the accuracy of the analysis.

ICP - Babington-Type-NebuHzer

Babington nebulizers include several specifically configured devices that are based on a common feature. This feature involves the process of passing the nebulizer gas through an orifice over which the sample is flooded in excess volume. This results in a nebulizer that is relatively immune to clogging or plugging due to suspended material or high dissolved concentration solids.

The USGS Babington nebuUzer is consists of a glass tube with a hemispherical end and a 0.1-mm-diameter orifice in the side. This tube is mounted in a PTFE housing with a sample delivery tube positioned directly above it. Argon gas passes through the orifice at about 600 mL/min.

Sample is introduced through the top delivery tube as 3-5 mL/min.

An aerosol is produced when a thin film of sample flows over the orifice from the top.
The primary advantage of this nebulizer is its ability to produce an aerosol from virtually any liquid material, irrespective of viscosity or suspended solids content.

The performance, in terms of precision and sensitivity is essentially equivalent to the concentric or cross-flow nebulizers.

ICP - Concentric Nebulizer

A very popular type of pneumatic nebulizer is the concentric or Meinhard nebulizer.

This nebulizer is a one-piece device, usually made of glass, that has an internal capillary tube (10-35 um in diameter) mounted in a concentric fashion axial to an external tube.

Nebulizer gas is passed through the external tube at a flow rate of about 1 L/min, which results in sample being aspirated (or preferably pumped) through the internal capillary at a rate of about 0.5-1 mL/min, with aerosol formation occurring at the tip.
As with the cross-flow nebulizer, samples with suspended matter or high dissolved solids concentration can result in partial blocking of the nebulizer, thus inhibiting aerosol formation.

The high-efficiency nebulizer (HEN) is a low-volume low-flow version of the concentric nebulizer.

It operates with a sample delivery rate as low as 10 uL/min. Therefore, this is a suitable nebulizer for use when low sample volume is available.

This nebulizer has good long-term stability, but because its internal capillary diameter is only 75-100 micron, it is prone to blockage.

The microconcentric nebulizer, which is made entirely of an inert material, is highly suitable for the analysis of corrosive materials such as hydrofluoric acid.

This nebulizer operates at a solution uptake rate of about 30 uL/min.

Detection limits achieved with this system are comparable to those obtained with other pneumatic nebulizers.
from suspended particulate matter.

ICP - Cross-Flow Nebulizer

Cross-Flow Nebulizer:
This type of nebulizer consists of two capillary tubes, usually made of glass or quartz, positioned at right angles to each other so that gas flowing through one capillary creates a pressure differential at the tip, which naturally aspirates solution from the other capillary tube at about 3 mL/min.
The efficiency and performance of the aerosol generation process is highly dependent on the alignment of the two capillary tubes.
The energy imparted by the flowing gas disrupts the liquid and results in the generation of an aerosol at the capillary tip.
The drop-size distribution is usually very large for aerosols generated by this type of nebulizer. Better precision can be obtained by forcing sample solution to the nebulizer by use of a peristaltic pump.
Pumping minimizes variations in sample delivery rate due tovariability of sample viscosity and surface tension.
Since the sample solution must pass through one of the nebulizer capillaries, partial clogging or blockage can result from suspended particulate matter in the sample or to a lesser extent high dissolved solids concentrations in the samples that precipitate at the tip as a result of solvent evaporation.This can be a particularly insidious problem because partial blockage can result in an irreproducible inhibition of aerosol formation.
This decrease in aerosol production results in a variable loss of sensitivity that may not be apparent to the analyst.
A modification of the cross-flow nebuHzer is the Meddings-Andersen-Kaiser (MAK) nebulizer. This nebulizer has "fixed" or nonadjustable capillaries that are constructed of heavy-walled glass. This nebulizer typically operates at a gas flow rate of about 500 mL/min. Operating precision of better than 0.5% RSD has been reported.

ICP - Pneumatic Nebulizer

The most common type of nebulizer for producing an aerosol of the sample is the pneumatic nebulizer, of which there are several different designs.
All pneumatic nebulizers share a common feature:

They use the force of a flowing gas, passing through an orifice or capillary tube, to create microdroplets from the liquid sample.

These droplets are transported via the flowing gas stream to the plasma for decomposition, atomization, and ionization.

ICP - Cyclonic Spray Chamber

popular type of spray chamber utilizes a cyclonic design. Instead of using two compartments as with the Scott-type spray chambers, this device uses a tangential rotary flow path in a single circular compartment.

The centrifugal force of the aerosol, as it ascribes a circular path in the spray chamber, results in the larger droplets being forced to the outside where they collide with the wall.

The smaller diameter droplets are swept through the spray chamber and exit to the plasma. Usually the internal volume of cyclonic spray chambers is small (20—40 mL), reducing the washout time by a factor of 2-3.

The use of a cyclonic-type of spray chamber benefits the analyst from the point of view of a simple design and ease of operation.

The nature of the operation of these spray chambers is that only a small percentage (<10%)>

The bulk of the sample is diverted to waste. Therefore, when limited sample is available, special modifications or techniques are required for accurate and sensitive analysis.

ICP - Scott-type spray chamber

Scott-type spray chamber
The aerosol enters the inner compartment and is forced to change direction 180°, returning through the outer compartment, where it exits the spray chamber and is directed to the plasma. An external drain is provided to remove condensed solvent.
For some specialized applications a water jacket or an electrical Peltier cooler is provided that acts as a thermostat for the spray chamber. By lowering the temperature, the vapor pressure of the solvent can be significantly reduced. This results in a substantial reduction in molecular oxide (in the case of water as the solvent) formation and potentially interfering peaks in the background spectrum of the plasma originating from solvent species. Many variations of this type of spray chamber are available.

ICP - Spray Chamber

To improve aerosol performance, techniques are used to classify the droplets before the aerosol is transported to the plasma.

This results in a narrow distribution of droplet sizes being introduced into the plasma, preferably with a small mean droplet diameter.

The most effective way to accomplish this classification is to use a spray chamber, which provides an expansion chamber and a circuitous route for the droplets to travel en route to the plasma.

The larger droplets, which have higher momentum, collide with the walls of the spray chamber where they are condensed.
Only the smallest diameter particles survive the process and are transported through the system for the use of a spray chamber to classify aerosol droplet size.

The bimodal distribution with a measurable quantity of large-diameter droplets. After passing through a spray chamber, the size distribution is much narrower and exhibits a smaller mean droplet diameter.
Several configurations and designs of spray chambers are prevalent and are used with different types of nebulizers.

ICP - Nebuliser

Nebulizers perform the function of converting liquid samples to an aerosol, consisting of finely divided droplets, which are suspended in the plasma carrier gas. This aerosol, which is presumably representative of the composition of the original sample, is transported to the plasma torch, where it is injected into the central channel of the plasma for subsequent atomization and ionization.

The liquid aerosol is introduced into the "thermal front" at the base of the plasma, the solvent is evaporated when the temperature of the aerosol reaches the solvent s boiling point.

The resulting "dry aerosol" continues to be transported into a higher temperature region of the plasma. When the melting point, followed by the decomposition temperature of the residue is reached, simple molecules and atoms are formed.

Finally, in the highest temperature region of the plasma, complete atomization occurs, and the resulting atoms are efficiently ionized prior to being sampled by the mass spectrometer interface and transported to the UV - Visible Spectrophotometer / mass spectrophotometer for analysis. In addition, molecules formed from the decomposition of the dry aerosol or recombination by collisions of the atoms can form ions also.
The aerosol formed by the nebulization process creates a population of droplets that have a distribution of sizes, ranging from a mean diameter of about 1 to 80 Micron.

The more uniform the droplet size (i.e., the narrower the size distribution) the more precise the results of the subsequent analytical determinations.

Larger droplets require more energy to evaporate the solvent and subsequently more energy to vaporize and atomize the post solvent removal residue, resulting in local instability in the plasma.

This instability is reflected in the measured ion currents of the analyte elements.

ICP - Torch

The torch is a device that is used to contain and assist in configuring the plasma.

Torches are typically made of materials that are transparent to the RF radiation. Therefore, they do not attenuate the field generated by the load coil/antenna. Torches can be made of materials such as ceramics or boron nitride; however, by far, most are made from quartz, which has a sufficiently high melting point to allow it to maintain its configuration when operated at temperatures commonly experienced with argon ICPs.
A simple quartz tube centered in the load coil with laminar flow argon gas passing through it at 10-20 L/min will form a simple plasma, when RF power is applied and seed electrons are provided.The plasma or "fireball" that forms will have the shape of a prolate spheroid. If an excessive amount of power is applied, the plasma will reach a sufficiently high temperature to melt the quartz tube, which confines it.

This configuration is not fully suitable for analytical spectrometric purposes. It is difficult to efficiently inject a sample aerosol into this type of plasma because of the "skin effect" created by the potential barrier at the surface of the plasma.

This barrier tends to deflect aerosol particles around the outside of the spheroid, rather than entrain them in the plasma. Unless appreciable sample can reach the optimal excitation region of the plasma, analytical sensitivity will be significantly limited.

This torch consists of three concentric quartz tubes.

A coolant gas (argon) is introduced into the space between the outer and center tubes at a tangential direction relative to the longitudinal axis of the torch, creating a vortical flow. This gas stream serves two purposes:

1. It isolates the plasma from the internal wall of the outer quartz tube preventing melting, and

2. It encourages the formation of a toroidal (annular)-shaped plasma.

The center tube is for the injection of sample aerosol into the plasma.

The space between the injector and the intermediate tube is used for the introduction of an auxiliary flow of argon gas to assist in the formation of the plasma and to ensure that the plasma is forced away from the tip of the injector, preventing it from melting.

This auxiliary gas flow is usually very low compared to the coolant gas flow and often is not used. Typical argon gas flow rates for the various inputs are listed below.

Typical Operating Conditions for an Argon ICP

Power-----------------------1-2 kW

Argon flow

Coolant-----------------------15 L/min

Auxiliary---------------------0-2 L/min

Nebulizer---------------------1 L/min
Sampling depth*--------------14-18 mm (* Beyond last turn of load coil).
These flow rates can vary depending on the specific dimensions and configuration of the torch being used.

LCP - Load Coil

The load coil, which is either an integral part of the RF oscillator circuit (free-running generator) or part of the tuning network in a crystal-controlled oscillator system, usually consists of two to three turns of 3-nimdiametercopper tubing wound in about a 3-cm-diameter spiral.

Cooling liquid or gas is circulated through the coil to dissipate thermal energy, which minimizes distortion of the coil from overheating.

The coil serves as an antenna to produce an electromagnetic field, which sustains the plasma.

It can be thought of as the "primary" winding of a RF transformer, with the "secondary" winding being the plasma itself, thereby transferring energy to maintain the plasma.

The load coil is usually grounded to earth potential at its front turn (closest to end of torch), rear turn (closest to the sample injector), or center turn. The grounding location influences the formation of a residual secondary discharge at the plasma sampling interface, which can have an significant impact on the formation of molecular oxide and doubly charged ion species in the ion beam of the mass spectrometer.

ICP - Solid-State Generators

Solid-state semiconductor circuitry has been utilized for the generation of high-power RF energy. Although these generators are significantly less expensive to manufacture, they are usually less efficient at producing higher power levels. The circuitry for these generators is the solidstate analogue for the crystal-controlled oscillator described previously.

Some solid-state systems can be operated at either 27.1 or 40.6 MHz at the operators discretion, with an impedance-matching tuning network functional for either frequency. These types of generators still require a highpower output amplifier tube to produce the energy required to sustain a plasma.

ICP - Generator

Two basic types of electronic circuits are commonly used to produce the RF energy required to operate an ICP:
1. The fixed-frequency crystalcontrolled oscillator and

2. The free-running variable frequency oscillator.

Either type of circuit is fully suitable for operating a plasma configured for ion generation. Radio-frequency generators that are used to operate ICPs are basically simple circuits with a limited number of components, which produce an alternating current at a specific frequency. These generators must be capable of operating with up to 2 kW of output power to adequately sustain an atmospheric pressure argon plasma.

The basic frequency of this generator is controlled by a piezoelectric crystal in the feedback circuit of the oscillator.

The crystal-controlled oscillators for ICPs typically operate at a frequency of 13.56 MHz; the power supply circuitry includes a frequency doubler to provide a typical plasma operating frequency of 27.12 MHz.

Although other frequencies have also been used, this is the most commonly utilized crystal-controlled frequency in plasma systems.

At the higher frequency of 27.12 MHz, the coupling between the generator and the plasma is more efficient, which makes the plasma more robust with regard to sample introduction stability.

A thermionic amplifier tube is used to provide the high-power output needed to operate an atmospheric pressure plasma.

Crystal-controlled systems usually require a servomotor-operated variable capacitor in an impedance-matching network to maintain tuning of the system and minimize reflected RF power, which extends the functional life of the power tube.

How ICP - Plasma generated ?

Inductively coupled plasmas are formed by coupling energy produced by a RF generator to the plasma support gas with an electromagnetic field.

The field is produced by applying an RF power (typically 700-1500 W) to an antenna (load coil) constructed from 3-mm-diameter copper tubing wrapped in a two- or three-turn 3-cm-diameter coil, positioned around the quartz torch assembly designed to configure and confine the plasma.

An alternating current field is created that oscillates at the frequency of the tuned RF generator. The plasma is initiated by the addition of a few "seed" electrons, generated from the spark of aTesla coil or a piezoelectric starter, to the flowing support gas in the vicinity of the load coil.

After the plasma is initiated it is sustained by a process known as inductive coupling. As these seed electrons are accelerated by the electromagnetic RF field, collisions with neutral gas atoms create the ionized medium of the plasma.

The mean free path of accelerated electrons in atmospheric pressure argon gas is about 1 um before a collision occurs with an argon atom. These collisions produce additional electrons.This cascading effect creates and sustains the plasma.
Once the gas is ionized, it is self-sustaining as long as RF power is applied to the load coil. The ICP has the appearance of an intensely bright fireball-shaped discharge.

What is Plasma?

Plasma is an electrically neutral gas made up of positive ions and free electrons. Plasmas have sufficiently high energy to atomize, ionize, and excite virtually all elements in the periodic table, which are intentionally introduced into it for the purpose of elemental chemical analysis.
There are many types of plasma (direct current, microwave induced, etc.), the inductively coupled plasma (ICP) has demonstrated the most useful properties as an ion source for analytical mass spectrometry.

Direct current plasma (DCP) is obtained when a direct current field is established across electrodes,
ICP is obtained when a high-frequency (hf) field is applied through a coil,
Microwave-induced plasma (MIP) is obtained when a microwave field is applied to a cavity.

DCP was the first described and commercialized plasma. However, the ICP is currently the most commonly used plasma because of some unique properties. Originally, the ICP was designed for the production of crystals. The first analytical applications of the ICP were published in 1964 and 1965.

Gases such as argon, helium, nitrogen, and air have been used to sustain plasma useful for analytical purposes; however, the inert gases give some advantages, because of their desirable ionization properties and their availability in relatively pure form.

Impurities in the plasma support gas can result in spectral interferences, leading to inaccurate quantitative measurements.
Inert gases, specifically argon, also have the advantageous property of minimal chemical reactivity with various analyte species, which can also result in undesirable analytical results.