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Thermo Scientific iCAP 7000 Series ICP-OES

Gas Control Systems

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Key Words

ICP gas control, Pressure control, Mass flow control

Introduction

The majority of commercial Inductively Coupled

Plasma-Optical Emission Spectrometers (ICP-OES) use aplasma torch based on the original Fassell design,

consisting of 3 glass tubes (Figure 1).

The three concentric quartz glass tubes of the torch are

precisely aligned to ensure that gas flows meet the

required velocity in a uniform fashion so that the plasma

formation is stable. Typical gas flows within the torch

are:

Coolant or plasma gas 12 l/min with some applications

requiring slightly higher or lower flows

Auxiliary or intermediate gas 0.5 l/min with high salt

or organic applications requiring up to 2 l/min

Nebulizer gas 0.5 l/min depending upon the type of

nebulizer

Figure 1. ICP-OES torch design

It is critical that these gas flows remain constant in order

to achieve optimum instrument performance, resulting inlong term stability of the analytical signal. As each gas

flow serves a different purpose, changes in their flow rates

affect both the plasma and signal differently. The

nebulizer gas flow affects the analytical signal, with only

small changes in the flow rate causing relatively large

changes in the signal; this effect is commonly referred to

as instrument drift. For example, a 0.1% change in the

gas flow can result in a 1% change in the analytical signal.

The overall shape of the plasma is affected by the coolant

gas flow, at higher flows the plasma becomes elongated

and has a larger annular space, which affects the axial

channel, squeezing it and reducing its diameter. Stabilityof the coolant gas flow is particularly important for axial

plasmas where the viewing zone may only be 1-2 mm

in diameter.

The auxiliary gas flow affects the relative position of the

plasma, which is particularly important for radial

plasmas. Small changes in the position of the plasma

affect where the radial viewing channel passes through

the plasma which alters the signal intensities differently

for each emission line, as higher energy lines (UV) are

more emitted more frequently in the centre of the plasma.

Two types of gas flow control systems are available forthe Thermo ScientificiCAP7000 Series ICP-OES.

Coolant Gas

Auxillary Gas

Nebuliser Gas

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2 Pressure Control System

A pressure control gas system operates using the

relationship between gas pressure and the corresponding

flow through a fixed orifice, as seen in figure 2.

Figure 2. Schematic of pressure control system

The high pressure input is first reduced with a precision

regulator to the operating pressure of the system.

Coolant and auxiliary gas flows are diverted to a series of

fixed restrictors each controlled with a solenoid valve. In

order to achieve the required flow, combinations of valves

are opened to allow the gas to pass through different

restrictors. The nebulizer gas is passed to a variable, high

precision pressure regulator. The regulator can be

adjusted to give the desired gas flow.

Pressure control gas systems are very robust, reliable and

simple to operate. However, the use of fixed restrictors

can mean a limited number of steps for the gas flow.

Also since gas density changes with temperature, if the

temperature of the incoming gas varies greatly the gas

flow rate will change, affecting the long term stability.

Pressure control of the gas flows is a cost effective design

and ideal for application where a limited number of

methods with similar parameters are used. If different

nebulizer flows are used with different methods then the

flow will need to be manually adjusted each time the

method is changed.

Mass Flow Control System

Mass flow controllers, as the name suggests, operate by

controlling the actual mass flow of the gas. A typical

mass flow controller consists of 3 main components: a

control valve, a flow sensor and the electronic control

circuitry as show in figure 3. The sensor consists of a

long, thin capillary tube with temperature dependent

resistors on the outside. When gas starts to flow the

temperature profile becomes asymmetric, with the

corresponding shift in resistance being proportional to

the gas flow rate. This changing resistance is converted toa voltage and used to operate the gas flow control valve.

Figure 3. Schematic of a typical mass flow controller

The main benefit of mass flow controllers is that since

they operate using electron sensing of the gas flow they

are not affected by changes in atmospheric pressure or

ambient temperature, giving better long term stability.

In addition they offer rapid response times, typically lessthan 1 second, and full computer control. The gas flow

settings are saved in the Thermo Scientific Qtegra

software LabBooks and are automatically set when

analysis is performed.

Due to this automated computer control, full mass flow

control is ideal for complex applications where multiple,

varying analytical methods are required.

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Conclusions

By utilizing mass flow control for the nebulizer gas flow,

significant benefits can be gained in both long term

analytical stability and ease of use. By adding mass flow

controllers to the auxiliary and coolant gas flows,

performance will be further improved when using axial or

radial plasma viewing. Figure 4 displays the long term

(over 16 hours) analytical stability achieved when using

full mass flow control.

Figure 4. Long term signal stability using full mass flow control

Table 1. Recommended configurations

* Thermo Scientific iCAP 7400 ICP-OES with additional MFC

# Thermo Scientific iCAP 7600 ICP-OES for increased sample

throughput with Sprint Valve as standard upgrade

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Model 7200 7400 7400* 7600 #

Nebulizer Gas Pressure Control Mass Flow Control Mass Flow Control Mass Flow Control

Auxiliary & Control Gas Pressure Control Pressure Control Mass Flow Control Mass Flow Control

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