Key Instrument Parameters

Gas Flow Rates

There are three gas flow rates for the common torch designs. The outer gas flow is sometimes referred to as the coolant or plasma gas flow; the middle or intermediate gas flow is sometimes referred to as the auxiliary gas flow; and the central gas flow is referred to as the sample or nebulizer gas flow. When working with aqueous solutions, the outer and intermediate gas flows do not have a great impact upon the performance characteristics and the values suggested by the manufacturer should be used for common applications. However, the sample gas flow rate will vary between nebulizers of the same design and require adjustment on a regular basis.

Sample Ar Gas Flow for ICP-OES

Assuming sample solution is not significantly limited, the main consideration when adjusting the sample Ar gas flow is that of precision. Increasing the sample Ar gas flow does not necessarily increase the emission intensity. The objective in setting this flow rate is to obtain the best detection limit. Noisy signals will typically result from higher flow rates that will serve to degrade the stability of the plasma, increase the short-term measurement precision and consequently give poorer detection limits.

The following considerations should prove helpful:

• Nebulizers of the same type (design) will not necessarily give optimum performance at the same Ar flow rates.
• Each new nebulizer should be optimized.
• Determining the standard deviation of a dilute solution of a common analyte at different Ar flow settings is a simple way of determining the optimum flow setting for a given nebulizer.
• Use the Ar flow that gives the lowest standard deviation i.e. best precision.
• Most nebulizers come in a box or container that can be labeled with the optimum Ar flow determined for your instrument.
• Regularly check each nebulizer to confirm that the Ar flow rate is indeed optimal. Partial plugging, chipping, corrosion, matrix deposition, or an ailing mass flow controller are possible causes for a change in the optimum setting or an inability to reproduce the same precision as when the nebulizer was new.
  

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