Key Instrument Parameters
The performance characteristics of an ICP is a function of a variety of instrumental parameters. Current instrumentation has many parameters that are fixed by the manufacturer and all instrumentation will come with recommended settings for those parameters that are not. The purpose of this section is to point out the key parameters that will require adjustment on a regular basis. This discussion will be limited to the introduction of the analyte as a nebulized solution and Ar as the plasma gas.
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.
Applied Power for ICP-OES
The second key parameter that the operator may wish to vary is the applied power. Higher applied power will increase the net signal intensity but not necessarily improve the detection limit.
The following information may prove useful:
- The net signal intensity will increase as the applier power is increased.
- The background signal intensity will increase as the applied power is increased.
- In the early days of ICP there was a lot of discussion about optimum power settings and observation heights (for radial view). Over the years manufacturers have determined the optimum power and observation height settings. Therefore, first try using the settings recommended by the manufacturer.
- If changes in applied power are made then determine the effect upon the detection limits of the analytes of interest.
- Higher net signal intensities will not necessarily result in lower (better) detection limits.
IMPORTANT: Sample Ar Gas Flow cannot be separated from Applied Power and Sampling Depth for ICP-MS.
The sample Ar gas flow for ICP-MS systems is a parameter that is more complex than with ICP-OES instrumentation. Assuming the goal is to obtain the maximum signal intensity, the Ar gas flow is closely related to the applied power and sampling depth. There is not a single set of optimum power, sampling depth, and sample Ar flow settings. For example, a higher applied power will increase the signal intensity but change the optimum sampling depth and sample Ar flow. However, the higher sample Ar flow rates required at high power bring about some degradation in other performance characteristics. If the applied power is constant for every method, then the optimum sampling depth will change as the sample Ar is changed. The consideration of MO (metal oxide) formation and different sensitivities at different mass ranges must also be made with increased sample Ar flow.
Here are some final observations that may prove useful:
- Higher applied power will require higher sample Ar flow to achieve optimum sensitivity.
- Increased sample Ar flow will shift the ion zone in the direction of the sampler cone orifice.
- As applier power is increased, the ion zone will shift away from the sampler cone orifice.
- At a constant applied power and sampling depth, MO formation will increase with higher sample Ar flow rates.
- At a constant applied power and sampling depth, M+2 formation will increase with higher sample Ar flow rates.
- Deposition on the sampler cone will increase with higher sample Ar flow rates.
- At a constant applied power and sampling depth, an increased sample Ar flow will boost the sensitivity of the lighter masses relative to the heavy masses.
- Determine the effect that changes in applied power, sample Ar flow, sampling depth, and peristaltic pump speed make on your particular model instrument using a suite of elements covering the mass range. A mixture of Mg, Rh, Ce, and U should suffice where the CeO and Ce+2 masses are measured as well.
The above observations may seem confusing, but in reality they give the operator a degree of flexibility that the ICP-OES operator does not have in that you can optimize the instrument for selected mass ranges. For example, we know that a higher applied power will increase the signal intensity. We also know that there is an optimum gas flow for each nebulizer. Therefore, if an applied power of 1.35 kW is selected and we know that our nebulizer performs best at an Ar gas flow of 1.0 L/min then the nest step is to adjust the sampling depth to give the optimum signal while aspirating a solution containing a combination of light, mid-range, and heavy elements such as Mg, Rh, Ce, and U. If the double ion or MO signals are higher than desirable, a reduction in the peristaltic pump tubing diameter or pumping speed should lower these signals. These initial adjustments will take a lot of time and patience but they are well worth the effort. As the operator makes adjustments in these key parameters, a pattern will begin to unfold allowing the operator to optimize the instrument for selected mass ranges.
It is suggested that new ICP-MS operators take the time to determine the trends when changes in applied power, sample Ar flow, sampling depth and peristaltic pump speed are made.