Loss of or Change in Agilent ICP-OES Sensitivity: Causes and Corrections

Answer

This Information Applies To: Agilent 5000 Series ICP-OES (5100/5110/5800/5900) and Agilent 700 Series ICP-OES (710/715/720/725/730/735)

Issue

Loss or change of sensitivity in ICP-OES can occur under normal operating practices. This discussion identifies factors that can contribute to sensitivity changes including calibration standards, memory effects, and the sample introduction system. Sample introduction components include:

Autosampler probe
Switching valve
Pump tubing
Nebulizer
Spray chamber
Torch

Calibration standards

Calibration standard preparation is a common and frequently overlooked source of sensitivity changes. Check the standard preparation history for:

Changes in sensitivity that coincide with preparation of new stock or working standards.
Recent preparation of standards using large dilutions. The error in the preparation increases with increased dilution factor.
Reduced sensitivity for one or two elements, indicating these elements may have dropped out of solution.

Prepare new standards if any of the preceding are observed.

Memory effects (carry over)

Memory effects occur when an element or elements in a single sample are retained in the sample introduction system. These elements bleed out of the system in subsequent sample runs and contribute to increased background. The degree of memory effect depends on the:

Element. The most common elements to cause memory effects are Ag, Au, B, Hg, Mo, Si, Sn, W, Zn, Zr.
Sample introduction component contamination from the sample matrix.
Inadequate routine maintenance.
Sample rinse cycle. Rinse cycle length impacts sample run time and throughput.

Memory effects can be found by examining the analyte background following a sample with a high concentration of the analyte. Memory effects are present if in subsequent samples, the background is highest in the first replicate analysis and reduces in following replicates.

Sample introduction system

The sample introduction system components can contribute to changes in sensitivity. Agilent recommends routine maintenance to reduce changes in sensitivity. The sample introduction components include:

1. Autosampler probe

The autosampler probe flexes during movement into and out of sample tubes. Over time, flexing can cause hairline internal cracks that retain elements and sample contamination. To check the sample probe:

Replace with a new or a known intact probe and reanalyze the standard.

Typically, maintenance and cleaning are performed on other sample introduction components before replacing the probe.

2. Inlet switching valve system

The switching valve system improves sample productivity. Over time, sample deposits can accumulate and block or partially block the valve. To check the switching valve for blockage:

Isolate the valve.
Run standards with manual sample introduction.

If sensitivity returns to normal levels, clean the valve.

3. Pump tubing

Peristaltic pump tubing delivers consistent sample flow to the nebulizer. It is a consumable item that stretches during normal use and when significantly stretched it delivers variable sample flow. Check tubing daily and look for the following signs of wear:

Loss of elasticity and stretching. Roll the tubing between your fingers. Stretched tubing will feel oval or flat in shape and will not reshape quickly after being pinched. Tubing in good condition will feel round and reshape quickly.
Cracks in the tubing, which can cause liquid on the outside or bubbles on the inside of the tubing.
Visible sample deposit blocking or partially blocking the tubing.

Tubing has a typical lifetime of one to two weeks based on an eight-hour working day. Replace tubing earlier if these problems are observed.

4. Nebulizer

The nebulizer operates under backpressure to turn the liquid sample into an aerosol. Monitor backpressure through software diagnostics and record the normal operating backpressure for each application. An increase in backpressure can indicate a blocked or partially blocked nebulizer, a decrease in backpressure can indicate a leak. When a change in backpressure occurs visually check the nebulizer for the following:

Blocked nebulizer tip.
Liquid trapped in the nebulizer.
Chips or cracks in the glass.
Aerosol formation in the spray chamber.

Replace or clean the nebulizer when these problems are observed.

5. Spray chamber

The sample mist created by the nebulizer expands in the spray chamber and is carried into the sample torch in an Argon stream. Contamination on the inside surface of the spray chamber can cause the mist to bead, resulting in incomplete sample transfer. Check the spray chamber for the following: