This Information Applies To: Agilent 5000 series ICP-OES systems
Issue
Poor or high relative standard deviation, or a noisy signal is an indication of poor precision during analysis.
Resolution
There are many potential causes for an instrument exhibiting poor precision, including but are not limited to:
The first step in troubleshooting poor precision is to identify the problem.
Identifying the problem
Poor or high relative standard deviations are usually caused by poor functioning of the sample introduction system.
Poor precision can also be a symptom of low sensitivity. Make sure that intensities are correct by running the Instrument Performance Tests and then comparing the observed Resolution, Sensitivity, and Precision values to values listed in the test before attempting to resolve precision issues. If results are comparable, it suggests the problem may be related to the method (application) and may not be a fault of the instrument.
If any of the results vary by ~20%, then rerun the test with a known good, preferably new, SeaSpray Nebulizer and cyclonic spray chamber to eliminate the nebulizer as a possible cause. Poor comparable Precision numbers will verify that a problem exists.
The following table will give you some direction as to failure mode and possible solution. Precision is a measure of signal repeatability over multiple replicates (typically 10 replicates for instrument performance test) and expressed as %RSD. A single outlier (high or low) can cause this test to fail.
If poor or high relative standard deviations are observed, it will need to be determined if it is caused by drift (for example, steady increase or decrease of replicate signals), noise, or an outlier.
| Failure Mode (Precision) | Possible cause | Possible cause (Next level) |
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All or most wavelengths have poor precision |
Sample introduction components contaminated |
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| Inconsistent sample delivery |
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| Optimization of parameters |
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| Wavelengths off peak |
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| Running partially soluble (salts) |
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| High salts |
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| Previously running organics |
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| Unstable plasma |
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Some wavelengths have poor precision |
Wavelengths off peak |
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V-groove nebulizer
Check the nebulizer for blockage by aspirating distilled water through the sample introduction system. Carefully remove the nebulizer from the spray chamber. Hold the nebulizer in its normal operating position, turn the pump to normal speed, and set the nebulizer flow rate to 0.75 L/min (see Figure 1).
Observe the spray. The spray should be even and consistent (see Figure 2). If an even spray cannot be achieved by adjusting the peristaltic pump, clean or replace the nebulizer.
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Tips: There will be some degree of pulsing visible, which is acceptable.
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Figure 1. Nebulizer flow set at 0.75 L/min
Concentric glass nebulizer
Inspect the nebulizer tip. If the tip is damaged, replace the nebulizer.
Check the nebulizer for a blockage. Aspirate distilled water through the sample introduction system, carefully remove the nebulizer from the spray chamber, and observe the spray emanating from the nebulizer tip.
The sample should spray evenly and consistently from the tip of the nebulizer, depending on the pump speed (see Figure 2). If the spray pattern is irregular, the nebulizer opening may be blocked in which case you will need to clean or replace the nebulizer.
Check the capillary within the nebulizer. An internal blockage may require an excessively high nebulizer flow to nebulize a solution.
Figure 2. Nebulizer aerosol formation with peristaltic pump on and nebulizer gas on
1. Good aerosol formation, 2. Poor aerosol formation, 3. Poor aerosol formation
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Tips: A leaking sample inlet capillary often signifies a blockage in the nebulizer capillary.
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Torch
Check the torch for deposits that may interfere with the instrument optics. A clean injector tube is essential. Clean the torch if necessary.
If the injector tube of the torch is broken or worn, replace the torch.
Contamination in spray chamber
Disconnect the spray chamber from the sample introduction system and attempt to clean the spray chamber to remove the contamination. If contamination is present after cleaning, then replace with a new spray chamber.
Pump/Pump tubing
To check the operation of the pump and pump tubing, clamp the peristaltic pump tubing, turn on the peristaltic pump to normal speed, and dip the sample tubing in and out of the solution and watch the sample uptake along the capillary. If the liquid flow path is not smooth, or no liquid is taken up it could be one or more of the following factors:
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Tips: There will be a small degree of pulsing visible, which is acceptable.
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Air leak
Check all tubing and connections in the sample introduction system for air leaks.
Instrument calibration
If the instrument is reading on the edge of a peak (due to an instrument calibration error) the signal may be noisy. In this case, perform an instrument calibration.
Aqueous analysis after non-aqueous analysis
When performing aqueous analysis after running non-aqueous organic samples (or vice versa), a mutually miscible solvent must be used to ensure a thorough washout.
Traces of organic samples can remain in the spray chamber or torch. Rinse and if necessary, clean the spray chamber or torch when switching from organics to aqueous samples.
Partially soluble salts
Salts that are only sparingly soluble may partially precipitate when nebulized, causing a noisy signal. Converting the salt to a more soluble form may help reduce the noise.
High salt content
Samples with a high salt content exhibit more noise than more dilute solutions. If better precision is required, dilute the sample.
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Learn how to effectively operate your Agilent 5000 Series ICP-OES:
ICPOES-5900-1200e - Agilent 5800/5900 ICP-OES with ICP Expert: Instrumentation Overview e-learning course available from Agilent education |
