Low Sensitivity Causes

There are a multitude of potential causes for an instrument exhibiting low sensitivity (low analytical signal intensity). These include, but are not limited to:

• Dirty, blocked or poorly setup sample introduction components (including accessories)
  • Blocked nebulizer
  • Blocked injector tube
  • Dirty torch
  • Dirty or blocked spray chamber
  • Pump tubing
  • Organic samples
• Incorrect method parameters
• Wet chemistry- matrix interferences and/or poor solution preparation
  • Incorrectly prepared standards
  • Inter-Element Correction (IEC)
  • Fast Automated Curve-Fitting Technique (FACT)
• Contaminated or wrong utilities such as gas supply
• Incorrect instrument setup and adjustment
  • Conditions not optimized
• Instrument hardware failure
• or a combination of the above

Determining the Cause of the Problem

The first step is to run an 'Instrument Performance' test from the 'Tests' tab in the ICP Expert software to validate the instrument. This will perform the Resolution, Sensitivity, and Precision tests. Compare results to typical values.

• 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 the results are ~20% less than typical values then rerun the test with a known good SeaSpray Nebulizer and cyclonic spray chamber to eliminate these components as a possible cause.

Poor Sensitivity results will verify a problem exists. Using the generated test report, check which wavelengths are low on sensitivity and by how much. Compare to the installation results or past installation qualification numbers. The table below will give some direction as to failure mode and possible solution.

Instructions on how to run the 'Instrument Performance' test are here.

Failure Mode (Intensity)	Possible cause	Possible cause (Next level)
All Wavelengths very low on sensitivity	Sample introduction major blockage	Nebulizer blocked
		Tubing kinked, blocked, dirty (including accessories)
		Torch injector blocked
	Leaking fitting	Nebulizer
		Pump tubing
		Autosampler
		Bubbles in tubing
	Sample delivery	Pump tube tension
		Pump tube stretched
	Standard concentration	Prepare new standard solutions
	Optimization of parameters	Adjust the nebulizer flow on the Conditions page
		Adjust the viewing height on the Conditions page for radial or SVDV instruments
		Adjust the Plasma flow or RF power on the Conditions page
		Pump speed
	Wavelengths off peak	Instrument warm up - allow 20 minutes for the instrument to warm up from an idle state
		Instrument calibration
	Dark current values high	Detector calibration
	Contaminated blank	Sample preparation - prepare a new blank solution
		Housekeeping
	Sample matrix	Interference from EIE’s
Random Wavelengths low on Sensitivity	Wavelengths off peak	Instrument warm up - allow 20 minutes for the instrument to warm up from an idle state
		Instrument calibration
	Peaks missing or low due to moisture forming on Detector Lens	Moisture in purge lines. Refer to Contaminated Purge Gas.
	Dark current values high	Detector calibration
Short Wavelengths low on sensitivity	Sample Introduction minor restriction	Nebulizer blocked
		Tubing kinked, blocked
	Air present in optics (lack of purging)	Optics purge - turned on automatically when the instrument is turned on. Check gas supply and lines for leaks or damaged tubing.
		Boost purge - provides a higher gas flow to the optics
		Snout purge
		Cone purge - turned on automatically when the plasma is turned on. Check gas supply and lines for leaks or damaged tubing. Check that the cone is clean and installed correctly.
	Window Contamination	Axial/radial windows
	Optimization of parameters	Adjust the nebulizer flow on the Conditions page
		Adjust the viewing height on the Conditions page for radial or SVDV instruments
		Adjust the Plasma flow or RF power on the Conditions page
All Wavelengths marginal Sensitivity	Wavelengths off peak	Instrument warm up - allow 20 minutes for the instrument to warm up from an idle state
		Instrument calibration
	Dark current values high	Detector calibration

Contaminated Purge Gas (Moisture)

Optics Purge gas contaminated with moisture is a potential cause of loss of sensitivity and in a number of cases random loss of peaks. Moisture from purge gas will collect on the CCD window resulting in loss of peaks for specific wavelengths.

To verify if this is a problem turn off the chiller, wait approximately 30 minutes and then switch chiller back on and run the Instrument Calibration. The moisture should have evaporated as the camera heats up and the calibration should again pass.

If this resolves the problem purge the gas lines and/or add a moisture filter to the purge lines.

Blocked Nebulizer

Check the nebulizer for a blockage and clean if necessary.

Blocked Injector Tube

Check for salt or carbon build-up (with organic work) on the injector tube. Clean the torch if necessary.

Pump Tubing

Pump tubing that has lost its elasticity will not deliver the sample to the nebulizer correctly. When the pump tubing appears limp or flattened, or does not require stretching to fit over the pump rollers, it should be replaced with new pump tubing.

Organic Samples

Lower the nebulizer flow, lower the pump rate and/or use a smaller ID pump tube.

Incorrectly Prepared Standards

Prepare a fresh set of standards at the correct concentrations.

Conditions Not Optimized

Develop and run a method for optimization and check that: 

• The peak appears within the search window; and that
• The peak is smooth (not ragged).

Optimize the operating conditions, including the nebulizer flow and torch viewing height (for radial only), until the optimum signal is obtained.

Developing and running a method for optimization

1. Turn on the ICP-OES external exhaust and then the ICP-OES.
2. Open ICP Expert software.
3. Turn on the plasma by clicking Plasma On or by pressing F5 on the keyboard.
4. Let the ICP-OES warm up for 20 minutes if it was turn on from an idle state.
5. Click New.
6. Click the Elements tab.
7. Select the appropriate element and wavelength and then click Add.
8. Click the Conditions tab.
9. Select Axial or Radial depending on which view is to be optimized.
10. Select Time scan in the 'View Graph' section.
11. Place the solution tubing from the peristaltic pump into the wavelength calibration solution (or other standard solution) and the drain tubing into the drain vessel.
12. Turn on the pump to begin aspirating the solution.
13. Click Time scan to start a trace.
14. Adjust the operating conditions one at a time, including the nebulizer flow, or torch viewing height (radial systems only), to optimize for maximum intensity.

Scan Not Displayed

• Identifying the problem
• Sample concentration
• Conditions not optimized
• Blocked nebulizer

Identifying the Problem

A number of things may result in a scan not being displayed. The following procedure may help identify the problem.

Perform a Read spectrum (using 1 ppm Mn solution using the 257.610 nm line) and check that a peak is visible within the scan window.

If no peak is displayed try performing a Read spectrum on another element.

1. Aspirate a yttrium solution and check the plasma for the characteristic red/blue/red emission; or
2. Repeat the scan (Read spectrum) for the element of interest. If a peak is still not displayed in the scan window aspirate a sodium sample and view the plasma. When the sodium yellow coloration appears in the plasma, perform a scan at the sodium 589.592 nm line. If a peak is still not observed, contact your local Agilent field service engineer.

Sample Concentration

Very low sample or standard concentrations may not produce an analytical signal. Increase the concentration of the analyte 10-fold or 100-fold.

Conditions Not Optimized

Perform a Read spectrum (using 1 ppm Mn solution) and check that: 

• The peak appears within the search window; and that
• The peak is smooth (not ragged).

Optimize the operating conditions until the optimum signal is obtained.

Blocked Nebulizer

Check the nebulizer for blockage and clean if necessary.