Elevated Doubly Charged Ratio in ICP-MS

Answer

This Information Applies To: Agilent ICP-MS (7700/7800/7850/7900/8800/8900)

Issue

Elevated doubly charged values that are greater than 3% for x-lens configuration or greater than 7% for s-lens configuration, can be caused by contamination, or an issue with the sample introduction system.

Background

The Doubly Charged Ratio (%) is defined as: mass 70 (Ce++) / mass 140 (Ce) x 100

Zn, Ge, ClCl, GaH, and FeO are some examples of contaminants/interferences that contribute to elevated counts on mass 70.

Germanium (Ge) is a common element used in internal standard mixes. Elevated doubly charged ratios will occur if the tuning solution or internal standard rinse solution is contaminated with Ge.

Hydrochloric acid (HCl) is a common acid used in tuning solution or rinse solution. If HCl is present, the doubly charged ratio will be higher due to the ClCl polyatomic interference on mass 70.

Items Required

• Tuning Solution for ICP-MS 1 ppb (p/n 5185-5959), or 
• Tuning Solution for ICP-MS 10 ppb (p/n 5184-3566)
• Type I de-ionized water
• Trace metal grade Nitric Acid
• Spare Sample Introduction Components

Steps to Follow

Check for Contamination 

1. Prep or pour fresh tuning solution in 1% HNO₃ (1 ppb tuning solution for standard configuration, 10 ppb tuning solution for ISIS 3 configuration)

2. Place the internal standard line into a fresh 1% HNO₃ rinse solution (for standard configuration) or place the carrier line into a fresh 1% HNO₃ rinse solution (for ISIS 3 configuration).

3. Light plasma and perform the startup tasks: Torch Axis, EM, Plasma Correction, Standard Lenses Tune, Resolution/Axis, and Performance Report.

4. Check doubly charged ratio from the startup performance report. If the doubly charged ratio has decreased, either the tuning solution or rinse solution was contaminated.

5. If the doubly charged ratio has not decreased, it is possible the de-ionized source, or HNO₃ source is contaminated. Prepare again the tuning solution and 1% HNO₃ rinse solution with a second source of de-ionized water and HNO₃, then retest.

6. If the high doubly charged ratio occurred after replacing the argon, the contamination could be in the argon gas. Contact your gas supply company and test a second source of argon.

7. It is also possible that the sample introduction system is contaminated. Clean the nebulizer, spray chamber, torch, cones, and lens stack as per the Agilent ICP-MS Hardware Maintenance Manual. See the ICP-MS MassHunter Help and Learning Center. (See Access Agilent ICP-MS User Manuals with MassHunter 5.x) 

Check for Sample Introduction Issue

1. If the high doubly charged ratio occurred after sample introduction maintenance, please confirm that the torch, spray chamber, nebulizer, cones, and lenses have been reinstalled properly.

2. Verify that the cones are in good condition. Poor orifice shape or size, or excessively dirty cones can cause elevated doubly charged ratio. Replace the skimmer and sampler cone and retest.

Learn how to effectively troubleshoot your Agilent ICP-MS System: Agilent 7800/7850/7900 ICP-MS with MassHunter e-learning course available from Agilent education