Internal Standard (ISTD) Mixing Issues on ICP-MS

Answer

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

Issue:

Poor internal standard (ISTD) mixing can cause high relative standard deviation (RSD). 

The recommendations in this article are applicable if:

• It takes a long time for the ISTD signal to settle down from fast peripump to normal peripump speed. Using signal monitor, you can observe large dips or rises in the ISTD signal even 2-3 minutes into stabilization.
• ISTD RSD on a calibration standard or sample is intermittently high. For example, RSDs of ISTDs might be 1-2% throughout most of a run, but for some unexplained reason, the ISTD RSDs jump to 20% or more randomly. It has nothing to do with matrix effects of the sample.

Items Required

• Nebulizer sample inlet tubing 0.25 mm id or 0.5 mm id.
  • See the Agilent Spectroscopy Supplies Guide for part numbers, or Table 1 for MicroMist U-Series Nebulizer Sample Tubing.
• Triton X-100
• Paperclip (mixing coil)

Steps to Follow

Ensure that the ISTD and sample peristaltic pump tubing tension are set properly

1. With the internal standard and sample line tension set properly, there is low restriction at the connection block, and both internal standard and sample travel consistently to the nebulizer (Figure 1; Item 1).
2. When the sample line is too tight, and the internal standard line too loose, sample can backflow into the internal standard line and limit internal standard delivery to the nebulizer (Figure 1; Item 2).
3. When the sample line is too loose, and the internal standard line too tight, internal standard can backflow into the sample line and limit sample delivery to the nebulizer (Figure 1; Item 3).
   

   
   
   Figure 1. Peri Pump Tubing Tension Diagram
   1. Proper tension, 2. Tight sample line and loose ISTD line, 3. Loose sample line and tight ISTD line
   

Maximize internal standard and sample mixing

1. Add 0.1% Triton X-100 to an internal standard bottle to enhance better mixing.  
2. Install either a short piece of 0.25 mm inner diameter (id) nebulizer tubing to add a few loose knots, or a long piece of 0.5 mm id nebulizer tubing serpentine looped around a paper clip.
3. See Table 1 for the advantages and disadvantages of each setup. Adjust uptake and stabilize time in batch after making adjustments.
   
   

   Advantages: Shorter tubing allows for short runtime, fast rinsing, and minimizes carry over.  Small inner diameter maintains same resistance. Loose knots in the line help with sample and internal standard mixing.	Advantages: Better sample and internal standard mixing than a shorter 0.25 mm id and knots setup.
   Disadvantages: Sample and internal standard mixing is not as good as the longer 0.5 mm id and paperclip setup.	Disadvantages: Longer run time. Increased rinse time needed to minimize carry over.
   MicroMist U-Series Nebulizer Sample Tubing 0.25 mm id (p/n G3266-80013)	MicroMist U-Series Nebulizer Sample Tubing 0.5 mm id (p/n G3266-80012)

   Table 1. Internal Standard Mixing Setup Comparison
   
   

Prevent dead volume issues in the connection block

When the autosampler probe raises out of the sample tube, air is taken into the connection block via the sample line. This can lead to random high RSDs for the internal standard.

1. Enable Stop PeriPump Rotation.
   

   Tip: This is applicable for non ISIS3 configurations only!

   
   
   1. From the Dashboard page, click the Sample Introduction icon and from the drop-down menu click Special Process for Sample Change (Figure 2).
      

      
      
      Figure 2. ICP-MS Instrument Control Dashboard
      1. Sample Introduction, 2. Special Process for Sample Change
      

   2. Select the Stop PeriPump Rotation check box, then click OK (Figure 3).
      

      
      
      Figure 3. Special Process for Sample Change
      1. Stop PeriPump Rotation, 2. OK button
      

2. Disable preemptive rinse. Preemptive rinse must not be used with stop peripump rotation. 
   1. On the batch Sample Introduction (MH 5.x) / PeriPump/ISIS (MH 4.x) page, clear the Preemptive Rinse checkbox, then click Save Batch (Figure 4).
      

      
      
      Figure 4. Batch Sample Introduction (PeriPump/ISIS) Page.
      1. Preemptive Rinse, 2. Save Batch button
      

Learn how to effectively troubleshoot your Agilent ICP-MS System:  Agilent 7800/7850/7900 ICP-MS with MassHunter Agilent 8900 Triple Quadrupole ICP-MS with MassHunter e-learning courses available from Agilent education.