During the performance of inductively coupled plasma (ICP) spectrometry, problems can occur that may affect the uptime and performance of the instrument. In ICP-optical emission spectrometry (ICP-OES) and ICP-mass spectrometry (ICP-MS), these problems may originate in the sample introduction system and the radiofrequency (RF) coil; in ICP-MS, problems can also be traced to the interface region.
Below are troubleshooting strategies to optimize ICP spectrometer performance, and calibration techniques to help resolve problems with measurement accuracy and sensitivity.
Common problems
The most common problems that arise in ICP spectrometry include poor precision, carryover, drift, degraded detection limits, and inaccuracy.
Poor precision
Poor precision is the inability to get the same result for the same sample when measured multiple times. It is usually measured as % relative standard deviation (RSD); the lower the RSD, the better the precision. Poor precision may be caused by problems in the sample introduction system.
Carryover
Carryover between elements greater than 0.1% may not be considered adequate washout.
Drift
Drift in signal may not exceed 10% and may be caused by several issues, such as a deposit buildup on the nebulizer and injector, poor temperature control, worn peristaltic pump tubing, or dirty interface cones.
Degraded detection limits
Degraded detection limits may result from an imbalance in signal-to-noise ratio.
Accuracy and sensitivity
The accuracy and sensitivity of a measurement are affected by interferences and by matrix effects generated from the elemental matrix components. These lead to bias in the signal intensity and measurement inaccuracy.
Troubleshooting
Troubleshooting the sample introduction system, the RF coil, and the interface region may isolate the causes of performance problems.
Sample introduction system
The sample introduction system includes the peristaltic pump, nebulizer, spray chamber, and torch.
1. Peristaltic pump
Fluctuations in uptake rate may be due to worn pump tubing, a worn pump roller, improper tension of the peristaltic pump, or a faulty peristaltic pump. A visual test can be performed to check for the source of the problem. The test consists of watching the flow of an air bubble in the capillary tubing to check if it is smooth. If it is not smooth and the worn pump tubing has been changed, the next step is to optimize the tension on the pump tubing and visually inspect the pump rollers. Worn-out pump rollers can be replaced. If jerky movements remain, the problem may be due to a faulty peristaltic pump, which can also be replaced. A digital flow monitor can also help to optimize the tension of the pump tubing and alert the ICP analyst to blockages or worn peristaltic pump tubing.
2. Nebulizer
Selecting the right nebulizer is critical. When choosing a nebulizer for a specific application, you should consider a number of characteristics, as follows:
- Tolerance to total dissolved solids (TDS)
- Tolerance to particulates
- Tolerance to hydrofluoric acid (HF)
- Precision
- Purity
- Robustness
- Material
Additional nebulizer accessories can further improve performance for more challenging applications. With high TDS applications, an argon humidifier adds moisture to the nebulizer gas before it comes in contact with the sample, decreasing the likelihood of salt deposits forming at the nebulizer and injector tip. A reuseable inline-particle filter eliminates the chance of a particulate blockage.
Whichever nebulizer type is used, the lifetime and performance can be prolonged by proper maintenance and troubleshooting. A nebulizer problem can be identified by looking for “spitting” or pulsations in the mist inside the spray chamber. You can also monitor the nebulizer backpressure, which is a common feature in most ICP instruments. A high backpressure will indicate a blockage in the nebulizer capillary or tip. Another way to test the nebulizer is to replace it with a new or proven nebulizer.
A blockage can be removed by using the proper tools and procedures as recommended by the manufacturer. Several nebulizer manufacturers provide tools to safely flush the blockage from the nebulizer capillary. Extra caution should be taken not to damage the nebulizer tip, which can greatly affect the performance of the nebulizer. One should also contact the nebulizer manufacturer to ensure the recommended solvents are used for cleaning.
Here are some practices that should be avoided to prevent damage:
- Do not insert anything through the orifice of the nebulizer. This is most likely to damage the nebulizer beyond repair.
- Do not use any concentration of HF to clean a glass or quartz nebulizer. Even dilute HF can alter the orifice of the internal capillary and deteriorate the performance of the nebulizer.
- Do not place a glass nebulizer in an ultrasonic bath as it may dislodge the internal capillary.
- Do not use hot liquid to flush the sample capillary of an inert nebulizer. The temperature can potentially deform the capillary and affect nebulizer performance.
3. Spray chamber
Similar to the nebulizer, it is essential that the proper spray chamber be selected for the application and sample matrix. For samples high in TDS, it is important to use a baffled cyclonic spray chamber to prevent large droplets from reaching the ICP torch. With volatile organic samples, a chilled baffled cyclonic spray chamber and small-bore injector combine to reduce the solvent load and improve plasma stability. A single pass cyclonic spray chamber can be selected for optimum sensitivity, while low-volume cyclonic spray chambers can be utilized for low-flow applications to improve sensitivity and washout.
Poor ICP performance can also be isolated to the condition of the spray chamber; for example, a dirty spray chamber can result in poor RSDs or carryover, and insufficient drainage of the waste from the spray chamber can cause poor sample transport and instability in the plasma.
For glass and quartz spray chamber maintenance, it is good practice to always start and finish using the spray chamber by nebulizing a mildly acidic blank solution for several minutes. This ensures that sample deposits or crystals do not form inside the spray chamber when the solvent inside the chamber dries out. Check that the drain line from the spray chamber is well sealed and not blocked to ensure good drainage. The waste tubing should be replaced if drainage is slow.
Some PTFE and PFA spray chambers have an internal surface that is specially treated to ensure that it wets evenly and provides consistent drainage. The treated surface may degrade after prolonged use, depending on the type of samples used. With these spray chambers, hydrogen peroxide must not be used, as this will accelerate degradation of the surface and no contact should be made with it. If degradation in performance is noted, the spray chamber may be cleaned with a special cleaning solution. If the surface degrades and does not recover after cleaning, it can be re-treated.
If cleaning the spray chamber with an appropriate cleaning solution does not improve precision, the spray chamber can be replaced with a new or proven spray chamber to verify that it is not the cause of the poor performance.
4. Torch
If the spray chamber is cleared, the problem may be due to the torch injector. If a deposit around the tip of the torch injector is the cause, it can be cleaned. Other causes of problems may be injector misalignment, torch misalignment, and devitrification of the torch.
If these problems are observed, the torch or just the outer tube with a demountable torch may be replaced.
RF coil
The RF coil is used to generate a plasma, and its condition and shape can have a major effect on its efficiency. The RF coil is susceptible to corrosion; the greater the corrosion, the larger the amount of energy is needed to produce a plasma of the same power. This places undue stress on the rest of the electrical components used in the RF generating system, possibly contributing to premature failure. Replacing the RF coil thus reduces the load on the RF generating system. RF coils come in several materials, including copper, gold, silver, and PTFE-coated silver.
Interface region
In ICP-MS, the most common types of problems associated with the interface region are blocking or oxidation of the interface cones. A blockage is not always obvious, and the interface cones have to be inspected and cleaned on a regular basis.
The frequency of cone cleaning depends on the application and the workload of the instrument. If the samples are clean and the usage is low, the cones may only need cleaning monthly. But if the instrument is in continuous use and/or the samples contain high levels of dissolved solids or are highly corrosive, the cones may need daily cleaning. The cones should be cleaned if there are visible deposits near the orifice or if the orifice is blocked or distorted. Deterioration in the performance of the ICP-MS—such as increased background signal, memory effects, loss of sensitivity, or distorted peak shapes—can indicate that the cones may need cleaning.
The cone needs to be replaced if the orifice becomes blocked and the vacuum increases, or if the orifice is worn out and the vacuum decreases.
Calibration techniques
Problems with lack of accuracy and sensitivity may be resolved using the calibration curve technique or the standard additions technique. With the calibration curve technique, matrix matching and internal standardization can be used. Matrix matching can be used to correct for matrix effects. With internal standardization, an internal standard that is compatible with the matrix and that does not introduce spectral interferences or trace impurities is selected to correct for plasma-related effects.
For the standard additions technique, a standard added directly to the sample solution is compared to the same sample solution without standard, and the difference is measured to allow for sample quantification. In ICP-MS, isotope dilution can be used with the standard additions technique.
Conclusion
Troubleshooting can have a significant impact on the uptime of the ICP spectrometer. It is therefore crucial to perform troubleshooting and preventative maintenance on a regular basis. For troubleshooting and preventive maintenance procedures, visit http://www.geicp.com.
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com