In the sphere of analytical instrumentation, effective communication is key, and the language of instruments serves as a signal. For chemists, ensuring that the signal received from the ICP-OES instrument is clear, consistent, and concise becomes an utter priority. The sensitivity of the signal must be finely tuned to be easily recognized, precision should be consistently maintained, and accuracy is non-negotiable. In the world of ICP-OES, challenges arise when sensitivity falters, precision becomes inconsistent, the instrument fails to produce accurate results, and the plasma extinguishes too quickly.
In this installment, we will delve into the most common problems faced in ICP-OES operation and explore effective techniques to ensure seamless communication with this analytical tool.
As an ICP-OES chemist, I find myself encountering four distinct categories of challenges, each presenting its own set of hurdles in the quest for seamless results. Join me as we navigate the realms of plasma, enhancing sensitivity, ensuring precision, and achieving the goal of accuracy.
Although ICP-OES method development has been improved by advances in detector versatility and instrumentation, problems still arise. In this installment I will walk you through a systematic means of isolating, identifying, and correcting many typical ICP-OES problems.
Unlock the full potential of your ICP-OES instrument with the four key considerations that can impact your analyses.
Sensitivity (signal/concentration)
It is the ability to detect and measure the levels of analyte in relation to their concentration in the sample.
It determines the instrument’s ability to detect and quantify trace levels of elements in a sample. Optimizing for optimal sensitivity allows for the detection of elements present at lower concentrations, enabling for accurate analysis. Additionally, sensitivity directly impacts the instrument’s detection limits, precision, and the ability to differentiate between elements and background noise.
Sensitivity is influenced by 4 factors:
- Sample introduction system
- Method parameters
- Cleanliness
- Quality of standards used for calibration
Use Table 1 to determine which component may be causing the problem. By systematically eliminating potential causes, you can effectively identify the specific issue and take the necessary steps to resolve it.
Precision:
Ever wondered why your results sometimes go haywire? Well, here’s a little secret: about 80% of the time, it’s the mischievous sample introduction system causing all the trouble! Just like a mischievous prankster sneaking into your lab, this system can wreak havoc on your precision by introducing all sorts of unexpected variations.
Precision is the closeness of results from the same homogeneous sample under the same conditions. It is measured as %RSD. For ICP-OES, 2 % RSD is expected. Poor RSD or noisy signal is an indication of poor precision during analysis.
Why precision is a good metric to track?
It is a good indicator for confidence in the system.
The potential contributors to precision:
- Plasma stability
- Sample introduction system (sample uptake tubing, flow rate, Nebulizer, Spray chamber contamination)
- Method parameters
- Partially soluble salts
Use Table 2 for some direction as to problems and potential solutions.
Accuracy
Imagine a pharmaceutical researcher formulating a life-saving medication. In this process, even the slightest deviation in chemical composition could render the drug ineffective or, worse, harmful to patients. This example underscores why accuracy is paramount in metrology. Every measurement, every reaction, hinges on the ability to reliably determine the composition and properties of substances. Without accuracy, potentially jeopardizing the safety and efficacy of countless pharmaceuticals, environmental assessments, and industrial processes.
Accuracy is the closeness of the measured value to the true value.
Accuracy can be confirmed by:
- Using a certified reference material (CRM/SRM)
- Using a QC checks, CCV
Use table 3 on the most common causes of poor accuracy and their potential solution.
Drift
One of the important parameters that ensures streamlined analysis is instrument performance stability in producing consistent signal. Fluctuation in producing sustainable signal is what refers to as instrument drift.
Drift is the gradual change in instrument performance over time. It can be displayed as shifts in signal intensity, baseline instability, or variations in analytical sensitivity.
What factors can cause instrument drift?
Change in temperature, fluctuations in gas flow rates, wear and tear in the uptake tubing, not enough time for the sample uptake to reach the plasma and for the signal to stabilize, nebulizer clogging, not so often if the spectrometer was turned off, or there is electronic issue.
Why is monitoring and correcting for instrument drift important?
It is essential for maintaining the accuracy and reliability of analytical results, as even minor fluctuations can impact the precision and reproducibility of measurements.
Use table 4 on the most common causes of drift and their potential solution.
QC Failure
It’s the check standard verification time in the lab. The prepared check standard solutions are analyzed under the same conditions as the samples of interest. It’s like a chemistry detective mission – we’re comparing the concentrations of elements we measure to what we know they should be. This helps us make sure our instrument is operating within the acceptable limits and that the calibration curve is functionable within its working range.
Check standards are prepared solutions containing known concentrations of target elements. They are used to verify the accuracy of the instrument’s measurements and to monitor its performance over time.
Check standard verification is one important metric that provides information on the accuracy and reliability of the analytical results by validating the performance of the instrument and the calibration curve.
In regulated labs, it’s a must to hit the brakes on analysis if our check standard verification doesn’t cut the mustard. Usually, when our check standard flops, it’s because our blank’s got some unwelcome visitors – contamination, anyone? Now, here’s where the chemist detective hat comes on: we got to ask ourselves a couple of key questions. First off, is every single element failing, or is it just a select few? And secondly, if they are failing, are they all off by the same amount? If only some elements are acting up, it might be down to how we prepped our samples or if certain elements are throwing a tantrum in certain pH environments (acidic ones, especially). But if everything’s off by the same degree, we might have an issue with our internal standards. And if some elements are being a bit unpredictable with their results, it could be related to a stability issue that can be caused by improper sample uptake flow or insufficient read delay time.
Example 1:
Example 2:
The above two examples are what we don’t like to see in the lab – QC Failed. Let’s examine the cause of each one. But first few questions to ask yourself:
The quality and the speed of your solution depends on the quality of the questions you’re going to ask yourself to solve the issue.
- Has the signal of Mn alignment solution changed from the previous day? No, good.
- Has the signal of the analytes changed from the previous run, and were you able to build a good calibration curve? Yes, good.
- Is the QC fails with all elements or a few?
- What is the RSD of the replicate readings?
- What is the chemistry of the element of interest and its matrix?
- Is the peak or background correction BGC point position set correctly?
- Is the correct wavelength being used?
- Is the method parameters set correctly?
Example 1: shows that K has a QC value above the upper limit. This could be broadly grouped into 3 categories: 1) the true K concentration in QC is above the upper limit due to incorrect preparation, or 2) the true K concentration is within the control limits, but the 766.490 emission line result is not accurate due to potential interferents, 3) stability issue.
For example, checking the RSD shows that K 466.490 has an RSD of 8.12%, Commonly expected RSD between 1 -2 %. This could be due to sample read/delay issue, not sufficient time to flush the system.
Example 2: shows that K has a QC value below the lower limit. The RSD is within the allowable range. therefore, precision issue is excluded. This could be a challenge when testing alkali metals using ICP-OES, as low recoveries can occur as they exhibit low spectral intensity. Addition of ionization buffer such as Cs could help in suppressing neighboring spectra and allowing for accurate estimation.
The following example illustrates how signal can be trending and the possible causes for that:
| Replicate | Intensity A | Intensity B |
| Replicate_1 | 460,000 | 535,000 |
| Replicate_2 | 489,000 | 489,000 |
| Replicate_3 | 535,000 | 460,000 |
As you can see in the above table:
- Increasing intensity with time indicates sample uptake/read delay times are insufficient. Sample reading is determined too quickly or taking longer to stabilize due to matric effect.
- Decreasing intensity with time indicates previous sample may still be washing out.
Use table 5 on the most common causes of check standard failure and their potential solution.
Plasma
Plasma is the heart of the ICP, due to its paramount role in in excitation and ionization of the sample. Ensuring the plasma generation all the time is utmost importance for analysts. The following displays the most common causalities of plasma disturbance and how to resolve them.
Use table 6 on the most common causes of check standard failure and their potential solution.
The ICP-OES uses a high-temperature plasma torch to atomize and ionize the sample, allowing the analysis of its elemental composition.
Why does a torch melt in ICP-OES?
Melting of the torch mostly happens during the ignition step. However, it can also occur due to excessive heat generated during the analysis process.
Use Table 7 to determine the common causes for the torch material to melt, with the remedial action.
Understanding the key areas of your system configuration is like having a roadmap to success. The sample introduction system is the beating heart of the instrument, orchestrating a delicate dance between various components. To troubleshoot effectively, it’s crucial to grasp how these components interact and where to focus your attention. As a general rule of thumb, 80% of ICP problems rotates around sample introduction, while drift and signal suppression tend to lurk, closer to the injector tube and torch.
Lastly, the goal of troubleshooting is not just to fix problems, but to create systems that are robust and resilient to issues. Prevention is the key to minimizing the occurrence of problems for ICP-OES. The following table provides a comprehensive system maintenance plan for ICP-OES.
| Maintenance Task | Frequency | Description |
| Flush Sample Introduction | Daily | Flush the sample introduction with 2% HNO3 |
| Clean Nebulizer | Weekly | Use fill Eluo neb cleaner with methanol or 2% HNO3 to remove any build ups |
| Replace Consumables | As needed | E.g. tubings, o-rings, torch parts, as they wear or become damaged |
| Clean Injector | Monthly | Rinse with 2% HNO3, or a surfactant, and rinse with DI H2O. Place in the oven for 10 minutes |
| Clean Radial and Axial surfaces | Annually | Clean lenses to maintain signal clarity |
| Inspect Torch Components | Quarterly | Inspect torch components for wear, damage, or contamination |
| Check Plasma Stability | Monthly | Monitor plasma stability and torch condition for consistent performance |
About Author
