ICP-MS (Inductively Coupled Plasma Mass Spectrometry) is a highly sensitive analytical technique that can detect and quantify a variety of elements with a detection limit as low as ppt (parts per trillion). It is widely used in many fields such as environmental science, geochemistry, biomedicine, and materials science. In the ICP-MS analysis process, sample preparation is a crucial step that directly affects the accuracy and reliability of the final analysis results. Good sample preparation ensures that the sample enters the ICP-MS instrument in a suitable state, thereby achieving accurate elemental analysis.
The ICP – MS instrument itself has the capabilities of high sensitivity, wide dynamic range, and simultaneous multi – element analysis. However, the exertion of these advantages largely depends on the quality of sample preparation. Original samples often have complex compositions and physicochemical properties. For example, the organic matrix in biological samples, the complex mineral components in environmental samples, and the high – concentration matrix in industrial samples. These factors may interfere with the normal analysis process of ICP – MS, such as causing matrix effects, polyatomic ion interferences, etc.
Therefore, sample preparation is a necessary step to transform the original sample into a form suitable for ICP – MS analysis. Through appropriate sample preparation, interfering substances can be removed, sample concentration can be adjusted, and solid samples can be converted into solutions, etc., thus ensuring that ICP – MS can accurately determine the element content in the sample.
Effective sample preparation involves a sequence of carefully executed steps tailored to the sample type and the analytical goals. Each step aims to ensure the sample is compatible with the ICP-MS system and free of interferences. Here are the key aspects:
The tools and equipment used during sample preparation must be meticulously cleaned and prepared to minimize contamination and ensure reliable results.
Use high-purity acids (e.g., nitric acid) to clean labware such as digestion vessels, centrifuge tubes, and pipettes.
Rinse all tools thoroughly with ultra-pure water to remove any acid residues or particulates.
Dry labware in a clean environment, such as a laminar flow hood, to avoid contamination from airborne particles.
Check equipment for cracks, scratches, or residues that could trap contaminants.
Replace disposable items, such as pipette tips or filters, after each use to maintain purity.
Ensure all equipment, such as microwave digesters, centrifuges, and balances, is calibrated according to manufacturer specifications.
Test filtration systems and nebulizers for clogs or inefficiencies before starting the preparation process.
Preparation of tools and equipment is a critical step that lays the groundwork for accurate and consistent ICP-MS analyses. Proper cleaning, inspection, and calibration help eliminate contamination risks and maintain system efficiency.
The integrity of a sample is directly influenced by how it is collected and stored. Using proper techniques ensures the sample remains uncontaminated and stable until analysis.
The methods and tools used to collect samples depend on the type of sample. For example, for environmental water samples, clean, uncontaminated sampling bottles are required to be collected at specific locations and depths, and should collect in a single motion to avoid stratification and contamination from multiple dips; for biological tissue samples, sterile instruments are required for collection. The amount of sample collected also needs to be determined based on the needs and detection limits of subsequent analysis.
In terms of sample preservation, the stability of the sample needs to be considered. For example, some water samples may require the addition of acid to inhibit microbial growth and prevent element precipitation; biological samples may need to be stored at low temperatures or with specific preservatives.
Labeling the sample is also important. This need clearly label containers with sample IDs, collection dates, and any other relevant information. At the same time, maintain detailed records of collection methods and storage conditions for traceability.
Proper sample collection and storage is the first step to ensure accurate analytical results, which can prevent the sample from deteriorating or losing elements before it arrives at the laboratory for preparation and analysis.
Sample pretreatment involves preparing the raw sample into a form compatible with ICP-MS analysis. This step is crucial for ensuring that the sample is free of particulates, dissolved completely, and within the detection range of the instrument. Key methods including:
Dissolution: For solid samples, such as soil, rock or metal materials, they need to be dissolved into a solution for ICP-MS analysis. Common dissolution methods include acid dissolution (such as nitric acid, hydrochloric acid, hydrofluoric acid, etc.) and alkaline dissolution. Acid dissolution can select a single acid or a mixed acid according to the nature of the sample. For example, nitric acid-hydrochloric acid mixed acid (aqua regia) can be used to dissolve precious metals. During the dissolution process, the reaction conditions, such as temperature, time and acid concentration, need to be controlled.
Digestion: For samples containing organic matter, such as biological tissues or foods, digestion is a common pretreatment method. The purpose of digestion is to destroy organic matter and release the elements in it. Common digestion methods include dry ash method, wet digestion and microwave digestion.
Separation and enrichment: When the target element content in the sample is low or there is severe interference, separation and enrichment are required. For example, ion exchange resin can be used to selectively adsorb the target ions, thereby achieving separation from interfering ions; co-precipitation can precipitate the target element together with the precipitant to achieve the purpose of enrichment.
Dilution: Dilute samples to bring element concentrations within the instrument’s dynamic range. Use ultra-pure water or matrix-matched diluents to maintain consistency and minimize matrix effects.
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Matrix Matching: Prepare calibration standards that closely match the matrix composition of the samples to compensate for potential matrix effects. This step minimizes interferences from the matrix and ensures more accurate quantification.
Sample pretreatment is an important step in converting the original sample into a solution suitable for ICP-MS analysis. Different sample types and analysis purposes require the selection of appropriate pretreatment methods.
Despite careful planning, sample preparation for ICP-MS can pose several challenges. Recognizing these obstacles and addressing them proactively ensures high-quality analyses.
Matrix effect refers to the influence of other components in the sample other than the target element on the target element analysis signal. For example, in a high-salt sample, a large amount of salt matrix may inhibit the ionization efficiency of the target element, resulting in a decrease in the signal; or generate polyatomic ion interference, making the analysis result inaccurate.
Methods to overcome:
In ICP-MS analysis, atoms, ions and molecules in the plasma may react to form polyatomic ions. These polyatomic ions may have the same mass-to-charge ratio as the target element, thereby interfering with the detection of the target element. For example, argon (Ar) is a commonly used plasma gas in ICP-MS, which may react with elements or other gases in the sample to generate polyatomic ions, such as ArO +, ArCl +, etc.
Overcoming methods:
Contamination from labware, reagents, and the environment can significantly impact results. Use high-purity reagents, clean laboratory practices, and proper sample storage to minimize contamination.
Ensure complete dissolution of samples by optimizing digestion conditions and using appropriate acid mixtures.
These occur when residual analyte from previous samples affects subsequent analyses. Thorough cleaning of the sample introduction system and use of appropriate cleaning solutions can help prevent memory effects.
Consistency is key to producing reliable ICP-MS results. The following tips help maintain uniformity across sample preparation batches.
Sample preparation is an indispensable and important part of ICP-MS analysis. From sample collection and preservation, to pretreatment, each step has a key impact on the final analysis results. Proper sample preparation can give full play to the high sensitivity and multi-element analysis advantages of ICP-MS, and provide reliable elemental analysis data for scientific research, environmental monitoring, quality control, etc. in various fields.
A wide range of industries, from pharmaceutical to environmental and QC for metals, materials, and chemical industries rely on inductively coupled plasma mass spectrometers (ICP-MS) for high-sensitivity elemental analysis. These instruments use 9-10,000°C argon plasma to atomize samples and cone interfaces to transport those ions into high vacuum where the mass analyzer separates a selected element and allows detection at parts per trillion levels.
This incredible sensitivity comes at a cost—cones, tubes, torches, tips, and lenses all require regular replacement. Replacement cones form a major operational expense for labs. Purchasing aftermarket replacements can help reduce costs, but the quality of replacements is critical to exacting performance. Here we examine cone design, maintenance, what to look for in replacements, and why.
ICP-MS cones form the interface between sample and plasma at atmospheric pressure and the high-vacuum mass spectrometer (MS) chamber. They extract ions from the plasma through narrow orifices and focus them into the MS through progressively higher vacuum. This is often a two-stage reduction—a sample cone with an approximately 0.5-1 mm diameter aperture gates the initial drop in pressure (down to 1-10 mbar), followed by a skimmer cone, typically with 0.2-0.5 mm diameter apertures, which leads into the detection chamber (10-6 mbar). Some MS instruments include a third cone, called a hyper skimmer cone, to provide a more gradual pressure drop through a three-stage reduction with larger orifices.
Samples are normally prepared with acidic or alkaline diluents to reduce salt and solvent loads. Over time, cones will wear down and/or accumulate buildup that obscure the signals with high background and may considerably impact performance. Cones must be regularly cleaned and replaced to maintain data integrity.
Cleaning frequency depends on cone design, instrument configuration, and sample type. Sample cones, exposed to plasma and a higher concentration of “dirty” samples, require more frequent cleaning than skimmer cones, while smaller apertures clog more readily than larger apertures. Three-stage instruments can usually run longer between cleans than two-stage instruments, sample dependent. Cones will accumulate common ions like lithium, boron, and magnesium, particularly if running any samples with high concentrations. Salts quickly build up—seawater samples, for example, can cause visible salt deposits within hours of running.
The manufacturer-recommended cleaning protocols should be followed. These may involve a combination of DI water, cotton swabs, metal cleaners, nitric acid, isopropyl alcohol, and ultrasonic treatment depending on the type and degree of contamination. Blockages should never be cleared with force, which is highly likely to deform or erode the orifice geometry.
Often, visual checks for damage, especially to orifice geometry, during cleaning can indicate whether the cone needs to be replaced. The ICP-MS is a harsh environment with extreme temperatures, acids, and damaging solvents and sample compositions. This leads to vitrification, corrosion, cracking, and other visible damage indicating replacement need. Other indicators include drops in sensitivity or performance, or increased background levels post-cleaning. Replacement intervals depend on application and materials. Generally, labs that operate around the clock will need to replace cones every month or two, while labs with more moderate operation of their ICP-MS are likely to replace cones a few times per year.
Cones are extremely delicate. User error is another major driver for replacement. Rough handling, aggressive cleaning, and dropping are some of the leading causes of damage to the tip, making adequate training worth the time and expense. Users should always wear gloves and use proper installation tools when manipulating cones.
How do you choose? The simplest answer is to consider your application need and then to match the manufacturer’s part number for your ICP-MS cone to suppliers’ offerings. While quality is always of utmost importance, reliability and performance can be achieved with aftermarket cones. Even with the precise requirements of ICP-MS cone geometry for analysis performance, aftermarket cones from reputable suppliers can offer a more cost-effective solution to the higher-priced OEM cones while still providing the needed quality and reliability to meet high performance standards.
Alpha Resources® offers OEM-quality cones, as determined by customer testing, at approximately 30-40 percent less than the cost of OEM ICP-MS cones. As a production lab with years of experience manufacturing quality aftermarket products for elemental analysis, Alpha’s cones are produced in an ISO : accredited quality system and are a guaranteed quality match to an OEM product. Alpha’s cones are equal in durability to OEM cones with consistently reliable analytical output throughout their life.
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