Dry argon is used as an ICP nebulizer gas to generate an aerosol and transport the sample to the plasma. However, samples containing high amounts of total dissolved solids (TDS) increase the likelihood of salt deposits forming at the tip of the nebulizer and injector, which can result in a failed analysis due to a drift in signal or an extinguished plasma. In order to handle a challenging sample matrix like one high in TDS, it is important to optimize the inductively coupled plasma (ICP) sample introduction system. This includes choosing a nebulizer with the ability to handle high TDS, a baffled cyclonic spray chamber to minimize droplet size and a large-bore injector to allow for longer run times. For best performance, the nebulizer gas can be humidified before it comes in contact with the sample, decreasing the likelihood of salt deposits forming at the nebulizer and injector tip. Adding an argon humidifier will reduce maintenance and the chance of an extinguished plasma due to a blocked nebulizer or injector.
Figure 1 – Elegra argon humidifier.The Elegra argon humidifier (see Figure 1) (Glass Expansion, Pocasset, Mass.) is an efficient, simple-to-use humidifier for ICP-OES (optical emission spectrometry) and ICP-MS (mass spectrometry) applications. A small footprint enables it to fit easily into an ICP without overcrowding the sample compartment. Highly efficient membrane technology is utilized to add moisture to the gas as it flows through the inert, metal-free construction. This membrane technology eliminates the need for the Elegra to be pressurized or heated (no electrical power), as required by some other humidifier designs. A unique bypass switch permits the operator to turn humidification on and off without connecting or disconnecting any tubing.
For easy installation on any ICP instrument and with any nebulizer, each Elegra kit is packaged with customized gas connectors. Figure 2 shows an example installation on an Agilent 5100 ICP-OES (Agilent Technologies, Palo Alto, Calif.) with Glass Expansion’s SeaSpray and DuraMist nebulizers. The layout of the gas connections to and from the humidifier is shown in Figure 2a. P/N 70-803-0911 is used to make the connection from the Agilent 5100 nebulizer gas port directly to the Elegra gas inlet (Figure 2b). A second gas line is used to make the connection from the Elegra outlet directly to the nebulizer. This is P/N 70-803-1278 for the DuraMist nebulizer or P/N 70-803-0911 for the SeaSpray (Figure 2c). Each Elegra kit is also supplied with an adapter kit for non-GE nebulizers (see Table 1 or visit www.geicp.com/intro/elegra).
Figure 2 – Elegra installation on an Agilent 5100 ICP-OES.Table 1 – Nebulizer adapter kits
ICP-MS is being used more often in place of ICP-OES and atomic absorption (AA) techniques for trace metals analysis of high-matrix samples such as seawater and geological samples. For this reason, a dilution gas is now standard on most ICP-MS instruments.1–3 Inline aerosol dilution of the sample prior to introduction into the plasma greatly improves the plasma robustness, enabling improved analysis of high-matrix samples by ICP-MS. High-matrix samples also need to be diluted for ICP-MS analysis to prevent the rapid buildup of salt deposits on the interface cones, which would otherwise result in signal drift and frequent cleaning. Use of aerosol dilution rather than offline dilution of the samples4 prevents errors associated with manual sample preparation. The Elegra Dual model allows for humidification of both the nebulizer gas and dilution gas for these types of ICP-MS setups. This helps to prevent nebulizer and injector blockages, while the moisture added to the dilution gas further improves plasma robustness.3
Experimental
As mentioned above, it is important to select the proper nebulizer when dealing with high TDS. The SeaSpray and DuraMist concentricstyle nebulizers are well-suited for handling high dissolved solids. A self-washing tip on the SeaSpray tolerates up to 20% for the most common salts. The DuraMist is a PEEK nebulizer capable of handling up to 30% TDS and is resistant to hydrofluoric acids (HF).5
To evaluate performance, a nebulizer stress test was performed by aspirating a 25% NaCl solution while monitoring the nebulizer gas flow with and without the Elegra (Figure 3). The NaCl solution was aspirated continuously with no rinsing. Typically the Conikal nebulizer (Glass Expansion) would not be recommended for use with a high-TDS matrix because it has a tolerance of up to 5% TDS. However, it was used here to demonstrate the Elegra’s performance. As expected, the nebulizer was completely clogged after only 5 minutes of aspirating the salt solution without the Elegra. In contrast, the nebulizer held a relatively constant gas flow throughout the entire test (over 30 minutes) with the Elegra; the added moisture from the humidifier prevented the nebulizer from clogging.
Figure 3 – Elegra nebulizer stress test (see text for details).Matrices like lithium metaborate fusions can pose a challenge even when a suitable high-TDS nebulizer is used. The high amount of dissolved solids in such matrices often requires frequent nebulizer maintenance. Extended analyses are often rerun due to a drift in signal from the slow buildup of salts. An easy way to eliminate drift and frequent nebulizer maintenance is to add the Elegra. A high-throughput contract laboratory evaluated the Elegra with its lithium metaborate method on an ICP optical spectrometer. The stability of three internal standard (IS) lines was compared with and without the humidifier (Figure 4). Without the Elegra, the IS signal began to drift after 35 minutes, eventually resulting in a failed analysis (>10% drift). With the Elegra there was virtually no variation in IS signal over a four-hour period. The laboratory reported a significant reduction in required nebulizer maintenance—more than two months without a cleaning compared to weekly cleaning without the Elegra.
Figure 4 – Stability comparison of three internal standard lines with and without Elegra (see text for details).With respect to the ICP torch injector tube, a larger bore (>2.0 mm i.d.) is recommended for high-TDS applications. Since the torch outer tube often suffers rapid devitrification, a demountable torch design such as the Glass Expansion D-Torch is suggested since it reduces torch consumable costs.6 Increasing the auxiliary argon flow will lift the plasma higher off the injector, slowing salt buildup at the injector tip. Extended rinses in between each sample are also recommended. However, lengthy rinses can be avoided by adding Elegra since it provides the greatest decrease in the rate of salt buildup at the injector tip.
Conclusion
The compact Elegra argon humidifier helps alleviate salt deposits in ICP nebulizers and torch injectors, allowing uninterrupted and maintenance-free operation. Efficient membrane technology eliminates the need for a pressurized water reservoir, heating and electric power. Inert, metal-free construction eliminates the possibility of contamination. An easy-to-use bypass switch allows the humidifier to be taken off-line without disconnecting any argon lines and the Elegra Dual channel model provides the ability to humidify both the nebulizer gas and a dilution gas simultaneously.
References
- Agilent Technologies Application Note#5989-7737EN. www.agilent.com/cs/library/technicaloverviews/public/5989-7737EN.pdf
- Agilent Technologies Application Note #5991-4257EN. www.agilent.com/cs/library/applications/5991-4257EN_AppNote7900_ICP-MS_salt.pdf
- Perkin Elmer Product Note #012394-01. www.perkinelmer.com/CMSResources/Images/44-175832PRD_NexION-AMS-Systems-Product-Note-012394_01.pdf
- Agilent Technologies Application Note #5989-8573EN. www.agilent.com/cs/library/technicaloverviews/public/5989_8573EN.pdf
- Glass Expansion Newsletter, June 2013, “A Nebulizer Update.”
- Glass Expansion Newsletter, October 2013, “How to Achieve High Accuracy with Difficult Samples.”
Dr. Ryan Brennan is marketing manager, and Mr. Jerry Dulude is president, Glass Expansion, 4 Barlows Landing Rd., Pocasset, Mass. 02559, U.S.A.; tel.: 508-563-1800; e-mail: [email protected]; www.geicp.com. Dr. Vesna Dolic is director of the ICP laboratory, Glass Expansion Pty., Melbourne, Australia.