Lawrence Neufeld

This month's "Atomic Perspectives" column takes a look at interface cones in inductively coupled plasma–mass spectrometry (ICP-MS), which are probably the most critical components of the entire ICP-MS system. The interface region gave the early pioneers of the technique the most problems to overcome. Although we take all the benefits of ICP-MS for granted, the process of taking a liquid sample, generating an aerosol that is suitable for ionization in the plasma, and then sampling a representative number of analyte ions, transporting them through the interface, focusing them via the ion optics into the mass spectrometer for separation and detection, is not a trivial task. Each part of the journey has its own unique problems to overcome, but probably the most challenging is the extraction of the ions from the plasma into the mass spectrometer. This article therefore focuses on how best to maintain the interface region and in particular, understanding when the sampler and skimmer cones need to be cleaned or changed to optimize performance and maximize instrument uptime. 

	The role of the interface region in inductively coupled plasma–mass spectrometry, which is shown circled in Figure 1, is to transport the ions efficiently, consistently, and with electrical integrity from the plasma, which is at atmospheric pressure (760 torr), to the mass spectrometer analyzer region at approximately 10-6 torr (1,2).


	Figure 1: The interface region (circled in yellow) of an ICP-MS instrument.

	This is first achieved by directing the ions into the interface region. The interface consists of two or three metallic cones (depending on the design) with very small orifices, which are maintained at a vacuum of ~1–2 torr with a mechanical roughing pump. After the ions are generated in the plasma, they pass into the first cone, known as the 

. From there, they travel a short distance to the 

, which is generally smaller and more pointed than the sampler cone, and typically has a much smaller orifice. In some designs, there is a third cone called the 

, which is used to reduce the vacuum in smaller steps and provide less dispersion of the ion beam. Whether the system uses two cones or three, the cones are usually made of nickel (Ni) or copper (Cu), but can be made of other materials such as platinum (Pt), which is far more tolerant to corrosive liquids. To reduce the effects of high-temperature plasma on the cones, the interface housing is water cooled and made from a material that dissipates heat easily, such as Cu or aluminum. The ions then emerge from the skimmer cone, where they are directed through the ion optics and, finally, guided into the mass separation device. A more detailed view of the interface region is shown in Figure 2.


	Figure 2: A more detailed view of the interface region for ICP-MS.

The Importance of Routine Maintenance of the Interface Cones

	All sample and skimmer cones, regardless of ICP-MS manufacturer or design, serve the same function. They are the differential apertures separating the sample at atmosphere from the mass filter and detector(s) at high vacuum. Though the fundamental dimensions are all essentially the same (sample cone with shallow angle, skimmer cone with acute angle), each manufacturer has designed the cone's critical dimensions to their own unique specifications, based on the design of the mass spectrometer (including, but not limited to, orifice diameter, hole depth, and material composition). The actual cone design may vary significantly from one instrument manufacturer to another. The analytical zone, the area most in need of maintenance, displays the most significant differences. These differences, together with sample type (for example, ground water, soil digests, or sea water) contribute to the rate at which oxides and salts accumulate in and around the cone orifice. They also affect the rate of cone degradation, and influence the cleaning methodology. Ultimately, the quality of the data is the best diagnostic tool for determining when to clean or change the cones. However, if significant buildup on the cone is observed and appears to be occluding the cone orifice, cone removal and cleaning is warranted. All instrument manufacturers prescribe a daily routine to evaluate the status of the system prior to an analytical run. These fast and easy tests can also be used to qualitatively evaluate cone condition. Let's take a closer look at how to recognize when the cones are beginning to negatively impact instrument performance (3).

As oxides and salts build up around and within a cone orifice, they will change the gas flow dynamics and perhaps electrical characteristics of the cone interface. Symptoms may include reduced sensitivity and a degradation of signal stability. In severe cases, there may even be a change in signal response across the mass range due to pressure changes (higher vacuum) between the sample and skimmer cone.

Like other components of the sample introduction system, sample cones are prone to certain memory effects, especially from elements like lithium, boron, and magnesium. This is particularly true when attempting trace analysis of elements that were previously in solution at high concentration. These effects may inhibit quality analysis well before visual buildup occurs around the analytical zone of the cones. In many, if not most, cases, memory effects are due to the sample introduction system (tubing, spray chamber, torch); therefore, thorough evaluation of all potential sources of memory effects should be carried out.

 Whether from extensive use, improper use, aggressive matrices, or blunt force trauma, over time cones will degrade beyond acceptable limits. Careful handling and proper cleaning will typically enable about 500 h (several months) of operational use from each set of Ni cones and about 1500 h (18 months) from Pt cones. Some common symptoms of cone degradation are elevated oxide levels, doubly-charged species, and in severe cases, poor vacuum levels.

 Pt-tipped cones offer greater lifetime than Ni cones, and usually require less day-to-day maintenance. Aggressive matrices (high in oxidizing conditions) may require Pt-tipped cones. Unlike Ni cones, Pt-tipped cones can be cost-effectively refurbished. In addition, refurbishing processes can more than double the normal lifetime of Pt-tipped cones. Pt usually stays cleaner because it runs hotter, and has a tighter grain in structure than Ni. The tip can usually be reshaped several times. After extensive use, Pt (as well as Ni) will suffer from high temperature embrittlement, characterized by large grain crystals that will crack when the tip is being reformed. We strongly advise that users not try to mechanically alter the tip in any way, because the tolerances are tight and the shape is critical to good performance. Such alteration can lead to the unnecessary, early retirement of a good cone, which in the case of Pt, could be extremely expensive.

	Whenever possible, users should clean cones under mild, non-mechanical conditions. This is especially true for Pt cones, where accidental damage is very costly. However, with certain applications, a refractory buildup is inevitable, and will require mechanical cleaning. Following are three recommended cleaning methods. Choose the one that best suits your needs.

	Note: An alternative to the more aggressive mechanical cleaning of Pt cones is to use a professional Pt cone refurbishing service, where the most severe refractory buildup can be removed and damaged Pt cones can be reshaped to their original specifications (5).

	The light cleaning-detergent method is recommended for cleaning cones where there is no severe buildup, and what little buildup exists is easily removed. It is primarily carried out to reduce elemental memory, and improve detection limits.

It is worth emphasizing that today's cones are available in many different materials, including pure Ni, Pt, Al, and Cu, as well as varying combinations of these metals in the cone base and tip.

		Soak in 1:20 metal cleaner (such as Citranox) (4):H

0 for 2 h (sonicate in an ultrasonic bath for last 15 min, if available).

O, and repeat soak for 10 min (sonicate).

		Dry thoroughly using argon or nitrogen, if available.

	Note: If using a bath sonicator (recommended), place cones in metal cleaner solution and sonicate for the final 15 min of the 2 h soaking period. Rinse thoroughly in tap water, then place in deionized H

O, and sonicate for 10 min. Replace deionized H

O and repeat. Dry with clean, dust free cloth, let air dry, and then finally blow dry with argon or nitrogen, or place in glassware oven (40-50 °C). Always take care not to place cone tips in a precarious position where they can fall or tip. For most sample cones, this is not a significant problem, because the base is large relative to the height. Skimmer cones, however, are more prone to tipping over and deforming, since they tend to have thinner walls and sharper edges.

	Cleaning cones with a Cu base requires a slightly modified technique so as not to unduly wear the Cu part of the cone, especially where it meets the Ni or Pt tip. Over time, metal cleaner will attack the Cu, and may degrade the weld between the two metals.

		Invert cone into a dilute metal cleaner solution, so that only the tip of the cone is immersed.

O, and repeat rinse (sonicate for 15 min).

	Note: Soaking and cleaning times may be modified, depending on need. The cone fixture, easily assembled from a spare cone container, enables more aggressive cleaning of the analytical zone without degrading the bimetallic weld. However, more exotic and functional fixtures would be advisable.

Standard Cleaning–Acid/Detergent Method

	Mild accumulation of salts and oxides may build up around the analytical zone of your cones. Over time, this will affect the quality of your analyses. In this case, it is recommended that a slightly more aggressive cleaning protocol be used.

		Invert cone into fixture containing 2% HNO

		Wipe off residue with cotton swab, then rinse in tap water

		Dry with clean, dust free cloth, and then let air dry, blow dry with argon or nitrogen, or place in glassware oven (40-50 °C).

	Cleaning cones with a Cu base requires a slightly modified technique, so as not to unduly wear the Cu part of the cone, especially where it meets the Ni or Pt tip. Over time, metal cleaner and nitric acid will attack the Cu, and may degrade the weld between the two metals.

		Invert cone into fixture containing metal cleaner:H

		Wipe off residue with cotton swab, then rinse in tap water.

		Dry with clean, dust free cloth, and then let air dry, blow dry with argon or nitrogen, or place in glassware oven (40-50 °C)

 solution directly onto the Pt tip. It is recommended, however, that application be accomplished with an acid-soaked swab. This should be done with great care to ensure the Ni or Cu base is not in contact with acid, because it could become degraded.

		Mix alumina powder with water to make a loose paste.

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A detailed look at the most critical component of an ICP-MS instrument: the interface cones

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