Finding the Right Instrument for the Job
The recent controversy surrounding lead contamination of the municipal drinking water in Flint, Michigan, has shone a light on the necessity of routine testing for toxic metal contamination not only of water but also of food, drugs, and other consumables. Lead contamination of the water supply in Flint was uncovered by the method of choice for detection of elemental metals: inductively coupled plasma mass spectrometry (ICP-MS), which can detect many trace elements at concentrations as low as one part in 1015.
ICP-MS-based analytical techniques cross disciplines and are used in the food, pharmaceutical, environmental, geochemical, nuclear, and medical fields to monitor test samples for trace elements and their isotopes. Samples are ionized by application of an argon torch and electromagnetic field, followed by mass/charge ratio separation via the spectrometer.
A basic ICP-MS unit with a quadrupole mass filter is sufficient for testing municipal water supplies, but as sample complexity and required sensitivity increase, system requirements quickly get complicated.
For example, the semiconductor and medical industries have special requirements.
“Analysis of impurities in ultra-high-grade chemicals used for the semiconductor industry requires an ICP-MS with a high purity sample introduction system and very low detection limits,” says Fadi Abou- Shakra, product manager for ICP-MS at PerkinElmer (Waltham, MA). “Similarly, detection of low levels of elements such as chromium and titanium in biological fluids would require an instrument with a reaction cell.”
Food and pharmaceutical samples often contain trace amounts of elements such as oxygen and nitrogen, which can react with elements of interest to create interfering polyatomic species that have the same mass as species of interest. Samples with the potential to form interfering species typically require the addition of a collision cell to the reaction chamber. Simply put, the collision cell contains an inert gas, usually helium, to slow the rate of movement of the larger interfering species, which can then be removed by a filter before entering the spectrometer. First introduced by PerkinElmer, the collision/reaction cell, known by the trade name “dynamic reaction cell,” uses a mixture of gases to help remove interfering ions prior to detection, both through collisions and through chemical reactions. A similar solution, introduced by Agilent Technologies (Santa Clara, CA), uses an octopole collision cell containing helium or hydrogen in the reaction chamber itself. For most applications, the inert helium mode removes potential interfering species, allowing it to be used on samples with unknown elements, as the collisions don’t introduce new reactive species. However, for industries requiring detection of extremely low concentration species, removal of interferences by both collision and reaction using a mixture of helium and hydrogen may be required.
“For certain environmental applications or food applications, you might need to differentiate the different oxidation states of different metals,” says Dan Davis, ICP-MS product manager at Shimadzu Scientific Instruments (Columbia, MD). “For example, the toxicity of arsenic and chromium is related to their oxidation states, so being able to separate out the oxidation states prior to analysis can allow you to speciate and quantify.”
For these applications, ICP-MS is often combined with high-performance liquid chromatography.
Laboratories that expect to combine analytical techniques should consider the ease of combining ICP-MS with other instruments and the reporting of results from multiple pieces of equipment, says Davis. Some manufacturers have developed software platforms to streamline data analysis and reporting from up to five or six other pieces of equipment. In addition, Shimadzu has introduced a software assistant that helps users set up their protocols based on their sample profiles to improve the reproducibility of data and reduce the need for frequent calibration, he says.
The United States Pharmacopeial Convention, which sets standards for medicines, food ingredients, and dietary supplement ingredients, is implementing new standards in 2018 limiting elemental impurities in these ingredients and specifying ICP-MS analytical techniques for testing that are likely to increase the need for new equipment and training in these industries, says Ryan Brennan, PhD, US marketing manager for Glass Expansion, Inc. (Pocasset, MA), maker of glassware and sample introduction systems for all the major ICP-MS manufacturers.
Run time and consumables
Factors to consider when choosing an ICP-MS system include cost savings that can be accrued by being able to control the run time of the equipment and the flow rate and purity requirements of the noble gases within the system, as well as how frequently consumables like glassware and sampling cones need to be replaced. Add-ons such as autosamplers can greatly increase the throughput rate but increase the cost of the systems, so users in, for example, the environmental field, should do a cost-benefit analysis of their expected sample volume before choosing an autosampler, says Brennan. In many fields, labs are paid by the sample. “If you can run the same number of samples in half the time, the value of those samples is much higher and the consumable costs on your instrument are a lot less.”
Among ICP-MS customers, the issue of equipment maintenance is often a topic of conversation, says Paul Gaines, PhD, CEO of Inorganic Ventures (Christiansburg, VA), maker of ICP-MS calibration standards and a heavy user of the technology itself. Laboratories interested in purchasing an ICP-MS system should seek out current users in their geographic area and ask about the cost of maintenance, as well as how long it takes to get a service engineer out to fix problems with the system that go beyond routine maintenance. One of the biggest complaints Gaines hears about is significant downtime due to a delay in getting a technician in to repair a system.
For routine maintenance, be sure to find out how easy it is to gain access to skimmer cones and sampling cones, says Davis. “Keeping in mind that these are basically the interface between something that is 10,000 degrees Celsius and ambient pressure to something that is at ambient temperature but very low pressure, you want to be able to access the interfaces without having to break the vacuum because that vacuum takes a lot to restore. Things like shutter mechanisms that allow you to access the skimmer cone and sampling cone while still maintaining your vacuum [are] very advantageous.”
“One of the biggest maintenance items with ICP-MS, if you throw a challenging sample in there, is the time to clean the cones,” adds Brennan.
Most important, though, Gaines says, may be making sure that ICP-MS is the right tool for the job. “Ask for a demo with a sample from your own lab,” he says. “Detecting rare earth elements with a molecular weight less than zinc is more challenging. Make sure the instrument is compatible with your needs.”
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