It’s well known that arsenic is toxic, but what’s less well known is that its toxicity depends on the chemical form in which it’s present. Inorganic arsenic compounds, such as arsenite and arsenate, are more toxic than organic species, such as dimethylarsinic acid and arsenobetaine. The effects of arsenic (III) range from vomiting and abdominal pain as a result of acute exposure and skin lesions and cancer induced by chronic exposure.
Exposure to arsenic is more common than you might think. Its presence in soil and water (at widely varying concentrations across the world) leads to incorporation of it into a variety of plants and food crops. Rice is particularly efficient at taking up inorganic arsenic and has been the subject of many research studies into the scale of the problem and its impact on human health. The problem is acute in Bangladesh, where millions of people are exposed to both rice and drinking water contaminated with arsenic naturally present at high levels in the groundwater, and thousands die each year.
Identifying and quantifying arsenic species involves separation of the species, using a chromatographic technique, followed by detection using a suitably sensitive detector. The chromatography techniques that most readily lend themselves to this application are high performance liquid chromatography (HPLC) and ion chromatography (IC), with inductively coupled plasma mass spectrometry (ICP-MS) as the detector. HPLC and IC are both suitable for arsenic speciation work, but which is better?
HPLC versus IC
HPLC methods for arsenic speciation are based on reversed phase chromatography with, usually, phosphate buffered mobile phases. This is relatively easy to set up, reasonably sensitive and provides the necessary chromatographic resolution of the target species. However, the need for a buffered, sometimes multi-component mobile phase leads to extra preparation work and more cost. In addition, the presence of phosphate in the mobile phase is not ideal for the ICP-MS instrument, as phosphates cause pitting of the interface cones, leading to more frequent replacement or necessitating the use of more robust (and much more expensive) Pt-tipped cones. Longer term operation of HPLC methods for arsenic speciation eventually leads to a drop off in sensitivity and peak retention time drift caused by other components in the sample, leading to the need to periodically exchange the HPLC column. For lower numbers of samples, HPLC is a cost-effective solution, but the less expensive instrument cost can eventually be offset by the higher column consumable costs in the longer term.
In contrast, IC offers the benefits of sharp peak shapes (even for late eluting species), retention time stability, column robustness, excellent long term peak area / height reproducibility and simpler mobile phases; only dilute ammonium carbonate is required, which is easy to prepare. In addition, dilute ammonium carbonate is fully converted to gaseous species in the plasma (NO2, H2O and CO2), so no damage or blocking of the interface cones occurs when using this mobile phase. Although not so much of an issue for arsenic speciation, IC also provides a metal-free sample path as standard, which provides lower backgrounds for other elements of speciation interest, such as chromium. Although IC instruments can be more expensive to purchase than HPLC systems, over time this can be offset by the lower consumables cost.
While HPLC and IC are both suitable for arsenic speciation, IC offers some distinct advantages for this application. Thermo Scientific™ provides a full range of innovative IC systems including the Aquion™, Integrion™ and ICS-6000 HPIC™ systems, but if HPLC is your method of choice, we also offer the UltiMate 3000 and Vanquish range of instruments.
Choosing the Right ICP-MS Detector for Your Arsenic Speciation Applications
Once you’ve decided which of HPLC or IC to choose for your speciation work, you then have a choice of ICP-MS system too. For speciation applications in general, interferences are not usually a significant problem, so a single quadrupole ICP-MS such as our iCAP™ RQ instrument is a suitable detector. For direct ICP-MS analysis of arsenic in solution, chloride in the samples gives rise to 40Ar35Cl+ interference on 75As in the plasma, but with speciation the chloride in the sample is usually fully separated from the species of interest, so the interference is removed. In the event that a chloride containing component co-elutes with an arsenic species, operating the ICP-MS in collision cell mode effectively removes ArCl+ so the problem is still removed. In practice, collision cell mode is usually used for arsenic speciation to ensure that any potential problems with ArCl+ interference don’t arise. If you’d like to learn more about the iCAP RQ, or about our trace element analysis solutions in general, head to this page.
If you have other analysis requirements in your laboratory in addition to arsenic speciation, you might benefit from the extra interference removal capabilities provided by a triple quadrupole ICP-MS system, such as our iCAP™ TQ. Triple quadrupole ICP-MS has the power to resolve difficult interferences such as CoO+ on 75As+, which is a problem when measuring arsenic in samples such as Vitamin B which contains a high level of cobalt (4% by weight). In addition, the sensitivity for arsenic is higher with triple quadrupole ICP-MS compared to single quadrupole instruments, so lower detection limits can be achieved. For more information about this and other applications of the iCAP TQ, take a look here.
Finally, integrating either HPLC or IC with ICP-MS to provide routine unattended speciation analysis requires the right software to complement the hardware. With the iCAP RQ and iCAP TQ instruments, the Qtegra™ Intelligent Scientific Data Solution™ (ISDS) software integrates seamlessly to control the chromatograph and automate the analysis. Qtegra also provides the tools necessary to identify, quantify and generate reports from speciation analyses.
If you’d like to find out more about what IC-ICP-MS can do for your arsenic speciation work, visit our dedicated web page on this subject where you’ll find applications information and links to other useful IC resources.
Thermo Scientific offers a wide range of other analytical solutions to help you achieve your food safety, authenticity and QA/QC objectives. If you have any questions about methods, workflows or products for these application areas, from trace elemental analysis and chromatography to organic elemental analysis and high resolution mass spectrometry, just let us know via the comments box below.