Heavy metals are metals with a specific gravity of at least five times that of water. Examples of heavy metals include arsenic, copper, cadmium, chromium, nickel, zinc, lead, and mercury. Because of their toxic, non‐biodegradable, and persistent nature, certain heavy metals become major pollutants of freshwater reservoirs.
In drinking water, the most common heavy metals are chromium, lead, copper, arsenic, cadmium, and mercury. While the majority of these heavy metals are a by-product or a waste product of industrial processes, lead and copper can leach from water pipes and soldered joints and contaminate the drinking water.
Health effects
The consumption of drinking water contaminated with heavy metals can cause adverse health effects. For example:
- One form of chromium, chromium 6, can cause cancer and serious health problems.
- Lead can cause heart or kidney problems; in children, exposure may result in a lower IQ, seizures, and potentially death.
- Copper can lead to anemia and digestive disturbances. At high exposure levels, copper can cause liver and kidney damage.
- Arsenic exposure has been linked to cancer and neurological, endocrine, and cardiovascular issues.
- Damage to the kidneys, liver, and bone can result from long-term exposure to cadmium.
- Mercury can harm the kidneys and nervous system. This metal can be transmitted to a developing fetus.
Of the heavy metals in drinking water, lead and mercury have the highest toxicity in children. Children exposed to lead are more likely to suffer from a shorter attention span and learning difficulties. Those exposed to high levels of mercury are likely to experience learning difficulties.
Testing methods
The U.S. EPA has developed methods to analyze heavy metals in groundwater, surface water, drinking water, and wastewater. Method 200.8 uses inductively coupled mass spectrometry (ICP-MS),1 while Method 200.7 uses inductively coupled optical emission spectrometry (ICP-OES).2 Both are suitable for the analysis of aluminum, antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, manganese, mercury, molybdenum, nickel, selenium, silver, thallium, thorium, uranium, vanadium, and zinc.
Methods 200.8 and 200.7 outline procedures for selecting analyte masses and wavelengths, instrument tuning and calibration, and interference corrections, and provide specific instructions concerning sample collection, preservation, and treatment.
Testing optimization
What both ICP methods have in common is the sample introduction system—namely, the nebulizer, spray chamber, and ICP torch. Selecting the appropriate components is thus key to optimizing the performance of either ICP method to meet the requirements of Methods 200.8 and 200.7.
Nebulizer
Several nebulizers are available from Glass Expansion (GE) to test drinking water samples. All glass concentric nebulizers from GE utilize the unique VitriCone construction, which has a sample channel constructed from a machined, heavy glass capillary. The benefits of VitriCone construction are uniformity of the thickness and surface of the sample channel, as well as resistance to vibration and clogging, resulting in the highest precision. With hand-drawn sample capillaries, the IDs can vary, preventing a laminar flow and creating points where particulates may lodge. In addition, all GE concentric nebulizers feature a Direct Connection (DC) gas line, providing an inert, metal-free, and instrument-specific gas fitting, ensuring a leak-free connection for consistent day-to-day performance.
Some criteria that nebulizer selection depends on include: sample volume, total dissolved solids (TDS), and particulate size. For ICP-MS, the standard recommendation is the MicroMist, a low-uptake, high-precision, and high-sensitivity nebulizer. For ICP-OES, the standard recommendation for environmental samples is the SeaSpray, which features a unique “self-washing” tip with smooth surfaces to avoid build-up of salt crystals. The SeaSpray provides outstanding transport efficiency, with tolerance up to 20% TDS, resulting in the best detection limits and short-term precision (RSD) among the GE glass concentric nebulizers.
The Conikal glass concentric nebulizer is a great-value, high-precision nebulizer for routine analysis of aqueous and organic samples with low (<5%) TDS.
The HF-resistant DuraMist concentric nebulizer is made of inert PEEK materials. It is highly sensitive, with excellent short-term precision and the highest tolerance to dissolved solids of any concentric nebulizer (30%). It is a great all-rounder and the choice for analysis of diverse sample types.
The Slurry glass concentric nebulizer is designed for analysis of slurries and suspensions. It provides the excellent sensitivity and short-term precision of a glass concentric nebulizer but is tolerant to undissolved particles of up to 150 um in diameter. Specifications for each type of nebulizer are available here.
With high TDS samples, salt deposits may form at the tip of the nebulizer. Adding moisture to the nebulizer gas before it comes into contact with the sample may lessen the formation of salt deposits. An argon humidifier such as the Elegra and dual-channel Elegra may be coupled with the nebulizer to add moisture and mitigate the effect of salt deposits.
Spray chamber
There two main types of spray chambers used with ICP-OES—the cyclonic spray chamber and the Scott spray chamber. The cyclonic spray chamber was introduced to the ICP world by Glass Expansion in 1989. The Scott spray chamber dates from the origins of ICP in the early 1970s. Both cyclonic and Scott spray chambers are made from various materials, which include borosilicate glass, quartz, or a polymer such as PTFE, PFA, or Ryton, with the choice of material depending upon the tolerance to the sample matrix.
Cyclonic spray chambers from GE are available in two configurations, a single-pass or double-pass. The Tracey cyclonic spray chamber is a single-pass design (50 mL volume), manufactured from high-quality borosilicate glass. It provides the best sensitivity and lowest memory effects for standard ICP analyses. The Twister cyclonic spray chamber is a double-pass design (50 mL volume), also manufactured from high-quality borosilicate glass. The most important feature of the Twister is the central transfer tube or baffle. This acts as a secondary droplet filter to reduce the mean droplet size, which in turn reduces matrix effects and improves precision.
For ICP-OES instruments with a solid-state detector (as virtually all are now), the most reliable figure of merit for detection limits is the signal to root background ratio (SRBR). With SRBR, the higher the value, the lower the detection limits. The Tracey spray chamber provides the best SRBR and hence lowest detection limits. Closely followed by the Twister spray chamber and finally the Scott spray chamber, providing the lowest SRBR and potentially poorest detection limit performance.
Details can be found here.
For some applications, maintaining a stable spray chamber temperature is key to optimum spray chamber performance. The IsoMist and IsoMist XR spray chambers maintain temperature in a controlled range. See http://www.geicp.com/cgi-bin/site/wrapper.pl?c1=Products_accessories_isomistXR.
ICP torch
Torches for ICP include the D-Torch demountable torch, the semi-demountable torch, and the fixed (one-piece) quartz torch.
The D-Torch is the most versatile design, as it allows for complete customization, and is the most economical in the long term. The D-Torch features interchangeable injectors, with varying IDs and materials of construction, an alumina inner tube, and a replaceable outer tube of either quartz or ceramic. Since the cost of the quartz outer tube is significantly less than the cost of a fixed, single-body torch, the D-Torch is the most economical over the lifetime of the instrument. For complete “indestructability,” a ceramic outer tube is available, which will last indefinitely under standard operating conditions.
For more information and available applications, click here.
Other components
For labs concerned with sample throughput, the Niagara Rapid Rinse valve is a low-cost solution for boosting productivity an average of 25%. For example, a lab that currently has a 3-minute analysis time per sample results in 20 samples per hour. A 25% increase results in an extra 5 samples per hour, which translates to an extra 10,400 samples per year (for an 8-hour, 5-day work week).
Components such as the radiofrequency (RF) coils and the ICP-MS cones impact the efficiency of the sample introduction system. GE offers cones for many of the ICP-MS instruments in the market, all of which are manufactured to exact OEM specifications. GE now has the largest cone manufacturing plant in the world, and, since the entire process is controlled by GE (from raw material to finished product), quality control of each individual cone is of the highest possible degree.
Information on these components is available at http://www.geicp.com.
References
- https://www.epa.gov/sites/production/files/2015-08/documents/method_200-8_rev_5-4_1994.pdf
- https://www.epa.gov/sites/production/files/2015-08/documents/method_200-7_rev_4-4_1994.pdf
Lina Genovesi, Ph.D., JD, is a technical, regulatory, and business writer based in Princeton, NJ, U.S.A.; e-mail: [email protected]; www.linagenovesi.com