By Stephen R. Tischler
The idea for this new column was born from an article I wrote for this magazine entitled “A Look Behind the Scenes of a Modern Drinking Water Laboratory” (August 2005, pp. 30-34), where I described the complexities involved in operating a modern drinking water laboratory. To illustrate that point, I described a seven-step process for the analysis of Haloacetic Acids (HAA’s) by EPA Method 552.2. Haloacetic Acids are in a class of compounds known as disinfection by-products and are considered contemporary ‘analytes of interest’. In future columns, I will explore other ‘analytes of interest’ such as arsenic and classes of compounds like metals, volatile organics, herbicides, pesticides, or radiologicals, paying special attention to topics that directly impact WC&P readers.
For this first column, the ‘analyte of interest’ to be discussed is perchlorate. Perchlorate is an anionic (negatively charged) compound comprised of one chlorine atom and four oxygen atoms. Its chemical formula is ClO4(-), and it can exist in the form of salts of potassium or ammonia. If you haven’t heard this by now, its primary use is as an oxidizer in rocket fuel (which probably explains why the media has latched onto it). Everybody loves a good rocket fuel story. This analyte has drawn serious media attention from Massachusetts to California. Federal, state, and local government agencies at all levels, including the U.S. EPA, FDA, and DoD have weighed in on topics from health effects to origins in the environment. Both the health effects and the origins of perchlorate are still being discussed. It is generally accepted that perchlorate disrupts the function of the thyroid gland, blocking the uptake of iodine. As a result of all this attention, laboratories started looking at the current methods of analysis and the development of new methods to look for smaller and smaller amounts of perchlorate in drinking water and milk, as well as foods such as lettuce and tomatoes.
There are several methods published by the EPA for the determination of perchlorate in drinking water. The one I’ll focus on in this article is Method 314.0, Determination of Perchlorate in Drinking Water Using Ion Chromatography. There are other methods, such as EPA 314.1, Determination of Perchlorate in Drinking Water Using Inline Column Concentration/Matrix Elimination Ion Chromatography with Suppressed Conductivity Detection, EPA 331.0, Determination of Perchlorate in Drinking Water by Liquid Chromatography Electrospray Ionization Mass Spectrometry and EPA 332.0, Determination of Perchlorate in Drinking Water by Ion Chromatography with Suppressed Conductivity and Electrospray Ionization Mass Spectrometry. These methods have been developed to look for perchlorate in the sub-parts per billion range. I would also like to point out that the EPA has not established a maximum contaminant level (MCL) for perchlorate; however, ranges from 1 ppb up to 25 ppb have been tossed around recently. Most analysis work conducted on commercial samples is being used for informational purposes, although some states are requiring the analysis.
A modern drinking water laboratory employs several forms of chromatography for the analysis of contaminants in drinking water, including gas and liquid chromatography. EPA method 314.0 is based on the analytical technique known as ion chromatography, a form of liquid chromatography. So, what is chromatography? Simply stated, chromatography is a separation technique used to isolate specific analytes from one another using columns packed with specific types of media (the stationary phase) that are pushed along by a liquid or gas (the mobile phase). Ion chromatography makes use of certain ion-exchange resins (stationary phase) to separate atomic or molecular ions based on their interaction with the resin. This should be a familiar concept to readers, as the same techniques are used in water treatment (anion and cation resin), albeit on a much larger scale. Water samples are introduced into these packed columns using an injection system and the sample is ‘pushed’ through the column using a pumped liquid (mobile phase). The power of this technique is based on the fact that different ions have different affinities (attraction) for the ion exchange resin, causing the various ions to emerge from the packed column at different intervals known as ‘retention times’. As these ions emerge, they are detected using various instruments. There are many different kinds of detectors used in chromatography, including mass spectrometers, electron capture, flame ionization and conductivity. EPA method 314.0 uses a conductivity detector that responds to the change in effluent conductivity as the various ions emerge from the packed column. The detector is used to produce a chromatogram of ‘peaks’ that indicates the specific retention time and quantity for each ion. Figure 1 shows a typical chromatogram.
The successful analysis of perchlorate (or any analyte for that matter) starts with the proper collection, preservation and storage of the sample. Perchlorate may be collected in a plastic or glass bottle that has been thoroughly cleaned and rinsed with reagent water. The volume collected should be sufficient to ensure a representative sample, allowing for replicate analysis. Perchlorate is typically sampled in a 250-milliliter plastic container. Some analytical methods require that certain preservatives be added to the sample container, either prior to or after the bottle has been filled. Perchlorate analysis by EPA Method 314.0 does not require any preservative. Some methods also require that samples be chilled and shipped in thermally insulated containers. Perchlorate by 314.0 does not require this; however, the method does recommend that samples be protected from temperature extremes during shipment. A perchlorate sample can be held for a maximum of 28 days prior to analysis. This is referred to as the holding time.1 Tracking holding times is critical in the analysis of contaminants in drinking water.
Method 314.0 can detect perchlorate at a method detection level (MDL) near 0.5 micrograms per liter (parts per billion) and a minimum reporting level (MRL) near 1.0 micrograms per liter. The MDL is the minimum concentration of an analyte that can be identified, measured and reported with 99 percent confidence that the analyte concentration is greater than zero. The MRL is the minimum concentration that can be reported as a quantitated value for a target analyte in a sample following analysis. The MRL can not be lower that the lowest calibration standard.1
In order to determine the amount of perchlorate in a drinking water sample, the laboratory must engage in extensive quality control to make sure that the ion chromatography instruments are working properly. Also, a calibration curve must be developed in a linear calibration range (LCR) that covers the expected concentration range of the sample being analyzed. For example, if the expected concentration of perchlorate in a sample is expected to be around 10 parts per billion, then a calibration curve from 1 to 25 parts per billion would be developed. The calibration curve would have the standard concentrations plotted against peak area responses from the conductivity detector (Figure 2).
After the calibration curve for perchlorate has been determined and quality controls have been established using initial calibration check standards (ICCS), the field samples are analyzed. During the analysis of field samples, continuing calibration check standards (CCCS) are analyzed to make sure the instruments are working properly. Also, at the end of the analysis of field samples, an end calibration check standard (ECCS) is completed to verify the previously established calibration curve and to make sure that the instrument was working properly at the end of the run . The amount of perchlorate in each field sample is determined based on the peak area results as compared to the calibration curve and reported by the laboratory in micrograms per liter. A commercial laboratory will generally charge between $100 and $200 per analysis by EPA Method 314.0.
- EPA Method 314.0, Determination of Perchlorate in Drinking Water Using Ion Chromatography, Revision 1.0 November 1999.
About the author
Stephen R. Tischler is Director of Sales and Marketing for National Testing Laboratories and a member of WC&P’s Technical Review Committee. A former analytical chemist at NASA, he has a long history in the aerospace industry with expertise in quality control and analytical testing method development. Tischler has a bachelor’s degree in chemistry from John Carroll University and a master’s degree in business from Baldwin-Wallace College. You may contact him at (440) 449-2525, or visit www.ntllabs.com