Analyzing Organic Compounds in WaterThe Next Generation of Standards
At the January meeting of ASTM Subcommittee D19.06 on Organics in Water, ASTM International standard D 3695, Test Method for Volatile Alcohols in Water by Direct Aqueous-Injection Gas Chromatography, was under discussion. It is one of a number of “orphaned” standards, so called because none of the active members of the subcommittee use the standard. One of the members had volunteered to review the standard and was reporting to the subcommittee. A discussion about his report ensued, in which the subcommittee realized that the standard reflects old testing technology no longer in use. We agreed to ballot the standard for withdrawal.
This discussion triggered a flood of memories of a generation of technology that was just being born when I was a student and in my early working years in the late 1960s and early ’70s.
For many years after the birth of organic chemistry in the 19th century, the separation and analysis of organic compounds had been carried out by classical techniques. (Visualize a laboratory filled with glassware clamped to steel frames some of the flasks contain boiling liquids that are being distilled. Large separatory funnels contain solvents. Cheap labor students and technicians keeps the operations running.)
Needless to say, classical techniques were far more time-consuming than the instrumental techniques of a generation ago. Thin-layer chromatography, the oldest chromatographic technique, was performed by hand on glass plates prepared (again, by hand) with a slurry of silica gel poured evenly onto the glass and dried. Gas chromatography columns were made (by students or commercially prepared) from thin copper tubing, unwound and stretched straight, mounted in a stairwell if necessary, packed by hand, treated, and recoiled. (The commercial production of columns was a great timesaver.) Components separated by the instrument were collected by the operator watching the peaks on the recorder produced by the detector.
These might have been analyzed by infrared spectroscopy or perhaps carried to the mass spectrometry lab, where a dedicated professional confirmed a definitive structural analysis of the compounds of interest. Liquid chromatography separations with high resolution required pouring a solvent into a reservoir on the top of a long glass column that had been packed with the separation material. Fractions of the solvent passing through the column were collected and analyzed to determine the compound of interest.
I remember climbing up a stepladder to stand on the lab bench to reach the top of a tall column. What excitement when my advisor showed me an article describing a tabletop instrument called an HPLC for performing high performance liquid chromatography. It was based on a concept similar to the GC, except that a solvent or a solvent mixture was forced through a coiled metal column. The separation of the organic components in the mixture was dramatically improved, and the detector was built in.
A New Day for Water Testing
The age of instrumental analysis was upon us, and its promise was fulfilled with standard test methods that lowered detection limits, improved precision and accuracy, decreased the time of analysis and saved operator time. It was fortunate that technology had developed sufficiently to support the analytical needs of the developing field of environmental chemistry.
Figure 1 Marine science technician Duane Wilson, USCG, adjusts the nitrogen flow to prepare oil samples for analysis at the USCG Marine Safety Lab.
ASTM Subcommittee D19.06 became and remains the place where industrial analytical chemists interact with colleagues from governmental agencies to reach a consensus on the technical aspects of the drafts of standard methods and then collaborate on the interlaboratory performance testing required to validate the methods. The stringent requirements of standard D 2777, Practice for Determination of Precision and Bias of Applicable Test Methods of Committee D19 on Water, now specifies at least six laboratories and three concentration levels for each chemical component of interest, with pairs of samples at each level.
Figure 2 Kristy Juaire, chemist at the USCG Marine Safety Lab, compares gas chromatograms for two oil samples as part of a Coast Guard pollution investigation.
So-called “hyphenated instruments,” such as gas chromatographs combined with mass spectrometry, have become small enough to fit on a benchtop. The high resolution of such an instrument separates components sufficiently to analyze potentially thousands of components within the same sample. The challenge for method developers has become the task of performing interlaboratory studies with many components of potential interest. The difficulty of preparing reference standards and unknown samples for interlaboratory studies has been exceeded only by the complexity of the data analysis. One cheerful aspect of this development has been that personal computers have become fast enough to automate and control the instrument as well as collect the data and cheap enough to be dedicated to one instrument.
Subcommittee D19.06 maintains standards in many areas. Here is an overview.
When the Clean Water Act of 1972 assigned the U.S. Coast Guard the responsibility of determining the source of unknown oil spills, the Guard turned to Subcommittee D19.06, where a group of petroleum company representatives were working on methods that would compare the chemical characteristics of oil spills with their source oils. The activity expanded and a separate subcommittee was established for this purpose.
From initial standards for thin layer chromatography, fluorescence and infrared spectroscopy, and gas chromatography with packed columns, a high-performance liquid chromatography standard was later added, the gas chromatography standard was revised to use new capillary columns and the thin layer chromatography standard was withdrawn. The adoption of a standard for gas chromatography-mass spectrometry brought the oil identification standards into maturity, and that new subcommittee rejoined D19.06. The review, reapproval and revision of the oil identification standards remain an ongoing activity within D19.06.
The robotic arm of an autosampler maneuvers a sample vial into position for injection into the GC-MS for analysis as part of an oil spill investigation at the USCG Marine Safety Lab.
Quantitation of Oil and Grease in Water
The measurement of oil and grease in water had seemed straightforward for many years: carbon tetrachloride was used to extract the sample and the solution was analyzed with infrared spectroscopy. It was a wonderful solvent that was transparent for infrared. When “carbon tet” was found to be highly carcinogenic, a new standard was developed using Freon. Freon was banned for environmental reasons, and the subcommittee began a long search for and evaluation of a substitute synthetic solvent.
In 2004, D 7066, Test Method for Dimer/Trimer of Chlorotrifluoroethylene (S-316) Recoverable Oil and Grease and Nonpolar Material by Infrared Determination, was approved. While the standard, for theoretical reasons, is not as good as its predecessor, we didn’t expect it to be, and it represents the best that can be done at this time. A gas chromatography standard has been proposed for development, but we know that it will perform differently. When something better comes along, standards development will begin again.
Although ASTM standards are available and are widely used for total organic carbon, a new task group has begun work on a proposed standard for total organic carbon in water by high-temperature combustion infrared detection.
This test method will be used for higher levels of total organic carbon that require dilution by more sensitive methods. It is also useful for samples containing refractory organic species, or suspended carbon species that are not completely recovered by chemical oxidation.
Nonylphenols and Bisphenol A in Water
One of the newest standards developed by D19.06 is D 7065, Test Method for Determination of Nonylphenol, Bisphenol A, p-tert-Octylphenol, Nonylphenol Monoethoxylate and Nonylphenol Diethoxylate in Environmental Waters by GC-MS. This was a close collaboration between the U.S. Environmental Protection Agency and industrial chemists.
Why was this standard needed? The same month that this standard was approved, an article appeared in the national press about the concern over bisphenol A. Although not thought to be directly carcinogenic, it and its relatives are now known to be endocrine disruptors. Look for the impact on the environment and on human health to be a focus of concern in the future and remember that the standard test method used to quantitate these compounds was developed by ASTM International members.
A new standard under development, WK10122, Test Method for Pore Water Analysis of Polyaromatic Hydrocarbons (PAHs) Using Solid Phase Microextraction Method, has been fast-tracked by the committee to be approved with a limited validation study to be followed later by a full interlaboratory study meeting the requirements of D 2777.
Why should pore water within sediments be an area of interest? As subcommittee members learned at a recent workshop, there is little relationship between the total PAH concentration in sediment and the toxicity of that sediment. Unfortunately, the present regulatory framework does not address this reality. There are challenges in developing such a method: most alkylated PAHs occur in clusters for which there are no calibration standards, although the EPA lists only parent compounds. How do you analyze compounds which because of their varying solubility occur over five, six or even seven orders of magnitude in concentration? The task group is addressing these concerns, and the results of a single laboratory study will be presented at the June meeting.
To those not in-the-know, cyanides seem simple: three of the four bonds on a carbon atom are attached to a nitrogen atom, leaving one free to bond to something else (-CN). It turns out to have been more complex than even the early ASTM method developers knew.
One of the oldest D19.06 standards still on the books, D 2036, Test Methods for Cyanides in Water, was recently revised. It is a classic colorimetric technique and needed to be transferred to modern technology. The foundation of this work is ASTM standard D 6696, Guide for Understanding Cyanide Species, which was finalized in 2005.
Even within the area of cyanide analysis, there had been confusion in the terminology and understanding of cyanides. The ASTM task group reached consensus on these matters. In D 6696, the user finds explanations and discussions of such terms as “aquatic free cyanide,” “available cyanide” and “total cyanide.”
Without this guide, it is impossible to understand the significance of standards D 6888, Test Method for Available Cyanide with Ligand Displacement and Flow Injection Analysis Utilizing Gas Diffusion Separation and Amperometric Detection (approved in 2004); D 7237, Test Method for Aquatic Free Cyanide with Flow Injection Analysis (FIA) Utilizing Gas Diffusion Separation and Amperometric Detection (approved in 2006); and new test methods under development for total cyanide in water by midi- or micro-distillation and the detection of total cyanide by segmented flow injection/on-line ultraviolet digestion-gas diffusion with amperometric detection.
The members of Subcommittee D19.06 share a common goal with those on other ASTM committees: to gather people with knowledge to reach a consensus and validate methods for the benefit of all, including those who, by purchasing standards from ASTM, are able to make accurate determinations. The descriptions of standards recently developed or under development seem amazing possibly overwhelming even to those of us on D19.06. Perhaps this article will stand as a milepost to measure how far we have come and to provide a marker that another generation will look back to for comparisons. What will they think of our technology and our standard methods? //