By Henry Nowicki, Ph.D.
Summary: Mass spectrometry (MS)—a method of chemical analysis used in water treatment, evaluation of activated carbon and other lab testing applications—is a billion-dollar-plus marketplace with huge growth potential. Here, we discuss how laboratories apply this technology to the benefit of water treatment professionals.
A mass spectrometer is an instrument that analyzes samples by separating molecular and fragment ions according to their masses and associated electrical charges (most mass ions in the MS technique conveniently have one charge). It’s used as a detector in gas chromatography, for example, where a sample mixture is injected and vaporized into a stream of carrier gas moving through a column containing a solid medium or one coated with a relatively nonvolatile liquid where one by one component compounds are separated, analyzed, identified and quantified. In MS, the compounds emerging from the chromatograph are ionized and fragmented by bombardment with a beam of electrons. An electromagnetic field separates the ions according to their individual mass-to-charge ratios into a characteristic mass spectrum of the molecule. An analog computer analyzes the spectrum and makes it possible to identify molecules even in cases of poor separation on the chromatography column, hence the advantage of mass spectrometry compared to selective chromatograph detectors.
Complex to common
Just as it sounds, this was fairly involved and a costly procedure in the past, requiring both complex equipment and highly trained staff. In the ’60s, only a few labs had MS capabilities. However, advances in technology and increased use have brought expenses down and it’s become more common. Instrument manufacturers are now able to provide a gas chromatography-mass spectrometer (GC-MS) unit at a competitive price with a gas chromatograph loaded with several detectors. This attractive unit price—$75,000-85,000—has made it possible for almost all laboratories to have this instrument. The MS technique is the best available technology to determine qualitative and quantitative organic and inorganic compounds in a wide variety of samples, including water.
This technique is easy to learn and apply today, thanks to work of instrument manufacturers. You only need to add, subtract and have a fair understanding of organic chemistry to have success with the MS technique. Typically today, individuals using the MS technique have not had formal education covering the basic or advanced principles of MS. Also, the MS technology keeps evolving with new techniques to solve real world problems.1 This article offers some background as well as an evaluation of present and future use of MS in water analysis.
Evolution of detection
Since the late ’70s, environmental scientists have provided volatile organic compounds (VOC) and semi-volatile (SV) analysis of water samples. The sample preparation for these two GC-MS protocols are: 1) Purge-and-trap for VOC determinations, whereby VOC are stripped from the water with a stream of gas such as nitrogen and trapped on solid absorbent before thermal desorption of trapped compounds for GC-MS analysis, 2) Liquid-liquid extraction from water for SV determination followed by GC-MS analysis. The VOC method consists of purging five milliliters (ml) of the water sample to purge low molecular weight compounds—those with one to three carbon atoms with differing halogen atoms and benzene and its related compounds—toluene, xylenes and chlorobenzene.
A typical sequence to provide GC-MS analysis of water for VOC is: 1) run a known organic compound to “tune” the MS to assure mass assignment and relative intensity are correct; 2) after the instrument performance verification, 3) a clean blank water is run to assure there are no chromatographic peaks from prior runs (it was shown3 that surrogates and internal standards could be added to the sample before GC-MS analysis for quality assurance of the data and increase the productivity of GC-MS analysis of water samples); 4) a standard solution containing the compounds of interest is run with internal standards, and 5) the sample is then run and compared to the standard runs to provide qualitative and quantitative determinations of VOC. These steps are intended to help you understand the GC-MS protocol for VOC needs a disciplined sequence to provide high quality information.
The sequence for instrumental running GC-MS for SVs is similar but the target compounds are extracted from water instead of purged in the VOC analysis. Extraction of SVs consists of making the sample acidic and liquid-liquid extracting with methylene chloride to transfer the acids and neutrals (A/N) from the water to the methylene chloride. The sample is then made basic and the bases (B) are extracted into the fresh solvent. The methylene chloride solvents (A/N and B) are combined and evaporated to 1 ml for GC-MS analysis. Typically, this extraction technique provides about a 1,000 cycles of target compounds concentration—1,000 ml of water concentrated to 1 ml—and changes the matrix to a solvent compatible to the GC column for separation of the complex mixture. Then a similar sequence of runs is conducted as the VOC to provide SV determinations with legally defensible data quality.
The age of disinfection
Much of the environmental GC-MS business started in the chlorine era. Disinfection of drinking water with chlorine increased human life span, but it was later discovered many toxic halogenated organics—such as trihalomethanes (THMs)—were formed during the chlorination process. In the last few years, vendors increasingly provided drinking water disinfection using ozone and ultraviolet radiation as a result. To develop knowledge about the by-products for these new disinfection technologies, new methods for GC-MS analysis need to be provided.4 These new disinfection technologies create polar metabolites from natural and synthetic organic matter in the water sample.5 Thus the GC-MS measurement technology must change to keep up with changing disinfection methods. For instance, electrospray ionization (ESI) interfaced with MS, a method introduced recently, provides direct introduction of the water sample—no sample preparation is required.4 Often in analytical chemistry determinations, the sampling and sample preparation steps take the most time and are the source of most errors in GC-MS water measurements. The U.S. Environmental Protection Agency (USEPA) has been getting closer to requiring halogenated acidic acid measurements. Currently approved USEPA methods only allow a few measurements to be made in a workday with safety concerns because toxic compounds are used to make the measurement. Data presented for the ESI-MS technique show it could make dozen of measurements easily in a workday compared to the few now possible with existing regulatory approved methods.
With advances in instrumentation and computerization as well as related lab analysis techniques, mass spectrometry remains the “king” when it comes to making defensible measurements on water samples. And with the total size of the MS marketplace and intelligent professionals working on this subject, we should expect more accelerated new methods to solve problems of faster, more efficient and accurate detection, identification and quantification of waterborne contaminants.
- The Pittsburgh Conference, New Orleans, La., March 12-17, 2000: http://www.appcluster05.com/pittconsplash.cfm
- D. Sauter, H. Nowicki, C. Kieda, and D. Devine; ASTM Symposium on Organic
Measurements in Water, Denver, 1980.
- H. Nowicki, C. Kieda, D. Devine, and D. Sauter, ASTM Symposium on Organic
Measurements in Water, Denver, 1980.
- W. Buddle, The Pittsburgh Mass Spectrometry Discussion Group, November 1999.
- H. Weinberg, “Disinfection by-products in drinking water,” Analytical Chemistry, December
1, 1999, p. 801A.
About the authors
Dr. Henry Nowicki, president of PACS Inc. of Pittsburgh, holds a doctorate from Missouri’s St. Louis University in organic and biochemistry. He was fortunate to work on the LKB-9000, the first commercial mass spectrometer as a doctoral candidate in the mid-’60s. PACS Inc. provides analysis of samples for organic and inorganic compounds, provides sorption services with an emphasis on activated carbon, ASTM laboratory testing, R&D on new processes to regenerate used carbons and impregnate carbons with chemicals, software programs, training courses and sponsors the annual September International Activated Carbon Conference in Pittsburgh. Nowicki also is a member of the WC&P Technical Review Committee. He can be contacted at: (724) 457-6576 or email: HNpacs@aol.com
A Brief History of GC-MS
The coupling of mass spectrometry with gas chromatography (GC-MS) was a major advance in environmental analysis of the 20th Century. Analytical instrumentation helps environmental scientists measure chemical compounds in a wide variety of sample matrices such as drinking water, wastewater, air and soils. Heavy metals, hydrocarbons, polynuclear aromatic hydrocarbons (PAHs), chlorinated compounds, dioxins, herbicides and pesticides are among the main chemical compounds scientists look for while conducting environmental analysis. One factor contributing to GC-MS success was it was less expensive to use than gas chromatography (GC) on a cost-per-analysis basis. With GC-MS, it’s literally possible to analyze for millions of compounds on the same analytical run. With GC you need to make several runs. The GC technique is often used when what’s in the sample has previously been determined by GC-MS and GC is used to determine the quantity of the known compounds (often referred to as target analysis) in the sample.
During the 1970s, many chemists relied on GC with a conventional detector such as flame ionization detector (FID) or the electron capture detector (ECD) to identify and measure organic compounds. Gas chromatography is a method used for separating gases, liquids, or dissolved substances that provides quick separation of a complex mixture into its individual compounds. A FID or ECD and other detectors with specificity for different types of organic molecules are also available. They’re employed in conjunction with the GC column to identify and quantify the separated components coming out in the column effluent. Detection is based on having the same retention time (time from injection to elution from the column into the detector) as a reference standard and quantification is based on the area or height of the eluting peak. Ideally, the chromatographic peak has a bell shaped or Gaussian distribution. Gas chromatography is advantageous as a means of analyzing minute quantities of compounds in complex chemical mixtures.
At the end of the ’70s, the Clean Water Act’s stricter water analysis requirements mandated development of analytical instrumentation with greater identification accuracy than classical GC techniques. Lawyers quickly recognized you couldn’t make chemical identification based solely on coincidence of sample retention time with a known reference compound with the retention time in a sample. After examining several analytical techniques, the USEPA selected the computerized GC-MS. Combination of these techniques for analyzing water was recognized by the USEPA in 1978 for its superior level of accuracy and methods were finalized in the Federal Register in 1984. Use of capillary columns were shown to be superior to packed columns to separate complex mixtures in environmental water samples using GC-MS.2 Typically, capillary columns reduce the analysis run-time by half and provide better resolution and lower detection limits of compounds in water samples compared to packed columns. Since then, this technique has become the gold standard for organic analysis in water.
The MS spectrum for dichloroethylene (DCE) is shown to demonstrate the power and limitations of MS for structure elucidation. The tabular and bar graph presentations (see Figure 1) are both provided for the DCE spectrum. In the bar graph the x-axis is mass and the y-axis is relative intensity of each nominal mass. A mass spectrometer presents data as relative intensity as a function of ion mass. The MS technique can be used to detect very low concentrations of a chemical in a sample, parts per billion (ppb). The resulting mass vs. relative intensity pattern from a sample is compared to those from a known compound run on the same GC-MS and a library in order to determine the identity of the compound. The GC-MS also had the advantage of being one of the first types of detection instrumentation to be computerized. The mass spectrometer is data intensive. In one hour of MS scanning, there are 1,458,000 mass-intensity-time relationships. Computerized instrumentation allowed users to perform quantitative as well as qualitative analysis and database library searches more quickly and efficiently.
The beginnings of the MS technique can be traced to research in the early 1900s by English physicist Joseph John Thomson, who also won the 1906 Nobel Prize in physics and is often called the father of mass spectrometry. After developing the first mass spectrometer, he discovered in 1913 that neon consists of a mixture of two different isotopes (masses 20 and 22) rather than a single mass. An isotope is any of two or more elements with related chemical properties and the same atomic number but different atomic weight or mass. Thomson’s observation of the existence of stable isotopes is perhaps the greatest achievement of mass spectrometry. Environmental scientists use isotopes to accomplish a lot of mass spectral interpretations. Figure 2 illustrates various combinations of chlorine and bromine atoms—often found in molecules of environmental samples. Note the pattern in the bar graph for the molecular ion for DCE contains two chlorine atoms (96, 98, 100 mass ions with relative intensity of 9:6:1) and one chlorine atom on a diagnostic fragment at (61 and 63 with relative intensity of 3:1). Typically, the MS cannot distinguish isomers (position of the chlorines) thus 1,1-DCE, cis and trans 1,2-DCE, would have different GC retention times, but the mass spectrum for these three isomers would not be differentiated. Isomer identification couldn’t be made solely on their mass spectral data, retention time data is needed to confirm structure elucidation.