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Mass Spectrometry
Mass spectrometry is an established analytical technique that identifies compounds by the mass (more correctly, mass to charge ratio) of the analyte molecule. Mass spectrometry is especially noteworthy among analytical techniques because the signals produced by a spectrometer are the direct result of chemical reactions such as ionization and fragmentation, rather than energy state changes that are typical of most other spectroscopic techniques. Because of this distinction, mass spectrometry is considered the only definitive analytical technique and detector. The first general application of molecular mass spectrometry occurred in the early 1940s in the petroleum industry for quantitative analysis of hydrocarbon mixtures in catalytic crackers. Coupling mass spectrometers with gas chromatographic systems allows separation and subsequent determination of components of highly complex mixtures with a high degree of certainty. Recently, manufacturers of mass spectrometers, particularly spectrometers coupled with gas chromatographs, have significantly reduced their overall size and increased durability. This allows what was once a laboratory bench top instrument to perform field analysis. In the environmental field, mass spectrometers are typically used as detectors for gas chromatographs. Because of the increased durability of modern instruments, field gas chromatographs/mass spectrometers (GC/MS) are capable of the same analyses as fixed laboratory instruments. EPA approved SW846 methods are then capable with field GC/MS instruments. Some SW846 methods include:
An Electron beam of 20 electron volts (ev) is used to extract an electron from a molecule. Excess energy from the beam will further fragment the molecular ion to fragment (daughter) ions with lower mass to charge ratio (mass). The positive ions produced by electron impact are attracted through the slits of the ion source and mass analyzer. These ions are mass analyzed for differentiation according to their mass-to-charge ratios. The mass sorted ions are detected by an electron multiplier and the resulting signal is sent to a data system for processing. A display of the electron multiplier signal generated by the sorted molecular ions is displayed as the mass spectrum. In the case of a coupled GC/MS, one type of data display is an integrated sum of signals generated by the molecular ions plotted as a function of elution time from the GC column. This representation is a total ions current (TIC) chromatogram. Mass spectra can be extracted from each chromatographic peak of the TIC. The mass spectrum is in the form of a bar graph that relates the relative intensity of mass peaks to their mass-to-charge ratio. The largest peak in each spectrum is termed the base peak. The heights of the remaining peaks are computed as a percentage of the base peak height. The spectrum may be compared to a spectral library for identification of the compound based on fragmentation pattern and peak ratios. The individual TIC and ion peak intensities are directly related to the concentration of analyte in the sample extract making GC/MS an extremely powerful analytical tool for positive identification and quantitation of organic compounds. Mass spectrometers consist of four basic components; ion sources, sample inlet systems, mass analyzers, and a transducer. A brief description of various types of each component is presented below. Ion Sources The starting point for a mass spectrometric analysis is the formation of gaseous analyte ions. Ionization methods fall into two categories: gas phase sources and desorption sources. Gas phase ionization relies on first vaporizing the analyte then ionizing it. Desorption sources ionize the analyte in a solid or liquid state. Click here for a list of ionization sources Sample Inlet Systems The purpose of the inlet system is to permit the introduction of a representative sample into the ion source. Types of inlets include batch inlets, direct probe inlets, and chromatographic inlets. Again, mass spectrometers used for environmental analysis are essentially detectors for gas chromatographs. The flow rate from capillary chromatographic columns is generally low enough that the column output can be fed directly into the ionization chamber of the mass spectrometer. For packed or magabore capillary columns, a jet separator is used to eliminate most of the carrier gas from the analyte. The removal of carrier gas is critical because the MS is operated under high vacuum. Introduction of carrier gas, therefore, would disturb the high vacuum. The function of the mass analyzer is analogous to that of the grating in an optical spectrometer. In mass spectrometers, dispersion is based upon the mass-to-charge ratios of the analyte ions rather than upon the wavelength of photons. Mass spectrometers are therefore categorized based on the type of mass analyzer in the instrument. Two types of mass analyzers are typically used for GC/MS analysis: (1) the quadrupole mass analyzer, and (2) magnetic sector analyzer. The quadrupole MS interfaced with capillary-column GC, is the most commonly used in an environmental laboratory and field analysis. The sample extract is injected onto the capillary column on the GC, where the individual compounds in the complex mixture are separated. The individual compounds elute through the column at different rates into the MS for detection. The second type of mass analyzer typically used in mass spectrometers is the magnetic sector. Magnetic sector mass spectrometers, also known as high resolution mass spectrometers, are significantly more sensitive than the quadrupole. For example, magnetic sector spectrometers detect dioxins at parts per trillion (ppt) levels in soil and parts per quadrillion (ppq) in water. Typically, high resolution MS is not used for field-based environmental analysis because of size and stability requirements of the instrument. Like an optical spectrometer, a mass spectrometer contains a transducer that converts the beam of ions into an electrical signal that can be processed, stored in computer memory, and presented in graphical form. Transducers commercially available for mass spectrometers include electron multipliers and the Faraday Cup. The electron multiplier is the transducer of choice for most spectrometers. Less common detection systems include photographic plates and scintillation counters. Mass spectrometers are operated under high vacuum to remove atmospheric interferences that would affect the ionization process. All MS have safety devices so that ion source cannot become operational until high vacuum is attained. Because most MS are operated in conjunction with a gas chromatography, the removal of carrier gas is critical because the MS is operated under high vacuum. Introduction of carrier gas, therefore, would disturb the high vacuum. Computers and microprocessors are integral in modern mass spectrometers. A characteristic of a mass spectrum is the wealth of structural data that it provides. For structural determination of a molecule, the heights and mass-to-charge ratios of each fragmentation peak in a spectrum is determined, stored, and displayed. Because the amount of information is so large, rapid acquisition and processing is essential. The microprocessor is essential for mass spectrometric data acquisition and manipulation. Several instrumental variables are controlled and monitored during data collection. First, the computer serves as the instrument controller. Operating parameters are set through communication via the keyboard. Secondly, the computer controls programs responsible for data manipulation and output. The digitized ion-current signal is processed prior to display. The peaks are normalized, that is, the height of each peak relative to a reference peak is calculated. The largest peak in the spectrum, the base peak, is arbitrarily assigned a peak height of 100 (sometimes 1000). The mass to charge (m/z) value for each peak must also be determined. This assignment is frequently based on the time of the peak's appearance and the scan rate. Data are acquired as intensity versus time during a controlled scan of the magnetic or electric fields. Conversion from time to m/z requires careful periodic calibration. Calibration, or often called tuning, is achieved with perfluorotri-n-butylamine (PFTBA). For high resolution analysis, the standard may be added with the sample. The computer is programmed to recognize and use the peaks of the standard as references for mass assignments. For low resolution instruments, the calibration is obtained separately from the sample due to the possibility of peak overlap. Mass spectrometry has been widely applied to the quantitative determination of one or more components of complex organic systems encountered in studies of environmental problems, as well as the petroleum and pharmaceutical industries. Currently, analyses are performed by passage of the sample through a chromatographic column and into the spectrometer. With the spectrometer set at a suitable mass-to-charge (m/z) ratio, the ion current is recorded as a function of time. This technique is called selected ion monitoring. Generally, the areas are directly proportional to the component concentrations. In this type of analysis, the mass spectrometer simply serves as a sophisticated selective detector for quantitative analysis. In another type of quantitative mass spectrometry for molecular species, analyte concentrations are obtained directly from the heights of the mass spectral peaks. For simple mixtures, it may be possible to find peaks at unique m/z values for each component. Under these circumstances, calibration curves of peak heights versus concentration are constructed and used for analysis of unknowns. More accurate results can be achieved by incorporating a fixed amount of an internal standard substance in both samples and calibration standards. The ratio of peak intensity of the analyte species to that of the internal standard is then plotted as a function of analyte concentration. The internal standard reduces uncertainties in sample preparation and introduction.
Performance specs include information on detection limits, calibration, and quality control. Detection Limits Part per billion detection limits have been reported for quadrupole mass spectrometers. Magnetic sector mass spectrometers, also known as high resolution mass spectrometers, are significantly more sensitive than the quadrupole. For example, magnetic sector spectrometers detect dioxins at parts per trillion (ppt) levels in soil and parts per quadrillion (ppq) in water. Calibration Analyses are performed by passage of the sample through a chromatographic column and into the spectrometer. With the spectrometer set at a suitable mass-to-charge (m/z) ratio, the ion current is recorded as a function of time. This technique is called selected ion monitoring. Generally, the areas are directly proportional to the component concentrations. In this type of analysis, the mass spectrometer simply serves as a sophisticated selective detector for quantitative analysis. In another type of quantitative mass spectrometry for molecular species, analyte concentrations are obtained directly from the heights of the mass spectral peaks. For simple mixtures, it may be possible to find peaks at unique m/z values for each component. Under these circumstances, calibration curves of peak heights versus concentration are constructed and used for analysis of unknowns. More accurate results can be achieved by incorporating a fixed amount of an internal standard substance in both samples and calibration standards. The ratio of peak intensity of the analyte species to that of the internal standard is then plotted as a function of analyte concentration. The internal standard reduces uncertainties in sample preparation and introduction. Quality Control Method blanks are periodically analyzed to monitor laboratory and instrument-induced contaminants. A method blank must not contain any analyte in a concentration higher than the practical quantitation limit. Matrix spike and matrix spike duplicate samples are analyzed to evaluate the efficiency of the sample preparation, precision of the analysis, and matrix effect. Laboratory control samples are used to evaluate the accuracy of the analysis. The control samples are obtained from outside sources and contain known amounts of analytes. The values obtained by analysis of the control samples are compared with the known true values. The supplier of the control samples usually provides control limits. The results obtained should fall within the published range of acceptance values.
Mass spectrometer costs vary significantly. Instrument design and accessories affect instrument prices. Manufacturers listed below should be contacted directly for cost information. Verification/Evaluation Reports Verification of the performance of site characterization and field analytical technologies is conducted through a variety of programs. Evaluation and verification reports from EPA's Superfund Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program, EPA's Environmental Technology Verification Program (ETV) program, along with links to certification statements from California EPA's (CalEPA) California Environmental Technology Certification Program, are provided below. Superfund
Innovative Technologies Evaluation (SITE) Measuring and Monitoring Program No reports available for this technology EPA's
Environmental Technology Verification (ETV) Program
California
EPA's California Environmental Technology Certification Program
Technologies that have been certified through this program are listed below. Links are provided to the web sites that provide the Certified Environmental Technology Transfer Advisory and Certification Notice for the technologies. No reports available for this technology. 
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