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◊ Gas Chromatography
◊ Graphite Furnace Atomic Absorption Spectrometry
◊ Immunoassay
◊ Infrared Spectroscopy
◊ Laser-Induced Fluorescence
◊ Mass Spectrometry
◊ Test Kits
◊ X-Ray Fluorescence
◊ Analytical Systems
◊ Direct-Push Platforms
◊ Geotechnical Sensors
◊ Groundwater Samplers
◊ Membrane Interface Probes
◊ Soil and Soil-Gas Samplers
◊ Explosives
◊ Ground Penetrating Radar
◊ Magnetics for Environmental Applications
◊ Passive Diffusion Bag (PDB) Samplers
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Gas Chromatography Description Chromatography is the science of separation which uses a diverse group of methods to separate closely related components of complex mixtures. During gas chromatographic separation, the sample is transported via an inert gas called the mobile phase. The mobile phase carries the sample through a coiled tubular column where analytes interact with a material called the stationary phase. For separation to occur, the stationary phase must have an affinity for the analytes in the sample mixture. The mobile phase, in contrast with the stationary phase, is inert and does not interact chemically with the analytes. The only function of the mobile phase is to sweep the analyte mixture through the length of the column. Gas chromatography can be divided into two categories: (1) gas-solid and (2) gas-liquid chromatography. Gas-liquid GC, developed in 1941, is the primary GC technique used for environmental applications. Gas-solid GC is not widely used for environmental applications. The stationary phase is chosen so that the components of the sample distribute themselves between the mobile and stationary phase to varying degrees. Those components that are strongly retained by the stationary phase move slowly relative to the flow of the mobile phase. In contrast, components that have a lower affinity for the stationary phase travel through the column at a faster rate. As consequence of the differences in mobility, sample components separate into discrete bands that can be analyzed qualitatively and quantitatively. Gas chromatography is the most widely used chromatographic technique for environmental analyses. Gas chromatography (GC) analysis is a widely used technique for field-based analysis. Analysis of organic compounds is possible for a variety of matrices such as water, soil, soil gas, and ambient air. Typical settings include:
GC analysis with photoionization detection has been used extensively to characterize and remediate sites contaminated with volatile organic chemicals (VOCs). Likewise, gas chromatographs coupled with an electron capture detector are used for analysis on sites contaminated with chlorinated pesticides. The recent development of truly field portable quadrupole mass spectrometers now permits GC-mass spectrometry analysis which provides definitive identification. This instrument and technique is being implemented during emergency response and counter terrorism situations that requires definitive identification of contaminants in near real-time. Because of the increased durability of modern instruments field gas chromatographs are capable of the same analyses as fixed laboratory instruments. EPA verified SW846 methods are then possible with field gas chromatographs and various detectors. Some SW846 methods include:
The theory of separation by GC is relatively simple and understanding the factors that affect separation allows more effective applications of GC analysis in the field. The purpose of separation is to allow identification and quantitation of individual components of a mixture and the theory of separation is detailed below. In addition to separation, detection of analytes after separation, which is an essential but separate aspect of chromatography, is presented in the section describing system components. Basic components of a complete gas chromatographic system include: (1) a carrier gas supply, (2) a syringe for sample introduction, (3) the injection port, (4) the column and oven, (5) the detector and data collection system. Components of a gas chromatograph are presented in greater detail in the section describing the system components. Schematic diagrams and photographs of instruments can also be accessed through the Systems Components section. Before separation occurs in the chromatographic column, the mixture of components in the sample is introduced into the chromatograph through the injection port with a syringe. At this point, the analytes are vaporized (if not already in the gas phase) by the high temperature maintained in the injection port. The analytes are kept in the gaseous state by maintaining all elements of the instrument at a temperature above the boiling point of the analytes. The gas phase analytes are then immediately swept onto the chromatographic column by the mobile phase. The mobile phase is comprised of an inert carrier gas, which usually is nitrogen, helium, or hydrogen. As the analytes are swept through the column by the mobile phase, separation occurs based on the affinity of each analyte for the stationary phase. The gas chromatographic column is composed of a coiled, tubular column and the stationary phase within the tube. GC columns are either packed or open-tubular. Early GC columns were packed with carbon or diatomaceous earth based solids which acted as the stationary phase. In modern open-tubular columns, the stationary phase is a liquid organic compound that is coated on the internal surface of the fused silica column. Polarities of the analytes dictate the choice of stationary phase. Components of the mixture with a high degree of affinity for the stationary phase are strongly retained while components with low affinity for the stationary phase migrate rapidly through the column. As a consequence of the differences in mobility due to affinities for the stationary phase, sample components separate into discrete bands that can be qualitatively and quantitatively analyzed. As individual components of the mixture elute the chromatographic column, they are swept by the carrier gas to a detector. The detector generates a measurable electrical signal, referred to as peaks, that is proportional to the amount of analyte present. Detector response is plotted as a function of the time required for the analyte to elute from the column after injection. The resulting plot is called a chromatogram. Detector response is generally a gaussian shaped curve representative of the concentration distribution of the analyte band as it elutes from the column. The position of the peaks on the time axis may serve to identify the components and the area under the peaks provide a quantitative measure of the amount of each component. System Components The primary components of a GC include:
Other parts include:
The carrier gas is introduced in the injection port where the sample is volatilized and swept through the column, and where the compounds are separated. The carrier gas/sample mixture then enters the detector where the compounds are identified. The signal from the detector then is amplified and displayed by the data system. A capillary column is an open tube made of fused silica with an outer coating of durable plastic and an inner coating of stationary-phase material. Some capillary columns have a second outer covering of stainless steel to withstand the higher pressure required to analyze alcohols, ketone, and VOCs by the purge-and-trap method. A lesser used column type is the packed column. Packed columns use a stainless steel or glass tube with a 1/8th inch inner diameter packed with a solid stationary phase. The effectiveness of a chromatographic column in separating solutes (analytes) is dependent on a number of variables. Understanding these variables is essential to the process of optimizing any chromatographic system and achieving resolution of analytes. Variables that affect separation include distribution equilibrium constants, retention time, retention (capacity) factors, and selectivity factors. A discussion of each of these is presented below. Distribution Constants The effectiveness of a chromatographic column in separating two solutes is dependent on the relative rates at which the species are eluted. These rates are determined by the degree the solutes distribute themselves between the mobile and stationary phases. Distribution equilibria in chromatography defines the transfer of an analyte between the mobile and stationary phases. The equilibrium constant, referred to in chromatography as the distribution constant or partition ratio, is the ratio of the molar concentration of the solute in the stationary phase to its molar concentration in the mobile phase. This is mathematically expressed as K = cS/cM. If K is constant over a wide range of solute concentrations, then Cs (concentration of solute in the stationary phase) is directly proportional to cm (concentration of solute in the mobile phase). When this holds true, chromatographic peaks are symmetrical, Gaussian distributions and retention times are independent of the amount of analyte injected. Retention time of an analyte is defined as the time it takes after sample injection for the analyte to elute and reach the detector. The time for unretained species to reach the detector is defined as the dead time. Click here to see a figure that illustrates each. This “rate of migration” of an unretained species is the same as the rate of motion of the mobile phase molecule. The linear rate (v) of a solute molecule is defined as the column length divided by its retention time and is mathematically expressed v = L/tR where L is the column length and tR is the retention time of the solute. The linear rate (u) of a mobile phase molecule is calculated by dividing the column length by the dead time (tM). Obviously, the distribution of a solute between the stationary and mobile phases will directly affect the linear migration rate of a solute in the mobile phase and ultimately its retention time. The migration rate (v) is expressed as v = u x fraction of time the solute spends in the mobile phase. Thus the less time a solute spends in the mobile phase, the smaller the fraction to multiply u, and ultimately lowering the migration rate. The lower the migration rate v, the higher (or longer) the retention time, tr. The fraction can also be expressed as moles of solute in the mobile phase divided by the total moles of solute. Total moles of solute in the mobile phase is equivalent to the concentration of solute times the volume of mobile phase. Likewise, the total moles of solute is the sum of moles in the mobile and stationary phase. This is expressed as v = u x (cMVM) /{(cMVM)+(cSVS)} or u x 1/{1 +(cSVS)/cSV S}. Substituting the equilibrium constant for the ratio of solute concentrations results in v = u x 1/{1+(KVS/VM)}, an equation expressing a solute migration rate as a function of its distribution constant. Retention Factor Another parameter used to describe migration rates of solutes on gas chromatographic columns is the retention factor, also referred to as the capacity factor. Mathematically, the retention factor is expressed as k'=KVS/VM. By substituting this expression for capacity into the equation for the migration rate, a relationship between column capacity and migration rates is established. The relationship is mathematically expressed as v = u x {1/(1+k')}. When the migration rate, for a solute and mobile phase molecule u are substituted with values measured from a chromatogram, an equation to determine the retention factor can be derived. The equation is k'=(tR-tM)/TM. Selectivity Factors The ability of a column to retain one analyte more strongly than a second is a function of the column's selectivity. The column's selectivity factor for two species, A and B is mathematically expressed as alpha = KB/KA. By definition the more strongly retained species is B and therefore A is always greater than one. Substitution for the distribution constants yields an equation that allows the selectivity factor alpha to be experimentally determined from a chromatogram. The equation is alpha = (tR)B - TM/(tR)A TM. Theoretical Plates and Height Equivalent Theoretical Plates While movements of solutes through a gas chromatographic column are described by distribution constants, retention times, retention (capacity) factors, and selectivity factors, column efficiency (performance) is described by a quantitative measure labeled theoretical plates. The number of theoretical plates (N) is calculated by dividing the column length (L) by the height equivalent theoretical plate (H). Plate height is experimentally calculated by dividing the variance of a Gaussian shaped chromatographic peak divided by the column length. This is graphically illustrated in this figure. A separate equation that provides the number of theoretical plates (N) is N = 16(tR/W)2 and is graphically illustrated in this figure. Resolution A chromatographic
column's ability to separate a mixture of compounds is defined as its resolution.
The mathematical equation for resolution, Rs, is RS = Where: Resolution can also be calculated using retention factor k' for two solutes, the selectivity factor, and the number of theoretical plates. The following equation is used to calculate resolution, or with simple rearrangement, to calculate the number of theoretical plates required to achieve a desired resolution. Resolution:
RS = N1/2/4{( The previous discussions all assume Gaussian distributions of analytes as they elute from the column. However, non-gaussian shaped peaks do occur and the peak shapes provide information relative to chromatographic variables. Two frequent phenomena related to non-gaussian peak shapes that occur are fronting and tailing. With fronting, the front side of a peak is drawn out while the tail (or backside) on the right is steep. The most frequent cause of fronting is too large of a sample introduced into the column. In contract, the more frequent phenomenon of tailing results in the right side of a peak (the tail) is drawn out. This usually occurs when the solute has c concentration dependent (non-linear) distribution coefficient. This can also be a cause of fronting. The result of fronting and tailing is poor separation (and thus resolution) and less accurate quantitative analysis. Optimizing Separations Stationary phase - An organic liquid compound that is either coated on or covalently bonded to the silica surface of a capillary column. Stationary phases are occasionally solids packed inside the column. The most widely used columns are the fused silica capillary columns due to strength and flexibility. The polarities of the compounds of interest dictate the choice of stationary phase, under the rule “like dissolves like.” Commonly used stationary phases include: polydimethyl siloxane
(commonly referred to as OV-1 of SE-30) for PCB or PAH, separation carbowax
- used for free acids, alcohols, and glycols OV-3 or SE-52 for halogenated organics. Carrier gas - The mobile phase is composed of an inert carrier gas, usually nitrogen, helium, or hydrogen. The choice of carrier gas is frequently determined by the type of detector used and subsequent purity requirements. The sample constituents are transformed into the gaseous phase and are carried along the column during separation. By increasing the speed (flow rate) of the carrier gas, the analysis time can be reduced; however, optimal resolution may be compromised. A faster flow rate also sweeps the injector more efficiently, improving introduction of the sample into the column. If resolution is not compromised, increased flow rates can also reduce analysis times. Length - Capillary columns vary in length from 15 to 100 meters is a coiled configuration to fit in the instrument oven. For environmental analysis, 30- to 60-meter columns typically are used. Shortening the length of the column can shorten the analysis time; however, resolution (separation) will be compromised. Again, if resolution is not compromised, analysis time can be reduced with a shorter column. Diameter - Diameters of open tubular capillary columns are typically between 0.32 and 0.25 millimeter, with high resolution columns having diameters of 0.20 to 0.15 millimeter. The smaller diameter columns require special injection splitting to reduce the sample size and prevent column overload. Columns referred to as mega bore open tube columns are also available and have a greater capacity but at the expense of resolution. However, these columns have better resolution than packed columns. Packed columns have diameters large as 2 millimeters. The smaller diameter produces better resolution and greater selectivity, but can handle only a small volume of sample (1 to 2 microliters). Field analysis is frequently performed in less than ideal conditions and samples can have concentrations or constituents that can ruin column performance. When this occurs, columns are replaced, often at significant expense. At this point, analysts need to insure that data produced with the new column is consistent with and comparable to data produced with the previous column. This is accomplished by analyzing quality control samples to demonstrate comparable separation and sensitivity is achieved. Numerous other techniques are available to the analyst, such as the use of internal standards. A discussion of these techniques is beyond the scope of this text but available in most analytical chemistry text. Another problem, associated with coated capillary columns and not exclusive to field analysis, is column bleed. This phenomenon is best described as the elution of the stationary phase. A frequent cause of column bleed is excessive oven temperature. The end result of column bleed is a fouled detector. More sensitive detectors such ac electron capture detectors are highly susceptible to column bleed. A variety of detectors for gas chromatographs are available. In general, each detector takes advantage of a unique characteristic of a molecule and uses that characteristic to generate a measurable electrical signal. A discussion of the most frequently used detectors is presented below. Principles of operation, classes of compounds providing optimal response, and detection limits are included. Click on the detector name for a schematic of the detector. Because of the unique and complex nature of mass spectrometry, the mass spectrometer as a GC detector is discussed under the mass spectroscopy entry in the encyclopedia. Click here to go directly to the mass spectroscopy section. Note
that the MS detector is the most versatile. The MS is used widely in place of
conventional GC detectors. Standard Operating Procedures (SOPs) are available
for the following GC/MS methods: Field analysis is frequently performed in less than ideal conditions and samples can have concentrations or constituents that can ruin column performance. When this occurs, columns are replaced, often at significant expense. At this point, analysts need to insure that data produced with the new column is consistent with and comparable to data produced with the previous column. This is accomplished by analyzing quality control samples to demonstrate comparable separation and sensitivity is achieved. Numerous other techniques are available to the analyst, such as the use of internal standards. A discussion of these techniques is beyond the scope of this text but available in most analytical chemistry text. Another problem, associated with coated capillary columns and not exclusive to field analysis, is column bleed. This phenomenon is best described as the elution of the stationary phase. A frequent cause of column bleed is excessive oven temperature. The end result of column bleed is a fouled detector. More sensitive detectors such ac electron capture detectors are highly susceptible to column bleed. A photoionization detector (PID) consists of a special ultraviolet lamp, ranging in energy from 9.5 to 11.7 eV, mounted on a low-volume flow-through cell. As constituents of the sample pass through the cell, they are energized and ionized. The ions are collected at positively charged electrodes, where the change in current is measured. The 10.2 eV lamp emits ultraviolet light at 121 nanometers (nm), which is sufficient to ionize BTEX compounds and hexane. A few halogenated compounds that have ionization potentials of less than 11.7 eV can be detected by the higher-energy PID. The PID is more selective than the FID. The PID can detect VOCs (aromatic and chlorinated) and petroleum constituents including BTEX. The PID can detect BTEX in the low ppb to high part per trillion range. The PID is a nondestructive detector that can be used in series before other detectors. Using multiple detectors extends the range of compounds that can be detected in one analysis. PID is sensitive to water and must be recalibrated more often than the FID. A flame ionization detector (FID) consists of a stainless steel jet constructed so that carrier gas exiting the column flows through the jet, mixes with hydrogen, and burns at the tip of the jet. Hydrocarbons and other molecules which ionize in the flame are attracted to a metal collector electrode located just to the side of the flame. The resulting electron current is amplified by a special electrometer amplifier which converts very small currents to millivolts. The FID is sensitive to almost all molecules that contain hydrocarbons. Examples include aromatic and chlorinated VOCs, petroleum constituents, SVOCs, and PCBs. The FID is a destructive detector that can be used in series only after nondestructive detectors. The FID is sensitive to water, but has a wider linear range of detection than the PID. The FID also can detect more compounds than the PID. The FID can detect compounds that contain the low ppb to high part per trillion range. SOPs are available
for the following GC/FID methods: An electron capture detector (ECD) consists of a sealed stainless steel cylinder that contains radioactive nickel-63. The nickel-63 emits beta particles (electrons) which collide with the carrier gas molecules ionizing them in the process. A stable cloud of free electrons thus forms in the ECD cell. When an electronegative molecule such as a halogenated molecule enters the cell, it immediately combines with one of the free electrons which temporarily reduces the number of free electrons. The detector electronics pulse at a variable rate to measure the electrons remaining in the cell. SOPs are available for the following GC/ECD methods: The ECD is highly sensitive to electronegative molecules (those capable of producing negatively charged ions) such as halogenated compounds or those that contain nitrogen. The ECD readily detects chlorinated pesticides, halogenated solvents, PCBs, and dioxins. The ECD is a nondestructive detector that can be used in series before other detectors. The ECD is sensitive to water that affects the condition of the Ni-63 foil that covers the detector. The foil must be reconditioned when its sensitivity diminishes. Because the ECD contains a radioactive source, users may be subject to licensing requirements. The Nuclear Regulatory Commission (NRC) is the agency in the United States that regulates radioactive materials. However, in certain states, the regulations are enforced by a state agency. There are two types of ECDs that require licenses; specific license ECDs and general license ECDs. A general license ECD is an ECD that customers in the United States can purchase without having their own radioactive material license. General License ECDs are typically covered under the distributor's distribution license. General License customers are required to complete and sign the General License Registration Card and comply with regulations. The owner of a General License ECD becomes a General Licensee when the ECD is purchased. The owner does not have to apply for a General License from the NRC or State Agency but could be required to register the ECD within the state. Specific License ECDs require the end user to have a Materials License from the NRC or local State Agency, which permits the owner to posses the applicable type and quantities of radioactive material. Holders of Specific Licenses have greater flexibility with their ECD and more responsibilities. For example, Specific License customers are allowed to clean ECDs and work with open sources. Distributors of ECDs are required to notify customers of licensing requirements. The ECD can detect halogenated compounds in the low ppb to part per trillion range. The more halogenated the molecule, the more sensitive the detector is to that compound. For example, the ECD is orders of magnitude more sensitive to carbon tetrachloride than to vinyl chloride. Electrolytic Conductivity Detector An electrolytic conductivity detector (ELCD) is a halogen-specific detector that operates on electrolytical conductivity principles. Organic compounds eluding from a GC column form combustion products as they are mixed with hydrogen gas over a nickel catalyst at 1,000C in a quartz tube furnace. For example, organic chlorides form hydrochloric acid (HCl). The HCl readily ionizes and changes the electrolytic conductivity which is monitored by the ELCD. Click here for a GC/ELCD method for halogenated volatile organics. The ELCD is a halogen-specific detector. The ELCD readily detects chlorinated pesticides, halogenated solvents, PCBs, and dioxins. The ELCD is a destructive detector that can be used in series only after nondestructive detectors. The ELCD requires a higher degree of maintenance than other detectors. It also has a wider linear range than the ECD. The ELCD can detect halogenated compounds in the low ppb to part per trillion range. The degree of halogenation has less effect on sensitivity than is the case with the ECD. Detection limits for compounds are similar, even though the degree of halogenation may vary. A thermal conductivity detector (TCD) consists of tiny coiled wires arranged in a wheat stone bridge configuration. Electric current flows through the filaments making them glow hot, while carrier gas exiting the column flows past the other two filaments. The gas flow carries away excess heat, and the filaments equilibrate. When a sample compound exits the column, the thermal conductivity of the gas flowing around the filaments is changed. Therefore, the filaments get hotter and the balance of the wheat stone bridge is altered. The TCD is used to detect gaseous compounds, such as nitrogen, oxygen, and other nonhydrocarbon compounds (for example, landfill gases). The TCD is a destructive detector that can be used in series only after nondestructive detectors. The TCD has limited target analyte list. Because the TCD detects nitrogen, nitrogen cannot be used as a carrier gas. The TCD can detect gaseous compounds in the ppm range. TCD is not used for trace analysis. Larger sample volumes are required to achieve increased sensitivity. To increase sample volume sufficiently, a large-diameter packed column must be used. The nitrogen-phosphorus detector (NPD) is similar to the FID, except that the hydrogen gas flow rate is reduced to 1 to 3 milliliters per minute (ml/min), and an electrically heated thermoionic bead is positioned just above the jet orifice. Analyte molecules exiting the column collide with the hot bead, and the nitrogen or phosphorus react and liberate an electron. The electron is attracted to the same collector electrode and electrometer amplifier used in the FID. The NPD is sensitive to nitrogen and compounds that contain phosphorus. The NPD typically is used in analysis for organophosphorus pesticides. In addition, it is used in analysis of nitroaromatics (explosives). The NPD is a destructive detector that can be used in series only after nondestructive detectors. The NPD is sensitive to water that affects the condition of the thermoionic bead. The active element on the bead eventually will become depleted and require replacement. The NPD can detect nitrogen and compounds that contain phosphorus in the ppb range. Reduced sensitivity often indicates the depletion of the active element on the thermoionic bead. Surface Acoustic Wave Sensors (SAWS) Surface acoustic wave sensors (SAWS) are chemical sensors that use a piezoelectric crystal as a transducer to detect and quantitate individual chemical compounds. A transducer is defined as an electric device that transmits energy from one system to another. This transmission of energy frequently occurs by converting the energy to a different form. Several types of transducers include:
A commercially available example of a GC/SAW is the Electronic Sensor Technology (EST0 4199), a handheld, portable (35-pound) gas chromatograph system. The EST 4100 was evaluated by the U.S. Environmental Protection Agency (EPA) and researchers from Oak Ridge National Laboratory during an Environmental Technology Verification (ETV) study in 1997. The SAWS operates on the principle of the piezoelectric effect. The piezoelectric effect is defined as the oscillation of a crystal, at a constant frequency characteristic of the mass and shape of the quartz crystal, when an appropriate electrical potential is applied across the face. In general, a SAWS operates by applying an alternating electric current through a piezoelectric crystal transducer, causing the crystal to vibrate. As a sample from the GC column effluent is exposed to the sensor (detector), chemical compounds from the sample are deposited on the crystal surface, changing the crystal's mass and oscillation frequency. The piezoelectric crystal transducer is the critical element of the SAWS detector. The crystal transducer is a quartz crystal that is coated with a compound that selectively adsorbs specific molecules. As the GC effluent passes over the transducer, the coating adsorbs specific compounds and the mass of the crystal transducer increases, while decreasing the resonant frequency of the crystal. Ultimately, the change in frequency is correlated to analyte mass. When the analyte desorbs, the crystal returns to its original frequency. A SAWS with a chemically selective coating is designed to respond to a specific compound or compounds, providing qualitative identification. Other SAWS are non-selective and may respond to a number of compounds; a GC is used to separate and qualitatively identify individual compounds. The observed change in acoustic energy from the crystal provides the quantitative measurement of the concentration of the analyte. Although the SAWS is much less sensitive to humidity than to organic compounds, water vapor also can cause a sensor to respond. Water uptake always will occur because no coating is perfectly hydrophobic. The use of adsorbants to trap water in a sample before it is introduced to the sensor can reduce the effects of humidity. Besides chemical interferences, temperature can have a direct effect on the frequency of oscillation by causing thermal expansion of the crystal and affecting the acoustic properties of the chemically selective coating. A technique that often is applied to compensate for temperature is the use of a second sensor, almost identical to the first, which is maintained at the same temperature but not exposed to the target chemical. Target analytes include:
Minimum detection limits are approximately 1 to 10 parts per million (PPM) for typical VOCs and can be reduced to 10 to 100 parts per billion (ppb) through the use of a preconcentrator. The majority of GC analyses for environmental applications can be divided into two basic groups: volatile organic chemicals (VOC) and semi-volatile organic chemicals (SVOC). The division is based essentially on the extraction technique used to separate the analyte from the sample matrix. VOC analysis relies on relatively low boiling points and high volatility of the organic chemical. Therefore, the extraction technique frequently is simply heating the sample matrix and driving the analyte into the headspace of the sample container. SVOC analysis, however, requires more sophisticated extraction techniques. SVOC analysis requires solvents to extract the analytes from the sample matrix. Each of the fundamental techniques are discussed below, with advantages and limitations of each. VOC Analysis Methods of analysis for VOCs include direct injection for ambient air and soil gas, and static headspace extraction or purge and trap extraction for soil and water. These techniques will be discussed in the following slides. Analytes of interest include: (1) halogenated VOCs, including vinyl chloride, methylene chloride, trichloroethene (TCE), tetrachloroethene (PCE), trichloroethane (TCA), chloroform, carbon tetrachloride, and ethylene dibromide; (2) nonhalogenated VOCs (solvents), including methyl iso-butyl ketone (MIBK), methyl ethyl ketone (MEK), and acetone; (3) aromatic compounds, including BTEX and chlorobenzenes; and (4) fuels, including gasoline, diesel fuel, jet fuel, and kerosene. Click here for purge and trap sample preparation methods coupled with GC/PID and GC/MS analysis for volatile organics. Ambient Air: On site air analysis can be used to test for VOCs in stationary source testing (emission inventory), hazardous waste site testing to determine appropriate levels of personal protective equipment (PPE), fence line monitoring during remediation activities, and emergency response testing. The preferred mode of sample collection for quick analysis is to directly draw a sample of ambient air into the on site GC or GC/MS using an internal pump in the analytical instrumentation. Air samples also may be collected in Tedlar bags, Summa® canisters, on Tenax tubes, or using solid phase microextraction (SPME) devices. Click here for Summa® canister sample collection methods coupled with GC/FOZ Trap analysis for volatile organics. The use of these other sampling containers will require that an air sample (1 to 5 ml) be withdrawn from the sample container or desorbed from the sorbent and injected into the GC system. (Only applies to Tedlar bags and Summa® canisters. Tenax and SPME will need some desorption technique.) On site analysis of air generally is conducted using a portable GC system or GC/MS configuration. Sops for GC methods
for vapor phase organics using field portable instruments include: Transportable GCs (larger, lab-grade instruments) can also be used but provide far more logistical problems. The detection limits for air analysis will range considerably depending on the method of sample collection and GC system. Typical detection limits will range from 5 to 200 parts per billion by volume (ppbv). Detection limits will be considerably lower if the analytes are concentrated on some type of adsorbent material and desorbed versus directly drawn into the analytical system via a sampling pump. Analytical times for VOCs should be less than 10 minutes per sample. A GC/MS system can either be used to provide qualitative to semiquantitative data in a survey mode or quantitative data in an analytical or selective ion monitoring (SIM) mode. The advantages of on-site air monitoring is the quick data which allows flexibility for on-site personnel and the project manager. When conducting emissions testing, samples that have high moisture or acid content must be pretreated prior to analysis. If a mass spectrometer is not used, exact analyte identification (coelution problems) may not be possible in complex mixtures. Soil Gas: Soil gas analysis commonly is used to identify “hot spots” or source areas of VOCs in the subsurface. It also can be used to approximate the extent of a subsurface plume. The detection limit for most VOCs is 10 nanograms per liter (ng/L). Calibrations consist of direct injection using an air standard mix or methanol-based standards. The concentration is reported in ng/L. One liter of air weighs approximately 1 gram. Therefore on a weight basis, ng/L is approximately equivalent to ppb. Typical sample containers include glass bulbs, Tedlar bags, Summa® canisters, syringes or 22 or 40-ml vials. Teflon®-coated syringes, plungers, or stop-cocks should be avoided, because some VOCs (for example, 1,1,1-TCA) are strongly sorbed to Teflon® surfaces. Because analyte concentrations are generally more dilute in soil gas samples, analysis usually requires larger sample volumes than liquid samples. Therefore, a 1 to 5 milliliters aliquot of air from the soil gas sample is injected into the GC column. In contrast, liquid sample injection volumes are usually a fraction of that volume (microliters) and contain no air. Because of the relatively large amount of air injected from soil gas samples, deterioration of the column stationary phase and oxidation of the ECD foil by oxygen in the sample is possible. The advantages of soil gas analysis are that it is rapid, inexpensive, provides real-time results, and causes minimal disturbance to the site. One limitation of soil gas analysis is that it does not always reflect a true soil concentration. The technique is limited to high volatility and low solubility compounds. Coelution problems can occur in complex mixtures if a mass spectrometer is not used. Sample carryover or cross contamination may be a problem in highly concentrated samples. Decontamination of syringes is critical, especially for chlorinated VOCs. Static headspace extraction is widely used in determining VOCs in waste water, soil, and drinking water. This extraction method is highly productive and cost effective, requiring minimal sample preparation. Efficiency of headspace extraction is based on soil or water partition coefficients of the volatile organic analytes. The principle follows Henry's Law, where the vapor pressure of the solute in the headspace is proportional to its mole fraction in solution. In other words, when a sample containing VOCs is sealed in a headspace vial, the vapor pressure of the VOC in the headspace is proportional to its concentration in solution. This phenomenon allows for analysis of the headspace gas to determine VOC concentrations of the sample matrix without time consuming solvent extractions. Henry's Law is mathematically expressed by the equation P=HX, where P = Pressure of
gas above the liquid (atm) The process can be simply described by two actions: (1) diffusion of analyte into the headspace and, (2) diffusion back into the matrix. A steady state equilibrium is reached when the concentration in the headspace is equal to the concentration in the matrix. Low viscosity liquids reach equilibrium faster. Solids take longer. A constant heat time is recommended for samples that do not reach equilibrium within a reasonable time. Analysis time ranges from 10 to 30 minutes depending on number and boiling point of analytes of interest. Detection limits for BTEX compounds and most chlorinated VOCs are in the range of 1 to 10 ppb; detection limits for TPH-purgeable compounds are 1 to 10 PPM in water and 10 to 50 PPM in soil. Water samples can be collected in 40 ml volatile organic analysis (VOA) vials or directly in a 22-ml headspace vial. Soil samples can be collected in a 4-ounce glass jar or directly into a 22-ml headspace vial. Mass or volume is measured into a headspace vial (generally 5 ml or 5 grams). No sample preparation is required for water samples. Sample preparation for soil samples may vary depending on the initial analyte concentration and soil type. For high concentration soil samples (PPM levels), studies have shown that methanol-flood achieves the greatest extraction efficiency. An aliquot of the methanol can then be analyzed using the static headspace technique. Methanol-flood techniques, however, are not appropriate for soil samples that contain less than 200 mg/kg of an analyte due to the dilution effect. For these low concentration soil samples, Method 5021 recommends the addition of an aqueous matrix modifying solution. For field analysis, the addition of a matrix modifier may not be necessary due to the quick analysis times. Listed below are the steps in the operational sequence of a typical automated headspace sampler. The sample is introduced to the platen heated zone and allowed to equilibrate at a fixed temperature for a fixed time period. Typical heating temperatures range from 40 to 80C. Heating times vary from 10 to 30 minutes. After the sample is heated, it is then mixed to help volatilization into the headspace. Next, the vial is raised onto a needle and pressurization gas (nitrogen) fills the vial to a pressure of 3 to 27 pounds per square inch (psi). The pressure in the vial is then allowed to equilibrate for 0.5 to 2.5 minutes. The vent valve is then opened and the pressure in the vial displaces the headspace through the sample loop, filling the loop at the proper loop fill time. Pressure in the vial will equal atmospheric pressure. Loop volumes can range from 0.1 to 5 ml Typical loop volumes used are 1 or 2 ml After the loop fill, the vent valve and pressure valve are closed, allowing the sample vapor to equilibrate and pressure and flows to stabilize. When the GC-ready signal is received by the headspace unit, the 6-port valve rotates and the sample loop contents are transferred to a heated line with column carrier gas. Carrier gas then back flushes the loop, sweeping through the heated transfer line into the GC injection port (0.3 to 6 minutes). Helium is bubbled through the solution at ambient temperature and the volatiles are transferred from the matrix to the vapor phase. The volatiles are then swept through the sorbent column where they are trapped. Then, the sorbent column is heated and backflushed with helium gas to desorb the components. The components are then transferred to a GC via a heated line, where they are separated using the appropriate column and detected using a mass spectrometer or other detector. Typically, a 5 ml sample is used for water analysis and a 5 gram sample is used for soil and sediment analysis. Purge and Trap:The primary advantage of purge and trap over static headspace is that it is a dynamic process. It is a more efficient extraction technique for those VOCs which have a higher octanol/water partition coefficient, especially in soils with high organic matter content. Purge and trap is the recommended VOC extraction technique used for GC analysis. The recommended extraction methods are 5030A and 5035. Comparison of Attributes of Headspace Versus Purge-and-Trap Analysis The headspace technique is a static process, while purge-and-trap is a dynamic process. Some of the differences between the two processes are listed in the table above. Use of organic solvents is comparable for the two techniques; however, for purge-and- trap analysis, water is used as a solvent for soil purging, increasing the amount of liquid waste produced in the field laboratory. For headspace analysis, samples can be collected directly into preweighed headspace vials; no transferring of sample is required. For purge-and-trap analysis, a measured amount of sample must be weighed and then transferred into a specially designed purge vessel. The purge vessels are reusable glassware and must be decontaminated. Use of such vessels increases the potential for carryover contamination. Because the headspace process is static, the headspace can become saturated before equilibrium is reached between the medium (soil or water) and the headspace concentration. Therefore, high concentrations of compounds may not be transferred efficiently to the headspace and subsequently detected. Results for high-concentration samples may be biased low. In purge-and-trap analysis, compounds can be purged continuously from the medium and collected onto the trap, allowing higher concentrations to be transferred and detected effectively. Because the headspace system does not allow for concentration of compounds onto a trap over a period of time (like the purge-and-trap system), trace amounts of compounds that are not mobilized effectively at equilibrium may not be detected. Both systems have the potential for loss of compounds. During pressurization of the headspace vial, compounds can be lost if the cap is not properly sealed. In the purge-and-trap system, loss of compounds will occur as a result of volatilization during the weighing of the sample and its transfer to the purge vessel. The cost of the purge-and-trap system is typically more than that of a comparable headspace system. The headspace system is an attachment to the GC that requires little additional bench space; however, the purge-and-trap system is a separate stand-alone system that may require twice as much bench space as the GC alone. The throughput of the headspace system is significantly larger than that of the purge-and-trap system, primarily because the purge-and-trap system requires more handling, transfer, purging, and decontamination of samples than the headspace system.
In the past, typical “formal lab” extraction methods included soxhlet, liquid-liquid, and sonication. More recently, the accelerated solvent extractor (ASE) has become the extraction method of choice because it is more rapid and uses less solvent. Microwave Assisted Extraction (MAE) is another extraction method that is promising for fixed laboratory analysis because it too is more rapid and uses less solvent compared to the “older” solvent extraction methods. Sops are available
for the following: The key in the field is to simplify the solvent extraction methods to minimize solvent waste, save time, and reduce cost. Typical solvents used in the field include hexane, methanol, methylene chloride, and methyl tert-butyl ether (MTBE). Simplified field, solvent extraction methods normally do not include a cleanup step. A concentration step may also be eliminated if elevated detection limits are acceptable. ASE and MSE are not as commonly used in the field (especially on small projects) because of the initial expense of the equipment and other logistical constraints such as power and space requirements. Extraction techniques that use a minimal amount of solvent are preferred for field applications. The techniques listed above fit into this category. Although supercritical fluid extraction (SFE) is an EPA-approved technique for fixed-laboratory applications, it has not gained wide use in the field because of the expense of the apparatus and limited portability. Solid-phase extraction (SPE) and solid-phase microextraction (SPME) are ideal techniques for field use because they are rapid, use little or no solvent, simple, and inexpensive. SPE is primarily limited to water samples, although it can be used as a cleanup technique for liquid extracts of solid samples. SPME has gained much more popularity in the last 2 to 3 years. Its advantage over SPE is that it can be used for both VOCs and SVOCs and no solvent is required. Thermal desorption for SVOCs is analogous to a more rigorous static headspace extraction for VOCs. It also is convenient for field use because it is simple, rapid, and requires no solvent.
Performance specs include information on detection limits, calibration, and quality control. Detection Limits Detection limits for GC analysis are highly dependent on the type of detector used. Part per million (PPM) and part per billion (ppb) levels are routinely achieved with most detectors. Detection limits for each respective detector are discussed in the systems components section. 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 Because GC analysis provides both qualitative and quantitative information, GC calibration requires that each be addressed. Qualitative: Calibration consists of injecting a known volume of a standard (at a known concentration as well) and measuring the time between injection and elution. This is known as the retention time. If all variables such as temperature, flow rate, and column length are all constant, the retention time of a given compound should remain the same. To eliminate variables in injection technique, retention time relative to an internal standard can be used. Thus, if a peak appears in the chromatogram of a sample at the sample retention time of a standard, the compound is tentatively identified, not definitively. A chromatogram provides only a single piece of qualitative information about each species in a sample, its retention time. It is important to note that while chromatograms may not lead to positive identification of species present in a sample, they can provide sure evidence of the absence of a given compound (or is present at a concentration below the detection limit of the method). A compound can be definitively identified by dual column analysis of mass spectrometry detection. Quantitative: Quantitative gas chromatography is based on comparison of either peak height or area of the analyte peak with that of one of the standards. Most modern chromatographs have digital electronic integrators that calculate accurate peak areas. The most common method for quantitative chromatographic analysis involves the preparation of a series of standard solutions that approximate the composition of the unknown. Chromatograms for the standards are generated and peak areas are plotted as a function of concentration. A plot of the data should yield a straight line passing through the origin. Frequent restandardization is necessary for the highest accuracy. The highest precision for quantitative chromatography is generated by using internal standards to eliminate uncertainties introduced by sample injection. In this procedure, a measured quantity of an internal standard is introduced into each standard and sample. Quantitation is performed by comparing analyte peakness to internal standard peak areas. It is essential that the internal standard peak be well separated from the peaks of all other components of the sample. Quality Control As stated above, field-based GC data of the same quality as fixed laboratory data can be generated. However, to do so, quality control (QC) measures comparable to the fixed laboratory must be used. Typical QC measures are defined below. External standard calibration generally consists of a three-point or five-point calibration. A medium level standard (continuing calibration) is analyzed once a day to check the response of the instrument compared to the average response of the initial calibration. Method blanks are analyzed to check for laboratory-induced contamination and instrument blanks are analyzed to check for contamination induced by the instrument (usually by sample carryover). Method blanks are especially critical in soil gas analysis when chlorinated VOCs are target analytes. Performance evaluation (PE) samples are spiked matrix samples that have certified concentrations of analytes and that can be purchased from reputable vendors (for example, Environmental Resource Associates or Absolute Standards). They are usually analyzed “blind” by the analyst (meaning the analyst does not know what analytes are present or their concentrations). The analyst must report the proper analytes and concentrations within the certified concentration ranges for the laboratory's accuracy to be acceptable. PE samples are normally not analyzed for soil gas or ambient air analysis. Matrix spike/matrix spike duplicate (MS/MSD) samples are analyzed to check for extraction efficiency of the analytical system. Percent recoveries are calculated and must fall within an acceptable range for the extraction efficiency to be acceptable. Percent recoveries are also compared to each other by calculating relative percent differences (RPD) to assess the precision of a method. MS/MSDs are not typically run for soil gas or ambient air analysis. Laboratory duplicates are analyzed to assess the precision of the method and homogeneity of the sample. The laboratory duplicates consist of the analysis of two aliquots of the same sample. The results from the laboratory duplicates are compared to each other through RPD calculations. Surrogate spikes are necessary to evaluate the extraction efficiency on a per sample basis. A surrogate compound is one that is chemically similar to the target compounds but does not coelute with any of them. A percent recovery of the known spiked concentration is calculated for each sample and compared to site specific control limits. (Not typically analyzed in soil gas or ambient air samples.) Laboratory control samples (LCS) consist of a sample matrix spiked with a known concentration of standard purchased from a separate vendor other than the one from which the calibration standards were purchased. The percent recovery for all analytes must be within an acceptable range for the accuracy of the calibration to be acceptable. Dual column analysis allows confirmation of the compound identity and concentration of a compound. This is especially important when MS is not used for confirmation. Advantages
Limitations
Cost Data Manufacturers of gas chromatographs occasionally establish elevated rental prices to encourage purchase rather than lease. However, scientific instrumentation suppliers such as HAZCO provide rental gas chromatographs capable of most analyses. Some manufacturers such as Bruker Daltonics, Inc., also offer a lease to purchase plan. Purchase price varies widely depending on instrument capabilities, auto-sampling accessories, and detectors. Simple chromatographs capable of analyzing simple mixtures of BTEX can be purchased for less than $10,000. Bench scale instruments with a mass spectrometer detector can cost over $100,000. Field GC/MS systems can be purchased for $60,000 to $85,000. Lease prices also vary according to capability. Monthly rental for a simple instrument designed to analyze simple BTEX samples is about $1,500. More sophisticated instruments lease for approximately $3,000 per month. 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 No reports available for this technology 
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