Gas Chromatography (GC) is a popular, reasonably inexpensive technique traditionally used mostly for the separation and further detection of volatile and semivolatile organic compounds in gases or liquids. It is one of the most important techniques in environmental analysis. Based on the type of sample (gas or liquid) and compound of interest, the gas chromatograph can be configured with different types sample introduction systems and detectors. The Material Characterization Laboratory has the following GC systems:

The GC detector is the brain of any GC instrument. All detectors work by sensing some physical or chemical property of the components passing through the detector cell. The choice of detector type depends on particular analytical purpose and compound of interest (see Table for details).

For most GC detectors, identification is based solely on retention times and is performed by comparing the retention time of the unknown peak to that of a standard. Since many compounds may possess the same retention time, there is always a doubt about the nature and purity of the compound in the separated peak. Among all detectors available in the lab, there are only two, which provide additional qualitative (structural or elemental) information about the sample lacking in other GC detectors, and, therefore, are used directly for compound identification. They are Mass Selective Detector (MSD or MS) and Atomic Emission Detector (AED). At present, Material Characterization Laboratory has three GC/MS systems.

. GC/MS/AED Agilent 5973N Benchtop GC/MSD/Atomic Emission Detector (AED) with HP Automatic Liquid Sampler. Agilent ChemStation for GC control and data handling. NIST MS Spectral Library and NIST Chemical Structures Database. This system is the most updated GC/MS (quadrupole) system available in the Material Characterization Laboratory. The instrument is equipped with turbomolecular pumps and is operated in either electron impact or chemical ionization (positive and negative) modes. Among the most important advantages of 5973N mass selective detector, crucial for our applications are: high sensitivity (1 pg of octafluoronaphtalene with a signal to noise ratio of 10:1 with EI-SCAN mode) and a wide mass range from 1.6 to 800 amu for expanded compound identification. These features allow the detection and quantitative measurements of PCBs, dioxins, pesticides and other pollutants in a variety of environmental matrices and at very low concentrations. The HP MSD ChemStation provides complete automation of an integrated system (GC, MSD and automatic liquid sampler) and, therefore, improves productivity. MSD tuning, sample introduction, signal acquisition, data processing are automated by ChemStation methods. The NIST 98 MS spectral library (107K compounds and 130K spectra) along with NIST 98 Chemical Structures Database (107K chemical structures) allow searching mass spectral database for the best spectral match of the analyte with the library and make mass spectrum interpretation easier. The GC-MS can be used alone or coupled with the atomic emission detector (AED). This AED can detect virtually all elements present in volatile and semi-volatile compounds with picogram sensitivity, and thereby determines empirical chemical formulae. This detector is a perfect complement to a mass selective detector for identification of target or unknown compounds by screening candidates for the correct composition of elements after a library search has identified possible matches. The system can detect carbon, sulfur, chlorine, bromine, iodine, fluorine, phosphorous, nitrogen, oxygen, organometallic species containing tin, mercury, lead, selenium and arsenic and isotopes 12C, 13C, 14C, 1H, 2H, 14N, 15N, 16O, 18O. For the analysis of environmental samples, the AED's high sensitivities make it easy to detect trace-level chemical pollutants.

 

. GC/MS/SPE Hewlett Packard 5972A Benchtop GC/MSD with HP Automatic Liquid Sampler/ Supercritical Fluid Extractor (SFE) and Automatic Sample Preparation Station (SPE). Hewlett Packard ChemStation for GC control and data handling. NIST MS Spectral Library. This GC/MS (quadrupole) system is earlier version of the Agilent GC/MS system. Its vacuum system is maintained by diffusion pump instead of turbomolecular in the above-mentioned GC/MS. The instrument is operated in either electron impact or chemical ionization (positive and negative) modes. This GC/MS system can be used alone or in tandem with SFE and/or SPE. SPE is designed to automate sample preparation before injection, such as aspirating, dispensing, diluting, heating, evaporation, solid phase extraction and filtration. This improves the precision and accuracy of results, eliminates errors resulting from repetitive preparative procedures. SFE module uses liquid carbon dioxide to extract solid samples. Supercritical fluid extraction besides to be environmental safe is more selective than conventional methods by allowing the adjustment of extraction conditions (e.g., pressure, temperature and flowrate). Extraction time is reduced from days to hours or minutes with SPE module. The use of combined system (SPE-SPE-GC/MS) makes such procedure, for example, as analysis of polynuclear aromatic compounds, from the point of solid sample extraction up to GC/MS qualitative and quantitative analysis, completely automated and controlled by software.

 

. High Performance Liquid Chromatography (HPLC)

If the compound is non-volatile or thermally fragile, it cannot be analyzed by the GC, and then the HPLC is an appropriate method of analysis. The Material Characterization Laboratory has three standard HPLC systems for analysis of UV-absorbing organics. They all have conventional gradient pumping system, autosamplers and variable wavelength UV detector. Fluorescence, refractive index and photodiode array (PAD) detectors are also available. At present, PAD is the most informative among available in the laboratory HPLC detectors, since it can collect full UV spectra for each component in the sample.

. HPLC Waters 600E Pump and Controller with HP 1050 Autosampler, 484 UV/Visible and Refractive Index detectors. The EzChrom Elite Chromatography Data System for data acquisition and processing. Waters 600E Pump and Controller with 717 WISP Autosampler, 996 Photodiode array and 470 Fluorescence detectors. Millennium software for instrument control, and data acquisition and processing. Alliance 2690 Separation Module with 484 UV/Visible detector. The EzChrom Elite Chromatography Data System for data acquisition and processing. This is relatively new addition to HPLC capabilities of Material Characterization Laboratory. Being a new generation of Waters HPLC instruments, it combines two instruments in one, i.e. solvent delivery system and autosampler.

. Spectroscopy Instruments

. UV/Visible Spectrophotometer A spectrophotometer is an instrument for measuring the transmittance or absorbance of a sample as a function of the wavelength of electromagnetic radiation. Used mostly for quantitative analysis of molecular or ionic species in solution. Hewlett Packard, Model 8453 UV-Visible Spectrophotometer Wavelength range: 190 - 1100 nm Resolution: 1 nm slit width Full control of the spectrophotometer is solely by the general purpose UV-Visible ChemStation Software.

 

. Infrared Spectrophotometer If sample absorbs infrared radiation, the variations of sample transmittance with frequency of radiation are recorded as an infrared absorption spectrum. The infrared spectrum contains basic information about the composition and structure of a compound. Organic compounds, for example, may contains groups such as -OH, -NH2, -CH3, -CO, -CN, -C-O-C-, -COOH, -CS, etc. These groups have characteristic absorption frequencies in the infrared which are usually relatively unaffected by the reminder of the molecule. An unknown compound, therefore, can be characterized by observing the presence of the absorption frequencies associated with such groups. Spectra-structure correlation charts provide a key to the location of characteristic absorption bands for most of the common functional groups. Mattson Research 1 Fourier Transform Infrared (FTIR) Spectrometer The term Fourier Transform Infrared Spectroscopy (FTIR) refers to a fairly recent development in the manner in which the IR data are collected and converted from an interference pattern to a spectrum. Its major application: for identification of types of chemical bonds (functional groups) in molecules. It can be applied to the analysis of solids, liquids, liquid and solids in solution, and gasses. FTIR can be also used to quantitate some components of an unknown mixture.

 

. Fluorescence Spectrophotometer One of the advantages of fluorescence detection over other light-based methods is high sensitivity. Fluorescence techniques can measure concentrations one million times smaller (pico and femtomolar) than absorbance techniques (several tenth of micromolar). Rad FTS40 FTIR/HP 5890 Gas Chromatograph Wavelength range: 220 to 730 nm for both excitation and emission sides To analyze the reflective fluorescence of solid samples (powder; flat sample such as plastic, painted plate, paper; soft samples such as fiber, cloth, etc.) or the surface fluorescence of high concentration solution samples, the solid sample holder is available.

. Miscellaneous

. TOC Analyzer Tekmar-Dohrman Phoenix-8000 UV -Persulfate TOC Analyzer with Autosampler Concentration range: 2 ppb- 10,000 ppm Sample size: 500 µl to 20 ml This is UV/Persulfate TOC analyzer used for measuring total carbon (TC), total organic carbon (TOC) and total inorganic carbon (IC) in aqueous samples.