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HP 5973 Gas Chromatography Mass Spectrometry The basic instrument consists of a gas chromatograph (GC) with a quadrupole mass spectrometer (MS) as a detector. In the normal mode of operation, a sample consisting of a mixture of organic compounds which are volatile at a temperature of 275 degrees C is introduced to the head of the column with a syringe. Helium serves as a carrier gas to move the sample through the column. During passage through the column (a fused silica capillary with a wall coating of a thermally stable, non-volatile liquid), the mixture is separated into individual components. As each component issues from the column, it is introduced to the mass spectrometer where a mass spectrum for that material is obtained and stored in a computer. The spectrum is interpreted, either manually or by a computer search, to identify the compound. An alternative mode of operation involves the use of the Universal Concentrator, which consists of either of two heatable furnaces connected to a trapping system. In this mode, the sample (liquid or solid) is placed in a furnace tube and heated to an appropriate temperature (800 degrees C maximum). As in normal mode, helium serves as the carrier gas. Volatile materials evolved from the sample are adsorbed on a series of traps and subsequently thermally desorbed onto the chromatographic column. The remainder of the process is identical to that previously described. Another alternative mode of operation uses a heated direct inlet probe (DIP) for the introduction of the sample. The sample (usually a solid or a very slightly volatile liquid) is placed in a glass capillary and transported into the ionization region of the mass spectrometer by means of the probe. The probe is heated; mass spectra are continuously collected during the heating process and identified by the usual procedure. Since the DIP method does not separate components with great efficiency, it is not well suited for the analysis of mixtures. HP 6890 Gas and Liquid Chromatography Flame Ionization (FID) TechniqueGas Chromatography (GC) is used for the qualitative and quantitative analysis of complex mixtures of gases, liquids, and sometimes solids. A sample is vaporized and transported by an inert carrier (usually He) gas through a column of sufficient length to provide the separation. There are many different columns with both different mobile and solid phases used with different carrier gases. The right combination has to be chosen such that the desired separation takes place with reasonable retention times. Capillary columns (0.25mm - 0.53mm id), have coatings of a partitioning liquid bonded onto the inner wall surface of the column. The different components of the vapor mixture are separated as a result of their different vapor pressures and relative affinities for the bonded liquid phase. As the components of the mixture are separated and elude from the column, they enter a detector, where a signal proportional to the concentration of the component is amplified and displayed. Identification is done by the retention time. High performance liquid chromatography (HPLC) is used for the qualitative and quantitative analysis of complex mixtures of liquids and/or solids. Samples are dissolved in a solvent suitable for analysis. The HPLC column contains a stationary phase bonded to a support medium. Separation of the mixture is based on differences in degree of association of each component with the stationary phase as it is transported through the column by a liquid mobile phase. After separation the components peaks elude from the column into a detector where a signal proportional to the concentration of each component is amplified and displayed. Sample InformationThe sample must be altered prior to instrumental analysis, typically by extraction or dilution of the sample into a suitable solvent which is compatible with the technique. Components in a mixture may be qualitatively identified by retention times obtained through the analysis of known references. Where possible, replicate analysis on dissimilar chromatographic columns can be performed with subsequent retention time evaluations. In liquid chromatography, UV spectra can be used for qualitative identifications by utilizing diode array detection. Both GC and HPLC provide quantitative analysis of the components in a mixture. Statistics / MiscellaneousThe data collection time is typically 1-2 hours, depending on the boiling point of the components and the complexity of the mixture being separated. The typical analysis time is dependent on the sample preparation which may be necessary for proper sample introduction into the instrumentation. The sensitivity of these techniques ranges from ppb to percent for GC and from ppm to percent for HPLC. The actual values are extremely dependent on the sample matrix. The accuracy is typically 2-10% (also matrix dependent) and the precision which is obtained is typically less than a 5% relative standard deviation. Ion Chromatography (IC) Ion chromatography is used for quantitative analysis of water soluble ionic species. Anions such as fluoride, chloride, nitrite, bromide, nitrate, phosphate, sulfate are normally analyzed. IC utilizes ion exchange to accomplish separation of analyte ions, followed by chemical suppression of eluent conductivity and conductivity detection. A liquid sample is introduced at the top of the separator column. The ionic eluent pumped through the column causes differential migration of the analyte ions down the column as a result of colligative properties (size, mass, and shape) and the affinity of the ions to the active sites of the column. The analyte ions are separated into discrete bands which are detected using a conductivity detector. The chromatograph output is a plot of conductivity versus time (a chromatogram). Each ion appears as a peak in the chromatogram. Identification and quantification is performed by comparing the retention times (time from injection to peak maximum) and peak heights to the standards of the sample. Gideon Analytical Labs 80 Loughran Ln Highland NY 12528-2838 845-255-5356 info@gideonlabs.com |
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