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Basic Principles of Gas Chromatography

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1 Basic Principles of Gas Chromatography
Martin R. Hackman NJ - DEP Office of Quality Assurance

(gas or liquid) MOBILE PHASE Sample out Sample in STATIONARY PHASE (solid or heavy liquid coated onto a solid or support system)

3 GC Methods Parameter Group Method Compounds
SDW ’s EPA VOC’s SDW ’s EPA Cl-VOC’s/Pests WPP ’s EPA 604 Phenols WPP ’s EPA 625 SVOC’s SHW ’s SW A Org-P Pests VOC’s - benzene, toluene, chloroform, isopropyl benzene, styrene (PID/ELCD) Cl-hydrocarbons/pesticides/disinfect. Byproducts - bromochloroacetonitrile, EDB, Endrin - ECD 604 - Phenols - Phenol, 2,4-DNP, 2-Cl-phenol - FID 625 - SVOC’s - Naphthalene, anthracene, pyrene, diethyl phthalate - GC/MS-CI Org-P Pests/Herbs - Diazinon, Malathion, chloropyrifos (Dursban), Atrazine - NPD/FPD

4 Instrumentation Manual - Direct Injection Automated - Autosampler
 Injection Port - Sample introduction Manual - Direct Injection Automated - Autosampler - Purge and Trap Direct Inj , 625, 551.1, 604 P & T

5 Instrumentation - Oven
Temperature Control Isothermal Gradient Isothermal - Keep oven at one temp thru run. Not very useful. Possibly useful for series of very similar compounds differing by boiling points such as alcohols ( MeOH, EtOH, n-PrOH, i-PrOH, BuOH, i-BuOH). BP Gradient - temp profile: 40 deg hold for 10 min then 10deg/min to 240 deg and hold there for 20 min. Advantages: 1- resolution and 2- analysis time.

6 Columns Packed Capillary
Packed - As suggested by the term, it is filled with a coated inert solid support such as fire brick, alumina, and graphite with a specific mesh size. The coatings are called phases and for best results are chemically bonded to the support. Chemical bonding provides for longer column life and less bleeding (major source of background noise) contributing to lower sensitivity. Column dimensions 1/8” - 1/4” ID x up to about 6’ using glass or stainless steel. Advantages - higher capacity (higher conc). Disadvantages: low resolution and low S/N. Capillary - Here the phase (film) is coated on the inside diameter of the capillary wall with film thickness range of 0.1 to 5μ where the ticker film provides for better resolution but also allows for more bleed. Typical dimensions .25mm - .53mm ID x up to 60m made of fused silica coated with polyamide. Advantages: high resolution and better S/N. Disadvantages: low capacity and cost.

7 Polarity - Non-polar Polar +
Non - Polar : Equal distribution of electrons over the entire molecule. Look at the structure of fluorene. Polar: Non-equal distribution of electrons in a molecule causing one size of the molecule to be more positive or negative thus creating poles of charges. Look at 2,4,5-T (2,4,5-trichlorophenoxyacetic acid).

8 Phases Here are some of the commonly used phases. They range in polarity from non-polar such as low polarity DMS to the higher polarity of mixtures with DPS with any combination available. There are specialty phases with very high polarity such as cyanopropylphenyl siloxane or trifluoropropyl methyl siloxane for fluorinated compounds. Some of these columns are used for separation confirmation such as the cyanopropylphenyl siloxane column for method

9 Instrumentation - Detectors
Destructive Mass Spectral (CI/EI) [625] Flame Ionization (FID) [604] Nitrogen-Phosphorus (NPD) [8141A] Flame Photometric (FPD) [8141A] Electrolytic Conductivity (Hall/ELCD) [502.2] MS - Mass Spectral - EI Electron impact - used for absolute confirmation. Molecules are ionized and their mass to charge ratio is plotted against its abundance. The resulting spectrum is unique to each molecule and can be looked up in a standard library and the % fit is noted. CI- chemical ionization is used for research and for structure elucidation. FID - Air/Hyd. Flame combusts the compound and the conductivity due to the ionization of the resulting carbon is determined and is presented as a signal. Very large range. NPD - Similar to FID except the combusted compound is passed over a heated bead of rubidium which provides for specificity in determining N and P where P is 500x more sensitive than N. NPD-P limited range, NPD-N broader range. FPD - Similar to FID but detector is light-tight where a PMT/filter assembly collects signal from emission for P at 393nm and S at 529nm. Haal/ELCD - furnace at about 900°C produces ionized acidic gases such as HCl or HF which is dissolved into a deionized solvent to produce conductivity proportional to the mass of the halogen in the org. compound.

10 Instrumentation - Detectors
Non-Destructive Thermal Conductivity (TCD) Electron Capture (ECD) [551.1] Photo Ionization (PID) [502.2] TCD -Not normally used. Low sensitivity but good range. Change in resistance due to cooling effect of effluent over the resistance wire. ECD - limited range but very sensitive mainly to Cl org samples. The sample passes thru a Ni-63 foil where the Ni-63 gives off a constant amount of β particles and the Cl captures these electrons and the resulting loss produces a signal. PID - sample passes thru chamber it is constantly bombarded with high energy, 10.2 eV, where resulting ionization is produced and the ion current collected.

11 Chromatograms Note peaks 15, & 18 on the DB-5 column and note the same peaks on the DB-1701 column. This shows the need for confirmatory columns (columns with different phases) so that separation of the compounds can be verified.

12 Chromatograms - 604 Supelco® PTE-5 Supelco® SPB-50
Note peaks 1 and 2 using the different columns. The separation is achievable because the compounds are different. Also note peaks 4 and 5 and 11 and 12. Even using different columns, positional isomers, o-, m- and p- cresol are difficult to separate. Supelco® PTE-5 Supelco® SPB-50

13 m- Cresol

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