Gas Chromatography and Supercritical Fluid Chromatography

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Gas Chromatography and Supercritical Fluid Chromatography

Transcript of Gas Chromatography and Supercritical Fluid Chromatography

  • 1Gas and Supercritical Fluid Chromatography

    Lecture Date: April 7th, 2008

    Gas and Supercritical Fluid Chromatography

    Outline Brief review of theory Gas Chromatography Supercritical Fluid Extraction Supercritical Fluid Chromatography

    Reading (Skoog et al.) Chapter 27, Gas Chromatography Chapter 29, Supercritical Fluid Chromatography

    Reading (Cazes et al.) Chapter 23, Gas Chromatography Chapter 24, Supercritical Fluid Chromatography

  • 2GC and SFC: Very Basic Definitions

    Gas chromatography chromatography using a gas as the mobile phase and a solid/liquid as a stationary phase

    In GC, the analytes migrate in the gas phase, so their boiling point plays a role

    GC is generally applicable to compounds with masses up to about 500 Da and with ~60 torr vapor pressure at room temp (polar functional groups are trouble)

    Supercritical fluid chromatography chromatography using a supercritical fluid as the mobile phase and a solid/liquid as a stationary phase

    In SFC, the analytes are solvated in the supercritical fluid

    SFC is applicable to a much wider range of molecules

    Review of Chromatography

    Column/separation performance:

    Plates: HLN /

    Selectivity: AB KK / Important concepts/equations to remember:

    Retention volume: tFV

    mtLu / Linear velocity of mobile phase:

  • 3Review of Chromatography

    Terminology and equations from Skoog:

    GC Theory

    Mobile-phase flow rates are much higher in GC (pressure drop is much less for a gas)

    The effect of mobile-phase flow rate on the plate height (H) is dramatic

    Lower plate heights yield better chromatography

    However, much longer columns can be used with GC

  • 4GC Instrumentation

    Basic layout of a GC:

    Injector

    Column

    Oven

    Detector

    Carrier Gas

    See pg 703 of Skoog et al. for a similar diagram

    GC Instrumentation

    A typical modern GC the Agilent 6890N:

    Diagram from Agilent promotional literature.

  • 5GC Instrumentation

    Typical carrier gases (all are chemically inert): helium, nitrogen and hydrogen. The choice of gas affects the detector.

    Injectors: most desirable to introduce a small plug, volatilize the sample evenly

    Most samples introduced in solution: microflash injections instantly volatilize the solvent and analytes and sweep them into the column

    Splitters: effectively dilute the sample, by splitting off a portion of it (up to 1:500)

    Ovens: Programmable, temperature ranges from 77K (LN2) up to 250 C.

    Detectors: wide variety, to be discussed shortly

    Headspace GC

    A very useful method for analyzing volatiles present in non-volatile solids and liquids

    Sample is equilibrated in a sealed container at elevated temperature

    The headspace in the container is sampled and introduced into a GC

    Needle

    Liquid/solid

    Headspace

  • 6Columns for GC

    Two major types of columns used in GC

    Packed Open

    Open columns work better at higher mobile phase velocities

    Columns for GC Open tubular columns: most

    common, also known as capillarycolumns (inner diameters of

  • 7Types of Columns for GC

    GLC: Gas-liquid chromatography (partition) most common

    GSC: Gas-solid chromatography (adsorption) FSWC: fused-silica wall-coated open tubular columns,

    very popular in modern applications (a form of WCOT column)

    WCOT (GLC): wall-coated open tubular stationary phase coated on the wall of the tube/capillary

    SCOT (GLC): support-coated open tubular stationary phase coated on a support (such as diatomaceous earth)

    More capacity that WCOT

    PLOT (GSC): porous-layer open tubular Packed columns

    Mobile Phases for GC Common mobile phases:

    Hydrogen (fast elution) Helium Argon Nitrogen CO2

    The longitudinal diffusion (B) term in the van Deemter equation is important in GC

    Gases diffuse much faster than liquids (104-105 times faster)

    A trade-off between velocity and H is generally observed

    This is equivalent to a trade-off between analysis time and separation efficiency

  • 8Columns and Stationary Phases for GC Modern column design emphasizes inert, thermally stable

    support materials Capillary columns are made of glass or fused silica

    The stationary phase is designed to provide a k and that are useful. Polarities cover a wide range (next slide).

    Stationary phases are usually a uniform liquid coating on the wall (open tubular) or particles (packed)

    When the polarity of the stationary phase matches that of the analytes, the low-boilers come off first

    Bonded/cross-linked phases designed for more robust life, less bleeding often these phases are the result of good polymer chemistry

    Adsorption onto silicates (via free silanol groups) on the silica column itself: avoided by deactivation reactions, usually leaving an OCH3 group instead.

    Stationary Phases for GC Target: uniform liquid coating of thermally-stable, chemically

    inert, non-volatile material on the inside of the column or on its particles.

    Polysiloxanes Polydimethylsiloxane

    (R = CH3) phenyl polydimethylsiloxane

    (R = C6H5, CH3) trifluoropropyl polydimethylsiloxane

    (R = C3H6CF3, CH3) cyanopropyl polydimethylsiloxane

    (R = C3H6CN, CH3) polyethylene glycol

    Chiral amino acids, cyclodextrins

    Backbone structure of polydimethylsiloxane

    (PDMS)

    HOO

    OH

    n

    R Si

    R

    R

    O Si

    R

    R

    O Si

    R

    R

    R

    n

    structure of polyethylene glycol (PEG)

  • 9Common Stationary Phases for GC

    High-temperature columns work to 400C, include Agilents DB-1ht (100% polydimethylsiloxane), DB-5ht (5% phenyl).

    Stationaryphase

    polarityStationary Phase Common Trade Name

    Maximum Temperature

    (C) Common Applications

    polydimethylsiloxane OV-1, SE-30 350 General-purpose nonpolar phase; hydrocarbons,

    steroids, PCBs

    5% phenyl polydimethylsiloxane

    OV-3, SE-52 350 Fatty acid methyl esters, alkaloids, drugs,

    halogenated compounds

    50% phenyl polydimethylsiloxane

    OV-17 250 Drugs, steroids, pesticides, glycols

    50% trifluoropropyl polydimethylsiloxane

    OV-210 200 Chlorinated aromatics, nitroaromatics, alkyl-substituted benzenes

    polyethylene glycol Carbowax 20M 250 Free acids, alcohols, ethers, essential oils,

    glycols

    50% cyanopropyl polydimethylsiloxane

    OV-275 240 Polyunsaturated fatty acids, rosin acids, free acids,

    alcohols

    Temperature Effects in GC Temperature programming can be used to speed/slow

    elution, help handle compounds with a wide boiling point range

  • 10

    Comparison of GC Detectors

    See pg. 793 of Skoog et al. 6th Ed.

    Detector Sensitivity Selective or Universal Common Applications

    Flame ionization (FID) 1 pg carbon/sec

    Universal Hydrocarbons

    Thermal conductivity (TCD) 500 pg/mL Universal Virtually all compounds

    Electron capture (ECD) 5 fg/sec Selective Halogens

    Mass spectrometry (MSD) 0.25 to 100 pg Universal Ionizable species

    Thermionic (NPD) 0.1 pg/s (P)1 pg/s (N)

    Selective Nitrogen and phosphorus compounds (e.g. pesticides)

    Electrolytic conductivity (Hall)

    0.5 pg/s (Cl)2 pg/s (S)4 pg/s (N)

    Selective Nitrogen, sulfur and halogen-containing compounds

    Photoionization 2 pg/s Universal Compounds ionized by UV

    Fourier transform IR (FTIR) 0.2 to 40 ng Universal Organics

    GC Detectors: FID The flame ionization detector

    (FID), the most common and useful GC detector

    Process: The column effluent is mixed with hydrogen and air and is ignited. Organic compounds are pyrolyzed to make ions and electrons, which conduct electricity through the flame (current is detected)

    Advantages: sensitive (10-13g), linear all the way up to 10-4g), non-selective

    Disadvantages: Destructive, certain compounds (non-combustible gases) dont give signals in the FID.

  • 11

    GC Detectors: Thermal Conductivity Thermal conductivity

    detector (TCD): a non-selective detector like the FID

    Also known as the katherometer (catherometer) or hot wire

    Works by detecting the changes in thermal conductivity (also the specific heat) of a gas containing an analyte

    About 1000x < sensitive than FID

    Non-destructive

    GC Detectors: Electron Capture Detector Electron capture: selectively detects halogen-containing compounds

    (e.g. pesticides) Works by ionizing a sample using a radioactive material (63Ni). This material

    ionizes the carrier gas but this ionization current is quenched by a halogenated compound

    Detects compounds via electron affinity e.g. I (most sensitive) > Br > Cl > F

  • 12

    GC Detectors: Other Atomic emission detector: plasma systems (like ICP, but

    often using microwaves) elemental analysis Sulfur chemiluminescence detector (SCD): reaction

    between sulfur and ozone, follows an FID-like process Thermionic detector: like an FID, optimized and

    electrically charged to form a low-temp (600-800 C) plasma on a special bead. Leads to large ion currents for phosphorous and nitrogen a selective detector that is 500x as sensitive as FID

    Flame photometric detector: specialized form of UV emission from flame products

    Photoionization detector: UV irradiation used to ionize analytes, detected by an ion current.

    And, of course, the mass spectrometer (MS)

    Examples of GC Detection: Petroleum Analysis

    An example of atomic spectrosc