6890/7890 GC Hardware and Technology Overview

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6890/7890 GC Hardware and Technology Overview
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In this Section, We Will Discuss

• The major components of the gas chromatograph.
• The typical chromatogram and the information it
  contains.
• The way a GC separation occurs.
• Considerations for use of gases and plumbing
  configuration.

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Sample Requirements for Gas Chromatography
Only 10-20 of all compounds are suitable for GC
analysis, But these compounds are worth billions
and billions of dollars annually.

• The Compounds must have
• Sufficient Volatility Large macro molecules
  generally do not have sufficient volatility,
  (i.e., they will not become gaseous under
  instrumental parameters). Large biological
  polymers are examples of inappropriate compounds
  for GC.
• Free of Residues This is an extension of the
  first requirement. Non-volatile impurities in
  the sample matrix can lead to inlet and column
  contamination that
• will quickly degrade the chromatography.
• Thermal Stability The compounds of interest
  must not degrade when introduced into the hot
  inlet (upwards of 300?C) or while in the heated
  column (upwards of 350?C).

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What is the deal with GC?

• Temperature driven
• This means that there is a molecular weight
  limit.
• Uses a gas as the mobile phase
• The most efficient way to separate organics.
• Pressurized instrument
• This means that leaks are bad news.
• Huge arsenal of detectors
• Ranging from universal to selective to
  confirmatory.

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Innovation of GC Technology

• Fused Silica Capillary Columns 1979. This
  material has allowed user-friendly operation and
  installation of capillary columns worldwide.

• Electronic Pneumatics Control (EPC) Board --
  1989 - perfected in 1995 with the 6890.
• EPC has made GC more precise in providing peak
  areas and retention times than ever before in
  the history of GC.

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Typical GC System Schematic
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Typical Gas Chromatograph
Fixed
Mol-Sieve
Traps
Restrictors
Injection
Port
Regulators
Detector
Electrometer
Flow
PC
Controller
Column
Carrier
Hydrogen
Air
Gas
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Definitions

• Gases
• Carrier Gas Pressurized gas used to transport
  the sample through the system.
• Detector Gases Support for certain detectors
  (i.e., FID).
• Sample Introduction
• Introduces the sample to the carrier gas stream
  with minimal disruption of the gas stream.
• Column
• Achieves separation of the components in the
  sample.
• Detector
• Recognizes and responds to sample components as
  they elute from the column.
• Data Acquisition
• Converts the detector signal to a picture
  chromatogram and provides manual or automated
  determination of the identity and amounts of the
  sample components .

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Innovation of GC Technology

• Highly reproducible GC ovens
• New data systems
• Fast automatic samplers
• New detectors
• New integration algorithms
• Generally more precise GCs...

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Role of the Sample

• The sample determines the instrument
  configuration
• Type of Carrier Gas
• Type of Sample Inlet
• Type of Column
• Type of Detector
• Type of Data Acquisition

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Typical Chromatogram

• Retention Time
• Parameter used to identify a sample component.
• Peak Area
• Parameter used to measure the quantity of the
  sample component.

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Model of the Chromatographic Process
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How Separation Occurs
Chromatography is a separation method achieved by
the distribution of substances between two phases
(a mobile phase and a stationary phase)
Mobile Phase Stationary Phase
Gas Solid Chromatography (GSC) Gas Solid
Gas Liquid Chromatography (GLC) Gas Liquid
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Separation is a Partitioning Process
Column
Carrier Gas
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Column Types
Packed
Open (Capillary)
Wall Coated
Open Tube
PACKED
SERIES 530
NARROW BORE
LENGTH (meters)
0.5-10
5-100
5-100
I.D. (mm)
2-4
0.530
0.1-0.25
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There are Two Common Types of Capillary Column
Stationary Phase Coatings
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Comparison of Column Types
Column Evaluation Sample (Kerosene)
Packed Column Analysis
5 OV101
on 80/100 Chromosorb
Megabore ( packed column replacement)
30m X 0.53mm X .88µ
Capillary
30m X 0.32mm X .25µ
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Carrier and Detector Support Gases

• Gases must be
• Chosen with the consideration of the type of
  detector used
• Inert
• Dry
• Pure

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GC Gases

• Generally, the carrier gas for a GC system will
  start at a cylinder holding the compressed gas.
• A regulator valve on the outlet of the cylinder
  controls the pressure of the gas in the supply
  lines.
• Compressed gases are available in different
  levels of purity. Gases of four nines
  (99.9999) or better are recommended.
• Clean supply tubing to transfer gas from the
  cylinder to the GC and gas purification traps
  are recommended for routine operations.

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Gas Regulator Valves

• The regulator valve is a very common site in
  analytical laboratories.
• The gauge on the left indicates gas pressure
  remaining in the cylinder.
• The gauge on the right indicates the set
  pressure of gas leaving the cylinder and flowing
  into the supply tubing.
• Most systems use 1/8 fittings however, an
  adaptor is available for use with ¼ tubing and
  fittings.

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Gas Regulator Valves

• Regulators are standardized and come with NPT
  style threads, which are common to most gas
  plumbing applications.
• The regulator valves are made of brass with
  stainless steel diaphragms.
• There are separate regulator valves for
• Air
• Hydrogen, Argon/Methane mix (P5 mix)
• Oxygen
• Helium, Argon, Nitrogen

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Regulators and Flow Controllers

• The carrier gas must be regulated to provide
  constant pressure as well as a constant mass
  flow. The pressure differential between
  controllers is recommended as 5 psi.
• Recommended Line Pressures
• Carrier Gas should be 60-150 psi depends on type
  of column used (60 psi minimum for large
  diameter, 150 for very small diameter and
  capillary columns).
• Air pressure should be 80 psi.
• Hydrogen should be 60 psi.

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GC Gas Purifiers

• Carrier gas purity is very important. Trace
  amounts of oxygen and water will damage and
  shorten column lifetimes, especially for
  capillary columns.
• The more polar the column (i.e. waxes like
  polyethylene glycol), the more susceptible it
  will be to degradation.
• Oxygen will also degrade ECD performance.

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GC Gas Purification Configurations
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Assembling the Gas Plumbing
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Different Gas Purifiers
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Different Gas Purifiers
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Tubing and Traps

• GC or instrument grade copper or stainless steel
  tubing should be used for all gases.
• Stainless steel tubing is recommended for
  hydrogen.
• Plastic tubing is permeable to O2 and other
  contaminants. It may also outgas detectable
  impurities.
• Precondition the tubing with solvent flush and
  carrier gas drying or purchase tubing prepared
  this way.
• Filters need to be changed at the manufacturer's
  recommended interval to prevent contamination
  breakthrough (i.e. every 3 cylinders).
• All external fittings should be checked on a
  routine basis for leaks (every 6 months).

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Gas Flow Meters
Volumetric versus Mass Flow Measurement What you
should know

• Volumetric
• As the name suggests, these meters measure the
  amount of gas which is passing through the
  system.
• The Optiflow, at right, forms a soap- bubble
  membrane in a glass tube. The gas flow carries
  the membrane through an optical sensor that
  calculates flow based on travel time.
• The ADM and Flow Trackers utilize changes in
  thermal conductivity to calculate how much gas
  is passing through a precisely calibrated
  orifice.

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Gas Flow Meters
Volumetric versus Mass Flow Measurement What you
should know

• Mass Flow Meters
• As the name suggests, these meters are
  calibrated to the mass specifications for a
  specific gas.
• Veri-Flow 500 On board mass specifications
  are stored for the 5 most common gases
• Helium, Hydrogen, Nitrogen, Air, Argon
  Methane Mix (P5).