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 300C) or while in the heated column (upwards of 350C).

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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

Column

FlowController

Regulators

Air

Hydrogen

Carrier

Gas

Mol-SieveTraps

Fixed

InjectionPort

Detector Electrometer

PC

Restrictors

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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 GC’s...

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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

Separation of compounds , and

FlowA

B

C

D

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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 PhaseGas Solid Chromatography (GSC)

Gas Solid

Gas Liquid Chromatography (GLC)

Gas Liquid

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Separation is a Partitioning Process

SAMPLEMOBILEPHASE

STATIONARYPHASE

Carrier Gas

Column

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Column Types

LENGTH (meters)

I.D. (mm)

0.5-10

2-4

5-100

0.530

5-100

0.1-0.25

PACKED SERIES 530 NARROW BORE

Packed Open (Capillary)

Wall CoatedOpen Tube

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There are Two Common Types of Capillary Column Stationary Phase Coatings

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Comparison of Column Types1 2 4

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9

1011&12

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1 2

4 5 6

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10117&812

Packed Column Analysis:

Megabore ( packed column replacement):

Capillary:

5% OV101on 80/100 Chromosorb

30m X 0.53mm X .88µ

30m X 0.32mm X .25µ

Column Evaluation Sample(Kerosene)

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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

Using Compressed GasSafely

Obtain safety information from your company's safety department or

from your local gas supplier.

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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

General Plumbing Diagram

Main Gas Supply

Hel

ium

6890 or 7890

Oxygen Trap

Moisture TrapOn/Off Valve

Two Stage Regulator

Main SupplyOn/Off Valve

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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 MetersVolumetric versus Mass Flow MeasurementWhat 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 MeasurementWhat 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).