TCDTheory

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6850/6890 Thermal Conductivity

Detector

Theory of Operation

May 4, 2003

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Title of Presentation

Date

Agilent Restricted Page 2

6850/90 TCD - Comparison to Other GC

Detectors

MSD

IRD

PID

FPD(S)

NPD(P)

NPD(N)

ECD

FID

TCD

AED

gr

10 10 10 10 10-15-12 -9 -6 -3

fg pg ng ug mg

1ppt 1ppb 1ppm 0.1% 100%

Sensitivity

(SIM)

(SCAN)

ELCD (X)

ELCD (SorN)

TCD

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6850/90 THERMAL CONDUCTIVITY

DETECTOR

Thermal Conductivity Values of Common Gases/Solvents

Compound Relative Thermal Conductivity

Carbon Tetrachloride 0.05

Benzene 0.11

Hexane

0.12

Argon 0.12

Methanol 0.13

Nitrogen 0.17

Helium 1.00

Hydrogen

1.28

Thermal Conductivity Relative to Helium

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6850/90 TCD - Theory of Operation -

Traditional Bridge Design

VFilament

DriveVoltage

Four Identical FilamentsMatched Reference/Sample FlowsResult - Balanced Bridge

Equal Voltages

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Traditional Bridge Design

VFilament

DriveVoltage

Sample Crosses S1 and S2S1 and S2 Resistance IncreasesV1 Increases - V2 DecreasesResult - Peak Detected

V1V2

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Single Filament Detector

Thermal ConductivityTheory

Step1:Carrieronly

Filament

Step2:Carrierandsample

Filament

Increasedtemperature

Increasedresistance

Filament

Constanttemperature

Constantresistance

Thermal Conductivitydetectionscheme.

Instep1. thecarriergaspassingoverthefilamentmaintainsaconstantte

andthereforeconstantresistanceinthefilament. Instep2, theadditionmoleculeswiththecarriergasresultsinincreasedtemperatureandincrease

filamentresistance.

Thischangeismeasuredandrecorded.

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Single Filament Detector

F

i

l

a

m

e

n

t

C

h

a

n

n

e

l

246

VENT(60ml/min)

36

24

30ml/minColumnflow

30ml/minReferenceflow

F

i

l

a

m

e

n

t

C

h

a

n

n

e

l

636

36

24

VENT(60ml/min)

30ml/minColumnflow

30ml/minReferenceflow

FilamentMeasures

COLUMNFLOW

Filamente

REFEREN

5Hz

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6850/90 TCD - EPC Flow Manifold

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6850/90 TCD -

5 Hertz Pneumatic Switching

2.51 2.52 2.53 2.542.49 2.56

One Second -0.0167 Minutes

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6850/90 TCD -

Filament Drive - Delta T Sensor

See Service Note G1530A-19

TCD Upgrade Kit AvailableUpgrade if no connections for PRT

sensor on the TCD electronics board.

Filament

wires

PRT sensor

wires (new units only)

Filament

Temp C

Detector Temp (body temp) C

500

400

300

200

100

Response

100 200 300 400

DT 50C

DT135C

Detector response vs. detector temperature Filament Temperature vs. Block Temperature

100 200 300 400

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6850/90 TCD - Theory of Operation

Column

Second Delta T Sensor

for Filament Drive

Control:

The filament current is

set to keep the filament

at a fixed temperature

above the block

temperature

Constant

Current

Supply

Reference

Filament

Filament

Drive -

Microvolts(Typically

250-500 uV)

Heater/Sensor for Block

Temperature Control

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6850/90 TCD - Theory of Operation

Column

Constant

Current

Supply

Reference

Filament

Filament

Drive -

Microvolts

As a peak elutes, the

filament heats up and the

resistance increases -

Filament drive voltage is

increased to keep current

constant

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6850/90 TCD - Component Identification

Makeup and ReferenceGas Lines

Block Heater/Sensor

Connection

EPC Flow Manifold

TCD Switching Valve

TCD Filament Wires

Delta T Sensor for

Filament Drive

TCD Filament Block

Assembly

TCD Electronics Board

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6850/90 TCD - Component Identification

Block Heater/Sensor

Connection

TCD Switching Valve

Connection to

Electronics Board

TCD Filament Wires

Delta T Sensor for

Filament Drive

TCD Electronics Board

TCD Filament Block

Assembly

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