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Speakers
John V Hinshaw
GC Dept. Dean
CHROMacademy
Tony Taylor
Technical Director
CHROMacademy
Moderator
Dave Walsh
Editor In Chief
LCGC Magazine
Understanding and Optimising Detectors for
Capillary GC
Aims & Objectives
1. Review of Common Detector Types for
Capillary GC
2. Principles and Practice of:
- Flame Ionization Detectors (FID)
-Nitrogen Phosphorous Detectors (NPD)
- Electron Capture Detectors (ECD)
- Thermal Conductivity Detectors (TCD)
3. Learn about gases, flow rates and
parameter settings
4. Maximising detector sensitivity
5. Troubleshooting detector problems
6. Relate detector issues to chromatographic
problems
The Flame Ionisation Detector (FID)
1. Carrier and detector ‘fuel’
gases mixed to support a
FLAME at the anode tip
2. Potential difference applied
between the Anode (Jet)
and Collector (Cathode)
which is electrically isolated
3. Analyte burns in the flame
to produce cations which
move toward the cathode producing a very small
current (pA)
4. Signal is amplified and
recorded as a
chromatographic peak
page_06.flv
FID Gases
1. Requires three gases
FUEL gas, typically Hydrogen
OXIDISER gas, typically Air
MAKEUP gas, typically Nitrogen or
Helium
2. Gas flow rates are important (e.g.)
Carrier Gas:
1ml/min.
Makeup Gas:
30ml/min.
Hydrogen:
30ml/min.
Air:
300ml/min.
3. Make-up gas required to obtain
optimum flow into detector from GC
column of 20-40 ml/min
page_08.flv
FID Detector Sensitivity (I)
1. Critical Factors Include
a. Combustion gas flow
rates (Gas Stoichiometry)
b. Makeup gas flow rate
too low or too high
c. Flame jet exit diameter
(0.3mm v’s 0.5-0.7mm)
d. Relative positions of jet
and collector (fixed by
manufacturer)
e. Position of the column
relative to the jet exit
(avoid dead volume)
FID Detector Sensitivity (II)
f. Carrier Gas Flow Rate
-Constant Pressure
- Constant Flow
- Ramped Make-up
g. Detector temperature
>150oC
20-30oC> highest oven
temperature
Constant Pressure
Constant Flow
page_11.flv
FID Detector Troubleshooting (I)
Baseline Wander
Baseline Noise
Consider Detector Gas Quality
FID Detector Troubleshooting (II)
Baseline Wander
Baseline Noise
Consider Detector Cleanliness
1. Breakdown of electrical
isolation of collector
(Cathode)
2. Dirt on the collector
insulation
3. Dirt in the collector
inside surface
4. Clean with solvent
(methanol)
5. Clear plugged jet tip
FID Detector Troubleshooting (III)
Baseline Spikes
Issues & Remedies
1.
2.
3.
4.
Incorrect positioning of column in jet (protruding)
Use correct jet tip diameter
Dirt in the collector inside surface
Clean with solvent (methanol)
FID Detector Troubleshooting (IV)
Peak Shape / Broadening
Issues & Remedies
1. Column too low in jet - leading to large dead volume
2. Makeup gas flow rate too low
3. Shouldered peaks may indicate blocked jet tip
FID Detector Performance
Compounds giving little or no FID response
Typical FID Performance
1. Minimum Detectability (LOD) – 10-11 g (~50ppb)
2. Linearity – 107 (very wide dynamic range)
3. Response – organic carbon containing compounds (exceptions
shown above)
The Nitrogen Phosphorous Detector (NPD)
1. Is an Ionising Detector like FID
but uses different principle
2. Shows enhanced sensitivity to
Nitrogen and Phosphorous
containing compounds
3. Main difference is the rubidium
silicate ‘BEAD’ which is
resistively heated to emit
thermionic electrons
4. Hydrogen flow much lower than
in FID detectors – too low to
sustain a flame at the jet tip
5. Plasma of Fuel, Oxidiser, Carrier
and makeup is formed which
partially combusts around the
bead heating filament
page_18.flv
The NPD Operating Principle
5. Partially combusted Nitrogen
and Phosphorous
compounds adsorb on the
bead surface
6. This reduces the beads ‘work
function’ – increasing the
electron density at a given
potential / temperature
7. Increased electron density =
increased current = detector
response
page_20.flv
NPD Operation and Optimisation
1. Run at higher temperatures (260 –
350oC) to extend bead lifetime
2. Run at the lowest acceptable bead
power (start at 2V and increase in
10mV steps if unsure)
3. Hydrogen flow MUST be low
enough to ensure a continuous
flame – otherwise Nitrogen
response will be zero
4. Sensitive to variation in Hydrogen
flow – ensure constant flow supply
5. Performance decreases with time –
requiring an increase in bead
voltage to obtain suitable
sensitivity
NPD Operation and Optimisation
6. Moisture formed during
combustion hydrolyses the alkali
silicate bead to the metal
hydroxide and silica (whitening of
the bead!!)
7. Rubidium hydroxide is volatile and
constantly lost from the bead
8. Conserve bead life by interrupting
hydrogen flow:
• When solvent peak elutes
• During Oven cool down
9. Detector is Mass / Flow sensitive –
recommend constant flow
operation to avoid drifting
baselines
NPD Detector Performance
1. Minimum Detectability
(LOD) – 10-12 g/ml (P),
10-11 g/ml (N)
2. X500 increase in
sensitivity v’s FID for N
and P compounds
3. 500pg of pesticide
easily achievable
4. Linearity 106
5. 400oC temperature limit
6. Requires constant
carrier flow and stable
hydrogen supply
The Electron Capture Detector (ECD)
1. Measures electrical conductivity of
the effluent after exposure to ionising
radiation
2.Especially sensitive to ‘Electron
Capturing’ species – Halogens
3.Radionuclide is 63Ni foil which emits
low energy electrons (Beta particles)
4.These Beta particles collide with the
carrier to produce higher energy
electrons
5.This establishes a high standing
current between detector body (foil)
and a centrally located collector
The Electron Capture Detector (ECD)
6. Applied potential difference ~ 20 –
100V (dc)
7. Halogenated analytes elute and
‘capture’ some of the electron
density
8. Negative ions formed are low energy
and are not collected by the cathode
9. Reduction in the standing current
occurs
page_26.flv
ECD Operation and Optimisation
1. Detector gases should be clean
(99.999%+) and dry as both oxygen and
water are electronegative and contribute
to noisy baselines
2.Nitrogen or Argon (5% Methane) are
used as make up gases and the make up
gas and flow can be critical to sensitivity
3.Better sensitivity and linearity is achieved
using the detector in ‘Pulsed’ mode
4.Square wave pulse applied to maintain a
constant current
5.As analyte elutes pulse frequency is
increased to maintain current
6.Signal is proportional to frequency
ECD Troubleshooting
Baseline Noise
1. Use of impure gases (especially make up gas)
2.Incorrect make up gas flow rate
Linearity / Peak Shape
1.ECD is non-destructive and foil /
collector become contaminated
2.Background current reduces,
but current reduction per
analyte molecule remains
constant
3. Contaminated detector may become MORE SENSITIVE!!
4. Contamination will case decrease in detector linear range
5. Contaminated detectors usually give rise to negative ‘dip’ after each
peak
ECD Detector Performance
1. Minimum Detectability
(LOD) – 10-9 g/ml
(10ppm)
2. Highly selective for
Halogenated analytes
3. Linearity 103 - 104
4. 400oC temperature limit
5. Susceptible to low grade
gases and leaks in the
detector body
6. Extreme caution
required when cleaning
and reconditioning
ECD at 250oC
Make up – Nitrogen at 30ml/min.
20ppb Chloroacetic acid
The Thermal Conductivity Detector (TCD)
1. Simple detector which will
respond to almost any analyte
2.Popular for permanent gas
and inorganic compound
detection
3.Two filaments in separate
blocks with Heated filaments
connected to a tradition
Wheatstone bridge
4.Pure carrier flows over one
cell whilst the column effluent
flows over the other and the
Wheatsone bridge is
‘balanced’ to produce a
baseline signal
The Thermal Conductivity Detector (TCD)
5. Analyte eluting into the
detector will change the
thermal conductivity of the
gas in the analytical cell
6. Rate of heat loss from the
filament changes
7. Current applied to balance
the Wheatstone bridge
8. Size of the cell is critical to
detector sensitivity
page_32.flv
TCD Operation and Optimisation
1. Dual cell detectors not suitable for
capillary GC due to large cell size (140ul)
2.Most manufactures offer a single cell
version (5ul) with ‘switchable’ gas supply
(5-10 Hz)
3.Reference gas flow is switched out when
analyte elutes
4.Filament temperature should be >
detector body temperature
5.Higher filament temperature means
larger thermal conductivity changes and
better sensitivity but shorter filament
lifetime
TCD Operation and Optimisation
6. Susceptible to
variation in detector
body temperature and
carrier gas flow
7.Detector requires
thermal cladding,
constant temperature
supply and carrier
operating in constant
flow mode
8. Sensitivity depends upon the magnitude of difference in
thermal conductivity between carrier and analyte
9. Hydrogen and helium preferred as carrier
10. Area normalisation should not be used for quantitative
measurement using TCD
TCD Detector Performance
1. Minimum Detectability
(LOD) – 10-9 g/ml
(10ppm)
2. Responds to all
compounds
3. Linearity 104
4. 400oC temperature limit
5. Susceptible to ambient
temperature change and
carrier gas flow variation
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