1. carrier sample
gas introduction heated
zones
column
detector data
system
b c
d
a - compressed gas cylinder
b - pressure regulator
a
c - valve
d - filter
2. Purpose of port is to flash evaporate your sample
septum and introduce it into the column.
carrier in TINJ > 50oC above Tcolumn
heat source Injection is through a septum.
liner Septum must be
stable at the Tinj
column
replaced regularly to maintain seal
Syringes are used to introduce a known volume
of a liquid or gas samples .
Adapters can be used to help control the volume
injected.
Various styles are available
Fixed needle When filling, you must
Removable needle insure that there is no
Several needle lengths and angles air in the syringe.
Sample volumes from < 1 µl an up
Body loading
Through the barrel plungers
3. Samples must be injected
rapidly so that your sample
is introduced as a small ‘plug.’
By pulling back on the syringe
after injection, you can measure
how much sample remains in
the needle.
Introducing a constant amount of a gas can
sample sample
be difficult with a syringe.
vent vent
Gas sampling loops and valves offer a
high precision (+/- 0.1%) means of
introducing gases.
column
Equipment is relative inexpensive and only
requires a constant temperature for easy
carrier carrier
use.
• Heart of the separation process.
• Vast number of materials have been
evaluated.
• It is usually best to refer to various
catalogs as an up to date reference.
• Can be classified by tubing diameter and
packing type.
4. Columns Packed open (capillary)
Packed Open tubular
(capillary)
porous non-porous
packing packing
Packed with coated with
porous layer porous layer
bead column Porous Wall
liquid bound Layer Coated
coated phase porous layer Open Open
packed liquid coated Tube Tube
capillary wall conventional
Packed Capillary Because peaks remain narrower, the sensitivity
length, M 0.5 - 5 5 - 100 is improved.
ID, mm 2-4 0.1 - 0.7 Packed
Both peaks have an area
flow, ml/min 10 - 60 0.5 - 15 of 5000 units.
head pressure, psig 10 - 40 3 - 40
Because the capillary
total plates 4000 250,000 Capillary peak is higher, you get
capacity 10µg/peak 100ng/peak a better S/N.
film thickness, µm 1 - 10 0.1 - 8
Unless you’re developing new packing materials
or methods, the best starting point is to consult
a chromatographic catalog.
They provide a wealth of information regarding
cost, temperature limits, sample applications.
Another factor to consider, you must use the
proper column called for by the ‘standard’
method (Ex. A specific EPA method.)
5. With homologues, the retention time increases
exponentially with the number of carbon.
As tR increases, width increases and the height
decreases, making detection impossible after a
few peaks have eluted.
Since solubility of a gas in a liquid decreases as
temperature goes up, we can reduce the
retention of a material by increasing Tcolumn.
isothermal
programmed
temperature
(c) General steps to create a program assuming that
the separation is possible.
(b) 1. Determine initial temperature and time based
(a) on best possible separation of first first few
peaks.
a - initial temperature and time
b - ramp (oC/min) 2. Repeat 1 for the last few peaks to find the best
c - final hold time and temperature final temperature and time.
Some GCs will allow for a more complex program. 3. Experiment with various ramps to account for
the rest of the components.
6. We need a way to measure our eluents as Each can be roughly classified based on
they evolve from the column.
Destructive vs. nondestructive
Virtually every method of directly or
General vs. some discrimination
indirectly observing eluents as been
vs. very discriminating
looked at.
Let’s start by reviewing some general concepts
We’ll cover some of the more common such as detection limit and sensitivity.
types.
AED
High sensitivity - possible selectivity
TCD
Rapidly respond to concentration changes FID
ECD
Large linear range
NPD (N)
Stable with respect to noise and drift NPD (P)
FPD (S)
Low sensitivity to variations in flow, FPD (P)
pressure and temperature FTIR
Produces an easily handled signal MS (SIM) MS (Scan)
10-15 g 10-12 g 10-9 g 10-6 g 10-3 g
General purpose Thermal conductivity
Nondestructive Species 105 cal/cm sec oC
Limit of detection ~ 400 pg/ml carrier hydrogen 49.93
Linear range ~ 106 helium 39.85
nitrogen 7.18
Mode of detection ethane 7.67
Change in resistance of a wire based on water 5.51
variations in the thermoconductivity of the gas benzene 4.14
evolving from a column. acetone 3.96
chloroform 2.33
7. carrier
While hydrogen has the largest TC value, reference zero
helium is commonly used - less reactive. carrier +
control
sample
Hydrogen will give a negative peak when
helium is the carrier gas. power
supply
Peak response is a function the the TC
value for a species so you must
standardize for each eluent of interest. detector block recorder
Dual channel detectors require both an
analytical column and a blank column.
- accounts for response changes due to
- variations in temperature
- column bleed
Single channel TCD systems are available
that correct for temperature variations.
Specific - sample must be combustible
Destructive Sample components
Limit of detection ~ 5 pg carbon / second enter at the base of
Linear range ~ 107 the detector. They
Mode of detection mix with hydrogen
and enter the flame.
Production of ions in a flame result in a current
that can be measured.
Ions are produced
A make-up gas may be required to maintain an that can be measured.
optimum flow - capillary columns
8. collector igniter Compounds with little or no FID response
air noble gases NH3 CS2
Nox CO O2
burner jet H2O CO2 N2
hydrogen
A make-up gas may perhalogenated compounds
also be present if formic acid
capillary columns
formaldehyde
are to be used.
End of column
Response is based on the number of carbon Specific - sample must contain a gas phase
and if other elements like halogens or electrophore
oxygen are present which reduce Non-destructive
combustion.
Limit of detection ~ 0.1 pg Cl / second
Linear range ~ 104
methane
ethane
butane
pentanoic acid
propane
butanoic acid
formic acid
acetic acid
propionic acid
Mode of detection
Absorption of ! particles by species containing
halogens, nitriles, nitrates, conjugated double
bonds, organometallics.
!- are emitted by
an 63Ni source. anode purge
vent
Electrophores anode
will absorb !- , 63Ni plated surface
reducing the
current.
This is the basis
makeup gas
for the response
column
