1
Carrier Gas Selection
Page 1
Carrier Gas Considerations and Utilizing the EPC
Carrier Gas Selection
Page 2
Column
FlowController
Regulators
Air
Hyd
rog
en
Ca
rrie
r G
as
Mol-SieveTraps
Fixed
InjectionPort Detector Electrometer
Recorder/Integrator
Restrictors
Cylinders or Generators
Typical Gas Chromatographic System
Selection of type and velocity
influences efficiency and retention time
2
Carrier Gas Selection
Carrier Gas
Affects resolution and retention time
Optimal range of velocities
Too low or high results in loss of resolution
Carrier Gas Selection
Page 4
A = Multi-path termB = Longitudinal diffusion termC = Mass transfer term
uh = A + B + Cu
h = Height equivalent to a theoretical plate
uh = B + Cu
Packed Columns
Open-Tubular Columns
van Deemter Equation
3
HE
TP
HETPmin
Uopt u
HETP
uA + C
B/u A
HE
TP
HETPmin
Uopt u
HETP
uCB/u
Packed ColumnsHETP = A + B/u + Cu
Open-Tubular ColumnHETP = B/u + Cu
van Deemter Curves
Carrier Gas Selection
Carrier Gas Selection
Page 6
van Deemter Curve
10 20 30 40 50 60
0.25
0.50
0.75
1.00
u (cm/sec)
h
ūoptOPGV
Excessive Diffusion
Poor Mass Transfer
4
Carrier Gas Selection
Page 7
uopt and OPGV
uopt: Optimum gas velocity (slowest velocity!)
Maximum efficiency
OPGV: Optimum practical gas velocity Maximum efficiency per unit time
(1.5 to 2) X uopt
Best u = Balance resolution and analysis time
Carrier Gas Selection
Page 8
Nitrogen = 0.15 cm2/sec
Helium = 0.4 cm2/sec
Hydrogen = 0.6 cm2/sec
© Walter Jennings, 1999
Diffusion Constants for Dodecane @ 150oc
5
Carrier Gas Selection
Page 9
10 20 30 40 50 60
0.25
0.50
0.75
1.00
u (cm/sec)
h
12-20 cm/sec
N 2
van Deemter CurveNitrogen
Carrier Gas Selection
Page 10
10 20 30 40 50 60
0.25
0.50
0.75
1.00
u (cm/sec)
h
He
28-40cm/sec
van Deemter Curve Helium
6
Carrier Gas Selection
Page 11
10 20 30 40 50 60
0.25
0.50
0.75
1.00
u (cm/sec)
h H2
38-60 cm/sec
van Deemter Curve Hydrogen
Carrier Gas Selection
Page 12
Carrier Gas - Hydrogen Comments
Hydrogen is extremely diffusive in air
Difficult to reach explosive level of ~4 %
Most GC's flow regulated with safety shutdown
Spring loaded/Explosion ready doors
7
Hydrogen as Carrier – Contamination?
Carrier Gas Selection
Page 13
Contamination of GC flow modules and lines.
Hydrogen acts as a scrubber.
On the MS this typically looks like Hydrocarbon contamination
Most people report that it takes 2-4 weeks to clean out, depending on flows.
The FID only sees a high background for that time.
Hydrogen seems to scrub the lines of that which Helium leaves behind.
Page 14
Contamination?! – scrubbing or vapor volume
Carrier Gas Selection
8
Carrier Gas Selection
Page 15
Hydrogen as Carrier – MS and Lower Pressures
Hydrogen is more difficult for the MS vacuum system to pump away as compared to Helium.
Be careful not to get to a “negative” head pressure situation. (pulling instead of pushing flow is not good).
Trace level work on the MS requires low flow rates.
Must use smaller ID columns so you don’t have higher flow rates. (length plays a role as well)
0.18mm ID or smaller is ideal for trace level MS work when using Hydrogen.
Page 16
Fast DHA Analysis – 33% Faster with Hydrogen
min0 50 100
min.0 50 100
100 m x 0.25 mm, 0.5µm HP-1helium
100 m x 0.25 mm, 0.5µm HP-1hydrogen
Carrier Gas Selection
9
Page 17
Fast DHA Analysis – Resolution Check
min.0 50 100
100 m x 0.25 mm, 0.5µm HP-1helium
0 50 100
100 m x 0.25 mm, 0.5µm HP-1hydrogen
Carrier Gas Selection
Page 18
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
21.0
22.0
23.0
24.0
25.0
26.0
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0
1
2
3
4 5
6
7
8
910 11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28a
28b
28c
29a 29b
29C
30
Helium as Carrier Gas
InjectorTemperature 225 CPressure 16.2 psiSplit Ratio 25Split Flow 37Total Flow 42.2Flow 1.5 mL/minMode Constant Flow
OvenC/min Temp. (C) Time (min.)
40 3.8413.01 200 1.87
DetectorFIDHydrogen Flow40 mL/minAir Flow 400 mL/minMkup 20 mL/min
1. Methanol2. Pentane3. Ethanol4. Diethylether5. Acetone6. Isopropanol7. Acetonitrile8. Methylenechloride9. t-Butanol10. Methyltertbutylether11. Hexane12. n-Propylalcohol13. Ethylacetate14. Tetrahydrofuran15. Cyclohexane16. Isobutylalcohol17. Isopropylacetate18. Heptane19. n-Butanol20. Methylcyclohexane21. 1,4-Dioxane22. Methylisobutylketone23. Toluene24. Isobutylacetate25. Ethyleneglycol26. n-Butylacetate27. Dimethylformamide28a,b,c. Xylenes29a,b,c. Dimethylacetamide30. N-Methylpyrilidone
DB-624 30m x 0.25mm x 1.4µmSolvents in Drugs
Carrier Gas Selection
10
Page 19
Solvents in Drugs
Carrier Gas Selection
Page 20
20.0
21.0
22.0
23.0
24.0
25.0
26.0
27.0
28.0
29.0
30.0
31.0
32.0
33.0
34.0
35.0
36.0
0.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
21.0
22.0
23.0
24.0
25.0
26.0
0.0
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 11.0 12.0 13.0 14.0 15.0 16.0 17.0
Helium to Hydrogen saves an additional 5 minutes (33%)
Carrier Gas Selection
11
Faster GC Analyses
Page 21
20.0
21.0
22.0
23.0
24.0
25.0
26.0
27.0
28.0
29.0
30.0
31.0
32.0
33.0
34.0
35.0
36.0
0.0
10.0
11.0
12.0
13.0
14.0
15.0
16.0
17.0
18.0
19.0
20.0
21.0
22.0
23.0
24.0
25.0
26.0
0.0
10 min
15 min
Resolution Check
Volatile Chlorinated Solvents on a Megabore
Carrier Gas Selection
12
Min
ute
s
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
Volt
0.0
0
0.0
1
0.0
2
0.0
3
0.0
4
0.0
5
0.0
6
0.0
7
0.0
8
0.0
9
Volt
0.0
0
0.0
1
0.0
2
0.0
3
0.0
4
0.0
5
0.0
6
0.0
7
0.0
8
0.0
9
Methyl Chloride 59172
Vinyl Chloride 85885
Vinylidene Chloride 88170
Methylene Chloride 35179
trans 1,2-Dichloroethylene 76057
A-Di 85299
cis 1,2-Dichloroethylene 77970
Chloroform 32298
Carbon Tetrachloride 29443
Ethylene Dichloride 74714
Trichloroethylene 70046
1,1,2-Trichloroethane (B-tri) 51838
Perchloroethylene (1,1,2,2) 53142
Tetrachloroethylene 47308
Are
a
Min
ute
s
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
10
.0
Volt
0.0
0
0.0
1
0.0
2
0.0
3
0.0
4
0.0
5
0.0
6
Volt
0.0
0
0.0
1
0.0
2
0.0
3
0.0
4
0.0
5
0.0
6
Methyl Chloride 60921Vinyl Chloride 84781
Vinylidene Chloride 85285
Methylene Chloride 36048
trans 1,2-Dichloroethylene 77501
A-Di 86539
cis 1,2-Dichloroethylene 80678
Chloroform 27134
Carbon Tetrachloride 20238
Ethylene Dichloride 78755
Trichlorethylene 74357
1,1,2-Trichloroethane (B-tri) 61556
Perchloroethylene (1,1,2,2) 60576
Tetrachloroethylene 52461
He
lium
Ca
rrier/H
eliu
m M
ake
-up
Hyd
roge
n C
arrie
r/Nitro
ge
n M
ake
-up
Carrie
r Gas S
ele
ctio
n
Page 2
4
Meg
ab
ore
with
Hyd
rog
en
Carrie
r -
co
uld
giv
e a
baselin
e ris
e a
s th
e to
tal H
2 in
cre
ases
150C
Carrie
r Gas S
ele
ctio
n
13
Page 25
Isothermal column bleed and Hydrogen increase
Carrier Gas Selection
Page 26
Temperature program column bleed with increasing Hydrogen
Carrier Gas Selection
14
Oth
er C
hlo
rinate
d S
olv
en
ts o
n a
Meg
ab
ore
Carrie
r Gas S
ele
ctio
n
23
45
67
89
10
0 20 40
D M E M e t h y l C h lo r id eV in y l C h lo r id eM e t h a n o l
E t h y l C h lo r id e
V in y l id e n e C h lo r id e
M e t h y le n e C h lo r id e
T ra n s
A -d i
C is
M 3a -T r i
M 4E D C
T c eP D C
(U n n a m e d P e a k s )
B -T r iP C E
M C B (M o n o c h lo ro b e n z e n e )1 , 1 , 1 , 2 - U n s y m . T e t ra s
manhead new
.met4/7/2008 3:09:53 PM
Name2
34
56
78
910
1112
2 . 2 0 02 . 2 7 02 . 6 5 02 . 7 1 72 . 8 6 7
3 . 1 2 3
4 . 4 8 74 . 6 0 34 . 9 7 0
5 . 1 3 35 . 5 2 7
6 . 1 0 3
6 . 9 0 7 7 . 3 3 0
7 . 6 1 77 . 8 5 7
8 . 1 3 7
9 . 0 0 39 . 3 0 0
1 0 . 9 3 31 1 . 0 9 71 1 . 1 7 7
1 1 . 9 0 31 1 . 9 6 71 1 . 9 9 31 1 . 9 9 3H
eliu
m 1
0 m
L/m
in
Hyd
roge
n 1
2.5
mL
/min
Carrie
r Gas S
ele
ctio
n
15
Page 29
DB-1, 60m x 0.32mm x 0.25um
Helium
Fast Analysis/Hydrogen
Translate to Hydrogen
Steam Cracked Naphtha
Carrier Gas Selection
Page 30
Steam Cracked Naphtha
Carrier Gas Selection
16
Carrier Gas Selection
Page 31
10 20 30 40 50 60
0.25
0.50
0.75
1.00
u (cm/sec)
h
He
N2
H2
van Deemter Curves
Carrier Gas Selection
Carrier Gas Comparison
Hydrogen is difficult to explode under GC conditions
Gas Advantages Disadvantages
Nitrogen Cheap, Readily available Long run times
Helium Good compromise, Safe Expensive
Hydrogen Shorter run times, Cheap Explosive
17
Carrier Gas Selection
Page 33
Carrier Gas - Selection Summary
Hydrogen is best especially for wide k range analyses
Helium is acceptable
Nitrogen is not recommended
Carrier Gas Selection
Page 34
Carrier GasProperties
Expands to fill the space it occupies
Expands with increase in temperature
Viscosity increases with temperature
Compressible
THEREFORE:Flow increases as it goes down the length of the column
AND At Constant Pressure:
T u
18
Page 35
EPC To The Rescue!
Carrier Gas Selection
Page 36
BENEFITS OF EPC
Reliability and Reproducibility of set-points
(ambient temperature and pressure compensation)
Choice of constant flow versus constant pressure
Ability to Pulse flow during injection to buffer expansion and increase speed of transfer and amount on column
Flow ramping available to reduce time at the end of the analysis or during Bake Out
Carrier Gas Selection
19
Page 37
EPC CAUTIONSThings to be aware of
EPC doesn’t measure flow or velocity, they are calculated, THEREFORE…
Dimensions of the column, length and especially internal diameter, must be entered precisely or an errant velocity will result.
Carrier Gas Selection
Page 38
F =16 η L
(pi2 - po
2)
po
π r 4
Volumetric flow is a quadratic to the fourth power function of radius
Linear velocity is a squared function of radius
Length has less of an impact
The Poiseuille Equation
Affects Of Column Radius And Length
Carrier Gas Selection
20
Page 39
Pulsed Splitless- sample containment more critical than in split injection- much sharper peaks than in traditional splitless injection
Pulsed Split- the most volatile components and solvent effected most- faster sample transfer not as critical since it’s already fast
EPC for Split/Splitless Pulsed Injection
Pressure Pulse contains sample expansion and transfers analytes to the column faster.
Carrier Gas Selection
Page 40
Splitless Injection
Column Flow1 mL/min
(Septum) PurgeVent Flow3 mL/min
Sample Injection
Split Vent Flow0 mL/min
Total Flow4 mL/min
Syringe Needle
Sample
Carrier Gas Selection
21
Page 41
Splitless Injection
Injection Overload = Backflash
Column Flow1 mL/min
Split Vent Flow
(Septum) Purge
Column Flow1 mL/min
Split Vent Flow
(Septum) PurgeVent Flow
3 mL/minTotal Flow
1 mL/min
Split Vent Flow
0 mL/min
(Septum) PurgeVent Flow
3 mL/minTotal Flow
4 mL/min
= Solvent Vapor
= Analyte Vapor
Carrier Gas Selection
Page 42
10
20
30
40
50
60
70
80
90
50 100 150 200 250 300 350 400
Injection Port
Set Point Temperature
350°C
35°C
Oven150°C
Oven300°C
Oven
Temperature in Gas Stream (°C)
Bottom
of Septum
Syringe
Tip
Base of
Injection
Port
Temperature Profile of a Typical Vaporization Injector vs Oven Temperature
Carrier Gas Selection
22
Page 43
Solvent Expansion Equation
Solvent Vapor Volume = 22,400 x A x B x C x I
A = solvent density/solvent molecular weight
B = 15/(15 + column headpressure [in psi])
C = (Injection Port Temperature[°C] + 273)/273
I = liquid injection volume [µL]
Example
1 µL of water injected at 250 °C under 15 psi headpressure
22,400 x 1/18 x 15/30 x 523/273 x 1 = 1,192 µL vapor
Carrier Gas Selection
Page 44
Pressure Flow Calculator &
Solvent Vapor Volume Calculator
Carrier Gas Selection
23
Page 45
Pressure Flow Calculator
Carrier Gas Selection
Page 46
Solvent Vapor Volume Calculator
Carrier Gas Selection
24
Minimizing Backflash
Large volume liner
Small injection volume
Low expansion solvent
Low injector temperature
High carrier gas flow rates (EPC?)
High head pressures (EPC?)
Carrier Gas Selection
Page 48
Benefits of the Pulsed Splitless Mode
20
30
40
50
60
70
80
90
100
110
120
70 psi
pulsed splitlesson column
22.5psi
pulsed splitless
Injection Type
% R
eco
very
(o
n c
olu
mn
=100)
Methamidophos
Acephate
Azobenzene
Omethoate
Diazinon
Dimethoate
Chlorpyrifos
High Column Flow
Normal Column Flow
% Recovery of Each Labile Pesticide Relative to Cool On-Column injection
Carrier Gas Selection
25
Select Pulsed Splitless Mode in Inlets
Carrier Gas Selection
Page 49
Check the Splitless Pressure
Carrier Gas Selection
Page 50
26
Double or Triple the Pressure for ~1 sec less than the Purge Activation Time
Carrier Gas Selection
Page 51
Page 52
Flow Ramping for Late Eluter with EPC
6.00 8.00 10.00 12.00 14.00 16.00
GC3-6707.D\ECD2B
With Flow Ramp
No Flow Ramp
Carrier Gas Selection
27
Page 53
EPC Flow Ramping forLate Eluters and Faster Bake Outs
Carrier Gas Selection
Using Method Translator for Calculating Pressure Needed for a Given Flow at a Given Temperature
Carrier Gas Selection
Page 54
28
Using Chemstation for Calculating Pressure Needed for a Given Flow at a Given Temperature
Carrier Gas Selection
Page 55
Conclusions
Carrier Gas Selection
Page 56
Carrier Gases have an optimum range of velocities -Too low or high results in loss of resolution
Hydrogen is the best carrier gas for high velocity work -Can be used safely in 6890/7890-Lighter molecule that Helium-Lower pressure considerations
EPC gives the ability to change flow reproducibly -Pulsed Injection (efficiency, backflash)-Flow Programming (bake out, late eluters)