Carrier Gases in Capillary GC Analysis-e-seminar
Transcript of Carrier Gases in Capillary GC Analysis-e-seminar
Carrier Gases in Capillary GC
Abby Folk
Capillary GC
yApplications EngineerFebruary 15, 2011
Configurations for Carrier Gas Purifiers
Indicating Oxygen TrapOxygen Trap
High CapacityOxygen Trap
GCIndicatingMoisture Trap
GC
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CARRIER GASMobile PhaseMobile Phase
Carries the solutes down the column
Selection and velocity influences efficiency and retention time
Must be inert to solutes and stationary phase
Must be free of detectable contaminants
Must have a leak free and very precise pressure delivery system
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delivery system
COMMON CARRIER GASES
NitrogenNitrogen
Helium
Hydrogen
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CARRIER GASPropertiesProperties
Compressible
Expands with temperature
Viscosity increases with temperaturey p
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CARRIER GASCARRIER GASConstant Pressure Mode
Head pressure is constantFlow decreases with temperature
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Flow decreases with temperature
CARRIER GASC t t Fl M dConstant Flow Mode
Carrier gas flow is constantPressure increases with temperature
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Pressure increases with temperature
CARRIER GASConsistent Temperature
Set the velocity at the same temperatureSet the velocity at the same temperature
Initial temperature is the most convenientInitial temperature is the most convenient
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van Deemter Equation
h = Height equivalent to a theoretical plate
Packed Columns
h = A + B + CuPacked Columns
uh A + B + Cu
O T b l C l
h = B + CuOpen-Tubular Columns
A = Multi-path termB = Longitudinal diffusion term
uh B + Cu
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Carrier Gas Selection
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C = Mass transfer term
Packed Columnsvan Deemter Curves
HET
P
HETP
Packed ColumnsHETP = A + B/u + Cu
HETPmin
uA + C
B/u A
Uopt u
B/u A
TP Open-Tubular ColumnH
E
HETP
pHETP = B/u + Cu
HETPmin
U t
HETP
uCB/u
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Uopt u
Carrier Gas Selection
van Deemter Curve
Excessive Diffusion
1.00
0.50
0.75h
Poor Mass Transfer
0.25
0.50
ūoptOPGV
10 20 30 40 50 60
ūopt
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Carrier Gas Selection
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u (cm/sec)
uopt and OPGV
uopt:Maximum efficiency
OPGV:Optimal practical gas velocityOptimal practical gas velocity
Maximum efficiency per unit time
1.5 - 2x uopt
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VAN DEEMTER CURVENitrogen
N2
1.00
0.50
0.75H
0.25
0.50
12-20 cm/sec
10 20 30 40 50 60u (cm/sec)
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u (cm/sec)
VAN DEEMTER CURVEHelium
1.00
He
0.50
0.75H
He
0.25
0.50
22-35 cm/sec
10 20 30 40 50 60u (cm/sec)
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u (cm/sec)
VAN DEEMTER CURVEHydrogenHydrogen
1.00
0.50
0.75H
0.25
0.50H2
35-60 cm/sec
10 20 30 40 50 60u (cm/sec)
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( )
VAN DEEMTER CURVES
N1.00
He
N2
0.50
0.75H
He
0.25
0.50
H2
10 20 30 40 50 60u (cm/sec)
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u (cm/sec)
CARRIER GASFlow Rate (mL/min)( )
"Volume"Volume
Measurement:
At column exitAt column exit
Calculate
Electronic Pressure Control (EPC)
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CARRIER GASFlow Rate (mL/min)( )
“Handy Gizmos” for Flow Measurement:
FID Flow Measuring Insert (p/n 19301-60660)
“Little Red Cap” (p/n 325-0506)Little Red Cap (p/n 325-0506)
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CARRIER GASAverage Linear Velocity (cm/sec)g y ( )
"Speed"
Lu =
Ltmm
L = column length (cm)tm = retention time of non-retained peak (sec)
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FLOW RATE CALCULATION
Provides average flow rate
Fπ r2 L
F = tm
r = column radius (cm)( )L = column length (cm)tm = retention time of a non-retained peak (min)
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NON-RETAINED COMPOUNDS
Detector CompoundDetector CompoundFID Methane, Butane
TCD, MS Methane, Butane, Air
ECD Vinyl chloride, SF6
Methylene Chloride (vapors)*
NPD Acetonitrile (vapors)*NPD Acetonitrile (vapors)
PID, ELCD Vinyl chloride
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* at elevated temperatures
CARRIER GASAverage Linear Velocity CalculationAverage Linear Velocity Calculation
Ltm =u m ____
L l l th ( )L = column length (cm)tm = retention time of non-retained peak (sec)u = desired average linear velocity (cm/sec)
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g y ( )
CARRIER GASCARRIER GASAverage Linear Velocity Calculation
Ltm =
3000 cm
u m
32 cm/sec3000 cmtm = = 93.8 sec = 1.56 min
tm = retention time of non-retained peak (sec)L = 30 meters = 3000 cm
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u = 32 cm/sec
Figuring Carrier Gas Flow Rate – the easy way
USER CONTRIBUTED SOFTWARE
GC Pressure/Flow CalculatorGC Pressure/Flow Calculator Software
http://www.chem.agilent.com/en-US/S /D l d /U ili i /US/Support/Downloads/Utilities/Pages/GcPressureFlow.aspx
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RESOLUTION VS. LINEAR VELOCITYRESOLUTION VS. LINEAR VELOCITYHelium
4 50 3 84 3 364.50 3.84 3.36
R = 1.46 R = 1.31 R = 0.9730 cm/sec 35 cm/sec 40 cm/sec
DB-1, 15 m x 0.32 mm ID, 0.25 um60°C isothermal
4.4 psig 5.1 psig 5.8 psig
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60 C isothermal1,3- and 1,4-Dichlorobenzene
Effect of dimensional tolerances in capillary GC columnscolumns
I understand why RT changes occur with “in use” columns, but why with new columns?, y
– Normal dimensional differences in capillary tubingLengthInner diameter
– Minor differences in film thickness (β)– Variability in phase selectivity (RI)Variability in phase selectivity (RI)
More of an issue with high polarity phases
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Capillary Tubing Dimensional Tolerances
Capillary columns manufactured by Agilent Technologies have a general dimensional specification of:specification of:
– Length within about 0.5 meter (≈ 1 “loop”)– ID ± 6 μmμ
Variation in internal diameter is a normal distribution around nominal. Approximating the range (12 m) as 6 times the standardthe range (12 μm) as 6 times the standard deviation, there is a 95.5% probability that tubing will be within ± 4 μm.
Let’s look at the effects of these actual tubing dimensions on the pressures required to
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maintain retention times for a method…
Relationship of Tubing Dimensions and Pressure:30m x 0.25mm ID column
L (m) ID (μm) P (psi) % of nominal P29.5 255 8.2 82%29.5 255 8.2 82%30.0 250 10.0 100%30.5 245 11.9 119%Conditions: vacuum outlet (MSD), helium carrier,
100°C oven temperture,maintained Tm at 1.38 min
L (m) ID (μm) P (psi) % of nominal PL (m) ID (μm) P (psi) % of nominal P29.5 255 9.3 93%30.0 250 10.0 100%30.5 245 10.9 109%Conditions: atmospheric outlet (FID), helium carrier,
100°C oven temperture,maintained Tm at 2.60 min
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Calculated using Column Pressure/Flow Calculator available from Agilent (www.agilent.com/chem)
Relationship of Tubing Dimensions and Pressure:12m x 0.20mm ID column
L (m) ID (μm) P (psi) % of nominal P11.5 205 6.9 69%12.0 200 10.0 100%12.5 195 13.5 135%Conditions: vacuum outlet (MSD), helium carrier,
100°C oven temperture100 C oven temperture,maintained Tm at 0.345 min
L (m) ID (μm) P (psi) % of nominal PL (m) ID (μm) P (psi) % of nominal P11.5 205 8.7 87%12.0 200 10.0 100%12.5 195 11.5 115%Conditions: atmospheric outlet (FID), helium carrier,
100°C oven temperture,maintained Tm at 0.651 min
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Calculated using Column Pressure/Flow Calculator available from Agilent (www.agilent.com/chem)
Comparison of Influence of Length and I.D. onComparison of Influence of Length and I.D. on Required Head pressure (.25 mm, MSD)
14
16
re (p
si)
I.D.length
10
12
ad P
ress
u lengthExtremes
81 2 3
Hea
“Extremes” denotes combination of smallest ID/longest l d i ( 1 l th 6 ID)
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column and vice versa. (±1 m length; ±6 µm ID)
Comparison of Influence of Length and I D onComparison of Influence of Length and I.D. on Required Head Pressure (.2 mm, MSD)
121416
re (p
si)
I.D.
68
10
d pr
essu
r
length
Extremes
24
1 2 3
head
“Extremes” denotes combination of smallest ID/longest l d i ( 1 l th 6 ID)
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column and vice versa. (±1 m length; ±6 µm ID)
Impact of Dimensional Differences on Required PRequired P
I t i i l ith l th d IDImpact varies inversely with length and ID- The relative percentage of impact increases as
nominal length and ID decrease.nominal length and ID decrease.
Additionally, vacuum outlet (MSD) greatly exaggerates pressure drop across theexaggerates pressure drop across the tubing. This in turn amplifies the differences in head pressure required to maintain Tm
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“I don’t use retention time windows. Why should I care if my retention times shift?”care if my retention times shift?
Retention time changes in temperature programmed analyses can also alter the
l ti f l telution sequence of solutes…
Solutes elute in an order mandated by their “net” vapor pressures - i.e., vapor
d th ipressures under their gas chromatographic conditions.
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Impact of Dimensional Differences on RNot only may absolute retention times change, but resolution may change as well due to changes in carrier gas linear velocity (efficiency; HETP)
MSD
MSD
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Net Vapor PressureThe net vapor pressure is the intrinsic vapor pressure reduced by the sum of all solute-stationary phase interactions(e g dispersion proton sharing and dipole interactions all of(e.g., dispersion, proton sharing and dipole interactions, all of which are influenced by temperature).
If considerations for the inter-effects of length, diameter and g ,gas velocity are not factored into the method’s Standard Operating Procedure (SOP) the end result can be:
• Loss of resolution• Complete reversal of elution
More common in mixtures of disparate functionalities; it does not occur– More common in mixtures of disparate functionalities; it does not occur with homologues.
– Also more common with solutes and stationary phases employing multiple modes of solute-stationary phase interactions
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multiple modes of solute stationary phase interactions.
Example -- Compounds of InterestExample -- Compounds of Interest
d Camphor Linalool Linalyl acetated-Camphor, b.p 176-180°Cketo functionWeak H-Bonding, Van der Waals
Linaloolb.p. 199°Chydroxy and alkene functionsStrong H-Bonding, weak dipole,Van der Waals
Linalyl acetateb.p. 220°Cacid ester and alkene functions2 x Weak H-BondingVan der Waals Van der Waals 2 x Weak H-Bonding,weak dipole, Van der Waals
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Effects of Head Pressure on Elution Order: 26 psi
N FID1 A, (072701A\005B0801.D)
6 6
linaloolLinalyl acetateColumn: 20m x 0.10mm ID x 0.2um, DB-WAX
Oven: 50C (0.33 min), 10C/min to 200C and hold
Norm.
100
120
12.
456
12.
706
Lavender Oil, Spiked, 26 psi Constant Pressure
80
4
camphor
40
60 12.
534
Camphre, 12.534 min
20
10.
029
10.
642
11.
038
11.
152
11.
566
1302
3
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min9 9.5 10 10.5 11 11.5 12 12.5
Effects of Head Pressure on Elution Order: 30 psi
Norm FID1 A, (072701A\005B0601.D)
4 7
Column: 20m x 0.10mm ID x 0.2um, DB-WAX
Oven: 50C (0.33 min), 10C/min to 200C and hold
Norm.
100
120
11.
954
12.
187
Lavender Oil, Spiked, 30 psi Constant Pressure
80
40
60
12.
782 1
2.91
6
Camphre, co-elutes with Linalool
20
9.5
43
10.
143
10.
550
10.
637
11.
045
12.
481
12.
568
12.
694
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min9 9.5 10 10.5 11 11.5 12 12.5
Effects of Head Pressure on Elution Order: 38 psi
FID1 A, (072701A\005B0501.D)
7 0
Column: 20m x 0.10mm ID x 0.2um, DB-WAX
Oven: 50C (0.33 min), 10C/min to 200C and hold
Norm.
100
120
11.
187
11.
400
.182Lavender Oil, Spiked, 38 psi, Constant Pressure
80 11.
096
086
12
Camhpre, 11.096 min
40
60
11.
986
12.
0
12.
750
20
9.3
77 9.8
03 9
.845
10.
248
11.
655
11.
733
11.
861
12.
332
12.
465
12.
781 1
2.90
4
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min9 9.5 10 10.5 11 11.5 12 12.5
Temperature Programming
COLUMN A
Effect Of Locking Software On Temperature Of Elution
Inner diameter = 0.240 mm, Head Pressure 9.70 psi
COLUMN B
Tm 1.52 min
COLUMN BInner diameter = 0.260 mm, Head Pressure 9.70 psi
f
Tm 1.29 min
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Temperature Profile
Temperature Programming Eff t Of L ki S ft O T t Of El ti
COLUMN A
Effect Of Locking Software On Temperature Of Elution
Inner diameter = 0.240 mm, Head Pressure 9.70 psi
Tm 1.52 min
COLUMN BCOLUMN BInner diameter = 0.260 mm, Head Pressure 5.97 psi
T t P filTm 1.52 min
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Temperature Profile
CARRIER GASCARRIER GASHydrogen Comments
Hydrogen is extremely diffusive in airy g y
Difficult to reach explosive level of ~4 %p
Many GC's are flow regulatedy g
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VAN DEEMTER CURVES
N1.00
He
N2
0.50
0.75H
He
0.25
0.50
H2
10 20 30 40 50 60u (cm/sec)
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u (cm/sec)
CARRIER GASCARRIER GASSelection Example
6 2 min
Nitrogen11.7 cm/sec
Nitrogen20 cm/sec
Helium23.2 cm/sec
Hydrogen48 cm/sec
24.5 min 17.4 min13.0 min 6.2 min
R = 1.76 R = 1.33 R = 1.67 R = 1.65
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Carrier GasHelium vs HydrogenHelium vs. Hydrogen
Helium (35 cm/sec) Hydrogen (73 cm/sec)1 3
21
4
5
6
78
9
2
3
4 6 7 89
59
DB-1, 15 m x 0.25 mm I.D., 0.25 µm
0 2 4 6 8 10 12Time (min.)
0 2 4 6 8Time (min.)
10.5 min 7.8 min
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50°C for 2 min, 50-110°C at 20°/min
Hydrogen as Carrier with MSD – Contamination?
Contamination of GC flow modules and lines.
Hydrogen acts as a scrubber.
On the MS this typically looks like Hydrocarbon yp y ycontamination
Most people report that it takes 2-4 weeks to clean out, depending on flowsdepending on flows.
The FID only sees a high background for that time.
Hydrogen seems to scrub the lines of that which Helium leaves behind.
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Carrier Gas Selection
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Carrier Gas - Summary
Gas Advantages DisadvantagesNitrogen Cheap Readily available Long run timesNitrogen Cheap, Readily available Long run timesHelium Good compromise, Safe ExpensiveHydrogen Shorter run times Cheap ExplosiveHydrogen Shorter run times, Cheap Explosive
Hydrogen is difficult to explode under GC conditions
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Hydrogen is difficult to explode under GC conditions
CARRIER GASSelection SummarySelection Summary
Hydrogen is best especially for wide k range analyses
Helium is acceptable
Nitrogen is not recommended
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* Select option 3 option 3 then option 1* Select option 3, option 3, then option 1.
866-422-5571 (fax)
www.agilent.com/chem
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