Converting Helium
Carrier Gas GC
Methods Nitrogen and
Hydrogen
1
Jim McCurry
Agilent Technologies
Wilmington, DE 18951 USA
October 12, 2012
Market Situation
 The world of He supply is not reliable, prices are increasing and
customers are seeking alternative carrier gases
Agilent Restricted
Page
2
2
October 12, 2012
The Carrier Gas Decision
No
Is the chemist willing to convert
to alternative gasses?
GC
Yes
Is the Application based on
GC or GC/MS?
Does the current GC method
have too much resolution?
GC/MS
No
Yes
He Conservation
Consider migration to N2
Consider migration to H2
GC/MS specific H2
considerations
Agilent Restricted
Page
3
3
October 12, 2012
Migration to H2: Considerations for GC & GC/MS
No
Is the chemist willing to convert
to alternative gasses?
GC
Yes
Is the Application based on
GC or GC/MS?
Does the current GC method
have too much resolution?
GC/MS
No
Yes
He Conservation
Consider migration to N2
Consider migration to H2
GC/MS specific H2
considerations
Agilent Restricted
Page
4
4
October 12, 2012
H2: Considerations for GC & GC/MS Application
H2 safety
Sources of hydrogen
Plumbing modifications
Chromatographic method migration
Method revalidation
Agilent Restricted
Page
5
5
October 12, 2012
H2 Safety
GC, GC/MS: Both offer H2 enabled features
• Agilent H2 safety letter and safety manuals available
• GC four levels of safety design
•
Safety Shutdown
•
Flow Limiting Frit
•
Oven ON/OFF Sequence
•
Explosion Test
• Newer version 6890, 7890 GC and 5773, 5975 MSD offer greater safety
than older versions of GC and GC/MSD.
Agilent Restricted
Page
6
6
October 12, 2012
Source of Hydrogen
H2 Generator – preferred
• Very clean H2, >99.9999% available
• More consistent purity
• Built-in safety considerations
• Make sure to buy a good one with a low spec for water and oxygen
• Parker’s H-MD are used in LFS and SCS
H2 Cylinder
• Consider gas clean filter
• Possible to add safety device
Agilent Restricted
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7
7
October 12, 2012
H2: Plumbing Modification Considerations
Tubing:
• Use chromatographic quality stainless steel tubing
• Do not use old tuning (H2 is known as scrubbing agent)
• Especially don’t use old copper tubing (brittleness is a safety concern)
Venting:
• Connect split vent and septum purge vent to exhaust
Leak checking:
• Recommend G3388B leak detector
Agilent Restricted
Page
8
8
October 12, 2012
Migration to N2 for GC Application
No
Is the chemist willing to convert
to alternative gasses?
GC
Yes
Is the Application based on
GC or GC/MS?
Does the current GC method
have too much resolution?
GC/MS
No
Yes
He Conservation
Consider migration to N2
Consider migration to H2
GC/MS specific H2
considerations
Agilent Restricted
Page
9
9
October 12, 2012
Use N2 as carrier gas
Many HPI methods suited to nitrogen
•
•
•
•
Readily available and less expensive gas
No safety concern
Suitable for simple routine analysis (w. enough resolution)
2-D GC ideally suited to nitrogen
– Resolution issues solved by using 2 different columns
Potential issues
•
•
Reduced separation resolution
Not suitable for GC/MSD and certain GC detector applications
Agilent Restricted
Page
10
10
October 12, 2012
Helium Carrier Gas Alternatives
• Hydrogen, nitrogen, argon, argon/methane
• Detector considerations can influence choice
• Hydrogen and nitrogen are the most common alternatives
11
October 12, 2012
Helium Carrier Gas Alternatives
Important Theoretical Considerations Relating To Peak Efficiency
Efficiency & Carrier Gas Linear Velocity
Calculating Efficiency
We would like to know the actual time the
component spends in the stationary
phase.
Start
t ' = t - t
R
R
m
Inert Gas
tm
t'R
Time
n=
5.545
tR = Corrected Retention Time.
'
t ' 2
R
Wh
MOLECULAR
DIFFUSION
tR
Let's relate “n” to the length of the
column.
B
HETP = A µ
+
The minimum of the curve
represents the smallest HETP (or
largest plates per meter) and thus
the best efficiency. "A" term is not
present for capillary columns.
B{
}C
RESISTANCE TO MASS TRANSFER
}A
EDDY DIFFUSION
n = effective theoretical
plates.
n
Plates per meter (N) =
or
L
L
Height equivalent to a theoretical plate (HETP) =
n
Thus, the more efficient the column, the bigger the "N" the smaller the
"HETP“.
•
•
12
+ Cµ
HETP
Solute
Efficiency is a function of the
carrier gas linear velocity or flow
rate.
µ ( opt
µ)
Plot of HETP versus linear velocity is know as the Van Deemter
plot.
The linear velocity value at the minimum of the curve is the optimum
value for achieving the best efficiency.
Helium Carrier Gas Alternatives
Let’s Make This Easy
Efficiency & Carrier Gas Linear Velocity
Calculating Efficiency
We would like to know the actual time the
component spends in the stationary
phase.
Start
t ' = t - t
R
R
m
Inert Gas
tm
t'R
Time
n=
5.545
tR = Corrected Retention Time.
'
t ' 2
R
Wh
MOLECULAR
DIFFUSION
tR
Let's relate “n” to the length of the
column.
B
HETP = A µ
+
The minimum of the curve
represents the smallest HETP (or
largest plates per meter) and thus
the best efficiency. "A" term is not
present for capillary columns.
B{
}C
RESISTANCE TO MASS TRANSFER
}A
EDDY DIFFUSION
n = effective theoretical
plates.
n
Plates per meter (N) =
or
L
L
Height equivalent to a theoretical plate (HETP) =
n
Thus, the more efficient the column, the bigger the "N" the smaller the
"HETP“.
•
•
13
+ Cµ
HETP
Solute
Efficiency is a function of the
carrier gas linear velocity or flow
rate.
µ ( opt
µ)
Plot of HETP versus linear velocity is know as the Van Deemter
plot.
The linear velocity value at the minimum of the curve is the optimum
value for achieving the best efficiency.
Helium Carrier Gas Alternatives
Let’s Make This Easy
• Goal: change carrier gas while keeping other method conditions the same
– use the same column
– use the same oven program
– adjust column flow or holdup time velocity to:
• same peak elution order
• same peak retention times
• For N2, test resolution of key components
• adjust GC conditions (temp, flow) if needed
• Easier method revalidation
– minimal changes to timed integration events
– minimal changes to peak identification table
• Use tools built into the Agilent Chemstation to guide us through the process
14
October 12, 2012
The Tyranny of Van Deemter
Why Nitrogen Gets a Bad Rap for Capillary GC
HETP (mm)
N2 C17 at 175º C
k' = 4.95
1.2
1.0
.8
OV-101
25m x 0.25 mm x 0.4µ
Nitrogen
58 cm/s
2.4 mL/min
Helium
58 cm/s
2.5 mL/min
Hydrogen
58 cm/s
1.7 mL/min
R = 1.17
R = 1.37
He
.6
.4
H2
.2
10 20 30 40 50 60 70 80 90
Average Linear Velocity (cm/sec)
• N2 actually provides the best efficiency, but at a slower speed
• Most helium methods have too much resolution
– Lower N2 efficiency at higher flows can still provide “good enough” resolution
• Most GC methods now use constant flow
– N2 efficiency losses with temp programming are not as severe
15
October 12, 2012
Helium Carrier Gas Alternative
Test Case: ASTM D6584 for Free and Total Glycerin in Biodiesel
Istd 2
1.
2.
3.
4.
Istd 1
2
Glycerol
Monoglycerides
Diglycerides
triglycerides
3
1
4
5
10
15
20
25
30
COC Inlet: Oven Track Mode
Pre-column: Ultimetal 2m x 0.53mm ID
Column: Ultimetal DB5HT, 15m x 0.32mm ID x 0.1 df
Column Flow: Helium at 3.0 mL/min (50 deg C)
Column Pressure: 7.63 psi constant pressure mode
Initial Column Temp: 50 oC for 1 min.
Oven Ramp 1: 15 oC/min to 180 oC
Oven Ramp 2: 7 oC/min to 230 oC
Oven Ramp 3: 30 oC/min to 380 oC, hold 10 min.
Detector: FID with 25 mL/min N2 makeup
16
35
Configure Inlet for Carrier Gas in Chemstation
17
Set the Control Mode: Flow or Holdup Time
18
October 12, 2012
Wider Retention Time Variation Using the Same
Flow
23.818 min
Helium
Flow: 3.00 mL/min
P: 7.63 psi
Tr: 0.472 min.
µ: 52.97 cm/s
23.469 min
Hydrogen
Flow: 3.00 mL/min
P: 3.85 psi
Tr: 0.420 min.
µ: 59.50 cm/s
23.705 min
Nitrogen
Flow: 3.00 mL/min
P: 7.09 psi
Tr: 0.464 min.
µ: 53.84 cm/s
18
19
19
20
21
22
23
24
October 12, 2012
Same Holdup Time (Tr) Gives Consistent Retention
Times
23.818 min
Helium
Flow: 3.00 mL/min
P: 7.63 psi
Tr: 0.472 min.
µ: 52.97 cm/s
23.862 min
Hydrogen
Flow: 2.64 mL/min
P: 3.43 psi
Tr: 0.472 min.
µ: 52.97 cm/s
23.776 min
Nitrogen
Flow: 2.94 mL/min
P: 6.98 psi
Tr: 0.472 min.
µ: 52.97 cm/s
18
20
19
20
21
22
23
24
October 12, 2012
Monoglyceride Resolution “Good Enough” Using
Nitrogen Carrier
Mono2
area: 1049
Mono1
Helium
Flow: 3.00 mL/min
Tr: 0.472 min.
Mono3
Mono2
area: 1050
Mono1
Hydrogen
Flow: 2.64 mL/min
Tr: 0.472 min.
Mono3
Mono2
area: 1049
Mono1
16.5
21
17
Nitrogen
Flow: 2.94 mL/min
Tr: 0.472 min.
Mono3
17.5
18
18.5
19
19.5
October 12, 2012
ASTM D6584 - Quantitative Results For Alternative
Carrier Gas
Weight Percent
Helium
22
Hydrogen Nitrogen
Glycerin
0.015
0.014
0.013
Monoglycerides
0.226
0.216
0.223
Diglycerides
0.114
0.115
0.110
Triglycerides
0.071
0.085
0.098
Total Glycerin
0.097
0.095
0.098
October 12, 2012
Analysis of Oxygenates and Aromatics in Gasoline
Using 2-D Gas Chromatography
ASTM Method D4815 – Oxygenated Additives
– Ethers and alcohols from 0.1 wt% to 15 wt%
– Usually only one or two additives in a sample
ASTM Method D5580 – Aromatics in Gasoline
– Measures benzene, toluene, C8 aromatics and C9 plus aromatics
– Two injections per sample for complete analysis
Preliminary separation removes light hydrocarbons from
sample
– Polar TCEP micro-packed columns retains polars and aromatics
– Back flush TCEP column to non-polar capillary column (HP-1) to
complete analysis
23
October 12, 2012
GC System for D4815 and D5580
Similarities between each method
– Same GC hardware requirements
• Inlets, detectors, plumbing and valve configuration
– Same separation scheme
• 2-D separation using polar TCEP micro-packed primary column
• 20% TCEP on 80/100 Chromosorb PAW, 22“ x 1/16“ stainless
• Only one difference in instrument requirements – non-polar capillary
column
– D4815 – 2.65 µm methyl silicone, 30m x 0.53mm
– D5580 – 5 µm methyl silicone, 30m x 0.53mm
24
October 12, 2012
Configuration and Operation for D4815 and D5580
Primary flow
split/splitless
inlet
TCEP column
S/S
EPC
HP-1 capillary column
7 6 5
Secondary flow
PCM
EPC
variable
restrictor
25
8
4
9
10
3
1
FID
2
TCD
October 12, 2012
Instrument Conditions
carrier gas
Inlet
inlet temperature
inlet presuure
TCEP column flow
split vent flow
split ratio
PCM pressure program
HP-1 column flow
FID temperature
oven temperature
Run time
26
D4815 Method
nitrogen
Split/Splitless
200 Deg C
9 psi (constant P)
5 mL/min
70 mL/min
15:1
13 psi for 14 min
99 psi/min to 40 psi
3 mL/min
250 deg C
80 deg C isothermal
16 min
D5580 Method
nitrogen
Split/Splitless
200 Deg C
25 psi (constant P)
10 mL/min
100 mL/min
100:1
23 psi for 12.1 min
99 psi/min to 40 psi
10 mL/min
250 deg C
60 C for 6 min
2 C/min to 115 C
115 C for 1.5 min
35 min
October 12, 2012
Analysis of MtBE and Ethanol in Gasoline using N2
Carrier Gas
MtBE
valve
reset
DME(IS) benzene
ethanol
aromatic and heavy
non-aromatic
DME(IS)
benzene hydrocarbons
2
27
4
6
8
10
12
14
16
October 12, 2012
ASTM Precision Specifications
D4815 Precision Measures
Compound Mass %
Ethanol
0.99
Ethanol
6.63
MtBE
2.10
MtBE
11.29
Repeatability
Spec Observed
0.06
0.01
0.19
0.03
0.08
0.01
0.19
0.05
Reproducibility
Spec Observed
0.23
0.01
0.68
0.04
0.20
0.01
0.61
0.08
Accuracy Evaluation
MtBE mass %
Sample
known found
SRM2294 #1 10.97 10.61
SRM2294 #2 10.97 10.60
AccuStd Check 12.00 11.81
28
October 12, 2012
D5580 Requires Two Runs per Sample for
Complete Aromatic Analysis
Good resolution with nitrogen carrier gas
3
Method D5580A
1. benzene
2. toluene
3. 2-hexanone (IS)
4. C8 aromatics, C9 plus aromatics,
and non-aromatic hydrocarbons
4
2
T1
T3
1
T2
Method D5580B
1. 2-hexanone (IS)
2. ethylbenzene
3. m,p-xylene
4. o-xylene
5. C9 plus aromatics
5
29
10
1
3
2
15
5
T4
4
20
25
October 12, 2012
Detector Response Precision for a Pump
Gasoline Sample – 5 Runs Over 5 Days
Run # Method
1
2
3
4
5
4815
5580A
5580B
4815
5580A
5580B
4815
5580A
5580B
4815
5580A
5580B
4815
5580A
5580B
Avg
Std. Dev
%RSD
30
MtBE
24077
23384
22807
22915
23053
23247
512
2.2
Benzene
FID Response (area counts)
Toluene Ethylbenzene m,p-Xylene
3306
19427
3244
19436
3178
19580
3048
19344
3099
19529
3175
105
3.3
19463
92
0.5
o-Xylene
5402
21825
8069
5402
21956
8105
5422
22342
8232
5432
22263
8201
5422
22253
8199
5416
14
0.2
22128
224
1.0
8161
70
0.9
October 12, 2012
Correcting Occasional Resolution Problems
m,p-xylene
Initial oven temp = 60 oC
T2 = 1.85 min.
ethylbenzene
m,p-xylene
Initial oven temp = 50 oC
ethylbenzene
T2 = 1.85 min.
5
10
15
20
25
– Some combinations of TCEP and HP-1 columns show less resolution using N2
carrier
– Fastest and easiest solution is to lower the initial oven temperature
31
October 12, 2012
Using Nitrogen With High Resolution Capillary
Columns
Test Case: EN14103 for Total FAME and Methyl Linolenate Content in B100
Biodiesel
• Expand method to include animal fat derived biodiesel
– Replace C17 FAMEs ISTD with C19 FAME ISTD
• Expand method to include tropical oil (palm) derived FAMEs
– Quantify C6 to C24 FAMEs and methyl linolenate (C18:3)
– Requires column temperature program to resolve all FAMEs
• Method updated to provide better precision
– Verify C19:0 ISTD purity
32
October 12, 2012
EN14103:2011 GC Configuration and Operating
Conditions
Standard 7890A GC Hardware
G3440A Agilent 7890A Series GC
Option 112 100 psi split/splitless Inlet with EPC control
Option 211 Capillary FID with EPC control
G4513A Agilent 7693 Autoinjector
19091N-133 HP-INNOWax Column, 30 m x 0.25 mm ID x 0.25 mm
Split/Splitless Inlet
Temperature 250 oC
Split flow 100 mL/min.
Column Flow He or H2 at 1 mL/min. constant flow
60 oC for 2 min.
10 oC /min. to 200 oC
Column temperature
5 oC/min. to 240 oC
Hold 240 oC for 7 min.
Flame ionization detector 250 oC
33
October 12, 2012
EN14103 – FAME Identification Standard
Helium at 1 mL/min Constant Flow
1
4
7
2
5
6
5
Peak #
Name
15
8
20
19
16
17
25
Name
18
20
30
RT (min.)
Peak #
RT (min.)
C6:0
6.031
11
methyl arachidate
C20:0
22.857
1
methyl hexanoate
2
methyl myristate
C14:0
15.878
12
methyl eicosonate
C20:1
23.166
3
methyl myristoleate
C14:1
16.275
13
methyl eicosadienoate
C20:2
23.808
4
methyl palmitate
C16:0
17.996
14
methyl arachidonate
C20:4
24.551
5
methyl palmitoleate
C16:1
18.311
15
methyl eicosatrienoate
C20:3
24.730
6
methyl stearate
C18:0
20.332
16
methyl behenate &
C22:0
25.582
7
methyl oleate (9)
C18:1
20.617
methyl eicosapentaenoate
C20:5
8
methyl oleate (11)
C18:1
20.697
17
methyl erucate
C22:1
26.031
9
methyl linoleate
C18:2
21.205
18
methyl lignocerate
C24:0
29.574
methyl linolenate
C18:3
22.052
19
methyl nervonate
C24:1
30.203
20
methyl docosahexaenoate
C22:6
30.365
10
34
10
12
14
910 11 13
3
0
15
October 12, 2012
EN14103 – FAME Retention Time Standard
Comparison of He and N2 Carrier Gas
pA
Helium at 1 mL/min
Constant Flow
160
140
120
100
80
60
40
20
16
18
20
22
24
26
28
30
min
30
min
140
Nitrogen at 1 mL/min
Constant Flow
120
100
80
60
40
20
16
35
18
20
22
24
26
28
October 12, 2012
EN14103 – Soya B100 Biodiesel
Comparison of Helium and N2 Carrier Gas
C19:0 (IS)
Helium at 1 mL/min
Constant Flow
16
18
20
22
24
26
28
30
min
30
min
Nitrogen at 1 mL/min
Constant Flow
16
36
18
20
22
24
26
28
October 12, 2012
EN14103 – FAME Content in Biodiesel
Comparison of He and N2 Carrier Gas
Sample
SRM 2772
Rapeseed
Coconut
Rape/Coco (50/50)
Run 1
96.9
96.7
87.4
91.2
FAME Content (wt%)
Helium
Nitrogen
Run 2
r
Run 1 Run 2
96.7
0.2
97.2
97.2
96.0
0.7
95.5
95.7
86.9
0.5
86.6
86.9
91.2
0.0
91.4
91.0
r
0.0
0.2
0.3
0.4
r (spec)
1.01
1.01
1.01
1.01
r
0.0
0.0
0.0
0.1
r (spec)
0.2
0.2
0.0
0.1
C18:3 (wt%)
Sample
SRM 2772
Rapeseed
Coconut
Rape/Coco (50/50)
37
Run 1
7.3
8.5
0.0
4.2
Helium
Run 2
7.3
8.4
0.0
4.2
r
0.0
0.1
0.0
0.0
Nitrogen
Run 1 Run 2
7.3
7.3
8.3
8.3
0.0
0.0
4.2
4.1
October 12, 2012
If You MUST Use Helium – Consider Conservation
No
Is the chemist willing to convert
to alternative gasses?
GC
Yes
Is the Application based on
GC or GC/MS?
Does the current GC method
have too much resolution?
GC/MS
No
Yes
He Conservation
Consider migration to N2
Consider migration to H2
GC/MS specific H2
considerations
Agilent Restricted
Page
38
38
October 12, 2012
Reduce Helium Consumption
• 7890 GC Gas Saver
– reduces split flow to conserve helium consumption
• Leak checking
• Turn off the system to avoid long term idle
– Not always recommended for some GC columns or traps
Agilent Little Falls Site was able to reduce
consumption by 50%
Agilent Restricted
Page
39
39
October 12, 2012
Summary
Migrating your GC method from helium carrier gas can be easily done
• For resolution critical methods, H2 offer the best alternative
– Agilent GC and GC/MS systems have many built-in safety features
• For many GC applications, N2 offers a cheap, easy alternative without any
safety worries
– Most existing helium methods have too much resolution
– N2 can be used without changing any of the existing GC conditions
• keep the holdup time the same as the original method
– 2-D methods have high resolution built-in, so N2 is ideally suited as a carrier
gas
• Valve-based or Deans switch, not GCxGC
• If helium alternative is not an option, consider conservation
40
October 12, 2012