2015-Choice of Carrier Gas in GC-rev-3121).pdfConsiderations for using a certain carrier gas • Availability / delivery • Price • Purity • Speed of analysis • Type of detection

Copyrights: Restek Corporation

Considerations for Choosing a different

Carrier gas in Gas Chromatography

Considerations for Choosing a different

Carrier gas in Gas Chromatography

Jaap de Zeeuw

Restek Corporation,

Middelburg, The Netherlands


Considerations for using a certain carrier gasConsiderations for using a certain carrier gas

• Availability / delivery

• Price

• Purity

• Speed of analysis

• Type of detection system (PDD, MS..)

• Sensitivity (TCD)

Helium supply is under

pressure


What are the alternatives?What are the alternatives?

Nitrogen or Hydrogen“Slow” carrier gas

Cheap

Can be generated

“ Fast” carrier gas

Cheap

Can be generated

Safety

If there is “enough”

resolution

If there is “Just enough”

resolution


Both gases can be generated using generators

• Unlimited supply

• Low cost

• Relative small investment

For use as carrier gas need to make sure the purity is very high;

Consider Gas filtration

Both carrier gases are relatively CheapBoth carrier gases are relatively Cheap


1.0

0.6

0.2

HE

TP

(m

m)

Van Deemter Plot

Average Linear Velocity (cm/sec)

N2

He

H2

10 20 30 40 50 60 70

Van Deemter curve for different gasesoptimum gas velocitiesVan Deemter curve for different gasesoptimum gas velocities

Slow

FAST


Impact of optimum velocity on analysis timeImpact of optimum velocity on analysis time

6


Changing carrier gasChanging carrier gas

If a different carrier gas is used in the optimum velocity, the

temperature program has to be changed to get the same peak

elution order;

For N2: slower temperature programs = longer analysis time

For H2: faster temperature program = shorter analysis time

We can also use carrier gases deliberately “outside” their

optimum velocity;

If we use the SAME linear velocity as with helium, we do not have to change program and injection parameters;


Using a different carrier gases OUTSIDE the optimumUsing a different carrier gases OUTSIDE the optimum

If there is sufficient resolution, we can afford to loose some “plates”..

Using N2 at a HIGHER average velocity

Using H2 at a LOWER average velocity


Option 1:

Use Nitrogen at the SAME velocity as Helium..

..work at approximate 2x HIGHER then optimal velocity for N2..


Operating Nitrogen at higher velocitiesOperating Nitrogen at higher velocities

Using Nitrogen at the same linear velocity as Helium

will result in an increase of HETP by approx. a factor 2

Average linear velocity [cm/s]


Impact on ResolutionImpact on Resolution

A factor 2 lower efficiency means

only a factor 1.4 lower

resolution..

Loss of efficiency by a factor 2 only results in a loss of 1.4 on resolution

R = resolution

Nth = theoretical Plate numberα = selectivity

K = retention factorNth is linear with Column length

Resolution is not..


Impact on ResolutionImpact on Resolution

A factor 2 lower efficiency means

only a factor 1.4 lower

resolution..

Loss of efficiency by a factor 2 only results in a loss of 1.4 on resolution

Nth = 200.000 Nth = 50.000Nth = 100.000


Example using Nitrogen under “Helium” conditionsExample using Nitrogen under “Helium” conditions


Important practical implications using N2 at higher velocitiesImportant practical implications using N2 at higher velocities

• Peaks are BROADER so they are LOWER

• Response (height) will be 20-25% Lower

• Because peaks are broader, they will run faster in each other upon AGING

• More frequent maintenance• Column life time will be shorter


Option 2:

Use Hydrogen at the SAME velocity as Helium..

..work at approximate 30% SLOWER then optimal linear velocity for H2..

15


Option 2: Use Hydrogen at the SAME velocity as Helium..Option 2: Use Hydrogen at the SAME velocity as Helium..

Average linear velocity [cm/s]

Using Hydrogen at the same linear velocity as Helium will

result in an increase of HETP by approx. a factor 1.2


Setting same linear velocity for Hydrogen / Nitrogen/ HeliumSetting same linear velocity for Hydrogen / Nitrogen/ Helium

Carrier gases can be compressed, meaning at SAME

linear velocity, the Flows will be different..

Example below for a 30m x 0.25mm capillary, set at 30 cm/s at 40ºC

Gas Average Inlet pressure Flow

linear velocity

[cm/s] [kpa] [mL/min]

Helium 30.0 98.17 1.28

Nitrogen 30.0 87.93 1.23

Hydrogen 30.0 43.05 1.02


Using similar linear velocitiesUsing similar linear velocities

• Allows you to use the same temperature programming and injection conditions;

• Result is the same chromatogram with the same retention times


Comparison Comparison

25 cm/s, 0.85 mL/min He, 25 cm/s, 0.85 mL/min

2, 25 cm/s, 0.69 mL/min H2, 25 cm/s, 0.69 mL/min

R = 2.80

R = 2.68


He carrier, 1.4 mL/min constant flow, 34 cm/sec at 80°C

80°C (0.1 min), 16.7°C/min to 320°C (0.5 min)

FWHH

1.1 sec

FWHH

1.1 sec

H2 carrier, 1.06 mL/min constant flow, 34 cm/sec at 80°C

80°C (0.1 min), 16.7°C/min to 320°C (0.5 min)

13.64 min

13.50 min

8.10 min

8.03 min


He carrier, 1.4 mL/min constant flow, 34 cm/sec at 80°C

80°C (0.1 min), 16.7°C/min to 320°C (0.5 min)

Heptachlor

epoxideDDE

DieldrinEndrin

DDD

Endosulfan II

DDT

trans-

Chlordane

cis-

Chlordane Endosulfan I

H2 carrier, 1.06 mL/min constant flow, 34 cm/sec at 80°C

80°C (0.1 min), 16.7°C/min to 320°C (0.5 min)


What about using MS? Running H2 at SAME speed as HeWhat about using MS? Running H2 at SAME speed as He

Despite of running hydrogen at a LOWER velocity, and using the SAME

programming conditions, there is almost no difference in separation.

22


Optimal conditionsOptimal conditions

If analysis is optimized, analysis time is reduced by 40% using H2

Note the higher oven temperature rate


Using Nitrogen and Hydrogen under non optimal conditionsUsing Nitrogen and Hydrogen under non optimal conditions

Result in a loss of efficiency..

What if there is JUST ENOUGH separation

between the peaks?

Can only be used if:

“there is “ENOUGH” separation between the peaks


Complex chromatogramsComplex chromatograms

T im e - - >

Need to maintain efficiency (keep plates)


Maintain the separation efficiency using N2Maintain the separation efficiency using N2

Using same column and nitrogen under OPTIMAL conditions (12 cm/s)

Impact

• 2 x slower then Helium, analysis time will be 2x longer

• Peaks will be lower (sensitivity)

• Need to change temperature program for same peak elution order


Important : linear gas velocity vs oven temperature program Important : linear gas velocity vs oven temperature program

• If a different carrier gas is used and the SAME linear gas

velocity, the temperature program can stay the same;

• If the linear gas velocity is CHANGED, and the oven temperature program is NOT CHANGED, the elution

temperatures of components will change.

• This can result in a different peak elution order.


If elution temperatures are changed, also the peak elution can change..


Changing the flow keeping SAME temperature program.Changing the flow keeping SAME temperature program.

Result: elution temperatures will change:

Higher flow = lower elution temperature

At 8.9 psi, u = 22 cm/s

At 26 psi, u = 61 cm/s

Tel. = 250ºC

Tel. = 210ºC

Ref.: Jason Thomas, restek corporation

4,4’-DDE

4,4’-DDE

Endosulfan I

Endosulfan I


29


Using a smaller column diameter

Replacing the 30m x 0.25mm for a 20m x 0.15mm and operate the 0.15mm at a little higher velocity

Analysis times are approximate 2x shorter



Using a 0.15mm ID instead of 0.25mmUsing a 0.15mm ID instead of 0.25mm

0 10 20

0 2 4 6 8 10 12 14

Time (min)

20

40

28 min

15 min

30m x 0.25mm Rxi-5Sil MS,

df = 0.25 μm

20m x 0.15mm Rxi-5Sil MS,

df = 0.15 μm


Calculating the impactUsing method translatorCalculating the impactUsing method translator

Can use the SAME oven program

Same analysis time

Helium Nitrogen

30/0.25 20/0.15

32 21.5 cm/s


Using a smaller column diameter

Using 0.15/0.18mm ID and operate at higher velocity

Impact

• Comparable analysis time;

• Can use same programming conditions

• Need to purchase a different column

• Lower loadability



The Practical test


ChallengeChallenge

• Take POLAR column : Stabilwax

• Use COMPLEX mixture : Fragrances

• Compare 30m x 0.25mm with 20m x 0.15mm

Operate with same temperature programming using pre-selected flow / linear velocities

Efficiency optimized flow setting



Optimized Flows for He and N2Optimized Flows for He and N2

Efficiency Speed

optimized Optimized

0.25mm He 1.4 mL/min 2.0 mL/min

0.15mm N2 0.27 mL/min 0.38 mL/min

0.15mm N2 0.36 ml/min 0.52 mL/min

Actual Flows used to use the same program and obtain same analysis time


Efficiency optimized flow



30m x 0.25mm x 0.25µm Stabilwax

He constant 1.4 mL/min

40°C (0.1 min), 6.4°C/min to 250°C

6.81 min 31.23 min

Efficiency-optimized flow

Optimal heating rateMen’s After Shave

20.2m x 0.15mm x 0.15µm Stabilwax

N2 constant 0.36 mL/min

40°C (0.1 min), 6.4°C/min to 250°C

6.75 min 31.24 min

Men’s After Shave


Detail of separation


Efficiency optimized flow



Note the Pressures and flows usedNote the Pressures and flows used

For Helium 15.27 psi 1.40 mL/min

For Nitrogen 17.42 psi 0.36 mL/min

Very small difference

4 times lower flow


Features/Benefits using N2 and 0.15mm columnsFeatures/Benefits using N2 and 0.15mm columns

• Separations are exact identical

• Analysis time is the same

• No change in oven temperature program

• Pressure required for N2 is just a little higher

• Nitrogen is always available

• Price advantage for nitrogen and volume used

• 0.15mm columns have the same OD as 0.25mm: can use

the same ferrules

• 0.15mm columns have proven to work for many years and can be manufactured with different phase technologies


LimitationsLimitations

• Loadability/capacity is about 5 times less: for real trace analysis this will be a challenge

• Nitrogen is not a good MS gas, maybe in future..


Or..Or..

Move to a Hydrogen as the carrier gas


Why is hydrogen of interest ?Why is hydrogen of interest ?

• Readily available and can be produced in the lab

• Economical

• Can be used with many applications

• Offers an option to have also a faster analysis

• Peaks are 2x higher (sensitivity increase)

• For same data, need 50% of injected amount;

32


What are potential issues with using hydrogen?What are potential issues with using hydrogen?

• Safety aspect

• Potential leaks in the oven

• Leaks in the lab

• Split vent flows

• High pressures

• Reactivity

• Hydrogen can react with unsaturated compounds

• Impact on detector response

• FID signal

• MS fragmentation and sensitivity

33


Use of H2: Readily available and can be produced in the labUse of H2: Readily available and can be produced in the lab

H2-generators

• Unlimited supply

• Low cost

For use as carrier gas need to make sure the purity is very high;

Consider Gas filtration

NOTE: Most labs already have H2-line for operating FID.. This gas can be

used also, but need to verify cleanliness..


Optimal linear velocities for different carrier gasesOptimal linear velocities for different carrier gases

Hydrogen Uopt. = 40 - 45 cm/s

Helium Uopt. = 25 - 33 cm/s

Nitrogen Uopt. = 8 - 14 cm/s

Hydrogen is the

fastest carrier gas..

The same separation at

approx 1.7 shorter run time


20m x 0.15mm x 0.15µm Stabilwax4.22 min

6.74 min

20m x 0.15mm x 0.15µm Stabilwax6.68 min

10.67 min

134.1°C

133.6°C

Helium

5°C/min

Hydrogen

8.5°C/min

Fragrance analysis: Hydrogen instead of HeliumFragrance analysis: Hydrogen instead of Helium


Oven temperatureOven temperature

If the oven temperature is NOT adjusted:

• Gain in shorter analysis time is not as big as expected

37


Impact of using hydrogen, Using the SAME oven temperature programImpact of using hydrogen, Using the SAME oven temperature program

2 4 6 8 10 12 14 16

Time (min)

0 2 4 6 8 10 12 14 16

Time (min)

30 cm/s; 1.0 mL/min

Elution temperatures are lower

Only 1.5 minute faster..

Peaks start to shift

Helium: 1.0 mL/min

Hydrogen: 2.0 mL/min

38


Oven temperatureOven temperature

If the oven temperature is NOT adjusted:

• Gain in shorter analysis time is not as big as expected

• Peaks may start to move:

• Some separations will improve

• Some separations will go worse

• Peak elution can change

Example:

Chlorinated

Pesticides

39


a -ChlordaneEndosulfan I

4,4'-DDE

Dieldrin

Program 9 ºC/minProgram 9 ºC/min

Impact of elution temperature on Separation

T el= 204ºC

T el= 223ºC

T el= 214ºC

T el= 248ºC


New Calculator for oven programmingNew Calculator for oven programming

Free On-Line & windows based

Method translator & Flow calculator



On-Line & windows based Method

translator & Flow calculator

46



On-Line & windows based Method translator & Flow calculator

44

Calculator is available for FREE available on USB or can

be downloaded or accessed from website


Temperature programs needed to get the SAME elution temperatureTemperature programs needed to get the SAME elution temperature

2 4 6 8 10 12 14 16

60ºC, 2 min → 250 ºC @ 10ºC/min

60ºC, 1 min → 250 ºC @ 20ºC/min

Elution temp E12= 185.9 ºC

He, 1 mL/min

H2, 2 mL/min

2x faster analysis

43


Industrial application: Solvent impurity analysisIndustrial application: Solvent impurity analysis

15 min

Ref: J vercammen, Interscience Belgium


Hydrogen : safety issuesHydrogen : safety issues

Need to make sure there is no accumulation of H2

possible in the oven..

Hydrogen is combustible over a concentration range of 4% to 74% by volume;

What can we do to reduce this risk?


RisksRisks

When the column breaks

Amount of H2 released depends on where the column breaks

• End

• Middle

• Front • Limit the flow of H2

• Reduce risk of column breakage

50


Flow restriction to set with GC (electronic flow setting)Flow restriction to set with GC (electronic flow setting)

• Using Flow-controlled operation, the amount of H2

flowing in the GC is controlled.

• When column breaks at inlet, only the volume in injector can enter the GC + column volume

If this happens the GC will jump immediate in “standby”

mode as it detects an error because the pressure is not correct..


Da Vincihttp://www.davinci-ls.com/

SIMhttp://sim-gmbh.de/en/


Use metal capillary columnsUse metal capillary columns

• Metal columns are virtually unbreakable

• Already widely used for Simdist and Biodiesel, as well as in process and portable systems

• Many phase technologies available

Today’s series metal columns are VERY inert and

compete very well with fused silica


Inertness Test for 10m x 0.32mm metal column @ 110 ºCInertness Test for 10m x 0.32mm metal column @ 110 ºC

This is just a 0.1 μm film!

54


Use of H2: how big is the risk?Use of H2: how big is the risk?

At Restek, Chrompack, Varian and J&W, millions of GC columns have been tested using hydrogen.

55


Split outlet: What are the chances that hydrogen built

up in the lab? Considerations

Split outlet: What are the chances that hydrogen built

up in the lab? Considerations

• Volume of the Lab

• Total split flow of Hydrogen

• Hydrogen is lighter then air

• Fast diffusion (20m/s)


What to do with the hydrogen that

is splitted in the lab?



This is how we did it 30 years ago






This is how we did it 30 years ago

58


Reality: Lab ventilationReality: Lab ventilation

• Normal lab ventilation

• Air conditioning

• Fume hoods

• Local hoods

One can also install “vent”

line at the split outlet..


Use of HydrogenUse of Hydrogen

Reactivity?


Hydrogen is a reducing gasHydrogen is a reducing gas

Styrene → ethylbenzene

Higher injector temperatures

more ethylbenzene is formed


Alumina columns and hydrogenAlumina columns and hydrogen

• There has been no proof that Hydrogen can cause reactions with unsaturated hydrocarbons in the alumina PLOT

• Studies were done 25 years ago and showed up to 200ºC no reactivity (using KCl deactivated columns)

Problem may be present as today there are:

• More polar alumina columns

• Using different deactivations and different types of alumina

• Higher Operation temperatures

62


Solvents and HydrogenSolvents and Hydrogen

Some solvents can form acids when evaporated in H2

Result: Faster activation of Injection port liners

• Methylene chloride

• Chloroform

• CS2

For Split : minimal impact

For Splitless : bigger impact

May consider to change solvent: FE use ethylacetate

63


Important additional advantage besides 2x fasterImportant additional advantage besides 2x faster

Peaks are 2x narrower, they are 2x higher

For same sensitivity only 50% has to be injected..

This means:

• 2x less liner contamination

• 2x less column contamination

• Reduced costs per analysis

64


Hydrogen and MSHydrogen and MS

• Higher flow rate (use 0.18mm columns)

• Mass spectra may show bias (polar compounds)

• Initial activity of ion source (need some conditioning)

• 3 times lower of sensitivity (compensated by lenses)

• Hydrocarbon background (contamination of EPC)

• H2 – air mixtures in MS = deadly for MS (need service)

65


Changing carrier gas from heliumChanging carrier gas from helium

If peaks are well resolved:

- Nitrogen at higher velocity, keep conditions the same, loose some

separation, deal with shorter column life;

- Hydrogen at slower velocity, keep condition the same, small impact on

separation, deal with safety issues;

If peaks are just resolved:

- Nitrogen at optimum speed: accept longer analysis time;

- Nitrogen and smaller bore capillary; need some method development;

- Hydrogen at optimum velocity: need method development, has benefits

on analysis time and sensitivity; deal with safety issues;


Thank you for your kind attention

Documents

2015-Choice of Carrier Gas in GC-rev-3121).pdfConsiderations for using a certain carrier gas • Availability / delivery • Price • Purity • Speed of analysis • Type of detection