Split / Splitless Injection for Capillary GC

Speakers

John V HinshawCHROMacademy GC Dean‘GC Connections’ editor

Tony TaylorCHROMacademy TrainerTechnical Director, Crawford Scientific

Moderator

Dave WalshEditor In ChiefLCGC Magazine

1. Overview of Injectors for Capillary GC2. Injector components3. Split / Splitless Injection Overview4. Gas flows in Split and Splitless Mode5. Critical operating parameters in Split

and Splitless mode 6. Optimising injector settings for

maximum sensitivity and repeatability 7. Inlet liners – the critical facts8. Troubleshooting Inlet Hardware 9. Investigating Irreproducibility and

Poor Peak Shape

Aims & Objectives

Overview of Injectors for Capillary GC

1. Injectors and Inlets are used to introduce the sample to the GC column

2. Different classes of Injectors and Inlets available:a. Vaporising Injector (including Split / Splitless)b. Cool-on-Column Injector (Thermally Labile or

Accurate Low Level Quant)c. Large Volume / Programmed Thermal Vaporising

Injectord. Headspace Inlet e. Purge and Trap Inletf. Thermal Desorption Inletg. Pyrolysis Inlet

Characteristics of GC Injectors

1. Low / No Contribution to Band Broadening

2. Introduces representative & homogenous sample

3. No discrimination based on differences in analyte b.pt., polarity, concentration

4. Avoids thermal / catalytic degradation

5. Good accuracy & precision with a wide range of analyte concentrations

Injector Anatomy

01_overview.flv

Split Injection Mechanisms I

1. Sample syringe pierces septum which seals around needle

2. Sample rapidly introduced into heated inlet

3. Liquid sample volatilises and the gaseous ‘plasma’ is contained within a quartz glass liner

Split Injection Mechanisms II

4. The sample gas is swept by the carrier gas through the liner and EITHER into the GC Column OR between the liner and inlet body and down the Split Line

5. % of sample reaching the column depends upon the relative flow rates in the column and split flow line

02_Split_Inj.flv

Setting the Split Ratio I

1. Split ratio is the ratio of gas flows through the column and split line

2. Represents the volume fraction of sample entering the column

3. Split Ratios from 1:1 to 500:1 are common

4. Higher split – smaller amount of sample on column

Setting the Split Ratio II

5. Avoids column overload –fronting peaks and poor area reproducibility

6. Column capacity depends upon film thickness, column i.d., polarity and retention

7. HIGHER split ratios give SHARPER (more EFFICIENT) PEAKS

03_Split_Ratio.flv

Effect of Split Flow on Peak Shape

1. As split flow increases Liner Flow increases

2. Liner flow is a combination of the column flow and the split flow

3. Gaseous sample is transferred more rapidly onto the column

4. Net result is a decrease is analyte band width (peak width) at the column head

5. The analyte band will disperse during elution but initial bandwidth has impact on peak effciency

05_peakShape.flv

Split Injection Discrimination

Normalised response of n-alkanes in Hexane

06_discrimination.flv

Split Injection Set-Up Summary

1. Used as the Default Vaporising Injector

2. Primarily used for non-trace analysis of volatile samples

3. Need to consider gas flows (particularly split flow) carefully / Don’t forget septum purge flow!

4. Increasing split flow:

a. Improves peak shape

b. Lowers column loading

c. Lowers analytical sensitivity

d. Decreases analyte inlet residence time – therefore reduces the opportunity for thermal degradation

5. Need to consider Discrimination effects

Split Injection Default / DevelopmentConditions

Temperature: 250oC

Mode: Split

Septum Purge flow: 1-3 ml/min (instrument dependant)

Split Flow: 100ml/min

Column Flow: 0.5 – 2 ml/min (note: depends upon column)

Injection Volume: 1ml (check for backflash)

Liner: Straight through (deactivated and packed if necessary)

Injection Solvent: Match to column chemistry

Column Temp.: 50 - 100oC (note: analyte dependant)

Split Injection Advantages &Disadvantages

Advantages1. Simple to Use

2. Rugged Design

3. Narrow analyte band on column

4. Protects column from involatile sample components

5. Easy to Automate

Disadvantages1. Not suitable for ultra-trace

analysis

2. Suffers from Discrimination

3. Liner geometry dictates injector settings

4. Analytes susceptible to thermal degradation

Splitless Injection Mechanism I

1. Same principle as Split Injection

2. DIFFERENCES INCLUDE

3. Initial injector state is SPLITLESS i.e. The split line flow is turned off

4. All sample reaches the column

5. Sample vapours trapped onto head of column (solvent and thermal effects)

Splitless Injection Mechanism II

6. Column temperature programmed to initiate elution

7. At some point after analyte transfer to the column the split line is turned on to empty the injector

8. Primarily used for trace and ultra-trace analysis

07_Splitless.flv

Splitless Injection Gas Flows I

1. Flow through the liner = Column Flow during Splitless phase

2. Analyte can take MINUTES to transfer to the column

3. If no action is taken, chromatographic peaks will be unacceptably wide

Splitless Injection Gas Flows II

4. The answer is to ‘FOCUS’ the analytes onto the head of the column using Thermal and Solvent focussing effects

5. The SPLITLESS (Split-On or Purge) time needs to be carefully considered

08_Liner_Flow.flv

Splitless Analyte Thermal Focussing

Rule of thumb: Initial Oven Temp 10oC < Solvent B.Pt.

09_thermal_focussing.flv

Splitless Analyte Solvent Focussing

N-alkanes in CS2

10_solvent_focussing.flv

Splitless Analyte Solvent Focussing II

Rule of thumb: Match solvent and column polarity

N-alkanes in CS2 with Wax Column

11_solvent_focus_2.flv

Splitless Injection - Optimising PurgeTime

Use the peak area of an early eluting peak to determine analyte transfer time

Splitless Injection Default / DevelopmentConditions

Temperature: 250oC

Mode: Splitless

Septum Purge flow: 1-3 ml/min (instrument dependant)

Purge Time: 30 seconds (NOTE: optimise empirically)

Split Flow: 100ml/min

Column Flow: 0.5 – 2 ml/min (note: depends upon column)

Injection Volume: 1ml (check for backflash)

Liner: Straight through (splitless liner)

Injection Solvent: Match to column chemistry

Column Temp.: <10oC below solvent B.Pt.

Split Injection Advantages &Disadvantages

Advantages1. Simple to Use

2. Rugged Design

3. Excellent for trace analysis

4. Less Risk of Analyte Discrimination than Split Mode

5. Easy to Automate

Disadvantages1. Need to carefully optimise

conditions

2. Risk of backflash

3. Analytes susceptible to thermal degradation

Optimising Injection Volume

1. Solvent vapour volume should not exceed liner volume

2. Blank solvent injection following sample will give a ‘mini-chromatogram’

12_optimising_injection.flv

Calculating Optimum Injection Volume

1. Vapour expansion volume is dependant upon:a. Solvent typeb. Injection Port Temp.c. Inlet Pressure

2. Backflash dependant upon:a. Liner Typeb. Vapour expansion

volumec. Injection Volume

13_backFlash_calc.flv

Liners for Split / Splitless Injection

Straight Through

Top Features

Middle Features

Bottom Features

Liner i.d.

Troubleshooting – Temperature / Activity

Thermal Degradation

If in doubt use 250oC as an inlet starting point

Peak Tailing

1. Consider Volumetric tailing (due to unswept dead volumes)

2. Chemically de-activate the Liner / inlet / column

Troubleshooting – Septa

Septum Bleed Septa Considerations

1. Temperature Limit2. Size3. Material4. Sandwich5. PTFE Faced6. Pre-Drilled

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