Split / Splitless Injection for Capillary GC
Speakers John V Hinshaw CHROMacademy GC Dean GC Connections editor Moderator Tony Taylor CHROMacademy Trainer Technical Director, Crawford Scientific Dave Walsh Editor In Chief LCGC Magazine
Aims & Objectives 1. Overview of Injectors for Capillary GC 2. Injector components 3. Split / Splitless Injection Overview 4. Gas flows in Split and Splitless Mode 5. Critical operating parameters in Split and Splitless mode 6. Optimising injector settings for maximum sensitivity and repeatability 7. Inlet liners the critical facts 8. Troubleshooting Inlet Hardware 9. Investigating Irreproducibility and Poor Peak Shape
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 Injector d. Headspace Inlet e. Purge and Trap Inlet f. Thermal Desorption Inlet g. 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
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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
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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
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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
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Split Injection Discrimination Normalised response of n-alkanes in Hexane
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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 / Development Conditions Temperature: Mode: Septum Purge flow: Split Flow: Column Flow: Injection Volume: Liner: Injection Solvent: Column Temp.: 250 o C Split 1-3 ml/min (instrument dependant) 100ml/min 0.5 2 ml/min (note: depends upon column) 1ml (check for backflash) Straight through (deactivated and packed if necessary) Match to column chemistry 50-100 o C (note: analyte dependant)
Split Injection Advantages & Disadvantages Advantages 1. Simple to Use 2. Rugged Design 3. Narrow analyte band on column 4. Protects column from involatile sample components 5. Easy to Automate Disadvantages 1. 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
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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
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Splitless Analyte Thermal Focussing Rule of thumb: Initial Oven Temp 10 o C < Solvent B.Pt.
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Splitless Analyte Solvent Focussing N-alkanes in CS2
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Splitless Analyte Solvent Focussing II N-alkanes in CS2 with Wax Column Rule of thumb: Match solvent and column polarity
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Splitless Injection - Optimising Purge Time Use the peak area of an early eluting peak to determine analyte transfer time
Splitless Injection Default / Development Conditions Temperature: Mode: Septum Purge flow: Purge Time: Split Flow: Column Flow: Injection Volume: Liner: Injection Solvent: Column Temp.: 250 o C Splitless 1-3 ml/min (instrument dependant) 30 seconds (NOTE: optimise empirically) 100ml/min 0.5 2 ml/min (note: depends upon column) 1ml (check for backflash) Straight through (splitless liner) Match to column chemistry <10 o C below solvent B.Pt.
Split Injection Advantages & Disadvantages Advantages 1. Simple to Use 2. Rugged Design 3. Excellent for trace analysis 4. Less Risk of Analyte Discrimination than Split Mode 5. Easy to Automate Disadvantages 1. 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
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Calculating Optimum Injection Volume 1. Vapour expansion volume is dependant upon: a. Solvent type b. Injection Port Temp. c. Inlet Pressure 2. Backflash dependant upon: a. Liner Type b. Vapour expansion volume c. Injection Volume
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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 250 o C 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 Limit 2. Size 3. Material 4. Sandwich 5. PTFE Faced 6. Pre-Drilled
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