Split and Splitless Injection in Capillary Gas Chromatography

Transcription

1 Split and Splitless Injection in Capillary Gas Chromatography With Some Remarks on PTV Injection 3rd enlarged and revised Edition By Konrad Grob Hüthig Buch Verlag Heidelberg 1993

2 A Split Injection 1 Introduction Principles of Split Injection Basic Injector Design Purposes of Sample Splitting Splitting to Avoid Column Overloading Splitting to Generate Sharp Initial Bands The Two Principles of Gas Supply Pressure Regulator - Needle Valve System Flow Regulator - Back Pressure Regulator System Comparison of the Two Systems Historie Background 7 2 The Split Ratio Definition Determination of the Split Ratio Which Flow Rates are Determined? Influence of the Column Temperature Determination of the Column Flow Rate Direct Measurement Soap Bubble Meters Measurement via Gas Velocity Determination of the Split Flow Rate Flow Meters with Floating Particles 15 3 Sample Concentration Suitable for Split Injection Split Ratios Commonly Applied An Example Range of Suitable Concentrations 19 4 Initial Band Widths in Split Injection Band Widths in Space and Time Factors Determining Initial Band Widths Experimental Observation of Initial Band Shapes Description of the Experiment Subjects of Interest Some Results Effect of the Initial Band Width on the Final Peak Width Isothermal Runs Chromatography Involving Temperature Increase 29

3 X Contents 5 Split Injection for Fast Analysis Prerequisites for Fast Analysis Initial Band Widths Initial Band Widths Required Limits to the Sharpness of Initial Bands Examples of Fast Analyses Concepts for Producing Extremely Sharp Initial Bands Splitting by Fluidic Logic Gates On-Column Injection with Liquid Splitting Micromonitor Miniature GC Intermediate Cold Trapping 35 6 Analysis Requiring Maximum Sensitivity Sharp Bands at Low Split Ratios Headspace Analysis Rapid Isothermal Runs at Elevated Column Temperature Optimized Split Ratio Peaks Growing "Fat" instead of "Tall" Dilution in the Injector Dilution in the Column Maximum Vapor Concentration in the Injector Sample Volume Optimum Insert Volume Position of the Column Entrance Injection Point Syringe Needles Column Flow Rate Low Split Ratios Resulting from High Column Flow Rates Selection of the Carrier Gas Selection of the Column Summary: Obtaining Maximum Sensitivity from Split Injection 48 7 High Split Ratios for Reducing the Sample Size A Hypothetical Sample The Maximum Split Flow Rate Unreasonable Gas Consumption Resistance in the Split Line Resistance in the Gas Lines Pre-Heating of Carrier Gas Speed of Sample Evaporation Small Sample Volume Plunger-in-Needle Syringes Standard 10 (5) uj Syringes Column Flow Rate Narrow Bore Columns Slow Carrier Gas Column Capacity 55

4 XI 7.6 Length of the Syringe Needle Summarizing Guidelines 58 8 Sample Evaporation in the Injector Problems Caused by Incomplete Evaporation Evaporation Time Available Band of Liquid Nebulized Sample Transferred by Carrier Gas Deposition on Surfaces Amount of Heat Required Sources of Heat Available Carrier Gas Packed Injector Inserts Heat from Glass Insert Time Required for Heat Transfer Heat Transfer Within the Wall of the Insert Heat Transfer Through the Gas Phase Residence Time in the Vaporization Zone Required for Evaporation Conclusions from Calculations Some Experimental Results Calculated and Measured Temperature Drop Measurement of Evaporation Time via Split Flow Rate The Perylene Experiment Principle "Transparent Injector" Sample Liquid Leaving the Syringe Needle Three Scenarios of Evaporation in an Empty Vaporizing Chamber Flash Evaporation Jet of Liquid Rushing Through Injector Liquid Splashing on Insert Wall Some Conclusions Fate of Sample Liquid "Shot" to Bottom of Insert Stopping the Sample Liquid Retention of Liquids on Surfaces Injector Insert with Baffles Inverted Cup or "Jennings" Tube Glass Wool Solute Evaporation Solute Evaporation in the Gas Phase Evaporation from Surfaces Do we Really Need Complete Evaporation? Conclusions: What are the Relevant Parameters 95 9 Vaporizing Chamber Inserts Designed for Improved Sample Evaporation Improved Heat Transfer Through Turbulence Obstacles Preventing Direct "Shot" into the Column 97

5 Baffles Inverted Cup Glass Bead Liner Minimum Surface Area Evaporation from a Surface: Glass Wool Inserts Promoting Mixing with Carrier Gas Dilution of Sample Vapors Evaporation of High Boiling Compounds Reduction of Recondensation Effect Minimized Dilution for Trace Analysis Dilution Achievable in an Empty Insert Dilution upon Evaporation from a Surface Homogenous Distribution of Sample Across the Insert Inserts Promoting Homogeneous Distribution An Experimental Result: Two Columns in Injector The lodine Experiment Internal Diameter of the Injector Insert Storage of Vapors Deviation of the Split Ratio Undiluted Samples Conclusions Length of the Vaporizing Chamber Sample Degradation and Adsorption in the Injector Drugs Requiring an Empty Insert Empty Insert for Methyl Esters of Hydroxy Fatty Acids Adsorption of Phenols on Glass Wool Various Kinds of Glass Wool for Methyl Esters Solute Degradation in Empty Inserts Degradation in the Injector or in the Column? Deactivation of Inserts and Packing Materials Deactivation of the Inserts? Unsatisfactory Deactivation of Commercial Glass Wool Silylation Packings Coated with Stationary Phase Deactivation by Sample Material Increased Activity Resulting from Sample By-Products Retention of Non-Evaporating Sample By-Products ("Dirt") Evaporation of Solute Material from Involatile Matrix Prevention of Column Contamination Cleaning of Injector Inserts Washing with Strong Acids or Bases Burning the Contaminants Gentle Cleaning Problems Concerning the Split Ratio More Purposeful Search for Errors Systematic Errors 126

6 XIII Recognition of Systematic Errors? Conclusion? "Pre-Set" versus "True" Split Ratio Mechanisms Causing the Split Ratio to Deviate The Pressure Wave Dependence of the Pressure Wave on the Gas Regulation System Recondensation in the Column Inlet Incomplete Evaporation Cool Split Line Cool Needle Valve Charcoal Filters Buffer Volumes Minimizing the Deviation of the True Split Ratio from that Pre-Set Large Injector Insert Long Distance Between Needle Exit and Column Entrance Small Sample Volumes Volatile Solvents Packed Insert Experimental Results Results Conceming Pressure Wave Data on True Split Ratios Working Rules which Help to Prevent Systematic Errors No Quantitation on the Basis of the Pre-Set Split Ratio Use the Internal Standard Method Apply the Extemal Standard Method with Caution Problems Concerning Linearity of Splitting What is "Linear" Splitting? Origins of Non-Linear Splitting Diffusion Speeds How are Different Molecules Redirected? Isokinetic Splitting Incomplete Sample Evaporation Vapors and Droplets Split at Different Ratios Non-Linearity if Components Evaporated to Differing Degrees No Typical Discrimination Patterns Fluctuating Split Ratio Variation of the Split Ratio Pre-Separation of the Sample in the Injector An Example: Cognac Danger of Systematic Errors Techniques for Improving Quantitative Analysis Finding the Best Conditions Determination of the Correct Result On-Column Injection Optimization Procedure 162

7 12.2 Flash Evaporation Concept, High Injector Temperature Injector Temperature = Solute Boiling Point? Maximum Boiling Point Enabling Complete Evaporation Injection of Diluted Samples Small Sample Volumes Easily Evaporating Solvent Hot Needle Injection Mixing Devices An Experimental Result: Determination of Sucrose Evaluation of Flash Evaporation Sample Evaporation in Packed Injector Inserts Concept: Evaporation from Surface Inertness of Injector Packings Seal between Insert and Injector Body? Regulation of the Column Head Pressure Initial Band Widths "Ghost" Peaks as a Result of Injector Bleed Loss of High Boiling Material About a Dispute: the Methanol - 2-Ethyl-1-Hexanol Mixture Fluctuating Split Ratio Another Example: Fatty Acid Methyl Esters High Boiling Samples Undiluted Samples with a High Boiling Matrix High Boiling Solvents Difference between Injector Temperature and Solvent Boiling Point Problems with Solvent Purity Broadened Peaks Eluted Before the Solvent Application to Herbicide Analysis Stop Flow Split Injection Negligible Solute Evaporation Linear Splitting in the Liquid Phase Residence Time in the Injector Advantages of Minimal Evaporation Recommendations Analysis of Alcoholic Beverages as an Example Problems with the Sample Results Interpretation of the Results? Injector Design The Vaporizing Chamber Internal Diameter Length of the Vaporizing Chamber Seal Between Insert and Injector Body? The Gas Regulation System 199

8 XV Back Pressure Regulation versus Needle Valve Pressure Regulators Charcoal Filters and Buffer Volumes in the Split Outlet? Charcoal Filters Buffer Volumes A Problem for Splitless Injection Heating of the Injector Temperature Distribution Injector Head The Bottom of the Injector Column Attachment General Evaluation of Split Injection 208 References A 212 B Splitless Injection 1 Introduction Concept Historical Background How to Perform Splitless Injection Basic Steps of Splitless Injection Closing the Split Exit Carrier Gas Regulation Systems Closing the Needle Valve at the Split Exit? Automated Closure "Ghost" Peaks from Septum Material Purging the Injector Purpose of Purging Duration of the Splitless Period Purge Flow Rate Required Septum Purge Septum Purge Open or Closed During the Splitless Period? Arguments in Favor of Closing Sample Material Entering the Carrier Gas Supply Line Arguments in Favor of Leaving the Septum Purge Opened Conclusions? Sample Volumes Suitable for Splitless Injection Minimum Sample Volume Maximum Sample Volume Calculated Volumes of Solvent Vapor 235

9 XVI Contents 3.4 Experimental Determination of Injector Capacity Determination from Peak Sizes Detection of Solvent Overflow by the Flame Test Quantitative Determination of Losses through Septum Purge Losses Resulting from the Pressure Wave Compared with Those Resulting from Diffusion Dependence of Losses on Vaporizer Volume Influence of the Needle Length Dependence of Injector Capacity on Inlet Pressure and Solvent Recondensation Dependence of Injector Capacity on the Solvent Inserts with a Constriction at the Top? Increase of Pressure during Splitless Injection? Slow Injection? Conclusions Splitless Injection of Large Volumes by the Overflow Technique Concept Flash Evaporation: Solutes Spread with Solvent Vapors Evaporation from Surface May Render Overflow Acceptable Description of the Evaporation Process Retention of Volatile Compounds at the Site of Evaporation Keeping Liquid in Place Desorption of Solute Material Column Temperature During Injection Experimental Results Cooling of the Evaporation Site Rate of Injection Required Flow Rate through the Septum Purge Retention of Liquid Effect of the Solvent on the Loss of Volatile Components Instrumental Requirements Injector Design Packing the Vaporizing Chamber Syringe Needles Maximum Sample Volume Maximum Volume of Liquid Retained Increased Loss of Volatile Components Example Injector Design Volume of the Vaporizing Chamber Minimized Dilution of Sample Vapors Length of the Vaporizing Chamber Position of Column Entrance Length of the Syringe Needle Arresting the Jet of Liquid 268

10 XVII Glass Wool Constriction in Insert Accessible Volumes around the Vaporizing Chamber Deactivation and Cleaning of the Injector Insert Sample Transfer into the Column Nearly Complete Transfer Required The Transfer Process Dependence of Transfer Efficiency on the Duration of the Splitless Period Acceleration of Sample Transfer by Solvent Recondensation Mechanism of Acceleration Efficiency of the Recondensation Effect Experimental Results Tests on Completeness of the Sample Transfer Rapid Check via Forced Transfer Conditions Check via On-Column Injection Testing the Injector Conclusions Concerning Injector Volume Some Guidelines on the Splitless Period Conclusions Concerning Column Diameters Increase of Inlet Pressure During Splitless Injection? Shape of Initial Bands Introduction Characterization of the Two Band Broadening Effects Band Broadening in Space Band Broadening in Time The Initial Band in Splitless Injection Shape of the Band Optimization of Carrier Gas Flow Rate Duration of the Splitless Period Reconcentration by Cold Trapping Reconcentration of Bands Broadened in Time Principles of Cold Trapping Reconcentrating Power and Difference in Column Temperature The "15 Degree Rule" Reconcentration Factor Required Rapid Analysis by Manual Injection Reproduction of Absolute Retention Times Problems with Disturbed Baselines "Ghost" Peaks "Ghost" Peaks Presuppose Reconcentration Sources of "Ghost" Peak Material Tests to Determine the Source of Contaminants Evaluation of Cold Trapping 314

11 XVIII Contents 9 Reconcentration by Solvent Effects Simplified Mechanism Recondensation of Solvent Requirements for Solvent Effects Amount of Recondensed Solvent Required Factors Determining Recondensation Guidelines on Column Temperature and Solvent Boiling Point Polarity of Solvent Effects on Retention Times Effect of Solvent Trapping Effect of Phase Soaking Phase Stripping by Recondensed Solvent Band Broadening in Space in Splitless Injection Flow of Sample Liquid The Length of the Initial Solute Bands The Terminal Band Length Shape of the Initial Band Solute Material in the Injector-Thermostatted Column Inlet Visual Observation Effect on Initial Band Shape Extent of Peak Broadening and Distortion Sample Wetting the Surface of the Stationary Phase Sample Not Wetting the Surface of the Stationary Phase Defocusing Effect of the Cool Injector Base Avoidance of Peak Distortion Cold Trapping without Solvent Recondensation Retention Gap Technique Uncoated Column Inlets Uncoated Precolumns Press-Fit Connections Examples of the Use of Reconcentration Effects Dioctyl Phthalate Traces of Tetrachloroethylene Extraction of Water with Pentane Semivolatile Components of Cigarette Smoke Solvent Residues in Pharmaceutical Preparations Headspace Analysis Solvent Effects at Elevated Column Temperatures Problems with Quantitative Analysis Data on Precision from the Literature Limited Utility of Literature Data Selective Evaporation from the Syringe Needle Injector Overloading Incomplete Transfer of Sample Vapors 352

12 XIX Observations with the lodine Experiment High Standard Deviations as a Warning Inaecurate Absolute and Relative Peak Areas Adsorption in the Vaporizing Chamber Retention in the Injector Degradation of Labile Solutes Degradation in the Injector or in the Column? Mechanisms of Solute Degradation Effect of Sample By-Products Countermeasures against Solute Degradation Some Examples Matrix Effects Experimental Data with DC Experimental Data with Triglycerides in the Sample Matrix Conclusions from Experimental Results Interpretation of the Experimental Results Effects on Quantitation in Splitless Injection Examples of Analyses with Systematic Errors Conclusions Concerning Matrix Effects High Column Flow Rate Long Splitless Period? High Injector Temperature Methods Suitable for Quantitation Glass Wool in the Injector Insert? Uncoated Pre-Columns Effects of "Dirt" Reasons for Using Uncoated Pre-Columns Length of the Contaminated Zone Particles Driven Far into the Column Triglycerides Passing through the Pre-Column Direct Injection Concept Injector Design Vaporizing Chamber Connection of the Column Septum Purge On-Column Injection? An Example Working Rules Injection of Large Volumes Evaluation of Direct Injection General Evaluation of Splitless Injection 392 References B 395

13 XX Contents C Problems Arising from the Heated Syringe Needle in Vaporizing Injection 1 introduction Sample Evaporation inside the Needle Short Description of the Problems Avoid Evaporation in the Needle or Render Elution as Complete as Possible Evaporation Inside the Needle Which Part of the Sample Evaporates? Liquid Passing Through the Needle Elution of Remaining Liquid Models of Elution from the Needle Distillation in the Needle Gas Chromatography in the Needle Ejection from the Needle Conclusions How Much is Really Injected? Different Interpretations of "Sample Volume" Communicating "Sample Volumes" Effects on Quantitative Analysis Syringe Needle Handling Techniques Definitions Experimental Determination of Losses in the Needle The Method with Two Instruments Experiment with a Single Instrument Rapid Test during Routine Analysis Comparison of Needle Handling Techniques Filled Needle Injection Slow Injection Cool Needle Injection Hot Needle Injection Solvent Flush Injection Method Optimized Parameters Solvent Flush or Hot Needle Method? Poor Suitability for Splitless Injection High Boiling Flushing Solvents Air Plug Injection Sandwich Injection Heating the Needle after Injection? 430

14 XXI 5 Dependence of Discrimination on Sample Volume Experimental Results Discussion of Results Conclusions Solvent and Solutes Volatility of Solvent Type of Solute Adsorption in the Syringe Needle Dependence on Polarity of Solvent Adsorption Suppressors "Memory Effects" Arising from the Syringe Injector Temperature Imposed Temperature Temperature Gradient towards the Septum Different Results from One Injector to Another Experiment Results Conclusions Actual Distribution of Injector Temperature Well Heated Injector Head Are Septa Sufficiently Thermostable? Possibilities of Avoiding Evaporation in the Needle High Boiling Sample Matrix Designing a Method Experiments with Split Injection Cooled Septum Plunger-in-Needle Syringe Accuracy of Sample Volume Premature Elution Losses through the Septum Purge Cooled Needle Technique Fast Injection by Autosampier Cold Injection Techniques Summarizing Guidelines 457 References C 458

15 XXII Contents D Programmed Temperature Vaporizing (PTV) Injection 1 Introduction Concept of PTV Injection Historie Background Intention of this Section Injector Design Minimum Thermal Mass Thermostatting of the Vaporizing Chamber Injector Inserts Packed Insert? Glass Wool Other Packing Materials Baff les Empty Inserts Conclusion Prevention of Sample Evaporation in the Syringe Needle Initial Temperature below Corrected Boiling Point Advantages PTV Split Injection The Split Ratio No Pressure Wave Possibility of Suppressing Recondensation Complete Sample Evaporation Experimental Results Linearity of Splitting Moment of Temperature Increase Duration of Injector Heating When to Start Programming of the Oven Initial Band Widths Data on Precision and Accuracy Reported in Literature PTV Splitless Injection Principles Prevention of Injector Overflow Solvent Transfer before Heating Solvent Recondensation in the Column Inlet Transfer of Solute Material Expected Advantages of Improved Transfer Efficiency 482

16 XXIII Experimentally Determined Transfer Times Retention Power in the Vaporizing Chamber Experimental Results with Labile Compounds Large Sample Volumes Retention of Liquid in the Insert Evaporation of Large Volumes of Solvent Column Temperature During Transfer Matrix Effects in PTV Splitless Injection Evaluation of PTV Splitless Injection PTV Direct Injection Nomenclature Construction of the Injector Injector Inserts High Performance Insert On-Column Insert Packed Insert Results PTV Solvent Split Injection Concept Reasons for Eliminating the Solvent Vapors Backflush of the Vaporizing Chamber Losses of Volatile Solutes Injector Insert as Pre-Column Optimization of Conditions Packings in the Vaporizing Chamber Retention by Co-Solvent Trapping? Injection of Large Volumes Volume of Liquid Retained by the Packing Injection Speed Examples Evaluation of PTV Solvent Split Injection PTV Vapor Overflow Starting Point: Difficulties With Solvent Split Injection Concept of PTV Vapor Overflow Evaporation under Vacuum? Miminum Losses of Volatile Compounds Reversed Column Flow Reduced Effect of Leaching by Water Injector Design Rates of Evaporation Features of the Technique Reconcentrating Headspace Analysis 516

17 11 Initial Bands in PTV Injection Dependence of Band Widths on Heating Rate Injector Insert as GC Pre-Column Initial Band Shapes Cold Trapping Solvent Effects Band Broadening in Space in PTV Injection? Concluding Remarks Summary of Important Characteristics Fields of Application Not a Universal Injector 525 References D 527 Appendix 1: Selection of the Correct Injection Technique 531 Appendix 2: Selection of Parameters for Classical Split and Splitless Injection 532 Appendix 3: Glossary of the Most Important Terms Used in the Text 534 Subject Index 539