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- Herausgeber: John Wiley & Sons
- Kategorie: Fachliteratur
- Sprache: Englisch
A guide to the fundamentals of applied gas chromatography and the process gas chromatograph, with practical procedures for design and troubleshooting This comprehensive resource provides the theory that underpins a full understanding of the fundamental techniques of gas chromatography and the process analyzer. Without relying on complex mathematics, the book addresses hands-on applications of gas chromatographs within process industries. The author - a noted expert on the topic - details both the scientific information needed to grasp the material presented and the practical applications for professionals working in the field. Process Gas Chromatographs: Fundamentals, Design and Implementation comprises 15 chapters, a glossary of terms and a series of self-assessment questions and quizzes. This important resource: * Describes practical procedures for design and troubleshooting * Contains concise chapters that provide a structured course for advanced students in process engineering * Reviews the fundamentals of applied gas chromatography * Details the operation and maintenance of process gas chromatographs * Offers a summary, and self-assessment questions, for every chapter * Is written by an international expert in the field with extensive industry knowledge and teaching experience in courses on process sampling systems and gas chromatography Written for process analyzer engineers and technicians, application engineers, and industrial environmental engineers, Process Gas Chromatographs: Fundamentals, Design and Implementation offers an essential guide to the basics of gas chromatography and reviews the applications of process gas chromatographs in industry today.
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Table of Contents
Cover
Preface
References
Contributors
Acknowledgments
Part One: PGC fundamentals
1 An introduction
Chromatographic separation
The gas chromatograph
The oven
The sample injection valve
The column
The detector
Did you get it?
Self‐assessment quiz: SAQ 01
Student evaluation test: SET 01
References
Figures
New technical terms
2 Peak shape
How columns work
How peaks form
Some conclusions
Did you get it?
Self‐assessment quiz: SAQ 02
Student evaluation test: SET 02
References
Figures
Equation
Symbols
New technical terms
3 Separation
How peaks get separated
Measurements from chromatograms
Did you get it?
Self‐assessment quiz: SAQ 03
Student evaluation test: SET 03
References
Figures
Equations
Symbols
New technical terms
4 Peak patterns
Predictable patterns in peak position
Predictable patterns in peak width
Predictable patterns in retention
Did you get it?
Self‐assessment quiz: SAQ 04
Student evaluation test: SET 04
References
Figures
Equations
Symbols
New technical terms
Part Two: PGC analytics
5 Industrial gas chromatographs
Process analyzers
Process gas chromatographs
Competing technologies
The PGC analytics unit
Did you get it?
Self‐assessment quiz: SAQ 05
Student evaluation test: SET 05
References
Figures
Table
Symbol
New technical terms
6 Carrier gas system
Choice of carrier gas
Carrier gas purity
Carrier gas supply system
Pressure regulation
Flow regulation
Did you get it?
Self‐assessment quiz: SAQ 06
Student evaluation test: SET 06
References
Table
Figures
Symbols
New technical terms
7 Sample injection
Introduction
Injecting gas samples
Injecting liquid samples
Other techniques
Did you get it?
Self‐assessment quiz: SAQ 07
Student evaluation test: SET 07
References
Tables
Figures
Symbols
New technical terms
8 Chromatographic valves
Valve technology
The strange effect of competition
Valve types
Valve leaks
Valve leak mitigation
Did you get it?
Self‐assessment quiz: SAQ 08
Student evaluation test: SET 08
References
Table
Figures
New technical terms
9 Column systems
Two fundamental issues
The temperature ramp solution
The multiple column solution
The choice
Delayed injection
Four types of column system
Type A: A single column
Type B: Multiple columns, single detector
Type C: Multiple detectors, single injector
Type D: Multiple sample injectors
Elemental column systems
Backflush column system
Distribution column system
Heartcut column system
The real power
Endnote
Did you get it?
Self‐assessment quiz: SAQ 09
Student evaluation test: SET 09
References
Table
Figures
New technical terms
10 Detectors
Introduction
Types of detector
Thermal conductivity detector
Flame ionization detector
Flame photometric detector
Other detectors
Did you get it?
Self‐assessment quiz: SAQ 10
Student evaluation test: SET 10
References
Tables
Figures
Equations
Symbols
New technical terms
11 Temperature control
Need for stability
The air‐bath oven
The airless oven
Direct column heating
PGC standardization
A closing thought
Did you get it?
Self‐assessment quiz: SAQ 11
Student evaluation test: SET 11
References
Table
Figures
New technical terms
Part Three: PGC control
12 Event scheduling
A sequence of actions
Timing mechanisms
Control of analyzer operation
Peak identification
Did you get it?
Self‐assessment quiz: SAQ 12
Student evaluation test: SET 12
References
Figures
New technical terms
13 Data display techniques
The chromatogram display
The bargraph display
The trend record
Digital signal processing
PGC function alarms
Did you get it?
Self‐assessment quiz: SAQ13
Student evaluation questions: SET‐13
References
Table
Figures
New technical terms
14 Peak area integration
Digital chromatogram processing
Quantifying the analyte peaks
Measuring overlapping peaks
Did you get it?
Self‐assessment quiz: SAQ 14
Student evaluation test: SET 14
References
Figures
New technical terms
15 Calibration
Measurement principles
Calibration methods
Did you get it?
Self‐assessment quiz: SAQ 15
Student evaluation test: SET 15
References
Figure
Equations
Symbols
New technical terms
Answers to self‐assessment questions
SAQ‐01
SAQ‐02
SAQ‐03
SAQ‐04
SAQ‐05
SAQ‐06
SAQ 07
SAQ 08
SAQ 09
SAQ 10
SAQ 11
SAQ 12
SAQ 13
SAQ 14
SAQ 15
BibliographyBibliography
Glossary
Index
End User License Agreement
List of Tables
Chapter 5
Table 5.1 Comparing the Process Photometer and the PGC.
Chapter 6
Table 6.1 Properties of Common Carrier Gases.
Chapter 7
Table 7.1 Sample Loop Tubing Sizes.
Table 7.2 Temperature, Pressure, and Liquid Density.
Table 7.3 Conversion of Percentage Units for Gas Samples.
Chapter 8
Table 8.1 Summary of PGC Chromatographic Valves.
Chapter 9
Table 9.1 Summary of Column Systems.
Chapter 10
Table 10.1 Key Features of the Thermal Conductivity Detector.
Table 10.2 Key Features of the Flame Ionization Detector.
Table 10.3 Key Features of the Flame Photometric Detector.
Table 10.4 Summary of PGC Detectors.
Chapter 11
Table 11.1 Summary of Oven Heating Methods.
Chapter 13
Table 13.1 The First Microprocessor‐Based PGC Control Units.
List of Illustrations
Chapter 1
Figure 1.2 Basic Gas Chromatograph.
Figure 1.3 Typical Gas Sample Injector Valve.
Figure 1.4 Typical Gas Chromatographic Columns.
Figure 1.5 A Simple Column Switching System.
Figure 1.6 Three Kinds of Capillary Column.
Figure 1.7 Typical Chromatograms.
Figure 1.8 A Real Chromatogram.
Chapter 2
Figure 2.1 Gases Dissolve in Liquids.
Figure 2.2 A Different Gas.
Figure 2.3 Forming an Equilibrium.
Figure 2.4 The Carrier Gas Moves.
Figure 2.5 The Second Equilibrium.
Figure 2.6 The Third Equilibrium.
Figure 2.7 The Fourth Equilibrium.
Figure 2.8 The Fifth Equilibrium.
Figure 2.9 Effect of Having More Equilibria.
Chapter 3
Figure 3.1 Effect of Component Solubility.
Figure 3.2 Draw Your Own Chromatogram.
Figure 3.3 Significance of an Air Peak.
Figure 3.4 Typical Chromatogram Measurements.
Chapter 4
Figure 4.1 The Doubling Rule.
Figure 4.2 Diagnosing Peak Identities.
Figure 4.3 Effect of Temperature Ramp.
Figure 4.4 Fast Analysis of Natural Gas.
Figure 4.5 Measurements for Separation Factor.
Figure 4.6 Measurements for Resolution.
Figure 4.7 Same Separation, Different Width.
Figure 4.8 Patterns in Resolution.
Chapter 5
Figure 5.2 A Very Old PGC with an Even Older PGC Engineer.
Figure 5.3 Process Gas Chromatograph Functions.
Figure 5.4 A Typical PGC Column Oven.
Chapter 6
Figure 6.1 Effect of Impure Carrier Gas.
Figure 6.2 A Vacancy Peak in a Real Chromatogram.
Figure 6.3 Automatic Changeover Regulator.
Chapter 7
Figure 7.1 Typical PGC Sample Injector Valve.
Source: ABB Process Analytics
...
Figure 7.2 Gas Sample Injection.
Figure 7.3 Atmospheric Referencing Systems.
Figure 7.4 Liquid Sample Injection.
Figure 7.5 Plunger Valve for Liquid Sample Injection.
Source: ABB Process An
...
Chapter 8
Figure 8.1 Function of a Spool Valve.
Figure 8.2 Early Six‐Port Slide Valve.
Figure 8.3 Slide Valve for Liquid‐Sample Injection.
Figure 8.4 Slide Valve for Gas‐Sample Injection.
Figure 8.5 Rotary Gas‐Sample Injector Valve.
Figure 8.6 Example of a Valve Rotor and Stator.
Figure 8.7 Six‐Port Rotary Liquid‐Sample Injector.
Figure 8.8 Miniature Multiport Rotary Valve.
Figure 8.9 Pressure‐Seal Diaphragm Valve.
Figure 8.10 Piston/Plunger‐Seal Diaphragm Valve.
Figure 8.11 Diaphragm Valve‐Switching Mechanism.
Figure 8.12 Plunger Valve for Liquid Sample Injection.
Figure 8.13 Typical Plunger Valve Operation.
Figure 8.14 Purging Grooves to Intercept Leakage.
Chapter 9
Figure 9.1 Type A Column System.
Figure 9.2 Type B Column System.
Figure 9.3 Type C Column System.
Figure 9.4 Type D Column System.
Figure 9.5 Ten‐Port Gas Injector and Backflush System.
Figure 9.6 Distribution Column System.
Figure 9.7 Heartcut Column System.
Figure 9.8 A Real Heartcut Chromatogram.
Source
: Author's collection.
Figure 9.9 Trap‐and‐Hold Column System.
Chapter 10
Figure 10.1 Peak Distortion by Slow Detector Response.
Figure 10.2 Principle of the Thermal Conductivity Detector.
Figure 10.3 Intrinsically‐Safe Thermal Conductivity Detector.
Figure 10.4 Typical Wheatstone Bridge Circuit.
Figure 10.5 High‐Sensitivity Thermal Conductivity Detector.
Figure 10.6 PGC Flame Ionization Detector.
Figure 10.7 Principle of the Flame Ionization Detector.
Figure 10.8 Typical PGC Flame Photometric Detector.
Figure 10.9 Principle of the Flame Photometric Detector.
Figure 10.10 Principle of the Electron Capture Detector.
Figure 10.11 Principle of the Pulsed Discharge Detector.
Chapter 11
Figure 11.1 1967 Beckman Series D Air‐Bath Oven.
Source: Beckman Historical
...
Figure 11.2 Typical Air‐Bath Heater.
Figure 11.3 Dual Oven in the Yokogawa GC1000 Mark II.
Source: Yokogawa Elec
...
Figure 11.4 The 1971 GEC Elliot Chromatograph Mark 6.
Figure 11.5 The 1987 Yokogawa GC8 PGC Oven.
Source: Yokogawa Electric Corpo
...
Figure 11.6 The 2008 ABB PGC1000 Transmitter.
Source: ABB Process Analytics
Figure 11.7 The 2014 Rosemount 370XA PGC.
Source: Rosemount Inc
.
Reproduced
...
Figure 11.8 The 2009 Rosemount 700XA PGC.
Source: Rosemount Inc
.
Reproduced
...
Figure 11.9 An Airless Oven and Separate Detector Compartment.
Source: Siem
...
Figure 11.10 Fast PGC Separation of Paraffins.
Source: ABB Process Analytics
Figure 11.11 A PGC With Directly‐Heated Column.
Source: ABB Process Analytic
...
Figure 11.12 The 2007 Teledyne
Calidus
Portable Analyzer.
Source: Teledyne
...
Figure 11.13 A Direct‐Heated Column Module.
Source: Teledyne Analytical Inst
...
Figure 11.14 Fast Isothermal Analysis of Hydrocarbons.
Source: Qmicro B.V
.
Figure 11.15 The 2002 Siemens MicroSAM PGC.
Source: Siemens Analytical Prod
...
Chapter 12
Figure 12.3 Circa 1964 Beckman Model 620 Programmer.
Figure 12.4 The 1978 AAI Optichrom 2100 Control Unit.
Figure 12.5 Typical Method of Setting Peak Parameters.
Figure 12.6 The 1985 Servomex Model CP 409 Programmer.
Figure 12.7 The 1985 Servomex Model CP 403 Programmer.
Figure 12.8 The 1979 Yokogawa Model GC6P Control Unit.
Figure 12.9 The 2010 Envent Model 131 Natural Gas PGC.
Chapter 13
Figure 13.1 Historical PGC Data Displays.
Figure 13.2 The 1960 Greenbrier Chroma‐Matic III‐A Programmer.
Figure 13.3 Principle of an Analog Peak Picker.
Figure 13.4 The 1975 Bendix 7000 Digital Programmer.
Chapter 14
Figure 14.1 Sampled‐Data Method of Digitizing a Peak.
Figure 14.2 The 2010 Rosemount 2350A Control Unit.
Figure 14.3 The 2002 Siemens Maxum™ Edition II PGC.
Figure 14.4 Error by Negative Peak in Peak Gate.
Figure 14.5 Area Allocation by Perpendicular Drop.
Figure 14.6 Error by Perpendicular Drop for 9:1 Peak Areas.
Figure 14.7 Error by Integrating to a Valley Point.
Figure 14.8 Error by Integrating Valley‐to‐Valley.
Figure 14.9 Angular Drop Method for 9:1 Peak Areas.
Figure 14.10 Tangent Skim for a Rider Peak.
Chapter 15
Figure 15.1 A Validation Control Chart.
Source: Siemens Analytical Products
...
1
Figure 1.1 A Classic Process Gas Chromatograph.
2
Figure 5.1 A Classic Process Gas Chromatograph.
3
Figure 12.1 The 1979 Beckman Model 6750 Chromatograph Data Processor.
Figure 12.2 The 2008 ABB Model PGC5000A Master Controller.
Guide
Cover
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Process Gas Chromatographs Fundamentals, Design and Implementation
Fundamentals, Design and Implementation
TONY WATERS
Copyright
This edition first published 2020
© 2020 John Wiley & Sons Ltd
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions.
The right of Tony Waters to be identified as the author of this work has been asserted in accordance with law.
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For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com.
Wiley also publishes its books in a variety of electronic formats and by print‐on‐demand. Some content that appears in standard print versions of this book may not be available in other formats.
Limit of Liability/Disclaimer of Warranty
In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages.
Library of Congress Cataloging‐in‐Publication Data
Names: Waters, Tony, author.
Title: Process gas chromatographs : fundamentals, design and implementation
/ by Tony Waters.
Description: Hoboken, NJ, USA : Wiley, 2020. | Includes index.
Identifiers: LCCN 2020001270 (print) | LCCN 2020001271 (ebook) | ISBN
9781119633044 (cloth) | ISBN 9781119633006 (adobe pdf) | ISBN
9781119633013 (epub)
Subjects: LCSH: Gas chromatography–Equipment and supplies.
Classification: LCC QD79.C45 W38 2020 (print) | LCC QD79.C45 (ebook) |
DDC 543/.850284–dc23
LC record available at https://lccn.loc.gov/2020001270
LC ebook record available at https://lccn.loc.gov/2020001271
Cover Design: Wiley
Cover Image: Process Gas Chromatographs at the INEOS Olefin Plant in Cologne, Germany. Photo © INEOS in Cologne, 2019.
To Marilyn
Process Gas Chromatographs installed in a prefabricated air‐conditioned analyzer shelter for an ethylene plant in Texas. Image © Yokogawa Corporation of America, 2018. Reproduced with permission.
Preface
Welcome to the world of Process Gas Chromatography!
This book focuses on the Process Gas Chromatograph (PGC). There are dozens of fine books on the science of gas chromatography but few on the technology of the process instrument. I found only two previous books dedicated to online gas chromatographs (Huskins 1977; Annino and Villalobos 1992).
Process gas chromatographs are complex instruments, and the people that design and operate them need special knowledge and unique skills. With that in mind, I designed the book to serve the needs of the journeyman analyzer technician, the process instrument engineer, and the process analyzer specialist.
PGC is a practical technology, and this is a practical book. It's an effective classroom training manual for those currently learning the art and a handy reference manual for those already practicing it.
Chapters are deliberately compact, suitable for a weekly reading program or as focused lessons in an educational course. Each chapter ends with a summary of knowledge gained and a self‐assessment quiz with answers provided. In addition, there are nine optional test questions for students; three easy, three moderate, and three challenging.
Why is such a book necessary?
Anyone working in the fluid processing industries knows that their knowledge base is in full flight. Due to staffing reductions and mass retirements our industry is losing decades of hard‐won experience.
Walter Jennings and Colin Poole recently expressed this situation rather well (Jennings and Poole 2012, 72):
This [automation of gas chromatographs] has led to a continuing decline in the expertise of the average practicing chromatographer from the mid‐1980s to the present time. This can be perilous, because everything from column selection to trouble‐shooting skills is based on a fundamental knowledge of chromatographic principles, the absence of which degrades the quality and usefulness of the information acquired by these instruments. To address these problems requires a massive educational effort before the knowledge is lost and the usefulness of gas chromatography to decision makers is called into question.
There can be no clearer call to justify this book. While the authors were writing to laboratory chemists, those working on process gas chromatographs also need a fundamental knowledge of chromatographic principles presented in a way that facilitates a massive educational effort. This textbook sets out to satisfy those needs. It's primarily written for process analyzer engineers and technicians but should be helpful to anyone using or maintaining a process gas chromatograph.
To succeed in its mission, a book needs to so excite readers that they want to read more. It should be so useful that they immediately return to it when they need information. Yet the average book on gas chromatography is abysmally boring and poses an intellectual challenge even to post‐doctoral scientists, let alone the lonely guy faced with fixing a broken process chromatograph at midnight.
This text teaches the fundamental knowledge of process gas chromatography by encouraging the reader to think critically about what is happening in the instrument, mostly without recourse to analogy or math. It also describes some practical procedures for design or troubleshooting.
So, here you have it. A clear yet detailed book that is ideal for classroom instruction, private study, or distance learning. Focused chapters unfold the technology of a process gas chromatograph to an engineer or technician who may have no previous experience of the technique. The content is basic, yet thorough, so it should meet the needs of many readers.
I'm glad that you're here. I hope you enjoy the book!
Tony WatersAtascadero, CaliforniaJanuary 2020
References
Cited
Annino, R. and Villalobos, R. (1992).
Process Gas Chromatography
. Research Triangle Park, NC: Instrument Society of America.
Huskins, D.J. (1977).
Gas Chromatographs as Industrial Process Analyzers
. New York, NY: Pergamon Press.
Jennings, W.G. and Poole, C.F. (2012). Milestones in the development of gas chromatography. In:
Gas Chromatography
(ed. C.F. Poole), 1–18. Oxford, UK: Elsevier.
Contributors
An international team of expert chromatographers has peer‐reviewed the technical content of this text. This Editorial Advisory Board comprised the experienced analyzer engineers listed below. We gratefully acknowledge their contributions. Culpability for remaining errors or omissions rests entirely on the author.
Jerry Clemons, PhD
Process Gas Chromatograph Consultant
Formerly, General Manager
ABB Process Analytics
Ronceverte, West Virginia, USA
Jerry has worked with gas chromatographs during his entire career starting at Virginia Polytechnic University where he earned his PhD with Dr. Harold McNair.
He has held many engineering and management positions at ABB Process Analytics and its predecessors, always focused on their process gas chromatographs. Now retired from active duty, he continues to provide his technical expertise as a consultant to that company.
Jerry has 50 years of experience working with process gas chromatographs.
R. Aaron Eidt, BSc
Process Analyzer Consultant
PEAK PERFORMANCE Analytical Consulting Ltd.
Delta, British Columbia, Canada
Formerly, Analyzer and PGC Manager
Dow Chemical Canada
Fort Saskatchewan, AB, Canada
Aaron is a chemist with 25 years of experience developing new GC methods for research and industrial chromatographs at Dow Chemical Canada. Aaron specialized in process analyzer validation, troubleshooting and performance improvement. For several years, he led the Dow Global Process Chromatography Technology Network.
Since retiring from Dow, Aaron has had process analyzer consulting engagements with the Sadara Chemical Company in Jubail, Saudi Arabia, and with MEGlobal.
Aaron has developed and instructs both introductory and advanced troubleshooting training courses in process gas chromatography for analyzer maintenance technicians.
Aaron has 30 years of experience practicing industrial gas chromatography.
Zoltán Hajdú, RNDr
Marketing Manager
Analyzer System Integration
Yokogawa Europe
Formerly, Analyzer Systems Consultant for Yokogawa
Central and East Europe
Responsible for analyzer system design and analyzer selection, including on site start up, and trouble‐shooting of Yokogawa process chromatographs and analyzer systems throughout Central and East Europe.
Previously, Supervisor of Process Analyzers at Slovnaft Refinery in Bratislava. Now responsible for analyzer system sales for Yokogawa in Europe.
Zoli has 12 years' experience working with various process gas chromatographs.
Phil Harris, BSc MSc
Process Analyzer Consultant
President
Insight Analytical Ltd.
Calgary, Alberta, Canada
Formerly, Engineering Manager AMETEK Western Research
Calgary, Alberta, Canada
Phil has a BSc in Physics and a Master's in Chemistry. His career began in the Research Chemistry branch of Atomic Energy of Canada, where he designed spectroscopic analyzers and built algorithms for numerical analysis of spectral and chemical data.
Phil has been an independent consultant since 1998, primarily on the development of process analytical solutions in the oil, gas, and petrochemical industry. He provided services to AMETEK for a number of years and developed most of the numerical analysis algorithms used on the 900 series of Analyzers.
He has published over 25 papers and has given training courses on spectroscopy and process analyzer sample systems all over the world.
Phil has 35 years of experience with industrial process analyzers, mainly with process spectrometers.
Michael Hoffman
Business Development Manager
Siemens Industry, Inc.
Analytical Products & Solutions
Houston, Texas USA
Michael started in industry at Phillips 66, and continued the journey with Standard Oil Chemicals, BP, Innovene, and INEOS.
His initial work was with laboratory chromatographs. After transitioning to process chromatographs, he focused on online analyzer reliability, advanced control support, materials handling, and analyzer data management technologies.
Michael joined Siemens in 2007. He now provides marketing and technical support for analytical solutions, communications, PGC applications, and sample handling system designs.
Michael has 37 years of experience working with laboratory and process gas chromatographs.
Dirk Horst
Process Analyzer Consultant
Heerhugowaard, Netherlands
Formerly Global QMI Consultant
Shell Global Solutions Team
Amsterdam, Netherlands
Dirk has long experience with process analyzers, including startup assignments at Shell jobsites in Germany, India, Nigeria, and Russia. He is also well known for his many classroom and practical training programs for analyzer maintenance technicians.
Dirk has 34 years of experience working with process gas chromatographs.
Dr. Daniel Kuehne
Process Gas Chromatograph Consultant
Siemens AG
Analytical Products and Solutions
Manufacturing Karlsruhe, Germany
Daniel studied Chemistry at the University of Bremen and did his diploma and doctorate thesis in Analytical Chemistry.
He joined Siemens in 2005 as method developer for process GCs. He stayed in method development for 11 years, whereof the last five years being the head of the PGC method developer team.
Since 2016 he has been making technical evaluations of PGC inquiries and working as a technical consultant for sales and customers and additionally as technical advisor for the GC method development team.
Daniel has 14 years of experience working with process gas chromatographs.
James Leonard, PhD
Process Analyzer Specialist
Eastman Chemical Company
Kingsport, Tennessee, USA
James received his PhD in Analytical Chemistry from The Ohio State University. He has 20 years of experience working in the field of Process Analytics at Eastman Chemical. During this time, James has designed, installed, and commissioned analyzer systems incorporating modern on‐line techniques throughout the world.
He has presented lectures on process analytics at universities and other organizations to promote the use of on‐line technologies to improve process control and reduce waste.
James has 17 years of experience working with process gas chromatographs.
Harald Mahler
Process Analyzer Engineer
Siemens AG
Analytical Products and Solutions
Karlsruhe, Germany
Harald studied chemistry at the University of Applied Science in Reutlingen. Since 1989 he has gained experience in process analytics in various engineering and management positions within Siemens AG. He has authored and presented many technical papers within the process analytical community.
Harald has held engineering and management roles in application and method development, project management, industry marketing, and product management. Currently he is Global Sales and Business Development Manager for process analytics within the Process Automation Division, serving mainly the petrochemical, oil and gas, and renewable energy markets.
Harald has 29 years of experience working with process gas chromatographs.
Gen Matsuno, ME
Product Manager
Quality Analyzer Systems
General Manager
IA‐PS Analyzer Center
Yokogawa Electric Corporation
Mitaka, Tokyo, Japan
Matsuno‐san was leader of the Yokogawa GC8000 PGC development team. In addition to his experience of designing process gas chromatographs, he has five years of experience as a laboratory GC user.
Gen‐san has 12 years of experience working with process gas chromatographs.
Takashi Matsuura, BE
Senior Field Engineer
Nippon Swagelok FST, Inc.
Yokohama, Japan
Formerly, Manager of Process GC Development
Yokogawa Electric Corporation
Taka designed the Yokogawa GC1000 PGC oven and was leader of the engineering team that developed the Yokogawa GC1000 Mk2 PGC. He also wrote the specifications for the GC8000 PGC.
Taka has over 25 years of experience working with process gas chromatographs.
Suru Patel, PhD
Process Analyzer Consultant
Patex Controls Ltd.
Calgary, Alberta, Canada
Formerly, Distinguished Engineering Associate for Process Analyzers
Exxon Chemical Company, Sarnia, Canada, and Singapore
In addition to his process analyzer engineering work, Suru developed PGC training courses for process analyzer technicians and PGC data users.
Previously, for several years, Suru was a PGC Applications Engineer at Servomex Company in the UK and was the Lead Analyzer Engineer in Houston for Exxon's Singapore Chemical Complex project. He was also the development engineer for a new flame ionization detector.
Suru has 40 years of experience working with process gas chromatographs.
Ivan Rybár, PhD
Head of Process Analyzer Group
Slovnaft MaO, a.s.
Bratislava, Slovakia
Formerly Research and Teaching Assistant, Department of Analytical Chemistry, Comenius University
Bratislava, Slovakia
For 10 years now, Ivan has been responsible for the reliability of all process analyzers at the refinery, including the maintenance of existing systems and the design of new installations. As the supervisor of 26 analyzers, he creates work procedures and provides training and support for his team.
In 2013 and 2015, he was twice awarded the accolade “Slovnaft Star.”
Previously he worked as an analyzer engineer for several companies providing engineering services to industrial plants, including the selection of analyzers and the design of complete sampling systems.
During his time at university, Ivan developed new methods and taught several graduate courses in liquid chromatography. He has recently published four scientific papers on this work.
Ivan has 15 years of experience working with process gas chromatographs.
Eric Schmidt, PhD
Principal Research Scientist
The Dow Chemical Company
Analytical Sciences
Freeport, Texas, USA
Eric Schmidt received his PhD in Analytical Chemistry from The University of Texas at Austin. He has worked at the Dow Chemical Company in Freeport, Texas, for over 20 years as a Research Scientist where he spends his time developing new on‐line process measurements for R&D and manufacturing. He is currently leading the On‐line Chromatography Strategic Capability Team at Dow.
Eric has 20 years of experience working with process gas chromatographs.
Acknowledgments
Hearty thanks all friends and associates who contributed material to this text. Many of those listed below audited the beta‐test of an online tutorial based on this textbook. Their contributions of time and knowledge are much appreciated.
Minh Anh
Vietnam
Brian Aplin
South Carolina
Dale Arstein
Ohio
Masafumi Awano
Japan
Ken Backus
Texas
Hesham El Banna
Saudi Arabia
Eddie Beezemer
Netherlands
Linda Bonnette
Texas
Mark Booth
Scotland
Danny van den Burg
Netherlands
Bruno Chaurand
France
Alice Chin
Malaysia
Alex Chu
England
Scott Cookson
Australia
Marcus Creaven
Ireland
Dave Demsey Sr.
Pennsylvania
Matt Dixon
Ohio
Ana Dominguez
Switzerland
Kevin Fajri
Indonesia
Mikhail Fedorets
Russia
Mike Frost
Australia
Victor Alberto Fuentes
Spain
Keisuke Fukada
Japan
Yves Gamache
Quebec
Udo Gellert
Germany
Roger Glass
England
Urich Gokeler
Texas
Matt Hasenohr
Oregon
Darryl Hazlett
Texas
Jack Holland
England
Tom Huddle
England
Damian Huff
West Virginia
Humberto Serrato Hurtado
Colombia
Joe Iveljic
Ohio
Nick Iverson
Minnesota
Samson Jacob
Abu Dhabi
Jayson Zhang Ji
Singapore
Kyle Juist
Ohio
Eric Kayla
California
Eric Kvarda
Ohio
Bert Laan
England
André Lamontagne
Quebec
Wilco Landkroon
Netherlands
Rudi Lehnig
Germany
Tim Lenior
Netherlands
Hank Liu
Singapore
Aldemar Figueroa Loza
Colombia
Karim Mahraz
Ohio
Rogério Matos
Brazil
Bill Menz
Ohio
John Meyer
Germany
Thomas Neuhauser
Oklahoma
Kentaro Nomura
Japan
Doug Nordstrom
Ohio
Tatsuya Ohkoshi
Japan
Bob Perusek
Ohio
Stacey Phillips
Alberta
Wouter Pronk
Netherlands
Venkat Rao
Dubai, UAE
Syed Jawwad Raza
Qatar
Reino van Rensburg
South Africa
Joe Rodriguez
Ohio
Zaffar Shariff
Singapore
Joel Siallagan
Indonesia
Trey Sinkfield
Texas
Charlie Smith
Louisiana
Sharon Sng
Singapore
Mike Strobel
Pennsylvania
Max Sukuma
Australia
Asad Tahir
Texas
Jorge Trillos
Colombia
Steve Trimble
Oklahoma
Kunawat Wattanakij
Thailand
Mark Welch
England
Henk van Well
Germany
Martin Wieser
Germany
Kenta Yamada
Japan
Norbert Zeug
Germany
Part OnePGC fundamentals
1975 Beckman Model 6800 Air Quality Chromatograph.
Figure 1.1 A Classic Process Gas Chromatograph.
Source: Beckman Historical Collection, Box 58, Folder 28. Science History Institute, Philadelphia. https://digital.sciencehistory.org/works/474299142 Reproduced with permission of Rosemount, Inc.
“We cannot teach people anything; we can only help them discover it within themselves.”
Attributed to Galileo Galilei1564–1642
Why study this?
Part One introduces the art and science of gas chromatography (GC) as applied to the industrial process instrument.
These four chapters explain how a GC column works, why the compounds in the injected sample form the characteristic peak shape, how one peak becomes separate from another peak, and how we can predict the position and shape of peaks on a chromatogram from known patterns of peak timing and width.
The text presents this information in an easy‐to‐read and mostly non‐mathematical manner. Yet it shuns simplistic analogies of what happens inside a GC column because they tend to mislead rather than to inform. Instead, it offers a challenging insight into real chromatographic behavior.
The knowledge gained here is a necessary preparation for understanding the function of the hardware devices and software techniques introduced in later chapters of the book. For those who aspire to be proficient in the application or troubleshooting of process gas chromatographs, mastery of these concepts is not optional.
1An introduction
“Books on gas chromatography, of which there are many, usually start by reviewing the historical development of the science, so we won't do that here. Instead, we'll start by understanding the basic technique: what a chromatograph does and how it does it. To read the fascinating history of chromatographic science, see the beautiful book by Ettre (2008)”.
Chromatographic separation
Let's start by looking briefly at the various forms of chromatography.
Chromatography by itself is not a complete analytical technique. It's just a way to separate one kind of molecule from another kind of molecule. Of course, for those reading this book, the reason for separating those molecules is to measure them alone, without interference from other molecules. This is the analytical use of chromatography.
While analytical measurement is the main use of chromatography, it is not the only one. Some laboratory‐scale and industrial‐scale processes use a chromatographic separation to isolate extremely pure batches of valuable chemicals. This usage is known as preparative chromatography, and it works with much larger quantities of material than analytical chromatography does. This textbook focuses on analysis, so it doesn't further discuss the preparative use of chromatography.
When used as part of an analytical technique, chromatography is a very effective way to separate the measured compounds from each other and from all the other chemical compounds present in the analyzed material. After all desired compounds have been isolated, another device measures each one independently.
Keep in mind, then, that chromatographic analysis is always a two‐stage process: first separation, then measurement.
There are many ways to produce a chromatographic separation, and they involve all possible combinations of gases, liquids, and solids. While quite different in practice, these various forms of chromatography share some common features. All practical chromatographic separations involve a fluid material moving across the surface of a stationary material.
In the formal terminology of chromatography, the moving material is the mobile phase, and the immobile material is the stationary phase.
The mobile phase may be a gas or a liquid, from which we derive the terms:
gas chromatography
, in which the mobile phase is a gas.
liquid chromatography
, in which the mobile phase is a liquid.
A few applications have used a supercritical fluid as the mobile phase.
This book is about the analytical use of gas chromatography for the online measurement of industrial processes. We won't be discussing liquid chromatography.
In gas chromatography, the mobile phase is always a gas, and it's common to call it the carrier gas.
The carrier gas flows through a long narrow tube called a chromatographic column, which contains the stationary phase. The stationary phase may be an adsorbent solid or a non‐volatile liquid. More about that later.
The gas chromatograph
The basic instrument
A gas chromatograph is an analytical instrument that uses the techniques of gas chromatography to measure the concentration of selected chemical compounds in a small sample containing a mixture of compounds.
In a gas chromatograph, the mobile phase is a gas carefully selected for the application. It's usually hydrogen, helium, or nitrogen, but any gas will do the job, as long as is doesn't react with the sample components or the column materials. The gas should not contain oxygen or water vapor, as these substances might damage the columns.
The pressure of the carrier gas is closely controlled, after which it flows continuously through the column.
The analyzed fluid can be a gas or a volatile liquid. A special valve injects a small volume of that fluid into the flowing carrier gas. A liquid sample usually vaporizes instantly upon injection, so it's all vapor by the time it reaches the column.1
Note that when a gas chromatograph accepts a liquid sample, it doesn't become a liquid chromatograph. A liquid chromatograph is an entirely different instrument that employs a liquid mobile phase and separates components in the liquid phase. Liquid chromatographs are rarely employed as industrial online analyzers and are not considered here.
After injection, the carrier gas carries the gas or vapor sample into the column, where it contacts the stationary phase. It's the contact with the stationary phase that accomplishes the desired separation.
It's common to use the word component for a chemical substance or a group of chemical substances that are present in the sample. The gas chromatograph may not measure every component, but each component measured is an analyte.
A gas chromatograph can separate and measure one, several, or all the components in a gas or liquid sample.
After separation, the carrier gas carries the components into a detector that provides a measurable signal to the data‐processing circuits.
When actuated, the sample injection valve transfers a minute aliquot of the sample fluid into the flowing carrier gas. Later chapters provide full details of the many varieties of sample injector valve used in process gas chromatographs.
Figure 1.2 Basic Gas Chromatograph.
Figure 1.2 introduces the essential hardware devices found in any gas chromatograph:
A column oven with one or more controlled temperature zones.
A carrier gas supply and pressure control system.
A sample injector to inject a repeatable volume of sample into the flowing carrier gas.
One or more separating columns.
One or more detectors.
All gas chromatographs have these basic functions, yet we see a large variation in their design and fabrication.
The process instrument
The early development of the gas chromatograph was unusual. After the invention of the technique in 1952, the oil and chemical companies soon recognized its potential for process control, and those industrial companies did much of the original development work. The contribution of the instrument manufacturers came later.
Consequently, gas chromatographs intended for process monitoring and control evolved differently from those intended for laboratory use. Although both types of instrument use the same core technology, their sphere of application is quite different.
For example, a process gas chromatograph performing a two‐minute analysis receives 720 samples per day. The laboratory chromatograph might only receive three.
Thus, the design specifications for a gas chromatograph installed in an industrial processing plant are quite different than for a gas chromatograph sitting on a laboratory bench. The main reasons for these differences are:
The process instrument operates in a potentially hot, cold, dusty, wet, windy, corrosive, or hazardous environment.
The process instrument operates continuously twenty‐four hours per day, seven days per week.
The process instrument must operate reliably with almost no human intervention – perhaps only one calibration check each month.
The process instrument can focus on measuring just a few of the components in a sample – the ones needed for process control.
The process instrument suffers from a fanatical quest to reduce analysis time, so its measurements are valid for process control.
For all the above reasons, a process chromatograph (PGC) may include devices not shown in Figure 1.2. Later chapters will further discuss those devices. To whet your appetite, expect to see:
Devices external to the instrument to condition the incoming process sample to make it compatible with the chromatograph;
i.e
. a
sample conditioning
system.
Multiple columns with special valves to switch analyte molecules from one column to another, thus maximizing the rate that separated components arrive at the detector. This is an additional complexity rarely found in laboratory instruments.
Housekeeping columns
that allow strongly‐retained components to quickly exit the column system. A laboratory instrument used only a few times each day has plenty of time to recover between sample injections.
Robust column systems and stable devices, all designed to operate for a long time without adjustment. In contrast, the laboratory staff can frequently check and adjust their instruments, as necessary.
Automatic validity checking and automatic calibration as necessary. Most laboratories analyze a quality control sample every day.
Hardened electronic devices to capture and process the detector signal and to schedule timed events.
An analyzer enclosure, shelter, or house to protect the analyzers and workers from the plant environment.
The following paragraphs introduce the basic function of the hardware devices. Later chapters detail their performance and technology.
The oven
Temperature control
The hardware devices used by a gas chromatograph and the separations that occur within its columns are sensitive to temperature change, so a gas chromatograph needs very fine temperature control.
In the first makeshift gas chromatographs the temperature‐controlled enclosure was literally a laboratory oven, and the name stuck; the column compartment of a gas chromatograph is still the column oven.
Early PGCs had a single isothermal oven that housed the sample injection valve, column, and detector; and sometimes the pressure regulator too. The temperature setting was then a compromise that didn't always satisfy the needs of the individual devices. More recent instruments include several temperature‐controlled zones for columns, valves, and detectors, thereby allowing individual temperature settings.
The chromatographic columns are very sensitive to temperature change. A change of column temperature will change the time that a component spends in that column, which might cause an error in analyte detection and measurement. Most columns today reside in a separate column oven often controlled to better than ±0.03 °C.
Temperature programming
A separate column oven may also support temperature programming, a sometimes‐useful technique that gradually increases the temperature of a column during analysis. When temperature programming is employed, the analyzer needs a reproducible cooling system to rapidly lower the column temperature to its original starting point.
Temperature programming is common in laboratory gas chromatographs and allows them to separate a wide range of components, but it's rare in process gas chromatograph due to cost and analysis time issues. This may change with the introduction of less complex methods of heating and cooling, as discussed in Chapter 11.
The sample injection valve
Laboratory and online practice
To produce a chromatographic separation, the instrument needs a small sample of the gas or volatile liquid for analysis. The introduction of this sample into the carrier gas stream is known as sample injection. After injection, the carrier gas carries the sample into the column. As used here, a volatile liquid is one that will rapidly and completely vaporize at the injector temperature.
It used to be a standard laboratory practice to inject samples manually, using a glass syringe, but this routine procedure is now automatic. In the laboratory, an autosampler accepts an array of small vials containing the liquids for analysis. Then, according to a preloaded time program, it pulls a sample from each vial in turn and injects it into the chromatograph.
In contrast, an online gas chromatograph needs to periodically extract a minute sample from a continuously flowing process fluid and inject that sample into the carrier gas flow. To do this, most PGCs use a mechanical sample injector valve having a pneumatic actuator powered by an air signal from the chromatograph control unit. A few use electric power.
Figure 1.3 shows a typical valve configuration for injecting gas samples. For clarity, the diagram shows a rotary valve, but there are several other types of valve in use, including slide valves, diaphragm valves, and plunger valves. Chapter 8 details the function, design, and usage of these valves.
PGCs use several types of valve. As an example, this sketch shows a rotary valve. The rotor turns 60° to inject a sample.
Figure 1.3 Typical Gas Sample Injector Valve.
Plug injection
The injector valve must inject the measured sample volume all at once, in the form of a compact plug. If the injection is slow and the sample starts to mix with the carrier gas, the sample molecules will start to spread out in time even before they reach the column. This would not be good because it's more difficult to separate a wide band of molecules than it is to separate a narrow band. Separation is easier when the injected molecules tightly pack together.
The sample volume is determined by the application. It's crucial to inject the same volume of sample each time because the detector output signal is proportional to the number of molecules it sees. Should the injected volume change, so would the output signal, even if the concentration of the analyte remained the same.
Gas sample injection
The injected volume of a gas sample is typically less than 0.25 mL.
The number of molecules in a fixed gas volume increases with pressure, so it's necessary to maintain constant sample pressure for each injection. Therefore, most PGCs block and bleed the sample line to allow the sample gas to come to atmospheric pressure, a technique known as atmospheric referencing. Chapter 7 discusses some valve systems to achieve this.
But that still leaves the normal variation of atmospheric pressure, which is quite small, as you can see by inspecting any barometer. Discounting stormy weather, the jobsite pressure variation should not be more than about ±2 %.
In practice, atmospheric referencing works well enough for most applications. If greater precision is desired, it's best to measure local barometric pressure and adjust the measurement values to compensate for any variation found.
Some PGCs have a sensor to measure the absolute pressure of the gas sample and use an algorithm to correct for detected changes.
Liquid sample injection
With liquid samples, the main challenge is to avoid gas bubbles in the injected sample as these will cause erratic measurements. To guard against bubbles, keep the pressure of a liquid sample as high as possible, consistent with the pressure rating of the sample injector valve.
A volume of liquid contains about 300 times as many molecules as an equal volume of vapor. Therefore, to inject the same number of molecules, a liquid sample volume needs to be very small, usually less than one microliter (1 μL). In such a small volume, even the smallest bubble will displace a significant amount of the sample volume and cause low measurement values.
It's easy to visualize a microliter since it's the same size as a one‐millimeter cube (1 mm3). A volume of one thousand microliters is equal to one milliliter (1 mL) and to one cubic centimeter (1 cm3), commonly called a cc.
Another challenge with liquid samples is getting a complete and instant vaporization without making the sample too hot, lest it start to react or decompose. The small volume is helpful, and most process liquids quickly vaporize without significant decay.
The column
The separating device
The chromatographic column is the heart of any gas chromatograph. It separates the analytes from the other sample components, and from each other, so the detector can measure them individually.
Figure 1.4 pictures some typical chromatographic columns.
Figure 1.4 Typical Gas Chromatographic Columns.
Source: Ohio Valley Specialty Company, Inc. Reproduced with permission.
The carrier gas carries the injected sample molecules into the column, where they touch the selected stationary phase. It's the contact with the stationary phase that causes separation. The stationary phase delays the sample molecules − some more than others − so different components end up with different transit times through the column. Each component emerges from the column after its own characteristic retention time.
It takes time
A chromatographic separation takes time. In most process applications, the analysis time is from one to ten minutes, depending on the complexity of the analyzed mixture. Some complex separations take longer.
It's important to realize that separation is just a prelude to analysis. The enormous power of the gas chromatograph comes from its ability to physically separate almost any chosen component from all other components, and then to measure it. Other analytical techniques attempt to measure the concentration of one substance in the presence of all the other substances, a goal that few accomplish well. Gas chromatographs separate the analytes first, and then measure them individually. When properly designed, a process gas chromatograph may be the only process analyzer that doesn't suffer interference from other stuff in the sample.
Of course, it's possible for two or more components to have about the same retention time in a column, so a column might not separate every component from every other component present in the sample. The task of a PGC column system is to separate the measured components from all the others. It's neither necessary nor desirable to separate everything.
Multiple columns
The choice of separating column is always the key to a successful analysis. In practice, it's difficult to achieve the desired separation using just one column, so process gas chromatographs usually employ multiple columns to achieve the necessary separation in the shortest possible time.
With multiple columns, certain partially separated components from one column must flow into another column to achieve the desired separation. To divert flows between columns, an online gas chromatograph typically uses one or more column valves that are usually similar in design to its sample injection valve. Figure 1.5 shows an example of a simple column system.
For simplicity, the figure shows a rotary valve that rotates 90° when actuated, thereby flushing later peaks to vent. Other valves have a similar function.
Figure 1.5 A Simple Column Switching System.
Intercolumn valves must not leak. They must also have very low internal volume and smooth flow paths, lest separated components start to remix. For the same reason, a PGC typically employs ‐inch o.d. tubing for all its internal plumbing.
PGCs are individually configured for a particular application. During this procedure, known as application engineering, the application engineer chooses a column system to perform the desired separation and decides on the stationary phase needed for each column. Refer to Chapter 9 for a review of some standard column configurations and the function of each column.
SCI-FILE: On Column Types
Introduction to SCI‐FILEs
The more theoretical and mathematical content of the book resides in separate segments called SCI‐FILEs. These contain optional reading that may or may not be part of a course of study.
Each SCI‐FILE is a supplement to the main text that you can safely omit if not of immediate interest. Treat them as reference sources to consult when needed.
Two kinds of column
The stationary phase must be secure inside the column so it doesn't move. The packed column and the open‐tubular column differ per the method they use to anchor the stationary phase in place.
Packed columns
A packed column most often uses several meters of ⅛‐inch o.d. stainless steel tubing, although early PGCs used larger diameters, and some PGCs now employ ‐inch o.d. “micropacked” columns.
In the traditional packed column, the packing is a granular porous solid with particles about the same size as granulated sugar. These particles pack tightly together inside the tube so that any sample molecules moving with the carrier gas are in intimate contact with them.
The type of column so produced depends on the role of the solid particles:
An
active‐solid column
contains solid particles having a large activated surface area to selectively
adsorb
certain molecules from the sample gas.
Since the stationary phase is solid, this technique is gas‐solid chromatography (GSC).
A
liquid‐phase column
contains solid particles having a coating of non‐volatile liquid to selectively
dissolve
certain molecules from the sample gas.
Since the stationary phase is liquid, this technique is gas‐liquid chromatography (GLC).
Many columns now use proprietary stationary phases, often made from specialized polymer material. These columns don't easily fit into the old classifications of GSC or GLC, so the terminology is becoming passé.
In a liquid‐phase column, we call the granular solid an inert support. In real life, an inert support might not be completely inert; it sometimes affects the performance of a column.
The thickness of the liquid film coated on the support is an important variable. The liquid loading gives the percentage by weight of liquid on support.
The first gas chromatographs used packed columns, and they are commonly found in PGCs today.
Open‐tubular columns
An open‐tubular or capillary column uses several tens of meters of capillary tubing having an internal diameter ranging from about 100 to 530 μm. The mode of operation differs: the stationary phase adheres to the inner wall of the tube, and the carrier gas flows down the middle. Figure 1.6 illustrates three versions (Harvey 2017):
A “wall‐coated open‐tubular” or
WCOT column
uses tubing made of fused silica. The stationary phase is a very thin layer of a non‐volatile liquid coated on the inside wall of the tube to selectively dissolve sample molecules from the sample gas. PGCs rarely use these columns as they are fragile and tend to be unstable in use.
The WCOT columns typically use fused‐silica tubing and tend to be too fragile for process use. The PLOT and SCOT columns mostly use steel capillary tubing.
Figure 1.6 Three Kinds of Capillary Column.
A “porous‐layer open‐tubular” or
PLOT column
uses stainless steel capillary tubing. The stationary phase is a very thin layer of solid material coated on the inside wall of the tube to selectively adsorb sample molecules from the sample gas.
A “support‐coated open‐tubular” or
SCOT column
typically uses stainless steel capillary
tubing. The stationary phase is a coating on very fine support particles in a uniform layer on the inner wall of the tube. These rugged columns have become quite popular in process gas chromatographs.
Open‐tubular columns have smaller diameters than packed columns and require special operating techniques. As in packed columns, the film thickness is an important variable, but we'll defer discussion on that. While they achieve better separations, the operating conditions of open‐tubular columns can be difficult to sustain in the process environment.
For more information about column types and column liquid phases, refer to the excellent detailed review by Rahman et al. (2015).
The detector
Making the measurements
A chromatographic separation cannot produce a measurement. Chromatography is merely a separating technique; it doesn't measure anything. To measure the concentration of the analytes, the analytical instrument must estimate the quantity of selected molecules as they elute from the column. It follows that every gas chromatograph needs a device to generate a signal proportional to the number of sample molecules exiting the column. This is what a detector does.
In any gas chromatograph, two things are happening in series. First the column separates the analytes, and then the detector measures them. To improve your troubleshooting ability, keep that distinction in mind.
Many detectors are available for gas chromatography, most developed for applications that require selective measurement or enhanced sensitivity.
The thermal conductivity detector (TCD) was the first gas chromatograph detector, and after much improvement it is still popular today. The TCD responds to the difference in thermal conductivity between pure carrier gas and carrier gas that contains sample molecules. So, when a TCD is used, the carrier gas is chosen to maximize the difference in thermal conductivity between the carrier gas and the analytes. The TCD is a general‐purpose detector that will respond to any analyte.
Most other detectors are selective; they respond only to certain kinds of molecules and often do so with very high sensitivity. For instance, the flame ionization detector (FID) responds only to compounds containing both carbon and hydrogen, so it's very useful in the analysis of hydrocarbons. The flame photometric detector (FPD) is also very sensitive, but only to sulfur or phosphorus compounds. It's most used to measure sulfur compounds in fuels and stack emissions to ensure compliance with environmental regulations.
Generally, detectors operate in the differential mode. When pure carrier gas is passing through a detector, its output signal should be constant. The analytical instrument reads that signal and offsets it to a value close to zero. We call that the baseline. Then, when the detector responds to the presence of analyte molecules, the instrument outputs a change in signal level proportional to the concentration of that component.
Chapter 10 provides a detailed review of the three detectors most used in process gas chromatographs (TCD, FID, and FPD) and briefly mentions some other detectors that are common in laboratory instruments but only occasionally deployed for online process applications.
The chromatogram
The chromatogram is a graphical display of the detector signal plotted against elapsed time. The PGC may print the chromatogram on a chart or display it on a computer screen. Note that it's also common to refer to the raw signal from a detector as the
