* The information on this page is based on an original manuscript by Dr Mike Hewins, with the permission of the author.
Chromatography is the science of separation invented in 1906 by the Russian botanist Mikhail Tsvet (1872-1919). The basic technique is deceptively simple and allows both qualitative and quantitative analysis of biological and chemical mixtures. The mixture to be analysed is transported by a carrier gas or liquid, called the mobile phase, over a stationary phase. This is either a simple adsorbent solid or a solid holding a liquid immiscible with the mobile phase.
The stationary phase absorbs and slows down the different components of a mixture to different degrees, depending on their affinity for the stationary phase, so they separate and emerge at different times in the flowing mobile phase. How quickly or otherwise different components or groups of components emerge is, for a given set of conditions, characteristic of each component. In liquid chromatography mobile phases or carriers are liquids of differing polarity. In the similar but different laboratory technique known as gas chromatography [GC] the mobile phase or carrier is a non-reactive gas, usually nitrogen but some early instruments used argon.
Chromatography of all types developed experimentally in the late 1940s and by the mid-1960s was a routine analytical tool in most chemical laboratories. As the chemical industry expanded and its range of products grew so did its analytical needs and many commercial companies entered the market to make gas chromatographs. This was the market two Cambridge companies, Pye and Unicam, entered in the late 1950s and this is a brief history of the GCs they developed, initially as two separate companies and later in 1968 as a combined company. The drivers for development in the early 1960s came from GC research; later there were some new GC techniques developed but market forces became the main drivers by the late 1960s.
New demand for routine GC analysis came from industries as diverse as hospital laboratories and the perfume manufacturers while the chemical industry in particular sought high accuracy combined with ease of operation. These trends increased significantly with the advent of the microprocessor. Pye-Unicam, later known as Philips Scientific, sometimes struggled to read the market clearly enough, as did other companies, and finally exited the GC market in the 1990s.
Terminology; the author refers above to gas and liquid chromatography, using current terminology to distinguish the two broad separation techniques. An earlier term ‘Gas Liquid Chromatography’ (GLC) exists in the literature and refers to the mobile gas phase passing over a stationary liquid phase immobilised on a solid support (see above). The term is no longer widely used.
In GC the stationary phase is typically packed into a long glass tube and fitted into an oven. The tube is called the column, even though it is usually wound into a coil so that it fits into an oven whose temperature, up to 450°c, is very accurately controlled. The sample or mixture is dissolved in a solvent and injected into one end of the column. The mobile phase is a non-reactive gas, usually nitrogen, which carries the sample as it flows through the column.
As the different components become adsorbed by the stationary phase they travel through the column at different speeds and so become separated, leaving the column one after the other to be detected and measured. The time taken for a component to travel through the column is called its retention time and a detector can be used to produce a graph where each component is represented by a peak as it emerges from the column.
- The number of peaks represents the number of components separated from the sample
- The position of each peak is the retention time for that component
- The area under a peak is a measure of the amount of component present in the sample
- Note that components with similar retention times will elute with overlapping peaks
When the author joined Pye Unicam Ltd in 1972 to project lead the design of a new gas chromatograph the company was already an established leader in the field of GC and had gained a second Queen’s award in 1969 for technological innovation in spectroscopy and chromatography. W G Pye & Company Ltd and Unicam Instruments Ltd had merged in 1968 to form Pye Unicam Ltd [a part of Philips] but prior to the merger Pye developed and manufactured gas chromatographs and accessories. In the following sections the author will identify gas chromatographs both by their catalogue name and their manufacturing company.
Under the leadership of FP Speakman W G Pye developed and manufactured the world’s first high sensitivity GC; the Pye Argon Chromatograph [12000 series], introduced at the 1958 Amsterdam Symposium.
Pye Argon Chromatograph
It was so very much more sensitive than any previous instrument that gas chromatographers had to develop new techniques to make full use of it. Unlike later GCs its columns were straight tubes, mounted vertically and the instrument was about 1.2 meters tall. Large quantities were sold all over the world. The principle advantages for the chromatographer were firstly, that a much smaller sample could be injected [0.1 - 0.025 micro litres] so the columns could work at higher efficiency resulting in a better quality of separation. Secondly, columns could be operated at lower temperatures which meant less degradation of the sample and hence better chromatography. The third major advantage was that many samples which had not been detectable by gas chromatography before now gave very good peaks.
These peaks were detected as they left the column to give retention times and the areas under the peaks [a measure of the amount present]. The detector was the Argon Ionisation Detector invented by Dr J E Lovelock [A Journal of Chromatography Volume 1, 1958, Pages 35-46] utilising the unique ionisation properties of argon to make a sensitive detector in which ionisation is initiated by strontium 90, a radioactive source. The ionisation current then changes when organic compounds elute from the column and the detector can respond to as little as 2 - 10-11 moles, with a similar linear response to different compounds. The detector fed its signal to a recorder and the area under a peak measured by hand and later by an integrator.
An industrial version of this chromatograph, the PAC Process Analyser (12800 series), was developed jointly with ICI Billingham for chemical process control and could operate automatically in hazardous chemical areas. A remote gas valve sampled the process stream, one of several gas and liquid sampling accessories developed by W. G. Pye. High boiling point samples could be decomposed in a Pye Pyrolyser and the products passed to the chromatograph for identification.
In the early 1960s leading research establishments in many parts of the world developed new and enhanced GC techniques and W. G. Pye responded by expanding and improving its range of gas chromatographs. It developed smaller, bench mounted instruments with a great emphasis on versatility. The basic bench layout was two separate units standing side by side; the analyser unit and the electronic unit. This became the standard form of most chromatographs for more than a decade, justifying the sales pitch "not just an analytical research tool but a vehicle for investigating and teaching GC techniques".
The first of this family of chromatographs was the Panchromatograph 12100 series and it could simultaneously accommodate for example any two of six different detectors in the smaller of two temperature controlled ovens. With improvements in detector sensitivity, dynamic range and linearity it had two detector supply units for any two of the six new detectors available; the argon ionisation, flame ionisation, cross section, electron capture, gas-density balance and thermal conductivity detectors. The range was widened by cooperation with the Gow-Mac Instrument Company, the leading manufacturers of hot-wire detectors in the United States.
Other improvements in detector sensitivity came from innovations such as the Pye Molecular Entrainer where a small fraction of the column eluent was withdrawn and further diluted before entering a highly sensitive detector. This enabled the wide sensitivity of argon and flame ionisation detectors to be still further extended. It was one of numerous optional features developed by Pye such as pyrolysis, preparative scale, radio-chromatography and a patented column effluent splitting device which permitted further analysis of the column eluent.
Columns improved in parallel; filled with a wide range of new stationary phases and no longer just straight tubes but folder or coiled to fit into the larger of two independent ovens of the Panchromatograph. This took two columns simultaneously and had isothermal and programmed control (known as ‘temperature programming’). These features were to become standard on most GCs in the future but were ‘top of the range’ in the early 1960s.
First introduced in 1964 with a UK price of £590 for a single Flame Ionisation Detector [FID] and temperature programmed column oven the 104 range was to become the workhorse of many, many laboratories in the UK and Europe. It represented a new approach to GC design by WG Pye and was its response to a rapidly developing market, enhanced GC techniques and a demand for lower cost instruments.
This new range was designed on modular principles with certain units common to all the versions in the range. One such basic module was the Analyser Oven to which the customer added detectors and accessories to form an individual high performance chromatograph tailored to analytical needs.
Laboratory bench footprint was minimised as the front of the oven opened forwards and downwards but this also allowed rapid cooling of the column when required, reducing analysis time. The top of the column oven was removable and contained the detector and the inlet system, with the column attached to its underside. The amplifiers, power supplies and programmed controllers were housed beside the oven unit and all of these components could be changed quickly by the user for different analyses. The oven had a top temperature of 500°C and an upper heating rate of up to 48°C per minute. At launch there were four models available: -
- Model 4: Flame Ionisation Detector Isothermal Chromatograph
- Model 14: Flame Ionisation Detector Temperature Programmed Chromatograph
- Model 24: Dual Flame Ionisation Detector Temperature programmed Chromatograph
- Model 34: Katharometer Isothermal Chromatograph
After the merger of W.G. Pye and Unicam in 1966 development of the 104 series continued to keep pace with new analytical techniques such as detector ovens separately heated from the column oven. Three new models were added to the range in 1966, making seven in total: -
- Model 44: Heated Katharometer Programmed Chromatograph
- Model 54: Heated Flame Ionisation Detector Programmed Chromatograph
- Model 64: Heated Dual Flame Ionisation Detector Programmed Chromatograph
Later versions (finally there were 14) offered specialised electron capture, nitrogen and phosphorous detectors, together with a comprehensive range of accessories. The S4 Autojector was one such accessory; fitted to a manually operated chromatograph it transformed it into a fully-automatic analyser capable of processing 100 samples without operator intervention.
Data handling too was becoming more automated and data from a 104 chromatograph could be sent either to an electronic integrator or to a computer to calculate the area and retention times of peaks.
In its most sophisticated version a 104 could be connected to a mass spectrometer so that mass spectral data could be obtained immediately a compound in a mixture eluted from the column.
The final members of the 104 range were both automated instruments; the 105 preparative GC and the 106 automated sample systems
Originally designed and marketed by W.G. Pye and later by Pye Unicam Ltd the former collected or prepared relatively large quantities of pure specimens from organic mixtures and was characterised by a red plastic punched tape controlling automatic operation. The latter were non preparative systems with varying types of automated sample injectors. Both GCs were built by the analyst from modules in the 104 series.
By the mid 1970s Pye Unicam was selling the 204 series of GC instruments, arguably an overdue development given the changed needs of analytical chemistry. Analysts were buying chromatographs with improved technical performance to give them faster analyses, more accurate analyses and greater sensitivity. Column performance had improved considerably and so they wanted greater temperature stability, more uniform temperatures, faster cool down and tightly controlled, fast temperature programming for higher analytical repeatability. The Series 204 sought to deliver these, along with other improvements in performance, but stayed with the modular principles familiar to users of the 104 series.
It was a complete range of analytical gas chromatographs of modular design which enabled a simple starting system to be extended as the analytical requirements expanded. The basic building block, the Oven Assembly, reflected the new thermal requirements and added enhanced versatility. In addition to an improved column oven it carried dual heated injectors for dual column operation and multiple detector ovens. The latter accepted two or three detectors simultaneously from a choice of five different detector systems.
The simplest system, consisting of the Oven Assembly, one detector and one amplifier, allowed isothermal column oven control and separate, independent temperature control of injectors and detectors.
Detectors and their amplifiers changed a little and the 204 detectors were the same as those developed for the GCV. A major change in gas chromatography was that it was now a routine process often operated by technicians rather than highly qualified chemists. The market looked for easier and more fault tolerant instruments than found in the 104 series and so the 204 chromatograph featured for instance automatic temperature programming to reduce operator learning time, speed up routine tasks and cut analysis time to a minimum.
Errors in analyses were reduced as switches, safety trips, signal-lights and more displays of operating conditions helped operators synchronise tasks. Soon automation of the 204 using the S8 Autojector allowed unattended operation of a temperature programmed analysis of up to 100 samples.
Basically a mechanical syringe, the S8 could simulate closely any manual methods of syringe injection, saving manpower and giving a much higher degree of repeatability. It was a self-contained accessory utilising punched tape to control a sequence of events during the sample injection, including washing the syringe before injection, control of integrators and identification of each analysis on the chromatogram. A wide viscosity range of samples could be handled and sample volumes as low as 2O micro litres. The sample turntable held a 1OO samples and an interval timer set the delay between analyses from 0 and 12O minutes. It controlled all the events in the analysis, washed syringes and triggered data processing, characterised again by the red plastic punched tape of the S4 - 104 automatic instruments.
Data processing itself was moving into the early computer age, bringing greater accuracy and feed back to control the S8 itself but was not yet integrated into the chromatograph and data was manipulated in free standing integrators such as the DP101 and CDP1.
Finally the 304 chromatograph was developed out of the 204 as Pye Unicam’s first microprocessor controlled instrument. It used the same oven and detector systems but new low mass insulating materials brought improved thermal performance and faster analyses compared with the 204. Like the 204 it used the S8 autojector and shared many other accessories. The microprocessor controlled most oven operations, making it more operator fault tolerant, but unlike later instruments output data was still manipulated in free standing integrators such as the DP101 and CDP1.
Gas chromatography analysis had become a standardised process and by the mid 1970s Pye Unicam, in common with other manufacturers, gave the market place dedicated lower priced, simple instruments for routine analysis and higher specification, versatile instruments for research into new analyses and techniques. This was a radical departure from the modular concepts of earlier instruments where the analyst configured either a simple instrument or a sophisticated research instrument from the same individually boxed modules, such as detectors and amplifiers, as required. Such modularity carried some additional costs and so Pye Unicam developed the non modular GCD for lower cost routine work and the modular GCV for higher specification work. These instruments were sold alongside the 204 and 304 series giving the market choice but meaning Pye Unicam and others were manufacturing a much broader range of GC instruments in anticipation of expanding markets.
The GCD gas chromatograph was a range of five compact, self-contained instruments, each dedicated to a particular detection system. This non modular system was more economical to manufacture and was sold at lower cost for repetitive, routine applications, with an emphasis on reliability and simplicity in use, yet yielding high performance chromatography. Five versions were available - lsothermal FlD (flame ionisation detector), Temperature Programmed and Auto Door FlD, Temperature Programmed FlD, isothermal TCD (thermal conductivity detector), lsothermal ECD (electron capture detector) - together with a wide range of accessories. Separate ovens housed heated injectors for manual or automatic on-column injection of samples and accurately controlled detector ovens optimised stability to give high analytical sensitivity. Simplicity, a minimum number of controls and ease of use by unskilled operators were seen as strong selling points.
The column oven used dual standard or capillary columns and its thermal specification was somewhat reduced, compared to a research instrument, but perfectly adequate for routine work. Interestingly although the final column oven was a conventional air blown design its development started with a different concept where each wall carried its own heating pad. It was expected this would give a more uniform and stable temperature zone around the columns but unfortunately wall heating pads proved very unreliable and had to be abandoned.
As with the 204 and 304 instruments the GCDs could be automated with the S8 Autojector, injector switching and sampling valves and data processed in separate integrators such as the DP88.
The GCV chromatograph, introduced to the market around 1976 and just before the 204 and GCD, was a high performance, computer compatible, modular instrument which could be expanded to cover research and other demanding applications. Although not modular in the 104 mould the basic chromatograph was a frame that held the column and detector ovens with digital temperature controls and manual gas controls for both carrier and detector gases. The column oven held both standard and capillary columns and was designed for automatic, fast temperature cycling and re-stabilisation. Thermal specification was high with low gradients and very uniform temperature profiles.
Flexibility and versatility were assured by modules that slid into the frame, such as amplifiers and accessory modules, independent injection heating and the multiple detec tor ovens which could operate three different detectors simultaneously. As in the GCD and 204 emphasis was on ease of operation, reliability, reduced operator error and low servicing costs.
Major innovations in detector and amplifier design brought increased sensitivity and linearity to the five detectors. The Flame lionization Detector’s linear range was extended (10-14 to 10-7A; <1% deviation) and its sensitivity increased to 2x1O-2 coulombs/gm. A new approach to the design of the Katharometer Detector and amplifier brought greater sensitivity (normally >9,5OO DPSU) and an increased linear range of detection. The latter was achieved by keeping the detector filaments at a constant temperature as the sample eluted.
The Electron Capture Detector was of conventional design but its pulse frequency modulated amplifier supply was a major innovation. It maintained a constant detector current by monitoring the pulse frequency needed to keep the current constant, thus improving the linear range of detection to typically 104.
The Flame Photometric Detector was a new and unique design, using air and hydrogen as auxiliary gases in trace analyses to measure organo-sulphur and organo-phosphorus compounds. The sample was introduced in a novel way to stop the flame being extinguished, a common problem, and the detector could be operated in either single or dual mode.
The fifth detector of the GCV, also a new design, was a Nitrogen Detector. This was based on an Alkali Flame Ionisation Detector using a well proven three electrodes design. A rubidium salt tip stimulated thermal emissions when organo-nitrogen compounds were burnt in a hydrogen flame, giving a selective response to organo-nitrogen compounds in pesticides, herbicides and drugs. The detectability was 10-10g nitrogen with a linear range of 104.
An extensive range of accessories extended performance and included automatic injection, capillary columns inlet systems, all glass surfaces to detectors, column switching systems, and sub-ambient operation. Automation was possible through the self-contained S8 Autojector accessory which could now operate with the GCV, GCD, 204 and 304 Chromatographs to give completely automated measurement and injection of liquid samples. Basically a mechanical syringe controlled by a punched tape it could simulate closely any manual methods of syringe injection, saving manpower and giving a much higher degree of repeatability.
By the start of the 80s Pye Unicam’s broad GC instrument catalogue was in need of revision. The PU 4500 was the first of the replacements to the catalogue but the major driver of change in the late 70s and later was the emergence of the microprocessor and the market’s recognition that control and data handling could be integrated to give fully automated analysis. Other important drivers of change were the expanding regulatory frameworks laid on analysts, for example in pharmaceuticals. High repeatability of analyses, long term storage of data and demands to detect ever lower and lower contaminants were now common. New columns, new techniques and new instruments made this possible but only via integrated control and data processing.
Controlled from the then new ‘keyboard and screen’ automated, error free routine analyses requiring little operator intervention became a 24/7 possibility. Computing power, still in its infancy, was developing rapidly. The potential for microprocessors and the then novel personal computers (PCs) to create more powerful instruments and later create new functions was on the horizon. The micro processor was ‘coming of age’.
Quantifying the results from a GC instrument was originally done using a chart recorder which traced out the change in the signal from the detectors. The time taken for a peak to emerge was diagnostic of the component in the sample and the area measured under the peak indicated how much of the component was present; finally a trace could be kept as a record. Computing integrators such as the DP 88 and DP 101 offered more. They not only recorded the time to emerge but signalled back to GC accessories to start a different operation, such as diverting a peak to a second GC for further separation (peaks were often a mixture of several components) or start the next analysis.
They also calculated the area under the peak in a much more sophisticated way, allowing for drifting baselines and asymmetric peaks using trapezoidal, tangential, fused peak and other mathematical functions. More powerful integrators such as CDP1, SP 4100 and PU 4810 offered still more mathematical techniques plus the facility to recalculate results ‘post event’ in different ways. Working alongside a microprocessor controlled 304 the potential to integrate control and data functions was obvious.
The ‘half way house’ to this was the PU 4850 Video Chromatography Control Centre, available from the early 80s. It was PUs first keyboard and VDU data handling centre, taking up to four channels of raw data with graphical display, floppy storage of both data and methods and real time and post run data processing. Now traces could be displayed on screen post event and then manipulated in different ways to assess the effect.
Released in 1984 the PU4500 was a range of low-cost, high performance chromatographs for the routine laboratory. It consisted of six dedicated chromatographs; five standard isothermal versions - thermal conductivity, flame ionization, electron capture, nitrogen and flame photometric - plus a specific capillary dedicated version.
All shared the same temperature setting and monitoring via the LED display. The detector oven could hold two plug-in detectors and the column oven held two columns of any type. It was compatible with the same wide range of accessories used in other Pye Unicam chromatographs. Although not modular a second free standing amplifier could stand alongside.
The following year saw the release of the PU4550 gas chromatograph, a microprocessor controlled modular instrument to complement the PU 4500 dedicated instruments. The basic oven assembly contained two injectors, a column oven for both capillary and conventional columns and a detector oven, along with an electronic control pack.
Further detectors, amplifiers and the usual wide variety of accessories could be added as needed.
Some of the detectors were improved versions of those used in the PU4500. There was a full digital display of the state of the instrument and in common with the PU304 a data protect system preventing unauthorised change of settings.
Both the PU4500 and the PU4550 were designed for ease of use, error free operation and improved technical performance. The PU4550 had a 365-day time-programmer controlling start-up so that ovens could stabilise at any point during any day, up to one year ahead. It could be combined with the PU4850 video chromatography control centre described above to give full reports and check operating integrity.
Sample injection could be automated by using the microprocessor controlled PU4700 autojector, the successor to the S8 autojector described earlier.
The final addition in 1985 to the new catalogue was the PU4900. Reference was made earlier to microprocessors and personal computers pointing the way to fully integrating instrument control with data processing and the PU4900 did that; a step forward from the PU4850 Video Chromatography Control Centre previously described. Pye Unicam in 1985 said of the PU4900;
"The capabilities of gas chromatography were now so extensive that they had created their own dilemma. How to maintain control of the many elements needed to achieve a highly accurate analytical result? Using microprocessor control the 4900 was not simply a compromise between existing chromatography and add-on data handling but an extension of chromatography into new dimensions. It used the same program that controlled the analytical separation to control data manipulation."
The rationale for this was more fully explained by G. M. Ogle, S. W. S. McCreadie and D. F. K. Swan, all of Pye Unicam, in the Journal of Automatic Chemistry of Clinical Laboratory Automation, Vol. 7, No. 3 (Jul-Sep 1985), pp. 130-135. They said
"Few inventions have revolutionized the laboratory analyst’s job as much as the microprocessor. It has been incorporated into nearly all laboratory equipment and instrumentation and the benefits have been widely felt; particularly so in the field of automatic chemistry. The requirements of automatic chemistry are that the instrument must be left to run automatically without operator attention: a task to which microprocessors are ideally suited. Not only can they control the entire analytical system, but, because they can act ‘intelligently’ with it, they can correct any deviation or errors detected..............With all of the instruments and many accessories being microprocessor controlled, it became possible to link them all together, and, by computer data links, to create a very powerful analytical system. Unfortunately, however, these complete analytical systems were very complex and definitely ‘user unfriendly’. A system may consist of a main oven unit, a satellite chromatograph, automatic injection system, data handling, disk and graphics capability and be required to output data to an external computer for archiving. Such systems would often have a keyboard on each part of the set-up and could require many hours of work to start up, only to find that the system ‘crashed’ because one of the units was incorrectly programmed.
Perhaps a more serious problem was that with the emphasis being placed on the electronics side of the package, the chromatography, which is really the heart of the system, was ignored or given second place (but)... The PU4900 is a new style of total analytical chromatograph that contains chromatography, data handling, disks and graphics and control functions of a satellite chromatograph ......."
The PU4900 used new designs for its capillary injectors, column and injector ovens and detectors. With these samples were analysed more accurately, with greater sensitivity and in shorter time. Three detectors could operate simultaneously along with a satellite column oven that mirrored the main oven. The sales brochure claimed "Outstanding results from sophisticated data handling routines generated by the world’s fastest 16-bit processor (86000 CPU) and 0.5Mbyte operating program". In the 1980s 0.5Mbyte was a big programme and 64-bit processors not even a dream! Innovative design features included;
- A sliding head assembly giving easier access to columns and couplings from five sides
- Storage on ‘floppy discs’ (the removable storage of its day) of both the methods and output data
- Aerodynamic air flow to give more stable column temperatures, so improving separation of components in the sample
- Short capillary columns could be used to give ultra fast analysis
- Multidimensional chromatography where individual components separated in one oven were further separated in a satellite oven controlled by the same 4900. It was claimed no instrument had previously given acceptable results from multidimensional analysis
Finally for the first time a VDU made it possible to simultaneously monitor up to three analyses as they happened, to expand on screen any trace area to view fine detail or manipulate the raw data as many times as necessary. Self diagnostics and monitoring were built in as standard.
Menu-driven user interfaces were displayed on the same VDU to set all control parameters. Full integration of control and data processing had arrived for Pye Unicam, complete with permanent storage of control parameters and raw data.
As with other GC instruments a complete range of accessories could be fitted to the PU4900 but additionally they could also be
controlled from the keyboard/VDU and the parameters stored, e.g. up to two PU4700 autojectors.
In 1988 Pye Unicam became Philips Scientific and the last GC instrument from Philips Scientific was the PU4400. It was similar to the PU4550 and marketed as providing an ‘economical solution to all types of analysis’. Five instruments were available and data handling added as required in a separate data station.
This is the end of Pye-Unicam/Philips GC story as Philips Scientific passed out of Philips hands in 1991 to be acquired by ATI (Analytical Technologies Incorporated), a US company and the name changed to ATI Unicam.
Sources: 1. Original manuscript Dr Mike Hewins