GB1603615A - Gas chromatography - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO GAS CHROMATOGRAPHY (71) We. BRITISH GAS CORPORA TION, of 59 Bryanston Street, London, WIA 2AZ, a British Body Corporate, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a chromatograph and particularly to a high-speed chromatograph for the analysis of gas or vapour mixtures which is capable of directly displaying the concentration of a selected component of a mixture.

Our British Patent Specification 1,325,733 discloses a high-speed chromatograph which operates automatically and provides repetitive separations of simple gas mixtures with analysis times of less than 10 seconds. For the study of rapidly changing gas compositions, the speed of operation gives such an instrument the advantage over continuous monitors, which provide timeaveraged compositions or component concentrations. Furthermore, the chromatograph can be so constructed that it will discriminate and measure specific components of a mixture without interference from other components.There is a disadvantage, however. in that the results of an analysis take the form of a conventional chromatogram from which the concentrations of the mixture components have to be derived by measuring peak heights and comparing them with those from the analysis of a standard gas mixture. Concentrations are not displayed immediately, therefore, but have to be calculated subsequently, which takes much longer than analysis. The consequence of this disadvantage is that the full potential of the chromatograph is not realised in such applications.

Recent legislation, throughout the world, to reduce the amount of pollutant materials in the ambient atmospheres has resulted in the need to have instruments which are rapidly able to detect and measure small amounts of pollutant materials in the air. To adapt techniques such as infra-red spectroscopy to cope with legistative requirements has meant that large cells are required to enable small concentrations of pollutants to be analysed accurately, resulting in the necessity of long purge times between one analysis and the next and consequently the inability to carry out a succession of analyses rapidly.

The invention proposes apparatus for carrying out rapid accurate analysis of components of gas or vapour mixtures wherein concentrations of a specific component or components may be displayed directly and immediately. The apparatus according to the invention is based on the high-speed chromatograph such as described in UK Patent Specification No. 1,325,733, the disclosure of which is incorporated herein by reference.

According to the invention there is provided apparatus for measuring and displaying the concentration of a selected component of a gas or vapour mixture, which apparatus comprises a gas chromatograph capable of separating and detecting the components of the mixture in samples taken automatically in rapid succession, electronic circuitry including a detector for an associated amplifier adapted to identify a chosen peak for a different selected component which peak is earlier in the signal and which exceeds a predetermined level in the signal produced by the chromatograph detector as such sample is analysed such that upon identification of said chosen peak electronic timing devices are activated to enable the peak in the signal associated with the selected component to be distinguished from the rest of the signal and means for displaying the peak in the signal thus distinguished as a direct indication of the concentration of the selected component, the display being maintained until the chosen peak is identified in the signal from the next sample.

A high-speed chromatograph suitable for incorporation into the apparatus of the invention has been described in our British Patent Specification 1,325,733. The provision of automatic operation and the use of extremely small samples result in a rapid succession of determinations so that, for practical purposes, measurement is virtually continuous. Subsequent stages of the invention include electronic devices, however, and the automatic mechanisms on which operation of the chromatograph depends are required to be of such a kind that they do not give rise to electrical interference.

Thus a chromatograph with pneumatically operated mechanisms is to be preferred.

Electrical interference from mains pick-up must also be eliminated, but its frequency is high enough to allow removal using a low-pass filter. Alternatively, electrical interference may be eliminated by utilising 'fully floating' inputs to the instrumentation amplifier.

The elimination of electrical interference, though it is necessary to ensure that electronic devices function satisfactorily, nevertheless leads to some advantages since the electronic components can be highly sensitive and of low Power. Electronic circuitry that only requires low levels of electrical power allows the system to be designed in a way that is safer than systems using high levels of power, permitting its use in hazardous environments. Moreover, low power consumption also enables the chromatographic equipment to be operated remotely, via low current conductors, or to be made completely portable by the use of battery supplies and small gas cylinders.

In applications of the high-speed chromatograph where it is required to measure the concentration of a particular component (or particular components) in a gas mixture, timing devices may be used to define the period during which the particular component is being detected so that the relevant discrete part of the signal from the chromatograph detector can be applied, freed from electrical interference and suitable amplified, to a separate measuring circuit.

Examples of such timing devices include both digital and analogue timers.

The component or components in the gas mixture or vapour may be detected by means of a hot wire filament which registers changes in thermal conductivity, by means of a flame ionisation detector or by means of a radiation detector.

An analysis is completed in a few seconds, however. Consequently this period is brief and, since it depends on physical variables, difficult to define with the degree of precision required to enable an accurate determination of the selected component concentration to be made. The timing device(s) must ensure that the maximum value of the part of the signal associated with the selected component is measured and that no part of the signal associated with another component interferes with measurement.

Both the duration of the set period and its commencement must be accurately timed, and this is best accomplished using an identifiable part of the signal, in each analysis, to control the operation of the timing device(s). Inaccuracies resulting from variability, from one analysis to another, of the period between initiation of an analysis and generation of the signal are thereby avoided. A convenient part of the signal to identify when some mixtures are to be analysed is a peak resulting from the detection of another component which is always substantially higher than that from the selected component and which occurs earlier in the signal. Alternatively, an earlier peak which always exceeds a predetermined level may be identified.

Once the relevant part of the signal has been selected, it may be applied to suitable electronic circuitry which enables the peak voltage to be displayed until the sequence of events is repeated with the succeeding analysis. By providing for a suitable calibration procedure, the display may be arranged to glve a direct indication of the selected component concentration, which may be presented in digital form on a meter or printed on paper, or analogue form as a trace on a chart.

The chief advantage of direct display, of course, is that the information required is immediately made available in its most useful form. In contriving this feature, however, a further advantage arises in that a signal of relatively high strength is maintained long enough to allow the use of a cheaper potentiometric recorder in place of the ultra-violet or other fast-response recorder that would otherwise be needed.

Apparatus in accordance with the invention will now be described by way of example with reference to the accompanying drawings.

Although a high-speed chromatograph can be used to analyse a wide variety of mixtures, many of its applications involve a simple mixture such as methane in air. The apparatus to be described was built for use with such mixtures and comprises three units - a chromatograph, a katharometer control unit and an assembly of electronic modules - which may be mounted on standard racking. Of these units, the chromatograph and katharometer control are substantially as described with reference to Figure 1 and Example 1 of British Patent Specification 1 325 733, but with pneumatic rather than electromechanical automatic features. The automatically operated sample valve is actuated by a regulated supply of compressed air controlled by an electronic timer working in conjunction with a piezoelectric device as an electro-fluidic interface.Low pressure pneumatic signals generated by the interface in response to signals from the timer produce alternate switching of an air flow from the outputs of a fluidic gate. These air flows control the operation of booster, valves which provide air to drive the sample valve alternately from one position to the other. No inductive interference is produced, such as would result from the use of solenoid valves.

The chromatographic signal is derived from a conventional Wheatstone bridge circuit containing the reference and detector filaments of the micro-katharometer which monitors the thermal conductivity of the eluant from the chromatographic column.

Two peaks result from the analysis of each methane-air sample, of which the larger and first to appear arises as the air in the sample is detected by the katharometer. Identification of this peak is the first of a sequence of electronic functions which precisely measures the peak voltage produced as the methane is eluted so that it can be appropriately scaled and displayed on a meter and recorder to indicate the methane concentration of the sample.

The electronic circuitry, which is shown as a block diagram in Figure 1, may be considered in three main sections.

Firstly, there is the signal processing section, which amplifies the low level signal from the katharometer control unti. Then follows a peak detection circuit, which charges a capacitor to voltages determined by the air and methane peaks in the signal.

Thirdly there is a control section.

The signal processing section comprises a high performance instrumentation amplifier 102 which receives its input signal from katharometer 101. The instrumentation amplifier provides extremely stable amplification with a voltage gain of approximately 10,000.

The signal is than fed to the peak detector section 103 which comprises a precision rectifier and a high impedance amplifier stage. The precision rectifier allows current to flow into an associated capacitor provided that the voltage at its input is higher than that on the capacitor. As the component to be measures passes through the katharometer, the capacitor is charged to the peak voltage, allowing what is in effect an measurement of the concentration of the component to be temporarily stored. The voltage on the capacitor is monitored by the high impedance amplifier to ensure that it is held essentially constant between peaks.

Feedback is applied to the input of the rectifier to ensure that, during peak detecting periods, the voltage at the output is the same as that at the input. In this part of the circuitry, low-leakage components are used to minimise the inevitable drain of charge from the capacitor. Operation of the detector 103 is controlled by a digital timer 105, which is itself controlled by the control section 104. The peak voltage is diaplayed on unit 107.

A principal function of the control section is identification of the chosen peak and this function is executed by a threshold detector which responds only to voltages above a preset value. When it senses a voltage above the threshold this detector energises the timer 105 and discharges capacitors associated with detector 103, which provides a pathway to discharge the capacitor. The timing circuit can be set to 'time out' at the valley between the chosen peak and the peak to be detected so that the reset circuit opens again and the capacitor is made ready to be charged to the peak voltage resulting from the detection or the component. To facilitate setting the threshold level at a suitable voltage a light-emitting diode is included in the circuit to indicate when the threshold is exceeded.In addition, there is provision for the output of the timer to be compared with the input to the precision rectifier. Displaying these two signals on a chart recorder or an oscilloscope enables the timer to be precisely set so that the capacitor is grounded long enough to avoid its being recharged by the decaying chosen peak but isolated from the reset circuit soon enough to allow it to charge to the component peak voltage.

The amplification circuit in 103 may be made immune to further changes in input signals by means of an analogue timer 106.

However, the capacitor still remains charged and thus the peak voltage for the component can be retained and displayed until such time as the capacitor is discharged.

Figures 2, 3 and 4 are circuit diagrams showing in detail the circuitry for detecting a number of peaks sequentially.

Amplification and Threshold Detection Initial signal processing is carried out in the Amplification Module (Figure 2), where the low-level katharometer signal is fed at points 7 and 8 to the high performance instrumentation amplifier (Burr-Brown 3620). As a matter of convenience, a threshold detector in incorporated in the same module. The threshold level is set using the 1.K. multi-turn potentiometer which has a connection to Meter Point 3.

The katharometer signal exists as a fully floating input to the instrumentation amplifier, which thus achieves a high rejection of "common mode noise".

Peak Detection (Figure 3) The Peakhold Circuit comprises two identical peak detectors (one for each component to be measured) which receive their signal from the Amplifier Module via points (7, 19).

Each peak detector includes a precision rectifier and high impedance amplifier.

Control Circuit (Figure 4) The Control Module circuit diagram shows duplicate timer circuits which use mains frequency to generate digital time pulses. Each timer circuit is responsible for the control of a peak detector circuit, and both are started by the arrival of a signal from the threshold detector at point (5, 17) of the timer reset circuit.

The timers are not initiated at the time when the present threshold voltage is first exceeded by the signal produced by the air peak but when this signal again falls below the threshold value.' The provision of two timer circuits permits the values of any two peaks subsequent to the chosen peak to be stored on two separate capacitors. The provision of electronic switches in the Peakhold circuit enables the input to each capacitor to be isolated under control of the timers, and thus provides the capacitors with immunity from each other, and from other features of the signal.

Outputs The values of the peaks stored in each capacitor are monitored by digital meters and a multi-pen strip chart recorder Advantages Two component concentrations can be recorded in a multi-component mixture.

Immunity is provided from interference with the measurements by other components.

Circuit design philosophy enables the instrument to be extended to measure as many components as required, by provision of additional peak hold and control circuits identical to those described.

The use of digital timing bases on mains frequency facilitates setting of time intervals, since these are calibrated in 0.01, 0.1, 1, 10, etc. seconds.

Although these developments were carried out simply to permit analysis of more complex gas mixtures, the facility to measure several components may have wider implications.

In atmosphere monitoring for instance, one is involved in analysing ambient air for hazardous materials which one hopes are not present, and the absence of instrument response may indicate this, or that the instrument has failed. If the sample stream is doped with a gas capable of separation from air and from the monitored component, then the instrument response is confirmed, and operation becomes fail safe.

In a similar way the ability to dilute a sample in a quantitative manner with a second responsive gas, and to monitor both peaks, provides the possibility of achieving calibration with an internal standard. Electronic ratio devices are known, and using these the instrument could be made selfcalibrating.

An alternative application would be to use such devices to provide a ratio of two components present in a mixture, in cases where this value was more significant than the concentrations themselves.

Tests have been carried out on the instrument using a Wostoff pump to provide mixtures of methane and nitrogen of known concentration. This device delivers mixtures of two gases from sources at equal pressure through two piston pumps which are interlinked and driven at known rates by a calibrated gear system. Its accuracy is about 1% of the smaller component concentration over the range of compositions from 1 to 10% vol.

For the purposes of these tests the output terminals of the instrument were connected to a digital voltmeter and a potentiometric recorder giving its full scale deflection at 10my. The Wheatstone bridge circuit was first balanced using the supply potentiometers in conjunction with the digital voltmeter to achieve zero output from the high performance amplifier. Suitable settings of the threshold detector and timer were then found and the amplifier gain adjusted so that the instrument output was just under 10 volts (i.e. not off the scale) when mixtures containing 10% vol. of methane were analysed. Mixtures with methane concentrations in the range 1 - 10% volume were then analysed without further circuit adjustment.

At each concentration two slightly different readings were obtained because of the small variation in sample volume that arises as the chambers of the automatic sampling valve are used alternately. Drift on the recording between successive injections, timed to occur at 30 second intervals, was less tha 1% f.s.d. The series of analyses extended over several hours and the results showed no evidence of long term drift.

WHAT WE CLAIM IS: 1. Apparatus for measuring and displaying the concentration of a selected component of a gas or vapour mixture, which apparatus comprises a gas chromatograph capable of separating and detecting the components of the mixture in samples taken automatically in rapid succession, electronic circuitry including a detector and an associated amplifier adapted to identify a chosen

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. floating input to the instrumentation amplifier, which thus achieves a high rejection of "common mode noise". Peak Detection (Figure 3) The Peakhold Circuit comprises two identical peak detectors (one for each component to be measured) which receive their signal from the Amplifier Module via points (7, 19). Each peak detector includes a precision rectifier and high impedance amplifier. Control Circuit (Figure 4) The Control Module circuit diagram shows duplicate timer circuits which use mains frequency to generate digital time pulses. Each timer circuit is responsible for the control of a peak detector circuit, and both are started by the arrival of a signal from the threshold detector at point (5, 17) of the timer reset circuit. The timers are not initiated at the time when the present threshold voltage is first exceeded by the signal produced by the air peak but when this signal again falls below the threshold value.' The provision of two timer circuits permits the values of any two peaks subsequent to the chosen peak to be stored on two separate capacitors. The provision of electronic switches in the Peakhold circuit enables the input to each capacitor to be isolated under control of the timers, and thus provides the capacitors with immunity from each other, and from other features of the signal. Outputs The values of the peaks stored in each capacitor are monitored by digital meters and a multi-pen strip chart recorder Advantages Two component concentrations can be recorded in a multi-component mixture. Immunity is provided from interference with the measurements by other components. Circuit design philosophy enables the instrument to be extended to measure as many components as required, by provision of additional peak hold and control circuits identical to those described. The use of digital timing bases on mains frequency facilitates setting of time intervals, since these are calibrated in 0.01, 0.1, 1, 10, etc. seconds. Although these developments were carried out simply to permit analysis of more complex gas mixtures, the facility to measure several components may have wider implications. In atmosphere monitoring for instance, one is involved in analysing ambient air for hazardous materials which one hopes are not present, and the absence of instrument response may indicate this, or that the instrument has failed. If the sample stream is doped with a gas capable of separation from air and from the monitored component, then the instrument response is confirmed, and operation becomes fail safe. In a similar way the ability to dilute a sample in a quantitative manner with a second responsive gas, and to monitor both peaks, provides the possibility of achieving calibration with an internal standard. Electronic ratio devices are known, and using these the instrument could be made selfcalibrating. An alternative application would be to use such devices to provide a ratio of two components present in a mixture, in cases where this value was more significant than the concentrations themselves. Tests have been carried out on the instrument using a Wostoff pump to provide mixtures of methane and nitrogen of known concentration. This device delivers mixtures of two gases from sources at equal pressure through two piston pumps which are interlinked and driven at known rates by a calibrated gear system. Its accuracy is about 1% of the smaller component concentration over the range of compositions from 1 to 10% vol. For the purposes of these tests the output terminals of the instrument were connected to a digital voltmeter and a potentiometric recorder giving its full scale deflection at 10my. The Wheatstone bridge circuit was first balanced using the supply potentiometers in conjunction with the digital voltmeter to achieve zero output from the high performance amplifier. Suitable settings of the threshold detector and timer were then found and the amplifier gain adjusted so that the instrument output was just under 10 volts (i.e. not off the scale) when mixtures containing 10% vol. of methane were analysed. Mixtures with methane concentrations in the range 1 - 10% volume were then analysed without further circuit adjustment. At each concentration two slightly different readings were obtained because of the small variation in sample volume that arises as the chambers of the automatic sampling valve are used alternately. Drift on the recording between successive injections, timed to occur at 30 second intervals, was less tha 1% f.s.d. The series of analyses extended over several hours and the results showed no evidence of long term drift. WHAT WE CLAIM IS:

1. Apparatus for measuring and displaying the concentration of a selected component of a gas or vapour mixture, which apparatus comprises a gas chromatograph capable of separating and detecting the components of the mixture in samples taken automatically in rapid succession, electronic circuitry including a detector and an associated amplifier adapted to identify a chosen

peak for a different selected component which peak is earlier in the signal and which exceeds a predetermined level in the signal produced by the chromatograph detector as such sample is analysed such that upon identification of said chosen peak electronic timing devices are activated to enable the peak in the signal associated with the selected component to be distinguished from the rest of the signal and means for displaying the peak in the signal thus distinguished as a direct indication of the concentration of the selected component, the display being maintained until the chosen peak is identified in the signal from the next sample.

2. Apparatus as claimed in Claim 1 including means for activating the distinguished peak signal display means after elapse of a predetermined period of time from when the chosen peak is identified.

3. Apparatus as claimed in Claim 2, wherein said activating means is a digital timer.

4. Apparatus as' claimed in Claim 1 including means for nullifying the display when the chosen peak is identified.

5. Apparatus as claimed in Claim 1 including means for temporarily storing the peak signal.

6. Apparatus as claimed in Claim 1, substantially as hereinbefore described with reference to the accompanying drawings.

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