A method of controlling flow into a back pressure inlet for a gas chromatograph

Abstract

A method of controlling a flow of gas into a back pressure inlet for a gas chromatography system including column 23. The method comprises: i) determining an accumulation parameter, the accumulation p

Description

This invention relates to gas chromatography and, more particularly, to a method of controlling flow into a back pressure inlet for a gas chromatograph. BACKGROUND Inlets are used to introduce samples into chromatographic columns within gas chromatography systems. In a gas chromatography system comprising a back pressure-controlled inlet having a split outlet (for example, a Split / Splitless (SSL) inlet or a Programmable Temperature Vaporizer (PTV) inlet), the flow rate through a gas chromatography column to which the inlet is attached is regulated by controlling the pressure within the inlet. A back-pressure regulator configured to control the inlet pressure is located on the split line of the inlet. In this type of system, gas (e.g. a carrier gas) is supplied to the inlet at a flow rate corresponding to the sum of the flows of the gas out of the inlet, namely the sum of the gas flow out through a purge line of the inlet (purge flow out), the flow of the gas out through the split line of the inlet (split flow out) and the flow of the gas out of the gas chromatography column (column flow out) i.e. Total flow into the inlet = Purge flow out + Split Flow out + Column Flow out When pressure in the inlet increases (for example, to maintain a constant column flow during temperature programming of the column) the total gas volume contained in the inlet changes (i.e. gas accumulates in the inlet). As the total flow supplied to the inlet is constant, this results in a reduced flow of gas out through the split line. An excessively reduced split flow may result in inefficient purging of the inlet and could result in a release of contaminants and / or diffusion of air into the column and detector. Further, it may be necessary to increase pressure of the inlet at a desired rate for execution of a specific method of operation of the gas chromatograph. In some conditions, the total flow into the inlet may be insufficient to increase the inlet pressure at the desired rate for the execution of the specific method. US5803951 discloses a gas chromatograph having a control unit which normally carries out a pressure control so as to keep the pressure inside the vaporization chamber at a specified target level while the flow rate of a carrier gas into the vaporization chamber is kept constant. When a liquid sample is injected to cause a sudden rise in the pressure, the normal pressure control is temporarily stopped, say, for keeping the split ratio unchanged. In order to keep unchanged the retention time for components being analysed although the pressure inside the vaporization chamber rises, the target value for the pressure control is reduced for an appropriate length of time after the temporary stopping of pressure control is discontinued. US11255828 discloses a device for a gas chromatograph (GC) system including an injector connected to an inlet gas line and a conduit assembly. The inlet gas line is configured to pressurize an input end of a column and to deliver a split or purge flow. The conduit assembly includes a conduit surrounding the input end of the analytical column and coupled to a carrier gas line and a controller. The controller, connected to the conduit, has a first mode delivering a flow of carrier gas which is less than the column flow during an injection period to effect a sample transfer to the column and a second mode delivering a flow of carrier gas greater than the column flow following an injection period to prevent the split or purge flow from entering the column. The present invention seeks to address the problems caused by gas accumulation during pressure changes in the inlet (for example, as a consequence of temperature changes of a column operated at constant flow) and therefore improve accuracy in the determination of the required flow of gas into the inlet. SUMMARY In one aspect of this disclosure, there is provided a method of controlling a flow of a gas into a back pressure inlet for a gas chromatography system, the method comprising: i) determining an accumulation parameter, the accumulation parameter being indicative of a gas accumulation within the gas chromatography system; and ii) adjusting a flow rate of the gas into the inlet based on the accumulation parameter. This aspect of the present disclosure may comprise any one or more of the following features: The flow rate of the gas into the inlet may be adjusted such that a flow of gas out of the inlet through a split line is maintained at a target value. The accumulation parameter may be a gas accumulation rate (F) defined as: dV p = __ dt in which: dV is a change in volume of the gas contained in the inlet; and dt is a period of time. The flow rate of gas into the inlet may be adjusted by an amount equal to the gas accumulation rate (F). The accumulation parameter may be determined based on a volume of the inlet and a temperature of the inlet, the volume of the inlet optionally comprising a volume of a liner inside the inlet. The gas accumulation rate (F) may be defined as: VinlTrefdP dt Pref Tinl dt in which: dV a change in volume of the gas contained in the inlet in time dt; dP is the pressure variation in time dt; Vmi is the inlet geometrical volume; Tim is the inlet temperature; Pref is the reference pressure; and Tref is the reference temperature Determination of the accumulation parameter may be further based on a volume of a manifold of the gas chromatography system and a temperature of the manifold. The gas accumulation rate F may therefore be defined as: r__dV _ (V-ml Tref Vmm Tref \ dP dt \Pref Pint Pref PmanJ dt in which: dV a change in volume of the gas contained in the inlet in time dt; dP is the pressure variation in time dt; Vini is the inlet geometrical volume; Tim is the inlet temperature; Pref is the reference pressure; Tref is the reference temperature; Vman is the manifold pneumatic circuit geometrical volume; and Tman is the manifold temperature. Determination of the accumulation parameter may be further based on a volume of a column of the gas chromatography system and a temperature of the column. The gas accumulation rate (F) may be defined as: F = dV = / Vini 'Pref । Vman Pref 1 Vcoi Tref w dt V re / Pinl Pref Pman Pref Pcol / dt in which: dV a change in volume of the gas contained in the inlet in time dt; dP is the pressure variation in time dt; Vini is the inlet geometrical volume; Tini is the inlet temperature; Pref is the reference pressure; Tref is the reference temperature; Vman is the manifold pneumatic circuit geometrical volume; Tman IS the manifold temperature; Vcoi is the column geometrical volume; and Tcoi is the column temperature The pressure variation dP / dt may be set by the user at a target value. The pressure variation dP / dt may be automatically determined by the system based on column flow and oven temperature rate parameters. The pressure variation dP / dt may be determined according to: dP _ 5KFMTrace(Tul + tTraa^ 6a / PqUi + KFcoi(Tjn + tT^'ate )5 / 3 wherein: dP is the pressure variation in time dt; K is the column fluidic resistance; Fcoi is the column flow; Trate is the oven temperature program rate; Tn is the initial oven temperature; t is the time elapsed from the start of the oven program; and Pout is the column outlet pressure. The method may further comprise repeating steps i and ii at predetermined intervals during operation of the gas chromatography system such that a flow of gas out of the inlet through a split line is maintained at a target value. In a second aspect of this disclosure, there is provided an inlet for a gas chromatograph comprising a controller configured to carry out the method of the first aspect. This aspect of the present disclosure may comprise any one or more of the following features: The inlet may comprise one or more temperature sensors configured to measure the temperatures of on or more parts of the inlet, the controller being configured to determine the accumulation factor using the measured temperatures. The inlet may be a Split / Splitless (SSL) inlet or a Programmable Temperature Vaporizer (PTV) inlet, or any other inlet having a split outlet. The inlet may comprise one or more of: an intake flow sensor; and one or more temperature sensors. In a further aspect of this disclosure, there is provided a gas chromatography system comprising the inlet according to the second aspect of the disclosure. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the disclosure will now be described, by way of example only, with reference to the following non-limiting figures in which: Figure 1 is a schematic diagram of an inlet according to the present disclosure; Figure 2 is a pneumatic diagram of a gas chromatography system according to the present disclosure; and Figure 3 is a flow diagram illustrating an example of a method according to the present disclosure. DETAILED DESCRIPTION Figures 1 and 2 show an inlet 1 for a gas chromatography system according to an embodiment of the present disclosure. The inlet 1 comprises a controller 21 configured to carry out any method according to the present disclosure. A flow sensor (30 in Fig. 2) may be connected to a gas intake line 12 arranged to supply a gas, for example a carrier gas, to the inlet 1, the flow sensor 30 being configured to measure the supplied flow rate of the gas into the inlet. As shown in Figure 1, the inlet 1 may comprise a body 10 defining a chamber 11, an intake line 12, a purge line 14 and a split line (split outlet) 15. A liner 16 may be arranged in the chamber 11. The inlet 1 may be connected to a manifold 13. One or more valves 13a, 13b, 13c may be provided within the manifold 13 for control of the purge line 14, intake line 12 and / or split line 15. The flow sensor 30 may be arranged in combination with a gas intake line valve 13b of the one or more valves, for example the flow sensor 30 may be mounted on, in or at the valve 13b. As shown in Figures 1 and 2, the inlet 1 may be part of a gas chromatography system 20. The inlet 1 may be fluidly connected to a gas chromatography (GC) column 23 arranged in an oven 24 of the gas chromatography system 20. The controller 21 may be configured to control the gas chromatography system 20, including the inlet 1. Any inlet 1 according to the present disclosure may comprise one or more temperature sensors 40 configured to measure the temperatures of one or more parts of the inlet 1. One or more additional temperature sensors may be arranged to measure a temperature of the oven 24. The one or more temperature sensors 40 and the one or more additional sensors can be connected to the controller 21. To perform a scientific analysis within the column 23 a material sample, typically in the form of a liquid, may be injected into the inlet 1. A carrier gas may be provided through the gas intake line 12. The sample may be heated by the inlet 1 and transferred into the column 23 of oven 24, which separates the sample into its individual components. The individual components then travel out through an outlet 25 for detection, for example by a mass spectrometer. The present disclosure provides a method of controlling a flow of a gas, for example a carrier gas, into a back pressure inlet (for example an inlet 1 as shown in Figure 1) for a gas chromatography system (for example, a gas chromatography system 20 as shown in Figure 2). The method may be a method of controlling a flow rate of gas into and / or a pressure within the inlet. The method comprises determining an accumulation parameter indicative of a gas accumulation within the gas chromatography system, and adjusting a flow rate of the gas into the inlet based on the accumulation parameter. The flow rate of the gas into the inlet may be adjusted such that a flow of gas out of the inlet through the split line is maintained at a target value. The accumulation parameter may be a gas accumulation rate (F) and may be defined as: dV f-Ti in which: dV is a change in volume of the gas contained in the inlet; and dt is a period of time. The gas accumulation rate may be the rate of variation of the total gas volume contained in the inlet and expressed as a volume in standard condition per unit of time (i.e. equivalent to a mass flow rate). The flow rate of gas into the inlet may be adjusted by an amount equal to the gas accumulation rate (F) such that the flow rate of the gas supplied into the inlet is calculated from the sum of the flows of the gas out of the inlet (i.e. the sum of the gas flow out through a purge line of the inlet (purge flow out), the flow of the gas out through a split line of the inlet (split flow out) and the flow of the gas out of a gas chromatography column to which the inlet is attached (column flow out)) and the gas accumulation rate according to the equation: Total flow into the inlet = Purge flow out + Split Flow out + Column Flow out + F In a first embodiment, the gas accumulation rate F is determined based on a volume of the inlet and a temperature of the inlet. The calculation of the gas accumulation rate F, and therefore the calculation of the total flow of gas into the inlet, may therefore take into consideration an inlet volume and temperature as follows: The ideal gas law defines an amount of gas (in moles) according to the formula: - pv n ~ Hf in which: n is the molar amount of the gas; P is Pressure; V is Volume of the amount of gas; R is the Ideal Gas Constant; and T is Temperature. Therefore, if the volume and temperature are kept constant, then a pressure variation of dP in time dt is related to an amount of gas variation of dn in time dt as follows: dn Vint dP dl" RTini II in which: Vini is the inlet geometrical volume; and Tini is the inlet temperature. The variation of gas amount per time dn / dt is a molar gas flow (mass flow) and the variation in pressure in time dP / dt is the pressure rate variation. The above equation therefore determines the mass flow required to increase pressure in the inlet at a rate of dP / dt. In defined reference conditions, the amount of gas variation of dn in time dt can also be defined based on the ideal gas law as follows: dn _ _ Pref dV "di “ RTref "dt in which: Pref is the reference pressure (for example, 1 atm); Tret is the reference temperature (for example, 20 degrees C); and dV is the change in gas volume at reference conditions in time dt. By combining these two equations for defining dn / dt, the accumulation flow F can therefore be calculated as: dV Vinl Tref dP r — --—--- dt Pref Tin[ dt in which: dV is the change in gas volume at reference conditions in time dt; dP is the change in pressure in time dt; Vini is the inlet geometrical volume; Tini is the inlet temperature; Pref is the reference pressure; Tref is the reference temperature; and F is the mass flow expressed as volumetric flow at reference conditions as typically used in Gas Chromatography (for example, at 20 degrees C and 1 atm), usually defined in mls / min (Standard Millilitre per Minute, also abbreviated as ml / min(s)) or seem (Standard Cubic Centimetre per Minute). In a second embodiment, the calculation of the gas accumulation rate F further takes into account a volume of a pneumatic manifold of the gas chromatography system and a temperature of the manifold such that the accumulation parameter F is defined as: dV dt Vj.nl Tref D 'T' .‘ref ‘ini ^nan Tref m ‘man dP dt in which: dV is the change in gas volume at reference conditions in time dt; dP is the change in pressure in time dt; Virii is the inlet geometrical volume; Tim is the inlet temperature; Pref is the reference pressure; Tref is the reference temperature; Vman is the manifold pneumatic circuit geometrical volume; and Tman is the manifold temperature. In a third embodiment, the calculation of the gas accumulation rate F further takes into account a volume of a column of the gas chromatography system and a temperature of the column such that the accumulation parameter F is defined as: v _ dV _ / Vnl Tref Knan Tref 1 Vcol Tref\dP r — ---- — I---1----1----I--- dt \Pref Tfni Pref Tman % Pref TCol J dt in which: dV is the change in gas volume at reference conditions in time dt; dP is the change in pressure in time dt; Vini is the inlet geometrical volume; Tini is the inlet temperature; Pref is the reference pressure; Tret is the reference temperature; Vman is the manifold pneumatic circuit geometrical volume; Tman is the manifold temperature; Vcoi is the column geometrical volume; and Tcoi is the column temperature It may be noted that the 1 / 2 factor associated with the column in the above calculation is included because the column pressure varies along its length as the column inlet is at the inlet pressure while the column outlet is at atmospheric pressure or vacuum. An average pressure equal to 1 / 2 of the inlet pressure is used as approximation. In alternative embodiments, a more precise calculation of column pressure may be applied. In each embodiment, the column, manifold and / or inlet temperatures may be measured using one or more temperature sensors. The temperature sensor(s) arranged to measure the inlet 1 temperature may be configured to measure a temperature of the body 10 or chamber 11 of the inlet 1. The inlet volume, manifold volume and column volume may be known or measured values, or may be determined from known or specified geometry of the components. For example, the volume of the column may be calculated from a known column length and column diameter. The column diameter may be an effective column diameter, for example accounting for a known coating film thickness on an inside of the column. The inlet volume may refer to a volume of a heated part of the inlet. For example, the inlet volume may be determined by introducing a known amount of gas into the inlet and measuring the resulting pressure increase. The volume of the manifold may be calculated based on a known total volume of the inlet minus the volume of a heated part of the inlet. The inlet volume, or volume of a heated part of the inlet, may include a volume of a liner of the inlet. The pressure rate variation (dP / dt) may be set at a target value, for example if an increase in pressure in the inlet is specified by the method in use (e.g. methods using pressure programming). Alternatively, the pressure rate variation (dP / dt) may be determined by the system / controller. For example, the pressure rate variation (dP / dt) may be determined based on the column flow (e.g. in ml / minute) and rate of temperature change (e.g. in degrees Celsius / minute) in the gas chromatography oven when a constant flow method with oven temperature programming is used. When the gas chromatography system is running in a constant column flow mode with oven temperature programming, pressure must be increased to maintain the constant flow through the column, due to the increase in column pneumatic resistance resulting from an increase in gas viscosity. The pressure rate variation dP / dt in this example is not constant, but instead varies during the temperature program based on the temperature variation rate. When the gas chromatography system is running in a constant column flow mode, the pressure rate variation dP / dt may be determined from the temperature variation rate (Trate). The pressure P required to maintain the column flow (Fcoi) is: Where: Pout is the column outlet pressure (typically atmospheric or vacuum); K is the column fluidic resistance (which may be calculated from the column geometrical parameters and gas viscosity; Tn is the initial oven temperature; t is the time elapsed from the start of the oven program; and Trate is the oven temperature program rate (which may be a linear temperature variation rate, for example 20 degrees Celsius / min). The pressure rate variation dP / dt is therefore: FcoiTrate(Tin + ^rate)2^3 dt ~ The pressure rate variation dP / dt may therefore be calculated for use in the calculation of the gas accumulation F at any instant t of the run of the gas chromatography system using the known temperature program rate in use and the column parameters as described above. The above may be also applied in case the column flow is not maintained constant but it is programmed during the run. In such case the column flow (Fcoi) will be a function of the t elapsed time (t). The method as described above may be repeated iteratively during operation of the gas chromatography system, for example such that a flow of gas out of the inlet through a split line is maintained at a target value. An example of the method according to the present disclosure is shown in Figure 3. The adjustment of the flow may be carried out according to the disclosed method during or throughout an operational run of a gas chromatography system according to the present disclosure. In step 101 the user may input the desired column flow (column flow out), purge line flow (purge flow out) and split line flow (split flow out) In step 102, the system may determine an initial value for total gas flow as a sum of the column flow, purge line flow and split line flow. At step 103, the system may initiate or set the flow of gas into the inlet at the calculated initial value for the total gas flow into the inlet. At step 104, the user may set an oven temperature program for the oven. At step 105 the user may initiate a run of the gas chromatography system. At step 106, the controller may determine whether the oven temperature is increasing. Determining whether the oven temperature is increasing may comprise measuring the oven temperature using a temperature sensor and comparing the measured value to a previously measured temperature and / or an initial or target temperature. Based on the determination made in step 106: - if the oven temperature is not increasing, the gas accumulation rate F may be set to zero (step 107a); or - if the oven temperature is increasing, the system may determine the pressure variation rate and calculate the gas accumulation rate F according to any method of the present disclosure (step 107b). In step 108, the controller may determine an adjusted value for the total gas flow into the inlet as a sum of the column flow, purge line flow, split line flow and the gas accumulation rate F set in step 107. In step 109, the controller may set the flow of gas into the inlet at the adjusted value for the total gas flow into the inlet. In step 110, the controller may determine whether the oven temperature program is complete. If yes, the method ends. If no, the method returns to step 106. While embodiments of the present disclosure have been described above and illustrated in the drawings, these are for example only and are non-limiting. It will be appreciated by those skilled in the art that alternatives are possible within the ambit of the disclosure. For example, the method, inlet, gas chromatograph and system of the present disclosure may comprise any combination of the following features. The invention is for use with any back pressure controlled gas chromatography inlet having a split line, for example a Split / Splitless (SSL) inlet or a Programmable Temperature Vaporizer (PTV) inlet. The inlet may have a flow control apparatus, for example one or more valves, on the intake flow line and a further flow control apparatus, for example one or more valves, on the split line. For example, the inlet may comprise two valves for flow control and one valve for pressure control, with associated sensors for controlling the valves. In the embodiments described above, determining an accumulation parameter indicative of a gas accumulation within the gas chromatography system may comprise calculating a gas accumulate rate F. In alternative embodiments, determining an accumulation parameter indicative of a gas accumulation within the gas chromatography system may comprise determining a system parameter, for example a temperature of the inlet. In such embodiments, a required amount of adjustment of the flow rate of the gas into the inlet may be set based on previous experimentation. For example, an amount of adjustment of the flow rate of the gas into the inlet for a particular inlet temperature may be selected from values in a look-up table, the required flow adjustments for various inlet temperatures having previously been established by experimentation. The gas chromatography system according to the present invention may comprise a pneumatic circuit manifold, inlet and column. The gas accumulation may occur within one or more of the pneumatic circuit manifold, inlet and column. The inlet may also be referred to as an injector, these terms being interchangeable. Any inlet or gas chromatography system according to the present disclosure may comprise a controller configured to carry out any method according to the present disclosure. Any inlet according to the present disclosure may comprise one or more temperature sensors configured to measure the temperatures of one or more parts of the inlet, the controller being configured to determine the accumulation factor using the measured temperatures. The one or more temperature sensors may be configured to measure the temperatures of the injector, the manifold and / or the column measured. Alternatively, the manifold temperature may be assumed to be room temperature, for example 20 degrees Celsius. In any embodiment, the manifold may be a pneumatic circuit manifold. The volume of the pneumatic manifold may therefore be a volume of a pneumatics circuit of the gas chromatography system at the pneumatic manifold temperature Tman. In any embodiment, the manifold may be replaced with pneumatic circuit channels formed from tubing and fittings. In such a system the “manifold volume” and “manifold temperature” described herein refer to the geometric volume and temperature of the pneumatic circuit. In any embodiment, the temperature(s) of the system may be measured using one or more temperature sensors or may be determined, for example based on a known temperature of or voltage applied to a heating element of the system. The method of the present disclosure may be applied during operation of the gas chromatography system in non-stationary conditions (when pressure is changing in the inlet). In any embodiment, when the gas chromatography system is in stationary conditions (i.e. when pressure is not changing within an inlet, dP / dt=O), the gas accumulation F is zero. In these conditions, the method may further comprise calculating total flow rate of the gas supplied into the inlet as the sum of the flows of the gas out of the inlet, namely the sum of the gas flow out through a purge line of the inlet (purge flow out), the flow of the gas out through the split line of the inlet (split flow out) and the flow of the gas out of a gas chromatography column to which the inlet is attached (column flow out) i.e. Total flow into the inlet = Purge flow out + Split Flow out + Column Flow out The calculation of the total flow into the inlet according to the present invention takes into consideration an inlet volume and the temperature of the gas therein, for example the volume of the inlet may comprise the volume of a liner of the inlet. By determining the amount of gas accumulation and adjusting the total flow rate of a gas into the inlet based on the accumulation, the invention avoids the problems of excessive reduction of split line flow and allows for efficient increase of pressure in the inlet. The increase of inlet pressure at a desired rate is generally desirable but may be particularly critical for execution of specific methods, for example when using fast temperature programming or fast pressure programming. The invention allows maintenance of a constant split flow during inlet pressure increase when part of the flow of carrier gas into the inlet is taken by pressurisation of the inlet. This may be achieved by calculating in real time the amount of gas required by the pressurisation and automatically adding it to the flow of carrier gas into the injector, for example using a digital gas control, so that the split flow remains constant during the pressure change. The method described herein may be applied in any back-pressure controlled inlet / injector for a gas chromatograph with intake flow control. The method may therefore be applied to any inlet of this type without modification of the inlet hardware.

Claims

1. A method of controlling a flow of a gas into a back pressure inlet for a gas chromatography system, the method comprising:i) determining an accumulation parameter, the accumulation parameter being indicative of a gas accumulation within the gas chromatography system; and ii) adjusting a flow rate of the gas into the inlet based on the accumulation parameter.

2. The method of claim 1 wherein the flow rate of the gas into the inlet is adjusted such that a flow of gas out of the inlet through a split line is maintained at a target value.

3. The method of any preceding claim wherein the accumulation parameter is a gas accumulation rate (F) and is defined as:dtin which:dV is a change in volume of the gas contained in the inlet; and dt is a period of time.

4. The method of claim 3 wherein the flow rate of gas into the inlet is adjusted by an amount equal to the gas accumulation rate (F).

5. The method of any preceding claim wherein the accumulation parameter is determined based on a volume of the inlet and a temperature of the inlet;wherein optionally the volume of the inlet comprises a volume of a liner inside the inlet.

6. The method of claim 5 wherein gas accumulation rate (F) is defined as: p_ dV^ VtnlTrefdP dt Pref Tini dtin which:dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Virii is the inlet geometrical volume;Tim is the inlet temperature;Pref is the reference pressure; andTref is the reference temperature7. The method of any of claim 5 wherein determination of the accumulation parameter is further based on a volume of a manifold of the gas chromatography system and a temperature of the manifold.

8. The method of claim 7 wherein the gas accumulation rate F is defined as:in which:dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Vini is the inlet geometrical volume;Tini is the inlet temperature;Pref is the reference pressure;Tref is the reference temperature;Vman is the manifold pneumatic circuit geometrical volume; and Tman is the manifold temperature.

9. The method of any of claim 7 wherein determination of the accumulation parameter is further based on a volume of a column of the gas chromatography system and a temperature of the column.

10. The method of claim 9 wherein the gas accumulation rate (F) is defined as:F = dV = f Vini d'ref Knan d'ref 1 ^coJ Prefwdt \Pref P'ml Pref Pman 2 Pref Tco[ J dtin which:dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Vini is the inlet geometrical volume;Tirii is the inlet temperature;Pref is the reference pressure;Tref is the reference temperature;Vman is the manifold pneumatic circuit geometrical volume;Tman is the manifold temperature;Vcoi is the column geometrical volume; andTcoi is the column temperature11. The method of any of claims 6 to 10 wherein the pressure variation dP / dt is set by the user at a target value.

12. The method of any of claims 6 to 10 wherein the pressure variation dP / dt is automatically determined by the system based on column flow and oven temperature rate parameters.

13. The method of claim 12 wherein dP / dt is determined according to: _ SKFcolTratetTin +dt ~ + KFcol(Tin + tTrate)^wherein:dP is the pressure variation in time dt;K is the column fluidic resistance;Fcoi is the column flow;Trate is the oven temperature program rate;Tn is the initial oven temperature;t is the time elapsed from the start of the oven program; and Pout is the column outlet pressure.

14. The method of any preceding claim further comprising repeating steps i and ii at predetermined intervals during operation of the gas chromatography system such that a flow of gas out of the inlet through a split line is maintained at a target value.

15. An inlet for a gas chromatograph comprising a controller configured to carry out the method of any preceding claim.

16. The inlet of claim 15 comprising one or more temperature sensors configured to measure the temperatures of on or more parts of the inlet, the controller being configured to determine the accumulation factor using the measured temperatures.5 17. The inlet of claim 15 or claim 16 wherein the inlet is a Split / Splitless (SSL) inlet or aProgrammable Temperature Vaporizer (PTV) inlet.

18. The inlet of any one of claims 15 to 17 comprising one or more of: an intake flow sensor; and10 one or more temperature sensors.

19. A gas chromatography system comprising the inlet according to any of claims 15 to 18.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS:20 05 25CLAIMS:

1. A method of controlling a flow of a gas into a back pressure inlet for a gas chromatography system, the method comprising:i) determining an accumulation parameter, the accumulation parameter being indicative of a gas accumulation within the gas chromatography system; and ii) adjusting a flow rate of the gas into the inlet based on the accumulation parameter.wherein the accumulation parameter is a gas accumulation rate (F) and is defined as:dVF = — dtin which:dV is a change in volume of the gas contained in the inlet; and dt is a period of time.

2. The method of claim 1 wherein the flow rate of the gas into the inlet is adjusted such that a flow of gas out of the inlet through a split line is maintained at a target value.

3. The method of claim 1 wherein the flow rate of gas into the inlet is adjusted by an amount equal to the gas accumulation rate (F).

4. The method of any preceding claim wherein the accumulation parameter is determined based on a volume of the inlet and a temperature of the inlet;wherein optionally the volume of the inlet comprises a volume of a liner inside the inlet.

5. The method of claim 4 wherein gas accumulation rate (F) is defined as:dV VinlTrefdPr — -- —---dt Pref Tini dtin which:dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Vini is the inlet geometrical volume;Tini is the inlet temperature;Pref is the reference pressure; andTref is the reference temperature5 6. The method of any of claim 4 wherein determination of the accumulation parameteris further based on a volume of a manifold of the gas chromatography system and a temperature of the manifold.

7. The method of claim 6 wherein the gas accumulation rate F is defined as:dVF = — dtVinl Kef | Vman Tref Pref Tini Pref TmandP dt10in which:20 05 25dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Vim is the inlet geometrical volume;15 Tini is the inlet temperature;Pref is the reference pressure;Tref is the reference temperature;Vman is the manifold pneumatic circuit geometrical volume; andTman IS the manifold temperature.

208. The method of any of claim 6 wherein determination of the accumulation parameter is further based on a volume of a column of the gas chromatography system and a temperature of the column.25 9. The method of claim 8 wherein the gas accumulation rate (F) is defined as:_ dV _ / Tref Knan Tref 1 Trep\dPr — --- — I-----j~---j---dt \Pref Tini Pref Tman 2 Pref Tcoi J dt30in which:dV a change in volume of the gas contained in the inlet in time dt;dP is the pressure variation in time dt;Vim is the inlet geometrical volume;Tini is the inlet temperature;20 05 25no<£•*Pref is the reference pressure;Tret is the reference temperature;Vman is the manifold pneumatic circuit geometrical volume;Tman IS the manifold temperature;Vcoi is the column geometrical volume; andTcoi is the column temperature10. The method of any of claims 5 to 9 wherein the pressure variation dP / dt is set by the user at a target value.

11. The method of any of claims 5 to 9 wherein the pressure variation dP / dt is automatically determined by the system based on column flow and oven temperature rate parameters.

12. The method of claim 11 wherein dP / dt is determined according to: dP 5KFcolTra^dt ^Pout + KFcol(Tin + tTraCe)^wherein:dP is the pressure variation in time dt;K is the column fluidic resistance;Fcoi is the column flow;Trate is the oven temperature program rate;Tn is the initial oven temperature;t is the time elapsed from the start of the oven program; and Pout is the column outlet pressure.

13. The method of any preceding claim further comprising repeating steps i and ii at predetermined intervals during operation of the gas chromatography system such that a flow of gas out of the inlet through a split line is maintained at a target value.

14. An inlet for a gas chromatograph comprising a controller configured to carry out the method of any preceding claim.

15. The inlet of claim 14 comprising one or more temperature sensors configured to measure the temperatures of on or more parts of the inlet, the controller being configured to determine the accumulation factor using the measured temperatures.5 16. The inlet of claim 14 or claim 15 wherein the inlet is a Split / Splitless (SSL) inlet or aProgrammable Temperature Vaporizer (PTV) inlet.

17. The inlet of any one of claims 14 to 16 comprising one or more of: an intake flow sensor; and10 one or more temperature sensors.

18. A gas chromatography system comprising the inlet according to any of claims 14 to 17.20 05 25

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