Gas flow control

Abstract

The present invention relates to a gas inlet system for an analytical apparatus. The gas inlet system comprises switchable flow restrictions for regulating gas flow rate. The invention also provides a system for calibrating gas flow rate in gas inlet systems, the system comprising a calibration line that comprises a gas flow meter, and that is arranged downstream of gas flow controllers in the gas inlet system. Methods of adjusting gas flow rates and methods of calibrating gas flow rates are also provided.

Claims

1. A gas inlet system for providing a stream of gas into an analytical apparatus, comprising: a gas inlet line, for delivering gas into the analytical apparatus, the gas inlet line being connectable to the analytical apparatus to introduce gas into the apparatus, and to at least one gas supply, to deliver gas into the gas inlet line; at least one gas flow restriction assembly that is arranged on, or fluidly connected to, the gas inlet line, the flow restriction assembly comprising at least two switchable flow restrictions; at least one control line that is fluidly connected to the gas inlet line at a control line junction, downstream from the at least one gas flow restriction assembly, the control line comprising at least one back pressure regulator and at least one valve for regulating gas flow into the control line, and at least one vacuum pump that is arranged downstream from the back pressure regulator; and at least one inlet gas flow restriction that is arranged on the gas inlet line, between the control line junction and the analytical apparatus.

2. The gas inlet system of claim 1, wherein the flow restriction assembly is provided as a parallel arrangement of flow restrictions, the flow restrictions being arranged on separate flow restriction gas lines that meet at a first restriction junction upstream from the flow restrictions and a second restriction junction downstream from the flow restrictions on the gas inlet line, upstream from the control line junction.

3. The gas inlet system of claim 1, wherein the flow restriction assembly is provided as a parallel arrangement of two flow restrictions, wherein one of the flow restrictions is arranged on the gas inlet line and wherein the second of the flow restrictions is provided on a bypass gas line that is arranged parallel to the gas inlet line and that meets the gas inlet line at a first restriction junction upstream from the flow restrictions and a second restriction junction downstream from the flow restrictions, upstream from the control line junction.

4. The gas inlet system of claim 1, wherein the flow restriction assembly is provided as a parallel arrangement of two flow restrictions, wherein one of the flow restrictions is arranged on the gas inlet line and wherein the second of the flow restrictions is provided on a bypass gas line that is arranged parallel to the gas inlet line and that meets the gas inlet line at a first restriction junction upstream from the flow restrictions and a second restriction junction on the control line, between the mass flow regulator and the control line junction.

5. The gas inlet system of claim 4, further comprising at least one further flow restriction, wherein each such further flow restriction is arranged on a separate further gas line that is arranged parallel to the first and second flow restrictions in the flow restriction assembly.

6. The gas inlet system of claim 4, wherein the flow restriction assembly comprises two or more parallel flow restrictions that are adapted such that the ratio of gas flow through any two of the flow restrictions, for the same pressure difference across the restrictions, is in the range of about 1:20 to 1:1.5.

7. The gas inlet system of claim 1, wherein the analytical apparatus is a mass spectrometer.

8. The gas inlet system of claim 7, wherein the gas inlet line is fluidly connected to a collision cell of the mass spectrometer.

9. The gas inlet system of claim 1, wherein the vacuum pump is fluidly connected to the control line and is part of a vacuum pumping system of a mass spectrometer.

10. A system for calibrating gas flow in a gas inlet system for an analytical apparatus, the system comprising: a gas inlet line, for providing gas into an analytical apparatus; a gas flow calibration line that is fluidly connected to the gas inlet line via a first calibration junction, the gas flow calibration line comprising at least one gas flow meter; and at least one valve, for selectively directing gas flow either into the analytical apparatus via the gas inlet line or into the gas flow calibration line via the first calibration junction; such that gas flow in the system can, in a first gas flow setting of the at least one valve, be directed into the analytical apparatus via the gas inlet line, bypassing the gas flow calibration line, and in a second gas flow setting of the at least one valve be directed via the gas flow calibration line into the analytical apparatus, whereby gas flow that is measured in the gas flow calibration line in the second gas flow setting is, given a constant gas flow in the gas inlet line towards the calibration junction, a measure of the gas flow into the analytical apparatus in the first gas flow setting.

11. The system of claim 10, wherein the gas flow calibration line is further connected to the gas inlet line at a second calibration junction, such that gas flow in the system is, in the second setting, directed through the gas flow calibration line, from the first calibration junction to the second calibration junction, and into the analytical apparatus via a gas inlet junction that connects the gas inlet line to the analytical apparatus, downstream from the second calibration junction.

12. The system of claim 10, wherein the gas flow calibration line is further connected to the analytical apparatus through a calibration inlet junction that is fluidly separate from a gas inlet junction, between the gas inlet line and the analytical apparatus, such that gas flow in the system is, in the second setting, directed through the gas flow calibration line, from the first calibration junction to the calibration inlet junction and into the analytical apparatus.

13. The system of claim 10, further comprising a housing that is adapted to enclose the system and that further comprises means for maintaining the system at a constant temperature.

14. The system of claim 13, wherein the housing encloses at least the gas flow calibration line and the portion of the gas inlet line that stretches from the first calibration junction to the analytical apparatus.

15. The system of claim 10, wherein the analytical apparatus is a mass spectrometer.

16. The system of claim 15, wherein the gas inlet line and the calibration line are fluidly connected to a collision cell of a mass spectrometer.

17. The system of claim 10, further comprising at least one controller, for controlling the position of valves and/or gas flow regulators or controllers in the system.

18. A method of adjusting gas flow in a gas inlet system of an analytical apparatus, the method comprising steps of: flowing gas at an inlet pressure (P.sub.in) from at least one gas supply into a gas inlet line that provides gas into an analytical apparatus; regulating flow rate in the gas inlet line by splitting away a portion of the gas flow in the gas inlet line into a gas control line that is arranged on the gas inlet line and that meets the gas inlet line at a control line junction, such that a portion of the gas flow in the gas inlet line flows through the gas control line, and wherein gas flow in the gas control line is controlled by means of a back pressure regulator, whereby at a first setting of the gas flow controller gas flow rate in the gas inlet line and/or a pressure (P.sub.A) at the control line junction reaches a constant first value; adjusting the flow controller to a second setting and while the flow controller is in the second setting, flowing gas from the gas supply at least into one or more bypass gas line that is fluidly connected to the gas inlet line, between the gas supply and the control line junction; and maintaining gas flow through the bypass gas line until gas flow and/or pressure (P.sub.A) at the control line junction has reached a constant second value.

19. The method of claim 18, wherein gas is allowed to flow through the at least one bypass gas line while simultaneously maintaining gas flow through the gas inlet line.

20. The method of claim 19, wherein the portion of gas flow that is split away from the gas inlet line into the bypass gas line is in the range of about 50% to about 98% of the total gas flow.

21. The method of claim 18, wherein the ratio of gas flow rate through the gas inlet line to the gas flow rate through the bypass gas line, for the same pressure difference across the restrictions, is in the range of about 1:20 to 1:1.5.

22. The method of claim 18, further comprising diverting gas flow through a first bypass gas line for a first period of time to achieve a first gas flow rate at the control line junction, and diverting gas flow through a second bypass gas line that is arranged in parallel with the first bypass gas line for a second period of time to achieve a second gas flow rate at the control line junction, wherein the first and second bypass gas lines comprise different flow restrictions so that flow through the two bypass lines is different for a given fixed gas pressure difference across the bypass lines, whereby gas flow at the control line junction can be adjusted from a first flow rate to a second flow rate by selectively allowing gas flow through the first bypass gas line, and gas flow at the control line junction can be adjusted to a third flow rate by selectively allowing gas flow through the second bypass gas line.

23. The method of claim 18, wherein the analytical apparatus is a mass spectrometer.

24. The method of claim 23, wherein the gas is delivered into a collision cell of the mass spectrometer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The skilled person will understand that the drawings, described below, are for illustration purposes only. The drawings are not intended to limit the scope of the present teachings in any way.

(2) FIG. 1 shows a gas inlet system that includes a gas control line on which a gas flow controller is arranged.

(3) FIGS. 2A-2B shows an embodiment of the gas inlet system of the invention and which includes a bypass gas line which downstream end in (A) is connected to the gas control line and in (B) connected to the gas inlet line.

(4) FIG. 3 shows the change in pressure at the control line junction A in the absence (A) or presence (B) of gas flow through a bypass junction as a function of time following a change in pressure setting.

(5) FIG. 4 shows change in mass flow at the collision cell of a mass spectrometer for a change in pressure as indicated in FIG. 3 of a gas inlet line that delivers gas into the collision cell.

(6) FIG. 5 shows a gas inlet line for delivering gas into the collision cell of a mass spectrometer that contains a calibration gas line, a portion of the gas inlet line and the gas calibration line being within a thermostated chamber.

DESCRIPTION OF VARIOUS EMBODIMENTS

(7) In the following, exemplary embodiments of the invention will be described, referring to the figures. These examples are provided to provide further understanding of the invention, without limiting its scope.

(8) In the following description, a series of steps are described. The skilled person will appreciate that unless required by the context, the order of steps is not critical for the resulting configuration and its effect. Further, it will be apparent to the skilled person that irrespective of the order of steps, the presence or absence of time delay between steps, can be present between some or all of the described steps.

(9) It should be appreciated that the invention is applicable for analytical methods, including isotope analysis, of gases in general, by optical spectrometry, mass spectrometry or other types of spectrometry techniques. In general, therefore, the gas that is being analyzed in the system will be variable. Further, the system and method according to the invention is illustrated in the embodiments that follow with a preferred embodiment of an optical spectrometer, but it should be appreciated that the invention is also applicable to other spectrometers, including mass spectrometers.

(10) Turning to FIG. 1, there is shown a gas inlet system for delivering gas from a gas supply 10 to a collision cell 16 of a mass spectrometer. The collision cell is pumped by a vacuum pump; as a consequence, the system delivers gas into the collision cell from a high to low pressure. A single gas supply 10 is shown although it will be appreciated that, in other embodiments, the gas supply 10 could be a plurality of different gas sources that are switchable in order to supply a selected gas to the collision cell. The system has a gas control line 2, that is connected to the gas inlet line 1 at a gas line junction A. A first flow restriction 12 and a second flow restriction 13 are arranged on the gas inlet line. There are also valves 11, 14 arranged on the gas inlet line and valve 19 on the gas control line 2. A back pressure regulator 20, which functions as a flow controller, is arranged on the gas control line, downstream from the valve 19. Downstream from the back pressure regulator 20 there is a vacuum pump 21. In other embodiments, the back pressure regulator may not be pumped and may instead simply exhaust to atmosphere through an exhaust line.

(11) If gas is not to be delivered into the collision cell, valve 14, and optionally valve 19 (or valve 11) are kept closed. Opening the valves results in gas flowing through the restriction 12 towards the control line junction A. The pressure at this point in the system (P.sub.A) is regulated by back pressure regulator 20. Gas flow through the restriction 12 is therefore determined by P.sub.in, the pressure from the gas supply, and the pressure P.sub.A at the control line junction A. Gas flow continues from the control line junction, through the second flow restriction 13 and into the collision cell 16. Since the pressure in the collision cell is very low, e.g. 0.01 mbar or less, flow rate through the second restriction is controlled by P.sub.A, in accordance with the Poisseuille formula.

(12) In general, mass flow in the gas inlet system can be calculated by the equations:

(13) ${\stackrel{.}{m}}_1=\frac{1}{\mathrm{RT}}{G}_1\left(P_{\mathrm{in}}^2-P_{\mathrm{in}}{P}_A\right)$${\stackrel{.}{m}}_2=\frac{1}{\mathrm{RT}}{G}_2\left(P_A^2-P_A{P}_0\right)$${\stackrel{.}{m}}_3={\stackrel{.}{m}}_1-{\stackrel{.}{m}}_2$

(14) Where {dot over (m)}.sub.1, {dot over (m)}.sub.2 and {dot over (m)}.sub.a are the mass flow at restrictions 12, 13 and 19, respectively as shown in FIG. 1, G.sub.1 and G.sub.2 is the conductance of the gas inlet line upstream and downstream of junction A, respectively; P.sub.in, P.sub.A and P.sub.O is the pressure at the gas source, control line junction and collision cell, respectively; T is temperature; and R is the gas constant.

(15) Flow rates in the system can be adjusted by altering P.sub.in and/or P.sub.A, and/or by changing the flow restrictions 12, 13. For example, doubling P.sub.A results in a roughly 4-fold increase in flow rate through the second flow restriction 13, a five-fold increase in P.sub.A results in more than a 20-fold increase in flow rate, and so on.

(16) Care must be taken when configuring the system that the flow rate through the control line 2 is always high enough so that no back diffusion into the gas inlet line occurs. However, this is achievable by adjusting the pressure and restrictions in the system, and by adjusting the restriction of the gas control line.

(17) The vacuum pump 21 can have an exhaust that is open to atmosphere. However, multiple vacuum pumps can also be used with the system, for example pumps that are sequentially arranged, with a final pump in the sequence having an exhaust to atmosphere. The vacuum pump 21 can also be a part of, or be connected to, the vacuum pump system of a mass spectrometer. The vacuum pumps 21 and 16 can, in some embodiments, be the same pump or part of the same pumping system.

(18) The system can be set up so that gas flow of multiple gases can be individually controlled using a single flow controller. As an example the gas control line 2 can be branched so that it has a first branch that has a first junction with a first gas line carrying a first gas and has a second branch that has a second junction with a second gas line carrying a second gas. Each branch would have a valve to control the flow from each gas line (e.g. to have gas flow into one branch from one gas line but not into the other branch from the other gas line). Although the skilled person will appreciate that such a setup can be equally applied for any number of gases, through additional gas lines.

(19) With the set-up of FIG. 1, making a transition from a low gas to a high gas pressure in the collision cell (i.e. a transition from a low gas flow rate into the cell to a high gas flow rate into the cell) can take longer than is desired. The invention aims to address this problem.

(20) In FIG. 2A, a gas inlet system is shown that is similar to the system of FIG. 1 and therefore like parts have like references. The gas inlet system includes a bypass gas line 3 that is connected to the inlet gas line 1 at a first junction 4 and to the control line at junction 5. The bypass gas line contains a restriction 18 and a valve 17 to regulate gas flow in the bypass line. In this way, it can be seen that the additional restriction 18 is provided on the bypass line in parallel to the restriction 12 on the gas inlet line. The restriction 18 is preferably provided so as to allow greater gas flow for any given pressure at the control line junction A than restriction 12 on the gas inlet line 1. However, even a restriction 18 that provides the same or less flow than restriction 12 will still provide an increased flow rate from the gas supply 10 to control line junction A when both valves 11 and 17 are open as the total gas conductance is the sum of the conductances through restrictions 12 and 18.

(21) The system illustrated in FIG. 2B differs from that illustrated in FIG. 2A only in that the junction 5 (second restriction junction) is located on the gas inlet line, upstream from the control line junction A. It will be appreciated that the two configurations will provide identical results in terms of the regulation of flow rates/pressure at the control line junction. Other parts and their functionalities are identical in the illustrated systems. The following description applies to the system as illustrated in both FIG. 2A and FIG. 2B.

(22) By allowing gas flow from the supply 10 through the bypass line 3 (by opening valve 17), there can be an increased gas flow to the control line 2 (and thereby control line junction A) when required. As a consequence, if the setting of the back pressure regulator 20 is changed to an increased pressure setting, so as to provide an increased pressure at the control line junction A (and thereby increased flow rate into the collision cell), the flow in the system will reach the new equilibrium faster when the bypass line is open compared to allowing flow through the gas inlet line 1 and restriction 12 alone. This is a consequence of the fact that the overall conductance will be increased, i.e. the total conductance G.sub.T in the two gas lines leading away from the gas source, G.sub.1 (the gas inlet line) and G.sub.bp (the gas bypass line) is an algebraic sum of the two conductances:
G.sub.T=G.sub.1+G.sub.bp

(23) If additional parallel bypass lines were provided in the system, the overall gas conductance is the sum of individual conductance of each gas line that is open to gas flow:

(24) $G_T=\underset{n=1}{\overset{N}{.Math.}}{G}_n$
where G.sub.n is the gas conductance of individual gas lines.

(25) To minimize gas consumption in the system, the bypass line can be kept open to gas flow until the gas pressure at A (P.sub.A) has reached or substantially reached a constant value (equilibrium). At this point, valve 17 can be closed and gas flow from gas tank 10 to the control junction A be determined by restriction 12.

(26) The advantage of the setup is illustrated by the data shown in FIG. 3. Gas pressure at the gas control line junction A (P.sub.A) is shown as a function of time, following the switch in back pressure regulator setting from 0.2 bar to 0.6 bar. The lower curve (A) shows the pressure change for a gas inlet system as shown in FIG. 1, i.e. a system that does not contain a bypass junction. Obviously, this behaviour will also be observed when there is no gas flow through the bypass gas line of the system in FIG. 2. Following change in setting of the back pressure regulator, the system responds rather slowly as shown in (A), because gas flow towards the control line junction A is determined by restriction 12. The upper curve (B) shows the advantage of the system as shown in FIGS. 2A-2B. By allowing gas flow through the bypass gas line, the system responds much faster to change in pressure settings, in the illustrated case by a factor of more than 3. In the example shown, the gas inlet line and the bypass line both have an inner diameter of 0.1 mm and a length of 10 cm and 5 cm, respectively. Alternatively, the equilibrium could be reached even faster if the restriction on the bypass line has a larger inner diameter than the restriction on the gas inlet line. The restrictions on the gas lines herein are effectively provided by the minimum inner diameters present on the respective lines. A portion of the gas line or the whole gas line could be provided with the inner diameter that provides the restriction.

(27) When the pressure at the control junction A has reached a constant value (equilibrium) (in the illustrated case after slightly less than 5 s), valve 17 can be closed and gas flow thereby reduced to minimize gas consumption in the system. The corresponding time to equilibrium when there is no flow through the bypass gas line (or when there is no bypass gas line in the system) is, by comparison, about 16 s. The significantly shorter time needed to reach the new equilibrium pressure in the collision cell when analytical measurements can be taken enables a more efficient use of the apparatus for measurements because the apparatus is spending less time adjusting between different collision cell pressures.

(28) For rapidly lowering the pressure at the control line junction A, the gas pump can be used to pump away excess gas. This will be most efficient if the bypass gas line is kept closed and thereby minimizing mass flow towards the control line junction, while the pump removes excess gas via the control line to reach a reduced steady state pressure.

(29) In FIG. 4, corresponding data are shown, monitoring mass flow with a mass flow meter (MFM) immediately upstream of the collision cell, i.e. downstream from the gas inlet system. Due to the measurement being performed downstream from the control line junction A, there is a slight time delay in the data shown in FIG. 4 compared with the data in FIG. 3. Nevertheless, the data shows clearly the advantage of using the bypass line; the mass flow into the collision cell has reached an approximately constant value about 6 seconds after the change in pressure settings, while in the absence of the bypass line, the system takes about 17 seconds to reach equilibrium. The advantage of using the bypass line is therefore abundantly clear.

(30) The system can be adapted to minimize gas consumption, while simultaneously allowing changes in flow rate within a significant range. Consider for example the case where it is desirable to be able to regulate flow rate between 1 and 10 mL/min into the collision cell 16. To achieve this with a single gas inlet line (no bypass gas line), there would need to be a gas flow from the gas source of at least 10 mL/min. At low gas flow settings, e.g. 1 mL/min, most of the gas (9 mL/min) is pumped away into the gas control line, and only 1/10 of the gas actually enters the collision cell. When the gas flow into the collision cell is set to 10 mL/min, there is no gas flow in the control line. Practically speaking, a minimal gas flow of about 1 mL/min is maintained in the gas flow control line, which means that for a total flow rate of 10 mL/min from the gas supply, a maximal gas flow into the collision cell of about 9 mL/min can be achieved. Nevertheless, if the system is used for significant periods of time at low gas flow rates into the collision cell, there will be significant and unnecessary gas consumption.

(31) The gas consumption in the system can be reduced by using one (or more than one) bypass gas line. For example, there can be two parallel gas lines (one of which is the bypass gas line), one allowing gas flow up to 5 mL/min, and the up to 10 mL/min. For low gas flow rates into the collision cell (for example flow rates ranging from 1 to 5 mL/min), the first gas line can be used exclusively. When higher flow rates are required, gas flow can be diverted into the other gas line (e.g., the bypass gas line) which allows flow rates up to 10 mL/min. Thereby, there is increased use of the gas that is delivered into the system, and the amount of gas that goes to waste (via the gas pump on the gas control line) is minimized. Alternatively, each of the parallel gas lines can allow a gas flow of up to 5 mL/min, which means that in a first setting, gas will be allowed to flow through one of the lines, for regulating gas flow rates up to 5 mL/min into the collision cell, and in a second setting, gas flows through both lines (the gas inlet line and the bypass line), for regulating gas flow rates into the collision cell from 5 mL/min up to 10 mL/min.

(32) Obviously, additional control lines can be provided to achieve any desired flow rate, by directing gas flow through one, or any combination of, the individual gas lines. The advantage resides in the possibility to minimize excess gas flow into the system, and thereby reduce gas consumption by minimizing the amount of gas that is pumped away through the back pressure regulator on the gas control line.

(33) In FIG. 5, a gas inlet system that comprises a calibration line for gas flow is shown. The gas inlet system shown comprises a gas inlet system as illustrated in FIG. 1, with an additional calibration line 26. There is a gas flow controller 24 on the gas calibration line, as well as two valves 23, 25. The gas calibration line meets the gas inlet line at a first calibration junction C.sub.1 and a second calibration junction Cz. The first calibration junction C.sub.1 is located downstream of the flow restriction 13 on the gas inlet line. Between the first calibration junction C.sub.1 and the second calibration junction C.sub.2 there is a valve 22 on the gas inlet line. Preferably, the gas calibration line is located downstream of any gas flow controllers on the gas inlet line (such as the back pressure regulator in FIG. 1 or FIG. 2), i.e. in this example downstream of the control line junction A; thereby gas flow calibration that is based on gas flow in the line will be representative of gas flow in the gas inlet line and that flows into the collision cell.

(34) The gas calibration line and the portion of the gas inlet line that stretches from the first to second calibration junction (C.sub.1, C.sub.2) are placed within a thermally insulated chamber 30. The flow restriction 13 on the gas inlet line, downstream of the control line junction A, is also placed within the thermally insulated chamber. Temperature in the chamber can be adjusted using additional heating or cooling means and/or other conventional means, such as by means of a thermostat housing. Preferably restriction 13 and valve 14 are also placed within the thermally insulated chamber 30, as indicated. This way, the effects of temperature fluctuations on mass flow rate downstream of the control line junction can be minimized, thereby providing a precise calibration.

(35) In the example shown, the calibration line meets the gas inlet line at a second calibration junction C.sub.2, just upstream from the gas inlet 27 into the collision cell 16. Alternatively, the calibration gas line may be separately connected to the collision cell, i.e. via a gas connection/gas inlet 28 that is fluidly separate from the gas inlet 27, as indicated by the dashed line in FIG. 5.

(36) Thus, by keeping the calibration line and the portion of the gas inlet line that is downstream of the control line junction (e.g. including the flow restriction 13 on the gas inlet line) at a fixed temperature, the effects of temperature fluctuations on mass flow can be minimized. As a consequence, gas flow in the gas inlet line can be determined by feeding the gas flow into the gas calibration line, by closing valve 22 on the gas inlet line and opening valve 23 (and valve 25 if previously closed), and determine gas flow rate using the mass flow meter (MFM 24) on the calibration line. Following calibration of the flow rate, valve 23 and valve 25 can be closed and valve 22 opened to allow gas flow along the gas inlet line into the collision cell that has been calibrated. The relationship of pressure versus gas flow rate as measured using the mass flow meter on the calibration line can be used to accurately set the flow rate into the collision cell, with the gas inlet line inside the thermostatic housing 30 kept at a constant temperature. A further advantage of this setup, with the mass flow meter on the separate calibration line, is that there is no large dead volume in the gas flow leading into the collision cell when, in use, the gas is flowed through the gas flow line into the collision cell. It will be appreciated that the calibration system shown in FIG. 5 can be used in combination with the restriction bypass assembly system of FIG. 2.

(37) The embodiments of FIG. 1 and FIG. 2 could be modified for calibration purposes to provide inside a thermostatic housing the gas inlet line 1 downstream of the control line junction A, including flow restriction 13, along with a flow meter MFM on the gas inlet line itself, between the control line junction and the collision cell. However, this setup would not have the advantage of avoiding the dead volume caused by the presence of the MFM.

(38) Obviously, additional components of the gas inlet system, for example the gas control line, can be temperature controlled, for example by placing the components within a thermally controlled housing. The housing can for example comprise heat-insulating walls and comprise one or more air thermostats that maintain the housing at a constant temperature.

(39) Further, the calibration function described in the above may be implemented on gas inlet lines in general, as long as the calibration line is placed downstream of any gas flow controllers in the gas inlet line and downstream of flow restrictions on the gas inlet line. For example, the calibration line may be implemented in any gas inlet system as described herein, e.g. gas inlet systems that contain at least one bypass line for achieving rapid steady-state gas flow following change in gas flow settings. However, the skilled person will appreciate that the calibration function may also be implemented in other gas lines of analytical systems.

(40) While the gas inlet system of the invention has been described above in the context of providing a gas flow into a collision cell, e.g. of a mass spectrometer, it will be appreciated that the invention can be used to supply gas into other types of analytical device, especially vacuum pumped devices where gas is to be provided at a number of different selected pressures.

(41) As used herein, including in the claims, singular forms of terms are to be construed as also including the plural form and vice versa, unless the context indicates otherwise. Thus, it should be noted that as used herein, the singular forms a, an, and the include plural references unless the context clearly dictates otherwise.

(42) Throughout the description and claims, the terms comprise, including, having, and contain and their variations should be understood as meaning including but not limited to, and are not intended to exclude other components.

(43) It will be appreciated that variations to the foregoing embodiments of the invention can be made while still falling with the scope of the invention can be made while still falling within scope of the invention. Features disclosed in the specification, unless stated otherwise, can be replaced by alternative features serving the same, equivalent or similar purpose. Thus, unless stated otherwise, each feature disclosed represents one example of a generic series of equivalent or similar features.

(44) Use of exemplary language, such as for instance, such as, for example and the like, is merely intended to better illustrate the invention and does not indicate a limitation on the scope of the invention unless so claimed. Any steps described in the specification may be performed in any order or simultaneously, unless the context clearly indicates otherwise.

(45) All of the features and/or steps disclosed in the specification can be combined in any combination, except for combinations where at least some of the features and/or steps are mutually exclusive. In particular, preferred features of the invention are applicable to all aspects of the invention and may be used in any combination.