A method and device for the control of gaschromatographic functions

Abstract: The functions of a gas chromatograph depending on the advancement stage of the analysis can be at least partly controlled by detecting the data relating to the carrier feeding conditions (pressure and/or flow) in a position upstream the injection point (2) of the sample to be analyzed, and by measuring the variations occurring in said conditions after said sample injection. This method is particularly convenient for the analysis of samples with high volumes of eluent, and one of its preferred embodiments foresees the use of a pressure sensor (9) in a point of the carrier feeding duct (1) or, alternatively, of a flow sensor positioned between two points of said feeding duct.

Description: The present invention relates to a method and a device to control those functions of a gas chromatograph which depend on the advancement stage of a sample analysis, and particularly the functions which can be correlated to the presence of eluent in the separation column or in the precolumn. As it is known, in a gas chromatographic analysis a number of the gas chromatograph functions depend on the advancement stage of the analysis itself. Examples of said functions are the on/off switching of valves in an HPLC/GC interface of the so-called "loop'′ type, switching which depends on the sample complete introduction; or the rise of the oven temperature after the eluent complete evaporation in analyses using a precolumn (retention gap). Another example, the following description will refer to, is represented by the commutation of the regulating valves of an exhaust duct, or splitter, mounted on a point whatsoever of the sample elution run, between the injector and the detector, in a gas chromatograph used to analyze large volumes of samples (e.g., samples containing up to several ml of eluent). As it is known, in said analyses the splitter has the function of avoiding that the whole considerable amount of eluent (solvent, supercritical fluid or the like) flows through the column and/or the detector, discharging most of it before arriving, while only a small fraction of same is actually sent to the analysis or the detector. Obviously, once the eluent has passed, all (or almost) the remaining part of the sample, which in the meantime has started to elute, will be sent to the detector and its components analyzed: a switching valve working on the splitter is provided for this purpose. Of course, before operating said valve, it is necessary to ensure the eluent has practically all passed through the precolumn or column, which can be done only by monitoring the solvent peak on the detector during the first part of the injection, or by performing a test injection and determining how long the solvent takes to elute completely. If the volume of the injected sample varies, said test injection must be repeated, in that the solvent elution time varies as well. V.M.A. Häkkinen et al., JHRC & CC, 11 (1988) Febr., No.2, disclose a system to carry out a constant flow GC analysis where a pressure regulator is provided in order to limit to a preset value the maximum pressure generated by the flow regulator when it increases the carrier gas pressure to avoid or limit the drop of the flow rate of the carrier gas. This document refers to K. Grob et al., JHRC & CC, 9 (1986) sept., No. 9. These two documents disclose that by the combined use of a flow regulator and a pressure regulator the following effect occurs: upon injection of a sample an increase of pressure is obtained during the evaporation of the sample, the pressure subsequently drops once the sample has evaporated. JP Application n. 57-114853 discloses a system for removing attached material from a GC sampling device in process GC. In this system, the flow rate of a flushing gas is detected and compressed air is fed to the sampling tube when the flow rate is less then a set point. Anyhow, in both cases, the time the operator requires for said controls is too long. Furthermore the time necessary for said operations is never exactly the same; if for instance it is considered the time elapsed between the sample injection and the eluent complete evaporation, it can be noticed that said time is conditioned by a lot of factors which render the moment of evaporation end not reproducible with sufficient precision. Similar problems arise in the other aforementioned examples of the gas chromatograph controllable functions. There is therefore the need to automate the control of those gas chromatographic functions which depend on the advancement stage of the analysis itself, but which cannot be precisely correlated to the time elapsed from the beginning of the analysis. An object of the present invention is to solve the aforementioned problems by providing a method for the automatic control of the functions of a gas chromatograph which depend on the analysis advancement stage. A second object of the invention is to provide a device to automatically or semi-automatically control the functions of a gas chromatograph which depend on the advancement stage of the analysis. These and other objects are achieved by the method and the device provided by the invention, which allow to automatically control, thanks to a feedback effect, said functions of a gas chromatograph in an easy, reliable and economic way. More in particular, the present invention relates to a method for the control of one or more functions of a gas chromatograph which depend on the advancement stage of the gas chromatographic analysis, characterized according to claim 1. Moreover the invention relates to a device for the control of at least one of the functions of a gas chromatograph which depend on the advancement stage of a gas chromatographic analysis, characterized according to claim 6. The invention will be now described more in detail with reference to the accompanying drawings, attached for illustrative and not limitative purposes, wherein: Figure 1 is a schematic diagram of a first embodiment according to the invention; Figure 2 is a graph showing the variations of a parameter of the carrier feeding in the device of figure 1; Figure 3 is a schematic diagram of an embodiment according to the invention, alternative to the embodiment of figure 1; Figure 4 is a graph of the variations of a parameter of the carrier feeding in the embodiment of figure 3; Figure 5 is a scheme of a preferred embodiment of a splitter valve; Figure 6 is a schematic diagram of the device of figure 1 utilizing the valve of figure 5; Figure 7 is a schematic diagram of the device of figure 3 utilizing the valve of figure 5; and Figures 8 to 10 are diagrams of possible alternative embodiments of the splitter valve. As previously mentioned, the examples in figures 1 and 3 relate to a gas chromatographic equipment fitted with splitter, specially designed for the analysis of samples containing large volumes of solvent such as those consisting of fractions obtained via HPLC (high pressure liquid chromatography). Nevertheless it is obvious that the device according to the invention can be used on any other chromatographic equipment, to which the principle of the invention can be applied, independently from the type of eluent used, from the type of activated function and, in the case of activation of a splitter, from the splitter position along the elution run between injector and detector. Said principle is based on the fact that the evaporation of the eluent and/or low-boiling components of the sample generates a vapour pressure which on its turn causes variations in the feeding conditions of the inert gas constituting the mobile phase, namely the so-called "carrier". The embodiments of figures 1 and 3 are two examples of a device, designed in function of the different entity of the variations of said carrier conditions. Referring to figure 1, the shown equipment has a feeding line for carrier 1 connected via a valve 2 to a pre-column 3. On its turn the precolumn 3 is connected to a separation colum 5, which leads to a detector 6. Along the terminal section of the separation colum 5 there is an exhaust duct, or splitter, 10 connected to the separation column 5 by means of a valve 11. On the carrier feeding duct there are provided, in the opposite direction to the gas flow, starting from the valve 2, a pressure sensor 9, a flow regulator 8, a manometer 7 and a pressure regulator 4; the carrier gas will then flow through said devices 4, 7, 8, 9 before reaching the valve 2 and then the precolumn 3. The valve 2 also connects the sample feeding duct 12 to the precolumn 3; for instance, according to the type of sample injection used, the valve 2 will constitute an interface of the so-called "loop" type or an interface of the so-called "on-column" type. The pressure sensor 9 is connected, via a line 14, to a processing unit 13 which on its turn is connected to the valve 11 via a line 15. The device according to the invention, namely the group consisting of the apparatuses indicated by 9, 13, 14 and 15 can be indifferently used with both types of interfacing valves. Once again it is hereby underlined that, besides the control of the switching valve of the splitter 11, the device according to the invention can be equally used to control any other functions of the gas chromatograph which depend on the advancement stage of the analysis, that is which are related to the time elapsed from the beginning of the analysis. Furthermore it must be stressed that, being the following description referred to a combined equipment HPLC-gas chromatograph, the term "solvent" is intended in its true meaning, but that the concepts described hereunder can be applied to any type of eluent, as for instance to a supercritical fluid. When the sample is injected into the precolumn 3 via the interface valve 2, the solvent coming into contact with the heated precolumn evaporates and generates a vapour pressure which opposes the passage of the carrier and even tends to push the sample backward. This phenomenon occurs both using the on-column technique and the "loop" interface, and in the last case it is particularly important. The flow regulator 8 tries to keep the carrier flow constant and therefore (also to avoid that the sample is pushed backward along the precolumn 3) provides to raise the carrier pressure; said increase in any case cannot exceed a predetermined value previously set on the pressure regulator 4. This variation of the carrier pressure upstream the sample injection point, namely upstream the interface valve 2, is detected by the pressure sensor 9 which transmits the detected data to the processing unit 13 through the line 14. As previously mentioned, the processing unit 13 is on its turn connected to the valve 11 through a second connecting line 15. When the solvent evaporation is over, the vapour pressure of the solvent disappears and the flow regulator 8, detecting said disappearance, makes the carrier pressure decrease down to the value present before the sample injection. In this case, too, the pressure sensor 9 records a variation in the pressure value and sends it to the processing unit 13. The variations of the relative pressure values in the carrier feeding duct 1 are illustrated in the graph of figure 2, where the values of the carrier pressure are plotted on the ordinates, while the time elapsed from the the beginning of sample injection is plotted on the abscissae. As it can be noted, pressure P undergoes a first variation with a positive ΔP in the graph area indicated by 16. Said area and said ΔP correspond to the rise of carrier pressure in the duct 1 as effected by the flow regulator 8 because of the vapour pressure generated by the solvent evaporation in the precolumn 3. The subsequent analytical stage, i.e. the time necessary for the solvent to completely evaporate, is represented by the constant pressure plateau present in the graph central area and generically indicated by 17. At the end of the solvent evaporation, a decrease of the carrier pressure in the duct 1 will take place, said decrease being controlled by the flow regulator 8. The ΔP in this case is negative and indicated by reference 18. It has been noticed that, as soon as the pressure variation with negative ΔP 18 is detected, no condensed solvent is present in the precolumn 3. Generally at the end of the negative pressure variation 18, almost all solvent vapours have been exhausted through the splitter 10 and the processing unit 13 can give the order to switch the valve 11 to a position connecting the separation column 5 to the detector 6. Said order can be given immediately after detecting the negative ΔP 18 or after a subsequent, predetermined, time interval to allow all the solvent still possibly present in the column 5 to be eliminated through the splitter 10. It must be outlined that said pressure variations take place independently from the volume of injected sample and/or eluent; in particular, though the ΔP can vary in width, the speed of pressure variation (that is the graph slope in points 16 and 18) is essentially the same for a wide range of volumes of injected sample. Preferably said interval is determined experimentally in advance. During the analysis of a generic sample containing an eluent giving way to a cycle of variations of carrier pressure similar to that of figure 2, in order to avoid possible errors, the order to switch the valve 11 is given by the processing unit 13 only after the latter has received from the pressure sensor 9 a complete series of pressure values corresponding to variations similar to those illustrated in the graph of figure 2. Said variations must substantially involve a pressure increase at the beginning corresponding to the variation in positive 16, a period of constant pressure corresponding to the plateau 17 and a pressure decrease at the end corresponding to the area 18 of the graph of figure 2. Obviously, as already mentioned, positive and negative pressure variations must take place within a determined time period, which means that the graph of said variations must show slopes similar to the ones indicated by references 16 and 18 in figure 2. On the contrary, the plateau 17 can have any value, in that the realization of the functions only depends on the graph trend and is independent from the time needed for the eluent complete evaporation. In the gas chromatograph of figure 1 the splitter 10 and the switching valve 11 are fitted to the end portion of column 5, immediately upstream the detector 6; gas chromatographs for analysis of samples with large volumes of solvent are however known, wherein the splitter 10 and valve 11 are fitted to the end of the precolumn 3, immediately upstream of the separation column 5, or between two different separation columns. This arrangement is designed to speed up the analysis time avoiding that the large mass of solvent vapour is forced to elute through the whole column 5 before being exhausted through the splitter 10. In this case a very large exhaust opening is provided immediately downstream the solvent evaporation area; said disposition involves as a consequence the fact that pressure variations taking place in the feeding duct 1 are smaller and quicker than the previous ones and are generally too small and too quick to be accurately and easily detected by the pressure sensor fitted to the feeding duct 1. Similarly, there are cases where the solvent vapours, for quality or quantity, are not able to generate variations of flow, and therefore of the carrier pressure, having sufficient intensity and/or duration to be satisfactorily measured by the pressure sensor 9. It has been ascertained that the best method to overcome this inconvenience is to measure the variations of the carrier flow instead of the pressure variations as in the case of figure 1. For this purpose a modified version of the device according to the invention is used, as shown in figure 3. As it can be noticed in said figure the device according to the invention comprises herein, besides a processing unit 13 connected to the switching valve 11 in a completely similar way to that previously shown in figure 1, a flow sensor as well. In the preferred embodiment of figure 3, said flow sensor consists of a differential pressure sensor 19 detecting the differential pressure at the two ends of a restrictor 20 mounted in series on the carrier feeding duct 1. Unlike the example of figure 1, in this case the carrier feeding pressure is kept constant at the value set in advance on the pressure regulator 4. The carrier pressure and the restrictor 20 are selected in such a way that the pressure difference, or ΔP, between its upstream end 21 and its downstrean end, indicated by 22, is sufficiently low as to be annulled or reduced significantly by even the least vapour pressure of the solvent which is present in the precolumn 3 immediately after injection. The ΔP variation in function of the time passed from the beginning of the analysis is shown in figure 4. As it can be noticed, in this case the graph trend is substantially reversed with respect to the one shown in figure 2. In fact, the formation of vapour pressure in the precolumn 3 causes a reduction in the difference of pressure between the ends 21 and 22 of the restrictor. Said negative variation is indicated by 25 in the graph of figure 4. During the whole period in which solvent vapour is present in the precolumn 3, the flow, namely the difference of pressure between the points 21 and 22 of the restrictor, remains at a constant value which is nil or in any case near to zero; said constant value forms a plateau indicated by 26 in figure 4. At the end of the solvent evaporation the flow increases again until it goes back to the initial value corresponding to the difference of pressure before the sample injection into the precolumn 3; said positive variation of the differential pressure is indicated by 27 in the graph of figure 4. It has been verified that, when the positive flow variation occurs, that is when the ΔP value goes back to the initial value, condensed solvent is no longer present inside the precolumn and generally almost all solvent vapours have been at this point exhausted through the splitter 10. During operation, the flow sensor, or differential pressure sensor 19, records the variations of the difference of pressure between the two ends 21 and 22 of the restrictor 20 and transmits them via the line 14 to the aforementioned processing unit 13. The unit 13 in turn processes said data to switch the valve 11 from a position connecting the precolumn 3 to the splitter 10 to a position connecting the precolumn 3 to the separation column 5. Similarly to what described for the method relating to the apparatus of figure 1, the commutation control will be given when the unit 13 has verified that the differential pressure variation (or flow variation) has followed a trend similar to that shown in figure 4; in this case as well the differential pressure variations have to take place in well determined time intervals, i.e. the graph resulting from said variations must have variations with a slope similar to the one indicated by 25 and 27 in the graph of figure 4. In the embodiments shown in figures 1 and 3, a splitter was shown, wherein a generic valve 11, actuated by the processing unit 13, does or does not run the eluent flow to the exhaust 10. Figure 5 shows a type of splitter which can be advantageously used in the gas chromatographic equipment comprising the device according to the invention. Said splitter has been described in principle by Pacciarelli et al. (Proceedings 8th Int. Symp. on Capillary Chromatography, Riva del Garda, 1987, Huthig, Heidelberg, page 1204) and essentially consists of a "T" fitting 28 located along the normal duct 30 of gases elution and optionally connectable either to an exhaust 29 having a much greater internal diameter than that of duct 30 (and therefore a much lower hydraulic resistance), or to an exhaust 31 having a much lower internal diameter than that of duct 30 and therefore a higher hydraulic resistance. Connecting the "T" 28 to the exhaust 29, most of what elutes along the duct 30 is deviated towards the reduced resistance exhaust 29, following the run indicated by arrow 32. On the contrary connecting the "T" 28 to the exhaust 31, most of the eluent continues to flow along the duct 30 following the run indicated by the broken arrow 33, while only a small portion is eluted through the high resistance exhaust 31. Figure 6 shows the application of a valve of the aforedescribed type to a gas chromatographic equipment of the type described with reference to figure 1. As it can be noticed, at the downstream end of the separation column 5 a "T" fitting 28 is provided connecting the column 5 to a terminal duct 35 which, due to its internal diameter, will form a first resistance R1. The "T" fitting 28 is in turn connected to a three-way electrovalve 34 (connected to the processing unit 13 via the line 15), from which two exhaust ducts 36 and 37 come out. In particular the duct 36 will have a much lower internal diameter than both duct 37 and duct 35, while the duct 37 has an internal diameter which is much higher than that of the ducts 35 and 36. If R2 is the hydraulic resistance offered to eluent run by duct 36 and R3 the one provided by duct 37, it will result that R2 > R1 > R3. Consequently, by appropriately switching the electrovalve 34 thanks to the signals sent by the processing unit 13, it is possible to guide the eluent flow through the exhaust with less resistance 37 or along the terminal duct 35 up to the detector 6. It is herein pointed out that the eluent always flows in any case through the duct 35, even if with different flowrates according to the position selected for the electrovalve 34. A similar arrangement can be used also in the apparatus described in figure 3. Said arrangement is illustrated in figure 7, where the same reference numerals of figure 6 are used for corresponding parts. As it can be noticed, in this case too there is provided a three-way electrovalve 34 connected to and worked by the processing unit 13 via the line 15. The electrovalve 34 is fitted onto a duct 38, which connects the precolumn 3 to the separation column 5, by means of a "T" 28. The electrovalve 31 is connected to an exhaust opening 37 with large internal diameter and small resistance and to an exhaust opening 36 with small internal diameter and high resistance. In particular, the internal diameter of exhaust 37 will be much greater than both the one of exhaust 36 and the one of duct 38, while the internal diameter of exhaust 36 will be much lower than both the one of duct 38 and (as already mentioned) the one of exhaust 37. Indicating by R4 the hydraulic resistance of the duct 38 and by R2 and R3 those of the exhausts 36 and 37 respectively, it will result R2 > R4 > R3. Similarly to what described hereinabove, by appropriately switching the valve 34 via the signals coming from unit 13, the eluent will be mostly discharged through the exhaust duct 37 or sent to the separation column 5, according to the switching controlled by the unit 13 on the basis of the signals coming from sensor 19. In the preceding examples the hydraulic resistance offered by the ducts 36 and 37 depended on the diameter of the the ducts themselves, however said resistance can be varied in function of other parameters, such as the duct length or the pressure value inside or at the outlet of said ducts. Figures 8, 9 and 10 illustrate three particular embodiments of splitter where the duct 37 with reduced resistance has been replaced by a source of reduced pressure 40. Said source, which can be connected to an outlet of the valve 34, directly or via a duct 39, does not only zeroset the relevant hydraulic resistance, but has also the function of speeding up evaporation of the eluent present in the column or precolumn. The embodiment of figure 8 is quite similar to the splitter shown in figures 6 and 7, with the aforementioned modification consisting in the presence of a vacuum source 40. Figure 9 illustrates a modification of the splitter of figure 8, wherein the electrovalve 34' is no more a three-way one but a two-way valve, one of its outlets being connected to the vacuum source 40 and the other to the "T" fitting 28. In fact the presence of the reduced pressure source 40 makes the outlet with the duct 36 at resistance R2 practically needless, in that the vacuum almost completely eliminates any trace of eluent from the valve 34', making its flowing back to the "T" 28 very unlikely. However, to be thoroughly sure, the embodiment shown in figure 10 can be adopted, wherein the electrovalve 34' is still a two-way one, but a duct 36' with a very reduced diameter and high resistance is provided on the duct 41 which connects the electrovalve 34' to the "T" fitting 28. As previously mentioned, the aforereported examples exclusively concern the control of the switching valve of a splitter in a gas chromatograph during the analysis of samples with large volumes of solvent; it is however evident that the device according to the present invention can be used also to control the other functions of the gas chromatograph which depend on the advancement stage of the analysis. In particular the device according to the invention enables to detect the variations caused by the presence or absence of the sample in the injection area and to use said variations just to regulate those functions. For instance the device according to the invention can be used to set up the beginning of a temperature program with resulting increase of the analytical temperature at the end of the solvent evaporation, independently from the fact that said solvent is eventually eliminated through a splitter or not. Similarly, the device according to the present invention can be used to operate the injection valve,for instance to switch it to the position previous to sample injection as soon as the desired variation of pressure and/or flow of the eluting gas takes place.

Claims: A method of controlling or actuating at least one of the functions of a gas chromatograph that are depending on the advancement stage of a gas chromatographic analysis and are correlated to the passage of a sample eluent in a gas chromatograph precolumn (3) or column (5), said method providing the detection of the values of the actual pressure and/or flow of the carrier gas fed upstream the injection point of said sample to be analysed, characterized in that it comprises the following steps: feeding said detected values to a processing unit (13) in order to detect the variations of said values and the time required for said variations to take place; and controlling or actuating said function(s) of the gas chromatograph, immediately or after predetermined time interval(s), only after said processing unit (13) has detected a sequence of variations of said pressure and/or flow values comprising a first values variation (16,25), a constant values period and a second values variation (18,27), opposite to the said first variation, said first and second values variations having taken place within a predetermined time. A method according to claim 1, characterized in that said gas chromatograph functions are actuated after having detected, in succession from the beginning of the sample injection, a positive pressure variation (16), a period of substantially constant pressure value (17) and a subsequent negative pressure variation (18), both said pressure variations (16, 18) having value and duration within predetermined ranges. A method according to claim 1, characterized in that said gas chromatograph functions are actuated after having detected, in succession from the beginning of the sample injection, a negative flow variation (25), a period of substantially constant flow value (26) and a subsequent positive flow variation (27), both said flow variations (25, 27) having value and duration within predetermined ranges. A method according to any of claims 1 to 3, characterized in that said functions comprise the on/off switching of a valve (11, 34, 34') controlling an exhaust duct for part of the sample vapours, said duct being located between the injector (2, 12) and a detector (86). A method according to claim 4, characterized in that said functions comprise the on/off switching of a valve (11) controlling an exhaust duct (10) for part of the sample vapours, said duct being located between the precolumn (3) and the column (5). A device for the control or actuation of at least one of the functions of a gas chromatograph that are depending on the advancement stage of a gas chromatographic analysis and are correlated to the passage of a sample eluent in a gas chromatograph precolumn (3) or column (5), said device comprising means (9; 19-22) to detect the actual pressure and/or flow values of a carrier gas fed upstream the injection point (2) of the sample to be analysed, characterized in that it further comprises: a processing unit (13); means (14) for feeding said detected values to said processing unit (13), said processing unit (13) comprising means to detect variations of said values and the time required for said variations to take place; and means to actuate said function(s) of the gas chromatograph, immediately or after predetermined time interval(s), only after said processing unit (13) has detected a sequence of values variations comprising a first values variation (16,25), a constant value period and a second values variation (18,27), opposite to the said first variation, said first and second values variations having taken place within a predetermined time. A device according to claim 6, characterized in that said means to detect the values of carrier gas pressure comprises a flow rate regulator (8) and a pressure sensor (9), fitted on a carrier gas feeding duct (1), upstream the sample injection point (2). A device according to claim 6, characterized in that said means to detect the values of carrier gas flow comprises a flow sensor (19 - 22) fitted on a carrier gas feeding duct (1), upstream the sample injection point (2). A device according to claim 8, characterized in that said flow sensor comprises a restrictor (20) and a sensor (19) detecting the pressure differential between the two ends (21, 22) of said restrictor (20). A gas chromatographic equipment, of the type comprising a splitter positioned along the elution path of the gases and essentially including an exhaust duct (10, 37, 39) for the solvent vapours and a valve (11, 34, 34') to regulate the passage of said vapours to said ducts, characterized in that it further comprises a control device for said valve according to one of claims 6 to 9. An equipment according to claim 10, characterized in that said valve consists of a three-way electrovalve (34) connected to a first outlet (37), to a normal elution path (35) of the sample and to a second outlet (36), and in that the hydraulic resistance (R2) of said second outlet (36) is higher than that (R1) of said normal elution path (35), which in turn is higher that that (R3) of the first outlet (37). An equipment according to claim 11, characterized in that said first outlet is connected to a source (40) of reduced pressure. An equipment according to claim 10, characterized in that said valve consists of a two-way electrovalve (34') connected to the normal elution path of the sample and to a source (40) of reduced pressure. An equipment according to claim 13, characterized in that on the connection between said electrovalve (34') and said normal elution path there is provided an exhaust duct (36') having hydraulic resistance much higher than that of said normal elution path.

Inventors: MUNARI FAUSTO, MUNARI, FAUSTO
Applicants: FISONS INSTR SPA, FISONS INSTRUMENTS S.P.A.
Application year: 1989
Main classification: G01N30
IPC codes: G01N30