WO1994022009A2

WO1994022009A2 - A method and equipment for the analysis of volatile organic compounds

Info

Publication number: WO1994022009A2
Application number: PCT/EP1994/000889
Authority: WO
Inventors: Fausto Munari; Dimitris Kotzias; Matthew Duane
Original assignee: Fisons Instruments S.P.A.
Priority date: 1993-03-25
Filing date: 1994-03-22
Publication date: 1994-09-29
Also published as: WO1994022009A3; ITMI930571A1; IT1270972B; ITMI930571A0

Abstract

The analysis of volatile organic compounds at low concentrations (ppb or ppt) is performed by making a sample flow to pass through a cold trap containing few mg of packing, at a sample feeding speed ranging between 50 and 1000 cm/sec; the sample thus concentrated is then desorbed by heating of the trap and conveyed for the analysis.

Description

"A METHOD AND EQUIPMENT FOR THE ANALYSIS OF VOLATILE ORGANIC COMPOUNDS" Technical Field

The present invention concerns a method and an equipment for the analysis of volatile organic compounds. Background Art

The growing attention paid to environmental problems has led to the need of systems for the analysis of volatile organic compounds and other compounds present in traces in the air. On this respect, it is necessary to point out that the concerned volatile organic compounds are extremely different as far as their physico-chemical properties are concerned; for instance, hydrocarbon compounds having low molecular weight are gaseous, whereas aromatic compounds are liquid.

These differences involve several difficulties in the sampling and analysis of compounds that are often present in extremely reduced amounts, in the order of few ppb or even ppt. These reduced concentrations and wide variety of compounds have led to the use of capillary gas chromatography as universal method of analysis, possibly coupled with mass spectrometry, as well as to the need of concentrating volatile compounds before analysing them. The concentration of the concerned volatile compounds is carried out for example by using a trap provided with a packing capable of adsorbing at room temperature volatile compounds and of releasing them at higher temperatures, transferring them to the column. This type of trapping, however, has the drawback of causing the decomposition of thermolabile compounds during the stage of trap desorption.

It was proposed to concentrate volatile compounds in a cool trap. Nevertheless this method involves a further drawback, caused by the presence in the air of even high amounts of moisture, that, condensing in the cool trap during the sample concentration, tend to obstruct the sampling means, moreover altering the air flow through the trap.

Two methods for the analysis of volatile organic compounds in air by concentration in a cool trap are known. According to a first method, all compounds present in the sampled air, including water present as moisture, are retained in a first cool trap of considerable size (to avoid blocking by ice) provided with a packing and kept at very low temperatures, in the order of - 150 °C. Subsequently, the trap is progressively heated until all concerned compounds are released and reconcentrated on top of the capillary column, that is kept at -50 'C for the whole sampling time and then heated with temperatures increments of 8 "C/min up to 150-200 'C. This method, besides not avoiding the loss of volatile compounds during the sample transfer to the trap, also involves problems of reproducibility, since the latter depends on the constancy of the sample flow through the cool trap and said constancy is hardly achievable because of the condensate forming inside the trap itself.

According to another method, first of all moisture is eliminated from the sample, that is passed through a cool trap having considerable size, wherein there is provided a packing of different materials capable of retaining volatile compounds at a temperature of about -20 ^*C. The compounds are then desorbed by heating and flowing of an inert gas therethrough, and then reconcentrated in a second trap, downstream of the first one, kept at -150 ^*C. The packing material used in the first trap must be constituted by different layers of materials having different adsorption power, both in order to try to retain volatile compounds that otherwise would not be retained at -20 ^*C, and not to retain excessively the heaviest substances. The consequence of this structure is that the transfer of the compounds from the first to the second trap must be carried out in backflow mode and possible losses of compounds are avoidable only as a function of the effectiveness of the second trap. Disclosure of the invention

An object of the present invention is to solve the aforementioned problems by means of a method and a device for on-line analysis of volatile organic compounds, which is of easy structure and avoids the loss of compounds, simultaneously ensuring a high analysis efficiency and reproducibilit .

Said object is achieved by means of the present invention, that concerns a method for the analysis of volatile organic compounds, of the type comprising a sampling and concentration step of a gaseous sample in a trap and a step of heating said trap and transfer the compounds retained therein to a gas chromatographic column, characterized in that during said sampling and concentration step of the sample, this sample is fed to said cool trap at a speed ranging between 50 and 1000 cm/sec .

The invention moreover concerns an equipment for the analysis of volatile organic compounds present in a gaseous sample, of the type comprising a trap for the concentration of said volatile compounds, means to feed said sample to the trap and a gas chromatographic device provided with at least a gas chromatographic column for their analysis, characterized in that said trap and said feeding means are sized in such a way to feed said sample to said trap at a speed ranging between 50 and 1000 cm/sec.

The term "speed" of the sample as herein used means the speed that can be substantially considered as the average speed of the sample, achievable by dividing the sample flowrate in ml/min by. the area of the inlet section of the trap 2 and performing the appropriate dimensional adjustments.

According to a preferred feature of the invention, gases coming out of the trap during the step of concentration of volatile compounds are simultaneously conveyed to the gas chromatographic column and to a purge line having a substantially nil fluidic resistance with respect to that of said column.

According to a further feature of the invention, the trap is of the type that can be cooled and is provided with an ejector for the feeding of the cooling fluid. The method and the equipment according to the invention offer a number of advantages versus the state of the art. It is actually possible to use a single cool trap with reduced sizes, avoiding to desorb and reconcentrate the initially trapped compounds, that are on the contrary directly sent to the column. Furthermore, it has been noticed that part of the water contained in the samples of ambient air is eliminated through the purge line during the initial step of volatile compound concentration in the trap, and that the residual water does not affect the subsequent separation on column and the analysis by means of gas chromatographic detectors or mass spectrometer. A further advantage is provided by the use of a particular trap with ejector for the cooling fluid feeding. This trap is described in the patents US-A-4 ,836,871 and EP-B-201632 to the content of which, when considered included in the present specification, reference is made for further details. In fact the geometry and reduced size of the trap allow to reduce to a minimum (up to about 60 ml/min) the consumption of liquid nitrogen for each analysis and to house the trap inside the gas chromatographic oven together with the column.

It must also be noticed that the time required for the analysis is extremely reduced, since it is not necessary to cool the oven at very low temperatures, as it occurs on the contrary according to the state of the art, and since the mass of the trap (packing material included) is very reduced and allows rapid variations of its own temperature. These features allow to perform a high number of analyses in an automatic way within the time unit. Another advantage of the invention comes from the adoption of an injector of the split-splitless type (as described in the patent application EP-A-92106518.1 , included herein as reference) mounted in series between a valve feeding the gaseous sample and the cool trap. Said injector, appropriately modified as in figure 2, allows in fact the injection of liquid samples into the system, thus greatly facilitating the calibration of liquid compounds at ambient temperature (e.g. aromatics) that can be injected in solution, even aqueous solution. The calibration of volatile gaseous compounds, too, is extremely facilitated with the present invention, thanks to the presence of one or more loops for sample loading, and a possibility to directley iniecting with syringe through the injection part of split-splitless injector.

The invention will be now described more in detail with reference to the accompanying drawings given by way of illustration and with no limiting purposes, wherein: figure 1 is a schematic view partially in cross- section of an equipment according to the invention; figure 2 is a sectional schematic view of a preferred injector for the equipment according to the invention; figure 3 is a cross-sectional and magnified schematic view of a detail of figure 2; figures 4 - 6 are schemes of an equipment provided with loading loop during the operative step; figures 7 and 8 are schemes of the equipment during its operation in absence of loading loop; figure 9 is a scheme of a further embodiment of the invention;

- figure 10 is a scheme of another embodiment according to the invention; and figures 11 - 14 are analytical diagrams obtained with the equipment according to the invention.

With reference first of all to figure 1, the equipment 1 according to the present invention comprises in a known way a cool trap 2 connectable to a source (not shown) of cooling fluid, means for sample feeding and a gas chromatographic device 3 provided with at least a gas chromatographic column 9 for the separation and the analysis of the volatile compounds contained in the sample. The detectors 4 of the gas chromatograph can be for instance of the FID or ECD type or they can consist of a mass spectrometer 5.

The means for feeding the sample to the trap, figures 2-4, comprise a line 6, a multi-position valve 15 and an injector of the split-splitless type 7.

According to the invention, said means and the trap 2 are sized in such a way that the speed of the sample feeding flow to the trap 2 (namely the speed of the gaseous sample at the trap inlet), during the step of the sampling and sample concentration, ranges between 50 and 1000 cm/sec. It has been surprisingly noticed that these speeds allow to perform the analysis of volatile compounds at a wide range of trap temperatures, according to the type of volatile compounds to be analysed and to the type of packing present (if any) in the trap itself. In particular, it has been noticed that in case of analyses of volatile compounds having a low boiling point, namely when volatile compounds are trapped at temperatures of the trap below zero and up to values in the order of -100 or -150 ^*C, the speed according to the present invention allows to analyse also samples with high moisture content, even mists, avoiding the risk of obstruction of the trap itself because of the water present in the sample and condensing in the trap. For this purpose the trap 2, figures 2 and 3, is preferably formed by a pre-column made of silanised fused silica 8 connected upstream to a liner 8a present in the injector 7 and downstream to the gas chromatographic column 9, preferably of the capillary type. The injector 7 is of the split-splitless type as described in the Italian patent application n. MI91A001140 filed by the Applicant, and is provided with a liner 8a removable together with the connected pre-column 8. In this way a substitution of precolumn 8 is made easy when, for instance, the packing present therein must be changed as a function of the volatile compounds to be trapped. In fact, inside the portion of pre-column 8 contained in the cool trap, there is preferably provided a packing 12 arranged between two layers 11 of glass wool or similar retaining material. In order to obtain the required high speeds, the pre-column 8 of the trap 2 has preferably a reduced section, of the order of tenths of mm, and the packing material 12 is present in the order of few mg. Preferential dimensions are: internal diameter of about 0.53 mm for the pre-column 8, of about 0.32 mm for the column 9 and a packed zone of about 3 - 5 cm in length. With a thus sized trap, the flow of gaseous sample, pure or diluted with carrier gas according to how its feeding is performed, is generally ranging between 10 and 100 ml/min and preferably between 15 and 60.

The choice of the packing material is made according to criteria known in the art (see for instance K. Grob et al. , Journal of Chromatography, 321, (1985) 45-58) as a function of the type of volatile compounds to be trapped and of the operational temperature. Generally said temperature ranges from -180 ^*C approx. during the concentration of volatile compounds in the trap, to a maximum of about 400 ^*C during their desorption and conveyance to the separation column.

Suitable packing materials are for example Tenax (TM), graphitized Tenax (TM) , Carbotrap (TM) and mixtures thereof. Of course, the temperature for the desorption of volatile compounds will proportionally increase with the retention power of the packing material.

Normal capillary columns suitable for the analysis to be performed can be used, and they will not be affected by the small amount of water entering the column. The water penetrated in the column can be temporarily retained, namely during the first part of the gas chromatographic analysis, by using a column provided with an appropriate stationary phase, e.g. an AI2O3 PLOT column deactivated with KC1 or

Said column is advantageously used in combination with a second non polar column, for instance of the SE-54 type, mounted in series with the first column, downstream of same. This combination allows to perfectly resolve aromatic compounds too, and to have a long life of the column itself, that is considered to be constituted by two different portions. In this configuration it is also possible to obtain reproducible retention times and reduced analytical times, since it is not necessary to cool the oven at low temperature and avoid that alumina particles reach the detector and alter its operation.

Figure 3 shows in detail a preferential embodiment of connection between pre-column 8 and column 9. As it can be seen, column 9 is positioned inside the pre-column 8 and its upper end is preferably outside and in the vicinity of the lower limit of the trap 2. The connection 13 is moreover provided with a purge line 14, having a fluidic resistance much lower than that of column 9. If this does not happen, it is possible to increase the column fluidic resistance by attaching a fused silica capillary having a little inner diameter (0,1-0,2 mm) at the column head. The line 14 is positioned downstream of the inlet end of the column 9 and is thus in communication both with said column 9 and pre-column 8.

As it can be seen in figures 1 and 4, the trap 2 is connected in series, through the injector 7 (that can also be absent) and the line 6, to an injection valve 15 that is generally of the type with six of more positions. The injection valve 15 is in turn connected through a line 21 to the selector 17, which has the function of selecting the samples coming on different lines. The injection valve 15 is connected through line 16 to a carrier gas source, not shown, that is generally He. The line 16 is also connected through a resistor R5 and an electrovalve EV3 to the purge line 14 and therethrough it is connectable to the line 18a leading to the vacuum pump 18. Injection and selection valves 15 and 17 are housed (figures 1 and 2) inside a small chamber 25 thermostatable at a temperature ranging between ambient temperature and about 240 °C, whereas valve EVg is positioned near the injector 7 and is heated by it. Furthermore, the line 6 (figure 2) is provided with an insulation 6b from the injector 7 up to the valve 15.

As it can be better seen from a comparison between figures 4 and 7, the injection valve 15 is constructed to be used alternatively in a configuration with direct sampling or in a configuration with sampling by loop. The basis of this flexibility of structure is given by the presence of a couple of connections arranged in correspondence to two adjacent ports 5 and 6 (figure 4). As shown in said figures, when the valve 15 is in sampling configuration, figure 4, the two connections are closed in a known way and are not used during the analysis. When on the contrary the valve 15 is in direct sampling configuration, said connections 5 and 6 are connected by means of two lines 15b and 15c, figure 7, to the ports 3 and 8 respectively and are used during the step of the sample concentration in the trap.

The operation of the equipment according to the invention will be now described with reference to the use of a sample loading loop, figures 4 - 6, and to the direct sampling configuration, figures 7 and 8.

In case of direct sampling, first of all the gaseous sample is passed, at the aforedescribed speed, through the selector 17, line 21, valve 15, line 6, injector 7 and trap 2, where volatile compounds are retained and concentrated. The majority of gases coming out of the pre- column 8, due to the different fluidic resistance of relevant ducts, will enter the purge line 14. Electrovalves EV3 and EV1 are switched in a way that the purge line 14 is connected to the line 18a only and to the vacuum pump 18 (figure 7).

A mechanical or electronic flow regulator 19 allows to obtain a defined value of treated volume.

At the end of sampling, vacuum is cut off and the valve EV3 is switched to connect the line 14, through the resistor R5, to the line 16 feeding carrier gas, that is simultaneously conveyed along the valve 15, line 6 and injector 7, also to the trap 2, heated, for the transfer of volatile compounds from the trap 2 to the column 9. A similar procedure is followed in the case of figures 4 - 6, where the valve 15 presents a loop 15a for sample loading. In this case the loop 15a is initially connected by means of the line 20 to the line 18a and the vacuum pump 18, by appropriately switching the valve EV2 (figure 4). In this way the gaseous sample is brought through the selector 17 to the loop 15a, that has a known and preset volume, e.g. 100-200 ml. Valve EV4 can be programmed to remain open for a period of time to allow the pressure inside the loop to equilibrate to atmospheric pressure before transfer the loop contents to the trap. In this way the transferred volume is always the same because the pressure is constant or known.

Subsequently, figure 5, the injection valve 15 and the valve EV3 are switched to put the carrier gas line 16 carrier gas feeding in communication with the loop 15a and the latter with the line 6, injector 7 and the trap 2. Under this conditions the purge line is in communication with the outside and the vacuum pump is not working. The flow necessary to achieve required speeds is obtained by means of carrier gas that, besides carrying to the trap 2 the sample present in the loop, dilutes it, thus making the trap obstruction because of sample moisture even more difficult.

Once the concentration step of volatile compounds in the trap is over, their transfer to the column is performed in a similar way as described with reference to figure 8, so that the trap 2 is heated, valves 15 and EV3 are switched to feed carrier gas from the line 16 to the line 6 and then to the trap 2, as well as, through the resistor R5 , to the purge line 14 and column 9. As previously mentioned, the presence of carrier gas at light pressure in the line 14 ensures that substantially all trapped volatile compounds are transferred to the column 9. It is to be printed out that the flow of the carrier gas in line 14, that works like a make-up gas, is lower than that of the carrier gas in the line 16, thanks to the presence of the fluidic resistor R5. In other words, carrier gas is prevalent in the direction from injector 7 to the column 9.

Figure 9 shows a further embodiment of the invention, where, besides the injection valve 15, another valve 22 is present and is provided with two loops 23 and 24 having different dimensions and connectable alone or in series to the trap 2.

By this embodiment it is therefore possible to select the loop having the most suitable volume for the analysis, according to expected concentrations of volatile compounds and to the percentage of moisture present in the sample to be analysed. The presence of a loop of reduced size is also advantageous in the step of instrument calibration. The configuration shown in figure 9 is that of transfer of the sample from loop 24 to the trap 2; the other configurations of the valve in the different analytical steps are similar, mutatis mutandis, to those previously described.

Figure 10 shows the scheme of pneumatics relating to an embodiment of 10-position valve 15, capable of feeding an internal standard during the analysis itself. For this purpose, the valve 15, shown herein during the step of transfer the sample and internal standard to the trap 2, besides the loop 15a is provided with a loop for the internal standard 27, with a line 26 for internal standard feeding and with an outlet line 25, controlled by a valve 28. In this case too, the other configurations of the valve in the different analytical step are similar, mutatis mutandis, to those previously described. Figures 11 to 14 show chromatograms as obtained in analyses performed following the method and with the equipment of the invention, according to the following modalities. Example JL_.

Analysis of hydrocarbons in air - figure 11. A 100 ml sample containing the following compounds at a concentration of 10 - 50 ppb was analysed operating with sampling by loop, at a transfer flowrate from the loop to the trap of 30 ml/min (He, 400 KPa), using a trap formed by a pre-column of fused silica with i.d. of 0.53 mm. The trap was of the ejector type and was provided with a 4 cm packing consisting of a 1/1 mixture of Carbotrap (TM)/graphitized Tenax (TM) and was cooled to -150 'C by means of liquid nitrogen. At the end of the sample transfer, the trap was heated up to 240 °C. Volatile compounds were conveyed with a He flow of 3 ml/min to the analysis on a column consisting of a first portion (50 m) of PLOT AL2O3 deactivated with KC1, serially connected to a following portion (25 m) of a non polar SE 54 column.

The gas chromatographic detector was of the FID type.

The resulting chromatogram is shown in figure 10, where numbered peaks correspond to the following compounds:

1 methane; 2 ethane; 3 ethene; 4 propane; 5 propene; 6 acetylene; 7 i-butene; 8 n-butane; 9 trans-2-butene; 10 1- butene; 11 cis-2-butene; 12 n-pentane; 13 1-pentene; 14 n- hexane; 15 n-heptane; 16 benzene; 17 toluene; 18 ethylbenzene; 19 m/p-xylene; 20 o-xylene; 21 1,3,5- trimethylbenzene.

Example 2_

Analyses of hydrocarbons in air at different percentages of moisture - figures 12-14

Figures 11 and 12 refer to analyses performed on samples containing 59% (figure 12) and 86% (figure 13) of moisture, respectively. Figure 14 refers to the analysis of a sample containing 70% of moisture with separation carried out according to what described in example 1. In all three chromatograms, the signals corresponding to eluted water coming out from the column towards the FID detectors were indicated.

Claims

C L A I M S

1. A method for the analysis of volatile organic compounds comprising a sampling and concentration step of a gaseous sample in a trap and a step of heating said trap and transfer the compounds retained therein to one or more gas chromatographic columns, characterized in that during said sample transfer and concentration step, the sample is fed to said trap at a speed ranging between 50 cm/sec and 1000 cm/sec.

2. A method according to claim 1, characterized in that during said sampling and concentration step of the gaseous sample, said trap is cooled at a temperature ranging between -180 ^CC and +30 'C.

3. A method according to claim 1 or 2 , characterized in that said gaseous sample is fed to said trap together with a carrier gas.

4. A method according to one of the preceding claims, characterized in that during the concentration step of volatile compounds in said trap, gases coming out of said trap are simultaneously fed to said gas chromatographic column and to a purge line having a substantially nil fluidic resistance in respect to that of said gas chromatographic column, and in that during the step of trap heating and conveyance of compounds from the trap to the gas chromatographic column by means of a carrier gas flow, said carrier gas is fed towards said column also along said purge line, the flow of carrier gas in said purge line being lower than the flow in said trap.

5. A method accordint to claim 4, characterized by an alternative and controlled connection of said purge line to a vacuum source and to a carrier gas feeding source.

6. A method according to any of the preceding claims, characterized in that said gaseous sample is fed to said trap from a loop having known and preset volume.

7. A method according to one of the preceding claims, characterized in that a trap with internal diameter of about 0.53 mm is used and in that said sample is transferred to said trap at a flow rate ranging between 10 and 100 ml/min.

8. An equipment for the analysis of volatile organic compounds present in a gaseous samples, of the type comprising a trap for the concentration of said volatile compounds, means for feeding the samples to said trap and a gas chromatographic device provided with at least a gas chromatographic column for their analysis, characterized in that said feeding means and said trap are sized in a way to feed said sample to said trap at a speed ranging between 50 and 1000 cm/sec.

9. An equipment according to claim 8, characterized in that said trap and said column are connected to each other and to a purge line having a substantially nil fluidic resistance in respect to that of the gas chromatographic column, said purge line being positioned in the proximity and downstream of the inlet of said gas chromatographic column and of the lower end of said trap.

10. An equipment according to claim 9, characterized in that said trap is serially connected to a gas chromatographic injector and to at least a valve for sample injection.

11. An equipment according to claim 10, characterized in that said valve is a valve having 1 to 6 or more positions and comprising at least one loading loop.

12. An equipment according to claim 10, characterized in that said injection valve is connected to a second valve provided with two or more loading loops having different sizes and alternatively connectable to said trap.

13. An equipment according to one of claims 10 to 12, characterized in that said injector is an injector of the split-splitless type for the injection of liquid samples.

14. An equipment according to one of claims 8 to 13, characterized in that said trap is a cold trap comprising an ejector for feeding cooling fluid.

15. An equipment according to claim 14, characterized in that said trap is positioned inside the oven of said gas chro atograph.

16. An equipment according to one of claims 8 to 15, characterized in that said trap comprises a packing.

17. An equipment according to any of claims 9 to 16, characterized in that said purge line is controllably and alternatively connectable to a vacuum source and, through a resistor, to a line for carrier gas feeding.

18. An equipment according to any of claims 8 to 17, characterized in that said gas chromatographic column comprises a first portion with stationary phase for temporary retention of water, and a second portion of non polar type serially connected to said first portion.