WO1995020165A1

WO1995020165A1 - Analyzer for monitoring volatile contaminants in liquid streams

Info

Publication number: WO1995020165A1
Application number: PCT/US1995/000258
Authority: WO
Inventors: Italo A. Capuano; William T. Lefebvre; Kenneth E. Creasy
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1994-01-24
Filing date: 1995-01-06
Publication date: 1995-07-27
Anticipated expiration: 1996-07-24
Also published as: AU1561395A

Abstract

An analyzer (10) and method for monitoring volatile contaminants in an aqueous stream in which the volatile contaminants are stripped (12) from a sample of the aqueous stream with a gaseous nitrogen or air stream (60). The gaseous stream (72) containing the stripped contaminants is fed to a sample loop (102) of fixed volume. A carrier gas is used to carry the gaseous stream through a chromatographic column (128) where the various species of contaminants are separated. After separation, the gaseous stream with the separated contaminants is carried to a photoionization detector (20) wherein the organic molecules of the contaminants are ionized and move into an electric field producing a current proportional to the concentration.

Description

ANALYZER FOR MONITORING VOLATILE CONTAMINANTS IN LIQUID STREAMS

This invention relates generally to a method and analyzer for monitoring contaminants, and more particularly, to a method and analyzer for monitoring volatile contaminants in a liquid stream. Industry is striving to take positive steps to ensure that contaminants such as carcinogens and other pollutants used or produced in many industrial processes are not released into the environment. In many instances the government has set regulations specifying the maximum levels of contaminants that may be present in the wastewater released to the environment. Accordingly, various types of treatment systems are used to remove the contaminants from the wastewater before it is discharged into the environment.

While such treatment systems are generally satisfactory, there is always a possibility that some contaminant might escape through the system due to an inadvertent malfunction or other error. To minimize any environmental impact of such escape, and to ensure continuous compliance with the government regulations, it is desirable that the presence of contaminants in the wastewater stream be detected. Accordingly, it is necessary that an analyzer be available which can be used on line to continuously monitor the presence of the contaminants in the wastewater stream at relatively low levels, below the maximum levels set by government regulation, and provide an alarm or other alert when elevated levels of the contaminant are present, or when the level of concentration of a contaminant begins to rise even before the maximum permissible level is reached. By having an early warning of such elevated contaminant levels in the wastewater, environmental damage may be prevented and possible plant shutdown avoided. Analyzers for detecting material using process chromatographic methods have been in use for many years. However, such analyzers are complex, sometimes unreliable and require a high degree of service and maintenance to keep operational. For example, such analyzers used flame ionization detectors (FID) which require hydrogen or helium for operation. Such gases are generally not readily available in sufficient quantities at a plant site and accordingly, such gases would have to be separately supplied, usually in the form of individual containers, if such an analyzer is to be used on-line to monitor a plant process. This results in a relatively costly installation for in- plant usage and also requires service and maintenance to ensure that the supply of gas does not run out. Additionally, the use of hydrogen in an in-plant environment might pose safety hazards due to its explosive nature.

In view of the above, it is an object of the present invention to provide an improved method and analyzer for detecting contaminants in a liquid stream.

It is another object of the present invention to provide a method and analyzer for detecting contaminants in a liquid stream which are capable of detecting relatively low levels of the contaminant.

It is a further object of the present invention to provide a method and analyzer for detecting contaminants in a liquid stream which are capable of monitoring the contaminants on a continuous basis. It is yet another object of the present invention to provide a method and analyzer for detecting contaminants in a liquid stream which can operate reliably and without service or maintenance for relatively long periods of time.

These and other objects and advantages of the present invention may be achieved through the provision of a method and apparatus of the present invention.

In one aspect, the present invention relates to a method for monitoring volatile contaminants in a liquid stream comprising: a. stripping the volatile contaminants from the liquid stream with a gaseous stream to provide a gaseous stream containing stripped volatile contaminants, b. separating volatile contaminants in said gaseous stream by passing a volume of said stream containing the stripped volatile contaminants through a chromatographic column to provide a gaseous stream containing separated volatile contaminants, c. passing said volume of said gaseous stream containing the separated volatile contaminants through a photoionization detector, and d. detecting the presence of a particular contaminant in said gaseous stream by ionizing said particular contaminant and measuring the current generated to identify said particular contaminant by means of said photoionization detector. In another aspect the present invention relates to an analyzer for monitoring volatile contaminants in a liquid stream according to the present invention may comprise a.stripping means for stripping the volatile contaminants from the liquid stream with a gaseous stream, a chromatographic column for separating the volatile contaminants in the gaseous stream as the stream passes therethrough, and a photoionization detector for detecting the presence of a particular contaminant which may be present in the gaseous stream as it passes through the photoionization detector.

The objects and advantages of the present invention will become more apparent by reference to the following detailed description and to the accompanying drawing in which:

Figure 1 is a schematic diagram of an analyzer constructed in accordance with the present invention. The Figure of the drawings shows a schematic diagram of an analyzer according to the present invention which is capable of being used to detect the presence of volatile organic contaminants in an aqueous stream such as a wastewater stream. Some specific examples of contaminants that may be detected and monitored with the present invention include benzene, ethylene oxide, and propylene oxide, although any volatile contaminant capable of being detected by a photoionization device may be monitored by the analyzer and method of the present invention.

Referring to the drawings, the analyzer 10 generally includes a stripping unit 12 including a stripper 14 and separator 16 for removing the volatile components from the wastewater stream in a gaseous phase, a chromatograph 18 which separates the specific contaminant-from other components which may be present, a photoionization detector 20 in which the contaminant is ionized causing a measurable current to flow which is proportional to the contaminant concentration, and a microprocessor and output means 22, which controls the operation and provides means for collecting data and providing an alarm.

More specifically, a sample from the wastewater stream, or any other aqueous or liquid stream to be monitored, is pumped by a suitable pump (not shown) , or otherwise provided to the analyzer, through a wastewater sample input line 24 to a filter 26. The filter 26 serves to remove any solid residue which may be present in the incoming wastewater stream sample. The filter 26 may be similar to a wire strainer, or be any other type of filter capable of preventing passage of the solid residue into the wastewater sample incoming flow line 28. A relative large portion of the incoming wastewater sample is discharged at the filter 26 to a drain manifold 30 through a wastewater drain line 32 having a flow controller 34 therein. A flowmeter 36 is positioned in the drain line 32 downstream of the flow controller 34. A pressure gauge 38 may be attached to drain line 32 upstream of the valve 34. The flow controller 34 may be any suitable type of adjustable flow controller such as an adjustable flow control valve. An example of a suitable valve is a needle valve or a micro-valve. As will be noted, the drain manifold 30 has an outlet line 40 which may be connected to a suitable drain (not shown) .

The portion of the incoming wastewater sample passing through the filter 26 is supplied to an overflow vessel 42 through the wastewater sample input line 28. The line 28 includes a flow controller 44. A flowmeter 46 is positioned in the line 28 downstream of the flow controller 44. The flow controller 44 may of the same type as the flow controller 34.

The overflow vessel 42 has an overflow drain line 48 communicating with the interior thereof at the upper portion of its sidewall. The other end of the overflow drain line 48 is connected to the drain manifold 30 to provide for the discharge of excess wastewater sample from the overflow vessel 42. The overflow vessel 42 may be a closed container of suitable material and includes a vent 50 in its top or cover 52. The overflow vessel 42 serves to hold a supply of wastewater sample at a constant head for introduction into the analyzer.

A pump 54 -in a wastewater sample output line 56 from the overflow vessel 42 supplies a sample of wastewater from the overflow vessel 42 through the wastewater sample output line 56 to the input line

58 to the stripping unit 12. The pump 54 may be any suitable type of metering pump which will transfer the sample to the stripping unit 12 at a fixed flow rate. A constant flow of a stripping gas is also introduced into the stripping unit 12 through the input line 58 by means of a stripping gas input line 60. The stripping gas is a gas that is readily available on-site in sufficient quantities so that continual service is not required in order to ensure a regular supply of the gas and also be one that is not hazardous. Nitrogen and purified air are preferred as they generally meet these requirements. The stripping gas input line 60 may be connected to any suitable source of the stripping gas and includes a pressure regulator 62 and a flowmeter 64 therein. The flowmeter 64 is positioned in the gas input line 60 downstream of the pressure regulator 62 with a pressure gauge 66 connected to the line 60 between the regulator 62 and the flowmeter 64 as shown.

The stripping unit 12 includes the stripper 14 which serves to cause the volatile contaminants in the wastewater sample to pass into the gaseous stream provided by the stripping gas while the nonvolatile components remain in the liquid phase. The stripping gas displaces the volatile contaminants dissolved in the liquid phase forcing these species into the gaseous stream. Any suitable type of stripper may be used such as a container in which the stripping gas is caused to bubble within the liquid sample. A preferred form of the stripper 14 may be a closed column packed with glass beads having an inlet in its top through which the input line 58 extends axially into the interior of the stripper 14. The addition of the glass beads increases the surface area of contact between the stripping gas and liquid, thereby increasing the efficiency of the stripping operation.

An outlet line 68 is connected between the stripper 14 and the separator 16 through which both the stripping gas and liquid flow from the stripper 14 to the separator 16. The separator 16 serves to discard the liquid to the drain manifold 30 through a separator drain line 70 and supply the stripping gas with the stripped contaminants to the chromatograph 18 though a stripping gas output line 72. The separator drain line 70 has a double loop or a sideways "S" configuration as shown forming a concave loop 74 and a convex loop 76, with a vent 75 positioned at the top of the concave loop 74 as shown. The looped drain line serves to trap a portion of the liquid therein which provides a seal at the bottom of the separator to prevent the gaseous phase form escaping to the drain.

The stripping unit 12, including the stripper 14 and separator 16, is housed in a single unit as indicated by the dashed lines in the drawing. The unit is thermostated using a hot air heater 77, although other types of heating units may be used such as an electric heater or the like. The temperature, liquid sample flow rate, and stripping gas flow rate all have an effect on the efficiency of the removal of the volatile contaminants from the wastewater sample stream (stripping efficiency) . By containing the stripping unit 12 in a thermostated housing which maintains the unit at a constant temperature, changes in stripping efficiency due to temperature variations is prevented.

The stripping gas output line 72 from the separator unit 12 is connected to one of the ports of a ten-port two-position rotary sampling valve 78. The ten ports are numbered clockwise in the drawing as 80, 82, 84, 86, 88, 90, 92, 94, 96 and 98 starting with the port 80 to which the stripping gas output line 72 from the separator unit 12 is attached. The sampling valve 78 is moveable between two positions, a first or deactivated position and a second or active position. When the valve 78 is in its first or deactive position, adjacent ports are connected as shown by the dotted lines in the drawing. When the valve 78 is in its second or active position, the ports of the valve 78 are connected as shown by solid lines in the drawing. When the sampling valve 78 is in its first or deactive position, the stripping gas containing the stripped contaminants flows from the stripping unit 12 through the stripping gas outlet line 72 into the port 80 of the sampling valve 78 through a block valve 100 in the line 72. The block valve 100 is an electrically actuated on-off valve such as a solenoid valve or the like. The stripping gas stream then travels inside the valve as shown by the dotted line to the port 98 where it exits into one end of a sample loop line 102 having a sample loop 104 therein. The sample loop 104 may be a fixed length of coiled tubing having a predetermined internal diameter which contains a predetermined volume of the stripping gas stream containing the stripped contaminants. The other end of the sample loop line 102 is connected to the port 84 of the sampling valve 78. The stripping gas stream containing the stripped contaminants enters the sampling valve 78 from the sample loop line 102 through port 84 and exits the valve 78 through port 82 to an exhaust line 106. The exhaust line 106 is vented to the atmosphere through a suitable scrubber 108 or other treatment system to remove any contaminants. A suitable type of scrubber is one using a carbon adsorbent filter.

The sampling valve 78 has two of its ports 90 and 96 connected to a source of a carrier gas. The carrier gas should meet the same criteria as the stripping gas and preferably, is nitrogen or purified air. A carrier gas infeed line 110 is connected at one end to the source of the carrier gas and contains a pressure regulator 112 to maintain the incoming carrier gas at a constant pressure. The other end of the carrier gas infeed line 110 is connected to a T-shaped connection 114 which branches into two lines, a backflush carrier stream line 116 which connects with the port 90 of the sampling valve 78 and a measuring carrier stream line 118 which connects with the port 96 of the sampling valve 78. Each of the lines 116 and 118 have a flow control valve 120 and 122, respectively, positioned therein with a pressure gauge 124 being connected to the measuring carrier stream line 118 upstream of the valve 122.

When the sampling valve 78 is in the deactive position, port 90 of the sampling valve 78 to which the backflush carrier stream line 116 is connected, is in communication with the port 92. One end of a chromatograph loop line 126, containing the chromatographic column 128, is connected to the port 92 of the sampling valve 78. The other end of the chromatograph loop line 126 is connected to the port 86 of the sampling valve 78. In the deactive position of the sampling valve 78, the port 86 is in communication with the port 88 to which is connected a vent line 130. The vent line 130 is exhausted to atmosphere through a flow control valve 132 and a suitable scrubber 134 similar to the scrubber 3.08.

The port 96 of the sampling valve 78, to which the measuring carrier stream line 118 is connected, communicates with the port 94 when the sampling valve 78 is in the deactive position. A sample feed line 136 is connected at one end to the port 94 and has its other end connected the photoionization detector 20, permitting a stream of carrier gas to flow through the photoionization device 20 and out its vent line 138 when the sampling valve 78 is in the deactive position.

The chromatograph 18 may be in the form of a chromatographic column 128 which serves to render the photoionization detector 20 selective by spatially separating the volatile components of the gas sample which may be present in the gas stream passing therethrough. The chromatographic column 128 may generally comprise a packed tubular column or a capillary column having a medium therein which will cause the spatial separation of the various volatile contaminants traveling therethrough. Packed columns generally contain a material such as diatomaceous earth on which an oil may be deposited as the stationary phase. A capillary column containing a coating of oil on its inner wall as the stationary phase may also be used. A preferred form of a chromatographic column is a capillary column comprising a 6 foot length of 0.159 i.d. (inside diameter) capillary tube in the form of a coil having Poropak Q (GS-Q) , a styrene divinylbenzene polymer, as the stationary phase in the form of a thin layer coated on the column's inner wall. Such a column may be purchased from the J & W Scientific Co. The components of the sample travel through the chromatographic column at different rates depending upon their solubility in-the stationary phase of the column. Gas molecules which are similar to the stationary phase will be more likely to stay dissolved in the coating and will elute at longer times. Volatile components which are very different from the stationary phase will not stay dissolved in the coating and as a result will travel through the column faster.

The chromatographic column 128, the sample loop 104 and the sample valve 78, as well as the loop lines 126 and 102 are all contained within a housing 140 as indicated by the dashed lines in the drawing. A suitable heater 142, such as a hot air heater may be provided to maintain these components at a constant temperature.

The detector employed for the quantization of the contaminant is a standard, commercially available photoionization detector 20. This type of detector measures the current produced when various organic molecules are ionized by ultraviolet light. Once ionized, the molecules move in an electric field producing a current proportional to the concentration of that particular contaminant. This current is amplified to produce an analytical signal. The standard photoionization detector 20 includes a lamp and controller mounted in a housing having an ionization chamber and heating means for maintaining the chamber at a constant temperature. A suitable detector is a 10.2 electron volt (eV) lamp detector (Model PI-52) manufactured by HNU Systems Inc. The photoionization detector 20 sends a signal to the microprocessor 22 which is proportional to the level of concentration of the volatile contaminant being sensed. The microprocessor and output means 22 may include a microprocessor or controller 144 for controlling the timing of the various events including the opening and closing of the block valve 100 and movement of the sampling valve 78 between its two positions. A display unit 146 may also be provided to provide a visual display of such items as time and date, the analysis step, most recent contaminant concentration, and alarm status. Additionally, if the concentration of the particular contaminant is too high, or a malfunction of the analyzer is sensed, the display unit may be set up to display a blinking red light, while a steady green light may appear if none of these conditions exist. The display unit 146 may also show the concentration profile of the contaminant analyses over a past period of time such as thirty hours, and the chromatographic trace and most recent chromatographic analysis.

The analyzer of the present invention is adapted for on-line continuous operation. Thus, wastewater is continuously supplied to the stripping unit 12 through the filter 26, overflow vessel 42 and pump 54. Additionally, the stripping gas may be continuously fed to the stripping unit 12 through the input lines 60 and 58.

In operation, with the sampling valve 78 in its first or inactive position, and the block valve 100 open, the stream of stripping gas containing the stripped contaminants flows from the stripping unit 12 into port 80 of the valve 78 and exits through port 98 into the sample loop line 102. The gaseous stream then passes through the sample loop 104 back into the valve 78 through port 84 and is exhausted to the atmosphere through port 82 and exhaust line 106 and scrubber 108. Thus, when the sampling valve 78 is in its deactive position, the gaseous stream from the stripping unit 12 is vented to the atmosphere through the sample loop line 102 and exhaust line 106.

Also, with the sampling valve 78 in its first or deactive position, the carrier gas from the carrier gas infeed line 110 passes into the two branch lines 116 and 118. The carrier gas in the branch line 116 enters the port 90 of the sampling valve 78 and exits through the port 92 into the chromatograph loop line 126. The carrier gas flows through the line 126, through the chromatographic column into the port 86 of the sampling valve 78 and then exits the valve 78 through port 88 into the exhaust line 130 where it is vented to the atmosphere through the scrubber 134. Thus, when the valve 78 is in its first or deactive position, carrier gas is backflushed through the chromatographic column 128 and vented to the atmosphere. This serves to backflush any species left in the chromatographic column 128, particularly the heavier components of the gaseous stream, to the exhaust line 130 preparing the chromatographic column 128 for the next injection of the gaseous stream. At the same time, the carrier gas in the line 118 passes into port 96 in the valve 78 and exits through port 94 into the sample feed line 136 to the detector 20 where it is vented through the vent line 138. Thus, at the same time the chromatographic column 128 is being backflushed with the carrier gas, the detector is being swept with the carrier gas to remove any residual sample.

When it is desired to analyze a sample, at periodic intervals controlled by the controller 144, the block valve 100 is closed while the sampling valve 78 remains in its first or deactive position. After a predetermined period of time after the block valve 100 has been closed, for example 5 to 10 seconds, the sampling valve 78 is actuated to rotate it into its second or active position. The time delay between closing the block valve and moving the sample valve to its closed position permits the stripping gas stream in the sample loop 104 to come to come to atmospheric pressure since the sample loop 104 remains vented to the atmosphere during that period of time without additional stripping gas flowing thereto.

When the sampling valve 78 is moved into its second or active position, the ports thereof are connected as shown by the solid lines in the drawing. At this time, the carrier gas entering port 96 of the sampling valve 78 from the line 118 exits the valve 78 through port 98 into the sample loop line 102 and sweeps through the sample loop 104 carrying a fixed volume sample of the stripping gas containing the stripped contaminants into port 84 of the valve 78. The carrier gas with the sample of stripping gas then exits the valve 78 through port 86 into the chromatograph loop line 126 where the stripping gas sample passes through the chromatographic column 128. The chromatographic column 128 spatially separates the particular contaminant desired to be analyzed from the other components in the sample stream. The separated gaseous sample stream then exits the chromatographic line 126 into port 92 of the sample valve 78 and then exits port 94 into line 136 wherein it travels to the photoionization detector 20. As mentioned above, the photoionization detector 20 measures the current produced when the various organic molecules are ionized by ultraviolet light and move in an electron field. The lamp ionization potential is set so that it will detect the particular contaminant desired. Species with ionizations potentials substantially above the fixed ionization potential of the lamp will not be sensed while species slightly above the fixed ionization of the lamp will be sensed only if present in very high levels.

By way of a specific example, when utilizing the analyzer for the detection of benzene, the operational parameters may be set such that the wastewater sample flow to the overflow vessel is between about 50 to 100 milliliters per minute

(mL/min) . The wastewater passing to the drain 30 through line 32 may have a flow rate of 5 to 10 gallons per hour (gal/hr) at a pressure of 5 to 10 psig. The flow rate of the wastewater sample passing to the stripping unit 12 through line 56 may be about 10 mL/min with the temperature of the stripping unit being maintained at about 40°C. by the heater having an air pressure of 10 psig. The stripping gas, which may be nitrogen, may be supplied to the stripping unit 12 at 250 mL/min. The chromatograph column 128 may be of the capillary type described-above with the solid phase being poropak Q. The column 128 may be maintained at 90°C. by the heater having an air supply of 40 psig and a heater air pressure of 10 psig. Nitrogen may be used as the carrier gas. The nitrogen supply pressure from the pressure regulating valve 112 may be 30 psig while the pressure of the nitrogen used as the carrier gas in the column 128 from the carrier line 118 may be 11 psig. The flow rate of the nitrogen backflush gas through line 116 set by the valve 120 may be 60 mL/min while flow rate of the carrier gas in line 118 as set by the valve 122 may 45 mL/min.

As indicated above, the photoionization detector 20 is a standard commercial unit. Its operation parameters for the sensing of benzene include a lamp ionization potential of 10.2 eV, a detector lamp intensity of 50%, a detector input attenuation of 1 and a recorder attenuation of 4. The internal heater supplied with the unit may maintain the detector temperature at 120°C. The analyzer and method as described above is capable of being used on line and performing an analysis at least once every five minutes without stoppage. It is capable of detecting very low levels of a particular contaminant, well below the levels set by government regulation, so that an alarm or other indication of the presence of unacceptable levels of a contaminant, or of a trend of increasing levels even though below the maximum permissible level, may be provided to appropriate personnel. Such early warning will permit appropriate action to be taken before any significant environmental damage can occur. While the invention has been described above with reference to a specific embodiment thereof, it is apparent that many changes, modifications, and variations can be made without departing from the concept disclosed herein. Accordingly, it is intended to embrace all such changes, modifications, and variations that fall with in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method for monitoring volatile contaminants in a liquid-stream characterized by: a) stripping the volatile contaminants from the liquid stream with a gaseous stream by passing the liquid stream and gaseous stream together through a stripper means wherein volatile contaminants pass into the gaseous stream and thereafter separating the gaseous stream containing stripped volatile contaminants from the liquid stream, b) separating the volatile contaminants in said gaseous stream by passing a volume of said stream containing the stripped volatile contaminants through a chromatographic column to provide a gaseous stream containing separated volatile contaminants, c) passing said volume of said gaseous stream containing the separated volatile contaminants through a photoionization detector, and d) detecting the presence of a particular contaminant in said gaseous stream by ionizing said particular contaminant and measuring the current generated to identify said particular contaminant by means of said photoionization detector.

2. The method of claim 1 characterized in that said volatile contaminants are separated by entraining a fixed volume of said gaseous stream containing the stripped contaminants in a carrier gas stream and passing said carrier gas stream containing said fixed volume of the gaseous stream with the stripped contaminants through said chromatographic column.

3. The method of claim 2 characterized in that said gaseous stream and said carrier gas stream are nitrogen or purified air.

4. The method of claim 2 further comprising passing said gaseous stream containing the stripped volatile contaminants through a sample loop of a predetermined volume and injecting said carrier gas stream into said sample loop to carry a predetermined volume of said gaseous stream through said chromatographic column and said photoionization detector.

5. The method of claim 4 further comprising controlling the flow of said carrier gas stream and said gaseous stream containing the stripped contaminants so that for a first period of time, said gaseous stream flows into said sample loop and, for a second period of time, said carrier stream flows into said sample loop and carries said fixed volume of said gaseous stream through said chromatographic column and said photoionization device.

6. An analyzer for monitoring volatile contaminants in a liquid stream characterized by: a) stripping means for stripping the volatile contaminants from the liquid stream with a gaseous stream; b) a chromatographic column for separating the volatile contaminants in the gaseous stream as the stream passes therethrough; and c) a photoionization detector for detecting the presence of a particular contaminant which may be present in the gaseous stream as it passes through the photoionization detector.

7. The apparatus of claim 6 including means for entraining a fixed volume of the gaseous stream containing the stripped contaminants in a carrier gas stream and passing said carrier gas stream containing the fixed volume of gaseous stream through said chromatographic column.

8. The analyzer of claim 7 wherein said gaseous stream and said carrier gas stream are purified air or nitrogen.

9. The analyzer of claim 7 including a sample loop for containing a fixed volume of the gaseous stream containing the stripped contaminants, and means for introducing said carrier gas stream into said sample loop for carrying said fixed volume of said gaseous stream through said chromatographic column and said photoionization detector.

10. The analyzer of claim 9, further including a sampling valve for controlling the flow of said gaseous stream and said carrier gas stream, said valve being periodically-moveable between a first position and a second position, said valve in said first position permitting said gaseous stream to flow into said sample loop, and in said second position permitting said carrier gas stream to flow to said sample loop and carry said fixed volume of said gaseous stream through said chromatographic column and said photoionization detector.