HEATED SAMPLE TRANSFER APPARATUS

Abstract: A heated sample transfer apparatus (2) for use in auto-sampler devices. The transfer apparatus (2) comprises a heated metal plate (10) which contacts a sampling needle (14), a valve (26) for conducting sample to an analyzing instrument (6), and transfer tubing (20) connecting the needle (14) and valve (26). The conductive heat from the metal plate (10) maintains a uniform temperature in the needle (14), valve (26), and transfer tubing (20), thereby eliminating hot and cold spots in the transfer apparatus (2).

Description: HEATED SAMPLE TRANSFER APPARATUS Field of the Invention The invention relates to a heating device for a gaseous sample analyzer which provides more uniform temperature control of sample flowing from the sample needle to a valve outlet conducting the gaseous sample to an analyzing instrument. The heater device minimizes temperature fluctuation, thereby virtually eliminating the existence of hot or cold spots along the gaseous sample transfer lines. Background of the Invention Auto-sampler devices are employed to remove •and transfer sample from multiple vials or other receptacles for evaluation by an analyzing instrument. One such analyzer is manufactured by the Tekmar " Company, Cincinnati, Ohio, and is designated the Model 7000 Headspace Analyzer. The operative elements of this analyzer were discussed in more detail in Patent Application Serial No. 487,583, filed March 2, 1990, which is expressly incorporated herein by reference. A headspace analyzer operates by drawing gaseous sample located in the headspace of a sample vial through a needle and transferring to an analyzing instrument, such as a gas chromatograph, mass spectrometer, or the like. It is desirable to transfer the headspace sample, typically diluted with carrier gas, from the sample vial to the analyzing instrument at a uniform temperature. Though the operating temperature of the sampling needle and transfer lines is adjusted either at the factory by the manufacturer or by the operator on site, it is possible nevertheless for fluctuations in the operating temperature of the needle or transfer lines to occur. For example, where the transfer lines are heated by a heat tape wrapped around the lines, those portions of the transfer lines which do not directly contact the heat tape may be slightly cooler than the covered portions due to more rapid heat loss to the air, or to the passage of air over the uncovered portions. Alternatively, where the needle and transfer lines are heated by a single source via ' convection, some portions will tend to be cooler than others depending on the distance of the heater from the transfer lines and needle. Fluctuations in the temperature along transfer lines will take the form of hot spots or cold spots. Gaseous sample passing through a hot spot is ore susceptible to decomposition or chemical modification because of the effect of elevated temperature. Where there are cold spots in the transfer lines, one or more of the components of the gaseous sample may condense inside the line, affecting the total composition of the sample and altering the chemical composition detected by the analyzing instrument. Also, the condensation of components from previous runs may lead to a carryover effect whereby condensed components from earlier runs are detected in later runs, leading to a loss of analytical precision and accuracy. Because the concentration of components of headspace gases can be in the parts per trillion range, the loss or addition of any component can materially affect the analysis of the sample. Finally, repetitive runs conducted on similar or supposedly identical samples could thus show uniform results but yet still constitute an inaccurate . analysis due to the consistent loss of one or more sample components during a run because of condensation, decomposition or chemical modification. • Summary of the Invention The invention relates to a heated sample transfer device comprising a heated metal plate in direct or virtual contact with the sampling needle, transfer valve, and the transfer lines connecting the sampling needle and valve. Instead of heating the transfer lines and sampling needle by convection from a remote heater source, the invention provides heat via conduction through the metal plate to the entire sample transfer portion of the analyzer. The sampling needle is connected directly to the metal plate and isolated from other metal parts in the auto-sampler to permit easy removal for servicing, and to minimize heat loss. Cold spots are virtually eliminated, as are hot spots, and a significant reduction in sample carryover from prior runs can be obtained relative to those sampler transfer lines heated by convection. It is therefore an object of the invention to provide a heated sample transfer apparatus which provides uniform temperature to the components comprising the transfer line. It is a further object of the invention to provide a heated sample transfer apparatus which provides a significant reduction in sample carryover. It is an advantage of the invention that the sampler needle can be easily accessed and removed for servicing. These and other objects and advantages of the invention are described in more detail below, and are depicted in the drawings, in which: Description of the Drawings Fig. 1 is a perspective view of the sample transfer apparatus. Fig. 2 is a partially cut-away side view of the sample transfer apparatus, further showing the relationship to the sample vial-containing platen of an auto-sampler unit. Detailed Description of the Invention The invention in its broader aspects relates to an apparatus for transferring gaseous sample from a sampling point to an analyzer, comprising a metal plate, a needle mounted on the plate for taking samples, a transfer valve mounted on the plate for conducting sample from the needle to the analyzer, tubing mounted on the plate connecting the needle to the transfer valve, and at least one heater in contact with the metal plate, the heater being sufficient to produce a uniform temperature throughout the plate. As depicted in the drawings, the metal plate is bored to accept heater elements, such as cartridge heaters. Alternatively, a heater could be mounted onto the upper or lower surface of the metal plate, so long as it is in direct or virtual contact with the plate itself. In the case of a headspace analyzer device, a relatively large volume of sample must be removed from the headspace of a sample vial to provide enough material for analysis by the analyzing device because of the low concentration of components in the sample. In one method of withdrawing sample from the headspace of a sample vial, carrier gas is directed into the headspace of the vial to increase the pressure therein. After a predetermined period of time, the flow of carrier gas into the headspace is stopped and the mixture of sample and carrier gas, now at a higher pressure within the vial, is allowed to flow back through the sampling needle into the transfer tubing and ultimately to the analyzing instrument. Particularly in the case of the headspace analyzer device, the low concentration of detectable components in the headspace of the vial requires that a relatively large volume of sample be removed from the headspace of the sample vial. This requires a sufficient volume of tubing or other storage type device to be located between the sampler needle and the valve which directs sample to the analyzing instrument. Preferably, the necessary sample is removed from the vial headspace and transferred to a ■ section of coiled tubing wrapped around a metal cylinder in contact with the metal plate. Preferably, the metal cylinder itself is bored to accept a " cartridge heater which will be set at the same temperature as the other heaters in the metal plate. It is preferred that an electric cartridge heater be used in each of these applications, though this is not required. Referring to the drawings, the heated sample transfer apparatus 2 is mounted above an insulation layer 4 which in turn is positioned on the top surface of a headspace analyzer device 6, a portion of which is shown in Fig. 1. Metal plate 10 of the heated sample transfer apparatus 2 is preferably produced from aluminum. Sampling needle 14 fits through a bore 16 in metal plate 10 which is threaded to mate with the threads on the sampling needle 14. Needle outlet tubing 20 is connected to the sampling needle 14 by a compression fitting 22. The needle outlet tubing 20 is maintained in contact with the metal plate 10 by retainers 24, and connects to six-port valve 26 at position F. The insulation layer 4 thermally isolates sampling needle 14 from the stripper plate support 28 to reduce heat loss of the needle 14. The needle 14, which is not connected to the stripper plate support 28, may be •easily serviced by unscrewing from metal plate 10. The insulation layer 4 is formed preferably from MARINITE I, and is available from Johns-Manville Corp. • The six-port valve is commercially available, and can be obtained from Valco Instruments, Inc., Model No. C6 T. Sample loop 30 is used to store a volume of sample removed via the sampling needle 14 which has been mixed with carrier gas, typically helium. The depicted sample loop 30 is a coil which makes contact with metal cylinder 32. Sample loop 30 is connected to the six-port valve 26 via sample loop tubing 34 at ports B and E, port E being located on the back side of the six-port valve 26 as shown in Fig. 1. The sample loop tubing 34 is maintained in contact with the metal plate 10 by retainers 24. Port A is an inlet for carrier gas, as is port C. Port D is an outlet for conducting to the analyzing instrument either sample mixed with carrier gas or purge gas only. Heat is supplied to the metal plate 10 by electric cartridge heaters 40 which are positioned in holes bored in the sides of the metal plate as shown in the figures. Preferably, a cartridge heater 40 is also inserted into metal cylinder 32 to help maintain sample loop 30 at a uniform temperature. The cartridge heaters utilized are commercially available. . One such cartridge heater is Model SC-25 distributed by Hotwatt, Inc., Danvers, Mass., having a watt density of 35 watts/inch. The heater is one inch in • length. The tubing and connections through which the carrier gas and sample flow are made from nickel alloy, copper, stainless steel, or other material which does not evolve any compounds which would affect the analysis of the sample, does not retain compounds by any adsorptive or chemical means, is chemically inert to the compounds of interest, and which is not permeable to compounds in the air which might diffuse through the material and be carried to the analyzing instrument. The needle 14 is preferably constructed from stainless steel, with either the 304 or 316 grades being acceptable for this application. The elements comprising the heated sample transfer apparatus 2 function in the following way to transfer sample from sampling unit 14 through six-port valve 26 and exit tubing 46, and thereby to the analyzing instrument. Before a sample of the headspace gas is withdrawn from the vial 50 shown in Fig. 2, carrier gas is directed out the sampling needle 14 via tubing 20, from the six-port valve at port F. The six-port valve 26 is set so that carrier gas enters the six-port valve 26 at port A. The gas flows then from port A into port B, through sample loop 30, into port E, out port F and through needle •14. This results in a constant purge of needle 14 and the transfer tubing to minimize contamination. In Fig. 2, sample vial 50 is located in ■ chamber 52 of platen 54. Prior to sampling, the sample vial 50 is raised from below so that a septum (not shown) in the top of vial 50 is pierced by sampling needle 14. After the septum of sample vial 50 is pierced by the needle 14, the carrier gas flowing out the tip of needle 14 pressurizes the headspace of the sample vial 50. After a programmed amount of time sufficient to pressurize the vial 50, the flow of carrier gas is terminated. Upon the programmed opening of a vent valve (not shown) , which is depicted as Reference Number 164 in co-pending application serial number 487,583 filed March 2, 1990, the gases in the vial consisting of sample and carrier gas flow back through the needle and toward the six-port valve 26 via the needle outlet tubing 20. The mixture of carrier gas and sample enters six-port valve 26 at port F to fill the sample loop 30 with the headspace contents from vial 50. After a programmed period of time, the six-port valve 26 is cycled. Carrier gas entering through port C is now caused to flow into port B to thereby flush the contents of the sample loop 30 into port E on the back side of six-port valve 26. This port after cycling is connected to port D which in turn is connected to the .exit tubing 46. The mixture of sample and carrier gas is thereby conducted through the six-port valve 26 through the exit tubing 46 mounted to the metal plate • 10 by retainer 24 which itself is heated by a tubing heater 58, and thereby to the analyzing instrument. The heated sample transfer apparatus 2 maintains the components in contact with the metal plate 10 at a substantially uniform temperature, typically within 10 β C of the setpoint throughout the apparatus. Typically, the metal plate 10 is heated by cartridge heaters 40 to a temperature in the range of about 15"C above ambient to about 250 β C. The more uniform temperature of the apparatus results in a substantial decrease in carryover effect, thereby improving the accuracy of analysis of the particular sample taken from sample vial 50. The improvement in temperature uniformity relative to existing temperature control devices is shown in the following comparison. A one microliter sample of a blend of 10% each of C 1Q , C , C. , and C. straight chain alkanes in ethylbenzene was placed in a 22 milliliter vial heated in a platen to 150*C. The sample transfer apparatus was heated to a setpoint of 180 β C. The invention was compared with a sample transfer device identically configured except that it was heated only by an electrical heater such as the Model HA-1T Heater manufactured by Valco Instruments, •Inc. mounted onto six-port valve 26. The components of the sample transfer apparatus in the comparison device were heated by convection. The headspace of " the sample vial was sampled in separate runs for the two versions, and a follow-up analysis for each version was also run on a blank sample to determine the amount of carryover from the previous run for each version of transfer apparatus. The data collected on sample runs passing through the separate transfer devices is provided below in Table 1. Samples were analyzed in a Varian 3300 gas chromatograph fitted with a Varion 3300 flame ionization detector. Sample was analyzed over a 30 minute run, with a sampling rate of 1.0000 points/second. Temperature was ramped from 100 β C to 200"C at a rate of lO'C/min, and then held at 200*C for 20 minutes. The carrier gas was helium. The term "sample count" in the table below correlates to the amount of each sample relative to the total amount detected. Table 1 12.16 364,604 c 1 1 2 0 14.592 17,229,470 0 17.875 14,029,314 0 18 25.450 16,083,218 0 Total Sample 49,272,030 21,621 044 The total carryover of sample conveyed through the sample transfer apparatus of the invention was only 23.4% of that conveyed through the conventional transfer apparatus having a convection heater. It is therefore apparent that there has been provided, in accordance with the invention, a heated sample transfer apparatus that fully satisfies the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, the invention is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims.

Claims: What is claimed is: (1) An apparatus for transferring gaseous sample from a sample container to an analyzer, comprising: a metal plate; a needle mounted on said plate for taking samples; a transfer valve mounted on said plate for conducting sample from said needle to the analyzer; tubing mounted on said plate connecting said needle to said transfer valve; and at least one heater in contact with said metal plate, said heater sufficient to produce a uniform temperature throughout said plate. (2) The apparatus of claim 1 having a reservoir mounted on said plate for storing sample. (3) The apparatus of claim 2 wherein said reservoir is a length of tubing coiled around a metal cylinder mounted to said metal plate. - (4) The apparatus of claim 3 wherein said cylinder has a heater mounted therein. (5) The apparatus of claim 1 wherein said metal plate is bored to accept at least one heater mounted therein. (6) The apparatus of claim 1 wherein said plate is heated in the range of ambient plus 15"C to 250°C. (7) The apparatus of claim 1 wherein said heater s a cartridge heater.

Inventors: GREEN THOMAS B, GREEN, THOMAS, B.
Applicants: TEKAMR COMPANY
Application year: 1992
Main classification: G01N1
IPC codes: B01L7, G01N1, G01N30, G01N35