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Note 80: Design, Development and Testing of a Microprocessor ControlledAutomated Short Path Thermal Desorption Apparatus

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By John J. Manura, Vinod T. Das, David J. Manura, Christopher Baker, Daniel Lieske, John C. Miller, John Manos, Roland Roadenbaugh, Thomas G. Hartman

Presented at PittCon 99, Orlando, FL, March 1999

Volatile and semi-volatile organic compounds (VOC's) are of interest in many industries. The Short Path Thermal Desorption (SPDT) technique has been widely used to introduce VOC's into the gas chromatograph (GC) capillary column for analysis. This method involves the purging and trapping of VOC's onto glass-lined stainless steel (GLT) desorption tubes which are packed with an adsorbent resin. These tubes are then placed into the SPTD system which thermally extracts the volatiles off the adsorbent resin and into the GC. In an alternate method called Direct Thermal Extraction (DTE), solid matrix samples are placed directly into the GLT desorption tubes and the VOC's are thermally extracted in a one-step process from the sample and into the GC for analysis.

A manual version of the SPTD System has been on the market for more than 8 years, and an automated version has been under development for the past three years. The new AutoDesorb System is the automated sampling version of the patented SIS Short Path Thermal Desorption System. It is designed for the automatic and unattended pickup, injection and thermal extraction of volatile and semi-volatile organics from solid, liquid and gas samples for analysis by GC and GC/MS. This system uses a 12 position carousel for loading samples and the operation is controlled by a PC Windows program that is integrated with the Hewlett-Packard ChemStation program.

INTRODUCTION

The AutoDesorb was designed to operate with the HP-6890 GC or HP-5973 GC/MS. The AutoDesorb System consists of three components, the AutoDesorb Tower, the AutoDesorb Interface and the AutoDesorb PC Software.

Figure 1 - The AutoDesorb System Components

The AutoDesorb Tower sits on top of the GC injection port of the HP 6890 GC, where it is utilized for the direct thermal desorption of both volatile and semi-volatile analytes into the GC injection port and column. It is NOT permanently mounted to the GC. It is held in place by gravity and can be lifted up and removed in seconds to restore the GC to its normal operation. The AutoDesorb System permits the setup and automatic analysis of up to 12 desorption tubes.

The AutoDesorb Interface is an electronics console that interfaces the AutoDesorb Tower to the PC. It consists of a microprocessor that contains software to control the various states of the thermal desorption process including the control of desorption temperatures, cryo-trap cooling and heating temperatures, carrier gas pressure measurement and the various desorption processes. An RS232 cable interfaces this microprocessor to the PC. A desorption system cable interfaces the microprocessor to the AutoDesorb Tower.

Figure 2 - AutoDesorb PC Windows are integrated into the Hewlett-Packard ChemStation System.

The AutoDesorb system is controlled by a PC Windows program that is integrated with the HP ChemStation GC or GC/MS program to control the automatic injection, timed desorption, temperature ramp of heater blocks, control of the GC Cryo-Trap accessory, and remote starting of the GC and Mass Spectrometer. A series of PC windows permits the set up of system parameters as well as the monitoring of the system states. A graphical screen displays the actual states of the various AutoDesorb system operations. The HP ChemStation program controls the analysis of single samples or multiple samples using the sequential running mode. Data for each sample is entered into the HP ChemStation program and downloaded automatically to the AutoDesorb PC software as each sample is analyzed. A log screen displays the results of each sample analyzed.

Figure 3 - Desorption Tube With Needle and Connecting Tube For Installation Into the AutoDesorb System

Figure 4 - Theory of Operation of the AutoDesorb System

A GC Cryo-Trap accessory is normally used with the AutoDesorb System. The Cryo-Trap mounts inside the GC oven at the bottom of the injection port. The Cryo-Trap permits the cryo-focusing or trapping of analytes at the front of the GC column during the thermal desorption process using either liquid nitrogen or liquid CO2. A heater circuit enables the rapid heating of the column to release the analytes in a narrow focused band at the front of the GC column for further GC separation and analysis. The AutoDesorb software and electronics module controls both the heating and cooling of the Cryo-Trap.

Description - Theory of Operation

Samples to be analyzed are collected on Glass Lined Stainless Steel (GLT) Desorption Tubes containing an adsorbent resin such as Tenax® TA or activated carbon. Alternatively, samples of small size (1 to 500 mg) can be packed directly into the desorption tube and subjected to direct thermal extraction.

When ready for analysis, the GLT desorption tubes are fitted with a syringe needle. A connecting tube is attached to the thermal desorption tube to permit the pick up of the desorption tube by the AutoDesorb Tower. The top of the connecting tube contains a spring loaded sealing ball to prevent contamination of the desorption tube.

After the desorption tube and needle have been attached to the connecting tube, they are then placed into the carousel on the AutoDesorb Tower. After the sample tube and connecting tube have been placed onto the carousel, the HP ChemStation software is set up with the GC or GC/MS method and the AutoDesorb screens are set up with the desorption method. When the GC or GC/MS sample analysis run is initialized on the ChemStation Window, the carousel is positioned to the correct desorption tube.

The desorption tube is then loaded into the pickup mechanism. This occurs by a pickup mechanism extending out to meet the desorption tube in the carousel. Next, the mechanism picks up the desorption tube. After the tube has been picked up, the arm retracts to position the desorption tube for injection into the GC.

The thermal desorption blocks are preheated to their initial temperature which was set up on the AutoDesorb PC window. As soon as the desorption block temperature is stabilized, the cooling gas to the Cryo-Trap is turned on and the Cryo-Trap is cooled to its preselected cooling temperature

After both the desorption block temperature and the Cryo-Trap temperatures are stabilized, the carrier gas through the desorption tube is turned on. The flow through the desorption tube can be adjusted via the flow controller on top of the AutoDesorb tower to between 1.0 ml/min and 120 ml/min. This purge gas will remain on for an initial purge time which has been previously selected.

The entire desorption process can be monitored by the AutoDesorb window on the PC. This window displays both graphically and digitally the status of the system. The graphic in this windows visually displays the operation of the desorption process as well as the current state.

Figure 5 - AutoDesorb System Status Window

Figure 6 - AutoDesorb Time Settings Window

An additional AutoDesorb Windows display the time status of the desorption system. The times are normally preset before desorption is started, but can be changed on this window while the system is running. Values for the purge gas time, injection time, desorption time, cryo heat delay, cryo heat time and the GC start time are set on this screen. A vertical black bar displays the current time status of the thermal desorption process.

Another AutoDesorb PC window displays the thermal desorption and cryo trap temperatures. These values are normally preset before the desorption process is started, but can be changed during a desorption run. The desorption block initial temperature, ramp rate (up to 100 degrees per minute) and final desorption temperature are set on this screen. The Cryo-Trap mode (CO2, LN2 or None) as well as the Cryo-Trap cooling and heating temperatures are set on this screen. A vertical black bar displays the current temperature status of the desorption blocks and cryo-trap at the current time.

After the purge gas has been on for the pre selected time interval, the thermal desorption tube is injected into the GC injection port. The desorption tube will pass through the opening in the middle plate of the desorption unit base to position the desorption tube in proper alignment with the GC injection port and the normally open desorption block assembly. When injection is complete, a time delay permits the injection port pressure to equilibrate and the system is checked for pressure leaks. If leaks are detected, the sample is unloaded, the error reported and the next sample in the sequence is analyzed.

Figure 7 - AutoDesorb System Temperature Settings Window

If there are no leaks detected, the desorption blocks close around the desorption tube and the desorption tube will ballistically heat up to the set temperature or the temperature program ramp for the heater blocks will begin. The combination of the heat applied to the desorption tube and the carrier gas flow through the desorption tube will purge the desired analytes into the GC injection port and onto the front of the GC column. The various parameters are set and utilized according to the application requirements. Normally, desorption temperatures between 70 degrees C and 250 degrees C are suitable for most applications. The maximum desorption temperature permissible with the system is 450 degrees C. The heater blocks can be temperature programmed at ramp rates up to 100 degrees/min. Normal desorption times vary from 3 minutes to 15 minutes; however, longer desorption times up to 100 minutes are possible. This permits the trapped samples to be heated by the desorption tube heater blocks, the analytes desorbed from the adsorbent resin and injected directly into the injection port of the gas chromatograph via the shortest path possible, i.e direct injection into the GC much like a syringe.

During the desorption process, the analytes are trapped at the front of the GC column in the Cryo-Trap. The Cryo-Trap temperatures can be set down to -70 degrees C using liquid CO2 or down to -180 degrees C using liquid nitrogen. Since the column inside the Cryo-Trap is maintained at sub ambient temperatures, the desorbed compounds of interest are trapped on the front of the GC column in a narrow band. Despite the long desorption times, the peaks eluted from the column are extremely sharp and well resolved when the analytes are released.

When the desorption time has finished and the sample has been fully desorbed into the GC column, the desorption tube heater blocks are turned off and the desorption tube is uninjected from the GC injection port. The Cryo-Trap is rapidly heated to its preset heating temperature to release the analytes from the Cryo-Trap and the GC program is started by the AutoDesorb system. The GC temperature programming is commenced to elute and separate the analytes into the desired components.

The purge gas remains on for an additional 5 minutes to cool the desorption tube, after which time the desorption tube is unloaded back to the AutoDesorb carousel. After a preset time the Cryo-Trap is turned off and allowed to cool.

When the GC analysis is complete, the next sample of the ChemStation sequence is loaded, the GC and AutoDesorb system parameters are reset and this next sample is analyzed. Software Control and Operation

Testing

Figure 8 - AutoDesorb Analysis of Hydrocarbons

To demonstrate the operation of the AutoDesorb system and to verify its precision, repeatability and accuracy two tests were conducted. A solution of n-dodecane and n-tetradecane at a concentration of 100 ng/ul in methanol was prepared. Then 1.0 ul of this solution was injected into a set of 8 desorption tubes containing 100 mg of Tenax TA. The desorption tube was purged with 120 ml of helium to remove the methanol solvent. The desorption tubes were placed into the AutoDesorb system and automatically and sequentially desorbed and analyzed via a HP 6890 GC and the analytes detected and quantified by a HP 5973 MSD. The resultant chromatograms are shown in Figure 8. The area under each of the two peaks was integrated for each of the 8 runs and the RSD was calculated. For dodecane the TSD was 3.2% and for Tetradecane the TSD was 2.8%. The error observed was due to a number of factors including sample loading, thermal desorption efficiency, GC injection port stability, split accuracy, chromatography, GC stability and MS reproducibility. This result was quite good especially considering that no internal standard was used in the sample preparation and analysis.

In the next test, a calibration curve was generated as would normally be done for the quantitation of samples in the laboratory. Eight solutions of butylated hydroxutoluene (BHT) were prepared in methanol using d-20 BHT as the internal standard. The concentration of BHT was increased from 5 ng/ul to 100 ng/ul, while the internal standard concentration was held constant at 138 ng/ul. One (1.0) ul of each solution was injected onto desorption tubes packed with 100 mg of Tenax TA, purged with 120 ml of helium to remove the methanol solvent and sequentially analyzed with the AutoDesorb system. The resulting calibration curve is shown in Figure 9 and produced a correlation coefficient value of 0.000 for all the samples analyzed.

In a subsequent study, the AutoDesorb system has been used for the analysis of perfumes. In this study, the GC run was more than 120 minutes. The automated unattended operation of the AutoDesorb system enabled the collection of data overnight which produced considerable time savings. Additional studies on volatiles in papers was recently completed and many other studies are planned.

Figure 9 - BHT Calibration Curve

Conclusion

A new Automated Short Path Thermal Desorption System has been developed by Scientific Instrument Services to permit the automated and unattended analysis of thermal desorption samples. The new AutoDesorb system is an invaluable tool for increased productivity using the thermal desorption technique while also producing accurate and reproducible analysis results. The AutoDesorb system can be used for the automated thermal extraction and analysis of volatile and semi-volatile organics from samples in applications including environmental (air, water and soil), flavors in foods, pharmaceuticals, cosmetics, forensic (arson), plastics, building materials and other industrial products.

Features of the AutoDesorb System

  • Automated Thermal Desorption Sample Introduction System
  • Carousel holds 12 samples for unattended operation
  • Uses the patented SIS 'Short Path Thermal Desorption" technology
  • Designed to Operate with the HP-6890 GC or HP-5973 GC/MS
  • PC Controlled Operation - User-friendly Graphical Interface
  • Fully integrated with the HP ChemStation Software
  • Permits the analysis of volatile and semi-volatile organics
  • P&T Thermal Desorption and Direct Thermal Extraction
  • No "memory effect" - individual flow path for each sample
  • Mounts overtop the GC injection port
  • Not permanently installed to the GC - easily removable
  • Desorb samples at temperatures from room temperature up to 450º C, either isothermal or at ramp rates up to 100º C per minute
  • Glass Lined stainless steel desorption sample tubes are inert and strong for sample handling
  • GC Cryo-Trap Accessory for cryo trapping volatiles during desorption
Tenax® is a registered trademark of Buchem BV.