Note 38: A New Micro Cryo-Trap For Trapping Of Volatiles At the Front Of a GC Capillary Column

John J. Manura, Chris Baker and John Manos
1999

INTRODUCTION

The new Micro GC Cryo-Trap was developed for the GC analysis of organics in gas samples with injection techniques, as headspace injection and thermal desorption. In both of these techniques, large gas volumes are injected into the GC capillary column and the volatile organics are cryo trapped or cryo-focused on the front of the GC capillary column. Studies have proven that this trapping region can be as little as 1.0 inch in length. The Micro Cryo-Trap utilizes a 1.0" cooling chamber which surrounds the front of the GC capillary column inside the GC oven just under the injection port. Normally, in order to cryo trap volatiles, the entire GC oven is cyro-cooled using liquid CO2 at temperatures that cannot go below -40 deg C. With the new Micro GC Cryo-Trap, volatiles can be trapped at temperatures down to -70 deg C utilizing CO2 (or -180 deg C with LN2) using less than 3% of the cooling gas required to cool the entire GC oven. This results in more efficient trapping of the volatiles as well as substantial cost savings in CO2. To release the volatiles from the Cryo-Trap a heater coil inside the Cryo-Trap rapidly heats the capillary column to release the organics to produce well resolved GC peaks. Features of the SIS Micro Cryo-Trap

• 1" Long x 0.75" Diameter Cooling Chamber
• Sealed Chamber to Prevent Cooling Gas Leakage into GC Oven
• Cools down to -70 deg. C with CO2
• Cools down to -180 deg C with LN2
• Heats to 400 deg C at 1000 deg/min Ramp
• Mounts on GC Injection Port Nut
• Easy Installation

Figure 1 - SIS Micro Cryo-Trap - Theory of Operation

The Micro Cryo-Trap consists of a small heating/cooling chamber which is 3/4" in diameter and 1.0" long (Figure # 1). The GC capillary freely passes through the center of this chamber via a 1/16" O.D. stainless steel capillary guide tube. Capillary columns or guard columns up to megabore sizes (0.53 mm I.D.) can pass through this stainless steel capillary guide tube. Around the stainless steel capillary guide tube, a heating coil is wound, to provide for the rapid heating of the capillary guide tube and GC capillary guard column. A thermocouple is attached to the stainless steel capillary guide tube to provide the feedback to regulate the heating and cooling temperatures and to display the temperature on the digital temperature display of the Electronics Control . Liquid CO2 or Liquid Nitrogen for the cooling of the Cryo-Trap is introduced into the top inlet via an electrically controlled valve. The bottom outlet vents the dispersed Cooling gas into the GC oven, or optionally it can be vented external to the GC via appropriate plumbing. The Micro Cryo-Trap mounts onto the bottom of the GC injection port using a specially designed injection port clamp (Figure # 2). No drilling or other GC modifications are required for the installation of the GC Cryo-Trap in any GC oven. Power requirements are 110 VAC, 3 amp max. An external supply of Liquid CO2 or LN2 is required for the cooling operation.

Figure 2 - Micro Cryo-Trap Mounts At Bottom Of GC Injection Port Inside Oven

Both heating and cooling of the Cryo-Trap are controlled by the Micro Cryo-Trap Electronics Control provided with the system (Figure # 2). The Electronics Control can be used to manually switch between cooling and heating or optionally can be controlled automatically via an input signal provided by the GC operating system or by a controlling system, as the S.I.S. Short Path Thermal Desorption System. This input switches the system from the normal cooling cycle to the heating cycle to release the trapped analytes for chromatography. Cryo-Cooling temperatures down to -70 deg C can be set via the controller using liquid CO2 as the cooling gas or down to -180 deg C with Liquid Nitrogen. Heating temperatures up to 400 deg C are achievable at a ramp rate of about 1000 deg per minute. This provides sufficient heating to release the volatiles and semi-volatiles from the trap to produce sharp and narrow GC peak shapes.

Comparison of 1" and 4" Micro Cryo-Trap

Tests were performed with the new Micro Cryo-Trap to compare its efficiency against the older 4.0" long version of the GC Cryo-Trap. For this study, the LEAP Headspace System was used to introduce gasoline samples into the GC injection port. Injections of 0.5 ul gasoline were made into headspace vials containing 5.0 ml of high purity distilled water. The headspace vials were capped and placed into the agitated sample vial block and heated to 80 deg C for 5.0 minutes. Then, 1.0 ml of the headspace volume above the liquid was slowly injected into the GC injection port to minimize excessive sample splitting. The injection port was set to 200 deg C and the GC Cryo-Trap to -100 deg C during the trapping cycle. The GC Cryo-Trap was maintained at -100 deg C for 5.0 minutes after injection was complete to permit the air peaks to exit the GC column, after which the GC Cryo-Trap was rapidly heated to 220 deg C at 1000 deg C/minute to rapidly release the trapped analytes for subsequent chromatography. A J&W DB5-MS capillary column (0.32 mm I.D. x 60 meter long x 0.25 u film thickness) was temperature programmed from 30 deg C to 220 deg C at 10 degrees per minute. After the analytes were chromatographed through the capillary column, they were detected and identified via the HP Engine Mass Spectrometer. The Mass Spec was used in the electron impact mode and was scanned from 25 to 300 daltons. The mass spectra of the peaks were identified using the Wiley NBS library of mass spectral data.

Figure 3 - Comparison Of Micro Cryo-Trap and Previous 4.0" Model Cryo-Trap

Figure 3 shows the comparison of a direct liquid injection of gasoline for both the new one inch Micro Cryo-Trap and the four inch GC Cryo-Trap. In the direct injection sample, butane was detected, which was not detected in the headspace cryo-trapped samples. This is because the melting point of butane is less than -100 deg C, the Cryo-Trapping temperature. Otherwise, the chromatograms are near identical. Only slight loss in resolution is observed between the direct injection and the cryo-trapped samples. The 1" Micro Cryo-Trap was just as effective as the much larger 4" Cryo-Trap, proving that the analytes can routinely and efficiently be trapped on 1.0" of cooling length of the Micro Cryo-Trap.

Selection Of Guard Column Inside the Micro Cryo-Trap

Figure 4 - Effect Of Guard Columns On Efficiency Of Trapping Inside Micro Cryo-Trap

The efficiency of the Micro Cryo-Trap can be influenced by the type of guard column used inside the Micro Cryo-Trap as shown in Figure 4. The top chromatogram is the chromatogram of the direct injection of the liquid gasoline sample (0.01 ul) as a comparison to the other injection techniques. The same headspace technique described above was used to inject headspace gasoline samples into the GC injection port. For this study, the cryo-trap temperature was set to -80 deg C. In the second chart, a fused silica guard column with no liquid phase was used inside the Micro Cryo-Trap. Analytes are trapped or cryo-focused based on their melting points. Analytes with melting point values less than -80 deg C are not trapped in the fused silica column in the Micro Cryo-Trap. When the guard column was changed to a thick film capillary column (0.5 u DB-VRX), the trapping efficiency of the Micro Cryo-Trap is greatly increased as demonstrated in the bottom chromatograph. PLOT columns have also been used as guard columns in the Micro Cryo-Trap to trap gases such as formaldehyde, ethylene oxide and ethane.

Selection Of Cryo-Trap Cooling Temperature

Figure 5 - Effect Of Cryo-Trap Temperature On the Efficiency Of Trapping

The cooling temperature of the Micro GC Cryo-Trap can be set at any value from room temperature down to -180 deg C. This temperature depends on the requirements of the user and the range of compounds that need to be cryo-trapped and subsequently chromatographed. Figure # 5 shows the efficiency of the Micro Cryo-Trap for the trapping of the gasoline volatiles at consecutively lower cryo-trapping temperatures. The GC capillary and guard column consisted of a 0.32 mm I.D. x 60 meter x 0.25 u film thickness DB-5-MS (J&W) capillary column. As the temperature of the Cryo-Trap is sequentially lowered, additional lower boiling analytes are trapped on the guard column inside the Cryo-Trap. The resolution of the eluted peaks is very sharp. This ability to accurately select and regulate the cryo-trapping temperature gives a high degree of flexibility to the user to select the range of analytes that can be analyzed in a sample.

Headspace Micro GC Cryo-Trap Application

Figure 6 - Headspace Analysis Of Latex Paint By Cryo-Trapping

A typical Headspace application of the Micro GC Cryo-Trap is shown in Figure 6. In this sample, 3.0 grams of a water based white Latex wall paint was placed into the headspace vial at 90 deg C and equilibrated for 5 minutes. Then 2.0 ml of the headspace gas was slowly injected into the GC column and the analytes trapped at -160 deg C for 5.0 minutes. The trap was then rapidly heated to 250 deg C and analyzed on the same DB5-MS column described above. A large number of analytes were detected and identified. This technique has proved to be a valuable method for the identification of volatile organics in Latex paints that can be utilized as both an identification tool as well as a quality control method.

Thermal Desorption Application

Figure 7 - Purge and Trap Thermal Desorption Analysis of Cranberries With Cryo-focusing

A large number of applications of the Micro GC Cryo-Trap have been developed utilizing the SIS Short Path Thermal Desorption System. A typical example is shown in Figure 7. For this study, 2.0 gram samples of cranberries were crushed, purged with 2.0 liters of nitrogen and purged volatiles trapped on Tenax® TA adsorbent traps. The trap was then thermally desorbed at 220 deg C into the GC injection port and the analytes were cryo-trapped at -70 deg C in the Micro GC Cryo-Trap. After 5.0 minutes, the trap was heated to 220 deg C to release the trapped volatiles for subsequent GC analysis. The two chromatograms demonstrate the differences between two different varieties of cranberries. This study is part of a continuing project at SIS in conjunction with the NJ Department of Agriculture to study varieties of cranberries throughout the growing season. Similar studies have also been done on blueberries and other food products.

Conclusion

The Micro Cryo-Trap has proven to be a useful technique for the analysis of volatile and semi-volatile organics in large gas volume samples on capillary GC columns via sample introduction techniques such as Thermal Desorption and Headspace GC. These volatile organics are efficiently trapped in a 1.0" band in the Micro GC Cryo-Trap at the head of the GC capillary column. Cooling temperatures down to -180 deg C with Liquid Nitrogen are routinely utilized with the Micro GC Cryo-Trap. This reduces the use of cooling gas by more than 97% as compared to cooling the entire GC oven. Both Cryo-Trap temperature and the selection of capillary guard column liquid phases permit the selection of system conditions to analyze a selected range of analytes based on their melting points. Applications of the technique have been demonstrated for the detection of volatiles in paints via headspace GC injection and the detection of volatiles in food products via the thermal desorption technique. The new Micro Cryo-Trap has proven to be superior in performance and efficiency of cooling gas use than previously manufactured GC cryo-traps.