Inlet system

Inlet System

General Design

The "inlet" or sample preparation system is designed to produce pure helium and neon fractions from the gas mixtures extracted from the water samples and stored in the glass ampoules. It is optimized for fully automated operation and high sample throughput. It consists of three main parts (Fig. 3):

a manifold of eight vacuum and cracking valves for attachment of the glass ampoules to a high-vacuum system and cracking of the glass seals to admit the gas samples into the purification part of the inlet system,

an air standard preparation system, and

three traps cryogenically cooled by a helium compressor for separation of helium and neon from water vapour and permanent gases.

The inlet system also contains a QPMS for measurement of neon.

Fig. 3: Schematic diagram of the sample preparation system.

Vacuum system

The vacuum system consists of electropolished stainless steal tubing (1/4" or 3/8" outer diameter) with all-metal fittings (mostly VCR). The 3/8" tubing is used in the sample inlet and air standardization parts for more efficient pumping, whereas 1/4" tubing is used in the cryogenic cold trap part of the system to minimize the volume (the ratio of the trap volume to the mass spectrometer volume determines the fraction of the helium that is admitted to the mass spectrometer).

We used all-metal valves and vacuum fittings wherever possible to minimize the helium background in the system. The only parts that contain material other than stainless steel are the O-ring fittings in the system that are used to attach the glass ampoules to the all-metal inlet system (Cajon Ultra Torr). The all-metal fittings are well suited for relatively high bakeout temperatures of the vacuum lines and keep the helium background levels low. The Pirani and Penning pressure gauges are also all-metal designs and are attached to the vacuum lines through CF flanges (16 and 40 mm for the Pirani and Penning gauges, respectively). The valves are equipped with copper stamps (Nupro SS-4BG-V51-CU3C). Most of the valves are pneumatically driven, allowing for fast manual or computer-controlled valve operation. We use normally closed valves to secure the system in case of power or compressed air loss.

Rough vacuum (ó 10^-3 mbar) is produced by rotary pumps (Leybold, Trivac D4B). Oil filters (Edwards, activated alumina) prevent back-flow of oil vapour from the rotary pumps in case of a pump failure. A diffusion pump (Balzers, DIF 063) backed up by a rotary pump achieves a finer vacuum (ó 10^-5 mbar). The diffusion pump is protected from the water vapour by a water trap (see below). The ultra-high vacuum section, which consists of the 10 K charcoal trap and the QPMS (Balzers, Prisma or QMG 112 with Faraday cup), is pumped by a turbomolecular/drag pump (Balazers, TMU 065) or by a combination of a diffusion pump (Balzers, DIF 063) and a turbomolecular pump (Balzers, TPU 60). Either combination is backed by a rotary pump.

The pressures of the five rotary pumps are monitored by Pirani pressure gauges (Balzers, TPR 250). In the all-metal section, three all-metal Piranis (Balzers, TPR 260) are used. The vacuum of the diffusion and turbo/drag pumps is controlled by two Penning pressure gauges (Balzers, IKR 260). The pressure readings of the ten gauges are displayed by five dual controllers (Balzers, TPG 252). They are also monitored by the system control computer.

Sample Inlet

To open the flame-sealed glass ampoules and bulbs for gas transfer into the inlet system, we use modified Nupro SS-8BK-TW-10 valves (normally open, pneumatcilly dirven). The modification of the valves includes drilling open the inlet port of the valve to accommodate the glass neck of the ampoules next to the modified stamps (stamps have been replaced by stainless-steel screws). Before cracking the glass ampoules or bulbs by closing the cracking valves, the inlet system is evacuated in two steps by a rotary and a diffusion pump to decrease the partial pressure of helium to a negligible level.

During the extraction procedure, water vapour is used as carrier gas to transport the gases released from the water samples into the glass ampoules. This water is utilized to flush the gases contained in the glass ampoules into the inlet system. More than 99 % of the He is in the gas phase of the head space of the ampoules or bulbs. A capillary mounted just above the cracking valves (Fig. 4; inner diameter: 0.069 cm, length: ÷ 2.5 cm) prevents back-diffusion of helium into the glass container and reduces the amount of water transferred into the inlet system. The transfer times through the capillary are set to 1 minute for helium isotope samples, 2.5 minutes for 40 cm³ tritium samples, and 5 minutes for 400 cm³ tritium samples, respectively. These transfer times are checked by repeat measurements to ensure complete transfer of the sample gas from the glass ampoules or bulbs into the inlet system.

Air standards

Fig. 4: Schematic diagram of inlet part of sample preparation system (for symbol legend, see Fig. 3).

The helium isotope ratio measurements, as well as the absolute ³He , ⁴He , and neon measurements are standardized against air. The He content of air (5.24 ppm; Glueckauf, 1946), as well as its ³He/⁴He ratio (1.384× 10^-6; Clarke et al., 1976) and He/Ne ratio (0.288, Ozima et al., 1983) are well known. Therefore, atmospheric air is well suited as a standard for helium isotope measurements. As working standard, we use a stainless steel container with known volume (13 l) filled with atmospheric air (Fig. 5). The container is filled sufficiently far away from laboratory buildings to avoid contamination with tank He. For precise determination of the amount of helium contained in the air standard, atmospheric pressure, temperature, and humidity have to be measured accurately. Determination of the helium content in the standard container with a precision of ñ 1 ‰ requires measurement precisions of ñ 1 mbar, ñ 0.3 K and ñ 2 % for atmospheric pressure, temperature, and relative humidity, respectively.

Besides the working standard, we use a master standard (3.9 l) to calibrate the individual working standards which are changed after a depletion of about 10 percent (Fig. 5). For tritium measurement by the ³He ingrowth method, very small amounts of helium are used for standardization. Therefore, the air standards are diluted in an expansion volume (volume of about 1 l) by a factor of about 160 (Fig. 5)

Fig. 5: Schematic diagram of the standardization part of the sample preparation system (for symbol legend, see Fig. 3).

The individual air standards are taken from the standard containers by pipettes, which consist of calibrated volumes mounted between two Nupro valves. The pneumatically operated (automatic) pipettes (custom-made by D. Doerflinger, Heidelberg, Germany) are equipped with counters mounted into the pneumatic air lines to account for the depletion of the standard. The depletion has to be known for correction of the absolute amount of helium contained in the volume of one air standard taken by a pipette. The air standards are treated in the same way as the extracted gases from water samples.

Cryogenic cold traps

Three different traps (custom-made by Leybold and Janis Research Co. for the helium isotope and tritium mass spectrometers, respectively) are cooled by two cryogenic pumps (Leybold, RGD 510, with compressor, Leybold, RW4200, for cool down times of traps see Fig. 6). They are used to separate helium and neon from all other gases contained in a water sample or air standard (Fig. 7). All traps are housed in a protection vacuum maintained by a rotary pump for proper thermal insulation and to avoid water vapour condensation on the cold surfaces. Their temperatures are measured by silicon diodes and set by several controllers (2 Leybold LTC 60 in the helium isotope system, and 4 Lakeshore, Temperature Controller 330 in the tritium system, respectively). The temperatures are stable within about ñ 0.1 K or better.

Fig. 6: Cool down times for the cryogenic cold traps.

Fig. 7: Schematic diagram of the cryogenic cold trap system (for symbol legend, see Fig. 3)

The first trap (stainless steel cylinder; 2.5 cm diameter, 10 cm length, 40 cm³ volume) is connected by copper wires to the first stage of one of the cryogenic pumps. Two silicon diodes and heaters are stabilizing the temperatures at 253 K at the trap inlet and 123 K at the trap outlet to distribute the freeze-out of the water over the entire length of the trap. An additional temperature controller keeps the inlet line before the water trap above the freezing point of water to avoid any freezing and clogging.

The second trap (stainless steel cylinder of 2.3 cm diameter, 2.0 cm length, about 4 cm³ volume) mounted directly onto the second stage of one cryogenic pump with a stabilized temperature of 24 K liquefies all gases except helium and neon. About 5 minutes are required for removal of the water vapour and condensation of the permanent gases.

The inner surface of the final cold trap (cylinder of 2.3 cm diameter, 2.0 cm length, about 4 cm³ volume) are covered with charcoal (DESOREX, F11) by using a cold-temperature epoxy glue for good thermal contact. The adsorption coefficient of gases on the activated charcoal is a function of the partial pressure in the cold trap, the volume of the trap, the amount and type of activated charcoal, and the temperature of the trap. The adsorption and desorption of helium (and neon) onto and from the charcoal can be measured directly by using the sector field mass spectrometer (SFMS). The same holds true for neon if the QPMS is used. Quantitative (> 99.5 percent) adsorption of helium on the charcoal requires temperatures below 14 K. Quantitative desorption is achieved at temperatures of ò 45 K (Fig. 8; tritium mass spectrometer ò 38 K). The small ³He to ⁴He isotope fractionation effect (Fig. 8) is negligible at temperatures < 14 K and > 45 K.

Fig. 8: 3He and 4He adsorption isotherms (measured on the helium isotope SFMS).

We determined the absorption of helium and neon onto the charcoal as a function of time using the QPMS. For this purpose, both helium and neon were adsorbed quantitatively onto the charcoal at 10 K. This process took about 2.2 minutes for helium. The following desorption of helium to >99 percent of the original signal (at 45 K) required about 1.5 minutes (Fig. 9). The equivalent numbers for neon are about 4.5 and 7 minutes (120 K), respectively.

Fig. 9: Desorption time of 4He from the charcoal for a temperature of 45 K (measured on the Tritium mass spectrometer).

There is discrimination of helium by neon in the QPMS (Balzers Prisma). At the helium inlet temperature of 38 K (tritium mass spectrometer), less than 0.5 % of the neon is in the gas phase (Fig. 10a). With the full neon signal in the QPMS (120 K), the helium signal is reduced by about 50 % (Fig. 10a). In the SFMS, the discrimination for a 0.2 cc air standard at 120 K is about 20 % (Fig. 10b).

Fig. 10a: 4He discrimination by neon in the Prisma quadrupole mass spectrometer.

Fig. 10b: 4He discrimination by neon in the SFMS (helium isotope MS).