Hardware; Sample Tubes; Tube Desorption Oven; Tube Filters And Seals - Markes International Unity 2 User Manual

QUI-1057
Page 10
2.1

Hardware

2.1.1 Sample tubes

UNITY 2 is compatible with industry standard sample tubes - 3.5 inches (89 mm) long by ¼-inch (6.4 mm)
O.D with 5 mm (stainless steel and coated steel) or 4 mm (glass) I.D. Sorbent is retained in stainless steel
(or coated steel) tubes using stainless steel (or coated steel) gauzes and a gauze retaining spring. Quartz
or glass wool is recommended for retaining the sorbent in glass tubes.

2.1.2 Tube desorption oven

The UNITY 2 tube desorption oven heats up rapidly (~150°C/min) at the start of elevated temperature
purge (see Section 3.6.4) or tube desorption. It begins to cool at the end of primary (tube) desorption and
reaches 50°C from 300°C within 10 minutes.

2.1.3 Tube filters and seals

When ready for analysis, sample tubes are placed into the ambient temperature desorption oven with the
sampling (grooved) end pointing to the rear of the instrument. Operation of the lever mechanism seals
the sample tube into the UNITY 2 flow path. Temperature resistant Viton O-rings seal onto the outer wall
of the sample tube, ~2 mm from either end. Each O-ring should last for >1000 tube-sealing operations. In
the event of failure, O-rings at both ends of the sample tube are readily replaced by the user (see
Technical Support Manual). A porous PTFE filter sits just behind the O-ring in both sample tube seals.
These prevent UNITY 2 flow path contamination in the event that sorbent particles or high boiling sample
materials migrate out of the tube. The filters are readily accessed for user replacement (See Technical
Support Manual).

2.1.4 The cold trap

The quartz cold trap contains a 2 mm diameter x 60 mm long bed of sorbent (30 to 100 mg depending on
sorbent density) supported by quartz or glass wool. Note that the length of the first plug of glass wool is
included in the total 60 mm sorbent bed.

2.1.5 Cold trap cooling and heating

UNITY 2 contains a 2-stage Peltier cell, which uniformly cools the entire 60 mm sorbent bed to a minimum
of -30°C in ambient temperatures as high as +25°C (see UNITY 2 Installation Manual Section 1). No
liquid cryogen is required. Dry air or nitrogen flows into the cold trap box creating a slight positive pressure
and minimising ingress of water from the laboratory atmosphere. If the cold trap box was not purged, ice
would quickly build up around the Peltier cell which is maintained at sub zero temperatures throughout
UNITY 2 operation.
NOTE: C hydrocarbons (ethyne, ethene, ethane) and the most volatile freons cannot normally be
2
sampled using sorbent tubes at ambient temperatures - breakthrough volumes are too small for practical
use, even with the strongest tube sorbents. These compounds must be collected in bags or canisters or
sampled on line. These whole air/gas samples are then introduced to the UNITY 2 cold trap using an Air
Server Accessory.
Once all the target analytes have been collected and focused in the cold trap, the trap oven heats rapidly
reaching rates in excess of 60°C/sec for the first critical stages of trap desorption. Uncompromised
Markes International Ltd
T: +44 (0)1443 230935 F: +44 (0)1443 231531 E: enquiries@markes.com
www.markes.com