Improving Productivity and Reducing Costs with Off-line Sorbent Tube Conditioning

Rigorous conditioning of sorbent tubes is an essential part of any sampling and analysis protocol. This application note will explore the cost savings and productivity enhancements that can be made by off-line conditioning, rather than on‑line with the thermal desorber itself. In particular, we will focus on the revenue resulting from running more analytical samples, the cost-effectiveness of increasing sample capacity by this approach, and the benefits that stem from using nitrogen rather than helium.

The Need for Sorbent Tube Conditioning
In any sampling campaign employing thermal desorption tubes, it is vital that the sorbent bed is free from contaminants before sampling commences, to avoid complications during analysis and minimize the potential for false-positive results.

There are a number of potential causes of such contamination (1),  but in the vast majority of cases tube cleanliness can be assured by rigorous conditioning whenever they are freshly packed with sorbent, stored without being properly capped, heavily contaminated during a sampling procedure, or required for trace‑level monitoring.
Tubes should be conditioned for at least as long as their standard desorption time, using clean carrier gas (for example, grade 5.0 oxygen-free nitrogen or helium, and ideally with a hydrocarbon filter in the gas line). Temperatures and gas flow rates should be higher than those used in the analytical method (2).

Many commercial thermal desorbers offer a dedicated tube conditioning mode. Whilst this is a useful feature, it can take up valuable analytical instrument capacity - especially because only one tube can be conditioned at a time. This note will explore the cost savings and increased productivity that can result from employing a dedicated multi-tube conditioner - the example chosen here is Markes International’s TC-20.

Increased Revenue Through Running More Analytical Samples
Markes’ TC-20 (Figure 1) is a stand-alone tube conditioner that can clean (or dry-purge) up to 20 industry-standard sorbent tubes simultaneously. The only requirements are power and a clean, high-purity gas supply (typically nitrogen rather than the more expensive helium). The TC-20 can also now accept tubes fitted with Markes’ TubeTAG tube-tracking system.

Figure 1: Markes' TC-20 multi-tube conditioner.

Using a tube conditioner like the TC-20 is an excellent way to free up analytical instrument time to run more samples, and thus increase revenue. To illustrate this, imagine conditioning 20 tubes using a typical 1-h method (3) - a process that would take more than 20 h on a thermal desorber. Conditioning these tubes on a dedicated 20-tube conditioner instead would free up 20 h to run more samples (4).

More generally, whatever proportion of time your analytical instrument(s) are used for conditioning, this is the time that will be released to run analytical samples. Imagine using those extra 20 h of instrument time to run samples using a 40-min GC run. Up to 30 additional runs could be completed in this time, resulting in $3000 additional revenue, at $100 per sample.

Even if a laboratory were to recondition just 20 such tubes per week on a tube conditioner rather than on their analytical system, this would result in $156,000 per year additional revenue (52 × $3000) from the extra analytical instrument time released. This is a striking amount, and even if the laboratory is only running at maximum capacity for a fraction of the time (as is quite probable), the financial advantage of purchasing a tube conditioner will still be considerable.

Equation 1 can be used to estimate the revenue that you might be able to generate through running more analytical samples in your laboratory.

Equation [1]

[1]

Cost-Effectiveness of Increasing Capacity
Purchasing a dedicated tube conditioner is also a much more cost-effective way of increasing capacity than buying a whole new analytical system.

To illustrate this, consider the cost per sample of a new TD–GC–MS analytical system, with or without a tube conditioner, over the lifetime of the instrument. Inserting approximate figures into Equations 2 and 3 (including an estimate for maintenance costs of 10%) suggests that the cost of system hardware and maintenance per sample with a tube conditioner would be about half the cost for an analytical system on its own.

Equation [2]

[2]

Equation [3]

[3]

The simple calculations above assume sufficient business for the laboratory to run at capacity, but even when this isn’t the case, it is far cheaper to have the tube conditioner idle than the analytical instrument idle.

Reducing Ecological Footprint and Lowering Costs By Using Helium Rather Than Nitrogen
When tubes are conditioned on the TD analytical instrument, it is necessary to use the instrument’s carrier gas, which is nearly always expensive grade 5.0 helium. This is unnecessary for tube conditioning, which can be carried out on the tube conditioner just as effectively with cheaper oxygen-free nitrogen. As well as reducing use of helium, which is an increasingly scarce and expensive global resource, this results in an additional cost saving. This is assessed in Equation 4 for conditioning using the 1-h method, with a gas flow of 100 mL/min per tube, and 2014 gas prices in Markes’ laboratory in Cincinnati, Ohio, USA.

Although the cost saving seems on the face of it to be quite modest, the overall saving over the course of a year might be quite significant, especially in the context of laboratory budgets for consumables as opposed to instrumentation. It is also more ecologically responsible to use nitrogen rather than helium, especially given growing demand for organizations to be “greener” and less resource-intensive.

Conclusions
The above-described calculations have shown that a tube conditioner - illustrated here with Markes’ TC-20 - is a cost‑effective investment for the majority of laboratories running TD–GC methods, particularly in terms of analytical instrument time released, but also with regards to reducing the use of helium carrier gas.

References

1. For more information, see Markes International’s Application Note 006.
2. Conditions recommended for specific sorbents are detailed in Markes International’s Application Note 005 and are provided with shipments of pre-packed tubes from Markes.
3. The calculation described assumed that a 1-h conditioning method was used, such as is applicable to previously-used tubes packed with graphitized carbon black or carbonized molecular sieve sorbents (15 min at 100 °C, followed by 15 min at 200 °C, followed by 15 min at 300 °C, followed by 15 min at 380 °C). The recommended conditioning method depends on sorbent type, and is more rigorous if the tubes have been freshly packed or heavily contaminated during sampling.
4. It is also worth noting that the speeding-up of the conditioning process, as well as improving turn-around times on re-conditioning, would mean that laboratories with large inventories of tubes may be able to reduce the number of tubes in service.

Markes International
Gwaun Elai Medi-Science Campus, Llantrisant, Wales, UK
Tel: +44 (0)1443 230935
E-mail: enquiries@markes.com  Website: www.markes.com