Analytical and Environmental Services Ltd (AES) uses gas chromatography for a wide range of analyses, many of them at parts-per-trillion levels. As well as improving analytical performance, Air Products' "GC" helium with its built-in-purifier has reduced downtime and saved money by extending the life of the GC columns.

High-purity carrier gases can improve the performance of gas chromatography (GC) systems, especially for analyses at the parts-per-billion (ppb) level. Low levels of impurities in the carrier gas mean greater sensitivity, less baseline noise and better peak resolution. And high-purity gases also bring another, less obvious, benefit.

Thanks to the built-in purifier in each cylinder, carrier gases in Air Products' "GC" range contain such low concentrations of oxygen and water that they can significantly increase column life. Analytical and Environmental Services Ltd (AES), a big user of GC gases, used to buy helium from another supplier containing up to 7 ppm oxygen and 5 ppm water. After changing to BIP helium, which is guaranteed to contain less than 10 ppb oxygen and 20 ppb water, one of AES's Newcastle laboratories reports big savings in time and money because GC columns need replacing much less often.

Big laboratory, big reputation

With around 190 staff, AES is one of the U.K.'s largest providers of analytical services. Formerly the analytical division of Northumbrian Water, the company became independent in 1991 following privatisation of the water industry in 1989. Since then AES has been expanding its business outside the water industry, which now accounts for around 65% of its £8 million turnover. AES has three laboratories: Howdon and Horsley, both near Newcastle, and Rugby.

AES provides organic, inorganic and microbiological analyses of water and wastewater, contaminated land and gaseous emissions. The company also collects samples, designs and carries out surveys of waterways and contaminated land, and performs risk assessments for water systems. AES is NAMAS-accredited for its analyses, notably of hard-to-measure contaminants such as dioxins and furans.

The trace organics section at Horsley specialises in analysing "clean" samples such as drinking water for contaminants including pesticides, herbicides, polycyclic aromatic hydrocarbons (PAHs) and phenols. "We need to be able to do accurate analyses at the levels specified in the drinking water regulations, which are in the range 0.001 µg/l," explains laboratory director Paul Knowles. "For this we use mainly GC, with high-performance liquid chromatography (HPLC) for PAHs and some herbicides."

The Horsley lab is a modern building with an airy yet businesslike layout that encourages good working practices and a logical flow of samples. "It's certainly the nicest lab I've ever worked in," enthuses Andrea Dawson, a trace organics analyst at Horsley.

From river to results

Many of the water samples analysed at Horsley are collected by AES's own sampling teams. Each one-litre sample in its glass bottle is logged into the lab's information management system as soon as it arrives, and is then refrigerated before further processing.

Methanol or propanol, buffers, salt solutions or acid may be added to the GC samples, depending on the analyses required. The next stage is solid-phase extraction onto a resin cartridge, whose contents are then eluted into a solvent such as ethyl acetate. The quantity of solvent depends on the determinands required, but is usually 1.5–3 ml. A final concentration step yields a 0.5-ml solvent sample ready for injection into the GC. Most samples are processed in a single weekly batch, which typically contains 5–25 samples.

Although the requirements for sensitivity and accuracy are often high, much of the analytical equipment at Horsley is fairly standard, notes Paul Knowles. The trace organics section has four Hewlett-Packard 6890 GCs—two configured for GC/MS and the others for GC/ECD. The columns are mostly standard types such as HP5/DB5, with helium as the carrier gas. One of the GC/ECD instruments has a high-volume injector to increase sensitivity, and there is a headspace sampling system for volatile organics.

The GC analysts check the performance of their instruments before every batch, and quality control standards make up 10% of the samples analysed. "With this amount of checking we can pretty quickly spot any fall-off in analytical performance," says Andrea Dawson. "The first step is then to trim off the front end of the column, and clean the inlet system and possibly the detector. If that doesn't improve performance, it's time to change the column." And column changes are expensive.

Oxygen and water: the enemies within

Modern capillary GC columns are very sensitive to oxygen and water, which destroy the stationary phase. A 1995 survey by LC/GC magazine found that contamination was the source of 28% of problems reported with GC. Other common problems were bad resolution (25%), column deterioration (11%), low reproducibility (7%) and ghost peaks (4%). All these effects—accounting for a total of 73% of all GC problems—are caused or aggravated by impurities in the carrier gas.

The traditional way to remove oxygen and water is to rely on traps or purifiers in the line between the gas cylinder and the GC. This DIY approach has its problems, though, as AES analyst Tony Rycroft admits. Since many of the purifiers are not self-indicating, it is impossible to check their performance except through a time-consuming process of elimination. The extra tube connections associated with in-line purifiers also introduces potential sources of leakage and contamination.

Air Products' BIP cylinder system eliminates the need for external purifiers by placing a patented purification system inside each cylinder. Because Air Products is able to maintain total control over the condition of cylinder and its contents, the company is able to guarantee that the gas leaving the BIP cylinder will contain less than 10 ppb oxygen and 20 ppb water. The built-in purifier means fewer potential leak points, no in-line purifiers to worry about, and absolute peace of mind on gas purity.

Built-In-Purifier: a winning technology

AES was originally attracted to "GC" helium because Air Products offered better prices and standards of service than the company's previous supplier, says purchasing manager Joanna Brannon. Performance tests on the new gas more than satisfied the analytical staff, so in April 1997 AES took the decision to switch to "GC" grade for all its helium—around 30 cylinders a year at Horsley, as well as a much larger quantity at the Howdon lab.

Pretty soon, though, Joanna Brannon realised that as a result of the switch to "GC" the Horsley lab was buying fewer new columns. From January 1996 to April 1997 the lab used five new columns on its GC/ECD instruments, or an average of four columns a year. In the 21 months following the change to "GC" helium, the same instruments needed only two new columns, or an average of just over one column a year. For one particular GC/MS the results were even more striking: until April 1997 consumption was running at one or two columns a year. Since then, not a single new column has been required.

The cost savings in columns alone have been significant: £360 for each Jones Chromatography DB5 column for the GC/ECD machines, and £390 for the HP 5MS columns used for GC/MS. "But when you take lost time into account, the savings are even bigger," points out Andrea Dawson. "A new column takes me about an hour and a half to install, including vent time for the MS. Then it needs to settle down: overnight for a GC/ECD column, and a full 24 hours for a GC/MS column. Having fewer column changes is a big advantage."

Andrea's colleague Tony Rycroft likes another handy feature of the BIP cylinders: the built-in low-pressure cutoff valve, which cuts off the gas when the cylinder pressure has fallen to just 3 bar. To meet health and safety requirements, he explains, AES has banished all gas cylinders from their original position near the GC instruments to an outside storage area. Since all the GCs are now connected to a single cylinder, there is a greater chance that a cylinder will run out unexpectedly.

With conventional cylinders, a total loss of pressure brings the risk that air, or contaminants desorbed from the cylinder wall, will leak into the column. With "GC" helium, the design of the BIP valve means that it is impossible to empty the cylinder. As well as preventing backflow into the cylinders themselves, this protects the GCs from contamination.

"All in all we're very pleased with the BIP technology and our 'GC' helium," says Paul Knowles. "We're getting better analytical performance at a very reasonable gas cost, and Air Products gives us excellent service. And on column replacement costs we have proven, repeatable savings in both time and money. It's been a very worthwhile change for us."

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