Up to 30% Higher Response in GC with Gas from Air Products' BIP Cylinders

The presence of minimal quantities of contaminants in the carrier gas, such as oxygen, water, and hydrocarbons, can influence test results and affect the lifetime of GC columns and equipment. Column bleed—the degradation of a polymeric stationary phase to volatile compounds which are carried by the mobile phase to the detector—can be caused by water and oxygen in the carrier gas. The polyethylene glycol and cyanopropyl siloxane phases are the most susceptible, but all polymeric stationary phases are affected to some degree. Ghost peaks and baseline perturbations can arise from volatile and semivolatile hydrocarbons and halocarbons in the carrier gas. Detectors can also be affected by the inorganic and organic impurities in the gases. These impurities have varying origins and can be removed by different techniques.

Suppliers of gas chromatography equipment specify minimal gas purity for their instrumentation, recommending the use of appropriate purifiers as insurance. Purifier manufacturers advise installing in-line purifiers to upgrade qualitatively lesser gases into purer laboratory gases. Dr. David is of the opinion that the latter is not a good solution for several reasons. From his experience, purifiers can sometimes produce interfering peaks due to the presence of hydrocarbons and phthalates. In addition, purifiers can have a short lifetime. In actual practice, GC operators often pay very little attention to the purifiers. It is difficult to check whether they are still working properly, and they are rarely maintained to specification.

R.I.C. tests purifiers often, at the request of suppliers. The lifetime of water and oxygen purifiers made by various manufacturers has been measured by R.I.C using gas of different qualities. Theoretical calculations conclude that purifiers, in combination with a good quality gas, should have a much longer lifetime than when they are used to upgrade industrial gas to a lab quality gas. R.I.C studies have shown, for example, that an in-line purifier exposed to pre-purified helium as carrier gas can be used up to 300 times longer than when using an industrial gas.

Consequently, the savings derived from the use of cheaper gas are completely negated by the higher costs incurred by the rapid consumption of the purifiers. &It is an illusion to think that by using purifiers you can use a lower quality gas and save money this way," P David.

But even when the instructions for installing and using the purifiers are precisely followed, the results, according to Dr. David, are never 100% satisfactory. As one of the causes, he points to the additional fittings in the gas line which are required for installing the in-line purifiers. Even minimal leaks in these fittings can introduce air into the gas stream.

Thus, R.I.C. selects a good quality gas, does not install purifiers, and uses as few fittings in the gas delivery systems as possible. Advice commensurate with these experimental results is freely given to chromatographers trained at the Institute.

Switchover to BIP Cylinders

Last year, Air Products introduced R.I.C to its new BIP technology for guaranteeing the purity of compressed gases. Air Products builds a purifier into the gas cylinder. Before the gas exits the cylinder valve, it flows through the purifier located immediately below the valve, and is thereby pre-purified or "polished."

The problems characteristic of in-line purifiers are thus eliminated. The concentrations of oxygen and water in the gas are reduced to almost parts-per-trillion levels. The specifications for gases with the BIP technology are 10 to 300 times lower for oxygen and 25 to 150 times lower for water, depending on the quality of the gas with which these values are being compared (research grade gas versus industrial quality gas). The maximum concentration of oxygen in Air Products' pre-purified gas is 10 ppb (parts-per-billion) and the maximum concentration of water is 20 ppb. Other impurities such as hydrocarbons, carbon monoxide, and carbon dioxide are also largely removed.

Because the purifier is isolated from the atmosphere and from inadvertent contamination, and is thoroughly quality controlled by Air Products, the purifier is always guaranteed to work well for the total contents of the cylinder.

Favorable Test Results with BIP Technology

R.I.C. conducted three different tests with the gas from the built-in-purifier-equipped cylinders prior to making the commitment to switch permanently to the new product.

Test 1 was the analysis of fatty acid methyl esters on a cyanopropyl siloxane column with FID and helium as the carrier gas. For a relatively short period of time (24 hours), no measurable difference was detected in the degree of column bleed or column quality. However, carrier gas coming from Air Products' BIP cylinders provided a more stable column quality over a longer period of time (2 weeks).

Test 2 was the analysis of PCBs by GC-ECD. The carrier gas was hydrogen. Nitrogen was used as make-up gas for the ECD. It is well known that an ECD detector is affected by oxygen and water. The difference in detector sensitivity between the nitrogen with BIP technology and 5.0 quality nitrogen was immediately noticeable. The results showed an increased stability of the baseline, a 20% lower background, and a higher detection sensitivity (approx. 30% higher response).

Test 3 was an analysis with GC-AED. Helium was used as the carrier gas. Atomic Emission Detection is a very sensitive multiple element detector which is also used for oxygen detection. The result of this test was a more stable baseline for carbon (3x lower background) and for oxygen (2x lower background).

Having conducted these three tests, Dr. David concluded:

• Gases using Air Products' BIP technology improve analytical results considerably by lowering the background and increasing the sensitivity of detectors such as the ECD and AED;
• Gases using Air Products' BIP technology may extend the lifetime of cyanopropyl siloxane columns;
• Gases using Air Products' BIP technology decrease the maintenance efforts in a GC lab;
• Air Products' built-in-purifier-equipped cylinders can be dropped into an existing gas handling system without any modifications.