Novel on-column and inverted operating modes of a microcounter-current flame ionization detector

Improved operating modes of a microcounter-current flame ionization detector (μFID) are demonstrated. By operating the flame inside the end of a capillary gas chromatography (GC) column, the effective cell volume enclosing the flame is considerably reduced and results in significantly lower gas flows being required to produce optimal sensitivity from the stable flame. For instance, in this mode the tiny counter-current flame is situated “upside down” inside the column on the end of a stainless steel capillary delivering 4 mL/min of oxygen and is stabilized by a counter flow of only 10 mL/min of hydrogen carrier gas. Under these approximately fourfold reduced gas flow conditions, the μFID carbon response is linear over almost 5 orders of magnitude and yields a detection limit of 6 × 10−10 gC/s. These figures agree well with those reported for the original μFID, which also similarly operated under hydrogen-rich conditions. To better simulate the oxygen-rich environment of a conventional FID flame, a novel “inverted” counter-current flow mode was also investigated. In this post-column μFID arrangement, a very lean flame is now situated on the end of a stainless steel capillary delivering 10 mL/min of hydrogen, which is opposed by a counter-current flow of only 20 mL/min of oxygen. The μFID detection limit obtained in this stable, oxygen-rich counter-current flame mode is 7 × 10−11 gC/s with a response that is linear over almost 6 orders of magnitude. These findings are more comparable to those of a conventional FID. Overall, the low-flow sensitive μFID operating modes presented demonstrate that this detector may be potentially useful for adaptation to portable devices and related GC applications.