Background
The LUMA detector has proven itself as a highly-capable detector for trace analysis in many applications, often as a complementary detector alongside others (like FID), or as an alternative where others cannot meet the application's requirements.
LUMA operates like any other spectrophotometer in the sense that it follows the Beer-Lambert law. This means it is a concentration-dependent detector, like the TCD and ECD, rather than mass-sensitive like and FID. Anything that impacts the concentration of the analytes to, or within, the detector will impact the sensitivity of the analysis.
A make-up gas (MUG) flow supplements the carrier gas column flow into LUMA's flow cell. This primarily helps maintain peak shape, but also prevents contamination depositing within the flow cell. The flows combine, pass through the cell and then exit at the exhaust (EXH) port on the side of LUMA.
Make-up gas (MUG) flow
One of the biggest influences on analyte concentration in the LUMA flow cell is the make-up gas (MUG) flow rate, which is delivered to LUMA via an Auxiliary Electronic Flow Controller (AuxEPC) as a fixed pressure. Increasing the pressure increases flow rate, and vice versa. How to measure this flow rate is covered in a separate article, here.
The standard setting for the MUG flow rate, which is typically the flow that the system will be left with after installation, is 2.5 mL/min. This is a good starting point for any method development work.
Increasing the MUG flow rate will:
- Increase peak resolution, by reducing band broadening in the flow cell.
- Reduce the risk of contamination, particularly when analysing compounds with a bpt. close to the upper detector temperature limit (275°C) NB: never run the GC column oven to a temperature higher than the LUMA transfer line and flow cell temperatures!
- Reduce analyte sensitivity, by reducing the effective analyte concentration in the flow cell.
Decreasing the MUG flow rate will:
- Reduce peak resolution, which might result in some critical separations co-eluting.
- Increase the risk (or rate) of contamination, particularly for high boilers and high concentration components.
- Increase analyte sensitivity, by increasing concentration in the flow cell.
The absolute minimum MUG flow rate that should be used with LUMA is 0.5 mL/min. So, when combined with 1.0 mL/min column flow (as an example), the volumetric flow rate at the EXH should read 1.5 mL/min.
There is no real maximum MUG flow rate that should be used with LUMA, but anything over 5.0 mL/min is probably excessive.
Other parameters
As the detector is concentration-dependent, things like inner column diameter will impact concentration - for best sensitivity, narrow bore columns (0.18-0.25 mm i.d.) are recommended, as these will utilise a lower column flow rate and exhibit less band broadening, meaning the analytes will enter the detector more concentrated than they would in a wider bore column (0.32-0.53 mm i.d.).
Beyond this, the normal rules of GC apply! Note that running splitless, rather than split, for highest sensitivity can be false economy as the longer loading time on to the column can result in peaks with a lower concentration at the apex vs. a well-optimised low split injection. Trying both routes during method development is recommended, provided analyte volatility and column selection doesn't rule out splitless as being impossible to begin with.