Optimizing the GC-AED for Petroleum Analysis

An important application for the GC-AED is the determination of elements like sulfur and nitrogen in petroleum streams. Due to the high levels of carbon in these samples and the wide boiling range of sample components, care must be taken in choosing experimental conditions that will achieve optimum performance.

Reagent gas pressures must be increased from the standard settings used for the AED checkout sample to prevent the formation of excessive "soot", which can potentially damage system components downstream from the plasma cavity. Higher reagent gas pressures have also been shown to improve the selectivity of AED recipes like sulfur-181.

Another important factor to consider is the selection of a GC-Column (length, diameter, and film thickness) that will provide adequate column capacity and low bleed: the low-bleed, "MS" versions of columns are generally preferred. An injection volume and split ratio that match the selectivity and dynamic range of all the elements of interest should be chosen. This will ensure that the analyte peaks of interest all elute in their linear dynamic range, and large carbon peaks don't cause "unsuppressible" interference in the elemental chromatograms. These considerations are of particular importance for Simulated Distillation and applications that utilize compound-independent calibration. Diablo's free MDL Calculator Software can be used to help you with this determination.

Another important, but often overlooked, consideration is the choice of injection-port liner for Split/Splitless injections. Agilent Technologies has determined that a single-taper liner fitted with a glass wool plug that is positioned 16-18 mm from the top of the liner reduces boiling-point discrimination and injection-to-injection variation (Agilent Part # 5183-4647). This "wiped-needle" configuration is particularly important with wide-boiling-range petroleum samples.

Agilent Technologies has completed a study to determine the optimum conditions to use for the determination of sulfur, nitrogen, oxygen, lead, manganese, and fluorine in gasoline- and diesel-range materials. They have published the results of this study in an application note. A summary of the optimum experimental conditions taken from the application note are listed in the table below. These conditions are an excellent place to start if you are developing your own GC-AED methods for gasoline- and diesel-range petroleum samples.

Summary of Experimental Conditions:

Gas Chromatograph
Column	30-m x 0.32mm, 1.0 um HP1-MS (Agilent Part # 19091S-713)
Injection Port	Split/Splitless
Injection Port Liner	Single Taper with glass wool plug positioned 16-18 mm from the top of the liner (Agilent Part # 5183-4647)
Injection Port Temperature	280 DegC
Oven Temperature Program	40 DegC (0 min) to 300 DegC (10 min) @ 10 DegC/min
Column Head Pressure	12 psi (Helium)
Split Ratio (Carbon and Sulfur)	50/1
Split Ratio (Nitrogen)	10/1 (Gas Saver, 200 mL/min at 0.3 min to reduce the impact of inlet leaks)
Injection Volume	1 uL (Neat)
Atomic Emission Detector
Transfer Line and Cavity Temperature	310 DegC
Data Rate	2.5 Hz
Method 1 (Carbon 179 and Sulfur 181)
Reagent Gases	Oxygen: 55 psi, Hydrogen: 45 psi
Make-up Gas Flow Rate	100 mL/min (on low make-up setting)
Method 2 (Nitrogen 388)
Reagent Gases	Oxygen: 80 psi, Hydrogen: 40 psi, 100% Methane: 50 psi
Make-up Gas Flow Rate	230 mL/min (on high make-up setting)