My colleagues and I have been extolling the virtues of split injection for a while now. We’ve demonstrated that fast sample transfer often yields better chromatographic results (especially for early eluting volatile compounds). We’ve established that starting oven temperatures can be elevated, shortening run time and increasing sample throughput. Most importantly, we’ve shown that inlet discrimination and adsorptive loss are minimized, even after running many dirty samples. Performing a head-to-head calibration evaluation on the same column while holding everything constant except inlet liner, oven program, and MSD gain factor clearly highlights the advantages of split analysis and the superior inertness of the Rxi-5ms.
Calibration Preparation and Evaluation
A 9-point calibration curve was prepared at 0.10, 0.50, 1.0, 5.0, 10, 20, 40, 80, and 120 μg/mL using Restek 8270 analytical reference materials (cat# 31886, 31888, 31063, 31850, 31852, 31879), establishing an on-column calibration range of 0.0091 to 11 ng for the split analysis calibration and 0.10 to 120 ng for the splitless analysis. The split and splitless calibration data were collected on the same instrument on sequential days using the same tune file, but with a gain factor of 3.0 with a precision split liner for split and a gain factor of 0.3 with a single taper with wool liner for splitless. The complete instrument parameters are listed in Table 1 for split analysis and Table 2 for splitless.
The average % RSD for the split calibration was 7.0, with only 2 compounds (2,4-dinitrophenol and benzoic acid) exceeding method criteria when evaluating by response factor (RF) % RSD. This was marginally better than the splitless calibration, which had an average % RSD of 9.5, and three compounds (2,4-dinitrophenol, 4,6-dinitro-2-methylphenol, and benzidine) exceeding method criteria for evaluation by response factor % RSD. Additionally, only 2,4-dinitrophenol failed to meet minimum response factor criteria (min RF ≥ 0.010) for the 1.0 µg/mL calibration level (0.091 ng on column); however, 2,4-dinitrophenol met method linearity requirements and minimum response factor requirements when evaluated from 5.0 to 120 μg/mL (0.45 to 11 ng on column).
A performance summary comparing split and splitless calibration performance data for a subset of compounds can be found in Table 3. The table was designed to make it easy to visualize dropped calibration points. If you are looking to maximize your dynamic range, split analysis has obvious benefits.
Table 4 highlights the injection-to-injection variability by evaluating the surrogate response factors. The surrogates are at the same concentration in each calibration point, minimizing the effects of activity on the results for the acids and bases. You’ll notice that aside from 2-fluorophenol, the % RSDs for the surrogates acquired under splitless conditions trend up as volatility goes down. This is likely due to sample transfer efficiency dropping as analyte volatility goes down. This is a symptom of inlet discrimination and is normally exacerbated as the inlet gets dirty as real sample extracts are run. This is exactly why split analysis is advantageous; using a split ratio of 10:1 with a column flow of 1.4 mL/min results in an inlet flow of approximately 15.8 mL/min. This is significantly faster than the inlet flow under splitless condition. The split analysis provides a very fast transfer of a narrow analyte band to the head of the column, minimizing inlet discrimination and loss of sample to the inlet surfaces due to activity or other mechanisms of adsorptive loss.
Table 3 - Head-to-head evaluation of calibration performance on a Rxi-5ms GC column using EPA Method 8270 semivolatile organic standards and calibration criteria. At a 10:1 split, mass on column is approximately 1/11th that of the splitless injection for the same calibration point. Solid blue blocks indicate dropped calibration points. This is to make it easier to visualize the dynamic range for each 8270 component