Split vs splitless injection GC-MS: A head-to-head evaluation of calibration performance on a Rxi-5ms GC column using EPA Method 8270 semivolatile organic standards and calibration criteria.

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 1 - Agilent 7890A-5977C GC-MS Split Acquisition Parameters

Table 1 – Agilent 7890B-5977A GC-MS Split Acquisition Parameters

Table 2 - Agilent 7890B-5977A GC-MS Splitless Acquisition Parameters

Table 2 – Agilent 7890B-5977A GC-MS Splitless Acquisition Parameters

8270 calibration comparison

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

Table 4 - RSD comparison of the surrogate response factors for each of the 9 calibration points. Precision is good for both techniques, but the split analysis precision is clearly better.

Table 4 – RSD comparison of the surrogate response factors for each of the 9 calibration points. Precision is good for both techniques, but the split analysis precision is clearly better.

6 Responses to “Split vs splitless injection GC-MS: A head-to-head evaluation of calibration performance on a Rxi-5ms GC column using EPA Method 8270 semivolatile organic standards and calibration criteria.”

  1. Dear Chris and Team Restek –

    Very nice work …!

    My experience is that for many quantification applications, splitless injection very often is better than split injection, getting lower average %RSD’s. Have you tried to optimize the pulsed splitless injection for this application? For example, less injection volumes or reduced pulse and purge times?

    With kind regards –
    Lars Kürstein, Copenhagen

  2. Lars, the splitless injection used is an optimized pulsed splitless injection. It’s optimized, in that the pulse and purge times are timed to end and begin once the inlet volume has been swept 1.5 times to insure full sample transfer to the analytical column. I wasn’t interested in reducing the volume injected because I would need to change to a 2 mm ID inlet liner (and 0.5 µL injections are less forgiving chromatographically). I agree that reducing the pressure pulse and purge hold times would result in lower %RSDs, but this is because I would be opening the split vent before sample transfer was complete; in effect, I’d be introducing a bit of split character to my splitless injection.

    I wrote a blog a couple years ago that goes into more detail on optimizing a splitless injection.

  3. Hatem Elgendi says:

    Hi chris,

    great work. only people working to optemize 8270 parameters apperciate your work. i have been optemizing 8270 for the last 15 years.
    i have a question for you is those paramerts works only with 5977A MS or it will work find with 5975C MS which i have.
    i will aperciate your response

    with kind regards
    Hatem Elgendi
    chemist

  4. Hatem,

    The parameters should work with the 5975 and 5973, though he instruments with the triple axis detectors may be a bit more sensitive.

  5. chris,

    the other question is the column i am using Rxi-5sil ms since they came out do i have to use back Rxi-5 ms and i have reg. chemstation not masshunter version do i have to upgrade to masshunter or not

    with best regards
    Hatem Elgendi
    chemist

  6. Greg Pronger says:

    A believer in “Have your cake and eat it too”. What I have experienced is a greater response with a low split injection over a splitless injection.

    To perform a ‘sweep’ of the inlet, I will use the gas saver backwards, and use it to increase flow after the split pulse to 50mL/min. So, if I have a pulsed split at 35PSI, for 0.2 minutes, I will set the gas saver to 50mLs/min at 0.21 minute.

    Good luck,
    Greg

    PS, so I have found this confusing, how can I get more analyte into my column with a split injection than a splitless?

    My rationalization would be that in spite of calculations on expansion volume of solvent at injection (roughly 1uL becoming 1mL gas volume), the injection is a very dynamic situation. Remember, in splitless we are simply switching the total flow from out the split vent to out the septum purge line. So, if any of the injection vapor cloud goes up, it goes out. Maybe, with a low split flow, this cloud is maintained nearer the column at the bottom of the liner better, increasing the mass transfer from the inlet to the column.

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