Split vs Splitless Injection
The Shimadzu GC-2025 uses a Split/Splitless (SPL) injection unit. Choosing the correct mode depends on your sample concentration.
Split Injection
Use for: High-concentration samples or unknown concentrations.
The sample is vaporized in a heated chamber. A split valve divides the vapor into two paths: a small fraction enters the column, and the majority exits through the split vent.
Why it exists: Capillary columns have very small inner diameters and low sample capacity. Injecting concentrated samples directly — even 0.1 µL at 10% concentration — would saturate the column, causing poor efficiency and bad peak shape.
Split Ratio
The split ratio is the ratio of split vent flow to column carrier gas flow.
Example: Split ratio = 25 (50 mL/min split flow ÷ 2 mL/min column flow). If you inject 1 µL, only 1/25 µL actually enters the column.
Set the split ratio in Set Injection Parameters.
📋 From course materials
Splitless Injection
Use for: Trace-level (very low concentration) samples.
- The split valve stays closed during injection (“sampling time”)
- Nearly the entire injected volume (up to ~3 µL max) enters the column
- After a set time (tens of seconds), the split vent opens to purge residual vapor
Focalization Technique
To prevent peak broadening when injecting large volumes in splitless mode:
- Set initial oven temperature lower than the sample solvent’s boiling point
- Vaporized sample condenses into a narrow band at the column inlet
- Oven temperature ramps up → sample re-vaporizes → separation begins
This concentrates the sample at the column head and preserves peak shape.
📋 From course materials
Hardware Differences
The SPL liner packing differs between modes:
| Component | Split Mode | Splitless Mode |
|---|---|---|
| Silica wool amount | ~10 mg | ~2 mg |
| Wool position | Near needle tip | 25 mm from top |
⚠️ If switching modes, the liner packing should be adjusted. See Maintenance for liner replacement procedures.
📋 From course materials
Decision Guide
Is your sample concentration known?
├── Yes, high concentration (>0.1%) → SPLIT
├── Yes, trace level (<0.01%) → SPLITLESS
└── Unknown → Start with SPLIT (safer for column)
See Also
- Overview — Instrument specifications
- GC_Separation_Theory — How separation works
- Method_Development — Setting injection parameters in GCsolution
- Sample_Preparation — Preparing samples before injection