Table 1. A comparison of sample-delivery characteristics using liquid jets, viscous jets and fixed targets.
Given the rough grouping of the methods and the fact that many of the parameters depend on other experimental settings (such as the crystal shape, mother-liquor composition, crystal symmetry etc., to name a few), this table provides a high-level overview.
| Parameters | Liquid jets | Viscous jets | Fixed targets |
|---|---|---|---|
| Sample-translation speed | ∼10–100 m s−1 | Up to several millimetres per second | Defined by motors |
| Jet diameter | ≤5 µm | Defined by the inner diameter of the capillary, ∼50–10 µm | N/A |
| Flow rate | ∼5–50 µl min−1 | Tens of nanolitres to several microlitres per minute | N/A |
| X-ray background† | Generally low | Matrix-dependent; generally higher than for liquid jets | Design- and sample thickness-dependent |
| Suitable crystal size | ≤∼10 µm | Smaller than the inner diameter of the capillary | If chip is patterned, feature-dependent; otherwise, any size |
| Sample consumption | Tens of milligrams or more | Sub-milligram possible‡ | Sub-milligram possible‡ |
| Sample efficiency | |||
| At ≤120 Hz repetition rate | Low | High | High |
| At megahertz repetition rate | High | Unfeasible | High |
| Beam-time efficiency | |||
| At ≤120 Hz repetition rate | High | High | High |
| At megahertz repetition rate | High | Repetition-rate decrease necessary | To be seen§ |
| Longest time delay with optical triggering | A few microseconds or less | A few seconds or less | Unrestricted |
| Suitability for mixing | Yes | No | Yes |
| Suitability for synchrotron use | Currently not¶ | Yes | Yes |
A detailed comparison is difficult as many parameters contribute to the measured background (sample, scattering environment etc.) and prevent such a cross-publications comparison. For details, see Sections 2.1, 2.2.1 and 3.
Optically triggered TR experiments require much more sample since not only the X-ray affected section but the entire optically illuminated section of the jet needs to be displaced between the exposures.
The challenge will be the acceleration and deceleration times of linear scanning. This may be eliminated with radial scanning.
May be feasible at diffraction-limited storage rings. The exposure must be short enough to capture the fast-moving crystal as essentially ‘still’ and must contain a sufficient number of photons for high-resolution diffraction.