Christopher Lydon

We worked with a sample in a capillary.Patent SENSOR FOR OBTAINING RAMAN SCATERING SPECTRA AND METHOD OF I...

Focusing the laser beam: Set the laser in a stable position, pointing it at the area where the sample is.

Use a collimator to convert the light flux into a flat beam. This will significantly improve focusing.

Use a lens (usually a collecting lens) to focus the laser beam on the sample. The optimal distance from the lens to the sample depends on the type of lens, but, as a rule, this distance should be chosen so that the beam is as focused as possible. Collecting scattered light: Position the optical fiber so that its end is in the zone of scattered light from the sample. When using low-loss fiber, make sure that its end is in the correct position to collect the maximum amount of scattered light. This is important, since any distance from the sample can reduce the collection efficiency. Adjusting the fiber:

If possible, use focusing optical elements (e.g. a small lens) in front of the end of the fiber to additionally collect scattered light.

Adjust the angle of the fiber entry so that it coincides with the angle of scattered light from the sample. This increases the efficiency of signal collection.

Performing Raman spectroscopy on liquid samples using capillary tubes involves a series of optimized steps to ensure high-quality spectral data acquisition. The process begins with careful sample preparation, where quartz or borosilicate glass capillaries with 0.5–1.5 mm inner diameters are preferred to minimize fluorescence background, with fused silica capillaries offering optimal Raman transparency. The liquid sample is loaded by either dipping one end of the capillary into the liquid or using a micropipette for small volumes, allowing capillary action to draw in the liquid until a 10–20 mm column height is achieved, after which both ends are sealed with clay, wax, or flame sealing for volatile samples, typically requiring 5–50 µL of sample volume depending on capillary size. Instrument setup requires attention to laser configuration, with 532 nm or 785 nm lasers preferred to reduce fluorescence interference, adjusted to 10–100 mW power to prevent sample heating or degradation, while optical alignment positions the capillary at a 90° scattering geometry with the laser beam focused through the capillary center using a 10×–50× long-working-distance objective, employing backscattering collection for translucent liquids. Polarization control may involve using a polarizer for VV or VH scattering geometries when studying orientation effects, with capillary rotation recommended to average orientation effects in isotropic liquids. Data acquisition begins with short exposures of 1–5 seconds to assess signal-to-noise ratio, followed by accumulation of 10–30 scans for weak signals, covering a spectral range of 100–3500 cm⁻¹ while adjusting for solvent cutoffs, with 785 nm lasers particularly useful for aqueous solutions to minimize water band interference, and requiring background correction through empty capillary and solvent spectrum subtraction. Special cases like volatile liquids demand quick sealing with epoxy or flame-sealing and potentially cooling stages, while viscous samples may require wider-bore capillaries or centrifugal loading with longer integration times, and photosensitive samples necessitate reduced laser power below 5 mW, 785 nm or 1064 nm excitation, or flow systems for continuous refreshment. Data processing involves subtracting capillary background spectra, applying cosmic ray removal algorithms, and normalizing to solvent or internal standard peaks for quantification, with calibration using known concentration standards in identical capillaries for quantitative work. The capillary method offers advantages including minimal sample volume requirements, reduced degradation risk compared to open containers, excellent reproducibility for concentration measurements, and compatibility with temperature-dependent studies when used with heating/cooling stages. Common troubleshooting includes verifying laser focus at the capillary center for weak signals, switching to 785 nm lasers or photobleaching for fluorescent backgrounds, checking for sample heating effects if peaks appear distorted, and ensuring capillary cleanliness through solvent rinsing and drying prior to use. This method proves particularly effective for organic solvents, ionic liquids, and aqueous solutions where traditional sample holders may introduce evaporation or scattering artifacts, with adjustments like reduced path length using flat capillaries or modified laser power recommended for highly absorbing samples to prevent thermal lensing effects.