InPhotonics: Criteria for choosing a Raman spectrometer.

Guide to Raman Instrumentation

The Raman marketplace can be confusing -- there are many companies offering products that span an order of magnitude in price range. Which type of spectrometer to choose depends upon your application needs. Here are some general guidelines for the novice buyer:

Spectrograph

Spectral Range: The Raman shift range is specified in wavenumbers (cm^-1). Most spectrographs cover at least the "fingerprint region" from 400 - 1800 cm^-1; this is where the majority of vibrational bands occur. Below 400 cm^-1, there are vibrations associated with heavier atoms, such as C-halogen and metal oxide stretches. Other bending modes can occur at these lower frequencies as well. On the other end, C-H stretches occur between 2900 - 3100 cm^-1, and the O-H/N-H stretches are visible up to 3600 cm^-1. There is little between 1800 and 2900 cm^-1, except for nitrile (CN) bands occuring near 2200 cm^-1. For many applications, the fingerprint region is sufficient for both quantitative and qualitative analysis.

Spectral Resolution: The resolution of the spectrograph determines how well individual bands can be separated in the spectrum. Resolution should be specified as full width at half height (FWHH) of a measured band. Some manufacturerers quote pixel resolution, which is the data point spacing (or digital resolution) of the spectrograph; this is not the same as spectral resolution. Raman bands of solids are typically 2 - 6 cm^-1 wide, and those of liquids can be 4 - 10+ cm^-1 wide. Having a spectrometer that can measure close to the natural linewidths of the vibrational bands provides the most information. For simple applications, lower resolution spectrographs may be applicable.

Spectral Throughput: Spectrographs are usually quoted with an f # -- the smaller the value, the more photons will pass through to the detector. However, a complete Raman system is a sum of its parts, from the laser, to the sampling optics, to the spectrograph, to the detector. Throughput values are relevant, but overall performance is more important when comparing systems.

Detector Sensitivity: Most dispersive Raman spectrographs employ CCD (charge coupled device) detectors. These are one- or two-dimensional arrays of silicon elements. In general, the dark noise of the CCD is reduced by 50% with every 5 degree drop in operating temperature. Most research-grade spectrographs employ vacuum-sealed arrays operating between -50 and -90C for highest sensitivity. Warmer, less expensive detectors can be used for applications that can afford less sensitivity.

Exctation Laser

Excitation Wavelength: Raman scattering intensity is proportional to v⁴, where v is the excitation frequency. Therefore, higher excitation frequencies (i.e. shorter wavelengths in the visible and UV regions) provide higher Raman intenstiy than longer wavelengths. Unfortunately, shorter wavelengths can also excite fluorescence backgrounds in many samples, obscuring the Raman bands. For many general applications, near-IR excitation at 785 nm is preferred.

Laser Characteristics: There are many types of lasers available to the Raman spectroscopist -- gas lasers, diode lasers, and solid-state lasers. Traditional gas lasers provide stable, narrow lines, but are becoming less commonly used. Diode lasers differ widely in the marketplace. Single-mode diode lasers provide narrow linewidths but are usually low in power and require temperature stabilization to prevent mode hopping. Multi-mode diode lasers can provide very high power, but the output can be as wide as 30 cm^-1 -- this results in all Raman bands being 30 cm^-1 wide, regardless of the resolving power of the spectrograph. Multi-mode lasers can also be stabilized to produce a single-mode output; these lasers are usually associated with higher cost. Raman signal is generally linear with laser intensity, however very high powers can damage a sample.

When evaluating Raman excitation lasers, note the center wavelength, output power, linewidth, stability (power and linewidth), and lifetime.

Summary

The performance of a Raman spectrometer depends upon all of its parts. Numerical specifications are helpful to compare basic criteria, such as spectral range and resolution, and the general features of different systems. To evaluate overall performance of different spectrometers, measured spectra should be requested and compared.