The ability of an optical system to efficiently collect light is known as its throughput or etendue. In an ideal world of homogeneous samples, a Raman spectroscopic system would need only a small laser spot with a similar-sized aperture to collect high resolution spectra. However, the world is more complicated than this. Identification of all the components in a heterogeneous sample requires a larger area of interrogation. To accomplish this, the laser spot size could be increased to cover more area. Unfortunately, high resolution requires a small aperture which reduces the amount of Raman light a system can collect, and thus sacrifices sensitivity. Alternatively, a small aperture could be used while quickly moving (rastering) a tightly focused laser over the sample to collect information from a large area of the sample. This is the principle behind the patented Orbital Raster Scanning (ORS^TM) method developed by Metrohm Raman.

ORS technology is Metrohm Raman’s proprietary way of overcoming low resolution, poor sensitivity, and sample degradation while still interrogating a large sample area. This is illustrated in a variety of applications detailed within this Application Note.

Figure 1 shows a graphical representation of Orbital Raster Scanning, which is defined as quickly moving a tightly focused laser beam over a large area.

Raman spectroscopy is used in many different applications and industries where high-quality data is essential. The abilities of Metrohm Raman’s MIRA (Metrohm Instant Raman Analyzer) and MISA (Metrohm Instant SERS Analyzer) systems with ORS are highlighted in three different applications below: pharmaceutical regulation, material identification, and sampling of SERS (Surface Enhanced Raman Scattering) substrates.

The European Pharmacopoeia (Ph. Eur, or EP) is a complete standard for the production of medicines, quality control of medicines and raw materials, and the instruments used to perform such tests. Raman spectrometers are increasingly used for quality control of medicines because they are user-friendly, flexible, and provide fast, non-destructive measurements. A newly revised chapter, Ph. Eur. 2.2.48 Raman Spectroscopy, focuses on aspects that enhance the reliability of results, and it specifically mentions ORS [1].
 

«When using Raman spectroscopy [...] care must be taken to ensure that the measurement is representative. This can be achieved by, for example rotation of the sample, performing multiple measurements on different preparations of the sample, using orbital raster scanning (ORS) [...]» [1]

Pharmaceuticals are mixtures of excipients and active pharmaceutical ingredients (APIs) in carefully controlled proportions. Representative sampling can be an issue with the small beam diameter of most Raman systems (40–200 μm) and the small particle size (averaging 36–39 μm) in tablets [2]. MIRA and MISA spectrometers with ORS raster the laser around a 2000 μm diameter circle to interrogate a large area in a very short time and capture all of the ingredients in a single scan.

All MIRA and MISA systems use low laser power to collect high resolution spectra. This results from the combination of 785 nm laser excitation and a patented high etendue «free space» spectrograph design. For example, ≤50 mW lasers are sufficient for good Raman signal acquisition at 785 nm in MIRA and MISA systems, while 1064 nm systems must employ 420 ± 30 mW lasers to compensate for poor signal-to-noise and reduced Raman scattering exhibited at longer wavelengths. The combination of low laser power and ORS is ideal for interrogation of sensitive materials, such as highly colored and volatile substances.
 

Raman Identification of Polystyrene

A comparison between conventional 1064 nm and MIRA XTR DS 785 nm Raman systems in the interrogation of the plastic barrel of a ballpoint pen clearly shows the benefit of combining low laser power and ORS (Figure 2).

The crater on the left in Figure 3 was created when a fixed-laser 1064 nm system at 100% power burned the sample. The second test at 60% laser power gave the same result (Figure 3, right) and both tests failed to identify the material.

MIRA XTR DS collected high quality data with no sample damage and identified the plastic as polystyrene, with a Hit Quality Index (HQI) of 0.91. This indicates high correlation between the sample spectrum and a library spectrum.
 

Raman Identification of Polyphenylene Ether (PPE)

A second example of non-destructive sampling and positive material identification with a MIRA system can be seen in Figure 4.

The first test with the fixed-laser 1064 nm system produced a low-resolution spectrum, albeit one with some identifiable Raman peaks. The second test burned the material (Figure 5) and yielded no useful information.

Again, MIRA XTR DS easily identified this material, giving a positive identification of PPE with HQI = 0.91.

It must be acknowledged that these results may be successful because of a combination of factors: low power 785 nm lasers and ORS. The following application will focus entirely on the power of ORS to improve results for heterogeneous materials.

P-SERS are paper-based substrates embedded with nanoparticles which enhance the Raman signal and permit very sensitive detection of trace analytes with SERS. P-SERS are used with MISA, a portable 785 nm Raman system equipped with ORS.

Inks prepared from silver and gold colloids are used to print P-SERS substrates, resulting in a heterogeneous distribution of nanoparticles on paper fibers [3]. This can be seen in the SEM images below (Figure 6).

Scanning a standard on P-SERS with the raster OFF and contrasting this with raster ON data is a perfect demonstration of the ORS advantage. BPE [1,2-Bis(4-pyridyl) ethylene] was solvated and dropped directly onto the printed portion of a P-SERS strip. This strip was analyzed with MISA using constant acquisition parameters but with raster ON and OFF (Figure 7).

Each set of five spectra in Figure 7, ORS either OFF or ON, contains the same signature peaks for BPE. However, spectral consistency is ideal with the raster ON. Each spectrum is the averaged data from a large number of signal-producing hotspots collected during a single ORS scan. The result is a better representation of the heterogeneous surface. The variability of SERS signal intensity (standard deviation over a set of five spectra = 1.4%) with ORS ON is significantly lower than with ORS OFF (SD = 10%.) This confirms the strength of ORS in obtaining consistent data from a sample.

All systems from Metrohm Raman feature ORS, which has three significant benefits over stationary-beam systems:

1. Confidence in sample preservation, because the laser spot does not remain in the same location and there is very low risk of sample burning.
2. Data consistency, even for heterogeneous samples. The rastered beam collects data over a larger area of the sample, increasing reproducibility.
3. Best quality data collection. ORS covers a large sample area without losing resolution.