Raman Spectroscopy - Advanced Optical Analysis Technology

Raman spectroscopy is a kind of non-destructive analytical method, which is a spectroscopic technique used in sophisticated and research laboratories. Raman effect is the shift in wavelength of the incident frequency radiation. Raman scattering happens as inelastically scattered radiation and the formation of Raman lines occurs. Raman lines can provide chemicals and structural information of the molecule. Raman spectra are the interpretation of recognition of molecular functional groups, which represents the different subunits of a molecule. In the Raman spectrum, the Raman shift is related to the chemical bond length of molecules.    

This article explains how the optical analysis technology utilizes Raman spectroscopy and gives superior performance.

Necessity for Low-Optics

The broad adoption of Raman spectroscopy necessitates sensitive, lower cost, and portable instruments. The lasers to be used should be safe for human skin and the human eye. Though portability has been attained in Raman spectrometers, basic entropic limits for light collection limits sensitivity and demand high-power lasers and cooled costly detectors. In comparison with portable dispersive Raman spectrometers, a swept-source Raman spectrometer enhances light collection efficiency by 1000X. High detection sensitivity is achieved by only uncooled amplified silicon photodiodes and 1.5 mW average excitation power. The lower optical power requisite permits exploitation of miniature chip-scale MEMS-tunable lasers, having close to eye-safe optical powers for excitation.

Raman spectroscopy should become broadly accessible. For this, it needs to be compact, low-cost, low-power (both optical and electrical). Concurrently, it has to retain the performance. Most of the Raman spectrometers and their variants do not offer these simultaneously. 

Rationale behind the use of High-Optics

The contest of building an accessible and sensitive Raman spectrometer (low power, low cost, high sensitivity, compact) arises from the inefficiency of the Raman scattering process as only one out of a million or billion incident photons experience Raman scattering. This problem is aggravated because of diffuse scattering of photons in most inhomogeneous samples, which need larger spectroscopic equipment in achieving higher collection efficiency for photons, which are over a solid angle and a larger area. The lower Raman signal along with the collection efficiency limit, demands the usage of powerful excitation lasers or low-noise cooled detectors or larger high-throughput spectrometers for achieving greater sensitivity and adequate signal to noise ratio. Practically, for achieving acceptable performance, most systems combine at least two of the elements. 

Usually, the lasers surpass the eye exposure limit by 100x. Also, the spectrometers are subject to size-throughput trade-offs. In addition, the cooled detectors are expansive and power-inefficient. These led to the usage of Raman spectrometers, which are large and expensive. Consequently, lasers are also not operated without any precaution.

Cons of FT-Raman and SERS

Fourier transform Raman spectroscopy (FT-Raman) and surface-enhanced Raman spectroscopy (SERS) have been followed for addressing the limitations caused by weak Raman signals. SERS permits Raman sensing with single-molecule sensitivity and increases the Raman signal by 10¹⁴ fold. At the same time, SERS is not contactless or reagentless, as it generally needs chemical bonding between the analyte and metallic nanostructure. Hence, it is not compatible with solid samples. 

FT-Raman spectroscopy is contactless and reagentless and preserves the pros of dispersive Raman spectroscopy. High light collection efficiency (throughput gain) and detection of full-spectrum at once (multiplexing gain) exhibited by FT-Raman improves the sensitivity. Despite that, the moving mirror in FT-Raman is less robust when compared with the dispersive Raman spectrometer, which has ruled the field in recent years.

Nonlinear Raman Spectroscopy

In nonlinear Raman spectroscopy like stimulated Raman spectroscopy (SRS) and coherent anti-stokes Raman spectroscopy (CARS), tunable lasers have been utilized for elimination of throughput limitations in Raman spectrometers. But high-power benchtop light sources and optically-pumped lasers are demanded by the high-peak powers required in nonlinear Raman spectroscopy. Still, systems using ultrafast pulse lasers and portable supercontinuum fiber sources cannot be handheld due to power consumption and size.

Spontaneous Raman Spectroscopy

From nonlinear Raman spectroscopy, the use of tunable light sources and elimination of spectrometers can be brought to spontaneous Raman spectroscopy, which permits low-power, sensitive and compact instruments. The swept-source approach enables a high optical throughput design, which lowers laser excitation requirement to milliwatt scope, permitting utilization of chip-scale tunable lasers. The high optical throughput permits the use of uncooled photodiodes instead of CCD, but still achieves higher detection sensitivity. This is a giant step in the reduction of the cost of Raman spectrometers.

Advances in Optical Analysis Technology

The advanced optical analysis technology exploits the power of Raman spectroscopy for performing real-time and continuous chemical measurements in any type of environment without the requirement to destroy, change, extract or prepare the sample. This allows analysts to obtain chemical insights, which simplify product development and analytical scalability. This also helps in ensuring product quality via process automation.          

In this, Raman spectroscopy is an optical analysis technology, which measures concentration and composition. This Raman spectrometer aids in bridging laboratory analysis with the process environment in up to 4 channels. The rugged and reliable Raman analyzer ensures 24/7 quality control and monitoring of processes. Due to the presence of fiber optic sampling probes, in any kind of environment, real-time, continuous, and non-destructive chemical composition measurements can be performed. 

The probes are reliable, robust, and high performing , which can be compatible with all phases of matter like gases, solids, and liquids. The Raman spectrometers permit easy transfer of knowledge and protocols from the research and development section to the manufacturing division.

Kinds of Probes in Raman Spectroscopy

There are various types of probes used in Raman spectroscopy. They are as follows:

AirHead Probe

It is used in the monitoring of gases in the laboratory and the analysis of the process

Pilot-E Probe

It is utilized in the monitoring of liquids in process analysis.

WetHead Probe

It is also exploited in overseeing liquids in laboratory and process analysis

bIO-PRO probe

It is utilized in the control of liquids in process analysis.

RamanRxn Probe

It is utilized in the supervision of liquids and solids in laboratory analysis.

P^hAT Probe

It is utilized in the monitoring of solids in process and laboratory analyses.

Benefits of Optical Analysis Technology

The following are the pros of the optical analysis technology in Raman spectroscopy:

Speedy Implementation

• The software is user-friendly and automation compatible
• It is a reliable partner for Raman spectroscopy
• A global support network is available

Optimized Efficiency of Process

• There is faster delivery of real-time data
• There is high accuracy in the measurement of data
• Lab-to-process scalability can be done in a simplified way.

Enhanced Safety

• The measurement technique is reliable and non-invasive
• It minimizes human intervention
• At the same time, it maximizes risk avoidance capacities

Increased Product Quality

• There is easier compliance with industry regulations
• Increased visibility in the process occurs
• Tighter control of parameters, which are of critical quality

Reduced costs

• Maintenance needed is less
• Product wastage is lowered
• Lesser cleaning burden 
• Increase in the yields
• More availability

 Conclusion

A precise introduction to Raman spectroscopy has been provided. Need of low-optics and motivation behind the usage of high-optics have been discussed. Cons of FT-Raman, SERS and nonlinear Raman spectroscopy have been stated. Use of swept-source Raman spectroscopy has been demanded. Then, the advances in optical analysis technology have been introduced. Then, the kinds of probes used in Raman spectroscopy are briefed. Finally, the benefits of optical analysis technology have been detailed.