Choosing the right laser for Raman spectroscopy

בשנים האחרונות ספקטרוסקופיית ראמאן הפכה לכלי בעל ערך רב בניתוח כימי איכותי וכמותי בניתוח תהליכים.

ראמאן כלי עבודה במגוון תעשיות, כולל תרופות, נפט וגז, ביוטכנולוגיה, כימיקלים מיוחדים, בקרת תהליכי מזון ומשקאות, פלסטיקה ופולימרים ועוד תחומי פיתוח אחרים.

ראמאן מהווה יתרון גם בכך שמדובר בטכניקה אנליטית לא הרסנית – בעזרתה ניתן לחקור את ההרכב המולקולרי של מוצקים, נוזלים וגזים מבלי לפגוע בתערובת הכימית או השלחת דגימות.

לאחר שכבר החלטנו שמערכת ראמאן היא היא המערכת המתאימה לתהליך שלנו, הלייזר המשמש כ”מקור עירור” הוא גורם שיש להתחשב בו ע”מ שנדע לבחור את מערכת הראמן המתאימה לנו.

במאמר זה נסקור את הלייזרים השונים המוצעים.

 

Introduction 

In recent years, Raman spectroscopy has become a valuable tool in qualitative and quantitative chemical  analysis for process analysis as the technology offers convenient in-situ sampling interfaces, continuous  in-line measurements, and high spectral specificity. Raman is used across a range of industries including  pharmaceuticals, oil & gas, bio processing, specialty chemical, food and beverage process control,  plastics and polymers, and other process development sectors. Raman is also advantageous because it’s  a non-destructive analytical technique – it can be used to investigate the molecular composition of  solids, liquids, and gases without damaging the chemical mixture or discarding grab samples.  

One of the most important considerations when selecting/evaluating a Raman analyzer is the laser  which serves as an “excitation source” illuminating the chemical sample to facilitate the Raman scatter.  The chosen laser wavelength will have an impact on Raman intensity and background fluorescence (and  in Raman microscopy applications, spatial resolution as well). Lasers have proven incredibly valuable as  light sources for Raman, as they are more intense and more easily focused on to a small region. Early  Raman measurements used gas emission lines as narrowband excitation sources, but it was not until  lasers started being used as the excitation source that Raman started becoming practical outside of a  specialized lab. 

Quality Raman measurements put very specific requirements on several aspects of laser performance but choosing the best excitation wavelength for a given application is not always obvious. This blog will  focus on some of the parameters one should consider evaluating when optimizing a Raman experiment or application.

Considerations for excitation wavelength 

It is important to note that Raman is a low energy technique. It relies on rare “inelastic” photon  interactions in the sample material. When performing a Raman experiment, the vast majority of photons that are scattered by target molecules undergo an “elastic” interaction and thus provide no  useful chemical information. 

Fluorescence interference associated with Raman spectroscopy challenges users, as it is a natural and  unavoidable phenomenon that impedes Raman analysis. Specifically for bioprocess monitoring  applications, fluorescence interference is a key challenge due to complex bio-matrices. The fluorescence  signal is often more intense than the Raman signal, so it conceals the Raman signal characteristics. It  also contributes noise without contributing useful information. However, if the Raman signal can be  detected over the fluorescence background, analyses can still be performed. Therefore, a Raman  spectrometer with sufficiently high sensitivity and measurement speed will mitigate these fluorescence  challenges. Tornado’s HTVS™ technology overcomes the key challenge associated with Raman  spectroscopy as it detects 10 to 30 times more Raman signal, enabling significantly faster measurements  or higher signal-to-noise ratio spectra than a conventional spectrometer. This advantage allows quicker  and more reliable decision-making in a process environment to circumvent excursions, therefore  eliminating (or at least minimizing) the disruptions that can occur because of those excursions. 

So how does one select the appropriate laser wavelength for a particular application? There are many  different excitation options, but the three most widely used are 532 nm, 785 nm and 1064 nm. 

532 nm lasers  

These lasers are commonly used for gas phase measurements and for other non-fluorescing samples. They are one of the oldest and most established type of diode pumped solid state lasers (DPSSLs). Excitation with a 532 nm laser is particularly good for studying metal oxides and inorganic materials as  low fluorescence is common for many of these sample types. Excitation with 532 nm yields a higher  Raman signal with 532 nm due to the details of the Raman scattering mechanism, and this higher  intrinsic signal strength is often needed when using a conventional spectrometer with an inefficient  physical slit. However, with Tornado’s High-Throughput Virtual Slit (HTVS™) you can get significant signal  boost without resorting to short laser wavelengths. 

1064 nm lasers 

Raman spectroscopy lasers with longer wavelengths are preferred for samples known to suffer from  high fluorescence background, as the amount of fluorescence drops with increasing wavelength and  decreasing photon energy. Diode lasers emitting at 1064 nm are often used for highly fluorescing  materials, although the Raman signal also drops with longer wavelengths, so 1064 nm Raman systems  require a longer amount of time to get adequate levels of Raman signal. Furthermore, detection of the  Raman-scattered photons from a 1064 nm laser require short-wave infrared sensors like InGaAs arrays,  as CCDs are effectively blind to wavelengths above 1100 nm, but InGaAs sensors are more expensive  and have greater intrinsic noise than CCD cameras. One other salient point is that higher laser powers 

are often needed to achieve adequate measurements with a 1064-nm laser, which could bring up safety  concerns in a production area if a Class IV laser is used. The higher energy density might also increase  the risk of chemical alteration of the sample even with transient exposure. 

785 nm lasers 

The 785 wavelength excitation is a great compromise delivering stronger Raman signal than 1064 nm  and weaker fluorescence than 532 nm. It is also one of the most popular and common wavelengths used  as it performs effectively for over 90% of active Raman materials with manageable fluorescence. Deep depletion back-thinned CCDs (like those used in Tornado Raman analyzers) have good detection  efficiency for both the fingerprint and CH-stretch regions of the Raman spectrum with 785 nm  excitation. A Raman measurement that is intended for quantification needs to be fast, accurate and  precise, encapsulating the best possible resolution in both wavelength and intensity. At Tornado, our  785 nm Raman analyzers offer the best balance between scattering efficiency, influence of fluorescence,  detector efficiency and availability of cost-efficient and compact, high-quality laser sources. 

Laser stability 

Much of the quantitative power of Raman methods comes from measuring changes in Raman spectral  data over time, assuming that this is due to chemical or physical changes in the sample under study.  However, a change in the Raman spectrum will also result if the excitation laser changes power or  wavelength during the measurement. Hence, it is pivotal to have a laser source which is highly stable in  both power output and wavelength, so that users have full confidence that any observed change in the  Raman peaks is due to true changes in the sample, not a side-effect of a misbehaving laser. Opinions  vary regarding the required level of stability for quality Raman measurements, and it certainly depends  on the details of the process application, but as a general rule of thumb, laser wavelength variations  should not cause Raman peak positions to vary by more than ±0.1 cm-1, and the laser power output  should not change by more than ±1%. A variety of data analysis techniques can be applied to  compensate for Raman excitation lasers which fluctuate in wavelength or power or both, but there is a  limit to the ability to compensate with chemometrics without introducing uncertainty. Inherently stable  lasers will usually provide better Raman data. 

How can you safely use a laser in a hazardous zone? 

Lasers can be hazardous, and must be used following best practices for eye safety and other potential  hazards. In some process scenarios, there may be explosive gases present in the atmosphere and all of  the laser energy focussed down onto a small area can serve as an ignition source.  

In the particular case of Raman spectroscopy, how do you ensure that the excitation laser energy does  not ignite the flammable gases in the atmosphere that surround the sample point? At Tornado, we very  much favour the regulatory category “<op is>” which means that the laser source is inherently safe, and 

thus easier and less expensive to install and maintain. As a result, the risk of an explosion is mitigated  due to the inherent safety of the laser.  

For customers looking for the ability to meet Zone 0 operating environments, Tornado’s OPIS 35 ATEX  Safe Laser provides unequalled Raman sensitivity, safety, and convenience even at low laser power  settings and ensures full compliance for ATEX Zone 0, 1, and 2 hazardous environments. 

For more information on hazardous zones and IECEx/ATEX guidelines, and Tornado’s turnkey solution, we encourage you to view our webinar, Inherently Safe Raman Measurements for Hazardous Zones

Conclusion  

Among the three standard wavelengths for Raman excitation lasers, the balance of Raman signal  strength and fluorescence reduction makes 785 nm the most versatile choice. When combined with the  unrivalled sensitivity of Tornado’s HTVS™-enhanced analyzers, 785 nm excitation is able to deliver the  best Raman spectral data for the vast majority of process applications. 

If you would like to discuss your application and are considering adopting Raman spectroscopy as a  process analytic technology, please contact us at info@tornado-spectral.com

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