People who are unfamiliar with near-infrared (NIR) spectroscopy frequently ask the question: «Why should I need to know more about this technique, and how can I benefit from it?»
In this first installment of in this series of posts, we focus on the main advantages of NIRS over conventional wet chemical analysis methods and will provide examples of the types of parameters that can be measured with NIR spectroscopy.
Solid vs. liquid samples
In order to understand the benefits of NIRS, a good starting point is to understand how the NIR spectrum is measured. NIR spectroscopy can be used to analyze different types of samples. However, different instrumentation is required depending on the sample type. Several measurement methods are available for samples ranging from clear liquids to opaque pastes and powders. Choosing the right measurement method, sampling module, and accessories is the most important step to developing robust NIR methods. Below, the different methods are shown for various sample types (diffuse reflection, diffuse transmission, transflection, and transmission).
Diffuse reflection: Cream, paste, granulates, coarse and fine powders
NIR light penetrates into and interacts with the sample, and the unabsorbed NIR energy reflects back to the detector. This method is most suitable to measure solid samples without sample preparation.
Diffuse transmission: Tablets and capsules
As with diffuse reflection, the NIR light penetrates into and interacts with the sample. This light is scattered throughout the sample due to interaction with the particles. The unabsorbed NIR light is transmitted through the sample prior to reaching the detector. This method is most suitable to measure solid dosage forms without sample preparation.
Transflection: Liquids and gels
This measurement method is a combination between transmission and reflection. A reflector is placed behind the sample, used to reflect the unabsorbed NIR light back to the detector. This method is most suitable to measure liquid samples.
Here, the sample is placed between the NIR light source and the detector. NIR light is transmitted through the sample, and any unabsorbed NIR energy continues to the detector. This method is most suitable to measure clear liquid solutions or suspensions.
Solid sample measurement
Solid samples (e.g., powders) must be placed on the window as shown here, secured within an appropriate container or vial. The instrument lid needs to be closed prior to starting the analysis so external light does not affect the results.
The NIR radiation comes from below, and is partially reflected by the sample to the detector, which is also located below the sample vessel plane. After 45 seconds, the measurement is completed and a result is displayed. As this reflected light contains all the relevant sample information, this measurement technique is called diffuse reflection.
Liquid sample measurement
As the image illustrates, for liquid analyses via NIRS, a vial or cuvette must be placed in the drawer of the instrument. After pressing start, the drawer closes automatically and a result is obtained after 45 seconds.
In this case, the NIR radiation travels through the solution before reaching the detector. This measurement technique is known as transmission.
Advantages of NIRS
The procedure for obtaining the NIR spectrum already indicates two main advantages of NIRS: simplicity regarding sample measurement and speed. These and other advantages of NIR are listed here:
- Fast technique with results in less than 1 minute.
- No sample preparation required – solids and liquids can be used in pure form.
- Low cost per sample – no chemicals or solvents needed.
- Environmentally-friendly technique – no waste generated.
- Non-destructive – precious samples can be reused after analysis.
- Easy to operate – inexperienced users are immediately successful.
How to quantify with NIRS
NIRS is a secondary technique, which means a prediction model will need to be created first. You can compare this, for example, to HPLC. If you want to identify or quantify a substance with that technique, you would need to prepare standard solutions of the substance and measure them to create a calibration curve.
This is similar with NIRS: first you need to measure a number of spectra with known concentrations or known parameter values gathered from a primary method such as titration. A prediction model is then created out of these spectra using chemometric software (e.g., the Metrohm Vision software). We will explain in more detail how prediction models are created in another installment of this series.
Application versatility in all industries
NIRS is a versatile technique and can be used for various applications, both for chemical and physical parameters. You can find many different application examples for NIRS in the Metrohm Application Finder. Here, we have listed representative examples for some industry segments.
- Polymers: Density of Polyethylene (PE); Melt Flow Rate; Intrinsic Viscosity
- Chemical: Hydroxyl number of polyols
- Petrochemical: Research Octane Number (RON) of gasoline; cetane index for diesel
- Oils and Lubricants: Total Acid Number (TAN)
- Pharma: Water content of lyophilized products; content uniformity in tablets
- Personal care: Moisture content and active ingredients in creams
Overall, near-infrared spectroscopy is a robust alternative technique for the determination of both chemical and physical parameters in solids and liquids. It is a fast method which can also be successfully implemented for routine analysis by staff without any laboratory education.
In the next installment we answer another frequently asked question: «Is near-infrared the same as infrared spectroscopy?». Click here to go directly to the next post in the series!