Near-infrared spectroscopy (NIRS): The perfect solution to monitor purity of recovered solvents

8 sie 2022

Artykuł

Solvent recovery is the process of extracting useful solvents and raw materials from waste or byproduct solvents that are generated during manufacturing processes. The solvents that are used in these situations are often not disposed of or incinerated, but instead recovered and purified, as this saves considerable costs. Used solvents are mostly purified by means of distillation. Solvent recovery processes are very common in the chemical industry as well as the pharmaceutical industry during the manufacture of APIs (active pharmaceutical ingredients).

Organic solvents

Organic solvents are highly lipophilic – able to dissolve oils, fats, resins, rubber, and even plastics. They are used for many applications such as paints, coatings, adhesives, and detergents. Additionally, they are used to produce cosmetics, agrochemical products, polymers, and rubbers, just to name a few. Despite environmental concerns and potential health hazards, organic solvents (e.g., hydrocarbon, chlorinated, oxygenated, as well as nitrogen- and sulfur-containing) are still widely used because of their unparalleled performance.

When using organic solvents, the most frequently found impurity happens to be the most common solvent – water. The presence of moisture interferes with many reactions, which is why the determination of the water content is crucial.

Major benefits from recovering solvents [1]

Reduced Operating Expenses:

Significantly reduced purchases of expensive replacement solvents
Reduced cost of hazardous waste disposal
Reduced inventory requirements of expensive solvents

Improved Environmental Impact:

Green Approach — recovery and recycle of solvents means preserving and restoring valuable resources vs. disposal and/or disintegration of solvent mixtures.
Removal of solvents from aqueous wastes often accompanies customer’s objectives, thereby purifying waste waters in the process.

Quality Assurance:

Self-recovery in dedicated equipment assures you of on-spec material with no foreign substances.

Supply Chain Assurance and Operations Continuity:

When solvents are not delivered on time or are unavailable due to supply shortages, strikes or supplier outages, the pharmaceutical company who recovers their solvents can continue to manufacture product, with no interruptions.

Near-infrared spectroscopy—the ideal tool to monitor the purity of (and impurities in) recovered solvents

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control of solvent recovery processes for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, an available solution to monitor the purity of methylene chloride solvent along with two major impurities (methanol and water) is outlined, developed according the NIRS implementation guidelines of ASTM E1655.

Read our previous blog posts to learn more about NIRS as a secondary technique.

What is NIR spectroscopy

NIR vs IR: What is the difference?

How to implement NIR spectroscopy in your laboratory workflow

NIR spectroscopy pre-calibrations: Immediate results

Monitoring the purity of (and impurities in) a recovered solvent with the DS2500 Liquid Analyzer

In this application example, samples of methylene chloride (or dichloromethane, CH₂Cl₂) solvent were obtained from the output of a solvent recovery distillation unit. The samples covered a range of typical purity levels as well as methanol and water impurities in the distilled solvent. Samples were scanned in 4 mm disposable glass vials by using the Metrohm DS2500 Liquid Analyzer (Figure 1).

To obtain the reference values, the samples were analyzed by gas chromatography (GC) for methanol and by Karl Fischer titration for water immediately after scanning to avoid any changes in the samples over time. The sample temperature was not controlled and varied with the ambient conditions in the laboratory for all of the NIRS measurements. The NIR analysis was successful due to a combination of stable NIR measurements with the DS2500 Liquid Analyzer and the Partial Least-Squares (PLS) modelling capabilities in the Vision Air Complete software package.

Learn more about the Metrohm DS2500 Liquid Analyzer and Vision Air Complete software here!

Metrohm DS2500 Liquid Analyzer

Vision Air 2.0 Complete

The NIRS results are obtained very rapidly, with no sample preparation required prior to scanning. This makes it possible to monitor and control the process, which was not feasible when only using the primary methods. Measurement with NIRS does not require highly trained analysts – only disposable glass vials are needed for the analysis!

Parameter	Reference method	NIRS Application Note	NIRS benefits
Impurities (water and methanol) Purity (CH₂Cl₂)	KF titration / GC	AN-NIR-021	Water, methanol, and CH₂Cl₂ are measured simultaneously within one minute without needing chemical reagents or sample preparation.

Figures 2–5 show the results of the Application Note mentioned in Table 1. The correlation plots for water (moisture, Figure 3) and methanol (MeOH, Figure 4) show that both models are robust. Furthermore, the correlation coefficient (R²) is close to 1 for both models and the Standard Error of Prediction (SEP) is in line with the Standard Error of Calibration (SEC).

A calibration model was developed as well for the overall CH₂Cl₂ purity (Figure 5). There were several other impurities within the samples aside from moisture and methanol, and all usable spectral regions were used to model the solvent bands as well as the bands of all impurities (Figure 2). The reference values were calculated from the GC results. The SEP value was very similar to the SEC value, indicating a good predictive accuracy comparable to the accuracy of the GC determination.

Summary

NIR spectroscopy is excellently suited for the analysis of various impurities in solvents as well as solvent purity itself based on the example application shown here with methylene chloride. In comparison to the primary methods (gas chromatography and Karl Fischer titration), the time to result is a major advantage of using NIRS—a single measurement is complete within one minute instead of one to two hours with GC or KFT.

There are several benefits for utilizing NIR spectroscopy as an alternative technology including the aforementioned short time to result. Additionally, no chemicals or any other expensive equipment are needed and NIRS is so easy to use that even shift workers can perform these analyses with minimal training.