The saying «cleanliness is next to godliness» has been around for centuries. The implication is that the act of keeping our clothing, living spaces, belongings, and of course our own bodies clean elevates us above other animals, and by extension, our own baser instincts. Aside from this, it is well-known that diseases flourish in unclean conditions, and it is in our best interest as a society to keep things clean. Therefore, cleaning agents (e.g., detergents and sanitizers) are constants in our daily lives.
Many cleaning agents come in contact with our skin and are generally disposed of by means of rinsing or drying. Accordingly, they must be tested for safety reasons and to ensure the key ingredients are present in the correct quantities for quality control purposes. This blog article explains the uses of detergents and hand sanitizers as well as how to perform quality control of these products with near-infrared spectroscopy (NIRS).
What are detergents?
Often, the words detergent and soap are used interchangeably even though these are two different products. Detergents are defined as «any of numerous synthetic water-soluble or liquid organic preparations that are chemically different from soaps but are able to emulsify oils, hold dirt in suspension, and act as wetting agents» . Soaps differ in that they are prepared using an alkali and a fat component to form a sodium (or potassium) salt of the fatty acid.
As the definition states, detergents are cleaning agents which emulsify oils and dirt. Several kinds of products are available on the market for various purposes: liquid detergent, powder, and even concentrated single-use pods. These detergents are not generally all-purpose, but can be specialized e.g., for dishwashing, laundry, and even as high efficiency concoctions for tough stains and odors.
What are detergents made of?
Detergent formulations can be quite complex. They are mainly comprised of various surfactants (ionic, non-ionic, and amphoteric), enzymes, preservatives, chelating agents, bleaching agents, as well as dyes and fragrances. Not all of these ingredients are found in every detergent, but a surfactant is a key requirement for the emulsification of oils, dirt, and other stain- and odor-causing compounds. Enzymes are also necessary for breaking up difficult compounds, like those found in pet urine. The combination of these different compounds results in clean surfaces and other materials.
Hypoallergenic detergents are a specialized product for people with sensitive skin. These are usually unscented (fragrance-free) formulations which also do not contain dyes. The active ingredients remain similar, but perfumes and other compounds are eliminated to limit the risk of producing hives or rashes. Hypoallergenic detergents must be thoroughly tested before being marketed as such. Those with sensitive skin should consider using specially formulated laundry detergent as clothing is in close contact with the skin for many hours at a time. Clean laundry does not need to come with the risk of skin irritation.
What is hand sanitizer?
Hand sanitizer, also known as handrub, hand disinfectant, or hand antiseptic, has become a staple in nearly every household since the COVID-19 pandemic began in early 2020. Hand sanitizer is a liquid (usually a gel, foam, or spray) designed to dry rapidly after application, thereby eliminating the need for soap, water, and towels. These products are applied and rubbed on hands to prevent the spread of harmful germs by inactivating pathogenic microorganisms such as viruses and bacteria. Using hand sanitizer will not remove harmful chemicals or kill certain resistant germs, so it should not be used as a complete replacement for hand washing.
What ingredients are in hand sanitizer?
Depending on the active ingredient used, hand sanitizers can be classified as one of two types: those based on alcohol, or alcohol-free. Alcohol-based hand sanitizers typically contain between 60–95% alcohol as the active ingredient, usually in the form of ethanol (ethyl alcohol), isopropanol (isopropyl alcohol), or n-propanol . At such concentrations, alcohol immediately denatures proteins, effectively neutralizing certain types of microorganisms. Glycerol, other skin moisturizers, and a small amount of water make up the inactive ingredients. The World Health Organization recommends a similar composition of ingredients . Alcohol-based hand sanitizers have emerged as an important tool in the fight against COVID-19. The convenience and portability of hand sanitizers helped lead to their widespread usage beginning in 2020.
Non-alcohol-based hand sanitizers contain the active ingredient benzalkonium chloride, usually around a concentration of 0.1%. Benzalkonium chloride is an antimicrobial agent, proven to protect against bacteria, viruses, and fungi . Both alcohol-based and alcohol-free hand sanitizers can also have hydrogen peroxide (up to 3% concentration), dyes, and fragrances.
Near-infrared spectroscopy (NIRS)—the ideal tool to assess the quality of detergents and hand sanitizers
Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control of personal care and cosmetics products 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, available solutions for the quality control of detergents and hand sanitizer are shown developed according the NIRS implementation guidelines of ASTM E1655: Standard Practices for Infrared Multivariate Quantitative Analysis. These practices cover a guide for the multivariate calibration of infrared spectrometers used in determining the physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 to 400 cm-1).
For more detailed information about NIRS as a secondary technique, please read our previous blog posts.
Benefits of NIR spectroscopy: Part 1
Benefits of NIR spectroscopy: Part 2
Benefits of NIR spectroscopy: Part 3
Which parameters and applications are generally possible for NIRS analysis?
Typically, key parameters for the quality control of detergents and hand sanitizer such as alcohol content, surfactants, TAED (tetraacetylethylenediamine, a bleach activator), and enzymes are determined by chemical and physical methods in the laboratory. Not only do these methods incur high running costs, they are also time-consuming. In contrast, near-infrared spectroscopy (NIRS) requires neither chemicals nor sample preparation, which saves on related costs for consumables and labor. NIRS is also easy enough to be used by non-chemists and provides results in less than a minute. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc atline analysis.
In the following sections, key parameters for the quality control of liquid laundry detergent and alcohol-containing hand disinfection products are determined with the Metrohm NIRS DS2500 Liquid Analyzer (Figure 1). The obtained Vis-NIR spectra were used to create prediction models for surfactant and ethanol content, respectively. The quality of the prediction models was evaluated using a correlation diagram which displays the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis.
Determination of surfactant content in liquid laundry detergent with NIRS
Laundry detergents contain fabric softeners, bleaching agents, surfactants, as well as enzymes. Out of these, the surfactant is the most important factor for the cleaning effect, as it breaks down the interface between polar and nonpolar compounds. This allows the detergent to be effective against oils and other greases as well as stains from soil or drinks.
Quantification of surfactant content in liquid laundry detergent is most commonly performed by two-phase potentiometric titration. Disadvantages of this method include manual sample preparation steps (e.g., dilution and pH adjustment) and the determination itself is time-consuming. In contrast, Vis-NIR spectroscopy delivers high quality results in less than one minute and does not require any sample preparation or chemicals. Spectra from an assortment of laundry detergent samples are shown in Figure 2 with the correlation diagram and respective figures of merit for the prediction of surfactant content given in Figure 3.
|Figures of merit||Value|
|Standard error of calibration||2.20 mmol/100 g|
|Standard error of cross-validation||2.38 mmol/100 g|
NIRS determination of ethanol content in alcohol-based hand sanitizer
In 2020, the demand for hand sanitizer skyrocketed due to the spread of COVID-19. Many companies shifted gears, streamlining their operations to produce hand sanitizer in large volumes. As in any product manufacturing process, accurate formulation enables good quality and reduces waste. Reagents used in these solutions include water, alcohol (commonly ethanol or isopropanol), small amounts of emollient (skin softener, e.g., glycerol), and an oxidizing agent (e.g., hydrogen peroxide) to minimize microbial contamination. The alcohol content in hand sanitizers must be greater than 60% (v/v) to be an effective antiseptic and must therefore be measured for quality control purposes.
NIRS analysis of sanitizers is an easy process – just add a small aliquot of sample to a disposable vial (Figure 4), begin the analysis, and spectra are acquired in seconds (Figure 5).
The correlation diagram and the respective figures of merit for the prediction of ethanol content via NIR spectroscopy are shown in Figure 6.
|Figures of merit||Value|
|Standard error of calibration||0.41 v/v%|
|Standard error of cross-validation||0.56 v/v%|
Detergents are used in so many situations to clean our cookware, to remove tough stains from clothing, and/or to eliminate odors. Quality control of these products is necessary to ensure their safety and effectiveness for consumers.
Hand sanitizer spray, gel, or foam is a great way to stay hygienic when soap and water are not an easy option. However, the power of disinfection is dependent on the concentration of active ingredients (whether alcohol-containing or alcohol-free formulations with benzalkonium chloride).
The application examples presented in this article demonstrate that near-infrared spectroscopy is excellently suited for the analysis of multiple constituents in detergents and hand sanitizer. Results are provided in less than one minute without the need for sample preparation. Chemical reagents are not required for NIRS analysis, meaning less waste is produced, leading to lower costs per analysis.
 Definition of DETERGENT. https://www.merriam-webster.com/dictionary/detergent (accessed 2023-03-14).
 Saha, T.; Khadka, P.; Das, S. C. Alcohol-Based Hand Sanitizer – Composition, Proper Use and Precautions. Germs 2021, 11 (3), 408–417. DOI:10.18683/germs.2021.1278
 World Health Organization. Guide to Local Production: WHO-Recommended Handrub Formulations, 2010. https://www.who.int/publications/i/item/WHO-IER-PSP-2010.5
 Kovač, B.; Piletić, K.; Kovačević Ganić, N.; et al. The Effectiveness of Benzalkonium Chloride as an Active Compound on Selected Foodborne Pathogens Biofilm. Hygiene 2022, 2 (4), 226–235. DOI:10.3390/hygiene2040020
Your knowledge take-aways
On-demand webinar: Faster & Easier: Quality Control of Personal Care Products by Near-Infrared Spectroscopy
Application Note: Surfactant in laundry detergent by Vis-NIR spectroscopy – Fast determination without using chemicals
Application Note: Quality Control of Hand Sanitizers – Fast and reagent-free ethanol content determination
Application Note: Quality Control of Hand Sanitizers – Multiparameter determination within one minute