Review of standards of near infrared spectroscopy methods

Introduction

Near infrared spectroscopy operates in the electromagnetic wave spectrum between visible light and mid-infrared light, at wavelengths between ca. 780–2500 nm. Near infrared spectroscopy is comprised of acquiring the overtones and combinations of fundamental vibrational transitions in the mid-infrared region and analysed with appropriate chemometrics methods to achieve nondestructive, rapid, and simultaneous measurement of various properties of samples. There are many kinds of near infrared spectrometers, from laboratory instruments to online instruments, from specialised instruments to portable instruments, from large to small to micro. It has been widely used in various fields such as agriculture, petrochemicals, pharmaceuticals, and food and has brought great economic and social benefits. A number of near infrared spectroscopy standards have been developed by countries and international organizations, which reflects the degree of recognition and application of the method to a certain extent. This article reviews the standards of near infrared spectroscopy in China and globally, in order to collate the myriad of existing standards in one listing in journal where they can be easily viewed.

International and foreign standards

There are 47 international standards, American standards, European standards, German standards, Japanese standards, French standards, and Russian standards related to near infrared spectroscopy currently retrieved.^1–48 The main contents and scope of application of the standards can be consulted in supporting materials with the specific numeration shown in Figure 1.

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Figure 1.

International and foreign standards formulation.

As shown in Figure 1, there are 10 standards of the American Society of Materials and Testing (ASTM),^1–11 which cover a wide range of aspects, including standards for general methods, such as: standard practices for infrared multivariate quantitative analysis ¹ and standard practice for near infrared qualitative analysis ² . They also include standards for near infrared spectrometers, such as: standard practice for validation of the performance of multivariate online, at-line, and laboratory infrared spectrophotometer, and Raman spectrometer based analyzer systems, ³ standard practice for transfer standards for reflectance factor for near infrared instruments using hemispherical geometry, ⁴ standard guide for establishing spectrophotometer performance tests, ⁵ standard practice for describing and measuring performance of laboratory Fourier transform near infrared (FT-NIR) spectrometers, ⁶ and standard practice for describing and measuring performance of near infrared spectrophotometers. ⁷ Some specific application methods are also included, such as: standard practice for polyurethane raw materials: determining hydroxyl number of polyols by near infrared (NIR) spectroscopy ⁸ .

There are eight standards of the International Organization for Standardization (ISO),^12–19 among which seven are jointly compiled and implemented by ISO and CEN (the European Commission for Standardization),^12–18 and two are TS (technical specifications) standards.^15,16 There are two guide standards: animal feeding stuffs, cereals, and milled cereal products-guidelines for the application of near infrared spectrometry, ¹² and milk and milk products—guidelines for the application of near infrared spectrometry. ¹⁹ In addition, it also includes the following hardware standards: standards of optical components for near infrared spectral ranges and standards of application methods, such as: leather-determination of surface reflectance, ¹³ soil quality-determination of carbon and nitrogen, ¹⁴ characterization of single-wall carbon nanotube,^15,16 and determination of hydroxyl number in polyols ¹⁷ .

There are eight standards of the American Society of Cereal Chemists (AACC),^20–27 in addition to one general rule standard: near infrared methods—guidelines for model development and maintenance ²⁰ the rest are all standards for the determination of grain components, including wheat hardness, ²¹ protein content in whole-grain wheat, ²² protein content in small grains, ²³ protein content in wheat flour, ²⁴ ash content of flour, ²⁵ protein or oil content in soybeans, ²⁶ and protein, oil, or moisture content in whole-grain soybeans ²⁷ . The International Society of Cereal Chemists (ICC) also issued two standards for determination of protein and a procedure for spectral analysis of ground wheat and milled wheat products.^28,29 In addition, there is a British standard which adopts the European standard (BS EN) for cereals-determination of moisture and protein-method in whole kernels. ³⁰ In terms of grain analysis, about 90% of the world’s wheat trade is based on determination of protein content in whole grain by near infrared spectroscopy. ³¹

There are six standards of the Russioan GOST,^32–37 concerning the quality determination of oil, feed, and marine products, including “Vegetable oils. Determination of quality and safety by near infrared spectrometry,” ³² “Compound feeds, feed raw materials. Method for determination of crude ash, calcium and phosphorus content by means of NIR spectroscopy,” ³³ “Fish, marine products and products of them. Method of determination the fraction of total mass of protein, fat, water, phosphorus, calcium and ash by the near-infrared spectrometry,” ³⁴ “Fodder, mixed and animal feed raw stuff. Spectroscopy in near infrared region method for determination of crude protein, crude fiber, crude fat and moisture,” ³⁵ “Fodder and mixed fodder. Spectroscopy in near infrared region method for determination of metabolizable energy,” ³⁶ and “Oil-cake and ground oil-cake. Determination of moisture, oil and protein by infrared reflectance”. ³⁷

The American Association of Analytical Chemists (AOAC) promulgated and implemented four standards,^38–41 including the determination methods of piperazine in drugs, ³⁸ fiber (acid detergent) and protein (crude) in forages, ³⁹ moisture in forage, ⁴⁰ and protein (crude) in wheat whole grain. ⁴¹

There are three Japanese Industrial Standards (JIS),^42–44 including two electrical and electronic standards of code C, respectively: test methods ⁴² and calibration methods of optical spectrum analyzers. ⁴³ It also includes one chemical standard of code K: general rules for near-infrared spectrophotometric analysis. ⁴⁴

There are two standards of NF^45,46 and one of DIN standard ⁴⁷ which are the following: content determination of vinyl groups in basic silicones for industrial use, ⁴⁵ determination of ratios phenyl/silicium and phenyl/methyl in basic silicones for industrial use, ⁴⁶ and paints, varnishes, and their raw materials-analysis by near infrared spectrometry-general working principles. ⁴⁷ The International Electrotechnical Commission (IEC) has formulated the standard for medical electrical near infrared spectroscopy equipment: particular requirements for the basic safety and essential performance of functional near infrared spectroscopy (NIRS) equipment. ⁴⁸

In addition, the USP, EP, CPMP & CVMP, EMA, RIVM, and FDA have all provided corresponding guidelines for the application of near infrared spectroscopy in the pharmaceutical industry.

Chinese standards

There are 79 national standards, industry standards, local standards, group standards, and normative documents for near infrared spectroscopy in China.^49–127 The main contents and scope of application of the standards can be consulted in supporting materials. Among them, there are 27 national standards,^49–75 involving general rules of methods, terminology, detection methods of equipment, and specific indexes. Among them, one modification adopts ISO standard, ⁴⁹ one non-equivalent adopts ISO standard, ⁵⁰ and two non-equivalent reference ASTM standard,^50,52 one equivalent adopts ASTM standard, ⁵¹ and three modifications adopt AACC standard.^53–55 There are twelve inspection standards of grain and oils in 25 national standards,^53–64 which are comparable to international and foreign standards.

In addition to national standards, several industry sectors have also formulated a large number of industrial standards for industrial applications. The industries involved include the textile industry, the public security industry, the supply and marketing industry, the chemical industry, the machinery industry, the forestry industry, the agricultural industry, the light industry, the electronic industry, and the commodity inspection industry. The details are shown in Figure 2.

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Figure 2.

The formulation of industry standards.

It can be clearly seen from Figure 2 that near infrared spectroscopy is most widely used in the agricultural industry. At present, eleven corresponding industrial standards^76–86 have been formulated, including feed, apple, milk, peanut, meat, vegetable oil, rapeseed, and so on. There are six commercial inspection industry standards,^87–92 which are the six parts of the near infrared spectroscopy methods for quantitative analysis of fiber of textiles for import and export. In addition, the textile, public safety, supply and marketing, and forestry sectors have two standards each^93–100 and the electronic, chemical, machinery, and light industries have one each.^101–104

With the development of near infrared spectroscopy technology, many local governments have begun to pay attention to the application of this technology. The local, regional or provincial governments of Tianjin, Inner Mongolia Autonomous Region, Liaoning Province, Jilin Province, Jiangsu Province, Anhui Province, Jiangxi Province, Shandong Province, Hunan Province, and Yunnan Province have formulated corresponding local standards,^105–122 as shown in Figure 3. Some local standards and national standards have made provisions in similar aspects, but they are not exactly the same. For example, the local standard “DB21/T 2048-2012 determination of crude protein, crude fat, crude fiber, moisture, calcium, phosphorus, crude ash, water-soluble chlorides, amino in feeds—near infrared reflectance spectroscopy” ¹⁰⁸ has a wider range of applications compared to the national standard “GB/T 18868-2002 method for determination of moisture, crude protein, crude fat, crude fiber, lysine, and methinione in feeds—near infrared reflectance spectroscopy”. ⁶⁶ DB21/T 2048-2012 is applicable to the determination of crude protein, crude fat, crude fiber, moisture, calcium, total phosphorus, crude ash, and water-soluble chloride in compound feed, concentrated feedstuff, supplemental concentrate, feed raw materials, and the determination of amino acids in feed raw materials, while GB/T 18868-2002 is applicable to the determination of moisture, crude protein, crude fiber, and crude fat in various feed raw materials and compound feed and the determination of lysine and methionine in various plant protein feed raw materials. In addition, they all cite some conventional methods which have been stipulated in national standards, such as “GB/T6432-2018 determination of crude protein in feeds—Kjeldahl method,” ¹²³ “GB/T 6433-2006 determination of crude fat in feeds,” ¹²⁴ “GB/T 6434-2006 feeding stuffs—determination of crude fiber content—method with intermediate filtration,” ¹²⁵ “GB/T 6435-1986 method for the moisture in determination of feedstuffs,” ¹²⁶ and “GB/T 18246-2019 determination of amino acids in feeds,” ¹²⁷ and so on. At the same time, the local standards reflect the near infrared spectroscopy of local characteristic products, such as cashmere in Inner Mongolia Autonomous Region, ginseng in Jilin Province, wine and tea in Anhui Province, tobacco in Yunnan Province, amongst others.

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Figure 3.

The formulation of local standards in China.

According to the “The Plan for Furthering the Standardization Reforms” issued by the State Council (Guo Fa [2015] No. 13), the reform measures pointed out that standards formulated under the guidance of the government include mandatory national standards and recommended national standards, recommended industrial standards, and recommended local standards. These standards focus on basic protection, while standards formulated independently by the market, including group standards and enterprise standards, focus on improving competitiveness. China formulated group standards “T/AHFIA 008–-2018 rapid determination method of physical and chemical indicators for brewing Daqu–near infrared spectroscopy” ¹²⁸ and “T/CIS 11001-2020 online determination of powder mixing uniformity in production process of traditional Chinese medicine” ¹²⁹ in 2018 and 2020, respectively.

In addition to the above standards, China also has pharmaceutical industry standards: “Chinese Pharmacopoeia 9104 Guiding principle of near infrared spectrophotometry” ¹³⁰ and two national metrological verification regulations: “JJG 178-2007 Verification Regulation of Ultraviolet, Visible, Near Infrared Spectrophotometers” ¹³¹ and “JJG 001-1996 Verification regulation for Fourier transform infrared spectrometer”. ¹³²

Development of standards

The earliest standard for near infrared spectroscopy was the near infrared method of piperazine in drugs formulated by AOAC. ³⁸ The standard was formulated in 1966 and was revised in 1971. In addition to this standard, the other standards were formulated after 1986. Near infrared spectroscopy has been explored and studied since 1938. In the late 1950s, Norris et al. determined the contents of moisture and protein in agricultural products through six-wavelength narrow-band interference filter-near infrared spectroscopy, and registered American patents in 1973, beginning a productive increase of near infrared spectroscopy applications. With the development of applied research, a series of standards have been formulated since 1986 (see Figure 4).

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Figure 4.

The development of standards internationally and within China.

It can be seen from Figure 4 that the formulation of relevant standards in China is lags behind the formulation of international and foreign standards, but the speed of the development is rapid, especially in the past decade. The development of industry standards and local standards has been particularly rapid since 2012, which is consistent with the trend that the application of near infrared spectroscopy is gradually moving toward subdivision applications.^133–135

There are many types of near infrared spectrometers, which have incomparable advantages over other technologies in terms of cost, performance, and application scenarios. In particular the development of small and micro instruments and the application of online near infrared spectroscopy in the process industry, the development of near infrared spectroscopy will be promoted to a greater extent. The application research and practical application of near infrared spectroscopy have achieved fruitful results. However, with the further development of practical applications and the development of technology and equipment, NIR spectroscopy will be more widely recognized and will further promote the formulation of relevant standards.