FTNIR屬於吸收光譜的一種，根據光源與樣品之間的關係可以分為穿透與漫反射兩種，前者適用於液體樣品，後者則適用於粉末與顆粒樣品。
FTNIR與FTIR的差異
『N』代表Near，NIR就是近紅外波段光譜的意思，波長範圍從900至2500奈米(nm)之間，換算成波數(cm-1)之後範圍在4000cm-1至10000cm-1之間。與FTIR相比，一般稱為FTIR光譜，實際上屬於Middle-IR，也就是中紅外光譜，波數範圍在400cm-1到4000cm-1之間。
兩者從振動光譜(Vibrational Spectroscopy)的原理來定義時，FTIR屬於基頻(Fundamental Frequency)，而FTNIR屬於基頻的合頻(Combination)或倍頻(Overtone)，可以從下圖來了解。
FTNIR雖然屬於紅外光譜，但是可以克服FTIR對於含水率較高的樣品無法分析的問題，此外FTNIR也可以透過玻璃罐或者包裝袋等，直接對內容物進行分析。
FTNIR的光譜訊號來源主要是FTIR的基頻經過合頻或是倍頻之後的結果，因此只有C-H、N-H及O-H等官能基的能量足以達到近紅外光譜的範圍內。
FTNIR主要觀察的官能基
由官能基的分布，我們可以初步的對NIR光譜進行分類，依據官能基的屬性，我們可以大概知道樣品中的水分(O-H)、蛋白質(N-H)及油脂(C-H)等含量。
當我們進一步將NIR導入至產業中，從光譜訊號中得知，NIR可以做為水份及其他相對應的有機物質分析方法。
下面舉例幾個石化產業標準方法，其中有許多NIR光譜可以協助加快分析穩定生產效能的測試項目，例如水分、酸鹼、鹼價、黏度、密度，針對燃料的RON、MON，以及針對聚合物的末端官能基檢測的項目。
| 標準方法編號 | 方法名稱 | 分析目標 | |
|---|---|---|---|
| ASTM E1655 | Standard Practices for Infrared Multivariate Quantitative Analysis | NIR方法開發 | |
| ASTM D6122 | Standard Practice for Validation of the Performance of Multivariate Online, At-Line, and Laboratory Infrared Spectrophotometer Based Analyzer Systems | NIR方法驗證 | |
| ASTM D6299 | Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance | NIR儀器驗證 | |
| ASTM D6342 | Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy |
Hydroxyl number |
|
| ASTM D4274 | Standard Test Methods for Testing Polyurethane Raw Materials: Determination of Hydroxyl Numbers of Polyols |
Hydroxyl number |
|
| UOP326-17 | Diene Value by Maleic Anhydride Addition Reaction | Diene value | |
| ASTM D2699 | Standard Test Method for Research Octane Number of Spark-Ignition Engine Fuel | RON | |
| ASTM D2700 | Test Method for Motor Octane Number of Spark-Ignition Engine Fuel | MON | |
| ASTM D5769 | Standard Test Method for Determination of Benzene, Toluene, and Total Aromatics in Finished Gasolines by Gas Chromatography/Mass Spectrometry | Aromatic content | |
| ASTM D664 | Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration |
Acid value |
|
| ASTM D2896 | Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration |
Base number |
|
| ASTM D445 | Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids (and Calculation of Dynamic Viscosity) | Viscosity | |
| ASTM D6304 | Standard Test Method for Determination of Water in Petroleum Products, Lubricating Oils, and Additives by Coulometric Karl Fischer Titration | Moisture | |
| ASTM D2270 | Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100 °C | Viscosity | |
| ASTM D789 | Standard Test Method for Determination of Relative Viscosity of Concentrated Polyamide (PA) Solutions | Viscosity | |
| ASTM D6979 | Standard Test Method for Polyurethane Raw Materials: Determination of Basicity in Polyols, Expressed as Percent Nitrogen | amine value | |
| ASTM D7409 | Standard Test Method for Carboxyl End Group Content of Polyethylene Terephthalate (PET) Yarns | CEG | |
| ASTM 6869 | Standard Test Method for Coulometric and Volumetric Determination of Moisture in Plastics Using the Karl Fischer Reaction (the Reaction of Iodine with Water) | Moisture | |
| ASTM D7252 | Standard Test Method for Polyurethane Raw Materials: Determination of Monomer and Isomers in Isocyanates |
NCO content |
FTNIR的潛力與效益
這些品管依據的傳統分析方法，大多採濕式化學的方式，因此樣品都需要前處理，分析過程會消耗化學溶劑，分析完之後的樣品也屬於化學廢液，除了有機溶劑與酸鹼溶液的危害之外，還有相關的處理與時間成本。重點是部分的方法無法轉變成線上分析方法，這對於需要即時取得產線狀態的工廠，無疑是非常大的限制，反觀FTNIR可以透過光纖探頭，深入到產線中的每一個角落，提供24/7的數據，藉由觀察大量的數據變化，也許可以找出產能優化的關鍵。
