Abstract

Compositional variations in the surfactants used to produce personal care products result in significant challenges during large scale manufacturing, for example errors in product viscosity. Characterisation of the surfactant can be completed using chromatographic techniques however these are time consuming and impractical during real-time manufacturing. Near infrared (NIR) absorption spectroscopy with a fibre-optic coupled transmission probe is proposed as an in-line method of determining the levels of unsulfated alcohol in sodium lauryl ether sulfate (SLES). NIR absorption spectra in the region of 4000 – 12000 cm−1 were collected for a range of supplier samples at three temperatures. Gas chromatography - mass spectrometry was used as a reference technique to quantify samples of SLES and quantitative chemometric data analysis was used to produce partial least squares (PLS) calibration models for the prediction of surfactant composition. PLS regression was performed on the data in the spectral regions between 7509 – 5334 cm−1 using a range of data pre-processing techniques to identify the best model. Models were evaluated using root mean square error of cross validation (RMSECV) and residual predictive deviation (RPD) as the primary indicator of model accuracy and robustness. A partial least squares regression model using a generalised least squares weighting data pre-processing approach was found to be the most robust in regards to sample non-homogeneity and temperature, producing a model with an RMSECV = 0.094 w/w% and RPD = 4.03. The model successfully predicted the unsulfated alcohol mass percentage in an external validation of unknown samples with alcohol levels within the model limits of 0.7–2.2 w/w%. Spectra acquired at a resolution of 8 cm−1 with 32 scans take just 16 seconds to obtain, proving that NIR spectroscopy can successfully be applied as an alternative analytical method to gas chromatography for the determination of low level impurities in viscous surfactant systems.

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  1. Torres OrtegaJA. Sulfonation/sulfation processing technology for anionic surfactant manufacture. In: Z Nawaz (Ed.) Advances in chemical engineering. InTech, Shanghai, China, 2012, pp.269–294. ISBN: 978-953-51-0392-9.
  2. DW. RobertsSulfonation technology for anionic surfactant manufacture. Org Process Res Dev 1998; 2: 194–202.
  3. EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.
  4. W. Herman De GrootSulfonation technology in the detergent industry. London: Kluwer Academic Publishers, 1991.
  5. M. SittigDetergent manufacture. New Jersey: Noyes Data Corporation, 1976.
  6. NC. FosterSulfonation and sulfation processes. Chemithon 1997; 1–36.
  7. G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.
  8. PatriatiAPutraESeok SeongB. Effect of a long chain carboxylate acid on sodium dodecyl sulfate micelle structure: a SANS study. In: AIP conference proceedings, 2010. American Institute of Physics, New York, pp.40–43.
  9. A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.
  10. JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.
  11. K. MoleverMonitoring the linear alkylbenzene sulfonation process using high-temperature gas chromatography. J Surfact Deterg 2005; 8: 199–202.
  12. C Bengoechea and A. FernándezAnalysis of minor products in linear alkylbenzene sulfonation. Comparison between the traditional gravimetric method and high performance liquid chromatography (HPLC) procedures versus high temperature gas chromatography (HT-GC). J Surfact Deterg 2008; 11: 103–109.
  13. BC. SmithFundamentals of Fourier transform infrared spectroscopy. 2nd ed. Boca Raton: CRC Press, 2011, 182 p.
  14. D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.
  15. M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.
  16. PL Walling and JM. DabneyProcess control of shampoo with near-infrared reflectance spectroscopy. J Cosmet Sci 1988; 39: 191–199.
  17. PL Walling and JM. DabneyApplication of near infrared reflectance spectroscopy to the quality assurance of surfactants. J Cosmet Sci 1986; 37: 445–459.
  18. L. BokobzaNear infrared spectroscopy. J Near Infrared Spectrosc 1998; 6: 3–18.
  19. S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.
  20. P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.
  21. T. FearnAssessing calibrations: SEP, RPD, RER and R2.NIR News 2002; 13: 12–13.
  22. PC Williams and DC. SoberingComparison of commercial near infrared transmittance and reflectance instruments for analysis of whole grains and seeds. J Near Infrared Spectrosc 1993; 1: 25–32.
  23. F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.
  24. F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.
  25. WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.
  26. E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.
  27. K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.
  28. H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.
  29. MillerCE. Chemometrics in process analytical technology (PAT). In: KA Bakeev (Ed.) Process analytical technology: spectroscopic tools and implementation strategies for the chemical and pharmaceutical industries. John Wiley & Sons, London, 2nd ed. 2010. pp.353–438.
  30. Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

2019 (1)

K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.

2017 (1)

P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.

2014 (1)

D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.

2008 (2)

M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.

C Bengoechea and A. FernándezAnalysis of minor products in linear alkylbenzene sulfonation. Comparison between the traditional gravimetric method and high performance liquid chromatography (HPLC) procedures versus high temperature gas chromatography (HT-GC). J Surfact Deterg 2008; 11: 103–109.

2007 (1)

Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

2005 (2)

E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.

K. MoleverMonitoring the linear alkylbenzene sulfonation process using high-temperature gas chromatography. J Surfact Deterg 2005; 8: 199–202.

2002 (1)

T. FearnAssessing calibrations: SEP, RPD, RER and R2.NIR News 2002; 13: 12–13.

2001 (1)

S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.

2000 (3)

H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.

F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.

WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.

1998 (5)

F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.

L. BokobzaNear infrared spectroscopy. J Near Infrared Spectrosc 1998; 6: 3–18.

DW. RobertsSulfonation technology for anionic surfactant manufacture. Org Process Res Dev 1998; 2: 194–202.

G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.

A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.

1997 (1)

NC. FosterSulfonation and sulfation processes. Chemithon 1997; 1–36.

1993 (1)

PC Williams and DC. SoberingComparison of commercial near infrared transmittance and reflectance instruments for analysis of whole grains and seeds. J Near Infrared Spectrosc 1993; 1: 25–32.

1988 (1)

PL Walling and JM. DabneyProcess control of shampoo with near-infrared reflectance spectroscopy. J Cosmet Sci 1988; 39: 191–199.

1986 (1)

PL Walling and JM. DabneyApplication of near infrared reflectance spectroscopy to the quality assurance of surfactants. J Cosmet Sci 1986; 37: 445–459.

1966 (1)

JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.

1958 (1)

EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.

Alcalá, M

M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.

Arafeh, A

K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.

Axelberd, T

A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.

Barr, EA.

JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.

Bautista, M.

M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.

Bengoechea, C

C Bengoechea and A. FernándezAnalysis of minor products in linear alkylbenzene sulfonation. Comparison between the traditional gravimetric method and high performance liquid chromatography (HPLC) procedures versus high temperature gas chromatography (HT-GC). J Surfact Deterg 2008; 11: 103–109.

Blanco, M

M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.

Bokobza, L.

L. BokobzaNear infrared spectroscopy. J Near Infrared Spectrosc 1998; 6: 3–18.

Carlson, EJ

EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.

Crull, G

D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.

Dabney, JM.

PL Walling and JM. DabneyProcess control of shampoo with near-infrared reflectance spectroscopy. J Cosmet Sci 1988; 39: 191–199.

PL Walling and JM. DabneyApplication of near infrared reflectance spectroscopy to the quality assurance of surfactants. J Cosmet Sci 1986; 37: 445–459.

Dardenne, P

P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.

De Noord, OE

F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.

H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.

Eriksson, L.

S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.

Fearn, T.

T. FearnAssessing calibrations: SEP, RPD, RER and R2.NIR News 2002; 13: 12–13.

Fernández, A.

C Bengoechea and A. FernándezAnalysis of minor products in linear alkylbenzene sulfonation. Comparison between the traditional gravimetric method and high performance liquid chromatography (HPLC) procedures versus high temperature gas chromatography (HT-GC). J Surfact Deterg 2008; 11: 103–109.

Flinn, P.

P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.

Flint, G

EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.

Førland, G

G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.

Foster, NC.

NC. FosterSulfonation and sulfation processes. Chemithon 1997; 1–36.

Gilbert, EE

EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.

Gjerde, M

G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.

Hansen, WG

WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.

Haroon, K

K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.

Herman De Groot, W.

W. Herman De GrootSulfonation technology in the detergent industry. London: Kluwer Academic Publishers, 1991.

Ibarra, E

E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.

Kays, SE.

Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

Kim, Y

Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

Kok, WT

F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.

F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.

Martin, P

K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.

Miller, CE.

MillerCE. Chemometrics in process analytical technology (PAT). In: KA Bakeev (Ed.) Process analytical technology: spectroscopic tools and implementation strategies for the chemical and pharmaceutical industries. John Wiley & Sons, London, 2nd ed. 2010. pp.353–438.

Molever, K.

K. MoleverMonitoring the linear alkylbenzene sulfonation process using high-temperature gas chromatography. J Surfact Deterg 2005; 8: 199–202.

Pan, D

D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.

Patist, A

A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.

Patriati, A

PatriatiAPutraESeok SeongB. Effect of a long chain carboxylate acid on sodium dodecyl sulfate micelle structure: a SANS study. In: AIP conference proceedings, 2010. American Institute of Physics, New York, pp.40–43.

Pérez, H.

E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.

Putra, E

PatriatiAPutraESeok SeongB. Effect of a long chain carboxylate acid on sodium dodecyl sulfate micelle structure: a SANS study. In: AIP conference proceedings, 2010. American Institute of Physics, New York, pp.40–43.

Roberts, DW.

DW. RobertsSulfonation technology for anionic surfactant manufacture. Org Process Res Dev 1998; 2: 194–202.

Samseth, J

G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.

Seok Seong, B.

PatriatiAPutraESeok SeongB. Effect of a long chain carboxylate acid on sodium dodecyl sulfate micelle structure: a SANS study. In: AIP conference proceedings, 2010. American Institute of Physics, New York, pp.40–43.

Shah, DO.

A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.

Singh, M

Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

Sittig, M.

M. SittigDetergent manufacture. New Jersey: Noyes Data Corporation, 1976.

Sjostrom, M

S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.

Smilde, AK.

F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.

Smith, BC.

BC. SmithFundamentals of Fourier transform infrared spectroscopy. 2nd ed. Boca Raton: CRC Press, 2011, 182 p.

Snieder, M

WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.

Sobering, DC.

PC Williams and DC. SoberingComparison of commercial near infrared transmittance and reflectance instruments for analysis of whole grains and seeds. J Near Infrared Spectrosc 1993; 1: 25–32.

Stirton, AJ

JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.

Swierenga, H

H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.

Torres Ortega, JA.

Torres OrtegaJA. Sulfonation/sulfation processing technology for anionic surfactant manufacture. In: Z Nawaz (Ed.) Advances in chemical engineering. InTech, Shanghai, China, 2012, pp.269–294. ISBN: 978-953-51-0392-9.

Valencia, O

E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.

Walling, PL

PL Walling and JM. DabneyProcess control of shampoo with near-infrared reflectance spectroscopy. J Cosmet Sci 1988; 39: 191–199.

PL Walling and JM. DabneyApplication of near infrared reflectance spectroscopy to the quality assurance of surfactants. J Cosmet Sci 1986; 37: 445–459.

Weil, JK

JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.

Wiedemann, SCC

WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.

Williams, P

P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.

Williams, PC

PC Williams and DC. SoberingComparison of commercial near infrared transmittance and reflectance instruments for analysis of whole grains and seeds. J Near Infrared Spectrosc 1993; 1: 25–32.

Wold, S

S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.

Wülfert, F

F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.

H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.

F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.

Yin, S

D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.

Anal Chem (1)

F Wülfert, WT Kok, and AK. SmildeInfluence of temperature on vibrational spectra and consequences for the predictive ability of multivariate models. Anal Chem 1998; 70: 1761–1767.

Anal Chim Acta (1)

H Swierenga, F Wülfert, and OE De Noord, et al. Development of robust calibration models in near infra-red spectrometric applications. Anal Chim Acta 2000; 411: 121–135.

Chemithon (1)

NC. FosterSulfonation and sulfation processes. Chemithon 1997; 1–36.

Chemom Intell Lab Syst (2)

F Wülfert, WT Kok, and OE De Noord, et al. Linear techniques to correct for temperature-induced spectral variation in multivariate calibration. Chemom Intell Lab Syst 2000; 51: 189–200.

S Wold, M Sjostrom, and L. ErikssonPLS-regression: a basic tool of chemometrics. Chemom Intell Lab Syst 2001; 58: 109–130.

Eur J Pharm Sci (1)

M Blanco, M Alcalá, and M. BautistaPharmaceutical gel analysis by NIR spectroscopy. Determination of the active principle and low concentration of preservatives. Eur J Pharm Sci 2008; 33: 409–414.

Food Chem (1)

Y Kim, M Singh, and SE. KaysNear-infrared spectroscopic analysis of macronutrients and energy in homogenized meals. Food Chem 2007; 105: 1248–1255.

Ind Eng Chem (1)

EJ Carlson, G Flint, and EE Gilbert, et al. Sulfonation with sulfur trioxide. Ind Eng Chem 1958; 50: 276–284.

Int J Cosmet Sci (1)

K Haroon, A Arafeh, and P Martin, et al. Use of in‐line near infrared spectroscopy to predict the viscosity of shampoo using multivariate analysis. Int J Cosmet Sci 2019; 41: 346–356.

J Am Oil Chem Soc (1)

JK Weil, AJ Stirton, and EA. BarrOxypropylation of fatty alcohols, and the sulfation products. J Am Oil Chem Soc 1966; 43: 157–160.

J Colloid Interface Sci (2)

G Førland, J Samseth, and M Gjerde, et al. Influence of alcohol on the behavior of sodium dodecylsulfate micelles. J Colloid Interface Sci 1998; 203: 328–334.

A Patist, T Axelberd, and DO. ShahEffect of long chain alcohols on micellar relaxation time and foaming properties of sodium dodecyl sulfate solutions. J Colloid Interface Sci 1998; 208: 259–265.

J Cosmet Sci (2)

PL Walling and JM. DabneyProcess control of shampoo with near-infrared reflectance spectroscopy. J Cosmet Sci 1988; 39: 191–199.

PL Walling and JM. DabneyApplication of near infrared reflectance spectroscopy to the quality assurance of surfactants. J Cosmet Sci 1986; 37: 445–459.

J Near Infrared Spectrosc (5)

L. BokobzaNear infrared spectroscopy. J Near Infrared Spectrosc 1998; 6: 3–18.

PC Williams and DC. SoberingComparison of commercial near infrared transmittance and reflectance instruments for analysis of whole grains and seeds. J Near Infrared Spectrosc 1993; 1: 25–32.

P Williams, P Dardenne, and P. FlinnTutorial: items to be included in a report on a near infrared spectroscopy project. J Near Infrared Spectrosc 2017; 25: 85–90.

WG Hansen, SCC Wiedemann, and M Snieder, et al. Tolerance of near infrared calibrations to temperature variations: a practical evaluation. J Near Infrared Spectrosc 2000; 8: 125–132.

E Ibarra, O Valencia, and H. PérezAnalysis of betaines using near infrared spectroscopy. J Near Infrared Spectrosc 2005; 13: 133–138.

J Pharm Biomed Anal (1)

D Pan, G Crull, and S Yin, et al. Low level drug product API form analysis – Avalide tablet NIR quantitative method development and robustness challenges. J Pharm Biomed Anal 2014; 89: 268–275.

J Surfact Deterg (2)

K. MoleverMonitoring the linear alkylbenzene sulfonation process using high-temperature gas chromatography. J Surfact Deterg 2005; 8: 199–202.

C Bengoechea and A. FernándezAnalysis of minor products in linear alkylbenzene sulfonation. Comparison between the traditional gravimetric method and high performance liquid chromatography (HPLC) procedures versus high temperature gas chromatography (HT-GC). J Surfact Deterg 2008; 11: 103–109.

NIR News (1)

T. FearnAssessing calibrations: SEP, RPD, RER and R2.NIR News 2002; 13: 12–13.

Org Process Res Dev (1)

DW. RobertsSulfonation technology for anionic surfactant manufacture. Org Process Res Dev 1998; 2: 194–202.

Other (6)

Torres OrtegaJA. Sulfonation/sulfation processing technology for anionic surfactant manufacture. In: Z Nawaz (Ed.) Advances in chemical engineering. InTech, Shanghai, China, 2012, pp.269–294. ISBN: 978-953-51-0392-9.

W. Herman De GrootSulfonation technology in the detergent industry. London: Kluwer Academic Publishers, 1991.

M. SittigDetergent manufacture. New Jersey: Noyes Data Corporation, 1976.

BC. SmithFundamentals of Fourier transform infrared spectroscopy. 2nd ed. Boca Raton: CRC Press, 2011, 182 p.

PatriatiAPutraESeok SeongB. Effect of a long chain carboxylate acid on sodium dodecyl sulfate micelle structure: a SANS study. In: AIP conference proceedings, 2010. American Institute of Physics, New York, pp.40–43.

MillerCE. Chemometrics in process analytical technology (PAT). In: KA Bakeev (Ed.) Process analytical technology: spectroscopic tools and implementation strategies for the chemical and pharmaceutical industries. John Wiley & Sons, London, 2nd ed. 2010. pp.353–438.

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