Technical Overview

Evaluation of Photometric Accuracy
in the Near Infrared Region Using
Metal‑on-Quartz Filters
Comparing Agilent Cary 7000 UV-Vis-NIR
spectrophotometer measurements against traceablereference material values

Authors

Abstract

Wesam Alwan and Travis Burt
Agilent Technologies, Inc.

UV-Vis-NIR spectroscopy technique provides quick and accurate characterizationof solid and liquid analytes in a wide range of applications. To achieve high-quality
measurements, photometric accuracy of the spectrophotometers can be verified
using certified reference materials. These measurements are usually performed in
the UV-Vis region, as required by global pharmacopeias. However, in this technicaloverview, photometric accuracy (absorbance) in the near infrared (NIR) region of theAgilent Cary 7000 universal measurement spectrophotometer (UMS) is evaluatedusing three metal-on-quartz filter reference materials.

The photometric accuracy of aspectrophotometer can be determinedby measuring certified referencematerials of known absorbance values
and concentration and comparing themeasured absorbance with the referencematerial absorbance values at eachspecific wavelength. Poor photometricaccuracy will produce incorrect results.
In transmission measurements,
the photometric response of thespectrophotometer follows theBeer‑Lambert law. According to thelaw (Equation 1), a linear relationshipexists between absorbance andsample concentration (as representedin Figure 1). The simple linear
relationship between absorbance andconcentration, and the relative ease ofmeasurement of ultraviolet-visible-near
infrared (UV-Vis-NIR) light have madeUV‑Vis‑NIR spectroscopy a fundamental
tool for many routine quantitative
analytical methods.
Equation 1.

A = εbc

The United States Pharmacopeia(USP) and the European Pharmacopeia(Ph. Eur.) guidelines describe how toverify that the analytical performance ofUV‑Vis spectrophotometers is suitable
for the intended operational rangeof the analysis. The guidelines relate
mainly to photometric accuracy in theUV‑Vis region. For measurements below
1 Abs and above 1 Abs, the absorbanceaccuracy must be ±0.01% and ±1% of theabsorbance measured, respectively.1,2ε = molar absorptivity (M–1cm–1)
b = pathlength (cm)Instrumentation
The Cary 7000 UMS UV-Vis-NIR
instrument was used to evaluate
photometric accuracy in the NIR region
using the operating parameters given in
Table 1.

0.6
0.4
0.2
0050

100150200

Concentration (mg/L)

Figure 1. The Beer-Lambert law states that a linear relationship exists between the absorbance of asolution and the concentration of that solution.

Table 1. Agilent Cary 7000 UMS operating parameters used to evaluate NIR region photometric accuracy.

c = concentration (M)2Experimental

0.8

Where:
A = absorbanceThis technical overview evaluates thephotometric (absorbance) accuracy inthe NIR region of the Cary 7000 universal
measurement spectrophotometer(UMS) using metal‑on-quartz filter
reference materials.

1.0AbsIntroduction

Parameter

Setting

Wavelength Range

1,000 to 3,000 nm

Signal Averaging Time

0.1 s

Data Interval

1 nm

Spectral Bandwidth

Auto (nm)Slit Height

Full

Number of Measurements (n)Reported results are the average of three successive measurements of the filter
reference materials

Samples

1%T (2 Abs), 3%T (1.5 Abs), and 50%T (0.3 Abs) metal on fused silica filter
reference materials

Solid filter reference materials (RMs)with different transmission properties
were used to evaluate the photometricaccuracy of the Cary 7000 in the NIR
region. A metal-on-quartz NIR reference
set, 1%T (~2 Abs), 3%T (~1.5 Abs), and50%T (~0.3 Abs), was bought from
Starna Scientific Ltd (product number
RM-NQ1NQ35NQ, Figure 2). The filter
RMs are traceable to NIST standard
reference material (SRM) 2031a (forthe UV-Vis wavelengths) and National
Research Council of Canada (NRC)
primary certified reference materials(CRMs) for the NIR wavelengths. Thefilters are orientated at 0 degrees to theincident beam in the spectrophotometerand are constructed with an opticalsandwich. The design minimizes thefringing that is often caused by internalreflection effects from metal-on-quartz
filters.3

Results and discussion
The NIR metal-on-quartz RMs weremeasured by the Cary 7000 using theoperating parameters described inTable 1. To assess photometric accuracy
in the NIR region, the absorbance atcertified wavelengths 1,100, 1,700, 2,210,2,500, and 2,800 nm of each filter wasdetermined. The measured values werecompared to the certified values, andthe percentage error was calculated
accordingly. As shown in Table 2, theerror between measured absorbanceand certified absorbance values was<1% for all filters and wavelength’s
measured, verifying the photometricaccuracy of the Cary 7000 for NIR
measurements. The data are also shownin Figure 3 (transmission) and Figure 4(absorbance). Good repeatability of thethree measurements of each filter RM
was also achieved by the Cary 7000.
Representative repeatability data for the2 Abs filter reference material is shownin Figure 5 (transmission) and Figure 6(absorbance).

Figure 2. Metal-on-quartz filters used for the NIR photometricaccuracy test of the Agilent Cary 7000 UMS. Photo reproduced withpermission from Starna Scientific Ltd.
Table 2. Photometric accuracy using three NIR metal-on-quartz filter reference materials. Air was used asa blank and the reference beam was unobstructed during measurements using the Agilent Cary 7000 UMS.
Wavelength(nm)

Certified Value(Abs)Expanded Uncertainty
Difference Betweenof the Filter
Cary 7000 ValueMeasured and(Abs)(Abs, n = 3)
Certified Values (Abs)

Percentage
Error (%)

Metal-on-Quartz Filter 0.3 Abs (50%T)
1,100

0.3688±0.0025

0.36940.0006

0.18941,700

0.3709±0.0025

0.37160.00070.21532,210

0.3676±0.0025

0.36800.00040.12652,500

0.3665±0.0025

0.36650.00000.00452,800

0.3786±0.0025

0.38110.00250.67041,100

1.5371±0.0058

1.53900.00190.12951,700

1.5068±0.0058

1.50860.00180.12162,210

1.5064±0.0058

1.50740.00100.06992,500

1.5124±0.0058

1.51270.00030.02372,800

1.5291±0.0058

1.53700.00790.5210

Metal-on-Quartz Filter 1.5 Abs (3%T)

Metal-on-Quartz Filter 2 Abs (1%T)
1,100

1.6564±0.0059

1.65910.00270.16381,700

1.6254±0.0059

1.62790.00250.15772,210

1.6274±0.0059

1.62840.00100.06172,500

1.6347±0.00591.63490.00020.01792,800

1.6521±0.0059

1.65760.00550.3347345Filter 1 (0.3 Abs)_R1
Filter 1 (0.3 Abs)_R2
Filter 1 (0.3 Abs)_R3
Filter 2 (1.5 Abs)_R1
Filter 2 (1.5 Abs)_R2
Filter 2 (1.5 Abs)_R3
Filter 3 (2.0 Abs)_R1
Filter 3 (2.0 Abs)_R2
Filter 3 (2.0 Abs)_R3

403530%T

25
20
15
10
50
1,000

1,500

2,000

2,500

3,000

Wavelength (nm)Figure 3. Spectra (n = 3) of the three metal-on-quartz filters measured using the Agilent Cary 7000
UV-Vis-NIR system in transmission. Absorbance values at 1,100, 1,700, 2,210, 2,500, and 2,800 nm of eachfilter were recorded and compared to the certified reference values.
1.8

Filter 1 (0.3 Abs)_R1
Filter 1 (0.3 Abs)_R2
Filter 1 (0.3 Abs)_R3
Filter 2 (1.5 Abs)_R1
Filter 2 (1.5 Abs)_R2
Filter 2 (1.5 Abs)_R3
Filter 3 (2.0 Abs)_R1
Filter 3 (2.0 Abs)_R2
Filter 3 (2.0 Abs)_R3

1.6
1.4Abs

1.2
1.0
0.8
0.6
0.4
1,000

1,500

2,000

2,500

3,000

Wavelength (nm)Figure 4. Spectra (n = 3) of the three metal-on-quartz filters measured using the Agilent Cary 7000UV-Vis-NIR system in absorbance. Absorbance values at 1,100, 1,700, 2,210, 2,500, and 2,800 nm of eachfilter were recorded and compared to the certified reference values.

4Conclusion

2.370

The photometric accuracy of theAgilent Cary 7000 UV-Vis-NIR
spectrophotometer was tested in
the NIR region by measuring three
metal-on-quartz filter reference materials
ranging from 0.3 to 2 Abs. Excellent
accuracy and repeatability results wereobtained for all wavelengths with anerror between measured absorbance andcertified absorbance values of <1% forall filters.

2.365
2.360

%T

2.355
2.350
2.345
2.340
2.335
2.330
1,400

1,600

1,800

2,000

2,200

2,400

Wavelength (nm)Figure 5. Zoomed in section of the 2 Abs filter measured on the Agilent Cary 7000 UV-Vis-NIR system intransmission, demonstrating repeatability (n = 3).

1.6290

References

1.6285

1. Pharmaceutical Analysis
Using UV‑Vis: Compliance with
USP Chapter, and European
Pharmacopoeia (Ph. Eur. Chapter
2.2.25), Agilent publication,
https://www.agilent.com/cs/
library/applications/application_
pharmacopeia_uv-vis_cary35005994-1188en_us_agilent.pdf

1.6280Abs

1.62751.62701.62651.62601.62551.62501,700

Testing the photometric accuracy andrepeatability of a spectrophotometer
across its operational absorbance range
using traceable reference materials
provides confidence in the quality of thefinal data.

1,750

1,800

1,850

1,900

1,950

2,000

Wavelength (nm)Figure 6. Zoomed in section of the 2 Abs filter measured on the Agilent Cary 7000 UV-Vis-NIR system inabsorbance, demonstrating repeatability (n = 3).

2. Absorbance Accuracy and Linear
Dynamic Range to 3 Abs andBeyond with the Agilent Cary 3500
UV-Vis Spectrophotometer, Agilent
publication, https://www.agilent.
com/cs/library/technicaloverviews/public/techoverview-linearityaccuracy-cary3500-uv-vis-59943931en-agilent.pdf
3. Metal On Quartz Filters for NIR
Photometric Accuracy, accessed
May 2022, https://www.starna.
com/nir-absorbance/metal-onquartzfilters-for-nir#descriptionanddiscussionwww.agilent.com
DE44836828
This information is subject to change without notice.
© Agilent Technologies, Inc. 2022
Printed in the USA, May 26, 2022
5994-4950EN