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Using Reference Materials, Part 2: Aligning the y-Axis
CHAPTER
USING REFERENCE MATERIALS, PART 2: ALIGNING THE Y-AXIS
125
CHAPTER OUTLINE Introduction to Photometric Accuracy .................................................................................................. 981 Example of Reporting Photometric Accuracy .......................................................................... 982 Photometric Correction for Absorbance-Based Spectrophotometers ....................................................... 983 Ultraviolet Photometric Standards ....................................................................................................... 984 Visible (Vis) Photometric Standards .................................................................................................... 987 Near Infrared Reflectance Photometric Standards ................................................................................ 987 Infrared Reflectance Photometric Standards ........................................................................................ 993 Raman Intensity Correction Standards ................................................................................................. 993 References ........................................................................................................................................ 999
INTRODUCTION TO PHOTOMETRIC ACCURACY Photometric accuracy is a formal term used to describe how well a measuring device (spectrophotometer or illuminance meter) is able to determine the total energy flux transmitted through (or reflected from) a Standard Reference Material (SRM)® [1]. The various references to NIST documents and SRMs are given throughout this chapter. Note that equations are described as referenced in Refs. [2,3]. Volume 14.01 of the ASTM International document [4,5] describes the measurement of the photometric accuracy for an instrument as follows (Sections 20.1 and 20.3): Select the appropriate Standard Reference Material and obtain ten successive readings of the apparent absorbance or transmittance at the specified wavelength. Average the ten readings. The photometric accuracy is the difference between the true absorbance or transmittance value and the average observed value.
It is further stated that the following apply to the reporting of photometric accuracy. Report the photometric accuracy in the following order: reference material, wavelength, true absorbance or transmittance, observed absorbance or transmittance plus or minus the standard deviation.
Chemometrics in Spectroscopy. https://doi.org/10.1016/B978-0-12-805309-6.00125-2 # 2018 Elsevier Inc. All rights reserved.
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
EXAMPLE OF REPORTING PHOTOMETRIC ACCURACY As an example the stated specifications for photometric accuracy for a test instrument might be written as: Photometric Accuracy 0.002 Au (for range of 0.0–0.5 Absorbance units) 0.004 Au (for range of 0.5–1.0 Absorbance units) 0.3%T SRM: Measured with NIST 930D filters Here is an evaluation of these specifications in greater detail. A photometric accuracy of “0.002 Au at 0 to 0.5 Au” indicates that after the instrument has been set to zero, a standard sample which transmits between 30.1%T (i.e., 0.5 Au) and 100%T (i.e., 0.0 Au) measures within 0.002 Au as compared to the specified value for that reference standard material at any wavelength position. The tolerance for each of the 930d filters is stated by NIST as 0.5% relative. Thus, a 10%T filter (which corresponds to 0.1 T or 1.0 Au) neutral density 930D-type reference standard would be expected to be within the range of 9.95–10.05%T. These values are equivalent to 1.0022–0.9978 Au. The difference in Au is that 1.0022 Au minus 0.9978 Au equals 0.0044 Au. The standard tolerance in Au is 1.0000 0.0022 Au, rounded to 1.000 0.002 Au. Conservatively for transmittance we state 10.00 0.03%T, and related to 0.0 Au or 100%T, we specify 100.0 0.3%T [19] (Table 125-1). If the stated tolerance for the NIST 930D filter is 0.002 Au at 1.0 Au, we are stating that our maximum deviation in photometric accuracy for our instrument at 1.0Au is also 0.002 Au. For the worst case measurement for the NIST filter, we would have the filter error of 0.002 added to the instrument error of 0.002, to result in a total maximum variation 0.004 Au error. (This is our stated specification for the example given.) And one can readily calculate the reflectance of transmittance of any measurement given the absorbance of that measurement as follows. For conversion of absorbance to transmittance we use Eqs. (125-1)–(125-4): A ¼ log 10
I I0 ¼ log 10 I I0
(125-1)
Table 125-1 Relationship of %T and Absorbance Unit (Au) Values T (as ppm) 10,000,000 1,000,000 301,000 100,000 10,000 1000 100 10 1
%T 1000 100 30.1 10.0 1 0.1 0.01 0.001 0.0001
T 10 1 0.301 0.1 0.01 0.001 0.0001 0.00001 0.000001
1/T
Au 1
0.1 ¼ 10 1.0 ¼ 100 3.01 ¼ 100.5 10 ¼ 101 100 ¼ 102 1000 ¼ 103 10,000 ¼ 104 100,000 ¼ 105 1,000,000 ¼ 106
1.0 0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
ABSORBANCE-BASED SPECTROPHOTOMETERS
983
I I0 1 ¼ 10A ) ¼ A I 10 I0
(125-2)
I ¼ 10A ¼ T I0
(125-3)
%T ¼ T 100
(125-4)
To convert absorbance to reflectance we use Eqs. (125-5)–(125-8): I I0 ¼ log 10 A ¼ log 10 I I0
(125-5)
I I0 1 ¼ 10A ) ¼ A I 10 I0
(125-6)
I ¼ 10A ¼ R I0
(125-7)
%R ¼ R 100
(125-8)
PHOTOMETRIC CORRECTION FOR ABSORBANCE-BASED SPECTROPHOTOMETERS For making photometric calibrations using known reference materials, the ratioed replicate spectral data are retained, along with the replicate spectra of a reference sample and dark background or dark sample. Eq. (125-9) represents Beer’s law for taking measurements using a spectrophotometer .To convert the light reflected or transmitted from a sample (I) ratioed to the incident energy (I0) to absorbance (A), as a linearized estimate of the spectral response as related to analyte concentration, the following Eqs. (125-9)–(125-13) may be applied. If only an internal reference is used for spectral collection, then the absorbance spectrum with respect to wavelength is computed as Eq. (125-9): A ¼ log 10
I I0 RI DRI ¼ log 10 ¼ log 10 I S DS I0
(125-9)
If an external standard material is used to calibrate an automated internal reference material, then the absorbance spectrum with respect to wavelength is computed as Eqs. (125-10) and (125-11): A ¼ log 10
I I0 RI + RΔ DRI ¼ log 10 ¼ log 10 I S DS I0
(125-10)
where RΔ ¼ ðRE DRE Þ=ðRI DRI Þ
(125-11)
where for Eqs. (125-9)–(125-11), S is the sample measurement; DS is the dark measurement for the sample; RI is the internal reference measurement; DR1 is the dark measurement for the internal
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
reference sample; RE is the measurement of the external standard reference material for calibration; DRE is the dark measurement for the external standard reference material; and RΔ is the ratio between the dark corrected RE and RI. The final spectrum is simplified to the ratio of internal reference corrected by external reference minus dark divided by sample—dark. Note that dark is a measurement where no energy from the source is allowed to the detector (it is blocked) or where a totally dark sample or light trap is measured. The dark measurement represents the dark current changes in the instrument during a measurement that relate to electronic noise where the detection electronics, computational electronics, and control electronics perform basic measurement functions with no imputed energy reaching the detector (i.e., the source is turned off or blocked using a shutter closure during the dark measurement). When a comprehensive method of photometric correction includes a light trap measurement to determine the dark response to correct for optical and window scatter, we use Eq. (125-12): 2 6ðRI DRI Þ 6 A ¼ log 10 6 6 4
!3 ðRE DRE Þ ðIW DW Þ 7 R0I D0RI 7 7 7 S DS 5
(125-12)
where the variable terms used include: R0I which is the photometric correction calibration measurement of the internal reference sample, D0RI which is the photometric correction calibration measurement of the dark signal intensity for the internal reference sample, IW is the intensity measurement of the light trap to determine the window scatter, and DW is the measurement of the dark signal intensity for the light trap measurement. Eq. (125-12) is simplified for understanding by removing the dark correction terms as Eq. (125-13): 2 6 A ¼ log 10 6 4
RI
3 RE IW 7 R0I 7 5 S
(125-13)
ULTRAVIOLET PHOTOMETRIC STANDARDS There are several available standards used for calibrating or verifying photometric alignment when using UV spectrophotometers, with potassium dichromate demonstrated. The UV region is generally considered to cover from 190 to 380 nm (nonvacuum UV region). The photometric values for UV standards are illustrated in Figs. 125-1 and 125-2, and the reference absorbance values are given in the accompanying Table 125-2. Reference spectra and data provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
985
ULTRAVIOLET PHOTOMETRIC STANDARDS
3.0000 200 mg/L 180 mg/L
2.5000
160 mg/L
Absorbance (A)
2.0000
140 mg/L 120 mg/L
1.5000
100 mg/L 80 mg/L
1.0000
60 mg/L 40 mg/L
0.5000
20 mg/L
0.0000 220.0
240.0
260.0
280.0
300.0
320.0
340.0
Wavelengths (nm)
FIG. 125-1 Potassium dichromate absorbance and linearity standard traceable to SRM 935a. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
4.00 3.50
Absorbance (A)
3.00 2.50
257 nm
2.00
235 nm
1.50
350 nm
1.00
313 nm
0.50
g/ L
g/ L
m 0 24
22
0
m
m
g/ L
g/ L 18 0
18
0
m
m
g/ L
L g/ 16 0
m
14 0
m
m
g/ L
g/ L 12 0
L g/
10 0
m 80
m
g/ L
g/ L 60
m 40
20
m
g/ L
0.00
Dichromate concentrations
FIG. 125-2 Potassium dichromate absorbance linearity as a photometric standard using five concentrations of standard. This is a test for photometric linearity throughout the ultraviolet region. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK. www.starna.com.
986
Wavelength (nm) 350.0 313.0 257.0 235.0
20 mg/L
40 mg/L
60 mg/L
80 mg/L
100 mg/L
120 mg/L
140 mg/L
160 mg/L
180 mg/L
200 mg/L
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
0.2104 0.0948 0.2807 0.2404
0.4137 0.1858 0.5550 0.4766
0.6289 0.2834 0.8469 0.7280
0.8497 0.3827 1.1468 0.9860
1.0591 0.4764 1.4303 1.2271
1.2713 0.5718 1.7177 1.4738
1.4834 0.6673 2.0051 1.7205
1.6956 0.7627 2.2925 1.9672
1.9078 0.8581 2.5799 2.2139
2.1199 0.9535 2.8673 2.4605
Values shown are the absorbance values for each measured wavelength at the designated concentration of potassium dichromate. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
CHAPTER 125 USING REFERENCE MATERIALS, PART 2
Table 125-2 Data Table for Potassium Dichromate Absorbance and Linearity Standard Traceable to SRM 935a
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
987
VISIBLE (VIS) PHOTOMETRIC STANDARDS The visible spectral region is generally considered from 380 to 750 nm. Two types of available standards are shown, namely NIST SRM 930d and Russian Opal Glass, which are used for calibrating or verifying photometric alignment of visible spectrophotometers. Photometric values for these standards are illustrated in Figs. 125-3 and 125-4, and the reference absorbance values are given in the accompanying Tables 125-3 and 125-4.
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS The NIR spectral region is generally considered to be from 780 to 2500 nm. Two types of available reflectance standards are shown, namely R50 (50% reflectance) and sintered Fluorilon (PTFE) (R99 or 99% reflectance), which are used for calibrating or verifying photometric alignment of NIR spectrophotometers. Photometric values for these standards are illustrated in Figs. 125-5 and 125-6, and the reference reflectance values are given in the accompanying Tables 125-5 and 125-6. Materials noted when provided by courtesy by Avian Technologies New London, NH. Note that in transmittance measurement mode various solvents or dry air may be used to calibrate or verify the transmittance accuracy. Transmittance of SRM-930d set #1255 0.4500 0.4000 0.3500
Filter 30-1255
Transmittance
0.3000 0.2500 Filter 20-1255
0.2000 0.1500
Filter 10-1255 0.1000 0.0500 0.0000 350
400
450
500
550
600
650
700
750
Wavelength (nm)
FIG. 125-3 Transmittance of SRM-930d standard set photometric standards; Filter 10-1225, Filter 20-1255, and Filter 30-1255. Data and spectrum used with permission from Avian Technologies, New London, NH.
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
8 degrees/hemispherical reflectance factor Russian Opal Glass ROG-02c Glossy
8 degrees/hemi. refl. factor
1.000
0.900
0.800
0.700
0.600
0.500 360
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720
740
760
780
Wavelength (nm)
FIG. 125-4 Russian Opal Glass Glossy reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry. Mean reflectance over the spectral region shown is 0.967. Measured photometric data is given in Table 125-4. Data and spectrum used with permission from Avian Technologies, New London, NH.
Table 125-3 Sit and Dwell Measurements for SRM930d at Different Concentrations (1 nm Bandpass) Sit and Dwell Measurements 1 nm Bandpass Transmittance at wavelength Sample 10-1225 20-1225 30-1225
440.0 nm 0.0948 0.1871 0.3070
465.0 nm 0.1106 0.2084 0.3411
546.1 nm 0.1034 0.1989 0.3319
590.0 nm 0.0920 0.1833 0.3085
635.0 nm 0.1007 0.1953 0.3166
465.0 nm 0.9562 0.6811 0.4671
546.1 nm 0.9855 0.7014 0.4790
590.0 nm 1.0362 0.7368 0.5107
635.0 nm 0.9970 0.7093 0.4996
Absorbance at wavelength Sample 10-1225 20-1225 30-1225
440.0 nm 1.0232 0.7279 0.5129
Averages of 16 measurements:
All nominally 0.0002 T
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
Table 125-4 Russian Opal Glass Glossy Reflectance Values Wavelength (nm)
Glossy Reflectance Factor
360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780
0.882 0.903 0.929 0.943 0.954 0.960 0.966 0.968 0.970 0.973 0.975 0.976 0.978 0.978 0.978 0.979 0.979 0.979 0.979 0.978 0.977 0.976 0.975 0.975 0.974 0.974 0.972 0.973 0.972 0.973 0.972 0.972 0.972 0.971 0.970 0.970 0.969 0.969 0.968 0.968 0.968 0.965 0.964
989
FIG. 125-5 Fluorilon and carbon black mixture as a 50% reflectance standard. Spectrum used with permission from Avian Technologies, New London, NH.
FIG. 125-6 Fluorilon as a 99% reflectance standard. Spectrum used with permission from Avian Technologies, New London, NH.
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
Table 125-5 R50 Fluorilon Reflection Data from NIST Traceable Instrument Wavelength (nm)
R50 Refl.
Wavelength (nm)
R50 Refl.
800 825 850 875 900 925 950 975 1000 1025 1050 1075 1100 1125 1150 1175 1200 1225 1250 1275 1300 1325 1350 1375 1400 1425 1450 1475 1500 1525 1550 1575 1600
0.464 0.463 0.461 0.460 0.461 0.461 0.462 0.461 0.459 0.460 0.458 0.458 0.457 0.457 0.456 0.456 0.454 0.454 0.453 0.452 0.452 0.450 0.450 0.449 0.448 0.448 0.449 0.448 0.447 0.447 0.447 0.446 0.447
1625 1650 1675 1700 1725 1750 1775 1800 1825 1850 1875 1900 1925 1950 1975 2000 2025 2050 2075 2100 2125 2150 2175 2200 2225 2250 2275 2300 2325 2350 2375 2400 2425 2450 2475 2500
0.446 0.445 0.444 0.444 0.444 0.443 0.443 0.442 0.442 0.441 0.442 0.439 0.439 0.440 0.439 0.441 0.441 0.441 0.441 0.441 0.442 0.438 0.437 0.437 0.434 0.434 0.436 0.436 0.436 0.436 0.436 0.438 0.438 0.436 0.437 0.435
991
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
Table 125-6 R99 Fluorilon Reflection Data from NIST Traceable Instrument (Three Samples) Wavelength (nm)
Calibrated (#1) R99 Refl.
Checked (#2) R99 Refl.
Checked (#3) R99 Refl.
1000 1025 1050 1075 1100 1125 1150 1175 1200 1225 1250 1275 1300 1325 1350 1375 1400 1425 1450 1475 1500 1525 1550 1575 1600 1625 1650 1675 1700 1725 1750 1775 1800 1825 1850 1875 1900 1925
0.981 0.980 0.982 0.982 0.980 0.980 0.978 0.980 0.979 0.980 0.979 0.979 0.978 0.978 0.977 0.976 0.975 0.978 0.977 0.977 0.976 0.976 0.977 0.977 0.977 0.975 0.975 0.975 0.974 0.974 0.972 0.974 0.975 0.972 0.971 0.969 0.967 0.967
0.981 0.983 0.981 0.982 0.983 0.981 0.984 0.980 0.981 0.981 0.980 0.981 0.981 0.978 0.979 0.979 0.979 0.979 0.978 0.980 0.978 0.978 0.978 0.979 0.979 0.977 0.977 0.976 0.975 0.975 0.976 0.976 0.976 0.971 0.969 0.971 0.970 0.969
0.981 0.980 0.982 0.981 0.981 0.980 0.980 0.982 0.980 0.981 0.979 0.980 0.980 0.979 0.978 0.978 0.977 0.979 0.978 0.977 0.978 0.977 0.977 0.977 0.978 0.976 0.977 0.975 0.976 0.976 0.973 0.976 0.976 0.973 0.971 0.970 0.969 0.968
RAMAN INTENSITY CORRECTION STANDARDS
993
Table 125-6 R99 Fluorilon Reflection Data from NIST Traceable Instrument (Three Samples)—cont’d Wavelength (nm)
Calibrated (#1) R99 Refl.
Checked (#2) R99 Refl.
Checked (#3) R99 Refl.
1950 1975 2000 2025 2050 2075 2100 2125 2150 2175 2200 2225 2250 2275 2300 2325 2350 2375 2400 2425 2450 2475 2500
0.967 0.964 0.962 0.957 0.948 0.945 0.941 0.934 0.935 0.946 0.954 0.954 0.955 0.954 0.948 0.943 0.937 0.934 0.929 0.933 0.933 0.930 0.939
0.968 0.966 0.965 0.959 0.954 0.949 0.948 0.940 0.940 0.951 0.958 0.960 0.959 0.954 0.950 0.943 0.940 0.943 0.935 0.933 0.938 0.936 0.931
0.968 0.966 0.964 0.961 0.950 0.949 0.944 0.938 0.939 0.948 0.956 0.957 0.958 0.956 0.952 0.946 0.940 0.939 0.935 0.935 0.934 0.932 0.937
INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
The IR spectral region is generally considered to cover from 2500 to 25,000 nm (i.e., 4000–400 cm1). Diffuse gold is a primary surface material used for calibrating or verifying reflectance photometric alignment of IR spectrophotometers. Photometric values for this standard are illustrated in Figs. 125-7 and 125-8, and the reference reflectance values are given in the accompanying Table 125-7. Materials noted when provided by courtesy by Avian Technologies New London, NH. Note that in transmittance measurement mode dry, purged air may be used to calibrate or verify the transmittance accuracy.
RAMAN INTENSITY CORRECTION STANDARDS The Raman spectral region is generally considered to include from 2500 to 25,000 nm (i.e., 4000–400 cm1), with the Raman spectral region referred to as the Stokes shift being from 2857 to 200,000 nm or from 3500 to 50 cm1. A variety of materials are used for calibrating or verifying
994
CHAPTER 125 USING REFERENCE MATERIALS, PART 2
1.000
8 degrees/hemi. refl. factor
0.950
0.900
0.850
0.800
0.750 2.000
4.000
6.000
8.000
10.000
12.000
14.000
16.000
18.000
20.000
22.000
Wavelength (nm)
FIG. 125-7 Diffuse gold photometric reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry with a calibrated FT-IR instrument. Mean reflectance over the spectral wavelength region (in μm) shown is 0.941. Measured photometric data is given in Table 125-7. Data and spectrum used with permission from Avian Technologies, New London, NH.
the intensity scale for Raman spectrometers. Materials for this purpose are listed in Table 125-8. Materials noted are provided by NIST. The NIST previously known as the National Bureau of Standards (NBS) provides materials designed as reference sources to verify the performance characteristics of Raman spectrometers of any design. The main standards are currently used for intensity standardization. Table 125-8 lists the current materials available for Raman intensity calibration. The materials are termed SRMs®. In the case of Raman there are five active NIST SRMs for measuring Raman intensity, depending upon the excitation laser wavelength used [6–11]. The Raman intensity standards are certified SRMs useful for the correction of the relative intensity of Raman spectra obtained with instruments employing specific laser excitation sources. Individual intensity standards have been provided for Raman excitation wavelengths of 488, 532, 633, 785, 830, and 1064 nm. For these standards, the relative intensity of the glass luminescence has been
RAMAN INTENSITY CORRECTION STANDARDS
995
0.980
0.960
8 degrees/hemi reflectance factor
0.940
0.920
0.900
0.880
0.860
0.840
0.820
0.800 5000
4500
4000
3500
3000
2500
Wavenumbers
2000
1500
1000
500
(cm–1)
FIG. 125-8 Diffuse gold photometric reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry with a calibrated FT-IR instrument. Mean reflectance over the spectral wavelength region (in μm) shown is 0.941. Measured photometric data is given in Table 125-7. Data and spectrum used with permission from Avian Technologies, New London, NH.
calibrated at NIST using a uniform white-light source, with an integrating sphere collection design. The shape of the mean luminescence spectrum of the SRM glass is described using a polynomial expression for relative spectral intensity versus Raman shift wavenumber (cm1) based on the specific excitation laser wavelength used (in nm). The spectral correction for any Raman spectrometer is determined by measuring the luminescence spectrum of the SRM, applying the polynomial model, computing the spectral intensity-response correction for any Raman instrument. The application of the spectral intensity correction eliminates the instrument-induced spectral artifacts to provide a more uniform calibrated Raman spectrum. In order for a Raman spectrometer to be calibrated for intensity the Raman wavenumber axis is corrected using ASTM E1840-96 [12]. The laser excitation is aimed at the frosted surface of the glass, to minimize variation in the scattering response. The intensity correction must be completed over the same Raman shift range as that intended for sample measurement.
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
Table 125-7 Diffuse Gold Reflectance Values in Wavenumber and Wavelength Space for Specimen Measured for Figs. 125-7 and 125-8 Wavenumber (cm21)
Wavelength (μm)
Reflectance
5000 4000 3333 2857 2500 2222 2000 1818 1667 1538 1429 1333 1250 1176 1111 1053 1000 952 909 870 833 800 769 741 714 690 667 645 625 606 588 571 556 541 526 513 500
2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00 12.50 13.00 13.50 14.00 14.50 15.00 15.50 16.00 16.50 17.00 17.50 18.00 18.50 19.00 19.50 20.00
0.944 0.928 0.936 0.947 0.940 0.941 0.944 0.937 0.954 0.957 0.953 0.940 0.942 0.954 0.954 0.947 0.954 0.942 0.947 0.947 0.948 0.944 0.943 0.939 0.943 0.939 0.942 0.928 0.931 0.937 0.943 0.951 0.946 0.929 0.916 0.919 0.922
RAMAN INTENSITY CORRECTION STANDARDS
997
Table 125-8 NIST SRMs for Raman Spectrometer Calibration [6] SRM No.
Laser Excitation (nm)
2241
785 nm excitation
2242
532 nm excitation
2243
2244
488 nm and 514.5 nm excitation 1064 nm excitation
2245
633 nm excitation
2246
830 nm excitation
Material
SRM Dimensions
Chromium-doped (mole fraction of 0.02% Cr2O3) sodium borosilicate matrix glass Manganese-doped (0.15 wt% MnO2) borate matrix glass Discontinued
10 mm in width 10 mm in length 1.65 mm in thickness 10.7 mm in width 30.4 mm in length 2.0 mm in thickness NA
Doped (mass fraction 0.7% Cr2O3) oxide in a borosilicate matrix glass Bismuth-doped (0.11% mole fraction) oxide in a phosphate matrix glass Chromium-doped (0.30% mole fraction) oxide in a borosilicate-matrix glass
0.7 mm in width 30.4 mm in length 2.0 mm in thickness 10 mm in width 10 mm in length 1.65 mm in thickness 10 mm in width 10 mm in length 1.65 mm in thickness
The relative intensity of the measured Raman spectrum of the sample can be corrected for the instrument-specific response by a computational procedure that uses a correction curve. This curve is generated using the certified model and the measured luminescence spectrum of the SRM glass. For the spectral range of certification, from Δυ ¼ 200 to 3500 cm1, the elements of the certified relative mean spectral intensity of SRM 2241, ISRM(Δυ), are computed according to Eq. (125-14): ISRMðΔυÞ ¼ A0 + A1 ðΔυÞ1 + A2 ðΔυÞ2 + A3 ðΔυÞ3 + A4 ðΔυÞ4 + A5 ðΔυÞ5
(125-14)
where (Δυ) is the wavenumber in units of Raman shift (cm1) and the As are the coefficients listed in Table 125-9. The elements of ISRM(Δυ) are obtained by evaluating Eq. (125-14) at the same data point spacing used for the acquisition of the luminescence spectrum of the SRM and of the Raman spectrum Table 125-9 Coefficients of the Certified Polynomial (a) and of the 95% Confidence Curves (b) Describing the Mean Luminescence Spectrum of SRM 2241 for 785 nm Excitation (Valid for Temperatures From 20°C to 25°C) [14–19] Polynomial Coefficient (c) of the 95% Confidence Curves (b)
Polynomial Coefficient
Certified Value Polynomial Coefficient (c)
+95% CC
299% CC
A0 A1 A2 A3 A4 A5
9.71937 E 02 2.28325 E 04 5.86762 E 08 2.16023 E 10 9.77171 E 14 1.15596 E 17
1.04276 E 01 2.39131 E 04 7.81489 E 08 2.32243 E 10 1.03769 E 13 1.23774 E 17
9.01111 E02 2.17519 E04 3.92035 E 08 1.99803 E10 9.16653 E 14 1.07417 E17
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
of the sample. ISRM(Δυ) has been normalized to unity and is a relative unit expressed in terms of photons s1 cm2 (cm1)1. The data sets that are the measured glass luminescence spectrum, SSRM, and the measured Raman spectrum of the sample, SMEAS, must have the units of Raman shift (cm1). The elements of the correction curve ICORR(Δυ), defined by Eq. (125-15), are obtained from ISRM(Δυ) and the elements of the glass luminescence spectrum, SSRM(Δυ), by: ICORRðΔυÞ ¼ ISRMðΔυÞ=SSRMðΔυÞ
(125-15)
The elements of the intensity-corrected Raman spectrum, SCORR(Δυ), are derived by multiplication of the elements of the measured Raman spectrum of the sample, SMEAS(Δυ), by the elements of the correction curve as Eq. (125-16) [13]. SCORRðΔυÞ ¼ SMEASðΔυÞ ICORRðΔυÞ
(125-16) 1
The certified model, Eq. (125-14), is certified for use between 200 and 3500 cm . The certified model is intended as a simple numerical descriptor of the spectral response observed over the wavenumber range studied. It is not intended as a physically meaningful model. The model coefficients listed in Table 125-9 cannot be used to extrapolate the limits of certification without incurring significant error. Extrapolation of the model outside the certification limits of 200 and 3500 cm1 is not a supported by use of this SRM. This SRM is not intended for use as a standard for the determination of absolute spectral irradiance or radiance. The equation describing the mean luminescence spectrum of the glass SRM is given in Eq. (12514), where (Δυ) is the wavenumber in units of Raman shift (cm1). For correction of spectra where the x-axis is in wavelength with units of nanometers, the same model coefficients can be used to calculate ISRM(λ) through the following coordinate transformation as Eqs. (125-17) and (125-18): ISRMðλÞ ¼ 107 =λ2 A0 + A1 Z1 + A2 Z 2 + A3 Z3 + A4 Z4 + A5 Z5
(125-17)
Z ¼ 107 ½ð1:0=λL Þ ð1:0=λÞ
(125-18)
where λL is the wavelength of the laser in nanometers and λ is the spectral wavelength in nanometers. The prefactor of 107 in the first term of Eq. (125-17) is needed only if it is desired to preserve the numerical value of spectral areas computed relative to the two x-axis coordinate systems. Definition is taken from Ref. [7], with NIST basic application of intensity standard. 1. An NIST-certified value represents data, reported in an SRM certificate, for which NIST has the highest confidence in its accuracy in that all known or suspected sources of bias have been fully investigated or taken into account [14]. 2. The consensus curve is a point-by-point weighted mean of the average responses of three instruments [15,16], fitted by the polynomial model. The uncertainty curves are polynomial models of point-by-point expanded uncertainties, with coverage factor k ¼ t0.975,2 ¼ 4.303 (95% confidence), calculated by combining a between-instrument variance incorporating interinstrumental Type B uncertainties with a pooled within-method variance12, following the ISO/JCGM guide [18,19]. 3. where ISRM(Δυ) ¼ A0 + A1 (Δυ)1 + A2 (Δυ)2 + A3 (Δυ)3 + A4 (Δυ)4 + A5 (Δυ)5, for Δυ ¼ 200 to 3500 cm1 Raman shift relative to 785 nm excitation. ISRM(Δυ) has units of photons s1 (cm2) (cm1)1.
REFERENCES
999
A comprehensive paper describing the complete process of calibrating Raman instruments for intensity using various laser sources and instrument types along with the application of NIST SRMs is given in Ref. [20] as an Applied Spectroscopy feature article. This paper is recommended for anyone that would practice intensity correction for analytical measurements and for understanding the details of this process.
REFERENCES [1] NIST SRM Website: URL at https://www.nist.gov/srm (Accessed September 9, 2017). [2] J. Workman, in: J. Workman (Ed.), The Concise Handbook of Analytical Spectroscopy: Physical Foundations, Techniques, Instrumentation and Data Analysis. In 5 Volumes, UV, Vis, NIR, IR, and Raman, World Scientific Publishing-Imperial College Press, New Jersey/Singapore, 2016. vol. 1, pp. 163–214; vol. 2, pp. 217–278; vol. 3, pp. 363–424; vol. 4, pp. 323–360; vol. 5, pp. 229–250, ISBN-13: 978-9814508056 (Permission from NIST for data and spectra related to SRM materials). [3] Each table refers to specific NIST SRM datasheets and published references and as referred to in Ref. [2]. [4] Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers, American Society for Testing and Materials: ASTM International designation E 275-08, Philadelphia, PA, 2013. [5] Standard Terminology Relating to Molecular Spectroscopy, American Society for Testing and Materials: ASTM International designation E 131-10, Philadelphia, PA, 2010. [6] NIST Raman SRM Catalog Website: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.260176.pdf, pp. 162–163 (Accessed September 9, 2017). [7] Standard Reference Material (SRM)® 2241 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 785 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 28 February 2014, 6 p. [8] Standard Reference Material (SRM)® 2242 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 532 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 22 October 2013, 6 p. [9] Standard Reference Material (SRM)® 2244 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 1064 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 03 December 2009, 6 p. [10] Standard Reference Material (SRM)® 2245 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 633 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 27 September 2011, 6 p. [11] Standard Reference Material (SRM)® 2246 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 830 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 31 August 2012, 7 p. [12] ASTM E1840-96(2007) Standard Guide for Raman Shift Standards for Spectrometer Calibration, ASTM International, West Conshohocken, PA, USA, 2007. [13] K.J. Frost, R.L. McCreery, Calibration of Raman spectrometer response function with luminescence standards: an update, Appl. Spectrosc. 52 (12) (1998) 1614–1618. [14] W. May, R. Parris, C. Beck II, J. Fassett, R. Greenberg, F. Guenther, G. Kramer, S. Wise, T. Gills, J. Colbert, R. Gettings, B. MacDonald, Definition of terms and modes used at NIST for value-assignment of reference materials for chemical measurements, NIST Spec. Publ. (2000) 136–260. http://www.nist. gov/srm/publications.cfm (Accessed February 2014). [15] A.L. Rukhin, Weighted means statistics in interlaboratory studies, Metrologia 46 (2009) 323–331.
1000 CHAPTER 125 USING REFERENCE MATERIALS, PART 2
[16] R. Dersimonian, N. Laird, Meta-analysis in clinical trials, Control. Clin. Trials 7 (3) (1986) 177–188. [17] S.D. Horn, R.A. Horn, D.B. Duncan, Estimating heteroscedastic variances in linear-models, J. Am. Stat. Assoc. 70 (350) (1975) 380–385. [18] JCGM 100:2008; Evaluation of Measurement Data—Guide to the Expression of Uncertainty in Measurement (GUM 1995 With Minor Corrections); Joint Committee for Guides in Metrology, 2008, Available at: http:// www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf (Accessed September 9, 2017). [19] B.N. Taylor, C.E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297, U.S. Government Printing Office, Washington, DC, 1994. Available at: http://www.nist.gov/pml/pubs/index.cfm (Accessed September 9, 2017). [20] S.L. Choquette, E.S. Etz, W.S. Hurst, D.H. Blackburn, S.D. Leigh, Relative intensity correction of Raman spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm excitation, Appl. Spectrosc. 61 (2) (2007) 117–129. http://ws680.nist.gov/publication/get_pdf.cfm?pub_id¼906524 (Accessed September 9, 2017).
USING REFERENCE MATERIALS, PART 2: ALIGNING THE Y-AXIS
125
CHAPTER OUTLINE Introduction to Photometric Accuracy .................................................................................................. 981 Example of Reporting Photometric Accuracy .......................................................................... 982 Photometric Correction for Absorbance-Based Spectrophotometers ....................................................... 983 Ultraviolet Photometric Standards ....................................................................................................... 984 Visible (Vis) Photometric Standards .................................................................................................... 987 Near Infrared Reflectance Photometric Standards ................................................................................ 987 Infrared Reflectance Photometric Standards ........................................................................................ 993 Raman Intensity Correction Standards ................................................................................................. 993 References ........................................................................................................................................ 999
INTRODUCTION TO PHOTOMETRIC ACCURACY Photometric accuracy is a formal term used to describe how well a measuring device (spectrophotometer or illuminance meter) is able to determine the total energy flux transmitted through (or reflected from) a Standard Reference Material (SRM)® [1]. The various references to NIST documents and SRMs are given throughout this chapter. Note that equations are described as referenced in Refs. [2,3]. Volume 14.01 of the ASTM International document [4,5] describes the measurement of the photometric accuracy for an instrument as follows (Sections 20.1 and 20.3): Select the appropriate Standard Reference Material and obtain ten successive readings of the apparent absorbance or transmittance at the specified wavelength. Average the ten readings. The photometric accuracy is the difference between the true absorbance or transmittance value and the average observed value.
It is further stated that the following apply to the reporting of photometric accuracy. Report the photometric accuracy in the following order: reference material, wavelength, true absorbance or transmittance, observed absorbance or transmittance plus or minus the standard deviation.
Chemometrics in Spectroscopy. https://doi.org/10.1016/B978-0-12-805309-6.00125-2 # 2018 Elsevier Inc. All rights reserved.
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EXAMPLE OF REPORTING PHOTOMETRIC ACCURACY As an example the stated specifications for photometric accuracy for a test instrument might be written as: Photometric Accuracy 0.002 Au (for range of 0.0–0.5 Absorbance units) 0.004 Au (for range of 0.5–1.0 Absorbance units) 0.3%T SRM: Measured with NIST 930D filters Here is an evaluation of these specifications in greater detail. A photometric accuracy of “0.002 Au at 0 to 0.5 Au” indicates that after the instrument has been set to zero, a standard sample which transmits between 30.1%T (i.e., 0.5 Au) and 100%T (i.e., 0.0 Au) measures within 0.002 Au as compared to the specified value for that reference standard material at any wavelength position. The tolerance for each of the 930d filters is stated by NIST as 0.5% relative. Thus, a 10%T filter (which corresponds to 0.1 T or 1.0 Au) neutral density 930D-type reference standard would be expected to be within the range of 9.95–10.05%T. These values are equivalent to 1.0022–0.9978 Au. The difference in Au is that 1.0022 Au minus 0.9978 Au equals 0.0044 Au. The standard tolerance in Au is 1.0000 0.0022 Au, rounded to 1.000 0.002 Au. Conservatively for transmittance we state 10.00 0.03%T, and related to 0.0 Au or 100%T, we specify 100.0 0.3%T [19] (Table 125-1). If the stated tolerance for the NIST 930D filter is 0.002 Au at 1.0 Au, we are stating that our maximum deviation in photometric accuracy for our instrument at 1.0Au is also 0.002 Au. For the worst case measurement for the NIST filter, we would have the filter error of 0.002 added to the instrument error of 0.002, to result in a total maximum variation 0.004 Au error. (This is our stated specification for the example given.) And one can readily calculate the reflectance of transmittance of any measurement given the absorbance of that measurement as follows. For conversion of absorbance to transmittance we use Eqs. (125-1)–(125-4): A ¼ log 10
I I0 ¼ log 10 I I0
(125-1)
Table 125-1 Relationship of %T and Absorbance Unit (Au) Values T (as ppm) 10,000,000 1,000,000 301,000 100,000 10,000 1000 100 10 1
%T 1000 100 30.1 10.0 1 0.1 0.01 0.001 0.0001
T 10 1 0.301 0.1 0.01 0.001 0.0001 0.00001 0.000001
1/T
Au 1
0.1 ¼ 10 1.0 ¼ 100 3.01 ¼ 100.5 10 ¼ 101 100 ¼ 102 1000 ¼ 103 10,000 ¼ 104 100,000 ¼ 105 1,000,000 ¼ 106
1.0 0.0 0.5 1.0 2.0 3.0 4.0 5.0 6.0
ABSORBANCE-BASED SPECTROPHOTOMETERS
983
I I0 1 ¼ 10A ) ¼ A I 10 I0
(125-2)
I ¼ 10A ¼ T I0
(125-3)
%T ¼ T 100
(125-4)
To convert absorbance to reflectance we use Eqs. (125-5)–(125-8): I I0 ¼ log 10 A ¼ log 10 I I0
(125-5)
I I0 1 ¼ 10A ) ¼ A I 10 I0
(125-6)
I ¼ 10A ¼ R I0
(125-7)
%R ¼ R 100
(125-8)
PHOTOMETRIC CORRECTION FOR ABSORBANCE-BASED SPECTROPHOTOMETERS For making photometric calibrations using known reference materials, the ratioed replicate spectral data are retained, along with the replicate spectra of a reference sample and dark background or dark sample. Eq. (125-9) represents Beer’s law for taking measurements using a spectrophotometer .To convert the light reflected or transmitted from a sample (I) ratioed to the incident energy (I0) to absorbance (A), as a linearized estimate of the spectral response as related to analyte concentration, the following Eqs. (125-9)–(125-13) may be applied. If only an internal reference is used for spectral collection, then the absorbance spectrum with respect to wavelength is computed as Eq. (125-9): A ¼ log 10
I I0 RI DRI ¼ log 10 ¼ log 10 I S DS I0
(125-9)
If an external standard material is used to calibrate an automated internal reference material, then the absorbance spectrum with respect to wavelength is computed as Eqs. (125-10) and (125-11): A ¼ log 10
I I0 RI + RΔ DRI ¼ log 10 ¼ log 10 I S DS I0
(125-10)
where RΔ ¼ ðRE DRE Þ=ðRI DRI Þ
(125-11)
where for Eqs. (125-9)–(125-11), S is the sample measurement; DS is the dark measurement for the sample; RI is the internal reference measurement; DR1 is the dark measurement for the internal
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
reference sample; RE is the measurement of the external standard reference material for calibration; DRE is the dark measurement for the external standard reference material; and RΔ is the ratio between the dark corrected RE and RI. The final spectrum is simplified to the ratio of internal reference corrected by external reference minus dark divided by sample—dark. Note that dark is a measurement where no energy from the source is allowed to the detector (it is blocked) or where a totally dark sample or light trap is measured. The dark measurement represents the dark current changes in the instrument during a measurement that relate to electronic noise where the detection electronics, computational electronics, and control electronics perform basic measurement functions with no imputed energy reaching the detector (i.e., the source is turned off or blocked using a shutter closure during the dark measurement). When a comprehensive method of photometric correction includes a light trap measurement to determine the dark response to correct for optical and window scatter, we use Eq. (125-12): 2 6ðRI DRI Þ 6 A ¼ log 10 6 6 4
!3 ðRE DRE Þ ðIW DW Þ 7 R0I D0RI 7 7 7 S DS 5
(125-12)
where the variable terms used include: R0I which is the photometric correction calibration measurement of the internal reference sample, D0RI which is the photometric correction calibration measurement of the dark signal intensity for the internal reference sample, IW is the intensity measurement of the light trap to determine the window scatter, and DW is the measurement of the dark signal intensity for the light trap measurement. Eq. (125-12) is simplified for understanding by removing the dark correction terms as Eq. (125-13): 2 6 A ¼ log 10 6 4
RI
3 RE IW 7 R0I 7 5 S
(125-13)
ULTRAVIOLET PHOTOMETRIC STANDARDS There are several available standards used for calibrating or verifying photometric alignment when using UV spectrophotometers, with potassium dichromate demonstrated. The UV region is generally considered to cover from 190 to 380 nm (nonvacuum UV region). The photometric values for UV standards are illustrated in Figs. 125-1 and 125-2, and the reference absorbance values are given in the accompanying Table 125-2. Reference spectra and data provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
985
ULTRAVIOLET PHOTOMETRIC STANDARDS
3.0000 200 mg/L 180 mg/L
2.5000
160 mg/L
Absorbance (A)
2.0000
140 mg/L 120 mg/L
1.5000
100 mg/L 80 mg/L
1.0000
60 mg/L 40 mg/L
0.5000
20 mg/L
0.0000 220.0
240.0
260.0
280.0
300.0
320.0
340.0
Wavelengths (nm)
FIG. 125-1 Potassium dichromate absorbance and linearity standard traceable to SRM 935a. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
4.00 3.50
Absorbance (A)
3.00 2.50
257 nm
2.00
235 nm
1.50
350 nm
1.00
313 nm
0.50
g/ L
g/ L
m 0 24
22
0
m
m
g/ L
g/ L 18 0
18
0
m
m
g/ L
L g/ 16 0
m
14 0
m
m
g/ L
g/ L 12 0
L g/
10 0
m 80
m
g/ L
g/ L 60
m 40
20
m
g/ L
0.00
Dichromate concentrations
FIG. 125-2 Potassium dichromate absorbance linearity as a photometric standard using five concentrations of standard. This is a test for photometric linearity throughout the ultraviolet region. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK. www.starna.com.
986
Wavelength (nm) 350.0 313.0 257.0 235.0
20 mg/L
40 mg/L
60 mg/L
80 mg/L
100 mg/L
120 mg/L
140 mg/L
160 mg/L
180 mg/L
200 mg/L
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
Abs
0.2104 0.0948 0.2807 0.2404
0.4137 0.1858 0.5550 0.4766
0.6289 0.2834 0.8469 0.7280
0.8497 0.3827 1.1468 0.9860
1.0591 0.4764 1.4303 1.2271
1.2713 0.5718 1.7177 1.4738
1.4834 0.6673 2.0051 1.7205
1.6956 0.7627 2.2925 1.9672
1.9078 0.8581 2.5799 2.2139
2.1199 0.9535 2.8673 2.4605
Values shown are the absorbance values for each measured wavelength at the designated concentration of potassium dichromate. Data and spectrum provided with permission by Starna Ltd., Hainault, IG6 3UT, UK, www.starna.com.
CHAPTER 125 USING REFERENCE MATERIALS, PART 2
Table 125-2 Data Table for Potassium Dichromate Absorbance and Linearity Standard Traceable to SRM 935a
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
987
VISIBLE (VIS) PHOTOMETRIC STANDARDS The visible spectral region is generally considered from 380 to 750 nm. Two types of available standards are shown, namely NIST SRM 930d and Russian Opal Glass, which are used for calibrating or verifying photometric alignment of visible spectrophotometers. Photometric values for these standards are illustrated in Figs. 125-3 and 125-4, and the reference absorbance values are given in the accompanying Tables 125-3 and 125-4.
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS The NIR spectral region is generally considered to be from 780 to 2500 nm. Two types of available reflectance standards are shown, namely R50 (50% reflectance) and sintered Fluorilon (PTFE) (R99 or 99% reflectance), which are used for calibrating or verifying photometric alignment of NIR spectrophotometers. Photometric values for these standards are illustrated in Figs. 125-5 and 125-6, and the reference reflectance values are given in the accompanying Tables 125-5 and 125-6. Materials noted when provided by courtesy by Avian Technologies New London, NH. Note that in transmittance measurement mode various solvents or dry air may be used to calibrate or verify the transmittance accuracy. Transmittance of SRM-930d set #1255 0.4500 0.4000 0.3500
Filter 30-1255
Transmittance
0.3000 0.2500 Filter 20-1255
0.2000 0.1500
Filter 10-1255 0.1000 0.0500 0.0000 350
400
450
500
550
600
650
700
750
Wavelength (nm)
FIG. 125-3 Transmittance of SRM-930d standard set photometric standards; Filter 10-1225, Filter 20-1255, and Filter 30-1255. Data and spectrum used with permission from Avian Technologies, New London, NH.
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
8 degrees/hemispherical reflectance factor Russian Opal Glass ROG-02c Glossy
8 degrees/hemi. refl. factor
1.000
0.900
0.800
0.700
0.600
0.500 360
380
400
420
440
460
480
500
520
540
560
580
600
620
640
660
680
700
720
740
760
780
Wavelength (nm)
FIG. 125-4 Russian Opal Glass Glossy reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry. Mean reflectance over the spectral region shown is 0.967. Measured photometric data is given in Table 125-4. Data and spectrum used with permission from Avian Technologies, New London, NH.
Table 125-3 Sit and Dwell Measurements for SRM930d at Different Concentrations (1 nm Bandpass) Sit and Dwell Measurements 1 nm Bandpass Transmittance at wavelength Sample 10-1225 20-1225 30-1225
440.0 nm 0.0948 0.1871 0.3070
465.0 nm 0.1106 0.2084 0.3411
546.1 nm 0.1034 0.1989 0.3319
590.0 nm 0.0920 0.1833 0.3085
635.0 nm 0.1007 0.1953 0.3166
465.0 nm 0.9562 0.6811 0.4671
546.1 nm 0.9855 0.7014 0.4790
590.0 nm 1.0362 0.7368 0.5107
635.0 nm 0.9970 0.7093 0.4996
Absorbance at wavelength Sample 10-1225 20-1225 30-1225
440.0 nm 1.0232 0.7279 0.5129
Averages of 16 measurements:
All nominally 0.0002 T
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
Table 125-4 Russian Opal Glass Glossy Reflectance Values Wavelength (nm)
Glossy Reflectance Factor
360 370 380 390 400 410 420 430 440 450 460 470 480 490 500 510 520 530 540 550 560 570 580 590 600 610 620 630 640 650 660 670 680 690 700 710 720 730 740 750 760 770 780
0.882 0.903 0.929 0.943 0.954 0.960 0.966 0.968 0.970 0.973 0.975 0.976 0.978 0.978 0.978 0.979 0.979 0.979 0.979 0.978 0.977 0.976 0.975 0.975 0.974 0.974 0.972 0.973 0.972 0.973 0.972 0.972 0.972 0.971 0.970 0.970 0.969 0.969 0.968 0.968 0.968 0.965 0.964
989
FIG. 125-5 Fluorilon and carbon black mixture as a 50% reflectance standard. Spectrum used with permission from Avian Technologies, New London, NH.
FIG. 125-6 Fluorilon as a 99% reflectance standard. Spectrum used with permission from Avian Technologies, New London, NH.
NEAR INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
Table 125-5 R50 Fluorilon Reflection Data from NIST Traceable Instrument Wavelength (nm)
R50 Refl.
Wavelength (nm)
R50 Refl.
800 825 850 875 900 925 950 975 1000 1025 1050 1075 1100 1125 1150 1175 1200 1225 1250 1275 1300 1325 1350 1375 1400 1425 1450 1475 1500 1525 1550 1575 1600
0.464 0.463 0.461 0.460 0.461 0.461 0.462 0.461 0.459 0.460 0.458 0.458 0.457 0.457 0.456 0.456 0.454 0.454 0.453 0.452 0.452 0.450 0.450 0.449 0.448 0.448 0.449 0.448 0.447 0.447 0.447 0.446 0.447
1625 1650 1675 1700 1725 1750 1775 1800 1825 1850 1875 1900 1925 1950 1975 2000 2025 2050 2075 2100 2125 2150 2175 2200 2225 2250 2275 2300 2325 2350 2375 2400 2425 2450 2475 2500
0.446 0.445 0.444 0.444 0.444 0.443 0.443 0.442 0.442 0.441 0.442 0.439 0.439 0.440 0.439 0.441 0.441 0.441 0.441 0.441 0.442 0.438 0.437 0.437 0.434 0.434 0.436 0.436 0.436 0.436 0.436 0.438 0.438 0.436 0.437 0.435
991
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
Table 125-6 R99 Fluorilon Reflection Data from NIST Traceable Instrument (Three Samples) Wavelength (nm)
Calibrated (#1) R99 Refl.
Checked (#2) R99 Refl.
Checked (#3) R99 Refl.
1000 1025 1050 1075 1100 1125 1150 1175 1200 1225 1250 1275 1300 1325 1350 1375 1400 1425 1450 1475 1500 1525 1550 1575 1600 1625 1650 1675 1700 1725 1750 1775 1800 1825 1850 1875 1900 1925
0.981 0.980 0.982 0.982 0.980 0.980 0.978 0.980 0.979 0.980 0.979 0.979 0.978 0.978 0.977 0.976 0.975 0.978 0.977 0.977 0.976 0.976 0.977 0.977 0.977 0.975 0.975 0.975 0.974 0.974 0.972 0.974 0.975 0.972 0.971 0.969 0.967 0.967
0.981 0.983 0.981 0.982 0.983 0.981 0.984 0.980 0.981 0.981 0.980 0.981 0.981 0.978 0.979 0.979 0.979 0.979 0.978 0.980 0.978 0.978 0.978 0.979 0.979 0.977 0.977 0.976 0.975 0.975 0.976 0.976 0.976 0.971 0.969 0.971 0.970 0.969
0.981 0.980 0.982 0.981 0.981 0.980 0.980 0.982 0.980 0.981 0.979 0.980 0.980 0.979 0.978 0.978 0.977 0.979 0.978 0.977 0.978 0.977 0.977 0.977 0.978 0.976 0.977 0.975 0.976 0.976 0.973 0.976 0.976 0.973 0.971 0.970 0.969 0.968
RAMAN INTENSITY CORRECTION STANDARDS
993
Table 125-6 R99 Fluorilon Reflection Data from NIST Traceable Instrument (Three Samples)—cont’d Wavelength (nm)
Calibrated (#1) R99 Refl.
Checked (#2) R99 Refl.
Checked (#3) R99 Refl.
1950 1975 2000 2025 2050 2075 2100 2125 2150 2175 2200 2225 2250 2275 2300 2325 2350 2375 2400 2425 2450 2475 2500
0.967 0.964 0.962 0.957 0.948 0.945 0.941 0.934 0.935 0.946 0.954 0.954 0.955 0.954 0.948 0.943 0.937 0.934 0.929 0.933 0.933 0.930 0.939
0.968 0.966 0.965 0.959 0.954 0.949 0.948 0.940 0.940 0.951 0.958 0.960 0.959 0.954 0.950 0.943 0.940 0.943 0.935 0.933 0.938 0.936 0.931
0.968 0.966 0.964 0.961 0.950 0.949 0.944 0.938 0.939 0.948 0.956 0.957 0.958 0.956 0.952 0.946 0.940 0.939 0.935 0.935 0.934 0.932 0.937
INFRARED REFLECTANCE PHOTOMETRIC STANDARDS
The IR spectral region is generally considered to cover from 2500 to 25,000 nm (i.e., 4000–400 cm1). Diffuse gold is a primary surface material used for calibrating or verifying reflectance photometric alignment of IR spectrophotometers. Photometric values for this standard are illustrated in Figs. 125-7 and 125-8, and the reference reflectance values are given in the accompanying Table 125-7. Materials noted when provided by courtesy by Avian Technologies New London, NH. Note that in transmittance measurement mode dry, purged air may be used to calibrate or verify the transmittance accuracy.
RAMAN INTENSITY CORRECTION STANDARDS The Raman spectral region is generally considered to include from 2500 to 25,000 nm (i.e., 4000–400 cm1), with the Raman spectral region referred to as the Stokes shift being from 2857 to 200,000 nm or from 3500 to 50 cm1. A variety of materials are used for calibrating or verifying
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CHAPTER 125 USING REFERENCE MATERIALS, PART 2
1.000
8 degrees/hemi. refl. factor
0.950
0.900
0.850
0.800
0.750 2.000
4.000
6.000
8.000
10.000
12.000
14.000
16.000
18.000
20.000
22.000
Wavelength (nm)
FIG. 125-7 Diffuse gold photometric reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry with a calibrated FT-IR instrument. Mean reflectance over the spectral wavelength region (in μm) shown is 0.941. Measured photometric data is given in Table 125-7. Data and spectrum used with permission from Avian Technologies, New London, NH.
the intensity scale for Raman spectrometers. Materials for this purpose are listed in Table 125-8. Materials noted are provided by NIST. The NIST previously known as the National Bureau of Standards (NBS) provides materials designed as reference sources to verify the performance characteristics of Raman spectrometers of any design. The main standards are currently used for intensity standardization. Table 125-8 lists the current materials available for Raman intensity calibration. The materials are termed SRMs®. In the case of Raman there are five active NIST SRMs for measuring Raman intensity, depending upon the excitation laser wavelength used [6–11]. The Raman intensity standards are certified SRMs useful for the correction of the relative intensity of Raman spectra obtained with instruments employing specific laser excitation sources. Individual intensity standards have been provided for Raman excitation wavelengths of 488, 532, 633, 785, 830, and 1064 nm. For these standards, the relative intensity of the glass luminescence has been
RAMAN INTENSITY CORRECTION STANDARDS
995
0.980
0.960
8 degrees/hemi reflectance factor
0.940
0.920
0.900
0.880
0.860
0.840
0.820
0.800 5000
4500
4000
3500
3000
2500
Wavenumbers
2000
1500
1000
500
(cm–1)
FIG. 125-8 Diffuse gold photometric reflectance standard measured using 8 degrees/hemispherical spectral reflectance factor geometry with a calibrated FT-IR instrument. Mean reflectance over the spectral wavelength region (in μm) shown is 0.941. Measured photometric data is given in Table 125-7. Data and spectrum used with permission from Avian Technologies, New London, NH.
calibrated at NIST using a uniform white-light source, with an integrating sphere collection design. The shape of the mean luminescence spectrum of the SRM glass is described using a polynomial expression for relative spectral intensity versus Raman shift wavenumber (cm1) based on the specific excitation laser wavelength used (in nm). The spectral correction for any Raman spectrometer is determined by measuring the luminescence spectrum of the SRM, applying the polynomial model, computing the spectral intensity-response correction for any Raman instrument. The application of the spectral intensity correction eliminates the instrument-induced spectral artifacts to provide a more uniform calibrated Raman spectrum. In order for a Raman spectrometer to be calibrated for intensity the Raman wavenumber axis is corrected using ASTM E1840-96 [12]. The laser excitation is aimed at the frosted surface of the glass, to minimize variation in the scattering response. The intensity correction must be completed over the same Raman shift range as that intended for sample measurement.
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Table 125-7 Diffuse Gold Reflectance Values in Wavenumber and Wavelength Space for Specimen Measured for Figs. 125-7 and 125-8 Wavenumber (cm21)
Wavelength (μm)
Reflectance
5000 4000 3333 2857 2500 2222 2000 1818 1667 1538 1429 1333 1250 1176 1111 1053 1000 952 909 870 833 800 769 741 714 690 667 645 625 606 588 571 556 541 526 513 500
2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00 10.50 11.00 11.50 12.00 12.50 13.00 13.50 14.00 14.50 15.00 15.50 16.00 16.50 17.00 17.50 18.00 18.50 19.00 19.50 20.00
0.944 0.928 0.936 0.947 0.940 0.941 0.944 0.937 0.954 0.957 0.953 0.940 0.942 0.954 0.954 0.947 0.954 0.942 0.947 0.947 0.948 0.944 0.943 0.939 0.943 0.939 0.942 0.928 0.931 0.937 0.943 0.951 0.946 0.929 0.916 0.919 0.922
RAMAN INTENSITY CORRECTION STANDARDS
997
Table 125-8 NIST SRMs for Raman Spectrometer Calibration [6] SRM No.
Laser Excitation (nm)
2241
785 nm excitation
2242
532 nm excitation
2243
2244
488 nm and 514.5 nm excitation 1064 nm excitation
2245
633 nm excitation
2246
830 nm excitation
Material
SRM Dimensions
Chromium-doped (mole fraction of 0.02% Cr2O3) sodium borosilicate matrix glass Manganese-doped (0.15 wt% MnO2) borate matrix glass Discontinued
10 mm in width 10 mm in length 1.65 mm in thickness 10.7 mm in width 30.4 mm in length 2.0 mm in thickness NA
Doped (mass fraction 0.7% Cr2O3) oxide in a borosilicate matrix glass Bismuth-doped (0.11% mole fraction) oxide in a phosphate matrix glass Chromium-doped (0.30% mole fraction) oxide in a borosilicate-matrix glass
0.7 mm in width 30.4 mm in length 2.0 mm in thickness 10 mm in width 10 mm in length 1.65 mm in thickness 10 mm in width 10 mm in length 1.65 mm in thickness
The relative intensity of the measured Raman spectrum of the sample can be corrected for the instrument-specific response by a computational procedure that uses a correction curve. This curve is generated using the certified model and the measured luminescence spectrum of the SRM glass. For the spectral range of certification, from Δυ ¼ 200 to 3500 cm1, the elements of the certified relative mean spectral intensity of SRM 2241, ISRM(Δυ), are computed according to Eq. (125-14): ISRMðΔυÞ ¼ A0 + A1 ðΔυÞ1 + A2 ðΔυÞ2 + A3 ðΔυÞ3 + A4 ðΔυÞ4 + A5 ðΔυÞ5
(125-14)
where (Δυ) is the wavenumber in units of Raman shift (cm1) and the As are the coefficients listed in Table 125-9. The elements of ISRM(Δυ) are obtained by evaluating Eq. (125-14) at the same data point spacing used for the acquisition of the luminescence spectrum of the SRM and of the Raman spectrum Table 125-9 Coefficients of the Certified Polynomial (a) and of the 95% Confidence Curves (b) Describing the Mean Luminescence Spectrum of SRM 2241 for 785 nm Excitation (Valid for Temperatures From 20°C to 25°C) [14–19] Polynomial Coefficient (c) of the 95% Confidence Curves (b)
Polynomial Coefficient
Certified Value Polynomial Coefficient (c)
+95% CC
299% CC
A0 A1 A2 A3 A4 A5
9.71937 E 02 2.28325 E 04 5.86762 E 08 2.16023 E 10 9.77171 E 14 1.15596 E 17
1.04276 E 01 2.39131 E 04 7.81489 E 08 2.32243 E 10 1.03769 E 13 1.23774 E 17
9.01111 E02 2.17519 E04 3.92035 E 08 1.99803 E10 9.16653 E 14 1.07417 E17
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of the sample. ISRM(Δυ) has been normalized to unity and is a relative unit expressed in terms of photons s1 cm2 (cm1)1. The data sets that are the measured glass luminescence spectrum, SSRM, and the measured Raman spectrum of the sample, SMEAS, must have the units of Raman shift (cm1). The elements of the correction curve ICORR(Δυ), defined by Eq. (125-15), are obtained from ISRM(Δυ) and the elements of the glass luminescence spectrum, SSRM(Δυ), by: ICORRðΔυÞ ¼ ISRMðΔυÞ=SSRMðΔυÞ
(125-15)
The elements of the intensity-corrected Raman spectrum, SCORR(Δυ), are derived by multiplication of the elements of the measured Raman spectrum of the sample, SMEAS(Δυ), by the elements of the correction curve as Eq. (125-16) [13]. SCORRðΔυÞ ¼ SMEASðΔυÞ ICORRðΔυÞ
(125-16) 1
The certified model, Eq. (125-14), is certified for use between 200 and 3500 cm . The certified model is intended as a simple numerical descriptor of the spectral response observed over the wavenumber range studied. It is not intended as a physically meaningful model. The model coefficients listed in Table 125-9 cannot be used to extrapolate the limits of certification without incurring significant error. Extrapolation of the model outside the certification limits of 200 and 3500 cm1 is not a supported by use of this SRM. This SRM is not intended for use as a standard for the determination of absolute spectral irradiance or radiance. The equation describing the mean luminescence spectrum of the glass SRM is given in Eq. (12514), where (Δυ) is the wavenumber in units of Raman shift (cm1). For correction of spectra where the x-axis is in wavelength with units of nanometers, the same model coefficients can be used to calculate ISRM(λ) through the following coordinate transformation as Eqs. (125-17) and (125-18): ISRMðλÞ ¼ 107 =λ2 A0 + A1 Z1 + A2 Z 2 + A3 Z3 + A4 Z4 + A5 Z5
(125-17)
Z ¼ 107 ½ð1:0=λL Þ ð1:0=λÞ
(125-18)
where λL is the wavelength of the laser in nanometers and λ is the spectral wavelength in nanometers. The prefactor of 107 in the first term of Eq. (125-17) is needed only if it is desired to preserve the numerical value of spectral areas computed relative to the two x-axis coordinate systems. Definition is taken from Ref. [7], with NIST basic application of intensity standard. 1. An NIST-certified value represents data, reported in an SRM certificate, for which NIST has the highest confidence in its accuracy in that all known or suspected sources of bias have been fully investigated or taken into account [14]. 2. The consensus curve is a point-by-point weighted mean of the average responses of three instruments [15,16], fitted by the polynomial model. The uncertainty curves are polynomial models of point-by-point expanded uncertainties, with coverage factor k ¼ t0.975,2 ¼ 4.303 (95% confidence), calculated by combining a between-instrument variance incorporating interinstrumental Type B uncertainties with a pooled within-method variance12, following the ISO/JCGM guide [18,19]. 3. where ISRM(Δυ) ¼ A0 + A1 (Δυ)1 + A2 (Δυ)2 + A3 (Δυ)3 + A4 (Δυ)4 + A5 (Δυ)5, for Δυ ¼ 200 to 3500 cm1 Raman shift relative to 785 nm excitation. ISRM(Δυ) has units of photons s1 (cm2) (cm1)1.
REFERENCES
999
A comprehensive paper describing the complete process of calibrating Raman instruments for intensity using various laser sources and instrument types along with the application of NIST SRMs is given in Ref. [20] as an Applied Spectroscopy feature article. This paper is recommended for anyone that would practice intensity correction for analytical measurements and for understanding the details of this process.
REFERENCES [1] NIST SRM Website: URL at https://www.nist.gov/srm (Accessed September 9, 2017). [2] J. Workman, in: J. Workman (Ed.), The Concise Handbook of Analytical Spectroscopy: Physical Foundations, Techniques, Instrumentation and Data Analysis. In 5 Volumes, UV, Vis, NIR, IR, and Raman, World Scientific Publishing-Imperial College Press, New Jersey/Singapore, 2016. vol. 1, pp. 163–214; vol. 2, pp. 217–278; vol. 3, pp. 363–424; vol. 4, pp. 323–360; vol. 5, pp. 229–250, ISBN-13: 978-9814508056 (Permission from NIST for data and spectra related to SRM materials). [3] Each table refers to specific NIST SRM datasheets and published references and as referred to in Ref. [2]. [4] Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers, American Society for Testing and Materials: ASTM International designation E 275-08, Philadelphia, PA, 2013. [5] Standard Terminology Relating to Molecular Spectroscopy, American Society for Testing and Materials: ASTM International designation E 131-10, Philadelphia, PA, 2010. [6] NIST Raman SRM Catalog Website: http://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.260176.pdf, pp. 162–163 (Accessed September 9, 2017). [7] Standard Reference Material (SRM)® 2241 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 785 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 28 February 2014, 6 p. [8] Standard Reference Material (SRM)® 2242 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 532 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 22 October 2013, 6 p. [9] Standard Reference Material (SRM)® 2244 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 1064 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 03 December 2009, 6 p. [10] Standard Reference Material (SRM)® 2245 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 633 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 27 September 2011, 6 p. [11] Standard Reference Material (SRM)® 2246 Certificate, Relative Intensity Correction Standard for Raman Spectroscopy: 830 nm Excitation, National Institute of Standards and Technology, Gaithersburg, MD, 31 August 2012, 7 p. [12] ASTM E1840-96(2007) Standard Guide for Raman Shift Standards for Spectrometer Calibration, ASTM International, West Conshohocken, PA, USA, 2007. [13] K.J. Frost, R.L. McCreery, Calibration of Raman spectrometer response function with luminescence standards: an update, Appl. Spectrosc. 52 (12) (1998) 1614–1618. [14] W. May, R. Parris, C. Beck II, J. Fassett, R. Greenberg, F. Guenther, G. Kramer, S. Wise, T. Gills, J. Colbert, R. Gettings, B. MacDonald, Definition of terms and modes used at NIST for value-assignment of reference materials for chemical measurements, NIST Spec. Publ. (2000) 136–260. http://www.nist. gov/srm/publications.cfm (Accessed February 2014). [15] A.L. Rukhin, Weighted means statistics in interlaboratory studies, Metrologia 46 (2009) 323–331.
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[16] R. Dersimonian, N. Laird, Meta-analysis in clinical trials, Control. Clin. Trials 7 (3) (1986) 177–188. [17] S.D. Horn, R.A. Horn, D.B. Duncan, Estimating heteroscedastic variances in linear-models, J. Am. Stat. Assoc. 70 (350) (1975) 380–385. [18] JCGM 100:2008; Evaluation of Measurement Data—Guide to the Expression of Uncertainty in Measurement (GUM 1995 With Minor Corrections); Joint Committee for Guides in Metrology, 2008, Available at: http:// www.bipm.org/utils/common/documents/jcgm/JCGM_100_2008_E.pdf (Accessed September 9, 2017). [19] B.N. Taylor, C.E. Kuyatt, Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results, NIST Technical Note 1297, U.S. Government Printing Office, Washington, DC, 1994. Available at: http://www.nist.gov/pml/pubs/index.cfm (Accessed September 9, 2017). [20] S.L. Choquette, E.S. Etz, W.S. Hurst, D.H. Blackburn, S.D. Leigh, Relative intensity correction of Raman spectrometers: NIST SRMs 2241 through 2243 for 785 nm, 532 nm, and 488 nm/514.5 nm excitation, Appl. Spectrosc. 61 (2) (2007) 117–129. http://ws680.nist.gov/publication/get_pdf.cfm?pub_id¼906524 (Accessed September 9, 2017).