American Journal of Analytical Chemistry, 2012, 3, 622-631
http://dx.doi.org/10.4236/ajac.2012.39081 Published Online September 2012 (http://www.SciRP.org/journal/ajac)
3D Fluorescence Characterization of
Synthetic Organic Dyes
Leonard J. Soltzberg1, Sandy Lor1, Nnennaya Okey-Igwe1, Richard Newman2
1Department of Chemistry & Physics, Simmons College, Boston, USA
2Scientific Research Lab, Museum of Fine Arts, Boston, USA
Received June 27, 2012; revised August 2, 2012; accepted August 13, 2012
ABSTRACT
The identification of dyes is important in research on museum artefacts as well as in forensic applications. UV-visible
absorption spectroscopy cannot unambiguously distinguish dyes with similar hues, while mass spectrometry may fail to
distinguish isobaric dyes. The detailed patterns produced by 3D fluorescence spectroscopy appear to be virtually unique,
even among dyes that are closely related positional isomers. We report these patterns for 65 dyes from the Schweppe
Library of Synthetic Organic Dyes as well as measurements suggesting both the capabilities and limitations of this
method.
Keywords: Fluorescence; Spectrophotometry; Dye Analysis; MALDI-TOF; Mass Spectrometry
1. Introduction
The identification of dyes and pigments is important in a
museum setting for authentication of artefacts, conserva-
tion, and for art historical research. For example, the re-
cent identification of the yellow lichen-derived dye vulpinic
acid in a Northwest-Coast ceremonial blanket dictated the
need to display this artefact in subdued light [1]. MALDI-
TOF (matrix-assisted laser desorption time-of-flight) mass
spectrometry has been used successfully to characterize a
library of synthetic organic dyes [2]. However, that method
is less useful for distinguishing isomeric dyes such as Acid
Yellow 36 (CI 13065) and Acid Orange 5 (CI 13080)
because the molecules are isobaric.
3D fluorescence spectra, obtained by measuring fluo-
rescence emission via scanning both the excitation wave-
length (EX) and the emission wavelength (EM), can be
presented as contour plots on X-Y axes defined by EM
and EX. It has been suggested that these fingerprint-like
patterns are better able to discriminate among similar dyes
than are individual numerical data, such as tabulated ab-
sorption maxima [3-5].
Interesting work has been done using fiber optic probe
reflectance 3D fluorescence spectrophotometry as applied
to dye analysis in situ in textiles, prints and manuscripts
[3-5]. While that approach has the advantage of being rig-
orously non-destructive, the quality of the spectra makes
them difficult to apply as an identification tool. In this work,
we used 3D fluorescence spectra to characterize water
solutions of dyes in an important standard dye collection
[6], supplemented by MALDI-TOF mass spectra. The use
of solutions improves the quality of the spectra, elimi-
nates some background artifacts and yet, because of the
sensitivity of fluorescence spectrophotometry, requires only
nanomole quantities of analyte. This same dye collection
has been the subject of analysis by HPLC [4] and MALDI-
TOF mass spectrometry [2].
2. Experimental
2.1. Instrumentation
We used a Hitachi F4500 fluorescence spectrophotome-
ter and a Hitachi 650-0116 microcell to obtain fluores-
cence spectra; 200 μL of solution was required for this
cell. The excitation wavelength EX was scanned from 250
- 380 nm and the emission wavelength EM was scanned
from 395 - 700 nm. These two ranges purposely do not
overlap in order to avoid exposing the photomultiplier to
1st order scattering from the Xe light source. The scan-
ning rate was 1200 nm/min, and the response time was
set at 0.1 sec. These conditions allowed us to obtain a
spectrum in less than 8 minutes with no apparent loss of
detail, compared with slower scan speeds.
For those dyes for which the extinction coefficients were
available, concentrations of the test solutions were de-
termined by measuring the absorbance using an Agilent
8453 diode array UV-visible spectrophotometer and an
Agilent 5062-2496 50 μL ultra-micro quartz cell.
MALDI-TOF mass spectra were obtained with a Bruker
Omniflex instrument in reflectron mode, using the matrix
C
opyright © 2012 SciRes. AJAC

L. J. SOLTZBERG ET AL. 623
9-aminoacridine and sample preparation methods described
elsewhere [2,7].
2.2. Materials
The original Schweppe Library of Synthetic Organic Dyes
[6] is housed at the J. P. Getty Museum in Los Angeles,
CA. The samples from this Library in the possession of
the Boston Museum of Fine Arts (MFA) are solutions in
methanol. For the current work, we diluted samples from
the MFA collection with Sigma-Aldrich HPLC-Plus wa-
ter (Sigma-Aldrich 34,877) to give concentrations suitable
for recording fluorescence spectra.
Since the original Schweppe collection is over 50 years
old, we also examined fresh commercial samples of some
of the dyes, especially those showing obvious decompo-
sition based on the appearance of the library solutions:
Acid Black 1 (CI 20470), Basic Green 4 (CI 42000), Acid
Blue 93 (CI 42780), Acid Black 2 (CI 50420), Murexide
(CI 56085), Mordant Red 3 (CI 58005), and Acid Blue
74 (CI 73015). For these dyes, the fluorescence spectra
shown in Table 1 were obtained using fresh dye samples.
3. Results and Discussion
The 3D fluorescence spectra of all 65 Schweppe reference
dyes are shown in Table 1 along with the corresponding
molecular structures, organized by structure type. We refer
to those spectra below using the Schweppe number; e.g.
“S10”.
The concentrations of the dyes in the Schweppe col-
lection are not known. However, for dyes for which we
could find the extinction coefficients [8], we checked the
concentration of the test solutions used for the fluores-
cence spectra by measuring their absorption spectra in
the range 200 - 800 nm with the Agilent diode array spec-
trometer. These concentrations ranged from 1.5 × 10−6 M
to 9.0 × 10−5 M.
3.1. Instrumental Artifacts
Certain artifacts are characteristic of fluorescence spectra
and need to be taken into account when examining these
3D fluorescence spectra. First, Rayleigh scattering of the
excitation beam produces a distinct diagonal feature, as
seen, for example, with S10 Acid Orange 6 (CI 14270)
(Table 1) and with the water blank shown in Figure 1.
We purposely chose the excitation and emission wave-
length ranges to exclude scattered light from the intense
1st order beam, but the Rayleigh scattering of the 2nd or-
der diffracted light from the Xe source is clearly seen in
many of the spectra along the diagonal line λEM = 2λEX.
This feature provides an approximate gauge for the fluo-
rescent intensity of the dye itself. For weakly fluorescent
dyes such as S10, the Rayleigh scattering is more promi-
nent than any features arising from the dye. With a strongly
fluorescent dye such as S50 Basic Red 1 (CI 45160), the
Rayleigh scattering is not even seen, relative to the inten-
sity of the fluorescence of the dye itself.
Another artifact is Raman scattering from the solvent.
This phenomenon produces a diagonal feature in which the
energy of a scattered photon is shifted by a constant amount
from the excitation photon energy. For water, the features
arising from Raman scattering appear at the EX/EM val-
ues shown in Table 2 and in the pattern at the upper left
of Figure 1. The Raman scattering is considerably weaker
than the Rayleigh scattering. A different Raman scatter-
ing feature arising from the ν2 vibration of water [9] and
excited by the 2nd order light from the instrument lamp,
appears below the Rayleigh scattering feature in Figure
1; this feature is seen only in a few of the dye spectra in
Table 1.
Finally, it has recently been reported that the fluores-
cence spectra of highly purified water show features at-
tributed to interaction among trace impurities, including
air gases, and the hydrogen bond network of the water
[10]. We believe that the feature seen in a number of our
dye spectra at EX = 260 nm and EM = ~440 nm is an
example of these background effects. This feature, as well
as the Rayleigh and Raman scattering features, are seen
clearly in Figure 1 (HPLC-grade water blank). Given the
age of the Schweppe library samples, it is expected that
the solutions may also include species arising from de-
composition of the dyes.
3.2. Decomposition of the Dyes
The effect of decomposition can be seen, for example,
with the decades-old Schweppe sample of Acid Blue 74
(CI 73015) (nominal mass 466 Da). Figure 2 compares
the 3D fluorescence spectrum of that solution with that of
a solution freshly prepared from commercial reagent-
grade dye. The fluorescence intensity as well as the cen-
troid of the principal peak is clearly different in the fresh
solution. These differences can be understood by reference
to the corresponding MALDI-TOF mass spectra, shown
in Figure 3. The mass spectrum of the decomposed sam-
ple shows only a small peak at m/z 421 (the singly-charged
molecular ion [M − 2Na + H]− compared with the large
m/z 421 peak from the fresh sample.
3.3. Uniqueness of the 3D Fluorescence Patterns
Apart from the few Schweppe dyes showing no fluore-
scence other than background features, the patterns of the
fluorescent dyes appear to be unique, even for structurally
very similar dyes. The observation of Clarke [3] and of
van Bommel, et al. [4], is borne out for some compa-
risons, such as S5 Acid Yellow 36 (CI 13065) vs S6 Acid
Orange 5 (CI 13080) shown in Figure 4: subtle differ-
ences in the overall patterns are more diagnostic than a

L. J. SOLTZBERG ET AL.
C AJAC
624
Table 1. 3D fluorescence spectra of dyes in the Schweppe
Library. Grouped by structure type.
opyright © 2012 SciRes.
S 4a Basic
Orange 2
N
N
NH3
H2N
S 9 Mordant
Yellow 1
C
OH
N
N
O
2
N
O
O
S 10 Acid
Orange 6 N
N
OH
OH
SO
3
S 5 Acid
Yellow 36
N
N
SO
3
NH
S 6 Acid
Orange 5
NH
N
N
SO
3
S 7 Acid
Orange 1
N
N
N
SO
3
NO
aSchweppe Library number.
S 11 Acid
Orange 20
N
N
OH
O
3
S
S 13Acid
Orange 7
O
3
S
N
N
HO
S 16Acid
Orange 12
N
N
HO
SO
3
S 19Acid
Orange 14
N
N
HO
O
3
SSO
3
S 20Acid Red
26
CH
3
N
HO
O
3
SSO
3
N
H
3
C
S 23Acid
Orange 10
N
N
HO
SO
3
SO
3
S 8Acid Red
74
NO
2
N
N
NH
2
O
3
S
S 27Acid Red
33
N
N
OH
O
3
S
NH
2
SO
3
S 28Acid Red
1
N
N
SO
3
H
N
OH
O
3
S
CH
3
C
O
S 29Acid
Violet 7
CH
3
HN O
N
N
O
3
S
H
N
OH
CH
3
SO
3
O

L. J. SOLTZBERG ET AL. 625
S 34 Direct Red
28
N
N
N
N
O
3
SNH
2
H
2
NSO
3
S 12 Acid
Brown 6
N
N
SO
3
OH
S 14 Acid Red
88
N
HO
N
SO
3
S 15 Acid Red
9
N
N
HO
SO
3
S 17 Acid Red
13
N
N
HO
SO
3
SO
3
S 18 Acid Red
25
N
N
HO
SO
3
SO
3
S 21 Acid Red
17
N
HO
O
3
SSO
3
N
S 22 Acid Red
27
N
HO
O
3
SSO
3
N
SO
3
S 24 Acid Red
44
N
N
HO
SO
3
SO
3
S 25Acid Red
18
SO
3
N
NSO
3
HO
SO
3
S 26Acid Red
41
SO
3
N
NSO
3
HO
SO
3
O
3
S
S 40Basic
Green 4
H
3
CCH
3
N
NCH
3
H
3
C
S 41Basic
Green 1
N H
3
CCH
3
CH
3
N
H C
3
S 42Acid Blue
3
Et Et
N
OH
Et NSO
3
O
3
S
Et
S 43Acid
Green 6
N
O
3
S N
H 3 C
H 3 C
SO
3
SO
3
S 44Basic
Violet 14
NH
3
H
3
C
H
2
NNH
S 45Basic
Violet 1
CH
3
H
3
CN
N
CH
3
H
3
CNH
CH
3
S 46Basic
Violet 3
CH
3
H
3
CN
N
CH
3
H
3
CNCH
3
CH
3

L. J. SOLTZBERG ET AL.
626
S 47 Acid Blue
93
HN
N
H
N
H
SO 3
SO 3 O 3 S
S 48 Basic Blue
11
N
N
CH
3
H
3
C
NH
Et
CH
3
H
3
C
S 49 Basic Blue
26
N
NH
N
H
3
CCH
3
H
3
C
CH
3
S 50 Basic Red
1
OCH 3
O
O
H
3
C
N
H
H 3 C
CH
3
N
HCH
3
S 51 Solvent
Red 49
ONN
O
O
E
t
Et
Et
Et
S 52 Acid
Yellow 73
O
O
O
OO
S 53 Acid Red
87
O
O
O
Br
O
BrBr
O
Br
S 54 Acid Red
51
O
I
O
IO
O
I
O
I
S 61
Mordant
Red
11
OOH
OH
O
S 62Mordant
Red 3
OOH
OH
SO
3
O
S 63Alizarin
Red PS
OOH
OH
SO
3
OOH
S 64Mordant
Red 2
OOH
O
SO
3
OH
HO
S 1Picric acid
OH
O
2
NNO
2
NO
2
S 2Acid
Yellow 24
OH
O
2
N
NO
2
S 3Acid
Yellow 1
ONa
O
2
NSO
3
NO
2
S 36Acid Red
151
NN N
NO
3
S
HO

L. J. SOLTZBERG ET AL. 627
S 37 Acid Red
115
N
NN
NCH
3
SO
3
H
3
C
SO 3
O
3
S
CH
3
S
38
Acid Red
73
N
NN
N
HO
SO
3
O
3
S
S 30 Acid
Yellow 11
NN
N
N
CH
3
OH
O
3
S
S 31 Acid
Yellow 23
NN
N
N
C
OH
O 3 S
SO 3
OO
S 32 Acid
Black 1
OHNH 2
N N
SO 3 O 3 S
NN
O 2 N
S 33 Basic
Brown 1
NN NN
H
2
NNH
3
NH
2
H
3
N
S 35 Direct
Yellow 4
OH
NNSO
3
O 3 SNN
OH
S 39 Basic
Yellow 2
NH
2
N
NCH
3
H
3
C
CH
3
CH
3
S 56Basic Red
2
H
3
C N
N
CH
3
H
2
NNH
2
S 57Mauveine
C
2
H
5
N
N
C
2
H
5
H
2
NNH
2
S 55Acid
Yellow 3
N
O
SO3O
S 59Basic Blue
9
S
N
CH
H
3 CN
CH
3
N
3
CH
3
S 60Murexide
O
O
HN
N
H
OO
N
N
H
NH
O
O
S 65Acid Blue
74
O
N
H
O3SHN
SO3
O
S 58Acid
Black 2
structure
uncertain
tabulation of peak positions would be. These two isobaric
dyes are not distinguished by MALDI-TOF mass spectr-
ometry [2]. Their 3D fluorescence spectra are, indeed,
very similar but not identical. If the contour patterns are
viewed as topographic maps, one sees a prominent “hill”
centered around λEX 290 nm and λEM 395 nm in both
spectra. However, the topographic profile of the “descent”
from the “summit” toward the “northeast” is noticeably
different for the two dyes.

L. J. SOLTZBERG ET AL.
628
Figure 1. 3D fluorescence spectrum of HPLC grade water
showing diagonal features resulting from (left to right) Ra-
man scattering of 1st order Xe lamp photons by water vi-
bration mode
ν
3, Rayleigh scattering of 2nd order Xe lamp
photons, and Raman scattering of 2nd order Xe lamp pho-
tons by water vibration mode ν
2. The mask-like artifact in
the lower left is a transient feature [10].
Table 2. Raman scattering from water seen in 3D fluores-
cence spectra.
λEX (nm) λEM (nm)
Raman shift from water ν3
(~3400 cm−1) [11] 248 271
313 350
365 416
405 469
436 511
λEX (nm) λEM (nm)
Raman shift from water ν2
(~1550 cm−1)* 510 553
530 578
550 600
570 625
590 649
610 674
*Calculated values.
3.4. Robustness of the 3D Fluorescence Patterns
In order to be useful analytically, the 3D fluorescence
patterns need to be relatively invariant over the range of
conditions likely to be encountered. Some of these dyes
are pH indicators, and we did not attempt to buffer the
experimental media. For the majority of the dyes, however,
the principal question is whether the 3D fluorescence
patterns are sensitive to concentration. We investigated
this issue for several of the dyes in the Schweppe Library.
In each case, we prepared a stock solution by weighing
out the solid dye with an Ohaus GA200D 0.01 mg bal-
ance and dissolving in Aldrich A452 HPLC grade metha-
nol. Spectra were run for several different dilutions of each
stock solution. The comparisons show that, aside from
small shifts in the centroids of peaks, the patterns are
essentially unaffected by concentration. One such com-
parison is shown in Figure 5.
Figure 2. Comparison of 3D fluorescence spectra of original
Schweppe sample (top) and fresh commercial sample of
Acid Blue 74 (CI 73015) (botttom).
Figure 3. Negative ion MALDI-TOF mass spectra of the
two Acid Blue 74 samples shown in Figure 2.

L. J. SOLTZBERG ET AL.
629
Figure 4. 3D fluorescence spectra of isomeric dyes Acid Yellow 36 (CI 13065) (left) and Acid Orange 5 (CI 13080) (right).
3.5. Sensitivity
S54 Acid Red 51 (CI 45430)
mg dye/L Concentration (mol/L)
5 7.1 × 10–9
40 5.6 × 10–8
In a previous study of this same dye library using MALDI-
TOF mass spectrometry, we found a limit of detection on
the order of 10−13 mol of dye on the MALDI target [2].
The dye solutions studied here typically correspond to
about 10−8 mol of dye in the cuvette, since the microcell
requires only 200 μL of solution. Using Acid Red 27 (CI
16185) as an example, the limit of detection for a recog-
nizable 3D fluorescence pattern is conservatively about 3
× 10−8 M, or 6 × 10−12 mol of dye in the cuvette. For the
highly fluorescent dye Basic Red 1 (CI 45160), the limit
of detection is about 3 × 10−9 M, or 6 × 10−13 mol of dye
in the cuvette.
3.6. Application to an Historic Sample
The development of synthetic dyes following Perkin’s 1856
discovery of mauvieine coincided approximately with the
opening of Japan to the West, beginning in 1852. It is of
interest to ascertain how quickly Japanese artists adopted
synthetic dyes for printing inks and other applications. In
this context, we extracted a purple dye from a fragment
of a Japanese block print from around 1900 (Figure 6).
The dye extracted readily with water. The fluorescence
spectrum of the dye extract is shown in Figure 7, compared
with an extract from unused white drawing paper. There
is evidently no fluorescence from the dye solution other
than that due to background features. None of the patterns
from the Schweppe Library dyes can be recognized in
Figure 7.
80 1.0 × 107
It has been noted elsewhere that natural dyes are typi-
cally not fluorescent, in contrast with most synthetic or-
ganic dyes [12]. Knowing that this ink is likely based on
natural dyes narrows the field of candidates to the plant
or animal sources known to have been used in Asia.
Figure 5. Effect of concentration on 3D fluorescence pattern
of Acid Red 51 (CI 45430). Note that the contour interval
for the most dilute sample is set to a smaller value than for
the other two spectra in order to emphasize the shape of the
pattern.
These are relatively few in number [5,13]. There are
no pure purple dyes among those candidates, so a purple
mixture would have been made from madder, safflower,

L. J. SOLTZBERG ET AL.
630
or redbud (red) and indigo or dayflower (blue). Figure 8
compares the MALDI-TOF mass spectrum of the purple
Figure 6. Fragment of early 20th C Japanese block print
(height = 58 mm). Ink sample analyzed is from purple area
of robe.
Figure 7. 3D fluoresce nce spectrum of pur ple ink (top) from
20th C Japanese block print compared with extract of un-
printed drawing paper (bottom).
(a)
(b)
(c)
Figure 8. Negative ion MALDI-TOF mass spectra of (a)
purple ink from Japanese block print; (b) blue dye from
Commelina communis (dayflower) petal; (c) ground Rubia
cordifolia (Indian madder) root.
ink extracted from the print with mass spectra of madder
root and dayflower petal extracts. This comparison, taken
together with the above observations, suggests that this
purple ink was prepared from a mixture of a red madder
dye plus the blue dayflower blue dye.
Because of the distinctive fluorescence patterns of the
synthetic dyes studied and the high sensitivity of this
method, it is very unlikely that this ink sample contains a

L. J. SOLTZBERG ET AL.
631
synthetic dye.
4. Conclusion
3D fluorescence spectra can provide an important tool for
the identification of dyes. These spectra serve as finger-
prints that are virtually unique, even among closely re-
lated dyes. The sensitivity of fluorescence spectrophotome-
try means that identification can be accomplished with
very small analyte samples. Supplemented by mass spec-
trometry when necessary, dye identification using 3D
fluorescence is a rapid and reliable analytical strategy.
5. Acknowledgements
We thank the Camille and Henry Dreyfus Foundation for
a Senior Faculty Mentor grant supporting the participa-
tion of Lor and Okey-Igwe. We also thank the National
Science Foundation for grant CHE-0216268. Mass spec-
tra in this manuscript were prepared using the application
mMass [14].
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