Photobleaching in cotton fibers dyed using red, yellow, and blue direct 

dyes during examination with microspectrophotometry (MSP)

Amanda L. Forster, Julie L. Bitter, Sydney Brooks, Samuel Rosenthal, and Stephanie S. Watson

National Institute of Standards and Technology, 100 Bureau Dr. M/S 8102 Gaithersburg, MD 20899-8102, USA

Introduction

The ballistic performance of ultra-high molar mass polyethylene

(UHMMPE) fibres for ballistic protection is predicated on the

development of a highly aligned molecular structure that allows the

polymer to exhibit a superior strength in the axial direction of the fibre.

However, even an ideal molecular structure will be subjected to

degradation during use, which can reduce the high strength of these

fibres, and impact their ability to protect the wearer.

Materials and Methods

Comparison of Dyes and Dyed Fibers

Dyeing Experiments:

• Direct dye was applied to cotton fibers in the

laboratory.

• Direct Blue 71,

• Direct Red 81

• Direct Yellow 27

• Various dyebath concentrations were used.

• 0.05 % m/v, 0.033 % m/v, 0.025 % m/v,

and 0.005 % m/v

Microspectrophotometery Measurements:

• Absorbance spectra of single fibers mounted in 

glycerol on quartz were obtained using a MSP 121 

microspectrophotometer from CRAIC 

Technologies.

• Analysis was repeated at specified intervals.

Introduction Observation of Photobleaching- Red Dye

• Microspectrophotometry (MSP) is a rapid, nondestructive technique for

analysis of color in textile fibers.

• MSP combines microscopy and ultraviolet/visible (UV-Vis) spectroscopy and

eliminates the need for time consuming and destructive extraction of dyes

from textile fibers when examining colored fibers with traditional techniques

such as thin layer chromatography (TLC) or high performance liquid

chromatography (HPLC).

• While MSP is generally expected to be a nondestructive evaluation method, a

loss of color during analysis, or photobleaching can occur.

Conclusions and Future Work

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• Photobleaching is readily observed on cotton fibers dyed with red, yellow, and blue direct

dyes, and forensic scientists should be aware of this phenomenon.

• Photobleaching can be minimized by careful control of measurements on susceptible fibers

(reduction in time of exposure, careful use of shutters, etc.)

• Additional measurements will be made to better quantify repeatability.

• Recommendations for exposure time and filtering procedures will be developed.

Direct Blue 71

Direct Red 81

Direct Yellow 27

Dyes

• A portion of each dyebath solution was

analyzed using UV/Vis spectroscopy.

• Allows for comparison of spectra between

the dye solution and the dyed fiber.

• The overall shape and position of

absorbance peaks was found to be

similar for each dye.

• Some slight differences, especially

between the yellow and red dye

around 400 nm.

• Overall, absorbance was lower, as

expected since MSP is a microanalysis

technique.

MSP of Dyed Fibers 

0.05 %

Photobleaching

• MSP generally expected to be a nondestructive

evaluation method.

• High intensity UV light sources can cause a reduction in

the intensity of absorption during analysis, a

phenomenon typically referred to as photobleaching.

• Historically, dye chemists have been concerned with

lightfastness of dyed fabrics.

• Fading of dyed fabrics after exposure to sunlight is wellknown and related, but the lamps used in the MSP

cause fading to occur rapidly.

• A recent review article found that approximately half

of the scientific papers on spectroscopic analysis of

textile fibers focused on UV-Vis MSP, but

photobleaching was only mentioned in a few papers

from Was-Gubala and Starczak.

http://craftyc0rn3r.blogspot.com/2010/05/patio‐

furniture‐reupholstering.html

Dyed fabric showing poor 

lightfastness.

2nd Red 1

Wavelength (nm)

200 400 600 800

Absorbance 0.1

0.2

0.3

0.4

0.5 Time = 0 min 

Time = 1 min 

Time = 2 min 

Time = 3 min 

Time = 4 min 

Time = 5 min 

Time = 6 min 

Time = 7 min 

Time = 8 min 

Time = 9 min 

Time = 10 min 

Time = 11 min 

Time = 12 min 

Time = 13 min 

Time = 14 min 

Time = 15 min 

Time = 16 min 

Time = 17 min 

Time = 18 min 

Time = 19 min 

Time = 20 min 

2nd Red 1 Change in Absorbance

Exposure Time (min)

0 5 10 15 20

Percent Loss 0.92

0.94

0.96

0.98

1.00

1.02

518nm

405nm

293nm

0 minutes: red 10 minutes: pink/white 30 minutes: almost 

completely white

3rd Red 1

Wavelength (nm)

200 400 600 800

Absorbance 0.1

0.2

0.3

0.4

0.5 Time = 0 min 

Time = 1 min 

Time = 2 min 

Time = 3 min 

Time = 4 min 

Time = 5 min 

Time = 6 min 

Time = 7 min 

Time = 8 min 

Time = 9 min 

Time = 10 min 

Time = 11 min 

Time = 12 min 

Time = 13 min 

Time = 14 min 

Time = 15 min 

Time = 16 min 

Time = 17 min 

Time = 18 min 

Time = 19 min 

Time = 20 min 

3rd Red 1 Change in Absorbance

Exposure Time (min)

0 5 10 15 20

Percent Loss 0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

518nm

405nm

293nm

• Poor repeatability between changes in absorbance in dyed fibers.

• Due to nonuniform dye uptake- common in natural fibers.

3rd Blue 1

Wavelength (nm)

200 400 600 800

Absorbance 0.00

0.05

0.10

0.15

0.20

0.25

0.30 Time = 0 min 

Time = 1 min 

Time = 2 min 

Time = 3 min 

Time = 4 min 

Time = 5 min 

Time = 6 min 

Time = 7 min 

Time = 8 min 

Time = 9 min 

Time = 10 min 

Time = 11 min 

Time = 12 min 

Time = 13 min 

Time = 14 min 

Time = 15 min 

Time = 16 min 

Time = 17 min 

Time = 18 min 

Time = 19 min 

Time = 20 min 

3rd Blue 1 Change in Absorbance

Exposure Time (min)

0 2 4 6 8 10 12 14

Percent Loss 0.82

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

587 nm

293 nm

Observation of Photobleaching- Blue and Yellow Dyes

2nd Yellow 1

Wavelength (nm)

200 400 600 800

Absorbance 0.00

0.05

0.10

0.15

0.20 T = 0 min 

T = 1 min 

T = 2 min 

T = 3 min 

T = 4 min 

T = 5 min 

T = 6 min 

T = 7 min 

T = 8 min 

T = 9 min 

T = 10 min 

T = 11 min 

T = 12 min 

T = 13 min 

T = 14 min 

T = 15 min 

2nd Yellow 1 Change in Absorbance

Exposure Time (min)

0 2 4 6 8 10 12 14

Percent Loss 0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

409 nm

• Similar trends were observed for the fibers dyed with blue and yellow dyes in

the highest concentration.

• Repeatability still an issue due to nonuniform dye uptake.

• Lower concentrations of yellow (not shown) did not demonstrate much of an

effect, possibly since there was so little dye on the fiber at time zero.

• Photobleaching can be minimized by using a glass slide or UV filter (results

not shown here) to protect the fiber while setting up a measurement.

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