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Section 12- Protein Bradford
Measurement Concentration Range
Unknown protein concentrations from ~100ug/ml up to several
thousand micrograms/ml (ug/ml) can be determined on the NanoDrop
1000 Spectrophotometer using the regular Bradford assay. The best
linearity is in the 100-1000 ug/ml range. A mini-Bradford assay
covers the approximate range of 15-125 ug/ml.
Coomassie dye-dye and Coomassie dye-protein aggregates are
frequently encountered in Coomassie dye-based protein assays.
Particulates often form with increasing development time, which can
cause significant fluctuations in Absorbance readings. It is also
important to note the total analyte (protein-dye) signal at 595 nm is
limited to ~ 0.150 A as a result of the 1.0mm pathlength of the
instrument, the Bradford (Coomassie dye) reagent concentration, and
the acidic pH. Making measurements in
triplicate
of standards and
samples (unknowns) is good practice, particularly with the limited
assay signal obtained with the Bradford Assay.
Assay
Type
Approx.
Lower
Limit
Approx.
Upper
Limit
Typical Reproducibility
(minimum 5 replicates)
(SD= ug/ml; CV= %)
Regular
Bradford
100
ug/ml
8000
ug/ml
sample range 100-500 ug/ml:
±
25 ug/ml
sample range 500-8000 ug/ml:
±
5%
Mini Bradford
15 ug/ml
100 ug/ml
sample range 15-50 ug/ml:
±
4
ug/ml
sample range 50-125 ug/ml:
±
5%
Bradford Kits, Protocols, and Sample Preparation
Commercial Bradford Protein kit manufacturers typically outline
procedures for two different concentration ranges:
•
A regular assay – using a 50:1 reagent / sample volume ratio. To
accurately prepare standards, we suggest using a minimum
sample volume of 4 ul in 200 ul of Bradford reagent (larger
sample volume is preferable).
•
A mini assay– using a 1:1 reagent / sample volume ratio. To
prepare sufficient volume of these 1:1 mixtures, we suggest using
a minimum of 10 ul of sample and 10 ul of Bradford reagent in a
PCR tube. Use the same pipettor for both volumes to eliminate
any pipette-to-pipette accuracy differences.
12-2