Journal of Tea Science Research. 2015, Vol. 5, No. 9, 1-7
5
suggested that the data on leaching of residues into
tea brew should be taken into account for fixing a
realistic MRL for pesticides in black tea. The MRLs
fixed in tea for bifenthrin is 5 mg kg
-1
but it is
evident that black tea processed from green shoots
collected after 7-10 days, after spraying at
recommended dose contained residues below MRL
values. The waiting period based on MRL in black
tea may be therefore fixed as 4 days for bifenthrin.
4 Materials and methods
4.1 Field trials and experimental design
The experiments were conducted tea fields at
Valparai and Gudalur (Tamil Nadu, India) situated at
an elevation of 1140 m and 1150m above MSL.
Plots measuring 100 sq.m, containing tea plants of
mixed cultivars with appropriate guard rows, were
used for the study. Tea plants had been planted in
double hedge, in triangular planting system at a
spacing of 0.75x0.75x1.25 m under the shade tree,
Grevillea robusta (6x6 m spacing). The tea plants
were last pruned at a height of 60 cm above ground
level. The treatments were bifenthrin 8 SC @ 1000
and 2000 mL/ha and an untreated control. The
acaricide was applied with hand operated knapsack
sprayer, using a spray volume of 400 ml ha-1. Tea
shoots consisting of three leaves and a bud were
harvested on 0 (3 hr), 1st, 3rd, 5th, 7th, 10th and
14th days after application of the chemical.
The shoots harvested on the specific day after
chemical application were processed in a miniature
manufacturing unit. Harvested shoots were spread in
a withering trough and allowed to wither with
natural air, blown underneath for 16-18 hrs.
Withered leaves were passed through a rotorvane for
crushing and mixing of leaves and juice. This was
passed four times through a roller cut CTC (Crush,
Tear and Curl) machine. The resulting cut “dhool”
was spread over the fermentation trays at a thickness
of about 2 cm, maintaining a relative humidity of
90-95% for one hr. Fermented (enzymic oxidation)
“dhool” was dried in a mini fluid bed drier to attain
a final moisture content of 2.5-3.0%. Black tea
samples thus obtained were analysed in a gas
chromatograph (Perkin Elmer Clarus 500 GC)
equipped with electron capture detector, following
standard procedure.
4.2 Chemicals and reagents
Pesticide standard, bifenthrin was purchased from
Dr.Ehrenstorfer, Augsburg, Germany. Hexane,
acetone, acetonitrile, diethyl ether, sodium chloride
and sodium sulphate were obtained from M/s. Merck,
Mumbai, India; all were of chromatographic purity.
Florisil (60 – 100 mesh) was obtained from M/s.
Sigma – Aldrich fine chemicals, Bangalore, India.
4.3 Instrument and calibration
Bifenthrin technical standard of 99.0% purity (as per
the certificate of analysis) was used for analysis.
Details of the instruments employed and the
conditions of operations for analysis of residues of
bifenthrin are given below. A Perkin Elmer Clarus
500 GC with an electron capture detector was used
for separation and quantitative analysis, and a DB-5
Cross linked 5% PH ME Siloxane capillary column
30 m length, 0.32 mm internal diameter and 0.25µ
film thickness) was used for gas chromatographic
determination. Nitrogen at a speed of 5 ml min-1
was used as the carrier gas. The temperature of oven,
injector and detector was set at 210, 180 and 300 °C,
respectively and 0.5 µl of sample was injected for
detection. Quantification was accomplished by using
a standard curve prepared by diluting the stock
solution in hexane. Good linearity was achieved in
the range 0.003–1.0 µg ml
-1
with a correlation
coefficient of 0.9996. The limit of detection was
estimated to be 0.003 µg ml
-1
of bifenthrin. The
column was conditioned by three repeated injections
of standard and sample extracts until GC peaks were
reproducible.
4.4 Analysis
4.4.1 Extraction
Twenty gram of black tea sample was extracted with
150 ml of acetonitrile:water (2:1, v/v) by keeping
it in a mechanical shaker for two hrs. The contents
were filtered and to the filtrate 200 mL of 4%
NaCl and 60 ml of hexane were added. After
partitioning, the hexane layer was pass ed
through anhydrous sodium sulphate layer to a 500
ml round bottomed flask.
