difference in the shift of regression straight line from zero is
observed, value b is involved otherwise b is unkept.
Linearity of the method was evaluated as an ability to
provide results proportional to concentration within the defined
interval [16]. A maximum allowing deviation from average
relative response was assigned, which was graphically represented
as a lower and upper limit of acceptable value of relative
response. Linearity of the method was provided to the point
with relative average response intervening between lower and
upper limit of acceptance value of relative response.
For the purposes of result evaluation using the standard
addition method, limit of detection as a triple and limit of
determination as a decuple of standard deviation from the
blanks are calculated. For the purposes of result evaluation
using the calibration curve method, limit of detection and limit
of determination are calculated from the upper limit approach
(ULA) [17,18]. The condition is equidistant distribution of
concentrations. Relation for detection limit calculation
LODucD ¼ kD:sy
m , kD is table value for (n-2; 0.01) in calibration
relation y=b+mc, or for (n-1; 0.01) in calibration relation
y=mc, sy is residual standard deviation. Relation for determination
limit calculation LOQ=3·LOD.
All mentioned validation characteristics were calculated in
Excel program using computing tables. Created computing tables
allow calculations of validation characteristics from measured
data. The results are submitted to numeral and verbal evaluation
according to pre-established requirements for this purpose.
All partial results of evaluation are summarized in one table
as the validation protocol.
Standard uncertainty of these methods is calculated as an
extended combined uncertainty according to Eurachem documents
in the PC program Metro2003, version 3.02 [19].
2.6. Sample and sample preparations
Samples were washed with demineralized water to remove
all the salts produced by soil, sweat, etc. Clean tomatoes were
homogenized. Homogenized fresh matter was submitted to
mineralization.
Digestion procedure: Fresh matter and dry matter (reference
material) samples were weighed directly into mineralization
autoclaves (type ZA-1) at 10 g and 1 g respectively.
Samples were digested in digestion mixture 5 ml conc.
HNO3 and 2 ml H2O2, and dry matter samples had an extra
addition of 1 ml deionized water to moisturize. Samples were
closed in iron mineralization autoclaves and digested in hot-air
sterilizers at 140 °C for 120 min.
Sample modification for HG-AAS determination: after
cooling the samples, mineralizates were quantitative transferred
to 25 ml volumetric flasks, 4 ml HCl (7 mol/l) was added and
than heated 30 min at 80 °C. After cooling the samples
volumetric flasks were filled by HCl (1.7 mol/l) to the match
mark and measured immediately.
Sample modification for ET-AAS determination: after cooling
the samples, mineralizates were quantitatively transferred to
25 ml volumetric flasks, and filled with deionized water to the
match mark.
3. Results
Comparison of the possibility to determinate Se was
performed from the results gained from Se determination in
blanks and in real samples. Measurements were performed with
the series of blank and tomato samples analysis of by the HGAAS
and both ET-AAStechnique respectively.Absorbance of the
blanks from AAS measurements was compared to absorbance of
the tomato samples. Results of measurements and statistical
comparison of three techniques is described in Table 1.
Results show clear distinction of the absorbances of the
blank and vegetable samples performed by ET-AAS technique
with Zeeman background correction and technique HG-AAS.
Means of absorbance (±sd) of ET-AAS technique with Zeeman
background correction as well as the technique HG-AAS are
significantly different, whereas the ET-AAS technique with
deuterium background correction is not significantly different at
5% level by Mann–Whitney's U-test.
Validation of the three techniques listed above was realized