Page 37 - Volume 2,Issue 2, July 2009

LC-MS/MS multiresidue method in peaches

83

b

the slope of the regression line.

The precision of the method was esti-

mated by assessing the relative standard de-

viation (RSD) values of the five replicates at

both levels. According to the Document No

SANCO/2007/3131, repeatability with RSD ≤

20% is considered to be acceptable. The lim-

it of quantification (LOQ) was established as

the lowest concentration that gave satisfac-

tory recovery (70–110%) and precision (RSD

≤ 20%) data.

Linearity

Good linearity was found for most pes-

ticides with correlation coefficients better

than 0.990 in most cases. However, deme-

ton-S-methyl, fenpyroximate and methio-

carb sulfone did not exhibit a linear behav-

ior at the concentration range under study,

but at a narrower linear range, as shown in

Table 3. The dynamic linear range in electro-

spray ionization can be limited and gener-

ally depends on the physicochemical char-

acteristics of the analyte (3). In Table 3, the

basic calibration line parameters for the 56

compounds are presented (13).

Accuracy and precision

Recovery and repeatability of the meth-

od were established in order to evaluate the

method’s accuracy and precision, respec-

tively. Recoveries of 70–120% with a repeat-

ability RSD ≤ 20% are considered acceptable

(5), while in routine analysis, the acceptable

recoveries are 60 - 140%. The recoveries were

calculated using matrix-matched single-lev-

el calibration standards. Table 4 shows the

detailed recovery and repeatability data of

the pesticides studied. The majority of the

pesticides produced recoveries and RSD val-

ues within the accepted ones.

The recoveries of the pesticides ranged

from 65.7 to 125.3% with relative stand-

ard deviations (RSDs) less than 24.7% at the

lowest concentration level and from 70 to

110.9%with RSDs less than 27.1% at the high-

est level. In the cases of alachlor, aldicarb,

aldicarb sulfone, aldicarb sulfoxide, bro-

muconazole, carbofuran 3-hydroxy, deme-

ton–S-methyl, hexaconazole, mepanipyrim,

methiocarb sulfone, methiocarb sulfoxide

and pyrimethanil, the recovery and repeat-

ability values at the lowest concentration

level were outside the acceptable levels.

Therefore, the method is not considered

suitable for the determination of these com-

pounds at the lowest level. For clofentezine,

fenpyroximate, indoxacarb and prochloraz,

the RSDs at the highest concentration lev-

el were above 20%, but with satisfactory re-

coveries.

Limit of quantification/Limit of detection

As limit of quantification (LOQ) of the

method, the lowest validated level that pro-

duced acceptable accuracy and precision

results, which in most cases was 0.01 mg/

kg, was selected. In the cases of alachlor,

aldicarb, aldicarb sulfone, aldicarb sulfox-

ide, bromuconazole, carbofuran 3-hydroxy,

demeton-S-methyl sulfoxide, hexaconazole,

mepanipyrim, methiocarb sulfone, methio-

carb sulfoxide and pyrimethanil, the LOQ

was set at 0.05 mg/kg.

The limit of detection (LOD) of the meth-

od is the minimum concentration or mass

of the analyte that can be detected with ac-

ceptable certainty, though not quantifiable

with acceptable precision. The LOD of the

method was calculated as three times low-

er than the LOQ (LOQ = 3xLOD).

5. Analysis of real samples

To evaluate its application, the proposed

methodology was applied to the analysis of

real stone fruit samples. All samples were

extracted and analysed as described above

and the results are presented in Table 5. A

total of 14 samples of various stone fruit

(apricots, cherries, peaches and plums) were

analysed. Pesticide residues were found in

five out of the 14 samples. The pesticides

found were: carbaryl, carbendazim, indox-

acarb, imidacloprid, thiamethoxam and

thiacloprid. Three fruit samples contained

only one pesticide and two samples con-

tained two pesticides of the pesticides stud-

ied. However, the concentrations of the an-

alytes found in the samples, except for one

sample, were lower than the MRLs estab-