20 food hygiene & safety
☞
pre-requisite for analysis of low concentration compounds A limited resolution of 15 000 results in two major
in the presence of higher abundant ones. The example limitations. Firstly, the detection of unresolved doublets
in Fig. 2 shows an approximate 1:3 mixture of both may result in significant mass errors which are outside
pesticides measured and simulated. the characteristic accuracy specification of the Exactive
Parathion
instrument.
100
292.04031
As a consequence, at lower resolution settings the
C
10
H
15
NO
5
PS
mass windows for elemental composition determination
90
Thiamethoxam
have to be increased, resulting in a much larger number
80
292.02656
of elemental composition proposals for the unknown
70
C
8
H
11
CIN
5
O
3
S
or targeted compounds. This can be seen for the
example (Fig. 3) of Pirimicarb at m/z 239.1503. Due to
60
the presense of an isobaric interference the peak at 239
50
shows a mass error of 6.5 ppm. At a resolution of 15 000
40
the underlying interference causes an apparent shift to
Relative Abundance
30
Measured
higher mass, whereas at higher resolution (here 80 000)
Simulated the doublet is clearly resolved and the mass accuracy is
20
well within instrument specifications.
10
In order to limit the number of candidate elemental
0 compositions to a single confident assignment, a
291.98 292.00 292.02 292.04 292.06 292.08
sophisticated software algorithm is used. It takes into
m/z
account the peak height and mass accuracy of the
100
Parathion
monoisotopic peak and its isotopes.
292.04031
90
C
However, in order to function correctly, all of the
10
H
15
NO
5
PS
80
accurate mass values for the isotopic peaks must be
70
within specified limits. The absence of interference
Thiamethoxam
peaks can only be assured by use of high resolution, (as
60 292.02656
C
can be seen in Fig. 4). Here the fungicide Azoxystrobin is
8
H
11
CIN
5
O
3
S
50
shown at resolutions of 15 000 and 80 000. The medium
40
resolution spectrum shows very good mass accuracy
Relative Abundance 30
for the monoisotopic peak, but gives unusually high
Measured
mass errors for the A+1 and A+2 ions. This is due to an
20
Simulated
interference at m/z 405.1452, which cannot be resolved
10
at medium resolution. Whereas the high resolution
0 spectrum shows exellent mass accuracy for all three
291.98 292.00 292.02 292.04 292.06 292.08 measured isotopes.
m/z
Determining elemental compositions using data
Fig. 2. Mass chromatogram of two isobaric pesticides acquired at ~15 000 resolution will result in misleading
measured at a resolution of 15 000 (top) and 50 000 or incorrect data.
(bottom). Superimposed is the simulated mass trace at Only sufficient high resolution allows the
each resolution setting. determination of the accurate mass of the complete
Fig. 3. Improved
mass accuracy
simply by
increasing the
resolution in
order to resolve
the doublet. The
peak at 15 000
resolution splits
up into 2 by
increasing the
resolution to
80 000.
www.scientistlive.com
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44