6. LC/MS Diskussionstreffen, 29. Nov. 2005, Wuppertal ABSTRACTS

Abstracts: 6. LC/MS Diskussionstreffen, 29. Nov. 2005, Wuppertal 13
Zuckerrübe, Hopfen & Co: Mit 2D-LC on-line gegen Matrixeffekte
Stephan Sack & Albrecht Glänzel
RCC Ltd, Zelgliweg 1, CH-4452 Itingen, Switzerland, sack.stephan@rcc.ch
API-MS/MS instruments now provide sufficiently high sensitivity for the majority of small
molecule analytes at a detection level clearly below 1 ng/mL, i.e., typical extracts at the ‘baby
food’ residue limit of 10 ppb are accessible without the need of analyte enrichment that is
typically achieved in combination with classical clean-up steps such as SPE or LLP.
However, extensive HPLC gradient separations and the injection of a few µL of an even
diluted crop extract cannot routinely avoid disturbing effects of co-eluted matrix fractions on
the atmospheric pressure ionization (typically seen as ion suppression in the case of ESI
type API interfaces or signal enhancement in APCI).
Undoubtedly the best measure to compensate for these effects is the use of isotopic labelled
internal standards. However, in everyday practice these standards are typically not available
for all analytes of a residue method. So as a widespread work-around matrix-matched
standards are used in residue analysis assuming that they are suppressed or enhanced to
the same extend as given in the samples to be analyzed. However, already minor changes in
e.g. the crop growing stage can cause different matrix effects making the choice of the best
matching matrix often to be a compromise.
In the present study the potential of 2-dimensional HPLC (LC-LC column switching) as
efficient tool for the reduction of matrix effects is successfully demonstrated for the
carbamate pesticide carbofurane and its plant metabolite 3-hydroxycarbofuran in various
plant substrates (among them sugar beets, sunflower seeds, hops). This can be considered
as a typical example of target residue analyses with harsh extraction (here 0.25 N HCl
solution has to be used for 3-hydroxycarbofuran).
The column switching method developed for the two analytes consists of two isocratic
reversed phase separations on an Intersil Phenyl-3 and an Inertsil ODS-3 column (2mm x
100 mm each) with on-column focussing of the injected volume and of the two eluent fraction
switched from column 1 to 2. This system combined with narrow (“heart”) cutting of analyte
peaks from column 1 provide significant better reduction of matrix effects than could be
reached with gradient separation that were up to about 3 times longer in cycling. Staggered
separations on columns 1 and 2 in LC-LC can further speed up the LC process without
loosing the clean-up effect.