Introduction

Nowadays, mass spectrometry (MS) is the most used technique to determine food contaminants [1-3]. Normally, this technique is combined with a separation one, such as gas chromatography (GC) [2,4,5], liquid chromatography (LC) [6-8] or capillary electrophoresis (CE) [9,10], because of the high number of organic pollutants and their low concentrations in food samples.

Among the different MS techniques and their combinations with chromatographic separation, GC-MS is the routine, preferred analytical method for determining food contaminants and residues since the end of the 1970s. A large number of publications have resulted from research on food applications of GC-MS [4]. This technique is frequently used to study the behaviour of contaminants and residues as well as to monitor their presence in food. GC-MS is currently a mature

Comprehensive Analytical Chemistry, Volume 51 © 2008 Elsevier B.V.

ISSN: 0166-526X, DOI 10.1016/S0166-526X(08)00007-X All rights reserved.

technique applied in the analysis of a significant number of pollutants [2,4,11,12]. The compounds most commonly analysed include alkanes, polycyclic aromatic hydrocarbons (PAHs), pesticides, polychlorinated biphenyls (PCBs), polychlori-nated dibenzo-p-dioxins and furans (PCDD/Fs), as well as other endocrine-disrupting chemicals such as phthalates and short ethoxy alkylphenol etoxylates [5,13-18]. GC-MS is also the technique of choice for the analysis of emerging pollutants, such as polybrominated diphenyl ethers (PBDEs) or polychlorinated alkanes [17].

However, the development of LC-MS in the past 20 years has made the detection of many harmful compounds possible. The primary advantage LC-MS has over GC-MS is that it is capable of analysing a much wider range of components. Compounds that are thermally labile, exhibit high polarity or have a high molecular mass may all be analysed using LC-MS, even proteins may be routinely analysed. Several recently published comprehensive papers summarized the current state-of-the-art in LC-MS analysis of specific classes of contaminants (e.g., veterinary drug residues, banned growth promoters, perfluorinated compounds, and pesticides and their degradation/transformation products and food packaging migrating materials) [6,19-27].

These two approaches can be considered the established ones. There are more emerging techniques, such as CE-MS or ambient MS, which are very promising, helping to improve MS features for the analysis of food contaminants, even though they are not well ascertained yet in the field of food contaminants and residues analysis [28-30].

The introduction of novel methods and expanding applications to diverse areas highlight truly impressive progress in MS. Table 1 summarizes the current state-of-the-art in MS for food contaminants and residue analysis and the expected improvements.

These developments are illustrated here by two seemingly different areas of research: innovative ionization methods and mass analysers.

Some emerging technologies in the field of the ionization sources have shown great potentials to further enhance the capabilities of high throughput analysis by MS, including CE-MS and ambient MS [31,32]. The recent development on CE-MS has been produced in the field of micro- and nano-electrospray ionization (ESI) ion sources, which have radically improved the CE/ESI-MS sensitivity level. In such devices the need of make-up flow is eliminated because the sample is sprayed into the MS either directly from the tip of the separation capillary or from a tapered micro- or nanoemitter butted to the CE column. In both cases, the achieved flow rates are situated in the range of nanolitre per minute . The effects of the use of this kind of micro/nanosprayers, which include higher analyte concentration, reduced spraying potential and closer positioning of the sprayer to the orifice of the mass spectrometer, render a substantially improved mass transfer into MS, better ionization and desolvation of the generated droplets and therefore, superior sensitivity [33]. Because of the rather low sensitivity provided by the sheath flow CE-MS, for applications where limited amount of material is available, sheathless on-line CE/ESI-MS is a reliable alternative, even if the tedious optimization procedures make it often a non-user-friendly approach. The lack of commercial

Table 1 Summary of the improvements of mass spectrometry in food contaminants and residues

GC-MS

LC-MS

CE-MS

MS

Current state-of-the-art in mass spectrometry

Well-established

Well-established

The dominant

Method for the

ionization

ionization

interfacing

analysis of

sources: EI

sources: EI

system is the

solids are still

and CI

and CI

sheath interface

under

development

(DART)

Variety of mass

Variety of mass

Variety of mass

MALDI is the

analysers MS,

analysers MS,

analysers MS,

well-developed

MS2 (quadrupole,

MS2 (quadrupole,

MS2 (sector,

techniques

ion trap, sector)

ion trap, triple

triple quadrupole,

always

quadrupole)

ion trap,

combined with

time-of-flight)

time-of-flight

and not

appropriate for

small

molecules

Routine technique

Routine technique

No real applications

Not appropriate

for food

contaminants

and residues

which are

small

molecules

Expected improvements in mass spectrometry

New mass

New mass

New interfacing

New interfacing

analysers (triple

analysers

system (sheathless

systems

quadrupole,

(time-of-flight,

interface, liquid

(desorption

time-of-flight,

linear traps,

junction interfaces,

electrospray

etc.)

quadrupole

etc.)

ionization)

time-of-flight,

orbitrap, etc.)

Already a reality

Already a reality

The available

First applications

approaches

of commercial

require

devices

improvements

Note: EI, electron ionization; CI, chemical ionization; DART, direct analysis in real time; MALDI, matrix assisted laser desorption.

Note: EI, electron ionization; CI, chemical ionization; DART, direct analysis in real time; MALDI, matrix assisted laser desorption.

sheathless interfaces or completely standardized procedures for smooth and fast in-house production, along with the deficit in methodological protocols for running different analytes, stimulated continuously the creativity of the CE-MS scientific community. So far, a remarkable number of sheathless setups were conceived and their potential for various applications tested [34]. These combinations are still little applicable to food contaminants and residue analysis because the sensitivity is not optimum yet and will not be covered in this chapter [9,10].

Recently introduced ambient MS represents a new family of ionization techniques that create ions under ambient conditions without vacuum constraints, including desorption electrospray ionization (DESI) [31] and direct analysis in real time (DART) [35], which have already been detailed in Chapter 1. In DESI experiments, a spray of charged solvent droplets hits the surface of the sample and ionizes the sample molecules (small molecules and large biomolecules) [35]. In DART experiments, a plasma of excited-state atoms and ions from nitrogen/ helium desorb/ionize small molecules from the surface of the sample. These techniques can be used to rapidly obtain mass spectra of compounds without any sample preparation [35]. Some encouraging results on analysis of food with ambient MS have recently been reported in the literature [36]. These techniques are very promising and further developments in this area will establish the use of ambient MS for high-throughput analysis, accurate mass measurement and other aspects of structural characterization of food contaminants and residues.

This chapter is, thus, aimed at discussing the progress of MS analysers in food contaminants and residue analysis and reporting the recent applications for the trace analysis of selected contaminants. Two emerging trends in the mass analysers design are valuable to get better quality and quantity of structural information:

• High-resolution (HR) mass spectrometers, which calculate elemental composition and nominal mass, because they can determine exact mass of analytes. They are a tool to identify unknown substances in the samples. These spectrometers are represented by time-of-flight (TOF).

• Tandem mass spectrometry (MS/MS), which allow the mass spectrometer to analyse an ion and obtain its mass spectrometer again. The mass spectrometers capable of carrying out MS/MS are QqQ, ion trap, quadrupole time-of-flight (QqTOF) and linear ion trap (LIT).

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