APCI pesticides

Figure 5.1 Pesticides included in the systematic investigations on APCI-MS signal response dependence on eluent flow rate the parameter IsTow represents the distribution coefficient of the pesticide between n-octanol and water. Reprinted from J. Chromatogr, A, 937, Asperger, A., Efer, 1., Koal, T. and Engewald, W., On the signal response of various pesticides in electrospray and atmospheric pressure chemical ionization depending on the flow rate of eluent applied in liquid chromatography-mass spectrometry , 65-72, Copyright (2001), with permission from Elsevier Science.

No APCI or ESI data had been previously reported for two of the five pesticides which were to be determined, i.e. imazalil and benomyl, and therefore although some information was available from the literature it was not possible to make a totally informed decision on the best methodology to employ. 

Crescenzi et al. developed a multi-residue method for pesticides including propanil in drinking water, river water and groundwater based on SPE and LC/MS detection. The recoveries of the pesticides by this method were >80%. Santos etal. developed an on-line SPE method followed by LC/PAD and LC/MS detection in a simultaneous method for anilides and two degradation products (4-chloro-2-methylphenol and 2,4-dichlorophenol) of acidic herbicides in estuarine water samples. To determine the major degradation product of propanil, 3,4-dichloroaniline, the positive ion mode is needed for atmospheric pressure chemical ionization mass spectrometry (APCI/MS) detection. The LOD of 3,4-dichloroaniline by APCI/MS was 0.1-0.02 ng mL for 50-mL water samples. 

As with GC/MS, LC/MS offers the possibility of unequivocal confirmation of analyte identity and accurate quantiation. Similarly, both quadrupole and ion-trap instruments are commercially available. However, the responses of different analytes are extremely dependent on the type of interface used to remove the mobile phase and to introduce the target analytes into the mass spectrometer. For pesticide residue analyses, the most popular interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI). Both negative and positive ionization can be used as applicable to produce characteristically abundant ions. 

Electrospray ionization (ESI) and APCI are the two popular API techniques that will be discussed here. The applications to the analysis of pesticides that will be discussed include imidazolinone herbicides, phenoxy acid herbicides, and A-methyl carbamate insecticides. Matrix effects with respect to quantitation also will be discussed. Eor the... 

Polymeric precolumns of styrene-divinylbenzene were used by Aguilar et al. to monitor pesticides in river water. Water samples (50 mL) were trace enriched on-line followed by analysis using LC combined with diode-array detection. LC atmospheric pressure chemical ionization (APCI) MS was used for confirmatory purposes. It was found that after the pesticides had been extracted from the water sample, they could be stored on the precartridges for up to 3 months without any detectable degradation. This work illustrates an advantage of SPE for water samples. Many pesticides which may not be stable when stored in water, even at low temperature, may be extracted and/or enriched on SPE media and stored under freezer conditions with no detectable degradation. This provides an excellent way to store samples for later analysis. 

Applications APCI-MS is often more widely applicable than ESI-MS to the analysis of classes of compounds with a low molecular weight, such as basic drugs and their metabolites, antibiotics, steroids, oestrogens, benzodiazepines, pesticides, surfactants, and most other organic compounds amenable to El. LC-APCI-MS has been used to analyse PET extracts obtained by a disso-lution/precipitation procedure [147]. Other applications of hyphenated APCI mass spectrometric techniques are described elsewhere LC-APCI-MS (Section 7.33.2) and packed column SFC-APCI-MS (Section 73.2.2) for polar nonvolatile organics. 

A method has been reported for the quantification of five fungicides (shown in Figure 5.39) used to control post-harvest decay in citrus fruits to ensure that unacceptable levels of these are not present in fruit entering the food chain [26], A survey of the literature showed that previously [27] APCI and electrospray ionization (ESI) had been compared for the analysis of ten pesticides, including two of the five of interest, i.e. carbendazim and thiabendazole, and since it was found that APCI was more sensitive for some of these and had direct flow rate compatibility with the HPLC system being used, APCI was chosen as the basis for method development. 

Liquid chromatography -APCI-MS is applicable to many different types of pesticide structures, such as triazines, phenylurea herbicides, acetanilides, and OPPs. A study of 12 pesticides and pesticide degradation products demonstrated the sensitivity of the technique for OPP determination, with detection limits for water samples of about 0.001-0.005 /zg/L (32). 

Liquid chromatography-APCI-MS with NI and PI was used for the trace determination of several OPPs in groundwater. The limit of quantification varied between 5 and 37 ng/L in PI. Under the NI mode of operation, only the parathion group (both parathions and fenitrothion) had a better sensitivity than with the PI mode, with quantitation limit of 5-15 ng/L, whereas the rest of the pesticides had 2-4 times higher limits as compared to those in PI mode (52). 

A study using APCI and PB-MS in both the NI and PI modes coupled to HPLC was used to determine four OPPs. The results demonstrated the higher sensitivity of HPLC-APCI-MS compared with HPLC-PB-MS and the potential of both techniques for confirming the presence of pesticide (51). 

Liquid chromatography with ISP or ACPI followed by tandem MS-MS was used to analyze a 17-pesticide mixture. Both approaches gave similar product ion spectra from protonated molecules and an MS-MS library was set up for more than 60 pesticides and their degradation products. The library was successfully used for searching product ion-ion spectra from SPE/LC-APCI-MS-MS at low levels (10 ng/L) in tap water (54). 

The River Ebre (Spain) was investigated under APCI and PB in order to confirm unequivocally the absence of pesticides (50,51). The study demonstrated the absence of OPPs and OCPs, although other classes of pesticides were present. 

Spliid and Kpppen described a method using LLE and LC-APCI-MS for the analysis of water. The method proposed was then used to investigate the contamination of Danish ground-water with pesticides. More than 200 samples of groundwater collected from various areas of the country were analyzed. Metramitron was detected one or more times in concentrations ranging from the detection limit level to 19 /rg/L (32). 

Compared with APCI, APPI is more sensitive to the experimental conditions. Properties of solvents, additives, dopants or buffer components can strongly influence the selectivity or sensitivity of the detection of analytes. Nevertheless, this technique allows the ionization of compounds not detectable in APCI or ESI, mainly non-polar compounds. For these last compounds, APPI is a valuable alternative. Thus, APPI is a complementary technique to APCI and ESI. However, for a given substance it remains difficult to predict which ionization source (APPI, APCI or ESI) will give the best results. Only preliminary tests will allow the choice of the best ionization source. APPI appears to be efficient for some compound classes such as flavonoids, steroids, drugs and their metabolites, pesticides, polyaromatic hydrocarbons, etc. [85],... 

R.B. Geerdink, A. Kooistra-Sijpersma, J. Tiesnitsch, P.G.M. Kienhuis, U.A.Th. Brinkman, Determination of polar pesticides with APCI-MS using methanol and/or acetonitrile for SEE and gradient LC, J. Chromatogr. A, 863 (1999) 147. 

This chapter is devoted to the analysis of pesticides and related compounds. LC-MS characteristics of various classes of pesticides are described, i.e., the mass spectral information obtained using electrospray ionization (ESI) and atmospheric-pressure chemical ionization (APCI). Next, typical strategies with the analysis of pesticides in environmental samples and in fmit and vegetables are discussed. 

Many papers on the LC-MS analysis of pesticides and related compounds deal with the characterization of interface and ionization performance, the improvement of detection limits by variation of experimental conditions, and the information content of the mass spectra. As far as ESI and APCI ate concerned, this type of information is reviewed for various pesticide classes in this section (see Ch. 4.7.4 for results with thermospray and Ch. 5.6.1 with particle-beam interfacing). 

Thurman et al. [9] evaluated the performance of APCI and ESI in both positive-ion and negative-ion mode in the analysis of 75 pesticides from various compound classes. Part of their results is summarized in Table 7.1. 

Barnes and coworkers pioneered in this area. They reported the analysis of diflubenzuron in mushrooms [39], and of diflubenzuron and clofentezine in various fmit drinks [73], and the development of a multiresidue study for ten pesticides in fmit, involving ionization polarity switching in LC-APCI-MS [123]. In these studies, significant attention is paid to matrix-dependent ion suppression or enhancement effects (Ch. 6.7), which is observed even in APCI. Matrix effects in food analysis must be studied in detail for each fmit or vegetable. Obviously, optimization of the sample pretreatment procedures plays an important role in method development for pesticide residue analysis in food. 

E.M. Thurman, 1. Ferrer, D. Barceld, Choosing between APCI and ESI interfaces for the LC-ATS analysis of pesticides, Peaal. Chem., 73 (2001) 5441. 

S. Kawasaki, F. Nagumo, H. Ueda, Y. Tajima, M. Sano, J. Tadano, Simple, rapid and simultaneous measurement of eight different types of carbamate pesticides in serum usingLC-APCI-MS, J. Chromatogr., 620 (1993) 61. 

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