Liquid chromatography-mass spectrometry detection steps

For method tryout, run a control sample and two fortifications from each site. One fortification should be done at the LOQ and the other at the highest expected residue level, perhaps 1000 x LOQ. If the recoveries are within the acceptable range of 70-120% and there are no interferences, proceed with the method validation. If interferences are present which prevent quantitation of the analyte, try additional cleanup steps with SPE or use a more selective detection method such as liquid chromatography/mass spectrometry (LC/MS). 

Liquid chromatography-mass spectrometry (LC-MS) combines the separating power of LC with the uniquely powerful detection capabilities of MS. With this technique, it is now possible to separate, identify and quantify components in a mixture. As with GC-MS, there is a choice of mass spectrometric ionisation techniques and analysers available. LC-MS can also be taken a step further as LC-MS" (otherwise known as LC-tandem MS) where ions can be subjected to fragmentation so as to gain information on their structures. 

Liquid chromatography-mass spectrometry is still difficult to use for routine analysis. However, the technique is essential for special projects, as was demonstrated for determination of the total alkyl and ethoxy distribution of alkyl sulfate and alkylether sulfate in wastewater (66). Because of the specificity of MS detection, a simple SPE technique could be used for concentration, and an equally simple one-step HPLC separation was adequate. 

Basic techniques for speciation analysis are typically composed of a succession of analytical steps, e.g. extraction either with organic solvents (e.g. toluene, dichloromethane) or different acids (e.g. acetic or hydrochloric acid), derivatisa-tion procedures (e.g. hydride generation, Grignard reactions), separation (gas chromatography (GC) or high-performance liquid chromatography (HPLC)), and detection by a wide variety of methods, e.g. atomic absorption spectrometry (AAS), mass spectrometry (MS), flame photometric detection (FPD), electron capture detection (ECD), etc. Each of these steps includes specific sources of error which have to be evaluated. 

Fluorescence detection at 284/310 nm (extinction/ emission wavelengths) leads to a detection limit of 1.3 mmol/L (0.14 mg/mL for / -cresol). Identification of phenol and /7-cresol may be confirmed by liquid chroma- tography/mass spectrometry. Because HPLC methods require only simple extraction, e.g., by ethyl acetate, and do not require further steps such as derivatization, they j are simple and rapid compared with gas chromatography or gas chromatography/mass spectrometry. Such methods I are useful for monitoring serum phenols in dialyzed patients as an index of hemodialysis adequacy. How- ever, the separation of the three isomers of cresol can only be performed by adding 3-cyclodextrin to the c liquid phase. q... 

Xie et al. [5] used vapor-deposited parylene-C to fabricate ESI tips on silicon microfluidic devices, enabling integrated liquid chromatography with mass spectrometry detection with comparable performance to conventional techniques. The drawback for these devices is the complexity involved in their fabrication, requiring many sequential photolithography steps in a clean room. However, parylene is a material with high chemical resistance and may be a useful choice for the construction of nanospray tips in future work. For example, Kameoka et al. [6] constmcted a nanospray tip comprising a parylene film sandwiched between two plastic plates (Fig. 2b). This device is relatively easy to... 

In most respects, the standard approach taken to analyze the fatty acids of functional foods is similar to that for conventional foods. The steps are to extract the total lipids or fatty acids, convert the fatty acids to a suitable derivative (often to fatty acid methyl esters (FAME)), and analyze the derivatized fatty acids by a suitable chromatographic technique, usually GC with flame ionization detection (FID), Other chromatographic techniques, including gas chromatography-mass spectrometry (GC-MS) and high-performance liquid chromatography (HPLC), may be required. Nonchromatographic techniques such as infrared (IR) spectroscopy may be used in some simations, perhaps because of the speed of analysis. 

A further extension of the DFG S19 method was achieved when polar analytes and those unsuitable for GC were determined by LC/MS or more preferably by liquid chromatography/tandem mass spectrometry (LC/MS/MS). Triple-quadrupole MS/MS and ion trap MS" have become more affordable and acceptable in the recent past. These techniques provide multiple analyte methods by employing modes such as time segments, scan events or multiple injections. By improving the selectivity and sensitivity of detection after HPLC separation, the DFG S19 extraction and cleanup scheme can be applied to polar or high molecular weight analytes, and cleanup steps such as Si02 fractionation or even GPC become unnecessary. 

Ylinen et al. [53] developed an ion-pair extraction procedure employing tetrabutylamonium (TBA) counter ions for determination of PFOA in plasma and urine in combination with gas chromatography (GC) and flame ionisation detection (FID). Later on, Hansen et al. [35] improved the sensitivity of the ion-pair extraction approach using methyl tertiary butyl ether (MTBE) and by the inclusion of a filtration step to remove solids from the extract making it amenable to liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) determination. Ion-pair extraction procedure has been the basis of several procedures for biota [49,54-58] and food samples [50,59,60]. However, this method has shown to have some limitations, such as (1) co-extraction of lipids and other matrix constituents and the absence of a clean-up step to overcome the effects of matrix compounds and (2) the wide variety of recoveries observed, typically ranging. 

---