Solid-phase extraction offline

Before compounds in biological matrices can be analyzed by LC/MS/MS, the samples must undergo a preparation procedure. There are a variety of techniques available for sample preparation including offline sample preparation techniques (liquid-liquid extraction, protein precipitation, and solid phase extraction) and on-line sample preparation... 

Samples rarely come in a form that can be injected directly into the instrument some form of sample preparation usually is required. Sample preparation includes any manipulation of the sample prior to analysis, including techniques such as weighing, dilution, concentration, filtration, centrifugation, and liquid- or solid-phase extraction. Sample preparation can be performed either on-line or off-line, but it is usually performed offline. Off-line preparation can be time-consuming and tedious, and the more steps that are required, the more susceptible the analytical method is to operator error and irreproducibility. 

Another advanced and novel sample preparation technique is online solid phase extraction (online SPE). Although the extraction mechanism is as the same as traditional offline SPE, online SPE offers several advantages. Because sample preparation is carried out during analysis, it eliminates the time needed for sample preparation thus increases throughput significantly. Additionally, because the entire sample is eluted to the LC-MS/MS system, it may increase assay sensitivity. Finally, because there is no manual extraction involved, it may reduce human error, potential contamination, and inconsistent recovery. 

More success was obtained using on-line solid phase extraction before offline analysis by capillary electrophoresis with matrix-assisted laser-desorption ionization (MALDI) MS [18]. Because microdialysis is a method used for analysis of small molecules and peptides, MALDI is not used frequently with microdia lysis sampling. Another disadvantage is that MALDI cannot be used on-line with the separation because it is a vacuum ionization technique. However, if the pep-tide is large enough (0 1000 Da) MALDI can be useful. For the analysis of peptides in dialysate an appropriate separation is important before mass spectrometric detection. In a comparison with direct sampling of dialysate in MALDI, capillary electrophoresis provides the high efficiency separations necessary to resolve all... 

Patsias, J. and Papadopoulou-Mourkidou, E., Rapid method for the analysis of a variety of chemical classes of pesticides in surface and ground waters by offline solid-phase extraction and gas chromatography-ion trap mass spectrometry, J. Chromatogr. A, 740, 83-98, 1996. 

Aguilar, C., Bomil, F., and Marce, R. M., Online and offline solid-phase extraction with styrene-divinylbenzene membrane extrachon disks for determining pesticides in water by reversed-phase liquid chromatography-diode array detection, J. Chromatogr. A, 754, 77-84, 1996. 

Offline extraction is the easiest and most common way for combining affinity columns with other analytical techniques. This typically involves the use of antibodies that are immobifized and packed into a disposable syringe or solid-phase extraction cartridge. After conditioning the affinity column with the necessary application buffer or conditioning solvents. 

For solid-phase extraction, the offline procedure, based on sample pretreatment on disposable SPE cartridges, and a totally automated online procedure, based on column switching, have been developed. The column switching procedure involves the use of a polystyrene-divinylbenzene column for sample concentration and cleanup. 

Automation of solid-phase extraction for riboflavin separation has been carried out in both offline and online modes, in the last case with direct coupling to the injection valve of HPLC equipment. 

Less than 1% of fhe albumin in plasma is modified by OP. A cleanup step is required fhaf separates and concentrates the OP-t5u-osine adduct, such as solid phase extraction (Williams et al., 2007 Read et al., 2010) or offline HPLC (Jiang ef al., 2012), before fhe sample is analyzed by LC-MS/MS. However, fhe use of MRM in an LC-MS/ MS protocol increases the sensitivity and specificity of defection, making if possible to identify OP-tyrosine in a pronase digest directly, without a cleanup step (van der Schans et al., 2013). 

Some techniques that combine the properties of extraction and cleanup are supercritical fluid extraction (SEE) and matrix solid-phase dispersion (MSPD). Supercritical fluids, i.e., at a temperature and pressure in excess of their critical point, have unique properties for selective extraction of analytes from a sample. Solid samples are mixed with an inert dispersant, such as hydromatrix, and the mixture packed into the cell of the SEE apparatus. The sample is extracted with supercritical CO2, with or without addition of organic modifier, and the extracted analytes may be collected inline or offline on suitable adsorbents (Figure 3). Further cleanup of the sample extract may be performed using SPE. MSPD is based on intimate mixing of animal tissue sample with a bonded silica, such as Cig, and packing of the blended material into a column from which interferences can be eluted by washing with solvents and the analytes eluted using a selective solvent. 

Another approach to circumvent poor CE CLOD is to undertake offline sample pretreatment and analyte concentration. This should be avoided, if possible, particularly for dilute protein solutions since losses to exposed surfaces (e.g., walls of microcentrifuge tubes, pipette tips, solid extraction phases, etc.) can be substantial. Furthermore, excessive handing of a concentrated solution of protein(s) can lead to denaturation, aggregation, precipitation, and, ultimately, poor analyte recovery. 

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