Miniaturized SPE

Compared with conventional particle-packed SPE cartridges, FIT-SPE provides an increased surface area for the extraction medium and a reduced pressure drop during extraction and desorption. Also, the undesirable plugging effect from insoluble materials in real samples can be very much diminished. Utilization of FIT-SPE has been discussed in a few review articles concerning the on-line coupling of miniaturized SPE to microcolumn liquid-phase separation techniques.24,25... 

Another approach to increase the loadabihty is to enrich the analyte at the capillary inlet by means of an adsorptive phase, as reviewed in detail in Ref. 10. Re-versed-phase materials such as octadecyl-bonded silica have regularly been used in the form of membranes or column materials, thus, in principle, performing miniaturized SPE ( SPE) in-line with CE, allowing injections of 10-15 /uL. More selective sample concentration is obtained when using antibodies or Fab fragments for coating the inner wall of the capillary inlet [3,5]. 

To make any MALDI microspot technique (e.g., the spot-on-a-chip method) to work optimal, the sample has to be devoid of contaminants. Purification and enrichment of proteins and peptides is conveniently performed by SPE. Previously, standard commercial SPE options for miniaturized SPE or tips packed with beads were used for purification and enrichment before the spot-on-a-chip ... 

Another recent possibility is to use online sample preparation techniques. Standard SPE cartridges can be used with commercial SPE automation coupled upfront with LC-MS/MS. Alternatively, microextraction by packed sorbent (MEPS) is a miniaturized SPE format intended to work with sample volumes as small as 10 pL. The MEPS sorbent bed is integrated into a syringe that allows for manipulations of low-volume samples, either manually or in... 

Micro-extraction by packed sorbent, miniaturized SPE sample preparation method using a packed syringe needle, and GC injection via liquid desorption in the GC injector. 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

The use of robotics can be adopted also in sample preparation steps, in particular on-line SPE [7], This necessity is particular evident when small quantity of starting materials is available and the target molecules are present at low concentration levels. With the advent of miniaturization and automated procedures for samples handling, treatments and analysis, the lost of analytes due to a laboratory steps can be reduced. The reduction of analyte losses and the possibility to analyze even a total sample (no loss) leads to lower limits of detection (and consequently lower limits of quantification). Smaller volumes bring to obtain adequate sensitivity and selectivity for a large variety of compounds. In addition, on-line SPE requires low solvent consumption without the need to remove all residual water from cartridges, since elution solvents are compatible with the separation methods. 

As a corollary to this, more direct sample preparation procedures have been the pursuit of many scientists, who believe that miniaturization of analytical techniques can be a key solution to many of the unwanted drawbacks of LLE and SPE. Currently, several miniaturized extraction systems have been investigated, which are based primarily on utilizing downsized liquid, solid, or membrane extraction phases. 

Further miniaturization of the SPE technique permits a reduction in the amount of organic solvent used, on-line coupling to analytical instruments, fast analysis times and excellent sensitivity. Downsizing of SPE has been focused mainly on the use of libers, beads, and adsorbents as extraction phases that are reproducibly packed in tubes, capillaries, syringes, needles, and even micropipette tips. 

Microfluidics and miniaturization hold great promise in terms of sample throughput advantages [100]. Miniaturization of analytical processes into microchip platforms designed for micro total analytical systems (/i-TASs) is a new and rapidly developing field. For SPE, Yu et al. [123] developed a microfabricated analytical microchip device that uses a porous monolith sorbent with two different surface chemistries. The monolithic porous polymer was prepared by in situ photoinitiated polymerization within the channels of the microfluidic device and used for on-chip SPE. The sorbent was prepared to have both hydrophobic and ionizable surface chemistries. Use of the device for sorption and desorption of various analytes was demonstrated [123]. 

An innovative way to perform a sample cleanup that is fast, accurate, and easy, is the use of solid-phase extraction (SPE) columns. In many ways these devices resemble miniature HPLC columns upon which a preseparation is done. Using these minicolumns for sample clean-up eliminates many of the drawbacks associated with traditional liquid-liquid extraction, such as (1) the use of large amounts of expensive organic solvent, (2) low recovery due to solvent emulsion formation, and (3) large requirements for labor, time, glassware, and bench space. 

The procedure to use an SPE column is outlined in Figure 6-16. Essentially, the process of extraction works on the basis of selective elution from these miniature LC columns. To successfully do an SPE procedure, four basic steps are involved conditioning, loading the sample, eluting the undesirable material, and finally isolating the desired material. 

Considerably milder conditions are necessary for concentration using SPE, a process that is also available in a miniaturized version, SPME. 

Solid-phase extraction (SPE) (Fig. 6) is a miniature version of the liquid chromatography experiment. It was commercialized and introduced in the late 1970s. With the availability of prepacked cartridges, SPE first became popular in the mid-1980s [19]. It has been widely applied to sample cleanup in the drug discovery... 

A recent shift toward the discovery and development of new chemical entities that have greater potency has required their dosing at lower levels popular sample-preparation techniques such as protein precipitation are less useful for analyte concentrations below 1 ng/mL. Additionally, the frequent use of mouse plasma necessitates the use of sample volumes <50 pL and effectively miniaturizes the sample-preparation process. In these situations, SPE is especially appealing due to packaging of sorbents within small-diameter, thin disks requiring dramatically lower solvent and elution volumes [54,78-80]. Also, the benefits of disks are often attainable with small sorbent bed masses packed in colunms that are now available in bed masses as low as 2 mg [81]. 

The future of SPE is quite exciting and will continue to involve miniaturization of SPE methods and more on-line use of both liquid and gas chromatography. As instruments such as GC/MS and HPLC/MS become more sensitive, smaller sample sizes may be used and on-line methods will become routine. Sampling handling will be minimized with automated systems that fit directly on the gas chromatograph, so that analysis proceeds directly from the water sample or biological sample to the instrument. 

Hyphenation of automatic continuous flow systems (such as SPE, dialysis, gas diffusion, evaporation, direct leaching, etc.) to CE and the coupling of automatic sample preparation devices into commercial CE equipments have been devised as a means to simplification and miniaturization of analytical procedures. An automatic online SPE device for the multiresidue extraction of seven pesticides has been described. Four river samples were spiked with the test mixture at three different levels presenting recoveries Irom 90% to 114%. 

In a more direct translation of current macroscale, silica-based, SPE protocols other, microchip-based purification systems have focused on utilizing a packed silica-bead bed or silica sol-gel matrix solid phase for purification. This type of extraction was first miniaturized in a capillary format, to demonstrate the utility of the proposed method in the microscale, by Tian et al., who utilized a 500 nL capillary-based chamber packed with silica particles to establish that PCR-amplifiable DNA (with 80-90% of proteins removed during the load and wash steps) could be obtained from white blood cells with high extraction efficiencies (70%). This demonstrated the feasibility of incorporating such silica-based column purification methods into microfabricated devices and the effectiveness of such methods for the purification of DNA from a wide variety of biological species (white blood cells, cultured cells, and whole blood). 

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