Mass-Sensitive Approaches

ICP/AES). The mass spectrometric approach has introduced a wider ranging and more sensitive system for estimating element types and abundances in a huge range of sample types. 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

Many authors claim that the dilution by the sheath flow would not significantly affect the detection sensitivity, because it is completely evaporated in the spray process. Moreover, it has been discussed that in this layered-flow approach, preferably the inner layer of the spray enters the collector opening. If this were true, the composition of sheath liquid would be less important. Anyhow, it has to be stated that there is a dilution problem in the sheath-flow approach. In addition, it has been proven many times that ESI is a concentration-sensitive, not mass-sensitive, process. Knowing this, it makes sense to reduce the sheath liquid flow rate to the minimum required for stable spray conditions. 

Nanoelectrospray ionization (nanoESI), also known as nanospray, nanoflow electrospray, and micro-electrospray, is a low flow/high sensitivity approach to ESI. NanoESI15 is a slight variation on ESI such that the spray needle has been made very small and is positioned close to the entrance of the vacuum of the mass spectrometer and the mass analyzer (Figure 6). This greatly reduces required sample amounts allowing nanoliter flow rates and femto-mole sample consumption. The end result is increased efficiency since the flow rates for... 

A direct mass spectrometric method for simultaneous detection of five benzimidazoles including levamisole, thiabendazole, mebendazole, fenbendazole, and febantel in sheep milk was reported (377). The method, which involves injection of crude milk extracts and selection and collision of the most abundant ionic species obtained under electron impact ionization, was highly sensitive and rapid. Another direct mass spectrometric approach for rapid and quantitative determination of phenothiazine in milk was also described (323). This method involves an extraction step using a Cig microcolumn disc, followed by thermal desorption of the analyte from the disc directly into an ion trap mass spectrometer. 

There are many tricks and shortcuts to this process. For example, rather than compiling all of the transformation rate equations (or conducting the actual kinetic experiments yourself), there are many sources of typical chemical half-lives based on pseudo-first-order rate expressions. It is usually prudent to begin with these best estimates of half-lives in air, water, soil, and sediment and perform a sensitivity analysis with the model to determine which processes are most important. One can return to the most important processes to assess whether more detailed rate expressions are necessary. An illustration of this mass balance approach is given in Figure 27.5 for benzol a Ipyrene. This approach allows a first-order evaluation of how chemicals enter the environment, what happens to them in the environment, and what the exposure concentrations will be in various environmental media. Thus the chemical mass balance provides information relevant to toxicant exposure to both humans and wildlife. 

Laser microprobe mass analyzers permit mass spectrometric analysis of very small volumes (0.01-1 pm3) of thin Sections. The method is based on laser induced ion production from a microvolume and analysis of the evaporated ions in a time-of-flight mass-spectrometer. The technique allows detection of all elements and isotopes with a sensitivity approaching the ppm range and an extremely low limit of detection 10 15 to 10-20 g. Transmission type instruments such as the LAMMA 500 are designed for the analysis of particles of 3 pm in diam. The lateral resolution is about 0.5-1 pm. Because the area to be analyzed is selected by an optical microscope, distribution of chemical constituents can be precisely correlated with morphologic structures (Hillenkamp et al., 1982 39), Simons, 198440), Kaufmann, 1984)41 >. 

In order to increase the resolution, a heterodyne circuit may be used, which consists of two oscillators the quartz crystal microbalance and a second reference crystal. The frequency difference in the kilohertz range can be measured with a universal counter and the same resolution and gate time discussed above are valid however if the period of the differential wave is measured, the accuracy can be highly increased and a 1 Hz resolution is easily attainable for a 1 ms gate time. This approach has been used by Sheng-li Chen et al. [19] to measure the kinetics of silver oxide formation on polycrystalline silver. These authors reported a resolution of 0.1 Hz in 1 ms which corresponds to 0.44 ng cm"2 in the mass sensitivity. 

Since micro-gravimetry with the EQCM lacks specificity only the difference of cation and anion fluxes can be obtained by microgravimetry and therefore an independent measurement of specific ions is needed. Scanning electrochemical microscopy (SECM) coupled with a quartz crystal microbalance with independent potential control of the tip and substrate has been recently done by Cliffel and Bard [28]. In this experiment generation at the substrate (EQCM crystaj) working electrode and collection at the tip of an ultramicroelectrode (UNE) that was approached perpendicular to the EQCM crystal was employed with measurement of A/. Hillier and Ward [8] had previously used a scanning microelectrode to map the mass sensitivity across the surface of the QCM crystal. Reflection of longitudinal waves at the UME tip limits these experiments due to oscillations. 

Figure 2 The two most frequently used approaches for metabolomics are NMR spectroscopy and mass spectrometry. To maximize the coverage of the metabolome, these techniques can be combined. This figure shows the analysis of the aqueous fraction of a section of heart tissue that uses NMR spectroscopy, gas chromatography mass spectrometry and liquid chromatography mass spectrometry. Although mass spectrometry approaches are inherently more sensitive than NMR spectroscopy, metabolite identification becomes more difficult.

Over the past few years we have been developing new mass spectrometric approaches for the rapid, sensitive, and, accurate characterization of proteins. Reported here are some of our findings on the interfacing of Biomolecular Interaction Analysis with mass spectrometry, and the use of enzymatically active - or bioreactive - mass spectrometer probe tips in the characterization of analytes. 

Often it is required to detect compounds with no or only very weak chromophores such as sugars and amino acids. Refractive index detectors and mass sensitive detectors can be used but they are relatively insensitive in the context of biological sample concentrations. Indirect detection using a UV or fluorescent eluent can also be employed. However, the most common approach is the use of derivatisation. Derivatisation of some chemically reactive moiety on the analyte can be performed in two modes. In post-column derivatisation the sample is separated first and then reacted with a flowing stream of derivatising reagent being pumped into... 

This method uses the high-performance liquid chromatography (HPLC) equipment for sample handhng and requires molar mass sensitive detectors (such as light scattering and/or viscometry) to obtain a mean property values from each detector (Mw and/or IV, respectively). The FIA result from a concentration detector yields polymer content in a sample, which can also be determined with other well-established methods. The FIA approach requires expensive and well-maintained equipment, and will not save much time or solvent furthermore, no distribution information is available. 

Some manufacturers have taken the minimum detectable concentration and multiplied it by the sensor volume and defined the product as the minimum detectable mass. This gives values that are very misleading. For example, a detector having a true sensitivity of 10 g/mL and a sensor volume of 10 /rL would be attributed to a mass sensitivity of 10 g. This is grossly incorrect, as it is the peak volume that controls the mass sensitivity, not the sensor volume. Conversely, if the peak volume does approach that of the sensor, then a very serious peak distortion occurs with loss of resolution thus, this way of specifying sensitivity remains meaningless. 

Borrebaeck, C. A. Ekstrom, S. Hager, A. C. Nilsson, J. Laurell, T. Marko-Varga, G., Protein chips based on recombinant antibody fragments A highly sensitive approach as detected by mass spectrometry, Biotechniques 2001, 30, 1126-1132... 

For studies at common magnetic field strengths such as 11.7 T (500 MHz) or 14.1 T (600 MHz), there are three main ways to improve S/N beyond a standard 5-mm probe. The simplest is to make the coils smaller, as the mass sensitivity of an NMR measurement increases roughly in inverse proportion to the diameter of the coil. A second very popular approach is to cool the entire coil and preamplifier in order to reduce the noise, thus increasing the S/N. A third approach utilizes material that conducts electricity more efficiently than copper wire. 

As advantages, capillary separation techniques demonstrate high separation efficiency. On occasion, the number of theoretical plates available from these approaches has exceeded 1 million [29]. Also, very small sample volumes, on the order of 100 to 0.5 nL, are needed for these techniques. This can be an advantage for sample-limited situations, which are often encountered in bioanalysis. High mass sensitivity (the absolute weight of analyte injected) can be achieved, as the narrow capillary concentrates the sample plug and allows less opportunity for band broadening. 

---