Analytical techniques electrostatic sampling

In 1951Castaing8 published results to show that an electron microscope could be converted into a useful x-ray emission spectrograph for point-to-point exploration on a micron scale. The conversion consisted mainly in adding a second electrostatic lens to obtain a narrower electron beam for the excitation of an x-ray spectrum, and adding an external spectrometer for analysis of the spectrum and measurement of analytical-line intensity. Outstanding features of the technique were the small size of sample (1 g cube, or thereabouts) and the absence of pronounced absorption and enhancement effects, which, of course, is characteristic of electron excitation (7.10). Castaing8 gives remarkable quantitative results for copper alloys the results in parentheses are the quotients... 

On-probe purification using derivatized MALDI probe surfaces has been described to simplify the sample preparation process. Various developments in this field have allowed the introduction of new techniques such as the surface-enhanced laser desorption ionization (SELDI) [42], The surface of the probe plays an active role in binding the analyte by hydrophobic or electrostatic interactions, while contaminants are rinsed away. In the same way, this technique uses targets with covalently coupled antibodies directed against a protein, allowing its purification from biological samples as urine or plasma. Subsequent addition of a droplet of matrix solution allows MALDI analysis. 

Using SEC, most accomplishments of the commonly used RP-LC technique can be employed, such as various detector options, high sample loading capacity, variability in stationary phases and up-scaling option. Often in SEC, non-size-exclusion effects such as electrostatic and hydrophobic interactions between the analyte and stationary phase may be observed. The separation efficiency can be improved by optimizing the mobile phase, flow rate, column length, and sample volume. Practical guidelines for SEC method development have been described [42]. 

One key difference between MS and optical detection techniques is that the sample material must be physically transferred out of the source. Analyte ions are extracted by electrostatic lenses and transferred through a mass analyser for eventual detection. Although ions are formed throughout the source cell, only those created very close to the exit orifice can survive to the high-collision environment and depart in the charged state. Both magnetic-sector and quadrupole-based instruments have been used in GD-MS, and commercial versions of each are available. The typical discharge operation conditions for GD-MS are 1-5 mA, 800-1500 V and 0.2-2.0 torr. 

The nano-electrospray (nanoES) source is essentially a miniaturized version of the ES source. This technique allows very small amounts of sample to be ionized efficiently at nanoliters per minute flow rates and it involves loading sample volumes of 1-2 pi into a gold-coated capillary needle, which is introduced to the ion source. Alternatively for on-line nanoLC-MS experiments the end of the nanoLC column serves as the nanospray needle. The nanoES source disperses the liquid analyte entirely by electrostatic means [27] and does not require assistance such as solvent pumps or nebulizing gas. This improves sample desolvation and ionization and sample loading can be made to last 30 minutes or more. Also, the creation of nanodroplets means a high surface area to volume ratio allowing the efficient use of the sample without losses. Additionally, the introduction of the Z-spray ion source on some instruments has enabled an increase in sensitivity. In a Z-spray ion source, the analyte ions follow a Z-shaped trajectory between the inlet tube to the final skimmer which differs from the linear trajectory of a conventional inlet. This allows ions to be diverted from neutral molecules such as solvents and buffers, resulting in enhanced sensitivity. 

In 1993, Hutchens and co-workers described surface-enhanced laser desorption/ionization (SELDI) technique, an affinity technology, which has progressed over the last decade to become a powerful analytical, an on-plate approach (Hutchens and Yip 1993). SELDI is a distinctive form of laser desorption/ionization (LDI) mass spectrometry in which the EDI probe plays an active role in the homogenization, preconcentration, amplification, purification, extraction, enrichment digestion, derivatization, synthesis, separation, and detection with complementary techniques, prior to the desorption and ionization of the analytes by MALDI (Merchant and Weinberger 2000). The principle of this approach is very simple. Biomolecules are captured by adsorption, partition, electrostatic interaction, or affinity chromatography on a solid-phase protein chip surface. Although SELDI provides a unique sample preparation platform, it is similar to MALDI-MS in that a laser... 

Analyte ions can also be efficiently generated when sample vapor or finely dispersed sample droplets transported by a carrier gas stream are admixed to the expanding electrospray plume. This technique, simple yet effective, has been introduced as extractive electrospray ionization (EESI) [37]. It utilizes two separate sprayers, one conventional ESI sprayer to provide the electrostatically charged mist and another to supply the sample vapor or mist (Fig. 13.13). While this approach is suggested for API interfaces with the heated transfer capillary design, the sample carrier stream may alternatively be passed into the desolvation gas of interfaces employing the heated curtain gas design (Fig. 13.14) [6,38]. 

A comparison of the low-frequency sensitivity drift is also difficult to do directly, but a reasonable analogy can be drawn. A variation in sensitivity over a series of sample spots is a situation similar to infusion and observation of baseline drift with ESI. Using ideal conditions of sample deposition and crystallization with pure standards and solvents establishes some idea of reproducibility limits. An example is shown in Figure 13.12, where a series of spots from a sample of the dmg Propanolol were created with an electrostatic deposition device." The spots were rastered at a speed such that each spot was traversed in 600 ms. Absolute area reproducibility of approximately 6% was achieved— that is, worse than the infusion data of NanoESI and the high flow techniques, but still reasonable, particularly if referencing the signal to an internal standard (not done here) in the same spot that is ionized at the same time as the analyte. Suffice it to say that good quantitation is possible when an internal standard is cocrystallized with the analyte. ESI or APCI do not require coelution of the internal standard and analyte. 

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