Mass detection limits

Electrothermal vaporization can be used for 5-100 )iL sample solution volumes or for small amounts of some solids. A graphite furnace similar to those used for graphite-furnace atomic absorption spectrometry can be used to vaporize the sample. Other devices including boats, ribbons, rods, and filaments, also can be used. The chosen device is heated in a series of steps to temperatures as high as 3000 K to produce a dry vapor and an aerosol, which are transported into the center of the plasma. A transient signal is produced due to matrix and element-dependent volatilization, so the detection system must be capable of time resolution better than 0.25 s. Concentration detection limits are typically 1-2 orders of magnitude better than those obtained via nebulization. Mass detection limits are typically in the range of tens of pg to ng, with a precision of 10% to 15%. 

Method3 Mass detection limit (moles) Concentration detection limit (molar)b Characteristics... 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

Mass detection limit Concentration detection Advantages/disadvantages... 

In a further application of MI-SPE, theophylline could be separated from the structurally related caffeine by combining the specific extraction with pulsed elution, resulting in sharp baseline-separated peaks, which on the other hand was not possible when a theophylline imprinted polymer was used as stationary phase for HPLC. A detection limit of 120 ng mb1 was obtained, corresponding to a mass detection limit of only 2.4 ng [45]. This combination of techniques was also used for the determination of nicotine in tobacco. Nicotine is the main alkaloid in tobacco and is the focus of intensive HPLC or GC analyses due to its health risk to active and passive consumers. However, HPLC- and GC-techniques are time-consuming as well as expensive, due to the necessary pre-purification steps required because the sample matrices typically contain many other organic compounds besides nicotine. However, a simple pre-concentration step based on MI-SPE did allow faster determination of nicotine in tobacco samples. Mullett et al. obtained a detection limit of 1.8 jig ml 1 and a mass detection limit of 8.45 ng [95]. All these examples demonstrate the high potential of MI-SPE to become a broadly applicable sample pre-purification tool. 

A further promising development is the implementation of CEIA on microchip devices. In this case the excellent mass detection limit makes it favorable, in contrast to classic ELISA. The use of whole cells and viruses for CEIA applications is also an emerging field. 

A more sophisticated mode of LIE detection is the multiphoton-excitation (MPE) fluorescence [47], which is based on the simultaneous absorption of more than one photon in the same quantum event and uses special lasers, such as femtosecond mode-locked laser [48] or continuous wave laser [49], This mode of LIE detection allows mass detection limits at zeptomole level (1 zepto-mole=10 mol) due to exceptionally low detection background and extremely small detection volume, whereas detection sensitivity in concentration is comparable to that of traditional LIE detection modes. A further drawback is the poor suitability of MPE-fluorescence detection to the on-column detection configuration, which is frequently employed in conventional LIE detection. 

PQQ was successfully separated from three closely related isomers by capillary electrophoresis with UV detection <2000JCH(876)193>. Rapid and efficient separation of all four compounds 17-20 as their negatively charged carboxylate ions with baseline resolution was achieved by the addition of 1-5 mM R4N salts to the capillary buffer. Detection limits of PQQ and its three isomers were in the range of 7-15 xM with mass detection limits of 98-210 fmol. 

METHOD MASS DETECTION limit (moles) CONCENTRATION DETECTION limit (molar) advantages/ disadvantages... 

As described in Chapter 3, mass detection limits for AW devices are typically at or below one ng/cm. These low detection limits translate into hundredths of a monolayer of atoms and film thicknesses of hundredths of nanometers. This... 

Using AW devices to monitor dynamic processes such as diffusion and corrosion can dramatically reduce the time required to quantify these processes. For example, as discussed in Section 4.2.2, diffusion equilibration times typically increase with the square of the diffusional length. For a thin film, this length scale, the film thickness (h), is very small. This enables the quantification of diffusion coefficients as low as 10 cm /sec in less than one day, whereas months would be required using many conventional techniques that use thick films or bulk samples. For corrosion monitoring, the dramatic decrease in mass detection limits obtainable using coated AW devices, as compared with conventional balances and sample coupons, allows detectable mass changes to be achieved in minutes or hours rather than days or months (Section 4.4.3). 

MECC-TOAD also provides high sensitivity. A typical performance of this instrument under the conditions described above is shown in Figure 2. The detection limits calculated from Fig.2 range from 0.5 to 1.7 pM, which is equivalent to 1.4 to 4.6 fmol listed in Table I. In contrast, the HPLC-UV analyzers had about 1 pmol of mass detection limit and 2 iM concentration detection limit, provided that the injection volume was 50 pL [24]. Unfortunately, the volume mismatch between MECC-TOAD and available sequencers have limited the use of this reproducible and high sensitive technology. Therefore, miniaturization of the protein sequencer is essential. 

Detection with a microchip is primarily through LIF, since this is eashy implemented with the planar configuration of the microchip (Figure 5-10). Limits of detection for fluorescein-like fluors have been easily demonstrated at the 10 M level and pushed as low as 10 M—a mass detection limit of a few hundred molecules. This allows for detection, for example, of polymerase chain reaction (PCR)-amplified DNA fragments at a level that competes with P-autoradiography jffom Southern blots. Typical microchip separation times are around 50 to 200 seconds. 

The mass detection limits for the above detectors range from 10 to 10 moles based on a lOnl injection. 

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