Detector response particle size

Theory. We will outline theory developed earlier (11,12) for converting the detector response F(v) from a turbidity detector into particle size information. F(v) is related to the dispersion-corrected chromatogram W(y) by the integral equation... 

The 1000 A column did not show any resolution between 312 nm and 57 nm particle sizes. Shown in Fig.2 are the calibration curves for the 2000 A and 3000 A columns and for their combination. The 57 nm particle standard appears to have been erroneously characterized by the supplier. This was subsequently confirmed by electron microscopy. The 2000 X column exhibited a sharp upturn in its calibration curve close to the exclusion limit. It is to be noted that while data points corresponding to 312 and 275 nm diameter particles appear on individual column calibration curves, they are not indicated for the calibration curve of the combination. This is because these larger diameter particles were completely retained in the packed colimms, generating no detector response. The percentage recovery for these particles from individual columns was considerably less than 100 resulting in their complete retention when the columns were combined in series. 

In addition to ELS, charged aerosol (CA) or corona detector has more recently been introduced as a very promising HPLC detection system [105] while the sensitivity of the two systems is quite close, CA detector offers the advantage of a nearly linear response factor, particularly crucial for the assessment of enantiomeric purities, whereas ELS provides a nonlinear response at very low or high levels of analytes, resulting from several light scattering mechanisms and particle size distribution. 

On-line measurements of the sulfur content of atmospheric aerosols have been made by removing gaseous sulfur species from the aerosol and then analyzing the particles for sulfur with a flame photometric detector (24) or by using an electrostatic precipitator to chop the aerosol particles from the gas so that the sulfur content could be measured by the difference in flame photometric detector response with and without particles present. These and similar methods could be extended to the analysis of size-classified samples to provide on-line size-resolved aerosol composition data, although the analytical methods would have to be extremely sensitive to achieve the size resolution possible in size distribution analysis. 

Particle Size Determination. The corrected detector response at retention volume v is given as... 

Ki, the detector response factor, describes the signal generated when particles are present in the eluant as it transits the detector flow-through cell. Detector response arises primarily from scattering of light by the latex particles (15), although a small contribution from light absorption by the sample may occur (16). Polystyrene latex standards of known size and concentration were used to determine Ki factors for conversion of detector response into mass concentration information. 

Figure 2. Particle size distribution of a commercially available polybutadiene latex calculated using different detector response functions.

FIGURE 11.6 UV and ICP-MS-based Sd-FFF fractogram of a colloidal (0.2 to 0.8 Urn) fraction detector responses versus time (a) particle size distribution (UV response) and element-based size distribution for Mn and U (b) and element ratios for Mn/Si and U/Si (c). 

Not always does the lowest particle size provide the best results. The decreased detector response obtained from a particle size < 36 pm relative to < 50 and < 77 pm was ascribed to the increased co-extraction of interferents with the anaiyte for sediments [16] or to the coagulation of particles in the case of plant material [17],... 

As mentioned, in addition to the MMD and PSD, various average molar masses, particle sizes, and polydispersity indexes can be calculated from the FFF frac-tograms. If the detector response, h, is proportional to the mass of the macromolecules or particles, the mass-average molar mass or mass average particle diameter can be calculated from... 

The dependences of the retention ratio R on the size of the fractionated species (molar mass for the macromolecules or particle diameter for the particulate matter) are presented for various polarization FFF methods in the entry Field-Flow Fractionation Fundamentals. The raw, digitized fractogram, which is a record of the detector response as a function of the retention volume, is represented by a differential distribution function h(y). i can be processed to obtain a series of the height values hi corresponding to the retention volumes as shown in Fig. 1. Subsequently, the retention volumes are converted into the retention ratios Rf. 

To deduce a particle size distribution, the detector response must be deconvoluted by means of a simulation calculation. The scattering particles are assumed to be spherical in shape, and the data are subjected to one of three different computational methods. One system uses the unimodal model-dependent method, which begins with the assumption of a model (such as log normal) for the size distribution. The detector response expected for this distribution is simulated, and then the model parameters are optimized by minimizing the sum of squared deviations from the measured and the simulated detector responses. The model parameters are finally used to modify the originally chosen size distribution, and it is this modified distribution that is presented to the analyst as the final result. 

For air pollution monitoring, it would be desirable to have detectors whose response Isa single-valued function of particle size (volume) and not of shape or refractive index, because these parameters may vary from particle to particle. Practical measurement systems fall fa ... 

Asphalt contains many different compounds that vary not only in molecular, or particle, size but also in UV absorptivity or refractive index. Figure 17 shows the relation between detector response per unit mass and apparent molecular size for some asphalts (12). Neither detector is uniform, as a mass detector would be. The UV detector is much less uniform than the RI detector. This is mainly because paraffinic hydrocarbons, known as saturates, which comprise roughly 10-20% of a typical asphalt, are very weak absorbers of UV light, and the aromatic components in the asphalt are strong UV absorbers. Consequently, a... 

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