The cross-sectional radius-of-gyration, Rc is a useful parameter for samples that are highly asymmetric or elongated (see Chapter 4, p155 of Glatter and Kratky 1982). A similar q x Rc < 1.3 limit applies to the upper q-region (yellow circle Figure 1); however, the lower q-region tends to be much more worrisome as it is severely corrupted by the transformation (Figure 1). To start, click "Rc (x-section)" button in the lower part of the panel (Figure 1). This will pop up the Rc plot which is divided into the upper fit and lower residuals plots.

Figure 1 |

The goal here is to find the first linear region on the righthand side of the hump in the upper plot. To start, trim back the high-q data from the end until q x Rc < 1.3. The hump originates from the first sinusoid of the Fourier transform. Sometimes, datasets may not contain this hump largely observed in samples with large Rg values implying more low q data would be required for a typical Guinier analysis. In these cases, an Rg will have to be determined using the P(r) distribution.

Figure 2 |

Here, the data was truncated to the first 188 points. This sets the putative limit q x Rc < 1.3. Now, we truncate the starting points to capture the first linear region. Please note, the Rc value is updated in the analysis tab (red arrow Figure 2).

Figure 3 |

After truncating the starting point to 95, more points were removed from the end (Figure 3 and 4) to maintain the q x Rc limit resulting in an Rc of 14.7. This would be a satisfactory fit. Having both Rg and Rc allows for the estimation of the maximum dimension (see Glatter and Kratky 1982, Chapter 8 page 258)

Figure 4 |