Xray diffraction, slits

The art of slit setting:

The length of the sample that is touched by X-rays depends on the slit settings and on the diffraction angle. At lower 2 theta a larger area on the sample is iluminated compared to higher 2 theta if the slit is left unchanged. Care must be taken not to irradiate the sample support as this leads to unwanted background effects and to changes in relative intensities of the peaks.
If automatic divergence slits are used, the irradiated sample length is kept equal over a certain range of 2 theta. However, some slits can not be closed to values lower than 0.1° angular opening (SIEMENS D5000). That means, that a lower limit does exist beyond that this equipment can no longer be used (see below).

Minimum sample length ("illuminated area") needed to give correct intensities for various slit apertures and diffraction angles:

Slits [mm]	0.1	0.2	0.4	1	2	4	6	12
Slits [°]	0.05	0.1	0.2	0.5	1	2	3	6
2theta = 2°	10 mm	20 mm	40 mm	100 mm	200 mm	400 mm	601 mm	1204 mm
2theta = 4°	5 mm	10 mm	20 mm	50 mm	100 mm	200 mm	300 mm	602 mm
2theta = 6°	3 mm	7 mm	13 mm	33 mm	67 mm	133 mm	200 mm	402 mm
2theta = 8°	3 mm	5 mm	10 mm	25 mm	50 mm	100 mm	150 mm	301 mm
2theta = 10°	2 mm	4 mm	8 mm	20 mm	40 mm	80 mm	120 mm	241 mm
2theta = 12°	2 mm	3 mm	7 mm	17 mm	33 mm	67 mm	100 mm	201 mm
2theta = 20°	1 mm	2 mm	4 mm	10 mm	20 mm	40 mm	60 mm	121 mm
2theta = 30°	1 mm	1 mm	3 mm	7 mm	13 mm	27 mm	40 mm	81 mm
2theta = 60°	0 mm	1 mm	1 mm	3 mm	7 mm	14 mm	21 mm	42 mm
2theta = 90°	0 mm	0 mm	1 mm	2 mm	5 mm	10 mm	15 mm	30 mm

Calculations are made for a goniometer radius of 20 cm. For others, the formula:
L = R * tan(alpha) / sin(theta)
should be used.
L = iradiated sample length [cm]
R = diffractometer radius [cm]
alpha = angular opening of the slits

The SIEMENS D5000 with automatic slits:

The SIEMENS D5000 can be equipped with automatic theta-compensating slits, that keep the same sample area iluminated over a wide angular range. However, the slits can be closed to an opening of 0.1°. That means, that at 2° 2theta and at maximum closing of the slit, a length of 20 mm is touched by the X-ray beam. At 12° 2theta and 0.1° slit aperture, this length is 3 mm. The following conclusion can be drawn from that (assuming standard sample holders):

with the V20 slit option the minimum angle useable is 2° 2theta
with the V6 slit option the minimum angle useable is c. 7° 2theta
the V3 slit option cannot be used below 12° 2theta!

The 1° "universal" slit and clay mineralogy:

The 1° divergence slit is the choice for alround diffraction work and many users of diffractometers may never have thought about slit settings or changing slits. However, for clay mineral analysis, especially if 00l intensity data is to be comparable, slit setting is crucial. Even for the large c. 5 cm slides a 0.5° slit must be used if the intensity data are to be obtained from about 6° 2theta (eg. the chlorite 001). If the 2 cm standard sample holders are used, the slit must be as narrow as 0.2°!