USGS Bulletin 1823 - Table 4 - Instrumentation and Operating Conditions

METHODS FOR SAMPLING AND INORGANIC ANALYSIS OF COAL
U.S. Geological Survey Bulletin 1823

Instrument or Function	Operating Conditions
Spectrograph	3.4-m Ebert (Jarrell-Ash Mark III). Grating has 600 grooves per mm, and is blazed for the first-order ultravlolet region. A 2-mm high central portion of the arc is focused on the collimator by a cylindrical quartz lens that has a focal length of 45 cm and is located at the slit. The slit is 0.025 mm wide and 2.0 mm high. The two-step neutral density filter used for emulsion calibration and a 14 percent transmission neutral density filter are located near the slit. The height of the mask at the collimator is 18 mm.
Dispersion	0.5 nm/mm reciprocal linear dispersion, first order.
Wavelength range	230.0 to 470.0 nm; first order.
Excitation	15-A direct-current arc; current is set with a set of graphite electrodes in place (the anode cup is empty). The open-circuit voltage from the power supply is 325 volts.
Arcing of sample	15 mg of sample, mixed with 30 mg of pure graphite powder, is arced for 20 s at a current of 5 A, then the current is switched (without interruption of the arc) to 15 A for a period of 130 s.
Arc gap	4 mm, maintained constant throughout arcing process.
Upper electrode	Cathode: ASTM type C-6, 3.2-mm diameter ´ 50-mm long graphite rod.
Lower electrode	Anode: Ultra Carbon No. 3170, 6.3-mm diameter, thin-walled graphite.
Atmosphere	Mixture of 70 percent argon plus 30 percent oxygen; flow of 6.6 L/min directed by a ceramic jet concentric to the anode (Helz, 1964). The top of the jet nozzle is positioned 2 mm below the top of the anode.
Photoplates	Eastman-Kodak III-0 emulsion, 102 ´ 508 mm glass plate. Exposed emulsion is processed in D-l9 for 3.25 min, fixed for 10 min, washed for 30 min, and dried by a flow of warm air for 30 min.