mt PROCEDURE FOR GUMMED FILM Sampling The sampling medium is a rubber base cement spread on a 13" x 13" cellulose acetate backing and covered with glassine paper for protection, For sampling, the glassine is removed and the film attached to a cadmiumplated steel frame having a one foot square opening. three feet above the ground on a stand, The frame is held Each paper is exposed for twenty- four hours, the sampling being started at 1230 Greenwich Civil Time at each station, At the end of the sampling period fresh films are put in place on spare frames and exchanged with the exposed films. The exposed films are stripped off and folded with the gummed sides together into a size suitable for mailing. Most sampling stations collect duplicate samples, and the two samples plus a field data card giving the location, date of sampling, and weather information are forwarded to the Health and Safety Laboratory. Sample Preparation An important point in processing is maintaining identification of the sample from receipt to the reporting of the analytical results. purpose a six digit serial number is assigned to each sample. For this oi The first three digits indicate the day of analysis on a counting house calendar basis, day. When the sample envelope is opened, a prenumbered label is placed on a while the last three digits indicate the number of the sample run during that the data card and the gummed films are placed in steel cans for ashing. These ashing cans are used as crucibles and are discarded after a single use, For convenience and speed of operation they are held in groups of ten in a stainless steel rack and the rack number and position within the rack are the actual indication of sample number for each can, - 32- a