REPORT FOR ANALYTICAL CHEMISTS
Courtesy Esso Research & Engineering Co.
THE IDEAL PRESS FOR MAKING KBR PELLETS FOR INFRARED SPECTROSCOPIC ANALYSIS I
A l s o for forming pellets for x - r a y a n d
I
other types of spectroscopic analysis 9
I I
2 0 Ton Capacity Hydraulic Press · Accurate alignment in o p eration • Guided m o v i n g platen • Sturdy 3 column construction
•
Even pressure distribution for uniformly dense pellets
•
Self-contained
9
Bench mounted
9
H a n d operated
• 9 " χ 9 " platen a r e a 9
2 2 " max. adjustable
vertical
opening,
A multi-purpose unit with q u i c k l y adjusted v e r t i c a l opening, by handwheel, per mits many different set-ups in minimum time. Used around the world for RESEARCH, CONTROL & TESTING. I I I I I
Available accessories include: Heating and Cooling Platens; Temp. Controis, Auxiliary low pressure gauges; Fast air closing; Extruding units; Testing units, etc.
I
A v a i l a b l e in 3 0 a n d 5 0 ton sizes.
Write for Bulletin.
LOOMIS ENGINEERING & MANUFACTURING CO. Dept. A , Route 4 6 , C a l d w e l l , N . J . Circle No. 58 on Readers' Service Card 28 A
·
Objectives of various cooperative programs may differ a great deal. For example, Willits (21) and Wernimont (20) describe contrast ing approaches that have widely different objectives. In one case reported by Willits, a number of laboratories made determinations on two types of samples represent ing two chemical species and two levels of the ingredient (element), using two basic procedures but with variations of their own choice (equivalent to many methods). The primary objective was to select the best method or methods from the ones examined. In a case cited by Wernimont, one sample was tested by eight laboratories using one basic procedure with no choice of variation of techniques or equip ment, and using two operators in each laboratory, each operator per forming two replications of the tests on each of three different days. In this example, the objectives were to measure the precision of the method for one common sample and to assess the laboratory, operator, and day effects. In the Joint ArmyNavy-Air Force Analytical Panel work, the approaches given in both of the references cited above have been used—sometimes first the sur vey approach described by Willits, followed by the more closely con trolled experiments advocated by Wernimont. At times, the design of a single round robin has combined features of both approaches.
ANALYTICAL CHEMISTRY
Samples Every chemist realizes that a test can be no better than the sampling. Yet, faulty samples have ruined many cooperative testing programs. Usually it is desirable that particle size distribution not constitute a variable. Hence, the santple should be uniform, and the particles suffi ciently fine to have no influence on test results. Instructions may need to take into account the possibility of size segregations that might oc cur during shipment. They may need to warn the participants to use the sample as received (omit ting some process, such as grinding, that would ordinarily be used), or to process the sample in some speci fied way before use—e.g., grinding
or drying. Packaging for materials that can either lose volatile ingredi ents or take on moisture during shipment is a critical item. Changes caused by the aging of samples must be considered and di rections given that will eliminate such effects or minimize them.
Chairman's Instructions to Participants The chairman should issue clear instructions covering such impor tant details of interlaboratory test ing as the following: Methods. I t is the chairman's re sponsibility to see that all partici pants are supplied with correct de scriptions of the method (s) to be tested. These descriptions must be uniform for all concerned and as well written as possible. Practice Period. Frequently in interlaboratory testing a new method does not meet expectations with respect to precision as com pared with older methods. I t also happens that sets of replicates by new procedures will need replace ment (alternative) values when a test for rejection of data is applied. Lack of experience with the new method is responsible for some of the variability encountered. Hence, it is desirable to ask participants to practice with the new technique un til some familiarity with it is achieved before making the deter minations to be reported for the in terlaboratory test. If the official samples are in short supply a prac tice sample should be used. Practi cal limitations of time and expense may determine the extent of the practice period according to the op erator's judgment. Moisture Determination. Fre quently also it is essential that the chairman instruct each participant to make a moisture determination on his samples by a prescribed method, and to calculate values for other ingredients to a moisture-free basis. Sample Weights. Sample weights for individual determina tions have sometimes been set by chairmen at an exact number of grams (no variation permitted) in order to avoid errors of recording weighings and to simplify the work
