# Table S1
#
# The table gives results obtained for potassium in an inter-laboratory reference
# material certification exercise. Two materials were circulated; a
# candidate drinking water reference material (“RM”) and a performance
# control material (“QC”) made up by spiking demineralised water. All values
# (shown to four decimal places) are in mg/L. Laboratories measured
# multiple replicates of each material; the data set comprises the
# means for each laboratory.
LabID	QC	RM
Lab01	 7.9367	5.1640
Lab02	 9.3400	5.9400
Lab03	 7.3969	4.7404
Lab04	 7.6350	5.1580
Lab05	 7.6700	4.9720
Lab06	 8.2500	5.4080
Lab07	 7.7600	5.0840
Lab08	 8.2700	5.1900
Lab09	10.1200	6.5580
Lab11	 7.9900	5.1620
Lab12	 7.9300	5.0980
Lab13	 8.7933	5.7520
Lab14	 7.8533	4.9440
Lab16	 7.8500	5.4060
Lab18	 7.6600	4.7000
Lab19	 7.7800	5.1800
Lab20	 9.0600	5.1960
Lab21	 7.6191	4.9121
Lab22	 7.4167	4.7480
Lab23	 8.1000	5.2800
Lab25	 7.8700	5.1660
Lab26	 9.0858	5.7634
Lab27	 6.7433	3.8200
Lab28	 7.8167	4.9400
Lab29	 5.2550	7.7900