Determination of the phosphorus content in sewage sludge: comparison of different aqua regia digestion methods and ICP-OES, ICP-MS, and photometric determination

Environmental Sciences Europe

Table 3 ILC phosphorus results for different method classes (Fig. 1) and by digestion or analysis method

Method (class)	l	n	Outliers	\(\overline{x }\)	U	CV_r	CV_R	HorRat	\(\frac{{CV}_{r}}{{CV}_{\mathrm{R}}}\)
Method (class)	l	n	Outliers	[g/kg]	[g/kg]	[%]	[%]	HorRat	[%]
A	67	19	1	26.0	0.7	1.5	5.8	1.7	25
B	59	17	3	25.4	0.7	1.3	5.7	1.6	22
C	33	11	1	25.4	0.7	1.3	4.6	1.3	28
D	29	8		25.0	1.2	4.1	7.5	2.1	55
E	22	6		24.2	1.5	3.3	8.1	2.3	41
F	22	6		23.1	1.2	2.3	6.5	1.8	35
ICP-OES	138	36	4	25.7	0.5	1.4	5.8	1.7	24
ICP-MS	63	18	2	25.2	0.7	2.1	5.8	1.7	36
Photometry	44	12		23.6	1.0	2.9	7.5	2.1	39
Microwave	130	36	2	25.0	0.3	1.8	6.3	1.8	29
Reflux	118	31	3	24.2	0.5	2.1	8.0	2.3	27
Total	248	67	5	25.3	0.7	1.9	7.2	2.0	26

l: Number of measurement values: replicates included
n: Number of datasets: different lab averages by method class (several labs performed more than one method class), no outliers
\(\overline{x }\): Mean phosphorus content
U: Expanded Uncertainty of \(\overline{x }\) with factor k = 2: \(U=2\frac{{s}_{R}}{\sqrt{n}}\)
CV_r: Relative repeatability standard deviation (s_r); variability of the replicate means per laboratory
CV_R: Relative reproducibility standard deviation (s_R); variability of the means of different labs
HorRat: Horwitz ratio or ratio of CV_R and predicted Horwitz coefficient of variation pCV_R; \({pCV}_{R}={2}^{(1-0.5 log\overline{ x })}\)
\(\frac{{CV}_{r}}{{CV}_{\mathrm{R}}}\): Ratio of repeatability and reproducibility in %; 50–66% is common according to Horwitz [13]