2.2. Analytical methods
All water samples, due to their turbidity, were filtered through
0.7-µm-pore-size glass fiber filters (Whatman GFF, pre-combusted in
400°C, 24 h).. Samples were kept in cool storage (+4°C) and analysed
approximately two months since collection time. Specific electric
conductivity (SpC), pH and dissolved oxygen were measured in the field
with a multiparameter YSI Pro Plus probe with a 4-sport Quattro Cable
(11m).
Dissolved organic carbon (DOC) was determined through a total organic
carbon (TOC) measurement in filtrate with a TOC-VCSH/CSN analyser
(Shimadzu; the method of catalytic oxidation with oxygen at 680°C and
nondispersive infrared spectroscopy detection). Calibration was
performed with a potassium phthalate standard (Shimadzu), with an
R2 = 0.998. CVs of five sample replicates were below
2%. Samples with DOC concentrations above the analytical range were
diluted with MilliQ water (Merck Life Sciences). Only a few samples with
a high suspended sediment content were analysed for POC (details inSuppl. Mat. 1 ).
Major ions (F-, Cl-,
NO2-, Br-,
NO3-,
PO43-,
SO42-, Na+,
NH4+, K+,
Ca2+ and Mg2+) were determined using
Dionex ICS 3000 (Dionex, USA), calibrated with dilutions of a 1000 mg
L-1 single-ion standard solutions. Analytical accuracy
was checked with the TraceCERT® Multi Anion Standard 1 for IC
(69734-100ML) and Multi Cation Standard 1 for IC (91286-100ML) (all
solutions by Merck Life Science).
Elemental analysis was performed in acidified (with high-purity nitric
acid, Suprapur, Merck Life Science) filtrate using inductively coupled
plasma optical emission spectrometry (ICP-OES 9820, Shimadzu, Japan) for
Be, P, S and Ti. Inductively coupled plasma mass spectrometry (ICP-MS
2030, Shimadzu, Japan) was applied for Ag, Al, As, Ba, Bi, Cd, Co, Cr,
Cu, Fe, Hg, Li, Mn, Mo, Ni, Pb, Sb, Se, Si, Sr, V and Zn. For
calibration of the ICP-MS, the ICP IV multielement standard (Merck Life
Sciences) and single-element standards of As, Be, Sb, Se, Mo, and V
(Sigma-Aldrich, USA) were used. Single-element 1000 mg
L−1 standards (Sigma-Aldrich, USA)were applied for the
ICP-OES. Internal standards controlling for the MS signal were solutions
of Sc, Rh, Tb, and Ge in suprapure 1% HNO3 (Merck).
Accuracy of the analysis was checked with Sigma-Aldrich CRMs (certified
reference materials) Trace Metals ICP-Sample 1 and Trace Metals
ICP-Sample 2, with 98-104% recoveries. High-concentration samples were
diluted with Milli-Q water (Direct 8 Purification System Water, Merck
Life Sciences, France). All metals, non-metals and metalloids
concentrations measured have been corrected for a procedural blank value
if it exceeded the method limit of quantification (LOQ); in all other
determinations, the blank correction was not required. The details of
all measurement conditions and QA/QC parameters, including method LOQ,
are given in the Supplementary material (Suppl. Mat. 1, Table
S3 ).