The deposited salts and brine from
salt lakes are important industrial
resources for many kinds of salts,
such as edible salt, potassium,
boron, lithium, and a few heavy metals.
Nearly 97% of potassium fertilizer made in
China is produced from the salt lake brines.
However, in the industrial process for
manufacturing the targeted products from
salt lake brines, organic substances in brine
have been found to influence the quality
of products and the manufacturing process.
An effective sample preparation and analysis
method is required in order to determine
the trace organics in brine, including volatile
organic compounds (VOCs) and semi-volatile
organic compounds (SVOCs).
The conventional methods of screening
organics are liquid–liquid extraction, solid-phase extraction (SPE),1,2 solid-phase microextraction,3 purge and trap,4 reverse osmosis,
and electrodialysis.5 Brown6 reported that
XAD-8 and XAD-4 could be used to absorb
dissolved organic material for isolating fuvic
acids and transphilic acids from Pony Lake,
a saline and hypereutrophic coastal pond
located on Cape Royds in the McMurdo
Sound area in Antarctica. GC-MS is typically
used to determine VOCs and SVOCs.
In the present study, routine analyses were
performed on a QP2010 GC-MS (Shimadzu Corp., Kyoto, Japan).
Brines in the Qarhan Salt Lake (Haixi,
China) are hypersaline water and usually
saturated by salts. XAD mixed resins
and LC-C18 cartridges were used to collect
and concentrate the organics in the
authors’ studies. The analytes were identified
Qarhan Salt Lake, located in south central
Qaidam basin between 94°28’ and 96°23’
north latitude and 36°43’ and 37°02’ east
longitude, is the largest playa in China.
The lake is about 168 km in length and
20–40 km in width. Typically, the brine is
neutral, with an average salinity of 249.5 g
L–1 and a specific gravity of 1.201.7
In this study, water samples were collected
from the carnallite pond of a potassium
fertilizer plant (Qinghai Salt Lake Industry
Group Co., Ltd., the largest potassium
fertilizer industry in China). The brines of
the pond are from Qarhan Salt Lake and
are concentrated by sunshine.
Reagents and materials
Naphthalene-D10 internal reference was
supplied by Acros Organics (Morris Plains,
NJ) with 98+ atom% deuterium. Working
standard solutions were a series of ethyl acetate
solutions containing naphthalene-D10
between 0.1 mg L–1 and 1 mg L–1. The standard
solutions were freshly prepared and
were stored in the refrigerator at 4 °C.
Methanol, acetone, methylene chloride
(DCM), and ethyl acetate (spectrograde)
were further distilled before use. Magnesium
sulfate (analytical grade) was heated in a 300
°C muffle furnace for 3 hr to remove organic
substances and water before use. Water used
throughout this work was freshly double distilled
from potassium permanganate (ddH2O).
LC-C18 solid-phase extraction cartridges
(Supelco, Bellefonte, PA) and XAD-2 (Rohm
and Haas, Philadelphia, PA) and XAD-7 resins
(Acros Organics) were used for solid-phase
extraction. The properties of the absorbents are
presented in Table 1. The resins were purified
by sequential solvent extraction with methanol,
DCM, and acetone in a Soxhlet extractor
(8 hr per solvent). The purified resins were
stored in glass stoppered bottles under methanol
to maintain their high purity.
Table 1 - Properties of absorbents
The solid-phase extraction equipment
was supplied by Sigma-Aldrich Corp. (St.
Louis, MO). A KL 512 nitrogen evaporator
(Kanglin Science & Technology Co., Ltd.,
Beijing, China) was used for concentration.
As stated above, routine analyses were performed
on the QP2010 GC-MS. A DB-5
MS capillary column (Agilent Technologies,
Palo Alto, CA) (30 m × 0.25 mm ×
0.25 μm film thickness) was employed for
the separation of the analytes. GC conditions
were as follows: injector temperature,
240 °C; flame ionization detector temperature,
240 °C, split 1:20; oven temperature,
50 °C, then 8 °C min–1 to 200 °C (hold 5
min), then 10 °C min–1 to 240 °C (hold
for 5 min). MS conditions were as follows:
electron impact ionization, ion energy to
70 eV; mass range, 35–350 m/z.
The original brines are usually saturated
with salts. If the original brines were passed
through the LC-C18 cartridge and XAD
column directly, salts might be precipitated,
which causes the jam of columns. To avoid
jamming the LC-C18 cartridge and XAD
column and to maintain a moderate flow
rate in the pretreatment procedure, 500 mL
brine sample was diluted with ddH2O to 1
L. The concentration of the internal standard
in the diluted sample was 0.5 µg L–1.
C18 cartridge solid-phase extraction procedures
were as follows. The diluted sample was
passed through the LC-C18 cartridge previously
conditioned by passing methanol (10 mL), ethyl acetate (8 mL), and ddH2O (10
mL) in sequence. After loading the sample, the
cartridge was washed with ddH2O (10 mL).
The cartridge was air-dried using vacuum for at
least 15 min, and then eluted with 10 mL ethyl
acetate and 10 mL DCM. The proper amount
of anhydrous magnesium sulfate was added to
the extracted solution to remove the residual
water until a clear solution was obtained. After
filtering, the collected solution was then evaporated
to 1 mL under a gentle nitrogen stream at
50 °C. The final extract obtained was analyzed
and identified by GC-MS.
The procedures reported by Junk8 were referred
to for the XAD resin solid-phase extraction. A
1.5-cm-i.d. × 20-cm-long glass tube was packed with XAD-2 and XAD-7 (v/v 4:1). The resin
was then washed with ddH2O to remove methanol.
The diluted sample was passed through
the XAD resin column by gravity flow. After
loading the sample, the column was washed
with 20 mL of ddH2O. Subsequently, the column
was dried by passing air, and the elutions
were performed by passing 15 mL ethyl acetate
and 15 mL DCM in sequence (each elution
solvent was allowed to equilibrate with the resin
for 10 min). Finally, an additional 5 mL of DCM
was added to the column and immediately
flowed through the resin. The residual water
was removed by adding the proper amount of
anhydrous magnesium sulfate to the extracted
solution until the solution became clear. After
filtering, the collected extract was evaporated to
1 mL under a gentle nitrogen stream at 50 °C.
The concentrated solution was analyzed and
identified by GC-MS.
Results and discussion
The authors selected naphthalene-D10
as the internal reference for the recovery experiments. The calibration curves were
obtained by analyzing a series of naphthalene-D10 standard solutions with a concentration
between 0.1 and 1 mg L–1.
Table 2 - Recovery and standard deviations (SDs) of the method
The concentration of naphthalene-D10
in the diluted sample was 0.5 μg L–1. The
measured results are given as recoveries
in Table 2. The mean recovery of the
LC-C18 cartridge was higher than the
XAD column. The low
recovery of the XAD column
may be due to the air
bubbles formed in the process
of solid-phase extraction.
These bubbles may
have caused a bypass effect
that reduced the efficiency.
This phenomenon was not
be found in C18 solid-phase
extraction because the sorbent
in the LC-C18 cartridges
was tightly packed.
It can be argued that the
different affinities between
LC-C18 and XAD resins to
naphthalene-D10 can also
affect the efficiency.