Page 13 - Shimadzu Journal vol.8 Issue2

Insight from customer / Report

the goal is to analyse the undiluted sample wherever possible. The This resulted in concentrations sulphur of 1 to 4 mg/L S for the
detection method of choice is sulphur chemiluminescence detec- lowest level (level 1), 16 to 40 mg/L S for the medium level (level
tion (SCD), which is a technique both sensitive and selective and 2) and 82 to 200 mg/L S for the highest level (level 3) investigated,
therefore common in petrochemistry to study the sulphur content respectively (Table 1).
of e.g. diesel or gasoline. Sulphur chemiluminescence detection To investigate possibly occurring matrix effects, the target sul-
offers increased selectivity and linearity when compared with flame phur mix was spiked into hexane at the same three concentration
photometric detection. Additionally, it shows the benefit of equi- levels to prepare pure solvent matrix solutions. By comparing the
molar response, which facilitates quantitation of especially com- response in diesel and hexane matrix, the recovery of the system
plex mixtures as it erases the need for an individual calibration of could be investigated.
2
each target compound .
The reason for the advantages of sulphur chemiluminescence Table 1. Standard mix of seven sulphur compounds,
concentrations given as sulphur
detection over FPD technique lies in its detection principle: The
use of an oxidation-reduction sequence followed by a reaction with Level 1 Level 2 Level 3
# Compound concentration concentration concentration
*
ozone creates excited SO2 molecules from sulphur containing com- (mg/L S) (mg/L S) (mg/L S)
pounds eluting from the column. These SO2 molecules can then be 1 Thiophene 4.02 40.18 200.89
*
detected based on their chemiluminescence at a given wavelength 2 Butyl mercaptan 3.62 36.25 181.23
range. The matrix entering the detection system mainly consists of 3 2-Methylthiophene 3.39 33.87 169.34
hydrocarbon compounds, which in contrast create CO2, that is not
4 tert-Butyl disulfide 3.97 39.72 198.61
seen by the detector, making sulphur chemiluminescence detection
5 Benzothiophene 2.56 25.59 127.97
a highly selective technique.
6 Ethyl phenyl sulfide 2.29 22.95 114.74
From a petrochemical perspective the second most important
7 Dibenzothiophene 1.65 16.54 82.69
point is the equimolarity of the detection principle. Especially with
a huge amount of different sulphur compounds being potential
targets, an individual calibration for each target substance is not
practical or even not possible. Due to the generality of the de- Instrumentation
tection process, all sulphur containing compounds produce SO2 *
molecules, their number being dependent solely on the number of The sulphur samples were analysed using a Shimadzu Nexis GC-
sulphur atoms in the molecule. Consequently, a disulfide produces 2030 system equipped with SCD-2030 sulphur chemilumines-
*
two SO2 molecules leading to twice the response of a monosulfide. cence detector (fig. 1). A high split ratio of 1:100 was chosen since
The response of different monosulfides is the same provided that 1 µL of the undiluted diesel samples was injected into the GC using
the same amount of sulphur (in moles) is present. Therefore, when an AOC-20i Plus autosampler. Compound separation was real-
working with SCD technique concentrations of the target sub- ized using a Shimadzu SH-Rtx-1 column, with dimensions of 30
stances are typically given as concentrations sulphur (e.g. mg/L S), m length, 0.32 mm ID, 0.1 µm df. The temperature program was
not as compound concentrations. chosen to provide a typical screening method, not only ensuring
good separation of the seven target sulphur compounds, but also
allowing a closer investigation of the remaining sulphur content
Samples and reagents from the blank diesel matrix. This led to a chromatographic runt-
ime of 63 min.
To investigate selectivity, reproducibility and equimolarity of the
sulphur chemiluminescence detector for petrochemical samples, a
matrix of desulphurized diesel with a remaining sulphur content
below 10 mg/L was chosen as blank matrix. This was spiked with
a mix of seven different sulphur compounds belonging to differ-
ent sulphur substance groups relevant for diesel samples: sulfides,
mercaptans and thiophenes. The investigation range was chosen as
10, 100 and 500 mg/L per target substance since higher levels of
Fig. 1. Gas chromatograph GC-2030 with
a single compound are highly unlikely to occur in diesel samples. sulphur chemiluminescence detector SCD-2030

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