Headspace Solid-Phase Microextraction following Chemical Vapor Generation for Ultrasensitive, Matrix Effect-Free Detection of Nitrite by Microplasma Optical Emission Spectrometry

by Microplasma Optical Emission Spectrometry

Article

Headspace Solid-Phase Microextraction Following Chemical Vapor

Generation for Ultrasensitive, Matrix Eﬀect-Free Detection of Nitrite

Rui Yang, Yao Lin, Jiahui Yang, Liangbo He, Yunfei Tian, Xiandeng Hou, and Chengbin Zheng*

Cite This: Anal. Chem. 2021, 93, 6972 −6979

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sı Supporting Information

ABSTRACT: A new chemical vapor generation method coupled

with headspace solid-phase microextraction miniaturized point

discharge optical emission spectrometry (HS-SPME−μPD-OES)

for the sensitive and matrix eﬀect-free detection of nitrite in

complex samples is described. In an acidic medium, the volatile

cyclohexene was generated from cyclamate in the presence of

nitrite, which was volatilized to the headspace of the container,

eﬃciently separated, and preconcentrated by HS-SPME. Con-

sequently, the SPME ﬁber was transferred to a laboratory-

constructed thermal desorption chamber wherein the cyclohexene

was thermally desorbed and swept into μPD-OES for its sensitive

quantiﬁcation via monitoring the carbon atomic emission line at 193.0 nm. As a result, the quantiﬁcation of nitrite was accomplished

through the determination of cyclohexene. The application of HS-SPME as a sampling technique not only simpliﬁes the

experimental setup of μPD-OES but it also preconcentrates and separates cyclohexene from N and sample matrices, thus

eliminating the interference from water vapor and N and signiﬁcantly improving the analytical performance on the determination of

−

nitrite. Under the optimum experimental conditions, a limit of detection of 0.1 μg L was obtained, which is much better than that

obtained by conventional methods. The precision, expressed as relative standard deviation, was better than 3.0% at a concentration

−

of 10 μg L . The proposed method provides several advantages of portability, simplicity, high sensitivity, and low energy

consumption and eliminates expensive instruments and matrix interference, thus retaining a promising potential for the rapid,

sensitive, and ﬁeld analysis of nitrite in various samples.

2−27

s part of the nitrogen cycle in nature, nitrite is one of the

important nitrogen species with adverse implications on

ods.

Although some of these methods provide enough

sensitivity for the quantiﬁcation of nitrite in complex samples,

most of them usually suﬀer from severe matrix interference

such as color and turbidity, thus limiting their further

application for the ﬁeld analysis of nitrite. However, the ﬁeld

analysis allows both immediate identiﬁcation of the toxicity

induced from nitrite and in situ evaluation of its seriousness,

which is conducive to making a correct decision as soon as

possible. Therefore, it is an urgent need to develop a simple

and sensitive method for the ﬁeld analysis of nitrite.

Due to their advantages of simple construction, low

temperature and power consumption, and atmospheric

operation, various microplasmas have been used as a promising

excitation source to construct miniaturized optical emission

spectrometers for the ﬁeld analysis of toxic elements. Recently,

both the public health and environmental security. Previous

studies revealed that the nitrite in the blood can react with the

iron(III) of hemoglobin to form methemoglobin and seriously

decrease its capability of carrying oxygen, thus causing health

problems. However, nitrite is often used as an additive and

preservative because it not only can improve the ﬂavor and

color of meat products but also inhibit the growth of several

undesirable bacteria such as Clostridium botulinum. Despite

the strict restrictions set by the government, the poisoning

accident of food caused by nitrite frequently occurs. On the

other hand, nitrite can combine with secondary amines and

form strong carcinogenic N-nitrosamine compounds in the

,6−8

human body and leads to hypertension and cancer.

As a

result, it is of great importance to detect nitrite in body ﬂuids,

the environment, and food samples.

Received: December 15, 2020

Accepted: April 15, 2021

Published: April 30, 2021

Numerous techniques have been developed for the

determination of nitrite, including colorimetry, spectropho-

−11

13,14

tometry,

chromatography, capillary electrophoresis,

15,16

ﬂuorescence assay,

methods,

chemiluminescence, electrochemical

Raman spectrometry,

8,19

20,21

and other meth-

2021 American Chemical Society

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Article

the application scope of microplasma optical emission

spectrometry (MP-OES) has been signiﬁcantly extended

8−34

from metal ions (Hg, Cd, As, Pb, etc.)

to nonmetallic

−

analytes (total organic carbon, NH , NO , NO , and

5−40

carboxyl group)

when chemical vapor generation

(

CVG) was used as a sample introduction means. Although

CVG coupling to MP has been successfully used to determine

nitrite via monitoring the molecular emission of NO, the

nitrogen contained in discharge gas resulted in a high blank,

41,42

thus deteriorating the analytical performance.

Recently, Chai et al. reported that sodium cyclamate was

speciﬁcally converted to volatile cyclohexene in the presence of

2,43,44

nitrite.

On the basis of this phenomenon, herein, a

simple CVG method coupling headspace solid-phase micro-

extraction point discharge optical emission spectrometry (HS-

SPME−μPD-OES) was developed for the highly sensitive

determination of nitrite in complex samples. In this method,

the generated cyclohexene was separated and preconcentrated

from sample matrices by HS-SPME and then thermally

desorbed and transported to the μPD-OES for its quantiﬁca-

tion via the detection of carbon atomic emission at 193.0 nm.

Consequently, the quantiﬁcation of nitrite was accomplished

through the determination of the generated cyclohexene. The

HS-SPME not only separates and preconcentrates cyclohexene

from the complex matrices but also reduces the water vapor

and N introduced to the microplasma, thus signiﬁcantly

Figure 1. (a) Schematics of the HS-SPME−μPD-OES experimental

enhancing the excitation capability of microplasma, alleviating

the interferences from sample matrices and improving the

analytical performance of μPD-OES on the determination of

nitrite. Moreover, the size of μPD-OES is also remarkably

reduced because both the sampling system and chemical vapor

generator are eliminated in the proposed method. To the best

of our knowledge, this is the ﬁrst report of the use of HS-

SPME−μPD-OES for the sensitive and matrix eﬀect-free

determination of nitrite in complex samples.

setup; (b) HS-SPME procedure.

power supply (NGB408BL, 14 cm length × 6 cm width × 5

cm height, Electronic Equipment Factory of Jinshi, Guangz-

hou, China). This miniature electric transformer can convert

5 V of direct current to several thousand volts of alternating

current. Thus, a portable power source can meet the power

required in this system. The optical emission from discharge

was transmitted through the hole and recorded with the CCD

spectrometer. The photograph of the whole setup is shown in

Figure S1 of the Supporting Information.

EXPERIMENTAL SECTION

Instrumentation. The instrumental setup is described in

Figure 1a and mainly consists of a commercial SPME device

■

A 7890B gas chromatography (GC, Agilent Technologies,

Inc.) equipped with an HP-5 MS capillary column (30 m

length × 0.25 mm i.d. × 0.25 μm ﬁlm thickness) coupled with

a 5977B mass spectrometer (MS, Agilent Technologies, Inc.)

was used to identify the products generated from the reaction

between nitrite and cyclamate. The SPME device equipped

with commercial extraction ﬁbers with 100 μm polydimethyl-

siloxane (PDMS), 75 μm Carboxen/PDMS, and 65 μm

PDMS/DVB was obtained from Supelco (Bellefonte, PA,

USA). All these ﬁbers were aged at 250 °C for 0.5 h before HS-

SPME. A 20 mL headspace vial (Guoxiang Glass Instrument

Co., Chengdu, China) was used as the reactor to generate

cyclohexene.

(

Supelco, Bellefonte, PA, USA), a laboratory-constructed

thermal desorption chamber, a μPD excitation device, and a

Maya 2000 Pro charge-coupled device (CCD) spectrometer

(

spectral resolution, 0.25 nm; detectable spectral range, 180−

00 nm; Ocean Optics Inc., Dunedin, FL, USA). The thermal

desorption chamber was fabricated by simplifying the

29,37

desorption chamber reported in our previous studies.

quartz T-tube (0.4 cm i.d. × 0.8 cm o.d.) wrapped with a

nichrome wire and penetrated through a polytetraﬂuoro-

ethylene (PTFE) holder (7.0 cm length × 5.0 cm width × 2.5

cm height) was used as the thermal desorption chamber in this

work. One end of the T-tube was covered with an Agilent GC

injection septum and subsequently screwed with a PTFE cap.

A hole with 0.5 mm diameter was drilled on the cap wherein

the SPME ﬁber can be inserted into the chamber for the

desorption of the analyte. The μPD excitation device was

constructed by inserting two tapered tungsten electrodes into a

PTFE discharge chamber (1.0 cm height × 0.8 cm i.d. × 2.2

cm o.d.). A hole (0.5 cm in diameter) was drilled on the wall of

the chamber and rightly faced the center of the gap between

the two electrodes. Finally, the μPD excitation device was

directly connected to the outlet of the thermal desorption

chamber. The discharge was generated immediately as a high

voltage from a compact ac neon sign electric transformer

Reagents and Materials. All reagents used in this work

were of at least analytical reagent grade. High-purity (18.2 MΩ

cm) deionized water (DIW) produced from a water

puriﬁcation system (Chengdu Ultrapure Technology Co.,

Ltd., China) was used throughout the work. A standard

−

−1

stock solution of NO₂(1000 mg L ) was obtained by

dissolving analytical reagent-grade NaNO (Aladdin, Shanghai,

China) with DIW. Calibration solutions were prepared daily by

serially diluting the stock solutions with DIW. Sodium

cyclamate (C H NNaO S, 99%) and cyclohexene (C H ,

99.7%) were obtained from Aladdin Industrial Co. (Shanghai,

China). Sulfuric acid, hydrochloric acid, ZnSO , Na B O , and

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stir bar to achieve the following reaction:

Article

other chemicals were purchased from Kelong Chemical

Reagents Co. (Chengdu, China). Argon (99.999%) was

obtained from Qiaoyuan Gas Company (Chengdu, China)

and used as both carrier gas and discharge gas in this work.

The certiﬁed reference material (CRM) of meat chop

After the reaction, the SPME needle was inserted into the

vial’s headspace and the ﬁber exposed to the generated

cyclohexene for 10 min (Figure 1b). Then, the ﬁber was

withdrawn into the needle and removed from the vial. The

SPME ﬁber was transferred to the desorption chamber to

desorb the extracted cyclohexene at 250 °C for 30 s.

Eventually, the desorbed cyclohexene was further swept to

the μPD excitation source for its determination via monitoring

carbon atomic emission at 193.0 or 247.8 nm.

(

CFAPA-QC351A-2) was obtained from the National

Research Center (Beijing, China). Two water samples were

collected from local rivers (Chengdu China). Four human

serum samples were collected from adult healthy volunteers,

stored at −20 °C, and analyzed within 1 week. Simulated

gastric content samples were prepared according to previous

RESULTS AND DISCUSSION

■

work. 0.62 g of NaCl, 0.22 g of KCl, 0.05 g of CaCl , and

Feasibility of HS-SPME−μPD-OES for Quantiﬁcation

of Nitrite. Actually, microplasma molecular emission spec-

trometry (MES) has been recently used for the determination

of nitrate, nitrite, and ammonium via the generation of their

.12 g of NaHCO were dissolved into 200 mL of DIW. Then,

−

.0 mL of CH COONa solution (1.0 mol L , pH 5) and 23.6

g of gastric pepsin were added to 100 mL of the gastric

medium and the pH was adjusted to 2.0 with 0.1 M HCl

solution. After that, rice (150 g), vegetable (150 g), pork (150

g), salt (5 g), monosodium glutamate (1 g), vegetable oil (20

mL), and soy sauce (10 mL) were homogenized and added to

the simulated gastric ﬂuid, and then, the simulated gastric

contents were obtained.

corresponding volatile species (such as N and NO) and

8,39

monitoring their molecular emission.

Due to the

generation of N₂from the reaction between nitrite and

cyclamate, initial experiments tended to quantify nitrite via the

detection of N molecular emission bands. Standard solutions

−

containing 20 and 50 mg L NO₂as well as a blank solution

were analyzed by the μPD-MES system, respectively. As shown

in Figure 2a,b, a series of typical NO (205.3, 226.9, 237.0, and

Sample Preparation. The river water samples were

ﬁltered through a 0.22 μm ﬁlter membrane and directly

analyzed by the proposed method. For the serum samples,

47.9 nm) and N (337.1, 357.7, 380.5, and 405.3 nm)

mL of the samples together with 5 mL of DIW was added in 25

mL centrifuge tubes. The mixtures were shaken, mixed evenly,

and then the proteins contained in the samples were removed

by adding 4 mL of 300 g L⁻zinc acetate solution.

Subsequently, the mixtures were centrifuged for 5 min at

emission bands could be clearly observed in both the

background and standard solution emission spectra. According

to a previous work, Ar discharge gas usually contains a high

concentration of nitrogen impurities, thus resulting in a high

blank value. Although this method can be used for the

detection of nitrite in complex matrices samples, the high

background seriously limits the analytical performance.

Naturally, the quantiﬁcation of the generated cyclohexene

was tried to realize the highly sensitive determination of nitrite.

It is well known that headspace injection is a sensitive,

convenient, and high throughput sampling technique. Thus,

direct headspace injection was used as a sampling technique

for the determination of cyclohexene by μPD-OES. 10 mL of

000 rpm and the supernatants of the mixtures were collected.

The supernatants were ﬁltered with a 0.22 μm ﬁlter membrane

and the ﬁrst 4 mL of the ﬁltrates was discarded. The residual

ﬁltrates were kept in a refrigerator at 4 °C.

For the determination of nitrite in the meat samples and the

simulated gastric contents, 2.5 g of subsamples of each

sample was accurately weighed into preclean glass vessels into

which 6.5 mL of borax saturated solution (50 g L ) was added

and stirred vigorously. The sample solutions were transferred

to 100 mL volumetric ﬂasks with 75 mL of hot DIW (75 °C).

−1

−

0 mg L standard solution of NO₂was tested, as shown in

Figure 2c. As expected, the speciﬁc carbon atomic emission

spectral lines (193.0 and 247.8 nm) can be easily isolated from

the blank spectra. To further demonstrate the feasibility of

Subsequently, the ﬂasks were heated in a boiling water bath for

5 min. After cooling, 2.5 mL of 106 g L⁻potassium

−

hexacyanoferrate (II) solution and 2.5 mL of 220 g L zinc

acetate solution were successively added to precipitate

proteins. These mixtures were diluted to 100 mL with DIW,

shaken well, and let stand for half an hour. The mixtures were

ﬁltered with a 0.22 μm ﬁlter membrane and the ﬁrst 15 mL of

each ﬁltrate was discarded. Finally, the residue ﬁltrates were

transferred to precleaned polyethylene screw-capped bottles

and stored in a refrigerator at 4 °C prior to analysis.

direct headspace injection μPD-OES on the quantiﬁcation of

−

NO , a series of standard solutions containing 5, 10, 20, 50,

−

and 100 mg L of NO₂were analyzed, with the analytical

results shown in Figure 2d. The results show that the response

of carbon atomic emission increased while increasing the

−

NO₂concentration but not linearly at the low concentrations.

It is probably because the simultaneously generated N₂

changes the composition of discharge gas, thus reducing the

excitation capability of μPD and resulting in such a dynamic

nonlinearity. As a result, direct headspace injection μPD-OES

cannot accomplish the accurate determination of nitrite. To

support this viewpoint, it is necessary to separate the generated

Analytical Procedure. The SPME ﬁbers were ﬁrst

conditioned at 250 °C for 0.5 h under nitrogen. A certain

volume of the standard or sample solutions was added into 25

−

mL headspace vials and purged for 5 min using 400 mL min

of Ar carrier gas to remove volatile organic compounds

VOCs) prior to their analysis. Subsequently, the vials were

cyclohexene from N prior to its μPD-OES analysis. HS-SPME

(

not only can preconcentrate analytes, but it can also eﬃciently

−

sealed and 0.4 mL of 10 g L sodium cyclamate solution and

separate the analytes from sample matrices and other

.2 mL of 2 mmol L⁻sulfuric acid solutions were injected into

47−49

products.

Consequently, HS-SPME was used as a

the vials. The vials were heated in a water bath at 80 °C while

stirring at 1500 rpm for 25 min with a Teﬂon-coated magnetic

sampling technique instead of direct injection for the

determination of nitrite by μPD-OES.

10 mL of standard solutions was analyzed by HS-

SPME−μPD-OES, as shown in Figure 3a. Although the

concentration of nitrite used in HS-SPME−μPD-OES was 2%

of that used in direct injection μPD-OES, the response of

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Anal. Chem. 2021, 93, 6972−6979

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calibration curve established by using direct headspace injection μ PD-OES.

Article

Figure 2. Typical optical emission spectra of the Ar PD microplasma recorded using 20 and 50 mg L⁻¹nitrite solution. (a) Emission bands of NO

and (b) emission bands of N ; (c) response of carbon emission at 193.0 nm and 247.8 nm obtained by direct headspace injection μPD-OES; (d)

Figure 3. (a) Response of carbon emission at 193.0 and 247.8 nm obtained by HS-SPME−μPD-OES; (b) temporal emission proﬁle of standard

solutions; and (c) calibration curve established by using HS-SPME−μPD-OES.

carbon atomic emission at 193.0 nm (Figure 3a) is 4.5-fold

than that obtained by direct headspace injection μPD-OES

HS-SPME−μPD-OES on the determination of nitrite at the

trace level. As shown in Figure 3b, the intensity of carbon

atomic emission at 193.0 nm evidently increased with the

increasing concentration of nitrite. A determination coeﬃcient

(

Figure 2c). Meanwhile, for HS-SPME, the responses of OH

283, 309, and 319 nm), N (316, 358, and 380 nm), and NH

337 nm) molecular emission bands were at least decreased 2-

(R ) for the typical calibration curve of the carbon atomic

fold compared to those achieved by direct headspace injection.

These results indicate that SPME simultaneously enriches

cyclohexene and reduces the interferences from the generated

N₂and water vapor. It is noteworthy that some strong

emission bands in the spectral range 230−270 nm belonged to

emission intensity versus the concentration of nitrite is better

than 0.99 (Figure 3c). These observations not only support the

aforementioned speculation but also conﬁrm the feasibility of

HS-SPME−μPD-OES on the determination of nitrite in

complex samples. These results also demonstrate that HS-

SPME is an ideal sampling technique for MP-OES.

28,37

NO molecule emission bands

and result in the overlap of

carbon atomic emission at 247.8 nm. Therefore, the character-

istic carbon atomic emission at 193.0 nm was ﬁnally chosen for

the quantiﬁcation of nitrite. Furthermore, a series of standard

solutions were analyzed to roughly validate the feasibility of

Optimization of Experimental Parameters for μPD-

OES and HS-SPME. It is necessary to undertake investigations

of the eﬀects of the ﬂow rate of discharge gas, discharge

voltage, discharge gap, and electrode diameter because this is

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cyclamate were analyzed by GC −MS. As shown in Figure 4 a,

Article

the ﬁrst report of the use of μPD-OES for the determination of

nitrite to the best of our knowledge. The eﬀect of the ﬂow rate

from VOCs was eliminated beyond 5 min of purging time with

400 mL min of Ar purging gas.

−

of argon discharge gas in the range of 50−400 mL min was

investigated and is summarized in Figure S2a of the Supporting

Information . With the increase in the argon ﬂow rate, the peak

area of the carbon atomic emission increased, and the

maximum response was obtained at a ﬂow rate of 250 mL

Species Identiﬁcation. Although this CVG has been used

for the determination of nitrite by GC coupled with a ﬂame

ionization detector, there is still an argument for the exact

43,52

volatile products of sodium cyclamate after the reaction.

To identify the products of the reaction, the volatile species

−

−1

collected from the reaction between 100 μg L^N^O2 and

min . Thus, an argon ﬂow rate of 250 mL min was selected

for all subsequent experiments. As shown in Figure S2b of the

Supporting Information, the response increased signiﬁcantly

with the discharge voltage increase from 1.6 to 3.6 kV, reaching

a plateau at a higher voltage. Finally, 3.6 kV was selected as the

optimal discharge voltage. The eﬀect of the discharge gap was

also investigated and the analytical results are shown in Figure

S2c in the Supporting Information. The intensity of the carbon

atomic emission initially increased, whereas it tended to slowly

decrease over 3 mm. Thus, a gap of 3 mm was selected in

subsequent experiments. The factor of the electrode diameter

for μPD generation was explored. As can be seen from Figure

S2d in the Supporting Information, the intensity of the carbon

atomic emission decreased as the diameter of the electrode

increased. This is probably because the charge density on the

surface of the electrode is reduced with increasing diameter,

thus deteriorating the excitation capability of microplasma.

An electrode with a diameter of 1.6 mm was chosen for the

subsequent experiment.

Figure 4. GC/MS chromatogram and negative chemical ionization

mass spectra obtained by analyzing the products from the reaction

between nitrite solution and cyclamate (a,b) and cyclohexene

standard solution (c,d), respectively.

In addition, the relevant experimental parameters aﬀecting

the generation of cyclohexene and the extraction eﬃciency of

HS-SPME, including reaction temperature and time, the

concentration of sulfuric acid, desorption temperature,

extraction time, and so on, were also optimized. All the

analytical results are summarized in Figure S3, S4, and S5 of

the Supporting Information . Eventually, 80 °C of reaction

only one obvious chromatographic peak was obtained and its

retention time was 2 min, which agrees well with that obtained

−

by analyzing a 10 mg L standard solution of cyclohexene

(Figure 4c). Moreover, the fragmentation pattern of the

product summarized in Figure 4b is consistent with the mass

spectrum of cyclohexene (Figure 4d). Therefore, it can be

concluded that the majority of the products of cyclamate

generated from this reaction is cyclohexene.

−

temperature, 25 min of reaction time, and 2.0 mmol L of

sulfuric acid were chosen. Meanwhile, 250 °C of desorption

temperature, 10 min of extraction time, and the ﬁber coated

with 100 μm PDMS were chosen in subsequent experiments.

In addition, the memory eﬀect was tested via successively

Interferences. According to previous studies, HS-SPME

−

analyzing several standard solutions of 10, 50, and 100 μg L ,

respectively. The results summarized in Figure S6 of the

Supporting Information show that the intensity (Figure S6a )

and peak area (Figure S6b ) of the carbon atomic emission

were stable; no obvious memory eﬀect was observed after

desorption of the analyte for 30 s under these experimental

conditions.

has excellent capability of anti-interference from sample

matrices. However, some coexisting ions may bring interfer-

ences in the determination of nitrite by HS-SPME−μPD-OES.

For example, the transition-metal ions may oxidize nitrite or

change the speed of the reaction between nitrite and cyclamate

because they usually act as catalysts in many chemical

reactions. In this study, the eﬀects of 18 diverse ions including

Eﬀect of Purging Procedure. Since the quantiﬁcation of

2−

Fe , Fe , and CO on the determination of nitrite by HS-

−

^N^O2 by the proposed method was accomplished through

SPME−μPD-OES were tested, with the results summarized in

section S7 of the Supporting Information. As expected, no

monitoring the characteristic carbon atomic emission, all the

VOCs contained in samples may result in a blank signal. To

investigate the interferences from the VOCs contained in

sample, a meat sample was analyzed with or without a pre-

purging step using Ar gas. As shown in Figure S7a of the

Supporting Information, the response obtained with the

purging step was signiﬁcantly decreased compared to that

achieved without the purging step, indicating that VOCs can

be eﬃciently removed with Ar purging, which is consistent

signiﬁcant interferences from these tested ions were detected

−

even at their concentration as high as 1000-fold of NO .

Analytical Performance. Analytical ﬁgures of merit of

HS-SPME−μPD-OES for the determination of nitrite were

evaluated under the optimal experimental conditions. Figure 5a

shows typical carbon emission signal proﬁles achieved by

−

analyzing 15 mL of a series of NO₂standard solutions.

Although both the peak height and peak area can be used for

the quantiﬁcation of nitrite, the latter was used in the current

work for a wider linear dynamic range. As shown in Figure 5b,

a typical calibration curve of the intensity of carbon atomic

with our previous study. Therefore, a purging step is essential

before sample analysis. Furthermore, the eﬀect of the purging

time was investigated and the results are summarized in Figure

S7b of the Supporting Information . It can be observed that a

minimum response plateau was achieved as the purging time

was longer than 5 min, demonstrating that the interference

−

emission versus the concentration of NO₂can be charac-

terized using the calibration function I = 26.15C + 123.9,

−

−1

where C is the concentration of NO₂(μg L ) and I is the

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Article

HS-SPME−μPD-OES provides a great potential for the ﬁeld

−

analysis of ultra-trace NO in complex samples.

Analysis of the CRM and Real Samples. The accuracy of

the proposed method was ﬁrst validated by analyzing a CRM

of meat (CFAPA-QC351A-2). The CRM sample was also

analyzed by a Chinese national standard method (GB 5009.33-

2016). Analytical results are summarized in Table 2. A t-test

showed that the values obtained by the proposed system

agreed well with the certiﬁed values at the conﬁdence level of

99%.

Table 2. Analytical Results for Nitrite in a CRM by the

Proposed Method and Spectrophotometry

detected by

detected by this

spectrophotometry

certiﬁed

−1

sample method (mg kg

)

(mg kg

)

(mg kg

)

meat 3.60 ± 0.65

3.15 ± 0.28

3.48 ± 1.18

Mean and standard deviation of results (t₀_.₀₅_,₂= 9.92; P = 99%; n =

3).

As we all know, point-of-care quantiﬁcation of nitrite in

gastric and/or serum is helpful to rapidly identify acute nitrite

poisoning of humans. However, it is not easy to directly detect

trace nitrite in these complex samples by conventional

methods due to the serious interferences arising from sample

matrices. Fortunately, HS-SPME not only alleviates interfer-

ences of sample matrices but also improves the sensitivity and

LOD, thus making the direct determination of ultra-trace

nitrite in complex samples by μPD-OES possible. Therefore,

two simulated gastric content samples and four serum samples

were analyzed to validate the applicability of the HS-

SPME−μPD-OES for the determination of ultra-trace nitrite

in complex matrices. The analytical results are shown in Table

Figure 5. (a) Plots of carbon atomic emission intensity as a function

of nitrite concentration from 1 to 100 μg L ; (b) typical calibration

curve obtained using HS-SPME−μPD-OES.

−

carbon atomic emission intensity recorded by the peak area.

The linear correlation coeﬃcient for the calibration curve is

−

better than 0.99 in the concentration range of 1−100 μg L .

The limit of detection (LOD), deﬁned as the analyte

concentration equivalent to 3 times the standard deviation of

and show that the satisfactory recoveries ranging from 86 to

04% could be obtained from all the tested samples. Moreover,

1 measurements of a blank solution divided by the slope of

−

the ﬁeld analysis of nitrite in two river water samples by HS-

SPME−μPD-OES was undertaken to demonstrate its

application in environmental analytical chemistry (see Movie

S1), with the result summarized in Table 3. These results

conﬁrm the applicability of the proposed method for the ﬁeld

analysis of nitrite in complex samples.

the calibration curve, is 0.1 μg L . Figure S9 in the Supporting

Information illustrates the replicate signals obtained for 25

−

measurements of a standard solution containing 10 μg L of

−

NO₂and 11 measurements of a CRM sample solution,

yielding precisions (relative standard deviations, RSDs) better

than 4.1%. The analytical performances achieved by the

proposed method and other frequently used techniques are

summarized in Table 1, indicating that HS-SPME−μPD-OES

provided much better analytical performance than those

frequently used techniques. These results demonstrate that

CONCLUSIONS

A new and simple CVG was utilized to quantitatively convert

nitrite to cyclohexene, which was easily separated and

■

Table 1. Analytical Performance in Comparison with Other Methods

method

colorimetry

linear range

LOD

0.0164 μM

10 μg L⁻¹

1.0 μM

RSD, %

refs

0.087−4.17 μM

−

50−1000 μg L

10−150 μM

spectrophotometry

ﬂuorescence

chemiluminescence

electrochemical

2.9

4.0

3.2

5.9

5.0

5.4

2.2

3.5

2.3

−

−1

10−1700 μg L

0.5 μg L

1.0 μg L

−

−1

10−160 μg L

0.063−2.0 μM

0.1−10 μM

0.01−25 μM

0.05−780 μM

0.018 μM

0.053 μM

0.003 μM

0.015 μM

0.46 μg L

−

−1

2.0−30 μg L

2000−100,000 μg L⁻

1−100 μg L⁻

260 μg L

0.15 μg L

0.1 μg L (0.002 μM)

CVG-APGD

GC/MS

this work

−

−1

4.1

977

Anal. Chem. 2021, 93, 6972−6979

Analytical Chemistry

Complete contact information is available at:

Article

Table 3. Analytical Results for Determination of Nitrite in

Real Samples

Yao Lin − West China School of Basic Medical Sciences &

Forensic Medicine, Sichuan University, Chengdu, Sichuan

610064, China

detected ,

added ,

found ,

−

−1

Jiahui Yang − Key Laboratory of Green Chemistry &

Technology of MOE, College of Chemistry, Sichuan

University, Chengdu, Sichuan 610064, China

sample

mg kg

recovery, %

simulated gastric

0.73 ± 0.08

0.5

1.22 ± 0.03

content 1

Liangbo He − Key Laboratory of Green Chemistry &

Technology of MOE, College of Chemistry, Sichuan

University, Chengdu, Sichuan 610064, China

Yunfei Tian − Analytical & Testing Center, Sichuan

University, Chengdu, Sichuan 610064, China

Xiandeng Hou − Key Laboratory of Green Chemistry &

Technology of MOE, College of Chemistry, Sichuan

University, Chengdu, Sichuan 610064, China; Analytical &

Testing Center, Sichuan University, Chengdu, Sichuan

610064, China; orcid.org/0000-0003-2488-2063

simulated gastric

content 2

serum 1

1.19 ± 0.05

0.5

1.67 ± 0.06

113 ± 10

76 ± 8.0

150 ± 36

93 ± 16

330 ± 50

n.d.

100

500

211 ± 30.2

177 ± 5.6

253 ± 36

101

103

100

serum 2

serum 3

serum 4

193 ± 0.04

water sample 1

760 ± 31

520 ± 44

water sample 2

104

Mean and standard deviation of results (t

). μg L . n.d., not detected.

= 4.30; P = 95%; n =

.05,2

−1

https://pubs.acs.org/10.1021/acs.analchem.0c05254

preconcentrated from sample matrices and other products by

HS-SPME, thus making it possible to sensitively determine

nitrite by μPD-OES. To the best of our knowledge, this is the

ﬁrst time that μPD-OES is used for the accomplishment of

ﬁeld quantiﬁcation of nitrite via monitoring carbon atomic

emission. Compared to conventional methods used for the

quantiﬁcation of nitrite, the proposed method has several

advantages, including simplicity, high sensitivity, low energy

consumption, elimination of expensive instrument, and

alleviation of the interferences from sample matrices. Most

importantly, due to the advantages of eﬃcient preconcentra-

tion of the analyte and the simplicity of sample pretreatment,

low power and gas consumption, and simple experimental

setup provided by HS-SPME and μPD-OES, the proposed

method provides promising potential for the highly sensitive

and ﬁeld analysis of nitrite in complex samples. Moreover, it is

anticipated that the use of the proposed technique will broaden

its scope to the determination of other nitrogenous

compounds such as nitrate, ammonium, and nitrogen oxide

Author Contributions

The manuscript was written through contributions of all

authors. All authors have given approval to the ﬁnal version of

the manuscript.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

The authors would like to gratefully acknowledge the National

Nature Science Foundation of China (no. 22076128 and

1622508) and Excellent Youth Foundation of Sichuan

Scientiﬁc Committee (no. 2019JDJQ0024) for their ﬁnancial

support.

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