1,Related standards:

Chromium is a silver-white hard metal. It mainly appears in three forms: metallic chromium, trivalent chromium and hexavalent chromium. All chromium compounds are toxic, of which hexavalent chromium is the most toxic. Chromium has a wide range of industrial uses, including metal processing, electroplating, and tanning industries. Wastewater and waste gas discharged from these industries are the main sources of pollution in the environment.

At present, the detection methods of hexavalent chromium mainly include diphenylcarbazide (diphenylcarbazide) spectrophotometry, ion chromatography ultraviolet post-column derivatization method, atomic absorption spectrometry, atomic fluorescence spectrometry and inductively coupled plasma spectroscopy, etc. Among them, GB 5750.6-2006 "Standard Test Method for Drinking Water Metal Indicators" adopts the diphenylcarbazide spectrophotometric method to detect hexavalent chromium, but this method is susceptible to the influence of sample turbidity, chroma, and interfering ions, especially with Colored samples are prone to false positive or false negative results and poor repeatability. Ion chromatography can effectively eliminate the interference of the sample matrix, and the separation effect of the chromatographic column makes the hexavalent chromium enter the detector alone. Compared with common conductivity detectors, post-column derivatization UV-Vis detection can specifically detect hexavalent chromium, but does not respond to other anions, so it has less interference and higher sensitivity in the determination.

In order to implement the "Environmental Protection Law of the People's Republic of China" and "The Air Pollution Prevention Law of the People's Republic of China", protect the environment, protect human health, and standardize the determination method of hexavalent chromium in ambient air particles, the Department of Science and Technology Standards of the Ministry of Environmental Protection has organized and formulated HJ779 -2015 "Determination of hexavalent chromium in ambient air by post-column derivatization ion chromatography", this article mainly shares the post-column derivatization of hexavalent chromium by ion chromatography. This method can not only detect hexavalent chromium in the ambient air, but also can be used for the determination of hexavalent chromium in soil, water quality, textiles, leather, metal processing parts, toys, electronic appliances, etc.

2,Chromatography equipment

2.1 IC6600 multifunctional ion chromatograph, equipped with post-column derivatization device

2.2 UV detector: UV3300

3, reagents and consumables

• Ammonium sulfate: pure superior grade
• Ammonia: Premium grade pure
• Diphenylcarbazide
• Methanol: Chromatographically pure
• Sulfuric acid: pure superior grade
• Potassium dichromate: benchmark reagent
• Sodium bicarbonate: excellent grade pure

4, Determination of hexavalent chromium by ion chromatography post-column derivatization test results

4.1 Chromatographic conditions

Column	IonPac AS7 4x250mm+Wayeal HS-5AG4.6x50mm
Eluent	250mM ammonium sulfate+100mM ammonia	Eluent flow rate	1.0ml/min
Derivative	Diphenylcarbazide 0.5g+100ml methanol+28ml sulfuric acid	Derivative fluid flow rate	0.33mL/min
Column temperature	35℃	Derivative cell temperature	35℃
wavelength	530nm	Injection volume	1000μl

4.2 Standard curve test

Standard curve series concentration gradient table（ng/L)
Name	1	2	3	4	5	6	7
Hexavalent chromium	0.00	0.10	0.20	0.50	1.00	2.00	5

Standard curve overlay chromatogram

Note: The test linearity result is 0.99998, which shows good linearity and meets the standard requirements.

4.3 Repeatability test

Reproducible results

/	Hexavalent chromium-0.1ppb		Hexavalent chromium-5ppb
	Retention time	Peak area	Retention time	Peak area
1	5.287	2.292	5.305	116.643
2	5.304	2.295	5.305	116.868
3	5.294	2.292	5.316	116.756
4	5.297	2.288	5.314	116.993
5	5.304	2.289	5.297	116.978
6	5.289	2.289	5.308	116.845
Average	5.296	2.291	5.308	116.845
RSD(%)	0.139	0.122	0.129	0.114

Note: Using this method to test, the repeatability of hexavalent chromium at both low and high concentrations is good, and the repeatability of this test is less than 0.2%.

4.4 Minimum detection concentration test

The standard stipulates that the detection limit of this method is 0.1ng/mL. In this test, hexavalent chromium is 0.1ng/mL. Repeat six injections and calculate the minimum detection concentration with three times the signal-to-noise ratio. The results are as follows:

serial number	Peak height（mAU)	noise（mAU)	Minimum detectable concentration（ng/mL)
Hexavalent chromium-0.1ppb	0.543	0.0032	0.00177
	0.542		0.00177
	0.542		0.00177
	0.538		0.00178
	0.536		0.00179
	0.546		0.00176
Average	0.541		0.00177

Note: The minimum detection concentration of hexavalent chromium in this test is 0.00177ng/mL, which is less than 56 times the detection limit required by the standard, which is far lower than the standard requirement and has high sensitivity.

4.5 Sample spike test

Standard recovery rate calculation

No.	Sample addition volume	Constant volume (mL)	Concentration before adding standard (ng/mL)	Concentration after standard addition (ng/mL)	Recovery rate(%)
Water sample spike 1	100	100	0	0.976	97.6
Water sample spike 2				0.977	97.7

Note: The recovery rates of hexavalent chromium in this test water sample are 97.7% and 97.6%, which meet the standard requirements.

5, Determination of hexavalent chromium by ion chromatography post-column derivatization summary

This article introduces the use of ion chromatography post-column derivatization method to study the determination of Cr(VI) in ambient air. This method has the characteristics of good precision, high accuracy, high sensitivity, linearity, repeatability, and detection limit. Test requirements higher than existing standards.

This method can not only detect hexavalent chromium in the ambient air, but also can be used for the determination of hexavalent chromium in soil, water quality, textiles, leather, metal processing parts, toys, electronic appliances, etc.