The Application of Portable GC-MS on the Petrochemical Wharf

It is very important to quickly identify the liquid chemicals in the petrochemical wharf when they volatile in the air. Since most of the liquid chemicals transported in the petrochemical wharf are harmful to people, and they spread very fast in the air. If the exact kind of liquid chemicals is undefined or is wrongly defined, it will lead to a big mistake in detection result. The mass spectrum is very effective in determining the gas kinds, but its big size and complex sampling mode are not convenient for quick detection. To resolve this problem, a method based on portable GC_MS for VOCs determination is developed. The VOCs are directly extracted by solid phase micro extraction head for 120 seconds. After that, they are analysed on the portable GC-MS, determined by retention time and MS peaks, quantified by external standard method. It only takes 6.4 min from sample extracting to get the result. And its linear correlation is larger than 0.98, and the minimum detectable concentration is lower than half of their occupational exposure limits. In the simulation test, the relative error is lower than 4.4%. This method can directly sample in the air, rarely depends on external conditions. At the same time, it can determine and quantify multiple kinds of VOCs at a time. So, it will be very suitable for the VOCs emergency detection in the leakage scene.


Introduction
There are many kinds of dangerous chemicals in the petrochemical wharf area. And most of them are volatile organic compounds(VOCs). The pollution accidents have occurred many times in the petrochemical wharf area [1,2]. The VOCs diffuses very fast in the air, and would pollute water and soil on a large scale. Also, there are many kinds of VOCs, and their toxicities are very complicated. Once the pollution accidents occurred, the people nearby would be in danger [3,4]. The identification of VOCs is not difficult, but there are few detecting method to quickly quantify the concentration of VOCs when they are identified. So, it is necessary to develop a quick detecting method to identify and quantify the concentration of VOCs in the petrochemical wharf area. The portable gas chromatograph-mass spectrometer (portable GC_MS) is quite small, and convenient to carry out. So, it is very suitable for quick detection on the scene. When it is used on the scene, the sampling method is very important. Solid phase micro extraction (SPME) is a sampling method without solvents. It can combine the sampling, concentration and injection [5][6][7]. Especially, the SPME could direct sample in the air, so it will save much time.

Sampling
The air sample is directly taken by the SPME sampling head for 120 s. Then it is injected into the portable GC_MS without further treatment.

The concentrations of standard series of VOCs
The most important purpose of quick detection is to evaluate whether the pollutant concentration will cause health damage and to determine the pollution scope. Here we could refer to the occupational exposure limits of each VOCs, set by national standard. When the pollutant concentrations in the field are higher than their occupational exposure limits, it should be identified as contaminated area. This requires that the minimum detectable concentration of each chemical hazardous agent must be lower than its occupational exposure limit. According to the <Occupational exposure limits for hazardous agents in the work place: Chemical hazardous agents> (GBZ 2.1-2007) [8], the occupational exposure limits of benzene, toluene, paraxylene, aniline, trichloroethylene, tetrachloro ethylene and cyclohexanone are 6, 50, 50, 3, 30, 200 and 50 mg/m 3 , respectively. Based on this, we set the concentrations of standard series gases as 0.5, 1, 2, 4, and 8 times of their occupational exposure limits. The specific concentrations are shown in Table 1.

The sampling time
The more gases the SPME sample hand collected, the higher precision will be obtained. correspondingly, the sampling time needed will be longer. and if the sampling time is too long, it will not match the demand of the quick detection. So, a balance between the sampling time and detecting precision will be needed, at which, the analysis result could be obtained quickly and the analysis precision is accuracy enough. The analysis precisions at different sampling time are compared using the detection of minimum concentration in the standard series gases. When the sampling time is 120 s, the analysis precision of every gas is good, and 120 s is short enough. So, the sampling time is set as 120 s. The separating effect is shown in Figure 1.

The standard curve
The standard mix gases of benzene, toluene, paraxylene, aniline, trichloroethylene, tetrachloroethylene and cyclohexanone are prepared with pure air. The concentrations are 48, 400, 400, 24, 240, 1600, 400 mg/ m 3 respectively. And it is diluted into the series concentrations shown in Table 1. Then the series gases are analysed on portable GC_MS. All the gases are determined by the mass spectrum peak and retention time, and the standard curve are plotted through the peak area. Also, the equations of linear regression and correlation coefficients are obtained. The minimum detectable concentrations are calculated through 3 times noise signals. The equations of linear regression, correlation coefficients and the minimum detectable concentrations are shown in Table 2.