雷射誘導電漿光譜法之液態取樣及關聯方法之開發

Development of Liquid Sampling and Correlation Method for Laser-induced Breakdown Spectroscopy

黃哲勳(Jer-Shing Huang)

指導教授 ： 林金全

國立臺灣大學/理學院/化學研究所/博士(2004年)

https://doi.org/10.6342/NTU.2004.00273

摘要

本研究利用雷射誘導電漿光譜法（LIBS）來偵測液態樣品水溶液中微量金屬之含量。為了克服液態樣品偵測時，訊號震盪嚴重之問題，我們首次採用電灑法(Electrospray)作為液態取樣方法。液態樣品由一自製的電灑針頭噴出，隨即與聚焦在泰勒錐 (Taylor cone)尖端之雷射脈衝作用產生雷射誘導電漿。由光電倍增管（PMT）或陣列式電荷偶合偵測器（OMA或CCD）偵測可得電漿放射光譜之強度衰減及頻率分佈圖譜，以各種方法分析則可獲的分析樣品中原子種類、濃度電漿溫度等資訊。本實驗主要貢獻在於利用電灑針頭與接地電極間收集到的電流訊號作關連校正，可抵銷來自多種因素所造成的原子螢光強度震盪，提高本法於液態樣品之應用性，並大幅降低絕對偵測極限。我們將兩百發雷射脈衝所得的原子螢光強度對極間電流作圖可得一線性關連（linear correlation），此回歸直線之斜率僅隨放光物種之濃度作線性增加，因此可以之建立檢量線。利用此法可得鈉、鉀和鋁的濃度偵測極限分別為0.63 ± 0.02 、1.15 ± 0.04 以及43 ± 5 毫克/升，代入雷射聚焦點之體積可得絕對偵測極限分別為0.3 pg 、0.5pg以及21pg。 我們也研究了鹽類基質對此方法的影響。結果顯示此法對鹽類基質具有相當高之容忍性，在2000 毫克/升的各種鹽類基質存在之下，此方法對鈉樣品之之檢量線幾無變化。
除此之外，我們發現連續背景放射（continuum background emission）之訊號震盪與原子螢光訊號之震盪亦具有相當程度的一致性，因此我們利用連續背景放射訊號來校正原子螢光訊號之浮動。比較雷射脈衝能量對兩種關連校正法之影響後發現，連續放射訊號與原子螢光之間的線性關係對雷射脈衝之改變相當敏感。相反地，電流訊號與原子螢光之間的線性關係則幾乎不受脈衝能量之影響，因此幾乎可在任何脈衝能量下使用，實用性較高。為了進一步瞭解雷射脈衝影響連續背景校正法之原因，我們假設電漿內部達到局部熱平衡，遵守波茲曼動能分佈，因此可利用二線法 (Two-line method）求得一原子於不同波長放射強度之比，以此作為電漿溫度參數，我們大幅地改進了在非最佳化條件之下所得到的連續背景輻射與原子螢光之間的關連校正圖形的線性。因此我們認為，雷射誘導電流訊號是一獨立訊號，能有效的校正原子螢光的震盪而不需使再另外考慮電漿溫度。由於此法使用之液態樣品取樣方式為電灑法，因此我們也嘗試將此法與線上注入（FI）系統結合，並利用一自製預濃縮管柱，對含低濃度氯化鋁之水溶液進行線上預濃縮分析。一般而言，游離能越高之金屬其偵測極限越差，文獻上大部分有關於鋁的偵測，其濃度偵測極限多在數十至數百毫克每升，與我們未採用預濃縮管柱之前的偵測極限相似（43 ± 5 毫克/升)，在結合線上預濃縮管柱之後，我們成它a將此法對鋁之偵測極限降低至十毫克每升以下，改變管柱填充物質即可對其他目標金屬進行預濃縮，對提高此法之靈敏性具有關鍵性意義。

關鍵字

電漿 ； 雷射 ； 電灑法 ； 光譜

並列摘要

Laser-induced breakdown (LIB) technique is applied to detect trace metal contained in liquid. The sample in liquid droplets is generated with an electrospray ionization needle. The microdroplets interact with an impinging laser pulse at 2 mm down stream from the needle tip. A sequence of single-shot time-resolved LIB emission signals of Na, K, and Al is detected, respectively. The signal intensity integrated within a gate is linearly correlated with plasma current obtained simultaneously on a single-shot basis. The correlation plot exhibits a straight line, of which the slope increases with the sample concentration, but appear to be irrespective of different matrix salts up to a 2000 mg/l concentration. Given the calibration curves, standard deviations of the blank measurement, and the focused cross sections of the incident pulse beam, the detection limits may reach 0.63 ± 0.02 (0.3pg), and 1.15 ± 0.04 (0.5pg), and 43 ± 5mg/l (21pg) for Na, K, and Al, respectively. The salt matrix effect on detection limits is studied with matrix salts having the same K+ cation but different anions. The current normalization might have probably taken into account the ablated amount of the sample and the plasma temperature. Accordingly, the LIB/current correlated analysis becomes efficient to suppress the signal fluctuation, improve the LOD determination, and concurrently correct the matrix effect.
In addition, the plasma continuum background (CB) emission signal is found to hold the same fluctuation pattern with the LIB emission signal, and therefore can be utilized to normalize the LIB signal fluctuation. The effects of laser pulse energy on the detection limits of two normalization methods, i.e. current correlation and continuum emission correlation, are examined as well. The correlation between continuum emission signal and LIB emission signal no longer exits if laser pulse energy is tuned away from the optimum. Nevertheless, correlation between LIB emission and current seems not sensitive to the change in laser pulse energy. The influence of laser pulse energy is discussed as well. Given a laser radiation emitting at 355 nm with the energy fixed at 23 mJ, the current normalization method may achieve a LOD of 1.0 mg/l for the Na analysis, while the CB emission normalization reaches only 12 mg/l. The current normalization probably has taken into account the ablated amount of the sample and the plasma temperature such that the obtained linearity of the correlation plot may extend to a much wider range of laser energy. In contrast, the CB emission normalization is sensitive to the plasma temperature, yielding a correlation linearity limited in a small range of laser energy.
Assuming a local thermal equilibrium (LTE) in the plasma, Saha-Boltzmann equation is applied and plasma temperature is estimated by Boltzmann plot method. Two-line ratio from OMA spectrum is considered as a factor of plasma temperature and is taken into account to explain the deviation from linearity in the correlation plot of LIB emission versus CB emission. Dramatic improvement in the linearity of correlation between the LIB emissions versus CB emission is obtained in the Ca and Sr experiment under the consideration of plasma temperature factor. On the contrary, correlation between Ca emission and current signal is usually perfectly linear and needs no further consideration of plasma temperature. We conclude that the laser-induced current signal is an independent signal, which correlates well with the emission signal and is capable of normalizing the emission signal fluctuation without further consideration of plasma temperature.
With electrospray sampling, we benefit not only from the current normalization of signal fluctuation but also its ability to connect to an FIA system. LIBS coupled with FIA system is firstly demonstrated in this work. With the a simple home made preconcentration column and FI manifold, solutions containing dilute Al(III) are successfully detected and the LOD is more than one order improved. The Al(III) cation is first absorbed by immobilized Chromotrope 2B, which serves as a strongly chelating agent to Al(III). A 0.5M HCl solution in half and half methanol-water mixture is then pump into the column to wash out the retained Al(III). Simple FI manifold is configured to facilitate washing, original sample detection, sample loading, and eluating the concentrated sample out. Detection limit with FI preconcentration is reduced to 2.9 mg/l. Compared with LOD without FI treatment, it’s more than one order improved. Though it’s yet not that low to detect trace Al in bio-material, we first time provide the possibility to connect LIBS with a simple on-line preconcentration system.

並列關鍵字

Plasma ； Laser ； Spectroscopy ； Electrospray

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