李鑫星,朱晨光,周 婧,孫龍清,曹霞敏,張小栓
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光譜技術(shù)在水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)中的應(yīng)用進(jìn)展及趨勢(shì)
李鑫星1,2,朱晨光1,周 婧1,孫龍清1,曹霞敏3,張小栓2,4※
(1. 中國(guó)農(nóng)業(yè)大學(xué)信息與電氣工程學(xué)院,北京 100083;2. 食品質(zhì)量與安全北京實(shí)驗(yàn)室,北京 100083; 3. 蘇州大學(xué)基礎(chǔ)醫(yī)學(xué)與生物科學(xué)學(xué)院,蘇州 215200;4. 中國(guó)農(nóng)業(yè)大學(xué)工學(xué)院,北京 100083)
水產(chǎn)養(yǎng)殖的水質(zhì)是關(guān)乎水產(chǎn)養(yǎng)殖經(jīng)濟(jì)效益和水產(chǎn)品品質(zhì)的關(guān)鍵因素,與傳統(tǒng)的水質(zhì)檢測(cè)方法相比,光譜技術(shù)具有無(wú)創(chuàng)性、快速性、可重復(fù)性、準(zhǔn)確性等優(yōu)點(diǎn),已成為水質(zhì)監(jiān)測(cè)的重要發(fā)展方向。該文總結(jié)和整理現(xiàn)有國(guó)內(nèi)外研究文獻(xiàn),對(duì)基于光譜技術(shù)的水質(zhì)重要參數(shù)監(jiān)測(cè)、數(shù)據(jù)預(yù)處理方法、特征波段提取、預(yù)測(cè)模型算法進(jìn)行了系統(tǒng)的分析與討論。綜述結(jié)果表明,實(shí)時(shí)在線的水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)將成為重點(diǎn)研究方向;多源光譜融合、多參數(shù)的水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)將會(huì)成為新的發(fā)展方向;對(duì)于光譜數(shù)據(jù)的處理,將多種數(shù)據(jù)處理算法相結(jié)合,仍將占據(jù)主導(dǎo);而非線性建模將成為水產(chǎn)養(yǎng)殖水質(zhì)數(shù)據(jù)分析的主流方法非線性數(shù)據(jù)建模,將成為光譜技術(shù)應(yīng)用于水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)的主流建模發(fā)方法。
光譜技術(shù);水產(chǎn)養(yǎng)殖;水質(zhì);監(jiān)測(cè)模型
水產(chǎn)養(yǎng)殖已經(jīng)成為中國(guó)發(fā)展最快的食品生產(chǎn)行業(yè)之一,為保障食物供給、促進(jìn)經(jīng)濟(jì)增長(zhǎng)做出了巨大貢獻(xiàn)。水產(chǎn)養(yǎng)殖與其水質(zhì)密切相關(guān)[1],近年來(lái),隨著經(jīng)濟(jì)的發(fā)展,工業(yè)廢水、生活污水的排放量大增,造成環(huán)境污染,養(yǎng)殖池塘水質(zhì)遭到污染的情況時(shí)有發(fā)生。作為智能農(nóng)業(yè)和農(nóng)業(yè)物聯(lián)網(wǎng)的重要研究?jī)?nèi)容,水產(chǎn)養(yǎng)殖水質(zhì)信息的快速、準(zhǔn)確獲取,以求在環(huán)保、節(jié)能的同時(shí)達(dá)到高產(chǎn)、安全養(yǎng)殖的目的,成為學(xué)者們關(guān)心的問(wèn)題?;诠庾V分析的水質(zhì)監(jiān)測(cè)技術(shù)是水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)的一個(gè)重要發(fā)展方向,與傳統(tǒng)的化學(xué)分析、電化學(xué)分析和色譜分析等方法相比,光譜分析技術(shù)更具有操作簡(jiǎn)便、消耗試劑量小、重復(fù)性好、測(cè)量精度高和檢測(cè)快速的優(yōu)點(diǎn),非常適合對(duì)水質(zhì)的快速在線監(jiān)測(cè)。本文綜述國(guó)內(nèi)外光譜技術(shù)在水產(chǎn)養(yǎng)殖水質(zhì)指標(biāo)快速監(jiān)測(cè)方面的應(yīng)用,總結(jié)并展望其未來(lái)發(fā)展。
水產(chǎn)養(yǎng)殖水質(zhì)參數(shù)變化將直接影響水產(chǎn)品的生長(zhǎng),對(duì)于水產(chǎn)養(yǎng)殖業(yè)來(lái)說(shuō),水體溶解氧pH值、水溫對(duì)水中生物的生存有著至關(guān)重要的影響。不同的養(yǎng)殖環(huán)境和養(yǎng)殖對(duì)象,對(duì)水質(zhì)參數(shù)的要求不同。針對(duì)魚(yú)類,水質(zhì)指標(biāo)控制范圍如下,pH值:淡水6.5~8.5,海水7.0~8.5。溶解氧連續(xù)24 h中,16 h以上必須大于5 mg/L,其余任何時(shí)候不得低于3 mg/L;氮元素,氨氮含量要低于0.2 mg/L,凱氏氮不高于0.05 mg/L,亞硝酸鹽低于0.1 mg/L,非離子氨不高于0.02 mg/L;對(duì)于磷元素,黃磷不高于0.001 mg/L;重金屬,汞不高于0.000 5 mg/L,鉻不高于0.005 mg/L,鉛不高于0.05 mg/L,銅不高于0.1 mg/L。
化學(xué)需氧量,簡(jiǎn)稱COD,是指在一定條件下,水體中還原性物質(zhì)被強(qiáng)氧化劑氧化時(shí),所消耗的氧化劑的量,是表征水中還原性物質(zhì)的綜合性指標(biāo)。COD是評(píng)價(jià)水質(zhì)極為重要的指標(biāo),它被用來(lái)衡量水體受還原性物質(zhì)污染的程度,是水質(zhì)檢測(cè)時(shí)必須要檢測(cè)的參數(shù)[2]。很多相關(guān)研究表明當(dāng)水體中COD的濃度超過(guò)一定的限值時(shí),會(huì)對(duì)水產(chǎn)品的生長(zhǎng)造成影響,而且會(huì)增加水產(chǎn)養(yǎng)殖廢水的處理成本。檢測(cè)COD的常規(guī)方法主要是高錳酸鹽指數(shù)法(CODMn)和重鉻酸鉀回流法(CODCr)。兩者的適用范圍不同,重鉻酸鉀法適用于生活廢水和工業(yè)廢水的測(cè)定。而高錳酸鉀法更適用于清潔的水質(zhì),這些方法存在操作復(fù)雜、耗時(shí)長(zhǎng)、消解時(shí)易造成附加污染等問(wèn)題[3]。
氮是水體中的主要營(yíng)養(yǎng)物質(zhì)之一,水環(huán)境中氮的形態(tài)有氨氮、硝酸鹽氮、亞硝酸鹽氮、有機(jī)氮和總氮,前四者通過(guò)生物化學(xué)作用可以相互轉(zhuǎn)化??偟獮榍八恼咧?,是衡量水體受污染程度的重要指標(biāo)[4]??偟獫舛葯z測(cè)方法主要有離子色譜法、化學(xué)滴定法、流動(dòng)注射法、離子選擇電極法以及光譜分析法等,其中,化學(xué)滴定法的分析精度最高,但此類方法過(guò)程繁復(fù),耗時(shí)長(zhǎng),不適宜大范圍使用[5]。
總磷是衡量水質(zhì)的重要指標(biāo),也是評(píng)定水質(zhì)富營(yíng)養(yǎng)化的重要指標(biāo)。在水體中磷類物質(zhì)含量過(guò)大會(huì)造成藻類過(guò)度繁殖,使水透明度降低,水質(zhì)變差,從而影響水產(chǎn)養(yǎng)殖產(chǎn)品的品質(zhì)。目前,中國(guó)總磷檢測(cè)一般按照原國(guó)家環(huán)保部發(fā)布的鉬酸銨分光光度法進(jìn)行,國(guó)內(nèi)外用堿性過(guò)硫酸鉀消解—離子色譜法、過(guò)硫酸鉀消解法、硝酸—硫酸消解法、硝酸—高氯酸消解法測(cè)量水質(zhì)中的總磷也有報(bào)道[6-7]。
重金屬是水環(huán)境中較為危險(xiǎn)的污染物,不僅不可降解,而且會(huì)在生物體內(nèi)長(zhǎng)期積累,引起多種疾病。常用的檢測(cè)方法包括:原子吸收光譜法、電感耦合等離子體原子發(fā)射光譜法、電化學(xué)方法、紫外-可見(jiàn)分光光度法、液相色譜法、熒光分析法、流動(dòng)注射分析、生物化學(xué)分析法[8-9]。
溶解氧是指溶解于水中分子狀態(tài)的氧,是水生物生存必不可少的條件。對(duì)于水產(chǎn)養(yǎng)殖業(yè)來(lái)說(shuō),水體溶解氧對(duì)水中生物的生存有著至關(guān)重要的影響,能夠反映出水體受到有機(jī)物污染的程度,它是水體污染程度的重要指標(biāo),也是衡量水質(zhì)的綜合指標(biāo)之一[10]。目前常用的溶解氧檢測(cè)方法有碘量法、電化學(xué)法(電流測(cè)定法、電導(dǎo)測(cè)定法)、熒光淬滅法等。
pH值作為水的最基本性質(zhì),它可以影響水體的弱酸、弱堿的離解程度,降低氯化物、氨、硫化氫等的毒性,對(duì)水質(zhì)的變化、生物繁殖的消長(zhǎng)、腐蝕性、水處理效果等均有影響,是評(píng)價(jià)水質(zhì)的一個(gè)重要參數(shù)。pH值的傳統(tǒng)測(cè)量方式有化學(xué)分析法、試紙法和電位法等。
與水質(zhì)檢測(cè)的化學(xué)方法相比,基于光譜分析的水質(zhì)監(jiān)測(cè)技術(shù)是一個(gè)重要發(fā)展方向,已有工作表明,幾個(gè)重要水質(zhì)參數(shù)在光譜區(qū)均有很強(qiáng)的吸收。在一定的條件下,有機(jī)物的吸光度與有很好的相關(guān)性,利用這種相關(guān)性,可以用光譜技術(shù)直接測(cè)定[11-13]。
1.7.1 光譜法水質(zhì)監(jiān)測(cè)的理論基礎(chǔ)
光譜法則是基于朗伯比爾定律,通過(guò)監(jiān)測(cè)水產(chǎn)養(yǎng)殖水質(zhì)對(duì)特定波長(zhǎng)的光的吸光度,然后對(duì)比存儲(chǔ)的標(biāo)準(zhǔn)曲線計(jì)算出水樣的值,屬于利用光譜學(xué)原理和試驗(yàn)方法確定物質(zhì)結(jié)構(gòu)和化學(xué)成分的分析方法。通過(guò)建立有機(jī)物污染綜合指標(biāo)與水樣的光譜數(shù)據(jù)之間的回歸模型,來(lái)預(yù)測(cè)有機(jī)污染綜合指標(biāo)。
1.7.2 光譜法水質(zhì)方法步驟及試驗(yàn)設(shè)備
COD在紫外254 nm處有很強(qiáng)的特征吸收相關(guān)性,利用這一選擇性吸收原理,可建立特定波長(zhǎng)處吸光度值與COD濃度值的關(guān)系,計(jì)算溶液中COD濃度??偟庾V監(jiān)測(cè)方法有:堿性過(guò)硫酸鉀紫外分光光度法和氣相分子吸收光譜法。硝酸鹽是最穩(wěn)定的無(wú)機(jī)氮化合物,是亞硝酸鹽、氨氮和含氮有機(jī)物轉(zhuǎn)化的最終產(chǎn)物。目前的主要方法是堿性過(guò)硫酸鉀紫外分光光度法,該方法是采用堿性過(guò)硫酸鉀氧化,使有機(jī)氮和無(wú)機(jī)氮化合物轉(zhuǎn)變?yōu)橄跛猁}氮后紫外分光光度法進(jìn)行測(cè)定[14]??偭自谥行詶l件下用過(guò)硫酸鉀(或硝酸-高氯酸)使試樣消解,對(duì)消解液用抗壞血酸溶液和鉬酸銨溶液處理,利用分光光度法進(jìn)行測(cè)量。重金屬光譜監(jiān)測(cè)技術(shù)有原子吸收光譜法、分光光度法、熒光分析法,其中原子吸收光譜法,具有靈敏度高、檢出限低、分析速度快、選擇性好、抗干擾能力強(qiáng)等優(yōu)點(diǎn),是目前測(cè)定重金屬含量最主要的方法。由于水產(chǎn)養(yǎng)殖水質(zhì)不同于廢水、地下水等,其對(duì)氧、氮、磷等元素特殊需求,使得其組成成分復(fù)雜,干擾監(jiān)測(cè)結(jié)果。針對(duì)易受干擾的指標(biāo),需對(duì)干擾物質(zhì)進(jìn)行光譜分析,并與需監(jiān)測(cè)的物質(zhì)進(jìn)行比較,確定利用光譜法測(cè)量水產(chǎn)養(yǎng)殖水質(zhì)該指標(biāo)的主要干擾物質(zhì),所遵從的原則是在不影響該指標(biāo)測(cè)量準(zhǔn)確度的前提下,盡可能減少干擾物質(zhì)種類?;诠庾V的水質(zhì)重要參數(shù)的監(jiān)測(cè)方法如表1所示,其中關(guān)于基于光譜技術(shù)的水質(zhì)COD監(jiān)測(cè)方面研究較多,技術(shù)較成熟、簡(jiǎn)便;關(guān)于總磷、總氮、重金屬的檢測(cè)需要采用化學(xué)試劑進(jìn)行預(yù)處理,操作有一定的復(fù)雜性。實(shí)現(xiàn)利用光譜技術(shù)對(duì)水產(chǎn)養(yǎng)殖水質(zhì)多參數(shù)的監(jiān)測(cè),并提高光譜法水質(zhì)多參數(shù)監(jiān)測(cè)精度是值得探討的研究難點(diǎn)。
表1 基于光譜技術(shù)的水質(zhì)監(jiān)測(cè)方法
應(yīng)用光譜法進(jìn)行水樣的定性或定量分析,提取待測(cè)水樣光譜信息需要進(jìn)行光譜數(shù)據(jù)的處理,光譜數(shù)據(jù)處理分為預(yù)處理和光譜特征波段選擇2部分。
光譜中常常包含一些與待測(cè)樣品性質(zhì)無(wú)關(guān)聯(lián)的干擾信息,為了使建立的定性或定量分析模型更加穩(wěn)健、可靠,常常需要對(duì)測(cè)定的光譜數(shù)據(jù)進(jìn)行預(yù)處理。常見(jiàn)的光譜特征波段選擇方法包括Savitzky-Golay平滑算法、小波分析、多元散射校正,3種常見(jiàn)的常見(jiàn)預(yù)算法的對(duì)比分析如表2所示。
2.1.1 Savitzky-Golay平滑算法
Savitzky-Golay算法是一種基本圖像處理方法,由Savitzky等在1964年首次提出[23],是一種在時(shí)域內(nèi)基于局域多項(xiàng)式最小二乘法擬合的濾波方法,通過(guò)卷積運(yùn)算對(duì)曲線鄰域的像素灰度進(jìn)行平均化,從而減少雜點(diǎn)、降低曲線對(duì)比度,該平滑算法做一種加權(quán)平均的過(guò)程。
表2 3種預(yù)處理算法的對(duì)比分析
SG平滑算法可用于對(duì)光譜數(shù)據(jù)作平滑處理[24],程長(zhǎng)闊等[25]建立了紫外吸收光譜海水硝酸鹽反演模型,試驗(yàn)結(jié)果顯示,SG卷積平滑能夠極大地降低模型預(yù)測(cè)誤差。李毛毛等[26-27]將SG平滑算法結(jié)合其它算法,以達(dá)到更好的去噪效果。喬星星等[28-29]對(duì)Savitzky- Golay平滑算法不同程度模式處理效果進(jìn)行了研究,結(jié)果顯示,所建模型預(yù)測(cè)效果較未處理前有很大改善。Savitzky-Golay平滑算法不受樣本數(shù)據(jù)限制,適用于各種信號(hào)的平滑去噪,與傳統(tǒng)算法相比,該算法具有更穩(wěn)定、誤差更小的平滑去噪效果[30]。因此,SG濾波器通常用于光譜分析數(shù)據(jù)預(yù)處理,對(duì)原始數(shù)據(jù)進(jìn)行平滑與去噪。
2.1.2 小波分析
小波分析是一種窗口大小固定但其形狀可變,時(shí)間窗和頻率窗都可以改變的時(shí)頻局部化分析方法[31-32]。該方法在低頻部分具有較高的頻率分辨率和較低的時(shí)間分辨率,在高頻部分具有較高的時(shí)間分辨率和較低的頻率分辨率,與傅里葉變換相比,小波變換是時(shí)間(空間)頻率的局部化分析,通過(guò)伸縮平移運(yùn)算對(duì)光譜信息逐步進(jìn)行多尺度細(xì)化,最終達(dá)到高頻處時(shí)間細(xì)分,低頻處頻率細(xì)分,能自動(dòng)適應(yīng)時(shí)頻信號(hào)分析的要求,因此非常適合分析突變信息和非平穩(wěn)信息,把噪聲信息從正常信息中分離出來(lái),達(dá)到去噪的目的[33]。
趙進(jìn)輝等[34-37]采用小波分析法對(duì)農(nóng)產(chǎn)品的光譜數(shù)據(jù)進(jìn)行去噪處理,相比于未去噪處理,均方根誤差明顯減小。Ma等[38-39]對(duì)小波分析方法進(jìn)行了改進(jìn),并成功應(yīng)用于水樣的光譜數(shù)據(jù)的去噪處理。小波包去噪方法是小波分解的推廣,它提供了更豐富的信號(hào)分析方法[40]。張瑤等[41-42]利用小波包光譜信息進(jìn)行去噪處理,結(jié)果表明,小波分析技術(shù)能夠有效地提高光譜預(yù)測(cè)效果。小波分析由于具有低熵性、多分辨率、去相關(guān)性和選基靈活性的特點(diǎn), 能夠滿足各種去噪要求,廣泛應(yīng)用于去除光譜背景噪音、儀器干擾方面。
2.1.3 多元散射校正
多元散射校正(multiplicative scatter correction)最早是由Naes和Isaksson在1988年提出[43]。多元散射校正算法的基本思想是:假設(shè)每條光譜曲線都存在一條與其具有高相關(guān)性的理想光譜。真正理想的光譜雖然沒(méi)有辦法獲取,但通過(guò)使用樣本建模集的平均光譜曲線可以近似的替代,實(shí)現(xiàn)光譜數(shù)據(jù)的散射校正。多元散射校正的實(shí)現(xiàn)步驟如下:1)計(jì)算光譜平均值2)進(jìn)行線性回歸運(yùn)算,得出樣品的均勻程度、樣品特有的光譜信息3)通過(guò)樣品的均勻程度、樣品特有的光譜信息,進(jìn)行光譜校正。
多元散射校正方法能夠剔除各樣品間由于散射影響所導(dǎo)致的基線變化影響[44-45],蘆永軍等[46]經(jīng)過(guò)試驗(yàn)驗(yàn)證得到的散射校正相關(guān)光譜有效地降低了散射的影響。湯斌等[47]運(yùn)用多元散射校正法對(duì)受濁度影響的水樣光譜進(jìn)行校正試驗(yàn),結(jié)果表明:該方法可在不影響水樣紫外-可見(jiàn)吸收光譜特征的前提下對(duì)其吸收曲線進(jìn)行有效的校正。多元散射校正算法可提高原吸收光譜的信噪比,對(duì)消除光譜數(shù)據(jù)的線性散射干擾有較好的效果,該算法多用于光譜數(shù)據(jù)和濃度信息線性相關(guān)性較好的情況。
光譜儀獲取的光譜數(shù)據(jù)量大,光譜矩陣大量的冗余數(shù)據(jù),光譜矩陣中的無(wú)關(guān)信息等因素,導(dǎo)致光譜分析的速度變慢、效率降低。因此,從采集到的光譜數(shù)據(jù)中提取有益于建模的波長(zhǎng)變量,去除冗余變量和無(wú)信息變量,可以提高光譜監(jiān)測(cè)的精度,優(yōu)化預(yù)測(cè)模型的性能。常見(jiàn)的光譜特征波段選擇方法包括連續(xù)投影算法、無(wú)信息變量消除、主成分分析法等[48]。3種常見(jiàn)的常見(jiàn)特征提取算法的對(duì)比分析如表3所示。
2.2.1 連續(xù)投影算法
連續(xù)投影算法(successive projections algorithm, SPA)是一種使矢量空間共線性最小化的前向變量選擇算法,其目標(biāo)是為了解決建模變量的共線性問(wèn)題,改善多變量的建模預(yù)測(cè)效果。SPA算法的思想是:采用對(duì)光譜數(shù)據(jù)投影進(jìn)行映射的方法構(gòu)造新的變量集,并對(duì)新的變量預(yù)測(cè)效果進(jìn)行評(píng)價(jià)[49]。SPA算法的步驟:假設(shè)提取的特征波段的數(shù)量為,1)隨機(jī)選取光譜矩陣中的一列;2)計(jì)算該對(duì)剩余列的投影;3)重復(fù)第二步,直到得到個(gè)波段,停止迭代。
表3 3種常見(jiàn)特征提取算法的對(duì)比分析
周竹等[50-52]采用SPA算法對(duì)農(nóng)產(chǎn)品光譜數(shù)據(jù)進(jìn)行特征波段的選擇,確定了最佳波長(zhǎng),降低了模型復(fù)雜度并提高了預(yù)測(cè)精度。國(guó)內(nèi)許多學(xué)者SPA光譜特征選擇算法進(jìn)行了改進(jìn)[53-56],郝勇等[57]引入蒙特卡羅方法,對(duì)SPA算法進(jìn)行改進(jìn),對(duì)葡萄酒和蘋(píng)果的原始光譜進(jìn)行酒精度和可溶性固形物信息的提取,解決了小樣本數(shù)據(jù)集變量選擇的問(wèn)題。連續(xù)投影算法廣泛應(yīng)用于光譜領(lǐng)域,是一種最常用的光譜特征波段選擇的算法。
2.2.2 無(wú)信息變量去除算法
無(wú)信息變量消除算法(uninformative variables elimination,UVE)是在偏最小二乘回歸系數(shù)的基礎(chǔ)上建立的特征波段提取算法,用于去除對(duì)建立模型沒(méi)有貢獻(xiàn)的變量,即去除無(wú)信息變量[58]。UVE算法流程如下:1)把相同于自變量矩陣的變量數(shù)目的隨機(jī)變量矩陣(等同于噪音)加入光譜矩陣;2)通過(guò)交叉驗(yàn)證的逐一剔除法建立PLS模型,得到回歸系數(shù)矩陣,分析回歸系數(shù)矩陣中回歸系數(shù)向量的平均值和標(biāo)準(zhǔn)偏差的商的穩(wěn)定性;3)根據(jù)該列光譜數(shù)據(jù)的商絕對(duì)值大小確定是否把改列變量用于PLS回歸模型中。
UVE算法能夠減少模型輸入變量的數(shù)量,降低建模的復(fù)雜性,廣泛用于光譜數(shù)據(jù)特征波段選擇[59]。Tan等[60]提出了基于無(wú)關(guān)信息變量消除多變量校正策略,經(jīng)驗(yàn)證,該方法準(zhǔn)確性高、魯棒性強(qiáng)。Cai等[61]在光譜定量分析中,根據(jù)蒙特卡洛原理對(duì)無(wú)關(guān)信息算法進(jìn)行優(yōu)化,消除穩(wěn)定差的變量的無(wú)關(guān)信息變量,該方法能夠光譜數(shù)據(jù)中選取重要波長(zhǎng),使預(yù)測(cè)結(jié)果更加可靠、準(zhǔn)確。Zhou等[62-63]將UVE與SPA結(jié)合,對(duì)光譜數(shù)據(jù)進(jìn)行特征波段的選擇,發(fā)現(xiàn)與直接采用SPA算法相比,該算法參考更少的變量達(dá)到更高的預(yù)測(cè)效果。無(wú)信息變量去除算法能夠剔除沒(méi)有貢獻(xiàn)的變量,以達(dá)到光譜特征波段選擇的目的。
2.2.3 主成分分析
主成分分析(principal component analysis,PCA)將原變量通過(guò)線性組合變換為新變量,變換后的新變量相互正交、互不相關(guān),以排除信息中重疊的多余部分,并盡可能的保持原變量的數(shù)據(jù)信息。主成分分析法分析水樣紫外吸收光譜的基本思想是:將原來(lái)具有一定相關(guān)度的個(gè)波長(zhǎng)的吸光度參數(shù),重新組合成一組較少個(gè)數(shù)的互不相關(guān)的吸收向量[64-69]。PCA算法的步驟:1)對(duì)光譜矩陣進(jìn)行中心化2)計(jì)算光譜信息的協(xié)方差矩陣3)對(duì)矩陣進(jìn)行特征值分解4)取出最大的個(gè)特征值對(duì)應(yīng)的特征向量,將所有的特征向量標(biāo)準(zhǔn)化后,組成特征向量矩陣5)對(duì)光譜矩陣中的每一個(gè)樣本,點(diǎn)乘特征向量矩陣,轉(zhuǎn)化為新的樣本。
主成分分析法可簡(jiǎn)化水質(zhì)成分多樣性等問(wèn)題,Assaad等[70-72]通過(guò)主成分分析,提取特征光譜數(shù)據(jù)解決水樣成分的多樣性和可變性等問(wèn)題的影響。PCA算法也常與其它算法結(jié)合對(duì)水質(zhì)光譜信息進(jìn)行簡(jiǎn)化[73]。趙友全等[74]采用主成分分析結(jié)合歐氏距離和偏最小二乘法對(duì)水樣分類對(duì)COD含量的預(yù)測(cè)進(jìn)行了定性和定量的分析。通過(guò)試驗(yàn)驗(yàn)證了該方法對(duì)實(shí)際水樣可以進(jìn)行有效分類。主成分分析法是線性降維方法的基礎(chǔ),是一個(gè)典型的高維數(shù)據(jù)的降維方法,該方法最大優(yōu)勢(shì)在于可極大地縮短分類時(shí)間,常用于定性分析。
光譜建模常用的算法有偏最小二乘、最小二乘支持向量機(jī)和人工神經(jīng)網(wǎng)絡(luò)等。3種常見(jiàn)建模方法的對(duì)比分析如表4所示,其中偏最小二乘算法是線性建模算法,通常用于建立光譜數(shù)據(jù)和待測(cè)物質(zhì)之間具有線性相關(guān)的模型;而最小二乘支持向量機(jī)和人工神經(jīng)網(wǎng)絡(luò)算法是非線性建模算法,通常用于建立光譜數(shù)據(jù)和待測(cè)物質(zhì)之間具有非線性關(guān)系的預(yù)測(cè)模型。
表4 3種常見(jiàn)建模方法的對(duì)比分析
偏最小二乘法(partial least squares,PLS)最早于十六世紀(jì)晚期由H.Wold在計(jì)量經(jīng)濟(jì)學(xué)領(lǐng)域提出,是一種最常用的光譜建模方法,從廣義上講,相當(dāng)于主成分分析、多元線性回歸和典型相關(guān)分析的組合,其數(shù)學(xué)基礎(chǔ)為主成分分析,但它比主成分回歸更進(jìn)了一步,主成分回歸只對(duì)自變量矩陣進(jìn)行主成分分解,而偏最小二乘法將因變量矩陣和自變量矩陣同時(shí)進(jìn)行主成分分解[72-73]。PLS算法步驟:1)同時(shí)對(duì)光譜數(shù)據(jù)矩陣和待測(cè)指標(biāo)矩陣進(jìn)行因子分析,提取出相應(yīng)的隱含變量2)將隱含變量按照其對(duì)建模的貢獻(xiàn)率大小進(jìn)行排序3)選擇最優(yōu)個(gè)數(shù)的隱含變量進(jìn)行回歸。
針對(duì)水質(zhì)重要指標(biāo),PLS模型具有較好的效果[74-79]。Song等[80,81]采用建立GA-PLS校正數(shù)學(xué)模型,試驗(yàn)表明,預(yù)測(cè)模型效果穩(wěn)健。楊鵬程等[79]利用紫外光譜技術(shù)結(jié)合偏最小二乘回歸(PLSR)方法,可很好地觀察長(zhǎng)海水中硝酸鹽濃度的變化,對(duì)水質(zhì)進(jìn)行監(jiān)測(cè)。Chen等[82-83]通過(guò)紫外可見(jiàn)光譜技術(shù)建立PLS模型,分別對(duì)水中COD和重金屬離子濃度進(jìn)行分析監(jiān)測(cè),結(jié)果顯示,預(yù)測(cè)值與真實(shí)值之間有極高的相關(guān)性。Dahlén等[84-86]采用PLS模型對(duì)COD、硝酸鹽等多個(gè)水質(zhì)指標(biāo)進(jìn)行同時(shí)測(cè)定。PLS算法能顯著壓縮高維數(shù)據(jù),有效消除變量之間的多重共線性,充分提取因變量矩陣與自變量矩陣中的有效信息,通過(guò)減少光譜數(shù)據(jù)計(jì)算量來(lái)提高模型性能,利用該算法可建立簡(jiǎn)便的光譜預(yù)測(cè)模型。
支持向量機(jī)(support vector machine,SVM)是二十世紀(jì)九十年代興起的一種機(jī)器學(xué)習(xí)方法[87],它遵循結(jié)構(gòu)風(fēng)險(xiǎn)最小化原則,能解決傳統(tǒng)機(jī)器學(xué)習(xí)中在小樣本、非線性等情形下常見(jiàn)的陷入局部最優(yōu)以及過(guò)學(xué)習(xí)等問(wèn)題,對(duì)于非線性建模、解決樣本量偏少和數(shù)據(jù)挖掘領(lǐng)域具有很強(qiáng)的能力。支持向量機(jī)思想:1)線性可分情況,把問(wèn)題轉(zhuǎn)化為一個(gè)凸優(yōu)化問(wèn)題,用拉格朗日乘子法簡(jiǎn)化,然后用既有的算法解決;2)線性不可分,用核函數(shù)將樣本投射到高維空間,使其變成線性可分的情形,利用核函數(shù)來(lái)減少高緯度計(jì)算量。最小二乘支持向量機(jī)(least squares support vector machine ,LS-SVM)是一種經(jīng)過(guò)改進(jìn)的支持向量機(jī)方法,將其約束條件由不等式改為等式,轉(zhuǎn)換為在對(duì)偶空間中對(duì)一個(gè)等式方程組進(jìn)行二次規(guī)劃問(wèn)題的求解,在高維空間里求解最小化損失函數(shù)[88]。
Choi等[89-91]建立了最小二乘支持向量機(jī)法對(duì)水質(zhì)進(jìn)行預(yù)測(cè)。國(guó)內(nèi)外許多學(xué)者對(duì)LS-SVM模型進(jìn)行改進(jìn),以提高水質(zhì)預(yù)測(cè)模型的性能。曹泓等[92-93]對(duì)紫外、紅外多源光譜特征組合建立LS-SVM模型,對(duì)化學(xué)需氧量進(jìn)行定量預(yù)測(cè),良好的預(yù)測(cè)精度。最小二乘支持向量機(jī)可以極大的提高模型的計(jì)算效率,可以發(fā)揮小樣本、泛化能力強(qiáng)等優(yōu)點(diǎn),在保證預(yù)測(cè)準(zhǔn)確的同時(shí),縮短了光譜分析預(yù)測(cè)模型的運(yùn)行時(shí)間。
人工神經(jīng)網(wǎng)絡(luò)(artificial neural network,ANN)是在現(xiàn)代神經(jīng)科學(xué)研究成果的基礎(chǔ)上提出的,是應(yīng)用類似于大腦神經(jīng)突觸聯(lián)接的結(jié)構(gòu)進(jìn)行信息處理的數(shù)學(xué)模型。該算法可以在輸入變量和輸出變量之間建立高度非線性的映射模型,在映射過(guò)程中能夠并行分布處理和自適應(yīng)學(xué)習(xí)。人工神經(jīng)網(wǎng)絡(luò)的種類有很多,包括感知器人工神經(jīng)網(wǎng)絡(luò)、反向傳播人工神經(jīng)網(wǎng)絡(luò)、人工神經(jīng)網(wǎng)絡(luò)和自組織人工神經(jīng)網(wǎng)絡(luò)等,目前在光譜分析和建模中得到廣泛的應(yīng)用。BP神經(jīng)網(wǎng)絡(luò)通常由一個(gè)3層網(wǎng)絡(luò)組成,分別稱為輸出層、隱含層和輸入層。BPNN的輸入層、輸出層和隱含層都是由神經(jīng)元構(gòu)成。信號(hào)從輸入層神經(jīng)元輸入后,傳至隱含層神經(jīng)元,經(jīng)過(guò)隱含層傳遞函數(shù)計(jì)算之后,將輸出的信號(hào)傳遞到輸出層,最終由輸出層得到模型的計(jì)算結(jié)果[94]。在建立BPNN模型的過(guò)程中,通過(guò)將樣本已知的結(jié)果和模型的輸出結(jié)果進(jìn)行對(duì)比,如果輸出結(jié)果的預(yù)測(cè)誤差沒(méi)有滿足設(shè)定的要求,則通過(guò)反復(fù)迭代的方法,直到限定的迭代次數(shù)達(dá)到或者預(yù)測(cè)均方根誤差小于設(shè)定的閾值。
Zakaluk等[95-96]利用人工神經(jīng)網(wǎng)絡(luò)算法來(lái)提高水質(zhì)預(yù)測(cè)精度的方法。分析對(duì)比多種人工神經(jīng)網(wǎng)絡(luò)模型,發(fā)現(xiàn)徑向基函數(shù)神經(jīng)網(wǎng)絡(luò)(RBFNN)和反向傳播人工神經(jīng)網(wǎng)絡(luò)(BP人工神經(jīng)網(wǎng)絡(luò))對(duì)水產(chǎn)養(yǎng)殖水質(zhì)預(yù)測(cè)效果更突出[97]。徑向基函數(shù)神經(jīng)網(wǎng)絡(luò)模型常用于水質(zhì)的定量與定性分析,Xie等[98]將NIR技術(shù)應(yīng)用于水摻入的楊梅汁的監(jiān)測(cè),采用最優(yōu)參數(shù)的RBFNN模型可分離純種楊梅汁樣品。Mesquita等[99-100]提出了一種紫外多波長(zhǎng)與BP神經(jīng)網(wǎng)絡(luò)相結(jié)合的有機(jī)廢水COD預(yù)測(cè)技術(shù),誤差分析數(shù)據(jù)顯示相對(duì)誤差控制在5%以內(nèi)。BP人工神經(jīng)網(wǎng)絡(luò)是目前應(yīng)用最廣泛的人工神經(jīng)網(wǎng)絡(luò)算法,Ji等[101]建立了一種利用BP神經(jīng)網(wǎng)絡(luò)和動(dòng)力學(xué)分光光度法同時(shí)測(cè)定自來(lái)水中鐵和鎂的分析方法。BP神經(jīng)網(wǎng)絡(luò)模型常用于對(duì)在水產(chǎn)養(yǎng)殖水質(zhì)的監(jiān)測(cè)與預(yù)警[102-103],Qu等[104]開(kāi)發(fā)一種可見(jiàn)的近紅外成像技術(shù),建立了BP人工神經(jīng)網(wǎng)絡(luò)模型,結(jié)果證明,該模型可快速預(yù)測(cè)在水產(chǎn)養(yǎng)殖環(huán)境中腐植酸鈉的含量,進(jìn)一步實(shí)時(shí)監(jiān)控水產(chǎn)養(yǎng)殖水的質(zhì)量。人工神經(jīng)網(wǎng)絡(luò)有自學(xué)習(xí)、高容錯(cuò)和高度非線性描述能、高速尋找優(yōu)化解的能力等優(yōu)點(diǎn),避免了光譜分析模型計(jì)算量大、計(jì)算速度慢等問(wèn)題。目前,相較于其它人工神經(jīng)網(wǎng)絡(luò),BP神經(jīng)網(wǎng)絡(luò)是應(yīng)用最廣泛的水質(zhì)預(yù)測(cè)建模方法。
基于光譜技術(shù)水質(zhì)預(yù)測(cè)模型對(duì)比如表5所示,通過(guò)分析可知,對(duì)于小樣本數(shù)據(jù)偏最小二乘算法預(yù)測(cè)效果最好,偏最小二乘支持向量機(jī)算法經(jīng)過(guò)改進(jìn),預(yù)測(cè)效果明顯增強(qiáng)。
表5 基于光譜技術(shù)水質(zhì)預(yù)測(cè)模型對(duì)比
注:PCA-PSO-ELM(principal component analysis-particle swarm optimization-extreme learning machine)是基于主成分分析聯(lián)合粒子群優(yōu)化極限學(xué)習(xí)機(jī)預(yù)測(cè)模型,PCA-PSO-LS-SVM(principal component analysis-particle swarm optimization-least squares support vector machine)是基于主成分分析聯(lián)合粒子群優(yōu)化最小二乘支持向量機(jī)預(yù)測(cè)模型,NMF-PSO-LS-SVM(non-negative matrix factorization- particle swarm optimization-least squares support vector machine)是基于非負(fù)矩陣分解聯(lián)合粒子群優(yōu)化最小支持向量機(jī)預(yù)測(cè)模型,RMSEP(root-mean-square error of prediction)是預(yù)測(cè)誤差均方根
基于光譜技術(shù)的水質(zhì)監(jiān)測(cè)突破了傳統(tǒng)檢測(cè)方法的操作復(fù)雜、不可重復(fù)、易造成附加污染等局限,成為了水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)的重要方法。
1)目前,隨著食品質(zhì)量安全問(wèn)題的日益突出以及水產(chǎn)養(yǎng)殖水質(zhì)污染頻繁發(fā)生,迫切地需要構(gòu)建一種在線、實(shí)時(shí)的水質(zhì)監(jiān)測(cè)系統(tǒng),實(shí)現(xiàn)對(duì)水質(zhì)異常狀況進(jìn)行預(yù)警?,F(xiàn)階段的水質(zhì)檢測(cè)往往需要結(jié)合一些實(shí)驗(yàn)室處理方法,如化學(xué)分析法等,在做檢測(cè)結(jié)果之前,已經(jīng)消耗了一定的時(shí)間,因此水質(zhì)檢測(cè)無(wú)法做到實(shí)時(shí)在線進(jìn)行。將光譜技術(shù)與實(shí)時(shí)在線監(jiān)測(cè)技術(shù)相結(jié)合,實(shí)現(xiàn)對(duì)水產(chǎn)養(yǎng)殖水質(zhì)進(jìn)行實(shí)時(shí)在線監(jiān)測(cè)和預(yù)警,將對(duì)水質(zhì)監(jiān)測(cè)領(lǐng)域具有更大的實(shí)際意義。
2)多源光譜融合的水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)將會(huì)成為新的發(fā)展方向?,F(xiàn)階段的水質(zhì)監(jiān)測(cè)多采用單一光譜,無(wú)法達(dá)到較高的監(jiān)測(cè)精度。而將信息融合技術(shù)應(yīng)用于光譜領(lǐng)域,融合存在一定的相關(guān)性和互補(bǔ)性的不同光譜,可提高預(yù)測(cè)模型的分析精度和魯棒性。
3)利用光譜技術(shù)對(duì)水質(zhì)多參數(shù)監(jiān)測(cè),是今后水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)研究的發(fā)展方向。由于水中懸浮物對(duì)不同波長(zhǎng)可見(jiàn)光的散射存在非線性關(guān)系,且水中懸浮物對(duì)影響水質(zhì)參數(shù)的部分有機(jī)物存在吸附,導(dǎo)致單一可見(jiàn)光波長(zhǎng)的濁度補(bǔ)償方法無(wú)法準(zhǔn)確地扣除濁度引起的散射干擾。因此,研究一種抵消濁度干擾,對(duì)測(cè)量光譜進(jìn)行有效地校正的方法成為水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)的關(guān)鍵技術(shù)問(wèn)題。
4)對(duì)于光譜數(shù)據(jù)的處理,將多種數(shù)據(jù)處理算法相結(jié)合,仍將占據(jù)主導(dǎo)。目前常見(jiàn)的數(shù)據(jù)處理方法是以2種或2種以上的算法融合的數(shù)據(jù)處理方法為主,在今后較長(zhǎng)一段時(shí)間內(nèi),這種方法仍會(huì)占據(jù)主導(dǎo)。常見(jiàn)的如蒙特卡羅方法結(jié)合連續(xù)投影算法(CARS-SPA)預(yù)處理算法,無(wú)信息變量消除算法結(jié)合連續(xù)投影算法(UVE-SPA)特征波段提取算法,等。將多種數(shù)據(jù)處理算法相結(jié)合,對(duì)傳統(tǒng)算法進(jìn)行改進(jìn),能夠更好地發(fā)揮這些算法的優(yōu)勢(shì),以實(shí)現(xiàn)精確、快速地提取水質(zhì)參數(shù)有效的光譜信息。
5)非線性數(shù)據(jù)建模,將成為光譜技術(shù)應(yīng)用于水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)的主流建模發(fā)方法。水環(huán)境是一個(gè)無(wú)序的、非穩(wěn)定的、非平衡的隨機(jī)系統(tǒng),不同元素之間往往存在著隨機(jī)性、協(xié)同現(xiàn)象和相干效應(yīng),非線性建模算法可增加監(jiān)測(cè)的準(zhǔn)確性、快速性、魯棒性。
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Review and trend of water quality detection in aquaculture by spectroscopy technique
Li Xinxing1,2, Zhu Chenguang1, Zhou Jing1, Sun Longqing1, Cao Xiamin3, Zhang Xiaoshuan2,4※
(1.100083,; 2.100083,; 3.215200,; 4.100083,)
The water quality of aquaculture is a key factor concerning the economic benefits of aquaculture and the quality of aquatic products. In recent years, with the development of economy, the discharge of industrial wastewater and domestic sewage has greatly increased, resulting in environmental pollution, for example, the water quality of aquaculture ponds has been polluted. In order to achieve the goal of high yield and safe breeding at the same time of environmental protection and energy conservation, scholars have paid attention to the rapid and accurate acquisition of aquaculture water quality information, which was the important research content of the smart agriculture and agricultural Internet of Things. Water quality monitoring technology based on spectral analysis is an important development direction of aquaculture water quality monitoring. Compared with traditional chemical analysis, electrochemical analysis and chromatographic analysis methods, spectral analysis technology is more simple and convenient, consumes a small quantity of reagents, and is reproducible. This article summarizes and sorts the existing domestic and foreign research literatures, and systematically analyzes and discusses the important parameters of water quality monitoring, data preprocessing methods, feature band extraction, and detection model algorithms based on spectroscopy. This article reviews the COD (chemical oxygen demand) water quality monitoring methods, total nitrogen water quality monitoring methods, total phosphorus water quality monitoring methods, heavy metal water quality monitoring methods, covering traditional chemical methods and spectral analysis methods of these parameters. This article compares and analyzes the spectral method and the traditional methods. We find that compared with the traditional water quality monitoring methods, the spectral technology is non-invasive, rapid rapid monitoring, repeatable and accurate. The sensitive spectral bands of the above parameters are summarized. The data preprocessing algorithm includes Savitzky-Golay smoothing, wavelet analysis, and multivariate scatter correction, the feature band extraction algorithm includes continuous projection algorithm, no-information variable elimination algorithm, and principal component analysis, and the model includes partial least squares algorithm, least squares algorithm, and artificial neural network. The advantages, disadvantages and scopes of application of these algorithms are summarized and compared. The spectrum detection process of these algorithms is analyzed. Among them, a detailed review of the application of model algorithms in water quality monitoring is conducted, and the prediction results of each water quality prediction model algorithm are statistically analyzed. The results show that online aquaculture water quality testing will be the focus of research. Multi-parameter monitoring is the development direction of aquaculture water quality monitoring. For the processing of spectral data, the combination of multiple data processing algorithms will still dominate. Nonlinear modeling will become the mainstream method for water quality data analysis of aquaculture and will become the mainstream method for the application of spectral technology to water quality detection of aquaculture.
spectroscopy; aquaculture; water quality; monitoring model
10.11975/j.issn.1002-6819.2018.19.024
S959
A
1002-6819(2018)-19-0184-11
2018-05-04
2018-09-03
國(guó)家重點(diǎn)研發(fā)計(jì)劃項(xiàng)目(2017YFE0111200);農(nóng)村領(lǐng)域國(guó)家科技計(jì)劃資助項(xiàng)目(2015BAD7B-5)
李鑫星,副教授,主要研究方向?yàn)檗r(nóng)業(yè)系統(tǒng)與知識(shí)工程。 Email: lxxcau@cau.edu.cn
張小栓,教授,主要研究方向?yàn)檗r(nóng)業(yè)經(jīng)濟(jì)和信息系統(tǒng)工程。Email: zhxshuan@cau.edu.cn
李鑫星,朱晨光,周 婧,孫龍清,曹霞敏,張小栓. 光譜技術(shù)在水產(chǎn)養(yǎng)殖水質(zhì)監(jiān)測(cè)中的應(yīng)用進(jìn)展及趨勢(shì)[J]. 農(nóng)業(yè)工程學(xué)報(bào),2018,34(19):184-194. doi:10.11975/j.issn.1002-6819.2018.19.024 http://www.tcsae.org
Li Xinxing, Zhu Chenguang, Zhou Jing, Sun Longqing, Cao Xiamin, Zhang Xiaoshuan. Review and trend of water quality detection in aquaculture by spectroscopy technique[J]. Transactions of the Chinese Society of Agricultural Engineering (Transactions of the CSAE), 2018, 34(19): 184-194. (in Chinese with English abstract) doi:10.11975/j.issn.1002-6819.2018.19.024 http://www.tcsae.org